Preparation And Properties Of Potassium Sodium Niobate Ceramics

The application fields of potassium sodium niobate ceramics have been continuously expanded with the development of basic research.At present,the frequency bands for the dielectric properties of potassium sodium niobate ceramics studied by the researchers are mainly concentrated in the low frequency bands?KHz,MHz?and rarely involve the high frequency bands of GHz and above.Considering the development status of potassium sodium niobate ceramics and the technical advantages of our group,the dielectric properties of the ceramics in the X-band?8.2¹2.4GHz?were explored and analyzed.The sintering characteristics,microstructure,phase structure evolution and mechanical properties of the ceramics were also studied.A series of potassium sodium niobate ceramics(K_xNa_1-xNbO₃)with different potassium and sodium ratios were prepared using traditional electronic ceramic preparation processes.The sintering characteristic of solid solution ceramics is obviously better than that of single component ones.The optimum sintering temperature of the ceramics with different component is quite different and the sintering temperature range of all the ceramics is narrow.There are large and small grains distributing alternately in the creramics.The mechanical properties of solid solution ceramics show little difference,and the average flexural strength and hardness are 78.94 MPa and 319.6HV,respectively.The complex permittivity of K_xNa_1-xNbO₃ solid solution ceramics in X-band increases first and then decreases with the increase of K content,which is probably related to the discontinuity of the lattice parameter near the K_0.5Na_0.5NaO₃ component.K_0.5Na_0.5NbO₃ ceramic was used for subsequent studies.When the sintering temperature increases to 1110°C,second phase appears in the ceramic.It was found that the density has a great influence on the complex permittivity of K_0.5Na_0.5NbO₃ ceramic.The compacter ceramic has larger complex permittivity.The complex permittivity of K_0.5Na_0.5NbO₃ ceramic also increases with the increase of test temperature.And the complex permittivity at 220°C has an abrupt change because of the orthogonal-tetragonal phase transition.The annealing process will reduce the complex permittivity of K_0.5Na_0.5NbO₃ ceramic and change the frequency response characteristics,which is caused by the change of the internal stress state and the number of movable charges in the ceramic.The real and imaginary part of complex permittivity of K_0.5Na_0.5NbO₃ ceramic?calcined at 950°C and sintered at 1090°C?are110¹40 and 60⁸0,respectively.The dielectric loss is 0.4⁰.7.And the reflection loss of the ceramic is higher than-5dB in the X-band,which means that the ceramic has poor microwave absorption property.Potassium sodium niobate/alumina?KNN/Al₂O₃?composite ceramics were prepared using a conventional electronic ceramic preparation process in order to adjust the complex permittivity of KNN.Because the solid line temperature of KNN and the melting point of Al₂O₃ are quite different,the sintering performance of the composite ceramics deteriorates with the increase of Al₂O₃,and the porosity increases at the interface of the two phases.As a result,the flexural strength of KNN/Al₂O₃ decreases with the increase of Al₂O₃ content in the ceramic.The main phases in the composite ceramics are KNN and Al₂O₃,but there are also other phases such as Nb₂O₅.The addition of Al₂O₃ effectively reduces the complex permittivity of the ceramics and improves the microwave absorption property.K₂CO₃,Na₂CO₃,Nb₂O₅ and Al₂O₃ powders were co-sintered according to the stoichiometric ratio of K_0.5Na_0.5Nb_0.7Al_0.3O₃ to prepare ceramic.The ceramic prepared by this method were found to have severe deliquescence.The reaction mechanism is composed of two parts:Nb₂O₅ and carbonates generate potassium sodium niobate ceramics,Al₂O₃ and carbonates generate meta-aluminate,and Al₂O₃ tends to combine with stronger alkaline K₂O.The meta-aluminate cannot stably exist in the air and the hydration reaction easily occurs,resulting in the deliquescence characteristic of the ceramic.It can be seen that the preparation process of potassium sodium niobate ceramics are very strict.Changing the mixing mode of raw materials will lead to very different experiment results.The KNLN/Al₂O₃ composite ceramics were prepared by doping Li element into KNN/Al₂O₃ ceramics in order to improve the sintering characteristic.At the same time,the complex permittivity of the ceramics could be further adjusted by the replacement of K and Na elements by Li elements,which would further improve the microwave absorption property of the ceramics.Li doping causes partial liquid phase sintering in the ceramics,thereby improving the sintering characteristic and increasing the density of the ceramics.Li doping increases the real part of the complex permittivity obviously but has little effect on the imaginary part.When the molar ratio of Al₂O₃ in the composite ceramic is large,the effect of Li doping on sintering characteristic and complex permittivity is no longer obvious,and the effect of Al₂O₃ on the complex permittivity increases.The composite ceramics contain KNLN and Al₂O₃ phases as well as a small amount of K₃LiNb₆O₁₇.The appearance of the third phase indicates that the presence of Al₂O₃ reduces the solid solubility of LiNbO₃ in KNN ceramics.When the mole ratio of Al₂O₃ is 0.4,the real and imaginary parts of the complex permittivity are 35⁴5 and 5¹0,respectively.The numerical simulation of refection loss shows that when the thickness of the sample is 1.35mm,the creramic has good microwave absorption property.The minimum reflection loss is-49dB and the reflection loss is below-10dB in the frequency of 8.2⁹.6GHz.