Thermal characteristics and crystallization behavior of zinc borosilicate glasses containing Nb2O5

Glasses based on the molar composition 60ZnO·(40-x)B2O3·xSiO2, x = 10 or 20, with small additions of Nb2O5 (2.5, 5, 7.5 and 10 mol%), were prepared by melt-quenching. Increasing the Nb2O5 content decreased the glass transition and softening temperatures. Thermal stability increased with Nb2O5 addition and it was higher for glasses based on the composition having the highest SiO2 content. The thermal expansion coefficient tended to increase with the Nb2O5 amount. Zinc borate and zinc niobate were identified in glasses containing Nb2O5 ≤ 5 mol%, however in glasses based on the highest SiO2 composition, an additional crystalline phase, willemite, was detected. At the highest Nb2O5 additions, willemite formation was suppressed and zinc niobate plus zinc borate were detected. The non-isothermal crystallization kinetics was investigated, and the effect of Nb2O5 addition and of the base glass composition on the crystallization activation energy and crystallization mechanism was discussed.