Microstructural evolution in triaxial porcelain

Microstructural evolution in a model triaxial porcelain was studied by X-ray diffractometry and electron microscopy of quenched samples after firing for 3 h at 600 degrees -1500 degreesC, The clay component dehydroxylated to metakaolin at similar to 550 degreesC. Metastable sanidine formed from decomposition 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-Al2O3SiO2 eutectic. Liquid content increased at 1000 degrees -1200 degreesC with further feldspar melting and additionally at >1200 degreesC because of quartz dissolution. Small (less than or equal to7 nm) mullite and gamma -alumina 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 regions 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 to shape, the mullite size changed, increasing from regions containing type-I to type-III mullite, because the increasingly fluid liquid enhanced crystal growth. Below 1300 degreesC, primary mullite was richer in Al2O3 than the secondary mullite, and the glass composition was inhomogeneous, with the K2O and Al2O3 contents varying throughout the microstructure. Above 1400 degreesC, mullite began to dissolve.