Solid-state reactive sintering mechanism for proton conducting ceramics

•BaCe0.6Zr0.3Y0.1O3 − δ was synthesized via the solid-state reactive sintering method.•The SSRS mechanism is clarified using a suite of metal oxide sintering additives.•+ 2 Ox. state additives size matched to Zr4 + result in the highest density pellets.•A useful guide to engineer optimal microstructures is derived.

The recently developed solid-state reactive sintering (SSRS) method significantly simplifies the fabrication process for proton conducting ceramics by combining phase formation, densification, and grain growth into a single high-temperature sintering step. The fabrication simplicity provided by SSRS greatly enhances the potential for deployment of proton conducting ceramics in a number of electrochemical devices. Nevertheless, the mechanisms behind SSRS are still poorly understood. In this report, the SSRS mechanism is clarified through a systematic study of the effect of a suite of metal oxide sintering additives on the phase formation and densification of prototypical proton conducting ceramics BaCe0.6Zr0.3Y0.1O3 − δ (BCZY63), BaCe0.8Y0.2O3 − δ (BCY20), BaZr0.8Y0.2O3 − δ (BZY20), and BaZr0.9Y0.1O3 − δ (BZY10).Sintering additives with metal ions having a stable oxidation state of + 2 and an ionic radius similar to Zr4 + (which easily occupy the Zr4 + site of the BaZrO3 perovskite structure, resulting in the formation of large amounts of defects) produce the best sinterability. Sintering additives with metal ions having multiple stable oxidation states (2 +, 3 +, 4 + etc.) and ionic radii near to Zr4 + (which also readily occupy the Zr4 + sites of BaZrO3 perovskite structure, but form fewer amounts of defects) can result in partial sintering by the formation of mechanically stable highly-porous microstructures with limited grain sizes. Sintering additives with metal ions having stable oxidation states ≥ 3 + or ionic radii far from Zr4 + are unlikely to form a solid solution with BaZrO3 and yield no effect on sintering behavior (virtually identical to the control sample without any additives). These findings provide a useful guide for the selection of sintering additives to engineer optimal microstructures in proton conducting ceramics and may have potential consequences in other ceramic systems as well.