A microwave absorbing honeycomb with a pyramidal electrical structure and a method for preparing the same

Abstract

The present invention proposes an absorbing honeycomb with a pyramidal electrical structure and a preparation method thereof, which is composed of an absorbing aramid paper coated with an absorbent and a resin impregnated on the surface of the aramid paper. According to the electrical performance design, a pyramidal absorbing layer pattern is coated on the surface of the aramid paper along the width direction to obtain different absorbing aramid papers with gradient electromagnetic properties; the absorbing aramid paper is prepared into a corrugated structure by a template method, and an absorbing honeycomb is prepared according to the stacking sequence designed for electrical performance. The absorbing honeycomb has a pyramidal electrical structure along the pore direction, and the electromagnetic properties change gradiently. It has excellent absorbing performance in an ultra-wide frequency range in the pore direction, meeting the requirements of structural / stealth integration in sandwich structures. The present invention prepares the absorbing honeycomb by the template method, which can ensure the uniformity of the pore size and overcome the problem of uneven expansion of the pores when the laminated plates containing different types of aramid paper are stretched under the existing preparation process.

Description

Technical Field

[0001] The invention relates to a wave-absorbing honeycomb with a pyramidal electrical structure and a preparation method thereof, belonging to the technical field of wave-absorbing material preparation. Background Art

[0002] Aramid paper honeycomb is a new type of lightweight structural sandwich material with the advantages of low density, high mechanical properties, and good heat resistance. It is mainly used in lightweight sandwich structures and has been widely used in aircraft. With the development of detection technology, aircraft have been asked to have a wider absorption band and a stronger absorption effect. Aramid paper honeycomb has excellent electromagnetic wave transmission performance, but does not have electromagnetic wave absorption performance, so it cannot be directly used in the field of wave absorption. The conventional idea is to achieve this by introducing electromagnetic wave absorbers on the aramid paper wave-transmitting honeycomb wall.

[0003] At present, most of the existing aramid paper absorbing honeycombs are made by first preparing ordinary aramid paper without electromagnetic loss function into aramid paper transparent honeycombs, then impregnating the honeycomb walls of the aramid paper transparent honeycombs with electromagnetic wave absorbent glue, and finally curing and forming. Alternatively, by gradient impregnation, the absorbent is gradiently distributed along the direction of the honeycomb grid to obtain an impedance matching structure and obtain better absorbing performance along the grid direction. However, the absorbing honeycomb obtained by the impregnation method has the absorbent attached to the hole wall, which is easy to fall off, and has poor environmental resistance. In addition, the impregnation process is difficult to control and has poor process stability. The impregnation process cannot apply large aspect ratio carbon fiber and magnetic fiber absorbents, resulting in low magnetic loss performance of the aramid paper absorbing honeycomb. In addition, the impregnation process has poor controllability, resulting in low stability of the electromagnetic performance of the aramid paper absorbing honeycomb.

[0004] The absorbing sponge pyramid and absorbing honeycomb pyramid have excellent absorbing performance due to their pyramidal electrical structure, but their pyramidal structure cannot bear loads, which greatly limits their application range. At present, most of them adopt the method of foaming in the pyramidal absorbing structure to obtain a flat physical appearance and a pyramidal electrical structure, so as to achieve the bearing capacity, such as patents CN209929492U and CN112143023A. This method is a two-step method, and the performance is difficult to stably control, and the obtained flat absorbing material contains two different materials, which will bring adverse effects on environmental resistance. CN112492867A discloses a absorbing honeycomb with a pyramid-like structure and a preparation method, in which honeycomb sheets with different electromagnetic properties are stacked and bonded, and then filled with non-absorbing honeycomb sheets to form a pyramid-like absorbing structure and a flat overall appearance. The absorbing honeycomb is also in the form of a multi-layer structure in physics, which will affect its interlayer mechanical properties, and has the disadvantages of complex molding process and unreliable mechanical properties.

[0005] At the same time, in order to avoid the shortcomings of the impregnated absorbing honeycomb, a method of adding a carbon fiber absorber to aramid paper is proposed in a patent for dielectric loss aramid paper, absorbing honeycomb and preparation method (CN112553942A) to obtain absorbing aramid paper, and then prepare absorbing honeycomb. The advantages of this method are that the absorber is in the aramid paper, the environmental resistance is good, the amount of absorber added can be accurately controlled, and the process is stable; however, this method is subject to the mixing papermaking process, and only fiber absorbers can be added, and the amount added should not be too high. It does not have excellent absorbing effect in an ultra-wide frequency range, and generally the low-frequency effect is poor; and using the traditional honeycomb preparation process, the characteristics and tension of different areas of the absorbing aramid paper are different, and it is difficult to ensure the uniformity of the honeycomb grid. Another method is to apply an absorbent on aramid paper to obtain absorbing aramid paper, and then prepare an absorbing honeycomb, such as patents CN109796624A, CN202111440881.5, and CN114214871A; this method can add various types of absorbents, and a large amount of absorbents can be added to meet different absorbing needs; the absorbent is applied to the surface of the aramid paper to obtain absorbing aramid paper, and then the absorbing honeycomb is obtained by the traditional honeycomb preparation method. However, when preparing the absorbing honeycomb by this method, it is necessary to stretch it to form a hexagonal grid. When the absorbent coating has insufficient adhesion, debonding is likely to occur. At present, both of the above methods can obtain absorbing honeycombs with stable performance and excellent environmental resistance, but the absorbent content in the honeycomb is fixed, and the absorbing structure is not designed. The absorbing honeycomb as a whole presents a single electromagnetic characteristic, and there is no impedance matching design, so it is difficult to obtain excellent ultra-wideband absorbing performance.

[0006] The traditional honeycomb preparation method has high requirements for aramid paper, which has the same tension. When the aramid paper is stretched after lamination, it must be stretched at a uniform speed to ensure the uniformity of the pores. However, adding different amounts of absorbent to the aramid paper will result in different tensions of the aramid paper in different areas. When stretched, some areas will have full pores, while other areas will not be stretched, which seriously affects the uniformity of the overall pores of the honeycomb and causes the failure of honeycomb preparation. Patent A method for preparing a polyimide film honeycomb (CN112223772A) discloses a template for preparing a honeycomb. First, a thermoplastic polyurethane composite film is molded to obtain a unit with a corrugated structure, and then multiple units are stacked and heated to obtain a polyimide film honeycomb product. This method provides another idea for honeycomb preparation, but aramid paper itself does not have the ability to be thermoformed, and this method cannot be used directly for honeycomb preparation. Summary of the invention

[0007] The purpose of the present invention is to overcome the differences in the prior art and provide an absorbing honeycomb with a pyramidal electrical structure and a preparation method thereof. The absorbing honeycomb has excellent absorbing performance, mechanical properties and environmental resistance, and has excellent absorbing performance in an ultra-wide frequency range.

[0008] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0009] A wave-absorbing honeycomb with a pyramidal electrical structure, comprising a plurality of wave-absorbing aramid papers B1, B2, B3...Bm and a cured resin impregnated on the wave-absorbing aramid papers; each wave-absorbing aramid paper is composed of aramid paper and a pyramidal pattern wave-absorbing layer composed of an absorbent and an adhesive coated on the surface of the aramid paper, the pyramidal pattern wave-absorbing layer is composed of strip wave-absorbing layers of different lengths connected in sequence along the width direction of the aramid paper with their side lengths from small to large; the strip wave-absorbing layers have A1, A2, A3...An in total according to their lengths from small to large, and the percentage content of the absorbent is gradient from low to high; the wave-absorbing aramid paper B1 contains a total of wave-absorbing layers A1, A2, A3...An, and the wave-absorbing aramid paper B2 contains only strip wave-absorbing layers wave layers A2, A3...An, the absorbing aramid paper B3 only contains strip absorbing layers A3...An, and so on; the absorbing aramid papers are all in a uniform semi-porous shape, and the absorbing aramid papers are pasted together in a half-space shape in the order of Bm...B3, B2, B1, B2, B3...Bm to form an absorbing honeycomb of a pyramidal electrical structure; the absorbing honeycombs of the pyramidal electrical structure are one or more, and when there are more than one, the absorbing aramid paper Bm is shared at the junction; each type of absorbing aramid paper in each absorbing honeycomb of the pyramidal electrical structure is one or more, and when there are more than one, the more than one are pasted together; the gradient direction of the absorbent percentage content of the absorbing honeycomb from low to high is the incident direction of the electromagnetic wave.

[0010] A method for preparing a microwave absorbing honeycomb having a pyramidal electrical structure comprises the following steps:

[0011] 1) Honeycomb structure design: Determine the type of absorbent according to the absorbing frequency band, and use electromagnetic simulation software to calculate the best gradient absorbing structure and pyramidal structure, determine the electromagnetic characteristics and pyramidal size of each gradient layer in the absorbing honeycomb, and then determine the number of various absorbing aramid papers, the length, width, thickness of the strip absorbing layer, and the content of the absorbent, and then proceed to the subsequent steps based on the calculation results;

[0012] 2) Preparation of absorbing aramid paper: different amounts of absorbent and adhesive are mixed to form absorbing slurry, which is then coated on the surface of aramid paper and cured to form strip absorbing layers with different absorbent percentage contents, thereby obtaining various absorbing aramid papers B1, B2, B3...Bm;

[0013] 2) Impregnating the absorbing aramid paper with resin: completely immersing the absorbing aramid paper in the resin, taking it out and placing it vertically, and vibrating to remove excess resin, and then performing a semi-curing treatment;

[0014] 3) Preparing the corrugated structure unit: using a pre-designed pressing mold to press the semi-cured absorbing aramid paper to obtain a semi-porous shaped absorbing aramid paper, i.e., the corrugated structure unit, and coating the core strip glue on the convex edges between the semi-porous grids;

[0015] 4) Corrugated unit stacking: in the order of Bm...B3, B2, B1, B2, B3...Bm, the absorbing aramid paper is placed in a half-space shape, the width of each corrugated structure unit is aligned and bonded by the core strip glue to obtain a absorbing honeycomb semi-finished product;

[0016] 5) Heating treatment: placing the semi-finished absorbing honeycomb into a mold, inserting rods of the same shape into the honeycomb cells, applying a certain pressure to the honeycomb cell walls to ensure the bonding strength of the core strip glue, and then heating and curing to obtain an absorbing honeycomb with a gradient distribution of absorbent perpendicular to the direction of the honeycomb cells.

[0017] Furthermore, the content of the absorbent is 0.1-90wt%, and the thickness of the wave absorbing layer is 0.01-0.3mm.

[0018] Furthermore, the absorbent has electromagnetic loss capability and is one or more of carbon black, graphite flakes, carbon fiber, silica, carbonyl iron powder, ferrite, sendust, etc., preferably at least one of carbon black, carbon fiber, carbonyl iron powder, sendust, etc.

[0019] Furthermore, for the electromagnetic wave frequency band absorbed below 8 GHz, the absorber is selected from a magnetic absorber, i.e., at least one of carbonyl iron powder, ferrite, and sendust; for the electromagnetic wave frequency band absorbed above 8 GHz, the absorber is selected from a dielectric absorber, i.e., at least one of carbon black, graphite flakes, carbon fiber, and silica; for ultra-wide or full-band absorption of electromagnetic waves, the absorber is simultaneously selected from at least one of the magnetic absorbers and at least one of the dielectric absorbers.

[0020] Furthermore, the resin is selected from phenolic resin and polyimide resin, preferably phenolic resin.

[0021] Furthermore, the immersion time of the wave-absorbing aramid paper is 5 to 30 minutes; after removing the excess resin by vibration, the weight gain ratio is 50% to 300%.

[0022] Furthermore, the semi-curing treatment conditions are: the semi-curing temperature is 120 to 400° C., and the semi-curing time is 0.5 to 3 hours.

[0023] Furthermore, the conditions for heating and curing are: heating temperature is 180-450° C., and curing time is 3-10 hours.

[0024] Furthermore, the shape of the honeycomb cells is one of hexagonal, rectangular, circular, and irregular, preferably hexagonal.

[0025] The beneficial effects of the present invention compared with the prior art are as follows:

[0026] (1) The present invention coats the absorbent on the surface of aramid paper, and the amount and pattern shape of the absorbing layer can be precisely controlled, thereby achieving precise control of the absorbing structure. The process is relatively easy to implement and stable, and the obtained absorbing honeycomb has excellent absorbing performance, mechanical properties and environmental resistance, thereby overcoming the shortcomings of the existing impregnated absorbing honeycomb, such as the performance of the corner cone is not easy to be stably controlled, easy to fall off, and cannot bear loads.

[0027] (2) The present invention coats the absorbent on the surface of aramid paper, and different types of absorbents can be added to meet different absorbing requirements; the absorbent can be added in large quantities, and has excellent absorbing effect in an ultra-wide frequency range, overcoming the shortcomings of the existing absorbent-doped aramid paper absorbing honeycomb, such as limited absorbent types (only fibrous) and low absorbent content.

[0028] (3)) The traditional honeycomb preparation process is suitable for situations where the properties of aramid paper are consistent, and it can ensure that the cells are stretched at the same time and have uniform shapes. The same absorbing aramid paper in the present invention has different properties at different positions (the surface of the aramid paper is coated with a pyramidal absorbing layer). When the traditional lamination and stretching process is used, due to the different properties such as tension at different positions of the aramid paper, it is very easy for the honeycomb cells at different positions to not be stretched at the same time, resulting in uneven cell shapes or stretching failure. The present invention prepares absorbing honeycombs through a template method, which can ensure the uniformity of cell size, and overcomes the problems of uneven cell expansion and easy debonding when stretching laminated plates containing different types of aramid paper under the existing preparation process.

[0029] (4) In the present invention, different pyramidal patterns are coated on the surface of aramid paper in the horizontal direction (width direction) to prepare an absorbing honeycomb, forming an electrical structure with a pyramidal shape in the pore direction (thickness of the absorbing honeycomb), which is physically in the shape of an integrated flat plate, thereby overcoming the shortcomings of the traditional absorbing honeycomb impedance matching structure, complex molding process, and unreliable interlayer performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 It is a schematic diagram of the preparation process of the wave-absorbing honeycomb with a pyramid-shaped electrical structure of the present invention;

[0031] Figure 2 The present invention is a flow chart for preparing a wave-absorbing honeycomb having a pyramid-shaped electrical structure;

[0032] Figure 3 Schematic diagram of the wave-absorbing honeycomb structure of Comparative Example 1;

[0033] Figure 4 Schematic diagram of the wave-absorbing honeycomb structure of Comparative Example 2. DETAILED DESCRIPTION

[0034] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below through specific embodiments and drawings.

[0035] The present invention proposes an absorbing honeycomb with a pyramidal electrical structure, comprising a plurality of absorbing aramid papers B1, B2, B3...Bm (m≥3) and a cured resin impregnated on the absorbing aramid papers; each absorbing aramid paper is composed of aramid paper and a pyramidal pattern absorbing layer composed of an absorbent and an adhesive coated on the surface of the aramid paper, the pyramidal pattern absorbing layer is composed of strip absorbing layers of different lengths connected in sequence along the width direction of the aramid paper with their side lengths from small to large; the strip absorbing layers have A1, A2, A3...An (n≥3) in total according to their lengths from small to large, and the percentage content of the absorbent is gradient from low to high; the absorbing aramid paper B1 contains strip absorbing layers A1, A2, A3...An in total, ... The paper B2 only contains strip-shaped absorbing layers A2, A3...An, the absorbing aramid paper B3 only contains strip-shaped absorbing layers A3...An, and so on; the absorbing aramid papers are all in a uniform semi-porous shape, and the absorbing aramid papers are pasted together in a semi-space shape in the order of Bm...B3, B2, B1, B2, B3...Bm to form an absorbing honeycomb of a pyramidal electrical structure; the absorbing honeycomb of the pyramidal electrical structure is one or more, and when there are more than one, the absorbing aramid paper Bm is shared at the junction; each type of absorbing aramid paper in each absorbing honeycomb of the pyramidal electrical structure is one or more, and when there are more than one, the more than one are pasted together; the gradient direction of the absorbent percentage content of the absorbing honeycomb from low to high is the incident direction of the electromagnetic wave.

[0036] The present invention also provides a method for preparing a microwave absorbing honeycomb having a pyramidal electrical structure. The preparation process is as follows: Figure 1 and Figure 2 As shown, it is implemented by the following steps:

[0037] 1) Honeycomb structure design:

[0038] Conduct electrical performance design and determine the type of absorbent according to the absorbing frequency band; use electromagnetic simulation software to calculate and optimize the optimal gradient absorbing structure and pyramid structure of the absorbing honeycomb, and determine the electromagnetic properties of each gradient layer and the size of the pyramid in the absorbing honeycomb; thereby determine the content of absorbent in each absorbing layer in the absorbing aramid paper and the thickness of the absorbing layer, the length and width of the absorbing layer in each aramid paper, and the stacking order of different aramid papers.

[0039] 2) Preparation of microwave-absorbing aramid paper:

[0040] The aramid paper used in this step is composed of aramid paper and absorbing materials. The absorbent in the absorbing layer has electromagnetic loss capability and is one or more of carbon black, graphite sheet, carbon fiber, silicon dioxide, carbonyl iron powder, ferrite, and sendust. The type of absorbent in the aramid paper is determined according to the absorbing frequency band requirements.

[0041] The wave-absorbing layer is coated on the surface of the aramid paper and has a pyramidal pattern in the transverse direction (the width direction of the aramid paper). Different pyramidal patterns can obtain different wave-absorbing aramid papers.

[0042] Different types and contents of absorbents are mixed with adhesives to obtain different absorbing slurries; different absorbing slurries are coated on the surface of aramid paper according to the designed pyramidal pattern, and an absorbing layer is formed after drying and curing, wherein the absorbent content is 0.1-90wt%, and the thickness of the absorbing layer is 0.01-0.3mm. Strip absorbing layers A1, A2, A3, A4...An with different electromagnetic properties are coated on the aramid paper along the width direction, and each absorbing layer forms a pyramid shape and is periodically distributed along the length direction of the aramid paper to obtain absorbing aramid paper B1. Reduce one absorbing layer A1 to obtain absorbing aramid paper B2; and so on, gradually reduce the number of absorbing layers until only the absorbing layer An is left to obtain absorbing aramid paper Bm. The coating process of the present invention is a well-known technology in the art, and the specific process parameters are determined according to actual production needs.

[0043] 3) Absorbing aramid paper impregnated with resin:

[0044] The resin in this step is a phenolic resin, one of the polyimide resins, preferably a phenolic resin. The resin impregnation time is 5 to 30 minutes, which is determined according to the state of the absorbing aramid paper to ensure that the absorbing aramid paper can be completely impregnated. The impregnated aramid paper is placed vertically and vibrated to remove excess resin to ensure that the resin increases in weight to control the density of the final absorbing honeycomb. The weight gain ratio is 50% to 300%, and the specific weight gain is determined according to the density of the absorbing honeycomb.

[0045] The aramid paper soaked in resin is then purified, the solvent is removed, and semi-cured to make the aramid paper thermoplastic. The semi-curing temperature is 120-400°C, the semi-curing time is 0.5-3h, and the specific process parameters are selected according to the resin system.

[0046] 4) Preparation of corrugated structural units:

[0047] The structural parameters of the corrugated structural unit are determined according to the shape of the honeycomb cells. The shape of the honeycomb cells is determined according to actual needs, and is generally one of hexagonal, rectangular, circular, and irregular shapes, preferably hexagonal.

[0048] The corrugated structural unit is designed in the shape of a pressing mold according to the determined honeycomb cell shape, and the semi-cured absorbing aramid paper is continuously pressed through the mold. During the pressing process, core strip glue is applied to the straight side or edge of the corrugated unit.

[0049] The corrugated structural unit determines the shape of the final honeycomb cell; by preparing corrugated structural units of different shapes through molds, the final honeycomb cell shape can be hexagonal, circular, square, irregular, etc.

[0050] 5) Corrugated unit superposition:

[0051] The prepared corrugated units are stacked, and the absorbing aramid papers need to be aligned during the stacking process to ensure that the gradient absorbing layers in each aramid paper can be aligned along the width direction of the aramid paper. When the corrugated units are stacked, the two corrugated units are aligned and glued by core strip glue to form a complete honeycomb grid with periodic arrangement. The stacking order of different aramid papers is Bm···B3, B2, B1, B2, B3···Bm, and the number of layers of each aramid paper is determined according to the electrical performance design. The straight edges of the two corrugated units are glued by core strip glue.

[0052] 6)Heat treatment:

[0053] Place the bonded and arranged corrugated units in the mold, insert rods of the same shape as the honeycomb holes into the honeycomb holes, and apply a certain amount of pressure to the honeycomb hole wall to ensure the bonding strength of the core strip glue. Perform a heating treatment at a temperature of 180 to 450°C and a curing time of 3 to 10 hours. The specific curing process parameters are determined according to the core strip glue and the resin. On the one hand, ensure that the core strip glue is fully bonded, and on the other hand, ensure that the impregnated resin is fully cured to obtain an absorbing honeycomb with a pyramidal electrical structure in the direction of the honeycomb holes (such as Figure 1 As shown), the absorbing honeycomb can obtain excellent absorbing effect in an ultra-wide frequency range in the gradient change direction.

[0054] According to the electrical performance design, the present invention coats different pyramidal patterns on the surface of aramid paper in the horizontal direction (width direction), and the electromagnetic properties of the absorbing layer change in a gradient; the type of absorbing slurry coated and the shape of the pyramid of the absorbing layer are changed to obtain different absorbing aramid papers with gradient electromagnetic properties; a corrugated structure is prepared by a template method, and an absorbing honeycomb is prepared according to the stacking sequence designed for electrical performance, forming an electrical structure with a pyramidal shape along the pore direction (thickness direction of the absorbing honeycomb), and the electromagnetic properties change in a gradient. The absorbing honeycomb has excellent absorbing performance in an ultra-wide frequency range in the pore direction. The absorbing honeycomb is in an integrated flat plate shape as a whole, and has a pyramidal electrical structure inside the absorbing honeycomb, and the electromagnetic characteristic impedance of the honeycomb pyramid changes gradually.

[0055] The innovation of the present invention is that an impedance matching design is performed in the width direction of the aramid paper to obtain an absorbing aramid paper with a gradient impedance of electromagnetic properties along the width direction, and the absorbing honeycomb is obtained by lamination; it has an electromagnetic gradient structure along the pore direction (honeycomb thickness direction) to meet the requirements of ultra-wideband stealth. After the absorbing aramid paper has different electromagnetic properties at different positions and is laminated, if the traditional stretching process is used, it will cause uneven stretching deformation of the honeycomb pores, thereby causing uneven honeycomb pores in the final formation. Therefore, the present invention adopts a template method to prepare an absorbing honeycomb with an impedance matching structure.

[0056] The present invention is described in detail below with reference to the accompanying drawings and specific examples.

[0057] Example 1

[0058] According to the demand, the electromagnetic wave frequency band targeted by the absorbing honeycomb is determined to be mainly 8-40GHz, and excellent absorbing performance is required along the honeycomb grid direction (honeycomb thickness direction) in this frequency band. According to the absorbing frequency band, the absorbent in the absorbing aramid paper is determined to be carbon black particles.

[0059] The electrical performance design was carried out, and the electromagnetic simulation software was used to calculate and optimize the best gradient absorbing structure and pyramid structure. The thickness of the absorbing layer was determined to be 0.01 mm, and the mass ratios of carbon black in the absorbing layer were 0.5%, 1%, 2%, 4%, and 8%, respectively. The width of the aramid paper was 230 mm, and the middle effective area was 200 mm. The aramid paper was divided into 5 areas in the transverse (width) direction, and the transverse width of each area was 40 mm. The absorbing slurry with absorbent content of 0.5%, 1%, 2%, 4%, and 8% was coated respectively. Strip absorbing layers A1, A2, A3, A4, and A5 were obtained on the absorbing aramid paper. The lengths of each absorbing layer were 10 mm, 20 mm, 30 mm, 40 mm, and 50 mm, respectively, forming a cone, and the absorbing aramid paper B1 was obtained, which had a structure with a gradient of electromagnetic properties in the transverse (width direction). On the basis of the B1 absorbing layer structure, the strip absorbing layer A1 is removed to obtain the absorbing aramid paper B2; the strip absorbing layers A1 and A2 are removed to obtain the absorbing aramid paper B3; the strip absorbing layers A1, A2 and A3 are removed to obtain the absorbing aramid paper B4; the strip absorbing layers A1, A2, A3 and A4 are removed to obtain the absorbing aramid paper B5.

[0060] Electromagnetic simulation software is used to calculate and optimize the optimal gradient absorbing structure and pyramid structure of the absorbing honeycomb, and the electromagnetic properties of each gradient layer and the size of the pyramid in the absorbing honeycomb are determined; thereby determining the content of absorbent in each absorbing layer in the absorbing aramid paper and the thickness of the absorbing layer, the length and width of the absorbing layer in each aramid paper, and the stacking order of different aramid papers.

[0061] The aramid paper was impregnated with phenolic resin for 10 minutes. After impregnation with the resin, it was placed vertically for 15 minutes, and the excess resin was removed by vibration, and the weight was increased by 50%. The aramid paper was then placed in a clean room and kept blowing and exhausting for 2 hours to completely remove the solvent in the resin. The aramid paper was heated to 120°C and kept warm for 30 minutes to obtain the aramid paper in a semi-cured state and obtain the ability to be thermoformed.

[0062] Prepare the corrugated structure unit. The absorbing structure is designed based on the electrical properties of the absorbing honeycomb, and the final honeycomb cell shape is determined to be a hexagon with a side length of 2.77 mm. The corrugated unit is half of the hexagonal cell and is distributed periodically. The absorbing aramid paper is pressed using a mold to obtain the corrugated structure unit; at the same time, the core strip glue is applied to the straight edge of the hexagonal corrugated structure unit.

[0063] Corrugated unit stacking. During the stacking process, the absorbing aramid paper needs to be aligned to ensure that the gradient absorbing layers in each aramid paper can be aligned along the width direction of the aramid paper. According to the electrical performance design, the stacking order of the corrugated units is B5, B4, B3, B2, B1, B2, B3, B4, B5, forming a group of corrugated units, and periodically stacking is performed in this order. When the corrugated units are stacked, the two corrugated units are aligned and glued by the core strip glue at the straight edge to form a complete hexagonal honeycomb grid, which is arranged periodically.

[0064] Heat treatment. Place the bonded and arranged corrugated units in the mold, insert hexagonal metal bars into the honeycomb holes, and apply a certain amount of pressure to the honeycomb hole wall to ensure the bonding strength of the core strip glue. Heat treatment is performed, heating to 180°C, and keeping warm for 3 hours to ensure that the core strip glue and phenolic resin can be completely cured. Cool down to below 60°C, remove the metal bars, and obtain an absorbing honeycomb with a pyramidal electrical structure in the direction of the honeycomb holes. Peel the absorbing honeycomb to obtain an absorbing honeycomb with an effective thickness of 200mm.

[0065] The pores of the absorbing honeycomb are hexagonal, evenly distributed, without any unopened pores, and the tolerance of the pore length is controlled within the range of ±0.15mm; along the pore direction, the compressive strength of the absorbing honeycomb is 3.8MPa; along the pore direction (thickness direction), the reflectivity is tested, 1-2GHz reflectivity ≤-15dB, 2-8GHz reflectivity ≤-20dB, 8-18GHz reflectivity ≤-35dB, 18-40GHz reflectivity ≤-40dB. The absorbing honeycomb has excellent absorbing effect in the required electromagnetic wave frequency band (8-40GHz).

[0066] Example 2

[0067] According to the demand, the electromagnetic wave frequency band targeted by the absorbing honeycomb is determined to be mainly 1-8 GHz, and excellent absorbing performance is required along the honeycomb grid direction (honeycomb thickness direction) in this frequency band. According to the absorbing frequency band, the absorbent in the absorbing aramid paper is determined to be carbonyl iron powder.

[0068] The electrical performance design is carried out, and the electromagnetic simulation software is used to calculate and optimize the best gradient absorbing structure and pyramid structure of the absorbing honeycomb. The thickness of the absorbing layer is determined to be 0.25 mm, and the carbonyl iron powder content in the absorbing layer is 5%, 10%, 20%, 40%, and 60%, respectively. The shape of the honeycomb cell is rectangular, with a long side of 6 mm and a short side of 3 mm. During the preparation process, the absorbing aramid paper is impregnated for 5 minutes; after vibrating to remove excess resin, the weight gain ratio is 100%; the semi-curing temperature is 200°C, and the semi-curing time is 3 hours; the heating temperature is 200°C, and the curing time is 10 hours; the rest is the same as in Example 1, and an absorbing honeycomb with a pyramidal electrical structure in the direction of the honeycomb hole is obtained.

[0069] The pores of the absorbing honeycomb are rectangular, the pores are evenly distributed, there is no unopened pore, and the pore length tolerance is controlled within the range of ±0.15mm; along the pore direction, the absorbing honeycomb has a compression strength of 4.0MPa; along the pore direction (thickness direction), the reflectivity is tested, 0.5~1GHz reflectivity ≤-10dB, 1~2GHz reflectivity ≤-25dB, 2~8GHz reflectivity ≤-30dB, 8~18GHz reflectivity ≤-18dB, 18~40GHz reflectivity ≤-15dB. The absorbing honeycomb has excellent absorbing effect in the required electromagnetic wave frequency band (1~8GHz).

[0070] Example 3

[0071] According to the demand, the electromagnetic wave frequency band targeted by the absorbing honeycomb is determined to be mainly 0.5-40GHz, and excellent absorbing performance is required in the direction perpendicular to the honeycomb grid within this frequency band. According to the absorbing frequency band, the absorbents in the absorbing aramid paper are determined to be carbon black and flaky iron silicon aluminum absorbent.

[0072] The electrical performance design is carried out, and the electromagnetic simulation software is used to calculate and optimize the best gradient absorbing structure and pyramid structure of the absorbing honeycomb. The thickness of the absorbing layer is determined to be 0.3 mm, and the carbon black content in the absorbing layer is 1%, 2%, 4%, 8%, and 12% respectively; wherein the absorbing aramid paper with a carbon black content of 2%, 4%, 8%, and 12% is added with sheet-like iron silicon aluminum magnetic material, and the mass fraction is 20%, 30%, 50%, and 70% respectively. During the preparation process, the absorbing aramid paper is impregnated for 30 minutes; after vibrating to remove excess resin, the weight gain ratio is 300%; the semi-curing temperature is 400°C, the semi-curing time is 2 hours; the heating temperature is 450°C, and the curing time is 5 hours; the rest is the same as in Example 1, and an absorbing honeycomb with a pyramidal electrical structure in the direction of the honeycomb hole is obtained.

[0073] The wave-absorbing honeycomb has hexagonal cells, evenly distributed cells, no unopened cells, and a cell side length tolerance within the range of ±0.15mm. Along the cell direction, the compression strength is 4.2MPa. The reflectivity is tested along the cell direction (honeycomb thickness direction), 0.5-1GHz reflectivity ≤-20dB, 1-2GHz reflectivity ≤-25dB, 2-8GHz reflectivity ≤-30dB, 8-18GHz reflectivity ≤-35dB, 18GHz-40GHz reflectivity ≤-40dB. The wave-absorbing honeycomb has excellent wave-absorbing effect within the required electromagnetic wave frequency band (0.5-40GHz).

[0074] Comparative Example 1: The absorbing layers in the absorbing aramid paper are uniformly distributed in gradients, completely covering the surface of the aramid paper. The rest is the same as in Example 1, and an absorbing honeycomb is obtained, and the structure is as follows: Figure 3 The honeycomb cells are hexagonal and evenly distributed, with no unopened cells. The cell side length tolerance is controlled within the range of ±0.15mm. Along the cell direction, the wave-absorbing honeycomb compression strength is 3.3MPa. The reflectivity is tested along the cell direction (thickness direction), 1~2GHz reflectivity ≤-5dB, 2~8GHz reflectivity ≤-12dB, 8~18GHz reflectivity ≤-25dB, 18~40GHz reflectivity ≤-30dB.

[0075] Comparative Example 2: The carbon black content in each absorbing layer is 2%, and the rest is the same as in Example 1, to obtain an absorbing honeycomb, the electrical structure of the honeycomb pyramid inside of which is as follows: Figure 4 The wave-absorbing honeycomb cells are hexagonal, the cells are evenly distributed, there is no cell opening, and the cell side length tolerance is controlled within the range of ±0.15mm; along the cell direction, the compression strength is 3.5MPa; along the cell direction (honeycomb thickness direction), the reflectivity is tested, 1~2GHz reflectivity ≤-5dB, 2~8GHz reflectivity ≤-15dB, 8~18GHz reflectivity ≤-23dB, 18~40GHz reflectivity ≤-32dB.

[0076] Comparative Example 3: Using the traditional preparation method of the absorbing honeycomb, the absorbing aramid paper is subjected to the process of gluing the core strip, laminating, hot pressing, stretching, dipping, drying and curing, and the rest is the same as Example 1 to obtain the absorbing honeycomb. The absorbing honeycomb has a hexagonal cell, but the cell morphology is unevenly distributed, there is a phenomenon that some areas in the honeycomb cells are not opened and some honeycomb cells are debonded, the cell side length fluctuates greatly, and the tolerance is within the range of ±2.0mm; along the cell direction, the compression strength is 2.2MPa; along the cell direction, the reflectivity is tested, 1~2GHz reflectivity ≤-3dB, 2~8GHz reflectivity ≤-12dB, 8~18GHz reflectivity ≤-20dB, 18~40GHz reflectivity ≤-20dB.

[0077] Parts of the present invention that are not described in detail are well known to those skilled in the art.

[0078] The specific embodiments and drawings of the present invention disclosed above are intended to help understand the content of the present invention and implement it accordingly. It can be understood by those skilled in the art that various substitutions, changes and modifications are possible without departing from the spirit and scope of the present invention. The present invention should not be limited to the contents disclosed in the embodiments and drawings of this specification, and the scope of protection of the present invention shall be subject to the scope defined in the claims.

Claims

1. A microwave absorbing honeycomb with a pyramidal electrical structure, characterized in that: The invention comprises a plurality of aramid papers B1, B2, B3...Bm and solidified resin impregnated on the aramid papers; each aramid paper comprises aramid paper and a pyramidal pattern absorbing layer composed of an absorbent and an adhesive coated on the surface of the aramid paper, wherein the pyramidal pattern absorbing layer comprises strip absorbing layers of different lengths which are connected in sequence along the width direction of the aramid paper with their side lengths from small to large; the strip absorbing layers have A1, A2, A3...An in total according to their lengths from small to large, and the percentage content of the absorbent is gradient from low to high; the aramid paper B1 contains strip absorbing layers A1, A2, A3...An in total, and the aramid paper B2 only contains strip absorbing layers A2, A3 …An, the absorbing aramid paper B3 only contains strip absorbing layers A3…An, and so on; the absorbing aramid papers are all in a uniform semi-porous grid shape, and the absorbing aramid papers are pasted together in a semi-space shape in the order of Bm…B3, B2, B1, B2, B3…Bm to form an absorbing honeycomb of a pyramidal electrical structure; the absorbing honeycomb of the pyramidal electrical structure is one or more, and when there are more than one, the absorbing aramid paper Bm is shared at the junction; each type of absorbing aramid paper in each absorbing honeycomb of the pyramidal electrical structure is one or more, and when there are more than one, the more than one are pasted together; the gradient direction of the absorbent percentage content of the absorbing honeycomb from low to high is the incident direction of the electromagnetic wave; The steps of preparing the wave-absorbing honeycomb include: 1) Honeycomb structure design: Determine the type of absorbent according to the absorbing frequency band, and use electromagnetic simulation software to calculate the best gradient absorbing structure and pyramidal structure, determine the electromagnetic characteristics and pyramidal size of each gradient layer in the absorbing honeycomb, and then determine the number of various absorbing aramid papers, the length, width, thickness of the strip absorbing layer, and the content of the absorbent, and then proceed to the subsequent steps based on the calculation results; 1) Preparation of absorbing aramid paper: different amounts of absorbent and adhesive are mixed to form absorbing slurry, which is then coated on the surface of aramid paper and cured to form strip absorbing layers with different absorbent percentage contents, thereby obtaining various absorbing aramid papers B1, B2, B3...Bm; 2) Impregnating the absorbing aramid paper with resin: completely immersing the absorbing aramid paper in the resin, taking it out and placing it vertically, and vibrating to remove excess resin, and then performing a semi-curing treatment; 3) Preparing the corrugated structure unit: using a pre-designed pressing mold to press the semi-cured absorbing aramid paper to obtain a semi-porous shaped absorbing aramid paper, i.e., the corrugated structure unit, and coating the core strip glue on the convex edges between the semi-porous grids; 4) Corrugated unit stacking: in the order of Bm...B3, B2, B1, B2, B3...Bm, the absorbing aramid paper is placed in a half-space shape, the width of each corrugated structure unit is aligned and bonded by the core strip glue to obtain a absorbing honeycomb semi-finished product; 5) Heating treatment: placing the semi-finished absorbing honeycomb into a mold, inserting rods of the same shape into the honeycomb cells, applying a certain pressure to the honeycomb cell walls to ensure the bonding strength of the core strip glue, and then heating and curing to obtain an absorbing honeycomb with a gradient distribution of absorbent perpendicular to the direction of the honeycomb cells.

2. The wave-absorbing honeycomb according to claim 1, characterized in that: The content of the absorbent is 0.1-90wt%, and the thickness of the wave absorbing layer is 0.01-0.3mm.

3. The wave-absorbing honeycomb according to claim 1, characterized in that: The absorbent has electromagnetic loss capability and is selected from at least one of carbon black, graphite sheet, carbon fiber, silicon dioxide, carbonyl iron powder, ferrite, and sendust; the resin is selected from one of phenolic resin and polyimide resin.

4. A method for preparing a microwave-absorbing honeycomb having a pyramidal electrical structure, used for preparing the microwave-absorbing honeycomb according to claim 1, characterized in that: The following steps are involved: 1) Honeycomb structure design: Determine the type of absorbent according to the absorbing frequency band, and use electromagnetic simulation software to calculate the best gradient absorbing structure and pyramidal structure, determine the electromagnetic characteristics and pyramidal size of each gradient layer in the absorbing honeycomb, and then determine the number of various absorbing aramid papers, the length, width, thickness of the strip absorbing layer, and the content of the absorbent, and then proceed to the subsequent steps based on the calculation results; 1) Preparation of absorbing aramid paper: different amounts of absorbent and adhesive are mixed to form absorbing slurry, which is then coated on the surface of aramid paper and cured to form strip absorbing layers with different absorbent percentage contents, thereby obtaining various absorbing aramid papers B1, B2, B3...Bm; 2) Impregnating the absorbing aramid paper with resin: completely immersing the absorbing aramid paper in the resin, taking it out and placing it vertically, and vibrating to remove excess resin, and then performing a semi-curing treatment; 3) Preparing the corrugated structure unit: using a pre-designed pressing mold to press the semi-cured absorbing aramid paper to obtain a semi-porous shaped absorbing aramid paper, i.e., the corrugated structure unit, and coating the core strip glue on the convex edges between the semi-porous grids; 4) Corrugated unit stacking: in the order of Bm...B3, B2, B1, B2, B3...Bm, the absorbing aramid paper is placed in a half-space shape, the width of each corrugated structure unit is aligned and bonded by the core strip glue to obtain a absorbing honeycomb semi-finished product; 5) Heating treatment: placing the semi-finished absorbing honeycomb into a mold, inserting rods of the same shape into the honeycomb cells, applying a certain pressure to the honeycomb cell walls to ensure the bonding strength of the core strip glue, and then heating and curing to obtain an absorbing honeycomb with a gradient distribution of absorbent perpendicular to the direction of the honeycomb cells.

5. The preparation method according to claim 4, characterized in that: The content of the absorbent is 0.1-90wt%, and the thickness of the wave absorbing layer is 0.01-0.3mm.

6. The preparation method according to claim 4, characterized in that: The absorbent has electromagnetic loss capability and is selected from at least one of carbon black, graphite flakes, carbon fibers, silicon dioxide, carbonyl iron powder, ferrite, and sendust.

7. The preparation method according to claim 4, characterized in that: For electromagnetic waves absorbed in a frequency band lower than 8 GHz, the absorber is selected from magnetic absorbers, i.e., at least one of carbonyl iron powder, ferrite, and sendust; for electromagnetic waves absorbed in a frequency band higher than 8 GHz, the absorber is selected from dielectric absorbers, i.e., at least one of carbon black, graphite sheets, carbon fibers, and silica; for ultra-wide or full-band absorption of electromagnetic waves, the absorber is selected from both at least one of the magnetic absorbers and at least one of the dielectric absorbers.

8. The preparation method according to claim 4, characterized in that: The resin is selected from phenolic resin and polyimide resin.

9. The preparation method according to claim 4, characterized in that: The immersion time of the wave-absorbing aramid paper is 5 to 30 minutes; after the excess resin is removed by vibration, the weight gain ratio is 50% to 300%.

10. The preparation method according to claim 4, characterized in that: The conditions for the semi-curing treatment are: the semi-curing temperature is 120-400° C., and the semi-curing time is 0.5-3 hours; the conditions for the heating curing are: the heating temperature is 180-450° C., and the curing time is 3-10 hours.

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