On the roles of BaY2NiO5 and BaNiO2 vs dissolved B-site Ni on sintering of acceptor-doped BaZrO3

The sintering behavior of NiO-containing, acceptor-doped BaZrO 3 is critical for processing dense proton-conducting electrolytes and composite electrodes. BaY 2 NiO 5 (BYN) has long been regarded as the primary effective sintering aid for Ni-promoted densification of Y-doped BaZrO 3 . However, the underlying general sintering mechanism remains unclear. Here, we reinvestigate this by directly comparing sintering of BaZr 0.8 Y 0.2 O 3−δ with NiO and BYN additions, prepared by both solid‑state reactive and conventional (pre‑calcined) sintering routes. Contrary to previous reports, we find that excess BYN results in less densification than NiO as the additive, which casts doubts on BYN’s role as the primary sintering aid. Instead, we propose that the B-site deficiency in the pristine perovskite, which results from the BYN formation, is compensated and stabilized by B-site Ni occupation. The enhanced densification correlates with Ni incorporation, which can be considered to be in equilibrium with BaNiO 2 vapor. This mechanism is substantiated by the densification of Ni-free BaZr 0.8 Y 0.2 O 3−δ sintered adjacent to NiO-BaZr 0.8 Yb 0.2 O 3−δ composites separated by a Pt mesh. The mechanism holds regardless of the specific secondary phases present, indicating that B-site Ni transport facilitated by BaNiO 2 vapor is what effectively enhances sintering.