关键词: 镍粉, 石墨, 涂层, 复合材料, 介电性能, 吸波性能

Abstract: Graphite has good electrical conductivity, thermal conductivity and corrosion resistance, and is widely used in graphite electrodes, electromagnetic protection materials, superconducting materials and so on. To improve the wave absorbing properties of graphite-matrix composites, and analyze the effect of the coating layer number on the dielectric properties and wave absorbing properties of composites, the nickel powder/graphite matrix composites were prepared and researched. The nickel powder/graphite functional particles were prepared by mixing nickel powder and graphite according to the mass ratio of 1:4. By using PU2540 polyurethane as the substrate and adding appropriate thickener, the nickel powder/graphite coating sizing was prepared. The mass fraction of functional particles in the coating sizing is 40% of the total mass, and the viscosity of the coating sizing is 30,000 mPa·s. With the common polyester/cotton plain fabric in the market as the coating material base cloth, by adopting the textile coating process (scraping coating method) and using the automatic coating machine (coating machine speed of 60 cm/min), the nickel powder/graphite base coating sizing was evenly coated on the base cloth and dried in the oven at 80 ℃ for 10 min. Nickel powder/graphite based coating composites with 1, 2 and 3 layers were prepared, respectively. The dielectric constant of the composite was measured by BDS50 dielectric impedance spectrometer, and the reflection loss was measured by ZNB40 vector network analyzer. The effects of the layer number on dielectric properties and wave absorption properties of the composite were analyzed. 
The results show that the layer number has a certain influence on the electromagnetic shielding properties of Ni powder/graphite coating composites. In the applied electric field frequency range of 30 –1000 MHz, the real part of the dielectric constant of the composites with different coating layers decreases. When the frequency is 30 MHz, the real part of the dielectric constant of the samples is the largest, and the composites with three coating layers is the largest, indicating that it has the strongest ability to polarize electromagnetic waves and store charges. This is because the ability of the real part to store charge is directly proportional to the conductivity of the coated composite. The electrical conductivity of the composite is related to the volume fraction of the conductive filler, the size of the conductive particle, the shape of the particle and the dispersion uniformity of the particle, which will affect the degree of contact between the conductive particles. In the applied electric field frequency range 1–1000 MHz, the imaginary part and loss angle tangent value of the dielectric constant of the composite with different layer number increase. When the frequency is 1,000 MHz, the imaginary part of the dielectric constant of the samples is the largest, and the imaginary part and loss angle tangent value of the dielectric constant of the composite with three layers are the largest, indicating that it has the strongest loss ability to electromagnetic wave. This is because the imaginary part of the dielectric constant depends on the electric dipole moment of the rearrangement, which is proportional to the conductivity of the composite sizing. In addition, in the conductive network skeleton formed by graphite, due to the addition of nickel powder, the space between the graphite particles is fully filled, and the conductive property of the composite material is further improved. In the applied electric field frequency range of 10–3,000 MHz, the 2-coating-layer composite achieves the lowest peak value of reflection loss, up to -26.460 dB, indicating that it has the strongest absorption ability of electromagnetic wave. This is because graphite absorbs electromagnetic waves through the interaction with the electric field, and its main feature is that it has a high dielectric loss angle tangent, relying on the dielectric electron polarization or interface polarization attenuation to absorb electromagnetic waves. The smaller the volume resistivity of the material, the better the absorption effect, but the reduction of the surface resistivity of the material also increases the reflective ability of the material. Electromagnetic waves in free space are difficult to enter the interior of the material and cannot achieve the purpose of absorbing waves. Therefore, there is an optimal conductivity range, and its absorption efficiency depends mainly on the conductivity and dielectric constant of the material. The results can be used as reference for designing and developing economical and practical coating composites.

Key words:  , Ni powder, graphite, coating, composite material, dielectric property, wave absorption properties