Massachusetts Institute of Technology
Thermoelectrics can directly convert heat into electricity and therefore have applications in waste heat recovery. These solid-state devices can also be integrated directly on chips and actively cool down hot spots of high-speed devices. In this talk, I will discuss modeling of electron and phonon transport inside thermoelectric legs to identify fundamental length scales such as carrier mean free path and momentum and energy relaxation lengths. Knowing the fundamental length scales, we can design nanostructured materials with enhanced hermoelectric figure of merit (Z=σS2/ K). I will discuss strategies to reduce the thermal conductivity via introducing interfaces and rattling atoms to scatter phonons, to increase the electrical conductivity by means of modulation doping and to improve the Seebeck coefficient by energy filtering and introducing sharp features in the density of states. In each strategy the challenge is to improve one property without deteriorating the other properties. We have fabricated and characterized bulk samples as well as superlattices, which were designed based on different strategies. The obtained experimental results are in agreement with theoretical predictions but there is still a lot of room for improvement in terms of materials designing. Finally, I will address the issue of heat management. By using a Monte Carlo algorithm, we have identified the energy relaxation length and the location of Peltier cooling/heating at heterointerfaces. We have also explored the nonlinearity of heat current when the applied electric field is strong and electrons are out of equilibrium with phonons. The nonlinearity of thermoelectric transport coefficients could be used to enhance the device performance significantly especially at low temperatures.
Location: Physics Bldg., Room 401