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

Abstract

Summary 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−δ (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.

Densification and Proton Conductivity of La1-xBaxScO3-δ Electrolyte Membranes

“…EDX analysis of the cross sections does not detect these phases, which may be due to their small size and uniform distribution. A similar situation was obtained by Shakel et al [56] for BaZr0.8Y0.2O3-δ with NiO sintering additive, indicating that even transmission electron microscopy does not detect impurity phases in the form of individual grains. The XPS results in Figure 8 indicate that the Co content on the grain surface of the La0.95Ba0.05ScO3-δ + 0.5 wt% Co3O4 material is negligible, confirming that Co atoms diffuse into the bulk of the grains.…”

“…Most likely, this is facilitated by an increase in the transport numbers of holes (Figure 12a). The negative effect on the specific GB conductivity of Ba(Zr,Y)O3 with the NiO additive [56,67] is associated with the blocking of ion transfers through the grain boundaries, whereas in our case, no such effect is observed.…”

“…The results of XRD analysis indicate partial decomposition of the material with the formation of Ba2Sc2O5 and BaO2 phases with a total mass fraction of about 1.5 wt%. Similarly, the formation of impurity phases in Ba(Ce,Zr,Y)O3-δ with Fe2O3 and NiO sintering additives was observed in a number of works [36,[55][56][57]. EDX analysis of the cross sections does not detect these phases, which may be due to their small size and uniform distribution.…”

“…Violation of the cationic stoichiometry can cause the formation of cationic vacancies. Being a charged defect, cation vacancies can form associates with oxygen vacancies, which significantly reduces their mobility [56,58]. Additional trapping of oxygen vacancies is possible by Co atoms, which are embedded in Sc positions.…”

Comprehensive analysis and optimization of Y-doped and Ni-added BaZrO3 proton conductor

“…It has been reported that the greater the Y doping content, the greater the proton concentration [7]. However, the bulk conductivity of the BZY is not linear with the Y dopant concentration due to carrier migration and concentration, which needs to be further investigated [8,9].…”