Prominent Nanocrystallization of 25K ₂ O·25Nb ₂ O ₅ ·50GeO ₂ Glass

Abstract

Some K 2 O‐Nb 2 O 5 ‐GeO 2 glasses are prepared, and their crystallization behaviors are examined. 25K 2 O·25Nb 2 O 5 ·50GeO 2 glass with the glass transition temperature T g = 622°3C and crystallization onset temperature T x = 668°3C shows a prominent nanocrystallization. The crystalline phase is K 3,8 Nb 5 Ge 3 O 20,4 with an orthorhombic structure. The sizes of crystals in the crystallized glasses heat‐treated at 630° and 720°3C for 1 h are °10 and 20–30 nm, respectively, and the crystallized glasses obtained by heat treatments at 620°‐850°3C 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°3C for 1 h is estimated to be ∼35%. The usual Vickers hardness and Martens hardness (estimated by nanoindentation) of 25K 2 O·25Nb 2 O 5 ·50GeO 2 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.