Recent progress in p-type thermoelectric magnesium silicide based solid solutions

Abstract

Thermoelectric materials can convert waste or process heat directly into usable electrical energy. They are therefore a fascinating opportunity to increase the energy efficiency of various industrial processes and thus reduce fossil fuel consumption. For the temperature range of 500 K–800 K – where a large fraction of the reusable heat is available – magnesium silicide based solid solutions are among the most promising thermoelectric materials. They combine very good thermoelectric properties with a high material availability, low cost of raw materials, and environmental compatibility making them suitable for large scale applications. The thermoelectric properties of the n-type material have been found superior to the p-type. However, a p-type material with good thermoelectric properties is indispensable for a practical application of magnesium silicide based thermoelectric generators. Due to progress in synthesis techniques and the identification of efficient dopants there has been significant progress in the optimization of the p-type, resulting in several reports with a thermoelectric figure of merit exceeding 0.5.We analyze experimental data and theoretical results and rationalize the recent progress. In particular we investigate the interplay between thermoelectric properties and composition, electronic band structure, and the effect of doping by various elements. Overall we find that the optimization of p-type Mg2X requires consideration of more parameters than the n-type material. Achieving the optimum carrier concentration is experimentally challenging and depends on the dopant species as well as the Si:Ge:Sn ratio in the compounds. Although debated frequently, the overwhelming majority of the data indicates a rigid electronic band structure for the most popular dopants Ag, Si, Li, and Na. On the other hand, the doping efficiency differs between the dopants as well as the corresponding hole mobilities. Further progress in the material development can be expected if the various defect densities can be controlled and the bipolar effect can be decreased further.