On the role of copper as a sintering aid in proton conducting Gd-doped barium cerate (BCGO)

Several orthorhombic perovskite products with nominal chemical formula BaCe0.85Gd0.15O3–δ (BCGO), doped with variable amounts (0–5 mol%) of CuO and characterized by high total ionic conductivities, were synthesized by a sol-gel method followed by a calcination step at 1200 °C to induce the formation of a perovskite single phase. The products were characterized by X-ray Diffraction (XRD), Rietveld refinement, Scanning Electron Microscopy (SEM), Raman spectroscopy and Electrochemical Impedance Spectroscopy analysis (EIS). The powders were sintered into highly dense bodies (greater than 99 % for the best sample) through a relatively mild sintering process, i.e. 1350 °C for 6 h. The positive effect of copper as a sintering aid was revealed when a threshold content value (around 2 mol%) was e. Indeed, beyond such a limit, by calcination at 1200 °C, a secondary phase, a mixed oxide of Cu, Ba and Gd (very likely with chemical formula GdBa4Cu3O9.18) appears in the precursor powders strongly enhancing the densification process, reasonably via partial liquid-phase sintering. Finally, remarkable grain growth also occurred, enhancing the overall conductivity of BCGO due to the positive effect of the reduction of grain boundary dimensions.