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

The kinetics of hexacelsian-to-celsian phase transformation in SrAl2Si 2O8 have been investigated. Phase-pure hexacelsian was prepared by hea t treatment of glass flakes at 990-degrees-C for 10 h. Hexacelsian fla kes were isothermally heat-treated at 1026-degrees, 1050-degrees, 1100 -degrees, 1152-degrees, and 1200-degrees-C for various times. The amou nts of monoclinic celsian formed were determined using quantitative X- ray diffraction. Values of reaction rate constant, k, at various tempe ratures were evaluated from the Avrami equation. The Avrami parameter was determined to be 1.1, suggesting one-dimensional growth with the i nterface rather than a diffusion-controlled transformation mechanism. From the temperature dependence of k, the apparent activation energy f or this reaction was evaluated to be 527 +/- 50 kj mol (126 +/- 12 kca l mol). This value is consistent with a mechanism involving the transf ormation of the layered hexacelsian structure to a three-dimensional n etwork celsian structure which necessitates breaking of the strongest bonds, the Si-O bonds.