【作者】 刘旭光；

【导师】 王应德；

【作者基本信息】 国防科学技术大学 ， 材料科学与工程， 2010， 博士

【摘要】 吸波材料在军事和民用领域均具有广泛的应用。结构吸波材料是一种既可作承力部件,又具有优良的电磁波吸收性能的复合材料,是吸波材料的重要组成部分。异形截面碳化硅纤维与圆形碳化硅纤维一样,具有优异的力学性能和耐高温性能,同时又具有良好的吸波性能,有望成为结构吸波材料理想的增强/吸波一体化纤维。本文设计了具有相应形状的异形孔喷丝板,以聚碳硅烷为原料,通过熔融纺丝、空气不熔化和高温烧成制备了条形、三叶形、四叶形、五叶形、六叶形、七叶形、三折叶形、T形和C形等多种异形截面碳化硅纤维。研究了纺丝工艺对纤维异形度和当量直径的影响,确立了异形截面碳化硅纤维的制备工艺。研究了不同截面形状碳化硅纤维的电磁性能变化规律,分析了异形截面碳化硅纤维电磁匹配、形态及含量对其复合材料吸波性能的影响,并以此为基础制备了异形截面碳化硅纤维增强结构吸波复合材料,探讨了异形截面碳化硅纤维的吸波机理。根据聚合物模塑性理论和聚碳硅烷熔体流变性能设计了条形、三叶、四叶、五叶、六叶、七叶、三折叶和T形等叶片形喷丝孔以及具有不同弧度的C形喷丝孔,并通过实验验证,发现在狭缝宽度为0.12-0.14,狭缝长宽比为3-4时,可以制备具有较大异形度和较低当量直径的异形纤维。通过对纺丝工艺的研究发现,纺丝温度升高,异形截面纤维的异形度降低,当量直径增大。挤出速度和收丝速度对纤维异形度的影响不大。随挤出速度降低和收丝速度增大,异形纤维的当量直径降低。合适的异形纤维的纺丝工艺为:纺丝温度为聚碳硅烷软化点以上75-95℃,纺丝压力为0.5-1.3MPa,收丝速度为150-300rpm。通过合适的不熔化和烧成工艺,制备了异形度为0.3-0.8、平均当量直径为25μm左右的条形、三叶、四叶、五叶、六叶、七叶、三折叶和T形等8种叶片形和3种不同弧度的C形碳化硅纤维。叶片形碳化硅纤维截面曲率半径沿周向呈正割函数形式的变化,纤维叶片顶端和C形纤维截面两端的的曲率半径最小。异形截面碳化硅纤维与圆形碳化硅具有相同的元素组成和内部结构,且表面富碳。对比研究了圆形、叶片形和C形碳化硅纤维的电磁性能。发现每增加一对叶片,叶片形碳化硅纤维的归一化介电常数值实部比圆形纤维提高约10%,虚部提高约15%;偶数型纤维的每个叶片对介电常数的贡献略高于奇数型纤维;随叶片数量的增加,纤维损耗角正切值呈增大趋势,吸收峰有向低频移动的趋势;随纤维截面弧度的增大,C形碳化硅纤维的介电常数和归一化介电常数虚部降低,实部也呈降低的趋势,损耗角正切值降低,其吸收峰向高频移动。研究了纤维长度、排列方式及含量等对异形碳化硅纤维复合材料的介电常数和吸波性能的影响,发现随着纤维长度增加,其介电常数增大,介电损耗效能先提高后降低;当纤维无规排列时,比有序单向排列具有更加明显的频散效应,并且无规排列纤维的介电损耗角正切值大于有序单向排列;随纤维含量增加,复合材料的介电常数和介电损耗角正切增大。使用具有不同截面形状和不同规格的SiC纤维,进行阻抗匹配,设计了多层结构吸波复合材料。发现不同截面形状SiC纤维增强的多层结构吸波复合材料可以集合不同形状纤维的优点,或者拓宽吸波频宽,或者在保证吸波性能的情况下减少材料厚度,特别是可以一定程度上提高低频段的吸波性能。制备了一种双层结构吸波复合材料,该复合材料厚度为3.5mm,最低反射率为-18.62dB,反射率低于-10dB的频宽超过8GHz。通过在上浆剂中添加羰基铁粉,制备了三叶形SiC/羰基铁粉复合纤维束。与三叶形SiC纤维相比,磁改性纤维束增强的复合材料的反射衰减大于10dB的频宽向低频移动了2GHz左右,有效改善了材料在低频段的吸波性能。初步探讨了异形截面碳化硅纤维的吸波机理。异形截面纤维表面曲率的变化使纤维在电磁波作用下的极化发生改变,叶片形碳化硅纤维叶片端和C形纤维两端具有较大的表面曲率,在电磁场作用下,容易聚集较大密度的电荷,形成宏观偶极子。偶极子的产生增强了纤维的极化能力,增大了纤维的介电常数。随纤维叶片数量增加,叶片形碳化硅纤维的偶极子数增加,纤维介电常数增大,纤维损耗角正切值呈增大趋势,纤维对电磁波的损耗效能提高,吸收峰有向低频移动的趋势,吸波频宽具有增大的趋势。随着纤维截面弧度的增加,C形碳化硅纤维偶极子的偶极矩减小,且C形纤维表面的有效导电表面积减少,介电常数降低,纤维的损耗角正切值降低,纤维的吸收峰有向高频移动的趋势。随纤维比表面积增大,异形截面SiC纤维的介电常数增大。更多还原

【Abstract】 Microwave-absorbing materials have found widespread applications both in the military and civil fields. Structural microwave-absorbing materials perform both as reinforcements to withstand strength and also act as absorbents with excellent absorption ability of electromagnetic wave. Besides having the same excellent mechanical property and resistance to elevated temperature as circular silicon carbide (SiC) fibers, non-circular section SiC fibers also have good microwave-absorbing properties, which make them have potential to be the ideal reinforcement and absorbent in structural microwave-absorbing materials.In this presentation, spinnerets with specific non-circular holes were designed. Starting from polycarbosilane (PCS), after the process of melt-spinning, air-curing, high-temperature treatment, ribbon-like, trilobal, quatrefoil, pentalobal, sexfoil, septfoil, swirl-shaped, T-shaped and C-shaped SiC fibers were elaborated. Influences of spinning procedures on degree of complex profile (Dp) and equivalent diameter (Ed) of non-circular section SiC fibers were studied, and the optimum fabrication processes of them were determined subsequently in this paper. Electromagnetic properties of SiC fibers with different cross-sections were studied, influences of form and content of non-circular SiC fibers as well as electromagnetic matching on microwave-absorbing properties of the composites were also analyzed. Based on these studies, structural microwave-absorbing materials reinforced by non-circular section SiC fibers were prepared, and the microwave-absorbing mechanism of non-circular section SiC fibers was discussed.Ribbon-like, trilobal, quatrefoil, pentalobal, sexfoil, septfoil, swirl-shaped, T-shaped and C-shaped spinneret holes were designed according to polymer moulding theory and rheologic behaviour of PCS. Through the experiments, it was discovered that fibers could have good Dp and suitable Ed when the wide of slit is 0.12-0.14mm and the length-width ratio of slit is 3-4.According to the study of spinning process, Dp of non-circular section SiC fibers reduces while Ed of fibers increases with higher spinning temperature. Extrusion speed and fiber taken-up rate have little influence on Dp of them, while Ed of non-circular fibers reduces if extrusion is slowed down or collection is speeded up. Spinning processes were established like these: the spinning temperature is 75-95℃higher than the softening point of PCS, spinning pressure is between 0.5MPa and 1.3MPa, taken-up rate is between 150rpm and 300rpm.Through suitable air-curing and high-temperature treatment, ribbon-like, trilobal, quatrefoil, pentalobal, sexfoil, septfoil, swirl-shaped, T-shaped SiC fibers and C-shaped fibers with different sectional radian were prepared. Radius of curvature of vanes-shaped SiC fibers changes following secant formula, and the radius of curvature of vane head as well as end of C-shaped fibers is least. With carbon-rich on the surface, non-circular section SiC fibers have the same elements composition and internal structures as circular fibers.Electromagnetic properties of circular, vane-shaped and C-shaped SiC fibers were studied. It was found that real part of normalized dielectric constant of vane-shaped SiC fibers increases 10% while imaginary part increases 15% compared with circular fibers when a pair of vanes is added. The celebration of each vane to the normalized dielectric constant of the fibers with even number vanes is higher than the fibers having odd number vanes. Fiber loss tangent increases with increasing number of the vanes, while absorption peak moves to lower frequency. Real part and imaginary part of normalized dielectric constant as well as loss tangent of C-shaped SiC fibers reduce when the sectional radian increases, the absorption peak moves to higher frequency at the same time.Influences of length, arrangement and content of fibers on electrical parameters and microwave absorbing properties of composites were studied. When length of the fiber increases, electrical parameters of the composites increase. Loss tangent of non-circular SiC fibers increases first along with increase of length, and then decreases with continuative increase. Non-circular SiC fibers distributing randomly have more obvious frequency dispersion effect and larger imaginary part of electrical parameters as well as larger loss tangent compared with order unidirectional fibers. Electrical parameters of composites increase along with increasing of content of fibers, dielectric constant and dielectric loss tangent of the composites also increase.Multilayer structural microwave-absorbing materials reinforced by non-circular section SiC fibers were designed using different SiC fibers through impedance matching. By this method, we can exploit the advantages of different SiC fibers at the same time. The band width of reflectivity can be widened, or the material thickness can be decreased when the microwave-absorbing property is not weakened. And in particular, the microwave-absorbing property at low frequency can be improved. A double layer structural microwave- absorbing composite was fabricated: thickness of the composite was 3.5mm, the minimum reflectivity was -18.62dB while the band width of reflectivity below -10dB was beyond 8GHz.Trilobal SiC fibers/carbonyl iron compound fiber bundles were prepared through adding carbonyl iron in sizing agent. The band width of reflectivity lower than -10dB of composite reinforced by fiber bundles modified by magnetic agent moves to low frequency 2GHz compared with composite reinforced by trilobal SiC fibers. With the additive, the microwave-absorbing properties of composites at low frequency were improved.The microwave-absorbing mechanism of non-circular SiC fibers was discussed preliminarily. Because of the evolution of the surface curvatures of the fibers, non-circular section SiC fibers have different polarization effect under electromagnetic wave. Vane head of vane-shaped SiC fibers and ends of C-shaped fibers have higher surface curvature, more charges gather there, macro-dipoles can be formed afterwards. The formation of macro-dipoles enhances the polarization capacity of the fibers, subsequently increases the electrical parameters of them. When there are more vanes in the vane-shaped SiC fibers, the number of dipoles inside them increases, the normalized dielectric constant and loss tangent increase, while absorption peak moves to lower frequency and microwave-absorbing band width increases. When the sectional radian of C-shaped SiC fibers increases, dipole distance and surface area of effective electric conduction reduces, thus dielectric constant reduces, loss tangent of C-shaped SiC fibers reduce, and absorption peak moves to higher frequency. Dielectric constant of non-circular section SiC fibers increases with surface area.更多还原