Thermal properties and glass formation in the SiO₂-B ₂O₃-Bi₂O₃-ZnO quaternary system

Lead-free glasses in the SiO₂-B₂O₃-Bi ₂O₃-ZnO quaternary system were studied. The glass formation region, as determined by XRD patterns of bulk samples, was limited to glasses having more than 40 mol% of the glass-forming oxides SiO₂ and B₂O₃. Crystalline phases of Zn₂SiO₄ (willemite) were detected in compositions of 30SiO₂ • 10B ₂O₃ • 20Bi₂O₃ • 40ZnO and 20SiO₂ • 10B₂O₃ • 25Bi ₂O₃ • 45ZnO. Glass transition temperatures (T _g), dilatometric softening points (T_d) and linear coefficients of expansion in the temperatures range of 25-300 °C (α_25-300) were measured for subsystems along the B ₂O₃ join of 10, 20 and 30 mol%. For these subsystems, T_g ranged from 411 to 522 °C, and T_d ranged from 453 to 563 °C, both decreasing with increasing Bi₂O₃ content. The measured α_25-300 ranged from 53 to 95 × 10^-7 °C^-1, with values increasing with increasing Bi₂O₃ content. The ZnO content had the opposite effect to the Bi₂O₃ content. It appears that Bi³⁺ acts as a glass-modifier in this quaternary system.