The main focus of this work is on exploratory investigation of thermoelectric (TE) materials. Thermoelectric devices are solid-state devices that convert thermal energy from a temperature gradient into electrical energy (Seebeck effect), or convert electrical energy into a temperature gradient (Peltier effect). Modifying existing materials and finding new materials with proper thermoelectric properties are the two approaches considered in this research. Good thermoelectric materials are usually narrow band gap semiconductors with large Seebeck coefficient, reasonably high electrical conductivity and low thermal conductivity. Early transition metal antimonides and arsenides, with unique structural features were chosen for finding high performance TE materials. During the investigation of group four antimonides, a series of new ternaries, ZrSi??Sb2-??, ZrGe??Sb2-?? and HfGe??Sb2-?? was developed. Single crystal X-ray diffraction was used for crystal structure determination, and energy depressives X-ray analysis (EDX) was used for compositional analysis. Metallic properties of these compounds were predicted by electronic structure calculations and confirmed by physical property measurements. It was revealed that Mo3Sb7 turns semiconducting by partial Sb/Te exchange. Similarly, isostructural Re3As7 was modified to become semiconducting by partial Ge/As exchange. Crystal structures were determined by single crystal X-ray and powder X-ray diffraction utilizing Rietveld method. Electronic structures were determined by using the LMTO method and confirmed the semiconducting properties of these ternary compounds. Physical property measurements showed exceptional TE properties for these compounds. It was also confirmed by the X-ray single crystal analysis that it is possible to intercalate different cations with the proper size into the existing cubic voids of the structure. The effect of cation intercalation on physical properties of these compounds were investigated and revealed the enhancement of transport properties as a result of this intercalation.

Thermoelectric materials convert temperature gradients into electricity and vice-versa. These materials utilize the Seebeck effect for power generation and function without moving parts and are highly reliable. The efficiency of thermoelectric devices is related to the dimensionless figure of merit for the constituent materials, defined as where S is the Seebeck coefficient, is the electrical conductivity, is the thermal conductivity and T is the temperature. Maximizing ZT is very challenging because of interdependence of parameters, for example, increasing the electrical conductivity by increasing the carrier concentration invariably lowers S and vice versa. Presently numerous thermoelectric materials are being investigated by different research groups. Despite having high thermal conductivity, half-Heusler materials are promising candidates for thermoelectric applications due to their relatively high power factor () and the ability to tune the thermal and electrical properties through substitutional doping. 2S In this research work, I have investigated the synthesis and transport properties of half Heusler series Zr 0.5Hf0.5Ni1-xPdxSn0.99Sb0.01 (0≤x≤1). Also the role of NiO and HfO2 nanoinclusions in half –Heusler matrix were studied. The half Heusler samples were prepared by solid state reaction. Resistivity, Seebeck coefficient and thermal conductivity were measured for all samples over a temperature range from room temperature to 750K. Hall effect measurements at room temperature were also performed. Addition of NiO inclusions did result in an improvement in ZT whereas addition of 3% vol HfO2 in Zr0.5Hf0.5Ni0.8Pd0.2Sn0.99Sb0.01 showed 19% improvement in ZT.

In this thesis the spark plasma sintering process (SPS) was used to press Mg2Si powder with Ni and Cu slices into alternating layer stacks. These stacks, once cut at an angle, are an artificially anisotropic material. This anisotropy provides transverse thermoelectric properties to the sample. The transverse transport properties were measured along with the individual component transport properties. The SPS process provided malleable samples that gave a power factors of for the Ni/Mg2Si stack and for the Cu/Mg2Si stack. These fall short of the theoretical calculations which would give the power factors as .0254 for the Ni/Mg2Si stack and .211 for the Cu/Mg2Si stack. It is theorized that eddy currents and interface resistances between the layers are the causes for these discrepancies.

Thesis advisor: Cyril P. Opeil / Thermoelectric (TE) materials are of broad interest for alternate energy applications, specifically waste heat applications, as well as solid-state refrigeration. The efficiency of TE materials can be improved through either the enhancement of the Seebeck coefficient and electrical conductivity, or through the reduction of the thermal conductivity, k, specifically the lattice portion of thermal conductivity, klatt. Nanostructuring has been proven to reduce klatt and therefore increase efficiency. The inability to accurately model the lattice and electronic contributions to k makes optimizing the reduction of klatt difficult. This work demonstrates that the lattice and electronic contributions to k in nanostructured materials can be directly measured experimentally by separating the contributions using magnetic field. We use this technique along with other characterization techniques to determine the effects of doping Ce, Sm, and Ho into Bi88Sb12. Along with enhancing the efficiency of the material, TE devices must be thermally stable in the temperature range of operation. Therefore we also study the effects of temperature cycling, annealing, oxidation, and diffusion barriers on TE devices. These studies are accomplished through both homemade and commercially available measurement equipment. / Thesis (PhD) — Boston College, 2013. / Submitted to: Boston College. Graduate School of Arts and Sciences. / Discipline: Physics.

Thermoelectric generators are solid state machines used to convert temperature gradients into electrical energy. They are formed by several thermoelectric units connected electrically in series and thermally in parallel. These units are made by creating a junction between a p-type and an n-type conductor. This investigation documents the characterisation of the thermoelectric properties of carbon nanotube (CNT) films and the fabrication process of carbon nanotube-based thermoelectric devices. The Seebeck coefficient is a intrinsic property of a thermoelectric material that correlates the voltage produced by a conductor and the temperature gradient applied to it. To measure the Seebeck coefficient of films, an experimental set-up was fabricated and calibrated using constantan as standard material. CNT films of aligned nanotubes fabricated using a chemical vapour deposition method were analysed. The Seebeck coefficient along and across the samples did not show significant variations, with values between 40$\mu$V/K and 80$\mu$V/K. Using these CNT films, thermoelectric cells were fabricated with the CNT as the p-type conductor and constantan as the n-type. As a proof of concept, two hand-made thermoelectric generators were assembled by connecting hundreds of these thermoelectric cells. These devices were subjected to a temperature gradient of $\approx$200K, which was enough to produce enough power to light an LED. Other analytical techniques were used to characterise the materials used in this work. Electrical conductivity measurements, thermogravimetric analysis, Raman spectroscopy and scanning electron microscopy were performed. Using a deposition technique, films of nanotubes were produced from a liquid phase. The impact of the production method on their properties was evaluated. Characterisation equipment was developed to measure the Seebeck coefficient and thermal conductivity. Thermoelectric devices made with the carbon nanotube films were fabricated and characterised. The values of thermal conductivity of the CNT films analysed in this work are between 0.86Wm$^{-1}$K$^{-1}$. The electrical conductivity of these materials is between 3500Sm$^{-1}$ and 14100Sm$^{-1}$. The maximum figure of merit of the carbon nanotube thermoelectric devices fabricated in this work is $ZT$=0.35.

Les polymères conducteurs ont attiré l'attention de la communauté scientifique en raison de leur utilisation potentielle dans les applications thermoélectriques [1, 2]. En particulier, il a été prouvé qu'un paramètre important pour accorder les propriétés thermoélectriques et le comportement de transport de charge du polymères, est la forme du DOS dans le bord de bande. Dans la présente étude, la corrélation entre la structure du matériau, la structure électronique et les propriétés électroniques / thermoélectriques, est étudiée par une conception soigneuse et rigoureux du matériau, vers un matériau polymère, thermoélectrique efficace. En outre, les dispositifs hybrides ont été fabriqués comme un moyen alternatif pour améliorer encore l'efficacité thermoélectrique du matériau. / Conducting polymers (CPs) have recently gained the attention of the scientific community due to their prospective use in thermoelectric applications [1,2]. Particularly, it has been proven that an important parameter for tuning the thermoelectric properties and the charge transport behavior of the CP is the shape of the DOS in the band edge, where a more steep band edge would be translated in a semi-metallic behavior for the system, with higher thermoelectric efficiencies. In the present study the correlation between material structure, electronic structure and electronic/ thermoelectric properties, is investigated through careful material design, towards an efficient thermoelectric polymer material. Additionally, the hybrid devices were fabricated as an alternative means to further enhance the thermoelectric efficiency of the material.

Thermoelectric devices allow for direct conversion of heat into electricity as well as solid-state refrigeration. The skutterudite family of compounds continues to be of considerable interest both scientifically and technologically due to their unique physical properties, in particular as promising thermoelectric materials. In this thesis, the basic thermoelectric phenomena and some background history on skutterudites will be reviewed. Rhombohedral derivatives of the cubic skutterudite CoSb3, namely Co4-xFexGe6Se6 with x=0, 1, 1.5 (p-type) and rare-earth filled Ce0.13Co4Ge6Se6 and Yb0.14Co4Ge6Se6 (n-type), were synthesized and their synthesis and low temperature transport properties will be discussed. Reitveld refinement and elemental analysis were used to identify the structure and stoichiometry of these compositions. Both Fe substitution and rare-earth filling reduced the thermal conductivity compared with Co4Ge6Se6 skutterudite derivative. In addition the electrical and thermal properties of these compounds are greatly affected by doping. This fundamental investigation reveals new insight and is intended as part of the continuing effort to explore different skutterudite compositions and structure types for potential thermoelectric applications.

Thermoelectric power generators can convert a temperature gradient into electrical energy, serving as a new energy resource by utilizing solar energy or by utilizing more waste heat. Thermoelectric coolers have the advantage of no moving parts, are quiet and release no gases that are harmful to the atmosphere, in contrast to compression-based refrigeration. While the low efficiency of “classical” thermoelectric devices limits their wide applications, the exploration of better thermoelectric materials is of great importance to improve the efficiency of thermoelectric devices. Good thermoelectric materials are usually narrow band gap semiconductors with a large Seebeck coefficient, reasonably high electrical conductivity and low thermal conductivity. This thesis deals with the exploration of new thermoelectric materials based on transition metal tellurides and the optimization of bulk materials based on oxides of low toxicity and high stability in air. In the first project, seven new ternary or quaternary tellurides, crystallizing in three different structure types, were synthesized and characterized. Single crystal X-ray diffraction was used for crystal structure determination; powder X-ray diffraction and energy dispersive X-ray analysis (EDAX) were used for phase and composition analyses. Physical properties of these compounds were predicted by electronic structure calculations and confirmed by physical property measurements. In the second project, two series of n-type doped perovskite SrTiO3 were prepared in a high temperature tube furnace under dynamic high vacuum of the order of 10–6 mbar, namely SrTi1-x(Nb,Ta)xO3, and Sr1-xLaxTi1-x(Nb,Ta)xO3. The phase purity was characterized by means of powder X-ray diffraction and electron probe micro analysis (EPMA). Rietveld refinements were performed to check for purity and symmetry reduction. The physical properties, such as Seebeck coefficient, electrical conductivity, and thermal conductivity, were measured at high temperatures for all the samples. Of the series of Nb/Ta-doped strontium titanates SrTi1-x(Nb,Ta)xO3, SrTi0.90Ta0.10O3 exhibits the highest ZT value, namely 0.17 for at 752 K. Of the double substituted series, Sr0.99La0.01Ti0.99Ta0.01O3 was best with ZT = 0.13 at 660 K. The rapid increases imply that higher ZT values are likely to occur at higher temperatures.

Thermoelectric power generators can convert a temperature gradient into electrical energy, serving as a new energy resource by utilizing solar energy or by utilizing more waste heat. Thermoelectric coolers have the advantage of no moving parts, are quiet and release no gases that are harmful to the atmosphere, in contrast to compression-based refrigeration. While the low efficiency of “classical” thermoelectric devices limits their wide applications, the exploration of better thermoelectric materials is of great importance to improve the efficiency of thermoelectric devices. Good thermoelectric materials are usually narrow band gap semiconductors with a large Seebeck coefficient, reasonably high electrical conductivity and low thermal conductivity. This thesis deals with the exploration of new thermoelectric materials based on transition metal tellurides and the optimization of bulk materials based on oxides of low toxicity and high stability in air. In the first project, seven new ternary or quaternary tellurides, crystallizing in three different structure types, were synthesized and characterized. Single crystal X-ray diffraction was used for crystal structure determination; powder X-ray diffraction and energy dispersive X-ray analysis (EDAX) were used for phase and composition analyses. Physical properties of these compounds were predicted by electronic structure calculations and confirmed by physical property measurements. In the second project, two series of n-type doped perovskite SrTiO3 were prepared in a high temperature tube furnace under dynamic high vacuum of the order of 10–6 mbar, namely SrTi1-x(Nb,Ta)xO3, and Sr1-xLaxTi1-x(Nb,Ta)xO3. The phase purity was characterized by means of powder X-ray diffraction and electron probe micro analysis (EPMA). Rietveld refinements were performed to check for purity and symmetry reduction. The physical properties, such as Seebeck coefficient, electrical conductivity, and thermal conductivity, were measured at high temperatures for all the samples. Of the series of Nb/Ta-doped strontium titanates SrTi1-x(Nb,Ta)xO3, SrTi0.90Ta0.10O3 exhibits the highest ZT value, namely 0.17 for at 752 K. Of the double substituted series, Sr0.99La0.01Ti0.99Ta0.01O3 was best with ZT = 0.13 at 660 K. The rapid increases imply that higher ZT values are likely to occur at higher temperatures.

Thermoelectric materials are the only known materials capable of direct conversion of a heat gradient into electricity (Seebeck effect) or vice-versa (Peltier effect). Thermoelectric (TE) devices are comprised of solid-state p-type and n-type semiconductors paired in an electrical circuit and exposed to a temperature gradient. The effectiveness of the materials is evaluated based on the mathematical term ZT=T∙S^2 σ/κ: S represents the Seebeck coefficient; σ represents the electrical conductivity; κ is the thermal conductivity; and T is the average of the coldest and hottest regions of the applied gradient. This ZT term is larger for better materials; most modern devices in use to-date display ZT values on the order of one. A large temperature gradient combined with a large Z term will lead to a high-performance TE material that involves no waste, no side product, and no requirement for moving parts. Discovery and optimization of new thermoelectric materials is a critical component of current thermoelectric research. As such, researchers are constantly searching for a new material that has the following properties: the ability to withstand higher temperatures, thus maximizing the T term; exhibit a large Seebeck coefficient and electrical conductivity through doping techniques; and present minimal thermal conductivity, κ. In recent years, research attention has moved from S^2σ to κ, which can be optimized through a variety of techniques including complex crystal structure, heavy element inclusion, and introduction of structural defects such as nanodomains/nanostructuring. Due to their tendency to form complex crystal structures and bonding, Te-based materials have become popular targets for TE research and optimization. Compounds with Te anions that also include other heavy elements such as alkali (A) metals, alkaline earth (R) elements, or heavy p-block elements including the triels (Tr), tetrels (Tt), or pnictogens (Pn) have become a principal source of new and ground-breaking thermoelectric materials. Likewise, optimization of existing TE materials with these aforementioned compositions has led to ZT values twice those of the materials' original reports. Of the known TE materials, Bi2Te3 is one of the staples in the field. It shows narrow band-gap semiconducting properties that can be tuned to p- or n-type values based on the impurities introduced, and its κ values are inherently low due to the presence of heavy elements and their structural layering motifs. A series of compounds, (SnTe)x(Bi2Te3)y, based on this idea can be produced via the alteration of x:y. In this work, several of these compounds are introduced and studied as potentially useful thermoelectric materials: SnBi2Te4, SnBi4Te7, and SnBi6Te10 are the major targets because of their systematic layering motifs and complex structures. Phase range studies, crystal structure (Rietveld) refinements, and synthesis optimizations were commenced to ensure that the materials were well-characterized and produced phase-pure before the attempted ZT improvements. By altering the quantity of active charge carriers in these systems, changes in ZT can be observed – this is achieved through doping with, primarily, heavy Tr elements Ga, In, and Tl. Thusly, the physical properties are measured and compared for a number of series: [Tr]xSn1-xBi2Te4, [Tr]xSnBi2-xTe4, [Tr]xSn1-xBi4Te7, [Tr]xSnBi4-xTe7, [Tr]xSn1 xBi6Te10, and [Tr]xSnBi6-xTe10. Of the triels, Tl is the largest useful element in the group and is known for showing both Tl+ and Tl3+ cationic states and, in thermoelectric applications, for possessing uniquely low κ values. Thallium telluride compounds such as Tl5Te3 are therefore quite relevant to this field. The family of compounds includes Tl9BiTe6 – one of the better materials with ZT = 1.2 (500 K) using a hot-pressed pellet. Herein, the system is expanded to include Tl10-xSnxTe6 which shows good TE potential with ZT(Tl7.8Sn2.2Te6) = 0.6 (617 K) with a cold-pressed pellet. The incorporation of tetrel elements is investigated through measurements on Tl10-x-ySnxBiyTe6 and also applies to the lesser-studied Tl9SbTe6 compound via research on the systems Tl9SnxSb1 xTe6 and Tl9PbxSb1 xTe6. Tl is studied in three concentrations with Tl10 x ySnxBiyTe6: Tl9…, Tl8.67…, and Tl8.33…, with varying Sn:Bi at each increment. Tt elements are systematically added to the Tl9[Tt]xSb1 xTe6 structure with 0.0 ≤ x ≤ 0.7. Crystallographic studies, electronic structure calculations, and physical properties are explored for each series. Due to Te’s ability to form complex Te–Te interactions in certain environments, the combination of alkaline earth metals, namely R = Ba, with the coinage metals (Cg = Cu, Ag), chalcogenides (Q = S, Se), and Te, form a plethora of previously unknown crystal structures. Many of these are Zintl-phase narrow-band gap semiconductors with complex Cg–Cg and Q–Q bonding schemes – combined with their heavy element incorporation, the family is of great interest to the thermoelectrics community. Within this thesis, three new crystal systems are presented: Ba3Cu17-x(Se,Te)11; Ba3Cu17-x(S,Te)11 and Ba3Cu17-x(S,Te)11.5; and Ba2Cu7-xTe6. All structures show Cu-deficiencies in their crystal structures with d10–d10 interactions and 3-dimensional networks of the Cg metal. The chalcogenide elements in the structures display unique Q–Q or Te–Te bonding of varying dimensionality. The electronic structures and bonding calculations are reported for each compound, as are the single crystal studies. The first two of the aforementioned compounds are narrow-band gap semiconductors, whereas the latter two display metallic behaviour.