Application of hot-stage microscopy to evaluating sample morphology changes on heating

Abstract

Investigations of the thermal properties of a raw material or a ceramic product under a hot-stage microscope allow, among others, determining the characteristic temperatures corresponding to the changes of the shape and the cross-section area of samples being heated. They are based on observing and analysing images of the sample recorded with the growth of its temperature. The pictures of changes allow establishing—for the raw materials with the ability of thermal expansion—the technological temperatures of sintering, softening, thermal expansion, melting and flowing. Their knowledge is necessary in determining optimum ranges of manufacture conditions for a number of ceramic processes. Considering some difficulties with precise determination of these characteristic temperatures and with the problem of measuring the coefficient of thermal expansion S of clay raw materials, the authors have proposed hereby a new method of measuring a sample cross-section area on heating under a hot-stage microscope. It has been invented and implemented in the Matlab software environment on the basis of an image analysis of the expanding sample. The changes of its cross-section area (i.e. an outline area visible under a hot-stage microscope) are continuously recorded on the film and can be converted after digital processing into sample volume changes. The method is applicable to precise measuring the coefficient of thermal expansion S of clay raw materials, among others those used in the manufacture of lightweight ceramic aggregates.