IN-SITU SYNTHESIS OF CERAMICS AT ELEVATED TEMPERATURES. 1M.A. Roberts, 1G. Sankar, 2G.N. Greaves, 1C.R.A. Catlow, 1J.M. Thomas, 3L.M. Colyer, 3R.H. Jones, 2E.G. Giulotto, 1The Royal Institution of G.B., 21, Albemarle Street, London. W1X 4BS, 2CCLRC Daresbury Laboratory, Daresbury, Warrington. WA4 4AD U.K. 3Chemistry Department, Keele University, Staffordshire. ST5 5BG U.K.
The formation of ceramic materials from precursor sol-gels and zeolitic systems has been investigated by combined QuEXAFS/XRD and SAXS/WAXS, measured in-situ, under isothermal or ramping temperature regimes upto 1200deg.C. A new high temperature environmental cell, with the advantage of measuring both fluorescence and transmission EXAFS data, and having a wide (10deg. - 70deg. 2[[Theta]]) angular range for the XRD, has been designed in order to improve temperature uniformity at 1000deg.C and sample support for fragile specimens, a similar furnace being used for the SAXS/WAXS measurements. A variety of materials have been studied, including PLZT, LSCF's, cordierite glasses, phase transitions in oxides, nanocrystalline materials, and zeolites as precursors to ceramics.
The crystallisation of PLZT (lead lanthanum zirconate titanate) from an alkoxide sol-gel shows a three stage evolution, an amorphous precursor, a pyrochlore/fluorite intermediate and the final perovskite phase. From the in-situ measurements it is possible to obtain quantitative kinetic information, the activation energy for crystallisation depending on the La concentration.
The combined QuEXAFS/XRD and SAXS/WAXS studies on the collapse of zeolites A and Y has enabled the dilute crystallography of Cd-exchanged sites, followed by Cd K-edge EXAFS, to be directly related to the altering mesostructure defined by the SAXS, on structural change from microporous crystalline to amorphous alumino-silicate as the glass transition is approached, by comparison between the WAXS diffraction profiles. Within the temperature regime investigated zeolite A is found to recrystallise into carnegieite and nepheline, whereas zeolite Y tends to remain amorphous.