Piezoelectric Properties And Thermal Stability Research Of 0.7BF-0.3BT High Temperature Lead-free Piezoelectric Ceramics

In order to improve the operating characteristics of piezoelectric devices in high-temperature environments,it is necessary to develop high-temperature lead-free pie-zoelectric ceramics with high piezoelectric properties and good thermal stability.However,combining both aspects simultaneously is difficult for existing high-temperature piezoelec-tric ceramics.Bi Fe O₃-BaTiO₃（BF-BT）ceramics have high Curie temperature and good piezoelectric properties,which is an important kind of high temperature lead-free piezoe-lectric ceramics.The materials have a wide application prospect in automobile,aerospace,geological exploration and other fields.BF-BT ceramics are usually evaluated for thermal stability by ex-situ depolarization methods combined with Curie temperature in the past literature.The piezoelectric properties of the ceramics were tested by ex-situ depolarization method until the depolarizing temperature of the ceramics was cooled to room temperature,so the data is not continuous.But the piezoelectric properties of ceramics at high tempera-ture can be measured synchronously by the in-situ depolarization test.The in-situ depolar-ization test can simulate the vibration of the ceramics in the actual working state,which is real-time and continuous.Through the in-situ depolarization test、XRD、SEM、XPS and other test methods,the piezoelectric properties and thermal stability of the 0.7Bi Fe O₃-0.3BaTiO₃ ceramics were studied by doping MnO₂ and Li₂CO₃ during the batching and doping MnO₂ after calcining in this paper.The research content is analyzed as follows:（1）The influence of MnO₂ doping on piezoelectric properties and thermal stability of0.7Bi Fe O₃-0.3BaTiO₃ ceramics was investigated.0.7Bi Fe O₃-0.3BaTiO₃+x MnO₂（x=0,0.001,0.002,0.003,0.004,0.7BF-0.3BT+0.003Mn+x LC for short）high temperature lead-free piezoelectric ceramics with nano-BaTiO₃ as raw material was prepared.The ex-perimental results show that R phase and T phase coexist in all ceramics.The piezoelectric properties d₃₃=182p C/N,the mechanical coupling coefficient k_p=0.242,the residual polar-ization intensity P_r=21.56μC/cm²,and the coercivity field E_c of all the ceramics are less than 32.5k V/cm at x=0.003.In-situ depolarization results show that with the increase of MnO₂ doping,two depolarization peaks appear gradually,which are related to electric do-main arrangement and relaxation phase transformation.While the ex-situ depolarization d₃₃gradually decreases with the increase of depolarization temperature,no depolarization peak appears,and the ex-situ depolarization temperature T_d reaches a maximum value of 450^oC at x=0.003.A series of results indicate that MnO₂ doping can improve the piezoelectric properties and thermal stability of ceramics.（2）0.7Bi Fe O₃-0.3BaTiO₃+0.003MnO₂+x Li₂CO₃（x=0,0.001,0.002,0.003,0.004,0.005,0.7BF-0.3BT+0.003Mn+x LC for short）high temperature lead-free piezoelectric ce-ramics with nano-BaTiO₃ as raw material was prepared.The experimental results show that the structure of ceramic phase consists of rhombic phase（R）and tetragonal phase（T）.The average grain size and relative density can be increased to the maximum（4.50μm and95.78%）by doping LC.The optimum sintering temperature of LC doped ceramics can be reduced to 930^oC.When x=0.001,the piezoelectric properties and thermal stability reach the maximum values:d₃₃=178p C/N,T_C=516.3^oC and T_d=490^oC.Therefore,by doping LC to the 0.7Bi Fe O₃-0.3BaTiO₃+0.003MnO₂ceramics,the reduction of sintering temperature is significant and the thermal stability is improved,which has a wide application prospect in the field of high-temperature piezoelectric ceramics.（3）0.7Bi Fe O₃-0.3BaTiO₃+0.001Li₂CO₃+x MnO₂（x=0,0.001,0.002,0.003,0.004,0.005,referred to as 0.7BF-0.3BT+0.001LC+x Mn）was used as the base formula.Using BaTiO₃ nanoparticles as raw materials,the effect of MnO₂ doping on ceramics after cal-cining was analyzed by in-situ depolarization test,XRD,SEM and other testing methods.The results show that the structure of ceramic phase consists of rhombic phase（R）and te-tragonal phase（T）.When x=0.001,the average grain size and relative density are 8.83μm and 95.56%,respectively,and the excellent electrical properties and thermal stability are d₃₃=203p C/N,k_p=0.380,P_r=20.47μC/cm²,T_C=490.1^oC,T_d=450^oC,respectively.There are two obvious depolarization peaks in the in-situ depolarization curve,which may be related to the electric domain arrangement and relaxation phase transition.Although the thermal stability is not improved as expected,the piezoelectric properties of the ceramics are im-proved to some extent.