Preparation and Properties of Potassium Sodium Niobate Ceramics

Author:Gao Lu

Supervisor:zhou wan cheng

Database:Doctor

Degree Year:2018

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Pages:114

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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(KxNa1-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 KxNa1-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 K0.5Na0.5NaO3 component.K0.5Na0.5NbO3 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 K0.5Na0.5NbO3 ceramic.The compacter ceramic has larger complex permittivity.The complex permittivity of K0.5Na0.5NbO3 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 K0.5Na0.5NbO3 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 K0.5Na0.5NbO3 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 K0.5Na0.5Nb0.7Al0.3O3 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.