Microstructure and crystal orientation of rapidly solidified (Bi,Sb) 2(Te,Se)3 alloys by the liquid quenching technique

Abstract

A rapid solidification process was found to form unidirectional crystal structures in (Bi,Sb)2Te3 and (Bi,Sb)2(Te,Se) 3 based thermoelectric alloys. This fine microstructure with its unidirectional crystal orientation is expected to yield high value thermoelectric properties because the fine grain size is extremely effective in decreasing thermal conductivity by boundary scattering of phonons and because this crystal orientation is extremely effective in decreasing the electrical resistivity (increasing the carrier mobility). The rapidly solidified alloys were prepared by a single-roller liquid quenching method. These alloys were shaped as a thin foil with thicknesses ranging from 5 to 20 μm. The crystal orientation was analyzed by the x-ray diffraction method and the microstructure was observed by optical microscopy and SEM of the thin foil samples. The crystal grains of the rapidly solidified foils were very fine and highly oriented. The thermoelectric properties were measured for p-type Bi0.4Sb 1.6Te3 and the n-type Bi1.9Sb 0.1Te2.6Se0.4 compacted alloys. The figures of merit of 3.5 χ 10-3 K-1 for p-type alloy and 3.3 χ 10-3 K-1 for n-type alloy were obtains. It is proposed that the rapid solidification is very useful technique for the improvement of thermoelectric properties.