Structure And Property Of Na_0.5Bi_0.5TiO₃-Bi₄Ti₃O₁₂ Piezoelectric Ceramics

Mixed piezoelectrics with bismuth layered structure and perovskite structure has been prepared and their physical properties have been investigated. Bismuth layer-structured piezoelectric ceramic has aroused great attention in the field of piezoelectric materials due to its high Curie temperature (T_c), notable anisotropic electromechanical coupling factor and low rate of aging. It is considered to be the candidate to replace the traditional lead-based piezoelectric materials in specific field. Bi₄Ti₃O₁₂ (abbreviated as BiT) with T_c=675℃is in monoclinic phase with a 3°-5°lean along the c axis at room temperature. It is often considered to be the motherboard structure of the bismuth layer-structured ceramics, and modified for new materials. Na_0.5Bi_0.5TiO₃ (abbreviated as NBT) is a kind of perovskite relaxor ferroelectrics with a relatively large remanent polarization (Pr=38μC/cm²) at room temperature. Because of its strong ferroelectric property, NBT is considered to be one of the best candidates to replace the widely used lead-contended materials. In this dissertation, the x(Na_0.5Bi_0.5TiO₃)-Bi₄Ti₃O₁₂ (abbreviated as xNBT-BiT) ceramics were prepared by conventional solid-state reaction method. The micro structure, dielectric property and piezoelectric property of the ceramics have been systematically studied. The main results are summarized as follows:(1) Optimized process conditions have been obtained. The xNBT-BiT series of piezoelectric ceramics can be well synthesized at 780-830℃. The sintering temperatures are found to be from 1050℃to 1130℃according to the increasing x with a period of soaking time 120-150 minutes. The piezoelectric properties of the xNBT-BiT ceramics highly depend on poling field and temperature. Prolonging the poling time can make the polarization better. Appropriate poling field is 8000-10000V/mm. poling temperature is 180-200℃and poling time is 30 minutes.(2) Their micro-structures have been characterized. The densities of the sintered xNBT-BiT ceramics reduce with the increasing x. The X-ray powder diffraction experiment show that the xNBT-BiT ceramics is of bismuth layered structure when x=1. Other compounds are bismuth layered structure coexistent with perovskite structure. The ceramic with x=2 exhibits property of bismuth layered structure strongly. There are flake-like grains in the surface microstructure of the ceramics among the whole series.(3) Dielectric properties have been studied. The dielectric constant and the dielectric loss of the xNBT-BiT ceramics increase as x increases. They are especially small when x=l and 2 implying bismuth layer-structured character. Poling process reduces the dielectric constant of the samples, and makes a slight increase of the dielectric loss except x=1. The Curie temperatures relating to the bismuth layered structure are around 665℃for all the samples and almost don't change with x. However, the value of the peak of the sample with x=3 is the largest at 1MHz. Then it gradually reduces with the increase of x. When x>3, there appears a new peak at 342℃and a step at 220℃on theε_r-T curve of each compound implying the existence of the perovskite structure. Below 220℃, there is frequency dispersion in the dielectric constant of the ceramics. The phase transition displays typical diffused phase transition feature at 342℃.(4) Piezoelectric properties have been studied. The frequency constants show an increasing trend and the piezoelectric property of the ceramics enhances at room temperature with the increase of x. After annealed at 300℃for 120 minutes, the piezoelectric constant d₃₃ of the ceramic with x=2 is the largest. Then the piezoelectric property appears a reducing trend as x increases. But they still maintain a strong piezoelectric activity while x≤8. After annealed at 600℃for 120 minutes, the piezoelectric ceramics with x=1, 2 are hardly depolarized and show a very good stability independent of temperature. Ceramics with x=3-8 show both the features of NBT and bismuth layer-structured ceramics. One hand, they have large piezoelectric constants at room temperature. For example, the dielectric constant d₃₃ is 32pC/N for the compound with x=8. The other hand, their piezoelectric properties show a good stability independent of temperature at high temperature between 300℃to 600℃. Their dielectric constants are still around 10pC/N after annealed at 600℃for 120 minutes. The anisotropy of the electromechanical coupling coefficients of the materials is very obvious, especially when x is small. The mechanical quality factors of the samples reduce as x increases.