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.