Study On The Preparation And Spectrum Characters Of High Frequency Microwave Attenuation Ceramics

Microwave attenuation materials are widely used in vacuum microwave electronic devices, negative electronic countermeasure and microwave measuring system. AlN exhibits good thermal properties, electrical insulation and good chemical properties. Mo and W are suitable as attenuation substance in AlN composite ceramics for their appropriate thermal expansion coefficient and high thermal conductivity. It is such excellent properties and its healthy attribute that makes this kind of material could be taken as the alternative microwave attenuation materials. However, in this system, for one thing, the percolation threshold is very sensitive to the raw materials and preparation process so that it results in the change of dielectric properties of AIN/Mo composite ceramics. For another thing, the electrical resistivity of composite ceramics is too sensitive to the change of Mo content, so it is so difficult to control the electrical resistivity to obtain good dielectric properties. Therefore, the problems mentioned above need prompt solutions.In this work, the purpose of the study is to investigate the influence of the raw materials and preparation process on the percolation threshold of AIN/Mo composite ceramics and on this basis, study the process that how it change the dielectric properties. Furthermore, by adding proper amount of SiC, AIN/Mo/SiC composite ceramics were produced with high relative density. The purpose of adding SiC was to widen the range of the percolation threshold so that the composite ceramics could have proper dielectric properties that we expected, and at last, achieve the objective that obtain the material with excellent microwave attenuation properties.During the process of sintering, the method of mixing and the sintering temperature both have influence on the relative density of AIN/Mo composite ceramics produced by pressureless sintering process. In addition, adding Mo particles is helpful to the process of sintering. When the sintering temperature reaches 1750℃, the relative density of AIN/Mo composite ceramics with different Mo content exceed 98%. Especially, when the volume fraction of Mo content reaches 32% and above, the relative density of this kind of ceramics approaches its theoretical value.For AIN/Mo composite ceramics which is produced by changing the radius of the raw materials and preparation process, its percolation threshold has reached 37.45%. Comparing with value of 23% obtained in some formal works, it has increased greatly. On this basis, the measurement of dielectric constant shows that the dielectric constant of AlN/Mo composite ceramics has increased at the point closed to the percolation threshold. However, the thermal conductivity of AIN/Mo composite ceramics decreases because of the increase of Mo content.The addition of SiC has great influence on the relative density of AIN/Mo/SiC composite ceramics. Therefore, both choosing proper method of sintering and preparation process and adjusting the SiC content are key points to prepare AIN/Mo/SiC composite ceramics with high relative density.Comparing to AIN/Mo composite ceramics, when adding some SiC particles, the electric resistivity of AIN/Mo/SiC composite ceramics decreases gently instead of showing no-linear abrupt change. This result states that the addition of SiC has break down the channel which was made by the joint of Mo particles and achieving the goal of widening the range of the percolation threshold.SiC content has great influence on the dielectric properties and the thermal conductivity of AIN/Mo/SiC composite ceramics. The dielectric constant and dielectric loss increase with the increase of SiC content. Its thermal conductivity decreases with the increase of SiC content, which is closely related to these factors:the relative density, the relative content of each phase, the thermal conductivity of each phase and the distribution model of each phase.