Low-temperature sintering and enhanced dielectric properties of alkali niobate ceramics prepared from solvothermally synthesized nanopowders

Sodium niobate nanopowders (N-NaNbO3) and submicron powders (M-NaNbO3) were synthesized by the solvothermal and hydrothermal route, respectively. Subsequently, their sintering activities were comparably investigated through a modified solid-state sintering process. It's found that N-NaNbO3 green bodies presented an apparent shrinkage after the calcination at 650 °C, and dense N-NaNbO3 ceramics can be obtained at 950 °C. Besides, the optimized sintering temperature of N-NaNbO3 ceramics was 100 °C lower, yet the shrinkage was four times larger when compared with M-NaNbO3 samples. Importantly, N-NaNbO3 samples showed improved dielectric properties in the whole frequency range, and the room temperature relative dielectric constant (εr) of N-NaNbO3 samples sintered at 950 °C was as high as ~8000. The improved permittivity of N-NaNbO3 samples was attributed to the compact microstructure, unique crystalline structure and less oxygen vacancies. At the same time, the effect of abnormal phase transition behavior originated from the metastable structure was considered to explain the enhanced room temperature permittivity. It's believed that the solvothermal route has great potentials in the synthesis of alkali niobate ceramics powders with high activity for low-temperature sintering.