Kinetic study on the hexacelsian-celsian phase transformation

The hexacelsian-celsian transformation from grains of 1/4 in. (0.635 cm) size is slow and erratic. This is because the rate of heterogeneous nucleation in such grains is low and is influenced by contamination present in the furnace. When the grain size is reduced to a -200 mesh powder, heterogeneous nucleation becomes a dominant factor and the transformation is accelerated. The transformation has three stages: the first is controlled by the rate of crystal growth, the second is controlled by the rates of nucleation and crystal growth, and the third is controlled by the rate of nucleation. The activation energy of crystal growth in the first stage of transformation is 20.1 kcal mole^-1 ±20%. This suggests that the hexacelsian-celsian transformation does not involve Si-0 and Al-O bond openings, since this would be expected to involve an activation energy of at least 60 kcal mole^-1.