Study On Microwave Absorbing Properties Of Fibrous Nickel-cobalt Alloy Powders

As a typical polycrystalline iron fibers, fibrous nickel-cobalt alloy has become a subject of intense interest presently among new absorbing materials, due to its morphology anisotropy, magnetic anisotropy, high axial permeability and low density. In the present work, fibrous nickel-cobalt alloy powders were prepared by a coordination coprecipitation-thermal decomposition method, and the thermal decomposition mechanism and microwave absorbing properties were also investigated for the first time. The major research achievements are as follows:The fibrous Ni-Co oxalate complex salt precursor containing ammonia was prepared by coordination coprecipitation method in the mixed solvent. TG-DTA technique was used to analyze thermal decomposition course of precursor in N2and CO2. The apparent activation energy of thermal decomposition reaction was calculated by Starink method, the type of most probable mechanism function was determined by the master curve method using Tang-Liu-Zhang-Wang-Wang temperature integral approximation, and the dynamic exponent of the kinetic mechanism function was calculated as well as pre-exponential factor. The results showed that there was a similar course of thermal decomposition of the precursor in different atmosphere, whereas variations of decomposition temperature was obvious. The thermal decomposition reaction of Ni-Co oxalate in N2was controlled by chemical reaction, E=160.38kJ/mol, G(α)=[-ln(1-α)0.29, A=2.96×1010s-1. The thermal decomposition reaction of Ni-Co oxalate in CO2was controlled by diffusion, E=194.65kJ/mol, G(α)=[1-(1-α)0.5]0.25, A=3.78×1012s-1.According to transmission line theory, the variation law of complex relative permittivity region and reflectivity with material parameters of absorber were analyzed. The results showed that the required value of both real and imaginary part of complex relative permittivity decreased with an incerese of thickness, frequency or the imaginary part of permeability, thus electromagnetic waves can be effective absorbed by the absorber. The influence of the real part of complex relative permittivity and complex relative permeability on absorption frequency is obvious, whereas the absorption intensity was influenced greatly by the imaginary part of complex relative permittivity and complex relative permeability.The effect of preparation process on the electrical conductivity and electromagnetic parameters of fibrous Ni-Co alloy powder was studied and the reflectivity of fibrous Ni-Co alloy/paraffin composite was analyzed for the first time. The results showed that the electrical conductivity of fibrous Ni-Co alloy decreased obviously with the increase of cobalt content or oxygen content, or the decrease of thermal decomposition temperature. The impedance matching of fibrous Ni-Co alloy powder containing0.5wt%oxygen was poor, thus wave-absorbing effect was not ideal. However, The higher impedance matching ability of fibrous Ni-Co alloy powder containing2.92wt%oxygen was observed. In2-18GHz band, the multi-peak resonance phenomenon of electromagnetic parameters was found. The attenuation of electromagnetic waves mainly came from electric loss for Ni-Co alloy/paraffin composite. The absorption peak shifted to lower frequency with the increase of cobalt content or coating thickness. The minimum reflectivity is-35.99dB at5.84GHz with a coating thickness of2.5mm, when thermal decomposition temperature was400℃and mass fraction of Ni1Co1in Ni1Co1/paraffin composite was67%, the bandwidth with reflectivity less than-10dB was1.68GHz.