Abstract
Abstract The DTA patterns of synthesized coprecipitated gels, SiO2–Al2O3–Ga2O3–H2O system, resembled that of allophane, and the exothermic peak temperature in the DTA was lowered continuously from 980 to 810 °C with an increase in the gallium content, while that on mixed gels scarcely lowered. The apparent activation energy for this exothermic reaction on the coprecipitated gels decreased with an increase in the gallium content, while that on the mixed gels hardly changed. The crystallization process during the heat treatment of the gels depended on the content and on the coexistence state of the gallium. A unit cell volume of a mullite crystallized at 1200 °C in the coprecipitated gels was enlarged by forming a solid solution with the gallium ion, in contrast to the small change in that formed in the mixed gels. Further, the exothermic peak in the DTA was considered to originate not only from the formation of the mullite, but also from that of the silico-alumina spinel in the gallium-bearing gels; the exothermic reaction products seem to depend on the composition or the coexistence state of gallium in the gels.
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