Study On The Microstructure And Electric Property Of Potassium Sodium Niobate Lead Free Piezoelectric Ceramics

Due to the high Curie temperature,large spontaneous polarization,excellent piezoelectric properties,the lead-free(K_0.5Na_0.5)Nb O₃（KNN）ceramics is considered as the potential environmental-friendly piezoelectric ceramics.However,the study of the microstructure in KNN ceramics is ignored in recent years and the mechanism of perfect piezoelectric performance is not clear.In this paper,the microstructure of KNN ceramics is regulated by the sintering process and element doping,and the electrical performance is affected by the microstructure.Firstly,the pure KNN ceramics was prepared by the traditional sintering,two-step sintering and multi-step sintering methods to improve the electric properties.The crystal cell volume and dielectric loss of ceramics are reduced with the increasing of the sintering temperature.The problem of the narrow sintering temperature is solved by the two-step sintering method,and the dielectric properties of KNN ceramics is improved.The dielectric constant is 460.730 and the Curie temperature is 436℃.The cell volume increase with the increase for the holding time of the first step and the prolong for the holding time of the second step.Compared with the traditional sintering method,the multistep sintering method can improve the dielectric property with the higher Curie temperature T_C=433℃.Secondly,the grain structure of pure KNN ceramics for different sintering methods is studied.The grain size is reduced by increasing the sintering temperature in the traditional method,and the highest piezoelectric constant is 105pC/N.The grain size of the pure KNN ceramics is ranging from 0.56 to 2.14μm by adjusting the sintering conditions of two-step sintering method.The effect of grain size in KNN ceramics is found that the piezoelectric property is enhanced with the decrease of grain size.When the grain size is higher than 1μm,the extrinsic contribution rate decreases with the of grain size.When the grain size is 1.01μm,P_ris 31.9μC/cm~2.However,the grain size is smaller 1μm,the non-intrinsic contribution rate increases,and the dielectric constant is higher than the coarse-grained samples.The samples have high electric properties,d₃₃=122p C/N,k_t=38.01%.The good temperature stability for the electromechanical coupling coefficient is in the temperature range of40-140℃.The multi-step sintering method reduces the grain size and improves the ferroelectric properties of ceramics,P_r=25.5μC/cm~2,P_max=27.8μC/cm~2.Finally,the microstructure of KNN ceramics is controlled by the doping of Mn or Bi elements to improve the electrical properties.The doping of Mn made the KNN ceramics with less porosity,and the Curie temperature is 427℃ at the sintering temperature 1040℃.The doping of Mn can improve the ferroelectric property,P_r=21.54μC/cm~2,E_c=9.2 k V/cm.The doping of Bi can reduces the grain size,and the Rhombohedral-Orthogonal phase transition was constructed near the room temperature.The highest dielectric property is 468.03.The KNN-0.1%Bi₂O₃ceramics has good piezoelectric properties,d₃₃=121 p C/N,k_t=30.58%,k_p=47.37%.The microstructure of KNN ceramics was controlled by the prepared process and element doped,and the mechanism on the macroscopic electrical properties is obtained.It provides a theoretical and experimental basis for the study of KNN based piezoelectric ceramics.