Abnormal Electromagnetic Properties Of Percolative Ferrite Composites

When the electromagnetic wave interacts with the medium during the propagation process,physical phenomena such as reflection and transmission will occur at the interface.At this time,the permittivity or permeability of the material is generally used to characterize the law of interaction between the electromagnetic field and the materials.Conventional electromagnetic materials have positive permittivity and permeability.In recent years,electromagnetic metamaterials with negative permittivity or negative permeability have attracted wide attention due to their unique electromagnetic properties,such as negative refraction effect,reversed Doppler effect and perfect lens imaging.By adjusting the shape,size and orientation of unit,the tunable electromagnetic properties were controlled.Therefore,fabricating random metamaterials based on the intrinsic properties of materials was a new research direction.Random electromagnetic metamaterials can be obtained by traditional process for material preparation,and their electromagnetic parameters can be conveniently adjusted by changing the parameters of experimental process.The research on such random composites has effectively promoted the development of random electromagnetic metamaterials.In this thesis,we have prepared the ferrite powders and percolative ferrite composites,and studied their electromagnetic properties.The effects of conductive phase and chemical compositions on the change of electromagnetic properties have been systematically investigated,revealing the realization and regulation mechanism of negative electromagnetic parameters.In addition,this thesis has studied the microwave absorption properties of materials with negative electromagnetic parameters.The following were main research content and the experimental results.(1)The barium ferrite powder was prepared by hydrothermal synthesis at different holding times.The effects of holding time on phase composition,microstructure and static magnetic properties were studied.The results showed that the static magnetic properties of the prepared barium ferrite were optimal when the holding time is 17 h.After determining the optimum holding time,the phase composition,microstructure and static magnetic properties of barium ferrite with La-doped were studied.The results showed that when the amount of La doping is 0.1,the La-doped ferrite is a multi-domain structure.It had high magnetization and low coercivity which was suitable for high frequency applications.(2)The metal silver particles were loaded into the porous La-doped ferrite ceramic by the impregnation-reduction process using La-doped barium ferrite and silver nitrate as raw materials.As the content of silver increased,the silver/La-doped barium ferrite composite changed from capacitive to inductive.When the silver content exceeded 19 wt%,the frequency dispersions of negative permittivity were well described by the Drude model.The electron concentration of the composite increased as the silver content increased,causing the plasmon frequency to shift to a high frequency.The decreased permeability of silver/La-doped ferrite composites was caused by magnetic resonance and eddy current effect.(3)The iron powder/La-doped barium ferrite composites were prepared by pressureless sintering process using La-doped barium ferrite and carbonyl iron powder as raw materials.The percolation threshold of the composite was between 70 wt%and 80 wt%.When the content of iron powder approached the percolation threshold(60 wt%and 70 wt%),the composite obtains a large permittivity.When the iron powder content was 80 wt%,which exceeded the percolation threshold,the value of the permittivity was relatively small,and changed from negative to positive at 650 MHz.At a frequency of nearby 650 MHz,the value of permittivity is close to zero.Such materials can be referred to as epsilon-near-zero metamaterials.Epsilon-near-zero metamaterials have broad application prospects in super-coupling,invisible,high-directional antennas and optical devices.The decrease in permeability of iron powder/La-doped barium ferrite composites was mainly caused by domain wall resonance,natural resonance and eddy current effect.In order to study the relationship between the negative parameters and microwave absorption properties,the microwave absorption properties of iron powder/La-doped barium ferrite composites at 2-18 GHz were studied.The iron powder/La-doped barium ferrite with iron content of 80 wt%obtained a near-zero permittivity in the frequency of 100 MHz-1 GHz,and the RL reached a maximum value of-41.3dB at a sample thickness of 2.5 mm,which had good microwave absorption properties.(4)Carbon nanotubes/yttrium iron garnet composites were prepared by hot pressing process using carbon nanotubes and yttrium iron garnet as raw materials.When the content of carbon nanotubes exceeded the percolation threshold,the permittivity showed the Lorentz resonance.With the increasing carbon nanotubes content,the negative-positive transitions for permittivity shifted to low frequency.The increase of the concentration of electrons in the composites resulted in an increase of plasma frequency.Using the K-K relational formula,the real part of the permittivity was calculated from the imaginary part.Comparing the calculated results with the experimentally measured real part of the permittivity,the results were found to be more consistent,indicating that the experimentally measured permittivity is reliable.The real part of the permeability of the carbon nanotube/yttrium iron garnet composite decreased with the increase of the frequency,and the imaginary part of the permeability showed the loss peak,which was mainly caused by the relaxed resonance and eddy current effect.