Microstructural effects on the phase transitions and the thermal evolution of elastic and piezoelectric properties in highly dense, submicron-structured NaNbO3 ceramics
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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 discs were measured from room temperature up to the depoling one, close to 300 °C. 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 370 °C was found when a bimodal distribution of grain sizes, with a population of average grain size of 110 nm in between much coarser grains, is observed. The appearance of a phase transition at 150 °C took place when Na vacancies are minimised. The occurrence of a non-centrosymmetric, ferroelectric phase, in the unpoled ceramic from room temperature to ~300 °C, highly polarisable resulting in high ferro–piezoelectric properties was also observed in the ceramic which presents grain size below 160 nm. Maximum values of k p = 14%, d 31 = −8.7 × 10−12 C N−1 and N p = 3772 Hz m at room temperature, and k p = 18%, d 31 = −25.4 × 10−12 C N−1 and N p = 3722 Hz m at 295 °C were achieved in the best processing conditions of the ceramics.
KeywordsDielectric Permittivity Grain Size Distribution Piezoelectric Property Ferroelectric Phase Piezoelectric Coefficient
Authors thanks the EC project “LEAF” G5RD-CT2001-00431, EC project “PIRAMID” G5RD-CT-2001-00456 and MAT2001-4818E (MCyT Spain), MAT2001-0561, CAM (07N/0076/2002), MAT2004-00868 and MAT2007-61884 projects, COST 528 and COST 539 Actions, Thematic Network CE (contract G5RT-CT2001-05024) and Network of Excellence, 6FP-CE (NMP3-CT-2005-515757). Drs. A. Moure and T. Hungría are indebted to the CSIC (MICINN) of Spain for the “Junta de Ampliación de Estudios” contracts (Refs JAEDOC087 and JAEDOC082, respectively). Thanks are also given to Ms. M. Antón (working under a FINNOVA2003-LEAF grant) for the powder samples preparation. Authors are indebted to late Dr. C. Alemany, for the implementation of the software used.
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