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

• Published in: Journal of electronic materials Vol. 43; no. 6; pp. 1865 - 1869
• Main Authors: Hong, Qin-Gang, Chang, Li-Shin, Hsieh, Huey-Lin
• Format: Journal Article Conference Proceeding
• Language: English
• Published: Boston Springer US 01.06.2014; Springer
• Subjects: Characterization and Evaluation of Materials; Chemistry and Materials Science; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Conductivity phenomena in semiconductors and insulators; Cross-disciplinary physics: materials science; rheology; Electronic transport in condensed matter; Electronics and Microelectronics; Equations of state, phase equilibria, and phase transitions; Exact sciences and technology; General studies of phase transitions; Growth from melts; zone melting and refining; Instrumentation; Materials Science; Methods of crystal growth; physics of crystal growth; Nucleation; Optical and Electronic Materials; Physics; Solid State Physics; Theory and models of crystal growth; physics of crystal growth, crystal morphology and orientation; Thermoelectric and thermomagnetic effects; Seebeck coefficient; grain size; figure- of-merit; Ga; Sn; Bridgman method; Ba; clathrate; Liquid solid interface; Nucleation; Wetting; Polycrystals; Convection; Clathrates; Thermoelectric properties; Wettability; Thermoelectric power; Grain size; Temperature dependence; figure-of-merit; Temperature gradients; Seebeck effect; Solidification front; Velocity; Growth mechanism; Instability; Crystal microstructure; Supercooling; Figure of merit; Crystal growth from melts
• ISSN: 0361-5235; 1543-186X
• DOI: 10.1007/s11664-013-2892-5

Type-VIII Ba 8 Ga 16 Sn 30 polycrystalline clathrates were grown vertically downwards from Ba 8 Ga 16 Sn 50 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 Ba 8 Ga 16 Sn 30 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 Ba 8 Ga 16 Sn 30 clathrate prepared by the vertical Bridgman method in this study were, respectively, 0.8 and 11.4 μW/cmK 2 at 200°C; the Seebeck coefficient was −260 μV/K.