SAXS WAXS STUDIES OF THE DEVITRIFICATION OF CR-DOPED CORDIERITE GLASSUSING SYNCHROTRON-RADIATION/

In situ simultaneous Small Angle X-ray Scattering (SAXS) and Wide Angl e X-ray Scattering (WAXS) experiments have been carried out on Cr-dope d cordierite (Mg2Al4Si5O18) glass. Samples were initially pre-treated at 875 degrees C close to the glass transition to promote nucleation a nd this was followed by further treatments at 920 degrees C and 1040 d egrees C to generate dispersed crystal growth. The initial treatment r evealed no changes in the SAXS or the WAXS data, but the subsequent tr eatments at higher temperatures revealed large changes in microcrystal linity developing with time. In particular taking the SAXS data for q < 0.05 Angstrom(-1) the average Guinier radius, R(g), was found to inc rease with heat treatment from initial values of similar to 10 Angstro m to final values of similar to 30 Angstrom. I(q) versus q(-n) plots ( for q greater than or equal to 1/R(g)) revealed increasing values of n indicative of changes in mesostructure from rough to smooth fractal a rchitecture. From the WAXS data that was obtained concurrently two sep arate crystalline phases were identified during devitrification-spinel (MgAl2O4) and a stuffed quartz (MgxAl2xSi1-xO2(1+x))-each growing at different rates. The growth of spinel correlates well with the SAXS in variant, reflecting the major contrast in electron density between thi s phase and that of the residual glass. For the heat treatment conditi ons employed, WAXS reveals that the stuffed quartz phase appears later than the spinel phase and also grows less rapidly. For prolonged anne aling, a SAXS feature develops at similar to 0.022 Angstrom(-1), corre sponding to an average interparticle distance of similar to 280 Angstr om. This is considerably longer than the statistical separation of Cr atoms in the glass (14 Angstrom) and suggests some clustering of Cr ta kes place as devitrfication progresses.