Microstructural observation of ZnAl2O4 formation affected by the physical nature of Al2O3 in the presence of LiF

Microstructures of ZnA₂O₄ formation in the presence of LiF were observed for specimens including both coarse and dense alumina and agglomerates of fine alumina, and their morphological changes were compared with each other. LiF formed an intermediate liquid phase with ZnO and Al₂O₃. In the specimen with agglomerated Al₂O₃, the porous ZnAl₂O₄ layer was developed from the Al₂O₃ agglomerates. However, on firing at 700° C, zeta-lithium aluminate developed just inside the ZnAl₂O₄ layer and interfered with the spreading of the liquid including Al₂O₃ to the ZnO phase. However, on firing above 800° C, the formation of ZnAl₂O₄ was promoted by the rapid spreading of the fluoride liquid including Al₂O₃ to the ZnO phase through the porous ZnAl₂O₄ layer before LiAl₅O₈ formation. In the specimen with coarse and dense Al₂O₃, the dense ZnAl₂O₄ layer grew around the coarse and dense Al₂O₃ and the fluoride liquid phase was confined between the ZnAl₂O₄ layer and the Al₂O₃ particles. The dense ZnAl₂O₄ layer interrupted the spread of the liquid phase to the ZnO phase and interfered with further formation of ZnAl₂O₄. The confined liquid phase gradually reacted with Al₂O₃ to form LiAl₅O₈. It is necessary for the fluoride liquid including Al₂O₃ to spread out to the ZnO phase through the porous ZnAl₂O₄ layer to form ZnAl₂O₄, before the dense ZnAl₂O₄ or LiAl₅O₈ layer grew around Al₂O₃. The use of fine Al₂O₃ powder and a high firing temperature were effective in the promotion of ZnAl₂O₄ formation in the presence of LiF.