Transformation of Ba-Al-Si precursors to celsian by high-temperature oxidation and annealing

Celsian (monoclinic BaO · A1₂O₃ · 2SiO₂) is being considered as a matrix material for ceramic composites used in high-temperature structural applications. The present article describes the synthesis of celsian by the oxidation and annealing of solid, malleable, metallic Ba-Al-Si precursors. The phase and microstructural evolution after various stages of oxidation at 300 °C to 1260 °C in pure oxygen at 1 atm pressure have been examined by X-ray diffraction (XRD) and electron microprobe analyses (EPMA). Barium peroxide, BaO₂, formed rapidly during oxidation at 300 °C, with aluminum and silicon remaining largely as unoxidized particles in a BaO₂ matrix. Between 300 °C and 500 °C, barium orthosilicate, Ba₂Si0₄, formed by a solid-state reaction between barium peroxide and unoxidized silicon. Further exposure to temperatures between 500 °C and 1200 °C resulted in the oxidation of aluminum and of residual silicon. The oxidized silicon reacted with the barium orthosilicate matrix to yield higher silica-containing barium silicates that, in turn, reacted with alumina or mullite to form metastable hexacelsian (hexagonal BaO-A1₂O₃ · 2SiO₂). Celsian was then obtained by further exposure to peak temperatures ≤1260°C.