Tailoring of porosity of yttria-stabilized zirconia tubes as supports for oxygen separation membranes

Pure oxygen gas supplied by ceramic oxygen transport membranes can facilitate reduced CO2 emissions through more efficient gasification processes and CO2 capture and storage. Tubular membranes have some advantages compared to planar membranes, such as better resistance to thermal gradients and more straightforward sealing. The active oxygen separation layer in the membrane should be as thin as possible and therefore supported on a highly porous tubular substrate. In this work tubular porous supports of yttria-stabilized zirconia have been manufactured using thermoplastic extrusion. Two types of poreformers (spherical graphite (d50 18 μm) and polymethyl methacrylate (d50 10 μm)) have been used to form connected macropores, since their spherical geometry limits preferential orientation during extrusion. Their difference in decomposition temperatures also allows a high volume fraction of pore formers without deformation during debinding. The influence of the amount of pore formers (relative to the amount of ceramic and thermoplastics) on the microstructure of sintered samples, as well as the extrudability and ease of debinding of the feedstock, has been studied. Ceramics with 1-20 μm pores, open porosities exceeding 55 % and gas permeabilities close to 10-14 m2 could be produced, demonstrating that thermoplastic extrusion is suitable for fabrication of porous and permeable tubes.