O composto SbxCoSb3-x foi produzido em altas pressões e altas temperaturas em uma reação de auto-inserção a partir da escuterudita binária CoSb3. A reação de auto-inserção é caracterizada pelo colapso de átomos de Sb para o sítio 2a, no interior das cavidades formadas pelos átomos de Co e Sb na estrutura da escuterudita. A reação inversa, de desinserção de Sb, ocorre quando o composto SbxCoSb3-x é aquecido à pressão ambiente. O acompanhamento desta reação de desinserção por meio de medidas de calorimetria exploratória diferencial (DSC), difração de raios X (DRX) e de resistividade elétrica constitui o objetivo principal deste trabalho. A amostra de CoSb3 foi sintetizada conforme rota proposta pela literatura. A síntese foi confirmada por meio de DRX, e não foi observada a presença de fases contaminantes. Amostras cilíndricas da fase SbxCoSb3-x foram obtidas submetendo CoSb3 a pressões de 7,7 GPa e temperaturas de até 550ºC, com o auxílio de prensas hidráulicas e câmaras toroidais disponíveis no Laboratório de Altas Pressões e Materiais Avançados LAPMA no Instituto de Física da Universidade Federal do Rio Grande do Sul IF/UFRGS. A presença da fase SbxCoSb3-x foi comprovada por meio de análises de DRX. Para determinação da resistividade elétrica de amostras ricas de fase SbxCoSb3-x foi desenvolvido um sistema DC, aplicável a amostras cilíndricas de pequeno volume tal como as obtidas em altas pressões e altas temperaturas. A aferição do sistema foi feita através de medidas de resistividade elétrica de materiais de referência (NIST-SRM 1461 e NIST-SRM 8426). As medidas de DSC revelaram a presença de dois eventos térmicos. Um pico endotérmico foi observado em 118ºC e não está associado a alterações estruturais e nem a variações significativas na resistividade elétrica. O evento exotérmico, que inicia em 180ºC, constitui a assinatura da desinserção dos átomos de Sb do interior da escuterudita, como verificado por análises de DRX e medidas elétricas. Após aquecimento até 350ºC, a amostra rica na fase SbxCoSb3-x retorna à fase estável, CoSb3. A reação de desinserção obedece a uma cinética de primeira ordem, cuja entalpia de transição é de aproximadamente 50 J /g e uma energia de ativação de 83 kJ/mol. A resistividade elétrica à temperatura ambiente de amostras ricas em SbxCoSb3-x é cerca de dez vezes inferior à do CoSb3. Este resultado, aliado possivelmente a uma baixa condutividade térmica, sugere que a fase de auto-inserção SbxCoSb3-x pode constituir um material termoelétrico de alto desempenho. / Submitted by Marcelo Teixeira (mvteixeira@ucs.br) on 2014-06-03T19:52:12Z No. of bitstreams: 1 Dissertacao Fernanda Miotto.pdf: 1202367 bytes, checksum: b66708b3e1d417eab6ba5104702a3458 (MD5) / Made available in DSpace on 2014-06-03T19:52:12Z (GMT). No. of bitstreams: 1 Dissertacao Fernanda Miotto.pdf: 1202367 bytes, checksum: b66708b3e1d417eab6ba5104702a3458 (MD5) / The compound SbxCoSb3-x was produced at high pressures and high temperatures in a self-insertion reaction from the binary skutterudite CoSb3. The self-insertion reaction is characterized by the collapse of Sb atoms to the 2a site, into the cage formed by the Co and Sb atoms in the skutterudite structure. The opposite reaction, i.e., Sb desinsertion, occurs when the SbxCoSb3-x compound is heated at room pressure. This desinsertion reaction was followed by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD) and electrical resistivity measurements, and its study constitutes the main objective of this work. The CoSb3 sample was synthesized as described in the literature. The synthesis was confirmed by XRD, and the presence of contaminant phases was not observed. Cylindrical samples of the SbxCoSb3-x phase were obtained by submitting CoSb3 at pressures of 7.7 GPa and temperatures up to 550ºC, with the aid of a toroidal high pressure cell available at the Laboratório de Altas Pressões e Materiais Avançados - LAPMA in the Instituto de Física of the Universidade Federal do Rio Grande do Sul - IF/UFRGS. The presence of the SbxCoSb3-x phase was confirmed by XRD analysis. In order to determine the electrical resistivity of samples rich in SbxCoSb3-x phase, a DC system was developed which is applicable to small volume cylindrical samples such as those obtained at high pressures and high temperatures. The calibration of the DC system was made by measurements of the electrical resistivity of reference materials (NIST-SRM 1461 and NIST-SRM 8426). The DSC measurements revealed the presence of two thermal events. An endothermic peak was observed at 118ºC which is not associated to structural changes neither significant variation in the electrical resistivity. The exothermic event that starts at 180ºC is the signature of the desinsertion of Sb atoms from the skutterudite cage, as verified by XRD analysis and electrical measurements. After heating to 350°C, the sample rich in the SbxCoSb3-x phase converts back to the stable phase, CoSb3. The desinsertion reaction follows a first-order kinetics, with a transition enthalpy of approximately 50 J/g and an activation energy of 83 kJ/mol. The electrical resistivity at room temperature of samples rich in SbxCoSb3-x is about ten times smaller than that of CoSb3. This result, along with a possible low thermal conductivity, suggests that SbxCoSb3-x may constitute a high performance thermoelectric material.

O composto SbxCoSb3-x foi produzido em altas pressões e altas temperaturas em uma reação de auto-inserção a partir da escuterudita binária CoSb3. A reação de auto-inserção é caracterizada pelo colapso de átomos de Sb para o sítio 2a, no interior das cavidades formadas pelos átomos de Co e Sb na estrutura da escuterudita. A reação inversa, de desinserção de Sb, ocorre quando o composto SbxCoSb3-x é aquecido à pressão ambiente. O acompanhamento desta reação de desinserção por meio de medidas de calorimetria exploratória diferencial (DSC), difração de raios X (DRX) e de resistividade elétrica constitui o objetivo principal deste trabalho. A amostra de CoSb3 foi sintetizada conforme rota proposta pela literatura. A síntese foi confirmada por meio de DRX, e não foi observada a presença de fases contaminantes. Amostras cilíndricas da fase SbxCoSb3-x foram obtidas submetendo CoSb3 a pressões de 7,7 GPa e temperaturas de até 550ºC, com o auxílio de prensas hidráulicas e câmaras toroidais disponíveis no Laboratório de Altas Pressões e Materiais Avançados LAPMA no Instituto de Física da Universidade Federal do Rio Grande do Sul IF/UFRGS. A presença da fase SbxCoSb3-x foi comprovada por meio de análises de DRX. Para determinação da resistividade elétrica de amostras ricas de fase SbxCoSb3-x foi desenvolvido um sistema DC, aplicável a amostras cilíndricas de pequeno volume tal como as obtidas em altas pressões e altas temperaturas. A aferição do sistema foi feita através de medidas de resistividade elétrica de materiais de referência (NIST-SRM 1461 e NIST-SRM 8426). As medidas de DSC revelaram a presença de dois eventos térmicos. Um pico endotérmico foi observado em 118ºC e não está associado a alterações estruturais e nem a variações significativas na resistividade elétrica. O evento exotérmico, que inicia em 180ºC, constitui a assinatura da desinserção dos átomos de Sb do interior da escuterudita, como verificado por análises de DRX e medidas elétricas. Após aquecimento até 350ºC, a amostra rica na fase SbxCoSb3-x retorna à fase estável, CoSb3. A reação de desinserção obedece a uma cinética de primeira ordem, cuja entalpia de transição é de aproximadamente 50 J /g e uma energia de ativação de 83 kJ/mol. A resistividade elétrica à temperatura ambiente de amostras ricas em SbxCoSb3-x é cerca de dez vezes inferior à do CoSb3. Este resultado, aliado possivelmente a uma baixa condutividade térmica, sugere que a fase de auto-inserção SbxCoSb3-x pode constituir um material termoelétrico de alto desempenho. / The compound SbxCoSb3-x was produced at high pressures and high temperatures in a self-insertion reaction from the binary skutterudite CoSb3. The self-insertion reaction is characterized by the collapse of Sb atoms to the 2a site, into the cage formed by the Co and Sb atoms in the skutterudite structure. The opposite reaction, i.e., Sb desinsertion, occurs when the SbxCoSb3-x compound is heated at room pressure. This desinsertion reaction was followed by means of differential scanning calorimetry (DSC), X-ray diffraction (XRD) and electrical resistivity measurements, and its study constitutes the main objective of this work. The CoSb3 sample was synthesized as described in the literature. The synthesis was confirmed by XRD, and the presence of contaminant phases was not observed. Cylindrical samples of the SbxCoSb3-x phase were obtained by submitting CoSb3 at pressures of 7.7 GPa and temperatures up to 550ºC, with the aid of a toroidal high pressure cell available at the Laboratório de Altas Pressões e Materiais Avançados - LAPMA in the Instituto de Física of the Universidade Federal do Rio Grande do Sul - IF/UFRGS. The presence of the SbxCoSb3-x phase was confirmed by XRD analysis. In order to determine the electrical resistivity of samples rich in SbxCoSb3-x phase, a DC system was developed which is applicable to small volume cylindrical samples such as those obtained at high pressures and high temperatures. The calibration of the DC system was made by measurements of the electrical resistivity of reference materials (NIST-SRM 1461 and NIST-SRM 8426). The DSC measurements revealed the presence of two thermal events. An endothermic peak was observed at 118ºC which is not associated to structural changes neither significant variation in the electrical resistivity. The exothermic event that starts at 180ºC is the signature of the desinsertion of Sb atoms from the skutterudite cage, as verified by XRD analysis and electrical measurements. After heating to 350°C, the sample rich in the SbxCoSb3-x phase converts back to the stable phase, CoSb3. The desinsertion reaction follows a first-order kinetics, with a transition enthalpy of approximately 50 J/g and an activation energy of 83 kJ/mol. The electrical resistivity at room temperature of samples rich in SbxCoSb3-x is about ten times smaller than that of CoSb3. This result, along with a possible low thermal conductivity, suggests that SbxCoSb3-x may constitute a high performance thermoelectric material.

Dans une première partie, une nouvelle méthodologie, précise et hautement sensible de caractérisation des paramètres thermiques de liquides par radiométrie photothermique est ici présentée. Deux configurations expérimentales sont proposées. Elles ont été testées et validées avec des matériaux liquides usuels aux paramètres thermiques connus. Par la suite, cette démarche a été utilisée pour l'étude de polymères dispersés dans des cristaux liquides. Les propriétés thermiques dynamiques de chaque échantillon ont été mesurées en fonction de l'amplitude du champ électrique appliqué à une fréquence donnée aussi bien qu'en fonction de la fréquence du champ électrique à une amplitude fixe. Cette étude a montré que les propriétés thermiques étaient sujettes aux effets du champ de dépolarisation aux basses fréquences. La seconde partie de ce manuscrit décrit la nouvelle technique photothermique basée sur l'effet thermoélectrique. Cette technique est utile pour caractériser thermiquement les matériaux thermoélectriques sans avoir à recourir à un capteur extérieur pour mesurer le changement de température. Une étude théorique et expérimentale est présentée. Ces expériences ont été réalisées avec des composites polyaniline/nanotubes de carbone par mesure de la tension générée par l'échantillon thermoélectrique chauffé par un faisceau laser. Des mesures additionnelles à l'aide de la radiométrie infrarouge sur ces mêmes échantillons ont été entreprises et les résultats sont en bon accord avec ceux précédemment trouvés. Enfin, la possibilité d'utiliser les matériaux thermoélectriques comme capteur photothermique au travers d'une cavité résonnante à ondes thermiques est évoquée. / Primarily, newly developed, high sensitive and accurate methods for thermal characterization of liquids using photothermal radiometry are presented. Two experimental configurations are suggested, tested and validated with usual liquid materials. These methods are used to study polymer dispersed liquid crystal samples. Dynamic thermal properties of samples are analysed verses amplitude varying applied electric field with constant frequency as well as versus frequency varying electric field with constant amplitude. Our results clearly show the thermal properties of the samples are prone to depolarizing field effects at the lower frequencies of the applied electric field. The experimental results are modeled against existing theories to predict electric properties of the sample composites. Second part of the manuscript describes the development of a novel photothermal technique based on thermoelectric effect. This technique is particularly useful for thermally characterizing thermoelectric materials without using a separate sensor for measuring induced temperature changes. A theoretical and experimental study is presented. The experiments are done on polyaniline - carbon nanotube composite pellets by measuring Seebeck voltage generated by the samples upon heating by a modulated laser beam. Additional infrared radiometry experiments are done on the same samples and the results are in good agreement with those previously found. Later on, the possibility of photothermoelectric materials to be used as sensors for finding thermal transport properties of materials with a thermal wave resonant cavity is suggested.

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.

<p>Developing the next generation soft optoelectronic materials is of great importance for achieving high-performance, low-cost electronics. These novel material systems bring about new chemistry, physical phenomena, and exciting properties. Organic inorganic hybrid two-dimensional perovskites and organic stable radical molecules are two exciting material systems that bear high expectation and await extensive exploration.</p> <p>Organic inorganic hybrid two-dimensional perovskites are considered one of the solutions to the pressing instability issue of halide perovskites toward commercialization. Moreover, dimension reduction of perovskites creates new opportunities for using two-dimensional perovskites as thermoelectric applications due to the ultralow thermal conductivity. However, two-dimensional perovskite thermoelectric is still at its’ incipient stage of development, therefore a timely proof of potential is required to draw further research interests.</p> <p>In earlier part of this work, the two-dimensional perovskites featuring π-conjugated ligands are synthesized and optimized for high thermoelectric performance. With material design, device engineering, intensive measurements, and careful data analysis, we successfully showed that two-dimensional perovskites are competitive candidate for the emerging thermoelectric materials. Furthermore, temperature and carrier concentration dependencies on thermoelectric properties were also established, giving future researchers a generalized optimization strategy. </p> <p>Organic stable radical molecules are promising for organic electronics as stable radicals don’t require high conjugation for efficient solids-state charge transport. Thanks to their unique redox capability and the unpaired electrons, organic radicals have many unique electronic and magnetic properties that could be useful in spin-related applications. However, the understanding in charge transport mechanisms as well as structure-to-properties correlation remain shallow.</p> <p>In later part of this work, we achieved the highest recorded long channel electrical conductivity of non-conjugated radicals. Meanwhile, the important role of close packing between radical sites was demonstrated by slightly changing chemical design that resulted in drastic change in electrical conductivity. Finally, we concluded that the solid-state charge transport in non-conjugated species is governed by variable range hopping mechanisms. </p>

Thermoelectric (TE) materials directly convert heat energy into electrical energy. The conversion efficiency of the TE devices depends on the performance of the materials. The conversion efficiency of available thermoelectric materials and devices is low. Therefore, the development of new materials for improving thermoelectric device performance is a highly essential. As the performance of the TE materials depends on TE figure of merit [zT=S2P T ] which consist of three material properties such as Seebeck coefficient (S), electrical resistivity ( ) and thermal conductivity ( ). Thermoelectric figure of merit can be improved by either increase of power factor or decreasing of thermal conductivity or by both. In the present thesis, Cu based chalcogenide compounds are chosen for the study of thermoelectric properties because of their complex crystal structure, which leads to lower values of thermal conductivity. Also, the power factor of these materials can be tuned by the partial substitution doping. In the present thesis, Cu based chalcogenide compounds quaternary chalcogenide compound (Cu2ZnSnSe4), ternary compounds (Cu2SnSe3 and Cu2GeSe3) and tetrahedrite materials (Cu12Sb4S13) have been prepared by solid state synthesis. The prepared compounds are characterized by XRD for the phase identification, Raman Spectroscopy used as complementary technique for XRD, SEM for surface morphology and EPMA for the phase purity and elemental composition analysis respectively. For the evaluation of zT, thermoelectric properties of all the samples have been studied by measuring Seebeck coefficient, resistivity and thermal diffusivity. In the chapter 1, a brief introduction about thermoelectricity and its effects is discussed. Thermoelectric materials parameters such as electrical resistivity, Seebeck coefficient and thermal conductivity for different class of materials are mentioned. The selection of thermoelectric materials and the motivation for choosing the Cu based chalcogenide compounds for thermoelectric applications are discussed. In chapter 2, the details of the experiments carried out for Cu based chalcogenide compounds are presented. In chapter 3, the effect on thermoelectric properties by the cation substitution on quaternary chalcogenide compound Cu2+xZnSn1 xSe4 (0, 0.025, 0.05, 0.075, 0.1, 0.125, and 0.15) is studied. The electrical resistivity of all the samples decreases with an increase in Cu content except for Cu21ZnSn09Se4, most likely due to a higher content of the ZnSe. All the samples showed positive Seebeck coefficients indicating that holes are the majority charge carriers. The thermal conductivity of doped samples was higher as compared to Cu2ZnSnSe4 and this may be due to the larger electronic contribution and the presence of the ZnSe phase in the doped samples. The maximum zT = 0.23 at 673 K is obtained for Cu205ZnSn095Se4. In chapter 4, the effect of multi{substitution of Cu21ZnSn1 xInxSe4 (0, 0.05, 0.075, and 0.1) on transport properties were studied. The Rietveld powder X-ray diffraction data accompanied by electron probe microanalysis (EPMA) and Raman spectra of all the samples con firmed the formation of a tetragonal kesterite structure. The electrical resistivity of all the samples exhibits metallic-like behavior. The positive values of the Seebeck coefficient and the Hall coefficient reveal that holes are the majority charge carriers. The co-doping of copper and indium leads to a significant increase of the electrical resistivity and the Seebeck coefficient as a function of temperature above 650 K. The thermal conductivity of all the samples decreases with increasing temperature. Lattice thermal conductivity is not significantly modified as the doping content may infer negligible mass fluctuation scattering for copper zinc and indium tin substitution. Even though, the power factors (S2 ) of indium-doped samples Cu21ZnSn1 xInxSe4 (x=0.05, 0.075) are almost the same, the maximum zT=0.45 at 773 K was obtained for Cu21Zn09Sn0925In0075Se4 due to its smaller value of thermal conductivity. In chapter 5, thermoelectric properties of Zn doped ternary compounds Cu2ZnxSn1 xSe3 (x = 0, 0.025, 0.05, 0.075) were studied. The undoped com\pound showed a monoclinic crystal structure as a major phase, while the doped compounds showed a cubic crystal structure confirmed by powder XRD (X-Ray Diffraction). The electrical resistivity decreased up to the samples with Zn content x=0.05 in Cu2ZnxSn1 xSe3, and slightly increased in the sample Cu2Zn0075Sn0925Se3 . This behavior is consistent with the changes in the carrier concentration confirmed by room temperature Hall coefficient data. Temperature dependent electrical resistivity of all samples showed heavily doped semiconductor behavior. All the samples exhibit positive Seebeck coefficient (S) and Hall coefficient indicating that the majority of the carriers are holes. A linear increase in Seebeck coefficient with increase in temperature indicates the degenerate semiconductor behavior. The total thermal conductivity of the doped samples increased with a higher amount of doping, due to the increase in the carrier contribution. The total and lattice thermal conductivity of all samples decreased with increasing of temperature, which points toward the dominance of phonon scattering at high temperatures. The maximum zT = 0.34 at 723 K is obtained for the sample Cu2SnSe3 due to a low thermal conductivity compared to the doped samples. In chapter 6, thermoelectric properties of Cu2Ge1 xInxSe3 (x = 0, 0.05, 0.1, 0.15) compounds is studied. The powder X-ray diffraction pattern of the undoped sample revealed an orthorhombic phase. The increase in doping content led to the appearance of additional peaks related to cubic and tetragonal phases along with the orthorhombic phase. This may be due to the substitutional disorder created by indium doping. The electrical resistivity ( ) systematically decreased with an increase in doping content, but increased with the temperature indicating a heavily doped semiconductor behavior. A positive Seebeck coefficient (S) of all samples in the entire temperature range reveal holes as predominant charge carriers. Positive Hall coefficient data for the compounds Cu2Ge1 xInxSe3 (x= 0, 0.1) at room temperature (RT) con rm the sign of Seebeck coefficient. The trend of as a function of doping content for the samples Cu2Ge1 xInxSe3 with x = 0 and 0.1 agrees with the measured charge carrier density calculated from Hall data. The total thermal conductivity increased with rising doping content, attributed to an increase in carrier thermal conductivity. The thermal conductivity decreases with increasing temperature, which indicates the dominance of Umklapp phonon scattering at elevated temperatures. The maximum thermoelectric figure of merit (zT) = 0.23 at 723 K was obtained for Cu2In01Ge09Se3. In chapter 7, thermoelectric properties of Cu12 xMn1 xSb4S13 (x = 0, 0.5, 1.0, 1.5, 2.0) samples were studied. The Rietveld powder XRD pattern and Electron Probe Micro Analysis revealed that all the Mn substituted samples showed a single tetrahedrite phase. The electrical resistivity increased with increasing Mn due to substitution of Mn2+ on the Cu1+ site. The positive Seebeck coefficient for all samples indicates that the dominant carriers are holes. Even though the thermal conductivity decreased as a function of increasing Mn, the thermoelectric figure of merit (zT) decreased, because the decrease of the power factor is stronger than the decrease of the thermal conductivity. The maximum zT = 0.76 at 623 K is obtained for Cu12Sb4S13. In chapter 8, the summary and conclusion of the present work is presented.

The growing need of energy worldwide has lead to an increasing demand for alternative sources of power generation. Thermoelectric materials are one of the ‘green energy sources’ which convert directly heat into electricity, and vice–versa. The efficiency of this conversion is dependent on ‘figure of merit’ (z T), which depends on the material’s Seebeck coefficient (S), electrical resistivity (ρ) and thermal conductivity (κ) through the relation z T=S2T/ρκ, where T is the temperature. High values of z T lead to high efficiency, and therefore, z T must be maximized. Lead telluride is well–established thermoelectric material in the temperature range 350 K and 850 K. The aim of this thesis is to improve the z T of the material by adopting two different approaches – (i) doping/alloying and (ii) introducing additional interfaces in bulk i.e. having two phase PbTe. In this thesis, first an introduction about the thermoelectric phenomenon is given, along with the material parameters on which z T depends. A survey of literature associated with PbTe is done and the current status of thermoelectric devices is summarized briefly. This is followed by a description of the synthesis procedure and the measurement techniques adopted in this work. The first approach is the conventional alloying and doping of the material by which carrier concentration of the material is controlled so that maximum power factor Sρ2 is achieved and a simultaneous reduction of thermal conductivity takes place by mass fluctuation scattering. Under this, two systems have been studied. The first system is PbTe1−ySey alloys doped with In (nominal composition: Pb0.999In0.001Te1−ySey, y=0.01, 0.05, 0.10, 0.20, 0.25, 0.30). The compounds were single phase and polycrystalline. Lattice constants obtained from Rietveld refinement of X–ray diffraction (XRD) data showed that Vegard’s law was followed, indicating solid solution formation between PbTe and PbSe. Compositional analysis showed lower indium content than the nominal composition. Temperature dependent Seebeck coefficient showed all the samples to be n–type while Pisarenko plots showed that indium did not act as a resonant dopant. Electrical resistivity increased with temperature, while mobility vs T fitting showed a mixed scattering mechanism of acoustic phonon and ionized impurity scattering. Thermal conductivity followed a T1 dependence, which indicated acoustic phonon scattering. At high temperature, slight bipolar effect was observed, which showed the importance of control-ling carrier concentration for good thermoelectric properties. A z T of 0.66 was achieved at 800 K. The second alloy studied under this approach was Mn doped Pb1−ySnyTe alloy (nominal composition Pb0.96−yMn0.04SnyTe (y=0.56, 0.64, 0.72, 0.80)). All the samples followed Vegard’s law, showing formation of complete solid solution between PbTe and SnTe. Microstructure analysis showed grain size distribution of <1 µm to more than 10 µm. Seebeck coefficient showed all samples were p-type and the role of two valence band conduction in p–type PbTe based materials. Electrical resistivity showed a de-crease possibly due to (i) large carrier concentration or (ii) increased mobility due to Mn2+ ions. Thermal conductivity decreased systematically with decreasing Sn content. Bipolar effect was observed at high temperatures. Accordingly, the highest z T of 0.82 at 720 K was obtained for the sample with Sn (y=0.56) content due to optimum carrier concentration and maximum disorder. The second approach of having additional interfaces in bulk focuses on reducing thermal conductivity by scattering phonons. Under this approach, three systems were studied. The first system is PbTe with bismuth (Bi) secondary phase. The XRD and Ra-man studies showed that bismuth was not a dopant in PbTe, while micrographs showed micrometer–sized Bi secondary phase dispersed in bulk of PbTe. Reduction in Seebeck coefficient showed possible hole donation across PbTe–Bi interfaces, while electrical re-sistivity and thermal conductivity showed that the role of electrons at the interfaces was more important than phonons for the present bismuth concentrations. For the parent PbTe, z T of 0.8 at 725 K was reached, which, however decreased for bismuth added samples. The second system studied under the two phase approach was indium (In) added PbTe. Indium was not found to act as dopant in PbTe, while micrometer sized indium phase was found in PbTe bulk. A decrease in the electronic thermal conductivity ac-companied by a simultaneous increase of the electrical resistivity and Seebeck coefficient throughout the measurement range indicated increased scattering of electrons at PbTe-In interfaces. Higher values of the lattice thermal conductivity showed that the PbTe–In interfaces were ineffective at scattering phonons, which was initially expected due to the lattice mismatch between PbTe and In. For PbTe with 3 at. % In phase, z T value of 0.78 at 723 K was achieved. Under the two phase approach, as a comparative study, PbTe with both micrometer sized Bi and In phases together was prepared, in which no improvement in z T was found. A comparison of both the approaches showed that the alloying approach is better than the two–phase approach. This is because micrometer sized secondary phase scatter the electrons more than the phonons, leading to adverse effect on the transport coef-ficients, and hence, on z T. Alloying, on the other hand, is more beneficial in reducing thermal conductivity by mass fluctuation scattering, along with a simultaneous reduction of electrical resistivity.

Thermoelectric generators (TEG) are re-emerging technology that can be used to recover heat waste from commercial and industrial processes to generate electricity, enhancing fuel utilization and lowering greenhouse gas emissions. TEG modules are solid-state heat engines that produce no noise or vibration during operation. Notably, TEG modules are also able to operate at low-temperature differences, which makes them ideal for a wide range of heat waste recovery applications. Annular thermoelectric generator (ATEG) modules are optimal in applications where either the heat source or sink are round in shape. Bi2Te3 solution-based compounds are of significant interest in the application of thermoelectric materials (TE) used in low-temperature cooling and power generation applications. The main objective of the current work is to design a mechanically reliable ring-shaped ATEG module with a predictable performance using spark plasma sintered Bi2Te3 TE material for low temperature waste heat recovery applications. In terms of structure, this work is divided into two parts. The first part investigates how the use of a powder pre-treatment technique affects the mechanical and thermoelectric properties of P- and N-type Bi2Te3. In addition, part one also presents the measurements of these materials’ mechanical and thermoelectric properties, which serve as inputs for the finite element models used to design thermoelectric modules with parallel and perpendicular configurations vis-a-vis the sintering pressing direction. The second part evaluates the thermoelectric performance and thermal stresses of a ring-shaped ATEG couple that has been integrated between hot-side and cold-side heat exchangers. To this end, two configurations are compared with respect to their heat/electrical current flow paths: one that allows for radial flow (radial configuration), and one that allows for axial flow (axial configuration). The P- and N-type Bi2Te3 powder was treated using a mechanically agitated fluidized powder reduction facility that was built in-house. The characteristic uniaxial tensile strength of the P-type Bi0.4Sb1.6Te3 increased from 13.9 MPa to 26.3 MPa parallel to the sintering pressure, and from 16.3 MPa to 30.6 MPa perpendicular to the sintering pressure following oxide reduction using 5% H2 ˗ 95% Ar at 380 ℃ for 24 h. The figure of merit, ZT, increased from 0.35 to 0.80 and from 0.42 to 1.13 at room temperature (25 ℃) in the parallel and the perpendicular directions, respectively, after the surface oxide reduction treatment. On the other hand, the annealing effects of the oxide reduction pr-treatment of the N-type (Bi0.95 Sb0.05)2(Se0.05 Te0.95)3 using 5% H2 ˗ 95% Ar at 380 ℃ for 24 h were found to be responsible for the majority of the mechanical properties and ZT enhancement. Additionally, the characteristic uniaxial tensile strengths for this material increased from 30.4 to 34.1 MPa and from 30.8 to 38 MPa in the parallel and the perpendicular directions, respectively. The ZTmax (150 ℃) increased from 0.54 to 0.63 in both the parallel and perpendicular directions due to oxide reduction, while annealing led to an increase to 0.58 and 0.62 in the parallel and the perpendicular directions, respectively. An analytical model was constructed to compare the thermoelectric performance of the two configurations under three different hot-side thermal resistances, and a 3D coupled finite element ANSYS model was constructed to study and compare the thermal stresses of the two configurations at different dimensions. The two models were then used to create 2D maps in order to investigate the effects of ATEG couple configuration and dimensions, as well as the hot-side thermal resistance, with the goal of identifying the optimum design. The optimization of module geometry requires a trade-off between performance and mechanical reliability. The results of these investigations showed that increases in the temperature difference across the ATEG couple (ΔT) led to increases in both power and thermal stresses in both configurations. When both configurations were generating the same power at ΔT = 105 ℃, the thermal stresses in the radial configuration were as much as 67 MPa higher than those in the axial configuration due to the formation of additional tensile hoop stresses. The lowest thermal stress obtained for the axial couple configuration was 67.8 MPa, which was achieved when the couple had an outer diameter of 16 mm, an axial thickness of 1 mm, a ΔT of 14.8 ℃, and power generation of 10.4 mW per couple. The maximum thermal stress values were located at the corners of the interface between the solder and the TE rings due to the mismatched coefficient of thermal expansion. This thesis makes a novel contribution to the state-of-the-art literature in ring-shaped ATEG modules, as it details a well-characterised spark plasma sintered Bi2Te3 TE material and a methodology for designing a ring-shaped ATEG module with reliable, robust, and predictable thermoelectric and mechanical performance. The details of the contribution made by this work have been disseminated in the form of three journal publications, which have been integrated into this sandwich Ph.D. thesis. / Thesis / Doctor of Science (PhD)

Thermoelektrische Bauelemente ermöglichen die Erzeugung von Elektrizität aus überschüssiger Wärme, wie sie in großen Mengen in Geräten und Prozessen entsteht. Effiziente Thermoelektrika benötigen eine hohe thermoelektrische Gütezahl, die durch elektronische und thermische Transporteigenschaften der Materialien bestimmt wird. Die Dissertation untersucht zunächst die elektronischen Transporteigenschaften zweier hochaktueller thermoelektrischer Materialien, des Schichtsystems SnSe und einer komplexen Klathrat-Legierung. Deren theoretische Beschreibung benötigt unterschiedliche Methoden, die während dieses Dissertationsprojektes implementiert, erweitert oder entwickelt wurden. Die Temperaturabhängigkeit der Leitfähigkeit von SnSe wurde mittels der Boltzmann-Transportmethode in Relaxationszeitnäherung untersucht. Wir zeigen, dass nur bei gleichzeitiger Einbeziehung von thermischer Ausdehnung des Kristallgitters und Elektron-Phonon-Streuprozessen eine gute Übereinstimmung mit Experimenten erreicht wird. Die Eigenschaften des Typ-I-Klathrats Ba8AlxSi46-x sind sowohl von der Stöchiometrie als auch von der Al-Konfiguration, d.h. der Anordnung der Al-Atome im Wirtsgitter, abhängig. Für x=16 wurde der Grundzustand als hableitend bestimmt, während Konfigurationen mit höheren Energien metallisch sind. Wir erhalten eine zuverlässige Beschreibung der elektronischen, strukturellen und Transporteigenschaften von Ba8AlxSi46-x bei endlichen Temperaturen durch Mittlungen über Konfigurationen. Mittels einer neu entwickelten Methode zur Berechnung der temperaturabhängigen effektiven Bandstruktur von Legierungen beobachten wir ein temperaturbedingtes Schließen der Bandlücke bei x=16, was mit einem Phasenübergang von partieller Ordnung zu Unordnung bei 582K einher geht. Basierend auf Gedächtnisfunktions-Modellen präsentieren wir ferner eine neue Ab-initio-Methode zur Berechnung der elektrischen Leitfähigkeit von Festkörpern mit einem Unordungspotential beliebiger Kopplungsstärke. / Thermoelectric devices convert heat into electricity, thus enabling the reuse of waste heat produced by all kinds of engines. To make this conversion process profitable, materials with a high thermoelectric figure of merit, ZT, are demanded. ZT depends on electronic and thermal transport properties. In this thesis, we study the electronic transport properties of two emerging thermoelectric materials, the layered material SnSe and a complex type-I clathrate alloy. Their reliable description requires different methodologies, that has been implemented, extended, or developed during this PhD project. For SnSe, the temperature dependence of the conductivity and the Seebeck coefficient is studied using the Boltzmann transport approach in the relaxation time approximation. We show that only by simultaneously accounting for thermal lattice expansion and electron-phonon coupling, a good agreement with experiment is reached. The properties of the type-I clathrate Ba8AlxSi46-x are determined, on the one hand, by its composition, and, on the other hand, by the configuration, i.e., the arrangement of the Al atoms in the host lattice. At the charge-compensated composition x=16, the ground-state configuration is found to be semiconducting, while configurations higher in energy are metallic. We obtain a realistic description of the electronic, structural, and transport properties of Ba8AlxSi46-x at finite temperature by using configurational thermodynamic averages. From a newly developed method to compute the finite-temperature effective band structure of alloys, we observe a temperature-driven closing of the band gap for x=16, which is concomitant with a partial order-disorder phase transition at 582K. We further present a novel ab initio memory-function approach for solids that enables the calculation of the electrical conductivity of solids in a disorder potential at arbitrary coupling strength. An application of the developed formalism is demonstrated with the example of sodium.

To support the growing global demand for energy, new sustainable solutions are needed both economically and environmentally. Thermoelectric waste heat recovery and energy harvesting could contribute by increasing industrial process efficiency, as well as powering stand-alone devices, microgenerators, and small body appliances.The structural complexity of quaternary chalcogenide materials provides an opportunity for engineering defects and disorder, to modify and possibly improve specific properties. Cu2ZnSnS4 (CZTS, often kesterite), valued for the abundance and non-toxicity of the raw materials, seems particularly suited to explore these possibilities, as it presents several structural defects and polymorphic phase transformations. The aim of this doctoral work is to systematically investigate the effects of structural polymorphism, disorder, and defects on the thermoelectric properties of CZTS, with particular emphasis to their physical origin. A remarkable case is the order-disorder transition of tetragonal CZTS, which is found responsible for a sharp enhancement in the Seebeck coefficient due to a flattening and degeneracy of the electronic energy bands. This effect, involving a randomization of Cu and Zn cations in certain crystallographic planes, is verified in bulk and thin film samples, and applications are proposed to exploit the reversible dependence of electronic properties on disorder. Low-temperature mechanical alloying is instead discovered stabilizing a novel polymorph of CZTS, which disordered cubic structure is studied in detail, and proposed deriving from sphalerite-ZnS. The total cation disorder in this compound provides an uncommon occurrence in thermoelectricity: a concurrent optimization of Seebeck coefficient, electrical and thermal conductivity. These findings, besides providing new and general understanding of CZTS, can cast light on profitable mechanisms to enhance the thermoelectric performance of semiconducting chalcogenides, as well as delineate alternative and fruitful applications.