Metal based sealing material (Ag+CuO) for use in Solid Oxide Fuel Cells
Authors: Chatzimichail Rallou A. 
Published: July 2018

For mobile applications, the rapid heating rates and the high operating temperatures ofsolid oxide fuel cells (SOFCs) lead to increased stress on the joining and sealing points of the materialcomponents used for the development of planar SOFC stacks. At the junctions of the metalliccomponents and alternatively to the currently used glass-ceramic solders the possible useof oxide ceramic as an insulation layer in combination with air braze filler metal was examined.The joining of the components in air, without the use of vacuum or inert gases, requires that thefiller metal forms strong interfacial bonds with both the ceramic (insulating layer) and the additionalsheet (ferritic steel). In addition, it should be resistant to oxidation at the high operatingtemperatures and its thermal expansion coefficient should match those of the materials to bejoined. When ceramic and metal are joined, the presence of a ductile interfacial phase compensatesthe differences in the thermal expansion coefficients of the phases involved. Also, it isnecessary for the mechanical stability of the bond, that the binding partners are well wetted bythe interfacial phase.Both Ag and Cu provide high mechanical strength, ductility, and thermal, as well as electricalconductivity. Although Ag is more expensive than Cu, it is preferred as a basis metal due tothe lower process temperature and the lower oxygen affinity. A problem in using pure Ag is thepoor wetting properties, at the liquid state, when in contact with oxide ceramic and steel. Thehigh values of the contact angle (θ&gt;120o) measured in oxide ceramic/Ag systems at oxygen concentrationsof 0-3 ppm is reduced in air, but overall, the systems remain non-wetting (θ&gt;90°).Good wetting (θ&lt;&lt;90o) is crucial for a strong interfacial bond between the phases in contact andsimultaneously ensures the mechanical stability and gas tightness of the joints. Wetting can beimproved by adding an interfacial active compound that is soluble in the noble metal solvent. Asuitable material is CuO, which forms a pseudo-binary alloy with Ag in the solid state, as theypresent mutual solubility in the liquid state [11]. Depending on the percentage of CuO in themixture, small contact angles (θ&lt;20o) can be achieved in oxide ceramics/Ag + CuO systems.Requirements on the ceramic insulation layer include a high electrical and thermal resistance,a high thermal expansion coefficient, stability under mechanical pressure, structuralstability and oxidation resistance at high operating temperatures. The most suitable ceramics forthese requirements are MgO, MgAl2O4 or a mixture of MgO and MgAl2O4.The proposed Ag + CuO brazes come in contact with the ferritic steel of the interconnect partand with the additional sheet, as well as with the SOFC's electrolyte, 8 mol% Yttria-stabilizedZirconia (8YSZ), in the cell periphery. Ferritic steels, which have a Cr content above 20 wt% and aMn content below 1 wt%, form a double outer layer in air that consists of Cr2O3 on the inside, towards the steel side, and a MnCr2O4 spinel phase on the outside. During the wetting experiment,the active CuO contained in the liquid Ag migrates towards the interface and a mixed oxideinterface layer can be formed by reaction with the diffused cations Fe, Cr and Mn from thesteel. The formation of the reaction zone improves the wetting behaviour (θ&lt;90ο), but due to itshigher brittleness, the mechanical interface stability of the composite can be reduced.In the present work, the amount of CuO additive in Ag filler metal and the way in whichthis additive is applied, varied to achieve good wetting properties, and stable braze’s joints. Theaim was to achieve a strong interfacial bond between the contacting phases and to prevent extensiveinterface reactions. Reaction products that form during the early stages of the brazingprocess must remain constant at the operating conditions. For this reason, the long-term stabilityafter heat treatment in air, of the material combination oxide / brazes/ steel was examinedafter wetting experiments.