THE RELATIVE STABILITY OF BETA-PHASE AND BETA''-PHASE IN NA2O-AL2O3 BETA ALUMINA

To reveal the relative stability of β and β″-phases in beta alumina, effects of Na20 content and calcination and annealing history on the relative content of β and β″-phases have been studied for compositions ranging from Na2O-5Al2O3 to Na2O-9.5Al2O3. The relative stability appears to be dependent on the calcination or annealing history, except holding time such as annealing or re-annealing time, but independent of the Na2O content and the holding time. From these results it can be demonstrated that the relative stability of β and β″-phases is sensitive to the heat treatment path, except the holding time. The "two stage model" on the β/α. polytypic transition in silicon carbide has been quoted to explain the heat treatment pathdependent characteristics. © 1990 Chapman and Hall Ltd.