Preparation And Characterization Of (Bi_0.5Na_0.5) TiO₃ Based Ferroelectrics For Lead-Free Piezoelecttric Ceramics

Since environmental issues concern directly our life and activity, developing new electric materials substituting for the present used materials need to safeguard the nature environment. In electric and electronic field, Lead-free piezoelectric ceramics thus become more and more importance because the reject and replacing of electric manufactures have affected badly our living environmental. It is well known that the BaTiO₃ is one of the earliest developing piezoelectric ceramics, and there is no lead in the BaTiO₃ based piezoelectric ceramics. In recent years, there has been evidence showing 0.94Bi_0.5Na_0.5TiO₃-0.06BaTiO₃ (abbr. BNT6BT) ceramics could be an excellent candidate as lead-free piezoelectric ceramics.In the present study, BaTiO₃ nanopowder was synthesized by Sol-Gel method, the sintered behavior of BaTiO₃ nanopodwer was studied, and a better sintered process was obtained by optimizing process parameters using Artificial Neuron Network (ANN) tool;BNT6BT ceramics were prepared by solid-state method. The amount of doped additive and processes of ceramics sintering were studied and the key parameters such as the amount of chemical, sintering temperature, maintaining time and the manners of adding chemical were established.The BNT6BT ceramics doped different amounts of Ce(NO₃)₃ were studies. The ceramics were sintered at different temperatures and for different holding time. The dielectric properties had not great change as sintering temperature increasing and holding time increasing. The dielectric properties at room temperature and dielectric -temperature behavior had obvious change, the dielectric constant and dielectric loss tangent at room temperature tended to decrease as the amount of doped Ce(NO₃)₃ increasing. Many secondary phases TiO₂ and Bi₂Ti₂O₇ were discovered on the ceramic surface when the amount of doped Ce(NO₃)₃ was above 0.3 mol%. We also studied the influence of different doped Ce(NO₃)₃ order on electrical properties of doped 0.5 mol% Ce(NO₃)₃ BNT6BT ceramics. Ce³⁺ ions entered into the lattice of the sample adding Ce(NO₃)₃ after calcining. The dielectric constant of sample adding Ce(NO₃)₃ after calcining was close to that of pure BNT6BT ceramics, was about 2 times than that of sample adding Ce(NO₃)₃ before calcining.The properties of 0.4wt% Ce⁴⁺-doped 0.94(Bi_0.5Na_0.5)TiO₃-0.06BaTiO₃ ceramics sintered at different temperatures have been investigated in order to improve the piezoelectric properties of BNT6BT ceramics. The sintering temperatures did not leadto an obvious change in the phase structure in Ce-BNT6BT system. The morphology of ceramic inside and surface indicated that a few TiCh grains existed on the outside of the bulk due to the Bi2C>3 and Na20 volatilizing at higher sintering temperature. The ceramics sintered at 1200°C obtained a relatively large remnant polarization, Pr of about 34.2 jiC/cm2, and a coercive field, Ec, of about 22.6 kV/cm at room temperature. The dielectric constant of the ceramics increased with the increase in sintering temperature, but depolarization temperature (Td) increased below 1160°C (Td =94°C) then decreased at higher temperature. A maximum value of the resistivity p was 3.125X1010 ohnrm for the Ce-BNT6BT ceramics sintered at 1180°C at room temperature.The phase structure, microstructure, piezoelectric and dielectric properties of 0.4wt% Sn4+ & 0.4wt% Ce4+-doped 0.94(Bio.5Nao.5)Ti03-0.06BaTi03 ceramics sintered at different temperatures were investigated. The temperature dependence of dielectric constant of the compositions exhibited strong dielectric dispersion with the increasing temperature. The dielectric loss tangent increased when the temperatures were beyond 225°C. The depolarization temperature Td of Sn&Ce-BNT6BT ceramics sintered at 1160°C was 92.6°C, which did not change remarkably as sintering temperature increased. But the remnant polarization Pr for this system at room temperature increased with the sintering temperature increasing.Finally, the influence of BNT6BT ceramics doped different ions on phase structure, ferroelectricity, and dielectric properties were studied. The Ce3+ ions tended to even more substitute for Na+ ions, which resulted in the secondary phase Bi2Ti2O? appearing in BNT6BT ceramics. However, the Ce4+ ions tended to even more substitute for Bi3+ ions, which resulted in the secondary phase TiC>2 appearing in BNT6BT ceramics. The Sn4+ ions replaced both A-site and B-site, which decreased the coercive field (ï¿¡c) and remanent polarization (Pr) of the samples. The dielectric constant and dielectric loss tangent of the samples became low after aged or polarized. It was possible that the defect dipoles were pinned by domains after aged, or domains were re-orientated, inner stress appeared and defects pinned and disappeared under strong electric field.The piezoelectric properties, time stability and temperature stability were studied in detail in BNT6BT system, these characters and data make Ce-BNT6BT based ceramics suitable for mid-frequency narrowband filters and ultrasonic.