Prominent nanocrystallization of 25K₂O·25Nb₂O₅·50GeO₂ glass

Some K₂O-Nb₂O₅-GeO₂ glasses are prepared, and their crystallization behaviors are examined. 25K₂O·25Nb₂O₅·50GeO₂ glass with the glass transition temperature T_g = 622°C and crystallization onset temperature T_x = 668°C shows a prominent nanocrystallization. The crystalline phase is K_3.8Nb₅Ge₃O_20.4 with an orthorhombic structure. The sizes of crystals in the crystallized glasses heat-treated at 630° and 720°C for 1 h are ∼10 and 20-30 nm, respectively, and the crystallized glasses obtained by heat treatments at 620°-850°C for 1 h maintain good transparency. The density of crystallized glasses increases gradually with increasing heat-treatment temperature, and the volume fraction of crystals in the sample heat-treated at 630°C for 1 h is estimated to be ∼35%. The usual Vickers hardness and Martens hardness (estimated by nanoindentation) of 25K₂O·25Nb₂O₅·50GeO₂ glass change steeply by heat treatment at T_g, i.e., at around 35% volume fraction of nanocrystals. The present study demonstrates that the composite of nanocrystals and the glassy phase has a strong resistance against deformation during Vickers indenter loading in crystallized glasses.