【作者】 王鹏；

【导师】 成来飞；

【作者基本信息】 西北工业大学 ， 材料学， 2018， 硕士

【摘要】 SiC陶瓷基材料由于其较高的力学强度,良好的高温热稳定性,耐酸碱腐蚀等特性而被广大科研人员认为是最合适的作为服役苛刻环境的航空航天飞行器的结构材料,但是其较窄的电磁波吸收频宽限制了其在关键部位的应用,所以提高SiC陶瓷基吸波材料的有效电磁波吸收频宽是一个亟待解决的问题。纳米尺寸效应、界面极化以及偶极极化损耗是介电型吸波材料损耗电磁波的主要机制,鉴于此有必要发展新型复相纳米陶瓷吸波剂,又一维纳米材料由于其较大的比表面积以及较高的力学强度表现出了极强的工程应用的潜力,因此我们制备了一种新型一维Si-C-N纳米纤维满足高速隐身飞行器对于结构功能一体化材料的需求。本文采用静电纺丝技术结合聚合物转化陶瓷工艺制备了电磁吸收性能优异的一维Si-C-N纳米纤维,实现了反射损耗值（RL）在X和Ku波段的四分之三频宽小于-10 dB（90%电磁波被吸收）的目标。主要研究内容和结果如下:1、研究了电纺丝工艺参数对所得纳米纤维的形貌影响（1）纺丝电压为20 kV时,随着纺丝流速逐渐从0.0035 mm/s增加到0.008mm/s时纳米纤维的形貌由规则的圆柱状逐渐转变为扁带状且夹杂着大量的块状物。（2）当纺丝流速为0.0035 mm/s时,随着纺丝电压从15 kV增加到19 kV纳米纤维由串珠状结构转变成均匀的圆柱状形貌然后继续随着纺丝电压的增加纳米纤维直径增加。2、研究了氩气气氛下不同热处理温度对SiC纳米纤维的微结构、介电及电磁波吸收特性的影响规律（1）热处理所得纳米纤维的微结构由自由C,SiC纳米晶,无定型SiO_xC_y相组成。随着纳米纤维的热处理温度的增加,纳米纤维中C相从无定型结构转变成涡层状同时SiC晶粒尺寸变大。纳米纤维的介电常数实部?’和虚部?’’的值随着热处理温度的增加的变化规律为先增加然后减小最后再变大。1300 ^oC热处理得到的纳米纤维的?’和?’’值在8到18 GHz范围内最大。（2）随着热处理温度的增加纳米纤维的反射损耗值RL逐渐变大最后再减小,其有效的电磁波吸收频宽从4.7 GHz减小到0然后又增加到2.6 GHz,1300 ^oC热处理得到的纳米纤维的阻抗和空气的阻抗匹配度最高,其衰减常数值也最大。3、研究了氩气气氛下相同热处理温度聚碳硅烷和聚乙烯吡咯烷酮的质量分数变化对C/SiC复合纳米纤维的微结构、介电及电磁波吸收特性的影响规律（1）当聚乙烯吡咯烷酮的质量分数为定值时,聚碳硅烷质量分数为10%时得到的纳米纤维具有最优的电磁吸收表现,其最小的反射损耗RL的值为-57.8 dB,当吸波涂层厚度为1.9 mm时,其有效电磁波吸收频宽为5.5 GHz。（2）当聚碳硅烷的质量分数一定时,聚乙烯吡咯烷酮的质量分数为5.6%得到的纳米纤维具有最优的电磁吸收表现,最小的反射损耗RL值为-42.5 dB。当吸波剂的厚度为2.5 mm时,其有效电磁波吸收频宽最大为7.3 GHz包括从10.7到18.0 GHz。（3）自由C的缺陷,自由C和SiC纳米晶、无定型SiO_xC_y之间的异质界面,纳米纤维表面的化学键,纳米纤维的大的比表面积都对纳米纤维的介电损耗能力以及电磁波吸收能力的提高起到了关键的作用。4、研究了氮气气氛下热处理温度、热处理时间对C/SiC/Si₃N₄复合纳米纤维的微结构、介电及电磁波吸收特性的影响规律（1）氮气气氛中1300 ^oC热处理得到的Si-C-N纳米纤维的最优的RL值为-57.8 dB,既几乎99.9999%的电磁波被纳米纤维吸收或损耗,其有效电磁波吸收频宽为6.4 GHz,Si-C-N纳米纤维电磁波吸收性能的提高主要是由于其中的自由C、SiC和Si₃N₄纳米晶的协同作用导致。（2）氮气气氛下热处理4h所得Si-C-N纳米纤维的最优反射损耗（RL）值为-53.1 dB,既超过99.999%的电磁波能量被吸收或损耗,其有效电磁波吸收频宽覆盖整个Ku波段（12.4到18.0 GHz）,其优良的电磁波吸收性能由自由C、SiC和Si₃N₄纳米晶的协同作用所致。更多还原

【Abstract】 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更多还原