Showing papers by "Daryoosh Vashaee published in 2007"

Thermionic power generation at high temperatures using SiGe∕Si superlattices

Abstract: Recent studies have predicted that heterostructure superlattices can enhance the effective thermoelectric power factor significantly through selective emission of hot carriers via thermionic emission. Here, we study the potential of SiGe∕Si superlattices for power generation at high temperatures. A detailed theory based on Boltzmann transport equation is developed which takes into account multiple valleys. We show that thermionic emission provides only a modest improvement in the power factor. This is due to the fact that SiGe is a multivalley semiconductor and it has a large density of states. With reasonable dopings, Fermi energy in SiGe alloy is very close to the band minimum so that the symmetry of the differential conductivity does not change very much with small barrier superlattices. Particularly at high temperatures when the thermal spread of the carriers is much larger than the Fermi energy in the band, superlattice energy filtering is not effective.

Nanocomposites to Enhance Zt in Thermoelectrics

Abstract: The concept of using “self-assembled” and “force-engineered” nanostructures to enhance the thermoelectric figure of merit relative to bulk homogeneous and composite materials is presented in general terms. Specific application is made to the Si-Ge system for use in power generation at high temperature. The scientific advantages of the nanocomposite approach for the simultaneous increase in the power factor and decrease of the thermal conductivity are emphasized along with the practical advantages of having bulk samples for property measurements and a straightforward path to scale-up materials synthesis and integration of nanostructured materials into thermoelectric cooling and power generation devices.