Synthesis And Electrical Properties Of Bi_0.5Na_0.5TiO₃-ZnO Lead-free Ferroelectric Composites

Bi_0.5Na_0.5TiO₃（BNT for short） is an A-site complex perovskite material with a rhombohedral symmetry at room temperature and possesses high Curie temperature, relatively large ferroelectric properties and large thickness mode electromechanical coupling factors. However, BNT ceramics possesses relatively high coercive field（Ec≈70kV/cm）, which makes poling process of BNT ceramics difficult. Moreover, BNT ceramics with pure rhombohedral phasere quire high sintering temperature, and possess a thermal depolarization temperature（Td）. When the working temperature is close to Td, the piezoelectric properties of BNT ceramics become less stable. Therefore, BNT-xZnO composite piezoelectric ceramics were prepared. The composite structure,which is embedding nanometer zinc oxide grains into BNT grains’ boundary, can increase Td of BNT-base material and effectively improve the thermal stability of piezoelectric properties. Phase structure, microstructure, dielectric, ferroelectric and piezoelectric properties of BNT-xZnO ceramic were discussed.Firstly, BNT ceramics and BNT-xZnO（x = 0.1, 0.2, 0.3, 0.4, 0.5） composite ceramics have been synthesized using the conventional solid state reaction method. The phase structure and surface microstructure were investigated. BNT ceramics were prepared at 1050 oC for 2h; BNT-xZnO were prepared at 950 oC for 30 min. The addition of zinc oxide can greatly reduce the sintering temperature and time.Secondly, piezoelectric, ferroelectric, dielectric properties of both BNT ceramics and BNT-xZnO composites ceramic were investigated. The results suggest that the introduction of zinc oxide can increase the depolarization temperature, and Td increases with the content of zinc oxide. Therefore, thermal depolarization phenomenon and temperature stability of ceramics has been improved.Finally, BNT ferroelectric thin film with perovskite structure were prepared by sol- gel method. The effect of ethanolamine on the phase structure of calcined BNT dried gel powder were investigated, then the sol concentration and heat treatment（heating rate, annealing temperature） on the phase structure and the morphology of BNT film were investigated. We have successfully synthesized the formulation of stabilized BNT sol, the optimal sol concentration which can prepare smooth and flat BNT film morphology, a relatively reasonable heat treatment process condition, and we finally analyze the probable causes of impure phase.