Solid state synthesis and properties of monoclinic celsian

Monoclinic celsian of Ba0.75Sr0.25Al2Si2O8 (BSAS-1) and Ba0.85Sr0.15Al2Si2O 8 (BSAS-2) compositions have been synthesized from metal carbonates and oxi des by solid state reaction. A mixture of BaCO3, SrCO3, Al2O3, and SiO2 pow ders was precalcined at similar to 900-940 degrees C to decompose the carbo nates followed by hot pressing at similar to 1300 degrees C. The hot presse d BSAS-1 material was almost fully dense and contained the monoclinic celsi an phase, with complete absence of the undesirable hexacelsian as indicated by X-ray diffraction, In contrast, a small fraction of hexacelsian was sti ll present in hot pressed BSAS-2. However, on further heat treatment at 120 0 degrees C for 24 h, the hexacelsian phase was completely eliminated. The average linear thermal expansion coefficients of BSAS-1 and BSAS-2 composit ions, having the monoclinic celsian phase, were measured to be 5.28 x 10(-6 ) degrees C-1 and 5.15 x 10(-6) degrees C-1, respectively, from room temper ature to 1200 degrees C, The hot-pressed BSAS-1 celsian showed room tempera ture flexural strength of 131 MPa, elastic modulus of 96 GPa and was stable in air up to temperatures as high as similar to 1500 degrees C. (C) 1998 K luwer Academic Publishers.