Microstructural study of opaque glazes obtained from frits of the system: SiO_2-Al_2O_3-B_2O_3-(P_2O_5)-CaO-K_2O-TiO_2

摘要

Opaque ceramic tile glazes can be obtained by using a type of frit based on the system SiO_2-Al_2O_3-B_2O_3-CaO-K_2O-TiO_2. In these glazes, opacification and whiteness are due to the presence of titanite (sphene) crystals, formed by devitrification during firing from the frit applied in the form of a glaze composition onto the ceramic body. Though not widely used in tile manufacture, this type of frit is considered an alternative to the zirconium-containing frits currently used for fabricating opaque ceramic glazes. The present study shows that the addition of small quantities (around 1 percent, w/w) of P_2O_5 to a frit composition of the above oxide system considerably modifies resulting glaze gloss, converting a matt glaze into a glossy glaze. In order to determine how the P_2O_5 addition contributes to increasing gloss in opaque white wall tile glazes, we determined the arising changes in glaze melt viscosity and glaze microstructure, establishing the latter by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The results show that the presence of P_2O_5 inhibits wollastonite devitrification in the glaze melt and gives rise to changes in sphene crystal morphology, modifying glaze microstructure.