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

Abstract

Type-VIII Ba₈Ga₁₆Sn₃₀ polycrystalline clathrates were grown vertically downwards from Ba₈Ga₁₆Sn₅₀ solution at furnace temperatures between 500°C and 800°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 650°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 650°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 μW/cmK² at 200°C; the Seebeck coefficient was −260 μV/K.