Oxidation of alkaline-earth-metal sulfide powders and thin films

Abstract

The oxidation of CaS and SrS powders and thin films was studied in situ up to 1300 °C by thermoanalytical techniques (TG, DTA and high-temperature XRD) while FTIR and powder XRD were used to analyse ex situ the solid reaction intermediates and products. CaS powder starts to oxidize to CaSO₄ around 500 °C but the oxidation is not complete because of a competing reaction, i.e. the decomposition of CaSO₄ to CaO, which is significant above 1000 °C. SrSO₄ is more stable and therefore SrS completely oxidizes to SrSO₄ before the decomposition starts. FTIR and XRD failed to detect CaSO₃ or SrSO₃ at any stage of the heating process. Particle size appears to have a marked effect on the decomposition of powders, while moisture plays only a minor role. Thin films appear to be more stable towards oxidation but they react around 1000 °C with the silicon substrate.