The Effect of Furnace Temperature on Evolution of the Microstructure of Type-VIII Clathrate Ba8Ga16Sn30 Polycrystals Grown from Ba8Ga16Sn50 Solutions

Type-VIII Ba8Ga16Sn30 polycrystalline clathrates were grown vertically downwards from Ba8Ga16Sn50 solution at furnace temperatures between 500A degrees C and 800A degrees C with an ampoule velocity of 0.36 cm/h. The microstructure, composition, crystal structure, and thermoelectric properties of crystals were investigated. Polycrystalline samples in which Ba8Ga16Sn30 grains were wetted by an Sn-rich phase were prepared. In general, grain size increases along the direction of growth. It was found that the sample grown at 650A degrees C had the largest grains. Smaller grains were observed for samples grown at lower temperatures, as a result of higher rate of nucleation, because of higher undercooling at the solid-liquid interface caused by the lower thermal gradient in the liquid. However, at furnace temperatures higher than 650A degrees C enhanced convection in the solution at higher temperature gradients and wetting phenomena may cause instability of the solid-liquid interface and solid nuclei may flow into the liquid to become new nucleation sites. This explains the decrease of grain size at higher furnace temperatures. The optimum ZT and power factor of the undoped Ba8Ga16Sn30 clathrate prepared by the vertical Bridgman method in this study were, respectively, 0.8 and 11.4 mu W/cmK(2) at 200A degrees C; the Seebeck coefficient was -260 mu V/K.