Application of hollow porous carbon fiber lightweight absorbing materials in military equipment stealth and civilian electromagnetic radiation protection fields

        AbsorberIt has a wide range of applications in both military and civilian fields, and has become a hot topic in the research of military equipment stealth and civilian electromagnetic radiation protection in various countries. With the development of science and technology, the requirements for absorbing materials are becoming increasingly high. Among them, the preparation of lightweight radar wave absorbers and their absorbing materials has become an important direction for the development of stealth technology.

This article is based on this backgroundLightweight fiber absorbentResearch on its absorbing materials. Starting from the composition of absorbing materials, Advanced Institute Technology analyzed the methods of lightweighting absorbing materials, selected the shape and material system of lightweight absorbers, and for the first time, selected polyacrylonitrile (PAN) - based hollow porous carbon fibers (HPCFs) as the main absorber for the development of lightweight absorbing materials. The influence of pre oxidation and carbonization process conditions on the microstructure of HPCFs was studied using hollow porous PAN precursor fibers as raw materials.

The results indicate that within the scope of the study, the pre oxidation and carbonization temperatures and times have little effect on the microstructure of the original fibers and pre oxidized fibers, while the heating rate has a greater impact on the microstructure of the fibers. High heating rates cause deformation of the fibers inward, leading to the fracture of the transition layer between HPCFs micropores, forming "voids" and reducing the number of micropores. The influence of pre oxidation and carbonization processes on the elemental composition of HPCFs was studied. As the pre oxidation temperature and time increase, the introduced oxygen content increases, and the degree of pre oxidation improves. Under the same conditions, the carbon content of HPCFs obtained by carbonization decreases with the increase of pre oxidation temperature and time, while the nitrogen content increases.

The carbon content in HPCFs increases with the increase of carbonization temperature, while the nitrogen content decreases, consistent with the pre oxidation process. The sintered fibers also contain a certain amount of oxygen. The system studied the effects of pre oxidation and carbonization process conditions on the bulk conductivity of HPCFs and the dielectric constant of HPCFs/paraffin composite materials. The effect of pre oxidation temperature, time, and heating rate on the bulk conductivity of HPCFsHPCFs/Paraffin Composite MaterialsThe dielectric constant has a significant impact. Increasing the pre oxidation temperature, prolonging the time, and increasing the heating rate can all reduce the bulk conductivity of fibers and decrease the real and imaginary parts of the dielectric constant of HPCFs/paraffin composite materials, especially significantly reducing the imaginary part of the dielectric constant.

The carbonization temperature is the main means to regulate the conductivity of HPCFs. Carbonization in the temperature range of 550-950 ℃ can achieve a variation in HPCFs conductivity between 10-3-103 Ω -1 · m-1. The carbonization time increases the bulk conductivity of HPCFs by microliters, and the carbonization heating rate has little effect on it. The real and imaginary parts of the dielectric constant of HPCFs/paraffin composite materials increase with the increase of carbonization temperature, especially the imaginary part of the dielectric constant significantly increases. By adjusting the pre oxidation and carbonization conditions, HPCFs with adjustable bulk conductivity and dielectric constant can be prepared.

The pre oxidation and carbonization processes of HPCFs indicate that the pre oxidation temperature, time, and heating rate are important means to regulate the elemental content and dielectric properties of HPCFs. The carbonization temperature is the main means of regulation, followed by carbonization time and heating rate. The relationship between the composition, structure, and dielectric properties of HPCFs indicates that changes in the electrical conductivity of the fiber body lead to the formation of fibers as the main componentAbsorberThe change in dielectric constant and the change in fiber conductivity are caused by both carbon content and microporous structure, but the change in carbon content plays a major role, while the effect of micropores is relatively small.

Within the scope of the study, the pre oxidation temperature has a significant impact on the apparent density of pre oxidized fibers. When the carbonization temperature varies from 550 ℃ to 950 ℃, the apparent density of HPCFs varies between 0.55-0.79g/cm3. The pre oxidation and carbonization temperatures can be fine tuned within a certain range to adjust the apparent density of HPCFs. Exploration was conducted on the molding process of absorbing materials mainly composed of HPCFs, and lightweight HPCFs absorbing materials with p ≤ 1g/cm3 were prepared by compression molding method. We investigated the effects of HPCFs volume fraction, aspect ratio, and absorbing material thickness on the dielectric constant and reflectivity of HPCFs absorbing materials.

The ε′ and ε″ of HPCFs absorbing materials increase with the increase of volume fraction and aspect ratio of bone marrow HPCFs. When they reach a certain value, the increasing trend weakens. By increasing the volume fraction and aspect ratio of HPCFs, as well as increasing the thickness of the absorbing material, the reflectivity peak of HPCFs absorbing material shifts towards lower frequencies, resulting in improved low-frequency absorption performance. The absorption performance of HPCFs and solid carbon fibers as radar wave absorbers was compared. Under the same conditions, the carbon content in HPCFs obtained is lower than that in the corresponding solid carbon fibers, and the bulk conductivity values of individual fibers differ by about an order of magnitude.

Carbonized at 850 ℃ and 950 ℃Solid fibers as absorbentsThe reflectivity bandwidth of the absorbing material below -10dB is OGHz. The reflectivity bandwidth of the absorbing material using carbonized hollow porous fibers as absorbers at 850 ℃ and 950 ℃ below -10dB is 3.05GHz and 2.62GHz, respectively, which improves the absorbing performance compared to solid fibers. From the perspective of multi-layer absorbing material structure design, explore the broadband of absorbing materials with HPCFs as the main absorbent.

Using the impedance matching principle and layered design, fiber absorbers prepared at different carbonization temperatures were layered. The results showed that the optimized three-layer design of the absorbing material had a minimum reflectivity of -15.33dB in the frequency range of 2-18GHz at d=3mm, corresponding to a frequency of 7.60GHz. The bandwidth with reflectivity ≤ -5dB was 12.36GHz. Compared with single-layer HPCFs radar absorbing materials, the bandwidth of reflectivity ≤ -5dB increases, and the peak reflectivity is lower than that of single-layer absorbing materials.

Using HPCFs as the main absorbent, matching layers were prepared by adding carbon black and carbon fiber as absorbentsDouble layer lightweight absorbing materialAnd its absorption performance was investigated. The results showed that the absorbing material with HPCFs as the absorbent had a minimum reflectivity of -9.25dB at 3.04mm, with a reflectivity bandwidth below -8dB of 2.15GHz. After adding a matching layer of carbon black and carbon fiber, the reflectivity bandwidth below -8dB was 10.60GHz and 10.26GHz, respectively. The influence of frequency selective surface (FSS) on the absorption performance of HPCFs absorbing materials was investigated.

Adding FSS can improve and enhance the absorption performance of HPCFs absorbing materials. The position and size of FSS have a significant impact on the absorption performance. Adjusting the position and size of FSS can prepare HPCFs absorbing materials with a bandwidth of 11.18GHz and a reflectivity below -10dB at a thickness of 3.0mm and a density of 1.0g/cm3. The above research indicates that HPCFs can be used as a lightweight radar wave absorber, and combined with the structural design of the material, lightweight and wideband absorbing materials can be prepared.