Nanostructured Semiconductors for Thermoelectric Energy Conversion: Synthesis and Transport Properties.

Abstract

Half-Heusler (HH) alloys are among the most popular material systems for high temperature thermoelectric energy conversion. Approaches to increase the thermoelectric figure of merit (ZT) of HH range from (1) chemical substitution of atoms with different masses within the same atomic position in the crystal structure to optimize carrier concentration and enhance phonon scattering via mass fluctuation and (2) embedding secondary phonon scattering centers in the matrix (nanostructuring) to further reduce thermal conductivity. This work focuses on three material systems. The first part describes the synthesis and properties of oxide nanostructures (NiO, Co3O4) which were subsequently used as inclusion phases in HH matrix to reduce the thermal conductivity. The effects of the synthesis conditions on the thermoelectric properties of compacted pellets of NiO and Co3O4 are discussed. The second part of the work discusses the development of synthetic strategies for the fabrication of p-type and n-type bulk nanostructured thermoelectric materials made of a half-Heusler matrix based on (Ti,Hf)CoSb, containing nanostructures with full-Heusler (FH) compositions. The role of the nanostructures in the regulation of phonon and charge carrier transports within the HH matrix is discussed by combining transport data and electron microscopy images. It was found that the FH nanoinclusions form staggered heterojunctions with a valence band (VB) offset energy at the HH/FH phase boundaries which discriminates existing holes with respect to their energy by trapping low energy holes and promoting the transport of high energy holes through the VB of the FH nanostructures. This results in large increase in the mobility and effective mass of high energy holes contributing to electronic conduction. The simultaneous reduction in the density and the increase in the effective mass of holes caused large enhancements of the thermopower. In the third part, the application of this concept of nanostructuring on the copper selenide material system is described. Various synthetic approaches such as liquid assisted solid-state reaction and mechanical alloying are utilized for the fabrication of copper selenide compositions. We found that the mechanical alloying clearly decreases the thermal conductivity of the composition as well increases the Seebeck due to decrease in carrier concentrations.