| In the development process of the times,piezoelectric materials as functional materials play an irreplaceable role,they have important applications in the emerging field of science and technology,such as nano-robots,medical imaging,electronic components etc.Most of the piezoelectric materials used are Pb matrix,however,lead based piezoelectric materials will cause environmental pollution in the use and processing process,Human health will also be threatened.In response to the national sustainable development policy and the adverse effects of environmental deterioration on human beings,the search for lead-free piezoelectric materials has become the focus of research.KNN based lead-free piezoelectric materials have attracted attention because of their excellent electrical properties,and the piezoelectric materials in the form of single crystal have better performance,which has deeper research significance and value.In this paper,（1-y）[0.997(KxNa1-x)Nb O3-0.003Ba Bi O3]-y Zn O piezoelectric single crystal was prepared by seedless solid phase growth technique.The growth process,crystal structure,surface morphology and electrical properties of the single crystal were systematically tested and studied,then,the single crystal was annealized.The effect of annealing process on structural morphology and electrical properties of single crystal was investigated.Based on the above research,the following results are obtained:First of all,（1-x）[0.997K0.35Na0.65Nb O3-0.003Ba Bi O3]-x Zn O（KNN-BB-x Zn O）with different Zn doping amounts（x=0,0.002,0.004,0.006,0.008,0.01）single crystal,the effects of different Zn content on the surface morphology,crystal structure and electrical properties of single crystal were studied.It is found that the addition of Zn plays a positive role in crystal growth,which is conducive to crystal growth,get a smooth,flat crystal,The size of single crystal reaches its maximum when x=0.008.The doping of Zn into the KNN-BB matrix to form a solid solution.The samples of Zn with different contents all have typical perovskite structure,and no other phases appear.In the process of the continuous increase of Zn content,The tetragonal phase gradually becomes orthorhombic phase.After Zn doping,the oxygen vacancy defect in the crystal is reduced,the lattice structure is changed,and the mass is improved,so the piezoelectric constant d33 is improved.When x=0.008,the maximum value of d33 is 283 p C/N,and the maximum value of mechanical quality factor Qm and electromechanical coupling coefficient Kt are 66.8 and 0.60 respectively.In addition,after the P-E test of each single crystal sample,it is found that the maximum residual polarization Pr=30.0μC/cm2 is obtained at x=0.008,which is significantly improved compared with the undoped Pr=19.1μC/cm2.The addition of Zn changes the phase transition temperature of the crystal.When x=0.008,the phase transition temperature TO-T increases to161℃,and the Curie temperature TC increases to 409℃.Secondly,on the basis of the above research,the ratio of K and Na in single crystal is changed.0.992[0.997KxNa1-xNb O3-0.003Ba Bi O3]-0.008Zn(KxNa1-xNb O3-BB-0.008Zn)（x=0.31,0.33,0.35,0.37,0.39,0.41）single crystals were prepared.The morphology,structure and performance of single crystals with different proportions of K and Na are tested.The results show that the crystal can be effectively promoted to grow further by changing the proportions of K and Na.When x=0.37,the size of single crystal is the largest,reaching 12×10×2 mm2,and the crystal behaves more smoothly and neatly.It is helpful to improve the performance of single crystal.After the ratio of K and Na is changed,the phase structure in the crystal basically does not change,and the orthointersection phase is still dominant,but the crystal plane spacing will increase due to the increase of K ratio.When x=0.37,the average size of single crystal ferroelectric domains increases slightly.In the studied scale range,the domain walls can be reversed better during the polarization process,and the piezoelectric properties are improved.When x=0.37,the single crystal obtains the best electrical properties,which are d33=294 p C/N,Qm=69.2,Kt=0.62,Pr=31.9μC/cm2,TC=410℃,and the lowest dielectric loss tanδ=0.89%after the quadratic phase is transformed into tetragonal phase.It shows that the appropriate ratio of K and Na is beneficial to prevent the deformation of crystal,reduce the defects in crystal and improve the electrical performance of crystal.Then,the process of single crystal was improved.0.992(0.997K0.37Na0.63Nb O3-0.003Ba Bi O3)-0.008Zn O single crystal was annealed at different temperatures to study the variation trend of crystal morphology and properties after annealing.The results are as follows:The crystal surface morphology is improved.When annealed at 600℃,the domain size becomes more uniform,the domain walls become very small,the domain structure becomes very compact,and the phase structure of each crystal changes little after annealing.When the annealing temperature is 600℃,the properties of single crystal are the best,d33=354 p C/N,Qm=88.7,Kt=0.68,Pr=47.2μC/cm2,TC=402℃,indicating that the internal stress and oxygen vacancy defects can be eliminated at the appropriate annealing temperature,and the crystal performance can be improved.Finally,based on the above research results,（1-x）[0.997K0.37Na0.63Nb O3-0.003Ba Bi O3]-x Zn O single crystals were prepared by using the optimal atomic ratio of K to Na（K:Na=0.37:0.63）and annealing temperature（600℃）.The growth,microstructure and properties of the single crystals of the system were tested and characterized.The results further show that the ferroelectric domain size is the most uniform,the domain walls are the smallest,and the piezoelectric and ferroelectric properties are the best when x=0.008.Through this study,the optimal doping ratio of Zn is verified again,which is consistent with the above research results. |
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