Gel synthesis and crystallization of lithium-germanate glass powders

Lithium germanate gels, whose compositions are expressed by the general formula Li₂O·xGeO₂ with x=3; 4; 7, were synthesized by hydrolytic polycondensation of germanium ethoxide with lithium methoxide (x=3 and 7) or lithium hydroxide monohydrate (x=4) in alcoholic medium. The values of glass transition temperature of the gels exhibit a maximum at x=4. Crystallization behaviour of the gels, examined by differential thermal analysis and X-ray diffraction, is reported and discussed. The terms x=4 and 7 crystallize in two steps. Microcrystallites of the same composition of the gel are initially formed in an amorphous matrix and are then converted at higher temperatures into well shaped crystals. In term x=3, Li₂GeO₃ and Li₂Ge₄O₉ crystals are directly formed. The values of activation energy for each crystallization step are consistent with the crystallization mechanisms and comparable with those reported with conventional melt glasses from oxides.