Borate Volatility from SOFC Sealing Glasses

The volatility of borate species from glasses developed for solid oxide fuel cell seals was studied using thermodynamic calculations and compared with experimental results. Vapor pressure diagrams were used to identify the most volatile compounds under a range of expected operational conditions, e.g. oxidizing and reducing atmospheres with water vapor, at temperatures in the range of 700°–1000°C. The species with the highest vapor pressures were BO2(g) under dry conditions and B3H3O6(g) under wet, reducing conditions. The depletion of boron from glass surfaces to depths beyond 100 nm was characterized using Auger electron spectroscopy depth profile analysis. Weight loss experiments were conducted on several different glass compositions. The cumulative weight loss from a glass with 20 mol% B2O3 (“glass #59”) was about 10 times greater than from a glass with 2 mol% B2O3 (“glass #27”), under the same conditions. The activation energy for volatilization from glass #59 was 371±86 kJ/mol and was 272±65 kJ/mol for “glass #27.” The cumulative weight loss of each composition in forming gas with 30% water vapor was greater than in dry air at 800°C. Volatile species were collected in a water trap, and these results confirmed predictions about the effect of atmosphere and B2O3 content on volatilization behavior.