Stress-induced β→α-cristobalite phase transformation in (Na2O+Al2O3)-codoped silica

Colloidal gel-derived silica (SiO2) powder codoped with (Na2O+Al2O3) was sintered at 1100 °C. The crystalline phase content and phase transformation of the sintered ceramics have been studied via X-ray diffractometry and scanning electron microscopy. The amount of β-cristobalite retained metastably in the mixture to room temperature is found to depend on the level of additives. Samples codoped with Na2O and Al2O3, both of 6.30 mol%, were found to contain only β-cristobalite in the crystalline mixture, and which is known as the chemically stabilized cristobalite (CSC). Multiple liquid phase separation is also observed in the codoped samples. The lattice spacing d101α of α-cristobalite increases with the doping level while d111β of the β-phase remains almost unchanged in all compositions studied. Surface grinding or pulverizing of the sintered samples into powder induces the β→α-cristobalite phase transformation. The mechanism of the high temperature β-cristobalite stabilization to room temperature associated with both the chemical and mechanical terms is discussed.