High-performance Ni-YSZ thin-walled microtubes for anode-supported solid oxide fuel cells obtained by powder extrusion moulding

Aiming at the fabrication of microtubular anode-supports for Solid Oxide Fuel Cell (SOFC) applications, this contribution deals with the production of Ni-YSZ thin-walled tubes (<1 mm thickness) via Powder Extrusion Moulding (PEM). The overall method has been optimized with an emphasis on the effect of NiO particle size using two commercial NiO powders with mean sizes of 0.7 and 8 mu m. A thermoplastic binder system based on polypropylene (PP), paraffin wax (PW) and stearic acid (SA) in volume ratios of 50, 46 and 4, respectively, was used along with corn starch as a pore forming agent. Different feedstocks with solid loadings varying from 45 to 65 vol% were processed and characterized to determine the optimal formulation. Typically, the mixtures exhibited a pseudoplastic behaviour from 100 to 1000 s(-1). Feedstocks with finer NiO particles had the most balanced properties for PEM purposes and an optimal powder volume content of 65 vol% was established. After extrusion and debinding steps, defect-free and constant cross-section tubes with 15 mm of length and 4 mm of nominal diameter were obtained. The final microstructure and DC conductivity were found to be closely linked to the NiO particle size, yielding a higher degree of open porosity and a better performance when using finer NiO powder. Based on this study, the packing mechanism was found to be likely limited by the contribution of steric hindrances when dissimilar and coarse powders are mixed, which may play a decisive role in order to set tailored formulations.