The sintering of porcelain stoneware tiles - a low porosity products with excellent technical performances - involves a complex evolution of both phase composition and chemistry of the liquid phase, that were investigated on seven body formulations (prevalently sodic or potassic and mixed Na-K) consisting of a mixture of ball clay, quartz and feldspars. Samples were characterized by isothermal and constant rate optical TDA (hot stage microscope) and quantitative XRPD on quenched specimens. In the early stage of sintering - when strong reduction of specific surface occurs without shrinkage - the main transformations are breakdown of clay minerals with formation of amorphous components, then over 1000 °C mullite and viscous phase. The intermediate step - accounting for most densification - involves a viscous flow driven by fast formation of abundant liquid phase, with feldspars-quartz eutectics composition. Already at 1100 °C, feldspars are mostly fused and the chemical evolution of melt proceeds with gradual enrichment in silica and constant Na/K ratio. At the temperature of industrial sintering (~1200 °C) the compositions of the viscous phase plot in a plateau in the SiO2-NaAlSi3O8-KAlSi3O8 system, corresponding to low temperature melts, that is extended from sodic to potassic terms. Overall, the behavioural difference of Na-rich and K-rich bodies depends more on kinetics of phase evolution (faster in sodic batches) and viscosity of the liquid phase (higher in potassic ones). In the final stage, a decreasing densification rate is contrasted by a coarsening mechanism, promoted by the wide range of pore and particle sizes of green compacts, and an important role is probably played by changes in the solubility of solids and gases in the liquid phase.
Phase transformations during liquid phase sintering of porcelain stoneware tiles: a petrological approach
BECCALUVA, Luigi;VACCARO, Carmela
2003
Abstract
The sintering of porcelain stoneware tiles - a low porosity products with excellent technical performances - involves a complex evolution of both phase composition and chemistry of the liquid phase, that were investigated on seven body formulations (prevalently sodic or potassic and mixed Na-K) consisting of a mixture of ball clay, quartz and feldspars. Samples were characterized by isothermal and constant rate optical TDA (hot stage microscope) and quantitative XRPD on quenched specimens. In the early stage of sintering - when strong reduction of specific surface occurs without shrinkage - the main transformations are breakdown of clay minerals with formation of amorphous components, then over 1000 °C mullite and viscous phase. The intermediate step - accounting for most densification - involves a viscous flow driven by fast formation of abundant liquid phase, with feldspars-quartz eutectics composition. Already at 1100 °C, feldspars are mostly fused and the chemical evolution of melt proceeds with gradual enrichment in silica and constant Na/K ratio. At the temperature of industrial sintering (~1200 °C) the compositions of the viscous phase plot in a plateau in the SiO2-NaAlSi3O8-KAlSi3O8 system, corresponding to low temperature melts, that is extended from sodic to potassic terms. Overall, the behavioural difference of Na-rich and K-rich bodies depends more on kinetics of phase evolution (faster in sodic batches) and viscosity of the liquid phase (higher in potassic ones). In the final stage, a decreasing densification rate is contrasted by a coarsening mechanism, promoted by the wide range of pore and particle sizes of green compacts, and an important role is probably played by changes in the solubility of solids and gases in the liquid phase.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
