Microstructural control of bismuth tellurium alloys by solidification with undercooling

Abstract

The solidification of a thermoelectric alloy melt with undercooling was studied experimentally with the aim of controlling the microstructure during solidification. These trials were conducted using bismuth tellurium alloys, and the crystal morphology, number density and growth direction were observed while varying the initial composition and temperature gradient. The solidified material consisted of bismuth telluride compound crystals having plate-like facets along with a eutectic matrix. The growth mechanism of these crystals was examined in relation to the temperature and concentration fields around the solid-liquid interface, based on microstructural observations and temperature measurements. The solidification process was found to consist of three elementary steps: (1) recalescence dissipation in association with thermal undercooling, (2) relaxation in association with constitutional undercooling and (3) conventional phase change via heat conduction. The directivity and fineness of structure improved as the temperature gradient was increased during undercooling. In addition, electron backscatter diffraction data showed that the c-axis of each hexagonal bismuth telluride crystal was oriented almost parallel to the cooling surface. As a result, the thermoelectric power factor was enhanced. These results suggest that controlling the microstructure in bismuth telluride-based alloys via the application of a non-uniform undercooling field has the potential to allow the fabrication of high performance thermoelectric materials with increased production efficiency.