Thermal stability and oxidation resistance of polycrystalline Ba8Al16Si30-based clathrates



Polycrystalline Ba8Al16Si30-based clathrate samples (nominal composition Ba8Al15Si31) were prepared by combining arc melting and spark plasma sintering methods. Heat treatments were conducted at 873 K for 0–480 h in air to examine the thermal stability and the oxidation resistance, which are of practical importance when the material is used at elevated temperatures. Thermogravimetry (TG) and differential thermal analysis (DTA) in the range 300–1473 K in air indicated that Ba8Al15Si31 bulk form possessed relatively good thermal stability in air below approximately 1073 K. From X-ray diffraction measurements, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and X-ray photoelectron spectroscopy, it was inferred that thin oxide (probably barium aluminates) layer with the thickness of approximately 0.6 µm was formed on the surface by the heat treatment. The decrease in the electrical conductivity was found to be relatively low (less than 15%) by the heat treatment for 480 h in air.