SAXS/WAXS studies of the devitrification of Cr-doped cordierite glass using synchrotron radiation

In situ simultaneous Small Angle X-ray Scattering (SAXS) and Wide Angle X-ray Scattering (WAXS) experiments have been carried out on Cr-doped cordierite (Mg₂Al₄Si₅O₁₈) glass. Samples were initially pre-treated at 875°C close to the glass transition to promote nucleation and this was followed by further treatments at 920°C and 1040°C to generate dispersed crystal growth. The initial treatment revealed no changes in the SAXS or the WAXS data, but the subsequent treatments at higher temperatures revealed large changes in microcrystallinity developing with time. In particular taking the SAXS data for q < 0.05 A ̊^-1 the average Guinier radius, R_g, was found to increase with heat treatment from initial values of ∼ 10 Å to final values of ∼ 30 Å. I(q) versus q^-n plots (for q ≥ 1 R_g) revealed increasing values of n indicative of changes in mesostructure from rough to smooth fractal architecture. From the WAXS data that was obtained concurrently two separate crystalline phases were identified during devitrification-spinel (MgAl₂O₄) and a stuffed quartz (Mg_xAl_2xSi_1-xO_{2(1 + x)})-each growing at different rates. The growth of spinel correlates well with the SAXS invariant, reflecting the major contrast in electron density between this phase and that of the residual glass. For the heat treatment conditions employed, WAXS reveals that the stuffed quartz phase appears later than the spinel phase and also grows less rapidly. For prolonged annealing, a SAXS feature develops at ∼ 0.022 Å^-1, corresponding to an average interparticle distance of ∼ 280 Å. This is considerably longer than the statistical separation of Cr atoms in the glass (14 Å) and suggests some clustering of Cr takes place as devitrfication progresses.