Ultra-high thermal expansion glass–ceramics in the system MgO/Al2O3/TiO2/ZrO2/SiO2 by volume crystallization of cristobalite

Glasses in the system MgO/Al2O3/TiO2/ZrO2/SiO2 with and without the addition of As2O3 and Sb2O3 were thermally treated. Up to a temperature of 950 °C, this resulted in the formation of ZrTiO4, sapphirine and high quartz solid solution. Annealing at higher temperatures led to the formation of low quartz solid solutions, ZrTiO4 and sapphirine. This resulted in a continuous increase of density, hardness, fracture toughness and thermal expansivity. In the glass doped with As2O5 and Sb2O5 annealing temperatures >1000 °C resulted in the formation of cristobalite instead of quartz. Then the density, hardness and strength decreased again, while the fracture toughness (up to 2.8 MPa m1/2) and the thermal expansion coefficient increased strongly. In the dilatometric curves, a steep increase in expansion is observed in the temperature range from 100 to 200 °C, which is attributed to the transformation of low cristobalite to high cristobalite. The mean linear thermal expansion coefficient (25–200 °C) is 20 × 10−6 K−1 and the largest up to now reported in the literature for alkali-free silicate glass–ceramics.