Wave-absorbing properties of non-woven fabric coatings
Wave-absorbing properties of non-woven fabric coatings
As the demand for practical applications of radar absorbing materials increases, the use of a single absorbing agent and a single layer of absorbing coating is far from meeting the requirements of thinness, lightness, width, and strength. Therefore, the design and preparation of absorbing materials with multi-component composite absorbing agents and multi-layer structures have become the focus of research. Multi-component composite absorbers are usually composites of two or more absorbers that complement or enhance each other’s electromagnetic properties to expand the absorption band, enhance absorption capabilities, and reduce material density, such as chopped carbon fiber prepared by Shen Guozhu et al.  – Ferrite absorbing material, thickness 2mm, bandwidth less than -10dB in 8~18GHz up to 35GHz. Zhang Xin et al.  prepared a composite absorber by doping carbon black particles with nano-manganese dioxide, which greatly improved the microwave absorption performance of carbon black.
Multilayer absorbing materials generally have a structure from the surface to the bottom layer from the matching layer to the loss layer. Current research reports mostly use two or three layers. The multi-layer structure solves the impedance matching and large attenuation conditions that cannot be satisfied by a single layer at the same time, effectively improving the absorbing performance of the absorbing material. However, the increase in the number of layers often increases the weight and thickness of the material, which brings inconvenience to engineering applications. Therefore, it needs to be improved in combination with other methods. Studies have shown that reasonably designed structures, such as the creation of microscopic holes , macroporous composite structures  or surface three-dimensional structures , can make electromagnetic waves easy to enter the interior of the absorbing material and be effectively attenuated. These geometric structures increase This reduces the probability of interaction between the absorbing material and radar waves, thus reducing the surface density and thickness of the absorbing material to a certain extent.
Fabric materials have the advantages of low surface density, good flexibility, easy processing, and low price. Therefore, compared with the coated absorbing materials on traditional aluminum plates, fabric-coated absorbing materials have a lower density and can be directly covered on various shapes. The complex equipment surface is easy to process and shape according to requirements, and the raw materials are abundant. At the same time, if the mesh pore structure of the fabric is directly used in the structural design of the absorbing material, the propagation route of the electromagnetic wave after entering the interior of the material can be changed, causing the incident electromagnetic wave to suffer a large loss through a series of scattering, reflection and absorption processes inside the absorbing material [ 6], so fabric-coated absorbing materials are expected to develop into a new type of absorbing material. At present, the main function of most electromagnetic wave-related fabric materials is to protect electromagnetic wave radiation, that is, to minimize the transmission of electromagnetic waves and reflect them out of the protected object, but it also causes secondary pollution. Therefore, from a long-term perspective, fabric materials with wave-absorbing capabilities must be developed. Existing fabric absorbing materials are still in the experimental development stage, and are mostly directly woven from fibers with absorbing capabilities . There are few research reports on fabric-coated absorbing materials.
This work uses non-woven fabrics as the test substrate to prepare non-woven fabric-coated absorbing materials, and studies the effects of absorber content changes and internal structure adjustments on non-woven fabrics at 935GHz (the center frequency of the test instrument, which is representative). The influence of absorbing properties of absorbing materials. 1 Sample preparation and test method 11 Raw material test base material is non-woven fabric; some kind of water-based binder; acetone; OP; LDT-20 carbonyl iron powder, Shaanxi Xinghua Chemical Co., Ltd. carbonyl iron powder factory; acetylene carbon black , powdery, Beijing Xinyi Borui Chemical Factory. 12 Sample preparation: Use acetone as the solvent, add an appropriate amount of dispersant OP to disperse different proportions of absorbers (carbonyl iron powder and acetylene carbon black) evenly, ultrasonicate for 10 minutes, or grind with a grinder 2 to 3 times, then add water-based binder The absorbing coating is made from the agent. The non-woven fabric is pre-processed by soaking it in 100% deionized water for 1 hour to remove possible residual glue substances on the surface. Adjust the absorbing coatings with different proportions of absorber content to the appropriate viscosity and apply them evenly on the non-woven fabric. After curing at room temperature, each group of non-woven fabric coated absorbing materials (hereinafter referred to as non-woven absorbing materials or absorbing materials) is obtained.
13 Test method: Cut each group of non-woven absorbing materials into 5mm5mm samples, and select uniformly coated samples for experimental testing. Surface density test: Randomly select 10 pieces from each group of samples, and calculate the average value after measurement. Absorption performance test: each group of samples is stacked in increasing order according to the number of layers, that is, non-woven laminated absorbing materials with different layers are obtained, and measured using the two-point method  on a 3cm waveguide (measured by the DH1121A solid-state 3cm signal source , isolator, attenuator, waveguide measurement line and DH388A0 frequency selective amplifier, etc.) The system tests the reflectivity, and the test frequency is 935GHz. Place the absorbing material between the load interface of the test system and the short-circuit plate, and tighten it with screws to reduce the error caused by the air between the short-circuit plate and the sample and between the layers of the sample . Thickness test: Each group of absorbing materials with increasing layers is sandwiched in the rectangular waveguide and short-circuit plate of the 3cm waveguide measurement system. When testing the absorbing performance, subtract the total thickness of the rectangular waveguide and short-circuit plate from the reading of the digital spiral micrometer. Thickness, that is, the total thickness of the fabric coating when the number of layers increases in sequence, and each measurement result is subtracted to the thickness of a single layer of fabric coating.
Adhesion strength test: Randomly select 3 pieces of each group of samples and fold them repeatedly 100 times. There is no peeling off on the coating surface, indicating that the bonding performance between the coating and the substrate is very good. 2 Results and Discussion 21 Non-woven fabric absorbing material Figure 1 shows the non-woven fabric base material and non-woven fabric absorbing material. After applying the absorbing coating on the non-woven base material (Fig. 1a) (Fig. 1b), it can be seen that the original diamond mesh area has an extra layer of absorbing coating, and the absorbing coating in other parts has the base material.Starting from ��, different laminated structures were used to analyze the impact of internal changes in the absorbing material on incident electromagnetic waves. 23 The influence of different laminated structures on the absorbing performance of materials. In order to study the influence of the internal structure of absorbing materials on the absorbing performance, non-woven absorbing materials with better absorbing ability were selected and interspersed in simple laminated structures without absorbing properties. Wave-capable non-woven fabric matrix, according to different stacking sequences (Figure 9): (a) is an alternating structure of (single layer absorbing coating + single layer matrix); (b) is (n layers of absorbing coating + n-layer matrix) structure; (c) is an alternating structure of (single-layer absorbing coating + double-layer matrix) to study the absorbing properties of different non-woven fabric laminate structures. Figure 10 shows the reflectance curves of three non-woven absorbing materials with different laminate structures. It can be seen that the (single-layer absorbing coating + single-layer matrix) alternating structure has a better absorbing effect than the (n-layer absorbing coating + n-layer matrix) structure. At the same time, the absorbing effect of the alternating structure (single-layer absorbing coating + double-layer matrix) is better than that of the alternating structure (single-layer absorbing coating + single-layer matrix), indicating that changes in the internal structure of the absorbing material can enhance Its ability to absorb waves. In addition, the reflectance curve of each alternating structure (single-layer absorbing coating + double-layer matrix) has an obvious peak (repeated measurements), indicating that the reflectivity is small and the absorption loss is large at this thickness, indicating that this structure The absorbing capacity of the absorbing material has been fully exerted at this thickness. As shown in Figure 10a (single layer absorbing coating + double layer matrix), the peak value of the reflectivity curve of the alternating structure is -118dB. The number of absorbing coating layers After more than 6 layers, the reflectivity can be reduced to below -5dB.
After the three groups of absorbing materials in Figure 10 are interspersed with the matrix material internally, compared with the reflectivity curves of the simple superposition structure of the three groups of absorbing materials in Figure 7, the absorbing capacity has improved. This may be due to the fact that the introduction of a matrix without absorbing ability extends the wave traveling distance of the incident electromagnetic wave inside the absorbing material and increases the probability of interaction between the electromagnetic wave and the absorbing material. In addition, inserting the non-woven matrix material reduces the number of layers of the non-woven absorbing coating under the same reflectivity. Since the non-woven fabric itself has a very small area density, about 0.0185kg/m2, this structure reduces the number of layers of the absorbing material. areal density.
Conclusion (1) A non-woven fabric coated absorbing material is prepared using a laminated structure. The mesh pores of this absorbing material itself can change the propagation route of incident electromagnetic waves, and consume more electromagnetic waves inside the absorbing material through the process of reflection, scattering and absorption. At the same time, the area density of this kind of absorbing material is small, and the area density of a single layer of material is about 02kg/m2. (2) Adjust the composition of the absorbing coating to improve the electromagnetic properties of the composite absorbing agent, thereby improving the absorbing ability of the material. The addition of trace amounts of acetylene carbon black or the increase in the content of carbonyl iron powder can reduce the thickness of the absorbing material under the same reflectivity. (3) Improve the structure of the non-woven absorbing laminate, from the original direct superposition to the non-woven matrix interspersed with no absorbing ability, to improve the absorbing ability of the non-woven absorbing material and at the same time reduce the material areal density. nryijGN8b
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