Glass–ceramic seal to join Crofer 22 APU alloy to YSZ ceramic in planar SOFCs

Abstract

This work describes the design and development of a glass–ceramic seal to produce a hermetic joint between the ceramic electrolyte and the metallic interconnect of planar SOFC stacks. The glass–ceramic composition was designed considering the chemical compatibility with the ceramic electrolyte (yttria stabilized zirconia) and the oxidation resistant metal alloy interconnect (Crofer 22 APU), the thermo-mechanical properties (characteristic temperatures, thermal expansion coefficient), and the wettability on the substrates. The designed glass–ceramic seal is a barium-free silica-based glass, which partially crystallizes during the heat treatment after being deposited on Crofer 22 APU substrate by slurry coating or by electrophoretic deposition. The sealing process of the glass–ceramic was optimized, also taking into account that the maximum processing temperature should be lower that 950 °C (upper limit for the metallic interconnect) and the maximum heating rate of about 5 °C/min (limit for ceramic components). The joined ceramic/seal/metal samples were morphologically characterized and preliminary tested for 400 h in air atmosphere at the fuel cell operating temperature of 800 °C. Thermal ageing in air caused a Cr-diffusion from Crofer 22 APU alloy to the seal only when the alloy was used in the as-received condition. The preoxidised one did not diffuse Cr ions through the seal under the same ageing conditions.