Crystallization behavior and microstructure of lithium-calcium aluminogermanate glasses

The crystallization behavior and microstructure of lithium-calcium aluminogermanate (LCAG) glasses have been studied by using differential thermal analysis (DTA), x-ray diffraction (XRD), scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), and electron diffraction (ED). Uniform crystallization of the LCAG glass was found to result from two stages of the heating process. The kinetics of crystallization of the LCAG glasses was studied by DTA using the nonisothermal method. The activation energy for 3CaO · Al2O3 · 3GeO2 crystal growth was 693 kJ/mol. The precipitated crystals determined by XRD analysis were mainly 3CaO · Al2O3 · 3GeO2, and minor phases of 2CaO · Al2O3 · GeO2 and Li2O · Al2O3 · 2GeO2. Morphology and microstructure of the glasses after heat treatment determined by SEM and STEM techniques are presented. Crystallization starts at the surface of the glass sample and then proceeds toward the interior of glass matrix. The morphology of 2CaO · Al2O3 · GeO2 is that of a subangular bell-shaped single crystal growing in a preferred orientation through the segregated phase matrix of fine dispersion of 3CaO · Al2O3 · 3GeO2 crystals. The Li2O · Al2O3 · 2GeO2 phase grows anisotropically in the fine fibrillar morphology and parallel to the [331].