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.

Valoriser l'énergie perdue sous forme de chaleur par les moteurs thermiques en électricité via desgénérateurs thermoélectriques permettrai de diminuer l'empreinte carbone des transports routiers. Unesélection des matériaux basée sur des critères de performance, de coût et de développement durable aconduit au choix du siliciure de manganèse MnSi (semi-conducteur de type p). En s'appuyant sur uneapproche couplant la métallurgie et la chimie du solide, ce travail revisite l'état de l'art sur ces alliageset révèle les relations entre la structure (inco/commensurabilité), la microstructure et le procédé. Unemeilleure compréhension de ces liens a permis d'acquérir un contrôle plus précis des microstructures,et par conséquent d'optimiser les propriétés thermoélectriques, et a conduit à la mise au point d'unenouvelle voie de synthèse pour MnSi. De plus, la production de matériaux purs et texturés a permisde mettre en évidence l'isotropie des propriétés de transport de la phase MnSi. Enfin, cette étudesuggère une relation entre la texture des joints de grains et la ségrégation dans des alliages dopés,ouvrant de nouvelles perspectives pour améliorer les propriétés thermoélectriques. / Generating electricity from waste heat by means of thermoelectric generators may represent a very interestingopportunity to significantly reduce the impact of road transportation. In this context, HigherManganese Silicide (HMS) based alloys are studied as p-type semiconductors to achieve a sustainablescale-up of this technology. Through a strategy coupling metallurgy and solid state chemistry, thiswork revisits the knowledge on HMS and reveals the relationship between the phases, the microstructureand the manufacturing process. This systematic study has lead to the establishment of designguidance to maximize the performance and thus, to a new synthesis route. In addition, the productionof grain oriented and highly pure HMS materials evidences the isotropy of the transport properties ofHMS. Finally, this study suggests a relationship between grain boundary texture and segregation indoped-HMS, opening new directions for enhancing thermoelectric properties.

MnSi(111) films were grown on Si(111) substrates by solid phase epitaxy (SPE) and molecular beam epitaxy (MBE) to determine their magnetic structures. A lattice mismatch of -3.1% causes an in-plane tensile strain in the film, which is partially relaxed by misfit dislocations. A correlation between the thickness dependence of the Curie temperature (TC) and strain is hypothesized to be due to the presence of interstitial defects. The in-plane tensile strain leads to an increase in the unit cell volume that results in an increased TC as large as TC = 45 K compared to TC = 29.5 K for bulk MnSi crystals. The epitaxially induced tensile stress in the MnSi thin films creates an easy-plane uniaxial anisotropy. The magnetoelastic coefficient was obtained from superconducting quantum interference device (SQUID) magnetometry measurements combined with transmission electron microscopy (TEM) and x-ray diffraction (XRD) data. The experimental value agrees with the coefficient determined from density functional calculations, which supports the conclusion that the uniaxial anisotropy originates from the magnetoelastic coupling. Interfacial roughness obscured the magnetic structure of the SPE films, which motivated the search for a better method of film growth. MBE grown films displayed much lower interfacial roughness that enabled a determination of the magnetic structure using SQUID and polarized neutron reflectometry (PNR). Out-of-plane magnetic field measurements on MBE grown MnSi(111) thin films on Si(111) substrates show the formation of a helical conical phase with a wavelength of 2?/Q = 13.9 ± 0.1 nm. The presence of both left-handed and right-handed magnetic chiralities is found to be due to the existence of inversion domains that result from the non-centrosymmetric crystal structure of MnSi. The magnetic frustration created at the domain boundaries explains an observed glassy behaviour in the magnetic response of the films. PNR and SQUID measurements of MnSi thin films performed in an in-plane magnetic field show a complex magnetic behaviour. Experimental results combined with theoretical results obtained from a Dzyaloshinskii model with an added easy-plane uniaxial anisotropy reveals the existence of numerous magnetic modulated states that do not exist in bulk MnSi. It is demonstrated in this thesis that modulated chiral magnetic states can be investigated with epitaxially grown MnSi(111) thin films on insulating Si substrates, which offers opportunities to investigate spin-dependent transport in chiral magnetic heterostructures based on this system.