A review of the new thermoelectric materials

Most of the materials that are used in today's thermoelectric generators and refrigerators were first developed many years ago. However, during the last decade promising results have been reported for several new systems of compounds and alloys. These include the skutterudites, the clathrates and the half-Heusler alloys. There are also a number of layer-structured compounds, some of which may be regarded as developments of the bismuth telluride system. Certain of these materials seem to possess the highest known figures of merit at least at elevated temperatures. Moreover, it is doubtful that any of the new systems have yet been fully optimized. The problem is that there are large numbers of compounds that remain to be investigated. Here we shall try to formulate some principles that will allow us to select those systems that are likely to yield the best long-term results. We discuss, for example, whether it is better for a material to have a high power factor or a low lattice thermal conductivity. We also consider the relative worth of a large or small effective mass for the charge carriers. Then, of course, one must take account of problems associated with the structure and preparation of the materials. The potential of the new materials for use as low-dimensional thermoelectrics will be included in our discussion.