Preparation Of Porous C/Magnetic Metals(Oxides) And Study On Its Electromagnetic Absorption Properties

Due to the rapid development of the electronics industry and radio communications technology,the impact of electromagnetic（EM）radiation on human health and the environment is becoming more and more serious.In order to effectively suppress the harm caused by EM pollution in the military and civil fields,research on strong and effective absorbing materials is of great significance to China’s national defense construction and people’s social life.Porous magnetic metal carbon-based composites,owing to lots of advantages,such as light weight,corrosion resistance,large specific surface area and excellent conductivity,which show a great application prospect in the field of absorbing materials.However,there are still some problems with the absorbing materials in the currently studies.For example,the combination of multi-phase materials increases the total mass of the absorbing material,and more filler ratios cause excessive economic costs and manufacturing difficulties so that they can’t have a wide range of application markets.Thus,the research idea of this paper is to use glucose as the carbon source,cobalt nitrate（Co（NO₃）₂）or nickel sulfate（NiSO₄）as the magnetic metal（oxide）raw materials,and use the methods of hydrothermal reaction and high temperature reduction to prepare porous C/magnetic metal（oxide）composites,respectively.Furthermore,the design about the composition and microstructure of the materials was studied by changing the conditions of annealing temperature and the presence or absence of atmospheric protection.Meantime,the absorbing properties and mechanism of the samples were investigated by preparing a lower filler magnetic metal（Oxide）-C/PVDF thin film absorber that used polyvinylidene fluoride（PVDF）as the matrix.The main results and contents of this paper are as follows:（1）Co（NO₃）₂ was chose as the raw material,and the Co（CO₃）（OH）·0.11H₂O-C precursor was prepared by a simple and clean hydrothermal reduction method.Next,the precursor was annealed in air and N₂ atmosphere protection conditions,respectively.The obtained products of Co（CoO）-C composites in different annealing temperature were expressed a double-structured flower configuration and a porous spherical configuration.Furthermore,the 5 wt%filler ratios of Co₃O₄-C and Co/CoO-C materials annealed at different temperatures were introduced into PVDF matrix.The result was shown that the best reflection loss（RL）value of the Co₃O₄-C sample annealed at400℃is-51.8 d B at 15.8 GHz,the corresponding thickness of the absorber layer is 2.0mm,and when the absorber thickness is 1.5-4.5 mm,the effective bandwidth（RL≤-10 d B）is 13.2 GHz（4.8-18.0 GHz）.Meantime,the minimal RL value of Co/CoO-C sample annealed at 900℃reaches-55.7 d B at 6.3 GHz when the thickness of the absorber layer is 3.8 mm.And when the absorber thickness is 1.5-4.5 mm,the effective bandwidth reaches 12.6 GHz（5.2-17.8 GHz）.The excellent EM wave absorption performance is attributed to good impedance matching,dielectric loss（interface and dipolar polarization）,magnetic loss（natural and exchange resonance）and geometric effects（spot discharge,multiple reflection and scattering）.（2）NiSO₄ was chose as the raw material,and the 3Ni（OH）₂·2H₂O/Ni-C precursor was prepared by hydrothermal reduction method,and then the precursor was annealed at different temperatures under air conditions or N₂ atmosphere protection.And the Ni（NiO）-C composites were obtained with a double-structured flower configuration and a porous spherical configuration.Furthermore,the 2 wt%filler ratios of NiO-C and Ni/NiO-C materials annealed at different temperatures were introduced into PVDF matrix.The result was shown that the best RL value of the NiO-C-annealed at 350℃sample is-43.8 d B at 7.0 GHz,the corresponding thickness of the absorber layer is 3.5mm,and when the absorber thickness is 1.5-4.0 mm,the effective bandwidth（RL≤-10 d B）is 12.6 GHz（5.4-18.0 GHz）.Meantime,the minimal RL value of Ni/NiO-C sample annealed at 800℃reaches-35.8 d B at 7.3 GHz when the thickness of the absorber layer is 3.6 mm.And when the absorber thickness is 1.5-4.5 mm,the effective bandwidth reaches 13.2 GHz（4.8-17.8 GHz）.The excellent EM wave absorption property is attributed to the porous structure in Ni（NiO）-C materials,which can effectively adjust the complex dielectric constant to improve the impedance matching.At the same time,the porous pattern structure and porous spherical structure will cause secondary reflection and scattering,thereby extending the propagation path to further attenuate and dissipating more microwave energy.In summary,with PVDF as the matrix,this unique dual-structure Co（CoO）-C and Ni（NiO）-C composites have an ultra-low filler content and can be used as a lightweight,strongly absorption and broad bandwidth absorbers.