Earliest stages of crystal growth in a silicate glass containing titanium and zirconium as nucleating elements — HRTEM and XAS study

Abstract

The earliest stages of crystallization have been studied for a SiO 2-Al 2O 3-Li 2O glass-ceramics containing a few percent of titanium and zirconium as nucleating elements. After heat treatments at temperatures ranging between T=750°C and T=860°C, the growth process has been studied by combining HRTEM (high resolution transmission electron microscopy) and XAS (X-ray absorption spectrometry). A fine phase separation has been found in the parent glass (≈ 7 nm in size) independent of the presence of the nucleating elements. From HRTEM, the earliest crystalline phase has been definitely stated as being TiZrO 4. The incorporation of titanium atoms into TiZrO 4 crystals is correlated with a change of their coordinance from 4 to 6, directly evidenced in XAS spectra by the “prepeak” analysis at titanium K-edge. The crystallization of the aluminosilicate occurs after full precipitation of TiZrO 4. In glass ceramics, the number of aluminosilicate and TiZrO 4 crystals per unit volume are nearly equal, in good agreement with heterogeneous nucleation of the silicate solid solution on TiZrO 4 crystals. For samples heat treated at 750°C and 780°C, the number of TiZrO 4 crystallites is constant for a given temperature. These crystallites grow until the matrix becomes free of zirconium atoms. Their growth activation energy has been found to be 400±50 kJ/mol.