Characterisation of clay sintering process using impedance spectroscopy

Impedance measurements were made in situ while a clay compact was being fired at different temperatures. The measured impedance spectra consist of a high frequency (HF) semicircle arc and a low frequency (LF) tail. By employing an equivalent circuit of the clay compact to simulate the impedance spectra, we have obtained values for electrical properties and the parameters of constant phase elements (CPEs) corresponding to both bulk specimen and electrode effects. Both Arrhenius plot of specimen conductance and the dielectric loss curve demonstrate a phase transition occurring in the clay at a temperature between 900 and 950 °C. Such a phase transition was believed to be the formation of a liquid phase, which was also confirmed by using XRD technique and dilatometer analysis. The variation of specimen conductance as a function of time, which was obtained from isothermal impedance measurements, was found to be in agreement with the shrinkage curve. An equation relating the electrical conductance to the density of the specimen has been established, which has been verified by examining the densification of clay during sintering. Therefore, it is promising to use impedance measurements for examining the sintering of a wide range of ceramics in situ.