The thermal cycles and high operating temperatures (800 – 1000 oC) of the solid oxide fuel cell (SOFC) stacks, used for electric power production, lead to increased stresses at the joint- and insulating- points of the material components of the stack. As an alternative to previously used glass–ceramic solders, the possible use of oxide ceramics as insulating materials in combination with Ag–based brazes is investigated. The goal is to achieve a stronger bond between the components of the stack, as well as gas tightness and longer resistance to temperature changes.
Contact angle, θ, measurements were performed in systems consisting of the oxide ceramics MgO and MgAl2O4 and 85wt%MgO + 15wt%MgAl2O4 composite with thermal expansion adjusted to the other joining components as well as Cr–containing ferritic steel (interconnect) in contact with Ag + 4wt%CuO (Ag4CuO) and Ag + 2wt%B (Ag2B) brazes, in order to determine the wetting and bonding conditions.
Preliminary wetting experiments, on the one hand, showed that pure liquid Ag does not wet neither the ceramics (θ>120o) nor the steel (θ>90o). On the other hand, the liquid oxides CuO(Cu2O) and B2O3 wet well the ceramics as well as the steel, showed contact angles far below 90o.
The wetting results in the tested systems, performed at the temperature range 980 oC to 1200 oC in air, have shown that the CuO and B additions in Ag braze act as interfacial active compounds lowering the interfacial energy and therefore the values of the measured contact angles. a) In the oxide/Ag4CuO systems, depending on oxide substrate and temperature, the contact angles have values in the range θ = 50o to 45o (MgO), θ = 85o to 75o (MgAl2O4) and θ = 80o to 70o (85wt%MgO + 15wt%MgAl2O4). In the case of MgO substrate and temperatures up to ~1050 oC the oxide may react with CuO to form a Cu2MgO3 compound. At higher temperatures this compound decomposes accompanied by a maximal solubility of ~ 20at%Cu in MgO. b) In the system ferritic steel/ Ag4CuO the contact angle takes values in the range θ = 55o to 35o accompanied by a strong interfacial reaction and formation of a brittle interface layer of mixed Fe-Cr-Cu-O oxides. c) In the Oxide/Ag2B systems, the oxidation of B to B2O3 in air and its good adherence to the oxide-substrates suppresses the measured contact angles to values between 90o and 80o. In the case of MgO-substrate, it can react with B2O3 to form different Mg-B-O compounds, as well as in the case of MgAl2O4 substrate a high melting Al-B-O phase. d) In the system ferritic steel/ Ag2B, contact angles varied between 84o and 74o, the B2O3 in contact with the Fe-Cr-O oxide layer formed on the surface of the steel lowered the interfacial energy of the system.