Thermoelectric Skutterudites: Why and How High zT Can Be Achieved

Abstract

Thermoelectric materials have been widely studied over the past few decades due to their ability to convert waste heat into useful electricity. Among various thermoelectric materials, skutterudite distinguishes itself in both space and terrestrial applications with its excellent thermoelectric performance, robust mechanical properties, and thermal stability. The thermoelectric excellence of skutterudites is mostly attributed to the low thermal conductivity due to the addition of filler atoms (R) into the void (one per primitive cell Co4Sb12). Essential though this is to high zT, the importance of the intrinsic electronic structure in skutterudites is often understated or ignored completely. In this thesis, by combining experimental and computational studies, the electronic origin of high thermoelectric performance of CoSb3-based skutterudites is investigated. The high zT was shown to be a direct result of the high valley degeneracy inherent to CoSb3, which is further enhanced by band convergence at high temperatures. This successfully explains why the optimum doping carrier concentration in n-CoSb3 skutterudites is independent on the type of fillers. With the electronic origin of high thermoelectric performance clarified, the thesis moves on to elaborate how to achieve high zT in skutterudite with the aid of phase diagram study. By mapping out the phase regions near the skutterudite phase on the isothermal section of the R-Co-Sb ternary phase diagram, the solubility region of the CoSb3 skutterudite phase can be determined along with the solubility limit of R, both of which are often determined in stable compositions resulting in a synthesis window. The temperature dependence of the filler solubility is also demonstrated experimentally. This overturns the general understanding that the filler solubility is a single value only dependent on the filler type. The temperature dependence of stable compositions enables easy carrier concentration tuning which allows the optimization of thermoelectric performance. High zT values are achieved in single In, Yb, Ce-CoSb3 skutterudites. The methodology applied here are not confined to n-CoSb3, but can be generalized to any other ternary systems.