Preparation and Properties of Microwave Absorbing Materials with Oriented Carbonyl Iron Particles

Author:Zuo Dan Dan

Supervisor:zhou wan cheng

Database:Doctor

Degree Year:2018

Download:13

Pages:124

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With the increasing application of microwave absorbing materials,the requirements for it have become higher and higher.At present,the microwave absorbing material is moving in the direction of“thinness,light weight,absorption bandwidth,and strong microwave absorption performance”.It has been found that the microwave absorbing properties of polymer-based composites are not only related to the nature of the absorbent itself,but also related to the distribution of the absorbent in the matrix,including material structure,electrical conductivity,magnetic permeability,size,and morphology.The distribution of the absorbent in the matrix(random distribution or orientation)is closely related.Under the premise of determining the absorbent and the matrix,the adjustment of the distribution state of the absorbent in the matrix provides a broader space for the comprehensive design of the microwaveabsorbing material.The material with a directional distribution structure has a wide range of application in organic electronic devices,tissue engineering,etc.The preparation of materials having a directional distribution structure has become a hot spot in the current material research field.The main research contents and results of this paper are as follows:The carbonyl iron/epoxy resin microwave absorbing coating was prepared by magnetic field,and the electromagnetic parameters of the coating were adjusted by adjusting the distribution state of the carbonyl iron powder in the resin matrix.The microwave absorbing properties of the coatingwere adjusted by adjusting the thickness.After the orientation of carbonyl iron powder,the permeability of the coating increases,the permittivity shows anisotropy with the direction of the orientation of the carbonyl iron.The permittivity in the direction with its normal direction of the aligned carbonyl iron parallel to the electromagnetic wave incidence is greater than the permittivity in the direction perpendicular to the incident direction of the electromagnetic wave.The calculated reflectionloss results show that the orientation of the carbonyl iron can reduce the matching thickness of the absorbing coating and widen the absorption bandwidth of the absorbing coating.When the content of carbonyl iron is 75%by weight and the normal direction of the oriented carbonyl iron powder is perpendicular to the incident direction of the electromagnetic wave,the bandwidth of the coating at a thickness of 1.4 mm can reach 12.5 GHz with a bandwidth of less than-10 dB.Oriented and randomly distributed carbonyl iron/carbon black,carbonyl iron/nanographite sheet,carbonyl iron/carbon fiber composites were prepared using carbonyl iron as magnetic loss absorbent,carbon black,nano-graphite sheet and carbon fiber as electric loss absorbent,respectively.These composites were preparedby tape casting method.The microstructure of these composites was observed.The electromagnetic parameters of the coatings were tested.The effects of distribution of the absorbent in the matrix,the content of the absorbent,and the thickness of the coatingon electromagnetic and absorbing properties of coatings were studied.For the carbonyl iron/carbon black/epoxy resin coating,the addition of carbon black increased the permittivity of the coating,and the permeability of the coating increased slightly with increasing carbon black content.The orientation of carbonyl iron and the addition of carbon black broaden the absorption bandwidth of the coating and improve the microwave absorbing properties of the coating.For the carbonyl iron/nano-graphite sheet/epoxy resin coating,the addition of nano-graphite sheets increases the permittivity of the coating,and the permeability of the coating increases with the increase of the content of the nano-graphite sheet.With the addition of the nano-graphite sheet,the absorption bandwidth of the coating increases,the matching thickness decreases,and the coating can obtain an ideal absorption bandwidth at a relatively thin thickness.The orientation of the carbonyl iron increases the permeability and magnetic loss of the coating,and the orientation of the nanographite sheet increases the electrical loss of the coating.After the orientation of the carbonyl iron and the nanographite sheet,the permittivity and the permeability further increase.Compared to randomly distributed carbonyl iron powder/epoxy resin coating,oriented carbonyl iron/nanographite sheet/epoxy resin coating can achieve wider absorption bandwidth when the thickness is thinner.For the carbonyl iron/carbon fiber/epoxy resin coating,the orientation of the carbonyl iron increases the magnetic loss of the coating,and the addition of the carbon fiber increases the electrical loss of the coating.The dielectric properties of the coating exhibited significant anisotropy due to the different orientation of the carbon fibers,and the permittivity of the coating when the carbon fibers were oriented parallel to the direction of the electric field of the electromagnetic wave was higher than in the vertical case.The carbonyl iron oriented coating can achieve a wider absorption bandwidth at thinner thicknesses.With the addition of carbon fiber,the coating can achieve greater bandwidth at thinner thicknesses.For example,when the coating thickness is 1.3 mm,the coating has a reflection loss of less than-5 dB with a bandwidth of 13 GHz and an absorption frequency range of 5-18 GHz.We designed the coating with different carbon fiber orientations as a double-layer absorber.Compared with a single-layer coating,the double-layered microwave-absorbing coating can achieve a wider absorption bandwidth at the same thickness.Ferroferric oxide coated carbonyl iron was prepared by surface oxidation(blueingreaction).As the surface of carbonyl iron coats a high-resistivity Fe3O4 crystal,the permittivity of ferroferric oxide coated carbonyl iron decreases,and the permeability of the coating also decreases,but it remains at a high level.The frequency of the reflectance absorption peak of the ferroferric oxide coated carbonyl ironcoating moves to a high frequency with respect to the original carbonyl iron powder coating,and at the same time,when the coating thickness is between 1.0-1.5 mm,the ferroferric oxide coated carbonyl iron coatings have a wider absorption band with reflection loss less than-5 dBthan the original carbonyl iron powder coating.The experimental results show that coating a layer of Fe3O4crystal on the surface of carbonyl iron powder particles can adjust the electromagnetic parameters of the coating,thereby improving the impedance matching of the coating and improving the wave absorbing performance of the coating.Magnetic field induction method was used to realize the orientation of carbonyl iron coated with ferroferric oxide in the matrix.The results of the study indicate that the permittivity and permeability of oriented ferroferric oxide-coated carbonyl iron coatings are increased relative to randomly distributed coatings,and that a wider absorption bandwidth can be obtained at thinner thicknesses.