Growth And Performance Optimization Of Multi-Element Codoped Potassium Sodium Niobate Based Piezoelectric Single Crystals

Piezoelectric materials are an indispensable functional material in today’s society,whether in national economy or defense science and technology applications.The development of environmentally friendly lead-free piezoelectric materials has become an important goal in the research of piezoelectric materials,as the concept of green and sustainable development is gaining more and more attention to the impact of lead-containing materials on human beings and the environment.Therefore,in this paper,piezoelectric single crystalsof（1-x）(99.7K_0.5Na_0.5Nb O₃-0.3Ba Bi O₃)-xLi₂CO₃and（1-x）(99.4(99.7K_0.5Na_0.5Nb O₃-0.3Ba Bi O₃)-0.6Li₂CO₃)-x MnO₂ series were prepared by seedless solid-phase growth technique,and their growth The growth state,process,structure and electrical properties were systematically studied in order to improve the electrical properties such as piezoelectric,dielectric and ferroelectric properties of the crystals while growing larger crystals.The main research results and innovations are as follows.The growth,structure and electrical properties of（1-x）（KNN-BB）-xLi₂CO₃piezoelectric single crystals were systematically studied.The results show that the doping of Li element makes the composition of the crystal more homogeneous,the surface porosity is reduced and the densities become better;meanwhile,the doping of Li can promote the transformation of the orthorhombic phase to the tetragonal phase,which is most obvious at the doping content of 0.6at%-0.8at%,thus realizing the ideal state of the coexistence of two phases;the doping with the amount of Li element of 0.2at%makes the Nb O₆ octahedral The internal interaction of the lattice is increased and the degree of distortion is somewhat alleviated.The piezoelectric ferroelectric properties of the KNN-based single crystals first increase and then gradually decrease due to the increase of Li doping,while the best or better overall electrical properties are obtained at Li doping levels of 0.4at%-0.6at%,as follows:d₃₃=252 p C/N,d₃₃^*=853 pm/V,k_t=0.56,Q_m=40.73,P_r=33.7μC/cm²,T_O-T=168℃and T_C=420℃.The effects of the co-solvent MnO₂ on the growth,structure,piezoelectric and dielectric properties of KNN-BB-Li piezoelectric single crystals were systematically investigated.The results show that the doping of Mn can significantly improve the growth quality of single crystals and make their surface flatter.Due to the increase of MnO₂ doping,the piezoelectric properties of the single crystal are also significantly improved.Under the same conditions,the crystal has the highest strain of 0.19%when the content of Mn is x=0.1%,and the crystal also obtains the maximum electromechanical coupling coefficient k_t and mechanical quality factor Q_m,which are k_t=0.65 and Q_m=35.5,respectively;the doping of Mn element can effectively reduce the crystal’s leakage current density and dielectric loss,and the crystal has the lowest dielectric loss of 0.012 at x=0.3%at room temperature and the Curie temperature T_C is about 418°C.The effects of the cooling rate during the cooling down phase of single crystal growth on the growth,structure and electrical properties of（KNN-BB-Li）-0.2MnO₂single crystals were systematically investigated.The results show that the growth of single crystals obtained at all cooling rates is generally good with no significant morphological changes,but the grain structure becomes more compact;when the cooling rate is lower,the material diffusion is more adequate during the growth process and the surface morphology is flatter,but at the same time the volatilization of K elements is more serious and the concentration of O elements is reduced accordingly.When the cooling rate increases to 2℃/min and above,the crystal structure of the single crystal gradually transforms from a single orthogonal phase state to a state where the orthogonal and tetragonal phases coexist,while the lattice distortion is higher.When the cooling rate is 2℃/min,the maximum value of positive piezoelectric coefficient d₃₃ is 362 p C/N;meanwhile,the coercivity electric field E_r is lower at 4℃/min and the value is 6.6 k V/cm.While the dielectric loss of the crystal obtains the minimum value tanδ=0.019 at the cooling rate of 1℃/min.The residual polarization intensity P_rshows a gradual decreasing trend,so the maximum value is obtained atat 0.5°C/min,P_r=24μC/cm².The effect of the change in the cooling rate on the two phase transition points of the crystal samples is not obvious,with T_O-T around 160°C and Curie temperature（T_C）around 420°C for all samples.