Structured porous Ni- and Co-YSZ cermets fabricated from directionally solidified eutectic composites

NiO-YSZ and CoO-YSZ eutectic rods were produced by directional solidification using the laser floating zone method (LFZ). This technique produces highly structured material consisting of alternate lamellae of transition metal oxide and zirconia with variable interlamellar spacing depending on growth conditions. We have chosen conditions for interlamellar spacing of about 1 μm. The microstructure is homogeneous and mechanically stable during thermochemical reduction. Complete reduction of the transition metal oxide produces a lamellar porous cermet with porous metallic lamellae alternated with the YSZ phase. The thermal expansion coefficients of the cermets are those of the YSZ skeleton. Reaction kinetics at different temperatures during the reduction process were studied by gravimetric methods. The reduction process within the complete temperature range studied for NiO-YSZ, and at high temperatures for CoO-YSZ seems to be controlled by the O2− diffusion through the YSZ phase. The amount of Ni2+ and Co2+ ions dissolved in the YSZ phase is 2 and 5 mol%. Resistivity values for the cermets along the solidification axis are 50 μΩ cm for Co-YSZ and 130 μΩ cm for Ni-YSZ. These materials are porous, ionic and electronic conductors and could be used as textured anodes for solid-oxide fuel cells (SOFC).