Commercial alkaline earth boroaluminosilicate glasses for sealing solid oxide cell stacks. Part I: Development of glass‐ceramic microstructure and thermomechanical properties

Abstract

AbstractSealing performance in solid oxide cell (SOC) stacks and the devitrification process of commercially available alkaline earth boroaluminosilicate glasses containing 48‐61 mol% SiO2, 18‐28 mol% CaO, 1‐7 mol% MgO, 7‐10 mol% Al2O3, 1‐11 mol% B2O3 plus minor amounts of Na2O, K2O, FeO, and TiO2 were investigated and quantified through analysis of phase assemblages as function of heat treatments above the glass transition temperatures using the electron microprobe and powder X‐ray diffraction. For two of these glasses devitrification behavior was compared to the devitrification behavior of similar glasses produced in the laboratory. Glasses were characterized after annealing in air at 800°C and 850°C for up to 6 weeks. Even though the glasses lie within a relatively narrow compositional range, sealing performance and the resulting microstructures differed significantly. Best thermomechanical properties was developed in one of the laboratory‐produced glasses, MCAS, which may be applied in SOC‐stacks by allowing for a slow solidification in the range 750‐800°C followed by crystallization at or slightly above 800°C. The relatively high thermal expansion coefficient (CTE) from RT‐800°C, 11 × 10−6 K−1, which was developed over ~1000 hours at 800°C, depends mainly on the formation of cristobalite and quartz as well as the presence of a residual glass phase. The glass ceramic sealant appears relatively stable over time, except for a slow transition of cristobalite to quartz, and can possibly show self‐healing behavior if later brought close to 850°C. Devitrification led to increases of the thermal expansion coefficients in all other glasses tested, but did not reach levels interesting for SOC‐stack sealing.