MICROSTRUCTURAL EFFECTS ON THE PHASE TRANSITIONS AND THE THERMAL EVOLUTION OF ELASTIC AND PIEZOELECTRIC PROPERTIES IN HIGHLY DENSE, SUBMICRON-STRUCTURED NaNbO3 CERAMICS

The dielectric, piezoelectric and elastic coefficients, as well as the electromechanical coupling factors, of NaNbO3 submicron-structured ceramics have been obtained by an automatic iterative method from impedance measurements at resonance. Poled thin disks were measured from room temperature up to the depoling one, close to 300oC. Dielectric thermal behaviour was determined also for unpoled ceramics up to the highest phase transition temperature. Ceramics were processed by hot-pressing from mechanically activated precursors. Microstructural effects on the properties are discussed. The suppression of the classical maximum in dielectric permittivity in unpoled ceramics at the phase transition at 370oC was found when a bimodal distribution of grain sizes, in which a population of average grain size of 110 nm in between much coarser grains is observed. The appearance of a phase transition at 150oC took place when Na vacancies are minimized. The occurrence of a noncentrosymmetric, ferroelectric phase, in the unpoled ceramic from room temperature to ~300oC, highly polarisable resulting in high ferro-piezoelectric properties was also