【作者】 谢炜；

【导师】 陈朝辉；

【作者基本信息】 国防科学技术大学 ， 材料科学与工程， 2008， 博士

【摘要】 吸波材料在军用及民用领域有着广泛的应用,已经成为各国军事装备隐身和民用防电磁辐射领域研究的热点。随着科学技术的发展,对吸波材料的要求越来越高,其中,制备轻质雷达波吸收剂及其吸波材料已成为隐身技术发展的一个重要方向。本文基于此背景开展了轻质纤维吸收剂及其吸波材料的研究工作。论文从吸波材料组成入手,分析了吸波材料轻质化的方法,对轻质吸收剂的形状和材料体系进行选择,首次选取聚丙烯腈（PAN）基中空多孔炭纤维（HPCFs）作为主体吸收剂来进行轻质吸波材料的研制。以中空多孔PAN原丝为原料,研究了预氧化和炭化工艺条件对HPCFs微观形貌的影响。结果表明,在研究范围内,预氧化和炭化温度及时间对原纤维及预氧化纤维微观形貌影响较小,升温速率对纤维微观形貌影响较大,升温速率高,纤维向内发生变形,导致HPCFs微孔之间过渡层断裂,形成“空洞”,减少微孔数目。研究了预氧化和炭化工艺对HPCFs元素组成的影响。预氧化温度及时间增加,引入的氧含量增加,预氧化程度提高,在相同条件下炭化所得到HPCFs的碳含量随预氧化温度和时间的增加而降低,氮含量则提高。HPCFs中碳含量随炭化温度的升高而增加,氮含量降低,与预氧化过程一致,烧成纤维中还含有一定量的氧。系统研究了预氧化和炭化工艺条件对HPCFs体电导率和HPCFs/石蜡复合材料介电常数的影响。预氧化温度、时间及升温速率对HPCFs体电导率和HPCFs/石蜡复合材料介电常数有较大影响,预氧化温度升高、时间延长、升温速率提高均可降低纤维的体电导率,并使HPCFs/石蜡复合材料介电常数的实部与虚部降低,尤其是可以使介电常数的虚部显著降低。炭化温度是调节HPCFs体电导率的主要手段,在550-950℃温度区间炭化,可以实现HPCFs体电导率在10^-3-10³Ω^-1·m^-1之间变化。炭化时间使HPCFs体电导率微升,炭化升温速率对其影响较小。HPCFs/石蜡复合材料的介电常数实部与虚部随炭化温度升高而增加,尤其是介电常数的虚部显著增加。调节预氧化和炭化条件可制备出体电导率和介电常数可调的HPCFs。HPCFs预氧化和炭化过程表明,预氧化温度、时间及升温速率是调节HPCFs元素含量及介电性能的重要手段。炭化温度是调节的主要手段,炭化时间与升温速率效果次之。HPCFs组成、结构及介电性能的关系表明:纤维体电导率的变化导致以纤维为主体的吸波材料介电常数的变化,纤维体电导率的变化由碳含量与微孔结构共同引起,但碳含量的变化起主要作用,微孔作用较小。在所研究的范围内,预氧化温度对预氧化纤维的表观密度影响较大,炭化温度在550℃到950℃变化时,HPCFs表观密度在0.55-0.79g/cm³之间变化,预氧化和炭化温度在一定范围内可微调HPCFs的表观密度。对以HPCFs为主体的吸波材料成型工艺进行探索,采用模压法制备了p≤1g/cm³的轻质HPCFs吸波材料。考察了HPCFs体积分数、长径比和吸波材料厚度对HPCFs吸波材料介电常数和反射率的影响。HPCFs吸波材料的ε′和ε″,髓HPCFs体积分数和长径比的增加而增大,当增加到一定值时,增加趋势减弱。提高HPCFs体积分数和长径比、增加吸波材料厚度,HPCFs吸波材料反射率峰位向低频移动,低频吸波性能改善。将HPCFs与实心炭纤维作雷达波吸收剂时的吸波性能进行了对比。在相同的条件下所得HPCFs中的碳含量低于相应实心炭纤维中的碳含量,单根纤维的体电导率数值相差一个数量级左右。以850℃和950℃时炭化的实心纤维作吸收剂的吸波材料低于-10dB的反射率带宽均为OGHz,以850℃和950℃时炭化的中空多孔纤维作吸收剂的吸波材料低于-10dB的反射率带宽分别为3.05GHz和2.62GHz,较之于实心纤维吸波性能得到了提高。从多层吸波材料结构设计角度出发,对以HPCFs为主体吸收剂的吸波材料宽频化进行探索。利用阻抗匹配原理和分层设计,对不同炭化温度下制备的纤维吸收剂进行分层设计,结果表明三层优化设计后的吸波材料在2-18GHz频率范围内,d=3mm时,最低反射率为-15.33dB,对应的频率为7.60GHz,其反射率≤-5dB的带宽为12.36GHz。与单层HPCFs雷达吸波材料相比,反射率≤-5dB的带宽增加,反射率峰值较单层吸波材料的更低。以HPCFs为主要吸收剂,分别添加以炭黑、炭纤维为吸收剂的匹配层,制备了双层轻质吸波材料,并考察了其吸波性能。结果表明,以HPCFs为吸收剂的吸波材料在3.04mm时,最低反射率为-9.25dB,低于-8dB反射率带宽为2.15GHz,添加炭黑和炭纤维匹配层后低于-8dB反射率带宽分别为10.60GHz和10.26GHz。考察了频率选择表面（FSS）对HPCFs吸波材料吸波性能的影响。添加FSS可以改善和提高HPCFs吸波材料的吸波性能,FSS的位置、尺寸大小对吸波性能影响较大,调节FSS的位置、尺寸大小可以制备在厚度为3.0mm,密度为1.0g/cm³时,反射率低于-10dB的带宽为11.18GHz的HPCFs吸波材料。上述研究表明,HPCFs可作为一种轻质的雷达波吸收剂来使用,结合材料的结构设计可制备出轻质、宽频的吸波材料。更多还原

【Abstract】 Radar absorbing materials（RAMs） have been widely used in stealth technology and electromagnetic compatibility technology, and that is why they are a researching hotspot all over the world. With the development of science and technology, lightweight RAMs have become one of the most important aspects of stealth technology. This dissertation will present thorough investigation on fibrous absorbers and their composites to meet the imminent demanding of lightweight RAMs.The methods for lightening RAMs were discussed according to their compositions and structures. Based on the analysis of the shapes and compositions of absorbers, polyacrylonitrile（PAN）-based hollow-porous carbon fibers（HPCFs） were firstly chosen as the main absorbers in RAMs.The influence of preoxidation and carbonization process on the appearance and micropores of HPCFs were investigated. The results show that the temperature and time have little influence on the appearance and micropores of HPCFs. It is found that the heating rate influences the appearance and micropores of HPCFs greatly. The intergradation layers of dual-layer were ruptured and the micropores reduced when the heating rate is too high.The changes of HPCFs compositions after the preoxidation and carbonization process were studied. The 0 content of hollow-porous cured fibers increased with the increase of curing temperature and curing time. The C content of HPCFs decreased as the curing temperature and curing time increased. The C content of fibers increased with the increase of carbonization temperature and carbonization time while the N content decreased.The volume conductivity of HPCFs and the complex permittivity of the resultant HPCFs/paraffin composites affected by the heating treatment were investigated. The preoxidation temperature, time and heating rate have great influence on the volume conductivity and the complex permittivity. The volume conductivity decreases with the increase of preoxidation temperature, time and heating rate. The real and imaginary parts of the complex permittivity （ε’ andε"） decrease with the increase of preoxidation temperature, time and heating rate. Carbonization temperature is the main factor in adjusting the volume conductivity and complex permittivity of HPCFs. The volume conductivity can be adjusted in range of 10^-3-10³Ω^-1·m^-1 when carbonization temperature changes from 550℃to 950℃. The carbonization time leads to a slight increase in the volume conductivity and complex permittivity while the heating rate has little influence. The volume conductivity of HPCFs and the complex permittivity of the HPCFs/paraffin composites can be adjusted by controlling the heating treatment process. The results of the preoxidation process indicate that the preoxidation temperature, time and heating rate can effectively adjust the composition, volume conductivity and complex permittivity of the HPCFs as well as their composites. Carbonization temperature is the main reason for the above change and carbonization time and heating rate are the secondary factors.The relationship among composition, structure and dielectric properties of HPCFs indicates that the changes of complex permittivity of HPCF composites depend on their volume conductivity. The C content and micropores both caused the change of the volume conductivity, among which the C content is the first factor.The preoxidation temperature has great influence on the apparent density of the cured fibers. The apparent density of HPCFs can be adjusted in range of 0.55-0.79g/cm³ when carbonization temperature changes from 550°C to 950°C. The apparent density of HPCFs can be adjusted by heating treatment conditions.In order to get lightweight RAMs, the HPCF composites were prepared by the mould-pressing technique based on the analysis and experiments. Effect of the volume and the l/a ratio of HPCFs, and the thickness of RAMs on the radar absorbing properties of HPCF composites were studied. Theε’ andε" of HPCF composites increase with the increase of the volume fraction and the l/a ratio of HPCFs firstly, then change slowly. The reflectivity apex moves to the lower frequency with the volume fraction and the l/a ratio of HPCFs and the thickness of materials increasing.The radar absorbing properties of hollow-porous and solid carbon fibers as radar absorbents were comparatively investigated. The results show that the C content of HPCFs is lower than that of solid fibers obtained at the same condition. The volume conductivity of single solid carbon fiber is nearly 10 times that of the HPCF. The -10dB bandwidths of solid carbon fiber composites carbonized at 850°C and 950°C are both 0GHz, while those of the corresponding HPCF composites are up to 3.05GHz and 2.62GHz, respectively. Results indicate that the radar absorbing properties of the HPCF composites are better than those of solid carbon fiber composites.The wideband RAMs based on HPCFs were investigated according to multi-layer structure design. In order to get wideband RAMs, the reflectivity was simulated using RAMCAD software based on resistance matching principles and the multi-layer structure. The optimal reflectivity of three-layered radar absorbing composites of HPCFs and paraffin was -15.33dB at 7.60GHz and the bandwidth under -5dB was about 12.36GHz. The radar absorbing properties of three-layered HPCF composites are better than those of single layer composites.Two-layered radar absorbing materials with the carbon black and the short carbon fibers were prepared based on HPCF composites. The obtained materials have the bandwidths of 10.60 GHz and 10.26 GHz under -8dB with the carbon black and the short carbon fibers as the matching layer, provided that the thickness is 3.04mm. The bandwidth under -8dB of HPCF composites has only 2.15GHz and the lowest reflectivity is -9.25dB at the same time.A series of HPCF composites embedded with frequency selective surface （FSS） were investigated. The results indicate that the radar absorbing properties of HPCF composites with FSS can be improved. The results also show that both the size of the element and locations of the FSS in the composites are critical for the reflection property of the HPCF composites. The HPCF composites embedded FSS has a bandwidth of 11.18GHz under -10dB in the range of 2-18GHz, provided that the thickness and the density are 3.0mm and 1.0g/cm³, respectively.HPCFs are proved to be light conductive absorbers in view of their hollow structure and radar absorption properties. The results in this dissertation indicate that the lightweight and wideband composites based on HPCFs can be gained with suitable structure design.更多还原