Low-dimensional Materials and Applications 9783110424751, 9783110430004

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Ying Jia, Guogen Xu, Xuanjun Wang Low-dimensional Materials and Applications

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Functional Materials. For Energy, Sustainable Development and Biomedical Sciences Leclerc, Gauvin (Eds.), 2014 ISBN 978-3-11-030781-8, e-ISBN 978-3-11-030782-5

Ying Jia, Guogen Xu, Xuanjun Wang

Low-dimensional Materials and Applications

Translated by Zhiyong Huang, Wenhui Dou, Shiqiang Jiang, Bo Li

Authors Professor Ying Jia Xi’an Reserach Institute of High Technology Section 603 Xi’an 710025 Shannxi People’s Republic of China [email protected] Associate Professor Guogen Xu Xi’an Reserach Institute of High Technology Section 603 Xi’an 710025 Shannxi People’s Republic of China [email protected] Professor Xuanjun Wang Xi’an Reserach Institute of High Technology Section 603 Xi’an 710025 Shannxi People’s Republic of China [email protected]

ISBN 978-3-11-043000-4 e-ISBN (PDF) 978-3-11-042475-1 e-ISBN (EPUB) 978-3-11-042485-0 Set-ISBN 978-3-11-042476-8 Library of Congress Cataloging-in-Publication Data A CIP catalog record for this book has been applied for at the Library of Congress. Bibliographic information published by the Deutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at http://dnb.dnb.de. © 2018 National Defense Industry Press and Walter de Gruyter GmbH, Beijing/Berlin/Boston Typesetting: Integra Software Services Pvt. Ltd. Printing and binding: CPI books GmbH, Leck Cover image: PragasitLalao/iStock/Getty Images @ Printed on acid-free paper Printed in Germany www.degruyter.com

Preface Lightweight carbon materials refer to the low-density carbon material with rich pore structure and large specific surface area, mainly referring to the activated carbon, graphite (including expanded graphite), carbon nanotubes, carbon fiber, organic polymer fabrics, and the composite materials based on these carbons. Lightweight carbon materials include not only the traditional materials of long history such as activated carbon, graphite, and other new materials, but also some other materials that are better than the traditional materials with excellent and special properties, such as carbon nanotubes and carbon fiber. Because of the large specific surface area, strong adsorption capacity, good chemical stability, high catalytic activity, and easy and excellent repeated application, they are widely applied in the fields of industry, agriculture, national defense, transportation, medicine, health and environmental protection, and so on. Their demand is increasing every year with the development of society and the improvement of people’s standard of living. Lightweight carbon materials are of great importance in the material field and play an outstanding role in the development of modern society. Material, energy, and information have become the three pillars of modern science and technology, and material science is the basis and guide of the social technological progress. The development of modern high technology closely depends on material development. A new material breakthrough is pregnant with the birth of a new technology, and even leads to a revolution in the field of technology. At present, material, energy and information are three columns of modern science and technology and the material science is the basis and guidance of technological improvement. In the twenty-first century, the level of development of material science has become an important symbol in measuring a country’s level of science, the national economy level, and the comprehensive national strength. Many countries put the research and development of new materials in the priority development position. With the development of science and technology, people’s demands toward the material properties are increasingly widespread and slashing, making people continue to develop all kinds of new materials. Because lightweight carbon materials have excellent properties and easily modified peculiarity, they can form composite materials of different properties by combining with other materials, which make the composite materials attract more and more interests and much more widely researched. To enable readers to understand more about the lightweight carbon materials and composite materials, authors collected and summarized lightweight carbon materials and their composite material data and our research results in recent years in the light of carbon materials and composite materials, to complete this book, so that readers can get some beneficial enlightenment in technology innovation. This book is a summary of the research results of the authors. In consulting some domestic and foreign literature data about the carbon materials in treatment of pollutants, electromagnetic wave and electromagnetic shielding of stealth and poison DOI 10.1515/9783110424751-202

VI

Preface

protection, combined with some results we got in these fields, this book systematically introduces the photocatalysts or absorbents for the special pollutants, electromagnetic wave absorbing materials, liquid propellant protection fabrics, the preparation, function and modification of electromagnetic shielding fabrics and other materials, electromagnetic stealth materials, the national defense environment protection, the electromagnetic protection, and the liquid propellant protection. Hope that this book will communicate with the readers about the external research results, the dynamic of lightweight carbon materials domestically and abroad, and the results that the authors have been studying for many years, offer the technology reference for the workers, teachers, and students who are engaged in the lightweight carbon materials and composite materials research and application or work for national defense, in order to further improve the basic research and the application of lightweight carbon materials. Because the lightweight carbon materials and composite materials are interdisciplinary, and new achievements and applications in this field appear continuously, the content of this book is just a small part. There must be some inevitable mistakes in the book due the confined level of the authors, so we sincerely hope to get your comments. The publication of the book is supported by the National Defense Industry Press, and gets a number of colleagues’ help, postgraduate Liang Liang, Ren Qiangfu, Feng Cheng, Zhang Ying, Xu Hu, Zhu Guangwei, Yang Yunling and Gou Xiaoli finished writing part of experimental work and content, also expressing sincere thanks! The authors

Contents 1 1.1 1.1.1 1.1.2 1.1.3 1.1.4 1.2 1.2.1 1.2.2 1.2.3 1.2.4 1.3 1.3.1 1.3.2 1.3.3 1.4 1.4.1 1.4.2 1.4.3 1.4.4 1.5 1.5.1 1.5.2 1.5.3 1.6 1.6.1 1.6.2 1.6.3 1.6.4 1.6.5

Basic properties of lightweight carbon materials 1 Activated Carbon 1 Physical Structure and Classification 1 Chemical Properties and Functions 6 The Main Detection Indexes of the AC Properties 9 The Use of Activated Carbon 11 Expanded Graphite 13 Brief Introduction to Graphite 14 Graphite Intercalation Compounds (GIC) 15 Expanded Graphite 16 Application of Expanded Graphite and Its Composites Activated Carbon Fiber 19 Structure of the AC Fiber 20 Performance of the Activated Carbon Fiber 21 Application of the Activated Carbon Fiber 22 Carbon Nanotubes 24 Structure and Morphology of Carbon Nanotubes 25 Preparation of Carbon Nanotubes 26 Modification of Carbon Nanotubes 27 Application of Carbon Nanotubes 29 Graphene 32 Structure of Graphene 32 Preparation of GE 33 Performance and Application of Graphene 36 Adsorption Theory 39 The Mass Transfer Process of Adsorption 40 Physical Adsorption and Chemical Adsorption 41 Adsorption Isotherms 41 Factors Affecting the Adsorption 44 Adsorption Equilibrium and Kinetic Theory 45 References 50

2

Preparation and properties test for lightweight carbon composite materials 52 Preparation of Lightweight Carbon Composite Material 52 Direct Packing Method 52 Sol–Gel Method 54 Electroless Plating 57 Precipitation Method 66 Micro-emulsion Method 67

2.1 2.1.1 2.1.2 2.1.3 2.1.4 2.1.5

17

VIII

2.1.6 2.2 2.2.1 2.2.2 2.2.3 3 3.1 3.1.1 3.1.2 3.1.3 3.1.4 3.1.5 3.1.6 3.2 3.2.1 3.2.2 3.2.3 3.2.4 3.3 3.3.1 3.3.2 3.3.3 3.3.4 3.4 3.4.1 3.4.2

Contents

Chemical Vapor Deposition 67 Characterization Technology of Composite Material Electron Microscopy 68 Thermal Analysis Technology 70 Spectroscopic Technology 72 References 74

68

Application of lightweight carbon material and its composite in national defense environmental protection 75 Disposal of Unsymmetrical Dimethylhydrazine Wastewater with TiO2 /porous Carbon and Its Composite Materials 75 Preparation of TiO2 /porous Carbon Composite Material 77 Preparation of the Composite Photocatalyst 78 Characterization of Composite Photocatalyst 79 UDMH Wastewater Disposal Effect 84 Analysis of Factors Affecting Disposal Effect of Composite Photocatalysts 87 Research on the Degradation Dynamic of UDMH 97 The Degradation of TNT by TiO2 /ACF Composite Photocatalyst 99 Preparation and Characterization of the Loaded Titanium Dioxide 102 The Influence Factors of the Degradation of Composite Photocatalyst on TNT Wastewater 103 The Function of Fenton Reagent for the Photocatalytic Degradation of TNT with ACF/TiO2 108 The Photocatalytical Degradation of TNT Wastewater with the Modified ACF/TiO2 111 The Study on the Decolorization and Adsorption of Printing and Dyeing Wastewater by the Expanded Graphite 116 Preparation of the Dye Standard Solution and the Expanded Graphite 118 The Decolorizing Effect of the Expanded Graphite 120 The Exploration of the Decolorizing Mechanism of the Expanded Graphite 131 The Fractal Analysis of the Adsorption of the Expanded Graphite for Dye 134 Study on the Adsorption Performance of the Unsymmetric Dimethyl Hydrazine (UDMH) with the Multi-Walled Carbon Nanotubes 138 The Adsorption Performance of Carbon Nanotubes on the Unsymmetric Dimethyl Hydrazine Solution 140 The Impact of the Carbon Nanotubes on the UDMH Adsorption 144

Contents

3.4.3 3.4.4

3.5 3.5.1 3.5.2 3.5.3 3.6 3.6.1 3.6.2 3.6.3

4 4.1 4.1.1 4.1.2 4.1.3 4.2

4.2.1

4.2.2

4.2.3 4.2.4

IX

The Adsorption Performance of UDMH by the Modified Carbon Nanotubes 145 The Dynamic Adsorption and the Desorption Properties of the Modified Multi-Walled Carbon Nanotubes on UDMH 156 Study on the Adsorption Performance of UDMH with Activated Carbon and Modified Activated Carbon 159 The Decoloring Efficiency of Different Types of Activated Carbon 160 Surface Modification of the Activated Carbon 161 The Adsorption Performance of the Modified Activated Carbon 164 The Adsorption Research of UDMH with the Activated Carbon Fiber (ACF) 173 The Determination of Adsorption Isotherm 173 The Calculation of Adsorption Thermodynamics Function 175 The Adsorption Law of UDMH with Activated Carbon Fiber 177 References 177 Application of lightweight carbon material and composite material in electromagnetic wave absorbing material 180 Expanded Graphite/Fe/Co/Ni Composite Materials and Their Absorbing Performance 182 Production of Expanded Graphite/Fe/Co/Ni Composite Materials 182 Characterization of Composite Materials 185 Electromagnetic Spectrum Analysis on Magnetic Metal-Expanded Graphite Composite 193 Expanded Graphite/Carbon Fiber/ Electroconductive and Magnetic-permeable Metal Composite Materials 205 and Their Absorbing Performance Fabrication of Expanded Graphite/Carbon Fiber/ Electroconductive and Magnetic-permeable Metal Composite Materials 206 Structure Characterization of Expanded Graphite/Carbon Fiber/Electroconductive and Magnetic-permeable Metal Composite Materials 207 Characterization of Ag/Magnetic Metal/EG Composite Material’s Magnetic Property 211 Electromagnetic Spectrum of Ag/Magnetic Metal/EG Composite Material 213

X

4.3 4.3.1 4.3.2 4.3.3

4.3.4 4.3.5

4.3.6

4.4 4.4.1 4.4.2 4.4.3 4.5 4.5.1 4.5.2 4.5.3 4.5.4 4.6 4.6.1 4.6.2 4.6.3 4.7 4.7.1 4.7.2 4.7.3

Contents

Chemical Plating Polyaniline/Expanded Graphite Metal Composite 221 Materials and Their Absorbing Performance Preparation of Polyaniline 222 Preparation of Polyaniline/Expanded Graphite Composite Materials 223 Structure and Performance Characterization of Polyaniline and Polyanilin/Expanded Graphite Composite Materials 223 Influencing Factors of Polyaniline Conductivity 226 Influencing Factors of the Conductivity of Polyaniline/Expanded Graphite Composite Materials 229 Electromagnetic Spectrum of Series Metal/Polyaniline/Expanded Graphite Composite Materials 231 Absorbing Coating Preparation and Its Microwave Absorbing Properties 238 Preparation of Absorbing Coating Material 239 The Coating of Paint 241 Microwave Absorbing Performance of Composite Absorbing Coating 242 Ultrathin Band-gap Absorbing Structure Based on Carbon Matrix Composite Material 244 Introduction EBG Structure and Its Application in Stealth Technology 244 Absorbing Theory of EBG Structure 247 Preparation of Carbon Matrix Composite Material 251 Composites of EBG Structure Design and Simulation 267 Preparation of Carbon Nanotube/Transition Metal and Oxide Composite Materials 278 Preparation of Carbon Nanotube/ZnO Composites 279 Preparation of CNTs/Cu Composites 281 Preparation and Characterization of CNTs/TiO2 Composite Materials 283 Carbon Nanotubes/Magnetic Metal Composite Materials 284 Preparation of Laboratory Sample 284 Result Characterization 285 Optimization of Wave-Absorbing Performance 285 References 290

Contents

5 5.1 5.1.1 5.1.2 5.1.3 5.1.4 5.2 5.2.1 5.2.2 5.2.3 5.2.4 5.2.5 5.3 5.3.1 5.3.2 5.3.3 5.3.4 5.3.5 6 6.1 6.2 6.3 6.4

Index

XI

Application of lightweight carbon materials and composites in protective materials 294 Electromagnetic Pollution and Fabrics of Carbonaceous Electromagnetic Shielding 294 Generation and Hazard of Electromagnetic Pollution 294 Materials of Lightweight Carbonaceous Electromagnetic Shielding 295 Electromagnetic Shielding Mechanism of the AntiElectromagnetic Radiation Materials 297 Types of Electromagnetic Fabrics 300 Study on the Preparation and Electromagnetic Shielding Performance of Carbon Fiber-Based Magnetic Composites 301 Technological Process of Plating Ni-Co-Fe-P 302 Solution of Ni/Co/Fe Chemical Plating 303 Characterization and Properties of Chemical Plating Ni-Co-Fe-P Alloy Coating on Carbon Fiber 304 Study on the Effect of RE Elements Ce and La on the Surface Modification of Carbon Fiber Electroless Plating 311 The Optimization of the Chemical Plating Technology of Ni-Fe-Co-P by BP Neural Network 322 Application of Light Carbon Materials in Liquid Propellant Protective Clothing Fabrics 328 Technique of Toxic Chemicals Proof 328 Research on Activated Carbon Fibers Affecting on UDMH Vapor Penetration Performance 330 Prediction of Antitoxin Time Based on the Artificial Neural Network Theory 335 The Regeneration of the Activated Carbon Fiber’s Activity 337 New Liquid Propellant Protective Clothing Fabrics 339 References 341 345 Future development Activated Technique and Modified Technique of Lightweight Carbon Materials 345 Composite Technology of Lightweight Carbon Materials 347 Develop New Lightweight Carbon Materials 347 Expand the Application of Lightweight Carbon Material 350 References 352 353

1 Basic properties of lightweight carbon materials Lightweight carbon is a kind of carbonaceous material with abundant pore structures and a large surface area, mainly referring to the activated carbon (AC), graphite, carbon nanotubes (graphite), AC fiber, organic polymer fabric, and so on. Lightweight carbon material possesses the following characteristics: strong adsorption capacity, good chemical stability, and high mechanical strength; they can be easily modified or combined with other materials to form different properties. Therefore, it has wide use in industrial applications, agriculture, national defense, transportation, medicine and health, and environmental protection. The demand shows a rising trend year by year with the development of society and the improvement of people’s living standard, and the research content and scope are also becoming increasingly widespread.

1.1 Activated Carbon AC is a carbon material with the developed pore structure, big specific surface area and the strong selective adsorption. It is made through the carbonization and activation process, taking charcoal, sawdust, various fruit shells and petroleum as the raw materials. As a noncrystalline material, AC is composed of micro graphite crystalline and some hydrocarbons.

1.1.1 Physical Structure and Classification 1.1.1.1 Basic Crystal and Pore Structure of Activated Carbon AC is usually considered as amorphous carbon and is thought to belong to the class of microcrystalline carbon [1–7]. Its structure is more complex. Neither is it like the molecular structure of graphite and diamond carbon, whose atoms are arranged according to a certain rule, nor is it similar to the normal carbon-containing material, which has a large complex molecular structure. X-ray diffraction analysis shows that the structure of AC contains microcrystalline graphite, the particles size of the crystallization is 1–3 nm; its network structure is similar to graphite, but is different from graphite in some degree such as in lamellar size, level in the improvement of the hexagonal arrangement of carbon atoms in degree, the degree of complication, layer spacing, and other aspects. Except the microcrystalline graphite, AC also contains 1–3 amorphous carbon and heteroatom. The multiphase material constituted by graphite and amorphous carbon determines the unique structure of AC. DOI 10.1515/9783110424751-001

2

1 Basic properties of lightweight carbon materials

At present, based on X-ray diffraction analysis, two microcrystal structures of AC were found. One is the microcrystal carbon, which is the class of crystallite carbon graphite structure. Its size varies with the carbonization temperature, and it is constituted by three parallel graphite layers; its width is about nine times that of a hexagonal carbon. Compared with the graphite, the arrange plane net in the microcrystal is disorder, known as the “turbostratic structure”. The comparison with the structure of graphite is shown in Figure 1.1. Another kind of microcrystal carbon is formed due to the difference of the axis direction among the graphite structure, disorder of the spacing between the surface nets, or the interlayer distortion of graphite. It may be stabilized by the invasion of the impure atoms (such as oxygen, nitrogen) and the disordered structural topology is formed by the space crosslink of the carbon hexahedron. Most of the carbon materials (including AC) contain these two structures, and the characteristics of AC depend on its structure. AC is rich in pore structure, which brings about the huge specific surface area for AC to adsorb gas and liquid molecules. Therefore, the pore structure plays a very important role in the adsorption properties of AC. The preparation of AC comes from different raw materials, different carbonization and activation process and methods, so the formation of the pore shape, size and distribution are also different. Generally, the pores of AC can be divided into large pore hole (more than 50 nm), the middle hole (or transition pore, between 2 and 50 nm), and the microhole (less than 2 nm). All these different pore sizes are interconnected, like a tree structure, as shown in Figure 1.2. Researches showed that the micropore of AC is a gap of a molecule size between the bent and deformed aromatic layers or bands in the minor structure of AC. Some pores have narrow entrance, some are open at both ends or one-end closed capillary pores, and some have regular more or less slit and V hole between two planes. Because there isn’t much large pores on AC, they are taken as channels of the adsorbed solute molecules into the adsorption sites in the process of adsorption, and control the adsorption speed. The function of transition hole is not pure, similar to

(a)

(b)

Figure 1.1: The structure comparison of (a) graphite and (b) activated carbon.

1.1 Activated Carbon

3

Pore

Figure 1.2: Pore structure of activated

carbon.

the large holes in many cases, dominating the adsorption speed. At the same time, it also plays a role in the adsorption of large molecules, which can’t enter the pore. Most of the adsorption effect of AC is decided by the micropore of AC. It should be noted that the adsorption process depends not only on the pore structure but also on the adsorbate and the interaction between the adsorbate and adsorbent. Both the pore structure and the pore shape have a great effect on the adsorption; there are several micropores in the pore structure: open hole, half-closed hole, and mesenchymal cage, as shown in Figure 1.3. (a) Slit Wedge (b) Slit

Wedge

(c)

— Microcrystalline structure; . sp3bond;

Interspace

Figure 1.3: The possible micropore models in the microcrystalline graphite structure. (a) Open hole, (b) Half-closed hole, (c) Mesenchymal cage.

4

1 Basic properties of lightweight carbon materials

Because of the special structure of carbons, the carbon adsorbent pore has slit-type characteristics, which are significantly different from other types of adsorption agent. There are four main types of adsorption state according to the relationship between the molecular scale and adsorbent: (1) When the molecular scale is bigger than the micropore diameter, due to the molecular sieve, the molecules can’t enter the pore, so the micropore has no adsorption; (2) When the molecular scale is approximate to the micropore diameter, the molecular diameter is equal to the pore diameter, and the ability of the adsorbent to capture the adsorption of molecules is very strong, so it is very suitable for the adsorption of low concentration; (3) When the molecular scale is smaller than the micropore diameter, the adsorbate molecules have capillary condensation in the micropore and the adsorption capacity is large; and (4) When the molecular scale is smaller than the micropore diameter, the adsorption of molecules are prone to undergo desorption, with fast desorption speed but low concentration of adsorption capacity. 1.1.1.2 Determine of the Appropriate Aperture There is a matching problem of the adsorption of adsorbent and adsorbate molecular geometry aperture size in the operation: the size is too large, the force field of the hole wall decreases, which will reduce the force on the adsorbate molecules and the performance of adsorption and separation; the size is too small, the adsorption benefit of the force field of the hole wall to the adsorbed molecules does not reach the maximum value, which will reduce the adsorption capacity of the adsorbate molecule, so the AC adsorption of a substance should have the best aperture, and there is an optimum aperture corresponding to different pressure, and the liquid phase adsorption and gas phase adsorption are also different. Therefore, different adsorbents have different best sizes, and the modification methods are the best means to produce different aperture.

1.1.1.3 Modification and Control of Aperture (1) The preparation of AC with different pore sizes: A series of ACs with different average pore size, the average pore size of which ranges from 0.69 to 0.81 nm, and pore capacity of 0.4 mL/g, can be prepared with carbon raw material processed with the oxidizing composite in different activation environment of KOH and NaOH. P. J. M. Carrott prepared a series of AC by impregnation activation method with cork as the raw material, and KOH, NaOH, and H3 PO4 as the activating agents. The effects on the size and distribution of pore by activating agent concentration,

1.1 Activated Carbon

5

dimension of the raw material, scale agent material ratio, and activation temperature were researched. The experimental results showed that different pore structure ACs, the average pore size distribution of which are 0.7–2.2 nm and pore volume of 0.5– 0.7 cm3 /g, can be prepared by controlling the impregnation conditions and the degree of activation. P. J. M. Carrott prepared a series of ACs, the average pore sizes of which are from 0.7 nm to 1.6 nm and pore volume, less than 0.64 cm3 /g, by using the same cork material through the combination of physical and chemical activation methods. (2) Aperture shrinkage technology: In addition to using the selection of raw material, activation medium, activation temperature, activation time, and reaction conditions to adjust the aperture to control the pore size of AC, we can also control the pore size of AC through the subsequent processing. The aperture shrinkage technology mainly includes thermal shrinkage and carbon deposition. High temperature treating process can reduce the heteroatoms (oxygen, hydrogen), break the bond between oxygen and the wafer of aromatic heteroatom to form larger aromatic wafer and decrease the hole spacing of graphite microcrystalline layer. On the other hand, the crystallite carbon level tends to be regularized, leading to the shrinkage of the layers’ hole, which is significant for the adsorption of small molecules. Commercial AC was treated thermally at different temperatures by someone, getting the AC with different pore size. The aperture of the original AC is 0.63 nm, and the aperture is reduced to 0.61 nm after treatment at 950∘ C. Song Yan et al. made a secondary carbonization processing in the inert gas environment to the superactivated carbon produced by taking the petroleum coke as raw material and KOH as the activating agent, and investigated the adsorption behavior of methane on the AC under different pressures and the pore structure change of superactivated carbon before and after treatment. The results showed that the BET surface area and pore volume of the AC decreased after the secondary carbonization treatment at 1200∘ C and the aperture distribution narrowed from 0.64 to 0.6 nm. Qiao Zhijun et al. studied the effect of modification treatment on the adsorption properties, the aperture distribution, the pore structure, and surface chemistry of the asphalt base AC fiber modified at 900∘ C. The experimental results showed that the micropore on the AC fiber larger than 1.0 nm significantly decreased and the aperture distributed in the scope of 0.5–1.0 nm. X-ray diffraction analysis showed that the AC fiber was a turbostratic graphite structure and the thermal treatment decreased spacing of carbon laminating of the activated carbon fiber graphite microcrystal to result in the aperture decrease. Carbon deposition includes gas carbon deposition (CVD) of pyrolytic carbon deposition and liquid phase impregnation. The AC or carbide is thermally treated in the hydrocarbon gas containing benzene to release the thermal decomposing carbon to decrease the aperture by the decomposition of hydrocarbon gas. Asphalt or resin is added to the AC or carbide before the thermal treatment, and then the micropore is coated with the thermal decomposing carbon to decrease the aperture. At present, the gas carbon deposition has more applications.

6

1 Basic properties of lightweight carbon materials

(3) Aperture expansion technology: The expansion of aperture for AC is actually to eliminate the carbon atoms or other atoms on the surface of the AC pore by oxidization or heating to increase the aperture. There are two methods: cyclic oxidation and oxidation and high temperature composite modification. The cyclic oxidation mainly uses oxygen. The oxygen cycle oxidation is originally “one-step cyclic oxidation.” This method is influenced by the molecular oxygen diffusion resistance, and the pore size distribution of AC is so wide that the performance of AC is decreased. Therefore, the “two-step cyclic oxidation” is also used. With oxygen as the activating agent, oxygen is first adsorbed chemically at 200∘ C, then the AC is thermally treated in the N2 atmosphere at 900∘ C. The modified AC sample with narrow aperture distribution is prepared through the cyclic oxidation, through which the average aperture can be increased by 0.1 nm every time. Then, the AC is treated with cyclic oxidation by NaOCl, through which the aperture can be increased by 0.1 nm every time. Some strong oxidizing agents, such as HNO3 , H2 O2 , and (NH4 )2 S2 O8 , can also be used to expand the aperture distribution of the AC. The specific surface area of the AC modified with nitric acid decreases slightly, while the other two significantly increase; the apertures increase in different degrees. The reason may be that nitric acid has the greatest degree of modification, and the aperture is obviously expanded; the surface modification with hydrogen peroxide is the minimum and the specific surface area increases in the maximum degree, but the aperture changes little. The degree of modification of (NH4 )2 S2 O8 is between hydrogen peroxide and nitric acid. In addition, after oxidation of the AC by HNO3 and H2 O2 , the surface of the AC will generate carboxyl acid groups and they can be removed in H2 , N2 and other inert gas at a high temperature (higher than 700∘ C), to achieve the purpose of expanding the hole. 1.1.1.4 Classification of AC According to the shape, manufacturing method, aperture, and functions, AC can be divided into different types. According to the raw materials, AC can be divided into wood, coal, petroleum coke, and resin AC; according to the morphology, AC can be divided into granular AC and AC powder, and granular AC can also be divided into two categories of amorphous and figurate; according to the manufacturing methods, AC can be divided into chemical, physical and the physical and chemical method of AC; according to the applications, the AC can be divided into gas adsorption, catalysis adsorption and adsorption liquid AC. Different AC has different raw material and different methods of preparation.

1.1.2 Chemical Properties and Functions 1.1.2.1 Surface Functional Groups The main component of carbon material is carbon, which is nonpolar itself, but the nature of material will change with the difference of production process and

7

1.1 Activated Carbon

use environment. The carbon surface can be easily oxidized by oxygen, water, and other oxidants, and thus more or less generates surface functional groups, Which will generate the diversity of the interface chemical properties of carbon materials. The functional groups can be measured through the methods of organic chemistry. In general, functional groups in carbon materials are mainly carboxyl, lactone carboxyl, hydroxyl and carbonyl, as shown in Figure 1.4. There are nitrogen containing functional groups on the surface of AC possibly, as shown in Figure 1.5. These nitrogen containing functional groups are mainly introduced from the preparation process using the nitrogen containing raw material and the reaction between AC and nitrogen containing reagents. 1.1.2.2 Function Due to the existence of the chemical functional groups, impure atoms and compounds on the surface, the chemical properties of AC is determined to influence the adsorption capacity of AV greatly. Different surface functional groups, heteroatom and chemical compounds have obvious differences in the adsorption of different adsorbents. The AC is nonpolar adsorbent, due to their hydrophobicity, which can only absorb nonpolar organic substances in aqueous solution, but does not have the function

(a)

O

O

O

O

COOH

(b)

(c) O

OH

(e)

O O

OH

(d) O

(f)

O

O

OH

(g)

(h)

Figure 1.4: Oxygen functional groups on the surface of AC. (a) Carboxyl group, (b) Acid group, (c) Lactonic group, (d) Mellow group, (e) Oxhydryl, (f) Carbonyl, (g) Quinonyl, (h) Ether group.

O H4N

O

O

N N

O

N H

Figure 1.5: Nitrogen functional groups on the surface of activated carbon.

N

8

1 Basic properties of lightweight carbon materials

of polar solute adsorption. Through the modification and introduction of surface functional groups, it has more abundant adsorption characteristics. In general, the AC surface oxygen compounds can easily absorbed allotted polar compounds, and alkaline compounds can adsorb less polar or nonpolar substances. By means of the surface modification of the surface functional groups of AC, the adsorptive property to the adsorbent is changed and the polarity of the surface functional group is increased to increase the adsorption capacity to the polar matter. On the contrary, if the nonpolarity of AC surface is increased, the adsorption capacity of AC to the nonpolar matter is also increased. Therefore, the changing of surface chemical property of AC can increase its adsorption capacity.

1.1.2.3 Determination of Surface Groups The main AC heteroatom is oxygen atom and the most common functional groups are carboxyl, lactone, hydroxyl and phenolic hydroxyl. These groups make AC have amphiprotic in water. Oxygen group on carbon surface can be determined by acidbase characteristics. (1) Boehm titration. It is based on the reaction possibility of alkali in different intensity and surface acidic functional groups to make qualitative and quantitative analysis of oxygen-containing functional groups on the surface. The general point of view is that NaHCO3 (Pkb = 6.37) neutralizes only the carboxyl groups on the carbon surface; NaCO3 (Pkb = 10.25) can neutralize carboxyl and lactone on carbon surface; and NaOH (Pkb = 15.74) can neutralize the total quantity of carboxyl groups, lactone and phenolic hydroxyl groups and surface acidic functional groups of AC on the surface. The amount of corresponding functional groups can be worked out according to the alkali consumption of different functional groups. Similarly, certain concentration of acid solution can react with the surface basic functional groups and the amount of AC surface basic functional group can be worked according to the amount of acid consumption. (2) The point of zero charge (pHPZC ). Point of zero charge (pHPZC ) is the pH value when the net charge on the surface of the solid in the aqueous solution is zero. pHPZC is an important parameter for characterizing surface acidity of AC. Isoelectric point (IEP) is the pH value when potential on the surface of solid in the aqueous solution is zero. If there are only adsorbed ions of H+ and OH– , thus pHPZC = pHIEP . pHPZC is closely related with the surface oxide of AC, especially the carboxyl group, and it has a good correlation with Boehm titration results. IEP is generally measured by electrophoresis assay. Studies have suggested that IEP was the outer surface characteristics of AC through electrophoresis methods. Because H+ and OH– are smaller than micropores of the AC, the pHPZC measured by titration corresponds to the all or most of the surface features of AC.

1.1 Activated Carbon

9

(3) Fourier transform infrared spectroscopy (FT-IR): IR can measure the rotational state and vibrational state of molecules, so as to get the bond property between the surface of carbon and the adsorbent and the adsorbed substance. The black AC has a strong absorption of infrared radiation; at the same time, the physical structure of uneven surface increases the light scattering, which makes it easy to be absorbed by the “background,” so we generally believe that the carbon content of more than 94% is not suitable to take the infrared spectrum analysis. FT-IR uses the optical interference device, so radiation from the full spectrum in the whole scanning period can always shine on the detector, thus it increases the flux and the resolution. The polarization of FT-IR is small, so it can be accumulated for many times and be recorded after quick scan. Now it has become a powerful tool for the qualitative analysis of surface functional groups on the AC. (4) Scanning electron microscopy (SEM) analysis: SEM can be used for solid surface appearance observation analysis, particle size analysis, layer analysis, and so on. The visual images of sample surface can be acquired on the readout device by means of scanning the sample surface with electron beam of fixed energy. For particle size analysis, when a focused electron beam irradiates to the particle surface, the particle surface will produce two electron radiation, and the yield of the two electron radiation is related to the particle electronic structure. When the electron beam scans the whole surface of the grating, we can get the image morphology of particles. The diameter and distribution information of particle are obtained from the known magnification times, so as to get the information of the morphology, composition, structure and defects in the microscale region.

1.1.3 The Main Detection Indexes of the AC Properties As a carbon-based adsorption material, AC is widely used in gas phase and liquid phase adsorption purification, so it is very important for the production and application of AC to detect accurately the physical properties, adsorption properties and chemical properties of AC. The detection performance of AC can be divided into performance test, micromechanism and simulation test and simulation evaluation and so on. The performance test is the most widely used among them, mainly including physical performance test, adsorption performance test, and chemical performance test. The main measures are iodine value, methylene blue, carbon tetrachloride, specific surface, pore size distribution, adsorption of benzene, strength, loading density, ash content, volatile matter, and so on. Nowadays, the performance detection methods of AC production and sales in China are China method (GB), American method (ASTM) and Japan method (JIS), and so on. Although the detection methods and the results are different, the basic principles are the same.

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1 Basic properties of lightweight carbon materials

1.1.3.1 Performance Test of Activated Carbon (1)

(2)

(3)

The physical performance test. The physical performance test mainly tests water, ash, strength (also known as mechanical abrasion resistance), particle size distribution, surface density (or loading density), floating rate, ignition point and volatile content, and so on. The application purpose of AC demands various detection indexes of the physical properties and the corresponding requirements. For example, the granular AC for water treatment generally needs to detect floating rate, moisture, ash, strength, loading density, particle size distribution and so on, and for the AC powder, the detection of the strength and floating rate are not necessary. The adsorption performance test. The adsorption performance test generally includes water adsorption capacity, adsorption value of methylene blue and iodine, phenol adsorption value, carbon tetrachloride adsorption rate, saturated sulfur capacity, sulfur capacity and desorption rate of carbon tetrachloride, protective time (for benzene vapor, chlorine oxide) and other determination items. The chemical performance test. The chemical performance test of the AC includes elements (including industrial analysis, elemental analysis, and analysis of harmful impurities), surface oxide (functional groups), and $ potential (equivalent potential, pH value, etc.).

1.1.3.2 Detection of Microstructure of AC The characterization of the microstructure of AC includes specific area, volume of hole (including micropores, medium and large pore volume, and even thinner), average pore diameter, and most probable diameter. Because of the complexity of the microstructure of AC, so far, no persuasive standard test method is available to characterize the microstructure of AC in the world. At present, the microstructure of AC is characterized with the liquid nitrogen adsorption method through the automatic adsorption instrument. But the test results are greatly different and the general error is about 10% due to the different selection of the instrument and data processing method. 1.1.3.3 The Applied Simulation Evaluation Test Among all the test standard methods of AC in the world, only the United States and some large company of AC specified the standard principle and instruction for the gas phase and liquid phase applications of AC, but these methods are only applicable to single component adsorbate, not to the practical application of AC. In order to accurately evaluate the effect of the AC used in the practical application, we need adsorption test of laboratory simulation, which mostly uses dynamic methods, and use the appropriate method to evaluate the results.

1.1 Activated Carbon

11

1.1.4 The Use of Activated Carbon As an adsorbent, the adsorption or catalytic performance of the AC has been developed to more fields up to now, and its use is broader. 1.1.4.1 The Application of Activated Carbon Used as Gas Adsorption Agent Absorbing gas with the AC is an important method of air purification, removing odor and recycling product. With the increase of people’s awareness of environmental protection, the demand of AC in the aspect of air pollution will be more and more. Usually, the granular AC is used for gas adsorption, and the developed pore structure makes it have a strong adsorption capacity. The AC absorption can adsorb many kinds of gas quickly, and most AC can be recycled and its waste has no chemical pollution. The application of AC in the deodorization and air purification indoor, the recycling of volatile solvent (such as ether, benzene, xylene, and acetone) and the separation and purification of industrial gases such as the removal of hydrogen sulfide odor, dioxin and other organic pollutants, desulfurization and denitration, nuclear radiation, biological agents and chemical agents and other aspects have been developed rapidly. 1.1.4.2 The Application of Activated Carbon Used as Liquid Phase Adsorption Agent As a liquid phase adsorbent, AC started in the industry as a decoloring agent of refined sugar. At present, as a liquid phase adsorbent, AC is used mainly in the food industry as the decolorization and adjustment of flavor of sewage treatment agent, in wastewater treatment, in the pharmaceutical industry as the medicament decolorization and purification agent (such as penicillin production) and in petroleum chemical and rubber production, which also have a very wide range of use, and almost all biological and chemical synthesis of all production processes will choose AC as refined adsorption material, so the development and application of AC for separation, refinement, and purification is one of the main topics of current studies. 1.1.4.3 The Application of Activated Used as Catalyst and Catalyst Carrier AC with unsaturated bond contains amorphous carbon and graphite, so it is similar to the phenomenon of crystal defects. Therefore, in many cases, AC is the ideal catalyst material, especially in the redox reaction. AC is widely applied in flue gas desulfurization, sulfur cyanide oxidation, phosgene synthesis, sulfuryl chloride synthesis, synthesis, hydrolysis, ester on cyanogen chloride two industrial batteries, oxygen depolarization, ozone decomposition, and so on. 1.1.4.4 Other Applications The application of AC in the protective clothing is to use the adsorption to absorb gaseous agents. The sort of adsorbents is large, and generally the carbon products

12

1 Basic properties of lightweight carbon materials

used in the protective clothing is AC, which has a variety of physical states, and the most commonly used are powder (PAC), granular activated carbon (GAC) and AC fiber adhesion in organic fiber (FAC). With different physical state, the mechanical properties and adsorption capacity of AC are different. Through its huge specific surface and the appropriate coated catalyst, the granular (or powder) AC can adsorb and remove various types of and different concentration of organic matters and it is an ideal protective agent. But when it is used in protective clothing, its performance is affected by the environmental temperature, humidity and other coexisting nontoxic substances and other factors, and the adsorption capacity varies greatly and is slightly low, which cannot meet the requirements of long-term and effective protection. Application of activated carbon fiber (ACF) began in the 1970s, and the recent emergence of microspheres carbon products can greatly improve the adsorption performance of carbon products, which can meet the long-term and effective protection of chemical warfare requirements. The incidents have always occurred, due to production by industrial activities caused by the local soil pollution, and the consequences are very serious, which can cause the “toxic” crops, groundwater and surface water pollution, and eventually pose a threat to human health through the transfer function of the food chain. There are three kinds of commonly used contaminated soil remediation technologies, including washing method, heat treatment, and steam treatment method, all of which require the use of AC adsorption as the final retention agent. A vehicle using the compressed natural gas as fuel needs only a cylinder of 47 L, filled with 19.6 MPa liquefied natural gas, which will cause unnecessary load rise and excessive fuel consumption. More importantly, because the cylinder is very dangerous, once there is a vehicle accident, the cylinder explosion may happen to cause a big disaster. In this situation, researchers around the world were forced to seek a natural gas storage method of low risk and low pressure. The use of AC to store natural gas is one of the methods. When the AC of high specific surface area is used to store natural gas, the cylinder pressure can be reduced to below 3.5 MPa, and the quality demand and the risk of the cylinder will be greatly reduced. Through continuous improvement, the cylinder pressure can be reduced to below 0.98 MPa at last. 1.1.4.5 Development Trend of Application In the past ten years, China’s AC industry has been developed greatly. According to the reports, China’s annual production of AC exceeded 90,000 tons in 1995, only less than American’s 15–17 million tons, and more than Russian’s 85,000 tons, Japanese and German’s 70,000 tons, and ranks at the second in the world. According to Chinese customs statistics, China’s activated ash export was 26,000 tons in 1994, 53,000 tons in 1995, which was more than that of America and Japan, and ranked at the first in the world. But compared with developed countries, there are still many problems in the quality and types of AC product in China, such as lacking AC products with low

1.2 Expanded Graphite

13

ash, high strength, high adsorption performance, and special use. Therefore, China’s AC products are uncompetitive in the international market, and the price is low. So new technology to improve the performance and quality of products should be used in China’s AC industry, so as to enhance the competitiveness of China’s AC products in the international market. At the same time, according to the situation of China, we should vigorously develop the production and application of low-cost AC. China’s annual production is 10,000 tons furfural, and each ton of furfural will produce 10–12 tons furfural residue. The AC produced with this residue has a very low sulfur capacity and a good effect for gas desulfurization. If furfural residue is totally used to produce AC, it can produce at least 50,000 tons AC. The char chip of the medium temperature can also produce cheap carbon adsorption (the carbon content of only 30–70%) to purify phenol containing wastewater. Petroleum coke is also a kind of good raw material of AC, and it can be made into printing and dyeing wastewater purification materials through simple filtration and water vapor processing. Some domestic people have used the method of catalytic activation to prepare AC, and the methylene blue adsorption is up to 480 mg/g and the adsorption properties of the iodine absorption is highly up to 2000 mg/g. AC is widely used in the field of industry, agriculture, and environmental protection. Compared with developed countries and even some developing countries, China is still relatively backward in the application of AC. Developing the application of AC is an important force to promote China’s development of AC. Taking the food industry as an example, as early as in the 1930s to the 1950s, the AC was widely used in the sugar industry in foreign countries. China is in the leading position of sugar production and consumption in the world; according to statistics, the annual consumption of sugar in the production is more than 6,000,000 tons, and the production of sugar in the sugar industry is 400–500 million tons, but China is still backward on sulfur smoked decolorizing steaming. The sulfur decoloration method has color reversion, is easy to be deliquescent caking and has other shortcomings; more importantly, the sulfur residing in sugar has carcinogenic effect. With the improvement of people’s living standard and self-care awareness, sugar decolorization with AC will come sooner or later. To develop China’s sugar refining with AC and decolorization process and device is a very meaningful work. Coupled with the application of environmental protection, the AC market will be more attractive.

1.2 Expanded Graphite Expanded graphite is a porous carbon material and it has rich internal porous structure and its specific surface area is up to 50–200 m2 /g. As a new type of adsorption material, expanded graphite has the irreplaceable role of other materials in wastewater

14

1 Basic properties of lightweight carbon materials

purification, polar hydrophobic adsorption, adsorption in nonaqueous medium, and high-temperature thermal decomposition of oil, oily water, and masking treatment of dyeing wastewater [9–23].

1.2.1 Brief Introduction to Graphite Graphite is an isotope of carbon and belongs to the layered crystal. In each layer, the carbon atoms arrange in hexagons, the bond between carbon atoms is SP2 covalent hybrid, and the length of bond is 0.142 nm; the bond between the layers is van der Waals force, and the layer space is 0.334 nm. Because the interlayer force is weak, it is always broken into graphite crystal powder with small size. The graphite structure diagram is shown in Figure 1.6. Graphite is one of the nonmetallic minerals existing widely in nature. The industry divides graphite into two categories, which are crystalline graphite ore (flake) and aphanitic graphite ore (earthy graphite ore). Crystalline graphite ore can be divided into flake and dense. China’s graphite ore mainly belongs to flake crystalline type, and secondly belongs to aphanitic type. The crystallization of flake graphite is good, and its grain size is greater than 1 mm, generally 0.05–1.5 mm, and the bigger one, up to 5–10 mm, mostly belonging to aggregates. Aphanitic graphite, whose shape can

Figure 1.6: The layered structure of graphite.

1.2 Expanded Graphite

15

be observed only in the electron microscope, normally belongs to microcrystalline aggregates, and the particle size is less than 1 ,m. Aphanitic graphite is not as good as flake graphite in process performance, and the industrial application scope is smaller. Graphite has high temperature resistance, electrical conductivity, thermal conductivity, lubricity, chemical stability, plasticity, thermal shock resistance, and other special properties, so it has been widely used in the fields of metallurgy, machinery, petroleum, chemical industry, nuclear industry, national defense, and so on.

1.2.2 Graphite Intercalation Compounds (GIC) Graphite crystal is a typical layered structure, and the bond of the level of carbon is covalent bond, whose energy is great; however, the connection between the layers is only weak van der Waals force. People put atoms, molecules, ions, and even its exotic atoms into the graphite layers by using different levels and interlayer force with physical or chemical methods, combining with carbon atoms in graphite and forming superlattices on the C axis; thus, it forms a layered composite material with a nanometer scale, and that is the graphite intercalation compounds. According to the different situation of intercalation, the GIC has different forms. As shown in Figure 1.7: G is for a carbon layer, I is for insertion layer, and n is called the layer order number between layers of carbon, whose order number is smaller, explaining the intercalation is more fully. So far, we have found more than 200 kinds of types GIC. The type of insert agent mainly includes metal element GIC, metal halide GIC, halogen elemental GIC and pure acid GIC. That which can rapidly expand at high temperature is called expandable graphite. Methods of preparing expanded graphite are chemical intercalation method, electrochemical method, ultrasonic oxidation, vapor diffusion method and molten salt method. G I nG

Intercalation

1 Layer Graphite carbon layer

2 Layer

Graphite intercalation compounds (GIC)

Figure 1.7: The structure of graphite intercalation compounds.

n Layer

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1 Basic properties of lightweight carbon materials

1.2.3 Expanded Graphite At a certain temperature, the flake crystal of graphite will swell, and the expanded graphite is fiber wadding wormlike, so it is called the expanded graphite. Generally, the puffing temperature is 800–1000 degrees. When expandable graphite is heated rapidly at high temperature, the expansion force produced by the gasified inserters between the intercalations can overcome the intermolecular force, so the graphite wafer expands tens to hundreds of times along the C axis. The expanded graphite was found hundreds of years ago, and its development process can be divided into three stages: the first stage is from 1841 to 1974. Schafautl was the first to mix graphite and the liquid of concentrated sulfuric acid and nitric acid and found that the graphite expanded almost two times as the original along the direction perpendicular to the dissociation; thus, he opened a prelude to the research of expanded graphite. In the early 1930s, Hoffmann and others determined the intercalation order by using X-ray technique, and it was the first time that the intercalation compound was studied systematically. Study on the first stage was to discover new material and research on the basic process of intercalation. The second stage is from 1974 to 1987, and it started in Japan when the high-energy battery was invented as electrode made of lithium fluoride and graphite, which developed a commercial application prospect for the intercalation compounds. Then in 1975, America found AsF5 graphite intercalation compound with high conductivity, and the conductivity was even higher than copper, so the world set off the climax of expanded graphite. In 1987, GIC superconducting materials attracted the attention of scientists, and the study on expanded graphite was relatively less. Now it is the third stage; the focus of the study is expanded graphite engineering technology and industrial application. Although the expanded graphite experiences the chemical physical action of intercalation and other chemical physical effects, the crystal structure of the initial state and final state are the same. So the physicochemical properties of graphite – high temperature resistance, low temperature resistance, corrosion resistance, thermal conductivity, safety and so on – still remain, and expanded graphite in high temperature expansion forms a loose porous structure of worm expanded, with a large number of unique network like porous structure in the form of shape. Figure 1.8 is the schematic structure of expanded graphite. The structure of expanded graphite and the structure of common porous materials have great difference, and the structure of expanded graphite is also very different because of different materials and different preparation conditions. However, from the essence of the form of the pore structure of expanded graphite, the structure of different types still has a lot in common. The expanded graphite particle is in the fourgrade-pore construction and the formation of the four-grade-pore is not completed simultaneously. The first sub-layer of the pore structure is surface V-shaped open hole of intermicellar of the expanded graphite particles, and the size is about several tens to hundreds ,m; the second sub-layer of the pore structure is willow leaf–shaped hole,

1.2 Expanded Graphite

17

Figure 1.8: Schematic structure of expanded graphite.

which is lateral hole, and the size is about several to dozens ,m; the third sub-layer of the pore structure is the second film inner polygon hole, which is random orientation and connected network, and the size is 0.1 to several ,m; the fourth sub-layer of the pore structure is microhole in nm scale, and the number is very small. From the number of the four kind of holes, the number of the third sub-layer of the pore is far more than the second sub-layer of the pore, and the number of the second sub-layer of the pore is far more than the first sub-layer of the pore, but from the total pore volume, the second sub-layer of the pore has absolute advantage. Compared with AC structure, graphite porous structure is mainly in large pores, while AC is mainly in small and medium pores. Therefore, expanded graphite has good adsorption properties for the nonpolar large molecules and some nonpolar organic molecules, and it is a kind of good adsorption material. The structure and properties of expanded graphite show that it is a kind of ideal, economical, widely used and advanced functional material, and it has been widely used in petroleum, chemical industry, light industry, electric power, metallurgy, machinery, instrument, automobile, atomic energy, aerospace and other industrial sectors.

1.2.4 Application of Expanded Graphite and Its Composites At the microlevel, the expanded graphite and the natural flake graphite belong to the same system, so it has many similar properties of the natural graphite; at the macrolevel, the expanded graphite is a soft, porous material. Because it is crystallized by the natural flake graphite along the direction of the C axis of expansion several times to hundreds times; thus, it forms many tiny holes on the surface and inside and increases the specific surface area, and it is a kind of good adsorption material.

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1 Basic properties of lightweight carbon materials

The expanded graphite and its products not only maintain the natural graphite six party crystal structure and self-lubrication, high temperature resistance, radiation resistance, corrosion resistance and anisotropic properties but also have properties of softness, elasticity, plasticity, self-sealing, low viscosity and density that the natural graphite does not have. Application of expanded graphite is mainly in the following aspects. (1) Catalysts of organic reactions. Compared with other catalysts in organic reactions, prominent characteristics of expandable graphite catalyst are small molar ratio of the reactant, short reaction time, high yield, and good product color. And it has no corrosion to the equipment; the preparation method is simple and can be used repeatedly after simple treatment and regeneration without loss of catalyst in the reaction process. (2) Sealing materials. The application of expanded graphite as sealing materials mainly is divided into two categories: one is used as a sealing filler and the other is used as a gasket on the flange pipe. The sealing material is the preforming filler directly formed in the presser through cutting the expanded graphite sheet into strip and wrapping in a metal mold. This material is suitable for all kinds of cut-off valve, gate valve, pressure gauge valve and high-pressure water supply system, the sealing of the steam reciprocating pump tank, gas tank and boiler room filler rod, the sealing of reactor shaft factory diameter, the sealing of furnace door sealing pump, the sealing of compressor piston rod and other mechanical sealing. The graphite gasket is divided into pure graphite gasket and spiral wound gasket. The pure graphite gasket can be directly pressed in mold in the metallic mode, and it can be also made by tailor and cutting. This gasket is soft and elastic, which can achieve the sealing effect with lower fastening force. Graphite spiral wound gasket is made by expanded graphite and metal band through the way of overlap roll, and the gasket can be used as sealing furnace and pressure vessel at low-temperature and low-pressure flange gasket. (3) Insulation and heat shielding material. Because of the anisotropic of thermal expansion graphite rate, the various plates and sheet made of expanded graphite can be used as heat shielding material for the induction furnaces, the vacuum furnaces, reactors and the pot covers and ingot riser insulation materials. In addition, it can also be used for high temperature smelting furnace of the impurity scattering barrier and shrink-proof hole steelmaking agent. (4) Conductive material. Graphite has low resistivity (3.5–8 * 10–4 ⋅ cm–1 ), and the electronic conductivity with good ability can be used as conductivity, conductive materials, anti-static materials, battery materials, special brush, electromagnetic shielding and conductive coatings, and so on. (5) Environmental protection. Because the expanded graphite has the sophisticated porous structure like network, high surface area and high surface activity, the marginal polar groups on the surface and the anisotropy of the adsorption in the direction of graphite crystallite C axis and a axis, it has not only good adsorption

19

1.3 Activated Carbon Fiber

Table 1.1: Average adsorption weight ratio of oil with the expanded graphite. Sample name

Fuel oil

Wax oil

Lube

Diesel oil

Petrol

Expanded graphite Activated carbon

59.3 3.2

54.9 3.2

43.6 2.7

34.4 2.5

29.1 2.5

properties for various nonpolar organic molecules but also certain adsorption properties of the polar molecules. For these reasons, it has great application prospect in the field of environmental protection.

Experiments showed that the expanded graphite had good adsorption for SOx and NOx , and the adsorption effect is more significant with the increase of temperature. The expanded graphite also has good adsorption effect on many organic vapors. Therefore, the expanded graphite can be used to solve the increasingly serious problem of air pollution. The research shows that the expanded graphite has great adsorption amount of oil (see Table 1.1), and the adsorption is still floating on the water surface, which is easy to separate and recover. Through the extrusion to adsorbate adsorbent, the adsorbent is easy to be regenerated. Oil pollution is the main form of marine pollution. Japan, Israel, and other countries are now studying the expanded graphite to remove marine oil pollution. A tanker in Fukuoka Japan spilled in 1997, and they used the expanded graphite to remove the clearance leakage of crude oil and achieved good results. This method has better economic and environmental benefits than traditional AC, cotton, magnesium oxide, and polyurethane powder oil adsorbent. The expanded graphite has a good performance to adsorb oil from water, and it is a very good purifying agent for oily wastewater. The concentration of oily wastewater treated with expanded graphite is less than 1 × 10–6 , fully meeting the standard of drinking water. Oily wastewater is an important pollution source of water pollution in China. The experiments show that the expanded graphite has a good ability in removing large molecules and is easy to be recovered. It has great significance for the removal of chemical, food, oil and other industrial wastewater pollution.

1.3 Activated Carbon Fiber Activated carbon fiber is the third-generation AC material, which was developed after the powder AC (Powdered AC, PAC) and the granular activated carbon (granulated AC, GAC), and its commercial production began in 1960. It is made of organic fiber through carbonization and activation, and the precursor is polymeric fibers (polyacrylonitrile,

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1 Basic properties of lightweight carbon materials

phenolic resin, poly two vinyl, etc.), fiber and asphalt. Compared with the traditional GAC, PAC, FAC, the AC fiber has its unique structure and properties [24–27].

1.3.1 Structure of the AC Fiber By means of X diffraction analysis, it can be seen that the form of the ACF carbon atoms in graphite is like microcrystalline layers of turbostratic stacking and the order of microcrystalline layers in the three-dimensional space is poor and the average size is very small. Figure 1.9 is the scanning electron microscope of AC fiber, which clearly shows its pore structures: a single AC fiber is columnar (the whole AC fiber is netlike). Through surface analysis, there are a series of oxygen functional groups on the surface of AC fiber, such as hydroxyl group, carboxyl, ester and so on, and some also contain amino, imino, and sulfhydryl groups. These functional groups have obvious influence properties on the AC fiber. With the difference of activation, the surface oxygen groups also show different structures. The surface chemical groups can be determined through chemical analysis and instrumental analysis method. The pore structure of the AC fiber is microporous, the pore distribution of which is concentrated. The surface of the AC fiber is covered with micropores, above 90% of them in interspaces are microporous, and the microporous aperture is almost similar, which is in a few nanometers to tens of nanometers. The traditional AC has a lot of large holes and transition holes, whose aperture distribution is scattered, and many microporous pores contained in a giant hole, which are long narrow holes and almost all are located in the internal pore structure of the deep area, and its open

20 μm

Figure 1.9: The scanning electron microscope of activated carbon fiber.

Vega©Tescan

1.3 Activated Carbon Fiber

21

Table 1.2: The morphological characteristics comparison of ACF and GAC. Type

Specific surface area/(m2 /g)

Pore volume/(ml/g)

Pore diameter/(nm)

Component unit

ACF GAC

2000 900

1.10 0.75

1.3

13 ,m fiber

holes are very wide. The morphological characteristics comparison of them is shown in Table 1.2. For different applications, the aperture can be necessarily controlled and adjusted through the controlling activation process, the catalytic activation and the deposition method. For example, certain amount of KOH is added into the carbon precursor of different types and different forms, and then ACs, and ACF with large surface areas is formed by carbonizing it under certain temperature, and its fine crystallization tendency is obvious. Another method of controlling the structure of ACF is activation after heat treatment. Based on the simple surface oxidation (gas phase oxidation and liquid phase oxidation), hydrogenation, amination, alkaline or high-temperature treatment of ACF, the quantity of nitrogen-containing functional groups and oxygen-containing functional groups and the hydrophobicity can be changed, and the adsorption capacity of different acid gas can be increased.

1.3.2 Performance of the Activated Carbon Fiber Similar to the AC, ACF belongs to the carbon-based adsorption material, and its performance is greatly improved compared with GAC and PAC. (1) Large specific surface area. The AC fiber has sophisticated microporous structures, uniform aperture distribution, strong adsorption capacity, high unit adsorption capacity and large specific surface area, which can reach more than 1000 m2 /g. The comparisons of adsorption capacity for the organic vapors with AC fiber and granular AC are shown in Table 1.3. (2) The fast speed of adsorption and desorption. The AC fiber surface is mainly occupied by the concentrated aperture distribution micropores, and the adsorbate is not like that of the AC, which arrives at the micropores after passing through big holes and transition holes, but directly to micropores. And the diffusion resistance of the intraparticle is small, the adsorption speed is fast, the path of desorption is short and the desorption speed is fast. (3) Good adsorption capacity for low-concentration substances. On the surface adsorption, the smaller the pore size of adsorbent is, the larger the adsorption capacity is. Especially, it has a good adsorption capacity for low-concentration chemical substances, even for the adsorbent of 10–6 (ppm). Because the

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1 Basic properties of lightweight carbon materials

Table 1.3: Equilibrium adsorption capacity of organic matter (the capacity of saturated vapor pressure at 20∘ C). Adsorbed matters

Blanket-shaped activated carbon fiber/(quality)%

Granular activated carbon/(quality)%

Butyl mercaptan Dimethyl sulfide Trimethylamine Benzene

4300 64 99 49

117 28 61 35

(4)

(5) (6) (7) (8)

(9)

(10) (11)

granular AC has a low micropore proportion, its adsorption capacity for the low-concentration substances is much lower than that of AC fiber. High mechanical strength. Because of the appropriate fiber strength of the AC fiber, it can be made into the structure of paper-shaped, non-woven fabric, honeycomb, or corrugated plate according to different requirements, and the adsorption performance will not be affected. The AC fiber paper surface area can be maintained at about 75% of the original AC fiber. Good mechanical properties contribute the convenience for engineering application and process simplification. It is not easy to be powdered, so it will not cause the second pollution to the treated matters. Because of the small density and low-pressure loss, it can also achieve good adsorption effect, even for short time contact adsorption. The adsorption layer is thin, so the processing device is easy to be made miniaturization and high efficiency. It is a good conductor of electricity and heat, and the heat storage is small, the operation is safe. The regenerative bed can use electric heating mode (electrothermal desorption). It is easy to be regenerated and has long service life, and the adsorption performance almost does not decrease. ACF regeneration process is different with the difference of the treatments; generally speaking, the speed of ACF regeneration rate is only several minutes. The change of iodine adsorption after the regeneration indexes is little, and it has good performance for repeated use. It can act as the catalyst carrier to adsorb odorous substances. It has good characteristics of acid and alkali, and the application conditions are wide.

1.3.3 Application of the Activated Carbon Fiber Because the AC fiber has good characteristics and certain mechanical strength, it overcomes the problem of GCF and PAC in the operation, which is easy to be channeled and deposited, and it does not have the second pollution. ACF is replacing the granular AC

1.3 Activated Carbon Fiber

23

and the powdered AC in the field of environment purification, treatment and other aspects, attracting the attention as a the basic material. 1.3.3.1 Application of Gas Treatment The industrial application of ACF is mainly in adsorption, which can deal with SO2 , H2 , NH3 , CS2 , NO, NO2 , H2 S, CO, CO2 , HF, SiF, and the exhaust gas of various organic vapors, and it also has a strong adsorption capacity for the generated ozone by the copier. It is not easy for ACF exhaust gas treatment device to produce leakage of suction phenomenon, and only one operation can achieve the treatment effect in the structure. As the indoor air pollution is increasingly serious, the air purifier with ACF equipped can be used for the indoor air purification, ventilation or smoke odor removal. The domestic or car ACF purifier is on sale now. 1.3.3.2 The Application of Water Treatment The purpose for ACF to be used for wastewater treatment is to remove COD, BOD, TOC, color, odor, oil, phenol and the matters interfering with the biological treatment, the matters difficult to be removed with the biological treatment, the mercury and other metals and radionuclides, and to prepare recycled water and so on. One of the distinctive features of ACF as the adsorbent is the selectivity, namely, it has the molecular sieve effect. In the water purification, it can remove the odor produced by microorganisms, pesticide, residual chlorine, and humus, and it can inhibit the growth of algae. Some experiments show that ACF loaded with Ag or iodine has the killing rate of nearly 100% to Escherichia coli, Staphylococcus aureus, candida bacteria and Bacillus subtilis in the dynamic state, and the reason is that the protein metal elements have the direct effect on the cell editing and the protein of ACF microorganisms. ACF can treat the second pollution made by residual chlorine in water and the organic chlorides caused by generate trace organic reactions with the residual chlorine. The application of ACF wastewater treatment has been greatly developed in the field of large water purification and small household cleaners. 1.3.3.3 Applications in Other Aspects The gas mask made of ACF, which has the function of the chemical protective clothing, is suitable for the chemical protection of poisonous gas. Compared with the granular AC, not only can ACF improve the filtering effect but it can also reduce the mass of the adsorption layer and miniaturize the equipment; in addition, it can prevent chemical radiation, so it can be used as the chemical-radiation-resistant equipment. ACF has a good adsorption performance for the trace substances in water and air, so the trace substances in the environment media can be concentrated and enriched before the corresponding instrumental analysis. The invented AFC environment monitor is convenient for carrying.

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Cleaning production requires zero emission of pollutants. CIP technology uses ACF as the microadsorbent, and has obtained the patent in China. This technology can greatly shorten the extraction time, lower the abrasive wear of carbon fiber and reduce the loss. Almost all solvents can be recovered efficiently by ACF, especially the lowconcentration solvents. The excellent heat resistance, acid and alkali resistance of ACF make it as an effective ingredient of catalyst. The catalytic activity of NO is higher than single component catalyst on the conversion of NO, and the catalyst with ACF carrier has a wider temperature application and longer life service. Because ACF has chemical stability, thermal stability and radiation stability, there will not be any change in using various disinfection methods in disinfection. The adsorption of ACF on exogenous toxins in the human body is very effective, and has important uses in blood filtration, detoxification and oral trauma dressing and treatment. Not only can ACF be used for solar energy collector, high efficiency, large-capacity capacitor electrode, the catalyst, metal enrichment, and recovery of Hg, Ag, Au, and so on, but it can also be used for freshening fruits and vegetables, refrigerator deodorant, household water purifiers, and other civilian areas. In the future, the production cost and the sales price of ACF should be vigorously reduced, and the current price of ACF in China is ten times more than GCF, which becomes the biggest problem to extend ACF; then we should improve the carbonization process and equipment of AFC, improve the surface structure and properties of ACF through pre-processing or post-processing, and make it suitable for application requirements; thirdly, we should develop new gas and wastewater treatment technology of ACF, which can solve the problem of pretreatment, pressure drop, the method of ACF packing and so on, in order to reduce energy consumption; fourthly, we should expand new areas of application. For example, Japanese expanded the application of ACF for adsorbing the waste gas of rubbish. It is also widely used in the automobile exhaust treatment. It also has certain effects in the application of ozone oxidation and AC adsorption in the dyeing wastewater. Using the AC fiber to replace the AC combining with ozone is tried to form a complex effect, and to see whether the adsorption efficiency for the pollutants in the dyeing wastewater can be improved and the processing cost can be reduced. This technology is being further studied at present.

1.4 Carbon Nanotubes Carbon nanotubes, as a new member of the carbon family, can be seen as a seamless hollow tubular structure rolled with the monolayer or multilayer graphite flake. In addition to the pores of hollow tube, the multiwalled carbon nanotubes have the interlayer pores and the single-walled carbon nanotubes have the intertube pores. All of these special structures make the carbon nanotubes have abundant pore structures. In

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addition, the carbon nanotubes are characteristic of the high density of atoms on the surface, large specific surface area, the modifiability of pore structure and the surface structure, and so on. These features make it be widely used in the field of catalyst carrier, hydrogen storage materials, super capacitor and lithium ion secondary battery and stealth material [28–31].

1.4.1 Structure and Morphology of Carbon Nanotubes In 1991, Dr. S.Iijima, the electron microscope expert of the basic research laboratory of NEC company, Japan, accidentally discovered the nano-scale coaxial tubular carbon fiber in the experimental product based on carbon electrode arc vaporization of graphite (C60 ), and named it BucKytube, which is widely known as carbon nanotubes (CNTs). The cross-section of carbon nanotubes was found from the observation of 500,000 times electron microscope, being composed of two or more coaxial tube layers and the interlayer distance is 0.343 nm, which is slightly greater than the distance between the layers of carbon atoms in graphite (0.335 nm). The X-ray diffraction and calculations show that the crystal structure of carbon nanotubes is a closely packed hexahedron (h.c.p), a = 0.24568 nm, c = 0.6852 nm, c/a = 2.786. Compared with graphite, a is slightly smaller and c is slightly larger, indicating that there is a stronger bonding force between the atoms of the same layer and the carbon nanotube has a high coaxial intensity. Carbon nanotubes are the ideal quasi-one-dimensional material. According to the number of layers of carbon atoms in the tube wall, carbon nanotubes can be divided into the single-walled carbon nanotubes (SWNTs) and the multiwalled carbon nanotubes (MWNTs). Their common features are that both of them are curled up by monolayer or multilayer graphite flake and both have the nanometer hollow tube with the high ratio of length to diameter. SWNTs is the ultimate form of carbon nanotubes. Its wall is composed of only one layer of carbon atoms and the diameter of the tube is usually 1–2 nm, the length, from 10 to 100 nm. Usually a nanometer tube bond is formed by 10–100 parallel single tubes. Each single-walled carbon nanotube is formed by the carbon atoms hexagon, the length ranges from tens of nanometers to micron scale, and both ends are covered with the carbon atoms pentagon. The single-walled nanotubes exist possibly in three kinds of structures, as shown in Figure 1.10. The formations of these types of carbon nanotubes depend on the way of the rolled cylinder with the six-angle-lattice two-dimensional graphite flake of the carbon atom, and different types of nanotubes can be expressed with chiral vector (m, n). Recently, Zhu et al. prepared SWNTs with the n-hexane catalytic cracking method (enhanced vertical floating technique), and its length can be up to 10–20 cm.

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(a)

(b)

(c)

Figure 1.10: Several different types of single-walled carbon nanotubes. (a) Armchair type, (b) Sawtooth type, (c) Chiral type.

MWNTs is composed of 2–50 layers of coaxial carbon tubes, and the distance between each layer and the distance between base plan of graphite is similar. Carbon atoms in each layer are in a spiral shape distribution along the tube axis. The internal diameter of the multiwalled tube is 2–10 nm, the external diameter, 15–30 nm and the length, not more than 100 nm.

1.4.2 Preparation of Carbon Nanotubes Nowadays, the main methods for preparing carbon nanotubes are arc discharge method, laser evaporation and chemical vapor deposition (chemistry vapor deposition, CVD), and so on. The preparation method of SWNTs is mainly the arc discharge method, and CVD method can also be used to prepare SWNTs. The preparation of MWNTs is mainly through catalytic pyrolysis of CO and hydrocarbon by CVD. MWNTs can also be prepared by the arc discharge method, and Iijima, the first one reporting CNTs, got the MWNTs by the arc discharge method. 1.4.2.1 Arc Discharge Method The arc discharge equipment is mainly composed of a power supply, graphite electrodes, a vacuum system, and a cooling system. To synthesize the carbon nanotubes effectively, the catalysts are necessary to be doped into the cathode, sometimes with the laser evaporation. In the arc discharge process, the reaction chamber temperature is up to 2,700–3,700∘ C. The carbon nanotubes prepared with this method is in good quality, of uniform diameter, of straight tube, of high degree of graphitization, close to or reaching the expected performance. But the carbon nanotubes prepared with this method is uncertain of spatial orientation, easy to be sintered, of high content of

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impurity and low output, so it is only limited in the application of laboratory and not suitable for high-volume continuous production. 1.4.2.2 Chemical Vapor Deposition (CVD) The CVD method is an extensive method in the application of preparing carbon nanotubes. This method, mainly taking the transition metal as the catalyst, suitable for large-scale preparation of carbon nanotubes, makes the high content of carbon nanotubes in the product and more defects for the carbon nanotubes. The preparation of carbon nanotubes with CVD method needs simple equipment and process, but the preparation and dispersion of catalysts are the key steps. The present study of preparing carbon nanotubes through the CVD method mainly concentrates in the following two aspects: the large-scale preparation of nondirectional and disordered carbon nanotubes and the preparation of the carbon nanotubes array of the discrete distribution and oriented arrangement. Generally the single-walled or multiwalled carbon nanotubes can be produced with the free carbon ions produced by the hydrocarbon cracking in the presence of catalyst at 530–1130∘ C, by taking Fe, Co, Ni and its alloy as the catalyst, silica, clay, diatomite, alumina and magnesium oxide as the carrier, acetylene, propylene and methane as the carbon source, nitrogen, hydrogen, helium, hydrogen or ammonia as the dilution gas. 1.4.2.3 Laser Evaporation Method Laser evaporation method is an effective method for the preparation of the singlewalled carbon nanotubes. The preparations of single-walled carbon nanotubes and single-walled carbon nanotube bundle use the high-energy CO2 laser or Nd/YAG laser to evaporate carbon target with Fe, Co, Ni, and its alloy, and the diameter can be controlled by the laser pulse. The main drawbacks of this method are that the produced single-walled carbon nanotubes are of low impurity and are easy to tangle. Other preparation methods are polymer method, solar method, electrolytic method, low-temperature pyrolysis, solid in situ catalytic method, soluble salt method and solid phase synthesis, and so on.

1.4.3 Modification of Carbon Nanotubes Dispersity is the primary problem of the carbon nanotubes in the application of polymer composites. Because the carbon nanotubes are of small diameter and large surface energy, they is very easy to agglomerate so that they cannot be uniformly dispersed in the polymer to make the performance of the composite material very bad. In order to improve the dispersion ability and increase the binding force of carbon nanotubes and polymer interface, the surface of the carbon nanotubes needs the

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modification to reduce the surface state, to improve the affinity with the organic phase, and to prepare the high-performance composite materials. The main modification methods of carbon nanotubes are the following. 1.4.3.1 Chemical Modification By means of the chemical treatment, some functional groups can be obtained on carbon nanotubes to change their surface property to meet certain requirements (such as preparation of soluble carbon nanotubes). Liu et al. successfully introduced carboxyl and hydroxyl groups to the carbon nanotubes based on carbon nanotubes for acid treatment on carbon nanotubes; Dai et al. introduced a large number of sulfonic acid groups to carbon nanotubes with the similar sulfonation reaction. In addition, the CNTs are modified in the outer membrane, evenly coated with a layer of other material on the surface of the film, which also can make the surface properties of carbon nanotubes different. For example, Tang Benzhong et al. used situ polymerization method to bond multiwalled carbon nanotubes and phenylacetylene together by catalytic polymerization and got the multiwalled carbon nanotubes packaged by the phenylacetylene, which can dissolve four hydrogen furan, toluene, chloroform and other organic solvents, and it has good optical limiting effect; Mioskowski et al. used the strong binding ability of water-soluble protein streptavidin and carbon membrane to immobilize the monolayer streptavidin on the surface of multiwalled carbon nanotubes through the induction effect and developed potential applications of the carbon nanotubes modified by protein in the nano biotechnology. 1.4.3.2 Physical Modification This method activates the surface of carbon nanotubes through mechanical stress by means of crushing and friction to change the surface physical and chemical structure. When the inter energy of CNTs increases, under the action of external force, the activated CNTs surface can react and be attached with other materials, so as to achieve the purpose of surface modification. A typical method is to mill CNTs using ball mill. In addition, CNTs can be treated with ultrasonic dispersion or a large shearing force to prevent from the agglomeration to obtain a good dispersion effect. J. Sandler et al. prepared epoxy resin /CNTs composite materials by means of ultrasonic method. 1.4.3.3 High Energy Modification High energy modification modifies the surface of CNTs through ultraviolet ray and plasma ray. Dai et al. introduced functional groups on the surface of CNTs by using plasma ray and successfully fixed the polysaccharide chains to the activated surface of CNTs by plasma. The study found that the surface of CNTs could also be modified with the non-deposition plasma treatment to activate it. This method can be regarded

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as an effective way of modifying CNTs, and its application scope includes the field of composite materials, biomedical materials and so on.

1.4.4 Application of Carbon Nanotubes The peculiar structure of carbon nanotubes makes it exhibit very special properties, and people have high expectations for its application in many fields. 1.4.4.1 Mechanical Properties and Applications Cylindrical carbon nanotubes have excellent mechanical properties. The short and strong carbon–carbon bond (0.14 nm) in the monolayer of carbon nanotubes on graphite prevents the impurities and the defects from entering carbon nanotubes to form an excellent tensile resistance. In addition, the carbon nanotubes failure strain is up to 5–20%, and it has good flexibility and elasticity in the axial direction of carbon nanotubes. Under the torsion effect, the carbon nanotubes exhibit strong anti-distortion capacity, and the carbon nanotubes will recover when the load is removed. Carbon nanotubes are the highest specific strength materials that can be prepared now. The tensile strength of carbon nanotubes can reach 50–200 GPa, which is 100 times that of steel, but the density is only one-sixth of steel, at least an order of magnitude higher than the conventional graphite fiber. It is the strongest fiber and the most ideal fiber in the aspect of the strength weight ratio. If the carbon nanotubes are used to produce composite materials with other engineering materials, they will strengthen the base body of the materials. 1.4.4.2 Electrical Properties and Applications The carbon nanotube is rolled up with the graphite flake. Three covalent bonds are formed with four valence electrons, and the delocalized bond with the metal bonding properties is formed by the electrons from every carbon atom, so the cylindrical carbon nanotubes have good conductivity in its axial direction, and the spiral theory predicts the conductivity depends on the helical angle of diameter and wall. When CNTs diameter is larger than 6 mm, the electrical conductivity will decrease; when the diameter is less than 6 mm, CNTs can be regarded as one-dimensional quantum wires with good conductive properties. For the carbon nanotubes of spiral shape, coil shape, or fishbone, the electrical conductivity will interrupt when the layer is bended or discontinuous. So the carbon nanotubes are divided into conductor and semiconductor. The conductivity is related with its diameter and structure, and these two are determined by the chiral vector (n, m, are integers). When n-m is an integer times of 3, the single-walled carbon nanotubes are metallic, or semiconducting.

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The electrical properties of carbon nanotubes are expected to be used in super capacitors, lithium ion secondary battery, fuel cell, and so on. 1.4.4.3 Thermal Performance and Applications Carbon nanotubes are of high length-diameter ratio, so that most of the heat conducts along the axial direction, and the thermal conductivity of cylindrical carbon nanotubes in parallel direction is similar to that of diamond, while the vertical direction of it is very low. The proper arrangement of carbon nanotubes can obtain high anisotropic heat conduction material. The higher the degree of graphitization of carbon nanotubes, the greater the thermal conductivity is. 1.4.4.4 Application of Catalysis Carbon is a kind of excellent catalytic material, and nanometer grade of carbon tubes also have properties different from the ordinary carbon catalyst. It can absorb any molecules, the size of which is suitable for its inner diameter. So people use the open tip activity of the carbon nanotubes as adsorbent to adsorb some highly reactive molecules at the molecular level. At the same time, because of the hollow structure of CNTs, they can be used in some special catalytic occasions. The nanometer diameter can make the load on the catalyst particle size refinement, which is conducive to improve catalytic activity and selectivity. After activation, the specific surface area of carbon nanotubes is improved and the special surface structure is formed to make surface modification more conducive to the load of catalyst. Therefore, the application of carbon nanotubes in catalysis has a broad prospect. 1.4.4.5 Application of Adsorption The carbon nanotubes have larger specific surface area than the AC, and have a large number of pores. In addition, the carbon nanotubes are highly graphitized (higher than AC), and there are high aromatic atoms on the surface of carbon nanotubes, which can obtain the high density of 0 ion, so they have better adsorption performance than the AC, especially as the gas adsorbent. It is believed that the carbon nanotubes are the best hydrogen storage materials. Generally, the physical adsorption inside tube is the main mechanism of hydrogen storage, and hydrogen storage capacity of carbon nanotubes can be compared with the best hydrogen storage materials currently. The carbon nanotubes, as the gas phase adsorption agent, have been used in the field of environmental protection. Richard Q. Long et al. used program temperature adsorption and desorption technique to adsorb low volatile organic compounds, such as dioxins, and pointed out that the bond between the surface of sorbates was stronger than that between sorbates, so the adsorption is limited to single molecular layer and in agreement with the Langmuir adsorption curve. At the same time, the desorption

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temperature, the desorption activation energy and Langmuir constant of dioxin on carbon nanotubes are much higher than that of AC and #-A12 O3 . In the Henry’s law region of the low concentration, the adsorption capacity of carbon nanotubes is 1034 times higher than that of AC, so the removal efficiency of dioxin with the carbon nanotubes is higher than that with the AC. So far, there isn’t many reports on the carbon nanotubes as the liquid absorbent. Li Yanhui et al. from Tsinghua University have made great efforts in this field. They used the carbon nanotubes in the water treatment and absorbed the lead ions in the water. It is pointed out that the adsorption of lead ions is greatly affected by the pH value of the solution. The pH value affects the surface charge of the adsorbent, the adsorbate ionic strength, and species. With the increase of the pH value, the adsorption capacity increases significantly. Geng Chenghuai et al. from East China Normal University Nanometer Center took different methods of treatment for the carbon nanotubes before they were used for adsorption of aniline in aqueous solution. The results showed that the untreated carbon nanotubes had the maximum adsorption rate and the treated carbon nanotubes with nitric acid had the maximum adsorption amount. 1.4.4.6 Application in Complex Materials The carbon nanotubes of special property can be used as additives to prepare composite materials, which can not only improve the mechanical properties of materials but also meet the needs of some special functions. Research on the preparation and performance of carbon nanotubes/polymer composite materials, carbon nanotubes/metal matrix composites, carbon nanotubes/ceramic composites has been widely reported, but the problems encountered in the application are the following two points: first, the dispersion of carbon nanotubes in the matrix material; secondly, the interfacial bond between carbon nanotubes and matrix, which is the key to the excellent mechanical property of carbon nanotubes. Usually, the preparation of composite materials also needs to adopt the oxidation treatment to make the smooth surface coarse and produce some functional groups. Carbon nanotubes have special hollow structure and electromagnetic properties, and through activation, they can release the closed pore structure and form abundant pore structure on the pipe wall. Owing to the special structure and dielectric properties, carbon nanotubes exhibit strong broadband microwave absorption properties, and they also have the characteristics of light weight, adjustable conductivity, high temperature and strong antioxidant properties and good stability; thus they are an ideal microwave absorbent and can be used for hidden materials, electromagnetic shielding materials or darkroom wave absorbing materials. Similar to fullerenes, carbon nanotubes are considered to be a new promising material in the future. Carbon nanotubes as a new type of functional nanomaterials will promote the development of physics, chemistry, and materials science, and may lead to a new scientific and technological revolution. Based on the knowledge accumulation in certain, carbon science has made great progress, especially

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in the aspects of adsorption properties. In addition, carbon nanotubes can also be used to make the military potential absorbing material, energy storage materials, and so on.

1.5 Graphene 1.5.1 Structure of Graphene Graphene (GE), a new type of two-dimensional carbon nano-composite material discovered recently, is a fundamental unit to construct many carbon materials, including graphite, carbon nanotubes, carbon nanometer fiber and fullerenes materials [32–35]. In 2004, Professors Geim and Novoselov et al. from University of Manchester first produced GE and received the Nobel Prize for physics in 2010. Till then, the discovery of GE provoked a new round of researching enthusiasm for carbon materials and another researching hot point and advancing front of carbon materials, nanotechnology and conglomeration state physics after the fullerenes and carbon nanotubes. Theoretically perfect GE refers to the single-layered graphite flake composed of the hexagonal lattice work of carbon atoms and the three-dimensional graphite structure is composed of the many layers of arranged in parallel in certain sequence. The basic structure of GE is shown in Figure 1.11. In GE, the carbon atoms of sp2 hybrid orbital are connected with other atoms by means of the strong 3 bonds. These strong 3 bonds make the GE excellent structural rigidity and high strength in the direction parallel flake. A carbon atom has four valence electrons and each atom contributes a non-bond 0 electron to form a big 0 bond above

Figure 1.11: Basic structure of GE.

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and below the carbon atoms on the same layer, thus forming the 0 orbital in parallel and vertical to the GE layer. This kind of delocalized 0 electrons can flow freely on the carbon net plane, similar to the free electrons, so GE has the electrical conductivity and thermal conductivity similar to the metals and its magnetic resistance is also outstanding. GE can be taken as the basic structural unit to form other carbon materials. When the number of the carbon atoms contained in the fraction of GE is bigger than 30 and smaller than 1000, the formed carbon layers have the suspended bonds, namely, the uncombined vacant bonds. To reduce the number of the suspended bonds, the fraction of GE shall roll up to form the curled structure and the six-membered ring on the edge tends to shrink into the five-membered ring. The closed cage-shaped carbon clusters, fullerenes, may form when the number of the five-membered ring reaches to 12 or more. In fullerenes, the C60 molecules formed by 20 six-membered rings and 12 five-membered rings are the most stable. Similarly, the carbon nanotubes can be seen as the cylinder of the nanometer-grade diameter rolled up by the GE layers and the carbon atoms on the surface of the cylinder are in the spiral shape and two ends of the cylinder are covered by the fullerenes hemispherical stopper. Three-dimensional graphite is piled and formed when the layers of GE are connected by the Van der Waals force. GE, the zero-dimensional fullerenes, one-dimensional carbon nanotubes and three-dimensional graphite formed with itself are shown in Figure 1.12.

1.5.2 Preparation of GE So far, the ripe preparation methods for GE are physical methods and chemical methods. The mechanical detaching method was the initial physical method to prepare the GE. British scientists Geim et al. produced a single-layered GE flake first with this method. Additionally the extensional growing is also one of the important methods to prepare GE. UStarker, et al. treated the nitrogen doped 4H-SiC and 6H-SiC monocrystal surface by means of oxygen etching first to remove the oxide film and to chemically inactivate its surface. They got an even gradient base of atom-grade width, and then they deposited Si on the treated base under the condition of super high vacuum; finally they got a very thin GE flake after the sample was annealed at 800∘ C. At present, the mechanical detaching method is the most simple and direct method to prepare GE with low-cost and high-sample quality. GE produced with this method can be 100 ,m thin. But the output with this method is low and uncontrollable and cannot satisfy the industrialization and mass production. It is difficult to find the single-layer GE from the thick layers of big flake, and there are a few glue blots affecting the cleaning of the sample, restricting its application. The chemical gas phase deposition method is the most widely used method currently to produce the semiconductor film of mass production and it provides an

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Figure 1.12: Structure of GE, the zero-dimensional fullerenes, one-dimensional carbon nanotubes and three-dimensional graphite formed with GE.

effective method to prepare GE controllably. With this method, the granular catalysts are not necessary to prepare GE. The base materials, such as plane metallic film and metallic monocrystal, are put in the precursor gases decomposed at high temperature, such as methane and ethane, to make the carbon atoms deposit on the surface of base through the high temperature annealing to form the GE. Finally the metallic base is removed with the chemical corrosion method to obtain the GE flake. The growing rate, thickness, and area of GE can be controlled by selecting the type of base, temperature of growing, the flow rate of precursor and other parameters. The single-layer GE or mutilayer GE of square centimeter class has been prepared with this method and its advantage is to prepare a large-area GE. For example, polycrystal nickel base was put in the hydrocarbon of low concentration at 900–1000∘ C to prepare the multilayer GE film of 20 ,m with the chemical gas deposition method at the atmospheric pressure. The prepared GE could be single layer at the thinnest place, and the GE deposited on the nickel can be transferred to any base by means of chemical etching, expanding the application of GE. Although the GE of different size and different thickness was prepared with these simple and repeated methods, only the marginal GE can be obtained with these methods, difficult to be applied for the mass production and application.

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Reduction oxidization graphite method is one of the widely used methods to produce GE of mass production. The reduction oxidization graphite method is to reduce and oxidize graphite with certain means to remove the oxygen-containing groups on the surface of graphite to restore the flat conjugation structure of GE. The advantages of this method are the available raw materials rich in flake graphite, the relatively simple producing equipment and the mass production. The currently reported reduction oxidization graphite methods to prepare GE are thermal reduction, chemical reduction, UV-light reduction, and so on. The thermal reduction method uses the microwave as the heat source. It is manifested from the experiments that the GE prepared with the microwave thermal reduction method can be stably dispersed in DMAc to form the organic suspension and be stored stably for months at room temperature and the conductivity of the processed “graphene paper” is 104 times of that of the GE oxide. An excellent effect of the thermal reduction is to destroy the flake structure of the GE. During the thermal reduction, almost 30% of the graphite oxide is lost and many holes and state defects are formed on its surface. Clearly, these defects can affect the electrical performance of the products, but its conductivity can still be 1000–2300 s/m. This means that the thermal reduction is a very effective reduction method. Chemical reduction method is to transform the graphite into graphene oxide and then to reduce the graphene oxide to prepare GE. This method is characterized by low price and availability of raw material and simple producing. It is the most possible way to produce GE of mass production at present. The conventional reducers of this method are NaBH4 and hyazine. The conductivity of the GE prepared with the chemical reduction method is almost the same as that of the original graphite, and the prepared GE has a high specific area. Furthermore, the GE oxide after the reduction can generate the unsaturated, conjugate carbon atoms to increase dramatically the conductivity of GE, so the reduced GE oxide can be used as the hydrogen storage material or filler of electrical conductance in the field of composites. In the chemical reduction method, more than two kinds of reducers can be used to reduce the GE oxide by steps. First NaBH4 is used to reduce preliminarily the GE oxide to remove the most oxygen-containing groups and to increase the number of the sp2 hybrid orbital atoms for sulfonation reaction in the structure of GE oxide. Then the preliminarily reduced GE oxide is slightly sulfonated arylsulphonate to increase the hydrophilic groups of GE. Finally the strong reducer hydrazine is used to reduce again to remove the remained oxygen-containing groups. The GE treated by these three steps has gotten rid of most of the oxygen-containing groups. The prepared slightly sulfonated GE with the concentration of 2 mg/ml can be stably dispersed in aqueous solution with pH from 3 to 10. The problem that GE cannot be dispersed stably in the aqueous solution is initially solved. Other reducers include hydroquinone, hydrogen and strong alkali solutions. Reduction with hydrogen has been proven very effective, with the proportion of carbon

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and hydrogen reaching 10:8.1. Sulfuric acid and other strong acids can also be used to catalyze the surface of GE to dehydrate. After the reduction for the GE oxide with reducer, there are still a great amount of light groups, so the catalysis dehydration with strong acids can reduce the GE oxide further to improve its performance. Chemical reduction method seriously destroys the electronic structure of GE and the integrity of its crystal by the strong oxidizers to affect the electronic properties and restrict its application in the precise microelectronic field, but this method is simple, is of low cost and is conducive to the preparation of GE derivative, and a great amount of GE can be produced with this method, widening the application field of GE effectively. The preparation of GE is the precondition of its development and application, and the preparation of sufficient high purity of GE is the basis of the property research and application. Although new preparation methods for GE have been developed continuously, there are still many problems, which cannot be solved in a short time and need further research.

1.5.3 Performance and Application of Graphene Graphene is a typical two-dimensional 0 electron substance. Its rare valence band structure makes it greatly different from other substances of the two-dimensional materials. The particle-like substances in the structure can be described as “massless chiral Dirac fermions,” unique in the flocculation physics, making the graphene many curious electrical properties, including room-temperature quantum hall effect found by Novoselov et al. in 2007. It is the 0 electrons and the 0* electrons in the valence band near fermi level that are related with its electric properties in GE. The unit cell of GE is composed of two unequivalent carbon atoms, where the energy bands between 0 and 0* are connected at the place of fermi level to form a semiconductor of zero energy interval. In addition, 0 electrons are very sensitive to light, electricity and magnet, and 0 electrons in the conjugate system respond to the physical stimulation of light and electricity more quickly than the silicon semiconductor, the single-layer GE, whose electron transferring rate exceeds 200,000 cm2 ⋅ V–1 ⋅ s–1 , can be got. The electron moving rate of this single-layer GE is much bigger than that in silicon, comparable with the supercrystal lattice in the compound of semiconductor. Research shows that GE is very stable and has excellent electronic property, optical property, thermal property and mechanical properties, including high Young’s modulus (1100 GPa), high breaking tenacity (125 GPa), high thermal conductivity (5000 W ⋅ m–1 ⋅ K–1 ), high electron migration (200,000 cm2 ⋅ V–1 ⋅ s–1 ), high specific area (2600 m2 /g), high light permeability and quantum hall effect and ferromagnetism at room temperature. These outstanding properties make it a good application prospect in the field of electronic equipment, energy transformation storage, sensor, catalysis and biomedicine and so on.

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1.5.3.1 Application in the Composite Material GE has excellent properties of electric conductance, thermal conductance, and mechanical property, and it is an ideal nanometer filler for the preparation of high-strength conducting composite. GE dispersed in solution can mix with the monomer of composite to form the composite system. The addition of GE makes the composite multifunctional, showing not only the outstanding mechanical and electric properties but also good processing performance, widening the application of composite. GE of integral structure is a two-dimensional crystal composed of benzene six-membered ring without containing any unstable bond, of high chemical stability, in the noble state on its surface and weak action with other media (such as solvent), and there exists a strong Van der Waals force between the flakes of GE, so the conglobation happens easily to make GE difficult to be dissolved in water and the normal organic solvent, restricting the further research and application of GE. There are many oxygencontaining groups on the surface of GE oxide, such as alcohol groups and carboxyl groups, which make the modification of GE possible. The oxide of GE is the start point to synthesize GE largely, and also the one of the most effective ways to realize the functionalization of GE, preparing the functional polymer nano-composite by taking the GE oxide as the filler, to improve the comprehensive property of nano-composite, such as mechanical property, thermal property and electric property. At present, the researched GE composites are GE/polymer composite and GE/inorganic composite, the preparation methods of which are blending, sol-gel, intercalation and original position polymerization. Ruoff R S prepared GE-polystyrene macromolecule composite by reducing the GE oxide which is modified by isocyanate and dispersed in the polystyrene. The addition of GE does not only improve the conductivity of polystyrene but also decreases the percolation threshold of polystyrene (in the polymer filled with conductivity particles, when the concentration of the filled particle attains certain value, the conductivity of the system change abruptly. This is called percolation phenomenon). When GE of 1% volume fraction is added, the conductivity of polystyrene can reach 0.15/m at room temperature, conducive to the application of this composite in the conducting material. The addition of GE can also affect other physical properties of polymer composite, such as glass temperature. After 1% functional GE is added in polyacrylonitrile, the glass temperature of composite is increased by 40∘ C, while the glass temperature of composite is increased by 30∘ C when 0.05% functional GE is added in polymethyl methacrylate.

1.5.3.2 Application in the Electrode Material Yoo et al. researched the properties of GE and its composites with CNTs, C60 and SnO2 applied for the cathode material lithium ion secondary battery. It was manifested from the research that the addition of GE could greatly improve the specific capacity and circular stability of the cathode material of lithium battery. Kong et al. coated the GE

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flake on the quartz base by vacuum filtering GE suspension, and then they immersed the base into HAuCl4 aqueous solution to synthesize Au/GE composite. By circular operation, the composite was acquired by the layer summation of GE and Au nanoparticle by turns. With the reducing role of GE of electronegativity, this method does not need other reducers. Luechinger et al. coated GE on the surface of Co without using surface active agent and synthesized with polymer (PMMA PEO) to acquire the GE/Co polymer composite. This material has the performance of metal and polymer, which provide a new application for GE. Wang et al. used chemically synthesized GE nano-flake as the cathode material. Its first electric discharge capacity was up to 945 mAhg–1 , and its first reversible capacity is up to 650 mAg–1 . Recently GE/TiO2 composite material was used on the photoelectric anode solar battery of dye sensitization. It is found that the light collecting efficiency and charge transferring rate of solar battery were increased and the charge coupling rate was decreased. The current density of battery was increased by 45%, 39% higher than that using TiO2 only, without affecting the open-circuit voltage and its energy transforming efficiency attained to 6.9%. There are still many problems in using GE as the electrode material, even though it has a wide application prospect. 1 the first irreversible capacity is too high; 2 the capacity can decrease continuously; 3 there is no platform in the charging curve. So there are still many problems to be solved for GE to be used as the cathode material. 1.5.3.3 Application in Photocatalysts TiO2 , which is of strong oxidizability, of complete degradation and of repeated using, is focused in the field of wastewater treatment, light-electricity transformation and the preparation of clean material. The band interval of TiO2 (3.2 eV) is wide, and its chemical activity can be exerted in the UV-light area with wavelength less than 378 nm, so its utilization ratio for sunlight is less than 10%; at the same time, its photoproduction electrons and cavity combine easily, thus its photocatalysis degradation efficiency is decreased. How to improve the photocatalytic activity of TiO2 is the key point to research its practical application. The electron-cavity pair of TiO2 activated by light combines easily, so it is a heat point to use the special electron transferring property of GE to decrease the recombination of photoproduction charge carrier, so as to improve the photocatalytical efficiency of TiO2 . Kamat et al. added the GE oxide into the TiO2 colloidal dispersion for ultrasonic vibration, and acquired the suspension of GE coating the TiO2 nanoparticles. The suspension liquid was illuminated with the UV-light under the protection of nitrogen, and then the TiO2 /GE composite material and TiO2 /Ag/GE three-member system were got. Wang et al. reported the stability of GE flake improved by the anionic surfactant and the method to improve the metallic oxides to grow by means of self-package on GE flake. Because of the hydrophobic nature of GE and the hydrophily of metallic oxides, the surfactant can be used to solve the problem of noncompatibility; at the same

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time, the surfactant provides the molecular mode for the nucleation and growing of inorganic nanoparticles. This team used sodium dodecyl sulfate (SDS) as the surfactant and mixed the modified GE flake with TiCl4 in different proportion to synthesize the composites of GE with rutile type TiO2 and anatase type TiO2 respectively. Zhang et al. used hot water to further synthesize TiO2 /GE composite material. By research, they found that this composite material can not only adsorb the organic dye very well but also expand the application scope of visible light and separate effectively the photoproduction electrons and cavity due to the introduction of GE. Zhang qiong et al. prepared TiO2 –GE oxide intercalation composite material of nanometer class at low temperature ( 7) Further oxidization of the adsorbed hydrogen atoms by means of electron release under the condition of acidity or alkalinity can be expressed as the above equations. Mn+ + ne → M The precipitation process of metal ion, Mn+ , on the surface of bases after gaining electrons is expressed as the above equation. 2H+ + 2e → H2 (PH < 7) 2H2 O + 2e → H2 + 2OH– (PH > 7) The generation process of hydrogen under the condition of acidity or alkalinity can be expressed as the above equations.

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It can be seen from the above reactions that the dehydrogenation reaction of the reducing agents may undergo three different reactions according to the pH value and solution system of the precipitation and their products are H2 , H2 O, and H+ , respectively: 2RH + 2OH– → 2ROH + H2 + 2e RH + 2OH– → ROH + H2 O + 2e RH + OH– → ROH + H+ + 2e It can be seen from the above three equations that no matter in what state the adsorbed Had exists, the reduced effects are the same and the utilization ratios of the reducing agents are different. Electrons of 1 mol are released to reduce the metal similarly, but the necessary mole numbers of the reducing agents are different. Van den Meerakker mechanism can be used to explain the oxidization process of all reducing agents, so it can be taken as the general mechanism of electroless plating. 2.1.3.1 Common Activation Methods of Electroless Plating Currently two activation methods of electroless plating are widely applied: the fractional activation and the colloidal palladium activation. Sensitization-activation Method. This method has two steps. First, the acidic solution of tin chloride is used to sensitize the surface of the base to adsorb a layer of stannous ions. The stannous ions shall be hydrolyzed into Sn2 (OH)3 Cl in the following washing process. Then the base shall be activated in the aqueous solution of 2–4% PdCl2 , AuNO3 , or AgNO3 . The essence of activation is to plant the catalytic metallic particles on the surface of the base for the reduction of metallic ions and the oxidization of hypophosphorous acid. At present the palladium chloride method is the most widely applied, the reaction equation of which is as follows: Sn2+ + Pd2+ = Sn4+ + Pd It is easy to compound the solution for the sensitization-activation method, which is of good effect, especially for the ceramic material. But this involves high costs and is a complicated process. Repeat washing with distilled water is necessary between the steps of the process. The stability of the sensitization solution is poor. So this method is used in the laboratory and in small batch productions. Colloidal Palladium Activation. The sensitization-activation invented by Shipley in 1961 was called the second-generation activation method, also called the colloidal palladium activation method. Currently the colloidal palladium solution applied domestically is compounded with the following method:

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Solution A: PdCl2 lg/L, HCl 200 mL/L, SnCl2 ⋅2H2 O 2.5 g/L, H2 O 100 mL/L Solution B: HCl (37%) 100 mL/L, SnCl2 ⋅2H2 O 70 g/L, Na2 SnO3 ⋅7H2 O 7 g/L Tin chloride needs to be added finally for compounding solution A. After solution A has been stirred for 12 min timed from the addition of tin chloride, we added solution B slowly to solution A. The compounded colloidal palladium solution is put in (45 ± 5)∘ C water for 2 hours for temperature holding. The colloidal palladium activation solution of good property is obtained after the deionization water is added for dilution to 1 L. Temperature holding can not only improve the catalytic activity of the palladium ions but also prolong the life of the activation solution. The activity and stability of the colloidal palladium solution depend on the concentration ratio of Sn2+ and Pd2+ in solution A. When the concentration ratio of Sn2+ and Pd2+ in solution A is 2:1, the activity of the activation solution is the best. When colloidal palladium is used to activate a nonmetal, the temperature of the solution is 18–30∘ C and the activation time is generally 3–10 min. After the activation and water washing, the sample is immersed into the aqueous solution of hydrochloric acid for colloidal elimination. The colloidal elimination is to pull off the two valence tin ions around the palladium particles and to disclose the metallic palladium particles with catalytic activity. 2.1.3.2 Development of the Activation Method of Electroless Plating In order to get a metal coating with high activity on the surface of the nonmetallic material, to decrease the cost of electroless plating and to make a quality coating after the pretreatment for the surface of the nonmetallic material, many modifications and innovations were made in the activation methods. Modification for the Activation with Noble Metal. Severin conducted three-step activation tests based on the two-step sensitization-activation method. The process is as follows. First, the acidic solution of SnC12 is sensitized and washed. Then the activation of the AgNO3 solution is conducted. The final step is the activation of the PdC12 solution. It is thought that Ag can improve the even nucleation of palladium on the surface of the base, so the effect of three-step activation is better for obtaining an even catalytic activation coating; it helps improve the quality of costing. But this method is too complicated to be applied for industrialization. Yuan Gaoqing, et al. modified the activation process of plastic surface. Palladium chloride was dissolved in glycol butyl ether and then proper quantity of acetic acid and water were added to the solution in sequence. Polyoxyethylated alkylphenol and butyl acetate were added to the solution under the condition of stirring. Stirring is continued for 20–30 min after complete dissolution of the solution. The activation process for the Acrylonitrile Butasdiene Styrene plastic (ABS) plastic is as follows. First, grease elimination for ABS plastic happens and then it is washed with water. The activation is conducted for 20–30 min at normal temperature. The final step is washing with water.

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Among these steps, glycol butyl ether was used as the carrier of palladium chloride, which adsorbs PdC12 tightly on the surface of the base without carrying the plating solution by means of its humidification and permeation on the surface of plastic. Polyoxyethylated alkylphenol and butyl acetate can coarsen the surface of the base to make the clad layer tight. WU Guohui, et al. added varnish made of an organic substance to hydrochloric acid to make the activating agent by using acetate fiber to adjust the viscosity. The produced activating agent was pasted on to the partial surface of a ceramic to make its surface attain good catalytic activity. Activation for the Non-noble Metal. The activation process of the nonmetal electroless plating mentioned above needs a noble metal, so a large amount of silver and palladium are consumed every year. In order to save the noble metal, it is meaningful to research the activation process by replacing the noble metal with non-noble metal to decrease the cost. Gao Deshu, et al. from XiangTan University compounded the hydroxide colloidal solution of metals, such as cobalt, nickel, and copper, by controlling different reaction condition to activate the base of ABS. They all have different catalytic activities. Among them, the mixed colloidal solution of copper and nickel has the best activation effect. When ABS plastic is activated at room temperature with the solution with a concentration ratio of Cu2+ and Ni2+ from 1 to 2 and pH value of the solution from 6.0 to 8.0, electroless copper plating after being reduced with NaBH4 has good effect. But the binding force of the clad layer is poor and its coating is not even and complete. Its activity is less than the activating agent produced with palladium salt. Noble metals silver and palladium were replaced with n-type semiconductor oxidizers (such as SnO2 , Ni2 O3 , MgO, and ZnO) to prepare activating agents and then the prepared activating agent was adsorbed on the surface of a material with the physical vapor deposition (PVD) method, chemical vapor deposition (CVD), and pasting. The catalytic activity of the semiconductor oxidizer of this activating agent is good for electroless copper plating but poor for electroless nickel plating. The process is complicated and the binding force of the clad layer is poor for electroless nickel plating. Li Bin, et al. from Chongqing University compounded a nickel salt activating solution of pH 8 or so with nickel salt, proper quantity of ammonia water, stability and distilled water to activate the ceramic or glass base. The valid composition of this activating solution is the easily adsorbed and decomposed nickel salt. The functions of ammonia water and stabilizer are to complex with Ni2+ to stabilize the nickel solution of high concentration, and to improve the activating solution to moist and permeate the surface of the base. The base body was activated for 20 min in the activating solution and then it will be thermally treated for 20 min at a temperature of 200–250∘ C. Nickel salt was decomposed into metal nickel and a little nickel oxide on the surface of the base body. Chemical deposition can be conducted after washing. It was proven that this activation method has good effect and an even clad layer.

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2.1.3.3 Electroless Silver Plating Silver, the resistance ratio of which is 1.60 × 10–3 ,K ⋅ m at room temperature, is the best material of conductivity. Silver cannot be oxidized at normal temperature, but it can be dissolved in nitric acid, easily dissolved in hot concentrated sulfuric acid and slightly dissolved in hot dilute sulfuric acid. Silver oxide film can be produced on its surface when it is in hydrochloric acid and aqua regia. When silver is exposed to a sulfide, black silver sulfide can be formed. The standard potential of silver is >0 = +0.7991 V, so the silver ions can be reduced easily and the reaction rate of electroless silver plating is rapid. Because silver has good conductivity, stable chemical property, and good reduction capacity, the powder produced from electroless silver plating is expected to be taken as an agent in electromagnetic shield and wave absorbing materials. Reaction Mechanism of Electroless Silver Plating. The reduction mechanism of Ag+ in the electroless plating process is still disputable. One explanation is that silver deposition is different from electroless plating of Ni, Fe, and Co. This process is not self-catalytic. The silver deposit is coagulated by the colloidal particle Ag in the solution. The basis of this explanation is that Ag can be deposited on an unactivated surface and the induction period can be observed sometimes. Another explanation is that this is still a self-catalytic process but the self-catalytic ability is not strong. The basis for this explanation is that Ag was deposited on the surface of the activated plating parts and the plating solution is stable for 10–30 min. The complex formed by silver and most of the complexant is not as stable as argentocyanides. Because cyanide is toxic, it cannot be used as a complexant generally. But the amino complex (Kd = 7.2) with less stability is used to slow down the bulk reaction. The reaction mechanism is as follows: 2AgNO3 + 2NH3 ∙ H2 O = Ag2 O + 2NH4 NO3 + H2 O 2Ag2 O + 4NH3 ∙ H2 O = 2[Ag(NH3 )2 ]OH + 3H2 O The reason for the unstable electroless silver plating bath is that the high standard potential of silver, greatly different from the potential of the reducing agent, makes reduction of Ag+ from the solution easy. Stable complex system is beneficial to slow down the bulk reaction. Composition of Electroless Silver Plating Solution. 1. Main salt: AgNO3 is used as the main salt for Ag supply. 2. Complexant: A complexant is used to form a complex with the silver ions to improve the stability of the plating solution and the quality of the clad layer. The common complexants are ammonia water and cyanide. Ag+ ions exist in the form of Ag(NH3 )+2 , Kd = 7.2, and Ag(CN)–2 , Kd = 7.2, in the solution. Ag(CN)–2 is not commonly used because of its toxicity and overstability.

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3.

4.

5.

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Reducing agents: As Ag+ has high potential, a lot of reducing agents can be used. The commonly used reagents are formaldehyde, dextrose, seignette salt, hydrazine sulfate, diformyl, borohydride, aldolactol, and triethanolamine. Modifier of pH value: Generally dilute nitric acid is used for decreasing the pH of the plating and sodium hydroxide for increasing the pH of the solution. Stabilizer: The electroless silver plating bath is not stable and its life is short, so a stabilizer should be added to prevent from turbidness and decomposing. The commonly used stabilizers are gelatin; iodide; inorganic salts of Cu2+ , Ni2+ , FiNi2+ , Hg2+ , and Pb2+ ; and sulfocompounds such as thiocarbamide and thiosulfate. Recently, cystine, cysteic acid, two methyl two sulfur amino acid vinegar, 12 alkyl ammonium acetate, diiodothyrosine, etc. were researched. Polymerization retardor: When formaldehyde is used as the reducing agent in the plating solution, white floccus paraformaldehyde deposit is liable to be formed at low temperature. So polymerization retardor is added to avoid the production the paraformaldehyde. The commonly used polymerization retardor is ethanol or methanol.

The Influence Factors for Electroless Silver Plating. The coating effect of electroless silver plating is affected by pH, loading quantity, concentration of plating solution, the choice of reducing agents (formaldehyde, hydrazine hydrate, etc.), the loading rate of reducing agents, and the reaction time. If the pH is high, then the precipitation ratio of silver ions is also high, and the silver mirror phenomenon may appear simultaneously. The effect of temperature is the same as that of pH. 1. Concentration of silver ions: If the concentration of silver ions is too high in the solution, the stability of plating solution shall decrease and silver is liable to precipitate. At the same time, the life of the plating solution is shortened and the clad layer is coarse and gray. 2. pH: The deposition rate increases with the increase of pH. If pH is too high, the oxidization trend of the metallic ions is increased and the deposition rate decreases. In the plating process, the fluctuation of pH is high and the plating solution decomposes sometimes. Therefore, the pH should be investigated and regulated during the plating process to maintain normal deposition. 3. Mixing ways: The electroless silver plating solution is very active, so the reducing solution and the main salt solution need compounding and the method of mixing the two solutions influences the coating effect. Generally the reducing solution is mixed with the base body material and then the main salt solution is dropped in. If the dropping is too fast, the partial concentration of silver ions is so high that the silver shall be deposited from the solution. If the dropping is too slow, the efficiency of coating reaction may be affected.

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2.1.3.4 Electroless Plating for Iron, Cobalt, and Nickel Nickel, iron, and cobalt are liable to be magnetized and characteristic of soft megnetism. For the electromagnetic shielding and wave absorbing, nickel, iron, and cobalt not only can consume the energy of the electromagnetic wave by means of magnetic eddy loss but also can shield the electromagnetic field by means of the electric loss of conductive metals to prevent the electromagnetic wave from penetrating. Alloy of nickel, iron, and cobalt is a kind of good magnetic material. Permalloy, the nickel content of which ranges from 35% to 90% and the saturated magnetic induction intensity of which ranges generally from 0.6 to 1.0 T, is the most commonly applied alloy of nickel, iron, and cobalt. It has high magnetic conductivity in the weak magnetic field and high initial magnetic conductivity, making it sensitive to electromagnetic wave absorption. The permalloy, widely used today, is made by adding some other elements, such as aluminum and copper, to the system of nickel, iron, and cobalt. The purpose of adding such elements is to increase the resistance ratio of the material to control the eddy loss of the produced iron core. At the same time, the added elements can also improve the hardness of the material, especially for the application of the magnetic head. Hardness of the alloy is not a main parameter for the absorbing material of electromagnetic wave, so it is not necessary to increase the hardness of the alloy by adding other materials. (1) The reaction mechanism of electroless plating for nickel, iron, and cobalt. The mechanism of electroless plating for nickel has been researched for a long time. Issues in the deposition of Ni-P were analyzed from different angles. The above-mentioned mechanism has been carried out by the research of deposition dynamics, catalysis, nucleation process, and the oxidization of H2 PO–2 . The reaction of electroless plating for nickel, iron, and cobalt is to add Fe2+ , Co2+ reducing agents, complexant, buffer solution, and pH regulator to the nickel plating solution, so the reaction mechanism of electroless nickel plating will be introduced hereafter. Generally, the electroless nickel plating process is thought to be divided into dehydration, oxidization, and phosphorous deposition. Dehydration Oxidization

H2 PO–2 → HPO–2 + H HPO–2 + OH– → H2 PO–3 + e

Recombination Oxidization

H + H → H2

H + OH– → H2 O + e

Metal deposition

Ni2+ + 2e → Ni

Hydrogen precipitation Ni2+ + 2e → Ni – – Phosphorous precipitation mNiL2+ 2 + H2 PO2 + (2m + 1)e → Nim P + 2mL + 2OH

The chemical potential of nickel is –0.25 V, iron –0.44 V, and cobalt –0.28 V. Taking the sodium phosphate as the reducing agent, seignette salt and sodium citrate as the complexant to control the stability of ions and regulating the temperature and pH, the

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electroless plating can take place, so long as the reduction potential of the reducing agent in the solution is higher than the chemical potential of the metals nickel, iron, and cobalt. Some catalytic metals are adsorbed onto the surface of the pretreatment powder and the plating silver powder, so nickel, iron, and cobalt metals can deposit on the surface of the base body powder. (2) Influence factors of the electroless plating process for nickel, iron, and cobalt. Like electroless silver plating, pH, temperature, loading quantity, concentration of the plating solution, the consumption of the reducing agent, the consumption of the complexant, the reaction time, and mixing method of the plating solution in the electroless plating process all affect the coating of electroless plating for nickel, iron, and cobalt. If the pH of the solution is higher, the deposition ratio of the particles of nickel, iron, and cobalt will be higher and the nickel mirror may occur too. The effect of the increase in temperature is the same as that of pH. The complexant affects the reaction rate and the stability of the plating solution and the reducing agent affects the ultimate deposition quantity of nickel, iron, and cobalt.

2.1.4 Precipitation Method The precipitation method is to add proper precipitant in the raw material solution to make the positive ions in the raw material solution form all kinds of precipitates, the shape and dimension of which is controlled by the reaction condition, and then the particles shall be filtered, washed, dried, and sometimes even decomposed by heating in sequence to acquire nanometer particles. There are direction precipitation, co-precipitation, and even precipitation and hydrolysis. The direct precipitation is to make some metallic ions in solution react to form the precipitate. But this method is hardly used because of its limitation. If there are many kinds of metallic ions in the raw material solution, and they all exist homogeneously in the solution, all kinds of homogeneous precipitate will be obtained after precipitation. This is called the even precipitation method. The precipitant is generally a hydroxide, hydrous oxide, oxalate, or carbonate. It is an important method to prepare the composite nanometer grain containing more than two metallic elements. For the homogeneity of the precipitate, the salt solution containing many kinds of positive ions is generally added to the overdosed precipitant and stirred violently to make the concentration of all the precipitation ions much higher than the equilibrium concentration of precipitation, so as to precipitate every component simultaneously according to the proportion for the even precipitate. The common precipitation is not balanced. If the concentration of the precipitant in the solution is controlled to increase slowly, then the precipitation in the solution can be kept at a balanced state. In order to avoid the partially uneven concentration because of adding the precipitant directly, the slow release formulation can be added into the solution to slow down the formation of precipitant by the chemical reaction in the solution. So long as the formation rate of the precipitant is controlled, the uneven concentration will be avoided. The degree of supersaturation is controlled properly,

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so as to control the formation rate of the particles for getting the nanometer composite of high purity and little flocculation. This process is called the even precipitation method. The representative of slow release formulation is urea. The aqueous solution of urea is decomposed at about 70∘ C in ammonia water, acting as the precipitant. One of the important methods in hydrolysis is the metal alkoxide hydrolysis method, by which some organic alkoxides can be dissolved in an organic solvent and be hydrolyzed to form the precipitate of hydroxide or oxide for preparing nanometer composite particles.

2.1.5 Micro-emulsion Method The micro-emulsion is a thermodynamically stable, isotropic, transparent, or subtransparent dispersion system formed by two insoluble liquids. In microscopic view, it is composed of one or two liquids, the interface of which is stabilized by the surfactant. Compare with the emulsion, the grain diameter of micro-emulsion is smaller (less than 100 nm). Used in the preparation of nanometer particles, micro-emulsion technology generally includes nanometer reactor technology and micro-emulsion polymerization technology. The nanometer reactor generally refers to the Water/Oil (W/O) micro-emulsion. Because the W/O micro-emulsion can provide a micro-water core, the needed nanometer grain can be obtained by means of the reaction of the water-soluble substance in the water core. The W/O micro-emulsion system consists of three phases. They are the grease continuous phase, the water cell, and the interface between the surfactant and the cosurfactant. The constituent parameters of micro-emulsion include the grain diameter and the average concentration class of the surfactant. After the micro-emulsion system is determined, the preparation of nanometer grains is conducted by mixing two micro-emulsions with different reactions. There are many nanometer grains prepared with opposite phase emulsion, like some functional and high added-value products, including nanometer magnetic composite material and semiconductor nanometer.

2.1.6 Chemical Vapor Deposition CVD prepares the nanometer material of all kinds of substances by taking the vapor of the volatile metallic compound or the organic metallic compound as the raw material to get the needed substance by means of chemical reaction, condensing rapidly in the environment of protection gases. CVD is to make the reaction products congregate into many nucleuses automatically when a high supersaturated vapor pressure is formed under the condition of far higher than the thermodynamic arithmetic reaction temperature. These nucleuses grow in the heating area and congregate into grains. With the gas flow entering the low temperature area, the growing, congregating, and crystallizing grain stop and nanometer composite grains are collected in the collection

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room ultimately. The composition, shape, size and crystallization phase of the grains can be controlled with this method by setting the process conditions, such as the proper concentration, flow rate, temperature and mixture ratio, etc. CVD makes the precursors decompose, congregate into nucleuses and grow into nanometer composite grains taking DC isotope, microwave isotope or laser as the heat source by means of high temperature fragmentation principle. With this method, the nanometer grain of less than 50 nm with even grain diameter and controllable size can be obtained. The raw materials of CVD are metallic oxides, metallic hydroxides, metallic alkoxides, alkylates, carbonyl compounds and their mixtures of easy preparation, high vapor pressure, and good reactivity. Besides the common electric oven, the heating styles are also chemical flame, isotope, laser, and so on. Especially the last two heating ways were used more.

2.2 Characterization Technology of Composite Material The chemical composition and the structure of the composite material are the key factors to determine its properties and application, especially for the characterization of the composite material in the scope of nanometer grade. The characterization technology of composite materials is classified as structure characterization and property characterization. The structure characterization mainly refers to the configuration, topography, grain size and distribution, and the material composition of the composite system. The property characterization refers to the description of the composite system. Only when all kinds of fine structure of the material are characterized and known can the composite system be controlled effectively, so as to design and synthesize composite materials according to the demand of property. The structure characters of composite material, including the structure of the surface atomic layer, can be characterized with X-ray diffraction photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and ion energy loss spectroscopy. The interface structure and interaction can be characterized with many technologies, such as XPS, AES, laser Raman spectroscopy and infrared spectrography, researching and characterizing the interactions of nanometer grains/polymers. The property of the interface of the polymer can be characterized with the thermoanalysis method and the dielectric spectroscopy.

2.2.1 Electron Microscopy 2.2.1.1 Scanning Electron Microscopy Scanning electron microscopy (SEM), a technology developed on the basis of a scanning electron microscope in the 1930s, has a resolution rate less than 6 nm, strong

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stereoscopic perception and big field of view. Its image principle is different from that of optical microscopy and transmission electron microscopy. SEM is mainly used to observe the topography of the grain and the distribution of the grains in the base body, widely used to observe the congregation state of the nanometer grains in the composite material and to reflect the size of the congregated nanometer grain. The general method can provide the size and topography of the micron congregated grains. Compared with the optical microscopy and transmission electron microscopy, the SEM has its unique advantages, including: 1. High resolution rate: Recently, with the application of field-emission electron gun and the development of superhigh vacuum technology, the resolution of the scanning electron microscopes was further improved and the resolution of some advanced scanning microscopes can be 0.6 nm. 2. The amplification is high and adjustable continuously. 3. Its depth of field is big and its image has a cubic effect. It can be used to observe directly the fine structure of the irregular surface of the samples. 4. It is multifunctional. Cooperated with wavelength dispersion X-ray spectrometer and energy dispersion X-ray spectrometer, it can be used to make the composition microanalysis while the topography is observed. 5. Simple sample preparation. The sample, untreated or slightly treated, lumpy or powdered, conductive or nonconductive, can be observed with the scanning microscope directly. Its sample preparation is simpler than that of a transmission electron microscope (TEM). Its image is closer to the true state of the sample. 2.2.1.2 Transmission Electron Microscope Technology The electron optical analysis technology, taking the electron beam as the illumination source, and imaging with electromagnetic lens, is called the transmission electron microscope technology, the resolution of which is about 1 nm. The TEM technology, one of the widely used characterization technologies of the microstructure of nanometer materials, is used to research the crystallization of a nanometer material, to observe the topography of grains and the distribution and to measure and assess the diameter of the grains. It can be used to observe the structure of aggregation state of grains, and even the distribution state of a single nanometer grain. The combination of TEM technology and image processing technology can be used to determine the shape, size, distribution, distance and fractal dimension of grains. Sample preparation is very important for the analysis with a TEM. The penetrability of electron beam is low, so the sample for the TEM analysis must be very thin. Except the powder, the sample should be made into film, that is, the sample should be “transparent” to the electron beam. According to the atomic ordinal number of the sample, the thickness of a thin area of the sample for the observation with TEM ranges from 50 to 100 nm. If a powdered sample is observed, the grains of the superfine powder should be dispersed and isolated without aggregation. For the superfine powdered sample, its diameter is generally less than that of a copper wire mesh, so it

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cannot be put on the copper wire mesh directly. A copper wire mesh with a support film should be used, like plastic film and carbon film. 2.2.1.3 Electric Acoustic Microscope Technology Electric acoustic microscope technology, generalizing the modern electronic optical technology, electric acoustic technology, piezoelectric sensor technology, weak signal detection technology, and image processing technology, is a new type of lossless analysis and microscopic imaging technology. Based on the change of the microelectricity and the thermoelastic property of a material, the imaging mechanism can be used to get some unique information. It is a comprehensive multifunctional microscopic technology, integrating the micro-area physical property research (calorifics, acoustic mechanics, electricity and magnetics, etc.), non-destruction internal defect detection (air hole, impurities, lamination, microcrack and dislocation, etc.) and the structure analysis not requiring the pretreatment for the sample (crystalline, grain boundary and domain), and it can be used to observe the sample’s secondary electron image, electric acoustic image and the thermoelectric-stimulated current image at the original position. The electron microscopic technology is widely used in the structure characterization of composite materials. For example, carbon nanotubes (CNTs) were first discovered through SEM by Sumio Lijima. Up to now, the characterization concerned with CNTs mainly uses the technology of SEM, TEM, and high resolution transmission electron microscopy (HRTEM) to observe. SEM is mainly used to observe the surface structure of the CNTs, while TEM technology can not only observe CNTs’ structure but also get some other internal structural information. Some important information, such as the number of layers of multi-wall CNTs and the position of a catalyst in the tube (CNTs made with CVD), can be clearly observed through HRTEM technology. It is very important to the research of the structure and the production of CNTs.

2.2.2 Thermal Analysis Technology Thermal analysis technology is used to measure the change of property of a sample according to time or temperature in a given environment, where the turbidity of the sample is program controlled. The thermal analysis is a general analysis technology. It has the following characteristics in measuring the thermal stability of the composite material. The stability of a sample can be researched under the condition of temperature change within a wide temperature range. The physical state of the composite material can be solid, liquid, or gel. The sample needed is very little, generally 0.1 ,g–10 mg. The measuring time is short, from several minutes to several hours. Much information can be obtained with this technology. The commonly used thermal

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analysis technology includes thermogravimetric (TG) analysis, differential scanning calorimetric (DSC) analysis and dynamic mechanic analysis. 2.2.2.1 Thermogravimetric Analysis TG analysis is a method to measure the relation between the weight of a sample and temperature or time at the program temperature. The temperature program includes temperature increase, temperature decrease and a constant temperature. The influence factors of the test accuracy are temperature increase rate, environment and sample state, and so on. Mass change for some substances goes with the heating process, so the TG analysis contributes to research the change of crystal property, such as melting, vaporization, sublimation, and adsorption, and also to the research of some chemical properties, such as dehydration, deionization, oxidization, and reduction. 2.2.2.2 Differential Scanning Calorimetric Analysis DSC analysis is to measure the relation between the power and the temperature of the input sample and the reference material under the program temperature control. This method is useful for the determination of Tg and Tm of the composite material. The nanometer grains confine the thermal movement of the segment of polymer to some extent, so Tg and Tm are increased. The influence factors for the calorimetric curve of this method are as follows. (1) Samples: Sample consumption is little and the dissolution of sample is high but the sensitivity of measurement is decreased. The sample consumption is regulated according to the thermal effect of the sample, generally 3–5 mg. In addition, the sample consumption affects the measurement of the conversion temperature. Besides the consumption of the sample, the grain size of the sample also affects the calorimetric curve. It is noted that the geometric shape of the sample also affects the calorimetric curve outstandingly. (2) Temperature increase rate: The general temperature increase rate range is 5–20∘ C/min. Generally speaking, the more the temperature increase rate is, the higher the sensitivity will be and the resolution will decrease. The sensitivity of the measurement is contradictory to the resolution. We generally set the higher temperature increase rate to keep a good resolution. But the sensitivity of measurement is improved by adding the sample quantity properly. (3) Environment: If inert gases, such as nitrogen and argon, are used, the oxidization reaction peak will not occur generally and the monitor erosion from the volatile substance of sample can be decreased. But the rate of gas flow should be constant. It should also be noted that the property of these gases affect the determination outstandingly. The DSC analysis can be used to determine the specific heat capacity, isenthalpic change, and purity of the sample.

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2.2.3 Spectroscopic Technology With the research of the characterization technology of the composite material, a series of spectroscopic technologies have been applied, including nuclear magnetic resonance (NMR) technology, laser Raman spectroscopic technology, Fourier transform infrared spectroscopic technology, and X-diffraction technology. 2.2.3.1 Nuclear Magnetic Resonance NMR spectrum is an absorption spectrum formed by the transition of the nucleus with magnetic moment radiated by the magnetic wave. When a magnetic wave of certain frequency is used to illuminate the sample, the nucleus in the specific chemical environment is forced to realize the resonant transition. The positions and strengths of the signal recorded in the resonance during the illumination scanning are the NMR spectrum, where the position of the signal reflects the partial structure of the molecule of the sample (such as the functional group and molecule conformation) and the strength of the signal reflects the quantity of the nucleus in the molecule of the sample. Up to now, the commonly used nucleuses are 1 H, 11 B, 13 C, 17 O, 19 F, 31 P, and so on. The configuration of an organic compound structure can be well identified with the NMR, combining with infrared analysis and elementary analysis. X.-P. Tang, et al. researched the electronic structure of single-walled carbon tubes with 13 CNMR and acquired two kinds spin lattice relaxation rates. The component with a fast relaxation rate is the carbon tube with metallic property and the component with a slow relaxation rate is the carbon tube with semiconductor property. Xu Min, et al. characterized the soluble multi-walled carbon tube with Octadecylamine (ODA) grafted with the solid NMR technology and did some concerned research on the structure of the aggregation state of the ODA chain grafted on the carbon tube by means of 13 C Cross Polarization/Magic Angle Spinning (CP/MAS) experiment. M. Schmid, et al. performed a series of research on the hydrogen-storing ability of the single-walled carbon tubes with the high pressure hydrogen loaded by means of proton magnetic resonance (1 H-NMR). Because of the paramagnetism of the carbon tube itself and the metallic catalyst introduced during the preparation of the nanotubes, it is difficult to use NMR technology in the research of carbon tubes. It is prospective to use NMR technology in the research of the modified carbon tubes grafted with the other substances or the composite of carbon tubes and polymers. 2.2.3.2 Infrared Spectrum Technology The infrared spectrum technology is a molecule vibration spectrum, concerned with the energy transition caused by the chemical vibration among molecules. When molecules are radiated by the infrared light and vibration energy transition takes place, the infrared light is absorbed by the molecules to form the infrared absorption spectrum. Its vibration frequency is characteristic of the special group in the molecule.

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Except the optical isomerism, each compound has its infrared absorption spectrum, the band of which is complex and fine, providing rich structural information. The applications of infrared absorption spectrum in the material characterization are as follows: (1) Research on the structural base of molecules: Bond length and bond angle are measured with infrared absorption spectrum to infer the cubic configuration of molecules. (2) The strength of the chemical bonds can be known according to the acquired mechanic constant and the thermodynamic function can be calculated with the normal frequency. (3) Analysis of chemical composition: The structure of unknown substances can be inferred with the positions and the shapes of the absorption peaks in the spectrum and the content of each component in the mixture can be determined according to the strength of the characteristic absorption peak. The infrared absorption peak of inorganic compounds, simpler than that of the organic compounds, has less bands, lower than 1,000 cm–1 mostly. The absorptions of inorganic substances in middle infrared are caused mainly by the lattice vibration of anions. The position of its absorption spectrum seldom has relation with the cathodes. When the atomic ordinal number of anions increases, the absorption position of anions will make the microwave displacement toward the low-frequency direction. Therefore, the vibration frequency of the anions group is emphasized when the infrared spectrum of inorganic compounds is identified. According to the infrared absorption spectrum, the types and quantity of the surface groups of carbon material and the changing of the surface group after modification can be determined. The carbon atoms in the nanotubes are mainly bonded with sp2 hybridization. When nanotubes are treated with oxidization purification and chemical modification, the polar functional groups, such as carboxyl groups and alcohol groups, are normally introduced on the surface of the carbon tube. The treated carbon tube can be characterized with infrared spectrum. 2.2.3.3 X-diffraction Technology When X-ray is used to illuminate crystals, the diffraction patterns produced are mainly affected by the crystal structure, besides the X-ray. Through the analysis of the diffraction pattern, the crystal structure can be determined and a series of issues concerned with the structure can be researched. The X-ray diffractometer manufactured according to the X-ray diffraction principle has been used widely. With the characteristic of convenience, rapidness and high precision, it is mainly used to analyze the structure of the crystal and to research the direction and size of crystal grain and the position of crystal cell in the atoms. The phase analysis of crystal materials, including qualitative analysis and quantitative

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analysis, can be made through X-ray diffraction. That is, the phases of the material and the content of each phase can be determined. X-ray diffraction technology can be used to observe the crystallization of nanotubes and research the phase of the catalyst in the product and in the process of the reaction of nanotubes. Chen Ping, et al. found through X-ray diffraction experiment that two kinds of manufactured CNTs, taking CH4 or CO as the carbon source, had the same phase structure as graphite. But the wide characteristic peak of X-ray diffraction shows that the long range order parameter of this nanometer structure is not as high as that of graphite and a higher reaction temperature is beneficial to the improvement of the degree of graphitization of the product. A. Bougrine, et al. used X-ray diffraction technology to characterize and research the catalyst in the purified nanotubes. The structure and property of a composite can also be characterized with other modern analytical technologies, which will not be shown here. Even though the analytical principle, investigation process and the corresponding analytical instrument of different characterization methods are different, the analysis and investigation process of each method can be divided into steps as signal generation, signal detection, signal processing, and reading. The corresponding analytical instrument is composed of signal generators, detectors, signal processors, and reading devices. The signal generators make the sample generate the analytical signal and the signal detectors amplify, calculate, and compare the original signal before it is read or displayed. According to the characteristic relation of the signals and materials, the material can be analyzed and characterized by analyzing and processing the read signals.

References [1] Xu G, Zhang L. Nano composite materials. Beijing: Chemical Industry Press, 2001. [2] Li F, Yang Y, et al. Function and application of nano composite materials. Beijing: National Defense Industry Press, 2003. [3] Liao X. Testing technology of modern materials. Beijing: Metallurgical Industry Press, 2010. [4] Li N, Tu Z. Chemical plating technology. Beijing: Chemical Industry Press, 2004:134–147. [5] Huang S, Qiu W. Progress and present situation of surface engineering. Surface Eng 2005. [6] Jiang X, Shen W. Theory and practice of chemical plating. Beijing: National Defence Industry Press, 2000.

3 Application of lightweight carbon material and its composite in national defense environmental protection Lightweight carbon materials, such as activated carbon, have wide industrial and agricultural use as an adsorbing material of high specific surface area. Activated carbon was used as the adsorbing material of gas mask originally. Its specific surface area was not large, and its adsorbing capability was also poor. The specific surface area and the adsorbing capability were greatly improved with deep research after the application of the new preparation process. The lightweight carbon material played a very important role in all fields with the modified carbon material or other carbon base composite of outstanding property. This chapter introduces the application of carbon base materials and their composites for disposal of some industrial and special pollutants in the field of national defense environmental protection.

3.1 Disposal of Unsymmetrical Dimethylhydrazine Wastewater with TiO2 /porous Carbon and Its Composite Materials At present, liquid propellant is widely used in the field of military and aerospace. Unsymmetrical dimethylhydrazine (UDMH) is an important incendiary agent. UDMH [1] is an inflammable, volatile, colorless, or yellow transparent liquid. It has an intensely fishy smell similar to that of ammonia. Its molecular formula is (CH3 )2 NNH2 ; molecular weight, 60.11; boiling point, 63∘ C; freezing point, –57.2∘ C; density, 0.7911 g/cm3 . It is water-soluble and can also be mixed with gasoline, alcohol, and other organic solvents. According to the international criteria of toxicity of chemicals, UDMH is a class 3 medium toxicant, a little bit higher. It was proved that UDMH could mutate the tumor. Roger et al. found that the mortality and the morbidity rates were related to the dosage through the test of stomach injection to the rats. Yu F Sasaki and Ayako Saga proved that UDMH, with specific genetic toxicity of organ, was the colon cancer– causing agents of rodent. Oru-Tamura proved that UDMH was the genetic toxicity cancer-causing agents with mutating tumor effect. It is found through the research that UDMH is harmful to liver and kidneys to some extent. The disease mechanism it that UDMH leads to the shortage of vitamin B6 in human body. UDMH can also affect the respiratory system. UDMH enters human body through three ways: skin permeation, breathing, and eating. Because the boiling point of UDMH is low and it is volatile, the skin suction is poor. Its main professional intoxication risk is from the breathing. DOI 10.1515/9783110424751-003

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Because of the toxicity of UDMH, the maximum permissible concentration of UDMH is defined as 0.1 mg/L in water and 0.5 mg/L in the air. United States National Academy of Sciences and National Research Council suggested that the limited values in short period under the condition of 25∘ C, 100 kPa are 50 mg/L within 16 min, 25 mg/L within 30 min and 15 mg/L within 60 min. A great amount of waste liquid and wastewater of UDMH may be formed during its manufacturing, transportation, transferring, storage, and using process. If the waste liquid or wastewater is discharged into the environment without disposal, the environment will be polluted. UDMH pollutes water through two ways. The first way is the leakage of UDMH in the store and the flushing of pipeline and tank; the second way is the firing product of UDMH and nitrogen dioxide entering the flow guiding groove with the cooling water during the launching of rockets or the unburned UDMH entering the flow guiding groove with the fire extinguishing water. The discharged UDMH wastewater contains not only UDMH itself but also the oxidized or decomposed products in the environment, including hydrazone, tetramethyl-tetrazene, nitromethane, monomethylamine, dimethylamine, formaldehyde, cyanide, and nitrosamine (dimethylnitrosamine, diethylnitrosaminedipropylnitrosamine,dibutylnitrosamine,nitrosaminepiperidine,nitrosopyrrolidine, nitrosomorpholine, etc.). Some of the products may be more toxic than UDMH, such as nitrosamine and cyanide. A series of problems caused by UDMH wastewater are concerned greatly, so many countries in the world are striving to the research of the disposal method of UDMH wastewater. At present, many methods for the disposal of UDMH wastewater have been developed, such as physical method, chemical method, and biological method, but each method has its own defect in the practical operation. Currently there is no rapid, economical, and complete method to dispose UDMH wastewater. Table 3.1 lists the advantages and defects of the current disposing methods. Multiphase photocatalysis is widely applied in the environmental protection field in recent years. Among the known photocatalysts, TiO2 photocatalyst is the most effective new mode photocatalyst so far [2–18]. Besides being used as the photocatalysis decomposition for the harmful pollutants in water and air, TiO2 can also be used to oxidize some inorganic substance by photocatalysis to form the harmless or inactive inorganic components. TiO2 is characteristic of safety, harmlessness, stability, high catalytic effectiveness, and no secondary pollution. It can also mineralize all kinds of organic substances selectively, so it is appreciated more and more. As for the application of TiO2 , there are lots of research results. As for the application of TiO2 in the disposal of UDMH wastewater, some corresponding researches have been conducted and the effects were not bad. The present research results mainly focused on the application of powdered TiO2 , which is hard to be reused, so the improvement of this technology is affected. Lightweight carbon materials, such as active carbon, activated carbon fiber, and expanded graphite, have the following characteristic features: plenty of micropores, large specific surface area, and good selective adsorbing property [19–21]; therefore,

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Table 3.1: Advantages and defects of the current disposing methods. Method

Advantages

Defects

Natural purification

Effective, economical, applicable simple and energy saving

Long disposing time; big energy consumption in the disposing process; ammonia and small amount of hydrazine may be formed on the surface of disposing pond.

Adsorbing with activated carbon

No new toxic substance

The concentration of UDMH in the original water cannot be more than 100 mg/L; Hot air of 200∘ C is needed to regenerate activated carbon; high-energy consumption; many defects in the engineering aspect.

Ionic exchange

No secondary pollution formed during the disposal

The regeneration of ionic exchange resin and the secondary pollution from the regenerating products confine the development of this method

Oxidization with ozone

strong oxidization ability

The produced toxic substances, such as nitromethane, formaldehyde, and dimethyl nitrosamine, can hardly be eliminated.

TiO2 photocatalysis oxidization

High disposal efficiency, simple, convenient, and low cost

The solid loading technology of TiO2 is not ripe

Photocatalysis oxidization with compressed air

Simple process and low cost

After disposal, the remained concentration of UDMH is high and the toxic substance, like formaldehyde may be produced in the disposal.

they are also good carriers, on which some little particles can be carried to make a composite with fixed shape convenient for use, especially for the regeneration, besides their use in the gas separation, environmental protection, textile, chemistry, electronic, medical care, food, atomic energy, and so on. By means of considering the ab ove three aspects comprehensively, the comprehensive photocatalyst TiO2 /porous material, which is made into a fixed shape by carrying the TiO2 photocatalyst on the porous carbon material, was used to degrade UDMH wastewater. First, the loading and reuse of photocatalyst TiO2 can be conducted well. Secondly, during the degradation of simulated UDMH wastewater, the porous carbon material can also adsorb UDMH in the wastewater to increase the concentration of UDMH partially, so as to improve the degradation efficiency of UDMH by TiO2 and to eliminate UDMH through the adsorbing effect [22, 23].

3.1.1 Preparation of TiO2 /porous Carbon Composite Material The pretreatment of lightweight carbon material is as follows: 1. Pretreatment of activated carbon. Prepare certain amount of activated carbon in a beaker. Add deionized water into the beaker and boil it. Pour out the turbid liquid

78

2.

3.

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on the upper layer. Repeat these operations three times. Filter it to eliminate the possible impurity introduced during the preparation of activated carbon. Dry it at 80∘ C and put it in the desiccator for use. Pretreatment of activated carbon fiber. Cut the activated carbon fiber into rectangles of 2 cm×3 cm (the activated carbon fiber can be cut into suitable shape according to the requirement of experiment.). Soak them with nitric acid of 1 mol/L and put the soaked fiber in the thermostatic water bath of 60∘ C for 1 h to eliminate the remained impurity produced during the preparation of activated carbon fiber. Finally wash the soaked carbon fiber to neutrality with distilled water. The washed carbon fiber will be dried in the thermostatic drier of 80∘ C for 3 h. Put the dried carbon fiber in a drier for use. Pretreatment of the expanded graphite. Put the expandable graphite in the high temperature oven of 1,000∘ C for expansion. Put the expanded graphite in the drier for use after it is cooled down.

3.1.2 Preparation of the Composite Photocatalyst 3.1.2.1 Preparation of the Unalloyed Composite Photocatalyst The flowchart of the preparation of the unalloyed composite photocatalyst is shown in Figure 3.1. The detailed steps are as follows and the dosage of every reagent depends on the real situation. 1. Take 20 mL alcohol into a container and drip 10 mL tetrabutyl titanate (TBT) into the alcohol under the condition of stirring. Keep stirring for 30 min to make the TBT dissolved completely. Then add 1.5 mL acetylacetone into the solution. Keep stirring for 30 min.

Stirring for 0.5 h Mix evenly

Adding carbon material

Composite photocatalyst with film

Stirring for 4 h

Sol

Adding: 1.5 ml acetylacetone

Stirring for 0.5 h hydrolytie polycondensation

Seasoning for 24 h

Wet sol /carbon material

353 k dry

Activation Dry sol /carbon material High temperature

Figure 3.1: Flowchart of the preparation of the composite photocatalyst.

Stirring for 0.5 h

10 ml TBT + 20 ml ethanol absolute

Drip: (3 ml water + 5 ml alcohol + 0.5 ml nitric acid) mixture

Composite photocatalyst

3.1 Disposal of Unsymmetrical Dimethylhydrazine Wastewater

2.

3.

4.

79

Take 3 mL deionized water, 5 mL alcohol and 0.5 mL nitric acid and mix them evenly. Drip the mixed solution into solution 1 slowly while keeping stirring. After dripping, keep stirring for 30 min. Get yellow solution. Under the condition of stirring, we add the necessary amount of dried carbon material into the prepared yellow sol 2. Keep stirring for 4 h. Seal it and settle statically. Keep aging for 24 h. at the room temperature. Then dry for 12 h at 80∘ C. The dried material can be loaded secondarily or time and again according to the above steps. Finally the needed loaded photocatalyst can be prepared after being activated for 2 h. at certain temperature.

3.1.2.2 Preparation of the Silver Mixing Composite Photocatalyst The preparation of the silver loaded composite photocatalyst is similar to the above process. The only difference is that 0.5 g AgNO3 was added after the mixing of 3 mL deionized water, 5 mL alcohol and 0.5 mL nitric acid in step (2).

3.1.3 Characterization of Composite Photocatalyst 3.1.3.1 SEM Analysis for Composite Photocatalyst k(1) SEM analysis for composite photocatalyst TiO2 /AC and Ag-AC/TiO2 . The SEM pictures of TiO2 /AC and Ag-TiO2 /AC are shown in Figure 3.2. Figure 3.2(a) is the SEM picture of activated carbon without TiO2 loading. The adhesion of other substance on the activated carbon is invisible, and the pore structure on the surface is visible. The small white dots on the picture should be some impurity particles in activated carbon. Figure 3.2(b) is the SEM picture of activated carbon with the TiO2 loading once. It is obvious that white substance TiO2 , which can be proved with X-ray spectrometer and X-ray diffractometer, covered the total surface of activated carbon base. Because the size of TiO2 particle is small, some big particles on

(a)

(b)

(c)

(d)

Figure 3.2: SEM pictures of TiO2 /AC and Ag-TiO2 /AC. (a) SEM picture of activated carbon without TiO2 loading, (b) SEM picture of activated carbon with the TiO2 loading once, (c) SEM picture of activated carbon with the TiO2 loading two times, (d) SEM of composite photocatalyst with silver mixing.

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the picture should be the agglomerated particle of TiO2 during the preparation of material. Figure 3.2(c) is the SEM picture of activated carbon with the TiO2 loading two times. Compared with the loaded TiO2 particles once in Figure 3.2(b), the loaded TiO2 particles two times in Figure 3.2(c) are much more, but the agglomeration and the accumulation of TiO2 are liable to take place. Figure 3.2(d) is the SEM of composite photocatalyst with silver mixing. The surface of activated carbon is covered by a layer of substance and the original surface structure of activated carbon is invisible. Cracks, caused by silver mixing possibly, can be seen on the adhere substance. (2) SEM analysis for TiO2 /ACF composite photocatalyst. TiO2 /ACF composite photocatalyst was analyzed with the scanning electronic microscope, as shown in Figure 3.3. Figure 3.3(a) is the SEM of the bunched activated carbon fiber. It can be seen that there is no adhesion of other substances and the pore structure on the surface of activated carbon fiber. Figure 3.3(b) is the SEM of activated carbon fiber with TiO2 loading once. It can be seen that there is the adhesion of white particles TiO2 on the surface of column-shaped activated carbon fiber. Through the analysis of XRD, the diameter of the particle is so small that its average grain size is 3.9 nm, at the nanometer grade. If the grain size is too small, the agglomeration may occur. Figure 3.3(c) is the SEM of activated carbon fiber with TiO2 loading two times. Compared with the

(a)

(b)

(c)

(d)

Figure 3.3: SEM of TiO2 /ACF composite photocatalyst. (a) SEM of the bunched activated carbon fiber, (b) SEM of activated carbon fiber with TiO2 loading once, (c) SEM of activated carbon fiber with TiO2 loading two times, (d) SEM of composite photocatalyst mixed with silver.

3.1 Disposal of Unsymmetrical Dimethylhydrazine Wastewater

(a)

81

(b)

Figure 3.4: SEM of TiO2 /EG composite photocatalyst. (a) SEM of expanded graphite, (b) SEM of graphite with TiO2 loading.

state of activated carbon fiber with TiO2 loading once, the amount of TiO2 adhered in Figure 3.3(c) is a little more. Because of the agglomeration, cracks appear on the surface of TiO2 adhered on the activated carbon fiber. Figure 3.3(d) is the SEM of composite photocatalyst mixed with silver. It is similar to the SEM taking the activated carbon fiber as the carrier that there are some cracks on the column-shaped surface of activated carbon fiber covered with TiO2 . The crack is caused by the change of the property of material because of the silver mixing (3) SEM analysis of TiO2 /EG composite photocatalyst. TiO2 /EG composite photocatalyst was analyzed with the scanning electronic microscope, as shown in Figure 3.4. Figure 3.4(a) is the SEM of expanded graphite. The expanded graphite of single grain, with rough surface and big specific surface area, is vermicular, so the expanded graphite is a good adsorbing material, especially for the adsorption of big organic molecules. At the negative area of expanded graphite, its pore structure can be seen clearly, which is beneficial to be used as the carrier. Figure 3.4(b) is the SEM of graphite with TiO2 loading. The appearance of the graphite does not change but the adhesion of grainy substance TiO2 can be seen on the surface. The adhesion of TiO2 on the surface of expanded graphite is not homogeneous, only a little part adhered with TiO2 less than that loaded on activated carbon or activated carbon fiber. 3.1.3.2 X-ray Spectrum Analysis for Composite Photocatalyst The X-ray spectrum analysis for TiO2 /AC and Ag-AC/TiO2 composite photocatalyst was shown in Figure 3.5. Two peaks in Figure 3.5(a) are the excitation peaks of two electrons Ka and Kb on the layer K of titanium. That means titanium exists on the surface of composite photocatalyst TiO2 /AC. Similarly, the existence of silver on the surface of composite photocatalyst can be concluded from Figure 3.5(b).

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Ti(Ka)

Ti(Kb) 1

2

3

4

5

6

7

8

9

10 11 12 13 14 15

(a) Ti(Ka)

Ag(La1)

1

2

3

4

Ti(Kb)

5

6

7

8

9

10 11

12 13 14

(b) Figure 3.5: X-ray energy spectrum of composite photocatalyst (a) TiO2 /AC and (b) Ag-AC/TiO2 .

The X-ray energy spectrums of composite photocatalyst TiO2 /ACF, TiO2 /EG are similar to that of activated carbon. The existence of titanium and silver can be proved as well. 3.1.3.3 XRD Analysis for Composite Photocatalyst The crystal form of TiO2 is related with the activation temperature. Only when it is activated at the higher temperature can the anatase titanium dioxide of good property be got. In order to verify the relation between the activation temperature and the crystal form of TiO2 , the prepared photocatalysts at 100∘ C and 400∘ C were selected for comparison when they were investigated with XRD, as shown in Figure 3.6. In Figure 3.6(a), the curve is in anarchic state and there is no obvious diffraction at the 2( angle which can characterize the anatase titanium dioxide, so it is proved that

Intensity (Counts)

3.1 Disposal of Unsymmetrical Dimethylhydrazine Wastewater

500 450 400 350 300 250 200 150 100 50 0 20

30

40

Intensity (Counts)

60

50

60

70

T

(a)

(b)

50

83

120 100 80 60 40 20 0 20

30

40

70

T

Figure 3.6: XRD of composite photocatalyst. (a) TiO2 /AC, 100∘ C activation, (b) TiO2 /AC, 400∘ C activation.

TiO2 prepared at 100∘ C is in amorphous state. Figure 3.6(b) displays the diffraction diagram of photocatalyst prepared at 400∘ C. It can be seen in Figure 3.6(b) that there are several obvious diffraction peaks, which are the diffraction peaks of anatase titanium dioxide known by the comparison with the standard spectrum of titanium dioxide. The prepared titanium dioxide activated at 400∘ C is the anatase titanium dioxide. The grain size of titanium dioxides prepared with sol-gel method is small. The average grain size of titanium dioxide in Figure 3.6(b) is checked as 3.9 nm, belonging to nanometer crystal. In the aerobic state, activated carbon fiber can be oxidized at high temperature to lose the function of carrier. It is known from the experiments that the structure of activated carbon fiber starts changing when the temperature exceeds 300∘ C and activated carbon is completely oxidized into white powder when the temperature exceeds 350∘ C. Therefore, the composite catalyst loaded with activated carbon fiber can be activated only below 300∘ C, at which temperature the titanium dioxide cannot be transferred into anatase titanium dioxide and the X-ray diffraction spectrum is the same as Figure 3.6(a). Because the expanded graphite is refractory, the prepared composite catalyst TiO2 /EG can be activated at 400∘ C and its X-ray diffraction spectrum is the same as Figure 3.6(b). It can be seen from the SEM that the loading quantity of TiO2 is limited and its homogeneity is also poorer than that loaded on the activated carbon when titanium dioxide is loaded on the expanded graphite, so the photocatalysis will be affected later.

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3.1.4 UDMH Wastewater Disposal Effect The composition of liquid propellant wastewater is complicated. Besides the original component of propellant itself, there are also all kinds of degradation products of propellant in the wastewater. For example, in UDMH wastewater, besides UDMH itself, there are other 15 components, such as hydrazone, formaldehyde, cyanide, and nitrosamine. When the simulated wastewater of UDMH is disposed of with photocatalysts, two pollutants, formaldehyde and cyanide, which are the representative and toxic intermediate products during the disposal of wastewater, are also determined, besides the determination of UDMH concentration. In addition, the change of chemical oxygen demand (COD) of wastewater is determined. 3.1.4.1 Experimental Steps (1) Compound 500 mg/L simulated UDMH wastewater. Weigh accurately 0.5 g UDMH with subtraction method and transfer it into 1,000 mL volumetric flask. Pour the distilled water into the volumetric flask to dilute the UDMH to the scale of the flask. Shake it evenly and preserve it at the dark place for use. Under normal conditions, the simulated wastewater can be preserved for 1 week or so. (2) Pure adsorbing experiment. In the real experiment, the porous carbon material with TiO2 loaded on has the adsorbing effect on UDMH. Considering this effect, before the disposal experiment of the simulated UDMH is conducted, the carbon material of the same mass and the same volume of the simulated UDMH wastewater are used to conduct the experiment for measuring the carbon material’s adsorbing capability to UDMH. (3) Photocatalysis disposal. All the experiments are conducted with the same volume of the simulated UDMH wastewater. Add photocatalysts prepared under different conditions into the same volume of simulated UDMH wastewater and keep illuminating the wastewater with UV-light. Take wastewater sample at 20 min, 40 min and 1 h to determine the concentration of UDMH and then calculate the degradation ratio of UDMH. The concentration determination of UDMH is conducted with amino ferrocyanide sodium spectrophotometric method (0.01 mg/L ∼ 1.0 mg/L). Figure 3.7 is the standard curve of UDMH determination. The concentration of formaldehyde was determined with acetylacetone spectrophotometric method. Cyanide is determined with pyridine-barbituric acid spectrophotometric method. COD was determined with potash dichromate method. The degradation ratio of UDMH wastewater is calculated with the following formula: D=

C0 – Ct × 100% C0

UDMH concentration (mg/L)

3.1 Disposal of Unsymmetrical Dimethylhydrazine Wastewater

85

600 500 400 300 AC ACF EG

200 100 0

0

10

20

30

40

50

60

70

Time (min) Figure 3.7: Comparison of three carbon materials’ adsorbing effect on UDMH.

where C0 – the initial concentration of UDMH in wastewater; Ct – UDMH concentration in the wastewater after being degraded for time t; D – degradation ratio 3.1.4.2 Disposal Results (1) The adsorbing effect on UDMH with three porous carbon materials. Take 2.5 g activated carbon, activated carbon fiber and expanded graphite to three beakers respectively and pour 25 mL simulated UDMH wastewater of 500 mg/L to each beaker respectively to conduct the pure adsorbing experiment of carbon material. Take the wastewater samples from each beaker 20 min. 40 min and 1 hour later respectively to determine the change of UDMH concentration. The adsorbing effect of three carbon materials can be determined by calculating the concentration decrease of UDMH in wastewater. The experimental results are shown in Figure 3.7. It can be seen that three carbon materials have a certain adsorbing effect on UDMH, but do not achieve the discharge criterion. The adsorbing effect of graphite is poorer than that of both activated carbon and activated carbon fiber. The adsorbing for UDMH with the activated carbon and activated carbon fiber is completed within the first 20 minutes. (2) The disposal effect of composite photocatalysts loaded on three porous carbon materials. Take three kinds of photocatalysts of the same mass into three beakers and add 25 mL UDMH simulated wastewater of 500 mg/L into each beaker. Conduct the photocatalysis degradation with the illumination of UV-light. Take samples from three beakers at 20 min, 40 min and 1 h after the beginning of degradation respectively and determine UDMH concentration of each sample. The experimental results are shown in Figure 3.8. It can be seen that all three photocatalysts have the degrading capacity for UDMH-simulated wastewater. Comparatively speaking, the photocatalysts loaded on activated carbon and activated carbon fiber have better degrading effect than those on the expanded graphite. The photocatalyst loaded on activated carbon has the best degrading effect. This result is caused by the different loading quantity of TiO2 on each

Degradation ratio (%)

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80 70 60 50 40 30 20 10 0

a b c 0

10

20

30 40 Time (min)

50

60

70

Figure 3.8: Comparison of degradation effect on UDMH wastewater with three catalysts.

carbon material. The loading quantity of TiO2 on activated carbon and activated carbon fiber is more than that on the expanded graphite and the size of the particles on activated carbon and activated carbon fiber is also much smaller. On the other hand, expanded graphite has a poorer adsorbing effect on UDMH compared to activated carbon and activated carbon fiber, so the partial concentration of UDMH contacting the surface of catalyst loaded on the expanded graphite is lower than that loaded on the other two carbon materials when UDMH simulated wastewater is degraded with photocatalysis method. The particle size of the expanded graphite is smaller and lighter than the other two carbon materials, so the particle agglomeration in the aqueous solution also decreases the light application ratio. All of these factors lead to the poorer degradation effect on the UDMH by the photocatalyst loaded on expanded graphite. (3) Reuse of the composite photocatalysts. The photocatalyst made by loading TiO2 on carbon materials can effectively resolve the problem of the agglomeration, tough separation, and reuse of the powdered photocatalyst. But the preconditions are that the loaded TiO2 has certain loading intensity and cannot be scoured by the liquid flowing and friction of photocatalysts readily. Three groups of comparative experiments were made. In each group of experiment, the used catalyst was flushed with distilled water and dried, and then the UDMH-simulated wastewater was degraded many times with the same experimental procedure. The changes of the catalysis efficiency on UDMH by each photocatalyst in all degradation experiments were compared first. The experimental results were compared, as shown in Figure 3.9. It can be seen from the figure that the relative degradation ratio of the simulated UDMH wastewater degraded by the catalyst loaded on activated carbon is 70% or so every time, activated carbon fiber, 65% or so, the expanded graphite, 50% or so. The experimental results show that the catalysis capacity and effect on UDMH wastewater by three catalysts are almost the same. That means first the quantity of the loaded titanium dioxide changes little and the cohesion intensity of titanium dioxide on the carrier is so high that there is little falling of titanium dioxide during the

Relative degradation ratio (%)

3.1 Disposal of Unsymmetrical Dimethylhydrazine Wastewater

87

75 70 65 a b c

60 55 50 45

0

1

2

3 Times

4

5

6

Figure 3.9: Reuse of catalysts.

degradation. Secondly, the catalysis activity of titanium dioxide changes little; that is to say, the degradation effect in one hour almost does not change under the condition of permissible error. Therefore, the reuse of all these three composite photocatalysts provide an economical basis for their practical application.

3.1.5 Analysis of Factors Affecting Disposal Effect of Composite Photocatalysts

Concentration of UDMH (mg/L)

Comparative experiments were made according to the following three disposal conditions: Activated carbon (AC); TiO2 loaded on activated carbon (TiO2 /AC) and UV-light+ titanium dioxide loaded on activated carbon (UV+ TiO2 /AC). 25 mL simulated UDMH wastewater of 500 mg/L was disposed of under the above conditions. The samples of UDMH were taken for analysis at the experimental time of 20 min, 40 min, and 1 hour. The results of experiment are shown in Figure 3.10. Curve c and curve d show the disposal effects on UDMH in the simulated wastewater with pure activated carbon and the activated carbon loaded with titanium dioxide. It can be seen from the curves that the pure activated carbon can adsorb 30% UDMH

550 500 450 400 350 300 250 200 150 100

c d e 0

10

20

30

40

50

60

Time (min) Figure 3.10: Degradation effect on UDMH in several blank experiments.

70

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3 Application of lightweight carbon material and its composite

from the simulated wastewater within one hour, better than the effect (22%) of the composite catalyst loaded with titanium dioxide. This result may be caused by some porous structure of activated carbon being filled with loaded TiO2 , decreasing the adsorbing capacity of activated carbon. Curve e reflects the degrading effect on the UDMH simulated wastewater by the activated carbon composite photocatalyst loaded with TiO2 . It can be seen from curve e that the degrading efficiency of UDMH was as high as 70% within one hour. 3.1.5.1 Effect of Activation Temperature on TiO2 Photocatalysis Efficiency

UDMH concentration (mg/L)

There are three kinds of crystal forms for TiO2 : anatase phase, rutile phase, and brookite phase. The structural transferring mechanism and dynamics of TiO2 in the heating process have been widely researched. Rutile phase is so stable that it does not transform and decompose even at high temperature. But anatase phase and brookite phase make irreversible exothermic reaction in the heating process to change into rutile phase. It is generally thought that anatase TiO2 is highly active. But it is not very precise to say that the catalytic activation of pure anatase TiO2 is stronger than that of rutile phase. Their activation is affected by some factors during its crystallization process. When the amorphous TiO2 is crystallized, the rutile phase may generally change into big crystal grain with poor adsorbing property so as to decrease the activation of rutile under the same condition. If the grain dimension and surface property of rutile can be kept as big as that of anatase during the crystallization, the activation of rutile is high too. Three groups of comparative experiments were made as follows. Three kinds of TiO2 /AC composite photocatalysts were weighed by 2.5 g, respectively, and they were activated at 100∘ C, 200∘ C, 400∘ C respectively. Under the same condition, the activated composite photocatalysts were used to dispose 25 mL UDMH simulated wastewater of 500 mg/L. The results of experiment are shown in Figure 3.11. It can be seen that the degrading capacity of catalyst activated at 400∘ C is much better than that activated at 100∘ C and 200∘ C. 550 500 450 400 350 300 250 200 150 100

a b c 0

10

20

30

40

50

60

70

Time (min) Figure 3.11: Effect of activation temperature on the photocatalytic efficiency.

3.1 Disposal of Unsymmetrical Dimethylhydrazine Wastewater

89

The composite photocatalysts prepared with sol–gel method is in the amorphous state. After being activated at certain temperature, the catalysts of anatase and rutile with high catalytic activation are prepared. The activated carbon, used as the carrier, is a kind of carbon material, so its activated carbon structure can be destroyed at high temperature in the air. It is found from the experiment that the combination compactness of TiO2 and base and the structure of activated carbon at over 400∘ C were affected to a certain extent, and it is not beneficial to the use of composite photocatalysts, especially for the recycling. So it is better to set the activation temperature of composite photocatalyst as 400∘ C. 3.1.5.2 Effect of Loading Frequency on the Photocatalytic Efficiency of TiO2 When the sol–gel method is used to prepare TiO2 , the loading weight of TiO2 is improved by the repeated loading process, especially for the preparation of the film TiO2 , the thickness of which is increased by repeated pulling to enhance its photocatalytic capacity. Two groups of comparative experiments were made here. TiO2 /AC composite photocatalysts prepared by loading once and twice were weighed 2.5 g respectively and they were used to dispose of 25 mL UDMH simulated wastewater of 500 mg/L under the same condition. The results of experiment were shown in Figure 3.12. It can be seen from Figure 3.12 that the loading times do not affect the photocatalytic effect of TiO2 greatly and the degradation efficiency for UDMH are all 70% or so in one hour. It is mainly because that there is no great difference of catalytic TiO2 grain quantity when the composite photocatalysts loaded once and twice were used to degrade UDMH. The total surface of activated carbon was covered completely by TiO2 after the first loading. The second loading only increases the thickness of TiO2 layer, and the quantity of TiO2 grain illuminated by the UV-light does not increase. The conglobation of TiO2 grain occupies lots of passage of activated carbon, decreasing the adsorbing effect of activated carbon. These two factors caused the results of the experiment.

UDMH concentration (mg/L)

550 500

500

500 0 min

450

20 min

400 350 300

40 min 310

296

250

60 min 200

198

200

150

148

150 100

1

2 Loading times

Figure 3.12: Effect of loading times on the photocatalytic efficiency.

90

3 Application of lightweight carbon material and its composite

3.1.5.3 Effect of pH on the Photocatalytic Efficiency of TiO2 The value of pH of reaction liquid makes a great impact on the polymerization degree of the semiconductor catalyst grain in the solution, the band-edge position of the valence band and conductor band and the adsorption of surface charge and organic substance on the surface of semiconductor. It is manifested from the research that the property of the solid and liquid interface (electric double layer) in TiO2 heterogeneous reaction system changes with the variation of the pH of solution. So the adsorption and desorption processes of electron hole pair are also affected obviously by the pH value. And the change of pH also makes different impact when the photolysis object is different. Prepare 6 groups of 25 mL UDMH simulated wastewater of 500 mg/L and regulate their pH into 3 to 8 respectively. Conduct photocatalytic treatment under the same condition. The experiment results are shown in Figure 3.13. It can be seen from Figure 3.13 that the photocatalytic efficiency of TiO2 changes little when pH ranges from 4 to 6; the change of pH affects the photocatalytic efficiency when pH is outside that range; when pH is 3 or 7, the degradation efficiency of UDMH is 64% in one hour and when pH is 8, degradation efficiency is 40%, smaller than that when pH is 4, 5 or 6. In the photocatalytic system, OH is the major free radical. The forming mechanism of OH is as follows. e – + O2 → O 2 – ⋅ O2 – ⋅ +H+ → HO2 ⋅ 2HO2 ⋅ → O2 + H2 O2 H2 O2 + O2 – ⋅ → ⋅OH + OH– + O2

UDMH concentration (mg/L)

Obviously, the increasing of pH is not advantageous to the forming of OH. When pH is smaller than the isoelectric point of TiO2 , the adsorption of UDMH on the surface of TiO2 increases with the increase of pH and adsorption enhances the photocatalytic degradation. So the photocatalytic efficiency of UDMH degradation changes little when 550 500 450 400 350 300 250 200 150 100

pH=3 pH=4 pH=5

0

10

20

30 40 Time (min)

pH=6 pH=7 pH=8

50

60

Figure 3.13: Effect of pH on the photocatalytic efficiency.

70

3.1 Disposal of Unsymmetrical Dimethylhydrazine Wastewater

91

pH is near the range from 4 to 6. But the forming of OH and the adsorption of UDMH on TiO2 surface all decrease rapidly to decrease the photocatalytic degradation efficiency gradually when pH continues to increase. It can be seen from the experiment that pH of the simulated UDMH wastewater is ideal from 5 to 6. Generally the pH of UDMH wastewater is in the range, so it is not necessary to regulate the pH of solution additionally. 3.1.5.4 Effect of the Initial Concentration of Simulated UDMH Wastewater Compound 6 groups of simulated UDMH wastewater with concentration of 100 mg/L, 200 mg/L, 500 mg/L, 600 mg/L, 800 mg/L, 1,000 mg/L respectively. Conduct photocatalytic degradation experiment for all compounded wastewater of 25 mL, respectively, with 2.5 TiO2 /AC composite photocatalyst under the same condition. The results of experiments are as shown in Figure 3.14. It can be seen from Figure 3.14 that the relative degradation efficiency of UDMH in every period keeps stable and the degradation amount of UDMH is in proportional relation to its concentration when the concentration of UDMH is less than 600 mg/L; when the concentration of UDMH in the simulated wastewater is higher than 800 mg/L, the degradation of UDMH decreases with the increase of its concentration. As far as the result is concerned, the concentration of the simulated wastewater should be controlled less than 800 mg/L. 3.1.5.5 Effects of Adding Dosage of Catalyst Conduct 5 groups of comparative experiments as follows. Put 1 g, 1.5 g, 2.5 g, 3 g, 3.5 g TiO2 /AC composite catalyst into 25 mL simulated UDMH wastewater of 500 mg/L

Relative degradation efficiency (%)

80 20 min

70

40 min

60

60 min

50 40 30 20 10 0

1

2

3

4 UDMH concentration

5

6

Figure 3.14: Relative degradation efficiency of UDMH of different concentration (abscissa is the number of sample, representing the different initial concentration).

UDMH concentration (mg/L)

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3 Application of lightweight carbon material and its composite

500 450 400 350 300 250 200 150 100

1 1.5 2.5 3 3.5

0

10

20

30 40 Time (min)

50

60

70

Figure 3.15: The effect of catalyst dosage on the photocatalytic efficiency.

respectively. Conduct the experiments under the same condition. The results of experiments are shown in Figure 3.15. It can be seen from Figure 3.15 that the concentration of UDMH in the simulated wastewater changes rapidly with the increase of the catalyst amount in the scope of catalyst amount less than 2.5 g, and the changing of UDMH is not obvious when the catalyst amount is more than 2.5 g. When the dosage of composite catalyst increases, on the one hand, the adsorption capability of the composite catalyst is increased; on the other hand, the amount of TiO2 is increased, with the illumination of the UV-light; the electron hole pairs formed are increased to enhance its oxidation capacity, so as to improve the catalytic degradation of UDMH. Because the adsorption ability of the composite catalyst has been weakened greatly, the second factor works. At the same time, a saturated photocatalyst dosage, concerned with the content of the container, can be found. When the amount of the photocatalyst exceeds the saturated dosage, the added photocatalyst will accumulate so as to affect the catalytic efficiency. 3.1.5.6 Effect of Silver Doping Prepare 2.5 g composite catalysts with silver doping and without silver doping respectively, 25 mL simulated UDMH wastewater of 500 mg/L. Conduct the photocatalytic degradation experiment under the condition, as seen from Figure 3.16. Curve a in the figure stands for the degradation result of composite photocatalyst without silver doping and curve b stands for degradation result of composite photocatalyst with silver doping. It is obvious that the degradation efficiency of UDMH is greatly improved after the silver doping for the catalyst. Because the radius of Ag+ (0.126 nm or so) is much bigger than that of Ti4+ (0.068 nm or so), Ag+ cannot enter the crystal lattice of TiO2 . But these Ag+ can diffuse gradually onto the surface of TiO2 grain during the sintering and form island-shaped Ag+ diffusion layer (0.1–1 nm) or Ti-O-Ag bond after the illumination and thermal reduction. And then they form the flaky metal ions on the surface of TiO2 grains, especially at the boundary of TiO2 grains.

UDMH concentration (mg/L)

3.1 Disposal of Unsymmetrical Dimethylhydrazine Wastewater

93

600 a b

500 400 300 200 100 0

0

10

20

30 40 Time (min)

50

60

70

Figure 3.16: The effect of silver doping on the photocatalytic efficiency of TiO2 .

Additionally, some contact points on the boundary may be occupied by the Ag grains, so the contacting accumulation among small TiO2 grains may also be affected. Ultimately the diameter of TiO2 grain decreases and the forbidden bandwidth of TiO2 increases, so as to improve the oxidization-reduction capability. The doped Ag ions also improve the charge transferring to the adsorbed substance on the film to increase the concentration of OH participating in the TiO2 photocatalytic oxidization reaction on the surface of film. The proper amount of Ag+ doping can improve the photocatalytic activity of TiO2 . 3.1.5.7 Effect of Added Oxidant Compound three simulated UDMH wastewater samples of 25 mL, of 500 mg/L, and add 2.5 g composite photocatalyst to each wastewater sample respectively. And then add H2 O2 to each sample by 0.1 mL, 0.2 mL, and 0.3 mL, respectively. All the wastewater samples are illuminated by the UV-light for the photocatalytic reaction. The concentrations of all wastewater samples at 20 min, 40 min and 1 h were determined and the CODs of the degraded wastewater are also determined at the same time. The results of the experiment are shown in Figure 3.17. Figure 3.17(a) is the changing curve of UDMH and Figure 3.17(b) is the changing curve of COD. It can be seen from the figures that H2 O2 has a big effect on the TiO2 photocatalytic degradation to the simulated UDMH wastewater and its degradation rate and the relative degradation efficiency all increase with the increase of H2 O2 adding, but the increase of the relative degradation efficiency of COD is less than that of UDMH. This is because UDMH is not oxidized directly into the small inorganic molecules, such as CO2 and H2 O, but first into some big organic molecules, such as hydrazone and tetraethyl tetrazene, which can be determined first by COD before being oxidized gradually into the small inorganic molecules. The increase adding of H2 O2 is advantageous to the photocatalytic degradation efficiency of TiO2 , but the excessive H2 O2 can affect the determination of UDMH (excessive H2 O2 affects the formation of amino ferrocyanide sodium) and the determination of COD as well. Additionally it is shown from the references that the increase

Concentration of UMDH (mg/L)

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3 Application of lightweight carbon material and its composite

600 0 0.04 mol/L 0.08 mol/L 0.12 mol/L

500 400 300 200 100 0

0

10

20

30 40 Time (min)

50

60

70

COD (mg/L)

(a) 1600 1400 1200 1000 800 600 400 200 0

0 0.04 mg/L 0.08 mg/L 0.12 mg/L

0

10

20

30 40 Time (min)

50

60

70

(b) Figure 3.17: The effect of H2 O2 on the photocatalytic efficiency of TiO2 . (a) The changing curve of UDMH, (b) The changing curve of COD.

of H2 O2 concentration led to more OH production and better degradation but there is an optimum value for the dosage of H2 O2 and excessive H2 O2 may decrease the photocatalytic efficiency. During the photocatalytic degradation, the electron hole pairs produced by the UV-light excitation play a very important role. According to the photocatalytic oxidization mechanism of the semiconductor grains, there are two competing procedures: capturing and compounding. As for the photocatalytic reaction, it is effective only when the captured photo-induced holes take actions with the donors or receptors. If there is no suitable captor of electron or hole, the isolated electron will recombine with the hole inside the semiconductor or the surface and release the thermal energy. If the given electron receptors or donors are adsorbed preliminarily on the surface of catalyst, the transferring and capturing of the interface electrons will be more effective and more competitive and the dissolved oxygen, water molecule and organic molecule can all be used as the captors of electrons or holes without adding oxidants. But the amount of such substances is limited and their reactivity is not so high, so the photocatalytic efficiency of TiO2 is affected. If the extra electron receptors, such as H2 O2 , O2 and persulfate, are added to capture the photo-induced electrons, to decrease the recombination of electrons and holes, the photocatalytic efficiency shall be increased.

3.1 Disposal of Unsymmetrical Dimethylhydrazine Wastewater

95

H2 O2 is a good receptor of electrons. It captures the photo-induced electron well in TiO2 photocatalytic system. After the electrons are captured, OH with high oxidizability shall be produced. On the other hand, H2 O2 itself can also form OH under the illumination of UV-light, the reaction of which is as follows: H2 O2 + e– → ∙OH + OH– h-

H2 O2 󳨀󳨀→ 2 ⋅ OH

3.1.5.8 Effect of Combined Action of Silver Doping and H2 O2

Concentration of UDMH (mg/L)

Prepare 25 mL simulated UDMH wastewater of 500 mg/L and add 2.5 g composite silver doped photocatalyst and H2 O2 with concentration of 0.12 mol/L. The results of experiment are shown in Figure 3.18. 600 500

0 min 20 min

400

40 min 300

60 min

200 100 0 1

2

(a)

3

Factors 1600 1400

COD (mg/L)

1200 a

1000

b

800 600 400 200 0 0

(b)

10

20

30

40

50

60

70

Time (min)

Figure 3.18: Effect of combined action of silver doping and H2 O2 . (a) Concentration changing of UDMH; Factors: 1 Ag doping; 2 adding H2 O2 ; 3 Combined action of silver doping and H2 O2 , (b) Changing of COD; Curve a: adding H2 O2 ; Curve b: Combined action of silver doping and H2 O2 .

96

3 Application of lightweight carbon material and its composite

It can be seen from the figure that there is no big difference between the combined action of silver doping and the single action of H2 O2 as for the concentration changing of UDMH. Under the condition of the combined action, COD of the wastewater is decreased to 20 mg/L in one hour, which is much smaller than 105 mg/L that is made in one hour under the condition of the single action of H2 O2 . It is manifested that the combined action does better. This result is caused by the effect of silver ions during the catalytic procedure. Although the oxidizability of the whole composite photocatalyst system is strong and the molecules of UDMH have been decomposed in this procedure after the adding of H2 O2 , some substances hardly degraded in the analogical wastewater, including some intermediate products of UDMH degradation cannot be oxidized in a short time by means of adding H2 O2 singularly. Silver ion is a good catalyst and it can decrease the activation energy of some reaction to make the reaction take place more readily when the composite photocatalyst is doped with silver, so the degradation effect of COD is better. 3.1.5.9 Change Regularity of Formaldehyde and Cyanide Ion in Degradation

Concentration of formaldehyde (mg/L)

It is researched that formaldehyde and cyanide ions exist with big toxicity, high content and long time among the main intermediate products of UDMH degradation. Formaldehyde is irritative, water-soluble and exists in various industrial wastewaters. The content formaldehyde is low in the untreated UDMH wastewater. As one of the intermediate products of photocatalytic degradation of UDMH, formaldehyde’s concentration changes with the beginning of photocatalytic reaction. Prepare 25 mL simulated UDMH wastewater of 500 mg/L and add some composite silver doped photocatalyst and H2 O2 with concentration of 0.12 mol/L. The solution was illuminated with UV-light for degradation. The change regularity of formaldehyde is shown in Figure 3.19. It can be seen from the figure that the concentration of formaldehyde changes with the gradual degradation of UDMH. At the beginning of the reaction, the concentration of formaldehyde increases in straight climb with the formation of formaldehyde and it reaches the peak in 10 min. Then the concentration of formaldehyde decreases gradually and it decreases below 5 mg/L in 30 min or so. The formaldehyde can hardly 30 25 20 15 10 5 0

0

10

20

30 Time (min)

40

50

60

Figure 3.19: Changing curve of the concentration of formaldehyde.

Concentration of CN-(mg/L)

3.1 Disposal of Unsymmetrical Dimethylhydrazine Wastewater

2.25 2 1.75 1.5 1.25 1 0.75 0.5 0.25 0

0

10

20

30

40

50

97

60

Time (min) Figure 3.20: Changing curve of CN– concentration.

be detected in the solution in 50 min. The result of the experiment shows first that formaldehyde is an intermediate product of UDMH photocatalytic degradation with TiO2 , and secondly that formaldehyde can also be degraded within 30 min after it is formed and the whole system can not only degrade the UDMH photocatalytically, but also degrade formaldehyde photocatalytically. It is manifested that the catalysis of TiO2 is non-selectivity. The changing of CN– is shown in Figure 3.20. It can be seen from the figure that the change of cyanide ion is not like that of formaldehyde, forming and increasing rapidly at the initial stage of the reaction. Cyanide ion can hardly be detected in the solution within 10 min and the concentration of cyanide ion increases rapidly in 10 min and reaches its peak, 1.98 mg/L, at 25 min. Then cyanide ions were oxidized quickly with the development of experiment and cyanide ions can hardly be detected after 40 min.

3.1.6 Research on the Degradation Dynamic of UDMH When TiO2 is used to degrade organic wastewater, there is a radical reaction mechanism. UDMH reacts with the radical to form the active intermediate, which will be degraded into small molecule substance. 1. UDMH reacts with ∙OH to form the active intermediate (CH3 )2 N=N. The reactions are as follows: (CH3 )2 NNH2 + ∙OH → (CH3 )2 NNH + H2 O (CH3 )2 NNH + ∙OH → (CH3 )2 N+ = N– + H2 O And there is the following equilibrium in the aqueous solution: (CH3 )2 N+ = N– + H2 O → (CH3 )2 N+ = NH + OH–

98

2.

3 Application of lightweight carbon material and its composite

The decomposition mechanism of active intermediate (CH3 )2 N=N is as follows: 2(CH3 )2 N+ = N– → (CH3 )2 N+ = NCH3 + CH3 N = N– (CH3 )2 N+ = NCH3 → (CH3 )2 NN = CH2 + H+ CH3 N = N– + H+ → CH4 + N2

3.

The produced (CH3 )2 NN = CH2 reacts as follows: (CH3 )2 NN = CH2 + ∙OH → (CH3 )2 NH + CO2 + H2 O + N2 + NOX The substances, like (CH3 )2 NH shall be degraded into small molecule substances finally. The above degradation mechanism of UDMH can be simplified as follows, k1

k2

UDMH ←→ UDMH* 󳨀󳨀→ P k1󸀠

Where UDMH* stands for the active intermediate; P stands for the degradation product: k1, k1󸀠 are the reaction rates. Radical ∙OH takes part in two steps, the forming of UDMH* and the forming of product. UDMH* is an active intermediate, so it is rapid process from the forming of UDMH* to the forming of a new substance; that is to say, the existence of the intermediate UDMH* is in short time and its concentration is taken as changelessness. Therefore, the steady-state approximate method is used to solve the UDMH degradation dynamical equation and the concentration of UDMH* is assumed to keep constant, namely, the forming rate of UDMH* is equal to its consumption rate. k1 [UDMH] = k1󸀠 [UDMH∗ ] + k2 [UDMH∗ ] [UDMH∗ ] =

k1 [UDMH] k1󸀠 + k2

The forming rate of product P stands for the reaction rate: r = k2 [UDMH∗ ] then: r = k2 [UDMH∗ ] =

k1 k2 [UDMH] k1󸀠 + k2

It can be seen from the above formula that the photocatalytic reaction of UDMH is a first-grade reaction and the reaction rate is in direct proportion to the concentration of UDMH. The rate equation is established with the result of the experiment which is made under the condition of 500 mg/L simulated UDMH wastewater, taking TiO2 /AC as the photocatalyst and setting pH value of the solution as 5. The experiment results are as shown in Table 3.2.

99

3.2 The Degradation of TNT by TiO2 /ACF Composite Photocatalyst

Table 3.2: Dynamical results of the react. Time (min)

0

10

20

30

40

50

60

Concentration of UDMH (mg/L) r(mg⋅L–1 ⋅min–1 ) lnCUDMH lnr

500 – 6.21 –

380 6.69 5.94 1.90

296 5.32 5.69 1.67

242 4.48 5.48 1.50

198 3.47 5.28 1.24

171 2.94 5.14 1.08

150 – 5.01 –

Under the above-mentioned experimental condition, the influencing factors for the degradation of UDMH include UV-light illumination, H2 O, OH– , dissolved oxygen and the concentration of UDMH. Among these factors, only the concentration of UDMH is variable and other factors are taken as the constants, so the exponential rate model can be used to establish the rate equation, presuming r = kCUDMH n Where CUDMH stands for the updating concentration of UDMH. The regression analysis is made to the data in Table 3.2 by means of the abovementioned formulas. The regression results are n = 1.0231, k = 0.0157, showing that the reaction is a first-grade reaction, in accord with the theoretical derivation. The degradation of UDMH is taken as a first-grade reaction, and then the half-life t1/2 can be solved as: t1/2 =

ln 2 = 44(min) k

3.2 The Degradation of TNT by TiO2 /ACF Composite Photocatalyst The scientific name of TNT is 2,4,6 trinitrotoluene, also known as symmetrical trinitrotoluene, which is made from toluene by means of three stages of nitrification. Its molecular formula is C7 H5 N3 O6 and the molecular weight is 227.13, and its structure is as follows: CH3

O2N

NO2

NO2

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3 Application of lightweight carbon material and its composite

TNT was first made by J. Wilbrand in 1863, but the explosion property was not discovered until 1891, and it replaced the picrate as military explosives in 1905. TNT is a kind of white or amaranth, odorless needle crystal, the color of whose industrial product is yellow, and it is squamous-shaped. It is characterized by moisture absorption, water fast, slight solubility in ethanol and being soluble in benzene, aromatic hydrocarbon, and acetone. Its relative density is 1.65 g/cm3 ; the melting temperature, 80.9∘ C; the outbreak point, 290–290∘ C; the detonation velocity, 6,800 m/s. Under the action of sunlight, its color becomes dim without affecting explosion. TNT has low sensitivity to impact and friction, and will not burn or explode even with bullet shooting. It will emit smoke but will not explode when it is fired in the air. If TNT is fired with a large number (more than 200 kg) or in a confined space, it might be transformed into explosion. It is safe for handling and storage, but it can cause subacute poisoning, chronic poisoning and irreversible damage to human being because of its high toxicity. The clinical symptoms of poisoning include dizziness, nausea, vomiting, abdominal pain, delirious, incontinence, pupil, corneal reflection disappearance, even death from respiratory paralysis. The production technology of TNT is mature. Its manufacturing reaction is stable and easy to control with simple equipment, but no vacuum high-pressure equipment. It can be produced intermittently or continuously and can be easily controlled automatically. It produces a large amount of TNT wastewater during its production, processing, transportation, loading and unloading, stacking, and destruction process, which causes serious environmental pollution. TNT wastewater consists of the manufacturing effluent and the bomb disposal wastewater. And the manufacturing effluent consists of the acidic wastewater and the alkaline wastewater: (1) The acidic wastewater is the wastewater produced by boiling and washing TNT, including the wastewater for boiling and washing raw TNT before the refining with sodium sulfite method to or the wastewater of using the nitric acid to wash the refined TNT. The waste water for washing TNT and flushing the workshop can also be included as the acid wastewater. The wastewater of washing the acidic TNT is a yellow aqueous solution, the temperature of which is above 80∘ C, commonly known as the yellow water with a suspension of TNT. The species of the nitration in the acidic wastewater are related to the content of TNT. The nitration in the wastewater is mainly TNT if the water is used to wash the refined TNT with nitric acid. The wastewater after washing raw TNT includes the nitro benzoic acid, the nitro cresol, the nitrobenzene, the nitrophenol, the nitro salicylic acid and a small amount of the four nitromethane and unknown things. (2) The alkaline wastewater: The red water of refining TNT with the sodium sulfite solution and washing TNT and the wastewater produced for washing TNT after being treated with sodium sulfate sulfite are both alkaline wastewater. They are

3.2 The Degradation of TNT by TiO2 /ACF Composite Photocatalyst

101

characterized by high pH value, high organic matter concentration, high CODcr, high chromaticity, the extremely complex composition, toxicity, and poor biochemical. In addition, after unpacking the abandoned shells and collecting the explosives, there is still some explosive sticking on the inner surface of artillery shells. In order to clean the shells thoroughly, the process of rinsing the inner wall of the shell with water vapor and then flushing the tap water is applied. The condensated water of vapor and flushing water together constitute the ammunition disposal wastewater. As far as the safety of national defense is concerned, the ammunition storage and the destruction of operating point are scattered and their scales are various and the produced wastewater is also different, so it is not easy to centralize the management. The ammunition destruction is seasonal, mainly in summer. Data show that the main component of the explosives in the shell of our army is still TNT and the main pollution composition of ammunition disposal wastewater is TNT. The concentration of TNT in the bomb disposal wastewater is about 60 mg/L, CODcr is about 120 mg/L and the pH value is 7.2 or so. According to rough statistics, in China in 1979, the discharged explosive wastewater was about 25 million tons, among which TNT wastewater was 450,000 tons, and about 110 tons aromatic nitro compounds and 1,200 tons nitrocellulose along with the wastewater flew into nature. The current weapon industrial water pollutant emission standards – the explosive emission standards – in our country are shown in Table 3.3. The explosive wastewater treatment generally includes the physical, chemical, and biological technology. The physical adsorption method mainly includes the extraction method, the membrane separation method, the flocculation method, the evaporation method, the reverse osmosis, and the flotation method. The chemical method mainly includes the Fenton reagent method, the ozone and the combination of ozone oxidation method, the liquid discharge method, the photocatalytic oxidation method, the plasma method, the burning method, the hydrolysis method, and the iron reduction method. The biological method includes the activated sludge

Table 3.3: Discharge standards of water pollutants in the explosive industry. Type

Displacement (m3 /t)

establishment production time

Daily maximum discharge of pollutants (mg/l, except chroma and pH) Chroma(dilution Suspended ratio) matter (SS)

BOD5

CODcr TNT DNT pH

Before the time t

4.0

80

70

60

150

10

After the time t

2.5

50

70

30

100

5.0

6–9

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3 Application of lightweight carbon material and its composite

method, the biochemical method, the anaerobic white-rot fungus method, the biological membrane method, the bacteria aerobic method, the biological rotary method, and so on. The operation of the physical treatment technology is simple and rapid, but the cost is high and the secondary pollution is serious. For example, the effect of the adsorption method is stable and reliable, but the disposal of the adsorbent of adsorbing the explosive charges has not been fully resolved; the flocculation method has high efficiency, but its complex process and the high cost of operation are not suitable for industrialization. The processing speed of the chemical treatment technology is fast and the tolerance concentration of pollution is high, but the energy consumption is large and its industrialization is difficult. Biological treatment technology is safe in operation, low in cost and able to realize pollutants completely mineralization, but it has defects as the low tolerance pollution concentration of microbe, slow degradation rate, cultivation of the special culture and so on. There are many TNT wastewater treatment methods, but the research on using the load modification of TiO2 photocatalyst and the load of activated carbon fiber of TiO2 -Fenton under the UV-light and the natural light to degrade TNT is rarely reported. The activated carbon fiber, which was loaded with metal ion and doped with the modified TiO2 photocatalyst, is used to degrade photocatalytically the simulated wastewater of TNT under the UV-light and the natural light to study the photocatalytic activity of the composite photocatalyst and the TNT wastewater degradation efficiency, to explore the optimal combination model of using the Fenton method to help TiO2 /ACF catalyze and degrade the simulated TNT wastewater under the UV-light and the natural light and finally to prepare the composite photocatalyst which can achieve rapid and efficient degradation.

3.2.1 Preparation and Characterization of the Loaded Titanium Dioxide Similar to the preparation method of the porous carbon materials/TiO2 composite materials in the former section, the gel and immersion-lift method is used to prepare the ACF/TiO2 light catalyst mixed with metal Ag+ and Cu2+ . The prepared photocatalyst is characterized, and its electron microscope scan is shown in Figure 3.21. Although the titanium dioxide agglomerates, the particle is still small. It can be seen from the result of X-ray energy spectrum analysis that doping silver and copper in TiO2 is successful.

3.2 The Degradation of TNT by TiO2 /ACF Composite Photocatalyst

20 μm

Vega©Tescan

20 μm Vega©Tescan

10 μm Vega©Tescan

(a)

103

(c)

(b)

Figure 3.21: The composite photocatalyst electron microscope scan. (a) TiO2 /ACF 9000 times, (b) Ag+ –TiO2 / ACF 5000 times, (c) Cu2+ –TiO2 /ACF 5000 times.

3.2.2 The Influence Factors of the Degradation of Composite Photocatalyst on TNT Wastewater The degradation of TNT wastewater experiment is according to the following steps: (1) Weigh 0.50 g refined pure TNT and dissolve it in 3 mL concentrated sulfuric acid first, and then slowly add water to dissolve it before filtering, and then move the solution into a 1,000 mL volumetric flask and dilute it with water to the standard line, and finally determine its concentration. (2) Take the same volume (50 mL) of simulated TNT wastewater into several small beakers, and add the prepared photocatalyst under the different conditions, and then illuminate the samples under different light irradiation, and take samples, respectively, from each beaker in a certain period of time, and then determine the concentration of TNT in the sample. Calculate the rate of the degradation of TNT with the following formula: D=

C0 – Ct × 100% C0

Where C0 stands for the initial concentration of TNT wastewater; Ct stands for the concentration of TNT in TNT waste liquid after the time T of degradation; D stands for the degradation rate. The concentration of TNT is determined with the spectrophotometric method. If the wastewater hardness is large, the ethylenediamine disodium tetraacetic acid (EDTA)

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3 Application of lightweight carbon material and its composite

is used to eliminate the influence of calcium and magnesium, together with the regulating of the pH of water. The potassium dichromate method is used to determine its COD. 3.2.2.1 Effects of the Loaded Amount of TiO2 Prepare 50 mL simulated TNT wastewater of the same concentration and use six groups of composite materials of different load rate to photocatalytically degrade the simulated wastewater with UV-light at room temperature for 90 min. Take the samples and measure the content of TNT and COD. The experimental results are shown in Figure 3.22. As shown in Figure 3.22, with the increase of the load amount, the removal rate of TNT and COD does not increase. It is because that if the loaded TiO2 is too much, the part of activated carbon fiber microholes will be plugged, which does not make the TNT molecules easy to spread to the inner surface of activated carbon fiber, thus hindering the TNT molecules adsorbed by the activated carbon fiber migrate to TiO2 . When the loaded amount of TiO2 is too large, the particle accumulation phenomenon reduces the activity location of TiO2 , and too much TiO2 will produce shielding effect to UV-light, which greatly influences the efficiency of light and reduces the photocatalytic effect. It can also be seen from Figure 3.21 that the degradation of TNT rate is declined, and the COD degradation rate is increased instead when the loaded TiO2 is greater than 55%. The speculated reason may be that too much loading of TiO2 and the obvious particle aggregation affect the catalytic effect of TiO2 . Although the TNT molecular structure is destroyed under UV-irradiation, the intermediate mineralization rate is reduced, therefore the COD value of the system increases instead. But if the load capacity is too small, it couldn’t play the photocatalytic efficiency of TiO2 . It can be seen from Figure 3.22 that the optimal load rate is about 20%. TNT COD

65

Degradation rate (%)

60 55 50 45 40 0

10

20

30

40

50

60

Load factor (%) Figure 3.22: The efficiency of the degradation of TiO2 of different load rate on TNT.

3.2 The Degradation of TNT by TiO2 /ACF Composite Photocatalyst

105

3.2.2.2 Effects of the Initial Concentration of TNT Prepare 50 mL simulated TNT wastewater with the concentration of 100 mg/L, 200 mg/L and 300 mg/L respectively, and then add the same quality and the same load rate TiO2 /ACF to conduct the ultraviolet catalytic degradation at room temperature. Take samples and determine TNT concentration of each sample once every 30 min. The experimental results are shown in Figure 3.23. It can be seen from Figure 3.23 that the initial concentration of TNT has significant effect on the photocatalytic degradation to the simulated TNT wastewater with ACF/TiO2 . When the initial concentration of TNT is 300 mg/L, the removal rate of TNT can be high to 73% after the catalysis of UV-light for two hours. When the initial concentration of TNT is 100 mg/L, the removal rate of TNT can be 57% after the catalysis of UV-light for two hours. At the same time, the biodegradation rate is faster at the beginning of the degradation of TNT, and the photocatalytic degradation rate of the different initial concentration of TNT has little difference in the middle and later periods of the degradation. The reason for that is analyzed as follows. At the beginning of the degradation, the degradation of TNT is mainly composed of the adsorption of the activated carbon fiber, and the titanium dioxide photocatalytic efficiency in the process is not obvious. With the increase of the TNT molecules adsorbed by the activated carbon fiber, the concentration of TNT molecules in the activated carbon fiber increases; at the same time, the effect of adsorption of the activated carbon fiber gradually decreases, and the photocatalytic efficiency of TiO2 in this phase is gradually reflected. With the reduction of TNT concentration in the simulated TNT wastewater, the removal rate of TNT in the system is leveled off. 3.2.2.3 Effects of Reaction Temperature on the Photocatalytic Efficiency Prepare 50 mL simulated TNT wastewater of the same concentration for three samples. Conduct photocatalytic degradation experiment with TiO2 /ACF of the same quality and the same loading rate at 20∘ C, 40∘ C and 60∘ C respectively. Determine the

300 100 mg/L 200 mg/L 300 mg/L

c/(mg/L)

250 200 150 100 50 0

30

60

90

120

t/(min) Figure 3.23: The influence of the initial concentration of TNT on the light catalytic efficiency.

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3 Application of lightweight carbon material and its composite

300 20 ˚C 40 ˚C 60 ˚C

c/(mg/L)

250 200 150 100 50 0 0

30

60

90

t/(min) Figure 3.24: The effect of the reaction temperature on photocatalytic efficiency.

concentration of TNT in the system once every 30 min. The experimental results are shown in Figure 3.24. It can be seen that the reaction temperature has a significant effect on the photocatalytic degradation efficiency, and 40∘ C is an appropriate reaction temperature, while at the temperature of 60∘ C, the effect is not obvious. This phenomenon indicates that rising temperature is helpful for the oxidation-reduction reaction on the surface of the catalyst and the photocatalytic activity increases in a certain temperature. If the temperature is too high, the adsorption on the surface of the catalyst is decreased and the concentration of dissolved oxygen is also decreased to lead to the unnoticeable change of the degradation efficiency. 3.2.2.4 Effects of Adding H2 O2 Prepare 50 mL simulated TNT wastewater of the same concentration in two containers respectively and two parts of composite material of the same mass and the same load rate to do the photocatalytic degradation after adding a small amount of H2 O2 (about 1 mL) to one container. The experimental results are shown in Figure 3.25. It can be seen that adding H2 O2 has little influence on the catalytic efficiency. From the degradation curve, the degradation rate of the solution with H2 O2 added is slower than that of the solution without H2 O2 in the first 30 min, but the degradation rate of the solution with H2 O2 added is better. But generally speaking, adding H2 O2 has little influence on the photocatalytic efficiency. 3.2.2.5 Effects of the Light Source Conduct the photocatalytic degradation for the two systems respectively in the UVlight and the natural light under the same reaction conditions. The experimental results are shown in Figure 3.26. It can be seen that the catalytic degradation efficiency for TNT in the UV-light is better than that in the natural light under the same

3.2 The Degradation of TNT by TiO2 /ACF Composite Photocatalyst

250

Adding perhydrol peroxide Not adding perhydrol peroxide

250 c/(mg/L)

107

200 150 100 50 0

30

60

90

t/(min) Figure 3.25: The effect of adding H2 O2 on the photocatalytic efficiency.

100 90

Natural light UV–light

c/(mg/L)

80 70 60 50 40 0

30

60

90

120

t/(min) Figure 3.26: The effect of light source on the photocatalytical efficiency.

reaction conditions. The removal efficiency of TNT from the simulated TNT wastewater in the natural light for 2 h is 46%, while the removal efficiency of TNT from the simulated TNT wastewater in the UV-light for 2 h is 57%. It can also be seen that there is little difference of TNT degradation rates between two conditions in the early stage of the degradation, but the degradation efficiency with the UV-light is better than that with the natural light in the middle and final stage of the degradation. This is because the removal of TNT in the system is mainly composed of the adsorption of the activated carbon fiber at the beginning of the degradation and the effect of light source on the system at this time is not large. Because the UV-light is only 3% in the natural light, the TNT catalytic degradation rate by the natural light slows obviously down in

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3 Application of lightweight carbon material and its composite

the middle and later stage of TNT photocatalytic degradation with TiO2 ; in the meantime, the concentration of TNT inside the activated carbon fiber decreases slowly and the adsorption of TNT in the simulated wastewater on the activated carbon fiber also obviously slows down. But in the ultraviolet photocatalytical degradation system, the destroying rate of the structure of the TNT molecules adsorbed by the activated carbon fiber under photocatalysis of the titanium dioxide is much more obvious. On the one hand, TNT is able to be quickly removed; on the other hand, more space is provided in the activated carbon for further adsorption. 3.2.2.6 Effects of the Reusing Frequency of Catalyst Wash the used composite catalyst with the distilled water and dry it. Conduct the photocatalytical degradation of the simulated TNT wastewater for repeatedly under the same condition. The experimental results are shown in Figure 3.27. It can be seen from the figure that the catalyst can be reused and the degradation efficiency changes little. After the catalysts having been used for five times, its degradation efficiency can all achieve higher than 65%, which indicates that the titanium dioxide combines strongly with the carrier and does not fall off easily in the using process.

3.2.3 The Function of Fenton Reagent for the Photocatalytic Degradation of TNT with ACF/TiO2 Recent studies show that it is dominant to use the combination of the Fenton reagent homogeneous photochemical advanced oxidation technology and the heterogeneous photocatalytic oxidation together to handle wastewater. It can overcome the problems of low H2 O2 utilization efficiency, imperfect degradation of the organic pollutants, and the simple Fenton reaction being conducted in the strong acid medium whose pH value is smaller than 3.

Photocatalytical efficiency (%)

80

70

60

50 0

1

2 3 Number of use

4

5

Figure 3.27: The effect of catalyst reusing on the photocatalytical efficiency.

3.2 The Degradation of TNT by TiO2 /ACF Composite Photocatalyst

109

3.2.3.1 The Photocatalytical Efficiency of Different Reaction System in the UV-light The following reactions take place in the UV/Fenton oxidation reaction system: H2 O2 + Fe2+ → Fe3+ + HO– + HO H2 O2 + h# → 2HO⋅ The produced HO in this system has a strong oxidation ability (the oxidation reduction potential is 2.80 V) to make the pollutants mineralized completely or partially decomposed and generally has a big reaction rate constant to the organic substances with bond C–H or C–C, even close to the control limit of the diffusion rate. In the oxidation system, the UV-light and Fe2+ are essential for to produce free radicals. In TiO2 photocatalytic system, if the irradiation energy is greater than the forbidden bandwidth of semiconductor materials, the electrons will transit to form electron/hole pairs on the surface of the semiconductor, and these holes can adsorb water molecules or hydroxyl ions to form HO. Prepare respectively 50 mL simulated wastewater of TNT of the same concentration to form the following four reaction systems: UV/Fenton/TiO2 /ACF, UV/Fenton/ACF, UV/Fenton/TiO2 and UV/Fenton, among which the Fenton reagent is compounded as 0.05 mL H2 O2 , 0.01 g FeSO4 ⋅ 7 H2 O. Conduct the photocatalytic degradation under the condition of illumination with UV-light. Sample and measure the concentration of TNT of each system every 30 min. The experimental results are shown in Figure 3.28. It can be seen that the degradation efficiency of UV/Fenton/TiO2 /ACF and UV/Fenton/ACF are obviously superior to that of UV/Fenton/TiO2 , and the difference of degradation efficiency between UV/Fenton/TiO2 and UV/Fenton is not big in the first 30 min of the degradation. This is because ACF shows its strong adsorption properties, and the catalytic reaction of Fenton system is predominant in UV/Fenton/TiO2 and UV/Fenton system, and the 140

UV/Fenton UV/Fenton/ACF UV/Fenton/TiO2 UV/Fenton/TiO2/ACF

120

c/(mg/L)

100 80 60 40 20 0

30

60

90

120

t/(min) Figure 3.28: The comparison chart of the UV-light efficiency.

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3 Application of lightweight carbon material and its composite

photocatalysis reaction of the surface of TiO2 is secondary at the beginning of the reaction. It is mainly influenced by slow diffusion of TNT molecules from the liquid phase to the surface of TiO2 . In the middle and late stages of the degradation, the degradation efficiency of UV/Fenton/TiO2 /UV/Fenton is obviously better than that of ACF/ACF. This is because the strong adsorption of ACF provides a high concentration environment for the TiO2 photocatalysis reaction, accelerating the reaction rate and realizing the rapid degradation of TNT, so that the TNT biodegradation rate can reach more than 80% after 120 min. At the same time, the degradation efficiency of UV/Fenton/TiO2 is better than that of the UV/Fenton in the middle and late stage of degradation. It is because the photocatalytical property of TiO2 is more and more significant with the increase of reaction time, and its degradation rate is much faster than that of UV/Fenton. To sum up, the order of the degradation efficiency of a variety of photocatalyst for TNT under the condition of the UV-light is: UV/Fenton/TiO2 /ACF > UV/Fenton/ACF > UV/Fenton/TiO2 > UV/Fenton.

3.2.3.2 The Analysis of the Photocatalytical Efficiency in the Natural Light HO⋅ can be produced through the reaction H2 O2 + Fe2+ →Fe3+ +HO– +HO⋅ in the Fenton system; therefore, it also has a strong ability of oxidation in the natural light. There is about 3% UV-light components in the natural light, so the TiO2 photocatalysis system also can play a role in photocatalysis. Prepare respectively 50 mL simulated wastewater of TNT of the same concentration to form the following four reaction systems: UV/Fenton/TiO2 /ACF, UV/Fenton/ACF, UV/Fenton/TiO2 and UV/Fenton. The photocatalytic degradation is under the condition of natural light (at noon in mid-June, the outdoor temperature is 37∘ C). Measure the concentration of TNT of each system every 45 min. The experimental results are shown in Figure 3.29. It can be seen from the figure that the degradation efficiency of Fenton/TiO2 /ACF and Fenton/ACF is obviously better than that of Fenton/TiO2 and Fenton in the previous stage of reaction, and this conclusion is the same as in the UV-light. In the middle and later states of reaction, as the TNT concentration is reduced, the degradation rates of Fenton/TiO2 /ACF and Fenton/ACF are reduced, but the catalytic efficiency of Fenton/TiO2 /ACF is superior to that of Fenton/ACF. This is because the photocatalytic efficiency of TiO2 is significantly strengthened with the increase of the reaction time in the Fenton/TiO2 /ACF system, and more pores are provided for the adsorption of ACF while the adsorbed TNT on ACF is mineralized. In the Fenton/ACF system, the adsorption rate is decreased obviously with the decrease of TNT concentration and the saturated adsorption of ACF. Compared with the UV-light condition, the adsorption efficiency of the UV/Fenton system is superior to that of the Fenton system, while the adsorption efficiency of the Fenton/TiO2 /ACF system is superior to that of

3.2 The Degradation of TNT by TiO2 /ACF Composite Photocatalyst

140

Fenton Fenton/ACF Fenton/TiO2 Fenton/TiO2/ACF

120 100 c/(mg/L)

111

80 60 40 20 0 0

30

60

90

120

t/(min) Figure 3.29: The comparison chart of degradation efficiency in natural light.

the UV/Fenton/TiO2 /ACF system. Its possible reason is that the effect of temperature in the reaction system is more significant than that of the light source. To sum up, under the condition of natural light, the order of the efficiency of the catalyst for the degradation of TNT is: Fenton/TiO2 /ACF > Fenton/ACF > Fenton/TiO2 > Fenton.

3.2.4 The Photocatalytical Degradation of TNT Wastewater with the Modified ACF/TiO2 In order to improve the photocatalytical property of TiO2 and to increase its utilization of the sunlight, researchers have used a variety of means to modify TiO2 , including the precious metal modifier, the compound semiconductor, the dye sensitization and the transition metal ion doping, and so on. The transition metal ion doping can introduce defect locations in TiO2 or change the degree of crystallinity to affect the compound of electrons and holes and the doping of some metal ions can also expand the scope of the light absorption wavelength, widely studied in recent years. At present, the metal ions mainly used in the study of doping of nanometer TiO2 are Fe3+ , Cu2+ , Zn2+ , Ni2+ , Ag+ , Cr3+ , Co2+ , and the rare earth ions, but influence factors of the photocatalytical reaction are too many. The diversity of the preparation of the catalyst and the degradation objects makes the study results different, some even in the opposite conclusion, and the transitional metal ion doping modification mechanism does not have a consensus, so it is necessary to do further research on it. 3.2.4.1 The Effect of the Doping Amount on the Photocatalytical Efficiency Ion doping amount is an important parameter of the nanometer TiO2 photocatalytical performance. According to reports in the literature, there is an optimal concentration

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3 Application of lightweight carbon material and its composite

of the doping ions. When the concentration of doping ion is lower than the optimal value, there is not enough charge carriers in the semiconductor to capture the trap, and the photocatalytical activity is creased with the increase of doping ion concentration. When the concentration of doping ion is more than the optimal value of doping ions, the compounding rate is increased with the shortening of the average distance between captures. Because the solubility of doping ion inside TiO2 is limited, and the high doping amount can cause doping ions enriched on the surface of the catalyst, and the generation of these heterogeneous phases also can make the photocatalytical activity reduced, the ion doping amount all has an optimal value, at which the photocatalytical activity of TiO2 is the best. Prepare respectively 50 mL simulated TNT wastewater of the same concentration, and prepare respectively four groups of TiO2 /ACF with silver ion doped and copper ion doped, the concentration of which is 0.01%, 0.05%, 0.1%, and 0.3%, respectively. Conduct the catalytic degradation experiment at room temperature under the UV-light and the natural light. The experimental results are shown in Figures 3.30 and 3.31. It can be seen from the diagram that there is an optimal doping existing amount of Ag+ and Cu2+ on TiO2 photocatalyst, all 0.05%. As far as the doped ions are concerned, the photocatalytical activity is more sensitive to the doping amount of copper ion. It is possibly because the doped Cu2+ ions can be used as the capture position of electron-hole pair to capture the electrons and holes at the same time and reduce the recombination rate of electrons and holes. The ability for Cu2+ to capture the electrons and holes is better than that of Ag+ . The jumping type growth of the light absorption performance of the doped Cu2+ samples from the visible light region to the ultraviolet region is more apparent that of Ag+ and there is an obvious absorption band edge and

140 130

0.01% 0.05% 0.1% 0.3%

120

c/(mg/L)

c/(mg/L)

110 100 90 80 70 60 50 40 0 (a)

30

60 t/(min)

90

120

140 130 120 110 100 90 80 70 60 50 40 30

0.01% 0.05% 0.1% 0.3%

0 (b)

30

60

90

120

t/(min)

Figure 3.30: The ultraviolet photocatalytical degradation of TNT with TiO2 /ACF of different doping amounts. (a) Ag doping, (b) Cu doping.

3.2 The Degradation of TNT by TiO2 /ACF Composite Photocatalyst

140

80

c/(mg/L)

0.1% 0.3%

60

(a)

0.1%

80

0.3%

60 40

20

20

0 1.5

3.0 t/(h)

4.5

0 0.0

6.0 (b)

0.05%

100

40

0.0

0.01%

120

0.05%

100 c/(mg/L)

140

0.01%

120

113

1.5

3.0

4.5

6.0

t/(h)

Figure 3.31: The natural light photocatalytical degradation of TNT with TiO2 /ACF of different doping amounts. (a) Ag doping, (b) Cu doping.

Cu2+ can keep more photosensitive properties of TiO2 semiconductor photocatalyst than that of Ag+ . It can also be seen from the diagram that the photocatalytic efficiency of TNT is obviously increased after the doped TiO2 powder is loaded on the activated carbon fiber. The removal rate of TNT can be close to 65% when the simulated TNT wastewater is degraded with 0.05% Ag+ –TiO2 /ACF under the illumination of the UV-light for 2 h and the removal rate of TNT can be 73% when the simulated TNT wastewater is degraded with 0.05% Cu2+ –TiO2 /ACF under the illumination of the UV-light for 2 h. This is because the activated carbon fiber itself does not have the ability to decompose the TNT but its strong adsorption provides a high concentration environment for TiO2 photocatalyst doped with the metal ions, so as to improve the photocatalytical efficiency of TiO2 photocatalyst, to speed up the reaction rate, and to achieve the rapid degradation of TNT. It can also be seen from the diagram that the difference between the degradation efficiencies of TNT with catalyst ACF/TiO2 doped with different amount of Ag+ and Cu2+ in the natural light is not big and they are both greater than 90% in 6 h photocatalytical degradation. This is because the strong adsorption capacity of activated carbon fiber at the beginning of the degradation provides a high concentration reaction environment for the photocatalytical degradation of TiO2 . The trace doping of Ag+ and Cu2+ expands the TiO2 photocatalysis absorption band and improves the utilization of natural light, so the TiO2 photocatalytical efficiency can be significantly improved under the reaction environment of high concentration to realize the degradation of TNT. Therefore, the modified TiO2 powder being loaded on the activated carbon fiber exerts cooperatively the strong adsorbability of the activated carbon fiber and the photocatalysis of TiO2 and the photocatalytical efficiency for TNT is significantly improved under the condition of the UV-light and the natural light. At the same time, the

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3 Application of lightweight carbon material and its composite

problem of desorption for the pure carbon material after the TNT wastewater degradation is solved; on the other hand, the problem of the TiO2 powder’s loss and recycling is solved too. 3.2.4.2 Effects of Loading Amount of the Modified TiO2 on the Photocatalytical Efficiency Prepare respectively 6 groups of 50 mL simulated TNT wastewater of the same concentration and conduct the photocatalytical experiment for the simulated TNT wastewater with six group composites of different loading factor (Ag+ –TiO2 /ACF and Cu2+ –TiO2 /ACF all with 0.05% loading amount, the loading factors is increased in turn) under the illumination of UV-light at room temperature using for 120 min. Sample and determine the content of TNT. The experimental results are shown in Figure 3.32. It can be seen from the diagram that different loading amount of Ag+ and Cu2+ on the modified TiO2 has significant effect on the catalytic efficiency, and there is an optimal loading amount. With the increase of loading ratio, the degradation of TNT rate declines, and the photocatalytical efficiency of the doped Cu2+ is better than that of Ag+ . 3.2.4.3 The Ultraviolet Spectrum Analysis of TNT Wastewater Photocatalytical Degradation with Cu2+ –ACF/TiO2 Prepare 50 mL simulated TNT wastewater of 100 mg/L. Conduct the ultraviolet photocatalytical degradation experiment for the simulated wastewater with Cu2+ – TiO2 /ACF composite material of 0.05% Cu2+ doping and 16% Cu2+ –TiO2 loading at 80 75 70 Degradation rate (%)

Degradation rate (%)

75 70 65 60 55

60 55 50 45

50 5

(a)

65

10

15 20 25 Load factor (%)

30

5

35

10

15

20

25

30

35

Load factor (%)

(b)

Figure 3.32: Effects of different loading amount of the modified TiO2 on the photocatalytical efficiency. (a) Ag doping, (b) Cu doping.

3.2 The Degradation of TNT by TiO2 /ACF Composite Photocatalyst

115

3.500 0 min

3.000

30 min 60 min 90 min

A 2.000 b s

120 min 150 min

1.000

0.000 190.0

300.0

400.0 Wavelength [nm]

500.0

600.0

Figure 3.33: The UV spectrum of the degradation of TNT with Cu2+ -TiO2 /ACF in the UV-light.

room temperature. Inspect the sample with ultraviolet-visible absorption spectrum every 30 min to study the photocatalytical efficiency of Cu2+ –TiO2 /ACF through the spectrum analysis. The experimental results are shown in Figure 3.33. It can be seen from the diagram that the TNT simulation wastewater without the photocatalytic degradation in B band has a multiple absorption peak (+max = 260.50 nm), which is caused by p – p * transition on the benzene ring of TNT. In the process of the photocatalytical degradation, the concentration of TNT is reduced and absorption peaks at this location is reduced too with the continuous destruction of the benzene ring. At the same time, the existence of the nitro substituent and the intermediate product from the destruction of benzene make the ultraviolet spectrum redshift and the absorption peak decrease in the system. This is because in the photocatalytic process, the p – p * transition band in the conjugate system compound formed after the damage of benzene ring in TNT forms the redshift due to energy decreasing and the intermediate product is continuously photographically degraded to make the absorbable photon concentration reduced to make the hypochromic effect. In the middle and late phases of the photocatalytical degradation, the continuous destruction of TNT benzene ring and the continuous mineralization of the intermediate product make the concentration of the intermediate product containing a double bond or conjugated double bond in the system decrease greatly to weaken the ultraviolet adsorption. When the concentration of TNT or the intermediate product is too low, the polar solvent (water) makes the ultraviolet absorption band of system form the serious redshift, which cannot be detected in the near ultraviolet band. The absorption at 400.50 nm in the degradation process, possibly caused by the system error or the light source and the instrument itself, has no effect on the photocatalytic system.

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3.3 The Study on the Decolorization and Adsorption of Printing and Dyeing Wastewater by the Expanded Graphite The dye is a kind of colored material, which can paint other materials in water or other solvents, mainly organic matter. And it is mainly used in textile printing and dyeing, accounting about 60–80% of total consumption of dyes. The non-textile dyes are mainly applied in the sector of leather, fur, food coloring, and so on, accounting about 20–40% of total consumption of dyes. According to chemical structures, the dye can be classified into the azo dye, the nitro and nitroso dyes, the aromatic methane dye (molecules containing methylene aryl methane and three aromatic structure), the anthraquinone dye, the indigo dye (containing indigo or a similar structure), the phthalocyanine dyes (containing metal phthalocyanine complex structure), the sulfide dye (made by the sulfur or sodium polysulfide sulfuration), the cyanine dye (containing poly methyl acetylene structure), the heterocyclic dye (containing five or six yuan heterocyclic structures), and so on. According to the application, it can be classified into about ten kinds of dyes, such as the acid dye, the direct dye, the cationic dye, the metal complex dye, the reactive dye, the oxidation dye, the sulfur dye, the mordant dye, the vat dye, the disperse dye, and the oil soluble and alcohol soluble dye. The textile dyeing industry is one of the most serious discharging sectors of industrial wastewater, accounting about 35% of all industrial wastewater emissions. Our country is the first big country of textile printing and dyeing and China’s printing and dyeing wastewater is about 3 × 106 –4 × 106 m3 /d according to the incomplete statistics. Dyeing factory produces 3–5 t wastewater for every 100 m of fabric processing, so the ecological environmental damage and economic loss arising is immeasurable. Printing and dyeing wastewater is mainly produced in de-sizing, scouring, bleaching, mercerizing, dyeing, printing, finishing process. And the wastewater contains the entrainment of cellulose raw material itself, as well as the pulp, oil, dye and chemical additives. The characteristics or the wastewater are of large quantity, of complicated composition, containing the color and luster of dense and toxic substances, of high chromaticity, of high pH change, of large change of water temperature, of big water quality change, of high chemical oxygen demand (COD), of relatively small biochemical oxygen demand (BOD5) (the ratio of the biochemical oxygen demand (COD) and chemical oxygen demand (COD) is less than 0.2) and of poor biochemical characteristics. In recent years, the development of the chemical fiber fabric dyeing and the progress of the arranging technology make the refractory organic pollutants, such as the PVA slurry additives and some new assistants, enter the printing and dyeing wastewater which is to be treated. The COD removal rate of the original biological treatment systems decreases mostly from 70% to 50%, even lower. The treatment of printing

3.3 The Study on the Decolorization and Adsorption of Printing

117

and dyeing wastewater has certain difficulties, and the commonly used methods are the physical treatment method (adsorption), the chemical processing method (coagulation method, oxidation method and electrolysis method), the biological treatment, etc. Actually, the combined of methods of the physical, chemical and biological methods are applied. At present, the biochemical method is predominant domestically and abroad in treating the printing and dyeing wastewater, some combined with the chemical method in series. Bleaching is an important part of the dyeing and printing wastewater treatment, as well as an important index of printing and dyeing wastewater treatment effect. In printing, the main factor of dyeing wastewater is chromaticity dye. The quantity of the world textile dyes is more than 40 ten thousand tons every year, and about 10–20% of dyes in wastewater discharges into the aquatic environment. The dye wastewater can absorb light, reduce the water transparency, affect the growth of aquatic organisms and microorganisms, and impede water self-purification. Its degradation products are mostly the carcinogenic aromatic compounds, such as benzidine class, causing the visual pollution at the same time; therefore, decoloring method research has become an important subject of printing and dyeing wastewater treatment. At present, there are many studies on the method of printing and dyeing wastewater decolorizing, mainly including the physical method, the chemical method, and the biological method. The physical method mainly includes adsorption, membrane separation, and extraction method. The chemical processing method mainly includes the coagulation method, oxidation, reduction process, and electrochemical method. The biological methods mainly include aerobic biological treatment, anaerobic biological treatment, anaerobic-aerobic treatment, high-efficient degradation bacteria, and engineering bacteria. Because the ingredient of printing and dyeing wastewater is very complex and a single decoloring technology often cannot achieve the ideal effect, especially for multiple types printing and dyeing wastewater, the decolorizing agent and a decoloring process with a wide application scope are necessary to be developed and researched. So far, all kinds of decolorization methods have certain defects from the economical, technical, and practical consideration. In recent years, many newly developed types of dyes are composed of stable cyclic organic compounds with poor biodegradability, and the decoloring effect of the biochemical method is worse than the physical-chemical method, so the general physical-chemical method is adopted to improve the decolorization. The adsorption method is a kind of an important physicalchemical method, but the method is rarely used domestically for cost reasons, which has an obvious contrast with the popularity of absorbent decolorization in developed countries. Although the adsorption method is costly, its operation is reliable and its disposal efficiency is outstanding. When the quality of the effluent is highly demanded, especially for the demanding of backwater, it is better to use this method. At the same time, the utilization of the backwater can compensate the high cost to a certain

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3 Application of lightweight carbon material and its composite

extent. But the disadvantages of currently used adsorbents are that the adsorption quantity is often not enough and not easy to be regenerated. The expanded graphite has a network pore structure, and most of the holes are slits or the polygon column-shaped holes or wedge-shaped holes derived from the slits. The surface holes are generally the open holes, and the internal interconnection holes conclude three kinds of holes, which are open holes, closed holes and semi-closed holes. The pore size distribution of the surface and internal holes in the expanded graphite is wide, ranging from 10 nm to 1,000 nm, mainly large and middle holes, which determines that the expanded graphite is suitable to adsorb large macromolecular material. It is theoretically possible to remove the big molecules in printing and dyeing wastewater by means of adsorption with the expanded graphite, which is proved by the current researches [24–28]. The graphite ore resources in China are quite rich. The graphite mineral has been found in about 20 provinces in China, among which about 91 mining areas have been exploited, totaled 173 Mt, the first in the world[19] (two-thirds of the reserves in the world are in our country), but the current study on the expanded graphite in China is much later than abroad. As a new type of material, the expanded graphite will have more and more applications in many areas because of its excellent physical and chemical forms. The sales of expanded graphite in international expanded graphite industries reach billions of dollars at present. If our country can take good use of the advantage of being the richest in graphite resources conservation and expand its application field, there will be significant economic benefits.

3.3.1 Preparation of the Dye Standard Solution and the Expanded Graphite Weigh accurately 1.0 g dried dye and dissolve it in the distilled water. Transfer the solution totally into a 1,000 mL volumetric flask and fill the flask with the distilled water to the scale line. The concentration of this standard solution is 1.0 g/L, which is used as the simulation printing and dyeing wastewater. The following four kinds of dyes are used in the experiments and their names and formulas are as follows [29]: 1. Acid orange II. The formula is C16 H11 N2 O4 S⋅Na, and its relative molecular weight is 350.33, golden yellow powder, easily soluble in water, and its structural formula is: (a)

HO NaO3S

N

N

119

3.3 The Study on the Decolorization and Adsorption of Printing

2.

Direct yellow R. It is a kind of organic polymer, the monomer molecular formula, C14 H8 N2 O7 S2 ⋅2Na, brown powder, easily soluble in water, and its formula is as follows:

(b) O NaO3S

SO3Na CH

O2N

O SO3Na

N

CH

N

NaO3S

SO3Na NaO3S

CH CH

N

N

CH

NO2

CH n

Cationic red X – GRL. Its formula is C18 H21 N6 ⋅ZnCl3 , the relative molecular weight, 493.13, dark red powder, easily soluble in water, and its formula is as follows:

3.

(c) CH3 CH3

N N

N

+ ̅ N

N

N . ZnCl– 3

CH2 CH3

4.

Direct blending yellow D – 3 RNL. Its formula is C68 H48 N16 O26 S8 ⋅8Na, the relative molecular weight, 1945.61, yellowish powder, easily soluble in water, and its formula is as follows:

(d) OCH3

SO3Na

N

N

NH CH3

SO3Na

SO3Na

N

CH

NH

N N NH

SO3Na 2

Compound the dye into a solution of 10 mg/L. Scan the dye solution in the wavelength scope of 300–800 nm with the UV-1601 ultraviolet-visible spectrophotometer, respectively, and choose the maximum absorption wavelength (+max ) as the working wavelength. Determine the absorbance of standard solution series under the working wavelength, and make a diagram with the relation between the absorbance A and the concentration C, and the drawn straight line is the working curve. The dye

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3 Application of lightweight carbon material and its composite

decolorization rate is measured with the change of the absorbance in the corresponding wavelength. Expand fully four kinds of expandable graphite respectively at 900∘ C constant temperature in a muffle furnace for use.

3.3.2 The Decolorizing Effect of the Expanded Graphite In the experiment, four kinds of typical dyes were studied, but the similarity of research and the limitation of the paper length just permit the representation of the decolorizing effects of acid orange II and direct yellow R. 3.3.2.1 The Study on Dye Decolorization of Acid Orange II (1) The adsorption equilibrium time. Figure 3.34 shows the experimental results of adding 0.2 g expanded graphite to the acid orange II dye solution, whose concentration is 0.3 g/L. It can be seen that the adsorptions of all the four kinds of expanded graphite attain to a balance in five hours. So the equilibrium time can be determined as 5 hours. The smaller the particle size of the expanded graphite is, the larger the expansion multiplication is and the faster the adsorption rate is. (2) The effect of the expanded graphite dosage on the decoloring rate. Figure 3.35 shows the experimental results of different dosage of the expanded graphite in the acid orange II with concentration of 0.3 g/L. It can be seen from the figure that the decolorizing rate is increased with the dosage increase of the expanded graphite. The expanded graphite with larger expansion ratio and smaller particle size has the bigger adsorption when the expanded graphite of the same quality is added into the dye solution of the same concentration.

Decolorization rate (%)

100 80 60

80 mesh 250 times 80 mesh 200 times 50 mesh 250 times 50 mesh 200 times

40 20 0 0

2

4

6

8

10

Time (h) Figure 3.34: The expanded graphite adsorption equilibrium of acid orange II.

3.3 The Study on the Decolorization and Adsorption of Printing

121

Decolorization rate (%)

100 80 80 mesh 250 times 80 mesh 200 times 50 mesh 250 times 50 mesh 200 times

60 40 20 0 0.0

0.1 0.2 0.3 Quality of expanded graphite (g)

0.4

Figure 3.35: The relationship of the dosage of expanded graphite and decolorizing rate.

100 Decolorization rate (%)

90

0.40 g/30 mL 0.20 g/30 mL 0.10 g/30 mL 0.05 g/30 mL

80 70 60 50 40 30 2

4

6 pH

8

10

Figure 3.36: The change of the decolorizing rate of the expanded graphite of 50 mesh and 250 times expansion ratio with the change of the concentration and the pH value.

Among all expanded graphite of different specification used in the experiment, the decolorizing result of the expanded graphite with hole-size of 80 mesh and 250 times expansion ratio is the best. But the difference of decolorizing rate and adsorption capacity of various kinds of expanded graphite for acid orange II is smaller and smaller with the increase of dosage. (3) The effect of the dosage of the expanded graphite and pH value on the decolorizing rate. Figures 3.36–3.39 show the experimental results of different dosage of the expanded graphite in 0.3 g/L acid orange II solution of different pH value. It can be seen from the figures that the decolorizing rates of all expanded graphite are reduced with the increase of pH value. Therefore, the decolorizing ability of the

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3 Application of lightweight carbon material and its composite

Decolorization rate (%)

100 90 80 0.25 g/30 mL 0.20 g/30 mL 0.10 g/30 mL

70 60 50 2

4

6

8

10

pH Figure 3.37: The change of the decolorizing rate of the expanded graphite of 50 mesh and 200 times expansion ratio with the change of the concentration and the pH value.

Decolorization rate (%)

100 95 90 0.15 g/30 mL 0.10 g/30 mL 85 80 2

4

6

8

10

pH Figure 3.38: The change of the decolorizing rate of the expanded graphite of 80 mesh and 250 times expansion ratio with the change of the concentration and the pH value.

expanded graphite to acid orange II dye solution is higher under the acid condition. To the same kind of expanded graphite, the influence of the pH value on the decolorizing rate increases with the decrease of the dosage. For different expanded graphite of the smaller particle size and the larger expansion times, the decolorizing rate is less affected by the dosage change under the condition of the same pH value change. (4) The effect of the initial concentration and pH value on the decolorizing rate. Figures 3.40–3.43 show the experimental results of adding 0.1 g expanded graphite to the acid orange II dye solution, with concentrations of 0.1 g/L, 0.2 g/L, 0.3 g/L and pH value from 2 to 9.

3.3 The Study on the Decolorization and Adsorption of Printing

123

Decolorization rate (%)

98 96 94 92 90 88

0.15 g/30 mL 0.20 g/30 mL

86 84 82 80 2

4

6 pH

8

10

Decolorization rate (%)

Figure 3.39: The change of the decolorizing rate of the expanded graphite of 80 mesh and 200 times expansion ratio with the change of the concentration and the pH value.

100 95 90 85 80 75 70 65 60 55 50

0.1 g/L 0.2 g/L 0.3 g/L 2

4

6

8

10

pH Figure 3.40: The change of the decolorizing rate of the expanded graphite of 50 mesh hole size and 250 times expansion ratio with the change of the concentration and the pH value.

It can be seen from the figures that the trend of influence of pH value on the decolorizing rate is consistent when the expanded graphite of the same dosage is added to the acid orange II solution with the same volume and different concentration. The trend is the decolorizing rate increases with the reduction of pH. For the same kind of expanded graphite, the higher the solution concentration is, the greater affected by the pH value. For different expanded graphite in the solution the same change of the pH value, the larger the expansion is and the smaller the particle size is, the distribution is the less affected by the concentration. (5) The effect of the initial concentration and the dosage of the expanded graphite on the adsorption quantity. Table 3.4 shows the experimental results of adding 0.1 g and 0.2 g expanded graphite to the acid orange II dye solution with the concentration

Decolorization rate (%)

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3 Application of lightweight carbon material and its composite

100 95 90 85 80 75 70 65 60 55 50 45

0.1 g/L 0.2 g/L 0.3 g/L 2

4

6

8

10

pH Figure 3.41: The change of the decolorizing rate of the expanded graphite with 50 mesh hole size and 200 times expansion ratio with the change of the concentration and the pH value.

Decolorization rate (%)

100 90 80 70 0.1 g/L 0.2 g/L 0.3 g/L

60 50 2

4

6

8

10

pH Figure 3.42: The change of the decolorizing rate of the expanded graphite with 80 mesh hole size and 250 times expansion ratio with the change of the concentration and the pH value.

respectively of 0.1 g/L, 0.2 g/L, 0.3 g/L and pH of all 2.0. With the increase of dye concentration, the adsorption of expanded graphite increases. With the increase of the expanded graphite dosage, the adsorption capacity is reduced. Because the expanded graphite in solution adsorbs not only the dye but also the water as the solvent, the determined adsorption capacity of the expanded graphite is generally the apparent adsorption capacity, and this adsorption quantity is affected by the granularity, the expansion ratio, and the dosage of the expanded graphite and the concentration of the dye solution as for the same dye. (6) The effect of the temperature on the adsorption capacity and the adsorption isotherm. Add respectively 0.1 g expanded graphite to 30.0 mL solution with the concentration of 0.1 g/L, 0.15 g/L, 0.2 g/L, 0.25 g/L, 0.3 g/L and the pH of 2.0 and keep them at the temperature of 30∘ C and 50∘ C. Then calculate the equilibrium concentration

125

3.3 The Study on the Decolorization and Adsorption of Printing

100

Decolorization rate (%)

95 90 85 80 75

0.1 g/L 0.2 g/L 0.3 g/L

70 65 60 55 50 2

3

4

5

6

7

8

9

pH Figure 3.43: The change of the decolorizing rate of the expanded graphite with 80 mesh hole size and 200 times expansion ratio with the change of the concentration and the pH value.

Table 3.4: The adsorption capacity of the expanded graphite of different dosage and specifications to the dye of different initial concentration (g/g). Specification for expanded graphite

80 mesh 250 times 80 mesh 200 times 50 mesh 250 times 50 mesh 200 times

The dosage of the expanded graphite (/g)

0.10 0.15 0.10 0.20 0.10 0.20 0.10 0.20

Initial concentration of dye (/g/L) 0.1

0.2

0.3

0.02964

0.05769

0.02918

0.05190

0.02970

0.05226

0.02970

0.05226

0.07663 0.05824 0.06447 0.04401 0.06555 0.04387 0.06555 0.04387

(Ce) and the equilibrium adsorption capacity (Qe), and draw diagram of the relation of the balance concentration to the adsorption capacity as shown in Figure 3.44. It can be seen from the figure that the adsorption quantity of the expanded graphite to the acid orange II dye is reduced with the rise of temperature, but in general, the influence by temperature is not large. 3.3.2.2 The Decolorizing Effect of the Expanded Graphite on the Direct Yellow R dye (1) The adsorption equilibrium time. Add 0.1 g expanded graphite to the direct yellow R dye solution with concentration of 0.1 g/L. The adsorption of the three kinds of expanded graphite all reach a balance eight hours later, and the experiment results are shown in Figure 3.45.

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3 Application of lightweight carbon material and its composite

80

Qe (mg/g)

70 60 50 40 30

30˚C 50˚C

20 10 0 0

10

20

30

40

50

Ce (mg/L ) Figure 3.44: The adsorption isotherm of 30∘ C and 50∘ C.

Decolorization rate (%)

80

50

40

80 mesh 250 times 80 mesh 200 times 50 mesh 250 times 50 mesh 200 times

20

0 0

2

4

6

8

10

Time (h) Figure 3.45: The adsorption curve of expanded graphite for the direct yellow R dye.

It can be seen from the figure that the rapid adsorption phase and the slow adsorption phase of the expanded graphite for the direct yellow R adsorption are quite obvious. At the beginning of the experiment, the curve increases rapidly in a short time, and then level off. This is mainly caused by the different structure of the direct yellow R and acid orange II. At the same time, it can also be seen that for the same mass of expanded graphite, the smaller the particle size is and the larger the expansion times is, the faster the adsorption rate is, but all achieve to their adsorption equilibrium in 8 h. (2) Effect of the expanded graphite dosage on the decolorizing rate. Add different dosage of the expanded graphite to the direct yellow R solution with the concentration of 0.1 g/L and regulate the pH value to 2.0. The experiment results are shown in Figure 3.46.

3.3 The Study on the Decolorization and Adsorption of Printing

127

100

Decolorization rate (%)

95 90 85 80 75

80 mesh 250 times 80 mesh 200 times 50 mesh 250 times 50 mesh 200 times

70 65 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 Quality of expanded graphite (g) Figure 3.46: The relationship between the dosage of expanded graphite and the decolorizing rate.

It can be seen from Figure 3.46 that the decolorizing rate becomes bigger with the increase of the dosage of expanded graphite, but the adsorption amount is reduced. With the same change of the expanded graphite dosage, the decolorizing rate of the lower expansion multiple expanded graphite increases more quickly for the same expanded graphite on granularity. As shown in the figure, the decoloring rate of the expanded graphite of the 80 mesh 200 times changes greater than that of the 80 mesh 250 times with the increase of the dosage. For the expanded graphite of the same expansion rate, the decoloring rate of the expanded graphite with the bigger particle increases more quickly. As shown in the figure, the decoloring rage of the 50 mesh 250 times change s greater than that of the 80 mesh 250 times with the increase of the graphite dosage. (3) The effect of expanded graphite dosage and pH value on the decolorizing rate. Prepare 30 mL dye solution with the concentration of 0.1 g/L and of different pH value and add 0.1 g of different kinds of expanded graphite to the solution respectively and keep vibration for 4 h. The experiment result is shown in Figure 3.47. It can be seen from the figure that the decolorizing rate of the expanded graphite is reduced with the increase of pH, the decolorizing rate under the acidic conditions is large, and the decolorizing rate changes little when the pH value gets to 7. For the expanded graphite with the same particle size, the decolorizing rate of the smaller expansion multiple is much more influenced by the pH value. For the expanded graphite of the same expansion ratio, the decolorizing rate of the bigger particle size is much more influenced by pH value. (4) The effect of the initial concentration and the pH value on the decolorizing rate. Prepare 30 mL the direct yellow R dye solution with the concentration of 0.1 g/L, 0.2 g/L and 0.3 g/L respectively and regulate them to different pH value and add 0.1 g

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3 Application of lightweight carbon material and its composite

Decolorization rate (%)

expanded graphite to each one. Keep vibration for 4 h. The experimental results are shown in Figures 3.48–3.51. The figures show that the changing trends of all the expanded graphite are the same. As for the direct yellow R solution of 0.1 g/L, its decolorizing rate changes with the changing of pH value is larger than that of direct yellow R of 0.2 g/L and 0.3 g/L after being added with 0.1 g expanded graphite, different from the regularity of the

100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

80 mesh 250 times 80 mesh 200 times 50 mesh 250 times 50 mesh 200 times

2

4

6 pH

8

10

Decolorization rate (%)

Figure 3.47: The change of the decolorizing rate of the expanded graphite with the change of pH value.

100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

0.1 g/L 0.2 g/L 0.3 g/L

2

4

6

8

10

pH Figure 3.48: The change of the decolorizing rate of the expanded graphite of 80 mesh 250 times with the change of the concentration and the pH value.

Decolorization rate (%)

3.3 The Study on the Decolorization and Adsorption of Printing

100 90 80 70 60 50 40 30 20 10 0

129

0.1 g/L 0.2 g/L 0.3 g/L 2

4

6

8

10

pH Figure 3.49: The change of the decolorizing rate of the expanded graphite of 80 mesh 200 times with the change of the concentration and the pH value.

Decolorization rate (%)

90 80 70 60 50 40 30

0.1 g/L

20

0.2 g/L

10

0.3 g/L

0 2

4

6

10

8

pH Figure 3.50: The change of the decolorizing rate of the expanded graphite of 50 mesh 250 times with the change of the concentration and the pH value.

90 Decolorization rate (%)

80 70 60 50 40 30

0.1 g/L

20

0.2 g/L

10

0.3 g/L

0 2

4

6

8

10

12

pH Figure 3.51: The change of the decolorizing rate of the expanded graphite of 50 mesh 200 times with the change of the concentration and the pH value.

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3 Application of lightweight carbon material and its composite

above-mentioned acid orange II. But in fact, the adding of 0.1 g expanded graphite is too small to be compared with 0.2 g/L, 0.3 g/L direct yellow R solution. Even if the adsorption capacity of the expanded graphite is very large with the change of the pH value, the decolorizing rate won’t change a lot because of the large concentration. For 0.1 g/L direct yellow R solution, 0.1 g expanded graphite is relatively moderate; therefore, the change of the adsorption capacity of the expanded graphite with the change of pH value can well reflect on the decolorizing rate. (5) The effects of the initial concentration and the expanded graphite dosage on the adsorption capacity. Prepare 30 mL the direct yellow R dye solution with the concentration of 0.1 g/L, 0.2 g/L, 0.3 g/L respectively and regulate their pH value to 2.0 and add 0.1 g, 0.2 g expanded graphite to them respectively. Keep vibration for 4 h. The experimental results are shown in Table 3.5. It can be seen from the table that the adsorption capacity of the expanded graphite is increased with the increase of dye concentration. With the increase of the dosage of the expanded graphite, the adsorption capacity is reduced. For the expanded graphite of the same dosage, the difference of adsorption capacity increases gradually with the increase of the concentration. For the expanded graphite of the same concentration, the difference of adsorption capacity is decreased with the increase of the dosage. Table 3.5: The adsorption capacity of the dye of the different initial concentration by the expanded graphite of different dosage and specifications (g/g). Specifications of the expanded graphite 80 mesh 250 times

80 mesh 200 times

50 mesh 250 times

50 mesh 200 times

The dosage of expanded graphite (/g)

Initial dye concentration (/g/L) 0.1

0.2

0.3

0.05 0.10 0.15 0.20 0.30

0.04609 0.02842 0.01960

0.04773 0.03871

0.06109

0.05 0.10 0.15 0.20

0.03806 0.02760 0.01975

0.10 0.15 0.20 0.25

0.02600 0.01925 0.01466

0.10 0.15 0.20 0.25 0.30

0.02368 0.01834 0.01416 0.01158

0.04364 0.02879 0.04340 0.03832

0.05660 0.04793 0.04135

0.04337 0.03538

0.05710 0.04647 0.03364

0.03790 0.02747

0.05075 0.03527 0.02879

Qe (mg/g)

3.3 The Study on the Decolorization and Adsorption of Printing

80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

131

30˚C 50˚C

0

20

40

60

80

100

Ce (mg/L) Figure 3.52: The adsorption isotherm of 30∘ C and 50∘ C.

(6) The effect of temperature on adsorption capacity and the adsorption isotherm. Add respectively 0.1 g expanded graphite to 30.0 mL solution with concentration of 0.1 g/L, 0.15 g/L, 0.2 g/L, 0.25 g/L, 0.3 g/L and pH value of 2.0 and keep the solutions respectively at the temperature of 30∘ C and 50∘ C. Determine the absorbance when they get the balance. Calculate the equilibrium concentration and the equilibrium adsorption capacity and draw the diagram as shown in Figure 3.52. It can be seen from the figure that the adsorption of direct yellow R dye by the expanded graphite is greatly influenced by the temperature, and the adsorption capacity increases with the rise of the temperature.

3.3.3 The Exploration of the Decolorizing Mechanism of the Expanded Graphite The adsorption property of expanded graphite to the organic macromolecules comes from its structure characteristics. The developed mesh structure of the expanded graphite makes it possess high specific surface area (50–200 m2 /g), high surface activity and non-polarity. The structure of the expanded graphite is different from the common structure of porous materials. The expanded graphite has mainly large pores and middle pores, the aperture of which is between 10 and 1,000 nm, mostly slits or the multilateral column-shaped pores or the wedge-shaped pores derived from the slits. The surface holes are generally the open holes, and the internal interconnection holes conclude three kinds of holes, which are open holes, closed holes and semiclosed holes, determining the advantage of the expanded graphite in the adsorption of macromolecular material [30, 31]. The oil adsorption mechanism of the expanded graphite has been reported, but its adsorption and decolorizing mechanisms for the dye have not been comprehensively

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3 Application of lightweight carbon material and its composite

acknowledged. The above experiments show that the decolorizing rate of the expanded graphite is influenced by the particle size of the expanded graphite, the expansion volume, the dosage, the adsorption time, the dye structure, the initial concentration of solution, the pH value, the adsorption temperature and many other factors, and among them, the influence of temperature is the smallest. 3.3.3.1 The Effect of the Particle Size and Expansion Multiples of the Expanded Graphite on the Decolorizing Rate The expanded graphite with the larger expansion graphite and the smaller particle size has the better adsorption effect for the dye solution when other factors are all the same. It is because that the specific surface area and pore size distribution of the same amount of the expanded graphite are determined by its expansion ratio and the particle size. The larger the expansion is and the smaller the particle size is, the greater the expanded volume will be. The opener the interlayer expansion of the expandable graphite is, the bigger the proportion of the large and middle holes and the specific surface area is. The contact area of dye and adsorbent is increased, and the adsorption quantity of unit mass of expanded graphite is also increased. Therefore, the expanded graphite with high expansion ratio and small particle size has the better adsorption and decolorizing effect for the dye. 3.3.3.2 The Effect of the Dosage of the Expanded Graphite on Decolorizing Rate The expanded graphite of larger dosage has the better adsorption effect and less effect of other factors when other factors are all the same. For the expanded graphite of the same expansion ratio and the same particle size, the adsorption decolorizing efficiency is increased with the increase of dosage. But the adsorption capacity of the unit mass of expanded graphite is decreased with the increase of the dosage. Because the interactions among adsorbents will increase if the quantity of adsorbent increases, so that the available area of the unit mass of adsorbent decreases, the unit mass of adsorbent adsorption quantity will reduce. Therefore, in order to make full use of the adsorption decolorizing performance of the expanded graphite, the dosage of the expanded graphite is not the more, the better. 3.3.3.3 The Effect of the Initial Concentration of Dye Solution and Adsorption Time on the Adsorption Decolorizing of the Expanded Graphite When all other factors are the same, the adsorption time of the expanded graphite is prolonged and the adsorption quantity of the expanded graphite will increase before the adsorption equilibrium of expanded graphite. And the decolorizing rate is also increased and remains unchanged after balance. With the increase of the initial concentration, the decolorizing rate of the expanded graphite will generally reduce, but the adsorption volume will increase. The adsorption of expanded graphite to the dye solution is influenced by the dye concentration and the adsorption time.

3.3 The Study on the Decolorization and Adsorption of Printing

133

The expanded graphite can adsorb both the dye molecules and the solvent water molecules. When the initial dye concentration increases, the expanded graphite adsorbs more dye molecules, so the apparent adsorption quantity increases. When the expanded graphite adsorbs the dye molecules, the concentration of dye solution is also its adsorption force. The higher the concentration of the dye, the faster the molecules spreads to the graphite surface to form a surface adsorption. This adsorption rate of this process is very fast. With the adsorption process going on, the concentration difference gradually reduces, the adsorption becomes slow, and then the dye molecules spread to a slot in the expanded graphite. Therefore, with the extension of time, the decolorizing rate is increased before the balance. After the balance, the decolorizing rate basically remains unchanged. 3.3.3.4 Effect of the Dye Structure on the Decolorizing Rate The expanded graphite’s adsorption capacity and decolorizing rate for the different dye solution are also different when all other factors are the same. As for the expanded graphite of the same condition, the order of the adsorption capacity and the decolorizing rate of the four kinds of dye solution are as follows from big to small: the direct blending yellow, the acid orange II, the cationic red and the direct yellow R. There is an adsorption limit of macromolecule with the expanded graphite, not the higher the molecular weight is, the better. But compared with the activated carbon, the adsorption of macromolecule with the expanded graphite is obviously advantageous. Although the adsorption of acid orange II with the activated carbon is more than that with the same amount of expanded graphite, its adsorption of the direct yellow R is only one-third of the adsorption quantity with the expanded graphite of the 50 mesh 200 times. The differences between them have relationship with the size of dye molecules. The expanded graphite is rich of pore structure. When some of the dye molecules are in conformity with the aperture, the dye molecules can enter the aperture and the adsorption amount is relatively large. If the dye molecule is too big or too small, the dye molecule cannot stay in a long time within the aperture and the adsorption capacity is relatively small. The pore network of the expanded graphite is mainly composed of the large and medium holes, and activated carbon is mainly composed of the microhole and the medium holes, so the specific surface area of the activated carbon is much bigger that than of the same mass of the expanded graphite. Therefore, the adsorption of activated carbon is better than that of expanded graphite as for the dye molecules of smaller molecular weight. The aperture of the activated carbon is too small to adsorb the large dye molecules; therefore, the adsorption capacity of the expanded graphite is much larger. 3.3.3.5 The Effect of the pH value of the Solution on the Decolorizing Rate The effect of pH value on adsorption is different for different dyes, and each dye has its appropriate pH value under the given condition. It can be seen that the effect of the change of pH of solution on the decolorizing rate is, in fact, closely related to the

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structure of the dyes. The reason is that the expanded graphite in certain pH environment can adsorb H+ and OH– , which may have action with the functional group of the dye molecules, to form the adsorption; Or because there exists the competitive adsorption between H+ or OH– and the dye molecules, and the expanded graphite’s adsorbing H+ or OH– makes the expanded graphite of nonpolarity carry certain charge, in favor of the adsorption of the dye carrying the opposite charges. Take the acid orange as an example. It can be seen from the structure that it is easy to produce the negatively charged anion (basement) in the aqueous solution, and the OH– in the solution is easy to form a competitive adsorption. When the alkalinity of the solution is enhanced, the surface of the expanded graphite surface will get negative charges due to the adsorption of OH– to repulse the colorization group of acidic orange II with negative charge. With the increase of pH value and the more adsorption of OH– , the adsorption of the negatively charged acidic orange II colorization groups will be reduced, so the adsorption and the decoloring rate of expanded graphite on acid orange II solution are both reduced with the increase of the pH value. When the acidity of the solution increases, the expanded graphite surface will get positive charges due to the adsorption of H+ . With the decrease of the pH value and the more adsorption of H+ , the adsorption of the negatively charged acidic orange II colorization groups will increase, so the adsorption and the decolorizing rate of the expanded graphite on acid orange II solution are both increased with the reduction of the pH value. In the same way, the cationic red ionizes cation-based colorization groups in aqueous solution and the adsorption of the expanded graphite on its surface is enhanced with the increase of pH. When there are multiple functional groups in the dye structure, the dominant position of different functional group in different pH will change with changing the pH value, so the adsorption property of expanded graphite for all dye solutions doesn’t monotonically increase or decrease with the change of the pH value.

3.3.4 The Fractal Analysis of the Adsorption of the Expanded Graphite for Dye It is manifested from the current research that the fractal geometry theory, presented and developed by Mandelbrot and Feder provided a powerful tool to depict the heterogenization of various particles in nature, such as the rock particles, the soil particles, protein, the anisotropy of flocs, the catalysts, and so on. Avnir D, Pfeifer P, Fumiaki Kano, et al. successfully applied the fractal geometry to the surface adsorption system. The results showed that most of the material surface could be divided into fractal forms and it is suitable to use nonintegral fractal dimension to describe the surface irregularity an effective measure. 3.3.4.1 The Fractal Theory of Solid Surface Adsorption In general, fractal refers to a random chaos and complex form, but its partial has some similarities to the whole system and its constituent part is similar to the overall form

3.3 The Study on the Decolorization and Adsorption of Printing

135

in a way. The change of system dimension is continuous, or a fraction, called fractal dimension [32–35]. The fractal adsorption isotherm model in a diluted aqueous solution can be expressed as follows: A = Am ⋅ Ce1/m/(bm + Ce1/m) Where A is the adsorption capacity of unit mass particles (mg/g); Am is the saturated adsorption capacity of unit mass particles (mg/g); Ce is the adsorption equilibrium concentration (mg/L); b is a constant; m is the number of adsorption active position occupied by one adsorbate molecule. According to the research of Avnir D, m is related with the radius or the cross-section area of the adsorbate molecule as follows: m ∝ !Ds/2–1 ∝ rDs–2 0 0 then : lgm = 1gk1 + (Ds /2 – 1) lg !0 = 1gk2 + (Ds – 2)1gr0 Where !0 is the cross-sectional area of the adsorbate molecule (nm2 ); R0 is the radius of gyration of adsorbate molecule (nm); Ds is the fractal dimension of particle surface. Additionally, it is shown from the research of Kopelman R that the heterogeneous reaction process does not conform to the classic dynamic law and the corresponding reaction rate constant k is no longer be a constant unrelated with time, but displays the relation with time t. Here the following formula is established: k(t) = k0 t–h h = 1 – ds /2 Where h is a parameter of describing the particle degree of regional heterogeneous medium; k0 is a parameter unrelated with the time t; ds is the spectrum of fractal dimension, which is an important parameter describing the dynamic behavior of fractal structure. The definition of the fractal dimension spectrum is as follows: P ∝ t – ds/2 Where P is the probability of returning to the starting point of the irregular walker after the time t. 3.3.4.2 The Fractal Analysis of the Adsorption of Expanded Graphite for Dye The determination of the fractal dimension on the expanded graphite surface (1) The sectional area of the adsorbate molecule of dye. The sectional area of dye molecules can be worked out by the molecular surface area. Research shows that the

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continuity index of the first-order molecule 17 has a good correlativity with the total surface area (TSA) and the sectional area (!0 ) of molecule. The corresponding formula is as follows: TSA = 24.61 7 + 57.7 R2 = 0.9139 TSA = 4!0 = 40r20 The data of the sectional area and 17 of three kinds of dye molecules are shown in Table 3.6. (2) The fractal dimension of the expanded graphite surface. Make a fitting for the isothermal adsorption data of the acid orange II, the cationic red and the direct blending yellow on the expanded graphite of 80 mesh 250 times with the fractal adsorption isotherm model, the results which are shown in Table 3.6. It can be seen from the correlation coefficient that the fractal adsorption isotherm model in the diluted solution can well conform to the adsorption process of three kinds of dye on the expanded graphite. According to the fitting results of the adsorption isotherm, plot a diagram with lgm and lg!0 as Figure 3.53. The slope K of the straight line is calculated as 0.1113 and the surface fractal dimension of the expanded graphite can be calculated accordingly: Ds = 2 K + 2 = 2.2226. 3.3.4.3 The Fractal Adsorption Kinetics of the Expanded Graphite The data of the adsorption time and the mass concentration of the dye during the adsorption with the expanded graphite of the 80 mesh 250 times can be fitted according to the empirical equation, as shown in Table 3.7. Where C is the concentration (mg/L) of dye in the clear liquid the of the lower level at a given adsorption time; T is the corresponding adsorption time (h); k󸀠 and C1 are constants. The good relativity of each equation in the table means the dye adsorption process conforms to the pseudo-first-order reaction kinetics. Table 3.6: The fractal adsorption parameters and calculated molecular sectional area of three kinds of dye. Number

1 2 3

Dye

Acid orange II Cationic red Direct blend yellow

continuity index of the first-order molecule

10.8022 11.6311 53.9676

Sectional area of molecule !0 /nm2

0.8086 0.8596 3.4633

Fractal adsorption isotherm 1/m

R2

0.3024 0.3066 0.2597

0.9894 0.9306 0.8473

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3.3 The Study on the Decolorization and Adsorption of Printing

0.60 0.59

lgm = 0.5253 + 0.1113 lgα0

0.58

R2 = 0.9874

0.57

lgm

0.56 0.55 0.54 0.53 0.52 0.51 0.50 –0.2 –0.1 0.0

0.1

0.2 0.3 lgα0

0.4

0.5

0.6

0.7

Figure 3.53: The linear relationship between Lgm with lg!0. Table 3.7: The fitting results of the four kinds of dye adsorption kinetics curve. Dye name

Empirical equation fitting results

R2

Acid orange II Cationic red Direct blend yellow Direct yellow R

C = 3.6934 exp (–0.1486 t) C = 12.7524 exp (–0.0844 t) C = 17.5823 exp (–0.1961 t) C = 43.9411 exp (–0.0627 t)

0.9978 0.9111 0.9573 0.9436

C = C1 exp(–k󸀠 t)

The above fitting equation shows the corresponding relationship of the concentration (mg/L) of the dye in the clear liquid of the lower level with the adsorption time. Herein the relation of the adsorption capacity with adsorption time can be gotten: q = q0 [1 – exp(–k󸀠 t)] Where q is the adsorption capacity (mg/g); t is the adsorption time (h); q0 is an empirical constant; k󸀠 is the speed rate constant (h–1 ). For the data of the adsorption time and the adsorption, the instantaneous adsorption reaction rate coefficient k is computed according to regularity of the first-order kinetics; then we can get: ln(q1 /q2 ) = k(t1 – t2 ) Where the subscripts 1 and 2 express the adjacent adsorption time. According to the above formula, a series of instantaneous adsorption reaction rate coefficient k at different time can be calculated. The relationship between lgm and lgt can be fitted as shown in Table 3.8 and Figures 3.54–3.57.

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Table 3.8: The simulation results of the adsorption kinetics curves of four dyes. Dye

Fitting results

Acid orange II Cationic red Direct blend yellow Direct yellow R

Fitting equation results

h

R2

lg k = –2.5550 – 0.7046 lgt lg k = –0.9341 – 2.3674 lgt lg k = –0.8537 – 1.6950 lgt lg k = –0.5535 – 1.5846 lgt

0.7046 2.3674 1.6950 1.5846

0.9031 0.8795 0.9704 0.8059

–2.70 –2.75 –2.80

lgk

–2.85 –2.90 –2.95 –3.00

lgk = –2.5550 – 0.7046 lgt R2 = 0.9031

–3.05 0.3

0.4

0.5 lgt

0.6

0.7

Figure 3.54: The linear relationship between the lgm and lgt when the acid orange II is adsorbed with the expanded graphite.

Through the analysis, the relationships between the instantaneous reaction rate and the reaction time during the adsorption of four dyes with the expanded graphite conform to the fractal isotherm relationship, which means that the adsorption dynamics of dye with the expanded graphite is of fractal-typed characteristic.

3.4 Study on the Adsorption Performance of the Unsymmetric Dimethyl Hydrazine (UDMH) with the Multi-Walled Carbon Nanotubes The carbon nanotube has a large specific surface area and its adsorption performance is superior to the traditional adsorbent. Even though some research results, on the hydrogen storage, dioxins and other organic waste gas removal and the adsorption of the

3.4 Study on the Adsorption Performance of the Unsymmetric Dimethyl Hydrazine

139

–1.0 –1.2

lgk

–1.4 –1.6 –1.8 –2.0

lgk = –0.5535 – 1.5846 lgt R2 = 0.8059

–2.2 –2.4 0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

lgt Figure 3.55: The linear relationship between the lgm and lgt when the cationic red is adsorbed with the expanded graphite.

–1.6 –1.8

lgk

–2.0 –2.2 –2.4

lgk = –0.9341 – 2.3674 lgt

R2 = 0.8795 –2.6 –2.8 0.3

0.4

0.5 lgt

0.6

0.7

Figure 3.56: The linear relationship between the lgm and lgt when the direct yellow R is adsorbed with the expanded graphite expanded graphite.

heavy metal ions, have been achieved currently, there are still many applications that need to be developed and researched. This work adopts the multi-walled carbon nanotubes (MWNTs) to study the adsorption performance on the unsymmetric dimethyl hydrazine wastewater solution and to explore its adsorption condition, which can provide the basis for wastewater treatment applications for carbon nanotubes in UDMH [36–40].

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3 Application of lightweight carbon material and its composite

–1.3 –1.4 –1.5

lgk

–1.6 –1.7 –1.8 –1.9

lgk = –0.8537 – 1.6950 lgt R2 = 0.9704

–2.0 –2.1

0.3

0.4

0.5

0.6

0.7

lgt Figure 3.57: The linear relationship between the lgm and lgt when the direct yellow R is adsorbed with the expanded graphite expanded graphite.

3.4.1 The Adsorption Performance of Carbon Nanotubes on the Unsymmetric Dimethyl Hydrazine Solution 3.4.1.1 The Static Equilibrium Adsorption Capacity Add 0.05 g carbon nanotubes and 25 mL UDMH solution with the concentration of 40, 70, 100, 140, 160, 190 mg/L to six 50 mL conical flasks respectively. Keep the solution statically at 301 K for 10 hours and then filter out the carbon nanotubes. Determine the content of UDMH (0.01 mg/L–1.0 mg/L) in the residual liquid with the ferrous amino sodium cyanide spectral method. The static adsorption experiment data are shown in Table 3.9. The static equilibrium adsorption calculation formula is qe = V(C0 – Ce )/W Where qe is the adsorption capacity (mg/g) of UDMH on the CNTs; V is the UDMH solution volume (L); C0 is the mass concentration (mg/L) of UDMH before the adsorption; Table 3.9: The static adsorption experiment data. Sample

1

2

3

4

5

6

UDMH concentration (mg/L) UDMH equilibrium concentration (mg/L) Static equilibrium adsorption (mg/g)

40 6.00 17.00

70 13.58 28.21

100 22.78 38.61

130 36.00 47.00

160 50.82 54.59

190 70.09 59.95

Equilibrium adsorption capacity (mg/L)

3.4 Study on the Adsorption Performance of the Unsymmetric Dimethyl Hydrazine

141

70 60 50 40 30 20 10 0

0

10

20 30 40 50 60 Equilibrium concentration (mg/L)

70

80

Figure 3.58: The adsorption isotherm of UDMH by the MWNTs.

Ce is the mass concentration (mg/L) of UDMH at the adsorption equilibrium; W is the quality of the CNTs (g). Plot a diagram with the equilibrium concentration of UDMH and the corresponding equilibrium adsorption capacity, as shown in Figure 3.58. The adsorption isotherm can intuitively reflect the adsorption performance of the carbon nanotubes on UDMH. It can be seen from the figure that this is a type I adsorption, belonging to the monolayer adsorption, and the equilibrium adsorption capacity increases with the increase of the initial concentration of the unsymmetric dimethyl hydrazine in the discussed scope. (1) Langmuir adsorption isotherm curve fitting. Langmuir thinks the adhesion strength between the adsorbent surface and the adsorbate is caused by the weak chemical adsorption force, and the adsorption capability of solid is caused by the unsaturated atomic force field on the surface of adsorbent, the remained valence force. The interaction scope of the adhesion strength is the thickness of the monolayer, and it will not cause adsorption beyond this scope. The adsorbed molecule does not affect each other, and the surface is uniform. This model is called the monolayer adsorption model, which can be expressed by the Langmuir adsorption isotherm: qe =

qm Kb Ce 1 + Kb Ce

The above type can be transformed into a linear expression as follows. 1 1 1 = + qe qm qm Kb Ce Where qe is the adsorption capacity (mg/g); qm is the saturated adsorption capacity (mg/g); Kb is the binding constant (L/mg); Ce is the equilibrium concentration (mg/L).

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3 Application of lightweight carbon material and its composite

0.06 0.055 0.05 1/qe (mg/g)

0.045 0.04 0.035 0.03 0.025 0.02 0.015 0.01 –0.05

0.05

0.15

0.25

0.35

0.45

0.55

1/Ce (L/mg) Figure 3.59: The Langmuir-typed linear regression curve.

Take 1/qe as the ordinate, 1/Ce as the abscissa to plot a curve and take the experimental data in the coordinate diagram and fit the isothermal adsorption curve of UDMH with the Langmuir equation, as shown in Figure 3.59. The fitting equation is as follows: 1/qe = 0.2761 (1/Ce ) + 0.0135 (r = 0.9984) (2) Freundlich adsorption isotherm fitting. The Freundlich isothermal adsorption equation is an empirical equation through analyzing and summing a large number of experimental data to describe the adsorption phenomenon of matter on the surface with the inhomogeneous energy distribution. The adsorption heat reduces with the decrease of the adsorption capacity in a logarithmic form. The expression form of the equation is: qe = kCe1/n Make logarithm at both sides of the equation: ln qe = ln k +

1 ln Ce n

Where n and k are the constants and others are the same as those in the Langmuir isotherm. Take lnqe as the ordinate, lnCe as the abscissa to plot a curve as shown in Figure 3.60 with the experimental data, and fit the data as the following equation: ln qe = 0.518 ln Ce + 1.963 (r = 0.9933) The fitting results indicate that the Langmuir isotherm and Freundlich isotherm both conform to the adsorption behavior of UDMH by the carbon nanotubes very well, with the correlation coefficient more than 0.99.

lnqe

3.4 Study on the Adsorption Performance of the Unsymmetric Dimethyl Hydrazine

4.5 4.3 4.1 3.9 3.7 3.5 3.3 3.1 2.9 2.7 1.5

2

2.5

3

3.5

4

143

4.5

lnCe Figure 3.60: The Freundlich-typed linear regression curve.

3.4.1.2 The Adsorption Equilibrium Time In the process of adsorption, the adsorbate and adsorbent have certain contact time, which makes the adsorption reaction close to the balance, making full use of the adsorption capacity. The adsorption equilibrium time depends on the adsorption rate. The higher the adsorption rate is, the shorter the time it takes to reach the adsorption equilibrium. Respectively add 25 mL UDMH solution of 105.14 mg/L and 0.05 g carbon nanotubes into six conical flasks of 50 mL. Adjust the pH value of the solutions to 7 and then fully vibrate it. Keep the solutions statically at room temperature for 1 h, 2 h, 3 h, 4 h, 5 h, 6 h and 10 h respectively before being taken 0.5 mL supernatant fluid for the absorbance determination. The results of the equilibrium balance time of the carbon nanotube adsorbing UDMH are shown in Figure 3.61. At the room temperature and 80

Concentration (mg/L)

70 60 50 40 30 20 10 0 0

2

4

6

8

10

12

Time (h) Figure 3.61: The relationship curve between the adsorption time and the concentration of UDMH.

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3 Application of lightweight carbon material and its composite

with the pH value of 7, the adsorption equilibrium time of the carbon nanotubes on UDMH is about 6 h, at which time the adsorption quantity of carbon nanotubes gets more than 98% of its static adsorption capacity. With the adsorption continuing, the adsorption quantity changes little.

3.4.2 The Impact of the Carbon Nanotubes on the UDMH Adsorption 3.4.2.1 The Influence of pH value on Adsorption The pH value is an important impact factor on the ionizable organic compound adsorption behavior in the wastewater treatment. Because such compounds have two forms in the aqueous solution, the ionic state and nonionic state, the adsorption behaviors of these two compounds are different. The pH value controls the dissociation degree and the solubility of the ionizable organic compounds, so the changing of pH will change the proportions of the two compounds in the aqueous solution. In addition, the changing of pH value also affects the surface properties of the solid adsorbent, and thus affects the adsorption of these compounds. Respectively add 25 mL UDMH wastewater with the concentration of 114 mg/L and 0.05 g carbon nanotubes in six conical flasks with the volume of 50 mL and adjust the pH value of the solutions to 2, 4, 6, 8, 10 and 12 respectively with the citric acid sodium and the hydrogen phosphate solution. Determine the concentration after the adsorption equilibrium, as shown in Figure 3.62. It can be seen that the equilibrium adsorption quantity of UDMH is small under the acid condition; the adsorption quantity increases gradually with the increase of the pH value; the adsorption quantity remains basically unchanged when

Equilibrium adsorption concentration (mg/g)

60 50 40 30 20 10 0

0

2

4

6

8

10

12

pH value Figure 3.62: The influence of pH value on the adsorption.

14

3.4 Study on the Adsorption Performance of the Unsymmetric Dimethyl Hydrazine

145

the pH value is greater than 7. This is because UDMH has a weak alkalinity and there is a balance of UDMH ionization in the aqueous solution, (CH3 )2 NNH2 + H2 O→(CH3 )2 NNH+3 + OH– . UDMH molecules are ionized a lot under the acidic condition and the ionized UDMH is not easy to be adsorbed by the carbon nanotubes. Under the alkaline condition, the ionization balance moves to the left, so UDMH exists in the form of molecule and UDMH molecules are the main forms of adsorption in carbon nanotubes. 3.4.2.2 The Effect of the Dosage of Carbon Nanotubes on the Static Adsorption Respectively add 0.08 g, 0.12 g, 0.16 g, 0.20 g, 0.24 g and 0.30 g carbon nanotubes into six conical flasks of 50 mL respectively, and then add 50 mL UDMH solution with the concentration of 160 mg/L into all flasks. Determine the concentration of UDMH after the complete adsorption. The results of the experiment are shown in Figure 3.63. For the fixed amount of UDMH, when the dosage of the adsorbent is less than 0.2 g, the removal rate of UDMH is increased rapidly with the increase of carbon nanotube quantity; when the dosage of adsorbent is 0.2 g, the removal rate of UDMH can reach more than 95%; the continuing increase of the adsorbent has little effect on improving the removal rate of UDMH. Therefore, the optimum dosage of carbon nanotubes in the static adsorption is 0.025 g (CNTs)/mg (UDMH).

3.4.3 The Adsorption Performance of UDMH by the Modified Carbon Nanotubes Carbon nanotube, which is a kind of universal adsorbent, can easily adsorb materials at room temperature. It acts as the adsorbent and as the porous carrier of chemical too, 120

Removal rate (%)

100 80 60 40 20 0 0

0.1

0.2

0.3

0.4

Adding amount (g) Figure 3.63: The effect of the additive carbon nanotubes on the adsorption.

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3 Application of lightweight carbon material and its composite

so the appropriate chemical reagent (modifier) can be added on the carbon nanotubes to make the absorbent of carbon nanotubes couple electronically with the modifier. Thus, the pore and the surface of carbon nanotubes are improved because of the adsorption of modifier, so that the adsorbing capability of carbon nanotubes is increased. But the pores of carbon nanotubes may be blocked and the surface of the micropore may also be affected because of the adding of modifier, so as to decrease the adsorbing quantity. However, if the base adsorbent is a kind of carbon nanotubes with appropriate pore ratio and the variety and concentration of modifier are also suitable, the synergistic effect of modifier and adsorbent can also improve the adsorption capacity of the modified carbon nanotubes. 3.4.3.1 Modification of Carbon Nanotubes and the Structure Characterization (1) Add 20 g NaOH into 60 mL water first, then add 5 g multi-walled carbon nanotubes into the NaOH aqueous solution and stir it. Conduct the ultrasonic concussion for 0.5 h and keep the solution statically for 12 h. Filter the solution and wash the carbon nanotubes with distilled water till the filtering solution into neutrality. Dry the carbon nanotubes in a baking box at 110∘ C for 12 h, recorded as MWNTs-2, while the original carbon nanotubes sample is recorded as MWNTs-1. (2) Soak 1 g multi-walled carbon nanotubes in the hydrogen peroxide solution whose volume fraction is 30% for 72 h. Remove the amorphous carbon on the surface of the tube bundle and the carbon graphite layer wrapped on the catalyst particles. Use concentrated sulfuric acid to remove iron catalyst particles. Wash the carbon nanotubes with distilled water till the washing solution turning into neutrality. Finally the pure multi-walled carbon nanotubes are acquired after being baked at 100∘ C, recorded as MWNTs-3. (3) Soak 0.5 g multi-walled carbon nanotubes in the hydrogen peroxide solution whose volume fraction is 30% for 24 h. Then add the concentrated hydrochloric acid into the hydrogen peroxide solution and keep it statically for 1 h. Filter the solution and wash the carbon nanotubes with the distilled water till the washing solution turning into neutrality. Then add FeCl3 solution of 1 mol/L to the carbon nanotubes stir with the magnetic stirrer for 2 h. Filter and dry the carbon nanotubes at 120∘ C, recorded as MWNTs – 4. The morphology of the multi-walled carbon nanotubes was characterized with the scanning electron microscope (SEM). Its surface functional group was analyzed with the NEXUS670 FT – IR analyzer of American Nicolet Company and its surface physical structure was analyzed with the Micromeritics 2000 ASAP. The purification process principles of H2 O2 and HCl are as follows: H2 O2 → H2 O + [O] 2[O] + C → CO2 ↑ Fe + 2H+ → Fe2+ + H2 ↑

3.4 Study on the Adsorption Performance of the Unsymmetric Dimethyl Hydrazine

147

When the multi-walled carbon nanotubes were soaked in the H2 O2 solution, a lot of gases produced on the surface of the carbon nanotubes were observed. After the concentrated HCl solution was added, the solution quickly becomes yellowish green, which shows that the iron catalyst has been oxidized into Fe2+ . Among the products, the graphite layers with much crystal defect and amorphous carbon particles are easy to be oxidized by H2 O2 , but the multi-walled carbon nanotubes are preserved in the process because of its high degree of crystallization, good lattice integrity and good chemical stability. It is observed with the transmission electron microscope that most of the catalyst particles on the carbon nanotubes have been removed, but there are some cystic nanometer carbon particles in the surface of multi-walled carbon nanotubes bundles after having been soaked for 24 h in H2 O2 solution. It is because part of the amorphous carbon coating on the surface of the catalyst particles is oxidized, and the catalyst particles remains after being dissolved by the hydrochloric acid. There is almost no impurity particle in the product after the carbon nanotubes have been soaked for 72 h in H2 O2 solution. The surface physical structure of the carbon nanotubes was characterized with the Micromeritics 2000 ASAP and the specific surface area and average pore diameter of carbon nanotubes are measured as shown in Table 3.10. It can be seen from the table that the modified carbon nanotubes have larger specific surface area than that of the unmodified carbon nanotubes, and the specific surface area of the carbon nanotubes modified with H2 O2 is smaller than that modified with NaOH; the average aperture of modified carbon nanotubes changes bigger. It is because that the inner walls of carbon nanotubes are carved after the activation treatment and the microporous structure decreases. It can be seen from the scanning electron microscopy Figure 3.64 that the untreated multi-walled carbon nanotubes tube have longer tubes and worse dispersion, and intertwine with each other, causing the surfaces of the multi-walled carbon nanotubes are covered to reduce the specific surface area of the multi-walled carbon nanotubes (Figure 3.62(a)). NaOH is a kind of good modifier, and it can react with some organic functional groups of the multi-walled carbon nanotubes. It not only avoids the reunion phenomenon due to the excessive functional groups, but also generates the materials with certain surface activity to separate the multi-walled carbon nanotubes and other nanometer carbon particles effectively, expanding the surface area

Table 3.10: The specific surface area and the average pore size comparison. Adsorbent

Specific area (m2 /g)

Mean pore size (nm)

MWNTs-1 MWNTs-2 MWNTs-3

503 675 626

7.6 9.8 16.4

148

(a)

3 Application of lightweight carbon material and its composite

(b)

(c)

Figure 3.64: The scanning electron microscope of the carbon nanotubes. (a) Not modified, (b) Modified by NaOH, (c) Modified by H2 O2 .

(Figure 3.64(b)). In the treating process of the multi-walled carbon nanotubes modified with H2 O2 , on the one hand, the impurities such as the amorphous carbon and the carbon nanoparticles are oxidized; on the other hand, the multi-walled carbon nanotubes are also oxidized because of the topological defects (5 membered rings and 7 membered ring) of the multi-walled carbon nanotubes itself, causing the opening, pipe diameter increasing and even being truncated (Figure 3.64(c)). The peak at 3,428 cm–1 in Figure 3.65 of the CNTs-3 IR spectra is wider than that of the untreated carbon nanotube, which indicates that –OH is introduced onto the surface. The absorption peaks appear at 1,702 cm–1 and 1,629 cm–1 , which is caused by the introduction of > C = O and –COOH. 3.4.3.2 The Adsorption Performance of Different Modified Carbon Nanotubes for UDMH Respectively add 0.05 g carbon nanotubes modified with NaOH in six conical flasks of 50 mL, and then add 25 mL UDMH solution whose concentration is respectively 40, 70, 100, 130, 160, 190 mg/L. Conduct the adsorption for 10 h in the constant temperature water bath at 301 K after the vibration. Then filter the solution, determine the concentration of the filterate and calculate it adsorption capacity. Repeat the above experiment with the multi-walled carbon nanotubes modified after H2 O2 and the original multi-walled carbon nanotubes respectively. Divide 12 conical flasks of 50 mL into two groups and add 25 mL UDMH solution whose concentration is 42.06, 73.60, 105.14, 136.68, 168.22, and 199.76 mg/L into each group respectively. Then add 0.05 g multi-walled carbon nanotubes modified with NaOH to the first group and 0.05 g multi-walled carbon nanotubes modified with H2 O2 +FeCl3 to the second group. Conduct the adsorption for 10 h in the constant temperature water bath at 301 K after the vibration. Then filter the solution, determine the concentration of the filtrate and calculate it adsorption capacity. The experimental data of UDMH adsorption quantity under the adsorption of the original multi-walled carbon nanotubes, the multi-walled carbon nanotubes modified

3.4 Study on the Adsorption Performance of the Unsymmetric Dimethyl Hydrazine

149

a 1588

T/%

1702

b

1629 2922

1589

3428 3500

3000

2500 σ / cm

2000

1500

1000

–1

Equilibrium adsorption capacity (mg/g)

Figure 3.65: IR chart of CNTs-1(a) and CNTs-3(b).

75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0

MWNTs–1 MWNTs–2 MWNTs–3

0

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 Equilibrium concentration (mg/L)

Figure 3.66: The adsorption isotherm of MWNTs on UDMH.

with NaOH and H2 O2 under the temperature of 301 K are shown in Figure 3.66. From the figure it can be seen that the adsorption quantity of the original multi-walled carbon nanotube is the least, that modified with H2 O2 is the biggest and that modified with NaOH is in between. The reason is that the adsorption of the original multi-walled carbon nanotubes on UDMH is the physical adsorption and the adsorption quantity depends on its specific surface area and pore volume. The original multi-walled carbon nanotubes are in poor dispersion and intertwine with each other, reducing

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3 Application of lightweight carbon material and its composite

Equilibrium adsorption capacity (mg/g)

the specific surface area, so the adsorption quantity is small. The adsorption effects of multi-walled carbon nanotubes modified with NaOH is better than that of the original multi-walled carbon nanotubes. It is because that NaOH is a kind of very good dispersing agent and it can react with part of the organic functional groups of the multi-walled carbon nanotubes, not only avoiding the reunion phenomenon due to excessive functional groups but also the generating material with certain surface activity to separate the multi-walled carbon nanotubes from other carbon nanoparticles effectively and to increase the surface area and the adsorption quantity. The multi-walled carbon nanotubes modified with H2 O2 has the best adsorption effect. The reason for that may be that the hydroxyl (OH), the carbonyl (> C = O), the carboxyl (—COOH) group and other acidic functional groups are introduced on the surface of the multiwalled carbon nanotubes oxidized by H2 O2 and the acid-base reactions between the weak alkaline UDMH and these acidic groups happen or nucleophilic addition reaction happens between the carbonyl (> C = O) and UDMH to form the chemical adsorption. The acidic groups on the surface of the carbon nanotubes are the adsorption center of the chemical adsorption. In addition, the specific surface area and average pore diameter of carbon nanotubes have a certain degree of increase after being oxidized and modified with H2 O2 , which is beneficial to the adsorption. The experimental data of UDMH adsorption with the multi-walled carbon nanotubes modified separately with NaOH and H2 O2 +FeCl3 are shown in Figure 3.67. It can be seen from the figure that the adsorption quantity of the multi-walled carbon nanotubes modified with NaOH is larger than that modified with H2 O2 +FeCl3 . It is because that after the multi-walled carbon nanotubes is modified with H2 O2 +FeCl3 , its surface is covered with a layer of Fe3+ , which blockade part of the micropores of the multiwalled carbon nanotubes, reducing the adsorption quantity instead. Thus, Fe3+ modification does not favor the UDMH adsorption with the multi-walled carbon nanotubes. 90 80 70 60 50

MWNTs–2 MWNTs–4

40 30 20 10 0

0

5

10

15

20

25

30

35

40

Equilibrium concentration (mg/L) Figure 3.67: The UDMH adsorption isotherm with MWNTs.

45

3.4 Study on the Adsorption Performance of the Unsymmetric Dimethyl Hydrazine

151

Respectively add 0.05 g modified multi-walled carbon nanotubes into six conical flasks of 50 mL, and then add 25 mL UDMH solution whose concentration respectively is 44.51, 77.89, 111.27, 144.65, 178.03, 211.41 mg/L. Conduct the adsorption for 10 h in the constant temperature water bath at 296 K and 301 K respectively after the vibration. Then filter the solution, determine the concentration of the filtrate and calculate it adsorption capacity. Taking the equilibrium adsorption quantity qe as the ordinate, the equilibrium concentration Ce as the abscissa to plot the UDMH adsorption isotherm of the multi-walled carbon nanotubes modified with NaOH under the temperature of 296 K, 303 K and 313 K respectively, as shown in Figure 3.68. It can be seen that the UDMH adsorption quantity of the multi-walled carbon nanotubes modified with NaOH increases with the increase of the temperature at the same equilibrium concentration, illustrating that the UDMH adsorption process with the multi-walled carbon nanotubes modified with NaOH is an exothermic process with the experimental temperature scope. In order to further analyze the adsorption thermodynamics behavior of UDMH on the carbon nanotubes, the isothermal adsorption equation and the adsorption thermodynamic functions shall be studied next. First the UDMH adsorption isotherms of the carbon nanotubes shall be analyzed with the Langmuir and Freundlich isothermal adsorption equation. The adsorption isotherm regression curve of Figure 3.68 fitted according to the Langmuir equation is shown in Figure 3.69, and the corresponding regression equation and the correlation coefficient r are listed in Table 3.11. It is clear from the table that the correlation coefficient of the regression equation is greater than 0.98, and the linear correlation is high, indicating that the adsorption process is a reversible

Equilibrium adsorption capacity (mg/g)

90 80 70 60 50 40 313 K

30

303 K

20

296 K

10 0 0

10

20

30

40

50

60

70

80

90

Equilibrium concentration (mg/L) Figure 3.68: Adsorption isotherm of the modified MWNTs at different temperature.

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3 Application of lightweight carbon material and its composite

0.055 0.05

1/qe(g/mg)

0.045 0.04 0.035 313 K

0.03

303 K

0.025

296 K

0.02 0.015 0.01 –0.05

0.15

0.35 0.55 1/Ce (L/mg)

0.75

0.95

Figure 3.69: The Langmuir linear regression curve. Table 3.11: The Langmuir isothermal adsorption equation of MWNTs for UDMH. T/K

Regression equation

qm /mg⋅qg–1

r

296 303 313

qe = 6.3735 Ce /(1 + 0.12556 Ce ) qe = 11.8765 Ce /(1 + 0.19002 Ce ) qe = 20.0803 Ce /(1 + 0.25301 Ce )

50.76 62.50 79.365

0.9920 0.9937 0.9888

and monolayer adsorption. Moreover, the saturated UDMH adsorption quantity of the carbon nanotubes increases with the increase of temperature. The Freundlich regression curves of the multi-walled carbon nanotubes for UDMH under different temperature are shown in Figure 3.70, and the regression equation, the constant k and n, and the correlation coefficient r are listed in Table 3.12. It can be seen from Table 3.12 and Figure 3.70 that the correlation coefficient r is greater than 0.98, showing that the adsorption of the multi-walled carbon nanotubes on UDMH can all meet the Freundlich equation very well under different temperature and all n are greater than 2, indicating that the adsorption of the multi-walled carbon nanotubes for UDMH is easy, belonging to “the preferential adsorption.” 3.4.3.3 Calculation of the Adsorption Thermodynamics Function (1) The calculation of the adsorption enthalpy BH. The adsorption enthalpy BH can be computed by the Clausius–Clapeyron equation. The Clausius–Clapeyron equation is: ln Ce =

BH +K RT

153

eqln

3.4 Study on the Adsorption Performance of the Unsymmetric Dimethyl Hydrazine

4.7 4.5 4.3 4.1 3.9 3.7 3.5 3.3 3.1 2.9 2.7

313 K 303 K 296 K 0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

lnCe Figure 3.70: The Freundlich linear regression curve.

Table 3.12: The Freundlich isothermal adsorption equation of MWNTs for UDMH. T/K

Regression equation

k

n

r

296 303 313

lnqe = 2.4754 + 0.3367 lnCe lnqe = 2.8156 + 0.3214 lnCe lnqe = 2.9475 + 0.4356 lnCe

11.8865 16.7032 19.0582

2.9700 3.1114 2.2957

0.9941 0.9878 0.9929

Where Ce is the adsorption equilibrium concentration (mg ⋅ L–1 ); T is the thermodynamic temperature (K); R is the ideal gas constant; BH is the equivalent adsorption enthalpy change (kJ/mol); K is a constant. By measuring the adsorption isotherms of the multi-walled carbon nanotubes for UDMH under different temperature, the adsorption isosteres ln Ce —T –1 of different adsorption quantity are plotted as shown in Figure 3.71. The slope corresponding to different adsorption quantity is calculated with the linear regression method to calculate the equivalent adsorption enthalpy change of UDMH of different adsorption quantity. (2) Calculation of the adsorption Gibbs free energy function change BG The adsorption Gibbs free energy function change BG can be got from the adsorption isotherm through the following Gibbs equation: x

BG = –RT ∫ qe 0

dx x

Where qe is the adsorption quantity (mol ⋅ g – 1); x is the amount-of substance fraction the adsorbate in the solution.

lnCe

154

5.5 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 3.15

3 Application of lightweight carbon material and its composite

40 mg/g 50 mg/g 60 mg/g

3.2

3.25

3.3

3.35

3.4

T–1(×103) / (K–1) Figure 3.71: The adsorption isostere of MWNTs for UDMH.

If qe and x are in accordance with the Freundlich equation, that is: qe = kx1/n . Substitute it in the Gibbs equation: BG = –nRT It can be found from the above equation that the adsorption Gibbs free energy function change BG is unrelated with q. 3.4.3.4 Calculation of the Adsorption Entropy Change The adsorption entropy can calculated according to the Gibbs–Helmholtz equation BS = (BH – BG) /T The calculation results of the equivalent adsorption enthalpy change, the adsorption Gibbs free energy function change and the adsorption entropy change under different adsorption quantity are given in Table 3.13. Table 3.13: The thermodynamic functions of the adsorption of MWNTs for UDMH. qe /(mg⋅g–1 )

40 50 60

BH/(kJ⋅mol–1 )

85.834 93.084 98.995

BG/(kJ⋅mol–1 )

BS(J⋅mol–1 )

296 K

303 K

313 K

296 K

303 K

313 K

–7.309 –7.309 –7.309

–7.838 –7.838 –7.838

–5.974 –5.974 –5.974

314.672 339.166 359.135

309.149 333.076 352.584

293.316 316.479 335.364

3.4 Study on the Adsorption Performance of the Unsymmetric Dimethyl Hydrazine

155

It can be seen from the thermodynamic data in the table that the BH being greater than zero means that the adsorption process of the multi-walled carbon nanotubes for UDMH is endothermic. A possible reason is that the multi-walled carbon nanotubes modified with NaOH has a large surface activity, and the multi-walled carbon nanotube adsorbs both the water molecules and the UDMH molecules at the same time in the dilute UDMH solution. But the rate of adsorbing the water molecules is much faster than that of adsorbing the UDMH molecules, causing the water molecules to be adsorbed by the multi-walled carbon nanotubes at the very beginning. When the multi-walled carbon nanotube adsorbs the UDMH molecules, the water molecules should be desorbed at the same time. The relative molecular mass of UDMH is three times larger than that of the water, and its structure is bigger than that of the water molecule, therefore, adsorbing a UDMH molecule on the multi-walled carbon nanotubes will occupy a larger space; at the same time, more water molecules must be desorbed. Because the desorption process is endothermic, and the adsorption process is usually exothermic, the adsorption process of the multi-walled carbon nanotube for UDMH is an exothermic process. Because adsorbing a UDMH molecule needs to desorb more water molecules, the adsorbed heat in the desorption process is greater than the released heat in the adsorption process and this eventually leads to the whole adsorption process of UDMH is endothermic. The increase of the adsorption enthalpy change BH with the increase of the adsorption quantity may be caused by the inhomogeneous surface of the multi-walled carbon nanotubes. Because the concentration of the experimental UDMH simulates the concentration of the propellant UDMH wastewater, whose concentration is low; at this time, the main action between the adsorbent and the adsorbate is the direct action between themselves. The inhomogeneity of the multi-walled carbon nanotube surface makes UDMH have the advantageous position for the energy, so the adsorption enthalpy change BH increases with the increase of UDMH adsorption quantity in the multi-walled carbon nanotubes. The adsorption Gibbs free energy change shows the adsorption driving force and the adsorption concession. It can be seen from the data in the table, BG is smaller than zero, which shows the adsorption process of the multi-walled carbon nanotubes for UDMH is spontaneous; that is to say, UDMH is easy to be adsorbed by the multi-walled carbon nanotube. The adsorption entropy change BS is greater than zero, which indicates that the entropy of the adsorption reaction is increased. The total entropy change of the adsorption process consists of two parts. According to the adsorption exchange theory, for the solid–liquid adsorption exchange, the solute molecules adsorbed and changed from the solution phase to the solid–liquid interface will lose part of the degrees of freedom (including the translation and the rotation), and it is a process that the entropy is reduced; When the multi-walled carbon nanotubes adsorbs the UDMH molecules, a large number of water molecules are also desorbed. Because the water molecules are small and the adsorption process of water molecules is from the closely arranged statue to the freedom statue, and its entropy change will naturally increase, the above two entropy change eventually is positive; that is to say, the

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3 Application of lightweight carbon material and its composite

adsorption process of the multi-walled carbon nanotubes on UDMH is a process of entropy increases.

3.4.4 The Dynamic Adsorption and the Desorption Properties of the Modified Multi-Walled Carbon Nanotubes on UDMH The dynamic adsorption and desorption refer to the adsorption or desorption action between the adsorbent and the adsorbate in liquid phase. And the adsorption and desorption velocity and the quantity of the adsorption or desorption are important parameters in the study on the adsorption desorption separation process, and important bases of evaluating whether the adsorbent can be used in the industrial process or not. The dynamic adsorption process is influenced and limited by many factors, including the technological conditions corresponded, that is, the temperature, pressure, concentration, feed flow rate, and the structure surface performance of the adsorbent, the structure of the adsorption column and the adsorbent filling condition, and so on. It is extremely difficult to describe the impact of these variables with the method of theoretical calculation. In order to get the required process equipment design data and infer the adsorbent, the necessary comprehensive data are got by the adsorption devices simulating the technological conditions. It is expressed with the dynamic adsorption and desorption curve. 3.4.4.1 The Influence of the Flow Rate Take 0.8 g multi-walled carbon nanotubes modified by NaOH to fill in the adsorption column whose diameter is 22 cm, and the length of the packed column is 3 cm. Do the breakthrough experiment with UDMH of 156 mg/L at 300 K, controlling the flow rate as 8.2 mL/min, 5.1 mL/min, 2.1 mL/min respectively. Determine the concentration with the ultraviolet spectrophotometry after every several effluent volume till the effluent concentration remains constant. Figure 3.72 shows the breakthrough curve under three kinds of flow rate. It can be seen from the figure that the penetration volume is 100 mL, 150 mL and 225 mL respectively and the saturated volume is 250 mL, 290 mL and 300 mL respectively with the decrease of the flow rate. In order to get the effective adsorption, it must be ensured that the solid phase and the liquid should have sufficient contact time during the operation, that is to say, the liquid flow rate should not be too fast; otherwise, the contact time of the adsorbate in liquid phase and the adsorbent in solid phase is too short to exchange to make the breakthrough curve flattening; But the operation cycle may extend and the cost may increase because of the too slow flow rate. 3.4.4.2 The Influence of Influent Concentration Take 0.8 g multi-walled carbon nanotubes modified by NaOH to fill in the adsorption column whose diameter is 22 cm, and the length of the packed column is 3 cm. Do the breakthrough experiment at 300 K with UDMH whose concentration is 156 mg/L,

3.4 Study on the Adsorption Performance of the Unsymmetric Dimethyl Hydrazine

157

UDHM concentration (mg/L)

180 8.2 mL/min 5.1 mL/min 2.1 mL/min

160 140 120 100 80 60 40 20 0 –20

0

50

100 150 200 250 300 Collection liquid volume (mL)

350

400

UDHM concentration (mg/L)

Figure 3.72: The breakthrough curve under different flow rate.

180 160 140 120 100 80 60 40 20 0 –20

156 mg/L 105 mg/L 42 mg/L

0

50

100 150 200 250 300 Collection liquid volume (mL)

350

400

Figure 3.73: The breakthrough curve under different concentrations.

105 mg/L, 42 mg/L respectively, controlling the flow rate as 4.5 mL/min. Determine the concentration of UDMH with the ultraviolet spectrophotometry after every 25 mL effluent till the effluent concentration remains constant. Figure 3.73 shows the breakthrough curve under different concentrations. When the concentration of effluent reaches the standard of the prescribed discharge concentration, this point is the breakthrough point. And when the concentration of the solute at the outlet reaches 90–95% of that at the inlet, the adsorption capacity of adsorption column can be thought to be exhausted, and this point is the adsorption ending point. The mass m1 of the remained UDMH in the effluent is calculated by the integral calculation of the breakthrough curve from the beginning point of the adsorption to the breakthrough point and the end points respectively, and the adsorbed UDMH by the multi-walled carbon nanotubes is calculated by subtracting m1 from the total mass m of UDMH in the simulated wastewater. The breakthrough capacity and the saturated capacity of the multi-walled carbon nanotubes under this experiment condition can

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3 Application of lightweight carbon material and its composite

be got by the proportion of the adsorbed UDMH mass to the mass of the multi-walled carbon nanotubes. It is calculated from the figure that the breakthrough capacities are 53.63 mg/g, 38.06 mg/g, 15.75 mg/g respectively and the saturated capacities are 53.63 mg/g, 38.06 mg/g, 15.75 mg/g respectively under the condition of three kinds of the original solutions. The breakthrough capacity and the saturated capacity both decrease with the decrease of the concentration. The dynamic adsorption capacity is larger than the static adsorption capacity theoretically. This is because the slow flowing UDMH can contact with the microporous better, at the same time, and makes full use of the concentration gradient to increase the driving force of adsorption. 3.4.4.3 The Regeneration of the Carbon Nanotubes The high price is one of the main factors to limit the wide use of the carbon nanotubes. If the carbon nanotubes of the saturated adsorption can be regenerated to restore its adsorption activity, it is of great significance in the industrial application. The regeneration can also concentrate the wastewater of UDMH. The desorption technology is different with the difference of the adsorption properties. Generally the solvent elution, the hot blow off steam and the high temperature heating regeneration method can make the carbon nanotubes adsorption performance regenerated. In this experimental study, NaOH solution was chosen as the eluent finally by analyzing the above results and taking into consideration the simplicity of operation and the industrial cost. For the saturated adsorption column, the non-adsorbed UDMH in the column was washed entirely with the distilled water and then the NaOH solution of 0.1 mol/L was used to flow through the column to desorb UDMH by controlling the flow rate as 1 mL/min. Collect the effluent and determine the concentration of UDMH. Conduct static adsorption with the carbon nanotubes which have been regenerated. Figure 3.74 is the desorption curve of the multi-walled carbon nanotubes modified with NaOH after dynamic adsorption of UDMH whose concentration is 156 mg/L. It can be seen from the figure that 50 mL NaOH solution can almost desorb all the UDMH. The mass concentration of UDMH in the desorption solution is calculated with the integral method as 3.1 g/L, which is 19.9 times as that of the previous adsorption. For the desorbed and regenerated multi-walled carbon nanotubes, wash it to neutral with the distilled water and dry it at 120∘ C. Make the static adsorption experiment with the UDMH whose initial concentration is 156 mg/L. twice adsorption and regeneration experimental data are determined respectively, as shown in Table 3.14. Results show that the adsorption capacity basically remains unchanged and the recovery rate can reach higher than 90% after the multi-walled carbon nanotubes of the saturated adsorption are regenerated by NaOH solution. 3.4.4.4 The Comparison with the Adsorption Effect of the Activated Carbon Add respectively 0.05 g carbon nanotubes and 0.05 g granular activated carbon to two 50 mL conical flasks, and then add 25 mL UDMH whose concentration is 156 mg/L to

3.5 Study on the Adsorption Performance of UDMH with Activated Carbon

159

UDMH concentration (mg/L)

16000 14000 12000 10000 8000 6000 4000 2000 0 –2000

0

10

20

30

40

50

60

Collection liquid volume (mL) Figure 3.74: The desorption curve. Table 3.14: The performance experimental data of the multi-walled carbon nanotube regeneration. Regeneration times

recovery ratio (%)

Static adsorption capacity (mg/g)

0 1 2

– 92.4 90.2

52.2 53.9 49.1

Table 3.15: The adsorption effect comparison of the two adsorbents. Adsorbent

UDMH concentration (mg/L)

UDMH equilibrium concentration (mg/L)

Static equilibrium adsorption quantity (mg/g)

Carbon nanotube Activated carbon

156 156

51.69 136.7

52.16 9.65

two flasks and keep them statically for 10 h at 28∘ C. Determine the concentration of UDMH after the filtering. The experimental data are shown in Table 3.15. Through the calculation, the static adsorption capacity of the carbon nanotubes is 5.4 times as that of the activated carbon. Under the same condition, the adsorption capacity of carbon nanotubes is much larger than that of the granular activated carbon.

3.5 Study on the Adsorption Performance of UDMH with Activated Carbon and Modified Activated Carbon The activated carbon adsorption method is a kind of important method for UDMH wastewater treatment. As for the market activated carbon for the adsorption of UDMH,

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3 Application of lightweight carbon material and its composite

there are some disadvantages, such as small adsorption capacity, high adsorption heat and easy penetration defect. The modification studies on the surface properties and the surface structure with the impregnation method, the microwave method and the high temperature reduction method were conducted to get one or several activated carbon materials suitable for the adsorption of UDMH, which lays the technological foundation for the engineering design and development of this method [41–43].

3.5.1 The Decoloring Efficiency of Different Types of Activated Carbon Take a certain amount of UDMH sample and add respectively suitable amount of the pretreated granular activated carbon, columnar activated carbon, powder activated carbon and activated carbon fiber, and stir till the adsorption equilibrium. Determine the change of the absorbance of the solution before and after the adsorption with the spectrophotometer at a certain wavelength to calculate the decolorization rates. The decoloring rates of the four types of activated carbons are shown in Figure 3.75. Figure 3.75 shows that the effect of adsorption of the yellow substances with activated carbon fiber is better than other activated carbons under the same experimental condition; the effect of the powdered activated carbon goes secondly and the decolorizing rate of the columnar activated carbon is the lowest. This is mainly related to the specific surface area and the pore structure of various activated carbons. It can be seen from the above specific surface area structure measurement that the order of specific surface area of the four activated carbons are: activated carbon fiber > powder 100 90

Decolorization rate (%)

80 70 60 50 40 30 20 10 0 Carbon particles

Powdered carbon

Columnar carbon

Carbon fiber

Figure 3.75: The decoloring effect of different types of activated carbon.

3.5 Study on the Adsorption Performance of UDMH with Activated Carbon

161

particles >granular activated carbon > columnar activated carbon, and the decoloring effects are in the same sequence. But practically, other factors, such as the price and the operational conditions should also be taken into consideration for choosing activated carbon.

3.5.2 Surface Modification of the Activated Carbon 3.5.2.1 Modification of the Activated Carbon In order to further improve the adsorption ability of activated carbon on UDMH, the selected activated carbon was modified correspondingly. Its technology process is shown in Figure 3.76. In order to enhance the effect of modification, the activity process needs to be optimized. (1) Modified experiment by impregnation method. The solvents used for the modification of activated carbon are various. the concentrated ammonia and sodium hydroxide solution were chosen in the experiment. Concentrated ammonia solution and sodium hydroxide solution are selected as the dipping solution. Respectively take 100 ml solutions mentioned above and 20 g activated carbon into 250 ml conical flask, and seal the bottle with a rubber plug. Take the conical flask into the constant temperature oscillator under a certain temperature for a certain period of time. Then wash the impregnated activated carbon with the distilled water to the neutrality and put them

Boehm titration

Impregnation method

Granular activated carbon

pHpzc determination

Decolorization test

Property determination

Microwave modification

FT-IR

SEM Figure 3.76: The modification technology process of activated carbon.

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3 Application of lightweight carbon material and its composite

into an air dry oven with a suitable temperature for certain period of time. Finally, take it out and enclose it into a ground glass stoppered bottle. The modification of activated carbon has a certain relationship with the concentration of solution, reaction temperature and time as well as the post treatment condition. So optimization experiments are needed to obtain the best conditions for modification. (2) Microwave modification experiment. According to the literature, the main factors that influence the microwave modification include microwave power, time, carrier gas flow rate and the quality of activated carbon, but the last two have little effect on the modification. Therefore, the main consideration of modification on activated carbon is the different microwave power and irradiation time. The microwave modification device is shown in Figure 3.77. The solution impregnation method and the microwave modification method can be used to modify the activated carbon. The solution impregnation method is to use an alkali solution of the appropriate concentration, such as NaOH, or a salt, such as NH4 Cl solution, to soak the activated carbon for some time, and then wash it up, dry it, and activate it. And the microwave modification method is to use a certain power of microwave to irradiate the activated carbon for some time.

3.5.2.2 The Chemical Property Analysis of the Surface of the Modified Activated Carbon According to the method proposed by Boehm and the principle of acid and alkali neutralization, the carboxylic acid group on the surface of the activated carbon is neutralized with NaHCO3 , the carboxylic acid and the lactone are neutralized with Na2 CO3 , and the phenol hydroxyl, carboxylic acid and the lactone are neutralized with NaOH. The determination results are shown in Figure 3.78. Figure 3.78 shows that the oxygen-containing carboxyl functional groups, the lactone and the phenolic hydroxyl group are decreased significantly on the surface of the modified activated carbon. But the alkaline groups are significantly increased,

Nitrogen

Pryogallic acid

Concentrated sulfuric acid

Figure 3.77: The microwave modification device.

Flowmeter

Microwave oven

Temperature indicator

3.5 Study on the Adsorption Performance of UDMH with Activated Carbon

163

0.5 1 2

0.4

3 4

0.3

0.2

0.1

0 The original carbon

Modified by ammonia chloride

Modified by ammonia water

Modified by microwave

Modified by NaOH

Figure 3.78: The content variation of the surface functional group of all kinds of the activated carbon.

especially for the modified activated carbon with the microwave modification and NaOH modification. The microwave modification decreases oxygen-containing groups, but increases nitrogen-containing groups. It’s mainly because that the microwave heating makes the activated carbon achieve a high temperature in a relatively short period of time, and the reactions of the surface groups of the activated carbon are as shown as Figure 3.79. The isoelectric point pHPZC is determined with the quality titration under certain ionic strength. The isoelectric point pHPZC is an important parameter to characterize the acidity on the activated carbon surface. The pHPZC values of the activated carbon modified with the microwave and NaOH are significantly greater than that of the original carbon, which indicates that the amount of oxygen-containing functional groups on the activated carbon surface modified with the microwave and NaOH reduces obviously. The surface property of the modified activated carbon is characterized with the FT–IR, as shown in Figure 3.80. This shows that these modified methods can all bring about changes for the surface group of activated carbon. The surface morphology of the activated carbon samples before and after modification is observed with SEM, as shown in Figure 3.81. It can be seen from the diagram that the surface morphology of the activated charcoal is changed, and the original pore structure of the activated carbon is more or less damaged after modification.

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3 Application of lightweight carbon material and its composite

COOH

O

OH

OH

Active site

Cl

O O O

600˚C~800˚C

O

OH

O

O

~1000˚C

OH

OH

O

O

OH

~1200˚C

O

reflectance (%)

Figure 3.79: The active center diagram produced after the decomposition of the oxygen-containing functional group in the process of the microwave modification.

90 85 80 75 70 65 60 55 50 45 4000

5 4 3 2 1

3500

3000

2500 2000 1500 Wavenumber (cm–1)

1000

500

Figure 3.80: The FT–IR figure of activated carbon and modified activated carbon. 1–Original carbon; 2–Microwave; 3–NaOH; 4–NH3 ⋅ H2 O; 5–NH4 Cl.

3.5.3 The Adsorption Performance of the Modified Activated Carbon 3.5.3.1 The Static Adsorption Performance The adsorption performance of all activated carbons modified with the impregnation modification method is determined, which is represented with the adsorption efficiency and decolorizing efficiency. It is known from the experiment results that

3.5 Study on the Adsorption Performance of UDMH with Activated Carbon

(a)

(b)

(c)

165

(e)

(d)

Figure 3.81: The SEM diagram of various kinds of activated carbons. (a) The original charcoal, (b) Microwave modification, (c) NaOH modification, (d) NH4 Cl modification, (e) NH3 ⋅ H2 O modification.

100 90 80 70 60 a

50

b

40 30 20 10 0 l

2

3

4

5

Figure 3.82: The adsorption performance of all kinds of activated carbon. a. The adsorption rate, b. The decolorization rate, 1. The original carbon, 2. Modification 1, 3. Modification 2, 4. Modification 3, 5. Modification 4.

the microwave modification effect is the best and NaOH is the best in the dipping modification. In order to further test the adsorption performance of the modified activated carbon, the adsorption test for the UDMH solution with the original charcoal (YT), the NaOH modified activated carbon (NT) and the microwave modified activated carbon (WT) are made to get the adsorption isotherm diagram and the corresponding fitting curves as shown in Figures 3.83–3.85. It can be seen from the figure that within the researched concentration scope the adsorption isotherm of the oxidized impurity in UDMH adsorbed on the activated carbon belongs to the “concessionary” adsorption isotherm. Compared with YT, the average adsorption quantity of the modified activated carbon all increase apparently. qm and 1/n values of the modified activated carbon are greater than that of the original activated carbon, and the increase of those of the modified activated carbon are more apparent. With the increase of temperature, the adsorption quantity of activated carbon YT and WT are reduced, which indicates the adsorption process is an exothermic process, and low temperature is conducive to the adsorption process, and the lower the temperature is, the greater the adsorption quantity will be.

166

3 Application of lightweight carbon material and its composite

0.3

q(g/g)

0.25 0.20 0.15 WT experiment value NT experiment value

0.10

YT experiment value Langmiur

0.05

Freundlich 0 0

10

30

20

40

C(g/L) Figure 3.83: The adsorption isotherm and fitting figure of different types of activated carbons.

0.25

q(g/g)

0.2

0.15 285 K experment value 295 K experment value 303 K experment value Langmiur

0.1

0.05

Freundlich 0

0

10

20

30

40

C(g/L) Figure 3.84: YT adsorption isotherm and the fitting figure.

3.5.3.2 The Dynamic Adsorption Performance The adsorption kinetics curve of a UDMH solution of the same concentration at different temperature adsorbed on the original carbon of activated carbon were determined as shown in Figure 3.86. It can be seen from the figure that the higher

3.5 Study on the Adsorption Performance of UDMH with Activated Carbon

167

0.3

q(g/g)

0.25 0.2 0.15

283 K experiment value 293 K experiment value 303 K experiment value Langmiur Freundlich

0.1 0.05 0

0

10

20

30

40

C(g/L) Figure 3.85: WT adsorption isotherm and the fitting figure.

1.2 1

q/q*

0.8 0.6 303 K q* = 0.3104 g/g 293 K q* = 0.3236 g/g 283 K q* = 0.34 g/g

0.4 0.2 0

0

60

120

180

240

300

360

420

480

t/min Figure 3.86: The adsorption kinetics curves of the partial oxidation impurity in UDMH on the activated carbon at different temperature.

the temperature is, the faster the adsorption rate of partial impurities in UDMH on activated carbon is, and the shorter the time needed to reach the adsorption equilibrium. Fit the adsorption rate model of the pseudo-first-order reaction and the pseudosecond-order reaction with the experimental data of Figure 3.86 and calculate the coefficient of mass transfer kf under different temperature to get the effective diffusion coefficient De .

168

3 Application of lightweight carbon material and its composite

The pseudo-first-order reaction kinetics model is: 𝜕q = kf (q∗ – q) 𝜕t where kf =

15De R2p

is the adsorption rate constant, De is the effective diffusion coefficient,

Rp is the average particle radius and q∗ is the adsorption quantity balanced with the gas adsorbate concentration or the liquid phase adsorbate concentration. If the phase main body concentration of adsorbate during the process of adsorption maintains a constant, for example, the adsorbent particles contact the fluid phase with infinite volume or contact the mobile fluid phase with constant concentration , then q∗ is a constant. Integrate the above formula and take the initial conditions as follows: t = 0, q = 0, ln (

q∗ – q ) = –kf t q∗

∗

Draw the figure of the time t to ln ( qq∗–q ), and get a straight line passing the origin. The coefficient of mass transfer kf can be calculated with the slope of the straight line. The pseudo-secondary reaction kinetics model is as follows: 𝜕q 2 = kfII (q∗ – q) 𝜕t where kfII is the secondary reaction kinetics adsorption rate constant. If the fluid phase concentration in the process of adsorption remains constant, q* is a constant. Integrate the above formula, taking the initial conditions as follows: t = 0, q = 0. 1 = 1 – kfII q∗ t 1 – qq∗ Draw a straight line with time t to 1 – qq∗ and the coefficient of mass transfer kfII can be calculated by slope. The pseudo first-order reaction adsorption rate model fitting results are shown in Figure 3.87. The pseudo-second-order reaction adsorption rate model fitting results are shown in Figure 3.88. It can be seen that the pseudo-first-order reaction adsorption rate model is better than the pseudo-second-order reaction rate model to describe the adsorption dynamics behavior of the oxidation impurities of UDMH on activated carbons. The coefficient of mass transfer kf can be calculated with the pseudo-first-order reaction kinetics model and the effective diffusion coefficient De can be estimated according to the given rate model kf = 15De /Rp 2 . The activation energy and the

3.5 Study on the Adsorption Performance of UDMH with Activated Carbon

169

250

303 K 293 K 283 K

200

1/(1–q/q*)

R² = 0.5146 R² = 0.4842

150

R² = 0.4728 100

50

t/min 0

0

60

120

180

240

300

360

420

480

Figure 3.87: The fitting curve of the pseudo-first-order reaction rate model.

0 303 K 293 K 283 K

–1

ln[(q*-q)/q*]

–2 –3 –4 –5 –6

R3032 = 0.9765

–7

R2 = 0.9803

–8

R2832 = 0.9716

–9

0

60

120

180

240

300

360

420

480

t/min Figure 3.88: The fitting curve of the pseudo-second-order reaction rate model.

pre-exponential factor can be calculated based on the Arrhenius equation by means of the diffusion coefficient under different temperature. The Arrhenius equation is as follows: De = D0 exp (–Ea /RT) where Ea is the activation energy in system; D0 is the pre-exponential factor.

170

3 Application of lightweight carbon material and its composite

3.5.3.3 The Equivalent Differential Adsorption Heat and the Calculation of Thermodynamic Functions According to the fitted Freundlich equation, the equilibrium concentration can be calculated in the same adsorption capacity under different temperatures; According to the Clausius–Clapeyron equation, the adsorption isosteres of different adsorption capacity can be got by mapping lnc-1/T, as shown in Figures 3.89 and 3.90, then the adsorption capacity can be calculated corresponding to the slope, and the isosteric adsorption enthalpy of different adsorption capacity can be calculated. 1.2

1

q/q*

0.8

0.6 303 K q* = 0.3104 g/g 293 K q* = 0.3236 g/g

0.4

283 K q* = 0.34 g/g

0.2

0

0

60

120

180

240

300

360

420

480

t/min Figure 3.89

250

1/(1– q/q*)

2

R303 = 0.5146 R2932 = 0.4842 R2832 = 0.4513

150

303 K 293 K 283 K

100 q/q* = 0.95 50

0 –1

303 K 293 K 283 K

–2 ln[(q*–q)/q*]

200

–3 –4 –5 R3032 = 0.9765 R2932 = 0.9803

–6 –7

R2832 = 0.9716

–8 0

0

Figure 3.90

60 120 180 240 300 360 420 480 t/min

–9

0

60 120 180 240 300 360 420 480 t/min

3.5 Study on the Adsorption Performance of UDMH with Activated Carbon

171

It can be seen from the figure that under the same adsorption capacity, the equilibrium concentration at low temperature is lower than that at high temperature. Under the same adsorption capacity, 1/T and lnc show a linear relationship, and with the increase of the adsorption amount, the absolute value of the linear slope increases and the absolute value of the calculated amount of differential adsorption heat is also the largest. The isosteric adsorption enthalpy and other thermodynamic functions under different adsorption conditions are shown in Tables 3.16 and 3.17. 1. The adsorption enthalpy (BH) of activated carbon on the oxide impurities of UDMH is less than zero, indicating that the adsorption process is an exothermic process, the same with the adsorption equilibrium. 2. The absolute value of the adsorption enthalpy decreases with the increase of the adsorption capacity; that is, the adsorption heat decreases with the increase of coverage, which is due to the uneven surface of the activated carbon. At the beginning of the adsorption, adsorbates are first adsorbed on the most active sites on the surface adsorption center of activated carbon, and the higher the adsorption activity is, the greater the heat release. With the adsorption going on, the adsorption heat slowly reduces as coverage increases, and drops to a certain value after the adsorption heat change tends to be gentle; at this time, adsorption occurs at a lower activity adsorption sites; that is, the micropore filling adsorption starts between adsorbate molecules in synergy; at this time, the adsorption heat is relatively small, indicating that the interaction between adsorbates is less than the force between adsorbates and activity.

Table 3.16: The equivalent differential adsorption heat and the thermodynamic functions of activated carbon YT. q/(g/g)

0.10 0.15 0.20

BH/(kJ/mol)

–48.05 –46.26 –44.09

BG/(kJ/mol)

BS/(J/mol)

283 K

293 K

303 K

283 K

293 K

303 K

–6.13 –6.13 –6.13

–5.71 –5.71 –5.71

–5.38 –5.38 –5.38

–148.13 –141.80 –134.13

–144.51 –138.40 –131.00

–140.92 –134.92 –127.75

Table 3.17: The equivalent differential adsorption heat and the thermodynamic functions of activated carbon WT. q/(g/g)

0.10 0.15 0.20

BH/(kJ/mol)

–44.46 –40.13 –39.77

BG/(kJ/mol)

BS/(J/mol)

283 K

293 K

303 K

283 K

293 K

303 K

–6.42 –6.42 –6.42

–6.17 –6.17 –6.17

–6.14 –6.14 –6.14

–134.42 –119.12 –117.84

–130.68 –115.90 –114.67

–126.47 –112.18 –111.00

172

3.

4.

5.

3 Application of lightweight carbon material and its composite

When the adsorption amount is 0.1 g/g, 0. 15 g/g and 0.20 g/g, the absolute value of adsorption enthalpies is the largest, 48.05 KJ/mol, larger than the physical adsorption heat, 40 KJ/mol, but much smaller than chemical adsorption heat, and it is thought to be the transition state of physical adsorption and chemical adsorption, considered to be the non-activated chemical adsorption, and the chemical ceiling attached can be instantaneously finished and the adsorption is reversible. The main reason causing the heat of adsorption is greater than that of physical adsorption heat may be due to the component interactions of the oxygen-containing functional groups on the surface of activated carbon and the components of UDMH, which can be realized from the adsorption heat size of the two kinds of activated carbons. The oxygen-containing functional groups on the surface of the modified activated carbon are less than the amount of oxygen-containing functional groups on the surface of the original carbon, and the adsorption enthalpy is smaller as well. Therefore, the surface modification can improve the adsorption performance and reduce the adsorption heat at the same time, which is beneficial to the practical application of the surface modification of activated carbon. BG less than zero indicates that substance in UDMH on activated carbon adsorption process is spontaneous. With the increase of the temperature, the absolute value of BG is decreased, which indicates that the higher adsorption temperature is not conducive to adsorption. BS less than zero indicates that the adsorption process is an entropy reduction. With the adsorption of the system going on, the confusion of the system decreases. And with the increase of temperature, the absolute value of BS decreases slightly. This is because the increase of temperature is not conducive to the adsorption, and caused by the decrease of the adsorption amount and the increase of the confusion of the system.

3.5.3.4 Adsorption Heat Determination of Microcalorimeter The adsorption heat of the thermal oxidation impurities of activated carbon YT on UDMH is determined by MicroDSC III microcalorimeter. The test temperature is 25∘ C 0.001∘ C; the adsorption equilibrium time is not less than 30 min; the adsorption reaction is carried out under the protection of nitrogen gas. Test results according to the equilibrium adsorption amount is converted into standard units are shown in Table 3.18. It can be seen from the test results that with the increase of the equilibrium adsorption amount, the adsorption heat decreases. But its heat value is much larger than Table 3.18: The equilibrium adsorption data of microcalorimeter. q/(g/g)

0.15

0.2

BH(KJ/mol)

62.23

56.55

3.6 The Adsorption Research of UDMH with the Activated Carbon Fiber (ACF)

173

the calculated amount of adsorption heat, and this is because the amount of heat measured by the microcalorimeter is the total heat, which contains the infiltration heat and the liquid evaporation heat, and so on. Owing to the inhomogeneity surface of activated carbon, the activated carbon, when in contact with UDMH, will produce a large number of infiltration heat, and due to the lower boiling point of UDMH, the heat will enable the volatilization of UDMH. Therefore, in view of this phenomenon, in the dynamic adsorption experiment, the clip must be used to the system to carry on the cold feedstock.

3.6 The Adsorption Research of UDMH with the Activated Carbon Fiber (ACF) Because the activated carbon fiber is of good characteristics, such as good mechanical strength, without sediment and channel flow during the operation of GCF and PAC, ACF has been replacing the granular activated carbon and the powdered activated carbon in the area of governance, purifying environment and the other area of broad application as a basic material. The structural characteristics of the activated carbon fiber are applied to study its adsorption performance on UDMH to provide the technical basis for the treatment applications of UDMH wastewater [44–49].

3.6.1 The Determination of Adsorption Isotherm Prepare UDMH solution of 1,000 mg/L and take 100 mL UDMH solution into 250 ml conical flask. Add certain amount of activated carbon fiber to the flask till the adsorption equilibrium. Filter the solution and determine the content of UDMH in the solution with the ferrous amino sodium cyanide spectrophotometry (GB18063-2000). Weigh accurately 0.6 g activated carbon fiber and put them respectively into four conical flasks with plug. Add these flasks into 100 ml UDMH solution sample whose initial concentration respectively is 1,000 mg/L, 500 mg/L, 250 mg/L and 125 mg/L. Vibrate them on the constant temperature oscillation for more than 3 h to make them get the adsorption equilibrium. Determine the concentration C of UDMH in the residual liquid and calculate the adsorption quantity according to the following equation. Finally get the adsorption isotherm at 298 k, 308 k and 318 k. q=

(c0 – c)V W

where q is the adsorption capacity (mg/g); c0 and c are respectively the concentration of UDMH solution before and after the adsorption (mg/L); V is the adsorption volume of fluid (L); W is the amount of activated carbon fibers (g).

174

3 Application of lightweight carbon material and its composite

160 140 120

q(mg/g)

100 80 60

318 K 308 K

40

298 K 20 0

0

50

100

150

200

250

C(mg/L) Figure 3.91: The adsorption isotherm at different temperatures.

Draw a diagram with q-c and get the adsorption isotherm at different temperatures of ACF on UDMH, as shown in Figure 3.91. It can be seen from the figure that the adsorption isotherm of ACF on UDMH is approximately a straight line, taken as the type I isotherm. The adsorption quantity is reduced with the temperature increases, which explains the process of the adsorption is exothermic. Fit the measured data with the Freundlich adsorption isotherm q=kf C1/n . We get: ln q = ln Kf +

1 ln C n

Plot a diagram taking lnq as the ordinate and the lnC as the abscissa and make regression with the method of the least square. The curve is shown in Figure 3.92. The correlation coefficients and parameters at its own temperature after the regression are shown in Table 3.19. It can be seen from Table 3.19 that the correlation coefficient of regression curve of adsorption isotherm at its own the temperature is greater than 0.99, which indicates that the experiment of ACF adsorbing the trace UDMH in water has good compliance to the Freundlich adsorption model, belonging to the “preferential” type adsorption. The adsorption of AFC on UDMH is easy to happen and suitable for the adsorption of different concentration solution.

175

3.6 The Adsorption Research of UDMH with the Activated Carbon Fiber (ACF)

5.5 5 4.5

lnq

4 3.5 Series 1

3

Series 2 2.5 Series 3 2 1.5

2

3

4

5

6

lnC Figure 3.92: The lnq–lnC linear regression curve.

Table 3.19: The correlation coefficients and parameters in the equation after the Freundlich model regression. Temperature (K)

Correlation coefficients R2

Kf

n

298 308 318

0.9994 0.9955 0.9916

1.616 0.746 0.269

1.028 0.949 0.874

3.6.2 The Calculation of Adsorption Thermodynamics Function According to the adsorption isotherm of Figure 3.91, the adsorption isostere under the same adsorption quantity is made with lnC–1/T, as shown in Figure 3.93. It is clear that lnC has a good linear relationship with 1/T. According to the Clausius–Clapeyron equation: Q = PT 2 (

𝜕 ln P ) 𝜕T n

the slope corresponding to the adsorption amount is calculated with the adsorption isostere lnC–1/T of different adsorption quantity. The equivalent adsorption enthalpy of activated carbon fiber on UDMH in different adsorption quantity is calculated with the following formula: ln c =

BH +K RT

176

3 Application of lightweight carbon material and its composite

6

40 mg/g 80 mg/g

5.5

120 mg/g 5

lnC

4.5 4 3.5 3 2.5 2 3.1

3.2

3.3

3.4

1/T(K) Figure 3.93: The adsorption isostere of activated carbon fiber on UDMH.

where C is the adsorption equilibrium concentration (mg/L) of UDMH; T is the absolute temperature (K); R is an ideal gas constant; H is the amount adsorption enthalpy (kJ/mol); K is a constant. The calculating formula of adsorption free energy B G is: BG = –nRT where BG is the adsorption free energy (kJ/mol); n is a constant of Freundlich equation; T is the absolute temperature (K). Its adsorption entropy BS calculation formula is: BS =

BH – BG T

The data calculated with the thermodynamic function are shown in Table 3.20. Table 3.20: Thermodynamic function values of ACF adsorbing UDMH. q (mg/g)

40 80 120

BH (kJ/mol)

–41.91 –38.52 –35.55

BG(kJ/mol)

BS(J/mol)

298(K)

308(K)

318(K)

298(K)

308(K)

318(K)

–2.55 –2.55 –2.55

–2.43 –2.43 –2.43

–2.31 –2.31 –2.31

–132.08 –120.70 –110.74

–128.18 –117.18 –107.53

–132.89 –113.87 –104.53

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177

3.6.3 The Adsorption Law of UDMH with Activated Carbon Fiber It can be seen from the data in Table 3.20 that the adsorption laws of UDMH with activated carbon fiber are as follows: 1. The partial adsorption enthalpy BH of ACF on UDMH is smaller than 0, showing that the adsorption process is a exothermic process. And the adsorption enthalpy is reduced gradually with the increase of the adsorption quantity, this is because the adsorption amount is proportion to the adsorption coverage rate (() of activated carbon fiber, the larger the coverage, the greater the adsorption capacity. There is a lot of higher energy adsorption center on ACF surface. UDMH is first adsorbed by the most active adsorption center of ACF surface, and at this moment, the adsorption activation energy needed is smaller, but the adsorption heat released is larger. With the increase of surface coverage rate (, the active adsorption center on the ACF surface is gradually reduced. UDMH can only be adsorbed in the less active center; at this time, the adsorption activation energy needed is larger and the heat released is less. In addition, with the increase of the surface coverage rate ( of UDMH on ACF, the mutual exclusion of UDMH molecules is also one of the causes of the decrease of the adsorption heat. Although the activated carbon adsorbent is a kind of hydrophobic adsorbent, it can also adsorb water. Therefore, part of the active sites is occupied and mutual repulsions between the UDMH molecules and the water molecules in the late adsorption period also exist, which can also make the adsorption heat reduce. 2. The adsorption free energy BG is negative, which indicates that the adsorption process is a spontaneous irreversible process; at the same time, BG is slightly reduced with the increase of temperature, which shows that the rise of temperature is not conducive to the adsorption. 3. The adsorption entropy BS is negative, this is because, the solute molecules in the adsorption process is exchanged to the solid-liquid interface from the liquid phase for the exchange of solid-liquid adsorption, the molecular freedom degree is reduced, and the system’s chaotic degree is also reduced. At the same time, the absolute value of adsorption entropy decreases as the temperature increases. Because the rise of temperature is not conducive to the adsorption process, the adsorption quantity of ACF on UDMH is reduced, which increases the chaotic degree of the whole system.

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4 Application of lightweight carbon material and composite material in electromagnetic wave absorbing material Absorbing material is an electromagnetic functional material that can absorb electromagnetic wave effectively, expend the electromagnetic energy by transforming it into thermal energy, or promote electromagnetic destructive interference to attenuate the target echo. The development of absorbing material began in 1930s and was experimented during the World War II. In the 1980s, it had made a great breakthrough represented by a series of achievements,such as F-117 and B-2 stealth bomber. With the rapid development of avionics technology, there will be all the future weapons. As an effective means to enhance the survival and penetration ability of weaponry system, stealth has been one of the most important and effective technologies to conduct integrated multidimensional modern warfare that combine land, naval, aerial, space, and electronic-magnetic warfare. Absorbing material plays an important role in it. High-performance absorbing material is characterized by the following four aspects: It suits to a wide qualified frequency range; it remains thin when its absorbency is satisfied; it has excellent attenuation and mechanical performance; and it is light due to its low density. Although China has a 20- to 30-year history in the development of absorbing material, there is a considerable gap while comparing with that of some other countries. Nowadays, the wave absorbing materials widely studied and used are mainly traditional magnetic nano-metal and ferrite powder materials. Traditional ferrite, magnetic metal powder, and ceramic wave absorbing material can hardly meet the requirement of actual practice because of their high density but limited absorbing frequency range and bandwidth. It is important to explore the absorbing material field and bring out new varieties, new concepts, and new technical approaches. The development of absorbing material displays the following tendencies [1–4]: 1. Composite components. One particular mono-component absorbing material can hardly be a satisfactory solution according to the four aspects mentioned above. In order to solve this problem, by taking density and impedance matching and physical and chemical comprehensive performance into consideration, we can make full use of the synergistic effects of composite materials and their adjustable electromagnetic parameter to conduct multielement composition on materials of different absorbing frequency and loss mechanisms (resistance loss, dielectric loss or magnetic loss). Thus, wide frequency range, lightness, effective absorbency, compatible microwave, infrared, and electromagnetic wave at different frequency can be reached. However, some key problems, such as production techniques, optimized designation, and compatibility between components, remain unsolved. This also involves functionally gradient composite absorbing material with graduated DOI 10.1515/9783110424751-004

4 Application of lightweight carbon material and composite material

2.

3.

4.

181

wave impedance and electromagnetic parameter that is favorable to impedance matching and electromagnetic wave absorbing. Low-dimensioned form. Some low-dimension materials with unique electric, magnetic, or photo effect are more effective in absorption than conventional materials. High efficiency means weight loss. At present, low-dimensioned absorbing materials, such as nanometer grain, nanofiber, nanotube, nanofilm, and nanometer multilayer film, catch more and more attention because they suit to wide frequency ranges and wavebands, and they have high absorption intensity, light weight, excellent comprehensive performance, and great potentials. Widened absorbing frequency. With the newly born advanced detector, centimeter wave radar absorbing material (RAM) cannot be adapted to technical requirement. The future development of absorbing material aims at its compatibility of multiple bandwidths and frequencies as meter wave, centimeter wave, millimeter wave, infrared, laser, and so on, with high absorption intensity and light weight. Materials innovation. New absorbing materials break through the framework of traditional materials with the emergence of stealth material with new concepts and new absorption system. There are new electromagnetic wave absorbing materials such as intelligent stealth material, chiral stealth material, nanocomposite stealth material, and so on. And others still follow up.

This chapter focuses on the application of light carbon material and its composite material in the electromagnetic wave absorbing stealth technique. Carbon matrix composite refers to a series of light carbon matrix composites with light carbon materials (such as conductive graphite powder, expanded graphite, carbon fiber, and carbon nanotube) as base material, combined with magnetic metals or their composite, transition metals with excellent conductivity, thulium metal, and so on. Carbon matrix composite reveals superior intrinsic characters, such as low specific weight, fine stability, high temperature resistance, corrosion resistance, resistance to sudden heating, excellent electric conductivity, strong high temperature strength, and excellent absorbing performance with relatively high electronic and magnetic loss as well. For the microstructure of light carbon material, see Figure 4.1. Its notable features are as

(a)

(b)

(c)

(d)

Figure 4.1: Porous microstructure of some nano carbon matrix composite materials. (a) carbon nanotube (b) porous structure of expanded graphite (c) laminated structure of graphite (d) carbon fiber of chemical plated metal.

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4 Application of lightweight carbon material and composite material

follows: (1) Its low composite density makes it more likely to be used in light composite material. (2) It performs outstanding electromagnetic wave absorbing and suits to a wide absorbing frequency. (3) It can enhance mechanical properties of composite materials while keeping high efficiency in absorbing ability. It represents one of the important orientations in the development of new absorbing materials.

4.1 Expanded Graphite/Fe/Co/Ni Composite Materials and Their Absorbing Performance Magnetic microwave absorbing materials, such as transition metals and their oxide compounds, are the main components of microwave absorbing materials. However, transition metals and their oxide compounds share the common defect; that their density is high, so that they are inappropriate for immediate application. As a new nonmetallic inorganic material, expanded graphite is unique and outstanding in some physical and chemical performance, such as light, soft, anti-corrosion, good electric conductibility, and cheap. So when recombined with magnetic metals, expanded graphite will be broadly used in electromagnetic loss absorbing composite materials [5–18]. The traditional methods of producing magnetic expanded graphite are co-precipitation, solution immersion, chemical intercalation, and so on. But the products contain few magnetic elements. The magnetic material has little bonding force with base materials, and it distributes irregularly. Those flaws mentioned above restrict the application of magnetic expanded graphite in radar absorbing stealth technology field. There are some unique advantages in the application of chemical plating in the production of composite materials. Metalizing expanded graphite by chemical plating may produce improved metal expanded graphite composite materials. With the coordination of the advantages of magnetic metals and expanded graphite, composite materials reduce their density, interplay electric and magnetic conductivity, to meet the requirement of absorbing material.

4.1.1 Production of Expanded Graphite/Fe/Co/Ni Composite Materials 4.1.1.1 Preprocessing of Expanded Graphite Because expanded graphite has unique micro-porous and nano-porous structure, and its surface lacks active group to generate autocatalysis of chemical plating, preprocessing the surface of expanded graphite is crucial in plating metal regularly on the surface of expanded graphite. The procedure is as follows: (1) Degreasing. Put a certain amount of expanded graphite (about 0.3 g) into a beaker, followed by the addition of 100 ml NaOH solution at a concentration of 15%. Place the

4.1 Expanded Graphite/Fe/Co/Ni Composite Materials and Their Absorbing Performance

Expandable graphite

900°C Expansion

Expanded graphite

Degreasing

Washing with distilled water

Roughening

Washing with distilled water

Sensitization

Washing with distilled water

Activation

Washing with distilled water

Reduction

Chemical plating of metal

Washing with distilled water

Drying

183

beaker in an ultrasonic vibrator for 20 min vibration degreasing, then in a water bath (60∘ C) for 20 min stirring degreasing, and finally, cool it down to room temperature. After filtrating the degreased expanded graphite, wash it with distilled water until it is neutralized. (2) Roughening. Roughening is to make the surface of expanded graphite exhibit microscopic roughness and increase the contact area between the metal plating layer and the expanded graphite, to increase the bonding force between metal ions and expanded graphite. Otherwise, the plated metal layer will fall off easily. Put the degreased expanded graphite into roughening solution containing K2 Cr2 O7 100 g/L, H2 SO4 400 g/L. Place it into ultrasonic vibrator at room temperature (23∘ C) for 30 min vibration roughening. Filter the roughened expanded graphite before washing it with distilled water until it is neutralized. It should be noted that roughening duration would affect the size of the expanded graphite powder. (3) Sensitization. Sensitization is to cover the surface of expanded graphite with a layer of easily oxidized substances. During sensitization, sensitizer is reduced and remains on the surface of the expanded graphite, on which chemical plating can be further performed. Sensitizer reaction formulas are as follows: SnCl2 + H2 O → Sn(OH)+ + H+ + 2Cl– SnCl2 + 2H2 O → Sn(OH)2 + 2H+ + 2Cl– The Sn(OH)Cl produced by the reaction combines with Sn(OH)2 , creating the watersoluble gel-like Sn2 (OH)3 Cl that adsorbs on the surface of the expanded graphite. Put the roughened expanded graphite in sensitizing solution containing hydrochloric acid (37%) 60 mL/L, SnCl2 ⋅ 2H2 O 30 g/L, then place it in an ultrasonic vibrator at room temperature (23∘ C) for 30 min vibration sensitization. Filtrate the sensitized expanded graphite before washing it with distilled water until it is neutralized. (4) Activation. Activation is to cover the surface of the expanded graphite with a catalytically active noble metal layer. Put the sensitized expanded graphite into a solution containing the catalytically active noble metal compound, and the expanded graphite will be covered with a layer of catalytically active noble metal. Like sensitization, ultrasonic vibration is required to obtain a homogeneous activation of expanded graphite, to ensure the quality of coating. The reaction of activator is as follows: Pd2+ + Sn2+ → Sn4+ + Pd Put the sensitized expanded graphite into activating solution containing hydrochloric acid (37%) 10 mL/L, PdCl2 0.5 g/L. Put it in an ultrasonic vibrator at room temperature

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4 Application of lightweight carbon material and composite material

(23∘ C) for 30 min vibration activation. Filtrate the activated expanded graphite before washing it with distilled water until it is neutralized. (5) Reduction. The purpose of reduction is to reduce the palladium chloride remained on the surface of expanded graphite after activation, to prevent its influence on stability of plating solution. Immerse the activated expanded graphite into reduction solution containing sodium hypophosphite 25 g/L. Place it in an ultrasonic vibrator at room temperature (23∘ C) for 10 min vibration reduction. Filtrate the reduced expanded graphite before washing it with distilled water until it is neutralized. 4.1.1.2 Preparation of Expanded Graphite/Fe/Co/Ni Composites by Chemical Plating Plating solution mainly consists of main salts, reducing agents, complexing agents, buffering agents, stabilizers, accelerators, surfactants, brightener, and other components. Based on the relationship between the various components of the chemical plating solution, we take Ni plating as the example of plating solution formulation. Other plating processes, such as nickel-cobalt (Ni-Co) plating, nickel-iron (Ni-Fe) plating, nickel-iron-cobalt (Ni-Fe-Co) plating solution formulation, are similar to the Ni plating, with the change in content and type of the main salt. Ni plating solution formulation is shown in Table 4.1. Table 4.1: Chemical solution formulation (g/L). Composition

A1

Ni sulfate Hypophosphite Sodium tartrate Succinate Sodium citrate Glycine Malic acid Thiourea Sodium dodecyl sulfate Temperature/∘ C pH

32 20 42

B1

B2

32 20

28 20 10

29.4

29.4

B3

B4

28 20 42 16

28 20 16 29.4

C1

C2

C3

28 20 10

28 20 42 8

28 20

18

18 2

80

90

16 18

24 0.03 80 9

0.001 0.03 85 10

0.001 70 11

Composition

D1

D2

D3

E1

E2

E3

F1

F2

F3

G1

Ni sulfate Hypophosphite Sodium tartrate Succinate Sodium citrate Glycine Malic acid Thiourea Temperature/∘ C pH

28 20 5

28 20

28 20

28 20

28 20 42

28 20

28 20

28 20

28 20

8 26

26

10

6

21

4 21

28 20 5 4 18 10

21

85

60

70

85

26 4

50 9

70

8 18 10 3 80–85 10

4 18 4 10.5

10 11.5

G2 28 20 5 4 9 11.5 0.001

G3 28 20 8 18 10.5 0.001

4.1 Expanded Graphite/Fe/Co/Ni Composite Materials and Their Absorbing Performance

185

According to the study of chemical plating formulation, the final resolutions of chemical plating of Ni-Co, Ni-Fe, and Ni-Fe-Co on the surface of expanded graphite are the best recipes as shown in Table 4.2. Put the preprocessed expanded graphite into the alkaline chemical solution made up according to the formulation, Then by stunning and choking intermittently or by gas agitation intermittently can the chemical deposition be accelerated to prevent decomposition caused by partial overheating, and to create a favorable condition for the deposition of metal on the surface of the expanded graphite. The reaction time is about 45 min, with temperature controlled at about 85∘ C. Drop in 0.5 mol/L ammonia solution to adjust the pH value at 10 during the reaction. When the reaction is completed, the coated expanded graphite products, such as Ni-Fe-P, Ni-Co-P, and Ni-Fe-Co-P, can be produced after a series of procedures of cooling. Experimental apparatus is shown in Figure 4.2.

4.1.2 Characterization of Composite Materials In this chapter, the expanded graphite before and after chemical plating through XL30E scanning electron microscope (SEM) (Philips company, the Netherlands), The EDS spectrum analysis is conducted on the sample of DX-4 energy dispersive X-ray spectrometer (Philips company, the Netherlands), in addtion, the tissue analysis is conducted 3on the samples by D/MAX-3C X-ray diffractometer (Rigaku Co.). 4.1.2.1 Composite’s Morphology Figure 4.3 displays the SEM image of expanded graphite before and after metal plating and after heat treatment. (a) in the figure is uncoated expanded graphite; it can be seen that its surface is rough, loose, porous, and curl. It is a wormlike substance with large surface area. (b–e), respectively, are Ni, Ni-Co, Ni-Fe, and Ni-Fe-Co plating expanded graphite. As can be seen from the figure, the surface of the expanded graphite is coated with a metal layer. The coating layer is continuous and smooth, with spherical metal fractions. The spherical fractions on the surface of the material may be the result of poor activation during the pretreatment of expanded graphite, resulting in a larger surface curvature of small amount of expanded graphite, and plated metal depositing in the form of spherical particles. The particle size of the coating metal is about 60–70 nm, the coating thickness is about 70–150 nm, belonging to nanometer scale coating. (f–i) respectively are the SEM images corresponding to (b–e) metal-coated expanded graphite after 1 h 400∘ C heat treatment with the protection of hydrogen. It can be seen that after the heat treatment the spherical metal on the surface significantly reduced and the coating metal becomes more continuous, dense, and smooth. The coating metal of expanded graphite after metal plating exists in the form of amorphous alloy. With the heat treatment, phosphorus atoms diffuse and migrate, causing lattice distortion, which, once reaching a certain level,

Nickel sulfate Cobalt sulfate

Nickel sulfate Ferrous ammonium sulfate Nickel sulfate Ferrous ammonium sulfate Cobalt sulfate

Nickel-cobalt plating

Nickel-iron plating

Nickel-ironcobalt plating

Nickel sulfate

Nickel plating

12

28 12

28 12

28 12

28

Main salt Composition Amount

Solution type

Sodium hypophosphite

Sodium hypophosphite

Sodium hypophosphite Sodium hypophosphite

20

20

20

20

Reducer Composition Amount

Table 4.2: Best formulation for plating solution (g/L).

Sodium citrate Potassium sodium tartrate Sodium citrate Potassium sodium tartrate Sodium citrate Potassium sodium tartrate

Sodium citrate

44 42.4

29.4 42.4

29.4 42.4

29.4

Complexing agent Composition Amount

Ammonium citrate

Ammonium citrate

Ammonium citrate Ammonium citrate

20

20

20

20

Buffer Composition Amount

Aqueous ammonia

Aqueous ammonia

Aqueous ammonia Aqueous ammonia

Appropriate pH ≈ 10

Appropriate pH ≈ 10

Appropriate pH ≈ 10

Appropriate pH ≈ 10

pH Adjusting agent Composition Amount

186 4 Application of lightweight carbon material and composite material

4.1 Expanded Graphite/Fe/Co/Ni Composite Materials and Their Absorbing Performance

187

Aqueous ammonia Thermometer

Thermostat water bath Pneumatic agitation device

Figure 4.2: Chemical plating device.

will cause the decomposition and dispersion of the solid solution. As a result, the plastic deformation resistance of the coating, so that the coating can be strengthened. The study of micromorphology of metal-expanded graphite sample series reveals that chemical plating can coat a continuous, even, dense metallic layer on the surface of expanded graphite. After 1 h 400∘ C heat treatment, the coating becomes more continuous. Comparing with the traditional doping, co-precipitation method, this treatment will produce magnetic expanded graphite composite material characterized by containing more magnetic metal, enhanced binding force, evenly distribution and so on. 4.1.2.2 Field Emission Energy Spectroscopy EDS Analysis of Composites In order to quantitatively analyze the composition and content of expanded graphite composite materials after being coated with metal, EDS energy spectrum analysis is conducted on expanded graphite before and after chemical plating. The results are shown in Figure 4.4. Figure 4.4 is EDS spectra of expanded graphite before and after metal plating, wherein (a) is the spectrum of expanded graphite before plating. (a) reveals that, before plating, the expanded graphite is more or less pure, containing only element C, free of other impurities. (b–e), respectively, are spectrums of metal plating expanded graphite coated with Ni, Ni-Co, Ni-Fe, and Ni-Fe-Co. It can be drawn from the figure that the surface of expanded graphite is indeed coated with expected metal layer. They all contain phosphorus because of the use of sodium hypophosphite as the reducing agent in the experiment. For plating nickel, the fraction of Ni and P are 45.76% and 3.28%; for plating nickel-cobalt, the fraction of Ni, Co, and P are 30.26%, 21.45%, and 3.66%; for plating Nickel-iron, the fraction of Ni, Fe, and P are 35.76%, 17.48%, and 5.68%; for plating Ni-Fe-Co, the fraction of Ni, Fe, Co, and P are 28.26%, 16.45%, 20.14%, and 4.66%.

188

4 Application of lightweight carbon material and composite material

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

Figure 4.3: SEM images of metal expandable graphite before and after plating. (a) uncoated expanded graphite metal; (b–e), respectively: Ni, Ni-Co, Ni-Fe, Ni-Fe-Co plating expanded graphite; (f–i), respectively: the SEM images corresponding to (b–e) metal coated expanded graphite after 1 h 400∘ C heat treatment with the protection of hydrogen.

4.1.2.3 X-ray Diffraction Analysis of Composites Analyze the organization of expanded graphite with D/MAX-3C X-ray diffractometer (Rigaku Co.) before and after chemical plating. The XRD pattern results are shown in Figure 4.5. Figure 4.5 is XRD spectra patterns of expanded graphite with 1 h 400∘ C heat treatment before and after plating. Wherein, (a) is spectrum of expanded graphite before plating. It can be seen that the 2( is a hexagonal graphite structure produced by the diffraction peak at the angle of 26.56∘ , 44.26∘ , and 54.7∘ . The strongest diffraction peak

189

4.1 Expanded Graphite/Fe/Co/Ni Composite Materials and Their Absorbing Performance

C

(a)

(b) Ni C Ni P Ni

1 2 3 4 5 6 7 8 scale span94cts cursor0.155kev (24cts)

9

0 1 2 3 4 5 6 7 8 scale span295cts cursor0.092keV (8 cts) C

C

(d)

(c)

CoNi

Ni Fe

Ni

P

P Ni

Co

FeFe

Ni 1 2 3 4 5 6 7 8 scale span206cts cursor0.125kev (91 cts) C

Co Ni Fe

9

9

Ni

1 2 3 4 5 6 7 8 scale span94cts cursor0.155kev (24 cts)

9

(e)

P Ni Co FeFe

Ni

2 3 4 5 6 7 8 1 scale span94cts cursor0.155kev (24 cts)

9

Figure 4.4: EDS spectra of expanded graphite before and after metal plating. (a) spectrum of expanded graphite before plating, (b) Ni plated on the surface of expanded graphite, (c) Ni-Co plated on the surface of expanded graphite, (d) Ni-Fe plated on the surface of expanded graphite, (e) Ni-Fe-Co plated on the surface of expanded graphite. S-scale span; C-rsor.

(26.56∘ ) is not shown in the other figures in order to clearly display the diffraction peak of metal phase of the expanded graphite. (b) shows XRD spectra patterns of expanded graphite with 1 h 400∘ C heat treatment before and after nickel plating. It shows an amorphous diffuse peak between the angles 41∘ and 54∘ . Combined with EDS analysis, it shows that the combination of Ni-P amorphous diffraction peaks, indicating that Ni-P plating is amorphous while plating. However, after 1 h 400∘ C heat treatment, the diffuse peak disappears. Besides diffraction peak, there appears a new crystallization phase Ni3 P (JCPDS: 34-0501) diffraction peak at the position 2( = 36.3∘ , 41.7∘ , 42.8∘ , 43.7∘ , and 46.6∘ , respectively,

190

4 Application of lightweight carbon material and composite material

26.56

(a)

(b)

chemical nickel plating 400 °C

after 44.26

10

20

30

40

50

54.7

before

60

70

10

20

30

2θ/°

40

50

60

(c)

(d)

Co

Fe 400 °C

400 °C

after

after

before 10

20

70

2θ/°

30

40

50

60

70

2θ/°

before 10

20

30

40

50

60

70

2θ/° (e)

Fe Co Ni 400 °C

after before

10

20

30

40

50

60

70

2θ/° Figure 4.5: XRD spectra patterns of expanded graphite before and after metal plating. (a) spectrum of expanded graphite before plating, (b) XRD spectra patterns of expanded graphite with 1 h 400∘ C heat treatment before and after Ni plating, (c) XRD spectra patterns of expanded graphite with 1 h 400∘ C heat treatment before and after Ni-Co plating, (d,e) XRD spectra patterns of expanded graphite with 1 h 400∘ C heat treatment before and after Ni-Fe, Ni-Fe-Co plating.

corresponding to each diffraction plane index (031), (231), (330), (112), and (141). The intensity of the diffraction peak decreases significantly, for the metal coating layer on the surface of expanded graphite weakens its diffraction peak contrast. (c) is XRD spectra patterns of expanded graphite with 1 h 400∘ C heat treatment before and after Ni-Co plating. It shows an amorphous diffuse peak between the angles 40∘ and 50∘ , indicating that Ni-Co-P plating is also amorphous while plating. After

4.1 Expanded Graphite/Fe/Co/Ni Composite Materials and Their Absorbing Performance

191

1 h 400∘ C heat treatment, the diffuse peak disappears. Besides diffraction peak, there appears some sharp peaks which is determined to be diffraction peaks of Ni3 P (JCPDS: 34-0501) and Co (JCPDS: O1-1254). The position of diffraction peaks of Ni3 P is the same as that of Ni. Co diffraction peaks are 2( = 44.4∘ , 47.3∘ , and 51.7∘ , corresponding to each diffraction plane index (111), (101), and (200). (d) and (e), respectively, are XRD spectra patterns of expanded graphite with 1 h ∘ 400 C heat treatment before and after Ni-Fe, Ni-Fe-Co plating. (d) shows that there is an amorphous diffuse peak between the angles 40∘ and 53∘ of Ni-Fe plating expanded graphite. (e) shows an amorphous diffuse peak between the angles 42∘ and 52∘ of Ni-Fe-Co plating expanded graphite, indicating that metal coating is also amorphous while plating. After 1 h 400∘ C heat treatment, the diffuse peak disappears. Besides diffraction peak of expanded graphite, there also appears some sharp peaks which is determined to be diffraction peaks of Ni3 P (JCPDS: 34-0501) and Co (JCPDS: O1-1254) and Fe (JCPDS: 38-419). Through analysis of the XRD spectra patterns of expanded graphite with 1 h 400∘ C heat treatment before and after metal plating, we can tell that the coating metal is amorphous while chemical plating and the coating metal will transfer into crystalline simple substance, so that the coating layer is more continuous and densified. 4.1.2.4 The Magnetic Properties of the Composites Expanded graphite belongs to inorganic nonmetal and nonmagnetic material. After the metal particles deposit on the surface of expanded graphite, the expanded graphite has certain magnetic properties, which broaden its applications. After 1 h 400∘ C heat treatment to magnetic plating expanded graphite composite materials, test its magnetic properties with CDJ-7400 vibrating sample magnetometer. Figure 4.6 shows magnetic hysteresis loops of expanded graphite after metal plating with 1 h 400∘ C heat treatment. As it can be seen, that the saturation magnetization 3s-Ni = 5.7 emu/g, 3s-Ni-Co = 8.5 emu/g, 3s-Ni-Fe = 9.4 emu/g, 3s-Ni-Fe-Co = 11.3 emu/g,

Ni–Fe–Co Ni–Fe Ni–Co

π/(emu/g)

10 5

Ni

0 –5 –10 –6000 –4000 –2000

0 H/Oe

2000

4000

6000

Figure 4.6: Magnetic hysteresis loops of expanded graphite after plating different metals.

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4 Application of lightweight carbon material and composite material

the order of magnetic strength of magnetic plating expanded graphite composite materials is as follows Ni-Fe-Co plated > Ni-Fe plated > Ni-Co plated > Ni plated. Therefore, by chemical plating the magnetic metal, nano ferromagnetic metal particles deposit on the surface of expanded graphite, which can increase the magnetic strength of composite magnetic material, so that the metal plating magnetic expanded graphite becomes ferrimagnetic material of good quality. 4.1.2.5 Thermal Stability of Composites Metallic modification on the surface of expanded graphite can improve the thermal stability of the composite material, which is of great significance for its practical applications. Therefore, it is necessary to study its thermal stability. Among the composite materials, nickel-plated expanded graphite is weaker in thermal instability. Its TG-DSC thermal curve is shown in Figure 4.7. Ni-P layer is amorphous while plating, which is a metastable state. With the rise of the heat treatment temperature, its structure will change as a result of phosphorus atom diffusion and migration, forming crystalline structure. In the DSC curve, it can be seen that in the vicinity of 338∘ C there is a small exothermic peak. Combined with the XRD spectra of the products, it suggests that during the heat treatment, the coating layer transfers from amorphous to thermodynamically stable crystalline. From the TG curve, it can be seen that within the vicinity of 548.7∘ C, the sample experiences a significant weight loss. At the same time, in the DSC curve of the sample in the corresponding temperature range a clear exothermic peak appears, indicating that DSC/mW/mg

TG% TG

exo 0

100 –2 90 –4 338°C

80

–6

70

–8 –10

60 DSC

–12

50

–14

548.7°C 100

200

300

400

500

40 600

Temperature/°C

Figure 4.7: TG-DSC curve of Ni plating metal expandable graphite.

700

800

4.1 Expanded Graphite/Fe/Co/Ni Composite Materials and Their Absorbing Performance

193

the composite material conducts a thermal decomposition. It can be explained that nickel/expanded graphite based composites will keep a good thermal stability before the temperature reaches 500∘ C, which is significant for practical application of composite materials.

4.1.3 Electromagnetic Spectrum Analysis on Magnetic Metal-Expanded Graphite Composite 4.1.3.1 Defining Complex Dielectric Constant and Complex Magnetic Permeability (1) Defining Complex Dielectric Constant. The dielectric will be polarized when it is in the electrostatic field. The total vector of the molecule’s electrical distance in the unit volume is called electropolarization strength, signified as P. For isotropic dielectrics, the relationship of P and external field strength E is as follows: P = 7e %0 E where, 7e is the polarizability, %0 is the vacuum dielectric constant, its value is 8.85 × 10–12 C2 N–1 m–2 . The electric displacement D is expressed as: D = %0 E + P = (1 + 7e )E0 = %r %0 E where, %r is the relative dielectric constant, which is nondimensional. When external field is an alternating field, with the increasing frequency of the external field, the polarization of the medium gradually lags behind the changes of the external field, the dielectric constant must be expressed in plural form. That is, %r = %󸀠 – j%󸀠󸀠 The real part of the complex dielectric constant represents the ability to store electricity or energy. The imaginary part corresponds to an equivalent resistance connected in parallel with a capacitor, representing the loss of energy. When dielectric is in the electric field E = Em ej9t , the dielectric loss of the unit volume is: 1 Pdh = 9%0 %󸀠󸀠 Em 2 2 (2) Defining Complex Magnetic Permeability. Magnetic medium can be magnetized in the external magnetic field. Its magnetization strength can be represented by M. Within the static magnetic field, the magnetization strength at any point inside most isotropic magnetic medium M and magnetic field strength H is directly proportional. That is, M = 7m H

194

4 Application of lightweight carbon material and composite material

where, the proportionality factor 7m is constant, called magnetic susceptibility. Magnetic induction B in medium is expressed as: B = ,0 (H + M) = ,0 (1 + 7m )H = ,r ,0 H where, ,0 is known as vacuum magnetic permeability, which is 40 × 10–7 H⋅m–1 . ,r is relative magnetic permeability. Affected by the alternating electromagnetic field, due to magnetic hysteresis effect, eddying effect, magnetic aftereffect, domain wall resonance, and natural resonance, the changes in terms of time of the magnetization state of the medium legs behind the changes in terms of time of the external field, so that magnetized time effect needs to considering. If the amplitude of vibration of external alternating magnetic field is Hm , angular frequency is 9, the magnetic field strength H can be expressed as: H = Hm cos(9t). The corresponding magnetic induction B also presents cyclical changes. But it is behind H in a time phase difference $. Set its amplitude as Bm , then B can be expressed as: B = Bm cos(9t – $) In the dynamic magnetization process, in order to make clear the relationship between B and H in alternating field, complex magnetic permeability must be introduced. That is, ,r = ,󸀠 – j,󸀠󸀠 B and H can be expressed as follows: H = Hm ej9t B = Bm ej9t The complex formulation of relative magnetic permeability can be obtained by ,r =

Bm –j$ Bm B = e = (cos $ – j sin $) ,o H ,o Hm ,o Hm

Thus, Bm cos $ ,o Hm Bm ,󸀠󸀠 = sin $ ,o Hm ,󸀠 =

4.1 Expanded Graphite/Fe/Co/Ni Composite Materials and Their Absorbing Performance

195

Average energy loss per unit volume of ferromagnetic in uniform alternating field is: Pch =

󸀠󸀠 1 T 1 1 2 ∫ HdB = 9Hm Bm sin $ = 9,0 , Hm T 0 2 2

The density of the stored energy inside the magnetic media is: 1 1 1 2 W = HB = 9Hm Bm cos $ = ,0 ,󸀠 Hm 2 2 2 According to the physical conception of complex magnetic permeability and complex dielectric constant, the dielectric constant %r and magnetic permittivity ,r of different materials will show different electrical and magnetic spectrum. %󸀠 , %󸀠󸀠 and ,󸀠 , ,󸀠󸀠 are different according to the change of frequency. And the real part of %r and ,r , that is, %󸀠 and ,󸀠 , bears electromagnetic energy storage function of medium. And the imaginary part %󸀠󸀠 and ,󸀠󸀠 bears electromagnetic wave absorption function of medium. If compared with the real part, the imaginary part is small and neglectable; this medium cannot absorb electromagnetic waves and can be called waves through medium; on the contrary, when the imaginary part cannot be ignored, this medium has the ability to absorb electromagnetic waves. Considering from the perspective of media’s absorption of electromagnetic waves, the bigger the %󸀠󸀠 and ,󸀠󸀠 , the better the effect is. Magnetic loss and dielectric loss can be expressed by dielectric loss tangent are: tan $ = tan $e + tan $m = 󸀠󸀠

where, tan $e = %%r 󸀠󸀠 and tan $m = r magnetic loss tangent of the material.

,r 󸀠󸀠 ,r 󸀠󸀠

%r 󸀠󸀠 ,r 󸀠󸀠 + %r 󸀠󸀠 ,r 󸀠󸀠

represent the dielectric loss tangent and

4.1.3.2 Mechanism of Microwave and Absorbing Materials In free space, when microwave incidents on the medium, it will reflect and transmit at the interface. The microwave reflection coefficient at the interface A depends on the difference between the input impedance Zin at the surface of the absorbing material and the characteristic impedance of air Z0 = √,0 /%0 : A=

Zin – Z0 Zin + Z0

According to this formula, the reflectivity of the radar absorbing materials (RAMs) can be defined as: R = –20 lg |A|

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4 Application of lightweight carbon material and composite material

According to the transmission line theory, the input impedance at the interface of composite materials and free space is determined by the characteristic impedance and terminal impedance of the composite material: Zin = ZC

ZL + ZC tanh(𝛾d) ZC + ZL tanh(𝛾d)

In this formula, d is the dielectric thickness, h is Planck constant, and Zc and ZL are characteristic impedance and terminal impedance of the composite material. # is the propagation constant. It can be expressed as: 𝛾=

j9 20 √,r ,0 %r %0 = j √,r %r c +

For the metal substrate, if the electric conductivity is assumed as 3 → ∞, so ZL = 0. Characteristic impedance Zc depends on the equivalent electromagnetic parameters of the material. That is, ZC = √

,r ,0 , = Z0 √ r %r %0 %r

where, %r and ,r represent relative complex dielectric constant and complex magnetic permeability. %0 and ,0 are the vacuum complex dielectric constant and vacuum complex magnetic permeability. Thus, it can be obtained: ,r 20d tanh (j √,r %r ) %r + 󵄨󵄨 󵄨󵄨󵄨 √ , % ) – 1 󵄨󵄨󵄨 󵄨󵄨 ,r /%r tanh ( j 20d + √ r r 󵄨 󵄨󵄨 󵄨 R = –20 lg 󵄨󵄨 󵄨 󵄨󵄨 √, /% tanh (j 20d √, % ) + 1 󵄨󵄨󵄨 󵄨󵄨 󵄨󵄨 r r r r +

Zin = Z0 √

From this formula, it can be seen that the design principle of plate absorbing material is to ensure the equality of the input impedance Zin on the surface of the absorbing material and the characteristic impedance of air Z0 as possible, so as to achieve impedance matching. According to microwave theory, with the decay parameter ! to represent the wave attenuation per unit length, the formula can be: 𝛾=j !=

20 √(,󸀠 – j,󸀠󸀠 )(%󸀠 – j%󸀠󸀠 ) = ! – j" +

9 √ 󸀠󸀠 󸀠󸀠 , % – ,󸀠 %󸀠 √(,󸀠2 + j,󸀠󸀠2 )(%󸀠2 + j%󸀠󸀠2 ) √2c

4.1 Expanded Graphite/Fe/Co/Ni Composite Materials and Their Absorbing Performance

197

where, characteristic electromagnetic wave ! attenuates in the medium and is called attenuation coefficient. " is the phase factor; 9 is the angular frequency; # is the propagation constant. If the electromagnetic waves penetrated into the material must be evanesced completely and quickly, it must be completed that: %󸀠󸀠 ≠ 0 or ,󸀠󸀠 ≠ 0, or %󸀠󸀠 ≠ 0 and ,󸀠󸀠 ≠ 0. Only when %󸀠󸀠 = 0 and ,󸀠󸀠 = 0, ! has a minimum value of 0, and: %󸀠󸀠 /%󸀠 = ,󸀠󸀠 /,󸀠 This indicates an extreme value for the formula of !. If the second derivative of %󸀠 and ,󸀠n is required, ! can be minimized. If %󸀠󸀠 /%󸀠 ≠ ,󸀠󸀠 /,󸀠 , ! will be greater. And the greater the difference value of both sides, the greater the value of !. We can conclude that the study of absorbing material should be focused on the components and structure of materials designed. The reflection of electromagnetic wave at the interface or the attenuation of electromagnetic wave in the medium is closely related to the electromagnetic parameter of the material. So in order to achieve maximum absorption of the incident electromagnetic wave, the electromagnetic parameter of material should be adjusted. Absorbent is used to adjust the electromagnetic parameters of the material to increase the absorption of electromagnetic waves in the material. Thus, the electromagnetic parameter performance is critical. 4.1.3.3 Testing Electromagnetic Parameters of Composites The testing method of absorbent electromagnetic parameters is to mix the adhesive and absorbent according to a certain ratio to prepare a model material. The test will be conducted in the instrument. The electromagnetic parameters obtained by the test equal to the electromagnetic parameters of the absorbent and adhesive mixture. The equivalent electromagnetic parameters primarily depend on the contents of each component in the mixture and the intrinsic electromagnetic parameters of the absorbent. When the type and content of the adhesive are set, comparison can be made between the electromagnetic parameters of different absorbent. And for the same absorbent, comparison also can be conducted between the effects on the electromagnetic parameters when the absorbent content varies. Measure the complex dielectric constant and complex magnetic permeability of expanded graphite after chemical plating of metal applying coaxial method with HP8722ES automatic vector network analyzer. Since the chemical plating expanded graphite is in powder form, it is not suitable for direct sample testing. Mix the coated expanded graphite with adhesive paraffin wax according to a certain proportion to make an coaxial sample with 3 mm inner diameter, 7 mm outer diameter and 4 mm in length to conduct measurement of its complex dielectric constant and complex magnetic permeability with 2–18 GHz sweep range, and once every 0.08 GHz. Paraffin wax adhesive is selected because, during the test of the complex dielectric constant

198

4 Application of lightweight carbon material and composite material

and complex magnetic permeability of adhesive paraffin, it is found that the real part of the complex dielectric constant of paraffin wax is small, so it does not generate large amount of electromagnetic reflection. The imaginary part of the complex magnetic permittivity is close to zero, so paraffin wax adhesive is nonmagnetic material, and will cause no electromagnetic waves loss. The complex dielectric constant and complex magnetic permeability of the tested sample can be approximated as the complex dielectric constant and complex magnetic permeability of a coating expanded graphite. Fix paraffin wax adhesive content during the sample test, conduct measurements to electromagnetic parameters according to different absorbent composites (wt5%, wt10%, wt15%) of chemical Ni plating expanded graphite matrix composite material, and Ni-Co, Ni-Fe, and Ni-Fe-Co plating expanded graphite matrix composite materials at fixed content (wt15%). On the basis of the results of electromagnetic parameters, simulation calculation programming was performed with MATLAB® to achieve the relationship between reflectivity (R) and frequency (f) and thickness (d) to study the effect of different parameters on absorbing performance to optimize designation for application.

14

a-5%

b-10%

Imaginary part of dielectric ε′

Real part of dielectric constant

(1) Electromagnetic Spectrum of Ni/Expanded Graphite Matrix Composites. Mix the Ni plating expanded graphite matrix composites with adhesive paraffin wax uniformly with the composites dosage wt5%, wt10%, and wt15%, and measure the complex dielectric constant and complex magnetic permeability. The test results are shown in Figures 4.8 and 4.9. As can be seen from Figure 4.8, generally, the real part of dielectric constant of Ni/expanded graphite matrix composites %󸀠 and its imaginary part %󸀠󸀠 increase as the mass fraction in the mixture increases. In the 2–6 GHz and 2–8 GHz frequency range, the real part of dielectric constant %󸀠 and its imaginary part %󸀠󸀠 fluctuate between a fixed range. The real part and imaginary part, %󸀠 and %󸀠󸀠 , of magnetic permeability both increase as the mass fraction of the mixture increases. In the 2–8 GHz and 2–9 GHz frequency range, the real part of magnetic permeability %󸀠 and its imaginary part %󸀠󸀠

c-15%

c

12 10

b

8 a

6 4 2 2

4

6

8

10 f(GHz)

12

14

16

18

8 7

c

a-5%

b-10%

c-15%

6 5

b

4

a

3 2 1 2

4

6

8

10 f(GHz)

Figure 4.8: %󸀠 - f and %󸀠󸀠 - f spectrum of Ni/expanded graphite matrix composites.

12

14

16

18

199

0.5

c

a-5% b-10% c-15%

0.4 0.3

b

0.2 a

0.1 0

–0.1 2

4

6

8

10 12 f(GHz)

14

16

18

Imaginary part of magnetic permeability μ′

Imaginary part of magnetic permeability μ″

4.1 Expanded Graphite/Fe/Co/Ni Composite Materials and Their Absorbing Performance

1.4 1.2

c

1.0

a-5% b-10% c-15% b

0.8

a

0.6 0.4 0.2 0.1 2

4

6

8

10 12 f(GHz)

14

16

18

Figure 4.9: ,󸀠 - f and ,󸀠󸀠 - f spectrum of Ni/expanded graphite matrix composites.

decrease as frequency rises, fluctuating between a fixed range within the frequency range of 9–18 GHz. Figure 4.9 shows that the imaginary part of the dielectric constant %󸀠󸀠 and the imaginary part of magnetic permeability ,󸀠󸀠 of Ni plating expanded graphite matrix composites are large, within the range of 1.5–7.8 and –0.15 to 0.47. Expanded graphite is inorganic nonmetal material without magnetic properties. After coated with Ni, the imaginary part of the magnetic permeability of the Ni/expanded graphite matrix composite material is not zero, indicating that it has a certain magnetic loss for electromagnetic waves. While the composite material has a dielectric loss and magnetic loss, it is double-complex medium. It also can be illustrated that it is feasible to increase the dielectric and magnetic loss by metallizing the surface of the expanded graphite, so as to produce the light multifrequency wave absorbing composite material. (2) Electromagnetic Spectrum of Ni-Co/Expanded Graphite Matrix Composites. Mix the Ni-Co plating expanded graphite matrix composites with adhesive paraffin wax uniformly with the composite dosage wt15%, and measure the complex dielectric constant and complex magnetic permeability. The test results are shown in Figure 4.10. Figure 4.10 (left) shows the relationship between the dielectric constant of NiCo/expanded graphite matrix composites and the frequency. The real part of its dielectric constant %󸀠 and its imaginary part %󸀠󸀠 increase as the frequency f increases. In the 2–6 GHz and 12–18 GHz frequency range, the real part of dielectric constant %󸀠 shows no significant change; and there is a peak in the 6–8 GHz frequency range; and it falls gradually in the 8–12 GHz frequency range. The imaginary part %󸀠󸀠 of the dielectric constant increases in the 2–6 GHz frequency range, shows a peak in the 6–9 GHz frequency range, and stabilizes in the 9–18 GHz frequency range. Figure 4.10 (right) shows the relationship between the magnetic permeability of Ni-Co/expanded graphite matrix composites and the frequency. The real part and imaginary part, ,󸀠 and ,󸀠󸀠 , of magnetic permeability both increase as the mass fraction of the mixture increases.

4 Application of lightweight carbon material and composite material

Magnetic permeability μ′ μ″

Dielectric constant ε′ ε″

200

ε′

21 18 15 12

ε″

9 6 3 2

4

6

8

10 12 f(GHz)

14

16

2.4 μ′

2.0 1.6 1.2

μ″

0.8 0.4 0

18

2

4

6

8

10 f(GHz)

12

14

16

18

Figure 4.10: Relationship between the dielectric constant % and magnetic permeability , of NiCo/expanded graphite matrix composites and frequency.

In the 2–7 GHz frequency range, the real part of magnetic permeability %󸀠 and its imaginary part %󸀠󸀠 decrease sharply as frequency rises; in the 7–18 GHz frequency range, the real part ,󸀠 appears two peaks at 11 GHz and 15 GHz, while the imaginary part ,󸀠󸀠 shows only one peak around 11 GHz. Comparing Figure 4.9 with Figure 4.10, it shows that the dielectric constant and magnetic permeability of Ni-Co/expanded graphite matrix composite are larger than that of Ni/expanded graphite matrix composite.

Magnetic permeability μ′ μ″

Dielectric constant ε′ ε″

(3) Electromagnetic Spectrum of Ni-Fe/Expanded Graphite Matrix Composites. Mix the Ni-Fe plating expanded graphite matrix composites with adhesive paraffin wax uniformly with the composite dosage wt15%, and measure the complex dielectric constant and complex magnetic permeability. The test results are shown in Figure 4.11. Figure 4.11 shows the relationship between the dielectric constant and magnetic permeability of Ni-Fe/expanded graphite matrix composites and the frequency. The figure on the left shows the real part of its dielectric constant %󸀠 decreases as the frequency increases in the range of 2–8 GHz, and stabilizes in the range of 8–18 GHz; while its imaginary part %󸀠󸀠 remains stable in the 2–16 GHz frequency range, with only partial increase in the 16–18 GHz frequency range. The figure on the right shows

21 18

ε′

15 12 ε″

9 6 3 2

4

6

8

10 f(GHz)

12

14

16

18

2.4 μ′

2.0 1.6 1.2

μ″

0.8 0.4 0 2

4

6

8

10

12

14

16

18

f(GHz)

Figure 4.11: Relationship between the dielectric constant and magnetic permeability of NiFe/expanded graphite matrix composites and frequency.

201

4.1 Expanded Graphite/Fe/Co/Ni Composite Materials and Their Absorbing Performance

the relationship between magnetic permeability of Ni-Fe/expanded graphite matrix composite material and frequency. The real part ,󸀠 decreases as the frequency increases, and the imaginary part ,󸀠󸀠 is stable with only a wave hollow in the 2–7 GHz frequency range. Comparing Figures 4.9, 4.10, and 4.11, it can be seen that the magnetic permeability of Ni-Fe/expanded graphite matrix composite and Ni-Co/expanded graphite matrix composite are close, and their dielectric constant and magnetic permeability are larger than that of Ni/expanded graphite matrix composite. (4) Electromagnetic Spectrum of Ni-Fe-Co/Expanded Graphite Matrix Composites. Mix the Ni-Fe-Co plating expanded graphite matrix composites with adhesive paraffin wax uniformly with the composite dosage wt15%, and measure the complex dielectric constant and complex magnetic permeability. The test results are shown in Figure 4.12. Figure 4.12 shows the relationship between the dielectric constant of Ni-FeCo/expanded graphite matrix composites and the frequency. The figure on the left shows that the real part of its dielectric constant %󸀠 increases first and then decreases as the frequency increases, until it reaches 10. The figure on the right shows the relationship between magnetic permeability of Ni-Fe-Co/expanded graphite matrix composite material and frequency. The real part ,󸀠 and the imaginary part ,󸀠󸀠 decreases as the frequency increases, until it reaches a constant value. Comparing Figures 4.9, 4.10, 4.11, and 4.12, it can be seen that the magnetic permeability of Ni-Fe-Co/expanded graphite matrix composite is the largest, especially with a sharp contrast at magnetic loss. The explanation is that the mass fraction of low magnetic Ni contained in Ni-Fe-Co/expanded graphite composites is the least (28%). 4.1.3.4 Theoretical Calculation of Reflectance According to the electromagnetic parameters obtained, using the theory of the absorbing wall calculation, the reflectance of material is: 󵄨󵄨 Z – 1 󵄨󵄨 󵄨󵄨 󵄨 R(dB) = 20 log10 󵄨󵄨󵄨 in 󵄨 󵄨󵄨 Zin + 1 󵄨󵄨󵄨 Magnetic permeability μ′ μ″

Dielectric constant ε′ ε″

30 25 ε′

20 15 10 5

ε″

0 –5 2

4

6

8

10 12 f(GHz)

14

16

18

4.8 4.0

μ′

3.2 2.4 μ″

1.6 0.8 0 2

4

6

8

10 f(GHz)

12

14

16

18

Figure 4.12: Relationship between the dielectric constant and magnetic permeability of Ni-FeCo/expanded graphite matrix composites and frequency.

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4 Application of lightweight carbon material and composite material

Zin is the normalized input impedance and can be obtained by , 20 Zin = √ tanh [j [ ] √,% fd] % c wherein, %r = %󸀠 – j%󸀠󸀠 , ,r = ,󸀠 – j,󸀠󸀠 , j is the imaginary unit, d is the thickness of the sample, f is the frequency, and c is the speed of light. When Zin = 1, it is close to the total absorption. Since the absorbing properties of the material is determined by six impedance matching parameters %󸀠 , %󸀠󸀠 , ,󸀠 , ,󸀠󸀠 , d and f , when %󸀠 , %󸀠󸀠 , ,󸀠 , ,󸀠󸀠 values are obtained of a test sample, according to the absorbing wall formula, the relationship between the theoretical reflectance (R), frequency (f) and the thickness (d) can be reached by applying MATLAB programming simulation. (1) Reflectivity of the Ni/Expanded Graphite Matrix Composite with Different Thickness. According to the result of electromagnetic parameters of the Ni/expanded graphite matrix composite material, and the absorbing wall formula, to conduct simulation calculation to the electromagnetic parameters of the Ni/expanded graphite matrix composite material dosage wt15%, on condition that the thickness d = 1 mm, 3 mm, 4 mm, 5 mm, so that the best matching thickness can be determined. The results are shown in Figure 4.13. As can be seen in Figure 4.13, with the frequency increasing, the reflection first increases. Once reaching the maximum value, it decreases as the frequency increases. When d = 1 mm, the reflectivity of Ni/expanded graphite composite material is relatively small within the frequency range of 2–18 GHz, with a maximum reflection attenuation of only –6.3 dB, corresponding with 11 GHz matching band. The reason could be the thickness of the composite material is too small to form electronic conductive net, so there is less electromagnetic wave attenuation within the composites. When the matching thickness d > 1 mm, maximum reflection attenuation increases 0

Reflectance (dB)

–2 –4 1 mm

–6

4 mm

–8

3 mm

5 mm

–10 –12 –14 2

4

6

8

10 12 f(GHz)

14

16

18

Figure 4.13: Relationship between reflectance and frequency of the same powder with different thicknesses.

4.1 Expanded Graphite/Fe/Co/Ni Composite Materials and Their Absorbing Performance

203

with the matching thickness of the absorbing layer while the absorption peak shifts to lower frequency, and displays a narrowing trend; when d = 3 mm, the reflection attenuation reaches its maximum –15.4 dB at the 12 GHz frequency, the frequency range of reflection attenuation < –10 dB is up to 4.2 GHz; when d = 4 mm, the reflection attenuation reaches its maximum –15.9 dB at the 10.3 GHz frequency, the frequency range of reflection attenuation < – 10 dB, 3.1 GHz; when d = 5 mm, the reflection attenuation reaches its maximum –15 dB at the 9.5 GHz frequency, the frequency range of reflection attenuation < – 10 dB, 2.7 GHz. Thus, the matching thickness can affect the absorption of composites. According to theoretical simulation calculation, the best matching thickness d = 3 mm. According to the analysis of electromagnetic parameters and theoretical absorbing ability of the composites, to conduct chemical plating of Ni on the surface of expanded graphite has a significant influence on the magnetic loss of the composites, and can reinforce electromagnetic waves absorbing properties of the composites. (2) Reflectivity of the Ni/Expanded Graphite Matrix Composite with Different Contents Dosage. With the result of electromagnetic parameters of the Ni/expanded graphite matrix composite material and applying the absorbing wall formula, simulation calculation to the electromagnetic parameters with Ni/expanded graphite matrix composite material powder dosage wt15%, wt10%, and wt15%, thickness d = 3 mm was conducted, so that its reflection can be determined. The results are shown in Figure 4.14. As can be seen from Figure 4.14, the reflectivity of Ni/expanded graphite matrix composites appears only one peak maximum in the frequency range of 2–18 GHz. With frequency increasing, reflectivity increases first. Once reaching the maximum, it decreases with frequency decreasing. And with the increase of dosage of powder, the maximum reflectivity increases; the corresponding frequency to the maximum 0

Reflectance (dB)

–2 –4

wt5%

–6 –8

wt10%

–10 wt15%

–12 –14 2

4

6

8

10 f(GHz)

12

14

16

18

Figure 4.14: Relationship between reflectance and frequency of the same powder with different dosages.

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4 Application of lightweight carbon material and composite material

peak has no significant change; and the wave range increases when the absorbing frequency is below –10 dB. When the powder dosage is wt5%, the reflectivity of Ni/expanded graphite matrix composite is small in the range of 2–18 GHz, with the maximum reflection attenuation of –7.1 dB, corresponding with a matching frequency of 13 GHz. The reasons could be the dosage of composite powder is so small that the absorbent particles have little contact between each other to effectively form an electronic conductive net, and the electromagnetic wave attenuation is less in this composite. When the powder dosage is wt10%, reflection attenuation reaches its maximum –11.2 dB at 12.5 GHz, reflection attenuation < – 10 dB with a frequency range of 2.8 GHz. When the powder dosage is wt15%, reflection attenuation reaches its maximum –15.4 dB at 12 GHz, with the frequency range of reflection attenuation < – 10 dB up to 4.2 GHz. (3) Reflectance of Different Composites. With the result of electromagnetic parameters of different composite material, applying the absorbing wall formula, to conduct simulation calculation to the electromagnetic parameters with different composite material powder dosage wt15%, thickness d = 3 mm, so that its reflection can be determined. The results are shown in Figure 4.15. Figure 4.15 shows that expanded graphite composites with different metal powder plating with the same thickness d = 3 mm show different reflectance under simulation calculation as frequency changes. Wherein the Ni plating expanded graphite composites display the least absorbing effect, while Ni-Fe-Co/expanded graphite composite displays the best absorbing effect. The absorbing effects of NiCo/expanded graphite composite and Ni-Fe/expanded graphite composite are in the middle, with quite similar absorbing properties. Ni/expanded graphite composite material shows only one peak within the frequency range of 2–18 GHz, with the maximum reflection attenuation –15.4 dB at 0

Reflectance (dB)

–5 –10 Ni

–15 Ni-Fe

–20

NI-Fe-Co

Ni-Co

–25 –30 –35 2

4

6

8

10 f(GHz)

12

14

16

18

Figure 4.15: Relationship between reflectance and frequency of the same thickness with different material powder.

4.2 Expanded Graphite/Carbon Fiber/Electroconductive and Magnetic-permeable Metal

205

12 GHz. The frequency range corresponding to reflection attenuation < – 10 dB is up to 4.2 GHz. Ni-Co/expanded graphite composite shows two absorption peaks within the frequency range of 2–18 GHz, the higher one, –31.7 dB, at 9.8 GHz, and a lower one, –11.7 dB, at 12.5 GHz. The frequency range corresponding to reflection attenuation < – 10 dB is 5.2 GHz. Ni-Fe/expanded graphite composite shows three peaks in the frequency range of 2–18 GHz, with the maximum one, –27.8 dB, at 9 GHz; a lower one, –13.4 dB, at 12 GHz; another lower one, –7.8 dB, at 15 GHz. The frequency range corresponding to reflection attenuation < – 10 dB is up to 6.5 GHz. Ni-Fe-Co/expanded graphite composite shows one absorption peak in the range of 2–18 GHz, with the maximum value –28.8 dB at 13.4 GHz. The frequency range corresponding to the reflection attenuation < – 10 dB is up to 8 GHz. Although the maximum absorption ratio of Ni-Co/expanded graphite composite is bigger than that of the Ni-Fe-Co/expanded graphite matrix composites, its frequency range corresponding to the reflection attenuation < – 10 dB is much smaller. Considering a wider frequency range is more favorable to the absorbing materials, Ni-Fe-Co/expanded graphite composite is better in absorbing performance. Based on the above analysis, it can be drawn that the magnetic permeability and dielectric constant of Ni-Co-EG, Ni-Fe-EG and Ni-Fe-Co-EG increase significantly comparing with that of Ni-EG, with expanded frequency range of the maximum absorption peak, and improved absorbing performance. This indicates that the process for preparing the composite material should take both electronic conductive and magnetic properties into consideration. Therefore, according to the requirement, we can control the content dosage of the different magnetic metal coating for the regulation of the maximum absorption frequency. However, the magnetic loss and dielectric loss of Ni-Fe-Co-EG are large, so its absorption frequency range is wider. In short, it is feasible to prepare light absorbing material with wide absorbing frequency range with the method of metalizing the surface of EG with chemical plating, increasing electromagnetic loss of the composite to electromagnetic wave, and the maximum absorbing frequency range of the composite can be regulated according to different requirements.

4.2 Expanded Graphite/Carbon Fiber/ Electroconductive and Magnetic-permeable Metal Composite Materials and Their Absorbing Performance Magnetic metal/expanded graphite composite materials, as the previous chapter illustrated, have excellent microwave absorption at high frequency. To develop its microwave absorption at a lower frequency, further investigation about how to improve the electroconductivity of the composite material is of great necessity. A new magnetic microwave-absorptive material with elevated electroconductivity, light weight, and

206

4 Application of lightweight carbon material and composite material

broad absorption frequency spectrum was successfully fabricated. It was prepared by non-electrodepositing Ag and magnetic metal nanoparticles on the surface of expanded graphite/carbon fiber-based material which was fabricated by adopting minimum carbon fiber into shattered expanded graphite. Such a synthetic route helped Ag and magnetic metal nanoparticles to coat the lightweight and expanded graphite materials [19–46].

4.2.1 Fabrication of Expanded Graphite/Carbon Fiber/ Electroconductive and Magnetic-permeable Metal Composite Materials 4.2.1.1 Preparation of Electroplating Solution As the reaction activity of silver ammonia (AgNO3 ) is relatively large, it is easy to react with formaldehyde and other reducing agents even when silver-ammonia complex was formed under the effect of ammonia. Thus, AgNO3 solution and reducing solution was prepared separately and used immediately in case the electroplating solution failed because silver-ammonia complex was decomposed after a long time. Preparation of 500 ml of Tollens’ reagent: 17.5 g of AgNO3 was dissolved in 300 ml of deionized water (marked as solution-1). 100 ml of ammonia was slowly added into solution-1 under stirring after AgNO3 was dissolved completely. The solution became feculent and then turned to transparent. Thus, solution-2 was obtained. 7.0 g NaOH was dissolved in another 100 ml deionized water to prepare solution-3 after it was completely dissolved and was cooled to room temperature to avoid the heat effecting the determination of pH value. Solution-3 was then added to solution-2 to elevate the pH value. 500 ml of reducing solution: 11 ml of formaldehyde was diluted to 500 mL by deionized water. 4.2.1.2 Ag Plating Pretreated expanded graphite/carbon fiber based material was added into reducing agent under stirring dispersion for 5 min. The beaker must be washed with dilute nitric acid to prevent metal from adsorbing on the wall of beaker. Tollens’ reagent was dropped into reducing agent under mechanical agitation. The position of dropping cannot be too close to the beaker wall, because the main salt concentration would become too high in the dropping point to cause the decomposition and produce a silver mirror. If there was a silver mirror phenomenon, another clean beaker was needed for electroplating again. The raising temperature would accelerate the reaction, but prone to cause the decomposition of chemical silver plating liquid at the same time. Therefore, the reaction should be conducted at room temperature under mechanical agitation, rather than

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ultrasonic oscillation which will raise the temperature up. Moreover, the speed of stirring should be controlled as well. Expanded graphite could not completely disperse at a too slow rate of stirring, and was easy to float on the surface of the plating solution. When there is a silver mirror reaction process on the beaker wall, the stirring speed should be appropriately increased. However, overfast stirring speed could cause the plating solution splashing out of the beaker. 4.2.1.3 Nickel, Iron, and Cobalt Plating of Silver Plated Expanded Graphite-Based Material Electroplate nickel, iron, and cobalt on the surface of silver plated expanded graphite based material. Because of the metal catalytic activity of the silver layer, there is no need for pretreatment process. The formula and methods of nickel, iron, and cobalt plating are as the same as the previous section. As the silver coating has catalytic activity, the nickel, iron, and cobalt plating react more acute. Plating solution would turn to light green quickly from dark green with obvious bubbles (hydrogen). 4.2.1.4 Silver Plating of Nickel, Iron, and Cobalt Plated Expanded Graphite Based-Material Electroplate silver on the surface of nickel, iron, and cobalt plated expanded graphite based material. Because of the metal catalytic activity of the nickel, iron, and cobalt layer, there is no need for pretreatment process. The formula and methods of chemical silver plating are as the same as the previous section.

4.2.2 Structure Characterization of Expanded Graphite/Carbon Fiber/Electroconductive and Magnetic-permeable Metal Composite Materials 4.2.2.1 SEM Research of Ag/Magnetic Metal/EG Composite Material Scanning electron microscope (SEM, VEGA\\XMU, TESCAN) was used to characterize the microstructure of expanded graphite before and after chemical plating, as shown in Figure 4.16. As shown in Figure 4.16(a–c), expanded graphite is a wormlike material with great specific surface area due to its rough, porous, and curled up surface. It can be seen that the surface of expanded graphite and doped carbon fiber were evenly coated with a layer of gray matter continuously and smoothly. There are some spherical materials on the surface because the expanded graphite obtained a bad activation effect in the early stage of the pretreatment. Then a small number of expanded graphite process a big surface curvature, leading plating metal to deposit in the form of spherical particles as a result. The plated silver particle size is about 90–100 nm, while cobalt nickel iron particle size is about 60–70 nm. The coating thickness is about 70–150 nm, which

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4 Application of lightweight carbon material and composite material

a

d

b

c

e

f

g

h

i

j

k

l

m

n

o

Figure 4.16: SEM images of expanded graphite before and after Ag/magnetic metal plating (a–c) Expanded graphite before metal plating. (d–f) Ag plated expanded graphite. (g–i) Nickel, iron, and cobalt plated expanded graphite/carbon fiber. (j) Ag plated expanded graphite plated by nickel, iron, and cobalt. (k–l) Nickel, iron, and cobalt plated expanded graphite plated by Ag. (m) Expanded graphite/carbon fiber composite material before metal plating. (n–o) Metal plated expanded graphite/carbon fiber composite material.

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belongs to nanoscale coating. As shown in Figure 4.16 (k–o), no single metal layer was observed in the composite metal layer, and the surface is smooth and continuous, so different size of the particles on the surface are easy to aggregate. It can be seen from Figure 4.16 (m) that before plating expanded graphite mixed evenly with carbon fiber that would improve the conductivity of the composite materials. As shown in Figure 4.16 (n, o), after metal plating expanded graphite/carbon fiber composite material was coated with a layer of gray matter uniformly and smoothly. 4.2.2.2 EDS Analysis of Ag/Magnetic Metal/EG Composite Material To analyze the component and content of metal coated expanded graphite quantitatively, energy dispersive spectrometer (EDS, VEGA\\XMU, Czech TESCAN company) was investigated on expanded graphite after and before chemical plating, and the result is shown in Figure 4.17. As shown in Figure 4.17(a), relatively pure expanded graphite before plating contains (c) only, and does not contain other impurities. Figure 4.17(b–e) shows the spectrum of silver, cobalt nickel iron, cobalt nickel iron/silver, and silver/cobalt nickel iron plated expanded graphite respectively. It can be concluded that the carbon fiber doped expanded graphite surface is plated with metal layer as expected. Coating metal contains cobalt, nickel, iron, and phosphorus, and phosphorus was from reducing agent (sodium phosphite). For silver plating metals, the mass fraction of Ag is 92.10%. For nickel iron cobalt plating, the mass fraction of Ni, Fe, Co, and P was 39.26%, 16.45%, 24.14%, and 16.45%, respectively. For nickel iron cobalt/silver plating, the mass fraction of Ni, Fe, Co, Ag, and P was 31.76%, 8.48%, 10.62%, 8.48%, and 3.71%, respectively. And for Ag/cobalt nickel iron plating, the mass fraction of Ni, Fe, Co, Ag, and P was 24.26%, 5.75%, 8.32%, 35.26%, and 3.66%, respectively. According to the metal content of compound plating, the expanded graphite with cobalt nickel (a)

(b)

(c)

(d)

(e)

(f)

Figure 4.17: EDS spectra of expanded graphite after and before plating (a) Expanded graphite before plating. (b) Ag plated expanded graphite. (c) Nickel-iron-cobait plated expanded graphite. (d) Nickel iron cobalt/Ag plated expanded graphite/carbon fiber. (e) Ag/Nickel-iron-cobait plated expanded graphite/carbon fiber. (f) Nickel-iron-cobait plated expanded graphite/carbon fiber.

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iron plated lining and silver outer layer has a higher silver content and a lower cobalt nickel iron content than that with a silver lining and nickel iron cobalt outer layer. This is because the inner metal provides catalytic activity for outer metal chemical plating, so that the quality of the inner metal decreased. Figure 4.17(f) is the EDS spectrum of Nickel-iron-cobait plated expanded graphite. By the comparison between Figure 4.17(c) and (f), the pure expanded graphite and expanded graphite/carbon fiber was plated by a metal layer on the surface as expected, but because of the existence of the carbon fiber, the composition and mass fraction of coating metal will be different to some extent. 4.2.2.3 XRD Analysis of Ag/Magnetic Metal – EG Composite Material X-ray diffraction (XRD, X, Pert PRO-MPD, PANalytical) was used to analyze the organization of expanded graphite before and after chemical plating, and the result is shown in Figure 4.18. As shown in Figure 4.18(a), peaks of 2( at 26.56∘ , 44.26∘ and 54.7∘ were produced by expanded graphite’s hexagonal graphite structure. In order to clearly show the diffraction peaks at 44.26∘ and 54.7∘ of expanded graphite and other metal phase, the

(a)

(b)

(c)

(d)

(f)

(e)

(g)

Figure 4.18: XRD pattern of expanded graphite before and after metal plating (a–b) expanded graphite. (c) Ag plated expanded graphite. (d) Nickel iron cobalt plated expanded graphite. (e) Nickel iron cobalt/Ag plated expanded graphite or Ag/nickel iron cobalt plated expanded graphite. (f–g) Expanded graphite/carbon fiber before and after nickel iron cobalt plating.

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strongest diffraction peak of expanded graphite (26.56∘ ) is not shown in Figure 4.18(b) and other metal material’s diagram. It can be seen from Figure 4.18(c) that the diffraction peak of expanded graphite at 26.56∘ decreased its intensity obviously, while the intensity of peaks at 44.26∘ and 54.7∘ is significantly enhanced. In addition to the diffraction peaks of the expanded graphite, there are two raised up sharp diffraction peaks. The plated Ag shell, which diffraction peaks correspond to Ag element, is proved to have formed as crystalline elemental Ag. As shown in Figure 4.18(d), Nickeliron-cobait plated expanded graphite exhibit a bread peak between 40∘ and 50∘ in its XRD spectrum, indicating that Ni-Fe-Co-P plating layer was in amorphous state. After calibration, the plating layer was proved to be Ni3 P, elemental Fe, and elemental Co. According to Figure 4.18(e), the main diffraction peaks in two XRD patterns of plated expanded graphite/carbon fiber were silver’s diffraction peak, and the amorphous cobalt nickel iron’s peak is neglected, which is proved to be elemental Ni3 P, elemental Ag, elemental Fe, and elemental Co. Figure 4.18(f) and (g) are XRD spectra of expanded graphite/carbon fiber before and after plating. By comparing the two figures, it can be seen that expanded graphite/carbon fiber after the chemical nickel iron cobalt plating have a mild gentle peak between 40∘ and 50∘ , indicating the existence of amorphous Ni-Fe-Co-P plated layer, which is proved to be amorphous diffraction peaks of Ni3 P, Fe and Co after calibration.

4.2.3 Characterization of Ag/Magnetic Metal/EG Composite Material’s Magnetic Property The vibrating sample magnetometer (CDJ – 7400) is used to test the magnetic property of the expanded graphite before and after plating metal, and the result is shown in Figure 4.19. The initial permeability of hysteresis loop can indicate the induction speed toward external magnetic field of this powder, which is illustrated as the sensitive degree to electromagnetic waves of the powder too. The greater the initial permeability, the powder can absorb more external electric field in the starting stage. The rectangular area of hysteresis loop is closely related to the absorbed energy per unit volume of the powder. Soft magnetic properties are sensitive to the fluctuated outside electromagnetic field loss because of the advantages of fast demagnetization. It can be seen from Figure 4.19(a) that the expanded graphite after cobalt iron nickel plated has excellent soft magnetic properties. Figure 4.19(b) shows that compared to iron cobalt and nickel plated expanded graphite, the coercive force (Hc) of cobalt nickel iron/silver plated expanded graphite is 0.9 kOe with good stability, but the saturation magnetization is about 6 emu/g, which is much smaller than iron cobalt and nickel plated expanded graphite. This is due to the decline of cobalt nickel iron in iron cobalt nickel/silver plated expanded graphite. For silver/nickel iron cobalt plated expanded graphite, although the permeability of outer silver layer is 1 theoretically, the addition of silver

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4 Application of lightweight carbon material and composite material

Magnetization intensity (emu/g)

Magnetization intensity (emu/g)

10

10

–10

–5

5 Hc

–10

10 Magnetic field H/k0e

–5

5 Hc

–10

10

Magnetic field H/k0e

–10

(a)

(b)

Magnetization intensity (emu/g)

10

–10

–5

5 Hc

10

Magnetic field H/k0e

–10

(c) Figure 4.19: Hysteresis loop of expanded graphite plated with different metal. (a) Nickel iron cobalt plated expanded graphite. (b) Nickel iron cobalt/Ag plated expanded graphite. (c) Ag/nickel iron cobalt plated expanded graphite.

reduces the content percentage of cobalt nickel iron and has a certain influence on its magnetism performance. Further compared with Figure 4.19(c), both silver/nickel iron cobalt plated expanded graphite and cobalt iron nickel/silver plated expanded graphite have a coercive force of 0.9 kOe, but the latter has a 6.3 emu/g of saturated magnetization intensity, which is much higher than 2.4 emu/g of the former. This is because the max consumption of energy in the form of magnetic eddy current of the iron cobalt nickel/silver on the surface is higher than that of silver/nickel iron cobalt. In addition, the initial permeability of iron cobalt nickel/silver plated expanded graphite is better than silver/nickel iron cobalt plated expanded graphite, due to the effect of outer silver to the magnetism sensitivity of inner nickel iron cobalt layer. Initial permeability differences will affect the sensitivity of material to the electromagnetic wave magnetic eddy current loss. The magnetic detection results of silver/nickel iron cobalt plated expanded graphite are shown in Figure 4.19(c). The coercivity of this composite material is 0.9

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kOe, which is approximately the same as cobalt nickel iron plated expanded graphite. However, the saturated magnetization intensity is 2.4 emu/g, much less than that of iron cobalt nickel plated expanded graphite (10.2 emu/g). This indicates that the maximum consumption of energy in the form of eddy current of this powder is lower than that of cobalt nickel iron/beads composite powder. In addition, its initial permeability fell as well, suggesting that the influence of outer silver layer on the magnetic sensitivity of inner iron nickel layer. These hysteresis loops, however, can only show the magnetism function of cobalt nickel iron layer in composite materials. In electromagnetic wave shielding absorption, the addition of silver layer improved the conductivity of materials greatly which can combine magnetic eddy current loss with electrical shielding in electromagnetic wave shielding absorption and worked as a compound consumption.

4.2.4 Electromagnetic Spectrum of Ag/Magnetic Metal/EG Composite Material 4.2.4.1 Electromagnetic Spectrum of Expanded Graphite/Carbon Fiber Composite Material According to the physical conception of the complex permeability and the complex dielectric constant, the real component of %r and ,r (%󸀠 and ,󸀠 ) bared the energy storage function of the media on the electromagnetic wave. While the imaginary part (%󸀠󸀠 and ,󸀠󸀠 ) assumed the absorption function of electromagnetic wave. When the imaginary part is quiet smaller than the real part and even can be ignored, the material, the so-called electrowave-permeable media, cannot absorb electromagnetic waves. On the contrary, when the imaginary part cannot be ignored, this kind of medium has the ability to absorb electromagnetic waves. Considering the perspective of media on the electromagnetic wave absorption, the bigger the %󸀠󸀠 and ,󸀠󸀠 , the better. Dope expanded graphite with moderate carbon fiber (expanded graphite substrate) and mix it with paraffin which works as an adhesive. Complex permittivity and complex magnetic permeability are tested on composite material containing wt5%, wt10%, and wt15% of expanded graphite, and the results are shown in Figures 4.20–4.23. It can be seen from Figures 4.20 and 4.21 that both the real constant: %󸀠 and the imaginary part epsilon: %󸀠󸀠 of expanded graphite substrate’s epsilon dielectric increase with the increase of mass fraction in the mixture on the whole. On the range of 2–6 GHz, %󸀠 and %󸀠󸀠 of dielectric constant decreased by the increasing frequency, and on the range of 6–18 or 8–18 they fluctuate a lot around a certain value. From figure 3–7 and 3–8, the real part ,󸀠 and the imaginary part ,󸀠󸀠 of the expanded graphite’s permeability are constant with the increase of mass fraction in the mixture. From Figures 4.22 and 4.23, it can be seen that the imaginary part of expanded graphite substrate’s epsilon permittivity is pretty big, ranging from 0.7 to 3.8.

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4 Application of lightweight carbon material and composite material

Real part of dielectric constant ε′

10 c

8

a-5%

b-10%

c-15%

b 6

a

4 2

2

4

6

8

10

12

14

16

18

f(GHz)

Imaginary part of dielectric constant ε′′

Figure 4.20: %󸀠 - f frequency spectrogram of expanded graphite.

4

c

a-5%

b-10%

c-15%

b

3

a 2

1

2

4

6

8

10

12

14

16

18

f(GHz)

Real part of magnetic permeability μ′

Figure 4.21: %󸀠󸀠 - f frequency spectrogram of expanded graphite.

1.4

a-5%

b-10%

c-15%

1.2 a

1.0

b

c

0.0 0.6 0.4 0.2 2

4

6

8

10

12

14

16

f(GHz) Figure 4.22: ,󸀠 - f frequency spectrogram of expanded graphite.

18

Imaginary part of magnetic permeability μ′′

4.2 Expanded Graphite/Carbon Fiber/Electroconductive and Magnetic-permeable Metal

0.2

a-5%

b-10%

215

c-15%

0.1 a

b

c

0

0.1 2

4

6

8

10

12

14

16

18

f(GHz) Figure 4.23: ,󸀠󸀠 - f frequency spectrogram of expanded graphite.

As to the expanded graphite belongs to the inorganic nonmetallic materials, no magnetic permeability was observed, so its imaginary part ,󸀠󸀠 is zero. Thus, expanded graphite poses a quiet extend of dielectric loss but no magnetic loss on the electromagnetic wave. 4.2.4.2 Frequency Spectrogram of Ag/Expanded Graphite Composite Material The test of complex permittivity and complex magnetic permeability for Ag/expanded graphite composite material is conducted when the addition is wt15%, and the test result is shown in Figures 4.24 and 4.25. As Figure 4.24 shows, the real part of dielectric epsilon constant decreases within the increasing frequency ranging from 2 to 8 GHz, and it remains the value at the range of 8–18 GHz. The imaginary part of dielectric epsilon constant decreases with the increasing frequency ranging from 2 to 12 GHz, and

Dielectric constant - ε′ ε″

21 18

ε′

15 12 9 ε″

6 3 2

4

6

8

10 12 f(GHz)

14

16

18

Figure 4.24: Dielectric constant and frequency spectrum of Ag/expanded graphite composite material.

Magnetic permeability - μ′ μ″

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4 Application of lightweight carbon material and composite material

2.4 2.0 1.6 μ′

1.2 0.8 0.4

μ″

0 2

4

6

8

10

12

14

16

18

f(GHz) Figure 4.25: Permeability and frequency spectrum of Ag/expanded graphite composite material.

remains the same within 12 to 18 GHz. It can be seen from Figure 4.25 that the real and imaginary part of the permeability remain their value with the increase of frequency in the range of 2 to 18 GHz, during which period, the real part of permeability is 1, and the imaginary part is 0. In a whole, silver expanded graphite composite material has bigger dielectric constant than expanded graphite composite. Because the two materials are not no magnetic, their magnetic permeability is the same. 4.2.4.3 Electromagnetic Spectrum of Nickel Iron Cobalt-Expanded Graphite Composite Material The complex permittivity and complex magnetic permeability was tested since the addition amount of composite material is wt15% for the cobalt nickel iron/expanded graphite samples, and the results are shown in Figures 4.26 and 4.27.

Dielectric constant - ε′ ε′′

21 18 15 12 ε′

9 6 3

ε′′

2

4

6

8

10 12 f(GHz)

14

16

18

Figure 4.26: Dielectric constant and frequency spectrum of cobalt nickel iron/expanded graphite composite material.

Magnetic permeability - μ′ μ″

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4.8 4.0 3.2

μ′

2.4 1.6

μ″

0.8 0 2

4

6

8

10 12 f(GHz)

14

16

18

Figure 4.27: Permeability and frequency spectrum of cobalt nickel iron/expanded graphite composite material.

As shown in Figure 4.26, the real part of dielectric epsilon constant increases first and then decreases with the increase of frequency, and eventually tends to be about 9. It can be seen from Figure 4.27 that the real and imaginary part of permeability decreases with the increasing frequency overall, and finally tends to be a constant value. Comparing Figures 4.24, 4.25, 4.26, and 4.27, it can be seen that cobalt nickel iron/expanded graphite has a much smaller dielectric constant than silver/expanded graphite composite material, indicating the excellent conductivity of Ag, but in terms of magnetic loss, cobalt nickel iron/expanded graphite performed much better than silver/expanded graphite composite material.

4.2.4.4 Electromagnetic Spectrum of Cobalt Nickel Iron/Ag/Expanded Graphite Composite Material The complex permittivity and complex magnetic permeability are tested since the addition amount of composite material is wt15% for the cobalt nickel iron/Ag/ expanded graphite samples, and the results are shown in Figures 4.28 and 4.29. As shown in Figure 4.28, the real part of dielectric epsilon constant increases first and then decreases with the increase of frequency, and eventually tends to be about 10. It can be seen from Figure 4.27 that the real and imaginary part of permeability decreases with the increasing frequency overall, and finally tends to be a constant value. Comparing Figures 4.26, 4.27, 4.28, and 4.29, it can be seen that cobalt nickel iron/Ag/expanded graphite has a much bigger dielectric constant than cobalt nickel iron/expanded graphite composite material, indicating the excellent conductivity of Ag, but in terms of magnetic loss, cobalt nickel iron/expanded graphite has a smaller value than silver/expanded graphite composite material.

Magnetic permeability - μ′ μ″

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4 Application of lightweight carbon material and composite material

4.8 4.0 μ′

3.2 2.4 1.6

μ″

0.8 0 2

4

6

8

10

12

14

16

18

f(GHz) Figure 4.28: Dielectric constant and frequency spectrum of cobalt nickel iron/Ag/expanded graphite composite material.

Dielectric constant ε′ ε″

30 25

ε′

20 15 10 ε″

5 0 –5 2

4

6

8

10

12

14

16

18

f(GHz) Figure 4.29: Permeability and frequency spectrum of cobalt nickel iron/Ag/expanded graphite composite material.

4.2.4.5 Electromagnetic Spectrum of Ag/Cobalt Nickel Iron/Expanded Graphite Composite Material The complex permittivity and complex magnetic permeability was tested since the addition amount of composite material is wt15% for the Ag/cobalt nickel iron/expanded graphite samples, and the results are shown in Figures 4.30 and 4.31. As shown in Figure 4.30, the real part of dielectric epsilon constant increases first and then decreases with the increase of frequency, and eventually tends to be

4.2 Expanded Graphite/Carbon Fiber/Electroconductive and Magnetic-permeable Metal

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Dielectric constant ε′ ε″

30 25 20 15

ε′

10 5

ε″

0 –5 2

4

6

8

10

12

14

16

18

f(GHz) Figure 4.30: Dielectric constant and frequency spectrum of Ag/cobalt nickel iron/expanded graphite composite material.

Magnetic permeability μ′ μ″

2.4 2.0 μ′

1.6 1.2

μ″

0.8 0.4 0 2

4

6

8

10

12

14

16

18

f(GHz) Figure 4.31: Permeability and frequency spectrum of Ag/cobalt nickel iron/expanded graphite composite material.

about 10. It can be seen from Figure 4.31 that the real and imaginary part of permeability decreases with the increasing frequency overall, and finally tends to be a constant value. Comparing Figures 4.28, 4.29, 4.30, and 4.31, it can be seen that Ag/cobalt nickel iron/expanded graphite has a much bigger dielectric constant than cobalt nickel iron/Ag/expanded graphite composite material, but in terms of magnetic loss, it has a smaller value than cobalt nickel iron/Ag/expanded graphite composite material. Herein, the magnetic property of composites may decrease when the magnetic metal is coated in the lining of the plating shell, but as a result of the action of Ag, its conductivity will increase somehow.

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4 Application of lightweight carbon material and composite material

To sum up, in a series of expanded graphite composite materials, the introduction of magnetic metal increases the permeability of composite materials, but do not influence their dielectric constant. The introduction of Ag increases the dielectric constant of the composite materials. And the order of Ag plating has impact on the permeability of composite material. For example, when the plated Ag is in the outer layer and the magnetic metal is in the inner layer, the permeability of the composite would be reduced.

4.2.4.6 Different Reflectivity of the Composite Materials Based on electromagnetic parameters of Ag/expanded graphite, cobalt nickel iron/expanded graphite, cobalt nickel iron/Ag/expanded graphite and Ag/cobalt nickel iron/expanded graphite composite materials, the theoretical reflectivity of different composite powder with wt15% addition in d = 3 mm thickness could be calculated according to the formula of electromagnetic wave absorbing screens, and the calculation results are shown in Figure 4.32. It can be seen that different composite materials in the same thickness of d = 3 mm exhibit different simulation calculated reflectivity with the change of frequency curve. Expanded graphite has a peak value in the range of 2–18 GHz, and reach a maximum reflection loss of 5.8 dB at 9.9 GHz. Ag/expanded graphite composite materials exhibit an absorption peak at 2–18 GHz,

0 EG –5

Reflectance (dB)

–10

NiFeCo—EG

–15

NiFeCo/Ag—EG

–20

Ag—EG

Ag/NiFeCo—EG

–25

–30 2

4

6

8

10

12

14

16

18

f(GHz) Figure 4.32: The reflectivity of different composite materials in the same thickness related to frequency.

4.3 Chemical Plating Polyaniline/Expanded Graphite Metal Composite Materials

221

and reach a maximum reflection loss of 26.7 dB in the 14.0 GHz with a 2.1 GHz of bandwidth of reflection loss smaller than –10 dB. Cobalt nickel iron/expanded graphite composite materials exhibit an absorption peak at 2–18 GHz, and reach a maximum reflection loss of –16.5 dB in the 8.9 GHz with a 3.7 GHz of bandwidth of reflection loss smaller than –10 dB. Cobalt nickel iron/Ag/expanded graphite composite materials exhibit an absorption peak at 2–18 GHz, and reach a maximum reflection loss of –18.0 dB in the 10.4 GHz with a 3.5 GHz of bandwidth of reflection loss smaller than –10 dB. Ag/cobalt nickel iron/expanded graphite composite materials exhibit an absorption peak at 2–18 GHz, and reach a maximum reflection loss of –21.2 dB in the 12.5 GHz with a 2.5 GHz of bandwidth of reflection loss smaller than –10 dB. It can be learned from the above analysis that the addition of Ag increases the dielectric loss of Ag/expanded graphite composite materials, with the maximum absorption peaks move to high frequency. The coating order of Ag plating in composite materials may also affect the absorbing properties of Ag/Fe/Co/Ni/expanded graphite composite materials. The best absorbing effect appear in high frequency when Ag plating is in the outer layer. And the maximum absorption peak was observed in the low frequency when magnetic metal is coated in the outer layer. This phenomenon is corresponding to the analytic result of the electromagnetic spectrum. It can be concluded that the addition of Ag can improve the electrical conductivity of the composite, and increase the dielectric loss, so the absorbing composites at high frequencies was improved. Therefore, the process conditions of absorbing materials can be well designed as required, and so does the structure of absorbing coatings, to achieve full frequency coverage and improving effect of the critical frequency.

4.3 Chemical Plating Polyaniline/Expanded Graphite Metal Composite Materials and Their Absorbing Performance By use of the electrical and magnetic conductivity of polyaniline, and the characteristic that the electrical conductivity can be adjusted by adulteration, recombine the expanded graphite/magnetic metal and macromolecule polyaniline to adjust the wave absorbing frequency band and strength of the material and improve the machinability of material and the adjustability of electromagnetic parameters, enabling the composite material to meet the requirements for both dielectric loss and magnetic loss. Meanwhile, the optimal matching between the complex dielectric constant and complex magnetic permeability can be achieved, thus the absorbing performance of material and the absorbing frequency bandwidth can be improved, satisfying the feature requirements of “being thin, light, strong and wide.” Then expanded graphite matrix composite absorbing material with better performance can be produced. In recent decades, the study on synthesis and properties of conductive polymer has become an important area of polymer and material science. Among the polymers,

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4 Application of lightweight carbon material and composite material

polyaniline (PANI), due to its advantages, such as structural diversity, unique doping mechanism, good stability, and cheap raw material, has been widely used in the fields of energy (second battery), electronics, light emitting diode (LED), sensor, newly fashioned electromagnetic shielding and absorbing materials, etc. The polyaniline has become a hotspot of research on polymer, and one of the most promising conductive polymer materials [47–51]. The conductivity of eigenstate polystyrene gel is extremely low, demonstrating the feature of electrical insulation. But through the doping of proton acid into polyaniline, the transition from insulator to semiconductor and even conductor can be achieved; therefore, doping is an effective way to endow polyaniline conductivity. Due to its higher melting point and boiling point, as far as environmental stability is concerned, the organic macromolecular sulfonic acid is better than the inorganic acid. The organic macromolecular sulfonic acid contains both nonpolar and polar groups, making the produced polyaniline have high conductivity and its solubility greatly improved. In this study, the emulsion polymerization is adopted, in which the organic modification of inorganic substance, the doping of macromolecular proton acid, and the polymerization of monomers are conducted simultaneously to produce the polyaniline/expanded graphite composite material with high conductivity, thus simplifying the experimental steps and reducing the reaction cost. Meanwhile, through the method of chemical plating, a series of metal/expanded graphite/polyaniline composite materials can be produced.

4.3.1 Preparation of Polyaniline In the experimental apparatus shown in Figure 4.33, mix a certain amount of sulfosalicylic acid, aniline monomer and deionized water and stir them at a certain speed for 30 min to mix them into a homogeneous white emulsion. Then drop slowly a certain

1

3 2 4

5

Figure 4.33: Experimental apparatus. (1) retort stand, (2) thermometer, (3) mechanical stirre, (4) condenser pipe, (5) water bath.

4.3 Chemical Plating Polyaniline/Expanded Graphite Metal Composite Materials

223

quality of initiator, ammonium persulfate solution, into the homogeneous emulsion within a controlled period of 2 h. After a period of reaction under the designed temperature, the color of the reaction system changes from khaki, to light green, to green, and to stable dark green. During the whole experimental process, nitrogen should be always ventilated. After the reaction is over, keep the emulsion until it layers, pour away the upper reddish solution, and conduct repeatedly vacuum suction filter. Wash the remaining emulsion with sulfosalicylic acid solution (PH < 3) and deionized water (PH < 3) in turn until it is neutral. Conduct vacuum drying 24 h under the temperature of 80∘ C. Grind with a glass grinding bowl and then conductive doping polyaniline can be obtained.

4.3.2 Preparation of Polyaniline/Expanded Graphite Composite Materials Take a certain amount of expanded graphite with different volumes and put them into sulfosalicylic acid solution respectively by means of ultrasonic dispersion for 30 min. Then add a certain amount of aniline monomer into the above solutions and stir them at a certain speed for 30 min to mix them into a homogeneous white emulsion. Then drop slowly a certain amount of ammonium persulfate solution into the homogeneous emulsion within a controlled period of 2 h. During the 12 h reaction period, the color of the reaction system changes from khaki, to light green, to green, and to stable dark green. After the reaction is over, stand the emulsion until it layers, and conduct vacuum suction filter. Wash the left emulsion with sulfosalicylic acid solution (PHCo>Fe>P. Without changing the stability of plating solution, EDS detection is processed on plating with different main salt concentration ratios. It can be found that the proportion ranges for different elements in the plating are Ni 60–75%, Co 8–20%, Fe 3–10%. The change of structural components in the plating may lead to the change of its property, so CF can be applied to various needs.

5.2.3.3 X-ray Diffraction Analysis of the Plating Organization analysis is carried on CF both before and after chemical sedimentation by Bruker-Axs D2 X-ray diffractometer from Brook Axs Company in Germany. The result is shown in Figure 5.7. Figure 5.7(a) shows that before chemical plating, CF has a diffraction peak like “the peak of steamed buns” between 20∘ and 30∘ , which is called dispersion peak. It can be seen from Figure 5.7(b) that within the whole scope, Ni-Co-Fe alloy only has one dispersion peak of Ni at 45∘ . Compared with other characteristic peaks, this one is narrower and relatively powerful. It is not sharp enough and covers a wide range. This illustrates that Ni-Fe-Co alloy plating is amorphous. But part of the plating has the tendency to become microcrystalline whose particles are tiny or incomplete with a poor crystallinity.

308

5 Application of lightweight carbon materials and composites in protective materials

5000

Intensity (a.u)

4000

3000

2000

1000

0 10

20

30

(a)

40

50

60

70

80

90

2θ (degree) Fe

Ni Co

3000 2800 2600

Intensity (a.u)

2400 2200 2000 1800 1600 1400 1200 1000 10 (b)

20

30

40 50 2θ (degree)

60

70

80

90

Figure 5.7: XRD photographs of CF (a) before and (b) after chemical plating.

5.2.3.4 Analysis of Magnetic Performance CF is soaked in saltpeter solution for two hours and then gets dried. VSM test is performed on CF by CDJ-7400 VSM vibrating sample magnetometer at room temperature. The result is shown in Figure 5.8. It indicates that CF has no magnetism.

5.2 Study on the Preparation and Electromagnetic Shielding Performance

309

0.08 0.06

M (emu/g)

0.04 0.02 0.00 –0.02 –0.04 –0.06 –20000

–10000

0 H (Oe)

10000

20000

0

10000

20000

Figure 5.8: Hysteresis loop of CF.

0.3

M (emu/g)

0.2 0.1 0.0 –0.1 –0.2 –0.3 –20000

–10000

H (Oe) Figure 5.9: Hysteresis loop of CF after Ni-Co-Fe-P plating.

Figure 5.9 shows the hysteresis loop obtained from the test on CF after Ni-CoFe-P chemical plating by CDJ-7400 VSM vibrating sample magnetometer at room temperature. It can be concluded that after Ni-Co-F chemical plating, the coercive force of CF is 42 Oe and residual magnetism is 0.025 emu/g. When the maximum magnetic field strength reaches 20,000 Tesla, the saturation flux density is 0.23 emu/g. 5.2.3.5 Surface Resistance The curve in Figure 5.10 shows the relationship between weight gain rate and the average value of area resistance. It can be seen that with the increase of the weight gain rate of CF, its average value of area resistance declines. When there is no plating

310

5 Application of lightweight carbon materials and composites in protective materials

Average value of area resistance

22 20 18 16 14 12 20 8 6 4 0

50

100

150

200

250

The weight gain rate (%) Figure 5.10: Relationship between weight gain rate and the average value of area resistance.

on the surface of CF, the average value of area resistance can be as high as 21.7K. When the weight gain rate reaches 100%, the average value of area resistance declines to 9.4K. Then the decline of area resistance value becomes slower. As the weight gain rate reaches 210%, the average value of area resistance is 5.2K. In conclusion, though there are many factors that influence the area resistance of CF with chemical plating, such as the unevenness of the metal plating and the incompleteness of chemical plating reaction, the thickness of plating has a relatively greater influence on the conductivity of CF. 5.2.3.6 Analysis of Electromagnetic Shielding Performance The electromagnetic shielding performance depends on the shield’s absorbing ability and reflectivity to electromagnetic wave. Besides the area resistance of the material, the reflective performance also changes with different types of source radiation and distance between the shield and the source of radiation, while the performance of absorbing is influenced by the electrical and magnetic conductivity of the shield material and the thickness of plating. Shielded room method is adopted to test CF cloth with Ni-Co-Fe-P chemical plating within 20–1,500 MHz. As seams exist after CFs are made into cloth, the electromagnetic performance will be greatly influenced. In order to make up for this disadvantage, three pieces of CF cloth are put at the gap of the shielding box during the test and one-third of the result is regarded as the electromagnetic shielding performance of CF cloth. Figure 5.11 shows the electromagnetic shielding performance curves of CF cloth when the weight gain rates are 50%, 77%, 100%, 140%, and 210%, respectively.

5.2 Study on the Preparation and Electromagnetic Shielding Performance

A B C D E

70 65 Shielding property (dB)

311

60 55 50 45 40 35 30 25 0

200

400

600

800

1000

1200

1400

1600

f (MHz) Figure 5.11: Electromagnetic shielding performance of CF with different weight gain rates within 20– 1,500 MHz.

This figure illustrates that the electromagnetic shielding performance of CF improves with the increase of weight gain rate. When weight gain rate is 210%, the average electromagnetic shielding performance may reach 55 dB and can even be as high as 67 dB. The electromagnetic shielding performance of CF with lower weight gain rate can be about 35 dB. The electromagnetic shielding performance weakens with the increase of frequency. The reason might be that at low frequency wave band, CF with Ni-Co-Fe chemical plating has good magnetic performance and electrical conductivity with a high wastage of electromagnetic wave. Its shielding performance is excellent. In conclusion, CF prepared in this experiment has electromagnetic shielding performance at 20–1,500 MHz wave band. In order to further improve its electromagnetic shielding performance, we can mix CF or CF cloth with Ni-Co-Fe-P chemical plating with other materials to make advanced composite material. In this way, the seams’ influence on the electromagnetic shielding performance will be reduced to the most extent.

5.2.4 Study on the Effect of RE Elements Ce and La on the Surface Modification of Carbon Fiber Electroless Plating In recent years, the addition of rare earth metal in new functional materials has been widely used in the preparation and modification of a variety of materials. The rare earth metal does not only have a great activity, but also has good magnetic properties. It can influence the electromagnetic radiation. Therefore, it has a broad application prospect in the field of electromagnetic shielding.

312

5 Application of lightweight carbon materials and composites in protective materials

Stability of plating solution min

200

Ce La

180 160 140 120 100 80 60 40 20 0 0.0

0.2

0.4 0.6 Density of RE elements (s/L)

0.8

1.0

Figure 5.12: Effects of the density of RE elements on the stability of plating solution.

5.2.4.1 The Effect of RE Elements Ce and La on Electroless Plating (1) Effects on the stability of plating solution. Prepare the chemical plating solution of RE elements Ce and La, respectively, with the density of 0.1 g/L, 0.2 g/L, 0.4 g/L, 0.6 g/L, 0.8 g/L, and 1.0 g/L to take a test of the stability of plating solution. Based on Figure 5.12, it can be concluded that the stability of plating solution first increases and then decreases with the gradual rising of the cerium nitrate density. When the addition of element Ce reaches 0.6 g/L, the stabilization time of plating solution mounts up to its utmost, obviously 180 min. Afterward, the stability of the solution declines. In general, the addition of RE element Ce will significantly improve the stability of the solution. The effect of RE element La on the stability of plating solution is not that obvious. With an increase in the density of the RE La, the stability of the solution gradually decreases. Reasons why the stability of the solution can be improved are as follows: on the one hand, their intersolubility with elements Fe, Co, and Ni will be increased and the formation of impurities and particles will be repressed while the activity of nonmetallic elements mixed, such as S and N, will be reduced after the addition of RE elements into the solution, which would restrain plating solution’s tendency of spontaneous decomposition; on the other hand, complex compounds such as RECl+2 and [RE(H2 O)n ]3+ will be formed in the solution with the participation of RE ion, which will further promote the equilibrium dissociation of metallic ion in the solution and strengthen the stability of the solution in the meantime. (2) Effects on the sedimentation rate. Figure 5.13 presents the relationship between RE elements Ce, La and sedimentation velocity. As can been seen, the deposition rate shows an overall declining tendency with the addition of RE element La. When the addition of element La reaches 0.1–0.2 g/L, the declining deposition rate is dramatic.

5.2 Study on the Preparation and Electromagnetic Shielding Performance

313

Ce La

22

Planning rate (me/cm2–h)

20 18 16 14 12 10 8 6 4 0.0

0.2

0.4 0.6 Concentrations of RE (g/L)

0.8

1.0

Figure 5.13: Effects of RE on the sedimentation rate of chemical Ni-Co-Fe-P alloy plating.

And then the dropping tendency of it slows down after the addition exceeds 0.2 g/L. In the meantime, it is discovered that the addition of RE element La adds much glossiness to the plating metals, reduces the deposition of plating solution, and enables the solution to be purer. When the concentration of element Ce is comparatively low, the deposition rate gradually grows with the addition of RE element Ce. And while it comes up to 0.6 g/L, the deposition rate will reach its maximum. Then with the continual increase in the content of element Ce, the plating rate decreases with great obviousness. It is probably that gel-like hydroxides are formed under alkaline conditions with an increase in the addition of element Ce, which consequently leads to large deposits of Ni2+ , Co2+ , Fe2+ in the form of complex compound and the lowering of deposition rate in the experiment. While the 4f electron of RE elements does not tightly seal up its nuclei, its adsorption capacity is comparatively high. After the RE elements are added into the solution, they will then be attached to places with crystal defects such as vacancies, dislocation outcrops, and grain boundaries, reducing the surface energy of the substrate. As a result, the nucleation rate of the plating will be remarkably boosted and the sedimentation velocity accelerated. In addition, part of those RE elements will form positive ions, accelerating the reduction of metal ions. At the same time, not only can RE elements increase their intersolubility with transition group Ni, Co, and Fe, but also they can consume part of the dentate and enlarge the potential difference between the density of free metal ions and the interface via formation of complex compounds. In this way the transition of reactant ions could be promoted and the sedimentation rate increased.

314

(a)

(c)

5 Application of lightweight carbon materials and composites in protective materials

(b)

(d)

Figure 5.14: Effects of RE element Ce on the surface morphology of chemical plating Ni-Co-Fe-P alloy. (a) Without Ce, (b) Ce 0.4g/L, (c) Ce 0.8g/L, (d) Ce 0.4g/L.

5.2.4.2 Plating Characterization and Performance Test of RE Co-deposition Alloy (1) Effects of RE elements on surface morphology of the plating. While the RE elements are adhered to the surface of the substrate, they could lower surface energy and critical nucleation energy, and refine crystal particles. Thus, it is necessary to probe into the influence of RE elements on the surface morphology of the plating. Figure 5.14 is the SEM image of CF chemical Ni-Co-Fe-P plating with the addition of RE element Ce in different portions. As can be seen from Graph-a, the surface plating of CF is comparatively rough and covered with plenty of nodular-shaped bulges. This is, as a matter of fact, the result of a large amount of hydrogen evolution during the plating process and the change of plating’s internal stress caused by high phosphorus content there. Graph-b is the SEM image of the Ce additive amount of 0.4 g/L. Compared with the one in which no Ce is added, the plating here is relatively smooth and the number of nodular-shaped bulges decreases as well. Graph-c is the SEM image of Ce additive amount of 0.8 g/L, from

5.2 Study on the Preparation and Electromagnetic Shielding Performance

(a)

(c)

315

(b)

(d)

Figure 5.15: Effects of RE element La on the surface morphology of chemical Ni-Co-Fe-P alloy plating. (a) Without La, (b) La 0.4g/L, (c) La 0.8g/L, (d) La 0.8g/L.

which it could be seen that the plating is much finer and smoother and barely any projections could be detected. Graph-d is the after-plating CF fractography of the Ce additive amount of 0.4 g/L, which evidently shows that the thickness of CF surface plating is about 1 ,m. By making a comparison between Graph-b and Graph-c, we could discover that the plating thickness of 0.4 g/L element Ce exceeds that of 0.8 g/L element Ce, which is consistent with the effect of RE element Ce on the sedimentation velocity. Figure 5.15 is the SEM image of CF chemical Ni-Co-Fe-P plating with the addition of RE element La in different portions. With the gradual addition of RE element La, the number of nodular-shaped projections and pits on the plating surface rapidly decreases and particles become finer and finer. The coating surface turns smoother and smoother as well, and moreover, it shows much more brilliance based on gross examination. As we can see from Graph-d, the thickness of the plating is about 0.5 ,m, which is significantly reduced compared with the thickness of the plating before the addition of RE element La.

316

5 Application of lightweight carbon materials and composites in protective materials

In conclusion, the addition of RE elements into the plating solution could dramatically improve the quality of plating and enable the plating to take on much metallic luster. This is principally due to the reason that RE elements could suppress the grain boundary migration through the reduction of the grain boundary energy, inhibiting the growth of crystal particles and enabling the plating to be denser and denser during the thickening process. (2) Effects of RE elements on plating composition. RE can achieve its selfreduction and the positive shift of the potential under the influence of complexing agent and accomplish its co-deposition with Ni, Fe, and Co under the combined effect of elements Ni, Fe, and Co. While the difference lying between the atomic radius of RE and that of Ni, Co, and Fe is relatively large, the integrating amount of RE is quite slight. Figure 5.16 is the EDS spectrogram of RE elements’ effect on the plating composition with the RE additive amount of 0.6 g/L. As can be seen, a small portion of RE elements Ce and La can be detected on the plating surface, which indicates that RE realizes its co-deposition with Ni, Fe, and Co. With the addition of Ce, Ni content increases significantly, while Co content decreases greatly instead and Fe content declines modestly. And with the addition of La, Co content grows up while both Ni content and Fe content hardly change. The addition of Ce and La reduces the content of P in the plating owing to the reason that some of the RE elements in the form of positive ions play a role as reducing agent. Even though Ce and La’s influence on the plating composition is not that evident due to lanthanide contraction, Ce’s effect on it is much more significant while Ce’s atomic radius is shorter than La’s and thus Ce could be more easily integrated into the plating. (3) Effects of RE elements on plating structure. Figure 5.17 is the X-ray diffraction image respectively with RE added, RE Ce added, and RE La added from top down. It can be observed from the three curves that the intensity of bread-shaped diffraction peak weakens a bit and the peak width narrows when 2( = 45∘ in the X-ray diffraction image due to the addition of RE elements in the plating. This is caused by the decrease in the P content of the plating and RE’s refining effect on other metallic solids. First, while the electronegativity difference among Ni, Co, Fe, and P is quite large and their mutual influence is comparatively strong, P content will dramatically decline after RE elements are added, further lowering the amorphous stability of the plating and boosting the plating to trend towards crystalline state and microcrystalline state. Secondly, while the reduced RE is capable of adsorbing the electrons of other atoms nearby and shows an inclination to be dependent on active sites for self-growth, it is guaranteed to restrain the growth of the active sites, promote the formation of crystal nucleus in other locations, and accomplish the nucleation and growth of metal layer in a uniform way during the sedimentation process. Finally, as the formed plating possesses some degree of catalytic capability, the number of crystal nucleus could be continuously increased in the plating process and their opportunity of mutual contact is raised as well after they are widely spread on the substrate plane, thereafter refining the crystals and promoting the formation of crystallites.

5.2 Study on the Preparation and Electromagnetic Shielding Performance

Without RE elements

N Co

c

Ni Co Fe

Co Fe Fe

P

0 1 2 3 4 S_10824 cm C -0.278 (0 cm)

5

6

N

7

8

9

10

11

12 keV

Ce

Ni Co

Ni Ce Co Fe

Fe Co Ce Fe

P

N Ce

c 0

Ce

2 4 6 S_6820 cm C_0.470 (0 pts)

8

Ni

10

12

14

16

18

20 keV

Co La

Ni La Co Fe

P C 0 2 S_4942 cts

Fe La La

Co Fe Ni

La

4 6 C_12.913 (23 cts)

8

10

12

14

16

18

Figure 5.16: Effects of RE elements on the plating composition of Ni-Co-Fe-P.

20 keV

317

318

5 Application of lightweight carbon materials and composites in protective materials

Ni-Co-Fe-P + 0.6 La Ni-Co-Fe-P + 0.6 Ce Ni-Co-Fe-P

10

20

30

40

50

60

70

80

90

2θ (degree) Figure 5.17: Effects of RE on the plating structure of Ni-Co-Fe-P.

6.0 Surface specific resistance (Ω)

Surface specific resistance (Ω)

10 8 6 4

5.5 5.0 4.5 4.0

0

20

40

60

80

100

T (H)

(a)

(c)

20

40

60

80

100

60

80

100

T (H) 9

Surface specific resistance (Ω)

Surface specific resistance (Ω)

7 6 5 4

0

(b)

0

20

40

60 T (H)

80

100

8 7 6 5 4 0

(d)

20

40 T (H)

Figure 5.18: Relationship between surface specific resistance and corrosion time when weight gain rate reaches about 100% with the addition of RE element Ce.

(4) Corrosion resistance test. Figure 5.18 shows the relationship between surface specific resistance and corrosion time of the CF cloth with 100% weight gain rate before and after the addition of RE element Ce. As can be seen from the figure, the addition of RE element Ce exerts great influence on corrosion resistance. With a gradual increase

5.2 Study on the Preparation and Electromagnetic Shielding Performance

6.0

Surface specific resistance (Ω)

10 Surface specific resistance (Ω)

319

8 6 4

5.5 5.0 4.5 4.0

0

20

40

(a)

60 T (H)

80

0

100

Surface specific resistance (Ω)

Surface specific resistance (Ω)

(c)

40

60

80

100

60

80

100

T (H) 9

7 6 5 4

20

(b)

0

20

40

60 T (H)

80

8 7 6 5 4 0

100

(d)

20

40 T (H)

Figure 5.19: Relationship between surface specific resistance and corrosion time when weight gain rate reaches about 100% with the addition of RE element La.

in the density of RE element Ce, the rising tendency of the CF cloth’s surface specific resistance slows down, which means that its corrosion resistance is improving. Figure 5.19 shows the relationship between surface specific resistance and corrosion time of the CF cloth with 100% weight gain rate before and after the addition of RE element La. As can be seen from the figure, the resistance value of the plating decreases first and then increases with the gradual addition of RE element La, which means that its corrosion resistance descends after ascending. When the density of element La is 0.2 g/L, the anticorrosion performance of the plating is the best. To sum up, the corrosion resistance of the plating will improve with the addition of RE to the plating. This sort of phenomenon could be explained from the following two aspects. On the one hand, corrosion resistance and densification degree of the plating are closely associated. With the addition of RE elements, the plating becomes even smooth and the sedimentary metal particles turn to be finer, consequently improving the anticorrosion performance of the plating. On the other hand, favorable anticorrosion performance depends on the formation of amorphous plating. While RE element has a positive effect on the formation of amorphous state and element La’s positive effect outweighs that of element Ce’s, the plating with a small portion of element La will have the best anticorrosion performance. (5) Magnetic property analysis. Figure 5.20 is the hysteresis loop diagram of the plating with no RE and different kinds of RE added into the plating solution. Saturation induction density is 0.23 emu/g with no RE added. With the addition of La, saturation magnetization rises up to 9.04 emu/g. And with the addition of Ce, saturation magnetization greatly grows to 32.7 emu/g.

320

5 Application of lightweight carbon materials and composites in protective materials

40 Without RE elements

30

0.3

20 M (emu/g)

0.2 M (emu/g)

Ce0.4

0.1 0.0

10 0 –10

–0.1 –20 –0.2

–30

–0.3 –20000 –10000

0 H (Oe)

10000

20000

–40 –30000 –20000 –10000 0 10000 20000 30000 H (Oe)

La0.4 10

M (emu/g)

5 0 –5 –10 –20000 –10000

0 H (Oe)

10000

20000

Figure 5.20: Hysteresis loop diagram of the plating with no RE elements and different kinds of RE elements added.

Based on comparison among those data, the addition of RE elements prominently improves the saturation induction intensity of the plating. Additionally, all of the three hysteresis loops are long and narrow hardly with any portion of residual magnetism, which is representative of the features of non-retentive alloy. It could be concluded that the addition of RE elements would consequently improve the plating’s soft magnetic property and element Ce works better than element La in the improvement of the plating’s magnetic performance. The effect of element Ce on the plating’s magnetic property is quite different from that of element La, probably owing to the differences lying in the structure, the magnetic moment’s size and orientation of the alloy film which is formed by elements Ce’s and La’s mixture with Ni, Co, and Fe. In the co-depositing process, a considerable portion of super paramagnetic particles and single-domain soft magnetic particles will be formed in the plating due to the refining of crystal particles by the RE elements. (6) Electromagnetic shielding property analysis. Figure 5.21 represents the effect of RE element Ce with different additive portions on the CF chemical Ni-Co-Fe-P plating.

5.2 Study on the Preparation and Electromagnetic Shielding Performance

58

No RE elements Ce0.2

56

60 No RE elements Ce0.4

58

54

56

52

54

50

52 SE (dB)

SE (dB)

321

48 46

50 48 46

44

44

42

42

40

40 0

(a) 60

200 400 600 800 1000 1200 1400 1600 F (MHz)

0

(b)

200 400 600 800 1000 1200 1400 1600 F (MHz)

No RE elements Ce0.6

58 56

SE (dB)

54 52 50 48 46 44 42 40 0

(c)

200 400 600 800 1000 1200 1400 1600 F (MHz)

Figure 5.21: Plating’s electromagnetic shielding property with no RE added and element Ce added in different portions.

With the gradual increase of the frequency, the electromagnetic shielding property shows a downward tendency, which could not be changed with the addition of element Ce. Compared with the shielding property without any RE added, the addition of RE element Ce enhances the electromagnetic shielding property of the plating, which displays an overall slow upward tendency with a gradual rise in the additive amount of element Ce. With the micro-morphology and the result of X-ray diffraction mentioned previously taken into account, it could be deduced that the alloy plating formed by the co-deposition of RE element Ce refines crystal particles, improves the plating’s density, and further reduces the transmission of the electromagnetic waves, which is not the case with conventional plating.

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5 Application of lightweight carbon materials and composites in protective materials

58

No RE elements La 0.2

56

58

No RE elements La 0.6

56 54

54 52

50

SE (dB)

SE (dB)

52

48

50 48

46

46

44

44

42

42

40

40 0

(a)

(b)

F (MHz) 58

0

200 400 600 800 10001200 1400 1600

200 400 600 800 1000 1200 1400 1600 F (MHz)

No RE elements La 0.4

56 54

SE (dB)

52 50 48 46 44 42 40 0

(c)

200 400 600 800 1000 1200 1400 1600 F (MHz)

Figure 5.22: Plating’s electromagnetic shielding property with no RE added and element La added in different portions.

Figure 5.22 represents the effect of RE element La with different additive portions on the CF chemical Ni-Co-Fe-P plating. Differing from the addition of RE element La which is capable of improving the plating’s electromagnetic property, the addition of RE element La exerts no great influence on the enhancement of the plating’s electromagnetic property, which could be clearly reflected from the figure. Taking into the consideration of the hysteresis loops, we may deduce that this kind of phenomenon is probably due to the fact that RE element Ce’s effect on the plating’s electromagnetic property is far effective than RE element La’s.

5.2.5 The Optimization of the Chemical Plating Technology of Ni-Fe-Co-P by BP Neural Network There are so many factors affecting the chemical plating that few change of a technological parameter will cause a big difference. If considering all the factors,

5.2 Study on the Preparation and Electromagnetic Shielding Performance

323

we must do fussy and wasteful experiments. Moreover, because of frequent operations, the operator will be too tired to pay attention, which will further decrease the efficiency of the experiment and the reliability of the result. Therefore, it is a trend to improve efficiency and cut waste by a simulated way with science and efficiency. As there is an increase in developing high-technologies, especially in computer, computer simulation programs are occurring and spreading to replace traditional ways in experiments. Artificial Neural Network (ANN), among them, is a typical one. It is an artificial intelligent system that can simulate programs of a human brain. With the help of its extensive interconnected neurons to form functions between the input and the output step by step, it can reflect its developing trend in general without a high relevance between the input and the output. Therefore, ANN is a suitable way in the researches of nonlinear systems, and it has a wide use in the predictions of the functions, the optimization of the technological parameters, and so on. 5.2.5.1 BP Neural Network BP neural network always includes the input, the hidden layer, and the output layer, which are connected by weights between them. The basic network topology is shown in Figure 5.23. The biggest trait of BP network is that it can find out the linear organization between the input and the output to finish the predictions and analyses through learning by itself, without a high relation or an exact function between the input and the output. k

k

c1 c1

k

cj Wp1

W1j

cj

cq Wpj

W1q

Wij

Wi1

cq

Output layer Lc

Wiq Wpq

W11

W

b1

Vn1 Vh1

V1i

bi

Vni

Vhi

V1p

bp

Vhp

Hidden layer LB

Vnp

V11

V

a1

ah

an

k

ah

k

an

a1

Figure 5.23: Basic network topology.

k

Input layer LA

324

5 Application of lightweight carbon materials and composites in protective materials

5.2.5.2 The Building and Training of the Model For building a whole BP neural network, the first step is to set input and output parameters. According to the following principles, the input parameters (feature vectors) are always set: 1. The amount should be proper. Less amount will not fully show the impacts on chemical plating; too much amount will make the training slow. 2. The input parameters must have deep impacts on the output parameters and could be measured easily and accurately; then, the parameters should have less linear dependence relation with the output parameters if they are still more than needed. 3. If the choosing of a parameter cannot be decided, it can be chosen at first; then, contrast the result to the one gotten by the network without the parameter. A better choice can be made. 4. The laws of the chosen systems should be shown accurately from the chosen input and output parameters. We can analyze the input parameters to decide the amount of them by methods such as rough set theory and genetic algorithm. In chemical plating technology, deposition rate is often chosen as the output parameter, and solution temperature (T), concentration ratio of major salts, content of reducing agents, PH of solution, concentration of reducing agents, and so on are chosen as the input parameters. BP neural network has high requirements on training sets so that all the input and the output parameters cannot develop ideal mapping relations. In the optimization of the experiments, the data of orthogonal experiment are always chosen. The laws of the systems can be fully shown by the data. Though it cuts down the workload, its data can fully show the inner relations between the parameters with typicality. By this kind of set, a neural network model with efficiency and reliability can be built. This experiment is waged by orthogonal layout L16 (45 ), BP neural network and its training set is the result of the experiments. Table 5.7 is a training sample. The setting of structural parameters of BP neural network is the setting of the amount of the hidden layers and neurons in each layer, transfer function, and convergence algorithm. As is proven in existing documents, increasing only the amount of the hidden layers cannot increase the precision and the ability of the expression of the network. Only a hidden layer is enough. So this set only set a hidden layer, that is, an input, a hidden layer, and an output layer, a BP network of three layers. There are five parameters in the input, which are T, concentration ratio of major salts, content of reducing agents, PH of solution, concentration of reducing agents; a parameter in the output, which is the deposition rate of alloy; because the amount of the hidden layers is related to the project, but the relation between the amount of the hidden layers and the type and scope of the project can’t be known, we can only set the amount by

5.2 Study on the Preparation and Electromagnetic Shielding Performance

325

Table 5.7: Sample of neural network training. S/N

T/∘ C

CFe2+ /CCo2+ +Ni2+

Sodium citrate (g/L)

pH

Sodium hypophosphite (g/L)

Rate of deposition V/*10–4 mg/cm2 h

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

60 60 60 60 70 70 70 70 80 80 80 80 90 90 90 90

0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8

10 20 30 40 20 10 40 30 30 40 10 20 40 30 20 10

8 8.5 9 9.5 9 9.5 8 8.5 9.5 9 8.5 8 8.5 8 9.5 9

20 25 30 35 35 30 25 20 25 20 35 30 30 35 20 25

51,268 62,187 160,000 171,563 202,187 254,787 124,688 101,563 252,500 206,875 135,148 116,375 127,500 115,734 164,187 131,250

simulating experiments. At earlier stage, we simulated the experiments in different amount and found that 14 neurons of the hidden layer are suitable, that is, the structure of BP network is 5 × 14 × 1. The hidden layer is a tansig function of the input; the output layer is a purelin function of the hidden layer; network training function uses conjugate gradient algorithm; the percentage of comparison error between ANN training and the training sets is controlled within 1%. Through the Neural Networks Toolbox of MATLAB, the calculation of BP Neural Network is finished. After training 43 times, the function of the training set reaches the one of the target error, as is figured in 5.24. Figure 5.25 shows the linear dependence between the calculation of ANN and the results of the experiments, R (coefficient of correlation) = 0.99943. So, the prediction can be made by the results of training.

5.2.5.3 The Test of the Model There are 4 × 4 × 4 × 4 × 4 = 1024 experiments in total, each of five parameters has four parallel experiments of chemical plating, and 16 of them are chosen as orthogonal experiments. In order to test the Neural Network, all the experiments are numbered and five of them are chosen randomly as test sets. After the competition of the result and the calculation of the error, we can know the versatile degree of the network after training; the sets and the results are shown in Table 5.8. From Table 5.8, the maximal error is only 1.1%, that is, the results of the experiments are reliable.

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5 Application of lightweight carbon materials and composites in protective materials

Best Training performance is night at epech 43

101

Train Best Good

100

Mean squared error

10–1

10–2

10–3

10–4

10–5 0

5

10

15

20 25 43 Epechs

30

35

40

Figure 5.24: Target deviation curve.

5.2.5.4 The Optimization of Technological Parameters on the Neural Network When orthogonal experiments are designed, the parameters are chosen between the same interval, which will lead the best technological parameters produced in these integral points only. But, in fact, there may be better ones around them. In order to further get the best factors for the optimization of the technology, based on the existing Neural Network models, under the method of small pace search, some interval points with typicality can be chosen, and the calculation can be done through the Neural Network to see if there are any better parameters. 1. Based on A0 , which is the best technological parameter point according to orthogonal experiments, we rearrange the new parameters which are gotten by increasing or decreasing a small pace on every technological parameters to produce a new set; 2. Based on the former ANN model that has been trained, we get a deposition rate by calculating the new set; 3. When the deposition rate reaches its highest, the technological parameter is set as A1 , contrast it to A0 , set the bigger one as a new A0 and ignore the former one; 4. Back to the step (1) till V (the deposition rate) reaches the demand. Based on the steps, small variables are made around the best technological parameters group result from the orthogonal experiments to get new factors of optimization, as is figured in Table 5.9.

5.2 Study on the Preparation and Electromagnetic Shielding Performance

327

Training: R = 0.99943 1 Data Fit Y=T

0.9

Output = 1 – Target + 0.0028

0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Target Figure 5.25: Linear correlation between ANN computation and experiment data.

Table 5.8: Test sample. S/N T/∘ C CFe2+ /CCo2+ +Ni2+

1 2 3 4 5

60 80 70 90 90

0.6 0.4 0.8 0.2 0.4

Sodium citrate (g/L)

pH Sodium hypophosphite (g/L)

Experiment data V/*10–4 mg/cm2 h

Value predicted V/*10–4 mg/cm2 h

Error

30 30 20 20 30

8.5 9.5 8 8.5 9

154,326 223,500 113,567 120,705 123,457

154,789 225,512 114,846 121,188 124,445

0.3 0.9 1.1 0.4 0.8

30 25 30 30 35

Arrange the experiments by orthogonal table and get the deposition rates by existing Neural Network models; the results are shown in Table 5.4. The Neural Network, around the optimal technological parameters group result from orthogonal experiments, still can get better technological parameters group, which is T = 88∘ C, concentration ratio of major salts = 0.46, the ratio of sodium

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5 Application of lightweight carbon materials and composites in protective materials

Table 5.9: Factors and level of optimization. S/N

T/∘ C

CFe2+ /CCo2+ +Ni2+

Sodium citrate (g/L)

pH

Sodium hypophosphite (g/L)

1 2 3 4

66 74 88 92

0.23 0.46 0.61 0.87

12 25 33 46

8.05 8.57 9.03 9.56

24 27 32 39

citrate = 46 g/L, PH of solution = 9.63, the ratio of sodium hypophosphite = 24 g/L, the deposition rate gotten by the Neural Network models is 29.3618 mg/cm2 h. What should be paid attention to is that the group is not the best one. To get the best one, the steps must be done again and again. Or optimization can be made by ANN-genetic algorithm. After getting the model of technological process by the Neural Network, the best technological factors can be gotten from the optimization by genetic algorithm. Among them, the technological parameters in the calculation of fitness function, which is in genetic algorithm, are predicted by the model of the Neural Network, the constraints are determined by the medium values of every factors and experience. For details, researchers can survey the relevant literature. 5.2.5.5 Examination of Optimum Solution In order to examine the optimum solution gotten by ANN model, we measure the deposition rate of the Ni-Fe-Co-P alloy layer prepared by the optimized technology. The result is 28.6592 mg/cm2 h, and the error rate is 2.39%. That is to say, the mapping relation of the technological parameters of chemical plating and the deposition rate, which is reflected by the Neural Network, is basically correct. The Neural Network is available in the optimization of the technology.

5.3 Application of Light Carbon Materials in Liquid Propellant Protective Clothing Fabrics The most important function of protective clothing is to ensure absolute security to its user; protective clothing should be able to resist toxic chemicals effectively. Protective clothing can resist two forms of toxic chemicals: liquid and vapor. And in this section, toxic chemical vapor proof is the main focus.

5.3.1 Technique of Toxic Chemicals Proof A very key index to measure the property of a protective clothing is how long it can resist toxic chemical vapor – the longer the time is, the better its property is.

5.3 Application of Light Carbon Materials in Liquid Propellant Protective Clothing Fabrics

329

To achieve a certain time of resisting poison, protective clothing should be able to stop the seepage and osmosis of toxic chemicals. There are many ways to resist toxic chemicals (chemical warfare agents and liquid propellant), but due to the particularity of protective clothing, the main techniques adopted in it are the two below. 5.3.1.1 Aperture Control Isolation Technique This technique controls the level of isolation between the inner surface and outer environment by the sizes of the apertures in the material, and the effect of isolation mainly depends on the sizes of the apertures in the material and the sizes of molecules of toxic chemicals. The smaller the apertures are, the better the effect of resistance is and the best when the inner surface is totally isolated from the outer environment. But, when the apertures are small, the heat and sweat would not get out easily, which makes the suit physiologically afflictive. And for that reason, the apertures should be limited in a proper range, to achieve a basic balanced point between the effectiveness and comfort. Some main materials with outstanding impermeability are butyl rubber, VC polyvinyl chloride, polyvinyl chloride. and some with small apertures. Those suits made of totally impermeable materials are called impermeable protective clothing. The early suits took the advantage of the impermeability and abrasive proof of rubber to resist toxic chemicals from the users by physical proof, and the toxic chemicals would be washed away by liquid detergent. But due to its air-proof and weight, it was then wed out. Different sizes of apertures are gained from raw materials or manufacturing techniques. For example, fabric coated with chloroprene rubber is totally impermeable because the rubber does not have micro apertures in it; plain fabric of low cotton yarn, when it is dry, has a relatively large gap between the warp and weft thread, about 10 microns, and it can let a certain wet and heat through; tight fabric made of polyester or nylon fine fibers, because the yarn fineness is small enough to make the fabric yarn clearance tiny, makes the water vapor about 0.0004 microns in diameter able to pass, but liquid water about 100 microns in diameter does not pass; the waterproof and moisture permeable high-performance Gore-Tex fabric is compounded with PTFE micro-porous membrane and fabric, with about 0.2 microns apertures. Using different surface coating agents, such as the variously structural hydrophilic modification of polyurethane polymer, to process the surface of fabric can also control the size of the aperture of the fabric. To ensure physiological comfort, openings in the fabric of the suit cannot be too small. But big pores will affect its performance. To settle this contradiction, on the basic foundation of the first resistant technique, the second technique which makes use of adsorbent materials to adsorb toxic vapor is adopted to achieve the balance between its performance and physiological comfort.

330

5 Application of lightweight carbon materials and composites in protective materials

5.3.1.2 Adsorption Technique This technique uses adsorbent materials to adsorb the vaporous toxic chemicals. There are various kinds of adsorbent materials, and among them carbon products (activated charcoal) are used in the protective suits. Activated charcoal has several statuses, and powdery activated charcoal (PAC), granulated activated charcoal and the fibrous activated charcoal adhering to organic matters are the most favored in practical use. In different physical status, the activated charcoal has different mechanical properties and adsorption capacity. With a vast specific surface and after soaked in proper catalyst, the granulated (or powdery) activated charcoal can adsorb and dissolve various organic matters of different concentrations, thus it is a rather ideal toxic gas protective agent. But when used in protective clothing, it will be affected by the temperature and humidity of the environment and the coexisting nontoxic matters, and its adsorbent capacity will descend, unable to serve long and effectively. Activated charcoal fiber which has been used since 1970s and micro-spheres carbon products appeared recently have increased the adsorbent capacity of charcoal products, and they can also serve the long need of effective protection against toxic chemicals. Theoretically, both activated charcoal fiber and micro-spheres are qualified to be used in protective clothing against liquid propellants. But producing microcharcoal balls costs rather much and it is a complicated technique to paste them on fabric; adhesive species and dosage of carbon directly affect the micro-spheres of bulk density and the size of the activity; the production process is not easy to stabilize. Activated charcoal fiber producing technique is mature and the price is cheaper, and secondary processed products can be easily woven to cloth material, providing great convenience for the production of protective clothing.

5.3.2 Research on Activated Carbon Fibers Affecting on UDMH Vapor Penetration Performance Considering the raw material source, production cost, and other factors, activated carbon fiber is chosen as the final gas liquid propellant protective clothing material. There is a variety of specifications for activated carbon fiber of (see Table 5.10). According to the requirements of the protective clothing toxic resistance and weight, activated charcoal fiber cloth 2 was chosen as the defense material of the fabric. UDMH gas penetrating test device: self-made, made of organic glass, and structural plane diagram is as shown in Figure 5.26. On the other side of the device, the penetration quantity of UDMH is detected by ionization liquid propellant steam light detector (homemade). Before testing, breeze clean air for half an hour into the UDMH gas penetration testing device. Crop 20 cm2 area of activated carbon fiber cloth and dry it at a

5.3 Application of Light Carbon Materials in Liquid Propellant Protective Clothing Fabrics

331

Table 5.10: Activated charcoal fiber fabric specifications. Standard

Activated carbon fiber cloth 1

Activated carbon fiber cloth 2

Activated carbon fiber cloth 3

Specific surface area/(m2 /g) (BET) The benzene adsorption saturation/(wt%) Iodine adsorption saturation/(mg/g) Gram weight/(g/m2 ) Thickness/(mm)

800±100 25–32 600–700 100 0.25

1,000±100 35–45 850–900 200 0.32

1,200±100 46–52 1,100–1,200 300 0.46

3.5 mm Dia Oulet of gas Sample Inlet of gas Flow meter

Figure 5.26: Diagram of UDMH gas penetrating the test device.

temperature of 100–120∘ C for 2 hours until it reaches a constant weight, then fix it to the test device. Make a certain concentration of gas (C0 , mg/L), control the gas’ different flow velocity (v, cm/min) when it passes one side of the test device; on the other side, detect the concentration of the UDMH gas with light ionization liquid propellant steam detector, and when the gas concentration (Cb , mg/L) is of certain times of the initial concentration, it is the ending of “gas-gas” defending time (t). When the penetration concentration is 1/10 of the initial concentration, namely the Cb /C0 = 0.1, 12 UDMH vapor penetration curves have been tested. See the results in Table 5.11. To determine the lasting time of defense of activated carbon fiber against UDMH gas, Klotz equation, Wheeler equation, and Yoon equation that have been widely used in adsorption dynamics for gas mask canister are used to deal with the “gas-gas” data. The dynamics condition for activated carbon fiber against gas is very special: it is carried out on very thin activated fiber cloth and at very low airflow speed. Generally in the actual application of protective cloth, the speed of the airflow through the activated carbon layer is 10 cm/min, much lower than that (usually 200–500cm/min) in the gas mask canister. Though activated carbon fiber cloth is “gas-gas” masking on the condition of low airflow ratio, the very special dynamic condition, the contact

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5 Application of lightweight carbon materials and composites in protective materials

Table 5.11: Lasting time of defense of activated carbon fiber against UDMH. No.

1 2 3 4 5 6 7 8 9 10 11 12

Test conditions

Real value (t/min)

Initial concentration C0(mg/L)

Flow velocity v(cm/min)

2.68 2.68 2.68 2.68 8.04 8.04 8.04 8.04 13.40 13.40 13.40 13.40

10.5 23.0 30.0 47.0 10.5 23.0 30.0 47.0 10.5 23.0 30.0 47.0

221.2 93.0 75.1 40.6 77.5 37.1 21.4 12.8 40.2 21.0 17.0 11.5

time between the airflow and the cloth in antitoxin, is almost the same as that inside the canister, both about 0.001 min (see Table 5.12). The contact time is a very important parameter for “gas-gas” antitoxin. It is, therefore, useful to apply Klotz equation, Wheeler equation, and Yoon equation for the treatment of “gas-gas” data due to their advantages of being simple, easy to compute and apply in actual work. Klotz, Wheeler, and Yoon, from different perspective, get similar expression for the activated carbon “gas-gas” time. Klotz equation: t=

c a (w – k1 v0.41 ln 0 ) c0 v cb

Table 5.12: Airflow contact time of different protective equipment in actual use. Equipment conditions to use

#82 permeable protective clothing

Carbon (g/m2 )

50

Portion of carbon stacking (g/cm3)

0.50

Thickness of carbon (cm)

0.1 (carbon cloth) 0.01 (carbon)

Speed of airflow (cm/min) Airflow contact time (min)

#65 gas mask

#69 gas mask

200

50

40

0.65

0.65

0.65

2.0

1.0 (board?) –0.4

2.0 (gauze) –0.9 (carbon)

10

–200

–150

–450

0.010 (carbon cloth)

0.010

0.070 (board?)

0.004 (gauze)

0.003 (carbon)

0.002 (carbon)

0.001 (carbon)

#64 gas mask

5.3 Application of Light Carbon Materials in Liquid Propellant Protective Clothing Fabrics

333

Wheeler equation: t=

c v a (w – ln 0 ) c0 v k2 cb

Yoon equation: t=

c – cb a w ln 0 ) (w – c0 v k3 cb

In the above equations, k1 is the function of the diameter of the carbon particle, vapor diffusion coefficient, airflow viscosity, density; k2 is related to vapor diffusion coefficient; k3 is a constant, not related with the speed of airflow and the concentration of vapor. a is the dynamic equilibrium absorbing capability (mg/mg) of activated carbon to chemical toxicant. C0 is the initial concentration of gas (mg/L); v is velocity of the airflow in test (cm/min); w is the content of carbon (mg/cm2 ); Cb is the transitive concentration. The parameters in the above equations could be regressed in accordance to the actual data of antitoxin time under different conditions. Though the above equations are the description of the time of “gas-gas” antitoxin of activated carbon to chemical toxicants, they can also be applied to the description of the antitoxin time of “gas-gas” by activated carbon fiber to UDMH vapor. The meaning of parameters will change accordingly. Klotz equation: tk =

357.86 (w – 0.4233v0.41 ln 10) c0 v

Wheeler equation: tw =

v 324.72 (w – ln 10) c0 v 30.71

Because k2 is directly proportional to v0.5 , so Wheeler equation can be written as: tw =

347.66 (w – 0.2741v0.5 ln 10) c0 v

Yoon equation: Because of the ill-conditioned Jacobian matrix in regression process, there would be obvious error for direct regression. The equation has to be transformed. According to Yoon’s hypothesis, a is in direct proportion to C0n (n can be determined by experimental data regression), Yoon equation can be rewritten as: ty =

aw cn0 v

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5 Application of lightweight carbon materials and composites in protective materials

Table 5.13: Prediction of antitoxin time and error of Klotz, Wheeler, and Yoon equations. No.

1 2 3 4 5 6 7 8 9 10 11 12

Actual measured Value (min)

Klotz equation Calculated value (min)

Error (%)

Calculated value (min)

Error (%)

Calculated value (min)

Error (%)

221.2 93.0 75.1 40.6 77.5 37.1 21.4 12.8 40.2 21.0 17.0 11.5

221.8 95.6 71.5 43.4 74.0 31.9 23.8 14.5 44.4 19.1 14.3 8.7

0.3 1.2 1.6 1.3 –1.6 –2.4 1.1 0.8 1.9 –0.9 –1.2 –1.3

221.8 95.7 71.5 43.3 73.9 31.9 23.8 14.4 44.4 19.1 14.3 8.7

0.3 1.2 1.6 1.2 1.6 2.4 1.1 0.7 1.9 0.8 1.2 1.3

216.8 99.0 75.9 48.4 73.7 33.7 25.8 16.5 44.7 20.4 15.6 10.0

2.0 2.7 0.4 3.5 –1.7 –1.6 2.0 1.7 2.0 –0.3 –0.6 –0.7

Mean error (%)

Wheeler equation

1.3

1.3

Yoon equation

1.6

Regress according to the above equation to get: ty =

299.54w c0.9816 v 0

By referring to the regression equation, a rough transit time under different conditions can be estimated, see Table 5.13 for the result. Table 5.13 and Figure 5.27 show that the three equations express the result closely. t From equations above, under certain conditions: C0 , v and transit content (∫0 cb dt, when transit content is low, it is used to confirm transmissivity time) are certain; “gas-gas” transit time is defined by w, a, and k(k1 , k2 and k3 ), k is the weight of activated carbon fiber cloth (ACFC) per unit area; it is used to confirm the manufacturing technique of ACFC; a is the dynamic equilibrium absorbing capability of the active carbon fiber, which is related to the property and manufacturing technique of active carbon fiber; the most interesting figure is k, especially k2 , it reflects the speed of absorption rate. When the transit content is low, making some appropriate mathematical manipulations and approximate treatments to those equations, and using total content D to replace Cb . The equations can be transformed as: t = A + B ln D A, B in different equations can be seen in Table 5.14. Equation in this form can be more easily used in practice.

5.3 Application of Light Carbon Materials in Liquid Propellant Protective Clothing Fabrics

250

250

250

Klotz

Wheeler

100

200 Calculated value (min)

150

150

100

50

0

Yoon

200 Calculated value (min)

Calculated value (min)

200

150

100

50

0

100

0

200

Experimental value (min)

335

50

0

100

0

200

Experimental value (min)

0

100

200

Experimental value (min)

Figure 5.27: The comparison of calculated value and experimental value. Table 5.14: Value of A and B in different equations. Equation

A

B

Klotz

ak1 c0 v 0.59

Wheeler

a k2 c0

Yoon

aw k3 c0 v

(

(

w k1 v 0.41

k2 w v

– ln

(k3 – ln

– ln

k1 a ) v 0.59

ak1 c0 v 0.59

a ) k2

a k2 c0

aw ) k3 v

aw k3 c0 v

According to the results, the “gas-gas” antitoxin time of activated carbon fiber to UDMH can be expressed in Klotz and Wheeler equations as: Klotz tk = 197.1 + 14.12 ln D Wheeler tw = 20.85 + 0.3731 ln D

5.3.3 Prediction of Antitoxin Time Based on the Artificial Neural Network Theory Klotz, Wheeler, and Yoon equations can be used to express the antitoxin time of protective fabrics. However, many factors are related to the protective effect. When we do deductions, we use over simplifications. Thus, three equations cannot provide all information of throughout curve. So the throughout curve is unmatched to the s-shape curve measured by physical truth.

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5 Application of lightweight carbon materials and composites in protective materials

Theoretically, through rigorous mathematical deduction of dynamic absorption process, we can reach all forms of partial differential equations. But they can be hardly solved by analytic method. Even though many simplified hypotheses are used, solutions are very complicated. At present, various ways of numerical solutions and statistical moments are used to manage the throughout curve, solving the antitoxin time. But they are quite complicated and the result cannot be accurate. The absorption process is very complex, so it is nearly impossible to obtain an accurate expression of the throughout curve by solving partial differential equations under terminal conditions. But according to the specialty of absorption, we can use the Artificial Neural Network Theory to stimulate the throughout curve and predict the antitoxin time. The Artificial Neural Network can stimulate the structure and function of human brain. It is composed of a huge number of nerve cells. When we use it to solve the problem, we don’t need the accurate model of the objective, but only a huge amount of primary data, to gradually adapt to the external function through the variable nature of its structure. Then, the internal causality can be figured out and, finally, the result can be described as not very accurate input and output values. Due to the specialty of the Artificial Neural Network, the complicated description of the relationship between the throughout time and influencing factors can be avoided, especially the expression of equations. The Network can automatically learn and memorize the relationship between input values and output values. For the data in Table 5.13, we can use MATLAB neural network toolbox’s function and the BP Network to make predictions of the throughout time. In order to eliminate the differences of dimensional data and magnitudes orders, we can normalize the original data, using the following formula: xij󸀠 =

xij max xij 1≤i≤n

Then normalizing the transformed data by Zij =

xij p

2 √ ∑ xim m=1

Choosing ten transformed and intensive data randomly and another two as test samples, we can test the reliability of the network. During calculation, we have to determine the structure of BP Network Model, based on two important guiding principles: 1. For normal pattern recognition problems, three-layer network can solve them effectively. 2. In the three-layer network, n2 = 2n1 + 1 (n2 is the number of nerve cells in hidden layer, n1 is the number of nerve cells in input layer.)

5.3 Application of Light Carbon Materials in Liquid Propellant Protective Clothing Fabrics

337

Therefore, we can design the network as follows: 10 nerve cells in input layer, 1 nerve cell in output layer, and nearly 21 nerve cells in hidden layer (the number of nerve cells in hidden layer is not settled, it can be changed by practical training) After the designing process, we need to use ten random sample values to examine the network. The examining process is used to constantly amend the weight and threshold values. By amendments, the output error can reach the minimum point to meet the demand of practical applications. After the network examining, the two groups’ surplus of data can be used as input data to have the test. Figure 5.28 shows the concrete result of a single test. From the results of simulation test, activated carbon fiber’s prediction error of throughout time is acceptable, which shows that the prediction of antitoxin time based on the BP Neural Network Theory is feasible. It is important to note that the actual data are not enough, and the spacial distribution is not ideal, so when randomly choosing different data to examine samples, there could be lots of errors. It is because test samples are not within the limits of training data. However, this cannot influence the application of prediction. This model can strengthen the capability of BP Neural Network Model, in order to play an instructive role in actual production.

5.3.4 The Regeneration of the Activated Carbon Fiber’s Activity The exterior material of protective clothing made from the activated carbon fiber should be used repeatedly and durable to be washed for many times. After adsorp14 12 10 8 6 4 2 0 –2

0

2

4

Stop Training

Figure 5.28: Error of simulation test result.

6

8

10

12

338

5 Application of lightweight carbon materials and composites in protective materials

tion saturation, as well as washing, the property of the activated carbon fiber will be changed and therefore influences the protective clothing’s effect of gas defense. Thus, the certain way is needed to deal with the regeneration of the activity of the activated carbon fiber which is adsorbed and saturated. Before the exterior material of protective clothing regenerates, it should be washed by clean water at first. UDMH is a kind of weak alkaline agent. And when we are washing the exterior material, we can add a certain amount of acetic acid whose quantity should be controlled in adding a spoonful of vinegar into a tank of laundry water. And then, by the neutralization of acetic acid, it can dramatically reduce the residual amount of UDMH and greatly diminish its specific fishy smell. After washing the exterior material for two or three times and spin-drying it fully, we use the heat to regenerate the activated carbon fiber in the exterior material of the protective clothing. There are concentrated micropores mainly distributing on the surface of the activated carbon fiber. Also, because of its short path of adsorption and desorption, the low resistance, and the rapid rate, generally, we use the vapor of 120–150∘ C or hot air to dispose the activated carbon fiber and it will regenerate. On the principle of relieving the burden of using the protective clothing to the greatest extent and according to the protective clothing’s actual usage, after studying, we are certain about using the temperature of vapor (about 135∘ C) generated from the iron to regenerate and activate the exterior material of protective clothing for 15 to 20 minutes. In other words, we adjust the temperature of the iron to 135∘ C to iron the exterior material for 15 to 20 minutes and then dry the protective clothing in the sun. By the process, we can reach the goal of regeneration. In this way, not only can we avoid using the regeneration device, but also regenerate the protective clothing when we are washing it. By doing these, we keep the protective clothing clean and tidy as well as increase its service life. The property of the exterior material of the protective clothing after regeneration is tested. The test result can be seen in Table 5.15. We can see that as long as there is enough activation time, the exterior material’s property of gas defense will not be greatly affected. Table 5.15: Antitoxin performance of protective clothes after regeneration. Composite protective cloth Concentration of UDMH vapor (mg/L)

2.68

Laundry Regeneration time (min) Washing times before Transit time (min) after

Water, washing machine 10 3 93 89

20 10 95 100

30 13 90 91

5.3 Application of Light Carbon Materials in Liquid Propellant Protective Clothing Fabrics

339

5.3.5 New Liquid Propellant Protective Clothing Fabrics On the basis of the results given, choosing activated carbon fiber can achieve UDMH “gas-gas” antitoxin time requirements. Based on this, considering the cost, production process, technical infrastructure, and other factors, we use a new protective clothing fabric which is made of composite technology, to meet all kinds of requirements of protective clothing. Fabric is made of two parts: the inner and the outer. The outer one is mainly used to meet both the physical and mechanical properties of the fabric, anti-static, flame retardant, etc., and it must have abilities for water and oil repellency, to prevent penetration agent droplets; inner fabric is used for protecting from liquid propellant vapor, and exporting sweat, heat produced by the body. Since UDMH can dissolve cotton fabrics of outer layer, its surface should be made suitable for oil-water repellent. To make it repellent for water and oil, we should coat it with a thin-layer or multilayer polymer on the surface, to make the front and back surfaces getting different functions. The agents we commonly use are as follows: polyacrylates, polyvinyl chloride resin, silicone elastomer, teflon, synthetic rubber, etc. The purposes for treatment of fabric outer surface are as follows. The first is to prevent liquid UDMH to disclose fabric and corrosion, to ensure the absolute safety of protective clothing; the second is to make the fabric repellent for water and retardant for flame; the third is to prevent the propellant liquid contacting with the activated carbon fibers directly. Antitoxin functionality of activated carbon fiber is based on “oleophobic-adsorption” mechanism, namely activated carbon fiber surface is not in direct contact with a high concentration of liquid propellants or propellant vapor liquid, but only a lower concentration of adsorbed vapor permeable. ACF has so many characters, such as thin, light, mass absorbing area. But if you use a single antitoxin layer, when poison concentration is high, especially when poison drops directly onto the surface, the droplets will make it lose the protective effect. Surface treatment can prevent the droplets penetrate the outer layer of the fabric and contact with the activated carbon fibers directly. As the role of surface coating, the dripped, sprayed agents on surface of protective clothing cannot penetrate the outer layer of the fabric while contacting carbon fiber, but soon drip into a “ball,” the residual liquid agent will soon evaporate completely, with only a small part of the vapor generated on the outer layer of fabric material to penetrate into the inner layer of protective clothing, but is absorbed by inner layer of activated carbon fibers. So we can utilize high adsorption for agents of activated carbon fiber effectively, ensuring long-term and safe; and finally to prevent the outer fabric from absorption of moisture and liquids, and to lighten the weight of protective clothing. In order to maintain the long-term antitoxin function, surface coating agent should not only have a good hydrophobic effect, but also long-term effectiveness of washing and friction properties, at the same time the activated carbon fiber layer may also be used for processing its adsorption properties without any affect, while activated carbon fiber layer gets a hydrophobic group and adsorption properties of

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activated carbon fibers to protect against human sweat and other impacts. By coating the surface, the outer layer material can prevent infiltration of fluid on the surface, and an inner layer, the activated carbon fiber material, may play a protective role. To make agent droplets on the fabric surface a “ball” and prevent any infiltration, selecting the appropriate coating agent must be done, which is to select a suitable surface tension agent. According to Figure 5.29, to make the droplets “balls” on the fabric surface, the contact angle is less than 90∘ . To make the contact angle ( less than 90∘ , the surface tension of the solid surface must be less than the surface tension of the droplets. Liquid propellant UDMH surface tension is 24.18 mN⋅m–1 , therefore, the surface tension of the finishing agent should be less than this value. We tested the effects of two kinds of objects, polyacrylate, and fluoride AG-710. Comparing the test results, the fluoride AG-710’s effect was better, so we chose it as the coating finishing agent. Fluoride AG-710 is used for surface finishing agent of China FFF-02-type protective clothing, for it takes less activated impact on antigas/antimephitis material and active material. The consistency of the outer fabric with UDMH is tested. The results showed that, the fabric irrespective of its weight after 24-hour soaking proved that it was suitable for UDMH protective clothing’s outer material. The outer layer fabric which is repellent to water and oil with liquid propellant droplets is tested, and it can be seen that its performance fully met the requirements. What’s more, we tested the waterproof and flame resistance of outer fabric which had been treated by fluoride AG-710, and the test results showed that water-resistance level was not less than 4; continuing burning and smoldering time are not more than 5 s; burning damage length not more than 150 mm, which showed it met the relevant requirements. Since the staff are nervous when wearing protective clothing during work. With a certain time and intensity of work, the staff is easy to sweat, and much heat and vapor are going to be produced in protective clothing. If the working place is in the south of our country, which is exactly right in the air-moist at this time, the quantity of heat and the yield of vapor will be more. If the fabric cannot export these heat and sweats (vapor) out of protective clothing, it will seriously affect the personnel physiological wearing comfort and reduce the time wearing protective clothing. Protective clothing fabric should have good air permeability and water vapor permeability, so the thermal and moisture staff produced, due to physiology, movement, and work, can be discharged timely through the fabric, and it makes the body remain in an appropriate thermal equilibrium.

Vapor Toxicant (liquid) Fabric surface (solid)

Figure 5.29: Interface diagram of protective clothes in three conditions.

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Within a certain range, water vapor condensation occurred in the fabric is mainly caused by the difference of the temperature distribution (temperature gradient). The existence of temperature gradient defines the existence of gradient of the actual water vapor pressure, the saturated vapor pressure, and water vapor concentration gradient at that temperature. The existence of these gradients determines the occurrence of cohesion, the transferring speed of heat flow and water vapor, and the changes of range in the “dry–wet–dry” zone inside the fabric. For a given fabric, both the transferring speed and condensation speed of water vapor increase with increase in the concentration of water vapor on thermal fabric surface; when condensation occurs, the heat flux out of the fabric increases with the concentration of water vapor, and with the increase in the concentration of water vapor of the thermal fabric side, so the difference of the turnover in heat flux on the fabric wet area is gradually increased. Changing certain physical and chemical properties of the fabric can improve the heat transferring and vapor condensation. Reducing the thickness of the fabric, or improving the thermal conductivity, can improve vapor transferring and condensation rate. Since activated carbon fiber is porous, although protective clothing is composted with multiple layers of fabric, tests proved that it could fully meet the physiological requirements for comfortableness.

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6 Future development Carbon materials, traditional as well as modern materials, are widely studied and used. Before the twentieth century, carbon materials, such as charcoal, carbon black, coke, natural graphite, and synthetic graphite, had contributed greatly to the development of ceramics, metallurgy, and printing industry. It was not only part of ancient civilization but also the main supportive material of steam engine during the first Industrial Revolution. With the progress of national defense, industry, and agriculture, particularly the fierce competition of space exploration between American and the former Soviet Union in 1950s, new types of carbon materials got booming development, involving carbon fiber (graphite) and its composite material, active carbon fiber, carbon molecular sieve, and carbon microbeads. Of special note is the appearance of C60 , CNT, and carbon alloy at the end of twentieth century, which marks the enormous progress of carbon science. Being a multidiscipline subject, carbon science has its characteristics as structural diversity, various applications, and continual emergence of new types. Just a small part of it is covered in this book. More attention will be paid to the future study of carbon materials in the following aspects:

6.1 Activated Technique and Modified Technique of Lightweight Carbon Materials Some features of the traditional lightweight carbon materials cannot meet the requirements of high-tech fields such as environmental protection, atomic energy, stored energy, chemical industry, electron, and automobile. It is necessary to modify or activate those materials so as to meet the requirements of “high-adsorption, large specific surface area (≥2,500 m2 /g), polymorphic type, high strength and low cost.” In this way, the application of carbon materials can be expanded in the fields of electron, medicine, and radiation protection. Owing to differences of raw materials, production methods, and equipment, the lightweight carbon materials made with different techniques show striking differences not only in physical forms of aperture, pore volume, and pore distribution but also in basic chemical features of the product surface, manifesting very distinct differences of adsorption capacity in practical application. For example, R.R. Bansode took pecan shell as the raw material to make activated carbon, using vapor, carbon dioxide, using phosphoric acid as activators. A comparison of the adsorption efficiency on VOC of the two granular active carbons with the commercialized active carbon (Filrtasorb 200, Calgon GRC-20, Water 206C AW) shows

DOI 10.1515/9783110424751-006

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that the amount of adsorbing capacity of vapor- and carbon dioxide–activated pecan outweighs that of flat walnut shell by phosphoric acid activation. In a series of experiments, the vapor-activated pecan shell has a higher VOC adsorbing capacity than the other activated carbons. There are two common activation methods: vapor physical activation and chemical activation. Vapor physical activation is based on the redox reaction between vapor and carbon under high temperature (800–100∘ C) to enhance the pore structure of carbides, which is the most common way to make activated carbon because of its low cost. Its adsorbing capacity can be improved by raising temperature, increasing the quantity of vapor, and extending the time of carbon activation. Chemical activation is a method to put chemicals into raw materials to heat with the protection of inert gases, and at the same time to do activation and carbonization. Zinc chloride, phosphoric acid, and potassium hydroxide are the most frequently used activators. Factors such as concentration, dipping time, temperature, and time of activation all have the effects on the property of carbon products. There are more or less hydrophilia and oxygen-containing functional groups on the surface of all kinds of carbon materials. The change of the quantity of oxygencontaining functional group and hydrophilia of its surface and hydrophobicity can bring about different adsorbing capacity to different acidic and alkaline gas by means of oxidation, ammonization, hydrogenation, alkalization, or high-temperature processing, and specific surface heteroatom- or compound-increasing. Generally speaking, carboxylic acid group on the surface of activated carbon can be decomposed in a lower temperature to release carbon dioxide, while the groups of phenolic hydroxyl, ether, and carbonyl require a higher temperature to decompose to release carbon monoxide. For instance, Mochida found that after the asphalt ACF of high specific surface area was processed under 1,100∘ C, its desulfurization capacity was improved highly. That is to say, the adsorbing capacity of SO2 , oxidative activity, and dissolution speed of sulfuric acid are greatly increased. Thus, it is easy to make desulfurization even at indoor temperature. If the temperature of desulfurization increases to 50–70∘ C, complete desulfurization will be a necessity. Then the required quantity of vapor and ACF will be greatly increased. By adding different metallics to its noumenon or surface, are enabled with some specific functions, . . . The silver-loaded or copper-loaded measures can give ACF the antibiosis function. The ability of desulfurization of ACF can be improved by adding manganese, copper, and potassium carbonate. The activated carbon with gold and palladium on the surface can be used as masks for chemical toxic. The adsorbing efficiency of ACF to NOx will be enhanced after loading !–FeOOH and !–Fe2 O3 on it. The copper-cobalt–loaded ACF can catalyze and reduce NO with NH3 effectively. If the surface of activated carbon is loaded with the catalysts made by precious metal, sulfide (MnS, MoS2 , WS2 , CuS), halide (AlCl3 , chloride of alkaline earth metals), and inorganic acid, it can be used in the hydrogenation reaction or compound of pesticides,

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medicine, and spices. It can also be used in the production of polyester, polyurethane of plastics, and chemical fiber as well as the dehydrogenation of alicyclic compound to make aromatic compound.

6.2 Composite Technology of Lightweight Carbon Materials There are many advantages of composites. They can function comprehensively to coordinate all the efficiency of different ingredients. They can design and produce materials according to the features of composites. They can process the shape of technology by meeting the requirements, thus can avoid repeat processing. Therefore, composites have a wide range of applications. Lightweight carbon materials possess the characteristics of small proportion, good stability, heat resistance, corrosion resistance, heat-shock resistance, electroconductibility, and high temperature strength. In order to bring all the features into full play, the composites can be recombined with other materials, as needed, to obtain a better function, such as magnetism and electricity. At present, composites of lightweight carbon materials are CNT (carbon nanotube)/polymer, metal or ceramic CNT (or other lightweight carbon material), metal or metallic oxide–filled CNT (or other lightweight carbon material) and hydrogen storage CNT, and the like. Besides the conventional in situ polymerization, solution composite, and fusion composite, composite technology includes coating and sintering process, plasma spray, bionic induction, electrodeposition, chemical vapor deposition, and chemical liquid phase homogeneous phase (or heterogeneous phase) sedimentation. It is believed that in the future, more composite technologies will emerge to make lightweight carbon composites with various functions.

6.3 Develop New Lightweight Carbon Materials It is practical and important to develop new lightweight carbon materials to satisfy the specific needs of national economy. Carbon material is made mainly from carbon. In general, carbon materials produced before the mid-1940s are called old-fashioned carbon materials, while the carbon products made by graphite with high-purity and high-density carbon fiber, pyrolytic carbon, and vitreous carbon after World War II are called new-type carbon materials. The new carbon materials sometimes can be combined with high polymer materials, metals, or ceramics to form composites, while the old carbon materials to form composites.

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With wide range of variety of carbon-carbon bound combination, the basic structure unit, the minute structure, and the assembly structure, new carbon materials are constantly emerging, among which nano-activated carbon fiber, fullerene, CNT, and activated carbon microballoon have been the research and development hotspot. (1) Nano-activated carbon fiber. Nano-activated carbon fiber is made by activating carbon fiber. It is a kind of nanoparticle on the surface, which is a system of irregular structures and nano space. Its fiber diameter is minute with the micropore structure. When the radius of the micropore is < 2 nm, its pore size distribution is narrow and some specific pores are distributed in a monodisperse way. Microholes in different sizes constitute its structure, and diffusing caused the way of multiple pore size distribution. There is no macropore in nano-activated carbon fiber. Only small amounts of transition pore and micropore spread on the surface of the fiber. Its outside specific surface areas are twice larger than its inside ones, so its rate of adsorption and desorption is fast, which is 10–100 times faster than that of grain size. The space of nano-activated carbon cellosilk beam serves as a macropore, which has a better adsorption to vapor phase and liquid phase. The larger the specific surface area is, the larger the average aperture of micropore is. With increase of specific surface area, micropore volume increased accordingly, adsorption happens in micropore, then filled with micropore. Nano-activated carbon fiber was viewed as one of the best eco-friendly materials in the twenty-first century, which has been successfully applied in gas and liquid purification, noxious gas and liquid adsorbing disposition, solvent recovery, functional electrode, and so on. The study of the structure, function, and application of nanoactivated carbon fiber is new to the world, so it is expected to expand to fields such as information, biology, and environment. (2) Fullerene. Fullerene is the third allotrope apart from diamond and graphite. Its discovery is a milestone in the history of human science and technology development, which greatly promotes the study of carbon cluster Cn . C60 , the core of fullerene family, is a globular cage structure, which resembles the balls used in soccer. C70 is an ellipsoidal cage structure, with a similar shape to an American football. Fullerene is also called buckyball because the other modules in the family are symmetrical and spherical and ellipsoidal in appearance, quite similar to the geodesic dome designed by a noted architectural modeler, Buckminster Fuller. Fullerene molecule is a new all-carbon molecule, with 2 × (10 + M) (M is the numerical value of six-membered ring) carbon atoms in every molecule, which correspondingly constitute 12 five-membered ring and M six-membered ring. C60 is the smallest and the most stable fullerene molecule, as shown in Figure 6.1. Compared with Hückel System, C60 ’s conjugated delocalization big 0 system is nonplanar. Its electronic structure has a smaller aromaticity and certain reactivity. Thus, various reactions such as hydrogenation, halogenation and oxidation can be achieved, on the basis of which a great number of derivatives are synthesized.

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Figure 6.1: Molecule diagram of C60 .

C60 molecules are neutral and non-conducting. But it is a semi-conductor with direct transition of a 1.5eV energy gap. Its low conductivity is similar to that of insulator. It can be transformed from insulator to conductor after mingled some alkalis in C60 or C70 . Its conductivity, at indoor temperature, can be compared with the one with n-mingled polyacetylene. C60 proves to be of certain stability to pressure. Molecule itself is stable under 20 Gpa. After the pressure is removed, it can recover to its original volume, which makes it an excellent lubricant. Fullerene molecule presents unique properties because of its unique structure. Its wide application prospect is expected in the study of superconductivity, microelectronics, optoelectronics, and battery. The following two basic researches are remarkable. One is the application in macromolecule field. It mainly covers three aspects. First, it is C60 polymerization, that is, to synthesize or prepare polymers with C60 . Secondly, it is the charge transfer compound formed by C60 and polymers. Thirdly, it is the generation of polymers through catalytic polymerization reaction by using C60 and its derivatives as catalysts. The other is the application in biology field. The breakthrough in compound water-soluble fullerene derivatives solves the problem of its hydrophobicity, which accelerates the development of C60 and its derivatives in biology. New progress has been made in the resistance to HIV and bacteria, in the splitting decomposition of DNA, and removing the free radical, and in the dual function to biological membrane. The research and application of fullerene involves many subjects, and it is a prospective multi-interdisciplinary field. Currently, most researches are about the preparation, separation, characterization, performance test, and application. Great advances have been made in many aspects, for example, chemical property, new chemical reaction and its rules, modified technique; connecting C60 , as a platform, with a radical group to improve its property of electricity, optics, and magnetic property; increasing the water solubility of C60 to highlight fullerene negative ion chemistry, fullerene hydrosol, its photophysical properties and the assembly of fullerene

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nanoparticles by using its antibiosis and sterilization properties. But further study is needed to solve other problems. (3) Metal/charcoal composite. If metallo-organic compounds and macromolecule complexes, stable and soluble in the air, are carbonized in inert atmosphere, metal/charcoal composites, with neatly arranged particle size and evenly dispersed nanometal ion, are obtained. In the material, metal completely separates from charcoal. The material is invulnerable to oxygen. Metal/charcoal composite material has the characteristics of secular stability and good heat resistance, adjustable ratio of metal. Owing to the solubility of precursors, the material is available in making thin films, powder, fiber, or its granular. With a universal application prospect, it can be used as electronic material, radio-screening material, and new adsorbing material. In addition, it provides a way to make metal ion stay stable, which is of significance in the study of oxidation resistance of metal. The existence of metallics in metal/charcoal composites greatly increases the surface area of activated carbon. Moreover, it is easy to generate the mesopore-dominated activated carbon, which is helpful to adsorb organic macromolecule. Metal can chemically react with acid gases, so it has a good adsorption to acid gases. It can potentially serve as decolorant, deoxidant, deodorant, and so on. Lightweight carbon materials, especially new carbon materials, do have good performance, but there is still a long way to go to popularize them in every field because of some factors, such as its price. It makes sense to develop low-cost adsorbing materials like activated carbon, particularly using biowaste (petroleum coke, furfural residue, coal ash, etc.), to prepare cheap adsorbents.

6.4 Expand the Application of Lightweight Carbon Material With the development of science and technology and the improvement of living standard, lightweight carbon material has been applied in more and more fields. A variety of special lightweight carbon material emerges, such as medical activated carbon, anti-radiation activated carbon, and electronic activated carbon. The application of charcoal in medicine is not new. Compendium of materia medica has record of this: calcined carbon shell can be used to cure dysentery and rotten sore. Now, with the advantages of favorable adsorption, controllable pore structure, and better biocompatibility, as well as nontoxic/side effect, activated carbon is widely applied in the treatment of cancer, gastrointestinal diseases, and the prevention of poison adsorption. The activated carbon anticarcinogen has the property of function release, affinity, lymph tropism, partial anelasticity and less toxic side effect, so it has been used to treat gastric cancer, esophageal cancer, colorectal cancer, and breast cancer, with well-satisfied results. The focus of current clinical research is in the following aspects:

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to adjust the diameter of carbonaceous granular to the requirements of lymphatic system flowing, to develop adsorption-type anti-cancer drug and its dose for different malignant tumor so as to decrease the occurrence of serious complications and lead the drug to kill cancer cells and control lymphatic metastasis. Blood purification is an important way to remove various noxious compounds in blood, including blood perfusion, hemodialysis, and so on. Blood purification concerns not only the treatment of the disease itself, but also the security of blood and blood products, especially virus infection after transfusion. It is a major trend to use activated carbon as the adsorbent in blood purification to remove the toxicants in blood products. Activated carbon is available in blood perfusion by adsorbing the toxicants in blood, and purifying the blood. It is generally accepted that activated carbon is a preferred method to save lives in the treatment of drug poisoning. Hemodialysis is used to dialyze by using dialysis membrane made with cellulose. The drawback of this approach is that it removes only the solutes passing through cellulose membrane, but not those with large molecular weight. The present adsorption-type hemodialyzer, using activated carbon as adsorbent, has been developed. The granular activated carbon used in the hemodialyzer is the spherical activated carbon made from petroleum asphalt and coated processing on the surface has no grain corners. After-coating-process makes it possible to avoid the hemolysis or blood coagulation which is resulted from the direct contact of blood and the activated carbon. Thus, it is not easy to lead to blood contamination because of the dust from mutual rubbing. Furthermore, activated carbon is available here to remove the carbamide and other metabolites in dialysates. In this way, the dialysates can be regenerated. Producing drug and its clinical medication based on activated carbon remains the number one concern in the field of medicine. Many carbon materials are used as conducting materials. As early as 1802 charcoal had been the electrode material in batteries. In 1930, activated carbon electrode battery was invented. Activated carbon is employed to adsorb inorganic or organic electrolyte as electrode to make supercapacitors. Supercapacitors, with proper discharging circuit design, revolutionized storage batteries. Double electric layer capacitors require excellent properties of electrode materials, such as high electric conductivity, large specific surface area, good compatibility, low cost as well as no electrolysis reaction or electrochemical reaction with electrolytes. CNT, as a new carbon material, is best in preparing double electric layer capacitors. With the expansion of double electric layer capacitors in their application trend both domestic and international. Carbon nanotube is special because of its hollow structure and electromagnetic property. The closed pore structure can be released through the way of activation to form abundant pore structures on the tube wall. When electromagnetic wave goes through these pore structures, it will be reflected or

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refracted by the tube wall. New reflection and refraction will occur when those waves encounter the tube wall again. becomes, as promising as the ideal stealth material, that has the property of light and broad adsorption frequency. The repeated reflection and refraction results in the energy loss of electromagnetic waves, and accordingly, activated carbon nanotube becomes, as promising as the ideal stealth material, that has the property of light and broad adsorption frequency. Activated carbon nanotube is a hotspot in energy studies, including supercapacitors, lithium ion secondary batteries, fuel cells, and hydrogen or methane storage.

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Index 0 electrons 36 activated carbon 1 – adsorption capacity 8 – adsorption performance 164 – Aperture expansion 6 – aperture size 4 – applications 11 – classification 6 – microstructure 10 – modification 161 – performance test 10 – pore shape 3 – pore structure 3 – structure 1 – surface modification 8 – surface morphology 163 activated carbon fiber 19, 173, 177 – application 23 – performance 21 – pore structure 20 activation energy 169 activation solution 256 admittance 249 adsorption enthalpy 155, 171 adsorption entropy 154, 155, 176, 177 adsorption equilibrium 45 adsorption free energy 176, 177 adsorption isotherms 41, 42, 135 adsorption theory 39 adsorption time 136 air purification 11 anatase 88 anticarcinogen 350 antitoxin function 339 arc discharge 26 Arrhenius equation 169 Artificial Neural Network Theory 336 avionics 180 band gaps 244 BET surface area 5 blood purification 351 Bragg scattering 244 brookite 88 butyl rubber 329 DOI 10.1515/9783110424751-007

capacitance 247 capacitors 351 carbon matrix composites 272 carbon nanotubes 24 – application 29 – composite materials 31 – crystallization 74 – dosage 145 – modification 28, 146 – MWNTs 26, 139 – preparation 26 – properties 29 – regeneration 158 – structure 25 – surface physical structure 147 – SWNTs 25 carbonyl 7 carboxyl 7 cellosilk 348 chemical adsorption 41, 53 chemical plating 182, 184, 253 chemical reduction 35 Clausius-Clapeyron equation 48, 152, 170 coating curing 239 coating thickness 243 condensation reaction 55 conductivity 225 contacting accumulation 93 corrosion resistance 318 critical frequency 221 curing temperature 242 CVD 5, 27, 67 cyanide ions 96 cyclic oxidation 6 decolorization 117, 120 dehydrogenation 60 depletion layer 250 dielectric constant 193, 220, 237 dielectric layer 276 dielectric loss 195, 205, 221 dielectric thickness 196 differential adsorption heat 47 direct packing method 52 dispergation 256 dispersion 53 drying treatment 239

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Index

DSC 70 dyes 116, 118 – bleaching 117 – decolorizing 121, 122, 123 – molecular surface area 135 – wastewater 116 dynamic adsorption 156 dynamic desorption 156 dynamic magnetization 194 eigenstate 222 electric acoustic microscope 70 electroconductivity 205 electrode material 37 electroless plating 57, 58, 59, 60 – activation 60, 62 – cobalt 65 – iron 65 – nickel 65 – palladium 60 – silver 63, 64 electromagnetic interference shielding 294 electromagnetic parameters 197 electromagnetic pollution 294 electromagnetic shielding 296, 297 energy dispersive spectrometer 209 expanded graphite 16, 131, 132, 133, 191, 211, 212, 230 – application 18 – crystal structure 16 – properties 17 explosives 101 Fenton reagent 108 fractal geometry 134 frequency band 301 Freundlich adsorption isotherm 142, 174 Freundlich isotherm 46 fullerene 348, 349 Gibbs free energy 153, 154 Gibbs-Helmholtz equation 48 graphene 32 – application 36 – photocatalysis 39 – preparation 33 – single-layer 33 – structure 32

graphite 13 – aphanitic 14 – crystalline 14 – intercalation 15 hemodialysis 351 Henry formula 45 High Impedance Surface 245 hydrolysis 55 hydroxyl 7 hysteresis 191, 212, 320 impedance 196, 202 impregnation method 160 impurities 148 information devices 295 infrared absorption 73 infrared spectroscopy 9 integral adsorption heat 47 intersolubility 312 ion doping 111 Jacobian matrix 333 Joule-Lenz’s law 297 Klotz equation 332 Langmuir adsorption isotherm 141 Langmuir isotherm 46 laser evaporation 27 lightweight carbon materials 52 – composite materials 52 – pretreatment 77 – surface modification 53 liquid propellant 75 – degradation 97 – disposal 85, 87 – surface tension 340 – toxicity 76 – UDMH 75 – wastewater 84 magnetic impedance 249 magnetic induction 194 magnetic loss 195, 205 magnetic permeability 193, 215, 237 metallo-organic compounds 350

Index

micro-emulsion 67 microwave modification 163 nano-activated carbon fiber 348 natural light 110, 112 neural network 323 nuclear magnetic resonance 72 orthogonal layout 324 oxidant(s) 94, 226 p – p ∗ transition 115 paint coating 241 permeability 220, 290 permittivity 215 pH 90, 134 photocatalysis 76 – activation 88 – composite photocatalyst 79 – degradation 94, 110, 111 – degradation efficiency 91 – dosage 92 – efficiency 104, 105 – loading times 89 – loading weight 89 – reuse of catalysts 87 – silver doping 92 photocatalysts 38 photonic crystals 244 physical adsorption 30, 41, 45, 53 plating composition 316 plating solution 304 plating structure 316 point of zero charge 8 polyaniline 221, 224 – preparation 222 polyvinyl chloride 329 precipitation method 66 printing 117 pseudo-first-order reaction 167 pseudo-second-order reaction 167

reflection phase 250 reflectivity 202, 203, 220, 237 regression analysis 99 resonance circuit 247 roughening 183 rutile 88 Salisbury absorbing screen 246 scanning electron microscopy 9, 68 sedimentation 312, 313 semiconductor oxidizers 62 silane 240 silver doping 96 silver plating 206, 253 sol–gel method 54 – concentration method 54 – coupling agent 57 – dispersion method 54 sulfosalicylic acid 228 surface plating 314 thermal analysis 70 thermal reduction 35 thermal stability 192 TiO2 38, 77, 102, 105, 110, 284 titration 8 TNT 99, 103, 104, 105, 113 Tollens’ reagent 206, 253 transmission electron microscope 69 – HRTEM 70 treatment of fabric 339 ultrasonic dispersion 280 unsymmetric dimethyl hydrazine 140 UV-light reduction 35 Van der Waals force 37, 41 vortex 298 wave absorbing materials 180 Wheeler equation 333 X-ray diffraction 73, 188, 210, 307

radar absorbing material 181 radical(s) 97, 109 rare earth metal 311 reflection coefficient 268, 271

Yoon equation 333 ZnO 62, 279

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