Laser and Plasma Applications in Materials Science [1 ed.] 9783038135982, 9783037851074

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Laser and Plasma Applications in Materials Science

Edited by El-Hachemi Amara Djamila Bennaceur-Doumaz

Laser and Plasma Applications in Materials Science Selected, peer reviewed papers from the LAPAMS 2010, Algiers from 27th to 30th November 2010.

Edited by

El-Hachemi Amara and Djamila Bennaceur-Doumaz

Copyright  2011 Trans Tech Publications Ltd, Switzerland All rights reserved. No part of the contents of this publication may be reproduced or transmitted in any form or by any means without the written permission of the publisher. Trans Tech Publications Ltd Kreuzstrasse 10 CH-8635 Durnten-Zurich Switzerland http://www.ttp.net Volume 227 of Advanced Materials Research ISSN 1022-6680 Full text available online at http://www.scientific.net

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Preface Lasers and Plasmas are among the technologies that have contributed to the enhancement of the people way of life in many fields. Since the fifties, metals such as stainless steels, aluminium and copper who were difficult to process by flame, were treated instead by plasma in cutting for instance. Since then, the developments reached a stage that allowed treating all kind of materials, and one can find sophisticated plasma machines with high resolution. Since the sixties, the use of the laser increased in various fields, such as scientific, industrial, medical and military applications. Although the processes were demonstrated it was necessary to await their associations with adapted machines so that to be established in industrial environment. These conditions were filled in the end of the seventies and the first industrial platforms were established in Europe from the eighties. The laser then becomes an efficient tool in a point of view of the speed tasks execution, and the accuracy and quality of the obtained result. This explains the development and the continuous improvement of the laser sources and the techniques of laser material processing. The applications related to these treatments cover a broad spectrum going from nano-applications to the heavy industrial applications. Within the framework of its research programs, the Ionized Media and Lasers Division of the Advanced Technologies Development Centre (CDTA) of Algiers, carries out studies for the development of laser and plasma applications in the transformation of materials for industrial issues. It is a field of interest since approximately ten years, during which an appreciable work using experimental and theoretical approaches has been achieved. The CDTA with its activity in the field of lasers and plasma and their applications, is a founding member of the African Laser Centre (ALC) which head office is in Pretoria in South Africa, it constitutes one of the major nodes of the ALC on a continental. The CDTA is also a member of the Laser Atomic and Molecular Network (LAM Network) affiliated to ICTP, and the International Union for Pure and Applied Physics (IUPAP). The CDTA organized in June 2008 the first international conference on Laser And Plasma Applications in Materials Science LAPAMS’08. The output of the conference, published in the American Institute of Physics (AIP) proceedings, Vol. 1047 (www.aip.org ), can give an overview of the level and quality of the invited speakers and papers. The second conference LAPAMS, was also organized by CDTA in Algiers from 27th to 30th November 2010. This conference called LAPAMS’10 had as main objectives the state of the art presentation, the trends, the fundamental aspects of the interaction between a laser beam and material, and the modelling and simulation of laser and plasma material surface transformation and processing. The main topics developed in LAPAMS’10 were: - Laser materials processing. - Applications of the plasmas produced by laser. - Plasma discharge applications. - Laser technology for materials sciences and environment. - Telecommunications. - Modelling of the treatment of materials and the microstructures. This conference gathered eminent researchers of international notoriety from countries around the world, where valuable plenary talks, oral communications and posters, beside cultural and social activities were presented. The selected manuscripts were proposed for publication and diffusion by Trans Tech Publications Ltd in ‘Advanced Materials Research’ periodical.

Dr El-Hachemi Amara Conference Chairman

THE SPONSORS

General Directorate for Scientific Research and Technological Development (DGRSDT) National Agency for the Development of the University Research (ANDRU) International Atomic Energy Agency (IAEA) Welding and Control Centre (CSC) Condor Electronics Algeria African Laser Centre (ALC) LAM Network French Embassy in Algeria Centre for Research of Scientific and Technical Information (CERIST)

Committees General Conference Chairman Brahim BOUZOUIA, CDTA, Algeria Conference Chairman El Hachemi AMARA, CDTA, Algeria Scientific Committee Chairman Tahar KERDJA, CDTA, Algeria International Scientific Committee M. ABDELHARITH, Egypt S. AIDA, Algeria K. AIT AMEUR, France E.H. AMARA, Algeria S.E. AMARA, Algeria H. AOURAG, Algeria A. BELASRI, Algeria Z. BEN LAKHDAR, Tunisia A. BENDIB, Algeria M.S. BENKADDA, France A. BOGAERTS, Belgium O. BOUAFIA, Algeria S. BOUDJEMAI, Algeria L. BOUFENDI, France J.D. COMINS, South Africa A. DE GIACOMO, Italy H. DJELOUAH, Algeria D. DOUMAZ, Algeria J.M. DOWDEN, UK R. FABBRO, France T. GASMI, Spain D. GRAVES, USA K. HENDA, Algeria J. HERMANN, France F.J. KAHLEN, South Africa M. KECHOUANE, Algeria A. KELLOU, Algeria

T. KERDJA, Algeria A. KHALFAOUI, Algeria O. LAMROUS, Algeria J. LASERNA, Spain D. LOUHIBI, Algeria M. MAAZA, South Africa H. MANAA, Kuwait G.MANK, Austria N. MELIKECHI, USA S. MESSACI, Algeria S. MESSAOUD, Algeria G. MAYNARD, France S. PITYANA, South Africa M. RICHARDSON, USA M. SABSABI, Canada D. SAIFAOUI, Morocco N. SAOULA, Algeria C. SMAL, South Africa S. SOUKANE, Algeria W. STEEN, UK A. TALEB, Algeria T. TOUAM, Algeria H.M.VON BERGMANN, South Africa F. VIDAL, Canada A. WAGUE, Senegal M. ZGHAL, Tunisia O. ZIANE, Algeria

Organizing committee E.H. AMARA S. BELGUEBLI N. BENZABA S. BOUDJEMAI F. BOUKERCHA K. BOURAI D. DOUMAZ H. HAMOU T. KERDJA

B. KHITER S. LAFANE K. MAHDI G. MENGUELLET S. MESSAOUD-ABERKANE N. SAOULA R. TOUATI-BOUBETRA K. YAHIAOUI

Table of Contents Preface, Sponsors and Committees

Invited Papers

Computer Simulations of Laser Ablation, Plume Expansion and Plasma Formation A. Bogaerts, M. Aghaei, D. Autrique, H. Lindner, Z.Y. Chen and W. Wendelen Light Scattering Techniques Applied to Materials Science J.D. Comins Numerical Study of Butt Joining by Coaxial Powder Injection E.H. Amara, T. Tamsaout, K. Kheloufi, H. Berger and S. Pityana

1 11 17

I. Laser Matter Processing

Microstructure and Wear Behaviour of Al/TiB2 Metal Matrix Composite A.P. Popoola, S. Pityana and E. Ogunmuyiwa Numerical Simulation of Laser Bending of Thin Plate Stress Analysis and Prediction T. Tamsaout and E.H. Amara Angular Distribution and Ion Time of Flight Produced on Silicon Target by Laser Irradiation Y. Belaroussi, T. Kerdja and S. Malek

23 27 31

II. Materials for Telecommunication Application

Investigation on Thin Films Deposited by PECVD from a DiPhenylMethylSilane (DPMS) Vapors or Mixed with Oxygen for Low-K Material Application L. Bouledjnib, S. Sahli, A. Zenasni, P. Raynaud and Y. Segui Enhancement of Blue Spectral Response Intensity of PbS via Polyethylene Oxide-Adding for the Application to White LEDs S. Kaci, A. Keffous, M. Trari, B. Mahmoudi and H. Menari Study of Optical and Structure Properties for Different Composition Tin-AntimonySelenium Thin Film F.M. Abdel-Rahim

35 39 43

III. Laser Induced Plasma

Analyses of Plasmas Produced by Laser Ablation of Fresh Aliments S.A. Beldjilali, J. Hermann, T. Baba-Hamed and A. Belasri On the Electron Distribution Effect of an Expanding Laser Ablated Plasma D. Bennaceur-Doumaz, D. Bara and M. Djebli High Intensity Laser Ablation of Titanium Target K. Yahiaoui, T. Kerdja and S. Malek Laser Ablation in Liquids: Colloidal Nanoparticles Synthesis S. Messaoud Aberkane, S. Boudjemai and T. Kerdja Characterization of CNx/Si Using RBS, NRA and AES Techniques M. Siad, S. Abdelli-Messaci, T. Kerdja, S. Lafane and M. Abdesselam Effect of Laser Fluence on the Properties of Sm1-XNd X NiO3 Thin Films Deposited by KrF Laser Ablation S. Lafane, T. Kerdja, S. Abdelli-Messaci, S. Malek and M. Maaza Characterization of Laser Induced Plasmas by Fast Imaging for Graphite Target C. Siouani, S. Abdelli-Messaci, T. Kerdja and S. Malek

49 53 57 62 67 72 76

IV. Laser Technology for Materials Science and Environment

A Review of the Laser Pyrolysis Technique Used to Synthesize Vanadium and Tungsten Oxide Thin Films L. Shikwambana, M. Govender, B. Mwakikunga, E. Sideras-Haddad and A. Forbes

80

b

Laser and Plasma Applications in Materials Science

In Situ Metal Matrix Composite Surfacing by Laser Surface Alloying J.D. Majumdar Laser-Based Additive Manufacturing of Metals S. Kumar and S. Pityana Comparison of the Capability of Peak Function in Describing Real Condensation Particle Counter Profiles A. Djebara, R. Khettabi, J. Kouam and V. Songmene

84 92 96

V. Modelling of Materials Processing and Microstructures

One-Dimensional Modeling of a Dielectric Barrier Discharge in NeXe Mixture, Application to Excimer Lamps K. Khodja, H. Sisabeur and A. Belasri Electromagnetic Modeling of Microwave Axial Injection Torch at Atmospheric Pressure Used for Thin Film Deposition N. Ikhlef, M. Mekidèche, O. Leroy and A. Tibouche Modelling Sequential Impact of Molten Droplets on a Solid Surface in Plasma Spray Process I.R. Kriba and A. Djebaili Fluid Model Simulation of DC Glow Discharges H. Bahouh, S. Rebiaï, F. Bouanaka and S. Sahli PIC-MC Simulation Method of DC Discharge Plasmas F. Bouanaka, S. Rebiaï, H. Bahouh and S. Sahli Modeling of Detector Radiations Response P-I-N in Technology Thin Film on ASIC (TFA) Intended for Digitalization in Medical Imagery A. Saouli and K. Mansour Effect of Argon Ambient Gas Pressure on Plume Expansion Dynamics F. Hamadi and E.H. Amara Simulation of Geometry and Heat Transfer in a Thin Wall Produced by Direct Laser Powder Deposition K. Kheloufi and E.H. Amara Modeling the Formation of Periodic Nanostructures on Solid Surface Induced by Femtosecond Laser Ablation by Particle-in-Cell Method M. Djouder, T.E. Itina and O. Lamrous

101 105 111 116 121 125 129 134 138

VI. Plasma Discharge

Numerical Modeling of an End-Hall Ion Source N. Oudini, G. Hagelaar, L. Garrigues and J.P. Bouef Optimization of the Plasma Display Panel Characteristics with PIC-MCC Method W. Benstâli and A. Belasri OES Diagnostics of HMDSO/O2/CF4 Microwave Plasma for SiOCxFy Films Deposition R. Chabane, S. Sahli, A. Zenasni, P. Raynaud and Y. Segui Effect of the Plasma Deposition Parameters on the Properties of Ti/TiC Multilayers for Hard Coatings Applications N. Saoula, K. Henda and R. Kesri Computation of the Net Emission Coefficient with the Overlapping Lines Consideration on the CH4-Ar Plasma Discharge R. Benallal and B. Liani Modeling of a Ne-Xe-HCl DC Discharge for Excimer Lamp S. Bendella and A. Belasri Study of the Adhesion and the Thermal Stability of CrN and CrAlN Thin Films F.Z. Mammeri, L. Chekour and N. Rouag Dry Sliding Wear of Stainless Steel Coating Obtained by Plasma on Aluminium Substrate N. Khanafi-Benghalem, K. Benghalem, K. Loucif, S. Aounnallah and A. Redjechta Study of API 5L X52 Carbon Steel Treated in Oxygen Plasma Discharge B. Demri and D. Mansour Influence of Plasma Parameters and Circuit Connecting on Harmonics Generated in Ar/O2 13.56 Mhz Plasma Discharge R. Tadjine, H. Lahmar and M.M. Alim

144 148 152 156 160 164 168 173 177 181

Advanced Materials Research Vol. 227 Electrical Characterization of Inductively Coupled Plasma Reactor Excited by RF (13.56MHz) M.M. Alim, M. Zekara, L. Henni, R. Tadjine and K. Henda Experimental Study of Evolution of NO and NO2 in a Positive Corona Discharge I.S. Medjahdi, M. Lemerini, F. Pontiga, H. Moreno, A.K. Ferouani and R.D. Medjahdi Diagnostic of a RF (13.56MHz) Magnetron in Ar/CH4 Discharge S. Djerourou, N. Saoula and K. Henda Optical Properties of a-C:H Films Deposited by Plasma Microwave Discharge with Controlling Substrate Temperature M. Kihel, S. Sahli, R. Clergereaux, P. Raynaud and Y. Segui Development of a Radiofrequency Plasma Diagnostic System with a Langmuir Probe and Study of a Capacitively Coupled Argon Plasma D. Mendil, H. Lahmar, D. Ouadjaout, L. Henni, L. Boufendi, D. Louhibi and K. Henda Photo-Detachment of H- in Magnetized Cascaded Arc Hydrogen Plasma S.M. Elhamali, A. Bouzed and A.M. Hashkel Monte Carlo Simulation for an Electrical Discharge in O2 L. Zeghichi, L. Mokhnache and M. Djebabra

c

185 189 195 200 204 208 211

Advanced Materials Research Vol. 227 (2011) pp 1-10 © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.227.1



           

    
 

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Fig. 3: Spectral response for PbS films at different PEO content and temperatures 5> From the data of the present work, we can conclude that from Iph measurements, we have deduced that the detector has a good linearity response. Also the Iph, decreases with increasing PEG content. The Spectral response of all Au/PbS/p-Si(100)/Al photodiodes structures exhibit the combined effect of photogeneration in the Si side and PbS side of the Heterostructure. The peak intensity at 1µm corresponds to the fundamental absorption in the Si substrate and hence to the Si band gap. ( [1] S. W. Chen, L. A. Truax and J. M. Sommer: Chem. Mater. Vol. 12 (2000), p. 3864 [2] A. Popa, M. Lisca, V. Stancu, M. Buda, E. Pentia and T. Botila: J. Optoelec. Adv. Mater. Vol. 8 (2006), p. 43 [3] [4]

M.J. Turner and E.H. Rhoderick: Solid State Electron. Vol. 11 (1968), p. 291 S.M. Sze: Physics of Semiconductors Devices, Metal–Semiconductor Contacts, second ed., Wiley, New York, (1981).

[5]

S. Kumar, T. P. Sharma, M. Zulfequar and M. Husain: Physica B Vol. 325 (2003), p. 8

[6]

A. Boukezzata, G. Nezzal, A. Keffous, K. Bourenane, T. Kerdja, M. Kechouane and H. Menari : Optics Communications Vol. 281 (2008), p. 2126



Advanced Materials Research Vol. 227 (2011) pp 43-48 © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.227.43

Study of Optical and Structure Properties for Different Composition Tin-Antimony-Selenium Thin Film ABDEL-RAHIM Farid M.1,2,a 1

Physics Department, Faculty of Science & Arts Khulais, King Abdulaziz University, K. S. A 2

Physics Department, Science Faculty Al-Azhar University, Assiut Branch , Assiut, Egypt a

[email protected], [email protected],

Key words: Optical parameters, Thin films, Chalcogenide, Se (Sn Sb) Composition

Abstract. Amorphous Se100-x (Sn Sb)x glasses with (0 ≤ x ≤ 20 at. %) were prepared by the usual melt quench technique. Thin films for these compositions were prepared by thermal evaporation onto ultrasonically cleaned glass substrates kept at room temperature. From the spectral dependence of the absorption coefficient, a direct electronic transition was mainly responsible for the photon absorption inside these films. The effects of composition on the optical properties of Se100-x(Sn Sb) x thin films was investigated. The refractive index, n, for the as-prepared and annealed films has been analyzed according to the Wwmple–DiDominico single oscillator model. The effect of addition (Sn Sb) on the nature and degree of crystallization has been investigated by studying the structure using transmission electron microscope, X-ray diffraction and scanning electron microscope. Introduction The chalcogenide are one of the most widely known families of amorphous and polycrystalline materials and have been studied extensively over the past few decades because of their interesting fundamental properties and wide commercial applications. These applications like, memory, xerography [1], Infrared optics [2], infrared detectors [3], acousto-optic devices [4], electronic [5] and optical switching devices [6] and phase change optical recording, PCOR [7]. Several methods have been employed to grow Se100-x(Sn Sb)x thin films such as thermal evaporation, PLD Technique [8], chemical path deposition [9], The present study was undertaken in order to investigate the influence of composition on the optical properties of the chalcogenide glass compositions Se100-x(Sn Sb)x thin films. The obtained results were tentatively interpreted in terms of the chemical bond approach [10] and the cohesive energies. Experimental details The bulk materials were prepared by means of the melt quench technique, from Sn, Sb and Se with high purity (99.999%). The appropriate quantities of the pure elements were weighed. The weighed materials were placed into cleaned evacuated (10-5 torr) silica tube, which was put in a furnace. The furnace temperature was raised by 4 K/min. up to 1300 K then holed constant for 24 h. During the course of heating, the tube was shaken vigorously at regular intervals to ensure the alloy homogeneity. The tube was then quenched in an ice cold water to avoid the crystallization. The prepared alloys were used as the source material for thin film deposition on to ultrasonically cleaned glass substrates using Edward E306 coating unit. The evaporation rates as well as the film thickness were controlled using glass crystal monitor FTM5. The nature of films were examined by X-ray diffraction using Philips diffractometer type 1710 with Ni filtered CuKa source 0.154 nm. The transmittance (T) and reflection (R) at normal incidence for thermally evaporated Se100-x(Sn Sb)x thin films were measured in the wavelength range (200–900 nm) using Shimadzu UV- 2101PC double-beam spectrophotometer attached with PC data acquisition system.

44

Laser and Plasma Applications in Materials Science

Results and discussions Structure examination The chemical composition of thin films were investigated using energy dispersive analyses of X-ray (EDAX). It was found that, the elemental compositions for the as prepared films in good agreement with that of the atomic weight of each element with small deviations (± 1%. at). Fig.1. Shows the (EDAX) spectrum of the Se100-x(Sn Sb)x thin films.

Elmt Se

Element Atomic % % 99.90 99.66

a

Elmt Se Sn Sb

Element Atomic % % 80.90 81.65 8.95 9.25 10.15 9.10

b

Fig.1. EDX analysis of as-deposited thin film for (a) Se thin film (b) Se80 (Sn Sb)20 thin film The samples were gold coated before SEM examination to study surface morphology. The scanning micrograph specimens of different compositions annealed at 450 K for 2 h are shown in Fig. 2a–c. The microstructure obtained for the annealed Se composition is shown in Fig. 2a. a

b

c

e

e

e

Se98 (Sn Sb)2

Se80 (Sn Sb)20

Se

Fig.1. SEM photograph for the compositions films annealed at 450 K for 1 h (a) Se (b) Se98 (Sn Sb)2 (c) Se80 (Sn Sb)20 Most of the morphology is still amorphous and a few particles are crystallized in semispherical shape. By increased (Sn Sb) thin film shows, clearly, A polycrystalline structure consisting of different crystalline phases with different shapes and sizes embedded in an amorphous Se84 (Sn Sb)16 matrix is clear as shown as Fig. 2b. As indicated, the crystallites are dispersed homogeneously in an amorphous matrix. The photomicrograph in Fig. 2c shows the surface microstructure of the annealed Se80 (Sn Sb)20 composition. A polycrystalline structure consisting of different crystalline phases embedded in amorphous matrix is observed. Some of these crystallized particles are interconnected and others are isolated.

Advanced Materials Research Vol. 227

45

Optical absorption at the fundamental edge. The spectral distributions of transmittance T(λ) and reflectance R(λ)for the as prepared Se100-x (Sn Sb)x thin films were recorded in the wavelength range 200-900 nm as shown in Figs.3 and 4. 1.0

1.0 Se Se98(Sb Sn)2

0.6

0.8

Se96(Sb Sn)4 Se92(Sb Sn)8 Se84(Sb Sn)16 Se80(Sb Sn)20

Reflectance (R)

Transmittance (T)

0.8

0.4

0.2

0.6

0.4

Se Se98(Sb Sn)2 Se96(Sb Sn)4 Se92(Sb Sn)8

0.2

Se84(Sb Sn)16 Se80(Sb Sn)20

0.0 300

400

500

600

700

800

0.0 300

900

400

500

600

λ (nm)

Fig.3.

700

800

900

λ (nm)

Transmittance (T) versus wavelength(λ) nm for Se100-x (Sn Sb)x thin films.

Fig.4. Reflectance (R) versus wavelength(λ) nm for Se100-x (Sn Sb)x thin films.

It can be seen from these figures that, both transmission and reflectance process there is an opposite behavior with variation of wavelength (or incident photons energy). Knowing the film thickness d, the absorption coefficient, α, was determined as a function of wavelength by using the measured reflectance R and transmittance T through the following equation [11] : 1 α = ln[(1 − R ) T ] (1) d For α values < 104 cm−1, many amorphous semiconductors show an exponential dependence on photon energy hν, and obey Urbach’s empirical relation [12]:

α = α 0 exp(hv E e )

(2)

where α0 is constant and Ee is a width of the band tail of the localized state in the band gap. It should be mentioned that this equation is applicable only in the low absorption region. Fig. 5 represents the linear dependence of ln(α) on the photon energy (hυ) for all investigated films. The slope of each line yields Ee values. The estimated values of Ee as a function of composition are given in Table 1. 400

12

(cm eV)

1/2

11

7

300

1/2

(α hν )

ln(α )

9

8

350

-1

10

Se Se98(Sn Sb)2

Se Se98(Sn Sb)2

250

Se96(Sn Sb)4

Se96(Sn Sb)4

Se92(Sn Sb)8

Se92(Sn Sb)8

200

Se84(Sn Sb)16

Se84(Sn Sb)16 6 1.6

Se80(Sn Sb)20

Se80(Sn Sb)20 1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

hν (eV)

Fig. 5. ln(α) versus (hν) nm for Se100-x (Sn Sb)x thin films.

150

1.8

2.0

2.2

2.4

2.6

2.8

hν (eV)

3.0

3.2

3.4

3.6

3.8

4.0

Fig. 6. (αhυ)1/2 versus (hυ) eV for Se100-x (Sn Sb)x thin films.

46

Laser and Plasma Applications in Materials Science

For higher values (α ≥ 104 cm−1), the absorption coefficient (where absorption is associated with inter-band transitions) takes the form [13]:

(α hν ) = B hν − EgOpt. 

n

(3)

Where B is an energy-independent constant (band-edge steepness parameter in Tauc’s equation), E gOpt . is the optical band gap (n= 1/2, 3/2, 2 and 3) for direct allowed, direct forbidden, indirect allowed, and indirect forbidden transitions, respectively. The value of E gOpt . for films determine by extrapolation of the linear part of this relationship and the interception from the abscissa (at α =0, E gOpt . = hν), the variation of the values of Eg for indirect as a function of the (Sn Sb) content as shown in Table 1. Table 1. Variation of optical constants with (Sn Sb) content in Se100-x (Sn Sb)x thin films i

Composition

Eο

Ed

E g (eV)

Ee (m eV)

n

ε∞

N/m* (m-3kg-1)

Se

6.92

4.22

2.73

12.17

1.26

2.92

1.22×1050

Se98(Sb Sn)2

6.41

4.32

2.68

12.79

1.41

3.70

1.40×1050

Se96(Sb Sn)4

6.40

5.81

2.35

16.13

1.50

3.85

1.52×1050

Se92(Sb Sn)8

6.12

6.31

2.25

17.31

1.51

4.05

1.57×1050

Se84(Sb Sn)16

6.37

6.53

2.13

21.14

1.52

4.20

1.74×1050

Se80(Sb Sn)20

5.90

8.07

1.99

28.22

1.62

5.05

2.36×1050

i

Table 1. The values of E g decrease from 2.73 to 1.99 (eV) in the indirect transition and the values of Ee increase from 12.17 to 28.22 (meV) with increase (Sn Sb) content this variation of Eg as a function of (Sn Sb) content may be interpreted in terms of the change in cohesive energy (CE) (stabilization energy) as a function of (Sn Sb) content. According to Mott and Davis model [14] the width of localized states near the mobility edges depends on the degree of disorder and defects represent in the amorphous structure. Dispersion energy parameter of Se100-x (Sn Sb)x films. Fig. 7. Show the variation of refractive index, n, as a function of the wavelength (λ) the observed refractive index shows expected increase the refractive index by the increase in (Sn Sb) content this behavior can attributed to increase tailing [15]. Fig. 8. Shows the extinction coefficient, k, as a function of wavelength for different compositions of Se100-x(Sn Sb)x films. All curves exhibit the same trend. 2 .4

1 .0

Refractive Index (n)

S e 96 ( S n S b ) 4 S e 92 ( S n S b ) 8

2 .0

S e 92 ( S n S b ) 16 S e 80 ( S n S b ) 20

1 .8

1 .6

1 .4

Extinction Coeffecient (k)

Se S e 98 ( S n S b ) 2

2 .2

Se S e98 (S n S b )2 S e 9 6 (S n S b ) 4

0 .8

S e 9 2 (S n S b ) 8 S e 9 2 (S n S b ) 1 6 S e 8 0 (S n S b ) 2 0

0 .6

0 .4

0 .2

1 .2

1 .0

240

260

280

300

320

340

λ (n m )

Fig.7. Refractive index n(λ) for Se100-x(Sn Sb)x thin films .

0 .0 300

350

400

450

500

550

600

650

λ (n m )

Fig.8. Extinction coefficient (λ) nm for Se100-x(Sn Sb)x thin films

They decrease with increasing the wavelength to reach a constant value, this decrease can attribute to decrease in absorbance of the film.

Advanced Materials Research Vol. 227

47

The refractive index dispersion of the Se100-x(Sn Sb)x films can be fitted according to the Wemple and DiDominico to drive the dispersion parameters (single oscillator energy Eo and dispersion energy Ed). The dispersion plays an important role in the research for optical materials, because it is a significant factor in optical communication and in designing devices for spectral dispersion. The relation between the refractive index, n, and the single oscillator strength below the band gap is given by the expression [12, 13]

n 2 = 1+

E0 Ed

(4)

E 02 − E 2

This model describes the dielectric response for transitions below the optical gap. To an approximation, the oscillator energy Eo scales with the optical energy Eg, Eo = 1.97Eg which can be expressed as Eo ≈ 2Eg , also that it's clear that both Eg and Eo decrease with increase (Sn Sb) content. This can be attributed to increase tailing, Since Ed changes are assumed to be due to the change of nearest neighbor atoms configuration, Determination of dielectric optical constants for Se100-x (Sn Sb)x films. The obtained data of the refractive index, n, can be analyzed to obtain the high frequency dielectric constant. The procedure describes the contribution of the free carriers and the lattice vibration modes of the dispersion. To obtain a reliable value for the high frequency dielectric constant ε∞. The following equation can be used to obtain the high frequency dielectric constant :

where

ε r = ε ∞ − Bλ2

(5)

e2 N 4π 2 c 2 ε o m ∗

(6)

B=

where ε r is the real part of dielectric constant, ε∞ is the lattice dielectric constant or (the high frequency dielectric constant), λ is the wavelength, N is the free charge carrier concentration, εo is the permittivity of free space (8.854 × 10-12 F/m), m* is the effective mass of the charge carrier and c is the velocity of light. The real part of dielectric constants ε r = n2 was calculated at different values of λ. Then, the obtained values of ε r are plotted as a function of λ2 as shown in Fig.9. It is observed that the dependence of ε r on λ2 is linear at longer wavelengths. Extrapolating the linear part of this dependence to zero wavelength gives the value of ε∞ and from the slopes of these lines, values of N/m* for the investigated Se100-x(Sn Sb)x films were calculated according to Eq. (5) of the constant B. The obtained values of ε∞ and N/m* are given in Table 1. As shown in this table the ε∞ and the N/m* ration increase this behavior would be expected due to increasing the (Sb Sn) content due to the both Sb and Sn more electropositive than the amorphous Se . 5 .0 Se Se 9 8 (S b Sn ) 2

4 .5

Se 9 6 (S b S n) 4

4 .0

n

2

Se 9 2 (S b S n) 8 3 .5

Se 8 4 (S b S n) 1 6 Se 8 0 (S b S n) 2 0

3 .0 2 .5 2 .0 1 .5 1 .0 8x10

4

9x10

4

21x10

5

λ (n m )

2

1x1 0

5

1x10

5

Fig. 9. Plot of n2 versus λ2 (nm)2 for Se100-x (Sn Sb)x thin films.

48

Laser and Plasma Applications in Materials Science

Summary The optical properties of Se100-x(Sn Sb)x thin films were investigated, it was found that, the optical i band gap exhibits indirect allowed transitions. The optical band gap E g was found decrease with the increase (Sn Sb) contents, this is due to the increase of the localized state density within the band gap, this trends was discussed in terms of the chemical bond approach. The values of Ed, E0, ε∞ and N/m* were Determined. It was found that Eo decrease with increase (Sn Sb) content while both the Ed and ε∞ increases. References [1] K. Tanaka: Phys. Rev. B Vol. 39 (1989), p. 1270 [2] S.K. Hsiung and R. Wang, Chin.: J. Phys. Vol. 15(3) (1977). [3] T. Rajagopalan, and G.B. Reddy: J. Mater. Sci. Mater. Electron. Vol. 9 (1998), p. 133. [4] A.M. Andriech, V.V. Ponimar, V.L. Smirnov and A.V. Mironos: Sov. J.Quantum Electron. Vol.16 (1986), p. 721. [5] J. Cofmenero and J.M. Barandiaran: J. Non-Cryst. Solids Vol.30 (1979), p. 263. [6] J.A. Savage: J. Non-Cryst. Solids Vol.47 (1982) p. 101. [7] W.A. King, A.G. Clare and W.C. Lacourse: J. Non-Cryst. Solids Vol.181 (1995), p. 231. [8] R.K. Pan, H.Z. Tao, H.C. Zang, X.J. Zhao and T.J. Zhang: Journal of Alloys and Compounds Vol.484 (2009), p. 645. [9] P. Paramanik, R.N. Bhattacharya and A. Mondal: J. Electrochem.Soc. Vol.127 (1980), p.1857. [10] B. Jozef, O. Stanford, S. Mahadevan, A. Gridhar and A.K. Singh: J. Non-Cryst. Solids Vol. 74 (1985), p. 75. [11] A.S. Abd-Rabo and K.A. Sharaf: International Journal of Pure and Applied Physics Vol. 3 (2007), p. 49 [12] F.Urbach: Phys. Rev. Vol.92 (1953), p. 1324. [13] P. Sharma, M. Vashistha and I. P. Jain: Chalcogenide Letters Vol. 2 (2005), p. 115 [14] N.F.Mott and E.A.Davis, Electronic processes in Non-crystalline Materials, Clarendon, Oxford, (1971) [15] K.A. Aly, M.A. Osman, A.M. Abousehly and A. A. Othman: Journal of Physics and Chemistry of Solids Vol. 69 (2008), p. 2514.

Advanced Materials Research Vol. 227 (2011) pp 49-52 © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.227.49


    
   
 
 

 





 




 
 


 



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This article describes the three dimensional electromagnetic modeling of a microwave (2.45 GHz) plasma device, based on an axial injection torch (AIT). The model solves Maxwell's equations by the finite integration technique (FIT). We are interested in obtaining the optimal position of the short circuit in order to obtain the best percentage of coupling at 2.45 GHz. and understanding the influence of various geometrical parameters on the physical distribution of the electromagnetic field and on the power transfer within the structure.

  
Industrial development can take advantage on the inherent advantages of plasma: in particular, they have no negative effects on the environment. They also have other specific advantages, compared to the flames: very high temperatures and energy densities, and very fast kinetics. Microwave plasmas generated at atmospheric pressure have potential applications for environmental control, which is a current issue of great importance [1, 2]. Among their applications, plasma torches are used for materials deposition and surface treatment, plasma assisted combustion, heating, etc. The objective of this work is to model a three-dimensional plasma torch operating at 2.45 GHz at atmospheric pressure. The torch with axial gas injection, it consists of a coaxial structure perpendicular to rectangular waveguide and some elements to ensure optimal power transfer to the plasma [3]. The TIA can, depending on the geometry of nozzle, waveguide. We use the CST microwave studio commercial code which solves Maxwell equations using the finite integration technique (FIT), to understand the electromagnetic aspects involved in the operation of the plasma torch. We intend to optimize the power coupled to the plasma and to obtain the distribution of the electric field in the torch. The choice of appropriate boundary conditions, the problems of meshing, and the use of adapted numerical techniques, are of great importance. A few works dedicated to the modeling of AITs can be found in literature, in particular those of Alves et al. [4]. They consider an input power at a given frequency and couple 2D electromagnetic and hydrodynamic modules to investigate the effect of different assumed plasma profiles on the electric field and absorbed power distributions. Our approach is different, as we focus on the 3D electromagnetic modeling on a frequency range to optimize the system dimensions in order to obtain a maximum of microwave power coupling at the generator frequency (2.45 GHz).   


 
 
The description of the electromagnetic fields in the AIT device, with and without plasma is based upon two of Maxwell’s equations [4, 5]:

106

Laser and Plasma Applications in Materials Science




∂B
∫ E .d l = − ∫S ∂t .d A, C


∂D

∫ H .d l = ∫S ( ∂t + J ).d A. C


(1)










Where: E
is the electric field, B
the magnetic flux density, H
the magnetic field, J the current
density and D the dielectric displacement. C = ∂S : is the boundary of the surface S, dA: is the area of a differential square on the closed surface A with an outward facing surface normal defining its direction, l: is the edge of the open surface A In order to solve these equations, we need the constitutive relations. These relations describe the macroscopic properties of the media. The considered computational configuration contains air and plasma (argon) at low pressure. These media are assumed to be isotropic, linear, time-invariant, non-permeable ( µ r = 1 ), instantaneously reacting and locally responding. The corresponding





constitutive relations are: B = µ H , D = ε 0 ε r E and J = σE , where ε 0 = µ 0− 1 c 0− 2 is the permittivity in vacuum, εr: the relative permittivity of the media, µ 0 the permeability in vacuum, σ the electric conductivity of the media and co the speed of light in vacuum. The electromagnetic formulation is based on a quasi-stationary analysis, which assumes that all field quantities depend harmonically on time with a common real angular frequency: ω = 2π f (f is the corresponding frequency). Note that in harmonic case, we can replace ∂ / ∂ t by j ω ). In the plasma: σ = σ e , where, σ e is the electronic conductivity of the plasma, given by: σe =

nee 2 1 m e jω + ν

(2)

Using a dielectric description of the plasma, and assuming an isotropic electron energy distribution function, the permittivity of the plasma can be written εr=εp, with: ε p =1−

ω 2p

1

ω 2 1 − jν / ω

(3)

Where, ω p = n e e 2 / m e ε 0 is the electron plasma frequency (ne is the electron density, ν is the electron-neutral collision frequency, e is the electron charge and me is the electronic mass) CST commercial software solves the Maxwell equations in three dimensions. All the media have to be drawn and are supposed homogeneous with a given permittivity. The strength of CST is to be able to take into account a plasma medium, which has a complex permittivity with both real and imaginary parts which may be negative at the same time. The plasma permittivity requires two parameters which are respectively the plasma pulsation ω P (and thus the electron density ne) and the electron neutral collision frequency ν. In this simulation, the plasma has been taken as homogeneous with an electron density of 1015 cm-3 and an electron-neutral collision frequency of 1010 s-1. The excitation signal is a Gaussian shaped pulse defined in order to get a frequency response of the system in a chosen range. A structure study requires two calculations. The first enables to determine the resonance frequencies of the system, and then it is possible to monitor some frequencies in order to calculate the electric field distributions for these frequencies. Calculation times for the studied structure depend on the mesh, but are typically less than one hour.  
  
The main interest of this type of microwave discharge is the absence of inner electrode, which might be a source of pollution. The microwaves are guided along the device and transmit their energy to

Advanced Materials Research Vol. 227

107

electrons in the plasma gas. The microwave plasma torch developed at LPGP (Laboratoire de Physique des Gaz et des Plasmas, in France) includes a 2.45 GHz microwave generator (magnetron and circulator to protect the magnetron from reflected power), a classical rectangular waveguide line with a movable short circuit at its end to perform the optimization of the power coupling (which is also one of the goal of this work), a vertical gas injection system, which is also a part of the coaxial vertical cylindrical coaxial line, ending at the top with a nozzle, where the plasma is created. Contrarily to some other microwave systems, there is no need to any ignition system, as soon as the optimization of the movable short circuit is good. The AIT produces a vertical cylindrical plasma with dimensions of few mm in diameter and several cm in length. It can be generated in ambient air or in controlled atmospheres (inert gas, nitrogen, or mixtures) [1]. Figure 1 shows the computational domain considered for the electromagnetic modeling of our atmospheric pressure AIT. Note that the excitation is performed through the excitation port (in red on figure), where the classical TE mode obtained in a rectangular waveguide is applied. Coaxial guide

Waveguide

Excitation port

Movable short-circuit











 A vertical section of the axial injection torch (AIT)  


 



(a)

(b)


 Electromagnetic wave (magnetic component) propagation in the AIT (a) without (b) with the coaxial line The vertical coaxial line plays a very important role in the structure of the AIT. Figure 2a shows the electromagnetic wave propagation at 2.45 GHz for the AIT configuration without the gas injection tube (and thus without coaxial vertical guide). In this case, the wave cannot propagate in the vertical cylindrical guide at this frequency and we only have stationary waves in the rectangular waveguide. As soon as the gas injection tube is added (with adequate diameter), we can see on figure 2b that when the short circuit located at the end of the waveguide is correctly placed, the incident wave totally goes up through the vertical coaxial line. The inner metallic cylinder presents the double advantage to enable the gas injection up to the nozzle and to create the coaxial vertical guide which allows the wave to propagate from the rectangular waveguide to the conic nozzle. As a consequence of combined high electric field and gas injection at the nozzle exit, the plasma can be ignited and sustained.

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Figure 3 shows the coupled delivered power (1 Watt corresponds to 100% of coupling) as a function of the frequency. The calculation has been performed in the 2 to 3 GHz range for an arbitrary short circuit position horizontally located at 200 mm from the coaxial line vertical axis. We can observe three zones of good coupling: one near 2.2 GHz, one near 2.5 GHz and the last near 2.9 GHz. The zone which has to be optimized with reasonable displacement of the short circuit is the one located around the generator frequency, i.e. 2.45 GHz. Moreover, contrarily to the zone near 2.2 GHz, it presents the advantage to consist in a large peak, and is thus less sensible to small variations of the conditions.

 Delivered power [1W corresponds to 100% of coupling] as a function of the excitation frequency, x (sc) =200mm. Figure 4 represents the variation of the power coupling coefficient at 2.45 GHz (C%) expressed in % as a function of the position of the short circuit (distance to the vertical coaxial line axis). As the calculation is performed for 1 Watt sent by the generator, C% is directly obtained by multiplying the calculated delivered power by 100. The geometrical parameters required to optimize the structure are not unique: the optimized short circuit positions are periodically separated by half the rectangular guide wavelength (λg/2 is around 90 mm at 2.45 GHz). From this point, we adopted the optimal short-circuit position at 90 mm. Note that the fact that the optimal short circuit position near 180 mm seems to have a less C% than the one at 90 mm is only due to our chosen step of 10 mm for x(sc). It is remarkable to note that comparisons with experimental results obtained to perform the best microwave power coupling give a very good agreement with the calculated short circuit valu

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Optimum (x (cc)=90mm, C%=79.29%) +-

109

Optimum (x (sc)=180mm, C%=71.66 %)

C8

C-

B8

B-

88

87-

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+-

--

-

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! Power coupling coefficient [%] according to the position of the short-circuit x (sc) Figure 5 has been obtained for the optimized short circuit position at x(sc)=90 mm. Figure 5a represents the electric field vectors distribution at 2.45 GHz and Figure 5b is a vertical cutting plane (x=0) representation of the electric field absolute value, in order to better appreciate the plasma influence. For this optimal geometric configuration, and in the presence of the plasma, the electric field is maximum around the plasma shape, but does not penetrate in the plasma core, due to high electron density. The fact to take into account a realistic radial density profile would not change a lot this result, as the radial density decrease is mainly located near the radius edge of the plasma.





" Electrical field vectors distribution (x=0)


" 2D electrical field distribution (for x (sc)=90mm, C%=79.29%)

Figure 6 represents the power absorbed by the plasma at 2.45 GHz for the optimal structure with x (sc) = 90 mm. We can note that the absorbed power is maximum at the bottom of the plasma, even with our supposed homogeneous electron density in the whole plasma. In reality, where there is a vertical decrease of the electron density, this result would be emphasized.

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Laser and Plasma Applications in Materials Science


# Power dissipation distribution in plasma for optimal AIT $
Electromagnetic modeling enables to understand the basic operating of an axial injection torch, and to provide general guidelines for device optimization. Even with some strong hypothesis such as an imposed plasma profile (with given radial and axial dimensions, and assumed homogeneity), the calculation has given geometric dimensions which turned out to be correct to optimize an AIT in working conditions. Of course, a complete modeling of the torch would include both the electromagnetic, the hydrodynamic and the plasma modeling aspects. Such works are still in progress. 
[1] C. Tendero, C. Tixier, P. Tristant, J. Desmaison and P. Leprince: A review, Spectrochimica Acta - Part B Atomic Spectroscopy Vol. 61 (2006) p. 2. [2] S. S. Asad, C. Tendero, C. Dublanche-Tixier, P. Tristant, C. Boisse-Laporte, O. Leroy and P. Leprince: Surf. Coat. Technol. Vol. 203 (2009), p. 1790. [3] M. Moisan, G. Sauve, Z. Zakrzewski and J. Hubert: Plasma Sources, Sci. and Technol. Vol. 3 (1994), p. 584 [4] L. L. Alves, R. Alvarez, L. Marques, S. J. Rubio, A.Rodero and M. C. Quintero: Eur. Phys. J. Appl. Phys. Vol. 46 (2009), p. 21001 [5] N. Ikhlef, M. R. Mékidèche, O. Leroy and A. Kimouche: COMPEL - The International
Journal for Computation and Mathematics in Electrical and Electronic Engineering Vol. 27 (2008), p. 1069

Advanced Materials Research Vol. 227 (2011) pp 111-115 © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.227.111


     
           


   

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Plasma spray deposition is one of the most important technologies available for producing the highperformance surfaces required by modern industry. In this process, powder of the coating material is fed into high-temperature plasma, which melts and accelerates the powder; the molten particles subsequently hit and solidify on the surface to be coated. To obtain good quality coating, the powder particle must be at least partially molten and hit the substrate with a high velocity. The flattening characteristics of the droplets impinging on a substrate are important determinants in governing the eventual quality of the plasma spray coating. Different codes have been developed in recent years to simulate the overall thermal spraying process, as well as the growth of the 3D coatings, in which entrained particles are modeled by stochastic particle models, fully coupled to the plasma flow. The present investigation was carried out to have an approach to systematize the atmospheric plasma spraying process in order to create a basis for numerically modeling the plasma dynamics, the coating formation mechanisms and to predict the particle thermo- kinetic state at impact.
 
Physical properties of thermal spray coatings, such as porosity, are sensitive to a large number of process parameters (e.g.: droplet size distribution, velocity, temperature and degree of solidification; substrate material and temperature) which are optimized by trial and error [1]. Better control of the process requires a fundamental understanding of the fluid flow and heat transfer that occurs during the impact, spreading, and solidification of molten droplets. Different techniques have been proposed up till now. For example, Harlow and Shannon [2] firstly used the Marker and Cell (MAC) method to simulate the droplet flattening on a flat substrate. The Arbitrary Lagrangian Eulerian approach (ALE) and the volume of fluid method are also the current methods applied widely in the field of fluid simulation [3-6]. In general, the finite element method and finite difference method are both widely used internationally with some commercial codes, for example, Flow3D, RIPPLE, and ANSYS. The volume tracking and surface tracking methods have been widely applied to solve the key problem of tracking the free surface. Compared with dynamic behaviour of a single droplet impact, interactions between multiple droplets are more complicated. Pasandideh-Fard [7] modelled the sequential impact of two molten droplets on a solid surface with the help of the numerical codes of RIPPLE. Ghafouri-Azar [8] described a joint experimental and numerical study to predict the shapes of splats formed by twodroplet interactions with a low impacting velocity of the droplets. They give information about the solidification/melting behaviour of droplets before the coating is totally manufactured. But some numerical difficulties occur while simulating impact of several droplets: fluid flow, heat transfer and interactions between particles need a large number of computational nodes to give accurate results [9-11].

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Laser and Plasma Applications in Materials Science

The aim of the present study is to propose a simple numerical model of successive depositions of several melted droplets onto a solid substrate to simulate the evolution of cooling and solidification by a commercial code ANSYS. It is useful to predict the time of process and the film solidification and to help to choose the optimum process parameters.
 

Fluid flow in an impacting droplet was modelled using a finite element solution of the NavierStokes equations in a 2D Cartesian coordinates assuming laminar, incompressible flow. The surface profile of the deforming droplet was defined using the ‘‘fractional volume of fluid’’ scheme. Details of the fluid flow model are given by Bussmann et al. [12].
 

 
In this study, the numerical simulation of a fully molten alumina droplet flattening was conducted. The initial conditions of spray material used in the numerical computation are listed in table 1:










 


Impinging droplet

Alumina

Impinging velocity

100-200 m/s

Droplet diameter

50 µm

initial temperature of droplets

2500°k

Substrate initial temperature

300°k



Initial conditions








Evolution of temperature fields during
nteraction between two alumina droplets impacting substrate
.


Fig.1 shows the 2D images of the flattening process of a 50 µm diameter molten two alumina droplets on a solid substrate. The first droplet with a velocity of 200 m/s falls onto the substrate. Due to the sudden deceleration, the droplet flattens and the high-pressure inside forces the melted droplet to spread out laterally into a disk-like shape, and the splat height decreases with the increase of spreading time. It is obvious that the liquid splat is symmetric around the droplet axis. When the second droplet with a velocity of 100m/s was introduced, it spread over the solidified splat. The temperature of the solidified splat increased where it contacted the impacting droplet. The solidification front moved inside the droplet at the surface of the solidified splat. Droplet splashing stopped after the second impact because of solidification and lack of kinetic energy.

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113



Evolution of temperature fields during
nteraction between molten droplets in vertical direction

Fig.2 shows the interaction between the two alumina droplets with a diameter of 50 µm in the vertical direction. The upper droplet with the velocity of 100 m/s, impacts on the lower one. It can be seen that the upper droplet lands on the flattening lower one, and then they join together and spread out radically into a disk-like shape. The final shape is quite alike with the single droplet impact. Fig.3 shows a similar set of simulations with almost identical conditions to Fig.2. The difference is that the upper droplet impacts after an interval time of dt. It can be seen that the change in the impact position affects the splat morphology in the dynamics of droplet spreading. The final shape predicted is more elongated than that of Fig.2.


Evolution of temperature fields during
nteraction two Alumina droplets with a time interval of dt.

Fig. 4 describes also the temperature field evolution during the impact of several molten Alumina droplets with velocity of 200m/s. The upper droplets impact after time interval of dt with the same velocity in order to form the second layer. In the substrate, the top temperature increases rapidly in contact with the first layer. Concerning the first layer, the bottom temperature decreases rapidly from the initial value to the phase change temperature. Then the value remains constant during solidification and decreases until another layer is deposited and the first layer temperature increases. The solidification is not stopped but delayed. Remelting occurs as a new layer brings sufficient heat to the previous one. When the second layer is deposited, the top temperature of the first layer exhibits a sudden and important increase, whereas the bottom temperature rises slowly. The increase of heat originates from the change of behaviour of the solidification evolution.


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Laser and Plasma Applications in Materials Science








 
Evolution of temperature fields during
nteraction between several molten droplets



!  
Existing models have successfully predicted many features of droplet impact and solidification during plasma-spray processing. The 2-dimensional models solve the governing equations for conservation of mass, momentum, and energy. In addition, the free surface of a drop is tracked via a volume-of-fluid (VOF) approach. The model has shown that: 1) The model, in general, is applicable to transient fluid flows and heat transfer including two moving boundaries: a liquid-gas free surface and a liquid-solid interface in case of two molten droplets. 2) When the droplets had different center of impacts. The second droplet jetted out as it spread over the irregularities of the solidified splat; as a result, liquid breakup and splashing occurred. When the droplet jetted out at the rim of the solidified splat, void spaces were made between the droplet and substrate. These spaces remained void until the end of the process. This may explain one cause of porosity formation in thermal spray coating. 3) Simulations during the deposition of several layers shown that the first layer is less and less influenced by the incoming of the others. Whereas the deposition of the second layer alters greatly the solidification and temperature evolutions of the first layer, the following layers less and less affect the first layer. Considering a long enough time, the first layer temperature evolution becomes independent of addition of new layers. 4) Thermal contact resistance is an important factor in determining splat shape. To date, this has been treated as an empirical parameter. A model for determining thermal contact resistance is needed. " 
[1] M. Vardelle, A.Vardelle#
A.C. Leger, P. Fauchais and D. Gobin: J. of Thermal Spray Vol. 4 (1995), p. 50

Tech.


[2] F. H. Harlow and J. P. Shannon: J. Appl. Phys. Vol. 38 (1967), p. 3855. [3] G. Trapaga and J. Szekely: Metal Trans B Vol. 22 (1991), p. 901. [4] H. Liu, E. J. Lavernia and R. Rangel: J. Phys. D. Appl. Phys. Vol. 26 (1993), p.1900. [5] C. W. Hirt and B. D. Nichols: J Compute. Phys. Vol. 39 (1981), p. 201. [6] Z. Zhao, D. Poulikakos and J. Fukai
International Journal of Heat and Mass Transfer Vol. 39 (1996), p. 2771. [7] M. Pasandideh-Fard, J. Mostaghimi and S.Chandra Modelling sequential impact of two molten droplets on a solid surface[C]//Proceeding of 12th Annual Conference on Liquid Atomization and Spray Systems. Indianapolis, Indiana, 1999: p. 265. [8] R.Ghafouri-Azar, S. Shakeri, S. Chandre and J. Mostaghimi: International Journal of Heat and Mass Transfer: Vol. 46 (2003), p.1395.

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[9] L. Pawlowski, M. Vardelle and P. Fauchais: Thin Solid Films Vol. 94 (1982) p.307. [10] L. Pawlowski: Thin Solid Films Vol. 81 (1981), p.79. [11] Q. Fan, L. Wang, F. Wang and Q. Wang: Surface & Coatings Technology Vol. 201 (2007), p. 6977. [12$ M. Bussmann, J. Mostaghimi and S. Chandra, 
 , in press.

Advanced Materials Research Vol. 227 (2011) pp 116-120 © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.227.116


     
   
 
   

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Abstract. The purpose of the present work consists in the elaboration of numerical computing program allowing the simulation of various species behaviour present in plasma reactor during films formation by cold plasma in DC glow discharge. After application of some simplifying hypotheses, we have developed a simulation model based on a fluid approximation. The elaborated model was based on finite differences method solved by MATLAB software. We applied the model to simulate the plasma in the case of atomic gases (Ar) and molecular (CH4) gases. The simulation results are given in terms of spatial distribution of charge densities, electric potential and electric field between electrodes space. The effect of some discharge parameters such as the pressure, and the density of the gas was also investigated. Introduction Plasma Enhanced Chemical Vapour Deposition (PECVD) is nowadays a key sector of the industrial production of silicon-based films, which is an important material in modern microelectronics devices, notably transistors, computer chips and solar cells. In plasma processing, precursor gases are dissociated near room temperature by high energy electrons to produce reactive radicals and other intermediates. The reactive species interact with the semiconductor substrate to yield volatile products (etching) [1] or to grow solid films (deposition) [2]. To obtain a better understanding of the discharge mechanisms (physical description and electrical properties), several modelling efforts were performed in the past decade. Many of these models are based on fluid approximation or on PIC-MC simulation [3]. In this paper, we use a fluid approach to describe the electrons and ions transport. The model analysis is presented for parallel plate geometry conducting electrodes placed in a vacuum chamber. We concentrate on an electropositive discharge in presence of atomic (Ar) [4] and molecular (CH4) [5] gases, in order to obtain the one-dimensional space variation of different species densities in the plasma (electrons, positive and negative ions), electric potential and electric field after DC bias voltage application. Physical model The fluid or continuum model is based on the first two moments of the Boltzmann equation, invoking the drift-diffusion approximation. These two moments are the continuity and momentum transfer equations. They are coupled to Poisson’s equation to compute the self-consistent electric field. A simple model containing only electrons, positive and negative ions can then be described by the following set of equations: ∂
 ∂

+ ∇ (Γ 

)=





Γ = 
 µ  − ∇
 ∆ = −

 ρ = − (
 −
 −

) ε0 ε0

(1 ) (2) (3)

 − 1  →  =   + 1 

 


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117

Where ng, Γg, Sg, E, V, Dg, µg, q and ε0 are respectively, the particle densities (g = (e); electrons, (p); positive ions, and (n); negative ions), the charged particle flux density, the source term which depends on the ionization, recombination and attachment constants, the field, the electric potential, the diffusion coefficient, the mobility, the elementary charge and the vacuum permittivity. The simplifying assumptions used for the resolution of the system of equations are:
 The ionization is the only process considered in case of atomic (Ar) plasma following the chemical reaction:  +  →  + + 2
 The process of recombination, attachment and ionization are also taken into account in case of molecular plasma (CH4).
 The charged species present in CH4 plasma are positive ions (CH4 +, CH3 +, CH5 +, C2H5 +), negative ions (H-, CH2-) and electrons. We use in our simulation the average values of their contribution through a density characterized by average values , and and the coefficients of those reactions [5].
 The process of secondary electrons emission is ignored.
 The electronic (Te) and ionic (Ti) temperatures are assumed uniform: in the isothermal approximation; Ti is equal to 500K for Ar and CH4.
 The pressure P of the system is constant and equal to 1 Torr for both gases. Numerical Solution The plasma is governed by a system of nonlinear equations and cannot be solved analytically. We therefore conducted the resolution using finite differences method [6]. We perform first, a 1D mesh which determines the points in which densities of species and the electric potential are calculated. For solving the transport equation, we have used a numerical scheme similar to that described by Scharfetter and Gummel [7]. The flows of ions and electrons are discretized by finite differences method using an exponential scheme [8]. The numerical treatment of this system of equations needs an implicit method to avoid numerical instabilities [9]. The time steps considered in our calculations verify the following condition [10]: ε0 . (4) ∆ <  (
 µ  +
 µ +

µ
) The discretization of the Poisson’s equation by finite differences method leads to the following equation:  ( −1) − 2 ( ) +  ( +1) ρ =− 2 ∆ ε0

.

(5)

We note that for the calculation of these distributions, we have used two different methods for solving Poisson’s equation. The method of matrix inversion, in the case of argon plasma, and the SOR method, in the case of methane plasma. For the transport equation we have used the SOR method for both cases, using the following conditions: •Boundary conditions: (δnp/δx) = 0, ne = nn= 0 at x = 0 and L, V(x = 0) = Va and V(x = L) =Vc. •Initial conditions (t = 0): ne + nn ≈ np = n0 and V= 0. with no being the initial density equal to 2.5x1012 cm-3 for Ar and 2.001x1012cm-3 for CH4. The data needed to carry out the calculations of the discharge are mentioned in literature [4, 5]. Simulation Results for Argon The simulation results of argon glow discharge plasma are given for plane-parallel electrodes separated by L=2cm and a negative DC voltage Vc of -305V.

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Laser and Plasma Applications in Materials Science

Fig.1-a represents the spatial evolution of electron and ion densities for a time increment dt =10-9s. This figure clearly shows three distinct regions which represent cathode sheath, anode sheath and the positive column. In the first two regions the ion density is significantly higher than the electron density. The region of the positive column (plasma area) is characterized by ion and electron density constant and almost equal to the plasma density no. Fig. 1-b shows the space evolution of potential during the discharge. Typically, in the plasma bulk, the potential is nearly constant (VP = 1.0177V) and two sheath regions where the potential is variable and drop near the electrodes. We note also in fig. 1-b that the value of electric field increases rapidly in the cathode and anode sheaths, but remains almost equal to zero in the positive column because of the constant potential in this region.


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Simulation Results for Methane The simulation results for CH4 plasma are illustrated in Fig. 3 for a distance between electrodes of 4cm and a Dc voltage of -100V applied to the cathode. The gas used in our model is an electropositive molecular gas containing electrons, positive ions and a negligible density of negative ions compared to that of electrons. The spatial evolution of charged particles is shown in fig. 3-a and 3-b for nn / ne=10-2 and nn / ne=10-1, respectively. It is clear showed that nP = nn + ne in the positive column in which negative ions are confined in the central part of the discharge and at the electrodes, a zero density for negatively charged particles and a density of 2.0526x1011 cm-3 for positive ions. We note that the higher the density of negative ions nn is negligible compared to the electron density closer you get to equality np ≈ ne in the positive column. This result is similar to that

Advanced Materials Research Vol. 227

119

obtained in the case of monatomic gases (Ar) where nn/ne=0 (the electronegativity of the plasma is zero). Spatial distributions of potential and electric field are illustrated in Fig. 3-c, with a plasma potential Vp of 9.7764 V. These results are similar to those obtained in the case of argon plasma. 6=5 >5 6

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