Le déploiement constant de nouveaux réseaux de capteurs dans des endroits confinés des turbomachines ou difficilement accessibles nécessite l'apport d'énergie pour alimenter ces capteurs. De nombreuses recherches s'orientent donc sur une alimentation à demeure des capteurs afin de monitorer de nouveaux composants. Une voie possible d'alimentation (à demeure) est l'utilisation de modules thermoélectriques afin de convertir l'énergie thermique issue de la combustion dans les turbines. Parmi les matériaux thermoélectriques utilisables dans ce genre d'applications (au-delà de 700 °C), les siliciures semblent constituer la famille la plus prometteuse. Le sujet de cette thèse est l'étude de siliciures connus pour leurs résistances à l'environnement en température afin de réaliser des modules thermoélectriques pouvant fonctionner à des températures de l'ordre de 700 °C. À cette fin, différents représentants de la famille des siliciures ont été étudiés par calcul ab initio à l'aide de la théorie de la fonctionnelle de la densité. Cet outil a permis de calculer leurs propriétés thermoélectriques potentielles et a montré que les deux meilleurs candidats à des applications bon marché étaient les disiliciures de manganèse et de fer. Le calcul a également permis de montrer le caractère métallique de nombreux siliciures ternaires. Les siliciures prometteurs ont été élaborés par frittage et leur vieillissement ainsi que les coefficients de dilatation thermique ont été étudiés. Ces connaissances ont permis de mettre au point des modules thermoélectriques à base de siliciures résistants à 900 °C sous air. Bien que possédant des propriétés thermoélectriques modestes, leur résistance à l'oxydation à haute température permet d'envisager la fabrication de modules thermoélectriques / The constant deployment of new sensors networks in confined areas of turbomachines or difficult to access, requires the input of energy to power these sensors. Many researches are thus focused on a permanent power supply of sensors to monitor new components. One possible way of permanent power supply is the use of thermoelectric modules to convert the thermal energy from combustion into the turbines. Among the thermoelectric materials, silicides seem to be the most promising family for high temperature applications (above 700 °C). The subject of this thesis is the study of silicides known for their high temperature oxidation resistance in order to produce thermoelectric modules that can operate at optimal temperatures around 700 °C. To this end, different representatives of the silicide family have been studied by ab initio calculation using the density functional theory. This tool allowed to calculate their potential thermoelectric properties and showed that the two best candidates for cheap applications were the disilicides of manganese and iron. The calculation also showed the metallic character of many ternary silicides. The promising silicides were developed by sintering method and their aging as well as their coefficients of thermal expansion were studied. This knowledge has made it possible to develop thermoelectric modules based on silicides resistant to 900 °C in air. Although possessing modest thermoelectric properties, their resistance to high temperature oxidation makes it possible to envisage the manufacture of thermoelectric modules

Atualmente, existe uma grande demanda por materiais caracterizados por um balanço adequado de propriedades, para aplicações estruturais em altas temperaturas. Superligas de última geração a base de níquel podem ser usadas sob carregamento em temperaturas próximas a 1150 ºC, mas informações indicam que materiais constituídos de microestruturas multifásicas apresentam maior potencial para estas aplicações, dentre os quais, os que contêm fase(s) intermetálica(s) em equilíbrio com um metal ou liga refratária. Na última década foram avaliados pelo Grupo Diagrama de Fases e Termodinâmica Computacional da Escola de Engenharia de Lorena os sistemas ternários metal refratário (molibdênio, nióbio, tântalo, vanádio, titânio, zircônio)-silício-boro, com o objetivo de serem determinadas as relações de fases em altas temperaturas e de ser desenvolvido um banco de dados termodinâmicos. Com o estudo experimental do sistema háfnio-silício- boro na seção isotérmica a 1600 ºC e sua Projeção Liquidus, na região rica em Háfnio, completa-se este ciclo de trabalhos. Saliente-se que o estudo deste sistema ternário exigiu a revisão dos sistemas binários háfnio-silício e háfnio-boro, através de sua avaliação experimental. Foram utilizadas no trabalho matérias-primas de elevada pureza (háfnio - mínimo de 99,8%, silício - mínimo de 99,998% e boro - mínimo de 99,5%). A metodologia experimental envolveu basicamente as seguintes etapas: (i) produção das ligas em forno a arco; (ii) tratamento térmico das ligas na temperatura de 1600 ºC; (iii) caracterização por difração de raios-X, microscopia eletrônica de varredura das ligas no estado bruto de fusão e tratadas termicamente e espectroscopia de energia dispersiva. Como resultado do estudo observou-se: (i) no sistema binário háfnio-silício a reação eutética L _HfSS + Hf2Si, na região rica em háfnio, as reações peritéticas L + Hf5Si3 Hf2Si, L + Hf3Si2 Hf5Si3, L + Hf3Si2 Hf5Si4, L + Hf5Si4 HfSi, L + HfSi HfSi2, a transformação congruente L Hf3Si2, e a reação eutética L SiSS + HfSi2, na região rica em silício, a estabilidade das fases intermediárias Hf2Si, Hf5Si3, Hf3Si2, Hf5Si4 a 1600 ºC, e a estabilidade de HfSi e HfSi2 a 1200 ºC, o que sugere alterações significativas em relação ao diagrama de fases atualmente aceito pela literatura; (ii) no sistema binário háfnio-boro a reação eutética L _HfSS + HfB, na região rica em háfnio, a reação peritética L + HfB2 HfB, a transformação congruente L HfB2 e a reação eutética L B-RhomSS + HfB2, na região rica em boro, e a estabilidade das fases intermediárias HfB e HfB2 a 1600 ºC, o que está de acordo com o diagrama de fases atualmente aceito pela literatura; (iii) no sistema ternário háfnio-silício-boro na região rica em háfnio, na projeção Liquidus, verificou-se as reações L _HfSS + Hf2Si + HfB, L HfB + Hf2Si, L HfB2 + Hf2Si, L HfB2 + Hf5Si3, L HfB2 + Hf3Si2, L HfB2 + Hf5Si4, L HfB2 + HfSi, L _HfSS + Hf2Si, L _HfSS + HfB, L + Hf5Si3 Hf2Si, L + Hf3Si2 Hf5Si3, L + Hf3Si2 Hf5Si4, L + Hf5Si4 HfSi e L + HfB2 HfB e na seção isotérmica a 1600 ºC, verificou-se a estabilidade das fases _HfSS, Hf2Si, Hf5Si3, Hf3Si2, Hf5Si4, HfSi, HfSi2, HfB e HfB2, e a existência dos campos trifásicos _HfSS + HfB + Hf2Si, HfB2 + HfB + Hf2Si, HfB2 + Hf2Si + Hf5Si3, HfB2 + Hf5Si3 + Hf3Si2, HfB2 + Hf3Si2 + Hf5Si4, HfB2 + Hf5Si4 + HfSi e HfB2 + HfSi + HfSi2. / Nowadays, there is a big demand for materials for structural applications at high temperatures. These materials must present a good properties balance. The last generation of the nickel-base superalloys can be used at temperatures close to 1150oC. However, information available so far shows that multiphase microstructure materials are potentially better for such application. Among these materials, the Group of Phase Diagrams and Computational thermodynamics in the Escola de Engenharia de Lorena has chosen those systems, which contains intermetallic(s) phase(s) in equilibrium with refractory metal or alloy for evaluation. Recently we have evaluated the phase stability at high temperature in the refractory metal (molybdenum, niobium, tantalum, vanadium, titanium, zirconium)- silicon-boron system, aiming at the development of a thermodynamic data base. The experimental study of the isothermal section at 1600 ºC and the Liquidus projection of the hafnium-silicon-boron system completes this cycle of works. The study of this ternary system demanded the revision of the hafnium-silicon and hafnium-boron binary systems, through its experimental evaluation. Alloys had been produced with blades of hafnium (minimum 99.8%), silicon (minimum 99.998%) and boron (minimum 99.5%), in the voltaic arc furnace under argon atmosphere, and heat treated at 1600 ºC under argon atmosphere. The phases had been identified by X-ray diffraction and contrast in backscattered electron imaging mode and spectroscopy of dispersive energy. The study determined: (i) in the binary system hafnium-boron the eutectic reaction L _HfSS + HfB, in the rich region of hafnium, the peritectic reaction L + HfB2 HfB, the congruent transformation L HfB2 and the eutectic reaction L B-Rhom + HfB2, in the rich region of boron, and the stability of the intermediate phases HfB and HfB2 at 1600 ºC, what is in agreement to the currently accepted diagram; (ii) in the binary system hafnium-silicon the eutectic reaction L _HfSS + Hf2Si, in the rich region of hafnium, the peritectic reactions L + Hf5Si3 Hf2Si, L + Hf3Si2 Hf5Si3, L + Hf3Si2 Hf5Si4, L + Hf5Si4 HfSi and L + HfSi HfSi2, the congruent transformation L Hf3Si2, and the eutectic reaction L SiSS + HfSi2, in the rich region of silicon, the stability of the intermediate phases Hf2Si, Hf5Si3, Hf3Si2, Hf5Si4 at 1600 ºC, and the stability of HfSi and HfSi2 at 1200 ºC, what suggests significant alterations in relation to the currently accepted diagram; (iii) in the ternary system hafnium-silicon-boron, in the rich region in hafnium, in the Liquidus projection, the reactions L _HfSS + Hf2Si + HfB, L HfB + Hf2Si, L HfB2 + Hf2Si, L HfB2 + Hf5Si3, L HfB2 + Hf3Si2, L HfB2 + Hf5Si4, L HfB2 + HfSi, L _HfSS + Hf2Si, L _HfSS + HfB, L + Hf5Si3 Hf2Si, L + Hf3Si2 Hf5Si3, L + Hf3Si2 Hf5Si4, L + Hf5Si4 HfSi e L + HfB2 HfB and in the isothermal section at 1600 ºC, the stability of the phases _HfSS, Hf2Si, Hf5Si3, Hf3Si2, Hf5Si4, HfSi, HfSi2, HfB and HfB2, and the threephase fields _HfSS + HfB + Hf2Si, HfB2 + HfB + Hf2Si, HfB2 + Hf2Si + Hf5Si3, HfB2 + Hf5Si3 + Hf3Si2, HfB2 + Hf3Si2 + Hf5Si4, HfB2 + Hf5Si4 + HfSi and HfB2 + HfSi + HfSi2.

by Li Chi Pui. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1999. / Includes bibliographical references (leaves [105]-[109]). / Abstracts in English and Chinese. / Abstract / Acknowledgement --- p.Page no / Chapter Chapter 1. --- Introduction / Chapter 1.1 --- Metal silicides --- p.1 / Chapter 1.2 --- Cobalt silicides --- p.3 / Chapter 1.3 --- Ion beam synthesis of metal silicides by metal implantation into silicon --- p.4 / Chapter 1.4 --- Feature of MEVVA implantation --- p.5 / Chapter 1.5 --- Motivation and organisation of this thesis --- p.6 / Chapter Chapter 2. --- Sample Preparation and Characterisation Methods / Chapter 2.1 --- MEVVA implantation --- p.7 / Chapter 2.2 --- Simulation by TRIM --- p.9 / Chapter 2.3 --- Sample preparation --- p.12 / Chapter 2.4 --- Sheet resistivity measurements --- p.14 / Chapter 2.5 --- Rutherford backscattering spectroscopy (RBS) --- p.17 / Chapter 2.6 --- Transmission electron microscopy (TEM) --- p.19 / Chapter 2.6.1 --- Transmission electron microscopy (TEM) sample preparation --- p.21 / Chapter 2.7 --- Atom force microscopy (AFM) and conducting AFM --- p.31 / Chapter Chapter 3. --- Characterisation of As-implanted Samples / Chapter 3.1 --- Experimental details / Chapter 3.1.1 --- Sheet resistance measurements --- p.33 / Chapter 3.1.2 --- Rutherford backscattering spectroscopy (RES) --- p.36 / Chapter 3.1.3 --- Sputtering depth measurements --- p.43 / Chapter 3.1.4 --- Transmission electron microscopy (TEM) --- p.44 / Chapter 3.1.5 --- Spreading resistance profiling (SRP) --- p.61 / Chapter 3.1.6 --- Atom force microscopy (AFM) and conducting AFM --- p.64 / Chapter 3.2 --- Results and discussion --- p.71 / Chapter 3.3 --- Summary --- p.81 / Chapter Chapter 4. --- Characterisation of Annealed Samples / Chapter 4.1 --- Experimental details / Chapter 4.1.1 --- Rutherford backscattering spectroscopy (RBS) --- p.82 / Chapter 4.1.2 --- Transmission electron microscopy (TEM) --- p.87 / Chapter 4.1.3 --- Sheet resistance measurements --- p.98 / Chapter 4.2 --- Summary --- p.101 / Chapter Chapter 5. --- Conclusion --- p.102 / Appendix / Reference

by Peng Qicai. / Thesis (Ph.D.)--Chinese University of Hong Kong, 1997. / includes bibliographical references (p. 187-206). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web.

Thermoelectric (TE) materials, which can convert temperature gradients directly into electricity and vice versa, have received renewed interest for waste heat recovery and refrigeration applications. Higher manganese silicides (HMS) are promising p-type TE materials due to the abundance of the constituent elements, environmental friendliness, and good chemical stability. The objective of this dissertation is to establish a better understanding of the structure-TE properties relationship of HMS with a complex Nowotny chimney ladder structure. The focus of this work is on the investigations of the phonon dispersion of HMS crystals and the effects of chemical doping and nanostructuring on the TE properties of polycrystalline HMS. HMS crystals have been synthesized by the Bridgeman method for inelastic neutron scattering measurements of the phonon dispersion. In conjunction with density functional theory calculations, the results clearly show the presence of numerous low-lying optical phonon branches, especially an unusually low-energy optical phonon polarization associated with the twisting motions of the Si helical ladders in the Mn chimneys. The obtained phonon dispersion can be used to explain the low and anisotropic thermal conductivity of HMS crystals. (Al,Ge) double doping was found to be effective in modifying the electrical properties of HMS polycrystals. The peak thermoelectric power factor occurs at an optimized hole concentration of 1.8~2.2×10²¹ cm⁻³ at room temperature. On the other hand, Re substitution can suppress the lattice thermal conductivity to approach the calculated minimum value corresponding to the amorphous limit. Meanwhile, the thermoelectric power factor does not markedly change at low Re content of x ≤ 0.04 although it drops considerably with increasing Re content. Hence, the peak ZT has been improved to ~0.6 in both systems. The effects of nanostructuring on the TE properties have been studied in the cold-pressed samples and ball-milled samples. The thermal conductivity was reduced remarkably by decreasing the grain size. It is found that the grain size effects are more significant at low temperature. However, it is difficult to reduce the grain size to less than 50 nm without the formation of impurity phases by ball milling. These facts limit the ZT enhancement of the nanostructured HMS at high temperatures in this study. / text

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Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Thesis (Ph. D.)--University of Missouri-Columbia, 1996. / Typescript. Vita. Includes bibliographical references (leaves 138-143). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 1996. / Typescript. Vita. Includes bibliographical references (leaves 138-143). Also available on the Internet.

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