Mullite-based composites can be made by an insitu reaction process using silica-coated alumina (SCA) powderas a mullite precursor. In this paper we present the combinedeffects of zirconia and premullite seeds on the crystallizationprocess and microstructure development. When zirconia isadded without seeding, mullite formation proceeds through theformation of transient zircon. This phase provides a lowerenergy barrier for mullite nucleation and thus lowers themullitization temperature. The presence of yttria as a stabilizerin zirconia reduces the activation energy for zircon formationand thus promotes the transient reaction. The addition ofpremullite seeds results in the nucleation of mullite fromalumina and silica, and zircon does not form. At low seedinglevels mullitization remains nucleation-controlled; however,once the seeding level exceeds 1-2 percent, this is no longer the case.
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