Study On Electrical Properties And Thermal Stability Of BiFeO₃-BaTiO₃-based Lead-free Piezoelectric Ceramics With High Temperature Large Piezoelectric Response

BiFeO₃-BaTiO₃（BF-BT）is a high Curie temperature lead-free piezoelectric ceramic with a perovskite structure,which does not have any phase transition from room temperature to Curie temperature.It is considered one of the most promising candidates to replace PZT.However,the system has some problems such as difficult sintering,high dielectric loss and low piezoelectric performance at room temperature.In order to obtain BF-BT ceramics with good density and low defects and to understand the change of its electric properties with the temperature at high temperature,this paper first started with from the low temperature sintering of BF-BT ceramics.Different sintering additives were used to reduce the sintering temperature,and dense BF-BT ceramics samples were obtained.Further,BF-BT ceramics with high temperature and large piezoelectric response were prepared by adjusting the ratio of BF-BT,A/B site doping substitution,the addition of(Bi_0.5Li_0.5)TiO₃ sintering aid and optimizing the ratio of BF-BT after doping.The phase structure,microstructure,piezoelectric properties,ferroelectric properties,dielectric properties and thermal stability,especially the piezoelectric response at high temperature,were studied.The piezoelectric response behavior of BF-BT ceramics at high temperature was analyzed by using the in-situ quasi-static piezoelectric constant“Berlincourt”test method.The specific research contents are as follows:（1-x）Bi_1.05FeO₃-x BaTiO₃-0.0035MnO₂-0.001Li₂CO₃（BF-x BT）ceramics were prepared by adding MnO₂ and Li₂CO₃ sintering additives and adjusting the ratio of BiFeO₃ to BaTiO₃.With the increase of BT content,the phase structure of BF-x BT ceramics changes from R to R phase and T phase coexistence,the grain size gradually decreases,and the piezoelectric properties gradually increases and reaches the maximum value of d₃₃=185 pC/N,k_p=0.367 at BF-0.32BT ceramics.Curie temperature decreased gradually with the increase of BT content,but maintained a high Curie temperature T_c=470℃ when x=0.32.PFM domain morphology analysis revealed a small number of fine domains in BF0.32BT ceramics,while TEM analysis revealed a large number of nanodomains and polar nanodomains（PNRs）in microstructure.In-situ d₃₃ analysis showed that when x≤0.30,the real depolarization temperature T_dr remained around 250 ℃,but when x=0.32,the T_dr of BF-0.32BT suddenly increased to 334 ℃.BF-0.32BT ceramics have high temperature piezoelectric response at 334 ℃:d₃₃=450.5 pC/N.BF-BT ceramics doped with BiGaO₃ have been studied by A/B substitution.0.7BiFeO₃-（0.3-x）BaTiO₃-x BiGaO₃+0.01MnO₂（BF-BT-xBG）ceramics were prepared by solid-state sintering method.XRD analysis shows that all ceramics are of perovskite structure with R phase and T phase coexistence.With the increase of BiGaO₃ content,the R phase content of ceramics increased first and then decreased gradually,and the grain size gradually increased.Adding a small amount of BiGaO₃ is helpful to improve the piezoelectric properties,ferroelectric properties and dielectric properties of BF-BT ceramics,and obtain high piezoelectric properties at x=0.003:d₃₃=141 pC/N,k_p=0.314,Q_m=56.813,P_r=18.3μC/cm²,ε_r=853 and tanδ=0.048.By adding MnO₂ after calcination,the ceramics still have good piezoelectric properties when polarized at room temperature.With the increase of BiGaO₃ doping amount,the Curie temperature and depolarization temperature of the ceramic first increased and then gradually decreased,and reached the maximum value at x=0.006:T_c=485.9 ℃,T_d=465 ℃.In-situ d₃₃ analysis shows that the thermal depolarization temperature T_dr increases gradually,and the high temperature and large piezoelectric response d₃₃=466 p C/N is obtained at x=0.003 and T_dr=340 ℃.0.7BiFeO₃-（0.297-x）BaTiO₃-x(Bi_0.5Li_0.5)TiO₃+0.003BiGaO₃+0.01MnO₂（BF-BT-x BLT） ceramics were prepared and studied with(Bi_0.5Li_0.5)TiO₃ as a low temperature sintering agent.(Bi_0.5Li_0.5)TiO₃ as a sintering agent,the sintering temperature decreased from 960 ℃ to 880 ℃ with the increase of doping amount.(Bi_0.5Li_0.5)TiO₃-doped BF-BT system of lead-free piezoelectric ceramics are all perovskite structure,the XRD results show that BF-BT-x BLT ceramics are R phase and T phase coexistence structure.With the increase of(Bi_0.5Li_0.5)TiO₃content,the R phase content in ceramics first increases and then decreases,and the grain size firstly increases and then gradually decreases.The grain size of BF-BT-0.0072BLT ceramics decreases with the decrease of sintering temperature.(Bi_0.5Li_0.5)TiO₃ doping improves the piezoelectric properties,ferroelectric properties and dielectric properties of the BF-BT system,and high electrical properties are obtained at x=0.0012:d₃₃=152 p C/N,k_p=0.321,Q_m=58.184,P_r=25.07μC/cm² and tanδ=0.046.Adding MnO₂ after calcination gives the ceramics have good piezoelectric properties when polarized at room temperature.BF-BT-0.0072BLT ceramics still have good piezoelectric properties:d₃₃=135 p C/N,k_p=0.279 and Q_m=63.713 when sintered at 880 ℃.As the doping amount of(Bi_0.5Li_0.5)TiO₃ increases gradually,the Curie temperature and depolarization temperature of the ceramic increase first and then decrease gradually,and reach the maximum value when x=0.0012:T_c=485 ℃,T_d=465 ℃.In-situ d₃₃ test shows that the ceramic has a high temperature piezoelectric response d₃₃=392 p C/N at T_dr=352℃when x=0.0012.（1-x）Bi Fe_0.97Ga_0.03O₃-x BaTiO₃-0.0024(Bi_0.5Li_0.5)TiO₃-0.0035MnO₂(BF₉₇BG₃-xBT) ceramics were prepared by further adjusting the ratio of BiFeO₃ and BaTiO₃ and the basic formula.The phase structure analysis of BF₉₇BG₃-x BT ceramics shows that when BT content is low,the ceramics have an R phase and T phase coexistence structure.With the gradual increase of BT content,the phase structure gradually changes to the C phase,and the grain size of the ceramics gradually decreases.By comparing piezoelectric properties at different sintering temperatures,it is found that BF₉₇BG₃-x BT ceramics have the best piezoelectric properties when sintered at 970 ℃ for 4h.When BT content is 0.31,the optimal electrical properties are d₃₃=167 pC/N,k_p=0.328,Q_m=35.335,P_r=19.78μC/cm²,ε_r=1081 and tanδ=0.048.The results of the dielectric temperature spectrum and ex situ depolarization show that the Curie temperature and depolarization temperature of the ceramics decrease with the increase of BT content and the maximum values of T_c=492 ℃ and T_d=480 ℃ are reached at x=0.3.TEM test results show that BF₉₇BG₃-x BT ceramics possess nanodomains and PNRs.In-situ XRD results show that the phase structure is a multi-phase coexistence structure,and the phase structure does not change with the increase of temperature.The maximum piezoelectric response d₃₃=533.2 pC/N is obtained by in-situ d₃₃ test at T_dr=314.7 ℃.These results indicate that the change of phase structure is not the source of the large piezoelectric response of the BF₉₇BG₃-0.34BT ceramics.However,the large piezoelectric response of the BF₉₇BG₃-0.34BT ceramics is the source of the multi-phase coexistence structure and the presence of nanodomains and PNRs.