Preparation Of Si-C-N Nanofibers And Their Electromagnetic Properties Study

Owing to their high mechanical strength,excellent thermal stability,corrosion resistant to alkaline or acid conditions,SiC based ceramics have been considerred as ideal candidates used in structural parts of aeroplanes or aricrafts et al.However,the narrow frequency band of its electromagnetic（EM）wave absorption has severely limited the application of SiC ceramics,therefore the critical point is to enhance their EM absorption band to meet the demands for highly effective EM absorbers.Nanosize effects,interfacial polarization and dipoles polarization are main absorption mechnism for dielectric absorbers,therefore it is necessary to develop nano composite ceramics.As well as one dimensional nanomaterials has high aspect ratio,good mechanical strength which has great potential to be applied in enginerring area.Consequently,we prepared a new kind of Si-C-N nan fibers to meet the demands of aerospace materials with intense EM absorption.We combined electrospinning technique with polymer derived ceramics method to prepare Si-C-N nanofibers with RL values below-10 dB in the frequency range of three quarters of X and Ku band.The main research contents and conclusions are as follows:1、The effects of electrospinning processing parameters on nanofibers morphology（1）When electrospinning voltage is 20 kV,the morpholgy of nanofibers changed form regular cylindrical shape to ribbon with lots of chunks as electrospinning feed rate increasing from 0.0035 mm/s to 0.008 mm/s.（2）When electrospinning feed rate is 0.0035 mm/s,the morphology of nanofibers changed from bead-in-chain structure to cylindrical shape as voltage increasing from15 to 19 kV,at last the nanofiber diameter enlarge.2、The effects of annealing temperature in Ar on SiC nanofibers microstructure,dielectric properties and electromagnetic wave absorption performance.（1）The nanofibers after annealing is composed of free carbon,SiC nanocrystals,and amorphous SiOC.Carbon changed from amorphous structure to turbostratic with size increas of SiC grains as annealing temperature increasing.The real（?’）and imaginary（?’’）part of dielectric constant of nanofibers firstly increase then decrease finally increase as annealing temperature.The?’and?’’values of nanofibers from1300^oC annealing treatment are largest in the frequency range of 8 to 18 GHz.（2）The reflection loss（RL）values of nanofibers gradually increas then decrease as annealing temperature increase,their effective absorption bandwidth decreased from 4.7 GHz to O then increased to 2.6 GHz。The impedance matching degree of nanofibers derived from 1300^oC annealing treatment is higher compared with others also with large attenuation constant.3、The effects of PCS and PVP weight ratio on C-SiC nanofibers microstructure,dielectric properties and EM absorption performance（1）The nanofibers derived from 10 wt%PCS display excellent EM absorption performance with minimum RL values of-57.8 dB and their effective absorption bandwidth covered the frequency range of 5.5 GHz at the absorbers coating thickness of 1.9 mm.（2）The nanofibers obtained from 5.6 wt%PVP exhibited best EM absorption performance with minimum RL of-42.5 dB also their effective absorption bandwidth covered the frequency range from 10.7 to 18.0 GHz at the absorbers coating thickness of 2.5 mm.（3）The defects in free carbon,the heterogenous interfaces among free carbon,SiC nanocrystals and amorphous SiOC,and their high aspect ratio with surface functional groups of nanofibers both contributes to enhancement of nanofibers dielectric properties and EM absorption performance.4、The effects of annealing temperature and time on C-SiC-Si₃N₄ nanofibers microstructure,dielectric properties and EM absorption performance under N₂atmosphere.（1）The SiCN nanofibers obtained from 1300^oC annealing treatment in N₂atmosphere displayed best EM absorption performance with optimized RL valuesof-57.8 dB suggesting that more than 99.999%EM wave absorbed.The effective absorption bandwidth is up to 6.4 GHz,the EM performance improvment of SiCN nanofibers is attributed to the synergistic effects of free carbon,SiC and Si₃N₄nanocrystals.（2）The SiCN nanofibers derived from 4h annealing at 1300^oC in N₂ atmosphere showed excellent EM absorption performance with minimum RL valuesof-53.1 dB suggesting that more than 99.999%EM wave being absorbed.The effective absorption bandwidth covered the whole Ku band,the EM performance improvment of SiCN nanofibers is attributed to the synergistic effects of free carbon,SiC and Si₃N₄nanocrystals...