Structural Design And Performance Research Of High-performance Microwave-absorbing Composite Materials Based On Polarization Loss Mechanis

With the rapid development of wireless communications,especially popularity of 5G technology,future 6G and THz technology,electromagnetic wave plays an important role in global mobile communications and positioning systems.while electromagnetic waves provide us with convenience,it also seriously increases the problem of electromagnetic interference and pollution.Electromagnetic wave absorption materials can solve the problem of electromagnetic pollution by dissipating electromagnetic waves,thereby attracting widespread attention.In addition,wave-absorbing materials can prevent electromagnetic pollution while achieving radar stealth,which plays a pivotal role in ensuring national defense security and the stealth of military projects.Generally speaking,according to the loss mechanism,electromagnetic wave absorbing materials can be divided into dielectric loss and magnetic loss based-absorbers.The high density and Snoek limit of magnetic materials hinder their applications.Therefore,dielectric loss type materials have a broader application prospect and are more worthy of in-depth study.Currently,most of the studies are devoted to the preparation of composite materials to enhance the electromagnetic wave absorption properties through synergistic interactions,and it is not possible to determine quantitative relationships and preferred loss mechanisms（e.g.,interfacial polarization,dipole polarization,and conductivity loss）in the materials.Therefore,it is necessary to elucidate main loss mechanisms which cause superior performance.Here,we take manganese-based materials as research topic,and optimize their interface polarization and dipole polarization by constructing heterogeneous interfaces and introducing defects to enhance their attenuation of electromagnetic waves.The main research contents and conclusions are as follows:（1）Interface design for composites:manganese oxides are investigated to enhance the microwave absorption performance by constructing heterogeneous interfaces to improve the interfacial polarization.Specifically,i)Mn-based nanoparticles（Mn_xO_y）have high surface free energy and tend to agglomerate,thus exhibiting poor polarization ability.The Mn_xO_y@C composites are prepared by hydrothermal and subsequent carbonization processes with the introduction of porous carbon.The construction of Mn_xO_y-C heterogeneous interface has achieved the transformation from homogeneous to heterogeneous interface.Under the effect of alternating electromagnetic field,the positive and negative charges at the heterogeneous interface are forced to rearrange to form a local dipole electric field.The polarization process could dissipate electromagnetic energy and reach the equilibrium state from relaxation through the heterojunction contact.The result shows that the composites exhibit excellent microwave absorption properties compared to Mn_xO_y.The reflection loss of Mn O@C recaches-76.0 d B at 2.6 mm.ii)Mn₂O₃ multi-shell spheres exhibit poor microwave absorption performance due to weak conductivity.Firstly,the Mo S₂@C@Mn S core-shell structure composites are fabricated by calcination and hydrothermal introducing carbon layer and Mo S₂.Schottky barrier is formed between the metal-like C and the semiconductors Mn S and Mo S₂.The presence of the barrier leads to a large interfacial resistance,which facilitates the charge accumulation,and large number of charges form a space charge region thus enhancing the interfacial polarization.Compared with Mn₂O₃,the composite with Schottky heterojunction exhibits excellent microwave absorption performance with an effective absorption bandwidth of 3.84 GHz at 2.3 mm.（2）Dipole design for composites:Anion and cation are wildly present in crystals,and these crystal defects can induce excess electrons under alternating electric fields,forming electric dipoles and enhancing polarization losses.However,specifying exactly which vacancies dominate the performance can provide guidelines to specifically design and enhance the microwave absorption performance of the crystal.Herein,we investigate monometallic Mn based metal-organic framework by ion doping and changing annealing temperature to change the concentration of anion（oxygen ions）and cation（manganese ions）vacancies.During ion doping,oxygen vacancies show a positive correlation with performance,while manganese vacancies show a negative correlation,and it is determined that cations（manganese vacancies）are not the predominant contributors.When changing the annealing temperature,the work function is insensitive to temperature,but the performance varies considerably.Therefore,we exclude the dominant role of interfacial polarization on the performance.The anionic vacancies act more than the cationic vacancies,in other words,the dipole polarization induced by oxygen vacancies is the primary mechanism causing the excellent microwave absorption performance.The results show that the sample with the most oxygen vacancies have an effective absorption bandwidth of 8.0GHz at 2.1 mm.（3）Interface/dipole design for composites:Based on the above research,single loss mechanism cannot impart excellent microwave absorption performance to the composites.Here,Mn O₂@LDHs bionic leaf structure composites with Mn O₂ nanowires as the leaf veins and LDHs nanosheets as the blades are prepared by hydrothermal and oil baths.The bionic leaf structure increases the contact area with air,optimizes the impedance matching,and boosts the microwave absorption performance.Therefore,under the synergistic effect of interfacial polarization induced at the Mn O₂-LDHs interface and dipole polarization caused at the oxygen vacancy and nitrogen species,the composite exhibits excellent microwave absorption performance,and the effective absorption bandwidth can cover almost the whole X-and Ku-band at a low matching thickness.