Heat Treatment: Conventional and Novel Applications 9535107682, 9789535107682

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HEAT TREATMENT CONVENTIONAL AND NOVEL APPLICATIONS

Edited by Frank Czerwinski

HEAT TREATMENT – CONVENTIONAL AND NOVEL APPLICATIONS Edited by Frank Czerwinski

Heat Treatment – Conventional and Novel Applications http://dx.doi.org/10.5772/2798 Edited by Frank Czerwinski Contributors Michal Pohanka, Petr Kotrbáček, Grzegorz Golański, Nguyen Viet Long, Cao Minh Thi, Masayuki Nogami, Michitaka Ohtaki, Wesley M. Dose, Scott W. Donne, Hiroki Nagai, Mitsunobu Sato, Manuel Pedro Fernandes Graça, Manuel Almeida Valente, E-Wen Huang, Yu-Chieh Lo, Junwei Qiao, Fathollah Sajedi, Anton Panda, Jozef Jurko, Iveta Pandová, A.M.A. Mohamed, F.H. Samuel, Frank Czerwinski, Angel Zumbilev, Anna Biedunkiewicz, Sonia Hatsue Tatumi, Alexandre Ventieri, José Francisco Sousa Bitencourt, Katia Alessandra Gonçalves, Juan Carlos Ramirez Mittani, René Rojas Rocca, Shiva do Valle Camargo, Jeffrey C. De Vero, Rusty A. Lopez, Wilson O. Garcia, Roland V. Sarmago, Rabah Bensaha, Hanene Bensouyad

Published by InTech Janeza Trdine 9, 51000 Rijeka, Croatia Copyright © 2012 InTech All chapters are Open Access distributed under the Creative Commons Attribution 3.0 license, which allows users to download, copy and build upon published articles even for commercial purposes, as long as the author and publisher are properly credited, which ensures maximum dissemination and a wider impact of our publications. After this work has been published by InTech, authors have the right to republish it, in whole or part, in any publication of which they are the author, and to make other personal use of the work. Any republication, referencing or personal use of the work must explicitly identify the original source. Notice Statements and opinions expressed in the chapters are these of the individual contributors and not necessarily those of the editors or publisher. No responsibility is accepted for the accuracy of information contained in the published chapters. The publisher assumes no responsibility for any damage or injury to persons or property arising out of the use of any materials, instructions, methods or ideas contained in the book.

Publishing Process Manager Marina Jozipovic Typesetting InTech Prepress, Novi Sad Cover InTech Design Team First published September, 2012 Printed in Croatia A free online edition of this book is available at www.intechopen.com Additional hard copies can be obtained from [email protected]

Heat Treatment – Conventional and Novel Applications, Edited by Frank Czerwinski p. cm. ISBN 978-953-51-0768-2

Contents Preface IX Chapter 1

Design of Cooling Units for Heat Treatment Michal Pohanka and Petr Kotrbáček

1

Chapter 2

Regenerative Heat Treatment of Low Alloy Cast Steel Grzegorz Golański

Chapter 3

Novel Pt and Pd Based Core-Shell Catalysts with Critical New Issues of Heat Treatment, Stability and Durability for Proton Exchange Membrane Fuel Cells and Direct Methanol Fuel Cells Nguyen Viet Long, Cao Minh Thi, Masayuki Nogami and Michitaka Ohtaki

21

41

Chapter 4

Monitoring the Effects of Thermal Treatment on Properties and Performance During Battery Material Synthesis 75 Wesley M. Dose and Scott W. Donne

Chapter 5

Heat Treatment in Molecular Precursor Method for Fabricating Metal Oxide Thin Films 103 Hiroki Nagai and Mitsunobu Sato

Chapter 6

Lithium Niobiosilicate Glasses Thermally Treated 129 Manuel Pedro Fernandes Graça and Manuel Almeida Valente

Chapter 7

Cooling – As a “Heat Treatment” for the Mechanical Behavior of the Bulk Metallic Glass Alloys 173 E-Wen Huang, Yu-Chieh Lo and Junwei Qiao

Chapter 8

Using "Heat Treatment" Method for Activation of OPC-Slag Mortars 191 Fathollah Sajedi

Chapter 9

Deformation Reduction of Bearing Rings by Modification of Heat Treating 215 Anton Panda, Jozef Jurko and Iveta Pandová

VI

Contents

Chapter 10

A Review on the Heat Treatment of Al-Si-Cu/Mg Casting Alloys 229 A.M.A. Mohamed and F.H. Samuel

Chapter 11

Thermochemical Treatment of Metals Frank Czerwinski

Chapter 12

Carbonitriding of Materials in Low Temperature Plasma 287 Angel Zumbilev

Chapter 13

Methodology of Thermal Research in Materials Engineering 309 Anna Biedunkiewicz

Chapter 14

Effects of Heat Treatments on the Thermoluminescence and Optically Stimulated Luminescence of Nanostructured Aluminate Doped with Rare-Earth and Semi-Metal Chemical Element 351 Sonia Hatsue Tatumi, Alexandre Ventieri, José Francisco Sousa Bitencourt, Katia Alessandra Gonçalves, Juan Carlos Ramirez Mittani, René Rojas Rocca and Shiva do Valle Camargo

Chapter 15

Post Deposition Heat Treatment Effects on Ceramic Superconducting Films Produced by Infrared Nd:YAG Pulsed Laser Deposition 371 Jeffrey C. De Vero, Rusty A. Lopez, Wilson O. Garcia and Roland V. Sarmago

Chapter 16

Synthesis, Characterization and Properties of Zirconium Oxide (ZrO2)-Doped Titanium Oxide (TiO2) Thin Films Obtained via Sol-Gel Process 381 Rabah Bensaha and Hanene Bensouyad

247

Preface Heat treatment and surface engineering represent crucial elements in the design and manufacture of strategic components in a wide range of market sectors and industries including air, sea and land transportation, energy production, mining, defense or agriculture. Their influence is broad and of major importance for economics, society and the environment. Although human metallurgy and heat treatment were practiced for millennia, an understanding of the science and associated principles has only been developed in the last century. The progress in heat treatment of bulk alloys was accompanied by an application of heating/quenching to a modification of metal surfaces and development of technologies such as case hardening, thermochemical treatments and coatings, leading in 1980s to a creation of modern surface engineering. Today, surface engineering is seen as a critical enabling technology underpinning major industry sectors. The generally accepted definition of heat treatment is “heating and cooling a solid metal or alloy in such a way so as to obtain specific conditions and/or properties”. Thus, heat treatment represents a combination of thermal and also thermochemical operations aimed at altering mostly physical and mechanical but also chemical properties of materials without changing the product shape. Its ultimate purpose is to increase service life of a product by increasing strength and hardness, or prepare the material for enhanced manufacturability. At a technical level, heat treatment is a technological process which is conducted in furnaces and involves thermal phenomena, phase transformations and mechanical phenomena, mainly stresses. The most pronounced beneficial effect of heat treatment in altering microstructure and modifying properties is to a range of ferrous alloys and nonferrous alloys of aluminum, copper, nickel, magnesium or titanium. Of all materials, steel as the most common and the most important structural material, is particularly suitable for heat treatment. Heat treatment of steel is inherently associated with an improvement in strength, ductility, machinability, formability and involves normalizing, annealing, stress relieving, surface hardening, quenching, tempering, cold and cryogenic treatment. The above definition excludes processes where heating and cooling are performed inadvertently such as welding or forming. A similar case is with metal heating for the purpose of diffusion bonding. Apart from metals, heat treatment used for non-metallic

X

Preface

materials is also excluded from this definition. However, controlled heating and quenching accompany many modern manufacturing technologies with bulk material precursors as well as thin films and coatings. Similarly as for bulk metallic alloys, thermal routes affect the microstructure and properties of bulk non-metallic materials, particulate forms and thin layers. Understanding and controlling these processes is of the same importance as in the case of conventional heat treatment. This book was created by contributions from experts in different fields of materials science from over 20 countries. It offers a broad review of recent global developments in an application of thermal and thermochemical processing to modify the microstructure and properties of a wide range of engineering materials. Although there is no formal partition of the book, chapters are split into two sections. Part 1, with chapters 1-8 covers the conventional heat treatment with processing of bearing rings, wrought and cast steels, aluminum alloys, fundamentals of thermochemical treatment, details of carbonitriding and a design of cooling units. Part 2, with chapters 9-17 describes a use of non-conventional thermal routes during manufacturing cycles of such materials as vanadium carbides, titanium dioxide, metallic glasses, superconducting ceramics, nanoparticles, metal oxides, battery materials and slag mortars. Each chapter contains a rich selection of references, useful for further reading. A mixture of conventional and novel applications, exploring a variety of processes employing heating, quenching and thermal diffusion, makes the book very useful for a broad audience of scientists and engineers from academia and industry. In order to benefit from opportunities created by heat treatment, its capabilities for each individual material and service conditions should be understood and implemented at the stage of a component design. Since the design stage requires often multidisciplinary knowledge, metallurgy and heat treatment may not be there the core expertise. Therefore, I hope that the book will also attract an audience from outside of metallurgy area not only to generate the genuine interest but also to create new application opportunities for modern heat treatment and surface engineering. Frank Czerwinski CanmetMATERIALS, Natural Resources Canada Hamilton, Ontario

Chapter 1

Design of Cooling Units for Heat Treatment Michal Pohanka and Petr Kotrbáček Additional information is available at the end of the chapter http://dx.doi.org/10.5772/50492

1. Introduction Microstructure and nature of grains, grain size and composition determine the overall mechanical behavior of steel. Heat treatment provides an efficient way to manipulate the properties of steel by controlling the cooling rate. The way of heat treatment depends on many aspects. One of the most important parameter is the amount of production. Another important parameter is the size of products. We focus here on large production such as interstand [1] and run-out table cooling of hot rolled strip, run-out table cooling of sheets and plates, cooling of long products at the exit from a rolling mill, cooling of rails, tubes and special profiles [2], continuous hardening and heat treatment lines for steel strips. Such a treatment is called in-line heat treatment of materials and has become frequently used by hot rolling plants. This method achieves the required material structure without the necessity of reheating. In-line heat treatment is characterized by running of hot material through the cooling section. However, many of discussed topics can be applied on smaller production as well. The design procedure of cooling sections for obtaining the demanded structure and mechanical properties is iterative research involving several important steps. We begin with the Continuous Cooling Transformation (CCT) diagram for the selected material. Numerical simulation of cooling follows to find appropriate cooling intensity and its duration. Knowing the desired cooling intensity new cooling section is designed and tested under the laboratory conditions [3]. From the laboratory experiments boundary conditions are obtained and tested using a numerical model. When the best solution is found it is tested on the real sample and the result structure is studied. In most cases the process must be repeated as the CCT diagram is aimed at a different size of the sample and the cooling rate in the designed section is not constant.

2 Heat Treatment – Conventional and Novel Applications

2. Strategy of design When preparing a design of the cooling system for the continuous heat treatment, we should know the optimum cooling regime for the material and the product. Any continuous heat treatment process needs to vary the cooling intensity with time. Moreover, the practice has shown that the results obtained by using small samples (usually for the CCT diagram) are usually different from the results achieved when using real products of a large cross-section because it is not possible to achieve the identical temperature regimes in the whole volume due to the low diffusivity. We cannot expect same behavior on product as on the small sample. Next considerations focus on technical means that can be used to achieve the demanded temperature mode. There are varieties of technical methods for hot steel cooling; one of them uses the spray cooling. The cooling section should ensure reaching a required temperature history in the cooled piece prescribed by the metallurgists. The nozzles applied allow controlling the cooling over a wide range. The cooling intensity of groups of the nozzles must be measured and then the results obtained can be used in a numerical model of the temperature field in the cooled material.

2.1. Leidenfrost effect and its impact It should be understand that intensity of cooling strongly depends on the surface temperature. So called Leidenfrost effect can be observed above certain temperature. During this effect a liquid, which is near significantly hotter object than the liquid's boiling point, produces vapor layer which insulates the liquid from the hot object and keeps out that liquid from rapid boiling. This is because of the fact that at temperatures above the Leidenfrost point, the part of the water, which is near hot surface, vaporizes immediately on contact with the hot plate and the generated gas keeps out the rest of the liquid water, preventing any further direct contact between the liquid water and the hot plate. The temperature at which the Leidenfrost effect begins to occur is not easy to predict. It depends on many aspects. Ones of them are velocity and size of droplets. As a rough estimate, the Leidenfrost point might occur for quite low temperatures such as 200 °C. On the other hand for high water velocity the Leidenfrost point can be even above 1000 °C. Fig. 1 shows measured heat transfer coefficients (HTC) for water-air mist nozzle. Graph shows three measurements for same nozzle using varying water and air parameters. Three regimes can be found. The first one is for low temperatures when the HTC is relatively high and decreasing slowly. This part is below Leidenfrost point. From certain temperature HTC decreases rapidly. This is a transient regime in which some droplets are above Leidenfrost point and some are below that point. For the last regime HTC is relatively low and is constant or may be increasing due to the increasing radiation with the increasing surface temperature. Designed cooling section should work in the first regime for low temperatures or in the third regime with almost constant HTC. It is strongly recommended to avoid the second transient regime as the product surface temperature is not usually at uniform temperature. Due to the strong dependency of HTC on surface temperature non homogeneous cooling is achieved and causes distortion of the product.

Design of Cooling Units for Heat Treatment 3

4500 Air 3bar Water 0.06l/s 4000

Air 2bar Water 0.06l/s

3500

Air 2bar Water 0.20l/s

HTC [W/m².K]

3000 2500 2000 1500 1000 500 0 200

400

600 800 T surface [°C]

1000

1200

Figure 1. Moving Leidenfrost point for water-air mist nozzle and various water and air conditions.

2.2. Nozzle types and controllability Nozzle produces usually one of three typical sprays: flat-jet, full-cone, and solid-jet (see Fig. 2). However, other shapes can be found such as hollow-cone, square, spiral etc. An important parameter is controllability of the cooling section and intensity of cooling. The water-air mist nozzles can be used for a soft cooling and a wide controllability range (see Fig. 3). The HTC can vary from several hundreds of W/m².K up to several thousand of W/m².K. Water-air mist nozzles are not the cheapest ones and also the pressurized air is expensive in terms of power consumption. Water-only nozzles can often provide a lower cost solution. Small full-cone nozzles with high pressure and bigger distance from surface can provide also very soft cooling. On the other hand with high pressure flat-jet or solid-jet nozzles at small distances HTC over 50000 W/m².K can be obtained even for high surface temperatures (see Fig. 4). This results in enormous heat flux above 50 MW/m². The distance of the nozzle form surface is very important in this case for flat-jet nozzles because for 100 mm the HTC can be 50000 W/m².K but for 1000 mm it can be similar to water-air mist nozzle. As conclusion we can say that for soft cooling air-mist or full-cone water only nozzles can be used and for hard cooling flat-jet nozzles with small distances are used. In some cases solid-jet nozzles are used for hard cooling but there are often two major problems: large amount of water generates a water layer on the product and the spray spot is small which causes non homogenous cooling. On the other hand, clogging is not big problem for solid-jet nozzles.

4 Heat Treatment – Conventional and Novel Applications

Figure 2. Flat-jet, full-cone, and solid-jet nozzles with computed water distribution on flat surface

3500 Air 3bar Water 0.06l/s 3000

Air 3bar Water 0.20l/s Air 2bar Water 0.06l/s

HTC [W/m².K]

2500

Air 2bar Water 0.20l/s

2000 1500 1000 500 0 -150

-100

-50 0 50 Position in the moving direction [mm]

100

150

Figure 3. Controllability of water-air mist nozzle for surface temperatures 1000 °C

2.3. Influence of product velocity on heat transfer coefficient Three measurements are compared when the only different parameter is the casting speed. The first experiment was stationary with no movement, the second experiment used a velocity of 2 m/s and the last experiment was done for a velocity of 5 m/s. These three experiments used the identical water-air mist nozzle, and the same pressure settings were

Design of Cooling Units for Heat Treatment 5

70000 Small at 15 bar

HTC [W/m².K]

60000

Small at 190 bar

50000

Small at 380 bar

40000

Big at 200 bar

30000 20000 10000 0 -100

-50 0 50 Position in moving direction [mm]

100

150

Figure 4. Distribution of HTC under spray for high pressure flat-jet nozzles for surface temperature 1000 °C.

used in all experiments - water pressure of 2 bar, air pressure of 2 bar. Fig. 5 shows the distribution of heat transfer coefficient in experimental group with a variation of velocity. HTC for the stationary case (not possible in mill) is symmetrical and the peak is narrow. The cooling intensity decreases with the increasing velocity. Heat transfer coefficient distribution is more non-symmetrical when product speed increases. The observed effect is caused by the flow on the surface and different vapor forming conditions in front and behind the impinging jet.

3. Cooling intensity and numerical models In order to design a cooling section, knowledge of the cooling intensity is required for a group of nozzles and nozzle headers. Exact knowledge of the heat transfer coefficient as a function of spray parameters and surface temperature is the key problem for any design work. The cooling intensity is a function of several parameters, mainly, nozzle types, chosen pressures and flow rates, surface temperature of a material, and velocity of a material movement whilst under spray. There is no function available which describes cooling intensity using all the mentioned parameters. This is the reason why real measurement is absolutely necessary.

3.1. Experimental procedure During the in-line heat treatment the product is moving so our testing sample should be also moving through the cooling section. Fig. 6 shows schematically a suitable experiment used for obtaining boundary conditions for a numerical simulation. The hot sample is moving at

6 Heat Treatment – Conventional and Novel Applications

Stationary 2 m/s 5 m/s

-500

-250

0

250

500

Figure 5. Influence of velocity at heat transfer coefficient.

Nozzle

Spray Hot material

Thermocouples

Insulation

Direction of sample movement

Figure 6. Moving sample with embedded thermocouples cooled down by water spray

Design of Cooling Units for Heat Treatment 7

1000

50000

900

45000

800

40000

700

35000

600

30000

500

T measured

25000

400

T surface

20000

Position 300

15000

HTC

200

10000

100

5000

HTC [W/m².K]; Position [mm]

T [°C]

prescribed velocity which is similar to real conditions. The sample passes under spray which cools down the hot sample. For a simple shape like plate it is recommended to insulate all surfaces excluding the one on which the cooling intensity is investigated. One or more thermocouples are embedded in the sample and measure temperature during the experiment. The installed thermocouples should not disturb the cooled surface. This is the reason why they should be installed inside the sample, not on the investigated surface. In principle when all surfaces are insulated except the one which is investigated one thermocouple is enough. However, this thermocouple should be as close to the investigated surface as possible. Otherwise the resolution of description of boundary conditions will be degraded. After the measurement an inverse algorithm is used to compute boundary conditions on the investigated surface from the measured temperature history inside the sample. An example of recorded temperature history is shown in Fig. 7.

0

0 25

50

75

100

125 150 Time [s]

175

200

225

250

Figure 7. Example of recorded temperature history by one thermocouple inside the sample, computed surface temperature above the installed thermocouple, recorded position of the thermocouple in the cooling section, and computed heat transfer coefficient. Cooling section equipped with five rows of flat-jet nozzles.

Heat transfer test bench presented in [4] is designed so that it enables progression of samples up to the weight of 50 kg with infinitely adjustable speed from 0.1 to 6 m/s (see Fig. 7). On the supporting frame there is a carriage moving, on which the sample under examination with embedded temperature sensors and measuring system is fixed (see Fig. 9). The carriage's progression is provided by a hauling rope through a drive pulley and a motor with a gearbox. The motor is power supplied by a frequency converter with the possibility

8 Heat Treatment – Conventional and Novel Applications

Spraying nozzles Tested sample Moving carriage Motor with gearbox

Supporting frame

Electric heater

Figure 8. Heat transfer test bench

Figure 9. Examples of boundary conditions measurements on steel plate, rail, and pipe.

Design of Cooling Units for Heat Treatment 9

of a smooth change of speed. The direction of the carriage can be reversed and passages repeated in a requiring number. The whole cycle is programmed and controlled through the superior PC. There is a spraying section in the central sector where arbitrary jets configuration can be arranged when distribution of heat transfer coefficients or heat fluxes must be measured. The sample is equipped with thermocouples connected to the data logger. The thermocouples are calibrated before use and the results of calibration are used to eliminate dynamic error in measurement of highly transient thermal processes. Before the actual experiment the carriage with the sample is positioned to the electric heater and it is heated to the required temperature inside the furnace. After the temperature in the sample is stabilized, the heating device is removed, the stand is turned to spraying position, the pump for the water gets going and the carriage's runs through the cooling section. The position of the cooled surface can be horizontal with spraying upper or bottom surfaces or vertical. Signals from the sensors are read by the data logger which moves together along with the sample. At the same time, the signal indicating the actual carriage's position is recorded as well. After performing the required number of passes through cooling zone, data are exported from data logger's internal memory into the computer for further processing.

3.2. Inverse computation of boundary conditions and numerical models Information from temperature histories in a particular depth under the investigated surface are used as entry parameters for the thermal conduction's inverse task. Inverse task outputs are surface temperature histories, heat flows, and heat transfer coefficients (HTC) as function of time and position. Most often, in mathematical models the boundary condition of the 3'd type is used where heat flow is specified by the HTC value and the cooling water temperature. If the boundary conditions of a solid must be determined from transient temperature measurements at one or more interior locations, it is an inverse heat conduction problem (IHCP) during which the dispersed impulse on boundary must be found. The IHCP is much more difficult to solve than the direct problem. Such problems are extremely sensitive to measurement errors. There are number of procedures that have been advanced for the solution of ill-posed problems in general. Tikhonov has introduced the regularization method [5] to reduce the sensitivity of ill-posed problems to measurement errors. The mathematical techniques for solving sets of ill-conditioned algebraic equations, called single-value decomposition techniques, can also be used for the IHCP [6]. There were extremely varied approaches to the IHCP. These included the use of Duhamel’s theorem (or convolution integral) which is restricted to linear problems [7]. Numerical procedures such as finite differences [8][9][10] and finite elements [11] were also employed, due to their inherent ability to treat non-linear problems. Exact solution techniques were proposed by Burggaf [12], Imber and Khan [13], Langford [14], and others. Some techniques used Laplace transforms but these are limited to linear cases [15]. Combined approach is also described in [16]. The improvement in artificial intelligence has brought new approaches, such as genetic algorithm [17] and neural networks [18][19][20].

10 Heat Treatment – Conventional and Novel Applications

All the mentioned algorithms need a precise mathematical model of the tested sample for computing the direct heat conduction problem. Analytical methods may be used, in certain cases, for exact mathematical solutions of conduction problems. These solutions have been obtained for many simplified geometries and boundary conditions and are well documented in the literature [21][22][23]. However, more often than not, geometries and boundary conditions preclude such a solution. In these cases, the best alternative is the one using a numerical technique. For situations where no analytical solution is available, the numerical method can be used. Nowadays there are several methods that enable us to solve numerically the governing equations of heat transfer problems. These include: the finite difference method (FDM), finite volume method (FVM), finite element method (FEM), boundary element method (BEM), and others. For one-dimensional model with constant material properties there exists nice similarity. All of the FDM, FVM, and FEM with tent weighting function equations can be put in a similar form:



q t   d   T1   T2   2 T1  T2    1c x , dt x







(1)





d    Tj 1  2  Tj   Tj  1  2 Tj  Tj 1  2 Tj 1  Tj , dt x x q t   d ,  TN 1   TN   2 TN 1  TN   N   c x dt x

(2)

(3)

where  and  have the values listed in Tab. 1. Equations (1–3) are restricted to temperature-independent thermal properties but the concepts can be extended to T-variable cases. In general for multidimensional models and temperature dependent material properties the simplest equations are obtained for FDM while the complexity of equation for FVM and FEM is several times higher.

FDM FVM FEM





 +

1/2 3/8 2/6

0 1/8 1/6

1/2 1/2 1/2

Table 1. Values of the  and  of Eq. (1–3)

3.3. Phase change implementation Physical processes, such as solid/liquid and solid state transformations, involve phase changes. The numerical treatment of this non‑linear phenomenon involves many problems. Methods for solving the phase change usually use a total enthalpy H , an apparent specific heat coefficient c A , or a heat source q . The nature of a solidification phase change can take many forms. The classification is based on the matter in the phase change region. The most common cases follow:

Design of Cooling Units for Heat Treatment 11

a. b. c.

Distinct: The phase change region consists of solid and liquid phases separated by a smooth continuous front – freezing of water or rapid solidification of pure metal. Alloy: The phase change region has a crystalline structure consisting of grains and solid/liquid interface has a complex shape – most metal alloys. Continuous: The liquid and solid phases are fully dispersed throughout the phase change region and there is no distinct interface between the solid and liquid phase – polymers or glasses.

In a distinct phase change, the state is characterized by the position of the interface. In such cases the class of the so called front tracking methods is usually used. However, in cases b) and c) the models use the phase fraction. The phase change process can be described by a single enthalpy equation H     gd H d sd  gl H l sl      kT  t

(4)

where g is the phase volume fraction, s is the phase velocity, and subscript d and l refer to solid and liquid phases (or structure A and structure B), respectively [24]. The k is (in this case) a mixture conductivity defined as k  g d k d  g l kl

(5)

H  gs   dcd dT  gl  l cl dT  l cl L

(6)

and H is the mixture enthalpy T

T

Tref

Tref

where Tref is an arbitrary reference temperature. To overcome the problem of the non-linear (discontinuity) coefficient of a specific heat a non-linear source term is used. The term H t can be expanded as g H T  cvol H l t t t

(7)

Neglecting convection effects in Eq. (4) and substituting Eq. (7) results in cvol

T     kT   q t

(8)

where q   H

gl t

(9)

Eq. (4) is non-linear and it contains two related but unknown variables H and T . It is convenient to reformulate this equation in terms of a single unknown variable with

12 Heat Treatment – Conventional and Novel Applications

non-linear latent heat. Song [25] and [26] Comini uses so called Apparent heat capacity. The apparent specific heat can be defined as cA 

where

dg dH  cvol   H l dT dT

(10)

cvol  gd  dcd  gl  l cl

H 

  lcl  dcd  dT  l L.

T

(11)

Tref

Neglecting convection effects and substituting into Eq. (4) yields apparent heat capacity equation cA

T     kT  . t

(12)

Another approach is total enthalpy. From Eq. (6) it can be written T   H cvol   H gl cvol .

(13)

Substitution in Eq. (4) will result in a total enthalpy equation

 k   k  H     H       Hgl  . t  cvol   cvol 

(14)

3.4. Sequential identification inverse method For measurements where installed thermocouple inside the investigated body disturbs also surface temperature as it is very close to investigated surface HTC must be computed directly by an inverse method. Classical and very efficient sequential estimation proposed by Beck [27], which computes heat flux instead of HTC, has several limitations. Thus new sequential identification method was developed by Pohanka to solve such inverse problems. The basic principle of time-dependent boundary conditions determination (heat flux, HTC, and surface temperature) from measured transient temperature history is based on cooling (or heating) of heated (or cold) sample with thermocouple installed inside (see Figure 6). Let us assume one-dimensional inverse problem with 3D model involving installed thermocouple for simplicity:

    

Known dimensions of the sample. Known thermal temperature-dependent material properties of the sample. Known temperature profile at the beginning of the cooling (usually constant). All surfaces are insulated except the cooled one. HTC is not dependent on position.

Design of Cooling Units for Heat Treatment 13

The sample is heated before starting the measurement. Cooling is applied on one surface and temperature response inside the sample is recorded. Time-dependent boundary conditions are computed using inverse technique from the measured temperature history (see Fig. 7). Cooling of more surfaces can also be investigated when more thermocouples is used. This new proposed approach computes step by step (time step) heat transfer coefficients (HTC) on the investigated surface using measured temperature history inside the cooled or heated solid body. However, this method can be very easily changed to compute any kind of boundary conditions, e. g. heat flux. The method uses sequential estimation of the time varying boundary conditions and uses future time steps data to stabilize the ill-posed inverse problem [28]. To determine the unknown surface HTC at the current time tm , the measured temperature responses Tm* are compared with the computed temperature Tm from the forward solver using n future times steps

SSE 



mn i  m 1

T

* i

 Ti



2

.

(15)

Any forward solver can be used e. g. finite volume method described by Patankar [29]. The computational model should include drilled hole, whole internal structure of the embedded thermocouple, and temperature dependent material properties. At time zero homogeneous temperature is in the sample and thereby zero heat flux and thereby zero HTC on all surfaces is assumed. Otherwise there cannot be homogeneous temperature. This can be done e. g. by heating in furnace after enough long time. If the initial temperature is not homogeneous some modification of the algorithm is necessary for the first time step. The algorithm starts at time index zero when the HTC is equal to zero (see Fig. 10). The algorithm uses forward solver and it computes temperature response at thermocouple position for linearly changing (increasing or decreasing) HTC (see HTC1 and T1 computed in Fig. 10) over few time steps. These time steps are called future time steps n and five of them are used in Fig. 10–Fig. 11. Determination of minimum number of necessary future time steps to stabilize sequential algorithm is described in [28]. The computed and measured temperatures histories are compared using Eq. (15) the same one as for sequential Beck approach. The slope of linearly changing HTC defined as v

h t

(16)

should be changed until the minimum of SSE function in Eq. (15) is found. Such a minimum says that the computed temperature history matches the measured temperature history the best for used linearly changing HTC during n future time steps. When the best slope of HTC is found the forward solver is used to compute temperature field in the next time step using the computed boundary conditions. The algorithm is repeated for next time steps until the end of recorded temperature history is reached (see Fig. 11). For k

14 Heat Treatment – Conventional and Novel Applications

120

6000 T measured

100

5000

T1 computed T2 computed

4000

HTC1

HTC [W/m².K]

T [°C]

80

HTC2 60

3000 Optimum is somewhere here

40

2000

20

1000

0

0 0

1

2

3

4

5 6 Time index

7

8

9

10

Figure 10. Measured temperature history and two computed temperature histories using two different slopes of HTC for n future time steps.

10000

i=0

9000

i=2

i=1

i=3

HTC [W/m².K]

8000 i=4

7000

i=5

6000 5000 4000 3000 2000 1000 0

0

1

2

3

4

5 6 Time index

Figure 11. Real HTC and six optimum linearly changing HTC.

7

8

9

10

measured time steps only k  n time steps can be computed owing to the use of future data. This method works perfectly when real HTC is almost linear in time. When the slope is abruptly changing the computed HTC curve is slightly smoother than the real one; the more future time steps are used the smoother is the computed curve of HTC (bigger difference between computed and real HTC) but the sequential identification inverse algorithm is more stable.

Design of Cooling Units for Heat Treatment 15

The SSE function described by Eq. (15) has only one minimum and is dependent only on one variable – slope of HTC (see Eq. 16). Even more the function is very close to parabolic function near the searched minimum because it is sum of square of temperature differences. Brent’s optimization method [30], which uses inverse parabolic interpolation, is perfect candidate for finding the minimum of the SSE function in Eq. (15).

SSE

Brent’s optimization method is based on parabolic interpolation and golden section. The searched minimum must be between two given points 1 and 2 (see Fig. 12). Convergence to a minimum is gained by inverse parabolic interpolation. Function values of the SSE function are computed only in few points. A parabola (dashed line) is drawn through the three original points 1, 3, 2 on the SSE function (solid line). The function is evaluated at the parabola’s minimum, 4, which replaces point 1. A new parabola (dotted line) is drawn through points 3, 4, 2. The algorithm is repeated until the minimum with desired accuracy is found. If the three points are collinear the golden section [30] is used instead of parabolic interpolation.

SSE parabola through 1, 3, 2

1

parabola through 3, 4, 2

2 3

4 slope of HTC Figure 12. Convergence to a minimum by inverse parabolic interpolation.

3.5. Evaluation of boundary conditions To demonstrate the procedure a real measurement is used. The cooling section consists of five rows of flat-jet nozzles. The heated sample passes repeatedly under the spraying nozzles. Several thermocouples in one row, which is perpendicular to the sample movement direction, were installed in the sample to be able to investigate also the cooling homogeneity across the sample. For simplicity we focus now on only one thermocouple, however, it is easy

16 Heat Treatment – Conventional and Novel Applications

to do the procedure for all thermocouples. Temperature record from such measurement is shown in Figure 7. Recorded position (zigzag line) of the thermocouple is shown as well and demonstrates the repeated passes through cooling section. Using the inverse method boundary conditions were computed: surface temperature and HTC. All the shown lines are function of time; however, for numerical simulation we need HTC as function of position and surface temperature. We start with surface temperature from the measurement. See Figure 13 with shown surface temperature drawn using green line as function of position. The green lines represents surface temperatures through which the plate pass during experiment. In the place where the green line is shown we also know HTC from the measurement. HTC values are shown using the color scale. HTC values between green lines are interpolated. This chart shows HTC distribution as a function of surface temperature and position and is the key point for accurate numerical simulation. HTC values above the most top green line are extrapolated and are not accurate as there are no data available from measurement. We should avoid usage of these values during numerical simulation. 42695 40000 35000 30000 25000 20000 15000 10000 5000 14 HTC [W/m².K]

Figure 13. Prepared boundary conditions for numerical simulation from measurement shown in Fig. 7. Chart shows HTC as function dependent on position in cooling section in the direction of sample movement and on surface temperature of the cooled sample.

3.6. Numerical simulation Having prepared boundary conditions we can do numerical simulation of cooling of products of various material properties and of various thicknesses. By repeating the boundary conditions we can simulate long cooling section with more rows of cooling nozzles. An example of such simulation is shown in Fig. 14 and is drawn in CCT. You can see computed temperature at the surface and at the center of the material. It is obvious that

Design of Cooling Units for Heat Treatment 17

the cooling rate for the surface temperature is higher than in the center. The results are drawn in CCT diagram, however, you should not that the cooling rate is far away from constant. This is very important because the CCT diagram is only informative and the final structure has to be verified by experimental measurement. There are three major reasons why cooling rate is not constant. One is caused by passing product under separate row of nozzles. The passes are obvious from T surface curve in Fig. 14. You can see drops of temperature when the product is passing under spray followed by reheating due to the internal capacity of the heat in the product. The second reason is mentioned Leidenfrost point. You can see low cooling rate in the center up to 20 s as the surface temperature is above Leidenfrost point and after that cooling rate is increasing and reaching maximum which is almost triple in comparison to value above Leidenfrost point. Decreasing of cooling rate is followed as the surface temperature is getting closer to the temperature of water. The third reason is low diffusivity for big products. The product cannot be cooled down at the same cooling rate on the surface as in the center. The lower is the diffusivity and the bigger is the product the bigger difference is between the cooling rate on the surface and in the center.

Figure 14. Simulation of cooling in the CCT diagram.

4. Verification conducted at pilot test bench It is important to understand that cooling rate in cooling section in industrial application is far away from constant value using which CCT diagrams are obtained. Verification

18 Heat Treatment – Conventional and Novel Applications

functionality of a newly designed cooling system prior to its plant implementation is essential. The design obtained by using the numerical model must be verified and finetuned by further full-scale cooling tests. Pieces of tube, rail, wire or plate of real dimensions with implemented thermocouples are tested in the designed cooling section. The length of a laboratory test bench shown in Fig. 8 and Fig. 9 is limited, hence the sample must be accelerated prior entering the cooling section, to a velocity normally used in a plant, and after pass through cooling section, the direction of movement is reversed. In this way, the sample moves several times under the cooling sections. This cooling process is controlled by computer to simulate running under the long cooling section used normally in the plant. Nozzles, pressures, and header configurations are tested. The design of the cooling and the pressures used are modified until the demanded temperature regime and final structure is obtained. The full-scale material samples are then cut for the tests of material properties and structure. When heat treatment is performed on larger product such as rail, mainly its head, it is not possible to achieve same cooling rate at surface and in the center of rail head. The cooling rate near surface are much faster and even more reheating can appear and can cause very different material properties (see Fig. 15). As the rail head passed under the spray the surface temperature dropped fast and was followed by reheating due to the heat stored inside the head. The reheating caused lower hardness near the surface as shown in Fig. 14. The center of the head is harder because no reheating occurred in the bigger depth. To avoid this problem the cooling section should be modified. One solution is to use more row of nozzles with smaller row pitch and also nozzles with lower HTC. This can be achieved by smaller pressure or smaller nozzles. Also replacement of flat-jet nozzle by full-cone nozzles can be considered. The Leidenfrost temperature should be also considered. We should be above Leidenfrost temperature or below but definitely not near to avoid big different cooling rates for small changes in surface temperature.

450 400

Temperature [°C]

350

Hardness HV 0.3

300 250 200 150 100 50 0 0

100

200 Time [s]

300

0

10 20 Depth [mm]

30

Figure 15. Measured temperature histories in a rail head in two depths and measured micro-hardness in rail head after heat treatment.

Design of Cooling Units for Heat Treatment 19

5. Concluding remarks The design of cooling sections used for in-line heat treatment for hot rolling plants is very extensive work. It utilizes laboratory measurement, numerical modeling, inverse computations, and also pilot mill tests. The first step is the search of the best cooling regime for steels for which this is not yet known. The second step is to obtain a selection of technical means in order to guarantee obtaining the prescribed cooling rates. Nozzle configurations and cooling parameters are selected and controllability of the cooling section is checked. The final step of the design is a laboratory test using a full size sample simulating plant cooling. Design based on laboratory measurement therefore minimizes the amount of expensive experimentation performed directly on the plant. Elimination of potential errors and enabling adjustment of control models in the plant is possible after the cooling process is tested in laboratory conditions.

Author details Michal Pohanka and Petr Kotrbáček Brno University of Technology, Heat transfer and Fluid Flow Laboratory, Brno, Czech Republic

6. References [1] Raudenský, M.; Horský, J.; Hajduk, D.; Čecho, L. Interstand Cooling - Design, Control and Experience. Journal of Metallurgical and Mining Industry, 2010, No. 2, Vol. 3, pp. 193-202. [2] Hnízdil, M.; Raudenský, M.; Horký, J.; Kotrbáček P.; Pohanka M. In-Line Heat Treatment and Hot Rolling. In proc. AMPT 2010, Paris, France, October 2010, pp. 563-568. [3] Horký, J.; Raudenský, M.; Kotrbáček, P. Experimental study of long product cooling in hot rolling. Journal of Materials Processing Technology, August 1998, Vol. 80-81, pp. 337-340. [4] Pohanka, M.; Bellerová, H.; Raudenský, M.; Experimental Technique for Heat Transfer Measurements on Fast Moving Sprayed Surfaces. Journal of ASTM International, Sept. 2010, Vol. 1523, pp. 3-15. [5] Tikhonov, A. N.; Arsenin, V. Y. Solution of Ill-Posed Problems. Washington, D.C.: Winston, 1977. ISBN 0470991240. [6] Mandrel, J. Use of the singular value decomposition in regression analysis. Am. Stat., 1982, Vol. 36, pp. 15-24. [7] Stloz, G. Jr. Numerical solutions to an inverse problem of heat conduction for simple shapes. Int. J. Heat Transfer, 1960, Vol. 82, pp. 20-26. [8] Smith, G. D. Numerical solution of partial differential equations. UK: Oxford University Press, 1978. ISBN 0-198-59650-2. [9] Beck, J. V. Nonlinear estimation applied to the nonlinear heat conduction problem. Int. J. Heat and Mass Transfer, 1970, Vol. 13, pp. 703-716. [10] Beck. J. V.; Litkouhi, B.; St. Clair, C. R. Jr. Efficient sequential solution of the nonlinear inverse heat conduction problem. J. Numerical Heat Transfer, 1982, Vol. 5, pp. 275-286. [11] Bass, B. R. Applications of the finite element to the inverse heat conduction problem using Beck’s second method. J. Eng. Ind., 1980, Vol. 102, pp. 168-176.

20 Heat Treatment – Conventional and Novel Applications

[12] Burggraf, O. R. An exact solution of the inverse problem in heat conduction theory and applications. Int. J. Heat Transfer, 1964, Vol. 86C, pp. 373-382. [13] Imber, M.; Khan, J. Prediction of transient temperature distributions with embedded thermocouples. Al AA J., 1972, Vol. 10, pp. 784-789. [14] Lengford, D. New analytic solutions of the one-dimensional heat equation for temperature and heat flow rate both prescribed at the same fixed boundary (with applications to the phase change problem). Q. Appl. Math., 1967, Vol. 24, pp. 315-322. [15] Grysa, K.; Cialkowski, M. J.; Kaminski, H. An inverse temperature filed problem of the theory of thermal stresses. Nucl. Eng. Des., 1981, Vol. 64, pp. 169-184. [16] Sláma, L.; Raudenský, M.; Horský, J.; Březina, T.; Krejsa, J. Evaluation of quenching test of rotating roll with unknown time constant of sensor using genetic algorithm. Int. Conf. Mendel, Brno, 1996. [17] Raudenský, M.; Pohanka, M.; Horský, J. Combined inverse heat conduction method for highly transient processes. In Advanced computational methods in heat transfer VII, Halkidiki: WIT Press, 2002, pp. 35–42. ISBN 1-85312-9062. [18] Dumek, V.; Grove, T.; Raudenský, M.; Krejsa, J. Novel approaches to the IHCP: Neural networks. In Int. symposium on inverse problems - Inverse problems in Engineering Mechanics, Paris, 1994, pp. 411-416. [19] Krejsa, J.; Sláma, L.; Horský, J.; Raudenský, M.; Pátiková, B. The comparison of traditional and non-classical methods solving the inverse heat conduction problem. In Int. Conf. Advanced Computational Methods in Heat Transfer, Udine, July 1996, pp. 451-460. [20] Pohanka, M.; Raudenský, M.; Horský, J. Attainment of more precise parameters of a mathematical model for cooling flat and cylindrical hot surfaces by nozzles. In Advanced computational methods in heat transfer VI. Madrid: WIT Press, 2000, pp. 627–635. ISBN 1-85312-818-X. [21] Incropera, F. P.; DeWitt, D. P. Fundamentals of Heat and Mass Transfer. 4th ed. New York: Wiley, 1996. ISBN 0-471-30460-3. [22] Kakac, S.; Yener, Y. Heat Conduction. New York: Hemisphere Publishing, 1985. [23] Poulikakos, D. Conduction Heat Transfer. Englewood Cliffs, NJ: Prentice-Hall, 1994. [24] Voller, V. R.; Swaminathan, C. R. Fixed grid techniques for phase change problems: A review. Int. J. for Numerical Methods in Engineering, 1990, Vol. 30, pp. 875-898. [25] Song, R.; Dhatt, G.; Cheikh, A. B. Thermo-mechanical finite element model of casting systems. Int. J. for Numerical Methods in Engineering, 1990, Vol. 30, pp. 579-599. [26] Comini, G.; Giudice, S. D.; Saro, O. A conservative alghorithm for multidimensional conduction phase change. Int. J. for Numerical Methods in Engineering, 1990, Vol. 30, pp. 697-709. [27] Beck, J. V.; Blackwell, B.; Charles, R. C. Inverse Heat Conduction: Ill-posed Problems. New York: Wiley, 1985. ISBN 0-471-08319-4. [28] Pohanka, M. Limitation of thermal inverse algorithm and boundary conditions reconstruction for very fast changes on boundary. In Engineering mechanics 2007. Svratka (Czech Republic), 2007, pp. 229–230. ISBN 978-80-87012-06-2. [29] Patankar, S. V. Numerical Heat Transfer and Fluid Flow. Hemisphere Publishing Corporation, 1980. ISBN 0-891-16522-3. [30] William, H. P.; Saul A. T.; William, T. V.; Brian, P. F. Numerical Recipes in C. 2nd ed. 1997. ISBN 0-521-43108-5.

Chapter 2

Regenerative Heat Treatment of Low Alloy Cast Steel Grzegorz Golański Additional information is available at the end of the chapter http://dx.doi.org/10.5772/50505

1. Introduction Cylinder and valve chamber castings of large power steam turbines are usually made of low alloy Cr - Mo - V and Cr - Mo cast steels. Forming of the microstructure and mechanical properties of cast steels takes place through heat treatment, thus far mostly consisting of normalizing and tempering. As a result of such a treatment the cast steels of diverse wall thickness reveal microstructures from ferritic – pearlitic to bainitic – ferritic with various ferrite, pearlite and bainite amount. Operation of the cast steels under creep conditions contributes to the occurrence of deformations, fractures and changes in the microstructure, decreasing their functional properties. The resistance to crack expressed by impact energy falls drastically. The value of impact energy of test pieces taken from cast steels after long-term service was considerably below the required level of 27J, frequently reaching the value of 6 - 10J (Fig. 1).

Figure 1. Impact energy of turbine cylinder cast steel in the post-operating condition

22 Heat Treatment – Conventional and Novel Applications

Along with the fall of impact energy there is also a growth of nil ductility transition (NDT) temperature, frequently rising above 50  60 oC. Large decrease in crack resistance is usually accompanied by a slight decrease in the strength properties (Fig. 2). Unfavourable changes in mechanical properties of the castings are related to the changes in microstructure which occur during long term service at elevated temperatures, first and foremost to:  

the preferential precipitation of carbides on grain boundaries, as well as changes in morphology and dispersion of precipitates; segregation of phosphorus and other trace elements to grain boundaries and near boundary areas; disintegration of pearlite or/and bainite areas.

Figure 2. Changes in tensile strength (TS) and yield strength (YS) depending on the time of service

Lowering of impact energy as a result of long-term service depends largely on the as received microstructure of a cast steel. The impact energy decrease is the smallest in the case of tempered bainite microstructure or bainitic – ferritic microstructure, with ferrite amount not higher than 5% (Fig. 3). High impact energy of quenched and tempered cast steel, considerably higher than 100J, guarantees that during long-term service of steel castings with low phosphorus volume fraction ( 0.015% P), the impact energy will not fall below the minimum required value of 27J. Similar tendency has been noticed in new low-alloy bainitic 7CrWVMoNb9 – 6 (P23) steel. Impact energy in the case of this cast steel, whose microstructure is of tempered bainite in the as-received condition, after around 10 years of operation at the temperature of 555 oC and pressure 4.2MPa, was on the level of 70 – 80 J/cm2. Degradation of the microstructure of castings and the related gradual decreasing of mechanical properties, however, do not limit the possibility of their further operation, especially as in most of the examined castings there were no irreversible creep changes observed. One of the conditions for extending the time of safe operation for cast steels above the calculated service time is running the process of revitalization of the castings.

Regenerative Heat Treatment of Low Alloy Cast Steel 23

Figure 3. Influence of phosphorus amount and microstructure on impact energy KV of the Cr – Mo – V cast steel after long-term operation at the temperature of 535 oC

The revitalization process consists in heat treatment of turbine cylinders in order to regenerate the structure to the extent which allows the improvement of plastic properties (increase in impact energy, decrease in the nil ductility transition temperature). Regenerative heat treatment of castings applied in industry so far consists in normalizing/full annealing from the austenitizing temperature with the following high-temperature tempering/under annealing. The ferritic – pearlitic or ferritic – bainitic structure, obtained as a result of the above-mentioned heat treatment, provides the required impact energy of KV > 27J, however, with the strength properties being comparable to those after operation. Modern hardening plants applying aqueous solutions of polymers as a cooling agent make the cooling of massive castings possible at programmed rate which provides an optimum structure throughout the whole casting section. Regenerative heat treatment at costs not exceeding 40% of a new casting’s price, allows obtaining functional properties (yield strength, impact energy, NDT temperature) comparable to the properties of new castings. Regenerated cylinder is fit for further operation at least for another 100 000 hours. In order to achieve an improvement in mechanical properties of cast steels after long-term operation, the following changes in the degraded microstructure are necessary:     

grain refinement – leads to an increase in crack resistance, decreases the NDT temperature and raises yield strength; eliminating irreversible brittleness caused by phosphorus segregation to grain boundaries and interphase boundary: matrix/carbides; removal of the needle shaped ferrite (Widmannstätten’s ferrite); removal of pearlite precipitated on ferrite grain boundaries; dissolving of carbides in austenite, especially the carbides precipitated on grain boundaries, in order to obtain the required strength properties in the regenerated microstructure (hardness and tensile strength).

24 Heat Treatment – Conventional and Novel Applications

The research aim: The aim of the performed research was to determine the influence of regenerative heat treatment on the microstructure and properties of Cr – Mo – V cast steel with its microstructure degraded by long-term service and mechanical properties being lower than the minimum ones expected in the new castings.

2. Material for research The material for study was Cr – Mo – V low-alloy L21HMF cast steel (designation according to Polish Standards) with its chemical composition given in Table 1. Test pieces for investigation were taken from an inner cylinder of a steam turbine serviced for around 186 000 hours at the temperature of 540 oC and pressure of 13.5MPa. C

Mn

Si

P

S

Cr

Mo

V

0.19

0.74

0.30

0.017

0.014

1.05

0.56

0.28

Table 1. Chemical composition of the L21HMF cast steel, % mass.

2.1. Microstructure and properties of the examined cast steel after service In the post-operating condition the L21HMF cast steel was characterized by a degraded ferritic-pearlitic microstructure (Fig. 4). The dominant phase in the microstructure after operation was quasi-polygonal ferrite. The size of ferrite grain in the cast steel was diverse and ranged from 88.4 to 31.2m, which corresponds to the grain size grade: 4  7, according to ASTM standard scale.

Figure 4. The microstructure of L21HMF cast steel after service

Long-term service of Cr – Mo – V cast steel contributed to the changes in microstructure, including: 

preferential carbide precipitation of M23C6 carbides on ferrite grain boundaries. In some areas the number of carbides precipitated on boundaries was so large that they formed the so-called “continuous grid” of precipitates;

Regenerative Heat Treatment of Low Alloy Cast Steel 25



the process of degradation of pearlite grains consisting in fragmentation, spheroidization and coagulation of pearlitic carbides. Performed identifications have revealed the occurrence of the M3C and M7C3 type of precipitations in those areas (Fig. 5);

M3C

M7C3

Figure 5. Morphology and type of carbides in pearlite grain



precipitation of compound carbide complexes called „H – carbides”. The compound complexes of precipitates are created by MC and M2C carbides, where the MC carbide is a “horizontal” precipitation, while M2C carbides are precipitations of “vertical” type (Fig. 6). This sort of compound precipitations is defined as „H – carbide”. During longterm operation the MC carbide is enriched in molybdenum as a result of diffusion. The growth of molybdenum concentration in the interphase areas of MC/matrix makes it possible for the “needle-shaped” precipitations of M2C (rich in molybdenum) to nucleate on the interphase boundary: MC carbide/ferrite. These processes run more intensely in the border areas of grains, which results in the occurrence of precipitation free zones. The appearance of such zones may be the cause of slow reduction of the strength properties, the yield strength in particular, during long-term operation. The occurrence of this type of complexes results in a decay of fine-dispersion MC carbides which may lead to the fall of creep resistance in the serviced materials. A similar phenomenon can be seen at present in the new high-chromium cast steels for power industry, where the Z phase is being formed and developed at the expense of fine dispersion precipitates of the MX type, which causes a drastic drop of creep resistance of these cast steels.

26 Heat Treatment – Conventional and Novel Applications

Figure 6. Precipitation of „H - carbide” type in the cast steel

Mechanical properties of L21HMF cast steel after long-term service are shown in Table 2. Material

TS MPa

YS MPa

El. %

KV J

HV30

DBTT oC

Microstructure

L21HMF

545

305

26

10

156

65

ferritic-pearlitic

Requirements of PN *

500  670

min. 320

140  197**

___

___

min. 20 min. 27

*- PN - 89/ H - 83157 , ** - hardness according to Brinell,

Table 2. Mechanical properties and microstructure of the L21HMF cast steels after service

Tensile strength and elongation of the examined cast steel after service were higher than the minimum values required for new castings, while the value of yield strength was lower than the minimum required by 15MPa. Hardness of the investigated cast steel after operation amounted to 156HV30. A significant feature of the material proving its strain capacity, apart from elongation determined in the static test of tension, is the value of impact energy. Knowledge of this factor gives the possibility of assuming the right temperature for the hydraulic pressure tests used in industrial practice, as well as the right conditions of start-ups and shut-downs of a boiler, adjusted to the material state after long-term service. After operation the examined cast steel was characterized by low impact energy amounting to 10J, and the cracking of samples occurred through the transcrystalline fissile mechanism (typical for brittle fractures) with little energy absorbed due to the limited plastic strain preceding the decohesion. Fissile cracking requires little energy supply which is necessary for crack propagation, hence the low impact energy of the cast steel after service (Fig. 7). Low impact energy of the examined materials is related to the nil ductility temperature (brittle temperature). The fracture appearance transition temperature determined for the examined cast steel amounted to 65 oC.

Regenerative Heat Treatment of Low Alloy Cast Steel 27

Figure 7. Transcrystalline ductile fracture with areas of microductility and secondary cracks

2.2. Influence of austenitizing parameters on the size of prior austenite grain Influence of austenitization parameters on the prior austenite grain size has been described in a quantitative way using chosen stereological and statistical parameters, such as: mean diameter and mean area of grain, and also the coefficient of variation of grain size ν was calculated. The ν coefficient is characterized by the inhomogeneity of grain sizes: the more heterogeneous grains in terms of size within the casting, the higher the values of variation coefficient. The tests were run for the austenitizing temperature range of - 910  970 oC with the „measurement step” – 15 oC and times of holding at the austenitizing temperature: 3 and 5 hours. The character of austenite grain distributions was determined using the λ - Kolmogorov test of goodness of fit with normal distribution for logarithmed values (Fig. 8). The assumed significance level was  = 0.01, with its limiting statistics value amounting to 1.63. Selected logarithm-normal layouts of mean diameters and mean surface areas of former austenite grains for austenitization option of 925 oC and holding time 3 hours, are shown in Fig. 8. Obtained results of the tests are presented in Table 3 and 4 and graphically shown in Fig. 9 ÷ 11. Heat Min. treatment Amount diameter n parameters, of grain, oC/h m

Max. diameter of grain, m

Diameter of grain, m

Standard deviation

emp

=0.01

λ emp

λ α  0.01

910/3 925/3 940/3 955/3 970/3

976 969 954 964 2024

2 2 2 2 2

29 30 31 38 297

11.34 9.84 10.16 14.08 22.14

6.36 5.34 6.22 9.34 17.73

1.54 1.36 1.56 1.36 2.26

1.63 1.63 1.63 1.63 1.63

0.945 0.834 0.957 0.834 1.387

910/5 925/5 940/5 955/5 970/5

946 915 959 937 2034

2 2 2 2 2

27 28 32 39 324

9.36 11.00 9.05 12.02 23.67

5.70 7.01 5.41 8.37 18.31

1.35 1.55 1.33 1.18 1.43

1.63 1.63 1.63 1.63 1.63

0.828 0.951 0.816 0.724 0.877

Table 3. The results of measurements and calculations of the size of prior austenite grains for the cast steel

28 Heat Treatment – Conventional and Novel Applications

Heat Min. area Max. area Mean area treatment Amount of grain, of grain, of grain, parameters, n m2 m2 m2 oC/h

Standard deviation

emp

=0.01

λ emp

λ α  0.01

910/3

976

4

695

142.67

204.58

1.74

1.63

1.067

925/3

969

3

741

104.27

173.60

1.27

1.63

0.779

940/3

954

3

810

121.34

172.77

1.40

1.63

0.859

955/3

964

3

1302

187.41

379.31

1.59

1.63

0.975

970/3

2024

3

95722

812.62

2138.56

1.99

1.63

1.221

910/5

946

3

608

101.07

162.93

1.07

1.63

0.656

925/5

915

2

741

144.72

248.95

1.61

1.63

0.988

940/5

959

3

842

94.03

147.10

1.00

1.63

0.613

955/5

937

2

1533

391.66

423.09

0.98

1.63

0.601

970/5

2034

3

102305

915.33

2408.43

1.53

1.63

0.939

Table 4. The results of measurements and calculations of the size of prior austenite grains for the cast steel

a)

b)

Figure 8. Logarithm – normal layout of grains for : a) mean diameter; b) mean surface area

Regenerative Heat Treatment of Low Alloy Cast Steel 29

M e a n d ia m e te r,  m

25 20

y = 0.0079x 2 - 14.687x + 6819.6 R2 = 0.8959

15 10 5

y = 0.0072x 2 - 13.388x + 6221.5 R2 = 0.9946

a) 0 900

910

920

930

940

950

960

970

980

o

Temperature of austenitization, C

M ean area,  m 2

1000 y = 0.4369x 2 - 811.88x + 377200 R2 = 0.9205

800 600

y = 0.4154x 2 - 768.36x + 355395 R2 = 0.9697

400 200

b)

0 900

910

920

930

940

950

960

970

980

o

Temperature of austenitization, C Figure 9. Influence of austenitization temperature on: a) mean grain diameter; b) mean surface area depending on the time of holding (3hrs – orange, 5hrs – black) of L21HMF cast steel

The mean diameters of grains and their mean areas change continuously and reveal lognormal layouts on the significance level of  = 0.01 (λemp./ λ0.01 < 1). The exceptions were the treatment variants as follows: austenitization temperature of 910 oC and time - 3h for the mean area (fulfilled for lower significance level of  = 0.001) and the temperature of 970 oC and time - 3h for both: mean diameter and mean area of prior austenite grain. Within the range of austenitization temperatures: 910 ÷ 940 oC for holding times: 3 and 5 hrs, the mean diameters and areas of prior austenite grain do not reveal any considerable differences. (Fig. 9). The values of the mean diameters and areas for this range of austenitization amounted to: 9.05  11.34 m and 94.03  144.72 m2, respectively. The above-mentioned measurements can be confirmed by the calculated values of  coefficient, specifying heterogeneity in terms of grain sizes, which are the lowest for this range of austenitizing temperatures (Fig. 10), and by the distributions of frequency of cumulated grains (Fig. 11).

30 Heat Treatment – Conventional and Novel Applications

Figure 10. Interrelation between the heterogeneity factor (ν) of former austenite grain size in the cast steel and the austenitizing parameters for: a) mean diameter; b) mean grain area

Figure 11. Distributions of frequency of cumulated grains

Regenerative Heat Treatment of Low Alloy Cast Steel 31

At the temperature of 970 oC the grain growth was observed – mean diameter increased over two times, while the mean area about five times in comparison with the temperature range of 910  940 oC.

2.3. Determining the influence of cooling rate on the microstructure and properties In order to determine the influence of cooling rates, allowing proper selection of parameters for the regenerative heat treatment, the TTT curves were plotted for L21HMF cast steel. On the basis of results achieved by means of dilatometric tests, a graph was drawn up, as shown in Fig 12. It illustrates the influence of cooling rate in the temperature range of 800 ÷ 500 oC on the structure and hardness of the investigated cast steel.

Figure 12. Influence of the cooling rate on structure and hardness of the cast steel

Analysis of the curves presented in Fig. 10 allows to state that in the case of L21HMF cast steel, whose chemical composition is given in Table 1, austenite cooled at 0.004 K/s ≤ v8-5 ≤ 0.017 K/s gets transformed into ferrite and pearlite. The rate of cooling for austenite: 0.023 < v8-5 ≤ 0.869 K/s makes it possible to obtain ferritic – pearlitic – bainitic structures. Whereas after cooling of the cast steel at the range of 0.869 K/s < v8-5 ≤ 14.630 K/s bainitic – ferritic structures were obtained, with an increasing bainite volume fraction as the cooling rate increased. Bainitic structure with around 6% volume fraction was received for the cooling rate of v8-5  14.630 K/s.

32 Heat Treatment – Conventional and Novel Applications

2.4. Influence of heat treatment on the microstructure and properties of L21HMF cast steel The L21HMF cast steel was subject to heat treatment consisting in three-hour austenitizing of test pieces at the temperature of 910 oC and the following cooling at the rate corresponding to the processes of: bainitic hardening, normalizing and full annealing. The test pieces, bainite-hardened and normalized, were then tempered in the temperature range of 690  730 oC and 690  720 oC, respectively. While the test pieces cooled slowly from the austenitizing temperature (fully annealed), were subject to ( + ) annealing (under annealing) at the temperatures of 780  860 oC. Examples of microstructure of the examined cast steel after heat treatment are illustrated in Fig. 13.

a

b

Figure 13. Microstructure of cast steel after: a) service; b) bainitic hardening and tempering; b) normalizing and tempering; c) full annealing and tempering

Bainitic hardening made it possible to obtain bainitic – ferritic microstructure in Cr – Mo – V cast steel. The amount of ferrite in the microstructure did not exceed 6%. In the tempered microstructure there were numerous precipitations of carbides on the lath boundaries, as

Regenerative Heat Treatment of Low Alloy Cast Steel 33

well as inside and on the boundaries of prior austenite grain. Matrix after heat treatment was characterized by high dislocation density, however, some sparse polygonized areas were observed as well - showing lower density of dislocations (Fig. 14). Presence of the polygonized areas in the cast steel after heat treatment can be caused by the difference in chemical composition of particular grains resulting from a dendritic micro segregation or from the lack of austenite homogeneity during heat treatment. Differences of chemical composition may cause local decrease in the temperature of recrystallization.

Figure 14. Microstructure of the L21HMF cast steel after heat treatment (bainitic hardening and hightemperature tempering)

Normalizing and tempering allowed obtaining tempered bainitic – ferritic structure with around 20% of ferrite in the Cr – Mo – V cast steel. The observed microstructures after bainitic hardening and normalizing, apart from ferrite amount in the structure, differed in bainite morphology as well. After bainitic hardening only the “needle-shaped” form of bainite was observed, and it was morphologically similar to martensite, which indicates lower bainite presence in the structure (it can also be proved by the characteristic arrangement of carbides illustrated in Fig. 14). After normalization, however, the Cr – Mo – V cast steel microstructure showed the “feathery” bainite form, which indicates the presence of upper bainite. Apart from the “feathery” bainite also some single areas of „needle-shaped” bainite could be seen. The identifications of precipitates performed by means of the extraction carbon replicas revealed in the investigated cast steel after heat treatment (in the tempered bainitic and bainitic – ferritic structure) the occurrence of the following carbide types: MC, M3C, M7C3 and M23C6. The study of mechanical properties at room temperature has shown that the structure of high-temperature tempered bainite provides the combination of high strength properties and impact energy. Tensile strength and yield strength after tempering exceeded the minimum requirements considerably, and similarly, the impact energy was several times

34 Heat Treatment – Conventional and Novel Applications

higher than the required minimum of 27J for the new castings (Table 5, Fig. 15). Tempering of L21HMF cast steel with bainitic structure at the temperatures which are 10 and 20 oC higher than the maximum tempering temperature recommended by the standard, i.e. at 720 and 730 oC, caused an increase in impact energy by 8 and 35%, respectively, with the hardness decrease by 2 ÷ 5% in comparison with the tempering temperature of 710 oC (Fig. 15). Therefore, it can be concluded that for the cast steels of bainitic microstructure it is possible to apply higher temperatures of tempering compared to the ones recommended by the standards, without concern that the strength properties can go down below the required minimum. Apart form obtaining high impact energy with the required strength properties maintained, it also allows to achieve the microstructure of higher thermodynamic stability, which can guarantee slower process of its degradation.

140

260 255

130

HV30

KV

250

HV30

110

240 100

KV, J

120

245

235 90

230

80

225

70

220 215

690

700

710

720

730

60

o

Tempering temperature, C Figure 15. Influence of the tempering temperature on hardness and impact energy of the L21HMF cast steel with bainitic structure

The cast steel of tempered mixed (bainitic – ferritic) microstructure was characterized by the strength properties on a similar level as the cast steels with bainitic microstructure. However, the crack resistance of those cast steels was almost two times as low compared to that of cast steels with bainitic microstructure (Table 6). High impact energy of the cast steel with the microstructure of tempered bainite is a consequence of large total amount of grain boundaries (boundaries of bainite packets) and high ductility of the tempered microstructure of lower bainite. Whilst, lower impact energy

Regenerative Heat Treatment of Low Alloy Cast Steel 35

of the cast steel with mixed bainitic – ferritic structure results from the presence of ferrite in the microstructure, which favours the fissile cracking, and from the presence of upper bainite, characterized by greater brittleness than lower bainite. Full annealing allows to obtain ferritic – pearlitic microstructure for the examined cast steel grade (Fig. 13d), with pearlite located mostly on ferrite grain boundaries. In pearlite the processes of fragmentation and spheroidization of carbides could be observed. The ferritic – pearlitic microstructure obtained as a result of repeated cooling from the austenitizing temperature was morphologically similar to the microstructure after longterm service.

Heat treatment parameters

TS MPa

YS MPa

El. %

KV J

HV30

Microstructure

after service

545

305

26

10

156

ferritic-pearlitic

bainitic hardening + 720 oC/4h

728

620

18

104

228

bainitic

normalizing + 720 oC/4h

721

594

17

62

220

bainitic-20%ferritic

full annealing + 720 oC/4h

558

336

27

26

153

ferritic-20%pearlitic

full annealing + 800 oC/4h

552

316

31

42

162

ferritic-20%pearlitic

full annealing + o 820 C/4h

550

324

28

42

164

ferritic-20%pearlitic

*PN requirements

500  670

min. 320

min. 20

min. 27

140 **  197

___

*- PN - 89/ H - 83157 ; ** - hardness according to Brinell

Table 5. Microstructure and properties of the L21HMF cast steel after heat treatment

Mechanical properties

36 Heat Treatment – Conventional and Novel Applications

180 160 140 120 100 80 60 40 20 0

162

161

164

165

KV, J

42

42

41

45

800

820

840

860

HV30

o

Temperature, C Figure 16. Change in the values of hardness and impact energy of the cast steel depending on the temperature of ( + ) annealing

For the L21HMF cast steel of ferritic – pearlitic microstructure it is required to apply ( + ) annealing (under annealing) instead of tempering which did not always provide the required impact energy. Applying under annealing causes: dissolution of carbides precipitated on grain boundaries during slow cooling from the temperature of austenitization, decrease of phosphorus segregation on ferrite grain boundaries and further reduction of austenite grain size. This allows to obtain the required strength properties and impact energy KV on the level ~ 40J. The influence of ( + ) annealing temperature on the value of impact energy and hardness is presented in Fig.16.

The performed heat treatment, apart from the changes in microstructure and properties of the examined cast steels, also caused a change in the mechanism of cracking (Fig. 17). In the cast steel of high-temperature tempered bainite structure, on the entire surface under the fracture, there was a transcrystalline ductile fracture initiated by fine-dispersion precipitates of carbides and sulfide inclusions (Fig. 17a). The characteristic feature of plastic cracking is its ability to absorb significant amounts of energy connected with plastic deformations preceding the decohesion. The cast steel of bainitic – ferritic structure was subject to decohesion through mixed mechanism. Directly under the notch, at a depth of about 1.0 ÷ 1.5 mm, cracking proceeded in plastic manner through transcrystalline ductile mechanism. Below the area of plastic strain, fissile cracking could be observed, running through a transcrystalline fissile mechanism with micro fields of ductile character. The cast steel with regenerated ferritic – pearlitic structure, obtained as a result of slow cooling and under annealing, was cracking through a mechanism similar to decohesion of the cast steel after service, i.e. transcrystalline fissile mechanism with micro fields of ductile character (Fig. 17b).

Regenerative Heat Treatment of Low Alloy Cast Steel 37

a)

b)

Figure 17. Cracking mechanism of cast steel: a) transcrystalline ductile for tempered bainitic microstructure; b) transcrystalline fissile for ferritic – pearlitic microstructure

3. Summary The research performed on the L21HMF cast steel, taken from a steam turbine cylinder serviced for around 186 000 hours at the temperature of 540 oC, has revealed that long-term service contributed to: the processes of recovery and polygonization of ferrite grains, preferential precipitation of M23C6 carbides on grain boundaries and formation of „H – carbide” complexes near the boundary areas of ferrite grains. During long-term operation the strength properties were decreasing slowly – yet faster in the case of yield strength than tensile strength, and the impact energy decreased drastically below the required minimum level of 27J. Changes in the microstructure and properties of the long-term serviced cast steel do not eliminate the possibilities of their further safe operation. Extending the safe operation time beyond the calculative time of 100 000 hours (with the target up to 200  250 000 hours) is possible thanks to regenerative heat treatment. Performed research has proved that applying bainitic hardening instead of normalizing/full annealing, thus far applied in the castings, allows to achieve the best combination of high strength properties and very high impact energy. Moreover, the bainitic microstructure makes it possible to apply high temperatures of tempering, amounting to 710  730 oC. This allows increasing the stability of microstructure of long-term serviced cast steels without concern for reduction in the strength properties below the required minimum. High impact energy KV > 100J of the cast steel with high-tempered bainite structure guarantees that after long-term operation the impact energy will not drop below the minimum required level of 27J. Applying normalizing for the castings allows to obtain bainitic – ferritic microstructure, which is characterized by similar strength properties as the cast steel with tempered bainitic microstructure, with the impact energy, however, being almost two times as low. What

38 Heat Treatment – Conventional and Novel Applications

seems evident here, is the negative influence of ferrite in the microstructure on the impact strength. The ferritic – pearlitic microstructure, obtained as a result of slow cooling of the castings from the austenitizing temperature (full annealing), allows to obtain the strength properties comparable to those after service and impact energy on the level of 40J. After the process of full annealing it is recommended to apply the ( + ) annealing (under annealing) instead of the process of tempering, which makes it possible to obtain the required impact energy.

Author details Grzegorz Golański Institute of Materials Engineering, Czestochowa University of Technology, Poland

4. References [1] Zieliński A., Dobrzański J., Krztoń H., Structural changes in low alloy cast steel Cr- Mo – V after long time creep service, JAMME, 25, 2007, 33 [2] Stachura S., Changes of structure and mechanical properties in steels and cast steels utilised in increased temperatures, Energetyka, 2, 1999, 109 [3] Balyts’kyi O.I., Ripei I.V., Protsakh Kh. A., Degradation of the cast elements of steam turbines of thermal power plants made of 20KhMFL steel in the course of long – term operation, Materials Science, 41, 3, 2005, 423 [4] Dobosiewicz J., Influence of operating conditions on the changes in mechanical properties of steam turbine cylinders, Energetyka, 1992, 1, 27 [5] Stachura S., Kupczk J., Gucwa M., Optimization of structure and properties of Cr – Mo and Cr – Mo – V cast steel intended for use at increased temperature, Foundry Review, 54, 5, 2004, 402 [6] Stachura S., Trzeszczyński J., The choice of the regenerative thermal treatment of the Cr – Mo and Cr – Mo –V cast steel, Inżynieria Materiałowa, 6, 1997, 227 [7] Lu Z., Faulkner R.G., Flewitt P. E. J., The role of irradiation – induced phosphorus segregation in the DBTT temperature in ferritic steel, Mater. Sc. Eng., A 437, 2006, 306 [8] Islam M. A.,. Knott J. F, Bowen P., Kinetics of phosphorus segregation and its effect on low temperature fracture behaviour in 2.25Cr - 1Mo pressure vessel steel, Materials Sc. and Techn., 21, 1, 2005, 76 [9] Molinie E., Piques R., Pineau A., Behaviour of a 1Cr – 1Mo – 0.25V steel after long – term exposure – I. Charpy impact toughness and creep properties, Fatique Fract. Eng. Mater. Struct., 14, 5, 1991, 531 [10] Stachura S., Golański G., Metallographic and mechanical properties of steel and cast steel after long term service at elevated temperatures, Report BZ – 202 – 1/01 unpublished researched [11] Golański G., Stachura S., Kupczyk J., Kucharska – Gajda B., Heat treatment of cast steel using normalization and intercritical annealing, Arch. Foundry Eng., 7, 2007, 123

Regenerative Heat Treatment of Low Alloy Cast Steel 39

[12] Komai N., Masuyama F., Igarashi M., 10 – year experience with T23(2.25Cr – 1.6W) and T122 (12Cr – 0.4Mo – 2W) in a power boiler, Transations of the ASME, 127, 2005, 190 [13] Trzeszczyński J., Grzesiczek E.,. Brunne W, Effectiveness of solutions extending operation time of long operated cast steel elements of steam turbines and steam pipelines, Energetyka, 3, 2006, 179 [14] Rehmus – Forc A., Change of structure after revitalization cylinders of a steam turbine, Inżynieria Materiałowa , 2006, 3, 265 [15] Golański G., Influence tempering temperature on mechanical properties of cast steels, Archives of Foundry Eng., 8, 4, 2008, 47 [16] Dobosiewicz J., Reasons for regenerating steam turbine cylinders, Energetyka, 1996, 11, 634 [17] Łukowski J., Modern installations for quenching with the application of aqueous polimer solutions, Przegląd Odlewnictwa, 51, 2001, 2, 57 [18] Bakalarski A., Nowarski A., Sus J., Wyszyński K., Theoretical investigation and experimental application of aqueous polymer solution for quenching products of low-alloy cast steel, Przegląd Odlewnictwa, 51, 2001, 2, 61 [19] Golański G., Stachura S., Gajda B., Kupczyk J., Influence of the cooling rate on structure and mechanical properties of L21HMF cast steel after regenerative heat treatment, Archives of Foundary, 6, 21, 2006, 143 [20] Zieliński A., Dobrzański J., Golański G., Estimation of residual life of L17HMF cast steel elements after long – term service, J. Achievements in Materials and Manufacturing Eng.., 34, 2, 2009, 137 [21] Golański G., Microstructure and mechanical properties of G17CrMoV5 – 10 cast steel after regenerative heat treatment, J. Pressure Vessel Techn., 132, 2010, 064503-1 [22] Golański G., Stachura S., Kupczyk J., Kucharska – Gajda B., Optimisation of regenerative heat treatment of G21CrMoV4 – 6 cast steel, Archives Mater. Sc. Eng., 28, 6, 2007, 341 [23] Murphy M.C., Branch G. D., The microstructure, creep and creep – rupture properties of Cr – Mo – V steam turbine castings, JISI, 1969, 1347 [24] Senior B. A., A critical review of precipitation behavior in 1Cr – Mo – V rotor steels, Mater. Sc. Eng. A, 1988, 103, 263 [25] Williams K. R., Wilshire B., Microstructural instability of 0.5Cr – 0.5Mo-0.25V creep – resistant steel during service at elevated temperatures, Mater. Sci. Eng., 1981, 47, 151 [26] Danielsen H. K., Hald J., Behaviour of Z phase in 9 – 12 %Cr steels, Energy, 1, 1, 2006, 49 [27] Golański G., Mechanical Properties of G17CrMoV5 – 10 Cast Steel after Regenerative Heat Treatment, Solid State Phenom. 147 – 149, 2009, 732 [28] Ryś J., Stereology of materials, FOTOBIT Publ., Cracow, 1995 [29] Taylor J.P., Blondeau R., The respective roles of the packet size and the lath width on toughness, Metall. Trans., 7A, 1976, 891 [30] H. Kotilainen, The micromechanisms of cleavage fracture and their relationship to fracture toughness in a bainitie Low Alloy Steel, VTT, Espoo, Finland, 1980

40 Heat Treatment – Conventional and Novel Applications

[31] Bhadeshia H. K. D. H., Bainite in steels, 2nd edition, The University Press Cambridge, Cambridge, UK, 2001 [32] PN - 89/ H – 83157 Cast steels for elevated temperature applications. Grades.

Chapter 3

Novel Pt and Pd Based Core-Shell Catalysts with Critical New Issues of Heat Treatment, Stability and Durability for Proton Exchange Membrane Fuel Cells and Direct Methanol Fuel Cells Nguyen Viet Long, Cao Minh Thi, Masayuki Nogami and Michitaka Ohtaki Additional information is available at the end of the chapter http://dx.doi.org/10.5772/51090

1. Introduction Traditionally, Pt and Pd based catalysts are widely studied in the continuous developments of next fuel cells (FCs) with the critical issues of energy and environment technologies. So far, Pt and Pd based catalysts have been mainly used in the anodes and the cathodes in FCs by a electrode-membrane technology. In spite of the large advantages of Pt based catalysts in electro-catalysis for FCs, many problems of high cost remain. In addition, so far Pt and Pd catalysts have still exhibited very good catalytic activity and selectivity of hydrogen and oxygen adsorption as well as hydrogen evolution reaction (HER), hydrogen oxidation reaction (HOR) for the dissociation of hydrogen into protons (H+) and electrons (e-), and oxygen reduction reaction (ORR). At present, FC technologies and applications are polymer electrolyte fuel cell (PEFC) or also known as proton exchange membrane FC (PEMFC), phosphoric acid FC (PAFC), alkaline FC (AFC), molten carbonate FC (MCFC), solid oxide FC (SOFC). The typical features include operating temperature (C) for low-temperature PEMFC and DMFC of about 50-80 C, power density ~350 Mw/cm2, fuel efficiency ~40-65%, lifetime >40,000 hr, capital cost >200$/kW [5,49,52,173], and other practical applications. According to hydrogen and oxygen reaction, electro-oxidation of carbon monoxide (CO) is intensively studied in low temperature FCs. In DMFCs, methanol oxidation reaction (MOR) in catalytic activity of Pt catalyst is very crucial to improve the whole performance. Therefore, scientists have considerably focused on the various ways of improving HOR, ORR, and MOR in the catalyst layers of various FCs, PEMFCs, and DMFCs [1-3]. So far, ORR has become an important mechanism investigated in PEMFCs and DMFCs for their large-scale commercialization. Recently, U.S. Department of Energy Fuel Cell Technologies

42 Heat Treatment – Conventional and Novel Applications

Program (DOE Program), and New Energy and Industrial Technology Development Organization (NEDO Program) in Japan have supported large Research and Development programs (R&D) of FCs and FC systems for stationary, portable and transportation applications, such as FC vehicles. In addition, FCs become promising technology to address global environmental challenges in energy, science and nature issues [4-8]. Now, various DMFCs can work at low and intermediate temperatures up to 150 C [9]. Thus, next fuel cells also can meet the urgent demands for green energy.

Figure 1. Features of fuel cells and heat engine in the direct or indirect conversion processes from chemical energy into electric energy. Excellent advantage of fuel cells is direct energy conversion

Today, the proton exchange membranes, typically such as perfluorosulfonic acid (PFSA) membranes or Nafion® for FC applications are presented [51-53]. Interestingly, charge carriers in FCs are various kinds of H+ (PEMFC), H+ (DMFC), OH- (AFC), H+ (PAFC), CO3= (MCFC), and O= (SOFC) [4-9,49,52,173]. Figure 1 shows various energy conversion processes from chemical energy into electric energy through both FCs and heat engine. The operation principle of simple low-temperature FCs mainly depends on the chemical reactions of hydrogen and oxygen with direct conversion into electricity without mediate conversions of thermal energy and mechanical energy.

Novel Pt and Pd Based Core-Shell Catalysts with Critical New Issues of Heat Treatment, Stability and Durability for Proton Exchange Membrane Fuel Cells and Direct Methanol Fuel Cells 43

Fuel cells AFC, PAFC, MCFC, SOFC PEMFC, DMFC PEMFC, DMFC

Applications AFC: Space, mobile; PAFC: Distributed power; MCFC: Distributed power generation; SOFC: Power generation; PEMFC and DMFC: Portable, mobile, stationary Special use in compact mobile devices and handphones in future

Table 1. Potential applications for various fuel cells [4-9,49,52,173]

2. Pt and Pd based catalysts In recent years, novel Pt and Pd metals have been known as the best electrocatalysts to important chemical reactions for synthesis of new chemicals as well as the FC reactions. The electrocatalytic properties are typically characterized by hydrogen evolution reaction/hydrogen oxidation reaction (HER/HOR), ORR, and electro-oxidation of CO at the surfaces of Pt(hkl) facets of the as-prepared catalysts as well as the typical oxidations of methanol and formic acid on the surfaces of Pt (hkl) facets of the prepared catalysts. The (111), (100) and (110) low-index facets were proved in high stability and durability in catalysis, and good re-construction in the catalytic FC reactions [10,81,82]. The HER on the Pt catalyst is known by the important Volmer, Tafel, and Heyrovsky mechanisms. In addition, VolmerTafel and Volmer-Heyrovsky mechanisms can occur in the complex combinations of the above basic mechanisms [10]. The surface kinetics and chemical activity occurring at the electrode surface containing Pt/support catalyst are characterized as follows [10-14,140]. Pt-Hads → Pt + H+ + e-

(1)

QDL (Charge) ↔ QDL (Discharge)

(2)

Pt + H2O → Pt-OH + H+ + e-

(3)

PtOH + H2O → Pt(OH)2 + H+ + e-

(4)

Pt-(OH)2 → PtO + H2O

(5)

2PtO + 4H+ + 4e- → Pt-Pt + 2H2O

(6)

Pt + H+ + e- → Pt-Hads

(7)

To evaluate catalytic activity of the pure Pt catalysts or Pt based catalysts, the electrochemical active surface area (ECSA) is used as ECSA = QH/(0.21×LPt) [10-14,140]. Therefore, ECSA can be significantly enhanced by the use of a low LPt loading, and a low content of CO intermediates generated, and new discoveries of highly strong hydrogen reactions in the improvements of the Pt based catalysts. Clearly, the particle size of Pt NPs of 10 nm is crucial in catalysis and FCs, PEMFCs, and DMFCs because the metal NPs showed very large quantum and size effects in the size range of around 10 nm. The ORR is observed in two main pathways in acidic electrolytes as follows [10-15].

44 Heat Treatment – Conventional and Novel Applications

O2 + 4H+ + 4e- → 2H2O

(8)

O2 + 2H+ + 2e- → H2O2 + 2H+ + 2e- → 2H2O

(9)

For the ORR, the relationship between kinetic current (i) and potential (E) can be investigated as rate expression i = nFkc(1 - θad)x exp(-βFE/RT)exp(-Gad/RT)

(10)

Where n, F, K, c, x, β, and γ are constants. In addition, n, F, c, and θad indicated the mole (n), Faraday’s constant (F), the concentration of O2 (c), and coverage of adsorbed species (θad), respectively. Here, Gad indicated the weak or strong adsorption. We can choose x=1 and =1 in the simplification. According to the adsorption degree, the rate may be changed. Thus, it may be change from positive (Weak adsorption) to negative (Strong adsorption). It means that the reaction rate declines when the coverage of intermediates (θad) rises [13,173]. At present, the phenomena of ORR kinetics and mechanisms occurring on the Pt catalysts are intensively investigated but a very high overpotential loss observed. Thus, the very high loadings of Pt must be supplied in the high requirements of the FCs operation with large current. It is known that the Pt catalyst has showed the highest activity to the ORR mechanism. Most of research has led to understand ORR on catalytic systems of Pt designed catalysts using the ultra-low Pt loading at minimal level. The issues of the low Pt-catalyst loading, high performance, durability and effective-cost design in FCs systems are very crucial for their large-scale commercialization. The CV results of various Pt NPs (sphere, cube, hexagonal and tetrahedral-octahedral morphology …) in H2SO4 showed the strong structural sensitivity of the as-prepared Pt NPs. The most basic (111), (100) and (110) planes were confirmed in the active sites of catalytic activity such as in the edges, corners, and terraces [15]. In particular, monolayer bimetallic surfaces were investigated in the experimental and theoretical studies of the surface monolayer, subsurface monolayer, and inter-mixed bimetallic structures, especially by DFT theoretical approaches [16,17]. So far, the characterization of size, structure, surface structure, internal structure, shape, and morphology has been discussed in various the asprepared metal NPs by various strategies of syntheses. The noble NPs (Pt, Pd, Ru, Ir, Os, Rh, Au, Ag), and their combinations with cheaper metals (Ni, Co, Cu, Fe …) as alloy and core-shell nanostructures can be used as potential Pt based catalysts for further studies of ORR and CO oxidation reaction in various FCs for long-term physical-chemical stability and durability, such as PEMFCs and DMFCs [18-21]. Besides, the investigations of both theory and applications of alloy clusters and nanoparticles showed potential applications in catalysis and FCs [22]. In various FCs, noble Pt metal is the key to large-scale commercialization of PEMFCs and DMFCs because of its unusually high catalytic properties. Therefore, scientists and researchers try to create highly active and stable catalysts with a low Pt loading. To enhance its catalytic activity, Pt catalyst NPs were supported on various high-surface-area carbon materials, such as carbon black (e.g. Vulcan XC-72) [23-27]. The Pt based catalysts of various nanostructures are discussed in the developments of PEMFCs and DMFCs. Interestingly, electricity is directly generated in PEMFCs by hydrogen oxidation and oxygen reduction reactions through membrane-electrode assembly (MEA) [1-14].

Novel Pt and Pd Based Core-Shell Catalysts with Critical New Issues of Heat Treatment, Stability and Durability for Proton Exchange Membrane Fuel Cells and Direct Methanol Fuel Cells 45

2.1. Preparation methods of Pt and Pd based nanoparticles Essentially, various top-down physical or bottom-up chemical methods, such as polyol method, and chemical-physical combined methods are widely used for making Pt and Pd based catalysts for homogeneous and heterogeneous catalysis, FCs, PEMFCs, and DMFCs involving in both theory and practice [28-33,173-188]. The core-shell nanostructures of bimetallic nanoparticles can be synthesized by phase-transfer protocol method [34]. The relatively facile method with microwave and ultrasound supports or sonochemical method in the synthesis of nanoparticles without focusing on much consideration of the basic issues of the homogeneity of typical size and morphology for catalysis and FCs has been very attractive to researchers and scientists [35,36]. In the methods, the nanosized ranges of the as-prepared nanoparticles are crucial to practical applications in catalysis, biology and medicine. So far, no comprehensive survey of the effects of heat treatments to achieve the significant enhancements of catalytic activity of Pt and Pd based catalysts has been presented in detail.

2.2. Size, shape and morphology At present, it is known that the as-prepared Pt nanostructures show a variety of particle morphologies and shapes in homogeneous and heterogeneous characterizations. The main morphology and shape were prepared in the broad forms of cube, octahedra, cubooctahedra, tetrahedra, prisim, sphere, icosahedra, decahedra, rod, tube, wire, fiber, dendrite, flower, plate, twin, belt, disk etc… in the non-polyhedral and non-polyhedral or irregular shapes and morphologies in the near same size range. The spherical and non-spherical morphologies and shapes are observed in the near same range of particle size. The particle size is discovered in various nanosized ranges of from 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, and 100 nm … to 1 m, 10 m, up to 100 m …, especially particle size of around 10 nm for potential promising applications in catalysis, biology, and medicine. In our proposals, catalytic activity and selectivity of interesting homogeneous and heterogeneous morphologies and shapes of the Pt based nanoparticles in the nanosized ranges of 10 nm and 20 nm become important topics for scientific research because their structural transformations in that certain ranges of 1-30 nm are difficult to understand transparently unknown phenomena and properties [37,38]. When the size of Pt nanoparticles is decreased into the range of 10 nm, the total fraction of Pt atoms on the Pt catalyst surfaces is very large. This leads a very significant enhancement of electro-catalytic activity. Most of sizes, shapes, morphologies, nanostructures of the as-prepared Pt nanoparticles are significantly changed in normal conditions after the in situ TEM and HRTEM measurements. Clearly, the important effects of temperature on characterization of the pure Pt NPs and Pt/supports need to be studied at different temperatures for one optimum temperature range while keeping their good catalytic characterization. The new discoveries of surface-structure changes of polyhedral Pt shapes and morphologies are crucial in the further catalysis investigations [39-41]. The influence of hydrogen on the morphology of Pt or Pd nanoparticles was found in the structural transformations [42,43]. Therefore, alloy and coreshell nanoparticles with various Pt metal compositions are crucial to practical applications

46 Heat Treatment – Conventional and Novel Applications

in catalysis and FCs. Nevertheless, most of the as-prepared metal nanoparticles possibly change their certain good shapes and morphologies (e.g. cube, tetrahedra, octahedra …) into hetero-shapes and hetero-morphologies. In fact, the issues of catalytic activity, durability, and stability of the as-prepared metal nanoparticles in various media have become very important to most of current scientific research. For example, non-platinum anode catalysts or without the use of Pt metal for DMFC and PEMFC applications were developed [44]. It has been known that they are transition metal carbides (e.g. WC and W2C ...) and the promoted transition metal oxides (e.g. TiO2, SiO2, CeO2, Zr2O3, CeO2-Zr2O3 ...) that have the advantages of low prices and strong resistance to poisonous substances such as carbon dioxide (CO) poisoning or CO adsorption on the catalysts.

2.3. Structure and composition Among metal noble nanoparticles (Pt, Pd, Ru, Rh, Ir, Os, Au, and Ag NPs) as well as various cheap metal nanoparticles, metals show a face centered cubic (fcc) structure. The strong emphasis is that they can be used as metal catalysts for catalysis and FCs. Thus, Pt and Pd based alloy and core-shell nanoparticles can be engineered in a variety of composition using various metals (Co, Ni, Fe, Cu …) or oxides, ceramics, and glasses in the next significant efforts of researches according to the discoveries and improvements of catalytic activity, selectivity, durability, and stability in catalysis.

3. Proton exchange membrane fuel cell At present, PEMFCs are used for mobile, portable, and automobile applications because of generated high power densities. For instance, they can operate at low and high temperatures of 60-100 oC or up to 200 oC [45-50]. In addition, the PEMFC is used for transportation applications when pure hydrogen as fuel can be used in PEMFCs for their operation. The conventional fuels are used as liquid, natural gas or gasoline. Therefore, the direct use of methanol can lead to develop PEMFCs into DMFCs. In particular, DMFCs proved that they can offer potential applications, such as cameras, notebook computers, and portable electronic applications [45-50]. The nanostructured membranes have been extensively reviewed in potential FC applications [50]. In addition, proton exchange membranes for PEMFCs operated at medium temperatures are discussed [51-53]. It is likely that the fast developments of new membrane technology can be realized in FCs, PEMFCs, and DMFCs.

3.1. Operation principle A simple hydrogen and oxygen PEMFC includes the catalytic anode, membrane electrode assembly (MEA), and the catalytic cathode. Fuel is hydrogen fed to the anode that generates protons (H+). They travel through proton exchange membrane and combine with electrons (e-) and oxygen at the cathode to form water (H2O). Electrons travel through an external circuit. This leads to that electricity is generated by a FC. Figure 2 shows chemical reactions on the anode and the cathode of a PEMFC. The electrochemical reactions typically occur in a PEMFC as follows.

Novel Pt and Pd Based Core-Shell Catalysts with Critical New Issues of Heat Treatment, Stability and Durability for Proton Exchange Membrane Fuel Cells and Direct Methanol Fuel Cells 47

At the anode: H2 → 2H+ + 2e-

(11)

At the cathode: ½ O2 + 2H+ + 2e- → H2O

(12)

The overall reaction is ½ O2 + H2 → H2O

(13)

Figure 2. Basic configuration and chemical reaction of PEMFC

3.2. Catalysts in proton exchange membrane fuel cell Of great interest is the study of the Pt nanoparticles with controlled size and shape around 10 nm because of its importance in electro-catalysis. So far, the Pt and Pt catalysts have showed the best catalytic activities in the HOR and ORR mechanisms for PEMFCs comparable to other metal catalysts. Therefore, new Pt and Pd based catalysts are developed by using various metals combined in alloy nanostructures. Now, various kinds of Pt and Pd core-shell nanoparticles or nanostructures are prepared in the proofs of improving catalytic activities of HOR and ORR. The cost of Pt and Pd catalysts is very high for the large-scale commercialization of FCs. Therefore, cheaper metals such Cu, Co, Fe, Ni … can be studied in the uses in the alloy and core-shell catalysts with the Pt shells for reducing the Pt loading [54,55]. The thermal cathodic treatments on Pt/C and Pt-Ru/C catalysts were used to enhance methanol electro-oxidation in sulfuric acid solution in electrochemical activation [56-58]. Clearly, Pt/support catalysts are preferred in many applications. Bimetallic catalysts such as PtNi, PtCo, and PtCu have been very important to the ORR activity at cathode in PEMFC. The as-prepared nanoparticles were studied in the de-alloying phenomena of Pt binary alloys with the different nanostructures [59-62]. The nanostructured catalysts were reviewed in various FCs. The next catalysts with the Pt loadings for low-temperature PEMFCs and DMFCs will be proposed in various alloy, multi-composition, and core-shell structures. Here, Pt bimetallic catalysts were prepared by impregnating a commercial Pt/C with various transition metals (Pt/M = 3, M: V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Ag and W), and sintering at 900 C. An increase in electrode activity in ORR tests using a half cell in phosphoric acid solution at 190 °C at the initial stage was observed for PtCr, PtFe and PtAg catalysts due to the surface roughening effect [63]. So far, the Pt catalysts of high cost have

48 Heat Treatment – Conventional and Novel Applications

been successfully used in both the anode and the cathode in PEMFCs with high cost. Therefore, binary alloy catalysts with Pt are developed for PEMFCs. Thus, Pt-Ni, Pt-Co or Pt-Cr bimetallic nanoparticles can be used as binary alloy catalysts for the significant enhancement of ORR. In particular, the Pt3Ni catalysts of the very high ORR activity are investigated [64-66]. A thin-film rotating disc electrode (TF-RDE) method was used for investigating the electrocatalytic activity of high surface area catalysts, and the highest catalytic activity on Pt/C catalysts towards ORR [67]. In PEMFCs, the catalytic activity and stability of the Pt based catalysts are very important. In the cathode of PEMFCs, Pt based alloy catalysts are developed for PEMFCs. Consequently, the issues of sizes, internal nanostructures, surface nanostructures, shapes and morphologies are also important to the stable operations of PEMFCs for the long periods. In addition, the pt based catalysts with the core-shell nanostructures of the thin Pt shell of around several nanometers have been developed for the next PEMFCs. In particular, the non-Pt catalysts without noble Pt metal are developed. Because noble Pt and Pd metals have the near same catalytic activity of HOR, the Pd based catalysts are studied for the improvements of ORR of PEMFCs [68]. The electro-active sites on carbon nanomaterials electro-catalyzed the reduction of peroxide intermediated from ORR on Pt [69]. The Pd based catalysts can be also used in a combination of Fe, Co, and Cu metals for the improvements of ORR, especially methanol tolerant ORR catalysts. It is clear that stability and durability of the catalysts in the cathodes have importance of the operations of PEMFCs. Therefore, PEMCs can be improved in high stability in the uses of the special supports such as various kinds of carbon supports (e.g. CNTs …). It is clearly admitted that Pt-Ru, Pt-Mo, Pt-Ni catalysts … are used as reformatetolerant catalysts or impure-hydrogen tolerant catalysts for good stability of next PEMFCs. In addition, Pt based ternary catalysts were discussed for the development of various low and high temperature FCs, PEMFCs, and DMFCs involving in their performance, durability and cost [70-73]. Now, an emergence of the hugely urgent demands of the Pt or Pd based catalysts after high heat treatment processes in both low and high temperatures less than 1000 oC, or up to more than 2000 oC offering better characterizations of catalytic activity and stability can be predicted in future due to the catalysts exhibiting the pristine surfaces, shapes and morphologies in the electrode catalysts. The re-constructions, e.g. (111), (110), and (100) planes [10-12], or collapses of the Pt or Pd based nanoparticles and nanostructures with or without heat treatment in the preparation will be very attractive to research and development of new catalysts for PEMFCs and DMFCs [10-12,41]. There are little evidences of the size, structure and morphology of Pt or Pd based catalysts after high heat treatments in media of H2 or N2/H2. These are major challenges in catalysis science, and Pt or Pd based catalysts for FCs, PEMFCs, and DMFCs. Our catalyst preparation gave a better catalytic activity and stability of HER, ORR, and MOR in an environment of mixture H2/N2 avoiding the formation of PtO by heat treatment. In future, the Pt catalysts can be treated at high temperature but their size, nanostructure and morphology in the 10 nm range kept [128130]. Therefore, the pure Pt or Pd based catalysts used in the electrodes of FCs, PEMFCs, and DMFCs can give better catalytic activity, stability, and good performance of the whole FC systems.

Novel Pt and Pd Based Core-Shell Catalysts with Critical New Issues of Heat Treatment, Stability and Durability for Proton Exchange Membrane Fuel Cells and Direct Methanol Fuel Cells 49

4. Direct methanol fuel cell 4.1. Operation principle In general, PEMFCs can be categorized into various kinds of hydrogen/oxygen FCs, DMFCs, and direct formic acid fuel cells (DFAFC) according to the use of liquid or gas fuels etc. [7476]. So far, the economic uses of Pt, Pt-Pd, and Pt-Ru based catalysts as well as catalyst supports have been very crucial to MOR at the anode in DMFCs. To date, methanol can be used in direct chemical-electrical energy conversion in a DMFC in figure 3. The electrochemical reactions occurring in a DMFC are: Anode: CH3OH + H2O → CO2 + 6H+ + 6e-

(14)

Cathode: 3⁄2O2 + 6H+ + 6e- → 3H2O

(15)

Overall: CH3OH + 3⁄2O2 → CO2 +2H2O

(16)

The DMFC is attractive because methanol, being a liquid fuel, is easy to transport and handle. In DMFCs, the MOR mechanism at the anode is crucial. Due to the low operating temperature ~60-150 oC, the Pt based catalysts are sensitive to poisoning. Since CO is formed during electro-oxidation of methanol, CO-tolerant catalysts are used for DMFCs. Nevertheless, these have a much lower power density despite the typically high noble metal loading of the electrodes. In addition, the energy efficiency of DMFCs suffers from high electrode over-potential (voltage losses), and from methanol losses by transfer (by permeation) through the membrane used in DMFCs [77-80].

4.2. Catalysts in direct methanol fuel cell So far, Pt and Pd catalysts have been known as the most important catalysts for the direct methanol oxidation in electrodes, such as anodes and cathodes. The interesting hydrogen adsorption on Pt or Pd catalyst was studied [81-83]. In this context, PVP and TTAB polymerPt nanoparticles were synthesized with the same cubic shape and similar particle size (8.1 and 8.6 nm, respectively). They can be used as the potential Pt catalysts for chemical synthesis [84]. The PVP-Pt nanoparticles of good cubic, tetrahedral, and octahedral morphology in the size range of 10 nm were prepared by polyol method with the use of commercial chemicals for potential catalysts for FCs [85-87]. At such very small scale, the well-controlled synthesis of Pd nanoparticles by polyol synthesis routes using PVP polymer proved that the shapes of Pd NPs provide a good opportunity of investigating their catalytic property [88]. The catalytic reactions of Pt and Pd catalyst have been studied in the different combinations of various metals such as Ru, Rh, and Sn etc. with support materials such as carbon nanomaterials or oxides and glasses in both homogeneous and heterogeneous catalysis [89-97]. The new Pt-monolayer shell electrocatalysts of high stability were developed for the FC cathodes. The role of the Pt shell is to reduce the Pt loading significantly in PEMFCs and DMFCs but synergic effects for enhancing catalytic activity and stability. However, it is very difficult to make the Pt monolayers on the core nanoparticles.

50 Heat Treatment – Conventional and Novel Applications

Therefore, scientists and researchers have tries to study the easy-to-use chemical and physical methods for highly homogeneous core-shell nanosystems with the use of the thin Pt shells of several nanometers in the size range of 10 nm for PEMFCs and DMFCs. In addition, the good characterization of the engineered nanostructure of the Pt based coreshell nanosystems need to be stabilized after the high heat treatments and preparation processes for obtaining the best catalysts for PEMFCs and DMFCs [98-103], leading to the FC durability up to 200,000 cycles with core-shell catalysts with the Pt-monolayers shells [99]. In all cases, the as-prepared Pt nanoparticles are supported on various carbon nanomaterials for catalytic enhancement. The improvements of Pt catalyst can be performed through by the use of the core-shell nanoparticles or core-shell catalysts that have been proposed. The systems of Pt based, binary, ternary, and quaternary catalysts are proposed in both homogeneous and heterogeneous catalysis [104,105]. Therefore, the Pt based multi-metal catalysts (PtRu or PtRuIr/C) exhibiting high catalytic activities are developed for the MOR [106,107]. A comparison of catalytic activity of Pt, Ru, and PtRu catalysts using Pt(NH3)2(NO2)2 and Ru(NO3)3 precursors for DMFCs was performed. The Pt50Ru50/C catalyst prepared at 200 °C has the maximum electrocatalytic activity toward MOR. The Pt-Ru catalysts have shown the excellent MOR in the stable operations of DMFCs but very high cost [108]. For methanol electro-oxidation, binary, ternary, and quaternary Pt alloy catalysts were prepared. As a result, quaternary alloy catalysts (Pt-Ru-Os-Ir) were considered to be far superior to Pt-Ru in DMFCs [109]. In particular, the successful process of Pt based cubic nanoparticles (Pt-Fe NPs, Pt-Fe-Co NPs), Pt-Fe-Co branched nano cubes, Pt-Fe-Co NPs of low and high Co content was presented. Overall, Pt-Fe-Co branched cubes were used as a good catalyst to show best activity and durability for electrocatalytic MOR [110]. In one study, the evidences of linear sweep voltammetry (LSV) tests indicated that the high peak current density on Pt2Rh/C is about 2.4 times higher than that of Pt/C in alkaline media direct methanol FCs [111]. So far, the Pt-Au catalysts engineering has become very important to the developments of PEMFCs and DMFCs [112]. For the continuous developments of DMFCs, the Pt-Ru alloy and core-shell electrocatalysts for FCs are discussed in a critical survey [113]. The as-prepared catalysts of PtRuMo NPs supported on graphene-carbon nanotubes (G-CNTs) nanocomposites were developed for DMFCs. The results showed that the catalytic activity and stability of the PtRuMo/G-CNTs catalyst are higher than those of PtRuMo/G and PtRuMo/CNTs catalysts. However, the new trends of using Pt based multi-component catalysts without considering the structural issues lead to the complexity of preparation and synthesis [114]. The Pd-Pt catalysts are recognized as potential candidates for PEMFCs and DMFCs operating at low temperatures [115]. Electrocatalytic activity for ORR and oxygen binding energy was intensively studied in the metal catalysts among the different combinations of W, Fe, Mo, Co, Ru, Ni, Rh, Cu, Ir, Pd, Pt, Ag, and Au. According to the oxygen binding energy by density functional theory (DFT), the volcano-type dependence of ORR was discovered in a high catalytic activity of Pt, Pt, Ir, and Ag metal, especially for the very thin Pt and Pd layers as the monolayers. The structural investigation of ORR and oxygen binding energy by the d-band model or calculation method of d-band center is very crucial to the issues of engineering a novel catalyst [116118]. In particular, a X-ray absorption spectroscopy (XAS) method was employed in the

Novel Pt and Pd Based Core-Shell Catalysts with Critical New Issues of Heat Treatment, Stability and Durability for Proton Exchange Membrane Fuel Cells and Direct Methanol Fuel Cells 51

characterization of a number of catalysts for low temperature FCs to determine the existent oxidation state of metal atoms in the catalyst, or in the case of Pt, the d-band vacancy per atom. It is a good tool for confirming the structures of the catalyst in situ [119]. There is an important discovery of a synergistic effect to being studied. In theory and practice, synergistic effect needs to be intensively investigated in quantum property in respective to the development of bimetallic catalysts of alloy and core-shell nanostructures. In this context, the synergistic effect of core-shell bimetallic nanoparticles gives an excellent catalytic enhancement. In catalytic activity, the shell provides strongly catalytic sites. The core element gives an electronic effect (a ligand effect) on the shell element because the surface atoms of the shell are coordinated to the core in their catalytic reactions. Therefore, the shell is an important factor to control the catalytic properties. In addition, the core-shell bimetallic structures cause better suppression of adsorbed poisonous species [120,140-143]. This effect is discussed in the metallic NPs of hetero-morphologies and hetero-structures that can be as new types of important catalyst [121]. Of all catalysts, the bimetallic nanoparticles have played important roles in promising applications of catalysis, and FCs [122]. The core-shell nanoparticles and nanostructures were intensively investigated [123124]. The Pt-Ni-graphene catalysts were prepared for the high MOR activity observed [125]. In particular, an important role on the MOR activity of Pt-Co and Pt-Ni alloy electrocatalysts for DMFCs has to be ascribed to the degree of alloying [126]. The interaction of Pd NPs and Pd(111) with CH3OH and CH3OH/O2 mixtures was examined from ultrahigh vacuum conditions up to ambient pressures [127]. Here, the particle size and structure dependent effects in methanol oxidation and decomposition are very crucial to the use of the supported Pd catalysts as well as the good incorporation of the pure Pt into various supports with high homogeneous distribution. In addition, we should find suitable heat treatment in good experimental conditions, and Pt based catalyst engineering to increase catalytic activity and durability.

Figure 3. Chemical reactions of direct methanol fuel cell (DMFC)

52 Heat Treatment – Conventional and Novel Applications

5. Catalysts and heat treatment Of interest to scientists, at present the Pt catalysts are used in both the anode and the cathode in DMFCs. However, the Pt NPs showed a variety of sizes, shapes, and morphologies in the different size ranges. The shapes and morphologies of Pt NPs are synthesized in the forms of cube, octahedra, tetrahedra, plate, wire, flower, rod, fiber etc. The certain issues of their particle sizes are studied in the nanosized ranges of 10 nm, 20 nm, 50 nm etc as well as the homogeneous nucleation, growth, and formation of polyhedral nanoparticles. In particular, the range of the Pt NPs of about 10 nm exhibit the highest catalytic activity with the good morphology and shape such as sphere, cube, octahedra, tetrahedra, and polyhedra etc. The high stability and durability of the shape-dependent catalytic activity are needed to be confirmed in the homogeneity of the particle size in a whole nano-system. In the heat treatments of the Pt NPs, the characterization of size, surface, structure, shape, and morphology of Pt NPs need to be preserved to obtain the Pt catalysts of good catalytic activity, stability, and durability for a long time. The new generations of the Pt-Pd core-shell catalysts with the thin Pd shells of less than several nanometers were developed. They showed the excellent catalytic activity. In our recent research, the Pt NPs of less than 10 nm in size were used as the Pt catalysts, which can be used as the standards for any comparison of catalytic activity of new Pt based catalysts. So far, the heat treatment procedures and methods of heat treatments of the as-prepared metal nanoparticles for catalyst engineering in catalysis have not been considered in their very crucial issues of the size, surface, structure, shape, and morphology. The transformations of structure and property of the as-prepared metal nanoparticles in the ranges of 10 nm and 20 nm through heat treatments or sintering of Pt nanoparticles are very critical to nanoparticles as electro-catalysts or catalytic nanosystems for PEMFCs and DMFCs [128]. We need to confirm that Pt nanoparticles with the rough curved surfaces exhibit catalytic activity much better than Pt nanoparticles with flat and smooth surface through the measurement results. The important effects of heat treatment and the removal of poly(vinylpyrrolidone) (PVP) polymer on electrocatalytic activity of polyhedral Pt NPs towards the ORR mechanism have been investigated. The methods of keeping the size, surface structure, internal structure, shape, and morphology were proposed in our catalyst engineering processes, especially heat treatments of the as-prepared PVP-Pt nanoparticles at 300 oC. The loaded electrodes were carefully dried in air for 3 h at 25 °C and heated with the heating rate of 1 °C/min up to 450 °C in air and a keeping time of 2 h in order to remove any organic species. The pure Pt nanoparticles need to be kept in their good characterization of size and morphology in the size range of 10 nm. We proposed that the polyhedral Pt NPs of around 10 nm can be used the standard Pt catalyst in all research of electrocatalysts in PEMFCs and DMFCs due to their higher concentration of surface steps, kinks, islands, terraces, and corners [129-137]. In our research, Pt-Au NPs were prepared by polyol method, which can be used as Pt-Au catalysts for DMFCs [138]. In particular, Pt-Pd alloy and core-shell NPs were synthesized by polyol method. The core-shell NPs can be used for low temperature PEMFCs and DMFCs for the excellent advantages of reducing the Pt total metal weight. Therefore, it is an economic solution of the suitable use of Pt based core-shell catalysts for next FCs. The new

Novel Pt and Pd Based Core-Shell Catalysts with Critical New Issues of Heat Treatment, Stability and Durability for Proton Exchange Membrane Fuel Cells and Direct Methanol Fuel Cells 53

Pt monolayer Pd-Au catalyst of a core-shell structure with double shells was found in a good stability and activity of ORR. The double shells have an outermost shell of Pt monolayer and a sub-layer shell of Pd-Au alloy [139,140]. In our new results and findings, new evidence of fast enhancement of ORR on the electrode of the new Pt-Pd core-shell catalyst in the size range of 25 nm is clearly observed in a comparison with the prepared Pt catalyst in the size range of 10 nm. Therefore, it is important to use core-shell bimetallic catalysts to increase the ORR rate in the electrode catalyst. For the case of Pt-Pd core-shell catalysts, the fast hydrogen-desorption response and high sensitivity in our results after reaching the stable characterization after only the first CV cycle in a comparison to Pt catalyst. This enables the realization of robust and efficient Pt- or Pd-based core-shell catalysts that are extremely sensitive to the fast hydrogen desorption. Most of the alternative method of improving the hydrogen reaction by the core-shell nanostructures can be clearly realized. During the measurement, the electrodes are swept from -0.2 to 1.0 V with respective to the kinds of saturated standard electrodes. There are the specific regions in the cyclic voltammogram (Figures 4 and 5). They show highly and good catalytic activity and surface kinetics for the case of both the Pt catalysts of 10 nm and the Pt-Pd core-shell catalysts of 25 nm. It is clear that the high heat treatments of our catalyst preparation in H2/N2 offer the good characterization of the size, surface, structure, and morphology. Therefore, the highly long-term catalytic activity, stability, and durability in chemical and physical Pt or Pd based catalysts are needed in the catalyst layers of FCs, DMFCs, and PEMFCs. The effects of using a suitable temperature range in heat treatment should be suitable to various FCs, for example the better ORR activity. This depends on the operating temperature of various FCs. They were characterized by the chemical activity occurring at the electrode surface. In the forward sweep, the first region assigned to hydrogen desorption is crucial to confirm catalytic activity of the Pt catalysts. The slow kinetics of hydrogen desorption of the case of Pt catalyst was confirmed in the cell before the stabilization of CV was achieved from the first cycle to the twentieth cycle, and the fast kinetics of hydrogen desorption of the case of Pt-Pd core-shell catalyst. Indeed, the results proved the good desorption and adsorption of hydrogen of both Pt catalysts and Pt-Pd coreshell catalyst as evidences of good catalytic activity of two kinds of important catalysts in the preparation process and heat treatment in figures 4 and 5. To evaluate the catalytic activity of the prepared catalysts, the electrochemical active surface area (ECSA) of the Pt catalyst is calculated to be (10.5 m2g-1) in a comparison with that of the Pt-Pd core-shell catalysts (27.7 m2g-1) in our catalytic investigations. Thus, suitable heat treatments or sintering processes of the Pt or Pd based catalysts for obtaining good catalytic activity and stability in the desirable nano and micro structures are very crucial to enhance, and improve the continuous operation of direct chemical-into-electrical energy conversion, high stability and durability of various FCs, PEMFCs and DMFCs for the urgent global challenges of energy and environment. Obviously, the heat treatments can lead to significantly reduce the effects of CO poisoning to the electrodes in PEMFCs and DMFCs. One of the most challenging goals in the heat treatment is to develop successful protocols for keeping the good characterization of the as-preparation Pt NPs such as surface, structure, size, and shape in their nanosized ranges. In our research, the polyol method was used for our

54 Heat Treatment – Conventional and Novel Applications

synthesis of the Pt and Pd bimetallic nanoparticles with alloy and core-shell structure. In comparison, we also can control the time and temperature of convenient heat-treatments to the pure Pt or Pt/support catalysts for their better catalytic activity. Clearly, the temperature plays an important role in heat treatments for making the better Pt based catalysts. The shape and morphology of polyhedral-like or spherical-like Pt nanoparticles are crucial in the further study of electro-catalytic activity.

Figure 4. TEM images of the polyhedral Pt nanoparticles. Scale bars: (a)-(d) 20 nm. (e) Cyclic voltammograms of the pure Pt catalysts in 0.5 M H2SO4 from -0.2 V to 1.0 V. Reprinted from: Long NV, Ohtaki M, Hien TD, Randy J, Nogami M, Electrochim. Acta. 56:9133-9143. Copyright 2011 with permission from Elsevier [140]

It means that the high heat treatment to the as-prepared Pt nanoparticles for the good catalyst can be performed in the ways with various solvents or pure water during their preparation. However, the desirable characterization of size, shape, morphology, and structure need to be kept for the better catalytic activity. Therefore, scientists need provide more research results of Pt based catalysts with heat treatment effects in electro-catalysis. Thus, new Pt based catalysts, electrodes, and membranes need to be considerably studied for the reduction of their high costs but the high whole performance. The effects of heat treatments on size, shape, surface, and structure of the pure Pt nanoparticles (pure Pt catalyst) or Pt based catalysts for catalytic activity, durability and stability can be intensively analyzed by in situ TEM and electrochemical measurements as well as the chemical reactions and their kinetics at the surfaces of the only Pt catalyst or Pt/support catalysts in fuel cells. In electrochemical measurements, we can use electrolyte solution of using 0.5 M H2SO4+1.0 M CH3OH (a scan rate of 50 mV s-1). Our comparisons of cyclic voltammograms were done between the pure Pt catalysts in the nanosized range of 10 nm, and the pure Pt-Pd core-shell catalysts in the nanosized range of 25 nm in the mixture of 0.5 M H2SO4 and 1 M CH3OH in figure 6. A stable voltammogram was attained after 20 cycles of sweeping between 0 and 1 V. Two oxidation peaks are observed. The good MOR was confirmed in the evidences of the typical peaks at 0.6 V and 0.8 V in the forward sweep, and the other peaks at 0.4 and 0.5 V in

Novel Pt and Pd Based Core-Shell Catalysts with Critical New Issues of Heat Treatment, Stability and Durability for Proton Exchange Membrane Fuel Cells and Direct Methanol Fuel Cells 55

Figure 5. (a)-(f) TEM and HRTEM images of the as-prepared Pt-Pd core-shell. The thin Pd shells protect polyhedral Pt cores. The nucleation and growth of Pd shells are controlled by a chemical synthesis. Scale bars: (a)-(c) 20 nm. (d) 5 nm. (e) 5 nm. (f) 2 nm. (h) Schematic of a standard three-electrode electrochemical cell. Reprinted from: Long NV, Ohtaki M, Hien TD, Randy J, Nogami M, Electrochim. Acta. 56:9133-9143. Copyright 2011 with permission from Elsevier [140]

56 Heat Treatment – Conventional and Novel Applications

Figure 6. (a) Cyclic voltammogram of Pt catalysts, and Pt-Pd core-shell catalysts on glassy carbon electrode in N2-bubbled 0.5 M H2SO4 electrolyte (scan rate: 50 mV s-1). (b) Cyclic voltammogram towards methanol electro-oxidation of Pt catalyst and Pt-Pd core-shell catalyst. (c) Simple pathways of MOR (A). Possible reaction pathways of MOR (B). (d) Chronoamperometry data of Pt and Pt-Pd catalysts in 0.5 M H2SO4+1.0 M CH3OH and polarization potential about 0.5 V. Reprinted from: Long NV, Ohtaki M, Hien TD, Randy J, Nogami M, Electrochim. Acta. 56:9133-9143. Copyright 2011 with permission from Elsevier [140]

the backward sweep. Our results of the MOR mechanisms showed that the two peaks are directly related to methanol oxidation and the associated intermediates. The fascinating high stable characterization was observed for Pt-Pd core-shell catalysts. They showed a very high initial current about 1.29×10-3 A cm-2 in comparison with the Pt catalyst of initial current about 4.33×10-4 A cm-2. In all our research, the samples with the Pt or Pt based catalysts were carried out in the heat treatment procedures at 300 oC and up to 450 oC. The loaded electrodes were dried in air for 3 h at 25 °C and heated with the heating rate of 1 °C/min up to 450 °C in air and a keeping time of about 2 h for the complete removal of any organic species. After heat treatment, the specific size, shape, structure and morphology of Pt nanoparticles were retained in the good conditions of the Pt based catalyst for the CV measurements [141-143]. New hydrogen absorption was found in the Pt-Pd bimetallic nanoparticles [144]. In our research, we suggested that there are the relationships and dependences of electro-catalytic properties on issues of size, internal structure and surface structure, shape and morphology, and composition of Pt or Pd based catalysts in catalysis, biology, and medicine. The homogeneous properties of size, internal structure and surface structure, shape and morphology, and composition of nanoparticles can create novel and

Novel Pt and Pd Based Core-Shell Catalysts with Critical New Issues of Heat Treatment, Stability and Durability for Proton Exchange Membrane Fuel Cells and Direct Methanol Fuel Cells 57

promising properties in catalysis, biology and medicine. According to our research, the effects of the nanostructures can be confirmed in the better catalytic activity. In general, the Frank-van der Merwe (FM), Volmer-Weber (VW), and Stranski-Krastanov (SK) growth modes show the formation of various nanostructures of core-shell nanoparticles [141], such as core-shell bimetallic NPs and oxide-metal NPs [104,141]. It is clear that the formations of novel Pt based core-shell bimetallic nanoparticles the thin-based metal or alloy shells can be controlled in both the FM and SK growth modes for the utilization of Pt metal. The FM overgrowth mode or the epitaxial overgrowth of the thin Pt shells on the metal or oxide cores due to the layer-by-layer mechanism are very crucial to discover most of new Pt based core-shell catalysts for FCs, such as PEMFCs or DMFCs. The core-shell engineered nanoparticles in the nanosized ranges of about 10 nm and 20 nm with the Pt metal shell or the Pt based bimetallic shells or the Pt based multi-metallic shells become an important topic of next research and scientific investigations for FCs. The thin shells can be noble metals such as Pt, Pt-Au, Pt-Pd, Pt based alloys with the use of Ag, Au, Rh, Ru, and Pd metal. The thick cores can be cheap metals such as Ni, Co, Fe, Cu .. and alloys, even and ceramics and glasses. With regards to low-cost issues, we suggest that typical core-shell configurations of bimetallic nanoparticles are proposed in the main goals of designing Pt and Pd based catalysts with the shells of using ultra-low mass of Pt metal or Pt-noble (Pd, Ru, Rh, Au, Ag) bimetals [140-143, 190]. Thus, a variety of the core-shell configurations with the thin Pt shell or the thin Pt based bimetallic shells controllably created in chemical and physical engineering is one of the best ways of the realization of large-scale commercialization and development of FCs, such as PEMFCs and DMFCs. We suggest that noble Au NPs can be used the good core for the Pt shells for DMFCs and PEMFCs. In most cases, Pt-based bimetallic NPs were supported on carbon nanomaterials such as MWCNTs for the catalytic enhancement in the operations of various DMFCs due to their large surface area and good conductivity [145,146]. In addition, the Co-Pt core-shell nanoparticles can be used as potential catalysts in cathode of PEMFCS because of their high stability and durability [147149]. In many efforts, the catalysts such as Pt-Pd, Pt-Ru, Pt-Rh and Pt-Sn/C exhibiting an improved performance in MOR as anode materials were prepared by ultrasonic method [150,151]. Importantly, the catalytic properties of well-characterized Ru-Pt core-shell NPs were demonstrated for preferential CO oxidation [152]. The metal-oxide nanostructures of Pd or Pt metals and TiO2 or Zr2O3 oxides by self-assembly are proposed for catalysis and FCs. The core-shell nanoparticles and nanostructures will offer potentially promising applications in homogeneous and heterogeneous catalysis, biology and medicine [153,154]. The recent developments of polymer electrolyte membranes for FCs were discussed for improving the long-term stability [155]. The new methanol-tolerant catalysts with the use of Pd nano cubes exhibiting high electro-activity for ORR in the 0.5 M H2SO4 solution were confirmed [156,157]. Thus far, the catalysts with the use of the Pt-Pd nanoparticles or the PtPd nanostructures of various size, internal structure, surface structure, shape, morphology, and composition by various preparation methods have been intensively studied for practical applications in various FCs, PEMFCs, and DMFCs. There are the very huge demands of high-surface area catalysts of Pt-Pd NPs supported on carbon nanomaterials that have been used in various FCs, PEMFCs and PEMCs [158-170,188,189]. The nano-porous SiO2 solids

58 Heat Treatment – Conventional and Novel Applications

with Pt or Pd metal nanoparticles were also used [171-172], and they offered higher electrooxidation current densities for both methanol and ethanol.

5.1. Heat treatment Before the TEM and HRTEM measurements, copper grids with the prepared nanoparticles were annealed in the heat treatment at 300 oC for 4 h, 6 h or even a half day. The evidences and results of the characterization of size, structure, shape, morphology with the only Pt composition by TEM and HRTEM show the uniform polyhedral Pt nanoparticles synthesized by polyol method with the controlled size in the nanosized range of 10 nm and 20 nm, and sharp shapes and morphologies through an introduction of low contents of AgNO3 as size, structure, morphology and shape-controlling reagent and the gradual addition of the precursors of PVP and H2PtCl6 in the suitable volume ratio. In the characterizations of homogeneous polyhedral morphology and shape, the appearances of the main sharp cubic, octahedral, and tetrahedral shapes of Pt NPs in the controlled growth of (100), (110), and (111) selective surfaces are very good for their applications in catalytic activity, e.g. the ORR and MOR chemical reactions because the polyhedral Pt nanoparticles have at a higher concentration of surface steps, kinks, islands, terraces, and corners [1012,188]. In addition, the TEM and HRTEM images of Pt NPs by polyol method show interestingly important phenomena of particle-particle surface attachment, self-aggregation, and assembly in the case of the use of the pure Pt NPs or the PVP-Pt NPs. Most of Pt NPs showed various large morphologies and shape after heat treatment at high temperature from 20 to 60 nm for the case of the PVP-Pt NPs [128-130]. The clear overgrowths and structural transformation in the nanosized range of polyhedral Pt nanoparticles annealed at 300 C for 4 h were observed. Clearly, the morphology of PVP-Pt nanoparticles was significantly changed by heat treatment. During their synthesis, PVP polymer is used to stabilize and control the size and morphology of Pt nanoparticles against their aggregation. In addition, the amount of PVP polymer can bind the nanoparticle surface after the catalyst synthesis. Therefore, PVP or other polymers for the protections of the as-prepared nanoparticles should be removed in the minimal content before the catalytic reactions. In our research, PVP polymer plays an important role to stabilize their morphology and size of these Pt nanoparticles. Despite the fact that these Pt nanoparticles were put on copper grids, they still have their interfacial interactions to their particle-particle surface attachments, aggregation, and self-assembly leading to form the larger and irregular Pt particles or the larger particles in the forms of hetero-morphologies and hetero-nanostructures in the final formation by the heat treatment. It is known that Pt nanostructures with the issues of size, shape and morphology under heat treatment procedures become important to further investigation in the confirmation of catalytic activity in catalysis. The critical issues of size, shape and morphology of colloidal nanoparticles need to be intensively studied in different media (condense, liquid, and gas) for the certain confirmations of high and long-term stability, high durability, and safety in their practical applications in catalysis, biology and medicine. In our experimental methods, in order to obtain the pure Pt catalyst for the standard catalyst of the standard nanosized range of 10 nm in investigations of catalytic

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activity, PVP polymer of the as-prepared product of PVP-Pt nanoparticles was removed. Therefore, the pure Pt nanoparticles for electrochemical measurements can be obtained by using centrifuge. The resultant solution of as-prepared nanoparticle was washed by using the suitable mixtures of acetone and followed by centrifugation at 5,000 rpm up to 15,000 rpm. Next, the black solid product was re-dispersed in the ethanol/hexane mixtures with a suitable volume rate. The resultant mixture was centrifuged in order to obtain the fresh product. Experimentally, the procedures of washing and clean Pt nanoparticles in the mixture of ethanol and hexane were done many times. After washing PVP polymer and contaminations in the as-prepared product, the nanoparticles were dispersed in milli-Q water in order to achieve the fixed density of colloidal Pt nanoparticles at 1 mg/mL with the aid of ICPS analyzer. The working electrode was a glassy carbon rod (RA-5, Tokai Carbon Co., Ltd.) with a diameter of 5.2 mm. Then, the electrode surface was cleaned and activated by using a kind of polishing-cloth (Buehler Textmet) with alumina slurry (Aldrich, particle size of 50 nm), followed by washing any contaminations with milli-Q water. The step-bystep procedures were repeated until the surface looked like a mirror. Then, a fixed weight of some μg (e.g. 5 μg or 10 μg) of the Pt loading was set onto the surface of the polished electrode for the electrochemical measurements. The loaded electrodes were dried in air for 3 h at 25 °C and heated with the heating rate of 1 °C/min up to 450 °C or 723 K in air and a keeping time of 2 h in order to remove organic species [128-130]. We used a very slow heating rate to avoid the serious problem of sintering of the pure nanoparticles so that the layer catalyst is the nanoparticles. The electrodes were allowed to cool normally and then exposed into the flow of the mixture of H2/N2 gases (20%,80%) at 100 °C for 3 h to reduce the existence of PtO and ensure a pristine catalyst surface. In order to improve the mechanical stability of electrode surfaces, Nafion® solution, e.g. 10 μL of 5 wt.%, was added onto the electrode and followed by drying in air for a long time, e.g. overnight, before the electrochemical measurements. The cyclic voltammetry experiment was performed at room temperature using a typical setup of three-electrode electrochemical system in figure 5(h) connected to Potentiostat (SI-1287 Electrochemical Interface, Solartron). The cell was a 50mL glass vial, which was carefully treated with the mixture of H2SO4 and HNO3, and then washed generously with milli-Q water. A leak-free AgCl/Ag/NaCl electrode (RE-1B,ALS) served as the reference and all the potentials were reported vs. Ag/AgCl. The counter electrode was a Pt coil (002234,ALS) [128-130, 140-143]. The electrolyte solution was bubbled with N2 gas for 30 min before every measurement. This N2 blanket was kept during the actual course of potential sweeping. For the base voltammetry, the electrolyte was a solution of 0.1 M HClO4 that was diluted from 70% concentrated solution (Aldrich) using milli-Q water. Additionally, 0.5 M H2SO4, H2SO4 and 1 M CH3OH are prepared for electrochemical measurements. The potential window between -0.2 to 1.0 V with a sweep rate of 50 mV/s was used. For the methanol oxidation, the electrolyte was added with 1.0 M methanol in milli-Q water. The system measurements were cycled until the stable voltammograms were achieved. The electrochemical surface area (ECA) or ECSA was estimated by considering the area under the curve in the hydrogen desorption region of the forward scan and using 0.21 mC/cm2 for the monolayer of hydrogen adsorbed on the surface of Pt catalyst [13,174]. The CO poisoning issues or phenomena of CO-stripping voltammetry of the Pt catalysts cause a

60 Heat Treatment – Conventional and Novel Applications

significant decrease in the overall efficiency of DMFCs. In all CV research, we did not observe CO poisoning in the CV data. The evidences proved that heat treatments are very important to engineer the best Pt catalyst or the better Pt based catalyst for FCs, PEMFCs, and DMFCs. In the developments of FCs, PEMFCs, DMFCs, novel Pt and Pd based coreshell bimetallic nanosystems or Pt and Pd core-shell catalysts will be the next catalysts with use of the thin Pt or Pt-Pd shells on the thick cores (metals, oxides, glasses, and ceramics) in the nanosized ranges of 10 nm, 20 nm and 30 nm. The thin Pt metal shells or Pt based noble bimetal (noble Pt-Pd, Pt-Ir, Pt-Ru, Pt-Rh, Pt-Au, Pt-Ag) thin shells can be tuned to be several nanometers on the metal or alloy cores in the ranges of 10 nm, 20 nm and 30 nm, and so on through the controlled synthesis and preparation processes. Unfortunately, quantum-size, structural and surface effects of metal nanoparticles around 10 nm in electro-catalysis are not fully understood in their chemical and structural changes.

5.2. Stability and durability With respect to the heat treatment, electrocatalytic characterizations of the pure catalyst with our as-prepare products of Pt nanoparticles showed highly good quality, long-term stability and durability. Thus, the significant effects of time and temperature during heat treatment for the pure Pt catalyst or Pt/support catalysts are crucial. In our research, the findings and results involve in the cyclic voltammograms of polyhedral Pt nanoparticles with the different removal of PVP polymer acquired at 50 mV/s in 0.1 M HClO4 solution (or in 0.5 H2SO4 or in 0.5 H2SO4 and 1 M CH3OH). The voltammograms data proved the typical shape for the base voltammetry for the high activity, stability and durability of our catalysts used [98,99,129]. The very high activity, durability and stability in FCs described were achieved up to 200,000 cycles with the use of core-shell catalysts with the Pt-monolayers shells in the cathode as important scientific evidences [99]. In our interesting research, the structural effects of low and high-index planes of the fcc structure of Pt catalyst were confirmed in electrochemical measurements, typical low-index planes of (111), (110), and (100) or more high index planes (hkl). The Volmer-Tafel and Volmer-Heyrovsky mechanisms are observed in hydrogen reactions. Most of ORR evidences showed good oxygen reduction. Therefore, our results showed the important evidences of the catalytic activity of the Pt catalyst or Pt based catalysts with alloy structures or core-shell structures. The ECA of the Pt catalyst the use of the Pt nanoparticles washed and heated showed the best catalytic activity (ECA=10.53 m2/g), better than Pt catalyst the use of the Pt nanoparticles heated only-Pt nanoparticles (8.56 m2/g), and better than Pt catalyst the use of the Pt nanoparticles of washed-only Pt nanoparticles (6.75 m2/g). The values proved that the high stability and durability of the catalyst preparation, processes of heat treatment, methods of washing and clean the asprepared Pt nanoparticles for the pure Pt catalyst. However, we should find a good process of heat treatment to avoid particle sintering in the electrodes at high temperature more than 450 °C and keep the good characterizations of size, internal structure, surface structure, shape and morphology of the Pt nanoparticles or the Pt based nanoparticles for the pure Pt based catalysts for FCs, PEMFCs and DMFCs. Therefore, we propose that homogeneous

Novel Pt and Pd Based Core-Shell Catalysts with Critical New Issues of Heat Treatment, Stability and Durability for Proton Exchange Membrane Fuel Cells and Direct Methanol Fuel Cells 61

polyhedral Pt nanoparticles under control in the size, shape and morphology in the nanosized range of 10 nm should be used the standard catalyst in the scientific investigations of catalysis for FCs, PEMFCs and DMFCs. Our electrochemical measurements were performed by the use of the pure Pt catalysts of polyhedral Pt nanoparticles or the Pt based catalysts in alloy and core-shell structure in the mixture of 0.1 M HClO4/1 M methanol (or 0.5 H2SO4/1 M methanol). In these CV measurements, the stable voltammograms are attained after about 10-20 cycles of sweeping between -0.2 to 1.0 V potential range. The typical two oxidation peaks are observed in the evidences of methanol oxidation. The first one is between 0.6 and 0.7 V in the forward scan, and the other at around 0.4 V and 0.5 V in the reverse scan [129]. The two peaks showed the good methanol oxidation and its associated intermediate species. Our results of methanol oxidation with the use of the pure Pt catalyst prepared are agreement with other scientific reports. The peak current density in the forward scan serves as benchmark for the catalytic activity of Pt nanoparticles during methanol dehydrogenation. For the prepared catalyst samples, its values are 7.62×10-4 A/cm2 (washed-only samples), 8.75×10-4 (heated-only samples), and 9.90×10-4 (washed and heated samples), respectively [129]. Therefore, the as-prepared Pt nanoparticles should be washed with organic solvents before heating them at a specific temperature. It is known that PVP can be polymer that only protects the as-prepared Pt nanoparticles in the solution products that seriously decrease the catalytic activity of Pt nanoparticles in MOR. Therefore, PVP or other polymers should be completely removed in the centrifugation processes by centrifuge systems for the pure Pt and Pd based catalyst before high heat treatments for to enhance catalytic activity of Pt nanoparticles for FCs, PEMFCs, and DMFCs, primarily towards methanol electro-oxidation (MOR) in DMFCs.

6. Prospects and conclusion In the development and commercialization of various FCs, PEMFCs, and DMFCs, the next Pt and Pt based bimetal nanoparticles of internal structure, surface structure, shape and morphology in the nanosized ranges, typically 10 nm, 20 nm, and 30 nm are the best potential catalysts for a significant reduction of the very high costs of various FCs, PEMFCs, and DMFCs. The aim of this chapter is to demonstrate that the urgent demands of studying and synthesizing for the next Pt based catalysts of highly long-term stability and durability are crucial to create low-cost products of PEMFCs and DMFCs. The low cost of low and high temperature FCs, PEMFCs and DMFCs is an extremely important factor for large-scale commercialization. The low-cost Pt and Pd based catalysts can be the good solution to the big challenges of the costs of low and high temperature FCs, PEMFCs, and DMFCs. The modifications and improvements of the catalyst layer materials, and their synthesis become important factors. It should be stressed that next low and high temperature FCs for direct conversion from chemical energy into electrical energy will be very crucial to offer potential applications in portable power supply for mobile use or portable devices or transportation vehicles. Thus, large-efficiency FCs for direct energy conversion via chemical reaction into electricity can be commercially realized in near future.

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Author details Nguyen Viet Long Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasugakouen, Kasuga, Fukuoka, Japan Department of Education and Training, Posts and Telecommunications Institute of Technology, Nguyen Trai, Ha Dong, Hanoi, Vietnam Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, Japan Laboratory for Nanotechnology, Ho Chi Minh Vietnam National University, Linh-Trung, Thu-Duc, Ho Chi Minh, Vietnam Cao Minh Thi Ho Chi Minh City University of Technology, Dien-Bien-Phu, Binh-Thach, Ho Chi Minh City, Vietnam Masayuki Nogami Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya, Japan Michitaka Ohtaki Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasugakouen, Kasuga, Fukuoka, Japan

Acknowledgement This work was supported by NAFOSTED Grant No. 104.03-2011.33, 2011. This work was supported by Laboratory for nanotechnology, Vietnam National University, Ho Chi Minh in Viet Nam. We greatly thank Kyushu University in the Global COE Program, Novel Carbon Resource Sciences for the financial support in our research and development of science and nanotechnology in Kyushu University, Japan. We greatly thank Nagoya Institute of Technology for kind supports in Japan.

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[183] Du S (2012) Pt-based nanowires as electrocatalysts in proton exchange fuel cells. Int. J. Low-Carbon Tech. 7(1):44-54. [184] Huaman J, Fukao S, Shinoda K, Jeyadevan B (2011) Novel standing Ni-Pt alloy nanocubes. CrystEngComm.13:3364-3369. [185] Gong K, Vukmirovic M, Ma C, Zhu Y, Adzic R (2011) Synthesis and catalytic activity of Pt monolayer on Pd tetrahedral nanocrystals with CO-adsorption-induced removal of surfactants. J. Electroanal. Chem. 662:213-218. [186] Taufany F, Pan C, Rick J, Chou H, Tsai M, Hwang B, Liu D, Lee J, Tang M, Lee Y, Chen C (2011) Kinetically controlled autocatalytic chemical process for bulk production of bimetallic core-shell structured nanoparticles. ACS Nano. 5(12):9370-9381. [187] Wei Z, Feng Y, Li L, Liao M, Fu Y, Suna C, Shao Z, Shen P (2008) Electrochemically synthesized Cu/Pt core-shell catalysts on a porous carbon electrode for polymer electrolyte membrane fuel cells. J. Power Sources. 180:84-91. [188] Wang ZL (2000) Transmission electron microscopy of shape-controlled nanocrystals and their assemblies. J. Phys. Chem. B 104:1153-1175. [189] Wang Y, Toshima N (1997) Preparation of Pd-Pt bimetallic colloids with controllable core/shell structures. J. Phys. Chem. B. 101:5301-5306. [190] Nguyen VL, Ohtaki M, Matsubara T, Cao MT, Nogami M (2012) New experimental evidences of Pt-Pd bimetallic nanoparticles with core-shell configuration and highly fine-ordered nanostructures by high-resolution electron transmission microscopy. J. Phys. Chem. C 116(22):12265-12274.

Chapter 4

Monitoring the Effects of Thermal Treatment on Properties and Performance During Battery Material Synthesis Wesley M. Dose and Scott W. Donne Additional information is available at the end of the chapter http://dx.doi.org/10.5772/50882

1. Introduction 1.1. Energy supply Utilizing an inexpensive, clean and sustainable supply of energy is one of the world’s foremost challenges heading into the future. The present energy supply scenario is dominated by fossil fuels, which are both a finite resource and a substantial contributor greenhouse gas emissions. Many renewable sources of energy (e.g., nuclear, solar, wind, etc.) exist for consumer use, although they all have associated pros and cons which means they cannot be used ubiquitously across the planet [1]. As a result, the future supply of energy is not likely to be centralized in a limited number of large power stations, but rather much more distributed as smaller scale renewable energy sources are utilized. Much has been made in the literature concerning solar energy harvesting, both in terms of photovoltaics and solar thermal. Of all the renewable forms of energy, solar energy has the capacity to completely replace society’s dependence on fossil fuels. However, of course, the challenge remains to make this a reality, particularly so with the high cost of photovoltaics [1]. Another perceived problem with the use of photovoltaics, and indeed with many other renewable energy sources, is their intermittency. Whether this be over short time frames, such as with cloud cover, or extended periods of time, such as overnight, steps need to be taken to ensure the consistency of power supply. In other words, some form of energy storage must be present to complement the primary source of energy.

1.2. Energy storage Energy can be stored in many different ways, some examples of which include [2]:

76 Heat Treatment – Conventional and Novel Applications

i. ii. iii. iv.

Thermal energy storage as heat; Chemical energy storage in the form of a fuel; Electrical energy storage in the form of charge separation; and Mechanical storage in the form of kinetic energy.

Each of these types of energy storage has its own set of performance characteristics in terms of the energy and power that they can deliver, ultimately meaning that they will be best suited in specific applications. Of course cyclability is also a key performance characteristic. Chemical energy, stored as a fuel, can deliver very high specific power and energy, with reasonable efficiency, particularly when it is used in an internal combustion engine [3]. However, such a combustion reaction does little to abate the demand for fossil fuels (which are used most commonly in this domain) or the contributions to greenhouse gas emissions. Chemical energy storage has an added advantage in the sense that it can also be released electrochemically (rather than thermally), through devices such as batteries, supercapacitors and fuel cells [4]. While the specific energy and power performance of these devices is much less than that of the internal combustion engine, their efficiency is much higher, in some instances approaching 100%, meaning that they ultimately utilize any fuel much better.

1.3. Batteries and battery materials There are many battery systems available for consumer use, and over the years since commercial introduction, their performance has improved due to a combination of advances in material design and cell engineering [5]. The various battery chemistries that are available can be categorized as either primary (single use) or secondary (rechargeable) systems. The battery system that is currently receiving the most attention in the literature is based on the Li-ion chemistry. Here Li+ ions are reversibly shuttled between the positive and negative electrodes in the cell, which with the use of an appropriate non-aqueous electrolyte, can achieve a potential of over 4 V [5,6]. Some of the commonly found materials in Li-ion batteries include: i.

Negative electrode materials: Li metal, carbon, Li4Ti5O12, Sn and its alloys, Si and its alloys; ii. Electrolytes: organic carbonate mixtures; e.g., 1:1 ethylene carbonate:dimethyl carbonate, ionic liquids; and iii. Positive electrode materials: LiMO2 (where M is a combination of Ni, Mn and/or Co), LiMn2O4 (and doped varieties thereof), LiFePO4, MnO2. Whatever the combination of positive and negative electrode that is used, the performance of the resultant cell is quite significantly determined by the properties of the electroactive materials used, which in turn are determined by the way in which they were prepared.

Monitoring the Effects of Thermal Treatment on Properties and Performance During Battery Material Synthesis 77

Material properties such as phase purity, crystallinity and particle size (or extent of agglomeration) all affect performance.

1.4. Common synthetic routes Many different synthetic routes have been used to prepare the materials mentioned in the previous section, so much so that to list them here would be excessive (note the recent review in reference [6]). Nevertheless, the more common approaches can be categorized as being based on either (i) thermal methods, (ii) solvothermal methods, (iii) mechanical methods, and (iv) electrochemical methods. It is also reasonably common to find that a combination of these methods has been used to produce the resultant material. As an example, LiFePO4 can be made by first using a solvothermal process to intimately mix the precursors, with the resultant mixture then subjected to a thermal treatment to make the final product [7]. Overall, the majority of the positive electroactive materials listed above involve a thermal processing step as the last step in their synthesis.

1.5. Pitfalls of thermal processing methods The overall objective of any synthesis method is to obtain the final product in the desired form for immediate use. This is particularly true for thermal synthesis methods where there is a delicate balance between heat treatment temperature and duration so as to produce the desired material. Of course the choice of these thermal parameters is also dependent on the effectiveness of precursor mixing, with various solvothermal methods being used to ensure appropriate mixing on the molecular level. Contrast this with some of the initial solid state mixing methods (grinding) used in some synthetic efforts, and the implications it has on the thermal conditions necessary [8]. Let us begin by assuming that we have sufficient mixing of our precursors, since the focus of the discussion here is on the actual thermal conditions to be used. Under these circumstances if we were to thermally treat this mixture the temperature and duration of heat treatment would determine the phase purity and crystallinity of the resultant material. Of course a higher heat treatment temperature, and a longer heat treatment duration would ensure phase purity, as well as lead to a more crystalline material. The question at this time then becomes: What is the preferred material crystallinity? Many positive electroactive materials in Li-ion batteries require very small crystallite sizes so as to minimize Li+ ion diffusion paths, which is commonly regarded as a key limiting factor in performance [7]. Therefore, excessive heat treatment temperatures and durations, while they may ensure phase purity, also lead to excessive crystallization, which is detrimental. Additionally from a commercial perspective, excessive material heating leads to a waste of energy, which can be costly. What is required, therefore, is a method for predicting the optimum heat treatment temperature and duration so as to ensure phase purity and small crystallite size.

78 Heat Treatment – Conventional and Novel Applications

1.6. This work What we will describe here in this chapter is a method for identifying the optimum thermal synthesis conditions, and then explore their effects on the resultant material properties. The system we will use to demonstrate the approach is the heat treatment process used to remove water from the structure of -MnO2 prior to its use in non-aqueous Li-MnO2 cells. While this will be used as a representative example, the general method is applicable to any other thermally based synthetic method.

2. Methods 2.1. Preparation of starting material

The sample of -MnO2 used in this work was prepared by anodic electrodeposition, and hence given the designation electrolytic manganese dioxide, or EMD. The cell used for electrolysis was based on a temperature controlled 2 L glass beaker in which two 144 cm2 (72 cm2 on either side) titanium sheets were used as the anode substrate, and three similarly sized copper sheets were used as the cathode substrate. The electrodes were arranged alternately so that each anode was surrounded on both sides by a cathode. The electrolyte was an aqueous mixture of 1.0 M MnSO4 and 0.25 M H2SO4 maintained at 97°C. Electrodeposition of the manganese dioxide was conducted with an anodic current density of 65 A/m2 according to the reactions:

Anode  Ti  : Mn 2   2H2O   MnO2  4H  2e  Cathode  Cu  :

2H  2e   H2

Overall : Mn 2   2H 2O   MnO2  2H  H2

(1) (2)

(3)

The overall process was carried out for three days, during which time the electrolyte Mn2+ concentration was of course depleted, while the H+ concentration increased. To counteract this, and hence maintain a constant electrolyte concentration over the duration of the deposition, a concentrated (1.5 M) MnSO4 solution was added continually at a suitable rate to replenish Mn2+ and dilute any excess H2SO4 produced. Under these conditions control of the solution conditions was typically maintained to within 2%. After deposition was complete, the solid EMD deposit was mechanically removed from the anode and broken into chunks ~0.5 cm in diameter, and then immersed in 500 mL DI water to assist in the removal of entrained plating electrolyte. The pH of this chunk suspension was adjusted to pH 7 with the addition of 0.1 M NaOH. After ~24 h at a pH of 7 the suspension was filtered and the chunks then dried at 110°C. After drying the chunks were then milled to a -105 m powder (mean particle size ~45 m) using an orbital zirconia mill.

Monitoring the Effects of Thermal Treatment on Properties and Performance During Battery Material Synthesis 79

The powder was then suspended in ~500 mL of DI water and its pH again adjusted to 7 with the further addition of 0.1 M NaOH. When the pH had stabilized, the suspension was filtered and the collected solids dried at 110°C. When dry the powdered EMD was removed from the oven, allowed to cool to ambient temperature in a dessicator and then transferred to an airtight container for storage.

2.2. Thermogravimetric (TG) analysis TG analysis was conducted using a Perkin Elmer Diamond TG/DTG controlled by Pyris software. Approximately 10 mg of -MnO2 sample was added to an aluminium sample pan and placed into the analyser. The same mass of -Al2O3 in a similar aluminium pan was used as the reference material for DTA measurements. With the sample and reference materials loaded, and the furnace closed, dry nitrogen gas was passed over the sample at 20 mL/min for 30 minutes prior and during the heating profile. The heating profile applied to the sample was essentially a linear ramp at rates ranging from 0.25°-10°C/min.

2.3. Material characterization The structure of the materials generated in this work were identified by X-ray diffraction using a Phillips 1710 diffractometer equipped with a Cu Kα radiation source (λ=1.5418 Å) and operated at 40 kV and 30 mA. The scan range was from 10° to 80° 2θ, with a step size of 0.05° and a count time of 2.5 s. Analysis of the diffraction patterns was carried out by fitting a Lorentzian lineshape to individual peaks, with the fitting parameters then used to calculate structural properties such as the fraction of pyrolusite (Pr; as described by Chabre and Pannetier [9]) and the unit cell parameters. Morphology was examined by gas adsorption using a Micromeritics ASAP 2020 Surface Area and Porosity Analyser. A representative 0.10 g sample of the manganese dioxide material was degassed under vacuum at 110°C for 2 h prior to analysis. An adsorption isotherm was then determined over the partial pressure (P/P0) range of 10-7-1 using N2 gas as the adsorbate at 77 K. The specific surface area was extracted from the gas adsorption data using the linearized BET isotherm [10] in the range 0.05110°C) 2.13 1.67 1.60 1.66 0.94 1.15

%H2O (> Cb than Ceq ≈ Cb. Therefore the increase of ' in the samples series treated at 750 ° C, with the increase of the external electric field amplitude may be associated with an increase in the number of dipoles present in the bulk region of the samples. In these samples it was observed a decrease in the LiNbO3 particle size, with the increase of the applied electric field amplitude. This reduction is also associated with the decrease of intensity of the Raman band centered at 630 cm-1 related to the presence of LiNbO3 particles [24], indicating a decrease in the number of dipoles associated with those particles. By increasing the electric field amplitude applied during the heat treatment, the dc increases. This behavior must be related to the presence of a higher number of Li+ and Nb5+ ions inserted structurally in the glass matrix. Thus it can be assumed an increase in the number of electric dipoles in the bulk zone with the increase of the amplitude of the electric field.

168 Heat Treatment – Conventional and Novel Applications

In the samples series TET at 700 °C, the behavior of ', with the increase of the external electrical field amplitude, is opposite to the one observed in the samples TET at 750 °C. The increase of the particle size in the samples TET at 700 °C, with the increase of the external electrical field amplitude, was observed by SEM and by Raman spectroscopy (increase of the intensity of the band centered at 630 cm-1). This fact will cause a reduction in the number of dipoles in the sample bulk zone, thereby reducing the ' value. The same occurs in the sample TET at 650 °C. Thus, these results suggest that in the samples TET at 650 °C and 700 ºC the Nb5+ and Li+ ions migrate from the bulk zone to the surface, contributing to the increase of the particles size. The dielectric response, using the impedance formalism (Z*), as a function of frequency and temperature for the 92Si composition samples was adjusted to the physical model consisting on the equivalent circuit shown in figure 43 [R1(RCPE1)].

Figure 43. Equivalent electric circuit model.

It is noted that this model fits the experimental data reasonably, showing for all the samples a value of the parameter n very close to 1 (> 0.9), indicating that CPE element behaves very close to a capacitor. The value of R1 ~0 was considered in all samples. In the 88Si composition samples it was detected two relaxation mechanisms, which were fitted, using the CNLLS algorithm, to the equivalent circuit model shown in figure 33 ([R0(R1CPE1)(R2CPE2)]). From the obtained values, it should be noted that the parameters n1 and n2, associated to the elements and CPE1 and CPE2, respectively, are always higher than 0.73, which shows that also in this composition, the elements CPE tend to exhibit a behavior similar to a capacitive element. The relaxation mechanism, found in the high frequency region (R2CPE2) is assigned to the characteristics of the vitreous matrix ("bulk"), i.e., with the relaxation of dipoles associated with the ions structurally inserted in the glass matrix. The relaxation at the lowest frequencies was associated with surface features, namely those related with the dipoles related with the particles detected at the samples surfaces. The presence of those particles, related with the LiNbO3 crystalline phase, which is characterized by possessing dipoles which electrical depolarization is difficult, justifies the higher relaxation time. The fact that the relaxation time associated with the first depolarization mechanism, observed in the 88Si composition, is of the same order of magnitude to that observed in the 92Si samples composition, suggests that the electrical units responsible for both are the same, i.e., the dipoles associated with lithium and niobium ions inserted structurally in the glass matrix. In the 88Si composition, the relaxation time Z1, related with the R1CPE1, and associated with the relaxation mechanism at the low frequency zone, is higher (~ 10-2 s) than the Z2, which also indicates that the units responsible for this relaxation are more difficult to depolarize.

Lithium Niobiosilicate Glasses Thermally Treated 169

In the 92Si composition, the Z* behavior as a function of frequency, for the TET samples treated at 700 ºC is opposite to that observed in the samples TET at 750 ºC, with the increase of the external electric field amplitude (the series TET at 650 and 700 ºC have a similar behavior). In the samples TET at 750 ºC, the increase of the amplitude of the external electrical field, promote an increase in the values of the R and Y0 parameters. This behavior should be associated with the increase of the number of particles in the sample bulk zone. In the samples TET at 650 and 700 °C, the number of electrical units within the sample (bulk region) decreases with the increase of the amplitude of the external electrical field, and therefore, increases the number of surface particles, justifying the Z*() opposite behavior.

The 92Si composition presents dielectric constant ( ') and dielectric loss (tan     '' / '  values, much lower than those observed in the 88Si composition samples. The justification for this value disparity is the larger number of electric dipoles existing in the 88Si composition sample. In this composition, the decrease of ', from the sample TT at 600 ºC to the sample TT at 650 ºC, measured at room temperature and 1 kHz, is due to the presence, in the sample TT650, of particle agglomeration and also to high size particles, which promotes the decrease of the number of dipoles associated with lithium and niobium ions structurally inserted in the glass matrix. Furthermore it is likely that the crystal orientation of the particles and agglomerates, do not present a preferential grow direction contributing to the decrease of the dipole moment [36;37]. The parameter tan  (92Si composition) increases in the samples TET at 750 °C, with the increase of the amplitude of the external electrical field, mainly due to the increase of the '' component. This behavior will cause an increase in the conductivity, which is corroborated by the R parameter behavior. The decrease in the value of Z, in the samples TET at 750 °C with the increase of the amplitude of the electric field, suggests that the electrical units, present in these samples, follow the ac electric field more easily. In the TET samples treated at 650 °C, the behavior of tan  with increasing the amplitude of the external electric field) is opposite to that observed in the samples TET at 750 °C. This can be attributed to a significant increase in the ac resistivity. Moreover ' decreases due to the increase of the surface particles amount and thus reducing the amount of ions in the glass bulk zone.

The presence in samples of the 88Si composition of two relaxation peaks, in the Z* spectrum, similar to those already observed in other glass containing LiNbO3 crystals [38;39;40] suggests that the dc conductivity at low temperature and the dielectric relaxation at highfrequency can be assigned to the ion conduction and ionic polarization, respectively. The dc behavior at temperatures higher than 300 K and the relaxation process in the low frequency range should be assigned to interfacial electrode-sample polarization [38;39;40].

10. Main conclusions 1.

The sol-gel method allows the preparation of glasses and glass ceramics with compositions that by the melt-quenching method are extremely difficult to prepare.

170 Heat Treatment – Conventional and Novel Applications

2.

The drying process keeps the 92Si composition gel transparent and the 88Si translucent. The heat treatment with or without the presence of the external electrical field, makes the 92Si glass composition translucent. Samples of the 88Si composition, after TT at temperatures above 750 ºC, became opaque. 3. The detection of LiNbO3 crystal phase is observed in the 92Si sample composition TT at temperatures above 650 °C and in the 88Si composition at temperatures above 600 °C. Increasing the annealing temperature it is promoted the appearance of secondary crystalline phases. 4. SEM revealed that crystallization in both 92Si and 88Si compositions are predominantly at the glass surface. 5. Samples of the 92Si composition, TET at temperatures below 650 ºC, promote the formation of LiNbO3 crystallites 6. Increasing the amplitude of the electric field in the 92Si sample series treated at 650 and 700 °C favors the increase in the particle size. The TET at 750 ºC presents the opposite behavior. 7. The decrease in the surface particle size associated with an increase in the number of dipoles within the sample, justifies the maximum value of ' observed (9,44). Thus, the study of the dielectric constant enables to establish if the crystallization occurs, preferably in the sample surface or in the bulk zone. 8. The high values of ', measured in the low frequency region (