Phase Boundary Construction And Properties Of Potassium Sodium Niobate Based Lead-free Piezoelectric Ceramics

With the development of society,lead-free piezoelectric materials become the inevitable requirement of green and healthy development.Among them,potassium sodium niobate(K_1-xNa_xNb O₃,abbreviated as KNN)piezoelectric ceramic has high piezoelectric coefficient and Curie temperature,which make it has great application potential.The modification of ferroelectric phase boundary is an important method to improve the piezoelectric properties of KNN based piezoelectric ceramics.In this paper,a series of KNN based piezoelectric ceramics were prepared by the solid state method.The effect of rare earth elements Tb,Tm and Eu on the ferroelectric phase boundary and temperature stability of KNN based piezoelectric ceramics was studied.The crystal structure,dielectric properties,impedance properties and piezoelectric properties of the doped KNN based piezoelectric ceramics were analyzed.The main contents of this paper are as follows:（1）K_0.5Na_0.5Nb O₃（KNN）,K_0.5Na_0.5Nbo₃-0.05%mol Tb₄O₇（KNN-Tb）,K_0.5Na_0.5Nbo₃-0.1%mol Tm₂O₃（KNN-Tm）ceramics were prepared by solid state method.It was found that the incorporation of Tb and Tm has little effect on the Curie temperature（T_C）of the ceramics,and the temperature of transition from orthorhombic to Tetragonal phase(T_O-T)decreased about 10℃.Tb significantly inhibited the growth of ceramic grains and decreased the activation energy of conductivity.The effect of Tm on these two parameters was not obvious.The addition of these two elements can increase the residual polarization strength,coercive field and unipolar strain of ceramics significantly.（2）0.96K_0.48Na_0.52Nb_0.96Sb_0.04o₃-0.04Bi_0.5(Na_0.82K_0.18)_0.5Zr O₃-x%mol Tb₄O₇（KNNS-BNKZ-x Tb）;0.96k_0.48Na_0.52Nb_0.96Sb_0.04o₃-0.04Bi_0.5(Na_0.82K_0.18)_0.5Zr O₃-x%mol Tm₂O₃（KNNS-BNKZ-x Tm）piezoelectric ceramics were prepared by solid state method.The doping of Bi_0.5(Na_0.82K_0.18)_0.5Zr O₃ and Sb⁵⁺successfully moved the T_O-T of KNN ceramics to room temperature.With the increase of Tb content,the Curie temperature of ceramics decreased,the proportion of tetragonal phase decreased,and the structure diference between O and T phases decreased.After doping 0.1%mol Tb,the ceramics convers piezoelectric coefficient d^*₃₃ decreased by 6.1%from 30℃to 150℃and electromechanical coupling coefficient k _P decreased by 1.6%from 30℃to 150℃,showing good temperature stability.It was found that the addition of Tm can increase the tetragonal content of ceramics.With the addition of Tm,The Curie temperature decreased and the KNN ceramics present diffuse phase transformation which greatly improved the temperature stability of the KNN ceramics.After doping 0.4%mol Tm,the residual polarization intensity P_r=12μC/cm² was almost unchanged during 30-150℃.Strain S=0.097%decreased only 4%from 30℃to 150℃.The results show that Tb and Tm can improve the temperature stability of the properties of KNN based piezoelectric ceramics.（3）The(K_0.43Na_0.52Li_0.05)Nb_0.91Sb_0.045Ta_0.045O₃-x%mol Eu₂O₃（KNLNST-x Eu）piezoelectric ceramics were prepared by solid state method.Piezoelectric ceramics had ferroelectric phase transition near room temperature.With the addition of Eu,the dielectric constant,dielectric loss increases and the coercive field decreased,which show the characteristics of"soft"doping.As 0.3%mol Eu was added,ceramics had good piezoelectric properties d₃₃=252±6 p C/N,P_r=23.4μC/cm²and S=0.167%.It was found that the piezoelectric coefficient obtained after ferroelectric test was similar to that obtained after DC polarization and all had good time stability.Moreover,the effect of AC polarization was related to the crystal structure of ceramics.As 0.5%mol Eu was added,the sample had strong red fluorescence.As an electro-optic multifunctional material,KNLNST-x Eu ceramics had potential application value.