Thermoelectrics: Physical Mechanisms

Direct energy conversion from thermal to electrical energy, based on thermoelectric effect, is attractive for potential applications in waste heat recovery and environmentally friendly refrigeration. The energy conversion efficiency of thermoelectric devices is related to the thermoelectric Figure of Merit ZT, which is proportional to the electrical conductivity, the square of the Seebeck coefficient, temperature, and the inverse of the thermal conductivity. Currently, the low ZT values of available materials restrict the large-scale applications of this technology. Recently, however, significant enhancements in ZT have been reported in nanostructures such as superlattices mainly due to their low thermal conductivities. According to the studies on heat transfer mechanisms in nanostructures, the reduced thermal conductivity of nanostructures is mainly attributed to the increased scattering of phonons at the interfaces. Based on this idea, nanocomposites are also expected to have a lower thermal conductivity than their bulk counterparts of the same chemical configuration. Nanocomposites are materials with dimensions of less than 100 nm. They can be fabricated at low cost by mixing nano-sized particles followed by consolidation of nano-sized powders.