| The rapid development of precise instruments,reconnaissance technology and advanced guided weapons has made “destroy upon discovery” a distinctive feature of modern warfare,and the use of electromagnetic wave(EMW)absorbing materials to achieve “stealth” is an important means to enhance the life of equipment.Breakthrough has been made in metal-based,carbon-based and polymer-based materials in the field of EMW absorption.However,the increasingly harsh stealth environment becomes more demanding for EMW absorbing materials,making it urgent to develop “thin,lightweight,broadband,strong” and high-temperature resistant absorbing materials.Polymer-derived Si CN ceramics(PDCs-Si CN)possess the advantages of low ceramic temperature,easy microstructure regulation,easy macro shape processing,light weight,high temperature resistance,chemical stability,excellent dielectric properties,etc.,making it a promising material for EMW absorption.However,PDCs-Si CN have some problems,such as weak electromagnetic attenuation ability and slightly poor EMW absorption performance.To expand the application prospects of PDCs-Si CN,which combine multiple advantages,in the field of stealth,it is of great research significance to enhance the EMW absorption performance of polymer-derived Si CN-based composite ceramics.This paper took polysilazane(PSZ)as the ceramic precursor polymer to prepare polymer-derived Si CN-based composite ceramics with different micromorphologies from the perspective of electromagnetic parameter regulation,and their components were designed to improve the electromagnetic attenuation ability and optimize the EMW absorption performance of composite ceramics.The main research contents are described below:(1)The mixed powder of Fe and Ni nanoparticles(Fe Ni)was introduced into polysilazane,and Si CN-based composite ceramics containing magnetic particles such as Fe3 N,Ni2Si were prepared by polymer derivation.The effects of the addition amount of Fe Ni on the phase composition,electromagnetic parameters,impedance matching ability and EMW absorption performance of composite ceramics were studied.The results show that the introduction of Fe Ni could catalyze the generation of carbon nanowires(CNWs)in the Si CN matrix,enhancing the dielectric loss capacity of composite ceramics;The presence of magnetic particles coordinated the electromagnetic parameters of composite ceramics,improving their impedance matching ability.When the addition amount of Fe Ni was 5 wt%,the generated amount of CNWs accounted for approximately 11 wt% of the total amount of composite ceramics.At this time,the ceramic material exhibited good EMW absorption performance: the sample at a thickness of 1.0 mm had a minimum reflection loss(RLmin)of-18.0 d B and an effective absorption bandwidth(EABW)of 2.5 GHz.(2)In order to further improve the dielectric loss capacity of Si CN-based composite ceramics,carboxylated multi-walled carbon nanotubes(MWCNTs)were introduced into PDCs-Si CN by combining the polymer derivation with vibration ball milling,to prepare MWCNTs/Si CN composite ceramics.The effects of MWCNTs addition amount on the permittivity and EMW absorption performance of MWCNTs/Si CN composite ceramics were investigated.The results show that the dielectric loss capacity of composite ceramics could be effectively improved by the introduction of an appropriate amount of MWCNTs;When the addition amount of MWCNTs was 10 wt%,the composite ceramics exhibited excellent EMW absorption performance: the sample at a thickness of 1.7 mm had a RLmin of-21.7 d B and an EABW of 4.7 GHz.In addition,in order to investigate the effect of the introduction of the integrally connected carbon network on the EMW absorption performance of Si CNbased composite ceramics,the 3D carbon fiber skeleton(3D-CFS)was introduced into PDCs-Si CN by vacuum impregnation and polymer derivation to prepare 3D-CFS/Si CN composite ceramics,and the effect of Ni electrochemical deposition duration of 3DCFS on the EMW absorption performance of 3D-CFS/Si CN composite ceramics was studied.The results show that the introduction of the integrally connected carbon network made composite ceramics EMW shielding materials.The EMW shielding performance of magnetic 3D-CFS/Si CN composite ceramics was enhanced with the increase of Ni electrochemical deposition duration of 3D-CFS;When the Ni electrochemical deposition duration of 3D-CFS was 50 min,the total shielding parameter(SETotal)of the composite ceramics in X-band was greater than 55.0 d B.(3)The micromorphology of ceramics has an important effect on its EMW absorption performance.The fiber structure has the characteristics of large lengthdiameter ratio and large specific surface area,which are conducive to enhancing the polarization loss of the material.Nickel acetylacetone was taken as the magnetic precursor to compound with polysilazane,and the Ni3Si/Si CN composite ceramic fiber(NSF)was prepared by electrospinning and polymer derivation.The EMW absorption performance of NSF was optimized by adjusting the pyrolysis temperature and the introduction amount of Ni.The micromorphology and EMW absorption performance and mechanism of NSF were studied.The results show that the prepared NSF had a special one-dimensional structure,and the volume density was only 0.2 g/cm3.When the pyrolysis temperature was 800 ℃,the carbon content and the degree of carbon graphitization inside NSF had a good synergistic relationship,and it had the widest EABW(4.6 GHz)at the thickness of 1.9 mm.The conduction loss and polarization loss of NSF were significantly improved with the introduction of an appropriate amount of Ni;When the introduction amount of Ni was 0.5 wt%,the composite ceramic fiber had the best EMW absorption performance: At a thickness of 2.64 mm,it achieved a RLmin of-22.5 d B,and its EABW(6.2 GHz)surpassed that of most reported bulk Si CN-based composite ceramics.The outstanding EMW attenuation capability was closely related to its excellent dielectric loss performance dominated by polarization.(4)The porous structure can not only make the material lightweight,but also cause multiple reflections and scattering of EMW,thereby enhancing the EMW absorption capacity of the material.Si CN-based directional porous composite ceramics(Si CNDPCCs)were prepared using natural basswood as a template through polymer derivation and vacuum impregnation,and subjected to oxidation treatment for different duration to optimize their EMW absorption performance.The micromorphology,EWM absorption performance and mechanism of composite ceramics were studied.The results indicate that composite ceramics had a special directional porous structure.The oxidation can transform the performance of directional porous composite ceramics from EMW shielding to EMW absorption.Composite ceramics(named PFSi CN05 and PFSi CN10,respectively)oxidized for 5 h and 10 h at 550 ℃ could effectively absorb EMW in the entire Ku band(12.4~18.0 GHz),with an EABW of 5.6 GHz.The excellent absorption performance of composite ceramics was the result of the combined action of good impedance matching,EMW multiple reflections and scattering,polarization loss,conduction loss and natural resonance of magnetic particles.In addition,due to the superimposed energy attenuation effects of the radially arranged pores on the incident EMW,the EMW attenuation ability of PFSi CN10 in the radial direction was obviously superior to that in the axial direction.To sum up,in this paper,the components of Si CN-based composite ceramics with different micromorphologies were designed from the perspective of electromagnetic parameter regulation,the relationship between different compositions of composite ceramics and electromagnetic parameters,EMW absorption performance was studied,the effects of conductivity,micromorphology and spatial structure of composite ceramics on EMW absorption performance were clarified,and the EMW absorption performance of Si CN-based composite ceramics was further improved,which provides a basic experimental basis and a new strategy for promoting the development of EMW absorption performance of ceramics and its application in the field of EMW absorption. |
PDF Full Text Request