Phase Transition Behaviors And Piezoelectric Properties Of Acceptor-donor Co-doped BaTiO₃ Ferroelectric Ceramics

The research on BaTiO₃(BT)has a long history.BT ceramic has been widely used in equipment such as thermistors,ferroelectric random access memories and multilayer ceramic capacitors.However,the piezoelectric properties of ordinary sintered BT ceramics are relatively poor(d₃₃?100-170 p C/N),and the dielectric properties are better,so BT ceramics are mostly used as dielectric materials.Researchers have prepared fine-grained BT ceramics through special preparation methods,however,the piezoelectric properties of this fine-grained BT ceramic come from the extrinsic effects brought by fine crystals and nano-domains,and whether the intrinsic large piezoelectric mechanism in terms of composition control or strain-driven that is similar to morphotropic phase boundary in the Pb(Zr_xTi_1-x)O₃(PZT)system can be obtained in the BT ceramic is still a problem to be solved.Based on the above problems,this thesis used traditional solid-phase synthesis to prepare Li⁺-RE³⁺(RE³⁺=La³⁺,Sm³⁺)and Li⁺-B³⁺acceptor-donor co-doped BT ceramics.The influence of doping on the microstructure and phase of the material at room temperature was analyzed by X-ray diffraction and scanning electron microscope;the dielectric,ferroelectric and piezoelectric properties of the doped sample were obtained by impedance analyzer,ferroelectric measuring instrument,and strain tester.Thermal-electrical treatment with different parameters were applied to the material,and the influence of different doping content and different thermal-electrical treatment parameters on the electrical properties of acceptor-donor co-doped BT ceramics was analyzed and discussed,and the physical mechanism combined with the changes in microstructure and phase were studied..For Li⁺-RE³⁺co-doped BT ceramics,with the increase of doping content,the grain size of the material decreases slightly,and its phase composition changes from pure T phase to T phase and M phase coexisting,and when the doping amount is higher than 2.0mol%,a new impurity phase appears,and the performance of the ceramic also deteriorates rapidly.Li⁺-RE³⁺co-doped BT ceramics obtained a huge dielectric response near MPB.The dielectric constant of LL:BT ceramics was increased from 2000 of pure BT to1.7×10⁵(0.9 mol%),and the dielectric constant of LS:BT ceramics was increased to1.6×10⁵(1.0 mol%),the large dielectric response comes from the polarization rotation near the MPB.Li⁺-RE³⁺co-doped BT ceramics also show different phase transition behaviors in different phase regions.In the pure T phase region,the phase transition behavior of the material is basically the same as that of the pure BT ceramic,in the MPB region,the material has abnormal phase change behavior,in the coexistence area of T phase and M phase,the phase change of the material becomes diffuse.For LL:BT ceramics in the T phase region,the intensity ratio of[002]and[200]diffraction peaks and the changes in lattice constants a(b)and c indicate that Li⁺and La³⁺ions tend to be preferentially arranged along the[001]direction.Thereby forming the Li⁺-La³⁺ionic pairs.LL:BT ceramics obtained the most excellent piezoelectric properties when the doping content is 0.6 mol%(d₃₃=246 p C/N,Q_m=570).Thermal-electrical treatments are applied to LL:BT ceramics,which further improves the piezoelectric properties of LL:BT ceramics and greatly improves the temperature stability of the material.After thermal-electrical treatments,0.6 mol%of LL:BT ceramics have obtained excellent piezoelectric constant(d₃₃=276 p C/N)and mechanical quality factor(Q_m=2010),and have very excellent temperature stability.Compared with other ABO₃ systems,it has excellent comprehensive performance,which has positive significance for the application of lead-free piezoelectric ceramics in high-power piezoelectric devices.For LB:BT ceramics,Li⁺-B³⁺co-doping significantly improves the piezoelectric properties of the material.When the doping content is 1.0 mol%,the material obtains the best piezoelectric properties,including large field-induced strain(0.26%),small hysteresis(13.7%),and large large-signal piezoelectric constant(460 pm/V)and huge electrostriction coefficient(0.067 m⁴/C²).Thermal-electrical treatment further improves the piezoelectric properties of the material.Compared with other systems,after the thermal-electrical treatment in the in-plane direction at 30 k V/cm and 200°C,1.0 mol%LB:BT ceramics have obtained excellent comprehensive properties,including extremely large field-induced strain(0.42%),huge large-signal piezoelectric coefficient(860 pm/V),large electrostriction coefficient(0.105 m⁴/C²),very small hysteresis(9%)and very excellent temperature stability,which has important meaning for the application of lead-free piezoelectric ceramics in actuators.The mechanism of thermal-electrical treatments to improve the piezoelectric performance is analyzed.The study found that thermal-electrical treatments promotes the pairing of doped ions,reduces the concentration of discretely distributed acceptor/donor ions,and reduces the generation of oxygen vacancies and cation vacancies.The ionic pairs are aligned along the direction of the applied electric field,and a self-built electric field is formed.The low symmetry structure and large lattice distortion near the ion pair allows the P_D and electric domains to rotate.For the sample after thermal-electrical treatments in the in-plane direction,when the electric field is applied to the material,the external electric field exerts a large torque on the P_D arranged in the in-plane direction,which causes the P_D and the electric domain to rotate in the direction of the applied electric field at the same time,so that the material possesses great,recoverable electric-field-induced strain.