Study On Preparation And Property Of Thermal Conductivity AlN Microwave Attenuation Composite 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. W and SiC are suitable as attenuation substance in AlN composite ceramics for their appropriate thermal expansion coefficient and high thermal conductivity. Carbon nanotubes (CNTs) have a wide band of microwave absorption, large amout of attenuation and low content to be added. CNTs and diamond have a high intrinsic thermal conductivity, therefore, microwave attenuation composite ceramics with high thermal conductivity will have better comprehensive properties.The effects of powder-mixed method, milling time of high-energy ball milling, sintering temperature, sintering holding time, microwave attenuation phase contents and starting powder microstructure and sizes on microstructure, thermal conductivity and dielectric properties of AlN composite ceramics were studied. AIN based composite ceramics with high thermal conductivity, better dielectric properties and higher relative density were prepared based on the above technique.Composite ceramics mixed by magnetic stirring method show higher dielectric loss, while their relative density is lower and CNTs are not disperse uniformly. Powders mixed by high-energy ball milling method and added W to AIN substrate can increase the relative density of the composite ceramics and improve the dispersion properties of CNTs. In addition, milling time also affects the properties of ceramics. The sample prepared with the powder milling for20min has the higherst relative density. With the increase of milling time dielectric loss of the sintered composite ceramics show a trend of decrease. The AlN-5vol.%W ceramics prepared with the powder milling for10min has the better dielectric properties, its permittivity constant approximate to25.Sintering temperature of SPS has influence on the relative density and the dielectric properties of AlN/W composite ceramics. With the increasing of sintering temperature from1400to1700℃, the crystal grain of AlN/W composite ceramics grow bigger, the dielectric loss tangent and permittivity constant of the samples decrease. Sintering holding time has a slight influence on the microstructure and relative density of AlN/W composite ceramics.W and CNTs content have great influence on the properties of the composite ceramics. If the content of CNTs is excessive, the porosity of the samples will increase observably. Adding the conductive phase W into AlN can increase the relative density of the samples. In addition, adding CNTs into AlN/W can improve the dielectric properties of the composite ceramics. AlN-10vol.%W-lvol.%CNTs composite ceramics has an excellent dielectric properties with and ε<17.5between70kHz and1MHz.The dielectric loss and permittivity of AIN-SiC composite ceramics are increasing with the increase of SiC content, but the relative density shows a trend of decrease. Nano-SiC is a good microwave absorber and has strong absorbing properties. The dielectric loss tangent of the AIN-SiC composite is more than0.2, and the permitiviy is lower than40at the frequency range of12.4-18GHz, which has been applicable to the high power tubes.The thermal conductivity of AIN-diamond composite ceramics is increasing with the increase of diamond content. The highest thermal conductivity is104.92W/(m·K) from AlN-lvol.%diamond. AlN-15vol.%diamond has an excellent dielectric properties with the tan5>13.1and s<9.592between70kHz and1MHz. Some conducting amorphous carbons have been induced with sp2and sp3hybrid of diamond, and the dielectric properties are increasing. Due to the relative density is low, the dielectric loss tangent of Al-15vol.%diamond and AlN-20vol.%diamond are higher. The self-excited oscillation has been hindered by the defects inside the composite ceramics which were passed through the electromagnetic wave. The phonon thermal conductivity is the heat conduction mechanism of AlN composite ceramics. The phonons free path is lengthened with the increasing of the grain boundary. The effect on the high intrinsic thermal conductivity of diamond is greater than the phonon.