Sintering of 4YSZ (ZrO2 + 4 mol% Y2O3) nanoceramics for solid oxide fuel cells (SOFCs), their structure and ionic conductivity

In this work 4YSZ nanoparticles were produced with a variant of the sol–gel method, the particles were sintered using two different heating schemes, and the properties of the resulting pellets investigated. XRD spectra show the crystal structure of the particles to be mainly cubic, and TEM pictures reveal their size to be around 20 nm. The relative densities of the sintered pellets are found with the Archimedes method to lie between 87 and 97% of the theoretical material density, and the grain size, using thermal etching and SEM imaging, found to lie between 0.2 and 5 μm, depending on the sintering temperature and the presence of impurities. The pellets were subjected to electrical impedance spectroscopy making it possible to distinguish the internal crystal and grain boundary impedances due to their different time-constants. With the “brick layer model” the approximate grain boundary thickness was found to lie between 4 and 8 nm, depending on the sintering conditions and on the presence of impurities. The specific resistances of the grain interiors and the grain boundaries, and the activation energies for ion diffusion in both, are presented. The ionic conductivities of the pellets are compared with literature values, and are found to compare favorably with these, at some temperatures even with data for 8YSZ, which is the standard material used for electrolytes due to its higher ionic conductivity, but which is also mechanically weaker than 4YSZ.