High-temperature Young’s moduli and dilatation behavior of silica refractories

Abstract

Silica refractories with cristobalite–tridymite ratios between 0.83–0.86 and 1.43–1.47 and porosities of 13–17% are characterized by impulse excitation up to 1200°C and by dilatometry up to 1300°C. During heating, Young’s moduli start to decrease from their room temperature values (9–12GPa) to about 5–7GPa at 200°C, followed by a very steep increase at around 230°C to values higher than the room temperature values and a nonlinear increase to their final high-temperature values. During cooling, Young’s moduli increase even further, exhibit a maximum and a nonlinear decrease that does not follow the heating curve, a very steep decrease at around 200°C and a final increase to the intial room temperature values, so that a closed loop results during thermal cycling. Dilatometric measurements confirm that the strong increase of the Young’s modulus at around 230°C (during heating) can be attributed to the phase transition between low- and high-temperature cristobalite, while the more gentle—but still very significant—changes in Young’s moduli below 200°C are due to the more diffuse transitions between tridymite subpolymorphs. Microcracks are identified as the primary cause of the low Young’s moduli and their increase with temperature.