A kinetic study of the quartz-cristobalite phase transition

Cristobalite is a common silica polymorph in ceramics, as it can crystallize in SiO2-rich systems during high temp. processes. Its occurrence in final traditional ceramic bodies remarkably affects their thermal expansion, thus playing an important role in the shrinkage upon cooling. The quartz-cristobalite transformation kinetics is investigated by in-situ isothermal X-ray powder diffraction expts. and then correlated to the av. particle size (〈d〉) of the starting quartz using a model here developed. An Avrami-like rate equation, i.e. α(t) = 1 - exp(- k × t)n, in which the n-term is assumed to account for the dependence on the av. particle size, has provided the best fitting of theor. to exptl. data, yielding activation energy values that range from 181 to 234 kJ mol-1, and exponential n-coeffs. from 0.9 to 1.5. Ex-situ observations have demonstrated that the formation of cristobalite from quartz after 50 min, 2, 4 and 6 h at 1200 and 1300 °C, exhibits a remarkable dependence on 〈d〉 of quartz, showing comparable behaviors in the case of 〈d〉 equal to 15.8 and 28.4 μm, but significant differences for 〈d〉 of 4.1 μm. The formation of cristobalite is boosted remarkably at temp. higher than 1200 °C, with an increase by wt. even of 500%, with respect to its content at lower temp. The method of sample prepn. (dry powder, wet powder and tablet of compressed dry powder) seems to influence the results only at temp. > 1200 °C and in the case of fine powder.