Microstructural observation of ZnAl2O4 formation affected by the physical nature of Al2O3 in the presence of LiF
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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.
KeywordsAl2O3 Liquid Phase Physical Nature Microstructural Observation Firing Temperature
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