The focus of this research is to explore high-efficiency thermoelectric materials, which can be put into daily application to mitigate the energy crisis. Some fundamentals and modern characterization techniques are briefly discussed. Due to their ability to convert waste heat into electricity, thermoelectric materials have drawn significant attention in the past two decades. The most widely used thermoelectric materials nowadays are still composed of Pb and Te. Due to the toxic nature of Pb, extensive work has been done on the GeTe system, an environmentally friendly replacement for PbTe. Unfortunately, the pristine GeTe suffers from a high carrier concentration originating from the low formation energy of Ge vacancies. Herein, the introduction of ZnO nanoparticles into the GeTe matrix to form ZnTe nanophase resulted in the suppression of carrier concentration. This simultaneously increased the average Seebeck coefficient by 40% and achieved a substantial reduction (33%) in electrical thermal conductivity below 600K when compared to a pure GeTe. As a result, the peak zT reached 1.44 at 690K in the 1.5wt.% ZnO sample, and an average zT value was increased by 23% to 0.79 in the 323-733K range. By adopting partial substitution of Fe at the Co site in the Co4Ge6Te6 ternary skutterudites, Co4-xFexGe6Te6 (x=0.04 and 0.12) was successfully tuned from an n-type material into a p-type one as proven by the positive Seebeck coefficient. An enhanced electrical conductivity was achieved by increasing the carrier concentration. / Thesis / Master of Science (MSc)

Thesis advisor: Zhifeng Ren / Silicon germanium alloys (SiGe) have long been used in thermoelectric modules for deep-space missions to convert radio-isotope heat into electricity. They also hold promise in terrestrial applications such as waste heat recovery. The performance of these materials depends on the dimensionless figure-of-merit ZT (= S2σ T/ κ), where S is the Seebeck coefficient, σ the electrical conductivity, κ the thermal conductivity, and T is the absolute temperature. Since 1960 efforts have been made to improve the ZT of SiGe alloys, with the peak ZT of n-type SiGe reaching 1 at 900 - 950 C. However, the ZT of p-type SiGe has remained low. Current space-flights run on p-type materials with a peak ZT ~ 0.5 and the best reported p-type material has a peak ZT of about 0.65. In recent years, many studies have shown a significant enhancement of ZT in other material systems by utilizing a nanostructuring approach to reduce the thermal conductivity by scattering phonons more effectively than electrons. Here we show, using a low-cost and mass-production ball milling and direct-current induced hot press compaction nanocomposite process, that a 50% improvement in the peak ZT, from 0.65 to 0.95 at 800 - 900C is achieved in p-type nanostructured SiGe bulk alloys. The ZT enhancement mainly comes from a large reduction in the thermal conductivity due to the increased phonon scattering at the grain boundaries and crystal defects formed by lattice distortion, with some contribution from the increased electron power factor at high temperatures. Moreover, nanocomposite approaches have been used to study the thermoelectric properties of other material systems such as bismuth telluride (Bi-Te), bismuth antimony (Bi-Sb), and half-Heusler phases. We observed a significant improvement in peak ZT of nanostructured p- and n-type half-Heusler compounds from 0.5 to 0.8 and 0.8 to 1.0 respectively. The ZT improvement is mainly due to the reduction of thermal conductivity. This nanostructure approach is applicable to many other thermoelectric materials that are useful for automotive, industrial waste heat recovery, space power generation, or solar power conversion applications. / Thesis (PhD) — Boston College, 2010. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Thesis advisor: Zhifeng Ren / Thermoelectric materials have attracted a lot of research interests because of their promising applications in solid-state cooling and power generation. The low ZTs of the current available thermoelectric materials have restricted the device efficiency, and thus the wide application of the thermoelectric technique. We propose a nanocomposite approach to improve ZT by reducing lattice thermal conductivity. The nanocomposite approach was first applied to n-type Si. Since there is no point defect scattering from Ge in pure Si, hence it provides an opportunity to study the scattering of grain boundaries. We found that the thermal conductivity is reduced by a factor of 10 in nanostructured Si in comparison with bulk crystalline Si. By adding 5 at% Ge, the thermal conductivity is further reduced by a factor of 2, thereby leading to a thermoelectric figure of merit 0.95 for Si95Ge5, similar to that of large grained Si80Ge20 alloys. Moreover, thermoelectric properties of In4Se3-x and Zn4Sb3 were investigated. Extremely low thermal conductivity values of 0.41 and 0.69 Wm-1K-1 were obtained in In4Se2.2 and Zn4Sb3 nanocomposites respectively, leading to peak ZTs of 1 and 1.3. / Thesis (PhD) — Boston College, 2011. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Tin halide perovskites are gaining interest as a replacement for lead perovskites for various device applications. However, compared to lead-based perovskites, the understanding of their charge transport properties has received limited attention. In particular, the effect of A-site cation on electronic properties of tin perovskites warrants further attention to design efficient material systems for various applications beyond photovoltaics. In the presented work, leveraging the composition tunability of halide perovskites, we establish a relationship between the A-site composition and electronic properties in tin perovskites (ASnI3). The effect of prototypical A-site cations such as Formamidinium (FA), Methylammonium (MA), Cesium (Cs), and their binary combinations on structure, morphology, and electronic properties are explored. MACs combination offers the highest electrical conductivity owing to enhanced mobility compared to mono-cations MA and Cs, resulting in an impressive electrical conductivity of ∼ 143 Scm−1 and thermoelectric power factor of ∼ 149 μW m−1K−2. The library of properties generated for Sn perovskites in this work will be helpful for their further development as an electronic material.

This dissertation is centered on investigation of metastable thermoelectric thin film materials and is split into 3 primary sections. Section 1 focuses on formation mechanisms of FeSbx compounds from layered precursors. It was found that a compositionally favorable and homogeneous nucleation environment allowed for the nucleation of a metastable phase, which surprisingly resembles the local coordination environment of the precursors, even in cases where they are compositionally unfavorable. Over the course of this work, the technique of normal-incidence thin film pair distribution function analysis is introduced, which allows for rapid acquisition and analysis of local structure data from intact thin films. Section 2 investigates changes in the stacking sequences of ([PbSe]1+δ)m(TiSe2)n nanolaminate materials, which consist of interleaved layers of each compound in the chemical formula, and how these changes effect the thermoelectric power factor. Homologous series of systematically varying m and n values are investigated and measured properties are correlated back to the designed nanoarchitecture of the laminate materials. It is found that the compounds are stabilized by electron exchange between constituents at the interfaces, and that ‘doping’ of the laminate structure by changing the relative amounts of each constituent is an effective means of optimizing their transport properties. It is also shown that interface density between constituents can be utilized to optimize performance. Section 3 moves from the case of PbSe layers, which maintain their structure, to SnSe layers that significantly distort as the layer size is changed. The distortions in SnSe are observed to occur from templating off TiSe2 layers. As the size of the SnSe layers increases, relatively fewer templated interfacial atoms exist and stabilization of interior atoms must also be considered. The coarse behaviors developed in ([PbSe]1+δ)m(TiSe2)n hold, but the structural distortions in SnSe likely change the band structure of this constituent and hence the composite material, complicating the analysis. In some cases, these changes allow for radically different behavior, best exemplified with high TiSe2 ratios in ([SnSe]1+δ)1(TiSe2)n displaying significant enhancement of the Seebeck coefficient at cryogenic temperatures over the low-n and PbSe-containing analogues. This dissertation includes previously published and unpublished coauthored material.

Polymers are intrinsically poor thermal conductors, which are ideal for thermoelectrics, but low electrical conductivity and thermopower have excluded them as feasible candidates as thermoelectric materials in the past. However, recent progress in polymer technology, particularly nanomaterial-polymer composites, can bring them into degenerate semiconductor or metallic regimes by incorporating a small amount of conductive filler. I demonstrate that such polymer nanocomposites can be viable for light-weight and economical thermoelectrics by using a segregated network approach for the nanocomposite synthesis. The thermoelectric properties were further improved by a change of stabilizer and drying conditions. The thermoelectric properties of the segregated network nanocomposites were measured for carbon nanotubes and the thermoelectric figure of merit, ZT, was calculated at room temperature. The influence on thermoelectric properties from filler concentration, stabilizer materials and drying condition are also discussed.

Electronic transport properties have been used to classify and characterize materials and describe their functionality. Recent surge in computational power has enabled computational modelling and accelerated theoretical studies to complement and accelerate experimental discovery of novel materials. This work looks at methods for theoretical calculations of electronic transport properties and addresses the limitations of a common approximation in the calculation of these properties, namely, the constant relaxation time approximation (CRTA). This work takes a look at the limitations of this approximation and introduces energy and temperature dependent relaxation times. This study is carried out on models and real systems and compared with experiments.

Abstract The main focus of this work was on exploratory preparation of thermoelectric materials and analyses of their physical properties. A thermoelectric material is capable of converting heat to electricity or vice versa. Usually, narrow band gap semiconductors are good candidates for thermoelectric applications, because such materials have large Seebeck coefficient, reasonably high electrical conductivity and low thermal conductivity. In this work, two different systems were studied, ternary layered tellurides and quaternary derivatives of Tl9BiTe6. I tried to prepare Pb1−xBi2+xTe4 with x = 0.30, 0.10, −0.10 and = 0.30 and Pb1−xBi4+xTe7 with x = 0.15, 0.00, −0.15 and −0.35, and two pure compounds, Pb0.8Bi2.2Te4 and Pb0.9Bi2.1Te4 were obtained. Powder X-ray diffraction was used to confirm the purity of the compounds, and physical properties were measured on cold-pressed samples with densities around 80% of the theoretical value. The figure of merit of the ternary tellurides is comparable to the published values of PbBi2Te4 (0.5 at 600 K). I also investigated the quaternary series Tl8.67PbxBi1.33−xTe6 with x between 0.50 and 1.00. The purity was confirmed by powder X-ray diffraction data, and physical properties were measured on Spark Plasma Sintered (SPS) samples. Low thermal conductivity was observed as well as competitive power factors. The highest ZT value was 0.57 for the compound Tl8.67Pb0.60Bi0.73Te6 at 575 K.

Abstract The main focus of this work was on exploratory preparation of thermoelectric materials and analyses of their physical properties. A thermoelectric material is capable of converting heat to electricity or vice versa. Usually, narrow band gap semiconductors are good candidates for thermoelectric applications, because such materials have large Seebeck coefficient, reasonably high electrical conductivity and low thermal conductivity. In this work, two different systems were studied, ternary layered tellurides and quaternary derivatives of Tl9BiTe6. I tried to prepare Pb1−xBi2+xTe4 with x = 0.30, 0.10, −0.10 and = 0.30 and Pb1−xBi4+xTe7 with x = 0.15, 0.00, −0.15 and −0.35, and two pure compounds, Pb0.8Bi2.2Te4 and Pb0.9Bi2.1Te4 were obtained. Powder X-ray diffraction was used to confirm the purity of the compounds, and physical properties were measured on cold-pressed samples with densities around 80% of the theoretical value. The figure of merit of the ternary tellurides is comparable to the published values of PbBi2Te4 (0.5 at 600 K). I also investigated the quaternary series Tl8.67PbxBi1.33−xTe6 with x between 0.50 and 1.00. The purity was confirmed by powder X-ray diffraction data, and physical properties were measured on Spark Plasma Sintered (SPS) samples. Low thermal conductivity was observed as well as competitive power factors. The highest ZT value was 0.57 for the compound Tl8.67Pb0.60Bi0.73Te6 at 575 K.

Cette thèse présente une étude théorique du transport de chaleur dans les matériaux composites nano poreux et nano fils ainsi qu'une étude théorique des propriétés thermoélectriques de l'alliage Si0:8Ge0:2 confrontée à des mesures expérimentales réalisées pour une partie, dans le cadre de l'étude.La première étude démontre que les alliages poreux affichent des réductions de conductivité thermique à des dimensions de pores beaucoup plus grandes que les matériaux poreux non alliés de même porosité nominale. Si on considère une taille de pores de 1000nm, la conductivité thermique de l'alliage Si0:5Ge0:5 avec 0:1 de porosité est deux fois plus faible que la conductivité thermique d'un matériau non poreux, alors que les pores plus petits que 100 nm sont nécessaires pour obtenir la même réduction relative dans le Si ou Ge pur. Nos résultats indiquent que les alliages nano poreux devraient être avantageux devant les matériaux nano poreux non alliés, et ceux pour les applications nécessitant une faible conductivité thermique, tels que les nouveaux matériaux thermoélectriques.La deuxième étude théorique sur la conductance thermique de nano fils révèle l'effet de la structure sur le transport des phonons. Avec un modèle théorique qui considère la dépendance en fréquence du transport des phonons, nous sommes en mesure quantitativement de rendre compte des résultats expérimentaux sur des nano fils droits et coudés dans la gamme de température qui montre qu'un double coude sur un fil réduit sa conductance thermique de 40% à la température de 5K. Enfin, nous avons procédé à une approche théorique des propriétés thermoélectriques des alliages SiGe frittés, en les comparant aux mesures expérimentales nouvelles et antérieures, tout en évaluant leur potentiel d'amélioration. L'approche théorique a été validée par comparaison de la mobilité prévue et la conductivité thermique prévues, en faisant varier la quantité de Ge et les concentrations de dopage, dans une gamme de température comprise entre 300 et 1000K. Nos calculs suggèrent qu'une optimisation par rapport à l'état de l'art actuel est possible pour le matériau de type n et type p, conduisant potentiellement à une augmentation de 6% (5%) du ZT _a 1000K et 25% (4%) _a température ambiante. Même des améliorations plus grandes devraient être possibles si la probabilité de diffusion des phonons aux joints de grains pouvait être augmentée au-delà de sa valeur actuelle de 10%. / This dissertation presents a theoretical study of heat transport in nanoporous composites andin nanowire and also theoretical study of thermoelectric properties of the Si0:8Ge0:2 alloywith some experimental new and old measurements.The first study on the porous alloys show that its can display thermal conductivity reductionsat considerably larger pore sizes than nonalloyed porous materials of the same nominalporosity. The thermal conductivity of Si0:5Ge0:5 alloy with 0.1 porosity becomes half thenonporous value at 1000 nm pore sizes, whereas pores smaller than 100 nm are required toachieve the same relative reduction in pure Si or Ge. Using Monte Carlo simulations, we alsoshow that previous models had overestimated the thermal conductivity in the small pore limit.Our results imply that nanoporous alloys should be advantageous with respect to nanoporousnonalloys, for applications requiring a low thermal conductivity, such as novel thermoelectrics.The second theoretical study on the nanowire thermal conductance reveals the structureeffect on the phonon transport. With a theoretical model that considers the frequency dependenceof phonon transport, we are able to quantitatively account for the experimental resultsof straight and bent nanowires in the whole temperature range which shows that due to andouble bend on the straight thermal conductance reduced by 40% at temperature 5K.Finally, we theoretically investigate the thermoelectric properties of sintered SiGe alloys,compare them with new and previous experimental measurements, and determine their potentialfor further improvement. The theoretical approach is validated by extensive comparisonof predicted bulk mobility, thermopower, and thermal conductivity, for varying Ge and dopingconcentrations, in the 300 �� 1000K temperature range. The effect of grain boundariesis then included for Si0:8Ge0:2 sintered nanopowders , and used to predict optimized valuesof the thermoelectric figure of merit at different grain sizes. Our calculations suggest thatfurther optimization of current state of the art n-type (p-type) material would be possible,possibly leading to 6% (5%) ZT enhancement at 1000K and 25% (4%) at room temperature.Even larger enhancements should be possible if the phonon scattering probability of the grainboundaries could be increased beyond its present value of 10%.