Kinetics of the β″‐to‐β Transformation in the System Na 2 O‐A1 2 O 3

Mixtures of α‐A1 2 O 3 and NaAlO 2 corresponding to the eutectic composition 65 mot % A1 2 O 3 were heated in sealed platinum crucibles above the eutectic temperature (measured to be 1572°C in this study). Once melting was complete, the liquid was quenched and the resultant solid examined by X‐ray diffraction. Sodium aluminate, beta alumina, and beta″ alumina were detected in all cases. However, the relative amounts of beta and beta″ present were a function of the cooling rate, with the amount of beta″ increasing with a decreasing cooling rate. Subsequent annealing (in platinum envelopes) of the solidified eutectic at 1400° and 1530°C showed that the amount of beta″ present decreased with increasing annealing time. Based on this observation, it is apparent that, in the temperature range studied, beta″‐is metastable. Analysis of the time‐transformation data shows that the reaction kinetics are consistent with the diffusion‐controlled growth of the beta phase at the expense of the beta″‐phase.