Study On (Na_0.5Bi_0.5)TiO₃-BaTiO₃ Of Lead-free Piezoelectric Ceramics

By the means of SEM,XRD and TG-DSC methods, with the help of the orthogonal design experiment, the experiments doped with BaCO₃, La₂O₃, NaNbO₃ and MnCO₃, the influence of various components on the piezoelectric properties and the influencing mechanism of various components have been investigated.The behavior of the BNT based on piezoelectric ceramics with low Ec and high Pr reaches the level as follows: the piezoelectric constant is 166×10^-12C/N, the dielectricconstant is 1509,the dielectric loss is 3.7%, the sintering temperature is 1140℃.It seems that Ba ions substitute for the A-site of the perovskite structure. The crystal distortion turns up as the differences of ions diameters. That reduces the difficult of poling, improving the piezoelectric and dielectric properties.According to the SEM photograph, the average grain size and Tc temperature becomes smaller with increasing the amount of BaCO₃.Under the condition of the same quantivalency, the doped ion sunstitutes for the similar ion. So La ions take the place of Bi-site, resulting in the confusion of A-site and the apparent properties of relaxation ferroelectrics.The crystal structure of BNT is rhombohedral. The rhombohedral and homocline morphotropic phase boundary doesn't turn up as the little amount of NaNbO₃ The interplanar distance enlarges with increasing the amount of NaNbO₃. As a donor dopant, Nb⁵⁺ promotes the grain growth and the formation of A vacancy, so the domain wall turns easily, improving the piezoelectric properties. When the amount of NaNbO₃ excesses, the excessive Nb₂O₅ segregates near the boundary, inhibiting the grain growth and the movement of intergranular, forming close grain, improving the properties. With increasing the amount of NaNbO₃, the sintering temperature appearing the grain adhesion reduces.The additive effects of Mn ions are similar to the rare earth oxide. It is supposed that Mn ions exist in the grain and substitute for A-site or B-site. The average grain size increases and the Tc temperature becomes lower with increasing the amount of MnCO₃. In the samples prepared by conventional technique, it seems that Mn ions eject Bi ions to the air as results from SEM, expecially on the outside surface.The (Na_0.5Bi_0.5)_0.94Ba_0.06TiO₃ powders are prepared by Sol-Gel method. The effects of NaNbO₃, MnCO₃ on the properties have been studied. The behavior of the BNT based piezoelectric ceramics reach the level as follows: the piezoelectric constant is 136×10¹²C/N, the dielectric constant is 1149,the dielectric loss is 2.6%, the sintering temperature is 1140℃.