Performance Enhancement in Bulk Thermoelectric Materials.

Thermoelectric energy conversion has great potential for both power generation and cooling; however, commercialization is currently limited by low energy conversion efficiency. This work explores several new methods to enhance performance in bulk thermoelectric materials. Theoretical calculations predict that Sn dopants create resonant energy levels in the valence band of the skutterudite CoSb3. This offers a potential means to improve thermoelectric power factor in p-type CoSb3, as such “bumps” in the electronic density of states have been shown to enhance the Seebeck coefficient without deteriorating electrical conductivity. Experimental and theoretical analysis in this work, however, shows that while Sn dopants improve thermoelectric efficiency in CoSb3 by reducing thermal conductivity, their solubility limit is insufficient to move the Fermi energy deep enough into the valence band to reach the resonant levels and thereby improve power factor. Incorporation of an additional p-type dopant (Fe) was studied as a means to move the Fermi energy further, but pinning by the heavy d-band in Fe was found to prevent access to the resonant levels. However, energy upshift of this band was found to provide an unrelated benefit for thermoelectric performance, as heavy holes contribute to a relatively high thermopower even at large carrier concentrations; the power factor was enhanced from 15 μWcm-1K-2 for Yb0.3FeCo3Sb12 to 25 μWcm-1K-2 for Yb0.8Fe3CoSb12. The engineering of secondary phases was also explored as a means to improve energy efficiency in bulk thermoelectric materials. In particular, a novel strategy for controlling carrier concentration was devised whereby different temperature dependences for the Fermi energy in the matrix and secondary phase enable optimization of matrix carrier concentration over a wide range of temperatures. An enhanced average power factor was demonstrated experimentally in GeTe-CuInTe2 composites. Finally, the influence of carrier band dispersion order was studied analytically and numerically to determine whether transport in higher order bands could lead to improvement in thermoelectric power factor. It was shown that higher-order band dispersion contributions to carrier transport could be beneficial under certain conditions due to enhancements arising from the energy dependence of the product of carrier mobility and density of states.PhDMechanical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/135915/1/huisi_1.pd