Study On The Microstructure And Electrical Properties Of BNT-BKT Lead-free Piezoelectric Ceramics

Recently,Bi_0.5Na_0.5TiO₃（BNT）-based piezoelectric ceramics have shown great potential to replace lead-based materials in practical applications because of their excellent electrical properties.Among them,0.8Bi_0.5Na_0.5TiO₃-0.2Bi_0.5K_0.5TiO₃（abbreviated as BNKT20）located at the morphotropic phase boundary（MPB）exhibits the enhanced piezoelectric properties,but it is far from meeting the requirements of practical applications.Therefore,the improvement of BNKT ceramic system by various processes to obtain better electrical properties is the focus of current research.Based on this,this paper investigated the effects of sintering process,B-site complex ion doping and solid solution modification process on the phase structure,microstructure and electrical properties of BNKT-based lead-free piezoelectric ceramics based on the microstructure-electrical properties of BNKT20ceramics as the selected object of study,and the main findings were as follows:1.The optimum sintering temperature for BNKT20 ceramics has been determined by studying the effect of sintering temperature（1100°C,1120°C,1140°C,1160°C,1180°C）on the microstructure and electrical properties of BNKT20 ceramics.XRD analysis showed that the changes in sintering temperature did not change the phase structure of BNKT20ceramics and all samples showed pseudocubic phase.SEM results showed that the increase in sintering temperature significantly promoted the grain growth.The dielectric analysis indicated that all samples were relaxor ferroelectrics,and an appropriate increase in sintering temperature was beneficial to the dielectric properties,withε_max=7690 and T_F-R=150°C in1160°C samples.Meanwhile,the P-E hysteresis loops of BNKT20 ceramics became saturated as sintering temperature increases and relatively good ferroelectric properties were observed in the 1160°C samples(P_max=50.1μC/cm²,P_r=41.8μC/cm² and E_c=29.0 k V/cm).Furthermore,the relatively good piezoelectric properties were obtained for BNKT20ceramics sintered at 1160°C:d₃₃=162 p C/N,k_p=0.35,Q_m=90.7,S=0.16%.2.The microstructure,dielectric,ferroelectric and strain properties of Bi_0.5Na_0.4K_0.1Ti_1-x(Al_0.5Ta_0.5)_xO₃（abbreviated as BNKT-x AT;x=0,0.01,0.02,0.03,0.04,0.05）ceramics prepared were investigated.The introduction of the AT composition had no significant effect on the overall phase structure of BNKT ceramics,and all samples exhibited a single perovskite structure.In addition,AT doping significantly inhibited the grain growth.From the dielectric and ferroelectric analyses,it can be seen that with the increase of AT content,T_F-R was effectively regulated to below room temperature,and P_max,P_r and E_c were significantly reduced,indicating that AT doping effectively disrupts the ferroelectric long-range orders,while the increase of P_bs(=P_max-P_r)with increasing AT content indicates the increase in the reversibility of ferroelectric domains under the electric field.This indicates that AT doping effectively triggers the NER-ER phase transition,which eventually exhibits the coexistence of NER/ER phases in BNKT-0.03AT,thus improving the strain characteristics of the ceramics.At x=0.03 composition,S=0.44%and d₃₃^*=732 pm/V.3.The microstructure,dielectric,strain and energy storage properties of（1-x）Bi_0.5Na_0.4K_0.1TiO₃-x Bi(Zn_2/3Ta_1/3)O₃（abbreviated as（1-x）BNKT-x BZT;x=0,0.02,0.025,0.03,0.035,0.04,0.05）ceramics were investigated.The results show that the（1-x）BNKT-x BZT ceramics have a single pseudocubic structure,and the lattice parameters and compositional disorder increase with the increase of BZT content.In addition,the introduction of BZT significantly affects the microscopic morphology of BNKT ceramics and leads to an increase in grain size.Dielectric analysis showed that the introduction of BZT effectively lowered the T_F-R to below room temperature,leading to the generation of dynamic PNRs.the introduction of BZT also effectively enhanced the strain and energy storage properties of the ceramics,in 0.975BNKT-0.025BZT ceramics,the bipolar strain S_bi=0.50%and the unipolar strain S_uni=0.56%,corresponding to d₃₃^* values of 833 pm/V and 933 pm/V,respectively.In addition,the 0.95BNKT-0.05BZT ceramic obtained a total energy storage density of 1.81 J/cm³,a recoverable energy storage density of 1.05 J/cm³,and an energy storage efficiency of 58%under an electric field of 90 kV/cm.