Thermoelectric Properties of Bismuth-Antimony Alloys

Abstract

The thermoelectric figure of merit (z), resistivity, and Seebeck coefficient have been measured between 20° and 300°K on single crystals of several alloys in the range from 1% to 40% antimony in bismuth. These materials are semimetals (0 to 5% antimony) or small energy gap intrinsic semiconductors (5 to 40% antimony) and all are n type. The Seebeck coefficients and figures of merit are anisotropic, the larger values being those measured parallel to the threefold symmetry axis. In the 12% antimony alloy the larger z rises from 1.0×10⁻³/°K at 300°K to a maximum of 5.2×10⁻³/°K at 80°K and falls rapidly at lower temperatures. All of the alloys between 3% and 16% antimony have a maximum z near 5×10⁻³/°K at a temperature between 70°K and 100°K. The 5% antimony alloy has the highest z at room temperature (z=1.8×10⁻³/°K). In this material, the Seebeck coefficient is practically constant (S=−110±10 μv/°K) between 77° and 300°K and the ratio of the thermal to electrical conductivities is close to the theoretical Wiedemann‐Franz ratio above 100°K. As a result, z is inversely proportional to the absolute temperature (zT=0.52±0.05) between 100° and 300°K. In the 12% antimony alloy, S rises from −110 μv/°K at 300°K to −220 μv/°K at 20°K. A specimen of this material, doped with 0.01% lead, is p type below 42°K. A qualitative explanation of these results is given in terms of mixed conduction by electrons and holes having properties similar to those in pure bismuth. The use of these alloys (and semimetals in general) in thermoelectric refrigeration at low temperatures is discussed.