Journal of Applied Crystallography

Volume 30, Part 4 (August 1997)

research papers

J. Appl. Cryst. (1997). 30, 467-475 [ doi:10.1107/S0021889897001672 ]

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

M. A. Butler and D. J. Dyson

Abstract: 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 -cristobalite, the second form has been designated as '-cristobalite. An internal standard approach was used to enable calibration curves to be obtained for the three crystallite phases, including the '- 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 '-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 '-cristobalite. This `standard' contained 32.9 wt% '-cristobalite, 1.6 wt% -cristobalite and 65.5 wt% mullite. The exact nature of the '- 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 -cristobalite than -cristobalite since it undergoes an ' [rightwards arrow] phase transition on heating. Compared with -cristobalite it has a different lattice size, and both the temperature and enthalpy of the ' phase transition are lower than for the phase transition. It has been proposed that the '-cristobalite observed is a defect form of -cristobalite but with a constant amount of defects/substituted cations which gives it a well defined and consistent structure.