Flash Sintering Of KNN Based Lead Free Piezoelectric Ceramics

In recent years,an emerging technology of flash sintering is widely known for ceramic materials.It is sintered with the aid of current/electric field,which has some merits of lower sintering temperature,low energy consumption,and rapid densification.Potassium sodium niobite KNN-based lead-free piezoelectric ceramic is a kind of functional ceramic material,which has the characteristics of environmental friendliness and excellent piezoelectric performance.However,preparation of KNN-based ceramics by the traditional solid-phase method requires long-term sintering at high temperatures,which not only causes high energy consumption,but also causes the volatilization of alkali metal elements K and Na,which will affect the compactness of KNN-based ceramics and lead to poor overall electrical properties.Therefore,it is of great significance to develop sintering technology for low energy consumption and low temperature rapid sintering of KNN-based ceramics.All the work in this article will use flash sintering technology to prepare KNN-based lead-free piezoelectric ceramics.It has been confirmed that flash firing significantly reduces the sintering temperature of KNN-based ceramics,greatly shortens the densification time of its formation,and reduces the volatilization of alkali metal elements K and Na.In order to obtain KNN-based ceramics with characteristics of dense,pure phase and comprehensive electrical properties.In addition,flash sintering can promote the simultaneous progress of solid-phase reaction and sintering densification,so that dense and pure phase KNN-based lead-free piezoelectric ceramics can be obtained by directly flashing oxide precursors.The specific research contents are as follows:（1）Using molten salt method to prepare pure phase Na Nb O₃,KNb O₃ ceramic powder,and then the oxide precursor was obtained by controlling the molar ratio of Na/K to 1:1.Zr O₂ doped Na Nb O₃-KNb O₃ mixed powder was pressed into dog-bone-shaped green body by uniaxial dry pressing technology.A DC electric field of 600-900 V/cm and limiting current of 30-150 m A was applied to the green body,and its reaction flash sintering behavior was studied in detail.The results show that the Zr O₂ doped Na Nb O₃-KNb O₃ can flash at 438-533℃,under the effect of electric field intensity,when the ambient temperature of the sample reaches a certain threshold,the conductivity of the sample increases rapidly,accompanied by rapid densification and strong luminescence,at which time the flash sintering occurs.The study found that with the increase of the electric field intensity,the initial furnace temperature of the flash sintering of the mixed powder of Na Nb O₃-KNb O₃doped with Zr O₂ is gradually reduced.Compared with undoped,when the electric field intensity is 600 V/cm,the flash temperature was reduced from 852℃to 533℃,and both dense,pure phase KNN ceramics were obtained.In this study,the solid-phase reaction and densification were achieved in one step.According to the black body radiation model and defect theory,the rapid densification mechanism of flash sintering KNN ceramics is explained.（2）KNN ceramic powders with orthorhombic phase were prepared by conventional sintering method at 800℃,and then the KNN green body was obtained by dry pressing.A DC electric field of 400-800 V/cm and limiting current of 21-150 m A were applied to the blank,and then the flash sintering behavior was studied.The study found that the KNN ceramics flash firing at 847-937℃,with the increase of electric field strength,the initial furnace temperature at which KNN ceramics undergo flash sintering decreases,the grain size of KNN ceramics decreases and the density increases;The size of the limiting current is positively correlated with the grain size and density of KNN ceramics;the KNN ceramics obtained after flash sintering have better ferroelectric properties.（3）The KNN ceramic powder doped with CaZrO₃ was prepared by the conventional sintering method and pressed into a dog-bone-shaped green body.Applying a 600-900 V/cm DC electric field and limiting current of 100 m A to the embryo and maintaining it for 30 s after flash firing,a dense KNN-based ceramic was prepared.The study found that KNN-CaZrO₃ flash at 742-842℃,with the increase of the electric field intensity,the initial furnace temperature of the KNN ceramic doped with CaZrO₃ decreased,the grain size decreased,and the density increased.At the same time,the phase structure of the KNN-based ceramics before and after the flash sintering was Orthogonal-Tetragonal mixed phase;the KNN ceramic sample doped with CaZrO₃ obtained after flash sintering has good piezoelectric properties.