| With the development of modern military detection technology and civil electromagnetic communication technology,the problems of electromagnetic wave damage become more and more prominent,so the requirement of microwave absorbing technology is higher and higher.The microwave absorbing coatings with high performance have wide application prospects in military and civil applications.In this paper,based on the traditional ferromagnetic absorber/polyurethane coatings,high performance dielectric/magnetic composite absorbing coatings were prepared by means of absorbent composite and morphology optimization.The electromagnetic properties were studied and the influencing factors for absorbing properties were discussed.In this experiment,we studied electromagnetic properties of all used materials by vibrating sample magnetometer?VSM?and vector network analyzer?VNA?,such as two carbonyl iron powders?CIP,SCIP?,ferrite?PX?,and flaky Fe-Si-Al powder?FFSA?,graphene?GNs?,and nano-silica?SiO2?.And the microstructure and phase structure of these materials were investigated by scanning electron microscopy?SEM?,X-ray diffraction?XRD?and transmission electron microscopy?TEM?.The mechanisms of microwave loss caused by different materials were analyzed.Ferromagnetic absorber/polyurethane absorbing coatings were prepared and the reflection loss ability of four ferromagnetic materials was analyzed and compared.Then,based on the two kinds of CIP/PU coatings with excellent performance,the absorbing properties of the coatings were further improved from the aspects of improving loss capability and optimized impedance matching.And based on high performance radar stealth coating to prepare the infrared/radar compatible stealth coatings.The results show that:The carbonyl iron powders?CIP,SCIP?,ferrite?PX?,and flaky Fe-Si-Al powder?FFSA?are commonly used ferromagnetic absorbers with high permeability and strong magnetic loss ability.However,due to the difference in impedance matching performance,different electromagnetic loss capabilities are exhibited in the range of 2-18 GHz.Among them,carbonyl iron powders have the best performance.But,the performance of coating filled a single absorber is difficult to meet the practical application requirements,we need to improve the stealth ability of the coating by various means.Graphene with strong dielectric loss properties has no magnetic loss ability,which makes it complementary to carbonyl iron.In addition,the unique structure of graphene and the interface effect of heterogeneous materials can also promote the performance of GNs/CIP composite coating.However,when the contents of absorbent were too high,the impedance mismatch would be caused and the absorbing performance would be weakened.Therefore,the microwave absorption performance can be adjusted by changing the content of GNs and CIP,and a wide band microwave absorbing coating with smaller thickness can be obtained.The peak value of reflection loss is-30.1dB and the effective bandwidth?RL<-10dB?is7.1GHz?10.9-18 GHz?.In addition,the addition of GNs plays an important role in improving the corrosion resistance of the coatings.The results of electrochemical impedance spectroscopy showed that the resistance of the coating increased from 1.653E6 to 2.012E6when the amount of graphene was 1%?CIP:PU:GNs=2:1:0.01?.But with the graphene contentfurther increased,The resistance of the coating decreases to 9.352E4,and the corrosion resistance of the coating decreases.By ball milling,the spherical carbonyl iron is formed into a sheet shape,which enables the material to break through the Snoke limit,obtain greater magnetic properties,and have stronger absorbing properties,the effective bandwidth?RL<-10dB?is 7.6GHz.However,in the process of flaking,the real part of carbonyl iron permittivity will increase from 6.1 to 8.2accordingly,which is not conducive to the realization of impedance matching.Therefore,the transparent material?SiO2?was added into the coating.The SiO2 can form a transmission channel so that the electromagnetic wave enters the coating and is attenuated by the absorber.In addition,nanoparticles size effect and the interfacial effects of CIP/SiO2/PU ternary also can enhance significantly the absorption bandwidth and the minimum absorption peak value.And nano-silica materials can cross-link with the substrate,effectively reduce the local defects of the coating,and then improve the corrosion resistance of the coating.And the maximal value of the effective absorption bandwidth?RL<-10dB?was in the frequency range of 8.5–18 GHz.The infrared coating layer is coated on the surface of the radar coating to prepare a stealth coating.The result of studying the influence of the infrared stealth layer shows that,Al/PU infrared stealth layer can reduce the infrared emissivity of the coating from 0.898 to0.462.However,the radar wave absorption performance also decreased obviously,and the effective absorption bandwidth decreased with the increase of Al content.This is because the permittivity of Al is too large,so the radar wave is reflected in the surface layer.By using the method of surface structure design,the electromagnetic wave can pass through the red outer layer effectively,reduce the infrared emissivity,and realize the broadband absorption of radar wave at the same time.The infrared emissivity of the compatible coating is reduced to 0.502,and the effective radar wave absorption bandwidth reaches 7 GHz?11-18 GHz?. |
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