Conversion efficiency and effective properties of particulate-reinforced thermoelectric composites

It is well known that nanoparticles have the ability to enhance the performance of thermoelectric materials. A comprehensive analytical model exploring the mechanism by which this enhancement takes place remains largely absent from the literature. We address this deficiency by introducing a simple model of a nanoparticle as a circular nanoinhomogeneity and analyze its effect on thermal-electric conversion efficiency and the effective properties of thermoelectric composites. Taking interface phonon scattering into consideration, the effective material parameters around a nanoinhomogeneity are derived explicitly, while the effective properties and optimal conversion efficiency are discussed in detail via numerical analysis. Our results show that the nanoinhomogeneity equally effects both the figure of merit and the conversion efficiency. A carefully selected inhomogeneity not only improves the thermoelectric power factor but also suppresses the effective thermal conductivity, and thus greatly improves the optimal conversion efficiency. Specifically, a nanoinhomogeneity with higher electric and thermal conductivities generates a higher thermoelectric figure of merit regardless of the magnitude of its Seebeck coefficient. Noting that the thermoelectric performance improved by the presence of the inhomogeneity is restricted by the intensity of interface phonon scattering, we further calculate the expressions for the effective figure of merit in certain extreme cases. Furthermore, we see that a smaller inhomogeneity has a stronger ability to improve the conversion efficiency for a given doping proportion, mainly because the interface phonon scattering of the smaller inhomogeneity has a relatively greater effect on thermal conduction.