Consolidation, microstructure and crystallography of dense NaNbO3 ceramics with ultra-fine grain size
The present study reports on the preparation and structural characterization of dense NaNbO3 ceramics with ultrafine grain size. Nanocrystalline raw powders obtained via microemulsion mediated synthesis were consolidated to green compacts, sinterforged to ceramic pellets with high density and ultra-fine microstructure and finally annealed under different thermal regimes in order to systematically increase the average grain size in the range from several hundreds of urn up to a few mu m. Structural characterization of these materials included electron-microscopic observation and X-ray diffraction in combination with temperature tuning Raman-spectroscopy in order to detect both the global and local crystallographic symmetry in dependence of the grain size. The results show that ceramics with an average grain size from approximately 360 urn to 1-2 mu m consist of a phase mixture of the anti-ferroelectric phase with the space group Pbcm, which is also observed for the conventional case of coarse NaNbO3, and secondly of a new polar polymorph described by the space group Pmc2(1). The volume fraction of the second polar modification appears to increase with decreasing grain size suggesting an enhancement of the piezoelectric response of ultra-fine grained NaNbO3 ceramics in comparison to coarse grained material of the same composition.