Environmental challenges, such as global warming, growing demand on energy, and diminishing oil sources have accelerated research on alternative energy conversion methods. Thermoelectric power generation is a promising method to convert wasted heat energy into useful electrical energy form. A temperature gradient imposed on a thermoelectric device produces a Seebeck potential. However, this temperature gradient causes thermal stresses due to differential thermal expansions and mismatching of the bonded components of the device. Thermal stresses are critical for thermoelectric devices since they can generate failures, including dislocations, cracks, fatigue fractures, and even breakdown of the entire device. Decreases in power-generation performance and operation lifetime are major consequences of these failures. In order to minimize thermal stresses in the legs without affecting power-generation capabilities, this study concentrates on structural solutions. Thermoelectric devices with non-segmented and segmented legs were modeled. Specifically, the possible effect of various leg geometries, configurations, and dimensions were evaluated using finite-element and statistical methods. Significant changes in the magnitudes and distributions of thermal stresses occurred. Specifically, the maximum equivalent stresses in the rectangular-prism and cylindrical legs were 49.9 MPa and 43.3 MPa, respectively for the temperature gradient of 100ºC. By using cylindrical legs with modified dimensions, decreases in the maximum stresses in legs reached 21.2% without affecting power-generation performance. Moreover, the effect of leg dimensions and coaxial-leg configurations on power generation was significant; in contrast, various leg geometries and rotated-leg configurations had very limited affect. In particular, it was possible to increase power output from 20 mW to 65 mW by simply modifying leg widths and heights within the defined range. It should be noted, however, this modification also increased stress levels. It is concluded that leg geometries, configurations, and dimensions can be redesigned for improved durability and overall performance of thermoelectric devices.

An experimental investigation of the electronic transport properties of bismuth telluride nanocomposite materials is presented. The primary transport measurements are electrical conductivity, Seebeck coefficient and Hall effect. An experimental apparatus for measuring Hall effect and electrical conductivity was designed, constructed and tested. Seebeck coefficient measurements were performed on a commercial instrument. The Hall effect and Seebeck coefficient measurements are two of the most important tools for characterizing thermoelectric materials and are widely used in the semiconductor industry for determining carrier types, carrier concentration and mobility. Further, these transport parameters are used to determine the thermal to electrical conversion efficiency of a thermoelectric material. The Boltzmann transport equation was used to analyze the Seebeck coefficient, carrier mobility and electrical conductivity as a function of carrier concentration for eleven samples. The relationship between the electronic transport and material/composite composition is discussed.

Anisotropic thermoelectric effects can be measured in certain materials. Anisotropy can also be simulated using a repeated, layered structure of two materials cut at an angle. Various aspect ratios and angles of inclination are investigated in device geometry in order to maximize the thermopower. Eddy currents have been shown to occur in thermoelectric devices, and evidence of these currents are revealed in finite element analysis of the artificially synthesized anisotropic Peltier effect.

The rapid increase in thermoelectric (TE) materials R&D is a consequence of the growing need to increase energy efficiency and independence through waste heat recovery. TE materials enable the direct solid-state conversion of heat into electricity, with little maintenance, noise, or cost. In addition, these compact devices can be incorporated into existing technologies to increase the overall operating efficiency. High efficiency TE materials would enable the practical solid-state conversion of thermal to electrical energy. Optimizing the interdependent physical parameters to achieve acceptable efficiencies requires materials exhibiting a unique combination of properties. This research reports two methods of thermoelectric enhancement: lattice strain effects in silicon-germanium alloy type I clathrates and the nanostructured enhancement of lead chalcogenides. The synthesis and chemical, structural, and transport properties characterization of Ba8Ga16SixGe30-x type I clathrates with similar Ga-to-group IV element ratios but with increasing Si substitution (4 < x < 14) is reported. Substitution of Si within the Ga-Ge lattice framework of the type I clathrate Ba8Ga16Ge30 results in thermoelectric performance enhancement. The unique dependences of carrier concentration, electrical resistivity, Seebeck coefficient, and carrier effective mass on Si substitution level, may imply a modified band structure with Si substitution. These materials were then further optimized by adjusting the Ga-to-group IV element ratios. Recent progress in a number of higher efficiency TE materials can be attributed to nanoscale enhancement. Many of these materials demonstrate increased Seebeck coefficient and decreased thermal conductivity due to the phenomenological properties of nanometer length scales. To satisfy the demands of bulk industrial applications requires additional synthesis techniques to incorporate nanostructure directly within a bulk matrix. This research investigates, for the first time, dense dimensional nanocomposites prepared by densifying nanocrystals synthesized employing a solution-phase reaction. Furthermore, the carrier concentration of the PbTe nanocomposites can be adjusted by directly doping the nanocrystals, necessary for power factor optimization. These materials were fully characterized using a low temperature TE transport measurement system, and exhibit enhanced power factors when compared to bulk polycrystalline PbTe with similar carrier concentrations.

Thermoelectric phenomena couple thermal and electric currents, allowing for solid-state conversion of heat into electricity. For decades Radioisotope Thermoelectric Generators have supplied power to NASA satellites and deep space probes. A more accessible application to consumers is the automotive industry's aspiration to incorporate thermoelectrics into active waste heat recovery systems. Higher power demands require these new thermoelectric devices to operate at higher temperatures and higher efficiencies, justifying new materials research. Recently, clathrates have gained interest for thermoelectric applications due to the unique properties they possess.These properties are directly related to their crystal structure. Therefore, clathrates are not only of interest from the standpoint of potential thermoelectric applications but are also of scientific interest as they presents an opportunity to investigate fundamental properties of group-IV elements in novel crystal structures. Clathrates are a class of novel open-structured materials in which molecules or atoms of one species are completely enclosed within a framework comprised of another species. This work presents a systematic investigation of the electrical properties of type I clathrate alloys, specifically Si-Ge alloys, for the first time. A series of Ba8Ga16-ySixGe30-x+y clathrates with varying Si content were synthesized and their structural and transport properties were studied. Two additional series of type I clathrates were also synthesized and characterized and their properties compared to those of the Si-Ge alloys in order to develop an understanding of their structure-property relationships. The increasing Si content correlates to a dramatic increase in Seebeck coefficient even as the resistivity decreases, suggesting the complex interaction between the Ba and the Si substitution within the Ga16Ge30 framework significantly modifies the band structure.

In recent years, Spin Seebeck effect (SSE) emerges as one of the efficient and easiest ways to generate pure spin current for spintronics devices. In this dissertation, we have systematically studied the SSE and related phenomena like spin Hall magneto-resistance (SMR), anomalous Nernst effect (ANE) in functional magnetic oxides for both fundamental understanding of their origins and practical ways to apply into technological devices. The research has been performed on three different systems of topical interest: (i) Y3Fe5O12 (YIG)/Pt and YIG/C60/Pt, (ii) CoFe2O4 (CFO)/Pt and CFO/C60/Pt, and (iii) Nd0.6Sr0.4MnO3 (NSMO). In case of the YIG/Pt structure, we have achieved a new consensus regarding the temperature dependence of the longitudinal SSE (LSSE). For the first time, we have demonstrated the temperature dependence of LSSE in association with the magnetocrystalline anisotropy (HK) and surface perpendicular magnetic anisotropy field (HKS) of YIG in the same YIG/Pt system. We show that on lowering temperature, the sharp drop in LSSE signal (VLSSE) and the sudden increases in HK and HKS at ~175 K are associated with the spin reorientation due to single ion anisotropy of Fe2+ ions. The VLSSE peak at ~75 K is attributed to the HKS and MS (saturation magnetization) whose peaks also occur at the same temperature. The effects of surface and bulk magnetic anisotropies are corroborated with those of thermally excited magnon number and magnon propagation length to satisfactorily explain the temperature dependence of LSSE in the Pt/YIG system. As a new way to reduce conductivity mismatch, promote spin transport, and tune the spin mixing conductance (G) at the YIG/Pt interface, we have deposited an organic semiconductor (OSC), C60, between ferrimagnetic material (FM) and Pt. Transverse susceptibility study on YIG/C60/Pt has shown that the deposition of C60 has reduced HKS at the surface of YIG significantly, due to the hybridization between the dz2 orbital in Fe and C atoms, leading to the overall increase in spin moments and G and consequently the LSSE. Upon lowering temperature from 300 K, we have observed an exponential increase in LSSE at low temperature (a ~800% increment at 150 K) in this system, which is attributed to the exponential increase in the spin diffusion length of C60 at low temperature. On the other hand, similar experiments on CoFe2O4 (CFO)/C60/Pt show a reduction in the LSSE signal at room temperature, due to the hybridization between the dz2 orbital in Co and C atoms that results in the increased magnetic anisotropy. Upon decreasing the temperature below 150 K, we have interestingly observed that LSSE signal from CFO/C60/Pt exceeds that of CFO/Pt and increases remarkably with temperature. This finding confirms the important role played by the spin diffusion length of C60 in enhancing the LSSE. A systematic study of SMR, SSE, and HKS on the YIG/Pt system using the same YIG single crystal has revealed a low-temperature peak at the same temperature (~75 K) for all the phenomena. Given the distinct origins of the SSE and SMR, our observation points to the difference in spin states between the bulk and surface of YIG as the main reason for such a low-temperature peak, and suggests that the ‘magnon phonon drag’ theory developed to explain the temperature-dependent SSE behavior should be adjusted to include this important effect. SSE and ANE studies on NSMO films have revealed the dominance of ANE over SSE in this class of perovskite-structured materials. The substrate-dependent study of the films shows that compressive strain developed due to the large lattice mismatch from LAO gives rise to the enhanced ANE signal. On the same substrate, ANE signal strength increases as the thickness increases. A sign change in ANE has been observed at a particular temperature, which explains that the Anomalous Hall effect (AHE) and ANE in these systems arise due to intrinsic scattering mechanisms. Overall, we have performed the SSE and related studies on the three important classes of functional magnetic oxide materials. We demonstrate the important role of magnetic anisotropy in manipulating the SSE in these systems. With this knowledge, we have been able to design the novel YIG/C60/Pt and CFO/C60/Pt heterostructures that exhibit the giant SSEs. The organic semiconductor C60 has been explored for the first time as a means of controlling pure spin current in inorganic magnetic oxide/metal heterostructures, paying the way for future spintronic materials and devices.

<p>The demand for more accurate measurements is increasing in today’s industry. One reason for this is to optimize production and thus maximize profits. Another reason is that in some cases government regulations dictate that supervision of certain parameters must be followed. At Preemraff Lysekil there are basically four reasons for measuring skin temperatures inside fired process heaters, namely; because of government regulations, in order to estimate the load of the fired process heater, to estimate the lifetime of the tubes inside the fired process heater and finally, to determine the need of decoking. However, only the first three of these reasons are applied to H2301/2/3. The current skin thermocouple design has been in use for many years and now the question of how well it measures surface temperature has risen. Furthermore a new weld-free design is under consideration to replace the old skin thermocouple design. Another question is therefore how well the new design can measure the surface temperature under the same operating conditions as the old one. In order to evaluate this, three–dimensional computer simulations were made of the different designs. As this thesis will show, the differences in calculated skin thermocouple temperature and calculated surface temperature is about the same for the two designs. However, the current design will show a lower temperature than the surface temperature, while the new design will show a higher temperature. Regarding the core of the skin thermocouple designs, namely the thermocouple, no hard conclusions can be drawn, although the industry appears to favor type ’N’ over type ’K’.</p>

Irreversible thermodynamics is a widely-applicable toolset that extends thermodynamics to describe systems undergoing irreversible processes. It is particularly useful for describing macroscopic flow of system components, whether conserved (e.g., particle number) or non-conserved (e.g., spin). We give a general introduction to this toolset and calculate the entropy production due to bulk and interfacial flow. We compare the entropy production and heating rate of bulk and interfacial transport, as well as interfacial charge and spin transport. We then demonstrate the power and applicability of this toolset by applying it to three systems. We first consider metal oxide growth, and discuss inconsistency in previous theory by Mott. We show, however, that Mott's solution is the lowest order of a consistent asymptotic solution, with the ion and electron concentrations and fluxes going as power series in t^-k/2, where k = 1, 2, .... We find that this gives corrections to the "parabolic growth law" that has oxide thickness going as t^1/2; the lowest order correction is logarithmic in t. We then consider the effect on spin of electric currents crossing an interface between a ferromagnet (FM) and non-magnetic material (NM). Previous theories for electrical potential and spin accumulation neglect chemical or magnetic contributions to the energy. We apply irreversible thermodynamics to show that both contributions are pivotal in predicting the spin accumulation, particularly in the NM. We also show that charge screening, not considered in previous theories, causes spin accumulation in the FM, which may be important in ferromagnetic semiconductors. Finally, we apply irreversible thermodynamics to thermal equilibration in a thin-film FM on a substrate. Recent experiments suggest that applying a thermal gradient across the length of the system causes a spin current along the thickness; this spin current is present much farther from the heat sources than expected. We find that, although the interaction between the separate thermal equilibration processes increases the largest equilibration length, thermal equilibration does not predict a length as large as the experimentally measured length; it does predict, however, a thermal gradient along the thickness that has the shape of the measured spin current.

Thermoelectric devices are capable of providing both localized active cooling and waste heat power generation. This work will explore the possibility of embedding thermoelectric devices within electronic packaging in order to achieve better system performance. Intel and Nextreme, Inc. have produced thin-film superlattice thermoelectric devices that have above average performance for thermoelectrics and are much thinner than most devices on the market currently. This allows them to be packaged inside of the electronic package where the thermoelectric devices can take advantage of the increased temperatures and decreased thermal lag as compared to the devices being planted on the outside of the package. This work uses the numerical CFD solver FLUENT and the analog electronic circuit simulator SPICE to simulate activity of thermoelectric devices within an electronics package.

No description available.