Bismuth -- The Thermoelectric Material of the Future?

Bismuth was the first material to display substantial thermoelectric coefficients but, in the nineteen-fifties, it was overshadowed by new compound semiconductors. Notable among these was bismuth telluride, with lead telluride superior at higher temperatures. Then these compounds were themselves improved by the formation of solid solutions. Meanwhile, it was demonstrated that bismuth would still have been pre-eminent for n-type thermoelements, if it were not for the presence of positive holes as minority carriers. In the nineteen-nineties, it was shown that the thermoelectric figure of merit could be improved by the use of nanotechnology. One of the effects of a reduction in the physical dimensions is a change in the electronic band structure and this can result in a semimetal displaying semiconducting properties. Nanostructured bismuth has indeed given high values of the Seebeck coefficient and it seems possible that it may eventually have a dimensionless figure of merit, zT, substantially larger than 4, the suggested limit for bulk material. This fact alone points towards a future in which bismuth is the best material for Peltier cooling but there is another important factor. Tellurium, which forms a substantial fraction of most present-day thermoelectric materials, is becoming increasingly scarce and expensive. There is, therefore, a strong incentive for finding thermoelectric materials that do not contain this element. Bismuth and bismuth-antimony alloys would, of course, meet this requirement