Development of barium boron aluminosilicate glass sealants using a sol–gel route for solid oxide fuel cell applications

Abstract

A key problem in the fabrication of planar solid oxide fuel cells is the sealing of the metallic interconnect to the ceramic electrolyte. The sealing material must be gas-tight, stable in different atmospheres at high temperature, chemically compatible with the other cell components and resistant to thermal stresses. Glass–ceramic sealants are good candidates because of their high mechanical properties and the possibility to use a wide range of chemical compositions to control some physical properties like viscosity or coefficient of thermal expansion (CTE). In this work, glass sealants were synthesized using a sol–gel route, which generally allows to obtain both homogeneity at a nanoscale and reduction of the processing temperature. The studied glasses were based on the system BaO–B₂O₃–Al₂O₃–SiO₂ with varying amounts of CaO and MgO additions. Dilatometry, differential thermal analysis and hot-stage microscopy were the techniques used to determine optimal thermal treatment for sealing operation (880 °C with a dwell time of 10 h). The thermomechanical properties of the sealants were improved after sealing by a thermal treatment transferring the sealant into a favourable partially polycrystalline state. Gas-tightness tests performed after joining and 100 h ageing treatment at 800 °C under air of steel-sealant-steel assemblies highlighted that 4 of the selected glass chemical compositions remained impermeable. Joining degradations, crystalline phases evolution and CTE of these glasses were analysed. Electrical resistivities were larger than 10⁵ Ω cm at 700 °C. On the basis of these results, four glasses were identified as promising candidates for this application.