ABSTRACT
In the previous chapter we discussed the importance of solid oxide fuel cells (SOFCs) and why it is important to make SOFC stacks [1]. However, for a planar SOFC stack, gas-tight seals must be applied along the edges of each cell and between the cell stack and gas manifolds to avoid intermixing of fuel gas (on the anode side) and air (on the cathode side). The selection criteria for a good sealant for SOFC are: (a) coefficient of thermal expansion (CTE) match with adjoining components, (b) high ρ (electrical resistivity), and (c) no harmful reaction with joining components [2-4]. Further, the sealant must exhibit (d) high chemical stability and low vapor pressure in both reducing and oxidizing atmospheres, (e) a nonspreading nature to the adjoining fuel cell components at the operating temperature, (f) deformability and the ability to withstand a slight overpressure, and (g) the capacity to survive several thermal cycles during operation at elevated temperature [2-5]. Glass or glass-ceramic sealants can, in principle, meet almost all of these requirements. Most of the works reported in the literature have focused on barium oxide-based borosilicate system, e.g., barium aluminosilicate (BAS) and barium calcium aluminosilicate (BCAS) glasses [6]. Both of these glasses have shown matching CTE values with Crofer 22 APU interconnect and 8YSZ electrolyte, producing a perfect sealing in the SOFC operating environment. But the high (≈35 mole%) BaO content of BAS glass leads to extensive formation of a BaCrO4 phase at 750-800°C. This causes CTE mismatch at the glass-metal interface. As a result, the BAS sealant spalls out. Hence, our aim in this chapter will be to develop an appropriate sealant glass [6-8] and to assess its nanomechanical properties.
