Processing and characterisation of nanostructure zirconia for application in solid oxide fuel cells

The fabrication and sintering of yttria-stabilised zirconia with varying yttria concentrations was carried out based on both wet and dry processing routes, utilising conventional single step and two-step sintering techniques. Results show that homogeneous, defect-free, high green density microstructures are essential in producing fully dense sintered samples of a desired grain size. Characterisation on morphology using HRTEM suggests that although there were no indications of impurities or secondary phases in the studies samples, there were signs of yttria segregation present in 3 mol.% YSZ, sintered at 1600C. This was explained to be due to the increased amount of cubic phase with increased sintering temperature. High yttria ion concentration regions that form within grains undergo phase transformation from tetragonal to cubic, allowing the cubic phase regions to be partitioned. However, the transformation was not detectable based on the techniques used in this study. The activation energies obtained from AC impedance spectroscopy demonstrated reduced activation energy at higher temperature. This suggests the presence of two different oxygen diffusion mechanisms within the material. These results are backed up by DC 4-probe ionic conductivity measurements. Reduced activation energy reported in nanostructured YSZ was reported to be due to large oxygen-ion vacancies. This phenomenon was also reported at lower yttria concentrations, due to the oxygen vacancies experiencing a binding energy to the dopant atoms caused by a combination of an electrostatic attraction and repulsive lattice relaxation energy. Overall, the conductivity results based on AC impedance and DC 4-probe showed that the finer grain structured 3 mol.% YSZ at ~100 nm exhibited slightly enhanced conductivity. On the other hand, 8 mol.% YSZ displayed a clear increase in conductivity with increasing grain size based on an intrinsic grain boundary blocking effect. High temperature thermal aging carried out on 3 mol.% YSZ at ~100 nm and ~500 nm was believed to have caused polarisation loss, a time dependent phenomenon.