Thermal Stability and Cyclic Sealing Performance of Glass Composite Seals

Abstract

Glass matrix composites as sealing materials for solid oxide fuel cells (SOFCs) were prepared with Al2O3 and/or 8 mol % Y2O3-stabilized zirconia (8YSZ) fillers. Their thermal stability was evaluated by measuring thermal expansion coefficients (TECs) after heat-treatment in the 700~790°C range for 500 hours. With a glass composite containing ~2% closed porosity, and 10 vol% of Al2O3 and 8 YSZ fillers, the TEC change was 2% after heat-treatment at 750°C for 500 h. The TEC change of a base glass was as low as 1% after heat-treatment at 730°C for 500 h, demonstrating superior thermal stability of the base glass. Meanwhile, a glass composite containing ~10% closed porosity, and 10 vol% of Al2O3 fillers, the TEC change was as high as 8% after heat-treatment at 700°C for 500 h. The high thermal expansion of the porous glass composite containing Al2O3 filler could be understood considering the effect of porosity on the thermal expansion coefficient of porous materials. Thermal stability of the glass composite containing 10 vol% of Al2O3 and 8 YSZ fillers was further investigated with long-term cyclic sealability. The glass composite to STS444 seal assemblies was subjected to thermocycling between 450°C and 750°C, and survived over 14,000 h of 37 consecutive cycles. The leakage rates during thermal cycles were below 0.004 sccm cm−1 at seal operating temperature of 750°C. In summary, glass composites with mixed fillers of Al2O3 and 8YSZ showed superior thermal stability and cyclic sealing performance required for SOFC sealing materials. In this presentation, we will discuss the effect of the two different fillers on the thermal stability of glass matrix composites.