Effect of Monovalent Cation Additives on the γ‐Al ₂ O ₃ ‐to‐α‐Al ₂ O ₃ Phase Transition

Abstract

The effect of monovalent cation addition on the γ‐Al 2 O 3 ‐to‐α‐Al 2 O 3 phase transition was investigated by differential thermal analysis, powder X‐ray diffractometry, and specific‐surface‐area measurements. The cations Li + , Na + , Ag + , K + , Rb + , and Cs + were added by an impregnation method, using the appropriate nitrate solution. β‐Al 2 O 3 was the crystalline aluminate phase that formed by reaction between these additives and Al 2 O 3 in the vicinity of the γ‐to‐α‐Al 2 O 3 transition temperature, with the exception of Li + . 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 + was concluded to be the most effective of the present monovalent additives for enhancing the thermal stability of γ‐Al 2 O 3 . Because the order of the phase‐transition temperature coincided with that of the formation temperature of β‐Al 2 O 3 in these samples, suppression of ionic diffusion in γ‐Al 2 O 3 by the amorphous phase containing the added cations must have played an important role in retarding the transition to α‐Al 2 O 3 . Larger cations suppressed the diffusion reaction more effectively.