Aqueous slurry processing of monolithic films for SOFC – YSZ, LSM and YSZ–NiO systems

The powder precursor for yttria stabilized zirconia (YSZ) was prepared by reverse strike co-precipitation method while those for lanthanum strontium manganite (LSM) and nickel oxide (NiO) were prepared by gel combustion method. The thermal decomposition and phase evolution behavior of these powder precursors was carried out by thermogravimetry and XRD. Phase pure cubic YSZ formed around 430 °C from the dried amorphous gel and exhibited a mass loss of 23%. Even though the as formed LSM and NiO precursors exhibited nano-crystallinity, they contained some amount of volatiles (up to 8%), elimination of which required a post heat treatment. The optimum calcination temperature for these powders to obtain sintered bodies with desired densities (viz. >95% T.D. for YSZ and 65–75% T.D. for LSM, YSZ–NiO) at the desired sintering temperatures (1350, 1400 and 1500 °C respectively), was found to vary in the range of 900 1350 °C. Fine YSZ powder with size (D50) 0.7–1.2 μm was used in formation of the electrolyte film while YSZ, LSM and NiO powder with size (D50) 3–5 μm along with carbon pore former (15 wt% in LSM) were used for formation of electrode films. The conditions for slurry formation for film casting were evaluated through surface charge and rheological studies. The study of the effect of pH of aqueous suspension on zeta potential showed that YSZ and NiO were charged to sufficient extent (>20 mV) in both acidic and alkaline media while LSM and pore former exhibited sufficient surface charging only in alkaline medium. The slurries for tape casting were formulated using a polyvinyl binder solution and the composition was optimized through rheological studies. Compositions were fixed to form slurries with desired amount of pseudo-plasticity that could exhibit controlled flow to form flexible films with desirable thickness. The process conditions were optimized to form flat sintered electrolyte films possessing about 95% T.D. and electrode films possessing 65–75% T.D. Sintered bodies of the electrolytes exhibited fine-grained microstructure while the electrodes exhibited composite structure of grains and inter-connected pores.

► Compositional and phase pure YSZ, LSM and NiO formation occurs around 450–700 °C.
► Calcination temperature and agglomerate size influence characteristics of sintered films.
► YSZ and NiO disperses in both acidic and alkaline pH while LSM only in alkaline pH.
► Tape cast slurry has to exhibit low viscosity but with pseudo-plastic flow behavior.
► Flat sintered tape formation requires sandwiching it in between flat fibreboards.