Study On Electrical Properties Of Bi_0.5Na_0.5TiO₃-BaTiO₃ Piezoelectric Ceramics By Complex Cation Doping

Piezoelectric ceramics have the ability to generate electrical potential in response to mechanical stress（direct piezoelectric effect）or mechanical motion in response to an electric field（inverse piezoelectric effect）.This type of smart material has been employed in a variety of applications,particularly in the disciplines of information and communications,medical diagnostics,industrial automation,etc.Lead-free piezoelectric ceramics have garnered a lot of interest as a bridge between electrical and mechanical energy because of the global requirement for sustainable economic development and environmental conservation.However,their lower energy conversion coefficients compared to conventional lead-containing piezoelectric ceramics greatly limit their application in devices.In recent years as composite cation doping in a variety of lead-free piezoelectric systems is a common strategy to obtain excellent electrical properties.Therefore,in this paper,bismuth Sodium Titanate-Barium Titanate（BNT-BT）ceramics were used as the substrate in this study,and the effects of doping on the microstructure and electrical characteristics,particularly the field-induced strain,were thoroughly examined by doping with(Nb_0.5Cr_0.5)⁴⁺and(Nb_2/3Zn_1/3)⁴⁺.In this paper,(Bi_0.5Na_0.5)_0.94Ba_0.06Ti_1-x(Nb_0.5Cr_0.5)_xO₃（BNT-BT6-x NC,x=0,0.01,0.015,0.02,0.025,0.03,0.035,0.4,0.045,0.05）ceramics and(Bi_0.5Na_0.5)_0.935Ba_0.065Ti_1-x(Nb_2/3Zn_1/3)_xO₃（BNT-BT65-x NZ,x=0,0.05,0.01,0.015,0.02,0.025,0.03）ceramics were prepared by the conventional solid-phase sintering method.The following are the study’s principal findings.1.The field-induced phase change,microscopic microstructure,piezoelectric and dielectric characteristics of BNT-BT6-x NC piezoelectric ceramics were investigated.With increasing(Nb_0.5Cr_0.5)⁴⁺doping content,the ceramics transitioned from a polycrystalline structure with rhombohedral R3c and tetragonal P4bm symmetry to a cubic-like phase with rhombohedral/tetragonal distortion.SEM results showed that the average grain size of the ceramics increased and then decreased with the increase of the x value.At x=0.03,the maximum size is 8.43μm.The large grain size could be beneficial for domain switching and excellent piezoelectric properties and the piezoelectric coefficient d₃₃ reaches a maximum of181 p C/N.The(Nb_0.5Cr_0.5)⁴⁺doping shifts the ferroelectric-relaxor transition temperature(T_F-R)to room temperature and drops to near room temperature at x=0.025.2.The field-induced strain characteristics of the BNT-BT6-x NC ceramic system were investigated in detail.At x=0.035,the reversible domain switching contribution during the reversible phase transition from the relaxor phase to the ferroelectric phase is greatly enhanced,effectively reducing the negative strain and increasing the positive output strain.Finally,a large field-induced strain of 0.41%was obtained at an electric field of 60 k V/cm with d^*₃₃=688 pm/V.The Rayleigh analysis showed that the portion of irreversible contribution dropped from 58.54%to 36.77%when the concentration of(Nb_0.5Cr_0.5)⁴⁺complex cations increased.It was therefore concluded that the rapid decline of1)₄0))andwere due to a lower contribution of irreversible domain wall displacements.3.Based on the previous study,the composite cation was replaced with(Nb_2/3Zn_1/3)⁴⁺and another composition of the morphotropic phase boundary（BNT-BT65）was selected as the substrate.The microstructure,field-induced phase transition,and valence of elements of BNT-BT65-x NZ piezoelectric ceramics were characterized mainly by XRD,SEM,XPS,and Raman diffraction.The average ionic radius of(Nb_2/3Zn_1/3)⁴⁺complex ions is 0.673?,which is slightly larger than that of the substituted Ti⁴⁺ions（0.605?）,resulting in a larger lattice constant.The SEM results show that the average grain size of ceramics increases and then decreases with increasing doping content,reaching a maximum of 2.80μm at x=0.015,and the excellent weak-field piezoelectric properties d₃₃=189 p C/N are obtained due to the grain size effect.displacement of cations and anions,resulting in a localized polarization region around the defective dipole and ion pair.4.The dielectric and ferroelectric properties of BNT-BT65-x NZ ceramics were investigated.The dielectric response of the ceramics with different compositions of superimposed small signals first increases and then decreases with the bias electric field,with the difference that it decreases rapidly when x<0.025 and decreases slowly when x≥0.025,and all of them can be maintained at a high level.This may be related to the formation of highly dynamic weakly correlated PNRs.When x=0.025,the bipolar asymmetric strain S≈0.72%and S_max/E_max≈1200 pm/V were measured under the circulating electric field;when x=0,the hysteresis of the S-E curveη≈55.20%,which has a considerable strain hysteresis;when x=0.025,η≈29.17%,and the hysteresis is also reduced while obtaining ultra-high strain 47.16%.This indicates that the doping of(Nb_2/3Zn_1/3)⁴⁺complex ions disrupts the long-range ferroelectric ordering and fosters the ergodic relaxation state,forming an ergodic/non-ergodic composite with highly reactive polar nano-microregions promoting reversible domain change contribution,which can effectively reduce the negative strain and hysteresis.The internal bias field due to oxygen vacancies and defective dipoles leads to asymmetric strain generation,which greatly contributes to field-induced strain and coordinates the balance of giant strain and low hysteresis.Thus,this work provides a design strategy for obtaining more efficient actuator applications.