Study On Preparation And Properties Of Na_0.5Bi_2.5Ta₂O₉ Base Bismuth Layered Structure Piezoelectric Ceramics

The bismuth-layered structure（BLS）lead-free piezoelectric ceramic has excellent properties such as high Curie temperature and low dielectric loss,making it highly promising in fields such as high-temperature and high-frequency sensing,as well as in harsh working conditions like aerospace and acoustic impedance.Na_0.5Bi_2.5Ta₂O₉（NBT）belongs to the m=2 type of BLS piezoelectric ceramic materials,which has high Curie temperature（T_c=760℃）and relatively high piezoelectric performance,making it highly applicable in the field of high-temperature piezoelectric sensing.However,due to its large coercive electric field strength,which leading to difficulty in polarization,as well as the volatility of bismuth elements in high-temperature sintering,leading to the formation of non-stoichiometry,causing an increase in leakage current during polarization and hindering its full polarization to obtain high piezoelectric performance.Additionally,the anisotropy of crystal growth in the BLS piezoelectric ceramic structure results in faster growth in the a-b plane than in the c-axis direction,leading to spontaneous polarization mainly concentrated in the a-b plane and resulting in lower macroscopic piezoelectric activity.These shortcomings limit its practical application in the field of high-temperature piezoelectric sensing.The crystal structure of a material has a very important influence on its piezoelectric performance.Ion doping is an important method for changing the crystal structure of a material and improving its piezoelectric activity.Therefore,this study uses ion doping technique to enhance the piezoelectric performance of the NBT-based BLS piezoelectric ceramics,by using solid-phase synthesis reaction and atmospheric pressure sintering process to prepare NBT-based BLS piezoelectric ceramics.In order to address the shortcomings of bismuth element volatility and polarization constraint in the a-b plane,this study systematically investigates the effects of excess bismuth element content and appropriate doping ions on the phase structure,microstructure morphology,electrical properties,and temperature stability and micro-mechanisms of NBT ceramics.In this study,Bi excess Na_0.5Bi_2.5Ta₂O₉+xwt%Bi₂O₃（NBT+x,x=0,1,2,3,5,8,10）ceramics were prepared by solid-state synthesis combined with normal pressure sintering.The influence of Bi content on the phase structure and electrical properties of NBT ceramics was systematically investigated.The experimental results indicate that moderate addition of Bi can promote the grain growth of NBT ceramics,reduce the degree of orthorhombic distortion,and effectively enhance the electrical properties of ceramics.When x=2,the ceramic exhibits the best electrical properties:T_c=760℃,d₃₃=13.4 p C/N,tanδ=0.0051,ε_r=165.9,k_p=7.11%,Q_m=2030,P_r=0.577μC/cm².Moreover,even after high temperature treatment at 700℃for 30 minutes,the piezoelectric coefficient d₃₃ of the ceramic remains at 87%of its initial value,demonstrating good temperature stability.In this study,La³⁺-doped Na_0.5Bi_2.5-xLa_xTa₂O₉（NBLT-x,x=0,0.1,0.2,0.3,0.4）ceramics were prepared by solid-state synthesis based on Na_0.5Bi_2.5Ta₂O₉+2wt%Bi₂O₃.The effects of La³⁺doping on the phase structure,microstructure,and electrical properties of NBT ceramics were investigated.The refined XRD results using Full Prof software show that all compositions are single-phase bismuth layer-structured compounds,with La³⁺entering the A site of the ceramics to partially replace Bi³⁺.Additionally,with the doping of La³⁺,the grain growth was inhibited,and the sintering temperature of the ceramics was reduced.Consequently,the degree of lattice distortion of NBT ceramics decreased,and their electrical properties were improved.When the doping amount was x=0.2,the ceramic exhibited the best electrical properties:T_c=680℃,d₃₃=17.8 p C/N,tanδ=0.00655,ε_r=148.592,k_p=9.95%,Q_m=2778,P_r=1.915μC/cm²,E_c=1.344 KV/μm.Moreover,after heat treatment at 600℃for 30 minutes,the piezoelectric coefficient d₃₃ of the ceramic remained at 91%of its initial value,reaching 16.2 p C/N,demonstrating excellent thermal stability.Finally,on the basis of Na_0.5Bi_2.3La_0.2Ta₂O₉ ceramics,the B-site of NBLT ceramics was doped and substituted with six-coordinate transition metal W⁶⁺.Na_0.5Bi_2.3La_0.2Ta_2-xW_xO₉（NBLTW-x,x=0.00,0.01,0.02,0.03,0.04）ceramics were prepared by solid-state sintering method,and the effects of La/W co-doping on the phase structure,microstructure,and electrical properties of NBT ceramics were systematically studied.XRD analysis and Full Prof refinement showed that all components were single-phase layered bismuth compounds,and the appropriate addition of W⁶⁺further reduced the degree of lattice distortion,making the NBLTW ceramics structure gradually approach a pseudo-tetragonal phase,and the Curie temperature of ceramics increased while the dielectric loss decreased.Meanwhile,the grain size of the samples was also moderately suppressed,and the sintering temperature of the ceramics was reduced.Among them,the NBLTW-0.03 ceramic sample exhibited the best electrical properties:T_c=684℃,d₃₃=19.4 p C/N,tanδ=0.00401,ε_r=191.682,k_p=12.93%,Q_m=3328,P_r=2.0645μC/cm².In addition,after the ceramic samples were subjected to a 600℃heat treatment for 30 min,the piezoelectric coefficient d₃₃ remained at 93.8%of the initial value(d₃₃=18.2 p C/N),indicating that the Na_0.5Bi_2.5Ta₂O₉ ceramic co-doped with La/W has great potential for application in the field of high-temperature piezoelectric ceramics.