Optimization of Thermoelectric Modules and Thermal Transport in Thin Film Semiconductors
The dissertation covers two research topics: optimization of thermoelectric devices for hotspot microcoolers and power generators applications, and study of ballistic phonon transport and superdiffusion in semiconductor alloys and superlattices. Minimizing cost and maximizing coefficient-of-performance (COP) are key factors in the design of thermoelectric modules. An analytic formulation is developed for thermoelectric (TE) devices for hotspot cooling and for power generation. A design constraint with a constant heat source temperature, maximizing the pumping power is considered. A systematic analysis is performed on the impact of external thermal resistances, the geometry of thermoelectric elements, and the material properties. In addition to hotspot micro-cooling, topping cycle thermoelectric generators are studied. Theoretical calculations show that solid-state TE modules on top of steam turbine cycle can generate significant amount of power with relatively low cost without the need for breakthrough in thermoelectric material’s figure-of-merit. Reducing thermal parasitic losses, the TE system can be designed to maximize output power while maintaining adiabatic fuel combustion temperature and the high pressure steam temperature. TE topping cycle reduces large amount of thermodynamic or exergy losses in fuel burning steam turbines. TE materials for high temperature operation can be optimized to maximize the overall energy production while keeping the fuel consumption constant. Furthermore a new TE module structure is analyzed, where thermoelectric generator is sandwiched with inexpensive insulator, polyamide as the gap fill materials. We study the impact of gap fill materials and thermal parasitic on the performance of wearable film-based thermoelectric power generators. Studies of nanoscale thermal transport are essential to optimize nanostructured thermoelectric materials as well as in the thermal management of electronic devices. Femtosecond laser, time-domain thermoreflectance (TDTR) is used to investigate thermal transport in high-quality Al0.1Ga0.9N thin films. Nitride alloys are important in power electronic device applications. The measured apparent thermal conductivity at room temperature is reduced by 30% when the laser modulation frequency is increased from 0.8MHz to 10MHz. This is due to the thermal penetration length approaching the mean-free-path of dominant phonons in thermal transport. Tempered Lévy analysis is used to study superdiffusion in Al0.1Ga0.9N thin films. A fractal random walk with an exponent α< 2 is obtained at room temperature which is an indication of quasi-ballistic heat transport. TDTR is also used to study thermal transport in nanoparticles embedded GaSb thin film. GaSb is one of the few non-alloy semiconductors, which shows a phonons ballistic effect in the thermal conductivity. The apparent cross-plane thermal conductivity of pure GaSb sample drops ~15% when the pump laser modulation frequency is increased from 0.8 MHz to 10 MHz at room temperature. However, the frequency dependence of the cross-plane thermal conductivity disappears if GaSb sample is embedded with 3%-20% ErSb nanoparticles. A strong anisotropic thermal transport can be observed in GaSb thin film. The ratio of in- to corss-plane thermal conductivity varies from ~0.2 to ~0.7 in GaSb with 0 to 20% ErSb nanoparticles volume concentrations. Temperature dependence of anisotropic thermal conductivity of ErSb: GaSb samples are described in the thesis. TDTR is also used to study heat transport in nanostructured metal/semiconductor superlattices. Detailed experimental characterization of (Ti,W)N/(Al,Sc)N metal/semiconductor superlattices as a function of layer thicknesses, number of layers and alloying in the metal or in the semiconductor layer show a distinct minimum in thermal conductivity as a function of superlattice period. Basic questions about heat transport in multilayer materials taking into account both lattice and electronic thermal contributions and achieving low thermal conductivity below the alloy limit are discussed.
Shakouri, Purdue University.
Electrical engineering|Mechanical engineering|Thermodynamics|Materials science
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