Solid state synthesis and properties of monoclinic celsian

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Monoclinic celsian of Ba0.75Sr0.25Al2Si2O8 (BSAS-1) and Ba0.85Sr0.15Al2Si2O8 (BSAS-2) compositions have been synthesized from metal carbonates and oxides by solid state reaction. A mixture of BaCO3, SrCO3, Al2O3, and SiO2 powders was precalcined at ∼900–940 °C to decompose the carbonates followed by hot pressing at ∼1300 °C. The hot pressed BSAS-1 material was almost fully dense and contained the monoclinic celsian phase, with complete absence of the undesirable hexacelsian as indicated by X-ray diffraction. In contrast, a small fraction of hexacelsian was still present in hot pressed BSAS-2. However, on further heat treatment at 1200 °C for 24 h, the hexacelsian phase was completely eliminated. The average linear thermal expansion coefficients of BSAS-1 and BSAS-2 compositions, having the monoclinic celsian phase, were measured to be 5.28 × 10−6 °C−1 and 5.15 × 10−6 °C−1, respectively, from room temperature to 1200°C. The hot-pressed BSAS-1 celsian showed room temperature flexural strength of 131 MPa, elastic modulus of 96 GPa and was stable in air up to temperatures as high as ∼1500 °C.

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