Recent developments in thermoelectric materials

The increased activity in attempts to develop improved thermoelectric semiconductors for use in the direct conversion of heat into electrical energy results mostly from research sponsorship by the US Military and NASA. Thermoelectric generators have no moving parts and are difficult to detect by visual, aural or thermal infrared means. Fossil multifuelled thermoelectric generators are the leading candidates for replacing standard US Military engine generator sets up to 1·5 kW under the SLEEP programme (Signature Suppressed Lightweight Electric Energy Plants). When coupled to an isotopic heat source, thermoelectric generators are able to operate reliably and unattended for long periods of time and have a proven performance record in supplying electrical power to the Lunar Experimental Package (Apollo Program) and in providing onboard electrical power to the Voyager spacecrafts. In both military and space applications any improvement in the thermoelectric generators' conversion efficiency would result in a saving in fuel--an important consideration. One way of improving the conversion efficiency is by increasing the so called [`]Figure of merit' of the semiconductor material employed in the fabrication of the generators' thermocouples. In this paper an assessment is made of current thermoelectric materials; recent attempts to improve the figure of merit of existing materials are discussed and a number of new thermoelectric materials described. Significant headway has been made in reducing the lattice thermal conductivity of thermoelectric materials through the use of additives, small grain sizes or combinations of both. This development will result in substantial improvements in the thermoelectric figure of merit, provided the electrical properties can be maintained close to single crystal values. It is concluded that, because in the past the development of new thermoelectric materials has occupied long periods of time, even during periods of intense research activity, it is likely that established or [`]modified' established materials will remain the mainstay of military and space applications at least for the forseeable future.