Doping Modification Of BiFeO₃-BaTiO₃ Based Lead-Free Piezoceramics

Environment-friendly lead-free piezoceramics have attracted much attention due to the enhancing awareness of environmental protection.Recently,BiFeO₃-BaTiO₃（BF-BT）based ceramics have been a study hotspot because of its high Curie temperature,strong ferroelectricity and good piezoelectric properties.However,the phase composition and chemical composition location of BF-BT based ceramics near morphotropic phase boundary（MPB）are still under debate.In addition,the piezoelectric performances of the ceramics need to be further improved.Herein,the relationship among chemical composition（including doping）,phase structure and electrical performances were systematically investigated.The main contents are as follows:（1）The relationship among chemical composition,phase structure and electrical properties of（1-x）BF-x BT（BF-x BT,x,0.24～0.38）ceramics were studied.The phase structure of the ceramics transforms from rhombohedral（R）to pseudocubic（p C）with increasing BT content,and the MPB of R-p C phase coexistence was determined in the range of 0.28≤x≤0.33.The R/p C ratio shows monotonic decrease from 75.3/24.7 to 36.9/63.1 as x increases from 0.28 to0.33,and the phase fraction of R（49.1%）and p C（50.9%）are almost equal to each other for the ceramics with x=0.31.With increasing x,both the dielectric loss and permittivity of the ceramics increase gradually while the Curie temperature decreases monotonically.Enhanced piezoelectricity and ferroelectricity were attained in the ceramics near MPB.Especially,the excellent piezoelectric performance(d₃₃=190 p C/N and k_p=0.359)as well as high ferroelectricity（P_r=24.09μC/cm²）are achieved in the BF-0.31BT ceramics.（2）Effects of Mn doping on the structure and electrical properties of BF-0.31BT ceramics were investigated.The ceramics with lower content（≤1.2 mol%）of Mn doping still locate near MPB of R-p C phase coexistence,while the ceramics with higher content（2 mol%）of Mn doping shows p C phase.An appropriate content of Mn doping facilitates the grain growth of the ceramics,but excess Mn doping inhibits the grain growth to some extent.Mn doping,exhibiting“hard”effects,increases the coercive field E_C and decreases both the dielectric permittivityε_r and piezoelectric coefficient d₃₃ of the ceramics.Meanwhile,improved dielectric loss tanδand mechanical quality factor Q_m were observed for the Mn-doped ceramics.（3）Influences of Bi compensation on the structure and electrical performances of B_1+xF-0.31BT（x,0～0.08）ceramics were studied.Bi compensation has slight influences on the phase structure and all the ceramics locate near MPB of R-p C phase coexistence,and R/p C ratio of all the ceramics fluctuates slightly in the range of 41.7/58.3-49.1/50.9.An appropriate content of Bi compensation could facilitate the densification and grain growth of the samples.Bi compensation,exhibiting“soft”doping effects,favors the decrease of E_C of the ceramics.Decreased percentage of oxygen vacancy was observed for x=0.04 compared with the no compensation ceramics as confirmed by XPS results.Optimization of Bi compensation content is beneficial for improving the piezoelectric property of the ceramics.The excellent piezoelectric performance(d₃₃=207 p C/N)was obtained for the B_1.04F-0.31BT ceramics.Besides,good thermal stability of piezoelectric property was observed for this series ceramics.（4）The relationship among chemical composition,phase structure and electrical properties of（1-x）BiFe_0.97Ga_0.03O₃-x BT（x,0.24～0.38）ceramics were investigated.A phase transition from R to p C phase was identified as x increases,and the MPB of R-p C phase coexistence locates in a broad range of 0.26≤x≤0.33.The addition of 3 mol%Ga could broaden the MPB region of the BF-BT ceramics.Enhanced piezoelectric and ferroelectric performances are achieved in the ceramics near the MPB.Compared with the results of undoped BF-BT ceramics mentioned above,the addition of Ga（3 mol%）could improve piezoelectric property for the ceramics with lower BT content（x,0.24～0.26）but deteriorate piezoelectric property for the ceramics with higher BT content（x,0.28～0.38）.（5）Effects of Ga content on the structure and electrical property of BiFe_1-xGa_xO₃-0.31BT（x,0～0.06）ceramics were studied.All the samples locate near MPB of R-p C phase coexistence,and Ga doping has distinct influences on the R/p C phase content ratio.An appropriate content of Ga doping favors densification and grains growth of the ceramics during sintering.With the increment of Ga content,the Curie temperature of the samples shifts towards lower temperature.Enhanced diffuse phase transition behavior（diffusion coefficientγ=1.85）was observed for the ceramics with higher content（x=0.06）of Ga doping.Optimization of Ga content favors enhancing both the piezoelectricity and ferroelectricity of the ceramics.Particularly,the optimal piezoelectric performance(d₃₃=206 p C/N and k_p=0.367)as well as strong ferroelectricity（P_r=25μC/cm²）are obtained in the ceramics with x=0.015.（6）Structural and electrical properties of a series of BiFe_1-xGa_xO₃-0.26BT（x,0～0.05）ceramics were systematically investigated.The ceramics show symmetries of R at x≤0.01,of p C at 0.04≤x≤0.05,and the MPB with R-p C coexistence was detected in the composition range of 0.02≤x≤0.03.The finer grains were observed for the ceramics with Ga doping.Both the remanent polarization P_r and coercive field E_C of the ceramics decrease gradually with increasing x.A proper content of Ga doping improves piezoelectricity while excess Ga doping deteriorates dramatically piezoelectricity of the ceramics.All the ceramics with x≤0.03 show relative high Curie temperature（513～522℃）and depolarization temperature（460～480℃）,presenting a new path to construct an MPB for BF-BT based ceramics with higher Curie temperature.