KINETICS OF HEXACELSIAN-TO-CELSIAN PHASE-TRANSFORMATION IN SRAL2SI2O8

The kinetics of hexacelsian-to-celsian phase transformation in SrAl2Si2O8 have been investigated. Phase-pure hexacelsian was prepared by heat treatment of glass flakes at 990-degrees-C for 10 h. Hexacelsian flakes were isothermally heat-treated at 1026-degrees, 1050-degrees, 1100-degrees, 1152-degrees, and 1200-degrees-C for various times. The amounts of monoclinic celsian formed were determined using quantitative X-ray diffraction. Values of reaction rate constant, k, at various temperatures were evaluated from the Avrami equation. The Avrami parameter was determined to be 1.1, suggesting one-dimensional growth with the interface rather than a diffusion-controlled transformation mechanism. From the temperature dependence of k, the apparent activation energy for this reaction was evaluated to be 527 +/- 50 kj mol (126 +/- 12 kcal mol). This value is consistent with a mechanism involving the transformation of the layered hexacelsian structure to a three-dimensional network celsian structure which necessitates breaking of the strongest bonds, the Si-O bonds.