Effect of pressure and temperature on thermopower of Bi–Sb alloys

Abstract

In this paper we present thermopower (TEP) studies on polycrystalline bismuth–antimony (Bi–Sb) alloys (containing 10, 12 and 15 at% antimony) as a function of pressure and temperature. The pressure and temperature dependence of thermopower of these alloys are dominated by the complex band structure (light hole L band and heavy hole T& H bands). The band structures of Bi rich Bi–Sb alloys depend on the Sb concentrations and also on pressure and temperature. The complexity of band structure leads to an unusual behavior of thermopower at high pressures and temperatures. The present range of Bi–Sb alloy compositions belongs to the regime of degenerate narrow band gap semiconductors (which are the best choice of materials for low temperature thermoelectric cooling applications). Pressure and temperature reduces the inter band gaps and eventually these materials behave as semimetals. Smaller magnitude of thermoelectric power and its decreasing magnitude with temperature confirm the decrease in the band gap energies between direct gap ( L a– L s) and also indirect band gap ( L a– T). The present experimental results give clear evidence to a temperature-induced crossover from direct gap semiconductor ( L a– L s) to an indirect band gap involving T and or H and L a bands. With the application of pressure, the temperature corresponding to this crossover shifts to lower temperature. These experimental results are discussed qualitatively based on the energy band structure proposed for these alloy systems involving both heavy hole T and H bands.