PHASE-FORMATION KINETICS IN A GLASS-CERAMIC STUDIED BY SMALL-ANGLE SCATTERING OF X-RAYS AND NEUTRONS AND BY VISIBLE-LIGHT SCATTERING

The kinetics of the primary phase formation in a glass ceramic base gl ass near the cordierite composition in the MgO-Al2O3-SiO2 system, with additions of ZrO2 and TiO2 as nucleating agents, have been investigat ed by combining different small-angle scattering techniques. Phase for mation in the base glass heated isothermally at 1093 K has been studie d in order to elucidate the structural processes that take place durin g controlled nucleation before volume crystallization begins, Compleme ntary results of X-ray diffraction, transmission electron microscopy a nd electron microprobe analysis reveal that in the final stages of ann ealing, different crystalline phases, which are dispersed in the resid ual glassy phase, are developed, The largest particles have sizes of t he order of several hundreds of nanometers and consist of magnesium pe talite as well as a high-quartz solid solution. Besides these crystals , two populations of smaller particles, the sizes of which can be reso lved by small-angle scattering, were detected and found to be enriched with TiO2 and/or ZrO2. The growth of particles of intermediate size w as studied in situ. The radius, R, of each of these particles increase s with increasing treatment time, t, according to the power law R prop ortional to t(1/2), indicating diffusion-limited Independent growth. T he scattering patterns are modified by diffusion zones surrounding the growing particles. The characteristics of the diffusion zones are eva luated by fitting appropriate model scattering curves to the experimen tal scattering data. The structural arrangement of the Ti and Zr ions was studied by contrast variation with the help of anomalous small-ang le X-ray scattering and small-angle neutron scattering, As revealed by the contrast variation experiments, Ti ions are concentrated in the i nterface between the Intermediate-sized particles and the glass matrix , whereas Zr is homogeneously distributed in these particles, Addition ally, the Zr ions participate in the formation of the smallest particl es, which are crystallites enriched with ZrO2. Finally the catalyzing effect of TiO2 and ZrO2 on the nucleation and volume crystallization o f the glass ceramic is discussed.