Kinetics of Hexacelsian‐to‐Celsian Phase Transformation in SrAl 2 Si 2 O 8

The kinetics of hexacelsian‐to‐celsian phase transformation in SrAl 2 Si 2 O 8 have been investigated. Phase‐pure hexacelsian was prepared by heat treatment of glass flakes at 990°C for 10 h. Hexacelsian flakes were isothermally heat‐treated at 1026°, 1050°, 1100°, 1152°, and 1200°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.