Thermoelectric properties of boron phosphide

Abstract

This paper describes the thermoelectric properties of boron phosphide wafers prepared using chemical vapour deposition and the thermal constants of a sintered boron phosphide specimen prepared using the r.f. hot pressing technique. The electric conductivity of the n-type boron phosphide wafers increases with temperature, becomes constant and then begins to decrease with temperature. In p-type wafers the conductivity increases with temperature and with a further increase in temperature the intrinsic conduction region is reached. The absolute thermoelectric power of n-type materials has an almost constant value of 500 μV K −1, but it begins to decrease at a temperature of 650 K owing to the creation of acceptors. The thermoelectric power of p-type materials decreases with increasing temperature, indicating that acceptors and donors contribute to the conduction. Thermal constants were obtained by the laser flash method and were measured using sintered discs as standards. Thermal diffusivity, heat capacity and the corresponding calculated thermal conductivity have an accuracy within ± 5% up to 700 °C. The calculated thermoelectric figure of merit is of the order of 10 −4 K −1. Therefore, boron phosphide is a promising material for high temperature thermoelectric devices.