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Refractory" Sr-Ca-Y-B-Si sealing glasses were developed for solid oxide fuel cells ͑SOFCs͒. The objective was to develop sealing glasses with appropriate thermal properties and minimal interfacial reactions with SOFC stack components, particularly ceramic electrolytes and metallic interconnects. The new glasses are described as refractory because their sealing temperatures ͑Ͼ950°C͒ are higher than traditional sealing temperatures for intermediate-temperature SOFC stacks ͑Ͻ850°C͒. Six glasses were formulated and made by a conventional glass-making process. The glasses exhibited rapid crystallization kinetics during intermediate temperature heat-treatments. Their thermal properties were characterized in both the initial glassy state and the final crystallized state. Candidate glasses were also aged for 1000-2000 h at elevated temperatures. Thermal expansion measurements showed minimal change after aging. One candidate glass ͑YSO-1͒ was used to seal anode-supported electrolyte membranes to metallic interconnects at temperatures ranging from 900 to 1050°C in air. Microstructure analysis revealed the presence of SrCrO 4 near the glass-ceramic/metal interface. The effect of composition on thermal properties and a possible reaction mechanism for the chromate formation are discussed.

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This paper examined the room-temperature mechanical properties of a mixed-conducting perovskite La 1-x Sr x Co 0.2 Fe 0.8 O 3 (x ‫؍‬ 0.2-0.8). Powders were made by the combustion synthesis technique and sintered at 1250°C in air. Sintered density, crystal phase, and grain size were characterized. Young's and shear moduli, microhardness, indentation fracture toughness, and biaxial flexure strength were determined. The Young's and shear moduli slightly increased with increasing strontium content. Young's modulus of 151-188 GPa and shear modulus of 57-75 GPa were measured. Biaxial flexure strength of ϳ160 MPa was measured for lower strontium content batches. Strength greatly decreased to ϳ40 MPa at higher strontium concentrations (x ‫؍‬ 0.6 -0.8) because of the formation of extensive cracking. Indentation toughness showed a higher value (ϳ1.5 MPa⅐m 1/2 ) for low strontium (x ‫؍‬ 0.2) content and a lower value (ϳ1.1 MPa⅐m 1/2 ) for the other batches (x ‫؍‬ 0.4 -0.8). Materials with fine and coarse grain size were also tested at various indent loads and showed no dependence of toughness on crack size. In addition, fractography was used to characterize the critical flaw and fracture mode.

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