Firing of illite-based ceramics followed by dynamical thermomechanical analysis

Abstract

Illitic clay (Tokai region, Hungary) is investigated using Young's modulus, which is determined by the dynamical thermomechanical analysis in the temperature cycle 20 °C → 1100 °C → 20 °C. Supplementary analyses are thermodilatometry and thermogravimetry. The development of bulk density with temperature is also evaluated. The bulk density decreases due to mass loss and weak thermal expansion. Despite these findings, Young's modulus doubled its values. It can be explained by the tighter position of clay crystals after a release of the physically bound water. In the temperature interval of 300 °C → 700 °C Young's modulus decreases linearly because no structural and microstructural changes took place. Dehydroxylation illite is presented at the temperature above 450 °C, but only a small decrease of Young's modulus at 700 °C could be ascribed to dehydroxylation of illite. Both, the solid-state sintering and following the liquid-state sintering increases values of Young's modulus at 700 °C → 1100 °C → 750 °C. Microcracking begins after reaching the glass transition temperature as a result of different thermal expansions of quartz and other minerals. The sharp V-shape minimum of Young's modulus and its short recovery is observed. They are caused by the reversal of radial and tangential stresses acting on the quartz grains between 600 °C → 500 °C. The microcracking continued and Young's modulus decreases its values at 500 °C → 20 °C.