Microstructural evolution of sealing glass materials with varied concentrations of B₂O₃ and SrO

The sealing glass plays a central role in fuel cells by providing critical sealing functions, preventing the leakage and mixing of fuel and oxidant. The structure of sealing glasses for fuel cells is of paramount importance in determining their stability and performance at high temperature. This study investigated the effects of B₂O₃ and SrO on the microstructure and thermal properties of MgO-SrO-B₂O₃-Al₂O₃-SiO₂-(Na₂O) systems using high-temperature quenching, Raman spectroscopy, XPS, and DSC. For B₂O₃, increasing the B₂O₃/SiO₂ ratio initially enhanced structural polymerization: Q³ and Q⁴ units increased (bridging oxygen, O⁰: 34.19 % → 37.14 %) at ratios ≤1.3. However, ratios exceeding 1.8 reversed this trend, regenerating Q² units (O⁰: 31.42 % at ratio 2.5), revealing a critical transition mechanism between ratios 1.3∼1.8. Thermally, optimal B₂O₃/SiO₂ ratios (1.33) maximized transition and softening temperatures (580.1 °C, 735.5 °C), while excessive B₂O₃ reduced network integrity and performance (570.1 °C, 722.2 °C at ratio 2.5).For SrO, increasing SrO/SiO₂ ratios (0.83∼1.85) linearly reduced structural polymerization: Q³/Q² ratio decreased (1.433→0.243), bridging oxygen (BO) declined (73.59 %→57.37 %), and non-bridging oxygen (NBO) increased (26.41 %→42.63 %). As a network modifier, Sr²⁺ induced the breakdown of Si-O-Si bonds, resulting in a decrease of the glass transition temperature from 654.2 °C to 630.3 °C and a concurrent reduction of the softening temperature from 799.0 °C to 769.5 °C. These results elucidate the dual structural roles of B₂O₃ and the depolymerization effect of SrO, providing insights for tailoring high-temperature sealing material performance.