Nanoengineered Materials for Thermoelectric Energy Conversion

In this chapter we review recent advances in nanoengineered materials for thermoelectric energy conversion. We start by a brief overview of the fundamental interactions between heat and electricity, i.e., thermoelectric effects. A key requirement to improve the energy conversion efficiency is to increase the Seebeck coefficient (S) and the electrical conductivity (sigma), while reducing the thermal conductivity (kappa). Nanostructures make it possible to modify the fundamental trade-offs between the bulk material properties through the changes in the density of states and interface effects on the electron and phonon transport. We will review recent experimental and theoretical results on superlattice and quantum dot thermoelectrics, nanowires, thin-film microrefrigerators, and solid-state thermionic power generation devices. In the latter case, the latest experimental results for semimetal rare-earth nanoparticles in a III-V semiconductor matrix as well as nitride metal/semiconductor multilayers will be discussed. We will briefly describe recent developments in nonlinear thermoelectrics, as well as electrically pumped optical refrigeration and graded thermoelectric materials. It is important to note that, while the material thermoelectric figure of merit (Z = S-2 sigma/kappa) is a key parameter to optimize, one has to consider the whole system in an energy conversion application, and system optimization sometimes places other constraints on the materials. We will also review challenges in the experimental characterization of thin film thermoelectric materials. Finally, we will assess the potential of some of the more exotic techniques such as thermotunneling and bipolar thermoelectric effects.