Studies on the Densification Process of Kaolin-Quartz-Feldspar Triaxial Porcelain Bodies

Abstract

Densification process of the triaxial porcelain bodies consisting of 40-55% kaolin, 15-30% quartz and 20-35% feldspar was investigated kinematically by means of isothermal shrinkage measurement, and microstructurally by X-ray diffraction method and observation with polarization and scanning electron microscopes.From the results of linear thermal expansion and shrinkage measurements of the specimens, it was found that the bodies showed distinct shrinkage of three stages occurred at about 500°, 900° and 1100°C or higher temperatures. It was inferred that the shrinkage occurred at about 500°C was caused by the decomposition of kaolin minerals. The bodies showed complicated expansion and shrinkage processes between 500° and 950°C, but these behaviours could be explained clearly with expansion and shrinkage characters of individual raw material and their content in the bodies. The shrinkage occurred at about 900°C seemed to accompany with diffusion sintering process at metakaolin grain boundary.The shrinkage observed at about 1100°C might be associated with the densification process occurred by viscous flow of spinel type intermediate phase grains with a small amount of melt formed from a little impurities contained in the kaolin and feldspar fine particles. In the range used in this investigation, regular relations between the apparent activation energy for the shrinkage process and compositions of the bodies could not be found. Further, the change of the firing atmosphere from oxidizing to reducing did not affect the shrinkage process and its kinetics.It was inferred that the shrinkage occurred at a higher temperature than 1100°C was also caused by viscous flow mechanism. The shrinkage diminished when almost all pores in the body were closed in the glassy matrix. Further, feldspathic melt spreaded through matrix phase, and its viscosity decreased with rising temperature, then coalescence and expansion of the pores were promoted, when the bodies were fired at 1500°C, all bodies showed bloating. Further, changes of the constituent minerals and microstructure of the bodies with the densification process were described.In general, the vitrification behaviour and microstructure of the triaxial porcelain bodies used in this investigation were not so markedly changed with variation of their compositions. From these results obtained, it was also confirmed that the composition range used was appropriate for the industrial usage.