Solid-state reactive sintering mechanism for proton conducting ceramics

摘要

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 SSRSmechanismis 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 BaCe_(0.6)Zr_(0.3)Y_(0.1)O_(3-δ) (BCZY63), BaCe_(0.8)Y_(0.2)O_(3-δ) (BCY20), BaZr_(0.8)Y_(0.2)O_(3-δ) (BZY20), and BaZr_(0.9)Y_(0.1)O_(3-δ) (BZY10). Sintering additives with metal ions having a stable oxidation state of +2 and an ionic radius similar to Zr~(4+) (which easily occupy the Zr~(4+) site of the BaZrO_3 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 Zr~(4+) (which also readily occupy the Zr~(4+) sites of BaZrO_3 perovskite structure, but form fewer amounts of defects) can result in partial sintering by the formation ofmechanically stable highly-porousmicrostructureswith limited grain sizes. Sintering additives with metal ions having stable oxidation states ≥3+ or ionic radii far from Zr~(4+) are unlikely to form a solid solution with BaZrO_3 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.