Study On Structural Distortion And Property Enhancement Of Ion-doped CaBi₂Nb₂O₉ Piezoelectric Ceramics

CaBi₂Nb₂O₉-based（CBN）bismuth layer piezoelectric ceramic material has many advantages of ultra-high Curie temperature（T_c～943℃）,low loss tangent,high insulation,and outstanding temperature stability,which is one of the candidate materials for high-temperature piezoelectric applications.However,the distortion of the oxygen octahedron of the bismuth layer material is limited to the a-b plane,resulting in a lower piezoelectric(d₃₃～5 p C/N)and ferroelectric（P_r～3μC/cm~2）properties.Therefore,in order to achieve the sufficient sensitivity that piezoelectric devices should possess in the high-temperature environment,it is necessary to improve the piezoelectric performance and temperature stability of CBN ceramics.The main purpose of this thesis is to improve the piezoelectric performance of CBN ceramics,systematically study the effect of ion doping on its comprehensive performance,and deeply analyze the internal mechanism of the effect of crystal structure on the improvement of piezoelectric performance.The exploratory experiment of this thesis is divided into the following three parts.Firstly,the modification experiments of Ca Bi₂Nb_2-xMg_xO_9-3x/2（CBN-x Mg,CBNM,0≤x≤0.04）ceramics were designed,and the effects of acceptor-doping at B-site of Mg²⁺on the crystal structure and electrical properties were carefully investigated.XRD and Raman show that Mg atomic enter B-site to form the solid solution of pure CBN phase.In addition,the XRD refinement results show that Mg²⁺doping increases the distortion of the Nb O₆octahedron,and enhances the contribution of the total P_sof each position along the a-axis,which increases from-28.278 to-31.768μC/cm~2.According to SEM,Mg²⁺doping strengthened the growth rate of CBN ceramic grains on the a-b plane,showing a more obvious plate-like structure.In addition,the reduced anti-site defects strengthen the structure of（Bi₂O₂）²⁺,and improved the resistivity of the CBN ceramics.When doping amount x=0.02,the comprehensive properties of the CBN ceramic sample reach the best,which include that d₃₃of 11.1 p C/N,P_rof 7.22μC/cm~2,tanδof 3.0%at 600℃,andρ_dcof4.5×10~7W·cm at 600℃.Secondly,using Cu/W composite ion as the dopant,the experiment of modified Ca Bi₂Nb_2-x(Cu_0.25W_0.75)_xO₉（CBNCW-x,CBNCW,0≤x≤0.15）ceramics was carried out.The effect of B-site Cu/W equivalent composite doping on the crystal structure,microstructure and electrical properties of CBN ceramics was systematically analyzed.The results show that all the ceramics have a bismuth layered structure of m=2,which is a single-phase of CBN.The ceramic grains show the typical plate-like structure of the bismuth layer.The refinement shows that the equivalent doping of Cu/W significantly enhances the distortion of the oxygen octahedron and effectively improves the polarization strength of the CBN ceramics,especially the ferroelectric properties of the samples are significantly improved.When x=0.075,it shows better comprehensive properties with an excellent d₃₃of 13.8 p C/N,a high remnant polarization of 12.10μC/cm~2,the T_cof 918℃,a dielectric loss value of 0.57%,and the Q_mvalue of 7099.Finally,on the basis of CBNCW-0.025 ceramic,a modification experiment of Ce-doped Ca_1-xCe_xBi₂Nb_1.975(Cu_0.25W_0.75)_0.025O₉（CBNCW-x Ce,0≤x≤0.09）was designed.The effects of co-doping of Ce ions at the A-site and Cu/W ions at the B-site on the crystal structure,oxygen vacancies and piezoelectric properties were investigated in detail.XRD patterns and Rietveld refinement show that the crystal structures of the samples transform from the orthorhombic phase into the pseudotetragonal phase and that the lattice distortion is weakened.Raman and XPS spectra indicate that Ce ions exist with+3 and+4 valences in the air sintered ceramics,in which Ce⁴⁺replaces Nb⁵⁺,causing the weakened Nb O₆octahedral vibration of torsional and tensile and an increase in oxygen vacancies in the doped ceramics.When x=0.04,it shows excellent comprehensive properties with the d₃₃value of 18.1 p C/N,the T_cvalue of 900℃,and theρ_dcvalue of 2.8×10~5W·cm at 500℃.The CBNCW-0.04Ce ceramic sample prepared in this experiment has an ultra-high Curie temperature and good piezoelectric properties,which can be regarded as one of the high-temperature piezoelectric candidate materials and has great application potential.