The Quantification of Different Forms of Cristobalite in Devitrified Alumino-Silicate Ceramic Fibres

When heat treated, amorphous alumino-silicate fibres will devitrify into crystalline phases which on cooling become stable at room temperature. X-ray diffraction has been used to identify and quantify the phases present in such products; mullite and two distinct forms of cristobalite have been observed. One form of cristobalite has been identified as α-cristobalite, the second form has been designated as α′-cristobalite. An internal standard approach was used to enable calibration curves to be obtained for the three crystallite phases, including the α′- cristobalite for which no commercially available standard exists. The accuracy of the methods was estimated as ± 5 wt% for all three crystalline phases present. This was achieved by quantifying the proportion of α′-cristobalite in a standard alumino-silicate fibre after heat treatment for 48 h at 1773 K and then using this material as the `standard' for α′-cristobalite. This `standard' contained 32.9 wt% α′-cristobalite, 1.6 wt% α-cristobalite and 65.5 wt% mullite. The exact nature of the α′- cristobalite phase has also been examined using high-temperature X-ray diffraction and differential scanning calorimetry. It has been shown to be more similar in nature to α-cristobalite than β-cristobalite since it undergoes an α′ → β phase transition on heating. Compared with α-cristobalite it has a different lattice size, and both the temperature and enthalpy of the α′ → β phase transition are lower than for the α → β phase transition. It has been proposed that the α′-cristobalite observed is a defect form of α-cristobalite but with a constant amount of defects/substituted cations which gives it a well defined and consistent structure.