Study On Microstructure And Electrical Properties Of Mixed-valent Complex Ions Modified BNT-BT Lead-free Piezoelectric Ceramics

Piezoelectric ceramics are an important part of many electronic devices owing to their excellent electrical,mechanical and temperature properties.Lead-based ceramics,represented by Pb（Zr,Ti）O₃,have dominated in commercial piezoelectric ceramics for a long time.However,lead-based ceramics cause serious health hazards and environmental pollution,making the development of lead-free piezoelectric ceramics to replace lead-based ceramics an urgent task.(Bi_0.5Na_0.5)Ti O₃-Ba Ti O₃（BNT-BT）lead-free piezoelectric ceramics have attracted much attention for their complex phase structure and good piezoelectric properties.In this paper,0.93(Bi_0.5Na_0.5)Ti O₃-0.07Ba Ti O₃（BNT-7BT）ceramic,which is in the morphotropic phase boundary（MPB）component of the BNT-BT system,is selected as the substrate to obtain more excellent electrical properties through sintering optimization,A-site and B-site mixed-valent complex ions modification.In this work,the temperature stability,piezoelectric properties and electrostrain of ceramics are investigated from the analysis of the crystal structure,microscopic morphology and electrical properties.The contents and results of the study are as follows:Firstly,the effect of sintering temperature on the piezoelectric response and depolarization temperature of BNT-7BT ceramics is investigated.The crystal structure and microstructure analysis show that both P4bm and R3c phases coexist in all samples.The optimization of sintering temperature can improve the content of R3c phase in the ceramics,and the high content of R3c phase can delay the ferroelectric-relaxor phase transition temperature.Meanwhile,increasing the sintering temperature promotes the grain growth of ceramics and enlarges the size of electric domains in the grains,which facilitates the domain switching and the orderly arrangement along the electric field direction during poling,improving the piezoelectric response of ceramics.At a sintering temperature of 1140°C,the ceramic achieves a trade-off between piezoelectric response(d₃₃)and depolarization temperature（T_d）,simultaneously achieving a large piezoelectric response(d₃₃=186 p C/N)and a high depolarization temperature（T_d=133°C）.Secondly,the crystal structure,microscopic morphology,piezoelectric properties,dielectric properties,and ferroelectric properties of A-site mixed-valent complex ions modificated[(Bi_0.5Na_0.5)_0.93Ba_0.07]_1-x(Li_0.5Sm_0.5)_xTi O₃（x=0,0.015,0.03,0.045,0.06）ceramics are investigated.Crystal structure analysis shows that the addition of(Li_0.5Sm_0.5)²⁺（LS）induces lattice distortions that cause local heterogeneities in the ceramics and produce short-range polar nanoregions.After poling,the analysis reveals that a small amount of relaxor phase is introduced into the strongly polar ferroelectrics to build the ferroelectric/relaxor phase boundary intentionally.In addition,the introduction of LS helps the grain growth which promotes domain switching and domain wall movement.The higher order of the domains along the electric field alignment contributes to the enhancement of the piezoelectric activity,and the maximum piezoelectric response d₃₃=230 p C/N is obtained at x=0.03.At the same time,the addition of an appropriate amount of LS retains a certain amount of domain wall so that the ceramic will not return to its original state after the electric field is removed due to the domain wall pegging.When x=0.03,the P-E loop and I-E loop of the ceramic have the largest maximum polarization(P_max=53.47μC/cm²),the remanent polarization（P_r=44.01μC/cm²）and the maximum current polarization(I_max=0.73 m A),demonstrating excellent electrical properties.Finally,the crystal structure,microscopic morphology,dielectric properties,ferroelectric properties,and electrostrain of B-site mixed-valent complex ions modificated(Bi_0.5Na_0.5)_0.93Ba_0.07Ti_1-x(Nb_0.5Ga_0.5)_xO₃（x=0,0.02,0.03,0.04,0.06）ceramics are investigated.The introduction of(Nb_0.5G_a0.5)⁴⁺（NG）shifts the ferroelectric-relaxation phase transition temperature of ceramics to room temperature,which induces the transition from nonergodic to ergodic relaxor ferroelectrics and builds the boundary between nonergodic and nonergodic relaxor phases.And the retention of moderate amount of polar phases inside the ceramics plays a positive role in promoting the polarization response of ceramics.Moreover,the introduction of non-equivalent NG ions creates defect dipoles in the ceramics,and by the polarization and aging treatment,the internal bias field is formed by poling and aging,which reduces the driving electric field and improves the electrostrain（S）and the piezoelectric strain coefficient（d₃₃^*）of the ceramics.At x=0.03,the ceramic achieves an ultrahigh electrostrain（S=0.73%）with giant piezoelectric strain coefficient（d₃₃^*=913pm/V）,as well as maintains excellent temperature and cycle stability.