Microstructure Design And Electromagnetic Properties Investigation Of SiC Based Nanofiber Materials

1D SiC materials have attracted great attention because of ideal mechanical properties(high strength and modulus),excellent chemical stability and high temperature resistance,as well as high surface area,designable microstructure and tunable dielectric properties.In the application field of electromagnetic(EM)absorption,1D SiC materials,meeting the requirement of “strong,wide,light and thin”,also have the potential to be served in harsh and high temperature environment.However,pure SiC phase shows relatively lower permittivity and conductivity compared with carbon and ferromagnetic EM absorbers,exhibiting more like EM transparent material.Therefore,defects,second phase additions,morphology and structure designing are usually introduced to enhance the dielectric loss ability of 1D SiC materials.To our knowledge,the fabrication of continuous SiC fiber-like materials with nanoscale size is rarely reported,indicating a large area for technique and process exploring.In this thesis,polycarbosilane is chosen as the precursor,electrospinning as the main fabricating method to prepare SiC based nanofiber materials with different components,microstructure,distribution and stacking structure.The process parameters are designed and optimized firstly,serving as the manufacturing knowledge foundation to obtain nanoscale and multiple components SiC based nanofiber materials.Moreover,the effects of second phase composition on EM absorption and shielding properties are investigated.And the EM absorption ablility of SiC based nanofiber materials are further promoted and manipulated through fibers' distribution and stacking designing.The main research contents and results are as follows:(1)The process parameters of spinning solution,electrospinning,curing and high temperature heat treatment,as well as functional additives are systematically investigated to optimize the microstructure of SiC nanofibers.Herein,PCS(0.1 ? 0.15 g/m L),chloroform and polycaprolactone(PCL,0.0875 g/m L)are selected as the precursor,solvent and spinning aid in the solution system.To obtain the spinning fibers with uniform morphology and stable microstructure,the electrospinning is in the range of 18 ? 30 k V,the pumping speed is around2.7 m L/h,the collector distance is 200 mm,the needle size is 7 ? 12#?The air curing process temperature is fixed at 200 ? for 1 ? 2 h in order to import oxygen as little as possible.As for the pyrolysis process,1400 ? is considered as an appropriate temperature to prepare SiC nanofibers with ideal crystallization degree.(2)The second phase modified SiC fibers were obtained by introducing magnetic elements(Fe and Co),naming as Fe/SiC and Co/SiC hybrid fibers.Furthermore,the modification effects on microstructure,conductivity,magnetic performance,dielectric property,EM absorption and shielding ability are also investigated.The results show that the elements of Fe and Co are existed as Fe3 Si and Co Si,distributing in the matrix of SiC fibers.The crystallization degree,electrical conductivity and dielectric loss have been dramatically enhanced by the introduction of magnetic phases.Particalarly,Fe/SiC hybrid fiber possesses outstanding EM absorption capacity in relatively low frequency range(4 ? 8 GHz).At an optimal Fe or Co content,the Fe/SiC hybrid fiber/silicone resin composite(35 wt.%)with a2.25 absorber thickness exhibits a minimal reflection coefficient(RC)of-46.3 d B at 6.4 GHz,and the Co/SiC hybrid fiber/silicone resin composite(10 wt.%)reaches a minimal RC of-20.6 d B(15.2 GHz and 2.5 mm)and a maximal effective absorption bandwidth(EAB)of 8.2GHz(9.8 ? 18 GHz,2.9 mm).However,pure Co/SiC fiber mat is more suitable to be served as EM shielding material with a total shielding effectiveness(SET)of 18.8 d B in room temperature,which improved 14% in 600 ?.(3)The High temperature refractory alloy phase(Hf C and Zr C)are imported into the SiC fiber system,and the influence on microstructure,phase components,conductivity,flexibility,dielectric property,EM absorption and shielding ability were studied.Hf C or Zr C hybrids with sizes of 5 ? 10 nm are uniformly distributed in the SiC fiber matrix,and the diameters of the hybrid fibers are greatly dropped to around 300 nm,as well as the conductivity enhances by 3 orders of magnitude.Meanwhile,the flexibility of the hybrid nanofiber mats are also greatly improved due to the nanometer size effect.By compositing the nanoscale Hf C or Zr C particles,heterogeneous interfaces and conductivity of the hybrid fibers are both increased,contributing to the enhancement of interfacial polarization loss and conduction loss abilities.At the optimal content of Hf C phase,the Hf C/SiC hybrid nanofibers/silicon resin composite(10 wt.%)shows excellent broad band EM absorption property in the frequency range of 10.6? 18 GHz.And the pure Zr C/SiC hybrid nanofiber mat exhibits ideal EM shielding performance with the SET value above 18 d B from room temperature to 600 ?.(4)Two structure designing proposals,including fibers alignment/distribution designing and fiber mats' stacking designing,are applied to optimize and manipulate the EM properties of SiC based nanofiber materials.It is found that fiber structure with continuous conducting path,shows higher permittivity than the uncontinuous whisker structure.Moreover,by aligning the SiC nanofiber in the plane of mat could further improve the EM absorption performance with the RC value of-53 d B and the EAB as wide as 8.6 GHz(9.4 ? 18 GHz).On the other hand,two kinds of Fe/SiC hybrid fiber mats with different Fe contents are fabricated as the high and low permittivity loss layer,respectively.By varying the stacking structure of multi-components stacking fiber mats composites,the EM absorption properties can be effectively promoted and manipulated.