Effects of Amorphous and Crystalline SiO2 Additives on γ‐Al2O3‐to‐alpha‐Al2O3 Phase Transitions

Abstract

This paper focused on the effects of various phases of SiO2 additives on the γ‐Al2O3‐to‐α‐Al2O3 phase transition. In the differential thermal analysis, the exothermic peak temperature that corresponded to the theta‐to‐α phase transition was elevated by adding amorphous SiO2, such as fumed silica and silica gel obtained from the hydrolysis of tetraethyl orthosilicate. In contrast, the peak temperature was reduced by adding crystalline SiO2, such as quartz and cristobalite. Amorphous SiO2 was considered to retard the γ‐to‐α phase transition by preventing γ‐Al2O3 particles from coming into contact and suppressing heterogeneous nucleation on the γ‐Al2O3 surface. On the other hand, crystalline SiO2 accelerated the α‐Al2O3 transition; thus, this SiO2 may be considered to act as heterogeneous nucleation sites. The structural difference among the various SiO2 additives, especially amorphous and crystalline phases, largely influenced the temperature of γ‐Al2O3‐to‐α‐Al2O3 phase transition.