Study On Microwave Absorbing Materials Of Nickel Coated Carbon Fiber And Graphite

A continuous and uniform nickel coating layer has been obtained on thesurface of carbon fiber (CF) and flake graphite (FG) by electroless platingtechnique. The reaction technique was optimized to achieve a high quality nickelcoating layer. The absorption characteristics of nickel-coated carbon fiber (NCF),FG and nickel-coated flake graphite (NFG) were measured over the frequencyrange of 4-18GHz by the RAM reflectivity far field RCS method.Enhancing the surface roughness of the CF (FG) and obtaining abundantfunctional groups on the CF (FG) can improve the adhesion between the CF (FG)and the nickel coating layer. Sufficient sensitization and activation is importantto gain sufficient nickel nucleation sites which are beneficial to the deposition ofnickel. Functional groups of –COOH and –OH were attained on the surface ofthe CF (FG) by the nitric acid treatment. The time of sensitization and activationtreatment plays a very important role to obtain a high quality, uniform andcontinuous layer of nickel coating on the carbon fibers. The nitric acid treatmentis effective in introducing functional groups to anchor Sn²⁺ for reducing Pd²⁺ toPd as nucleation site to the CF surfaces. By 75min sensitization, 75minactivation and 20 min electroless plating, sufficient and dense nucleation sites(900-1000 particles / μ m²) are obtained and the quality of coating layer isdistinctly improved. The mean size of nickel particles in the coating layer isabout 80 nm. The coating thickness is controlled by the plating time, from0.5μm for 10min to 1.0μm for 15min to 1.2μm for 20min. The nickel/phosphorus in the coating layer are ca.1: 0.047 and ca.1: 0.046 in weight ratiofor NCF and NFG respectively. The microcrystalline of Ni3P phase emerges atabout 300℃ and grows distinctly with the annealing temperature increasing,keeping strongly up to 1200 ℃. The saturation magnetization of NCF is 6.09emu/g and 4.73emu/g for NFG. After annealing treatment the saturationmagnetization of NCF was increased, which can be attributed to the increase ofnickel particles. The coercivity and residual magnetization of NCF were notobviously changed after annealing treatment.NCF and NFG exhibit good absorption performances in the high frequencyand their absorption ability increase upon the increase of the frequency, whichowe to the increase of eddy-current-loss with frequency. The absorption abilityof the NCF increased as the absorber of NCF increased. It is noted that the R.L.of NCF (30%) less than -10dB begins from 15GHz to 18GHz. The absorptionfor the NFG (30%) is higher than 5dB from 15.5GHz. The FG exhibits goodabsorption ability in the low frequency. The bandwidth in the low frequency is2.1GHz (4.6-6.7GHz) and 5.3GHz (12.7-18GHz) for the high frequency. Theabsorption ability in the high frequency increases with the time of grind. Whenthe time of grind is 12h, the absorption is 25.4dB at 14.5GHz which is 15dBhigher than FG. When the time of grind increased, the size of flake graphitedecreased. The radar reflects many times between the smaller graphite.Therefore the absorption performance of FG enhanced. The absorption peak ofFG shifted as the content of the FG increased. When the content was 35%, theabsorption ability was obviously poor and when the content was below 30%, theabsorption peak shift toward the low frequency from 14GHz (15%) to 5.4GHz(30%). It was noted that the absorption peak for 30% was ? 19.6dBat 5.4GHz,which can be put in practice.