| Radar is a key part of weapons and equipment to undertake the military mission of communications and electronic reconnaissance in a new-style war.In order to ensure normal communication without being identified,tracked,or attacked within the detection range of the enemy's radar,weapons and equipment need to enable efficient transmission of own radar waves and dissipate or scatter the enemy's radar waves to other directions.Therefore,improving the stealth performance of weapons and equipment is of great strategic significance.The key technologies to realize the dual working mode of "signal transmission and stealth" mainly include shape design,frequency selective surface(FSS),and skin antenna.The shape design can scatter the radar waves received on the surface of the structure to other directions and reduce the radar echo in a specific direction.However,the aerodynamic performance of the equipment is mostly sacrificed,and the shape design gradually reaches the bottleneck.Implanting antennas into weapon skins can also effectively reduce the stealth problem of weapons.However,a large number of communication antennas in aircraft and other equipment cannot be completely hidden through skin antenna technology.Loading FSS on the equipment can transmit electromagnetic waves in the passband and stealth outside the passband.However,the weapon system is monitored by the enemy's multi-band,multi-static radar,and it is difficult to achieve multi-band,full-spectrum,and omnidirectional stealth only by relying on FSS.Therefore,facing the complex and changeable battlefield environment,stealth technology will inevitably develop in the direction of intelligent tunable stealth.Magnetic absorbing particles have the characteristics of magnetically induced selforientation,and can be made into intelligent and tunable stealth/absorbing materials by compounding with high molecular polymers.The electromagnetic parameters and structure(microscopic,external form)of the tunable absorbing material are continuously variable induced by the magnetic field,and the electromagnetic stealth characteristics are also tuned by the magnetic field,which helps the weapon system to switch between "signal transmission and stealth" during mission execution.Aiming at the potential engineering application of intelligent tunable stealth structure in weapon stealth,this paper conducts research work from the preparation and characterization of magnetic absorbing particles,electromagnetic stealth structure and tunable mechanism research.The main research contents of this paper are summarized as follows:Magnetic absorbing particles are the basis of tunable absorbing material,which determines the absorption intensity and frequency band of absorbing material.Four kinds of magnetic absorbing particles,such as dendritic Co,Fe@CF,flakey CIP and Fe@F-CIP,were prepared for the problems of eddy current and skin effect,high density and low magnetic permeability in magnetic absorbing materials,and the electromagnetic absorption characteristics of magnetic absorbing particles were deeply discussed and analyzed.The hyperbranched dendritic Co particles prepared can effectively inhibit the skin effect and eddy current due to their special morphology and nanoscale.When the thickness is 1.5-3mm,the minimum RL reaches-36.3d B and the effective bandwidth covers 4-18 GHz,showing super strong electromagnetic wave absorption capacity.The microwave absorption peak of the highly anisotropic patchy carbonyl iron(F-CIP)prepared is about-27 d B,and the frequency band of absorbing more than 90%electromagnetic wave ranges from 10 GHz to 18 GHz,with the bandwidth up to8 GHz.Compared with CIP,the bandwidth is doubled,and the microwave absorption characteristics of CIP are significantly improved.This part of the work provides ideas for the preparation of high-performance magnetic absorbing materials and enriches the database of absorbing materials in engineering applications.At the same time,it provides a material basis for further research on the magnetic-induced microwave absorption characteristics and tunable stealthy structure of magnetic absorbing materials.The magnetic induction tunable mechanism is the theoretical basis for the design and preparation of tunable stealth materials.Based on the simulation of multiple physical fields and magnetic dipole theory,the magnetically induced self-orientation arrangement of absorbing particles is discussed and verified by experiments.Using the equivalent medium theory,the variable mechanism of magnetic-induced microwave absorption is expounded.In this paper,the magnetic absorption characteristics of spherical CIP,flakey CIP(F-CIP)and Fe@CF are studied,with emphasis on electromagnetic parameters,reflection loss,impedance matching,frequency-shift characteristics and so on.It is found that the saturation magnetization and arrangement mode of magnetic absorbing particles directly affect the electromagnetic parameters range,electromagnetic wave intensity,effective absorption bandwidth,frequency shift and other microwave absorption characteristics of magnetic absorbing particles.The magnetic field regulates the minimum reflection loss of Fe@CF to shift to low frequencies.However,the magnetic field induced the central absorption frequency of F-CIP absorbing material to move to the high frequency band,the central absorption frequency moved from 13.6GHz of 0m T to17.4GHz of 300 m T,and the bandwidth moved to 3.8GHz.The regulation of spherical CIP by magnetic field is bidirectional,then its central absorption frequency first moves left and then right with the magnetic field.These work provides guidance for the study of tunable stealth structures and can select magnetic particles to fill according to specific needs.Magnetic field induces the orientation arrangement of magnetic absorbing particles,which can not only change the microscopic structure of absorbing materials,but also change the appearance of the materials.The periodic lattice cloaking structure was prepared by using the magnetically induced self-orientation of the magnetic particles and characterized by the arch method.It is found that with high magnetic field intensity and high particle concentration,more magnetic particles are involved in the formation of abundant lattice elements driven by magnetic field.Electromagnetic wave enters into lattice structure to reflect and scatter so that its energy is consumed as much as possible.For example,the minimum reflection loss of the periodic lattice stealth structure prepared under 0-300 m T is-6d B,-9.2d B,-14.7d B and-22.9d B respectively.The corresponding bandwidth is expanded from 0GHz to 1.2 GHz,3.8 GHz and 8.2 GHz respectively.Combined with Comsol software platform,the experiment shows that the tunability of the structure is easier to realize the broadband and strong absorption of the stealth structure.This work has important guiding significance for the preparation of periodic lattice stealth structures and their tunable stealth structures.Based on the study of magnetically induced variable absorption mechanism and the absorbability of microscopic and macroscopic structures,tunable stealthy materials were prepared and a series of studies were carried out.Firstly,an excitation device is designed and assembled to provide control signals for tunable stealth materials.The excitation signal can change the aggregation state of magnetic absorbing particles and the macroscopic structure of the material repeatedly and continuously reversibly,so as to realize the tuning of absorption intensity,effective bandwidth and frequency shift characteristics.Under zero field,the absorption curve of stealth material tends to be flat,the minimum reflection loss is only-11.4d B,and the absorption bandwidth is not ideal.After the excitation signal is applied,the minimum reflection loss of tunable absorbing material is significantly increased,which is as high as-39.7d B,effectively reducing28.3d B.The number of loss formant increases from 1 under zero field to 3-4 formant when there is field,realizing multi-frequency absorption in C,X and Ku bands.The frequency band absorbing more than 90 % of electromagnetic waves has been increased from 1.3GHz to 10.2GHz,which is expected to realize weapon stealth in broadband.In addition,the influence factors such as particle content and matrix curing degree were also considered.The tunable absorbability of tunable stealth materials is also investigated by using magnetic dipole theory model and equivalent circuit.This part of research provides theoretical guidance for the further research of tunable stealthy materials and provides the possibility for the engineering application of tunable stealthy materials. |
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