| Alkaline environments such as seawater and the high temperature conditions of high-speed flight limit the further application of traditional alloys as absorbent materials.High entropy alloys(HEAs)are widely studied as the next generation of electromagnetic wave absorbing materials because of their excellent electromagnetic properties,high temperature resistance and corrosion resistance.Ferromagnetic elements such as Fe,Co and Ni are the key to achieve high magnetic loss of HEA,which explained the reason why they are widely used in HEA microwave absorbing materials.Mn is a metal with rich reserves and mature refining technology,but its antiferromagnetic properties limit the electromagnetic wave absorption performance of Mn containing HEAs.Therefore,it is urgent to realize the antiferromagnetic ferromagnetic transformation of Mn and reveal its influence on the magnetic and electromagnetic parameters of high entropy alloys.In this paper,FeCoNiMnAl and FeCoNiMnSiAl HEAs are designed.By using the first principles calculation,the effects of Al and Si elements on the antiferromagnetic to ferromagnetic transformation of Mn in HEAs are studied,the magnetostatic properties and the variation mechanism of electromagnetic parameters are explored,further investigate the static magnetic properties of high entropy alloys and the effect on eddy current losses and natural resonance losses.In addition,in order to verify the feasibility of absorbent in practical production and application,this paper applies HEA powders as electromagnetic wave absorbent to microwave absorbing coating,tests the relevant performance and analyzes the microwave absorbing mechanism of HEA coatings.Using femtosecond ultrafast laser to process the coating surface,the influence of structured surface on the performance of radar absorbing coating and the compatible stealth characteristics of infrared wave and radar wave is explored.X-ray diffractometer,scanning electron microscope,vibrating sample magnetometer,vector network analyzer,absorbing anechoic chamber and infrared emissivity tester were used to test and analyze samples.The following are the main conclusions of this paper:(1)By calculating the electron spin state,it is proved that the induced element Al can make Mn atoms produce different magnetism in different chemical environments.When the Mn content is low,the weakening of the coupling effect between ferromagnetic elements will lead to the reduction of magnetism.After further increasing the Mn content,the Mn atom with ferromagnetic transformation will compensate the total magnetic moment and improve the magnetism.High saturation magnetization can produce high initial permeability,which makes the high entropy alloy have satisfactory electromagnetic wave absorption ability.(2)Si element can promote the production of BCC phase in high entropy alloy and inhibit its transformation to FCC phase.The calculation of electron spin state shows that when the induced Elements Si and Al exist at the same time,Mn can complete the antiferromagnetic ferromagnetic transition to a great extent and enhance the magnetism of the alloy.The study of electromagnetic characteristics shows that the highest dielectric properties ε′ and ε′′ of flake HEA powder are 10.9 and 0.95,respectively,and the highest magnetic properties μ′ and μ′′ are1.52 and 0.32,respectively.The reflection loss at 2 mm thickness reaches effective absorption of less than-10 d B at 10.5-12.7 GHz,and the peak value is about-18 d B.(3)The spherical powder HEA absorbing coating has gratifying absorption performance.It can achieve effective absorption of less than-10 d B at 6.9-12.2 GHz,with an absorption peak of about-15 d B,and slightly improved in the high temperature environment of 300 ℃.The flake powder HEA absorbing coating achieves-10 d B effective absorption at 8.7-11.2 GHz.After laser etching the periodic structure on the coating surface,the absorption peak and effective absorption bandwidth of the coating are improved.After etching the 1mm hexagonal structure,the effective absorption bandwidth of spherical powder coating is increased from 5.3GHz to 8.5 GHz,the absorption peak is increased from-15 d B to-16 d B,the effective absorption bandwidth of flake powder coating is increased from 2.5 GHz to 2.8 GHz,and the absorption peak is increased from-13 d B to-42 d B.(4)The infrared stealth layer processed by femtosecond ultrafast laser can achieve low infrared emission below 0.2,and can effectively control the microwave absorption performance of the double-layer structure.It is found that the super surface with hexagonal periodic unit tends to affect the absorption performance of high frequency,but has a weak effect on the absorption of low frequency.There is a certain fit relationship between the size of the periodic structure and the coating performance of the bottom layer.The higher the frequency of the peak value of the coating and the larger the scale of the periodic structure,the more obvious the improvement of the microwave absorption performance.The coating with a filling ratio of 0.5:1can be increased to about-38 dB at most. |
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