Research On Growth、Structure And Properties Of K_0.5Na_0.5NbO₃-MnBiO₃-Based Lead-free Piezoelectric Single Crystal

Piezoelectric single crystals of perovskite ferroelectric system（K,Na）Nb O₃have been extensively studied in the past 20 years due to their large piezoelectric coefficient,high Curie temperature,low density and nontoxic chemical composition.The most widely used characteristic is piezoelectricity,which can realize the coupling between electrical and mechanical signals,and make perovskite ferroelectrics become an important part of electronic applications,such as actuators,sensors and converter.In addition,piezoelectricity is the basis of many emerging technologies,including electronic human implants,micro/nano robots,energy harvesters and Internet sensors.Therefore,it is not surprising that piezoelectric ceramics are considered as one of the market segments of electrical ceramics.Perovskite-type ferroelectric single crystals have great potential from both application and scientific perspectives.In this paper,KNN-based single crystal samples were prepared by seed-free solid-state method,by using Li,Ta co-doped（1-x）[（99.6KNN-0.4MB）-x LiTaO₃]（KNN-MB-xLT）（x=0-0.01）andCuO doped（1-x）（[99.7（99.6KNN-0.4MB）-0.3B₂O₃]-xCuO）（KNN-MBB-xCu）（x=0-0.007）single crystals as the research objects.The single crystal modification was optimized from the aspects of process,composition and structure.The relationships between the contents of LiTaO₃and CuO and the morphology,structure and properties of KNN-based single crystals were explored.The important results obtained from the experiment are as follows:Firstly,the effects of the introduction of trace LiTaO₃on the grain size,macroscopic microstructure,electrical properties and leakage current conduction mechanism of99.6KNN-0.4MB single crystal were studied.The results show that doping a small amount of LiTaO₃has the effect of solubilization,which can promote the further growth of single crystal particles.Compared with the undoped single crystal,with the increase of LiTaO₃content,the single crystal surface is more smooth and smoother,and the centimeter level large single crystal with better performance can be obtained.At the same time,the addition of LiTaO₃forms a complete solid solution in KNN.All the crystal samples have perovskite structure,and basically the coexistence structure of orthogonal phase and tetragonal phase.With the increase of LiTaO₃content,the contents of orthophase and tetragonal phase in KNN-based single crystal showed a certain range of fluctuation,which generally promoted the increase of tetragonal phase content in the crystal.When the crystal structure is within the morphotropic phase boundary and the ratio of orthogonal phase to tetragonal phase reaches the best ratio,the piezoelectric properties of single crystal can be greatly improved.In addition,the addition of appropriate amount of LiTaO₃can also reduce the concentration of oxygen vacancies in the crystal,reduce the lattice distortion caused by lattice mismatch,and thus obtain great piezoelectric response.The addition of LiTaO₃has little effect on the intrinsic contribution of the piezoelectric response of the crystal,but the extrinsic contribution increases with the increase of the electric field.When the LiTaO₃content is0.9 at%,the average size of KNN single crystal is the largest,which is 19.9×13.1×1.7mm³.At the same time,the electrical properties are also the largest,the piezoelectric coefficient d₃₃=792 pC/N(the largest piezoelectric coefficient d₃₃=1994 pC/N),relative dielectric constantε=549,strain S=0.51%,mechanical coupling coefficient K_t=0.83,residual polarization P_r=64μC/cm,dielectric loss tanδ=0.01,inverse piezoelectric coefficient d^*₃₃=4698 pm/V（10 kV/cm）,transition temperatures T_O-T=135℃,curie temperature T_C=415℃。Secondly,the effects of different annealing temperature on the crystal structure,domain structure,ferroelectric,piezoelectric properties and leakage current conduction mechanism of 99.1（99.6KNN-0.4MB）-0.9LiTaO₃single crystal were investigated.It is found that with the increase of annealing temperature,the crystal structure of the single crystal do not change significantly,and the perovskite structure coexisted with orthorhombic and tetragonal phases.However,the morphology changed obviously.With the increase of annealing temperature,the single crystal particles is refined,and the single crystal gradually became smoother from the original rough and uneven surface.After annealing,the average size of ferroelectric domains in the single crystal decreased first and then increased,and the overall change range was 123～650 nm.When the annealing temperature is 700°C,KNN-MB-0.9 LT single crystal showed excellent electrical properties,the values are:d₃₃=591 pC/N,ε_r=580,tanδ=0.01,K_t=0.715,T_C=417°C,when the electric field E=2.5 kV/cm,a smaller leakage current density of～10^-6A/cm²can be obtained in KNN-MB-MB-0.9LT single crystal,indicating that suitable annealing temperature can significantly reduce oxygen vacancies,defect dipoles and other defects,can greatly improving the performance of KNN-based single crystal.Finally,the effects of CuO content on the natural growth surface,microstructure,crystal structure,piezoelectric response,ferroelectric dielectric properties,leakage current density and conductive mechanism of KNN-MBB-xCu crystal were also studied.The experimental results show that CuO has played a good solubilization effect in the growth process of single crystal,which can promote the growth of single crystal.With the increase of CuO content,obvious growth steps appear on the surface of KNN single crystal,indicating that CuO has been fully integrated into the lattice and effectively promoted the growth of crystal.In addition,with the increase of CuO content,the crystal still maintained the structure of orthogonal and tetragonal phase in general.With the increase of CuO content,the piezoelectric,ferroelectric and dielectric properties of the crystal first increase and then decrease,when the CuO content is 0.3 at%,the best comprehensive properties are obtained,and the value are d₃₃=563 pC/N,ε=496,S=0.29%,K_t=0.63,P_r=64μC/cm（30kV/cm）,P_r=69μC/cm（35 kV/cm）,tanδ=0.01,d^*₃₃=1450 pm/V（20 kV/cm）,T_O-T=155°C and T_C=416°C.With the increase of CuO content,the leakage current conduction mechanism of KNN-based single crystal has not changed,and it is still a space charge conduction mechanism.In summary,the addition of CuO promoted the improvement of electrical properties of KNN-based crystals.