Effect of melt-spun powder additions on the thermoelectric properties of bismuth and antimony chalcogenides

Abstract

We have studied the thermoelectric properties of p-type Bi_0.5Sb_1.5Te₃ and n-type Bi₂Te_2.4Se_0.6 solid solutions prepared by vacuum hot pressing of mixtures of powders differing in particle size composition. The powders were prepared by mechanically grinding ingots and by ultrarapid melt cooling (melt spinning). Fracture surfaces of samples were examined by scanning electron microscopy. The samples consisted of large, layered particles of the major component (up to hundreds of microns in size) and small flakes (a few to tens of microns in size) of the component prepared by melt spinning. The microstructure of the materials was examined under an optical microscope. On grain boundaries in the p-type materials, we observed telluriumbased eutectic precipitates in the form of a white phase. In some of the n-type samples, the Bi₂Te_2.4Se_0.6 solid solution was found to undergo ordering, resulting in the formation of the ternary compound Bi₂Te₂Se. The Seebeck coefficient, electrical conductivity, and thermal conductivity of the materials were measured in the temperature range 100–700 K. The addition of 40 wt % powder prepared by melt spinning to hot-pressed p-type samples was shown to have no effect on their thermoelectric figure of merit (ZT)_max, which was 1.0 at 350 K. On the addition of 20 wt % powder prepared by melt spinning, we obtained (ZT)_max = 1.1 at 550 K in a hot-pressed n-type sample.