Microstructural analysis (Voigt function method) of mullite in whiteware triaxial porcelains

Abstract

Triaxial porcelains were produced of two whiteware blends containing kaolin B (of “high-crystallinity” kaolinite) or halloysitic kaolin M (also with “low-crystallinity” kaolinite) in two different industrial firing cycles (fast or slow) with final temperatures 1270 °C, 1300 °C, 1320 °C and 1340 °C. The crystalline microstructure of mullite in all porcelain samples was studied by X-ray diffraction through analysing 110 and 001 reflections using the Voigt function method and by optical and electron microscopy. Mean crystallite sizes were determined independently for both 110 and 001 diffraction directions as principal semi-quantitative characteristics for all sixteen specimens. They illustrated well the influence of kaolin kind, firing cycle and processing temperature on crystalline microstructure. Significant differences were observed as functions of each of these three parameters for most pairs of specimens. The differences of mean crystallite sizes were significant for 110 diffraction direction and smaller for 001 (especially for samples fired in higher temperatures). Additionally, the influence of fluxes of a triaxial composition on growth of mullite crystallites was shown by comparing crystallite sizes of mullite in the studied industrial porcelains to those produced by firing a reference kaolin. In most samples positive correlation of mean crystallite size estimated for [001] direction with length of crystalline prism estimated from electron microscopy images was noted, and this can be considered relevant for the investigation of formulations and processing conditions, since microstructural observations by electron microscopy require much longer sample preparation times, and are less representative of the studied whole sample.These results are significant in formulation of industrial porcelains in order to select both raw materials and firing cycles for obtaining required mullite development. That will improve the technical properties of final industrial porcelain.