Preparation And Properties Of A/B Doped CaBi₄Ti₄O₁₅ High Temperature Piezoelectric Ceramics And Research On High Temperature Conductivity Mechanism

At present,piezoelectric ceramics with high Curie temperature are urgently needed as sensitive functional materials for vibration monitoring and fault warning of key structural components in major technical equipment such as aerospace,nuclear power generation and oil exploration in China.Ca Bi₄Ti₄O₁₅（CBT,m=4）is a typical bismuth layered structure ferroelectric（BLSF）.It has many advantages such as high Curie temperature（T_c=790°C）,low dielectric loss（tanδ=0.69%）,low aging rate,small leakage current and anti-fatigue.It is expected to be used as a sensitive element for piezoelectric ultrasonic transducers,acceleration sensors and other electronic devices with an operating temperature exceeding 500°C.It is a high-temperature piezoelectric material with great application potential.However,the low sintering density,low piezoelectric coefficient(d₃₃<10 p C/N)and unsatisfactory high temperature resistivity of pure CBT ceramics limit their practical application.In this paper,pure Ca Bi₄Ti₄O₁₅ was selected as the basic formula,and CBT-based high-temperature piezoelectric ceramics doped with different ions at A/B sites（here in after referred to as CBT ceramics or materials）were prepared by traditional oxide solid-state reaction method.The effects of ion doping concentration and sintering temperature on the microstructure and electrical properties of ceramics were studied.Combined with dielectric temperature spectrum analysis,high-temperature resonance-anti-resonance test,high temperature conductivity research and other means,the dielectric relaxation characteristics and ion conductivity mechanism of ceramics at high temperature were revealed,which provided an optimized material formula and performance improvement method for the practical application of CBT ceramics.The main contents of this paper are as follows:（1）A-site doped Gd³⁺doped CBT ceramics(Ca_1-xGdx Bi₄Ti₄O₁₅+0.2wt.%Mn O₂)with Mn O₂ as sintering aid were prepared.The effects of Gd³⁺doping concentration（x=0～0.2）on the microstructure,dielectric properties,piezoelectric properties,high temperature resonance characteristics and high temperature conductivity behavior of CBT ceramics were studied.The XRD results show that all the doped components have an orthogonal phase structure and no impurity phase is formed,indicating that Gd³⁺diffuses into the perovskite layer of CBT to replace Ca²⁺at the A site.SEM observation shows that the microstructure of the ceramics is composed of flake grains,and the grain size decreases with the increase of Gd³⁺doping concentration.When x=0.11,the average grain size of the ceramic is the smallest（D_λ=2.30μm）.As the temperature increases,the resonance-antiresonance peak（f_r-f_a）of CBT ceramics gradually shifts to the low frequency direction.The introduction of Gd³⁺can increase the Curie temperature（T_c=805°C～810°C）,reduce the dielectric loss（tanδ=0.2%）and improve the piezoelectric coefficient(x=0.11,d₃₃=22 p C/N)of CBT ceramics.The DC conduction behavior of CBT ceramics at high temperature is related to the long-range migration of oxygen vacancies.Among all the doped components,the sample with x=0.06 exhibits the highest E_dc value of～1.87 e V and the lowest conductivity(σ=1.8×10^-5 S/m at 600°C).Based on the Bergman formula,the temperature dependence of the electrical modulus of x=0.06 sample was fitted and analyzed.The results show that when the temperature increases from 500°C to 700°C,the interaction strength（βvalue）of carriers increases first and then decreases.The maximum value ofβis～0.967 at 600°C,indicating that the dielectric relaxation behavior of the sample at this temperature is closest to the ideal Debye relaxation type.（2）CBT ceramics(Ca_0.89Gd_0.11Bi₄Ti_4-y（Nb Ta）_yO₁₅+0.2wt.%Mn O₂)doped with Gd³⁺/(Nb⁵⁺Ta⁵⁺)at A/B sites and Mn O₂ as sintering aid were prepared.The effects of(Nb⁵⁺Ta⁵⁺)doping concentration（y=0.03～0.07）on the structure and properties of CBT ceramics were studied.The results show that(Nb⁵⁺Ta⁵⁺)can replace Ti⁴⁺at the B site and play a donor doping effect,thereby improving the piezoelectric properties of the ceramics.Thanks to the synergistic doping effect of Gd³⁺and(Nb⁵⁺Ta⁵⁺)ions at A/B sites,the Curie temperature of CBT piezoelectric ceramics can be stabilized at about T_c=809°C.Moreover,the dielectric loss of CBT ceramics can be reduced to less than 0.3%by doping more(Nb⁵⁺Ta⁵⁺)（x=0.007）.When y=0.005,the sample Ca_0.89Gd_0.11Bi₄Ti_3.9875（Nb Ta）_0.005O₁₅+0.2wt.%Mn O₂ exhibits the highest relative density(ρ_relative=97.65%)and the highest piezoelectric coefficient(d₃₃=23 p C/N).（3）The effects of sintering temperature（950°C～1100°C）on the microstructure,dielectric and piezoelectric properties of Ca_0.89Gd_0.11Bi₄Ti_3.9875（Nb Ta）_0.005O₁₅+0.2wt.%Mn O₂ceramics were studied.With the increase of sintering temperature,the degree of orientation of ceramic grains along the c-axis gradually decreases.The ceramic grains show obvious lamellar structure,and the grain size increases with the increase of sintering temperature.In particular,when the sintering temperature exceeds 1000°C,the grain growth and development is good,the grain boundary is clear,the structure is dense,and the orientation is random.However,the sintering temperature has little effect on the Curie temperature of ceramics（T_c=803°C～810°C）,but prolonging the sintering time（2 h～4 h）can reduce the dielectric loss of ceramics（tanδ=0.90%～0.25%）.The sample sintered at 1000°C for 4 h has a higher relative density(ρ_relative=97%)and the highest piezoelectric coefficient(d₃₃=24 p C/N),which is 2 times higher than that of pure CBT.