Preparation And Electromagnetic Properties Of Iron Based Composites For Magnetron Absorbing Coatings

With the rapid development of modern electronic information and radar detection technology,various kinds of electronic equipments,especially the key electronic devices with X frequency band?8¹2GHz?,are widely used in military and civil applications,leading to people's living environment to be flooded with all kinds of electromagnetic waves.How to solve the electromagnetic radiation pollution caused by electromagnetic waves as well as the realization of weapon equipment radar stealth has received widespread attention,and one of its core technologies is related to the development of electromagnetic wave absorbing materials.The ferromagnetic absorber particles have excellent impedance matching characteristics,high Snoek's limit frequency,saturated magnetic moment and the Curie temperature show excellent performance of electromagnetic wave loss,which has always been an important direction for the development of absorbing materials.However,the current ferromagnetic absorber can not meet the requirements of the ideal microwave absorbing material with wide bandwidth,strong absorption,thin thickness and light weight.At the same time,intellectualization is the inevitable trend of radar stealth technology to cope with the complex and changeable battlefield environment.To this end,the purpose of this paper is to prepare Fe absorber particles with excellent microwave absorbing properties by simple method,especially for the x-band,effectively expand the absorption frequency band,enhancing absorption intensity.The main method is synthesis anisotropic structure of flower shaped micron grade Fe particles and mulberry Fe nanoparticles by simple,rapid and easy-to-control in situ reduction method under normal pressure,to effectively suppress the general existence of the eddy and skin effect of the Fe absorber particles,and thus improve the electromagnetic performance and the effect of absorbing.At the same time,to improve the antioxidation,corrosion resistance and monodispersity of the mulberry-like nano-particles,and enhancing the attenuation of electromagnetic waves by utilize the complementarity of magnetic loss and dielectric loss material,the mulberry-like nano Fe particles were coated with SiO₂ and carbon dielectric shells by sol-gel and in-situ polymerization to formation of core-shell structure particles.In addition,using the soft magnetic properties of the Fe absorber particles,the Fe-based particles having both excellent microwave absorbing properties and high saturation magnetization are selected to preparation of Fe-based composites by the technology of magnetorheological?MR?materials.Based on the prepared Fe-based composites to research influence of the anisotropy distribution of the particles on the absorbing properties of composites and analysis of the law.Finally,set up the corresponding testing device,to test and verify the controllable microwave absorbing properties of Fe-based composite coatings by magnetic field.The main research contents and results are as follows:?The flower-like carbonyl iron particles?CIP?was successfully prepared by depositing Fe nanosheet on the surface of micron CIP.On the basis of study the effect of reactant concentration,composition and conditions on the morphology of the synthesized product,the synthetic mechanism of flower CIP particles is proposed,that is under the space steric hindrance of the surfactant polyvinylpyrrolidone?PVP?,CIP and the Fe nucleus-H₂ bubbles which denerated by reaction collide with each other,combined and layer by layer deposition.The results of wave absorption test show that the flower-like CIP has better electromagnetic wave absorption performance.Compared to CIP,the minim?m reflection loss?RL?of the flower-like CIP is reduced by 46.05%to-35.49dB,and the effective absorption bandwidth in X band?RL<-20dB?expanded57%.By improving the process of preparation the flower-like CIP,the flower-like?-Fe particles?average particle size is 3?m?with three-dimensional layered structure were prepared by self-assembly of Fe nanosheet under the induction of magnetic field,and the prepared flower-like?-Fe particles exhibits excellent electromagnetic absorption performance.The RL value which less than-10dB is covers the entire X-band and part of the Ku-band wide up to 6.2GHz at a thickness of 3 mm.In particular,the minimiu RL value of-33.1 dB is obtained with a layer thickness of 5.5 mm.The reason for the flower-shaped Fe particles can obtain excellent absorbing performance is that the flower-like morphology is conducive to forming discontinuous network,enhancing diffuse scattering of the incident microwave and polarizing more interface charges,thus enhancing the incident and absorption of electromagnetic waves.?On the basis of the preparation technology of flower-like?-Fe particles,the mulberry shape Fe nanoparticles is prepared successfully by injection method added metal salt solution to into the reaction solution.The microstructure analysis showed that the particle size of mulberry-like nano-Fe particles are several hundred nanometers,and it was self assembly by small particles which particle size is several tens nanometers.The growth mechanism of the as-synthesized particle is that,due to the use of injecting feeding,the large amount of Fe nucleus-H₂ bubbles are locally aggregated to form tens of nanometers,then began to self assembly and combined with each other to formation of mulberry-like nanoparticles under the main driving force of magnetic force,gravity and buoyancy.The key to the preparation is the dynamic equilibrium and adjustment process of the reduction,nucleation,aggregation,self-assembly and sinking in the reaction solution.Since the size of the nanoscale mulberry-like Fe particles is below the skin depth,and also has the quantum effect,the small size effect and the surface effect which are favorable for the interface polarization and scattering properties,thus exhibits excellent absorbing properties.At a thickness of 3mm,the minimum RL reaches-29.57dB,and the bandwidth of RL less than-10dB reaches 8.38GHz,which covering the entire X-band,most of the C-band and part of the Ku-band.The electromagnetic properties test found that the introduction of the dielectric shell increases the dielectric loss of the particles and decreases the permeability.After coating the SiO₂ just enhanced the absorption intensity,while due to the large number of defects in the carbon shell,after coating the carbon shells not only the minimum RL value decreased by 34.65%,and the frequency band which the RL value is less than-10dB also expanded 38.4%.?Based on the theory of dipole,it is found that the soft magnetic particles will be arranged in the magnetic field along the direction of the magnetic field to form a chain structure,and the sturdy degree of the particle chain is positively correlated with the saturation magnetization of the particle as well as the applied magnetic field.To this end,choose the flower-like CIP as ferromagnetic particles,based on the preparation process of magnetorheological elastomer?MRE?,the MRE composite absorbing materials with anisotropic structure were prepared by applying the magnetic field of different sizes and angles in the solidification process of the matrix.The results of the wave performance test show that,with the increase of the magnetic field intensity,the dielectric constant of the MRE composite absorbing material increases and the permeability decreases.The effect of the wave absorption performance is mainly to the absorption peak frequency to move to high frequency and expand the effective absorption bandwidth.While,with the increase of the magnetic field angle,the dielectric constant and the magnetic permeability of the MRE composite absorbing material show an increasing trend,results in the enhancement of the absorption intensity,effective bandwidth expansion and the absorption peak frequency shift to the low frequency.?Based on the magnetron property of MRG,the MRG composite coating was fabricated and the test device based on the arch method was established.Wave absorbing properties tests show that the MRG composite coating has better absorbing properties of magnetron,and the absorption peak frequency shift with the magnetic field.In addition,the particle content has a greater effect on the magnetron absorbing properties of the MRG composite coating than the martix curing degree.