SYNTHESIS OF CELSIAN (BAAL2SI2O8) FROM SOLID BA-AL-AL2O3-SIO2 PRECURSORS - I, XRD AND SEM EDX ANALYSES OF PHASE EVOLUTION/

An intimate Ba-Al-Al2O3-SiO2 powder mixture, produced by high-energy m illing, was pressed to 3 mm thick cylinders (10 mm diameter) and hexag onal plates (6 mm edge-to-edge width), Heat treatments conducted from 300 degrees to 1650 degrees C in pure oxygen or air were used to trans form these solid-metal/oxide precursors into BaAl2Si2O8. Barium oxidat ion was completed, and a binary silicate compound, Ba2SiO4, had formed within 24 h at 300 degrees C. After 72 h at 650 degrees C, aluminum o xidation was completed, and an appreciable amount of BaAl2O4 had forme d. Diffraction peaks consistent with hexagonal BaAl2Si2O8, BaAl2O4, be ta-BaSiO3, and possibly beta-BaSi2O5 were detected after 24 h at 900 d egrees C, Diffraction peaks for BaAl2O4 and BaAl2Si2O8 were observed a fter 35 h at 1200 degrees C, although SEM analyses also revealed fine silicate particles, Further reaction of this silicate with BaAl2O4 at 1350 degrees to 1650 degrees C yielded a mixture of hexagonal and mono clinic BaAl2Si2O8. The observed reaction path was compared to prior wo rk with other inorganic precursors to BaAl2Si2O8.