Effects of Amorphous and Crystalline SiO 2 Additives on γ‐Al 2 O 3 ‐to‐alpha‐Al 2 O 3 Phase Transitions

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Abstract
This paper focused on the effects of various phases of SiO 2 additives on the γ‐Al 2 O 3 ‐to‐α‐Al 2 O 3 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 SiO 2 , such as fumed silica and silica gel obtained from the hydrolysis of tetraethyl orthosilicate. In contrast, the peak temperature was reduced by adding crystalline SiO 2 , such as quartz and cristobalite. Amorphous SiO 2 was considered to retard the γ‐to‐α phase transition by preventing γ‐Al 2 O 3 particles from coming into contact and suppressing heterogeneous nucleation on the γ‐Al 2 O 3 surface. On the other hand, crystalline SiO 2 accelerated the α‐Al 2 O 3 transition; thus, this SiO 2 may be considered to act as heterogeneous nucleation sites. The structural difference among the various SiO 2 additives, especially amorphous and crystalline phases, largely influenced the temperature of γ‐Al 2 O 3 ‐to‐α‐Al 2 O 3 phase transition.

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