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°–۱۵۰۰°C. The clay
component dehydroxylated to metakaolin at ;550°C. Metastable
sanidine formed from decomposition of the feldspar at
>600°C and dissolved at >900°C. Liquid formation at
;۱۰۰۰°C was associated with melting of feldspar and silica
discarded from metakaolin formation via the K2O–Al2O3–
SiO2 eutectic. Liquid content increased at 1000°–۱۲۰۰°C with
further feldspar melting and additionally at >1200°C because
of quartz dissolution. Small (<7 nm) mullite and g-alumina
crystals precipitated in pure clay relicts and larger (<30 nm)
mullite crystals in mixed clay-feldspar relicts at 1000°C. 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°C, 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
۱۴۰۰°C, mullite began to dissolve.