Thermoelectric figure of merit of semimetal and semiconductor Bi1–xSbx alloy foils

Abstract

The temperature dependencies of thermal conductivity χ(Т), electrical conductivity σ(Т), and thermopower α(Т) in foils of Bi1–xSbx alloys in the semimetal and semiconductor states, in a temperature range of 4.2–300 K, were experimentally studied. Foils of Bi1–xSbx alloys were prepared by high-speed crystallization of a thin layer of the melt on the polished inner surface of a rotating copper cylinder. High crystallization rates (v = 5 × 105 m/s) enabled a uniform distribution of the components throughout the volume. The thickness of the foils was 10–30 μm with the texture 101¯2 parallel to the foil plane and the С3 axis coinciding with the normal to the foil surface. It was shown that, in the low-temperature range (T < 10 K), the thermal conductivities of the semimetal (Bi–3 at.% of Sb) and semiconductor (Bi–16 at.% of Sb) foils are, respectively, two orders of magnitude and an order of magnitude lower than the thermal conductivities of the bulk samples of the same composition. This effect is interpreted from the viewpoint of combined additional phonon scattering on both the surface and boundaries of the foil grains. The thermoelectric figure of merit of the foils ZT = α2σ/χ was calculated from the ρ(Т), α(Т), and χ(Т) dependencies in the temperature range of 5–300 K. It was found that the thermoelectric figure of merit ZT in the semiconductor foils of n-type Bi1–xSbx alloys at 100 K is 2 times higher than that of the bulk samples of the same composition and crystallographic orientation, which may be used in low-temperature thermoelectric energy converters.