Microstructural evolution in triaxial porcelain

Microstructural evolution in a model triaxial porcelain was studied by X-ra y diffractometry and electron microscopy of quenched samples after firing f or 3 h at 600 degrees -1500 degreesC, The clay component dehydroxylated to metakaolin at similar to 550 degreesC. Metastable sanidine formed from deco mposition of the feldspar at >600 degreesC and dissolved at >900 degreesC, Liquid formation at similar to 1000 degreesC was associated with melting of feldspar and silica discarded from metakaolin formation via the K2O-Al2O3S iO2 eutectic. Liquid content increased at 1000 degrees -1200 degreesC with further feldspar melting and additionally at >1200 degreesC because of quar tz dissolution. Small (less than or equal to7 nm) mullite and gamma -alumin a crystals precipitated in pure clay relicts and larger (less than or equal to 30 nm) mullite crystals in mixed clay-feldspar relicts at 1000 degreesC . In the evolving microstructures, three regions were observed. These regio ns were derived from pure clay relicts containing primary (type-I) mullite; feldspar-penetrated clay relicts, also containing secondary (granular type -II) mullite; and the matrix of fine clay, feldspar, and quartz, containing secondary (granular type-II and elongated type-III) mullite, In addition t o shape, the mullite size changed, increasing from regions containing type- I to type-III mullite, because the increasingly fluid liquid enhanced cryst al growth. Below 1300 degreesC, primary mullite was richer in Al2O3 than th e secondary mullite, and the glass composition was inhomogeneous, with the K2O and Al2O3 contents varying throughout the microstructure. Above 1400 de greesC, mullite began to dissolve.