Thermal Properties and Joinability Investigation of BaO–SrO–SiO₂–B₂O₃ Glasses for Oxygen Transport Membrane Application

Three new BaO–SrO–SiO₂–B₂O₃ (BS) glasses with different SrO contents (6–25 mol%) are developed for oxygen transport membrane (OTM) joining application. The content of strontium is investigated first in terms of its effect on the glass-forming tendency, thermal expansion coefficient, crystallization, shrinkage behavior, and viscous flow properties. Differential scanning calorimetry (DSC) is carried out. Dilatometric tests are performed to obtain coefficients of thermal expansion (CTEs) of BS glasses. The crystallization behavior of the BS glasses is investigated by X-ray powder diffraction (XRD). Sinking dilatometric measurements simulate the joining procedure and observe the shrinkage behavior of the BS glasses. The viscous flow behavior of the BS glasses is examined via hot stage microscopy. The glass with 15 mol% SrO (BS15) glass shows the best glass-forming tendency, most matching CTE (11.9 × 10⁻⁶ K⁻¹), densest microstructure, highest shrinkage rate (24%), and good viscous behavior at high joining temperatures compared with other BS glasses. BS15 glass is chosen for helium leak test and assembly test joining with Aluchrom and SrTi_0.75Fe_0.25O_3−δ membrane (STF25). The sandwiched sample with two Aluchrom plates sealed by BS15 glass at 1075 °C for 5 min achieves good gas-tightness with low helium leakage rate <10⁻⁹ mbar·l s⁻¹.