Effect of monovalent cation additives on the gamma -Al sub 2 O sub 3 -to- alpha -Al sub 2 O sub 3 phase transition

• Published in: Journal of the American Ceramic Society Vol. 83; no. 5; pp. 1233 - 1236
• Main Authors: Okada, K, Hattori, A, Kameshima, Y, Yasumori, A, Das, R N
• Format: Journal Article
• Language: English
• Published: 01.05.2000
• ISSN: 0002-7820

The effect of monovalent cation addition on the gamma -Al sub 2 O sub 3 -to- alpha -Al sub 2 O sub 3 phase transition was investigated by differential thermal analysis, powder x-ray diffractometry, and specific-surface-area measurements. The cations Li exp + , Na exp + , Ag exp + , K exp + , Rb exp + , and Cs exp + were added by an impregnation method, using the appropriate nitrate solution. beta -Al sub 2 O sub 3 was the crystalline aluminate phase that formed by reaction between these additives and Al sub 2 O sub 3 in the vicinity of the gamma -to alpha -Al sub 2 O sub 3 transition temperature, with the exception of Li exp + . The transition temperature increased as the ionic radii of the additive increased. The change in specific surface area of these samples after heat treatment showed a trend similar to that of the phase-transition temperature. Thus, Cs exp + was concluded to be the most effective of the present monovalent additives for enhancing the thermal stability of gamma -Al sub 2 O sub 3 . Because the order of the phase-transition temperature coincided with that of the formation temperature of beta -Al sub 2 O sub 3 in these samples, suppression of ionic diffusion in gamma -Al sub 2 O sub 3 by the amorphous phase containing the added cations must have played an important role in retarding the transition to alpha -Al sub 2 O sub 3 . Larger cations suppressed the diffusion reaction more effectively.