BiSb alloys for magneto-thermoelectric and thermomagnetic cooling

Abstract

The thermoelectric and thermomagnetic properties of undoped Bi-Sb alloys have been investigated with highly homogeneous single crystals throughout the entire alloy composition as functions of temperature (77–300°K), magnetic field (0–7·5 kilo-oersteds), and crystallographic direction (∥ and ⊥ to the trigonal axis). The undoped, n-type Bi 85Sb 15 alloy gave the largest magneto-thermoelectric figure of merit (11 × 10 −3 deg −1) at 100°K and the same value at 80°K in transverse fields of 3 and 1·3 kilo-oersteds, respectively. Within the range of field intensity used, the largest thermomagnetic figure of merit (4·1 × 10 −3 deg −1) was obtained at 100°K with undoped n-types Bi 99Sb 1 in a field of 7·5 kilo-oersteds. The possibility has been explored of obtaining p-type Bi-Sb alloys through additions of acceptor impurities and by applying transverse magnetic fields. The results showed that a Sn-doped Bi 88Sb 12 alloy yields a p-type figure of merit of 2·3 × 10 −3 deg −1 at 85°K in 7·5 kilo-oersteds. The difference between the adiabatic and isothermal conditions was reviewed in some detail in connection with measurements of various transport coefficients that enter into the calculation of the figures of merit. Also, a comparison was given on the difference between the magneto-Peltier and the Ettingshausen cooling. With a hybrid cooling unit based on the combination of the Peltier and magneto-Peltier effects in telluride alloys and BiSb alloys, respectively, a continuous cooling as large as 172°K, from room temperature down to 128°K, has been achieved.