Thesis (M.S.)--Bowling Green State University, 2008. / Document formatted into pages; contains viii, 32 p. : ill. Includes bibliographical references.

Thesis (M.S.)--University of Wisconsin--Madison, 1964. / eContent provider-neutral record in process. Description based on print version record. Bibliography: l. 41-42.

Thesis (Ph. D.)--Northeastern University, 2008. / Title from title page (viewed May 21, 2009). Graduate School of Engineering, Dept. of Mechanical and Industrial Engineering. Includes bibliographical references (p. 184-186).

Thesis (M.S.)--University of Wisconsin--Madison, 1962. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaf 58).

Thermoelectric materials can provide solutions to power generation and refrigeration challenges. Layered chalchogenides are of particular interest, with bismuth telluride and lead telluride being the most common compounds. Bismuth telluride is often used for room temperature applications, while its solid solutions with antimony or selenium as well as lead tellurides show better thermoelectric properties at elevated temperatures. Regrettably, the efficiency of the known thermoelectric materials is still low. Evidently, bringing thermoelectric energy harvesting to commercial viability is a materials challenge: How can we obtain materials with figure of merit above 3? This question drives the research community since the successes of nanoengineering in the 1990s. Nowadays, high-pressure technology is a promising frontier for making further advances in thermoelectric material performance. The main goal of this thesis is to understand the electronic and thermoelectric properties of selected materials using density functional theory and semi-classical Boltzmann transport theory. Bulk and monolayer CuSbS2 and CuSbSe2 are studied to clarify the role of the interlayer coupling for the thermoelectric properties. The calculated band gaps of the bulk compounds turn out to be in agreement with experiments and significantly higher than those of the monolayers, which thus show lower Seebeck coefficients. Since also the electrical conductivity is lower, the monolayers are characterised by lower power factors. Therefore, the interlayer coupling, even though it is weak, is found to be essential for the thermoelectric response. We study Cu (Sb,Bi)(S,Se)2 under hydrostatic pressure up to 8 GPa, considering the van der Waals interaction, as these compounds have layered structures. We find an indirect band gap that decreases monotonically with increasing hydrostatic pressure. Only CuBiS2 shows an indirect-indirect band gap transition around 3 GPa, leading to conduction band convergence with a concomitant 20% increase in the Seebeck co-efficient. This enhancement results from a complex interplay between multivalley and multiband effects as well as changes of the band effective masses. The variation of the electronic band structure of AB2Te4 (A = Pb, Sn and B = Bi, Sb) under hydrostatic pressure up to 8 GPa is analyzed in detail and its consequences for the material properties are explained.

A test system was commissioned to characterize commercial thermoelectric modules to be used in the Pizza Oven Waste Energy Recovery (POWER) system for Pizza Pizza restaurants. The objective of this testing was to obtain and classify the thermoelectric material parameters of the Bi2Te3 within commercial thermoelectric generator (TEG) modules. These parameters consisted of the Seebeck coefficient, the thermal conductivity and the electrical resistivity. Together they provide the normalized figure of merit for the thermoelectric material which is a performance indicator for energy efficiency at a given operating temperature. From this research, a two phase methodology was developed that was able to extract the desired values from these modules. Material quality and device composition was first assessed with tools such as SEM and EDS. During this phase, dimensional and elemental data was gathered and a finite element model was constructed to ensure the validity of the primary selected test method for this research which was the Harman technique. The results obtained with this method were all three of the aforementioned thermoelectric parameters as well as a direct measurement of the figure of merit. Thermal and electrical losses for the TEG1B-12610-5.1 module were characterized from room temperature to 200°C using this process. It was determined that the thermal losses were more dominant and could be approximated using a function of T4 to within 1% of their calculated values. This process can be applied to any model of TEG to forecast these losses. To assist with future research, a secondary test method known as the Parallel Thermal Conductance technique was researched and a proposed model of it was designed for use in temperatures up to 300°C. Due to the relatively short test time of the Harman Technique, it was also used to effectively bin incoming groups of TEGs used in the POWER system so that they could be placed strategically in different areas of heat flow based on their measured performance. An increase of 13.2% was observed in the electrical output of the system after the binning had occurred. / Thesis / Master of Applied Science (MASc)

The goal of this project is to develop a fabrication process for an annular thermoelectric module using a powder methodology that can potentially later be automated for high volume manufacturing. Prototypes were produced and experimentally tested to study and characterize thermal and effective Seebeck performance. Manufacturing procedure parameters were changed systematically to characterize the impact on key performance parameters and develop the fabrication process. Parameters investigated were sintering temperature, pressing pressure, oxide reduction and geometry. A novel design for an annular thermoelectric generator geometry has been proposed. The new geometry utilizes more of the module material into power production making the geometry more efficient than the typical ring-structured modules similar to that proposed by Min & Rowe (2007). Experimental results tests highlighting only geometry differences showed V-shaped modules with higher effective Seebeck coefficient compared to ring-structured modules. Experimental results showed the proposed V-shaped annular thermoelectric generator prototype with a Seebeck coefficient of 190.75 µV/K compared to (Min & Rowe, 2007)’s earlier ring-structured prototype measuring a Seebeck coefficient of 145 µV/K. A numerical simulation model was created to compare electrical and thermal behaviour for different TEG module geometries. ANSYS Workbench® simulation results show that V-shaped TEG module outperforms the ring-structured design similar to Min et al.’s design by 7% to 9% under different conditions. / Thesis / Master of Science in Mechanical Engineering (MSME)

Thermoelectric devices (TEDs) are useful in a variety of niche applications, but low efficiencies limit their broader application. Semiconductor nanowires (NWs) could be the key to efficient thermoelectrics, through the benefits of one-dimensional band structures and a greatly reduced thermal conductivity. This thesis explores the transport fundamentals, experimental characterization, and computational approaches relevant to prospective III-V NW TEDs. Predictive electronic transport models are outlined for NWs and bulk III-Vs. These models are used to determine the optimum carrier concentration for maximizing the thermoelectric figure of merit (𝑍𝑇) in the bulk and in NWs of arbitrary size. We demonstrate the physical mechanisms underlying electronic thermoelectric improvements in NWs and confirm the superior performance of InSb and InAs, among other III-Vs. Next, thermal conductivity reduction in structurally complex NWs is investigated as a means of improving 𝑍𝑇. We compare polytypic and twinning superlattice (TSL) GaAs NWs in measurements obtained by a novel application of the 3𝜔 method. We find thermal conductivities of 8.4 ± 1.6 W/m-K and 5.2 ± 1.0 W/m-K for the polytypic and TSL NWs, respectively, demonstrating a significant difference and an almost ten-fold reduction compared to 50 W/m-K of bulk GaAs. We employ molecular dynamics simulations and the atomistic Green’s function method to address phonon engineering in generalized GaAs NW structures. In comparing twinning NWs, we find that a TSL period of 50 Å minimizes the lattice thermal conductivity across all the diameters considered. Our results also illustrate the importance of NW surfaces versus the internal crystal structure. Phonon coherence lengths are obtained by analyzing thermal conductivity trends in periodic and aperiodic structures. Transmission spectra are calculated to reveal the phonon frequencies targeted by structural engineering in NWs. These findings explain the range of thermal conductivities obtained for GaAs NWs with various crystal phases. Finally, to inform future growths of TSL NWs, we study the influence of the substrate temperature and V/III flux ratio on TSL formation in Te-doped GaAs NWs. The crystal structure of several NWs is investigated using transmission electron microscopy, revealing a range of polytypic and TSL morphologies. We find that periodic TSLs form only at low V/III flux ratios of 0.5 and substrate temperatures of 492 to 537 °C. To explain these trends, we derive a phase diagram for TSL NWs based on a kinetic growth model. / Thesis / Doctor of Philosophy (PhD) / In a circuit of dissimilar conductors, temperature differences create voltage differences that can drive electrical currents. Similarly, electrical currents in such circuits inherently lead to heating and cooling. These phenomena are known as thermoelectric effects because they couple heat and charge transport (electricity) in a symmetric and reversible way. The goal of some thermoelectric devices (TEDs) is to exploit these effects to generate electrical power or to provide controlled cooling. However, greater conversion efficiencies are required to compete against other existing technologies. With the advent of nanofabrication, semiconductor nanowires (NWs) have emerged as an attractive material system for efficient TEDs. In this thesis, we explore their thermal and electronic properties. We demonstrate a novel way to measure the NW thermal conductivity and employ computational methods to examine heat transport in NWs with various crystal structures. Finally, we examine how synthesis conditions can determine the morphology of NWs.

According to different sources, from forty to sixty percent of the overall energy generated in the world today is squandered in waste heat. The existing energy conversion technologies are either close to their efficiency limits or too costly to justify their implementation. Therefore, the development of new technological approaches for waste heat recovery is highly demanded. The field of thermoelectrics can potentially provide an inexpensive, clean and efficient solution to waste heat underutilization, given that a new type of thermoelectric materials capable of meeting those requirements are available. This thesis reports on strategies to optimize a thermoelectric efficiency (ZT) of conducting polymers, more specifically poly(3,4-ethylenedioxythiophene) (Pedot). Conducting polymers constitute a special class of semiconductors characterized by low thermal conductivity as well as electrical conductivity and thermopower that can be readily modified by doping in order to achieve the best combination of thermoelectric parameters. Conducting polymers that have never previously been regarded as hypothetically compatible for thermoelectric energy conversion, can exhibit promising thermoelectric performance at moderate temperatures, which is a sought-after quality for waste heat recovery. A rather substandard thermoelectric efficiency of Pedot-Pss can be markedly improved by various secondary dopants whose addition usually improves polymer’s morphology accompanied by a drastic increase in electrical conductivity and, consequently, in ZT. In order to enable further enhancement in thermoelectric properties, the optimization of the charge carrier concentration is commonly used. The oxidation level of Pedot-Pss can be precisely controlled by electrochemical doping resulting in a tenfold increase of ZT. In contrast to Pedot-Pss, another conducting polymer Pedot-Tos exhibits superior thermoelectric performance even without secondary doping owning to its partially crystalline nature that allows for an improved electronic conduction. With the aid of a strong electron donor, positively doped Pedot-Tos gets partially reduced reaching the optimum oxidation state at which its thermoelectric efficiency is just four times smaller than that of Be2Te3 and the highest among all stable conducting polymers. The downsides associated with chemical doping of Pedot-Tos such as doping inhomogeneity or chemical dopants air sensitivity can be surmounted if the doping level of Pedot-Tos is controlled by acidity/basicity of the polymer. This approach yields similar maximum thermoelectric efficiency but does not necessitate inert conditions for sample preparation. Optimized Pedot-Tos/Pedot-Pss can be functionalized as a p-type material in organic thermogenerators (OTEG) to power low energy electronic devices. If printed on large areas, OTEGs could be used as an alternative technique for capturing heat discarded by industrial processes, households, transportation sector or any natural heat sources for electricity production.

The goal of this research was to design, build and test a small burner and heat exchanger system that could be used as a source of heat for thermoelectric modules (TEMs) for the purpose of generating portable electric power for soldiers in the field. The project was conducted as a subcontract to Marlow Industries Inc. which was under contract from the U.S. Army. The scale of the burner thermal output was to be in the approximate range of 2 kW of heat production and it was to be able to operate on a liquid fuel, specifically JP8. The first burner investigated was a custom burner designed and built at UT. It was tested with various fuel and air delivery systems. Different methods to start it, with the goal of developing an electrical starting system, were also investigated. It was capable of operating at outputs over 1 kW, but was difficult to start reliably and fuel vaporization characteristics were sensitive to operating conditions. Two commercial burners were also studied, each with somewhat different designs. One of those burners, manufactured by MSR, was chosen to be further tested in conjunction with a heat exchanger and thermoelectric modules. The performance of the thermoelectric modules used in this study was determined to be very dependent on an attached resistive load, with a peak power output occurring at approximately 3 ohms. Power output was also determined to increase linearly with increasing temperature difference between the hot and cold sides of the module. Power output followed similar trends as open circuit voltage. The temperatures of the heat exchanger across its width were very uniform, but the accuracy in centering the heat exchanger over the burner could significantly affect temperatures. The time to reach steady state temperatures was relatively insensitive to the length of the heat exchanger. The presence of attached thermoelectric modules reduced the temperature of the heat exchangers and exhaust gas slightly. Reducing the heat exchanger length resulted in higher metal temperatures. Without cooling the cold side of the thermoelectric modules, performance increased while the system was heating up, but then dropped after reaching a peak. Cold side cooling improved thermoelectric performance by increasing its temperature difference. Active cooling with a blower and heat sink provided even better performance than passive cooling using just a heat sink at the expense of a larger parasitic load. The TEMs on the 5 inch long heat exchanger could generate 6.32 W with passive cooling, but active cooling would produce no net power. The 11 inch long heat exchanger could generate 12.8 W with passive cooling, and 16 W net could be generated with active cooling. A heat exchanger efficiency calculation showed that the 16, 11 and 5 inch long heat exchangers were about 94.4%, 93.4%, and 90.7% efficient respectively. This efficiency was defined as the ratio of the heat transferred to the heat exchanger to the heat released in the flame. / text