Preparation And Properties Of Sodium Bismuth Titanate-based Piezoelectric Ceramics Research

Sodium bismuth titanate, (Na_0.5Bi_0.5) TiO₃ (abbreviated as NBT), is a kind of perovskite-type relaxor ferroelectrics with a relatively large remanent polarization (Pt =38μC/cm²) at room temperature and a relatively high Curie temperature (Tc =320℃). Because of its strong ferroelectric property, NBT is considered to be the best candidate to replace the widely used lead-contented perovskite materials. In this dissertation, the ceramic samples have been prepared by the conventional solid-state reaction method and by powders from sol-gel method respectively. The microstructure, dielectric and ferroelectric properties of the (Bi_0.95Na_0.75K_0.2-xLi_x)_0.5TiO₃Ba_0.05TiO₃ (abbreviated as NBT-KBT-BT+xLi) ceramics have been investigated and ceramic with excellent piezoelectric property has been obtained. The main results are summarized in the followings:(1) NBT-KBT-BT+xLi ceramics prepared by conventional solid-state reaction method can be well sintered at 1155℃ and its crystalline grain increase slightly with the increasing of the additive amount of lithium. The temperature dependence of the relative dielectric constants ε_r and dielectric loss tan8 of the NBT-KBT-BT+xLi ceramics show that they are typical relaxor ferroelectrics. The ε_r of the samples reaches the maximum value between 300℃ and 350℃, and the maximum ε_r temperature Tm increase with the additive amount of lithium. The sample has the largest remanent polarization Pr=36μC/cm² and piezoelectric constant d₃₃=188 pC/N when x=0.05, and the coercive field Ec is 2.6 kV/mm.(2) NBT-KBT-BT+xLi ceramics prepared by sol-gel method can be well sintered at 1145℃, and the crystalline grains are larger than that of the samples prepared by conventional solid-state reaction method. Many clubbed crystalline grain has been observed in the samples by scanning electric microscope and the number of clubbed crystalline grain increase with increasing of the amount of lithium, and the reason for this may be that the Li⁺ generate other phase in the samples. The remanent polarization and coercive field of NBT-KBT-BT+xLi ceramics are smaller than that ofthe ceramics by conventional method. When x=0.05, the sample has largest remanent polarization Pr=24.5 |aC/cm2 with the smallest coercive field ï¿¡c=l.l kV/mm, and the piezoelectric constant is 197 pC/N.(3) The additives of Co2C^ MnC03 and CuO obviously influenced the properties of the NBT-KBT-BT+0.05Li ceramics. Co2O3 doped facilitate the growth of the crystalline grain. The average grain size of this ceramic sample sintered at 1160°C can reach up to 6 ^m. The piezoelectric constant d^ of CO2O3 doped samples is about 120-135 pC/N and the dielectric loss is only 20% of non doped samples. MnCC>3 doped seriously reduced the ferroelectric property of the NBT-KBT-BT+0.05Li ceramics, and this maybe that MnCO3 increase the samples conductance. The dielectric constant of CuO doped samples show distinct frequence dispersion from room temperature to high temperature.