Study Of Structure And Giant Electrostrain Properties Of BiFeO₃-BaTiO₃ Based Lead-Free Piezoelectric Ceramics

Pb（Zr,Ti）O₃（PZT）-based ceramics dominate the global ferroelectric materials market due to their significantly enhanced piezoelectric properties in the quasi-homotypic phase boundary（MPB）region.However,as the world became more environmentally conscious in the late 20th century,the large-scale application of traditional PZT ceramics was restricted,so lead-free piezoelectric ceramics gained widespread attention and importance.Bi Fe O₃-Ba Ti O₃-based ceramics have both high Curie Temperature and strong piezoelectricity and have become one of the research hotspots in the lead-free field in recent years.In this paper,starting from the MPB of the BF-BT ceramics,the electrostrain applied to the driver field and the energy storage performance applied to the energy storage were studied.The relationship between the chemical composition,structure,and electrical properties of（1-x）BF-xBT（BF-xBT,x=0.25～0.4）ceramics were studied.With the increase of BT content,the crystal structure of the ceramics gradually changed from the rhombohedral phase to the tetragonal phase.The rhombohedral phase decreased from 95.54%at x=0.25 to 17.14%at x=0.4,and when x=0.3,the content of the rhombohedral phase（53.6%）was almost equal to that of the tetragonal phase（46.4%）.With the increase of x,the grain size of the ceramic samples decreased,the dielectric constant increased first and then decreased,reaching a highest value at the MPB,the Curie temperature continued to decrease.The piezoelectric and ferroelectric properties of the ceramics were significantly improved within the MPB region.The 0.7BF-0.3BT ceramic sample showed excellent piezoelectric and ferroelectric properties:d₃₃=165 p C/N,k_p=0.31,P_r=23.5μC/cm²,S=0.21%.The crystal structure,micromorphology,dielectric,ferroelectric and strain properties of0.7BF-0.3BT-xSn（x=0,0.2%,0.4%）ceramics were studied.With the increase of Sn content,the ceramic samples undergo a transition from（R-T）two-phase coexistence to（R-T-P_c）three-phase coexistence and finally to（R-P_c）two-phase coexistence.The three-phase coexistence structure has a significant contribution to the electrical properties of the ceramics.When x=0.2%,P_max=35.8μC/cm² and P_r=25.8μC/cm² and an asymmetric strain profile was induced.Significantly enhanced electrostrain was obtained after poling and ageing of ceramic samples with S=0.7%and d₃₃^*=1166 pm/V.The enhanced electrostrain response originated from the combined effect of multiphase coexistence and the internal bias field generated by the defective dipole.The microstructure,dielectric properties and energy storage characteristics of（0.7-x）BiFeO₃-0.3BaTiO₃-xBaSn_0.11Ti_0.89)O₃（x=0,0.1,0.2,0.3）ceramics were studied.From the XRD and SEM analyses,it can be seen that with the as the increasing content of BST doping,the sample changes from rhombohedral and tetragonal coexistence phases to a pseudo-cubic phase dominated structure,and the average grain size is also reduced.With the increase of BST content,the dielectric constant of the samles decreases and the relaxation is obvious.At the same time,the residual polarisation strength（P_r）of the ceramic decreases.When x=0.1,the ceramic samples exhibit an extremely strong strain response,with S=1.5%at 70 k V/cm and d₃₃^*=2150 pm/V,exceeding the majority of reported electrical strain values for lead-free ceramics.When x=0.2,the maximum polarisation(P_max)of the ceramic is34.77μC/cm² and P_r is 4.22μC/cm²,corresponding to an energy storage density（W）is 1.8J/cm³ and an effective energy storage density(W_re)is 1.4 J/cm³,with an energy storage efficiency（η）is 76.6%.The（0.7-x）BF-0.3BT-x BST ceramic system shows excellent promise for application.