MS, University of Cincinnati, 2024, Engineering and Applied Science: Mechanical Engineering

The underlying physics principles governing thermoelectricity (TE) were uncovered in the nineteenth century, laying the foundation for the development of TE devices. These devices, which convert temperature differences directly into electrical voltage or vice versa, have found diverse applications in the aerospace, medical, and sensor industries. Their solid-state nature offers significant advantages, including a longer lifespan and reduced maintenance requirements compared to mechanical systems. Despite these advantages, thermoelectric coolers (TECs) have a lower coefficient of performance (COP) compared to other cooling technologies, and thermoelectric generators (TEGs) exhibit relatively low efficiency. These limitations have affected their widespread adoption and confined their use to specific areas where they can be effectively utilized. Efforts to overcome the drawbacks of TE have mostly focused on optimizing individual variables while keeping other parameters fixed. However, the TE field lacks comprehensive optimization techniques. Nonlinear programming, a form of constraint optimization developed in the mid-twentieth century, could address this issue but has not been widely adopted. My goal is to bridge this gap. In my project, I propose an optimization algorithm that includes a three-step methodology to implement nonlinear programming as a technique for optimizing TE devices. The implementation of this methodology is detailed through a case study using an air-to-air thermoelectric air conditioner (TEAC) that operates with two thermoelectric modules (TEMs). Key parameters in the TEM are varied simultaneously in order to achieve a TEAC with an optimized COP without compromising its cooling capacity. The last part of this section is dedicated to analyzing the metrics of the optimized TEMs and uncovering the reasons behind their superior COP.

Committee: Sarah Watzman Ph.D. (Committee Chair); Milind Jog Ph.D. (Committee Member); Kishan Bellur Ph.D. (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy, The Ohio State University, 1977, Graduate School

Committee: Not Provided (Other) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 1976, Graduate School

Committee: Not Provided (Other) Subjects: Physics

Doctor of Philosophy, The Ohio State University, 1964, Graduate School

Committee: Not Provided (Other) Subjects: Physics

MS, University of Cincinnati, 2015, Engineering and Applied Science: Aerospace Engineering

Thermoelectric techniques are sensitive to the slightest variations in material properties regardless of the surface finish, shape and size of the material to be tested. This makes it a very promising candidate to characterize materials in scenarios where the changes in the material properties are so small that they cannot be measured otherwise because of variations in size and shape. Unfortunately, inherent measurement errors caused by the imperfect interfacial contact between the specimen and the hot electrode in the widely used two-point contact TEP measurements adversely affects the accuracy and call for the development of an improved TEP measurement technique for reliable material characterization. Permanently fixed electrodes yield the best results and are most suitable for long term structural health monitoring. In this work, a novel three-point measurement technique is introduced for reliable thermoelectric power measurement. The values of the absolute TEP obtained using the deployable three-point configuration is compared with the permanently attached configuration to determine the accuracy of the measurement. Finally, a computational model is developed to study the effect of the imperfect interfacial layer on the three-point measurement technique and better understand the obtained experimental results.

Committee: Peter Nagy Ph.D. (Committee Chair); San-Mou Jeng Ph.D. (Committee Member); Francesco Simonetti Ph.D. (Committee Member) Subjects: Aerospace Materials

Doctor of Philosophy, University of Akron, 2015, Mechanical Engineering

Thermoelectric materials exhibit a significant coupling between thermal and electrical transport. Devices made from thermoelectric materials can convert between thermal and electrical energy. System reliability is extremely high but widespread use of the technology is hindered by low conversion efficiency. To increase the practicality of thermoelectric devices improvements are required in both (i) device design and (ii) thermoelectric materials. Advanced thermoelectric analysis developed in this work provides general guidelines for device design by introducing a new set of design factors. The new analytic factors include Device Design Factor, Fin Factor, Inductance Factor, and Thermal Diffusivity Factor. The advanced analysis is applied to two material systems developed in this work. The first system investigated was a composite of WSi2 precipitates in a Si/Ge matrix. The composite was investigated through both solidification techniques and powder processing. The system has a 30% higher figure of merit, a material parameter relating to conversion efficiency, than traditional zone-leveled Si/Ge. The second system investigated was a novel quaternary CoxNi4-xSb12-ySny skutterudite. The system was found to achieve both n- and p-type conduction with tuning of the Co level.

Committee: Celal Batur Dr. (Advisor); Alp Sehirlioglu Dr. (Committee Member); Minel Braun Dr. (Committee Member); Guo-Xiang Wang Dr. (Committee Member); Jerry Young Dr. (Committee Member); Bob Viellette Dr. (Committee Member) Subjects: Aerospace Materials; Materials Science; Mechanical Engineering

Doctor of Philosophy, The Ohio State University, 2015, Mechanical Engineering

Thermoelectric effects can be used for fast response temperature sensing, active solid state cooling, and solid state electricity generation from heat. It is favorable to have a material with large thermopower alpha, high electrical conductivity sigma, and low thermal conductivity kappa. The criterion for good thermoelectric materials that have a high energy conversion efficiency is the dimensionless thermoelectric figure of merit, ZT=(alpha^2 sigma)/kappa T. A ZT value of 1 is the current commercialization criterion. Typically, good thermoelectric materials are semiconductors with large electron effective masses. Doping is a common way to manipulate carrier concentration within a semiconductor class to optimize thermoelectric properties. Bi2Te3 is the paradigm for the class of (Bi1-xSbx)2(Te1-ySey)3 thermoelectric alloys that have been successfully commercialized. The ZT values for this class are of order 1. Recently, theorists have predicted that p-type Bi2Se3 could have comparable ZT to the Bi2Te3, but without the expensive element tellurium. In this dissertation, p-type Bi2Se3 was prepared by Bi substitution doping with Ca, Mn, and Pb. Then two independent experimental methods, the transport properties and the Shubnikov–de Hass measurements, were used to establish the effective mass of Bi2Se3. The results show that the effective mass of the Bi2Se3 valence band is lower than theoreticians have calculated. This work represents the first experimental study of the valence band of Bi2Se3, and concludes that p-type Bi2Se3 does not have a large enough effective mass to have good thermoelectric properties. In the second part of this dissertation, thermoelectric metallic alloys are discussed. Metals are the materials used in temperature-sensing, thermocouple applications. An accurate thermocouple requires metal alloys with large thermopower. Copper-nickel alloys have been widely used as the n-type leg of thermocouples under the name “constantan” due to their larg (open full item for complete abstract)

Committee: Joseph P. Heremans (Advisor); Roberto C. Myers (Committee Member); Sandip Mazumder (Committee Member); William Rich (Committee Member) Subjects: Materials Science; Mechanical Engineering

Doctor of Philosophy, The Ohio State University, 2014, Mechanical Engineering

This dissertation consists of three interrelated research themes that encompass the engineering and science of thermal material properties. The primary goal is to contribute to solving the world's energy crisis through solid-state conversion of waste heat into usable electrical energy. To achieve this goal, it is necessary to extend our understanding of underlying physics of different heat carriers in solids: electrons, phonons, and magnons. The three research subjects presented here attempt to address this challenge. The first subject is the development of thermoelectric materials for cryogenic cooling applications, including solid-state cooling for far infrared, x-ray, and gamma-ray detectors in space applications. Bismuth and bismuth-antimony alloys were chosen as the materials of study. We aim to improve the thermoelectric efficiency (i.e. figure of merit) of these material systems through the development of new doping approaches and other mechanisms. The second part of the dissertation focuses on thermal conduction and the spin-Seebeck effect in ferromagnetic metallic glasses (Metglas). In the last portion, we provide evidence for a new property of phonons, namely, diamagnetism in phonons, which hints at a new approach for magnetically manipulating phonon heat transfer in materials.

Committee: Joseph Heremans Dr. (Advisor); Walter Lempert Dr. (Committee Member); Vish Subramaniam Dr. (Committee Member); Roberto Myers Dr. (Committee Member) Subjects: Condensed Matter Physics; Materials Science; Mechanical Engineering

Doctor of Philosophy, The Ohio State University, 2012, Mechanical Engineering

The dimensionless thermoelectric figure of merit, zT, is used to characterize the conversion efficiency of thermoelectric materials. In this dissertation, we include experimental results on new p-type semiconducting alloys based on lead telluride that have higher zT values than historical materials. Through alloying PbTe:Tl with sulfur, we demonstrate an increase in zT over the parent material PbTe:Tl. Next, we remove the toxic element Tl from the PbTe/PbS alloy and retain the high efficiency via doping heavy valence band in PbTe, a separate mechanism than the high-zT resonant level doping achieved by the impurity Tl. We present experimental evidence relevant to the valence band structure of PbTe alloys at elevated temperature and demonstrate that these alloys remain direct gap semiconductors at temperatures relevant to automotive thermoelectric waste heat recovery (<850K). Secondly, we report the first confirmation measurement of a new effect – the spin-Seebeck effect – in thin films of GaMnAs, of work by researchers at Tohoku University two years prior on NiFe. The spin-Seebeck effect is a thermally driven spin distribution – the spin analog to the charge-Seebeck effect, and is measured using the inverse spin Hall effect in platinum transducers that are attached to the spin-polarized material. We report extensive measurements over temperature and at various positions along the sample. We show that this effect is phonon driven, and that a phonon-magnon drag is capable of enhancing the magnitude of this effect, much like phonon-electron drag in charge Seebeck. Lastly, by using a non-magnetic material with large spin orbit interaction, we show the magnitude of this effect can reach the order of mV/K, whereas in ferromagnets it is order µV/K. Here, external magnetic field generates the necessary spin splitting. The discovery of this effect may allow for solid state heat engines based off spin, as an analog to thermoelectricity heat engines.

Committee: Joseph Heremans PhD (Advisor) Subjects: Condensed Matter Physics; Mechanical Engineering

Master of Science, The Ohio State University, 2010, Mechanical Engineering

As demands for energy increase throughout the world, the desire to create energy efficient technologies has emerged. While the field thermoelectricity has been around for well over a century, it is becoming increasingly popular, especially for automotive applications, as new and more efficient materials are discovered. Thermoelectricity is a technology in which a temperature difference can be applied to create a potential difference for the application of waste heat recovery or a potential difference can be used to create a temperature difference for heating and cooling applications. Materials used in thermoelectric devices are semiconductors with high Figure of Merit, zT. The dimensionless thermoelectric Figure of Merit is a function of Seebeck coefficient, S, electrical resistivity, ρ, and thermal conductivity, κ. Experimental testing is used to determine the properties of these materials for optimized zT. This thesis covers a new class of thermoelectric semiconductors based on rocksalt I-V-VI2 compounds, which intrinsically possess a lattice thermal conductivity at the amorphous limit. It has been shown experimentally that AgSbTe2, when optimally doped, reaches a zT=1.2 at 410 K.3 Unfortunately, there is a metallurgical phase transition at 417 K (144 °C). The phase transition at 417 K was identified to be a potential problem in thermal cycling, and is expected with all heavy chalcogenides of group Ib elements. This issue has to be resolved before I-V-VI2 can be considered practical. After examination, two routes to avoid the phase transition were planned: (1) alloying AgSbTe2 with Na, and (2) using off-stoichiometry formulations Ag1-xSb1+yTe2. Experimental results show that with increased sodium concentration, resistivity increases substantially, causing this method to be impractical for optimizing zT, however it did eliminate the phase transition at 417K. Using off-stoichiometric silver antimony telluride shows promising results with high S, on the order of 25 (open full item for complete abstract)

Committee: Joseph Heremans PhD (Advisor); Walter Lempert (Committee Member) Subjects: Materials Science; Mechanical Engineering

Master of Science, The Ohio State University, 2008, Mechanical Engineering

With the global interest in cutting back dependence on non-renewable energy sources, the field of thermoelectricity has recently seen renewed interest. Additionally, several exciting breakthroughs in both nanoscale and bulk materials have led to increased figure of merit zT. Figure of merit is a indirect measurement of a material's thermoelectric efficiency; its ability to convert a thermal heat flux to electrical power, and vice versa. Material zT has been limited to approximately 1 since the 1950's; in the past decade there have been several reports of zT>1.4. The major drawback for the development of thermoelectric devices is historically low material efficiency. However, it has been predicted that only a zT=2 is necessary to reach widespread usage of thermoelectric materials. A high zT material requires high thermopower, low electrical resistivity, and low thermal conductivity. These three material parameters are all interrelated. Therefore, recent research has focused on decoupling these parameters from each other. In this thesis, the underlying theory that is necessary for characterizing thermoelectric materials is outlined. We investigate the zT increase in the material system PbTe-PbS that was achieved through nanoprecipitation of PbS rich regions in a bulk PbTe matrix. After performing a full galvanomagnetic and thermomagnetic characterization where electrical resistivity, Seebeck, Hall, and Nernst-Ettingshausen coefficients are measured we calculate Fermi level, carrier effective mass, and scattering parameter. The introduction of the second phase of PbS in PbTe does not lower the product S2σ but is reported to reduce thermal conductivity. It is this decrease in thermal conductivity that leads to a gain in zT. The second system studied is Bi2Te3:Sn. It has been reported by Kulbachinskii that Sn possibly forms a resonant level in Bi2Te3, and in this thesis, we investigate this claim further. In order to do so, we calculate a Seebeck coefficient vs. carri (open full item for complete abstract)

Committee: Joseph Heremans PhD (Advisor); Yann Guezennec PhD (Committee Member) Subjects: Mechanical Engineering

Doctor of Philosophy in Engineering, University of Toledo, 2013, College of Engineering

This research focused on the formation mechanism of TiO2 nanotubes on pure Ti foil and the development and improvement in performance of thermoelectric multi-components. For the formation mechanism, based on our experiments and observations, oxygen formed on the anode determines the final dimension of the TiO2 nanotubes. The length of the TiO2 nanotubes achieved was 15 µm in the electrolyte containing ethylene glycol and water (98:2 vol. %) + 0.3 wt. % NH4F for 24 hours. Bent anode was employed to show that there were no nanotubes formed on the bent part. Different anodization times were used to examine the action of fluorine ions. We also used different types of Ti foils, cold rolled and hot-rolled, to evaluate the effect of preprocess condition on the oxygen formation at their surfaces. Electrochemically and chemically treated Ti foils with exposed grain boundaries were used to reveal that the nanotubes grow along the grains of the Ti substrate. Finally, a dissolution model was established to calculate the dissolved TiO2 mass. The primary strategy to improve the performance of thermoelectric materials was employing low-dimensional materials to reduce the lattice thermal conductivity as described by the Wiedemann-Franz law. Rattling structures, point defects, vacancies and multi-components were used to efficiently scatter phonons within or between the unit cell crystals. And complex crystalline structures were used to decouple the electrical conductivity and thermal conductivity to achieve this goal. Based on such considerations, we developed TiO2 nanotubes/polyaniline, TiO2 nanotubes/Te-Bi-Pb nanoparticles and TiO2 nanotubes/CoO coaxial nanocables. Firstly, TiO2 nanotubes/polyaniline (PANI) multi-components were synthesized. The experiments of how the time, voltage, concentration of F- ions and concentration of H3PO4 were associated with the formation of TiO2 nanotubes were conducted. The formation of polyaniline was confirmed by both Raman Spectroscopy (open full item for complete abstract)

Committee: Yong X. Gan Dr. (Advisor); Maria Coleman Dr. (Committee Member); Matthew Franchetti Dr. (Committee Member); Joseph Lawrence Dr. (Committee Member); Arunan Nadarajah Dr. (Committee Member) Subjects: Energy; Engineering; Materials Science