Preparation And Electromagnetic Properties Of Flaky Carbonyl Iron Powders With Large Aspect Ratio And High Grain Orientation

Microwave absorbing material is an important material for military and civilian use.It is widely used in electromagnetic radiation protection,electromagnetic compatibility,electromagnetic pollution control and military stealth.With the miniaturization of electronic and communication devices and the development of highly mobile stealth weapons,ultra-thin broadband absorbing materials have become the focus of research.The flaky carbonyl iron(CI)is the preferred absorbent for ultra-thin broadband absorbing materials due to its high magnetic permeability and magnetic loss.In view of the main problem that the aspect ratio(a_r)and grain orientation of CI are difficult to increase in recent years,this paper proposes to use a high liquid-to-material ratio(L/M)and low medium viscosity heating and agitating technology to prepare the large a_r CI,and the grain orientation is improved by heating and magnetic field post-heat treatment,in order to greatly increase the magnetic permeability,thereby improving the electromagnetic and absorbing properties of the absorbent.The specific research contents and results are as follows:Firstly,in order to obtain an absorbent having a large ar,it is proposed to adjust the L/M,the agitation temperature and the viscosity of the grinding aid in the agitating process to prepare CIPs.The results show that the high L/M can reduce the strain rate of the particles,so that it maintains a relatively ordered crystal structure during plastic deformation,achieving uniform grain strain to slow the fracture process;the low-viscosity process control agent maintains high mechanical stress because of viscosity and internal friction.In proportion,high viscosity will increase the loss of mechanical energy,and small force on the particles will hinder plastic deformation.The ar of the sheet CIPs stirred at 250 is up to 65.8,which is 1.94 times as high as that obtained by stirring at room temperature.This is because the elevated temperature causes the atomic thermal motion to be intensified,and the grain boundary strength is weakened,which is favorable for grain boundary slip and grain rotation during plastic deformation,and at the same time prevents the particles from being broken due to stress concentration.A paraffin-based composite consisting of 28%by volume of flaky CIPs heated and stirred at a thickness of only 0.35 mm achieved a reflectance of less than-2.5 dB at 10-18 GHz.Secondly,in order to obtain a large sheet-like absorbent having high grain orientation,the heating agitation technique combining pre-heat treatment is proposed,and the grain orientation degree and electromagnetic properties of the CIPs are further improved by the post-magnetic field heat treatment.The initial grain size of the spherical CIPs after heat treatment at 400°C rises to 75.8 nm,and the grain boundary slip and grain rotation are more fully achieved by the dual action of mechanical force and thermal effect during heating and agitation,achieving high grain orientation degree.The test results show that the grain orientation degree of the CIPs with the ar max preheated at 400°C and heat-crushing can reach 29.8%,while the room temperature stir is only 16.81%.Subsequently,the sheet-shaped CIPs were heat-treated at 350°C in a magnetic field to obtain the higher grain orientation degree and magnetic permeability by directional growth of crystal grains.When the thickness of the material is 0.35 mm,the absorption is less than-3 dB in the range of 6-14 GHz,and the minimum reflectance is slightly over-3.5 dB.