Study On Preparation Technology, Structure And Properties Of Na_0.5Bi_0.5TiO₃-BaTiO₃ Lead-free Piezoelectric Ceramic

Piezoelectric ceramic is a function material which can exchange energy between mechanic and electricity. Traditional piezoelectric ceramics give off Pb vapor in the preparation. The evaporation of Pb caused not only deviation of the composition of ceramic but also the pollution to environment. (l-x)Na_0.5Bi_0.5BaTiO₃ has strong Ferroelectric performance, It is supposed to be a hopeful lead-free piezoelectric ceramic. But it is difficult to pole net (l-x)Na_0.5Bi_0.5BaTiO₃ because of the high coercive field and conductivity, so the piezoelectric properties can not be exploited completely. The coercive field can be decreased by dopand. In this paper, the (l-x)Na_0.5Bi_0.5TiO(3-x)BaTiO₃ samples were prepared in solid synthetic way. The Ba2+ substituted for the instrinsic cations at the A sites of the preovskite structure. The optimum preparation condition, piezoelectric, ferroelectric, and relaxor properties of the samples were investigated.Samples with various compositions were prepared through different calcined temperature, sinter temperature and different maintained time at sinter temperature. The structure, density, shrinkage, and piezoelectric properties were investigated. It is concluded that optimum calcined temperature is 900℃ with x<0.06 and 950℃ with x 0.06. Liquid phase is indispensable in the sinter procession. It contributes to the growth of the crystalline grain and the density. But too much liquid phase decreases the piezoelectric properties. The amount of liquid phase is decided by the sinter temperature. Suitable sinter temperature is 1170℃ or so, and maintained time is 2 hours.The samples were poled at different temperature and voltage and maintained different time at poling voltage. Piezoelectric properties were detected, it is suggested that the optimum poling condition is maintaining 15 minutes under 4kV/mm, poling temperature is 70℃XRD results indicate that the fabric transited from rhombohedral to tetragonal gradually with the substitution of the Ba2+. Morphotropic phase boundaries (MPB) of the ceramic exist in the composition range of x=0.06⁰.10.SEM results show that the growth of the crystalline grain was restrained and thedimension of it turned littler.The properties of all samples were investigated, It is the same to the PZT ceramics, samples in the composition range of MPB exhibited best piezoelectric performance. The hysteresis loops were detected, it is found that the remanent polarization and the conceive field of the ceramics deceased with the dope of Ba2+. In the composition range of MPB, the conceive field exhibit the lowest value.The temperature dependence of dielectric constant between room temperature and 500 under 1 kHz, 10kHz 100kHz frequency conditions were measured, it is found that the ceramics are relaxor ferroelectric. The relaxor properties are decreased by the substitution of Ba2+. The ceramics exhibit normal ferroelectric properties at low temperature and relaxation properties at high temperature with the composition of x 0.10. It can be concluded that there are two phase transitions take place in the heating process by the phenomenon of two abnormal dielectric peaks.Hysteresis loops of sample were measured at different temperature, the double hysteresis loops were detected which suggest that the anti-ferroelectric phase exist in the heating process of ceramics, through the analysis of the temperature dependence of dielectric constant, the phase transition diagram was concluded.This work was supported by the National Natural Science Foundation of China (Grant No. 50272044), Japanese NSG Foundation and Hubei Province Natural Science Foundation (Grant No. 2002AB076).