Effect of Heat Treatment in Air on the Surface of a Sintered Ba8Al16Si30-Based Clathrate

Abstract

A polycrystalline silicon-based clathrate of nominal composition Ba8Al15Si31 was prepared by a combination of arc melting and spark plasma sintering. Its thermal stability in air and the effect on the surface of heat treatment in air, which are of practical importance for use of the material at elevated temperatures, were examined for different temperatures (873 K, 973 K, and 1073 K) and heating times (0–480 h). Thermogravimetry and differential thermal analysis in air in the range 300–1523 K indicated that Ba8Al15Si31 in the bulk form had relatively good thermal stability in air at high temperatures. X-ray diffraction measurements, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy revealed that a thin layer of oxide (probably a barium aluminate, BaAl2O4) was formed on the surface by heat treatment in air. The thickness of the oxide layer increased with increasing temperature and heating time. Growth of the oxide layer can be explained well in terms of a diffusion mechanism. The activation energy for oxidation was estimated to be approximately 201 kJ/mol, which is comparable with that for thermal oxidation of silicon. The chemical composition of the interior of the Ba8Al15Si31 was found to be stable to heat treatment in air.