Effect Of Ionic Pair (Nb⁵⁺-Cr³⁺) Doping On Microstructure And Electrical Properties Of 0.93Bi_0.5Na_0.5TiO₃-0.07BaTiO₃ Ceramics

Bismuth sodium titanate（BNT）based ceramic has excellent piezoelectric properties and ferroelectric properties,which is a promising material for lead-free piezoelectric materials to replace lead-based ceramiacs.It has broad application prospects in piezoelectric actuators,sensitive sensors,pulse energy storage capacitors,and other fields.The composition of 0.93Bi_0.5Na_0.5TiO₃-0.07Ba TiO₃ceramics（abbreviated as BNT-7BT）based on morphotropic phase boundary has better piezoelectric and ferroelectric properties,and it’s properties are further improved by ionic pair modification.In this paper,using BNT-7BT ceramics with peroveskite structure as the matrix material,the ionic pair(Nb⁵⁺-Cr³⁺)modified ceramic[(Ba_0.5Na_0.5)_0.93Ba_0.07Ti_1-x(Nb_0.5Cr_0.5)_xO₃,x=0,0.5%,1%,2%,2.5%,5%)（abbreviated as BNBT-xNC）]were prepared by the conventional solid-phase synthesis method.The microstruture,weak/high field pizeoelectric properties,dielectric properties,ferroelectric properties,and strain properties of BNBT-xNC ceramics are systematically studied though X-ray（XRD）,scanning electron microscopy（SEM）,Raman scattering spectroscopy,impedance analyzer,and intergrated ferroelectric test system.The main results are as follows:（1）The microstructure and week-field piezoelectric properties of BNBT-xNC ceramic materials were studied.XRD and Raman show that the BNBT-xNC ceramic is pseudo-cubic before poling,and the BNT-7BT ceramic has a tetragonal phase;In the range of 0.5%≤x≤2.5%,the pseudo-cubic and the tetragonal phase coexist;when x=5%,it is a pseudo-cubic phase.The surface morphology analysis of BNBT-xNC ceramics shows that BNT-7BT ceramics decreases from 8.74μm to 3.98μm.The weak-field piezoelectric performance test shows that the piezoelectric coefficient d₃₃and the planar electromechanical coupling coefficient k_pincrease in the range of0%≤x≤2%,and decrease in the range of 2.5%≤x≤5%.x=2%,d₃₃and k_preach the maximum,which are 200p C/N and 0.49 respectively.The mechanical quality factor Q_mdecreases as the doping composition increases.（2）The dielectric temperature spectra of BNBT-xNC ceramics before and after polarization were studied.The results show that the BNBT-xNC ceramics before polarization exhibit the relaxor characteristics of frequency dispersion and dispersive phase transition.The maximum value of dielectric constantε_mand the temperature T_mcorresponding to the maximum value decrease monotonously with the increase of ionic pair(Nb⁵⁺-Cr³⁺).Comparing the dielectric temperature spectra before and after polarization,it is found that the modification of the ionic pair(Nb⁵⁺-Cr³⁺)significantly changes the ferroelectric relaxor characteristics of the ceramic,and it is non-ergodic relaxation in the lower doped concentration（0%≤x≤1%）.As the content increases（1%≤x≤2.5%）,the non-ergodic-ergodic relaxor state coexists,and finally it changes to the ergodic relaxor state（x=5%）.（3）The ferroelectric properties and high-field strain properties of BNBT-xNCceramics under different electric fields and temperatures are studied.The results show that the ionic pair(Nb⁵⁺-Cr³⁺)has a significant effect on the initial ferroelectric properties and strain performance of BNBT-xNC at room temperature.When the ionic pair content is low,the hysteresis loop,current curve,strain curve and the second electric field of the initial state ceramic are significant different from those under the second electric field.With the increase of ionic pair content,the difference weaken finally disappears,which corresponds to the non-ergodic—non-ergodic/ergodic coexis—ergodic relaxor ferroelectric transition.With the increasing of ionic pair content,the maximum strain S_maxand the strain piezoelectric constant d*₃₃first increase and then decrease.The composition of coexistence of non-ergodic and ergodic ferroelectric relaxor state（x=2.0%）reaches the maximum value S_max=0.22%,d*₃₃=431pm/V.The results show that the ferroelectric properties and strain properties of ceramics increase with the increasing of the electric field.S_max,positive piezoelectric coefficient d*₃₃S+,negative piezoelectric coefficient d*₃₃S-increase with the increasing of electric field.The results show that the properties of the bipolar hysteresis loop,current density curve,and strain curve have obvious differences with the icreasing temperature.When the temperature is lower than T_F-R,d*₃₃increases with the increasing temperature;near T_F-R(slightly lower than T_F-R),a ceramic phase transition occurs,d*₃₃S+increases greatly and d*₃₃S-decreases;When the temperature is higher than T_F-R,ceramics exhibt ergodic relaxation,S_max,d*₃₃decreases.The results of the unipolar strain of piezoelectric constant d*₃₃show that d*₃₃,increases with the increasing of temperature in the composition range of non-ergodic relaxation state with low ionic content（0≤x≤0.5%）;in the range of coexistence of non-ergodic and ergodic ferroelectric relaxation states（x=2.5%）,d*₃₃first increases and then decreases;While in the composition range of ergodic ferroelectric relaxation states（x=5.0%）,the d*₃₃always decreases.