Synthesis And Microwave Absorption Properties Of Manganese Oxide Composites

With the exponential growth of various miniaturized,integrated and high-frequency electronic equipment,electromagnetic pollution issues have become increasingly prominent.As wave-absorbing materials can fundamentally eliminate the harm of electromagnetic waves,they have become the current research hotspot.With good dielectric loss ability,moderate impedance matching,excellent weather resistance and relatively light weight,the manganese oxide has attracted much attention as a kind of absorbing material.The manganese oxide is characterized by rich crystal form,changeable morphology,simple preparation method,large output,low cost,abundant natural resources and environmental friendliness,which enjoys great application potential in the microwave absorption field.However,their single loss mechanism,low conductivity and large matching thickness make them still far from the absorbing materials with ideal broadband,light weight and strong absorption.Therefore,how to improve the absorption strength and achieve broadband effective absorption under the thin matching thickness is the key to make breakthrough in the performance of manganese oxide absorbing materials.In order to solve the problems of weak absorption strength and narrow absorption band of the manganese oxide,the strategy of "doping optimization" and "composite enhancement"is adopted to gradually improve the electromagnetic loss ability in this paper.Based on the design concept of dual optimization of composition and structure,the physical and chemical properties of the manganese oxide are regulated,the magnetic loss is increased,the polarization types are enriched and the conductivity loss is optimized to enhance the electromagnetic attenuation ability.In this paper,the design above is adopted to break through the performance limitations of manganese oxide absorbing materials so that they have broadband absorption performance at low matching thickness,and to reveal the microwave loss mechanism of manganese oxide composites.The research results are as follows:The manganese dioxide doped with different magnetic elements is prepared with the one-step hydrothermal method to realize the dual regulation of morphology and crystal structure.The magnetic elements play a guiding role during the formation of manganese dioxide crystals.The doping of Fe,CO and Ni transforms the rod-shaped ?-MnO₂ into granular ?-MnO₂,flower-like microsphere ?-MnO₂ and ?-MnO₂.The heterogenous doping results in more lattice distortion left in manganese dioxide crystal,which leads to the decrease of conductivity.The conductivity of P-MnO₂,Fe-MnO₂,Co-MnO₂ and Ni-MnO₂ is 1.39×10^-3S·cm^-1,1.71 × 10^-5S·cm^-1,2.32× 10^-5S·cm^-1 and 2.56× 10^-5S·cm^-1 respectively.The weakened conductivity has a greater influence on the overall dielectric loss ability and impedance matching of the doped manganese dioxide,resulting in weak ability of overall dielectric loss,but impedance matching has been improved to some extent.Meanwhile,the dipole moment generated in the manganese dioxide crystal induced by doping and its defects will produce resonance due to the electromagnetic field,which increases the magnetic loss ability.The preparation process is optimized and the one-step hydrothermal method is adopted to prepare a series of fiber ball shaped Ni-doped manganese dioxides of stable phase structure with different specific surface area.When the molar ratio of Mn and Ni in the sample is adjusted,the oxygen vacancy in the crystal changes accordingly,and the microstructure of the material changes slightly.The layered structure of the mesoporous manganese dioxide guarantees that electromagnetic wave can be incident into the material as much as possible without being reflected,so that the polarization loss is enhanced,multiple reflection and scattering effect are realized,and microwave absorption performance is strengthened.The absorption intensity can reach-32.7dB at 15.7GHz,and the effective absorption band can selectively fully cover the bands of X and Ku,so as to realize dynamic tuning of resonance frequency.Besides,effective absorption of the electromagnetic wave(RL<-10dB)can be achieved at the lower S-band(2.95GHz).Carbon,magnetic iron nanoparticles and zinc oxide dielectric materials with excellent dielectric properties are introduced into the manganese oxide to construct the binary composite material system,which enriches the interface polarization and loss mechanism of different materials,and improve the absorption performance of the composite.When the volume ratio of methane to argon in the reaction chamber is 2:1,the absorption performance of the composite MnO₂@C is enhanced.When the matching thickness is only 1.5 mm,the reflection loss at 13.2GHz reaches-17.76dB,and the effective absorption bandwidth reaches 3.52GHz.The MnO₂/ZnO composite has good electromagnetic absorption performance.When the thickness is only 1.8mm,the reflection loss reaches-22.5 and the effective absorption bandwidth is 11.70-17.63GHz.In order to further enhance the absorption performance of the manganese oxide,magnetic Fe nanoparticles and conductive carbon materials are introduced into the manganese oxide according to the mechanism of magnetic dielectric synergistic coupling.The crystalline Fe/MnO@C core-shell capsule composite embedded in the porous amorphous carbon matrix restrains the growth of magnetic Fe particles,interrupts the agglomeration of the iron core and hinders the accumulation of the carbon conductive network.The mass ratio of Fe to MnO₂ is regulated from 9:1 to 6:4,the thickness of crystalline graphite layer of the sample is reduced from 11 layers to 2-3 layers,and the conductivity is reduced from 10.16S·cm^-1 to 4.72S·cm^-1.The increase of interface polarization and the decrease of conductivity loss jointly make the dielectric properties adjustable.The absorption peak is increased from-11.23dB to-45dB,and the effective bandwidth is 5.1GHz when the thickness is 2.0mm.In the research,a variety of loss modes is introduced into manganese oxide microwave absorbing materials through doping and compounding strategies,the broadband absorption and strong absorption strength are realized under thin thickness(?2.0mm),which provides the foundation for perfecting the theory of manganese oxide microwave absorbing materials.