The unification of paste rheologies for the co-extrusion of solid oxide fuel cells

Current methods used to manufacture solid oxide fuel cells (SOFCs) involve multiple steps, applying successive layers to produce laminated components associated with repeated sintering steps. This work aims to cut the number of processing steps involved by co-extruding a complete SOFC tube from pastes in a single step, which is then co-sintered. Several aspects must be overcome in order to co-extrude such a fuel cell. Crucially the co-extrusion process requires that the rheological properties of the materials are well matched. This prevents the formation of flow defects and allows for the continuous operation of the co-extrusion process. This work has investigated the manipulation of the rheological parameters through design of the paste formulation as well as the manufacturing process.Two pastes, one made from a fine yttria stabilised zirconia (YSZ) and the other from a combination of a fine YSZ, a coarser nickel oxide and activated carbon, have been characterised using the Benbow/Bridgwater model and their parameters have been unified through adjustment of their solids loading. Using packing theory and the assumption that pastes will only flow when the powder void space has been filled with a slight excess of liquid to overcome inter-particulate friction, an intermediate paste has been designed to possess similar rheological properties.