Effects of NiO addition on sintering and proton uptake of Ba(Zr,Ce,Y)O3-δ

The effects of 0.125-2.0 wt% NiO added as a sintering aid for highly refractory Ba(Zr,Ce,Y)O3-delta proton conducting ceramics are investigated. The complex nature of the solid state reactive sintering method shows up in the phase evolution (quenched samples exhibit up to three different perovskite phases) and the densification kinetics. For >= 0.25 wt% NiO addition the characteristic changes in the sintering behavior indicate that a (Ba,Ni,Y)O-x transient liquid phase forms which simultaneously facilitates densification, grain growth, and homogeneous (Zr,Ce,Y) cation distribution (indicated by decreased microstrain). However, thermogravimetry shows a clear decrease of proton uptake and effective acceptor concentration with increasing amounts of NiO. This can be attributed to a withdrawal of Ba from the perovskite lattice, which decreases the effective acceptor concentration. A NiO content of 0.4-0.5 wt% is suggested as a balance between the benefits for phase formation and sintering, and its drawback for proton uptake.