Tailoring the properties of dense yttrium-doped barium zirconate ceramics with nickel oxide additives by manipulation of the sintering profile

Liquid phase sintering (LPS), by the addition of transition metal additives, has become one of the most preferred approaches to lower the sintering temperature of yttrium-doped barium zirconate (BZY) proton-conducting ceramics. Nonetheless, this approach is known to negatively impact the bulk conductivity of the parent perovskite phase, due to the significant incorporation of the sintering additive into the bulk composition. In the current work, we study the typical proton conducting material BaZr0.8Y0.2O3-delta ( BZY20), prepared by One or Two-step sintering strategies using 4 mol% NiO of sintering additive, compared against a control sample of pure BZY20. The main results demonstrate that the Two-step approach is shown to minimise Ni interreaction with the BZY material, allowing this sample to maintain a higher hydration capacity than the One-step material, as confirmed by X-ray diffraction (XRD), scanning transmission electron microscopy/energy-dispersive spectrocopy ( STEM/ EDS) and thermogravimetric (TG) measurements. Despite these benefits, the Two-step approach presents a diminished bulk conductivity, which was ascribed to depletions in proton mobility due to "proton trapping" effects. In addition, the specific grain boundary conductivity was revealed to be higher in the Two-step sample, as a possible result of a higher Ni accumulation in this region. Overall, this work opens an exciting new debate for further studies, in that prevention of Ni-incorporation in the bulk material, as typically suggested in current literature, may not actually be the best method to improve the conductivity of samples densified by Ni-additives and that control of potential defect associations may be the more critical method for conductivity tailoring.