Ordered coalescence of nano-crystals in alkaline niobate ceramics with high remanent polarization

Lead-free alkali niobates Na0.5K0.5NbO3 (NKN) ceramics, with significantly enhanced ferroelectric remanent polarization (Pr), were prepared using Spark Plasma Sintering (SPS). Three types of boundaries were observed in the ceramics, being grain boundaries between faceted grains, domain boundaries that separate ferroelectric domains inside individual grains, and nanoscale sub-grain boundaries that reveal the nano-scale mosaicity of individual grains. Part of the sub-grain boundaries were from initial powder particles. The other sub-grain boundaries were built by ordered coalescence of nano-crystals during rapid SPS process. It was worthwhile to emphasize that the ordered coalescence of nano-crystals in bulk ceramics during sintering takes place and completes within minutes. These sub-grain features would disappear at higher temperature by long time sintering. Rapid Spark Plasma Sintering allowed us to capture this transient microstructure. The significantly enhanced ferroelectric Pr of NKN was attributed to nanoscale sub-boundaries, which stimulated the dynamics of ferroelectric domain formation and switching.
QC 20171124