Controlled crystallization and ionic conductivity of a nanostructured LiAlGePO4 glass–ceramic

A Li1.5[Al0.5Ge1.5(PO4)3] glass composition was subjected to several crystallization treatments to obtain glass–ceramics with controlled microstructures. The glass transition (Tg), crystallization onset (Tx) and melting (Tm) temperatures of the parent glass were characterized by differential scanning calorimetry (DSC). The glass has a reduced glass transition temperature Tgr = Tg/Tm = 0.57 indicating the possibility of internal nucleation. This assumption was corroborated by the similar DSC crystallization peaks from monolithic and powder samples. The temperature of the maximum nucleation rate was estimated by DSC. Different microstructures were produced by double heat treatments, in which crystal nucleation was processed at the estimated temperature of maximum nucleation rate for different lengths of time. Crystals were subsequently grown at an intermediate temperature between Tg and Tx. Single phase glass–ceramics with Nasicon structures and grain sizes ranging from 220 nm to 8 μm were then synthesized and the influence of the microstructure on the electrical conductivity was analysed. The results showed that the larger the average grain size, the higher the electrical conductivity. Controlled glass crystallization allowed for the synthesis of glass–ceramics with fine microstructures and higher electrical conductivity than those of ceramics with the same composition obtained by the classical sintering route and reported in literature.