Preparation and crystallization kinetics of silica-based ceramic cores modified by alumina
Alumina is a common mineralizer in silica-based ceramic cores,but its promotion or inhibition effects on the crystallization of fused-silica are in contrary conclusions.To explore the effects of different alumina mineralizers on the property and crystallization behavior of silica-based ceramic cores,silica/alumina ceramic cores were fabricated using high-pure alumina and common commercial alumina powders as mineralizers,respectively.The high-pure alumina powders hinder the crystallization of cristobalite,but the common alumina powders promote the crystallization behavior.The common alumina powders have larger amounts of impurities than that of high-pure alumina powders,including alkali oxides and alkaline-earth oxides,which highly decrease the crystallization activation energy and promote the crystallization of cristobalite.There is significant difference in the effects of two alumina mineralizers on the linear shrinkage rate,densification,and mechanical properties of ceramic cores.Due to the common alumina powders promoting the crystallization and sintering densification of silica glass,the resistance of creep deformation is improved.When the content of common alumina powders is 5％ (mass fraction),the ceramic cores exhibit superior performance,including linear shrinkage of 0.63％,room-temperature flexture strength of 20.2 MPa,and deflection of 0.23 mm via double cantilever beam method at 1540 ℃/0.5 h,meeting the requirement for precision casting.
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