TRANSFORMATION OF BA-AL-SI PRECURSORS TO CELSIAN BY HIGH-TEMPERATURE OXIDATION AND ANNEALING

Celsian (monoclinic BaO.Al2O3.2SiO(2)) is being considered as a matrix material for ceramic composites used in high-temperature structural a pplications. The present article describes the synthesis of celsian by the oxidation and annealing of solid, malleable, metallic Ba-Al-Si pr ecursors. The phase and microstructural evolution after various stages of oxidation at 300 degrees C to 1260 degrees C in pure oxygen at 1 a tm pressure have been examined by X-ray diffraction (XRD) and electron microprobe analyses (EPMA). Barium peroxide, BaO2, formed rapidly dur ing oxidation at 300 degrees C, with aluminum and silicon remaining la rgely as unoxidized particles in a BaO2 matrix. Between 300 degrees C and 500 degrees C, barium orthosilicate, Ba2SiO4, formed by a solid-st ate reaction between barium peroxide and unoxidized silicon. Further e xposure to temperatures between 500 degrees C and 1300 degrees C resul ted 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.Al2O3.2SiO(2 )). Celsian was then obtained by further exposure to peak temperatures less than or equal to 1260 degrees C.