Preparation And Microwave Absorbing Performance Study Of C/BN/Metal Magnetic Composites

With the broadening application of electromagnetic wave field, the development and use of electromagnetic wave absorbing materials have received widespread attention. Preparation of "thin, light, wide, strong" characteristics of microwave absorbing materials is not only to reduce the harm of electromagnetic wave in the daily life, but also as a kind of special military defense materials to enhance weaponry stealth characteristics. Due to its low density and good electrical conductivity, carbon materials are considered as a kind of good microwave absorbing material. However the pure carbon absorbing material is usually caused by the strong polarization effect that leads to the unbalance of the impedance matching characteristic, which makes the electromagnetic wave reflection ratio increase. How to adjust the impedance matching and make full use of the high loss of carbon materials to attenuate electromagnetic wave has become one of the hot spots of domestic and foreign scholars. In this paper, the impedance matching characteristics of graphite and CNT were adjusted by mixing with metal magnetic material and wave-transmitting material to improve the wave absorbing properties.Firstly, the complex calculation formula of the reflectivity was compiled by the window, two-dimensional and three-dimensional image through the MATLAB software. Through the simulation of the relative dielectric constant, relative permeability, thickness and frequency, the influence of the six variables on the reflection attenuation was explored. By means of numerical simulation, which laid a theoretical foundation for the following experiment.The impedance matching characteristics of graphite were adjusted by the method of mixing with the wave-transmitting material. Graphite/BN composite absorbing material was prepared by thermal decomposition techniques and ammonia borane as precursor. The means of XPS, SEM, EDX proved the formation of BN uniform coating on the surface of graphite sheet. Compared with pure graphite, when the added amount and filling amount was 40 wt.%, the graphite/BN composite absorbing material achieved the most obvious enhancement of the wave absorption performance. When the thickness was 2mm, the reflection attenuation reached-33 d B and the bandwidth was 3.8GHz. It can be proved that the mixing of BN wave phase is beneficial to improve the wave absorbing property of graphite.The impedance matching characteristics of graphite were regulated by mixing metal magnetic materials. Respectively using three different electromagnetic properties of metal particles just as Fe Ni Alloy（magnetic loss greater than the dielectric loss）, micrometer Fe powder（magnetic loss close to dielectric loss） and iron oxide（electrical losses greater than magnetic loss） to mix with graphite. Decentralized agent, ultrasonic time and powder ratio of absorbing wave properties was investigated. By means of back scattered electron image analysis, it was concluded that the metal particles with larger atomic number are dispersed in carbon matrix homogeneously. Microwave absorbing performance test showed that graphite and metal magnetic materials in accordance with appropriate proportion could effectively regulate the impedance matching characteristics. Compared with graphite and metal magnetic material, the microwave absorbing properties of composites were greatly improved after mixing.The CNT/BN microwave absorbing composite was prepared by the thermal decomposition techniques methods. The results showed that when the sintering temperature is up to 1200°C and 1300°C, the change of wave performance was not obvious. When the temperature reached 1400°C, there were some B4 C nanoparticles on the surface of the product. Compared with CNT, the microwave absorbing properties enhanced at a certain degree.CNT/Fe₃O₄ microwave absorbing composite was prepared through the method of mixed with Fe₃O₄ to adjust the impedance matching characteristics of CNT. It was concluded that the metal particles were dispersed in CNT homogeneously, the reflection loss can achieve-50 d B at 16.8 GHz, when the thickness was 1.5 mm. Compared with CNT, the absorbing performance increase significantly. BCNFe absorbing material with ammonia borane and ferrocene by CVD method was prepared. The results showed that the experimental products are mainly CNT, the compounds containing Fe have weaker magnetic properties, and the wave absorbing performance of the product was similar to CNT. Referring to physical mixing method, the products could be mixed with Fe₃O₄. When the thickness was 3mm, the reflection attenuation properties reached-44.2d B at 8.9GHz frequency, the corresponding bandwidth was 3.1GHz. The wave absorbing performance was significantly improved. The feasibility of mixing metal magnetic material with carbon material for improving the microwave absorbing performance was further verified.