Effects Of Mn And B Doping On The Growth,Structure And Properties Of Lead-Free Piezoelectric K_0.5Na_0.5NbO₃-BiAlO₃ Single Crystals

With the speedy improvement of economic,cultural and cutting-edge information technology in society,and in particular the growing material and spiritual lifestyle of people,piezoelectric materials are turning into an indispensable functional material in people’s daily life.In the previous few decades,many new piezoelectric ceramic materials have been developed developed through continuous efforts,all of which have extraordinary piezoelectric properties and outstanding physical and chemical properties such as high mechanical quality factor(Q_m).Lead zirconate titanate（PZT）is presently the most competitive and extensively used ceramic material in the world.It has extremely good piezoelectric properties and high Curie temperature,and is broadly used in industrial,medical and martial fields with splendid results.However,due to the scarcity of resources and environmental pollution of PZT,it is urgent to develop a high-performance and environmentally friendly lead-free piezoelectric material.Lead-free piezoelectric materials based on potassium sodium niobate(K_xNa_1-xNb O₃,KNN)with a perovskite structure are of excellent interest due to their splendid piezoelectric properties and high Curie temperature.In this paper,the seed-free solid-state crystal growth（SFSSCG）technique was used to prepare series of lead-free piezoelectric single crystals（1-x）K_0.5Na_0.5Nb O₃-x Bi Al O₃,（1-x）(99.6K_0.5Na_0.5Nb O₃-0.4Bi Al O₃)-x Mn O₂,（1-x）(99.6K_0.5Na_0.5Nb O₃-0.4Bi Al O₃)-x B₂O₃and（1-x）[99.7(99.6K_0.5Na_0.5Nb O₃-0.4Bi Al O₃)-0.3Mn O₂]-x B₂O₃.The doping impact of fluxing agent was systematically investigated in terms of crystal growth,crystal structure,chemical composition,domain structure and electrical properties.The major research contents and consequences are as follows:Firstly,the impacts of Mn O₂doping on the growth,microstructure and electrical performance of K_0.5Na_0.5Nb O₃-Bi Al O₃lead-free piezoelectric single crystals had been investigated.The consequences exhibit that the crystal growth regularity is diffusion growth from the edge of the ceramic to the center,and the grown single crystal samples have regular geometric shapes.The addition of Mn O₂makes the crystal surface more smooth and the grain size gradually increases.When the doping amount of Mn O₂is x=0.005,the average size of KNN-based piezoelectric single crystal reaches the maximum value of about 18×17×2 mm³.X-ray diffraction evaluation indicates that the addition of Mn O₂does not alter the single crystal perovskite structure,but the addition of Mn O₂contributes to the phase transition from orthorhombic to tetragonal.Adding an appropriate amount of Mn O₂contributes to the deflection of ferroelectric domains,reduces oxygen vacancies and lattice distortion inside the material,and improves the overall electrical properties of single crystals.When the doping amount of Mn O₂x=0.003,the overall electrical properties of the single crystal is the best:d₃₃=568p C/N,P_r=32.2μC/cm²,Q_m=55.3,k_t=0.36,k_p=0.39,T_C=418°C,ε_r=430,tanδ=0.015.Secondly,the impacts of trace B₂O₃doping on the growth,structure and performance of 99.6K_0.5Na_0.5Nb O₃-0.4Bi Al O₃primarily based piezoelectric single crystals had been investigated.The consequences exhibit that B₂O₃has an analogous fluxing effect as Mn O₂.Both of them can significantly reduce the sintering temperature of KNN-based single crystals,reduce the internal defects of single crystals,produce more liquid phases,and grow high-quality single crystal samples.With the augmentation of B₂O₃content,the surface of single crystal turns into more flat,and the average grain dimension of single crystal will increase first and then decreases.Trace B₂O₃doping can promote the transformation of the tetragonal phase,the phase structure of the single crystal transformed to the morphotropic phase boundary（MPB）,and the performance of the crystal also changes positively.With the augmentation of B₂O₃concentration,the piezoelectric performance,ferroelectric performance and dielectric performance of KNN-BA-B single crystal all exhibit the characteristics of increasing first and then decreasing.When the doping amount x=0.005,the performance of single crystal sample is the best:d₃₃=421p C/N,P_r=29.6μC/cm²,k_t=0.34,Q_m=46.3,T_O-T=159°C,T_C=416°C,tanθ=0.018.By comparing the doping effects of Mn O₂and B₂O₃,it can be seen that both of them can reduce the internal defects of KNN-BA-based single crystals,promote the growth of single crystals,do not change the perovskite structure of single crystals,and improve the piezoelectric,ferroelectric and dielectric properties of materials.However,through performance comparison,it is found that Mn O₂doped KNN-BA single crystal shows better doping effect than B₂O₃in terms of piezoelectric properties,ferroelectric properties,dielectric properties and electromechanical coupling properties.In addition,based on 99.7(99.6K_0.5Na_0.5Nb O₃-0.4Bi Al O₃)-0.3Mn O₂single crystal system,we co-doped it with B₂O₃.The specific research is as follows:The surface of manganese and boron co-doped KNN-BA-Mn-B single crystal is a uniform and flat layered structure.The single crystal sample co-doped with manganese and boron is a single perovskite structure,and element doping does not change the phase of the material.When0.3 at%Mn O₂and 0.3 at%B₂O₃are co-doped,high-quality KNN-BA-based lead-free piezoelectric single crystals with large grain size can be grown,and they show the best piezoelectric and ferroelectric properties(d₃₃～220 p C/N,P_r（max）～26.7μC/cm²).However,the electrical properties of Mn and B co-doped KNN-BA single crystal samples are lower than those of Mn O₂or B₂O₃single doped KNN-BA single crystal samples.Finally,the impacts of annealing temperature on the microstructure,crystal structure,domain engineering,chemical composition and electrical performance of99.7(99.6K_0.5Na_0.5Nb O₃-0.4Bi Al O₃)-0.3Mn O₂single crystal had been investigated.The consequences exhibit that the surface of the single crystal is a flat layered structure,and as the annealing temperature increases,the average grain size of the single crystal gradually increases,and the surface of the crystal becomes flatter,showing a two-dimensional layered growth.However,when the heat treatment temperature is too high,the single crystal size will decrease and the crystal surface will become rough.The ferroelectric domain has the same change rule.When the annealing temperature is 650°C,the average domain size of the single crystal sample is the lowest,about 180 nm.At this time,the single crystal will show higher piezoelectric properties,its d₃₃～764 p C/N.With the change of annealing temperature,the proportion of orthorhombic phase and tetragonal phase of the sample changes.When the annealing temperature is 650°C,the content of tetragonal phase is the highest,and the ratio of its content to orthorhombic phase is close to1:1,but the phase structure composition remains unchanged.Appropriate annealing temperature can effectively reduce the oxygen vacancy concentration and domain pinning effect,reduce or eliminate the internal stress of single crystal,and improve the electrical properties of single crystal.With the increase of annealing temperature,the electrical properties of KNN-BA-Mn O₂single crystal samples increase first and then decrease.When the annealing temperature is 650°C,the single crystal shows the best comprehensive electrical properties.The specific electrical properties are as follows:d₃₃=764 p C/N,P_r=31.6μC/cm²,E_C=8.8 k V/cm,ε_r=665,tanδ=0.005,T_C=417°C,Q_m=68.7 and k_t=0.476.The highest strain value S is obtained under the action of 15 k V/cm electric field.