Depuis les années 90, dans le contexte de la pénurie annoncée d'énergies fossiles et de modifications climatiques, un regain d'intérêt s'est fait sentir pour les énergies renouvelables et, parmi elles, pour la thermoélectricité. Cette dernière permettant la conversion directe entre énergies thermique et électrique devrait permettre la récupération de l'énergie perdue sous forme de chaleur, durant les processus industriels. Cependant, à l'heure actuelle, l'efficacité de la conversion thermoélectrique, évaluée à partir du facteur de mérite, ZT, n'est pas suffisante pour des applications à grande échelle. Cependant, pour évaluer l'impact potentiel de la thermoélectricité, il est nécessaire de prendre en compte d'autres critères que celui de l'efficacité relativement faible de la thermoélectricité, qui la cantonne dans des niches spécifiques, et tels que la faible toxicité des matières premières, leur abondance, leur faible coût et leur facilité de mise en œuvre. Diverses familles de matériaux répondent à ces critères parmi lesquelles les plus favorables sont les siliciures pour les matériaux inorganiques et les polymères conducteurs pour les composés organiques. Dans ce travail de thèse, nous nous sommes intéressés au disiliciure de chrome (CrSi2) et au polymère PEDOT:PSS.Parmi les siliciures, CrSi2 est un candidat prometteur mais sa grande conductivité thermique est un handicap par rapport aux matériaux conventionnels. Elle peut cependant être réduite par nanostructuration. Nous présentons une étude CrSi2 nanostructurés (10-15 nm) obtenus soit par four à arc suivie d'un broyage mécanique, soit par mécanosynthèse. Ces poudres sont chimiquement stables jusqu'à 1073 K. A plus hautes températures, des phases secondaires apparaissent, notamment CrSi. Grâce à la technique SPS, CrSi2 a pu être densifié jusqu'à 94% en maintenant une taille nanométrique (30-45 nm) ce qui a conduit à une réduction de la conductivité thermique d'un facteur 2 par rapport au composé CrSi2 massif. Malheureusement, le facteur de mérite ZT n'a pas été amélioré en raison de l'accroissement de la résistivité électrique.Guidés par des calculs DFT, nous avons explorés de nouvelles voies de dopage pour améliorer les propriétés thermoélectriques de CrSi2. Des alliages Cr1-xTixSi2, Cr1-xZrxSi2 et Cr1-xMoxSi2 ont ainsi été synthétisés et nous avons étudié leurs propriétés thermoélectriques. A Tamb, le dopage au Ti semblerait de ne pas significativement améliorier le facteur de puissance (PF) pour une porosité de 11% par rapport à nano-CrSi2. Par contre, pour 2% Zr, PF a été amélioré d'un facteur 1.7 pour une porosité de 30%. En plus, il semblerait que le PF de Cr0.98Zr0.02Si2 avec une porosité de 0% est amélioré d'un factuer 1.9 par rapport au CrSi2 massif. Néanmoins, à hautes températures il existe encore un potentiel d'amélioration pour le dopage au Ti. Bien qu'il soit métastable, l'alliage Cr1-xZrxSi2 a pu être obtenu jusqu'à 5% Zr. Une optimisation supplémentaire est encore nécessaire pour les alliages dopés au Ti et Zr. L'alliage Cr1-xMoxSi2 est le plus prometteur avec une amélioration significative du facteur de Mérite (ZTmax > 0.2) pour 10% et 20% de Mo entre 500 et 700 K. Ces valeurs sont supérieures à celle du massif bien que toujours du même ordre de grandeur que le meilleur ZT obtenu pour CrSi2.Enfin, nous avons élaboré des composites à base de CrSi2 associé à du Polypyrrole (PPy) ou du PEDOT:PSS. Aucune amélioration significative n'a été obtenue pour le composite PPy-CrSi2. Dans le cas du PEDOT:PSS, nous avons testé la fonctionnalisation de CrSi2 pour améliorer sa dispersion dans le polymère. Nous avons montré qu'un agent de fonctionnalisation avec un groupement phosphonique réagissant avec Cr est le plus efficace. Cette étape est très prometteuse car il s'agit de la première étude de ce type effectuée dans le cas de l'élaboration de composites à base de siliciures intermétalliques pour des applications thermoélectriques. / In the context of renewable energies, which is related to the shortage of fossil energies and climate change, thermoelectricity has regained interest in recent years (1990s). The concept of this technology is the direct conversion between thermal and electrical energies. This can contribute to a progress in the industrial sector. However, the efficiency of the thermoelectric materials, denoted ZT (figure of merit), is not sufficient for large scale applications. Nevertheless, thermoelectricity can be found in the niche market sector where other criteria are taken into account such as the abundance of the raw elements, their price and toxicity. Several families meet these criteria with the silicides as inorganics materials and conductive polymers as organics materials being the most favorable. The objective of this dissertation was to investigate the chromium disilicide (CrSi2) and PEDOT:PSS.Among the silicides, CrSi2 stands as a promising candidate for thermoelectric applications. However, its relatively high thermal conductivity compared to the conventional materials is considered as a drawback. Therefore, we were interested in nanostructuration in order to reduce this thermal conductivity. Nanostructured CrSi2 is synthesized by arc melting followed by mechanical milling or mechanical alloying with a grain size of 10-15 nm. These powders have a good chemical stability up to 1073 K. Above this temperature, secondary phases, such as CrSi, are formed. SPS processing permits to efficiently densify our pellets up to 94% while maintaining nanometric grain size (30-45 nm). This leads to a reduction of the thermal conductivity by a factor of 2 compared to bulk CrSi2. However, the Figure of merit (ZT) does not improve as the electrical resistivity increases.DFT calculations predict that some doping elements have potentiality to improve CrSi2 electronic structure. Therefore, Cr1-xTixSi2, Cr1-xZrxSi2 et Cr1-xMoxSi2 alloys are synthesized in order to enhance the thermoelectric properties of CrSi2. At RT, The power factor of Cr0.9Ti0.1Si2 seems not to be improved compared to nano-CrSi2 for a porosity of 11%. On the other hand, 2% Zr doping improved the power factor by 1.7 at RT for a porosity of 30%. It also seems that for 0% porosity, the power factor of Cr0.98Zr0.02Si2 is improved by a factor 1.9 compared to bulk CrSi2. Nevertheless, an improvement could be predicted at high temperatures for Cr0.9Ti0.1Si2. Although Cr1-xZrxSi2 alloy is metastable, we were able to synthesize it by mechanical alloying up to 5%. Further optimization is needed for improving the thermoelectric performances of Ti and Zr- doped CrSi2 . On the other hand, Cr1-xMoxSi2 alloys seem to be the most promising as an increase of the Figure of merit is observed with a ZTmax reaching values larger than 0.2 for Cr0.9Mo0.1Si2 and Cr0.8Mo0.2Si2 in the temperature range of 500-700 K. These values are higher compared to the bulk but still in the same order of magnitude as for the highest ZT value report for CrSi2.We then elaborated CrSi2 based composites with Polypyrrole (PPy) and PEDOT :PSS. No significant enhancement was observed for PPy-CrSi2 composites. In order to improve the dispersion of CrSi2 in the PEDOT :PSS, we have tested the functionnalization. Preliminary tests show that grafting agents with phosphonic acid group is the most efficient by reacting with Cr. This step is very promising as no study till date reported the functionnalization approach for the elaboration of silicides based composites, especially for thermoelectric applications.

Transition Metal (TM) silicides are potential materials for different high temperature applications due to their high melting points and chemical stability at elevated temperatures. In the present work, the possible use of Gr (VIB) disilicides: MoSi2 and CrSi2 for high temperature structural application and thermopower generation respectively are investigated. Literature reports on MoSi2 indicate this material to have excellent mechanical and thermal behaviors at temperatures greater than 1273 K. The major problems limiting its use are the low temperature brittleness and oxidation at intermediate temperatures and form the scope of this work. Also, CrSi2 is reported to be a narrow band gap semiconductor. Its feasibility as a thermoelectric material for power generation is investigated. The first chapter briefly summarizes the literature on MoSi2 and CrSi2 relevant to structural and thermoelectric applications respectively. Based on the available literature, the scope of further work is discussed. The second chapter describes the methods of synthesis employed for these materials and the characterization techniques adopted. Some experimental setups like thermal conductivity and hot pressing unit that were fabricated as part of the work are described in detail. The thermal conductivity apparatus is based on the principle of parallel heat flow technique. It allows accurate measurement of K and S in the temperature range 300-700 K. The induction based hot-pressing unit allows compaction of polycrystalline powders to near theoretical densities thereby allowing quantitative evaluation of the physical properties. In the third chapter, an understanding of ductility/brittleness based of electron charge density distribution is attempted. The electron charge density in Tin and simple metals (BCC and FCC) is analyzed using Bader’s Atoms in Molecule (AIM) theory. Also the relevant surface and dislocation energies in these materials are calculated according to the Rice Model. It is found that the electron densities at the critical points correlate in a simple way with the relevant stacking fault and surface energetics. Based on these results, a ductility parameter (DM odel) based on electron charge distribution, to predict the effects of chemical substitutions on ductility/brittleness in materials is proposed. In the fourth chapter, possible elements to impart ductility in MoSi2 are identified based on the DM odel values. Calculations indicate, Nb, Ta, Al, Mg and Ga to be suitable candidates for improving ductility in MoSi2. Also oxidation studies based on present experiments and reported literature data reveal, Al to improve the intermediate temperature (773-873 K) oxidation behavior. Thus to simultaneously improve the low temperature ductility and oxidation resistance, Nb and Al were identified as suitable candidates. In the fifth chapter, the experimental data of Nb and Al co-substituted MoSi2 samples are reported. Oxidation studies carried out by thermogravimetry show improved oxidation resistance in Nb and Al co-substituted samples compared to pure MoSi2 in the temperature range of 773-873 K. Mechanical characterization was carried out for (Mo0.99Nb0.01)(Si0.96Al0.04)2 co-substituted composition. Compression testing at room temperature show plastic deformation at low strain rates (10−3 /sec). Indentation experiments show a reduction in the hardness and stiffness compared to pure MoSi2. There is also an increase in the fracture toughness (K1C ) value with the fracture modes being predominantly transgranular. The sixth chapter describes the structural, thermal and transport properties of CrSi2. Structural refinement was carried out by Rietveld method and the positional, thermal parameters and occupancy were fixed. Thermo-gravimetric analysis shows oxidation resistance in powdered samples upto 1000 K. Thermal expansion (α) studies reveal anisotropy in the α values with an unusual decrease in the average αV values between 500 and 600 K. Measurements of electrical resistivity and seebeck coefficient indicate a degenerate semiconducting behavior. Electronic band structure calculations indicate a narrow indirect band gap (EG) material with EG~0.35 eV. Thermal conductivity (K) measurements show a decrease in K value with increasing temperature. Calculation of the thermoelectric figure of merit (ZT) show a maximum value of 0.18 at 800 K for the temperature range studied. Based on an analysis of the experimental and theoretical results, it is identified that further improvements in ZT of CrSi2 may be possible by reducing the lattice thermal conductivity and optimization of the carrier concentration. In chapter seven, the effect of particle size on ZT of CrSi2 is studied. Nano powders of CrSi2 were prepared by mechanical milling. Contamination is found to be a major problem during milling and the different milling parameters (milling speed, atmosphere, dispersant etc) were optimized to minimize contamination. The milled powders were further hot pressed to achieve high densities in a short duration thereby minimizing the grain growth. It is observed that the lattice thermal conductivity is reduced significantly with decreasing grain size. Measurements of ZT show a maximum value of 0.20 in the milled sample compared to 0.14 in arc melted CrSi2 at 600 K. In chapter eight the effect of chemical substitutions on ZT of CrSi2 is studied. Mn substitutions in Cr site were carried out to study the effect of atomic mass on lattice thermal conductivity (KP ). Al substitutions in Si site were carried out to tune the Fermi level. Results of Mn substitution show a large decrease in KP but also a reduction in the thermoelectric power factor (S2σ). The maximum ZT observed in the Mn substituted samples was 0.12 at 600 K. Al substitution results in an increase in the thermoelectric power factor and a subsequent increase in ZT. The maximum ZT observed was 0.27 at 700 K for 10% substitution of Al in Si site. The work reported in the thesis has been carried out by the candidate as a part of the Ph.D. training programme at Materials Research Centre, Indian Institute of Science, Bangalore, India. He hopes that this work would constitute a worthwhile contribution towards (a) basic understanding of ductility/brittleness in materials and understanding the effects of chemical substitutions, (b) Suitability of chemically substituted MoSi2 to overcome the problems of low temperature brittleness and oxidation. (c) Development of CrSi2 as a high temperature thermoelectric material.

Transition Metal Silicides (TMS) are well known refractory materials because of their high thermal and structural stability at elevated temperature. In addition TMS materials are known for their moderate thermoelectric applications at high temperature since they exhibit superior semiconducting behavior. But TMS materials have relatively higher thermal conductivity which limits their applications in the field of thermoelectrics. So it is important to reduce their thermal conductivity to enhance conversion efficiency. In this regard, the work is performed to reduce the thermal conductivity of selected silicides such as CrSi2, MnSi2, and β-FeSi2 through alloys scattering and nano-structuring by mechanical alloying. A brief introduction about basic principles of thermoelectricity and related parameters are described in the chapter 1. Thermoelectric material’s figure of merit (zT) depends on the ratio of carrier charge transport and thermal energy transport. The conversion efficiency can be significantly enhanced by increasing the zT value. This chapter discusses the methods to increase the zT and list out some of the state-of-art of thermoelectric materials which possesses high zT value. Chapter 2 covers the preparation of selected silicides, such as CrSi2, MnSi2 and β-FeSi2, and the characterization techniques used to define the thermoelectric performance. In this chapter the suitability and the performance of transition metal silicides for high temperature thermoelectric application are discussed. In summary, the objective of the thesis has been framed. Chapter 3 deals with thermoelectric properties of pure and Mn, Al doped chromium disilicide (CrSi2). This chapter has been divided into three parts and discussed the effect of composition variation (CrSi1.90-2.10), point defects (by introducing Al at Si site), and mass-fluctuation scattering (by co-substitution of Mn and Al) on thermoelectric properties of polycrystalline CrSi2 in the temperature range of 300K-800K. In the first part, it is observed that CrSi2 has a homogeneity range of CrSi1.95-CrSi2.02. The secondary phases evolve above and below this homogeneity range. These secondary phases significantly scatter phonons and reduce the thermal conductivity. In the second part, Al has been introduced at Si site in CrSi2 and creates the point defects which is also scatter the short wavelength phonons and lead to low thermal conductivity. The third part explores the influence of co-substitution of Mn at Cr site and Al at Si site on lattice thermal conductivity. Here, substitution of Al creates point defects and addition of Mn leads to mass fluctuation scattering. These combined effects result in huge reduction in lattice thermal conductivity and thereby enhanced the zT. Chapter 4 deals with efforts of nano-structuring the CrSi2 through Mechanical Alloying (MA) using SS (stainless steel) and WC (Tungsten Carbide) milling media. The effects of two milling media on crystallite size reduction are discussed. It is seen that as milling time increases the rate of crystallite size reduction also increases. The X-ray diffraction studies of hot pressed pellets show the formation of secondary metallic phase like Cr1-xFexSi from SS milled samples and CrSi from WC milled samples. It indicates that CrSi2 gains metallic Fe atoms during mechanical alloying and the secondary phases are formed. As milling time increases it is observed that weight loss from the milling balls also increases. The Fe content coming from SS ball forms a solid solution with CrSi phase. The transport properties like resistivity, Seebeck coefficient and thermal conductivity were measured for milled samples from 300K-800K. It is observed that formation of the secondary metallic phase reduces resistivity and Seebeck coefficient of overall ceramics. Very large reduction in thermal conductivity was found for samples which were 15hrs-WC-milled (7.4 W/m.K at 375K) due to increased phonon scattering by grain boundaries. The 15hrs-SS-milled samples show thermal conductivity ~10 W/m.K which is considerably low as compared to the as-cast CrSi2 (13.5 W/m.K). This chapter explores the structural studies and mechano-chemical decomposition of CrSi2. In addition, the influences of mechanical milling media and micron size secondary phase on transport properties of CrSi2 are also discussed. Chapter 5 deals with the influence of microstructures of MnSi2 densified by hot uni-axial pressing (HP) and spark plasma sintering (SPS) on thermoelectric properties. The effects of these densification processes on arresting the grain growth during sintering are explored. The powder X-ray diffraction studies show higher manganese silicide (HMS) with secondary Si phase. The SEM and EPMA results confirmed the presence of Si phase. The TEM micrographs are shown the particle size distribution of HMS to be <200nm with fine precipitates of Si, of 5-10nm size, in the HMS matrix. The ball milled samples of MnSi2 showed increase in resistivity and Seebeck coefficient with large reduction in total thermal conductivity as compared to that seen in as-cast sample. The SPS densified samples show lower thermal conductivity, with reduction by about 52%, as compared to HP sample’s (45%) reduction for same conditions. An enhancement in zT by 73% could be achieved for the SPS densified for 2 min at 1060˚C. Chapter 6 examines (i) the decomposition of α–FeSi2, generally known as α-Fe2Si5, (eutectoid reaction) into β-FeSi2 with Si dispersoids (ii) formation of β-FeSi2 from ε-FeSi and α-Fe2Si5 (peritectoid reaction). This is accompanied by a discussion of the microstructural effect on thermoelectric properties. Prolonged annealing of peritectoid composition decomposes the α– FeSi2 phase, replaces the ε–FeSi phase, and forms pure β-FeSi2 whereas eutectoid composition of α–FeSi2 decomposes into lamellar structure of β-FeSi2 and Si dispersions. The aging heat treatment carried out for composition prepared from eutectoid reaction at various temperatures (600°C, 700°C, 800°C and 850°C for duration of 100hrs, 10hrs, 4hrs and 10hrs, respectively) below the equilibrium eutectoid temperature were found to have fine and homogenous dispersions of Si particles. The XRD and SEM studies confirmed the presence of a secondary Si phase on the matrix of β-FeSi2 for the heat treated eutectoid composition. The excess Si phase in β-FeSi2 increases the resistivity and Seebeck coefficient by the reducing carrier concentration of system as compared to those that of pure β-FeSi2, which is prepared from peritectoid composition. The samples heat treated at 600°C showed relatively low thermal conductivity as compared to that of β-FeSi2. This chapter gives a route map for reducing the thermal conductivity by micro structural engineering through Si dispersions on β-FeSi2. In addition, this comparison of two the decomposition processes and its influence on the microstructure and thermoelectric properties is made. Chapter 7 summarizes the key conclusions of the work performed in this thesis. The work reported in this thesis has been carried out by the candidate as a part of Ph.D training programme. He hopes that this would constitute a worthwhile contribution to the field of thermoelectrics for understanding the (i) effect of alloy scattering, (ii) mass fluctuation scattering, (iii) and nano-structuring of transition metal silicides for high temperature thermoelectric materials.