An absorbing honeycomb with an impedance matching structure and its preparation method

Abstract

The present invention provides a wave-absorbing honeycomb with an impedance matching structure and a preparation method thereof, which is composed of wave-absorbing aramid paper coated with absorbents with different contents in the width direction and a resin impregnated on the surface of the aramid paper. According to the electrical performance design, different types and contents of absorbents are coated on the surface layer of the aramid paper in the width direction to obtain wave-absorbing aramid paper with a gradient change in transverse electromagnetic characteristics; the wave-absorbing aramid paper is prepared into a wave-absorbing honeycomb by a template method to form a structure with a gradually changing impedance gradient in the cell direction, and a wave-absorbing honeycomb with excellent wave-absorbing performance in a super-wide frequency range in the cell direction is obtained. The wave-absorbing honeycomb prepared by the present invention has an impedance matching structure in the cell direction, overcoming the disadvantages of the traditional wave-absorbing honeycomb impedance matching structure, such as complex forming process and unreliable interlayer performance.

Description

Technical Field

[0001] The present invention relates to a wave-absorbing honeycomb with an impedance matching 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, which has the advantages of low density, high mechanical properties, good heat resistance, etc. It is mainly used in lightweight sandwich structures and has been widely used in aircraft. With the development of detection technology, there is a stealth requirement for aircraft with a wider wave-absorbing frequency band and stronger absorption effect. Aramid paper honeycomb has excellent electromagnetic wave transmission performance and does not have electromagnetic wave absorption performance, so it cannot be directly used in the wave-absorbing field. The conventional idea is to achieve it by introducing an electromagnetic wave absorber on the wave-transparent honeycomb wall of aramid paper.

[0003] At present, most of the existing aramid paper wave-absorbing honeycombs are realized through a process of first preparing ordinary aramid paper without electromagnetic loss function into an aramid paper wave-transparent honeycomb, then impregnating the honeycomb wall of the aramid paper wave-transparent honeycomb with an electromagnetic wave absorber glue solution, and finally curing and forming. Or through the method of gradient impregnation, the absorber is gradient-distributed in the direction along the honeycomb cell to obtain an impedance matching structure and better wave-absorbing performance in the direction along the cell. However, for the wave-absorbing honeycomb obtained by the impregnation method, the absorber adheres to the pore wall, is easy to fall off, has poor environmental resistance, and the impregnation process is difficult to control and has poor process stability. The impregnation process cannot apply carbon fibers and magnetic fiber absorbers with a large aspect ratio, resulting in low magnetic loss performance of the aramid paper wave-absorbing honeycomb. In addition, the controllability of the impregnation process is poor, resulting in low electromagnetic performance stability of the aramid paper wave-absorbing honeycomb.

[0004] To avoid the disadvantages of the impregnated wave-absorbing honeycomb, in the patent Dielectric Loss Aramid Paper, Wave-Absorbing Honeycomb and Preparation Method (CN112553942A), a method of adding carbon fiber absorbents to aramid paper is proposed to obtain wave-absorbing aramid paper, and then a wave-absorbing honeycomb is prepared. The advantages of this method are that the absorbents are in the aramid paper, with good environmental resistance, the addition amount of the absorbents can be precisely controlled, and the process is stable. However, this method is restricted by the blending papermaking process, can only add fibrous absorbents, and the addition amount should not be too high. Another method is to coat absorbents on aramid paper to obtain wave-absorbing aramid paper, and then prepare a wave-absorbing honeycomb, such as in patents CN109796624A, CN202111440881.5, and CN114214871A. This method can add various types of absorbents and can add a large amount of absorbents to meet different wave-absorbing requirements. The absorbents are coated on the surface of the aramid paper to obtain wave-absorbing aramid paper, and then a wave-absorbing honeycomb is obtained using the traditional honeycomb preparation method. However, when preparing the wave-absorbing honeycomb by this method, stretching is required to form hexagonal cell grids, and when the adhesion of the absorbent coating is insufficient, debonding is likely to occur. Currently, both of the above methods can obtain wave-absorbing honeycombs with stable performance and excellent environmental resistance, but the absorbent content in the honeycomb is fixed, no wave-absorbing structure design is carried out, the wave-absorbing honeycomb as a whole presents a single electromagnetic characteristic, without impedance matching design, and it is difficult to obtain excellent ultra-wideband wave-absorbing performance.

[0005] In the patent Wave-Absorbing Paper and Its Preparation Method and Application (CN104404814A), wave-absorbing paper with different absorbent contents is first prepared to make a wave-absorbing sandwich core, and then the wave-absorbing sandwich core is combined with a wave-transparent layer and cured by impregnating resin to obtain a wave-absorbing material. The wave-absorbing honeycomb obtained by this method has a multi-layer structure physically and has the disadvantages of poor and unreliable interlayer mechanical properties. In the patent A Paper-Based Material with Transverse Gradient Distribution of Electromagnetic Characteristics, Its Manufacturing Method and Application (CN114606794A), wave-absorbing aramid paper with a gradient distribution of absorbents in the cross-machine direction is first prepared, and then a wave-absorbing honeycomb is prepared, and the obtained wave-absorbing honeycomb has a gradient distribution of electromagnetic characteristics in the thickness direction. This method uses absorbent-doped aramid paper to prepare wave-absorbing aramid paper, which limits the types of absorbents to fibrous ones only, and the addition amount should not be too large. It does not have excellent wave-absorbing effects in the ultra-wide frequency range, generally with poor low-frequency effects. Moreover, using the traditional honeycomb preparation process, the characteristics and tensions of different regions of the wave-absorbing aramid paper are different, and it is difficult to ensure the uniformity of the honeycomb cell grids.

[0006] Traditional honeycomb preparation methods have high requirements for aramid paper, with the same tension. When stretching the stacked aramid paper, it is necessary to proceed at a constant speed to ensure the uniformity of the cell pattern. However, when different amounts of absorbent are incorporated into the aramid paper, the tension of the aramid paper in different regions will be different, and when stretching, the cell pattern in some regions will be fully stretched, while the cell pattern in other regions has not been stretched yet, seriously affecting the overall uniformity of the honeycomb cell pattern and resulting in the failure of honeycomb preparation. A method for preparing a polyimide film honeycomb (CN112223772A) discloses a method for preparing a honeycomb using a template. First, a thermoplastic polyurethane composite film is molded by pressing to obtain units with a corrugated structure, and then multiple units are stacked and heat-treated to obtain a polyimide film honeycomb product. This method provides another idea for honeycomb preparation, but aramid paper itself does not have thermoplasticity and cannot directly use this method for honeycomb preparation. Summary of the Invention

[0007] The object of the present invention is to overcome the deficiencies of the prior art and provide an absorbing honeycomb with an impedance matching structure and a preparation method thereof. The absorbing honeycomb has excellent absorbing performance and environmental resistance performance and is easy to perform impedance matching type design, and has excellent absorbing performance in an ultra-wide frequency range.

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

[0009] An absorbing honeycomb with an impedance matching structure includes multiple sheets of absorbing aramid paper and cured resin impregnated on the absorbing aramid paper; an absorbing layer composed of an absorbent and an adhesive is coated on each sheet of absorbing aramid paper. The absorbing layer is composed of strip-shaped absorbing layers arranged along the width direction of the absorbing aramid paper, and the absorbent content of each strip-shaped absorbing layer shows a gradient difference along the width direction; each sheet of absorbing aramid paper has a unified semi-cell shape, and adjacent sheets of absorbing aramid paper are aligned and pasted; the absorbing honeycomb shows a gradient change in electromagnetic characteristics along the cell direction, and the direction along the cell direction and with the absorbent content increasing from low to high is used as the electromagnetic wave incident direction.

[0010] A preparation method of an absorbing honeycomb with an impedance matching structure includes the following steps:

[0011] 1) Honeycomb structure design: Determine the type of absorbent according to the absorbing frequency band, use electromagnetic simulation software to calculate the optimal gradient-changing absorbing structure, determine the thickness of the honeycomb along the electromagnetic wave incident direction, and determine the thickness, width and absorbent content of each strip-shaped absorbing layer, and then perform subsequent steps based on the calculation results;

[0012] 2) Preparation of absorbing aramid paper: Mix different amounts of absorbent and adhesive to make an absorbing slurry, and then coat it on the surface of the aramid paper in strip form and cure it to form an absorbing layer with a gradient change in absorbent along the width direction of the aramid paper to obtain absorbing aramid paper;

[0013] 3) Impregnating the wave-absorbing aramid paper with resin: Completely immerse the wave-absorbing aramid paper in the resin, take it out and place it vertically, vibrate to remove the excess resin, and then perform semi-curing treatment;

[0014] 4) Preparing the corrugated structure unit: Press the semi-cured wave-absorbing aramid paper with a pre-designed pressing mold to obtain the wave-absorbing aramid paper with a semi-hole lattice shape, i.e., the corrugated structure unit, and coat the core strip adhesive on the convex edges between the semi-hole lattices;

[0015] 5) Stacking the corrugated units: Align each corrugated structure unit along the width direction of the wave-absorbing aramid paper and bond them with the core strip adhesive to obtain a semi-finished wave-absorbing honeycomb;

[0016] 6) Heat treatment: Place the semi-finished wave-absorbing honeycomb in a mold, insert rods with the same shape into the honeycomb holes, apply a certain pressure to the honeycomb hole walls to ensure the bonding strength of the core strip adhesive, and then perform heat curing to obtain a wave-absorbing honeycomb with an absorber gradient distribution perpendicular to the honeycomb hole direction.

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

[0018] Further, the absorber has electromagnetic loss ability and is selected from at least one of carbon black, graphite flakes, carbon fiber, silica, carbonyl iron powder, ferrite, and iron silicon aluminum, preferably at least one of carbon black, carbon fiber, carbonyl iron powder, and iron silicon aluminum.

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

[0020] Further, the resin is selected from one of phenolic resin and polyimide resin, preferably phenolic resin.

[0021] Further, the impregnation time of the wave-absorbing aramid paper is 5 - 30 min; after vibrating to remove the excess resin, the weight gain ratio is 50% - 300%.

[0022] Further, 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 h.

[0023] Further, the conditions for heat curing are as follows: the heating temperature is 180 - 450°C, and the curing time is 3 - 10 h.

[0024] Further, the shape of the honeycomb cell is one of a hexagon, a rectangle, a circle, and an irregular shape, preferably a hexagon.

[0025] Advantages of the present invention compared with the prior art:

[0026] (1) In the present invention, the absorbent is coated on the surface of the aramid paper, the addition amount can be precisely controlled, the process is stable, and the obtained wave-absorbing honeycomb has excellent environmental resistance, overcoming the disadvantages of the existing impregnated wave-absorbing honeycomb that its performance is not easily stably controlled and is prone to falling off.

[0027] (2) In the present invention, the absorbent is coated on the surface of the aramid paper, and different types of absorbents can be added to meet different wave-absorbing requirements; a large amount of absorbent can be added, and it has excellent wave-absorbing effects in an ultra-wide frequency range, overcoming the disadvantages of the existing aramid paper doped with absorbent wave-absorbing honeycomb that the types of absorbents are limited (only fibrous) and the content of the absorbent is not high.

[0028] (3) The traditional honeycomb preparation process is by stretching after laminating, which is applicable to the case where the properties of the aramid paper are consistent, and can ensure that the cells are simultaneously opened and have a uniform shape. In the same wave-absorbing aramid paper in the present invention, the properties at different positions are different (different wave-absorbing layers are coated on the surface of the aramid paper). When using the traditional lamination and stretching process, due to the different properties such as tension at different positions of the aramid paper, it is very easy to occur that different positions in the thickness direction of the same honeycomb cell cannot be simultaneously opened, resulting in non-uniform cell shapes or stretching failure. The present invention prepares the wave-absorbing honeycomb by the template method, which can ensure the uniformity of the cell sizes at different positions in the thickness direction, overcoming the problems of non-uniform 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 types and contents of absorbents are coated along the transverse direction (the width direction of the aramid paper) to obtain a wave-absorbing aramid paper with a gradient change in transverse electromagnetic characteristics. The prepared wave-absorbing honeycomb has an impedance matching structure in the cell direction (the thickness direction of the honeycomb) and has excellent wave-absorbing effects in an ultra-wide frequency range, overcoming the disadvantages of the traditional wave-absorbing honeycomb impedance matching structure, complex forming process, and unreliable interlayer performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] Figure 1 is a schematic diagram of the preparation process of the wave-absorbing honeycomb with an impedance matching structure of the present invention;

[0031] Figure 2 is a flowchart of the preparation of the wave-absorbing honeycomb with an impedance matching structure of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0032] In order to make the above objects, features and advantages of the present invention more obvious and understandable, the present invention will be further described in detail below through specific embodiments and drawings.

[0033] The present invention provides a wave-absorbing honeycomb with an impedance matching structure, which includes multiple sheets of wave-absorbing aramid paper and cured resin impregnated on the wave-absorbing aramid paper; a wave-absorbing layer composed of an absorbent and an adhesive is coated on each sheet of wave-absorbing aramid paper. The wave-absorbing layer is composed of strip-shaped wave-absorbing layers arranged along the width direction of the wave-absorbing aramid paper, and the absorbent content of each strip-shaped wave-absorbing layer shows a gradient difference along the width direction; each sheet of wave-absorbing aramid paper has a unified semi-hole lattice shape, and adjacent sheets of wave-absorbing aramid paper are aligned and pasted; the wave-absorbing honeycomb shows a gradient change in electromagnetic characteristics along the hole lattice direction, and the direction along the hole lattice direction and with the absorbent content increasing from low to high is used as the incident direction of electromagnetic waves.

[0034] The present invention also provides a preparation method of a wave-absorbing honeycomb with an impedance matching structure. The preparation process is as Figure 1 and 2 shown, and is specifically realized through the following steps:

[0035] 1) Honeycomb structure design:

[0036] Conduct electrical performance design, use electromagnetic simulation software to calculate and optimize to obtain the best gradient-changing wave-absorbing structure, determine the electromagnetic characteristics of each gradient layer in the wave-absorbing honeycomb and the dimensions along the incident direction of electromagnetic waves (honeycomb thickness direction), so as to determine the thickness of each wave-absorbing layer and the content of the absorbent, and the width of each wave-absorbing layer along the width direction of the aramid paper.

[0037] 2) Preparation of wave-absorbing aramid paper:

[0038] The wave-absorbing aramid paper in this step is composed of non-wave-absorbing aramid paper and a wave-absorbing layer. The absorbent in the wave-absorbing layer has electromagnetic loss ability and is one or several of carbon black, graphite flakes, carbon fiber, silicon dioxide, carbonyl iron powder, ferrite, iron silicon aluminum, etc. According to the requirements of the wave-absorbing frequency band, determine the type of absorbent in the wave-absorbing aramid paper.

[0039] Mix absorbents of different types and contents with an adhesive to obtain different wave-absorbing slurries; respectively coat different wave-absorbing slurries on the surface of the aramid paper, and form a wave-absorbing layer after drying and curing, where the absorbent content is 0.1-90 wt%, and the thickness of the wave-absorbing layer is 0.01-0.3 mm. Strip-shaped wave-absorbing layers A1, A2, A3, A4···An with different electromagnetic characteristics are coated on the aramid paper along the width direction, forming a structure with a gradient change in electromagnetic characteristics in the transverse (width direction). After drying and curing, wave-absorbing aramid paper is obtained. The coating process of the present invention is a well-known technology in the art, and specific process parameters are determined according to actual production needs.

[0040] 3) Impregnating the wave-absorbing aramid paper with resin:

[0041] The resin in this step is one of phenolic resin and polyimide resin, preferably phenolic resin. The impregnation time of the resin is 5 - 30 min, which is determined according to the state of the wave-absorbing aramid paper to ensure that the wave-absorbing aramid paper can be completely infiltrated. Place the infiltrated aramid paper vertically and vibrate to remove the excess resin to ensure the resin weight gain and control the density of the final wave-absorbing honeycomb. The weight gain ratio is 50% - 300%, and the specific weight gain is determined according to the density of the wave-absorbing honeycomb.

[0042] Then, the wave-absorbing aramid paper impregnated with resin is subjected to purification treatment. After removing the solvent, it is semi-cured to endow the wave-absorbing aramid paper with thermoplasticity. The semi-curing temperature is 120 - 400 °C, and the semi-curing time is 0.5 - 3 h. The specific process parameters are selected according to the resin system.

[0043] 4) Preparing the corrugated structure unit:

[0044] The structural parameters of the corrugated structure unit are determined according to the shape of the honeycomb cell. The shape of the honeycomb cell is determined according to actual needs, generally one of hexagon, rectangle, circle, and irregular shape, preferably hexagon.

[0045] For the corrugated structure unit, according to the determined shape of the honeycomb cell, design the shape of the pressing die, and continuously press the semi-cured wave-absorbing aramid paper through the die. During the pressing process, apply the core strip adhesive at the straight edges or edges of the corrugated unit.

[0046] The corrugated structure unit determines the final shape of the honeycomb cell; by preparing corrugated structure units of different shapes through the die, the final obtained honeycomb cell shapes can be hexagon, circle, square, irregular shape, etc.

[0047] 5) Stacking of corrugated units:

[0048] Stack the prepared corrugated units. During the stacking process, align the wave-absorbing aramid paper to ensure that each gradient wave-absorbing layer in each aramid paper can be aligned along the width direction of the aramid paper. When stacking the corrugated units, two corrugated units are aligned and glued through the core strip adhesive to form a complete honeycomb cell, arranged periodically.

[0049] 6) Heat treatment:

[0050] Place the corrugated units after adhesive arrangement into the mold, insert rods with the same shape as the honeycomb holes into the honeycomb holes, and at the same time, a certain pressure can be applied to the honeycomb hole walls to ensure the bonding strength of the core bar adhesive. Perform heat treatment, with the heating temperature being 180 - 450 °C and the curing time being 3 - 10 h. The specific curing process parameters are determined according to the core bar adhesive and the resin. On the one hand, ensure that the core bar adhesive is completely bonded, and on the other hand, ensure that the impregnated resin is completely cured to obtain a wave-absorbing honeycomb with an absorbent gradient distribution in the direction of the honeycomb holes (such as Figure 1 shown), and this wave-absorbing honeycomb can obtain excellent wave-absorbing effects within an ultra-wide frequency range in the gradient change direction.

[0051] Combining the characteristics of the electromagnetic modified aramid paper wave-absorbing honeycomb, the present invention proposes a wave-absorbing honeycomb with an absorbent coated on the surface of the aramid paper and its preparation method. According to the electrical performance design, wave-absorbing layers with different absorbent types and contents are coated on the surface of the aramid paper along the transverse direction (width direction) to obtain a wave-absorbing aramid paper with a gradient change in transverse electromagnetic characteristics; the wave-absorbing aramid paper is prepared into a wave-absorbing honeycomb by the template method to form an electrical structure with a gradually changing impedance gradient along the cell direction, and a wave-absorbing honeycomb with excellent wave-absorbing performance within an ultra-wide frequency range in the cell direction (honeycomb thickness) is obtained, meeting the requirements of structure / stealth integration in the sandwich structure.

[0052] The innovation of this invention patent lies in that impedance matching type design is carried out in the width direction of the aramid paper to obtain a wave-absorbing aramid paper with a gradually changing impedance in electromagnetic characteristics along the width direction, and then laminating to obtain a wave-absorbing honeycomb; it has an electromagnetic gradient structure along the cell direction (honeycomb thickness direction), meeting the requirements of ultra-wideband stealth. It is not achieved by the method of gradient impregnation, but by the integrated preparation of the honeycomb. Different positions of the wave-absorbing aramid paper have different electromagnetic characteristics. If the traditional stretching process is used after lamination, it will cause uneven stretching deformation of the honeycomb cells, resulting in uneven honeycomb cells finally formed. Therefore, the template method is adopted in this invention patent to prepare a wave-absorbing honeycomb with an impedance matching structure.

[0053] The following further details the present invention with reference to the drawings and specific examples.

[0054] Example 1

[0055] According to the requirements, it is determined that the electromagnetic wave frequency band targeted by the wave-absorbing honeycomb is mainly 8 - 40 GHz, and excellent wave-absorbing performance is required in this frequency band along the cell direction (honeycomb thickness direction). According to the wave-absorbing frequency band, it is determined that the absorbent in the wave-absorbing aramid paper is carbon black particles.

[0056] Conduct electrical performance design, use electromagnetic simulation software to calculate and optimize to obtain the best gradient-changing wave-absorbing structure. Determine that the thickness of the wave-absorbing layer is 0.01 mm, and the mass ratios of carbon black in the wave-absorbing layer are 0.5%, 1%, 2%, 4%, and 8% respectively. The width of the aramid paper is 230 mm, the middle effective area is 200 mm, and it is divided into 5 regions in the transverse direction. The transverse width of each region is 40 mm, and wave-absorbing slurries with absorbent contents of 0.5%, 1%, 2%, 4%, and 8% are respectively coated to obtain strip wave-absorbing layers A1, A2, A3, A4, and A5 on the wave-absorbing aramid paper. The wave-absorbing aramid paper has a structure with a gradient change in electromagnetic characteristics in the transverse (width direction).

[0057] Immerse the wave-absorbing aramid paper in phenolic resin for 10 min; after immersion, let it air-dry vertically for 15 min, vibrate to remove the excess resin, and the weight increases by 50%. Then place the wave-absorbing aramid paper in a purification room, keep blowing air and exhausting for 2 h to completely remove the solvent in the resin. Heat the wave-absorbing aramid paper to 120 °C and keep it warm for 30 min to obtain the wave-absorbing aramid paper in a semi-cured state and obtain the thermoplastic forming ability.

[0058] Prepare the corrugated structure unit. According to the wave-absorbing structure designed based on the electrical performance of the wave-absorbing honeycomb, determine that the final honeycomb cell shape is hexagonal with a side length of 2.77 mm. The corrugated unit is half of the hexagonal cell and is distributed periodically. Use a mold to press the wave-absorbing aramid paper to obtain the corrugated structure unit; at the same time, apply core strip glue outside the straight edges of the hexagonal corrugated structure unit.

[0059] Stack the corrugated units. During the stacking process, align the wave-absorbing aramid paper to ensure that the gradient wave-absorbing layers in each aramid paper can be aligned along the width direction of the aramid paper. When stacking the corrugated units, align and bond two corrugated units through the core strip glue at the straight edges to form a complete hexagonal honeycomb cell, which is arranged periodically. The number of stacked layers is 1000 layers.

[0060] Heat treatment. Place the bonded and arranged corrugated units in a mold, insert metal bars in the honeycomb cells in the shape of hexagons, and at the same time, a certain pressure can be applied to the honeycomb cell walls to ensure the bonding strength of the core strip glue. Conduct heat treatment, raise the temperature to 180 °C, and keep it warm for 3 h to ensure that the core strip glue and phenolic resin can be completely cured. Cool down to below 60 °C, and the metal bars can be taken out to obtain a wave-absorbing honeycomb with a gradient distribution of absorbent in the honeycomb cell direction. Peel the wave-absorbing honeycomb to obtain a wave-absorbing honeycomb with an effective thickness of 200 mm.

[0061] The wave-absorbing honeycomb cell is hexagonal, with the cells evenly distributed and no unopened cells. The dimensional tolerance of the side length of the cells is controlled within the range of ±0.15 mm. Along the cell direction, the compressive strength of the wave-absorbing honeycomb is 3.0 MPa. When measuring the reflectivity along the cell direction (thickness direction), the reflectivity is ≤ -5 dB in the range of 1 - 2 GHz, ≤ -12 dB in the range of 2 - 8 GHz, ≤ -25 dB in the range of 8 - 18 GHz, and ≤ -30 dB in the range of 18 - 40 GHz. The wave-absorbing honeycomb has excellent wave-absorbing effect in the required electromagnetic wave frequency band (8 - 40 GHz).

[0062] Example 2

[0063] According to requirements, it is determined that the electromagnetic wave frequency band targeted by the wave-absorbing honeycomb is mainly 1 - 8 GHz, and excellent wave-absorbing performance is required along the honeycomb cell direction (honeycomb thickness direction) within this frequency band. According to the wave-absorbing frequency band, carbonyl iron powder is determined as the absorber in the wave-absorbing aramid paper.

[0064] Electrical performance design is carried out. Using electromagnetic simulation software, the optimal gradient-varying wave-absorbing structure is calculated and optimized. The thickness of the wave-absorbing layer is determined to be 0.3 mm, and the carbonyl iron powder contents in the wave-absorbing layer are 5%, 10%, 20%, 40%, and 80% respectively. The cell shape is rectangular, with the long side being 4 mm and the short side being 2 mm. During the preparation process, the impregnation time of the wave-absorbing aramid paper is 5 min; after vibrating to remove the excess resin, the weight gain ratio is 100%; the semi-curing temperature is 200 °C, and the semi-curing time is 3 h; the heating temperature is 200 °C, and the curing time is 10 h; the rest is the same as in Example 1, and a wave-absorbing honeycomb with a gradient distribution of the absorber along the honeycomb hole direction (honeycomb thickness direction) is obtained.

[0065] The wave-absorbing honeycomb cell is rectangular, with the cells evenly distributed and no unopened cells. The dimensional tolerance of the side length of the cells is controlled within the range of ±0.15 mm. Along the cell direction, the compressive strength of the wave-absorbing honeycomb is 3.4 MPa. When measuring the reflectivity along the cell direction (honeycomb thickness direction), the reflectivity is ≤ -5 dB in the range of 0.5 - 1 GHz, ≤ -20 dB in the range of 1 - 2 GHz, ≤ -25 dB in the range of 2 - 8 GHz, ≤ -15 dB in the range of 8 - 18 GHz, and ≤ -10 dB in the range of 18 - 40 GHz. The wave-absorbing honeycomb has excellent wave-absorbing effect in the required electromagnetic wave frequency band (1 - 8 GHz).

[0066] Example 3

[0067] According to requirements, it is determined that the electromagnetic wave frequency band targeted by the wave-absorbing honeycomb is mainly 0.5 - 40 GHz, and excellent wave-absorbing performance is required perpendicular to the honeycomb cell direction within this frequency band. According to the wave-absorbing frequency band, carbon black and flaky iron silicon aluminum absorber are determined as the absorbers in the wave-absorbing aramid paper.

[0068] Conduct electrical performance design, use electromagnetic simulation software to calculate and optimize to obtain the best gradient-varying wave-absorbing structure, determine that the thickness of the wave-absorbing layer is 0.3 mm, and the carbon black contents in the wave-absorbing layer are 1%, 2%, 4%, 8%, and 16% respectively. Among the wave-absorbing aramid papers with carbon black contents of 4%, 8%, and 16%, flaky FeSiAl magnetism is added with mass fractions of 40%, 60%, and 80% respectively. During the preparation process, the impregnation time of the wave-absorbing aramid paper is 30 min; after vibrating to remove the excess resin, the weight gain ratio is 300%; the semi-curing temperature is 400 °C, and the semi-curing time is 2 h; the heating temperature is 450 °C, and the curing time is 5 h; the rest is the same as in Example 1, and a wave-absorbing honeycomb with a gradient distribution of absorbents along the honeycomb hole direction (honeycomb thickness direction) is obtained.

[0069] The cells of this wave-absorbing honeycomb are hexagonal, the cells are evenly distributed, there is no phenomenon of unopened cells, and the dimensional tolerance of the cell side length is controlled within the range of ±0.15 mm; along the cell direction, the compressive strength of the wave-absorbing honeycomb is 3.5 MPa; the reflectivity is measured along the cell direction (honeycomb thickness direction), the reflectivity at 0.5 - 1 GHz is ≤ -15 dB, the reflectivity at 1 - 2 GHz is ≤ -20 dB, the reflectivity at 2 - 8 GHz is ≤ -25 dB, the reflectivity at 8 - 18 GHz is ≤ -30 dB, and the reflectivity at 18 - 40 GHz is ≤ -32 dB. The wave-absorbing honeycomb has excellent wave-absorbing effects within the required electromagnetic wave frequency band (0.5 - 40 GHz).

[0070] Comparative Example 1: The carbon black content in the wave-absorbing slurry is 0, and the rest is the same as in Example 1 to obtain a honeycomb. The cells of this honeycomb are hexagonal, the cells are evenly distributed, there is no phenomenon of unopened cells, and the dimensional tolerance of the cell side length is controlled within the range of ±0.15 mm; along the cell direction, the compressive strength of the honeycomb is 3.3 MPa; the reflectivity is measured along the cell direction (honeycomb thickness direction), and there is no wave-absorbing performance.

[0071] Comparative Example 2: The carbon black content in the wave-absorbing slurry is 2% for all, and the rest is the same as in Example 1 to obtain a wave-absorbing honeycomb. The cells of this wave-absorbing honeycomb are hexagonal, the cells are evenly distributed, there is no phenomenon of unopened cells, and the dimensional tolerance of the cell side length is controlled within the range of ±0.15 mm; along the cell direction, the compressive strength is 3.6 MPa; the reflectivity is measured along the cell direction (honeycomb thickness direction), the reflectivity at 1 - 2 GHz is ≤ -2 dB, the reflectivity at 2 - 8 GHz is ≤ -4 dB, the reflectivity at 8 - 18 GHz is ≤ -10 dB, and the reflectivity at 18 - 40 GHz is ≤ -12 dB.

[0072] Comparative Example 3: An electromagnetic wave absorbing honeycomb prepared by a traditional method. The electromagnetic wave absorbing aramid paper is processed through processes such as sticking core strip glue, laminating, hot pressing, stretching, dipping in glue, drying, and curing. The rest is the same as in Example 1 to obtain the electromagnetic wave absorbing honeycomb. The cell of this electromagnetic wave absorbing honeycomb is hexagonal, but the morphological distribution of the cells is uneven. There are phenomena that some areas in the honeycomb holes are not opened and some honeycomb holes are debonded. The side length dimension of the cells fluctuates greatly, and the tolerance is within the range of ±2.0 mm. Along the cell direction, the compressive strength of the electromagnetic wave absorbing honeycomb is 2.4 MPa. When measuring the reflectivity along the cell direction (thickness direction), the reflectivity is ≤ -2 dB at 1 - 2 GHz, ≤ -6 dB at 2 - 8 GHz, ≤ -12 dB at 8 - 18 GHz, and ≤ -15 dB at 18 - 40 GHz.

[0073] The parts not detailed in the present invention are well-known technologies to those skilled in the art.

[0074] 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. Those of ordinary skill in the art can understand 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 content disclosed in the embodiments and drawings of this specification, and the protection scope of the present invention shall be subject to the scope defined by the claims.

Claims

1. An absorbing honeycomb with an impedance matching structure, characterized in that, it includes multiple sheets of absorbing aramid paper and cured resin impregnated on the absorbing aramid paper; an absorbing layer composed of an absorber and an adhesive is coated on each sheet of absorbing aramid paper, and the absorbing layer is composed of strip-shaped absorbing layers arranged along the width direction of the absorbing aramid paper, and the absorber content of each strip-shaped absorbing layer shows a gradient difference along the width direction; each sheet of absorbing aramid paper is in a unified semi-cell shape, and adjacent sheets of absorbing aramid paper are aligned and pasted; the absorbing honeycomb shows a gradient change in electromagnetic characteristics along the cell direction, and the direction along the cell direction and with the absorber content increasing from low to high is used as the electromagnetic wave incident direction; The steps of the absorbing honeycomb include: 1) Honeycomb structure design: Determine the type of absorber according to the absorbing frequency band, use electromagnetic simulation software to calculate the optimal gradient-changing absorbing structure, determine the thickness of the honeycomb along the electromagnetic wave incident direction, and determine the thickness, width and absorber content of each strip-shaped absorbing layer, and then perform subsequent steps based on the calculation results; 2) Preparation of absorbing aramid paper: Mix different amounts of absorber and adhesive to make an absorbing slurry, and then coat it on the surface of the aramid paper in strip form and cure it to form an absorbing layer with a gradient change in absorber along the width direction of the aramid paper to obtain the absorbing aramid paper; 3) Impregnating the absorbing aramid paper with resin: Completely immerse the absorbing aramid paper in the resin, take it out and place it vertically, and vibrate to remove the excess resin, and then perform semi-curing treatment; 4) Preparation of corrugated structure units: Press the semi-cured absorbing aramid paper with a pre-designed pressing mold to obtain the absorbing aramid paper with a semi-cell shape, that is, the corrugated structure unit, and coat the core strip glue on the convex edges between the semi-cells; 5) Stacking of corrugated units: Align each corrugated structure unit along the width direction of the absorbing aramid paper and bond them with the core strip glue to obtain a semi-finished absorbing honeycomb; 6) Heat treatment: Put the semi-finished absorbing honeycomb into a mold, insert bars with the same shape into the honeycomb cell holes, apply a certain pressure to the honeycomb cell walls to ensure the bonding strength of the core strip glue, and then perform heat curing to obtain an absorbing honeycomb with a gradient distribution of absorber perpendicular to the honeycomb cell direction.

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

3. The absorbing honeycomb according to claim 1, characterized in that, the absorber has electromagnetic loss ability, and at least one of carbon black, graphite flakes, carbon fiber, silica, carbonyl iron powder, ferrite, and iron silicon aluminum is selected; the resin is selected from one of phenolic resin and polyimide resin.

4. A preparation method of an absorbing honeycomb with an impedance matching structure for preparing the absorbing honeycomb according to claim 1, characterized in that, it includes the following steps: 1) Honeycomb structure design: Determine the type of absorber according to the absorbing frequency band, use electromagnetic simulation software to calculate the optimal gradient-changing absorbing structure, determine the thickness of the honeycomb along the electromagnetic wave incident direction, and determine the thickness, width and absorber content of each strip-shaped absorbing layer, and then perform subsequent steps based on the calculation results; 2) Preparation of microwave-absorbing aramid paper: Absorbents and adhesives in different amounts are mixed to form a microwave-absorbing slurry, which is then coated on the surface of the aramid paper in a strip form and cured to form a microwave-absorbing layer with a gradient change of the absorbent along the width direction of the aramid paper, obtaining the microwave-absorbing aramid paper; 3) Impregnation of the microwave-absorbing aramid paper with resin: The microwave-absorbing aramid paper is completely immersed in the resin, taken out and placed vertically, and the excess resin is removed by vibration, and then semi-cured; 4) Preparation of corrugated structure units: The semi-cured microwave-absorbing aramid paper is pressed using a pre-designed pressing mold to obtain a microwave-absorbing aramid paper with a semi-hole lattice shape, i.e., the corrugated structure units, and a film core bar adhesive is coated on the convex edges between the semi-hole lattices; 5) Superposition of corrugated units: Each corrugated structure unit is aligned along the width direction of the microwave-absorbing aramid paper and bonded through the core bar adhesive to obtain a semi-finished microwave-absorbing honeycomb; 6) Heat treatment: The semi-finished microwave-absorbing honeycomb is placed in a mold, bars with the same shape are inserted into the honeycomb holes, and a certain pressure is applied to the honeycomb hole walls to ensure the bonding strength of the core bar adhesive, and then heat-cured to obtain a microwave-absorbing honeycomb with a gradient distribution of the absorbent perpendicular to the honeycomb hole lattice direction.

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

6. The preparation method according to claim 4, characterized in that, the absorbent has electromagnetic loss ability and selects at least one of carbon black, graphite flakes, carbon fiber, silica, carbonyl iron powder, ferrite, and iron silicon aluminum.

7. The preparation method according to claim 4, characterized in that, for the electromagnetic wave frequency band absorbed below 8 GHz, the absorbent selects at least one of magnetic absorbents, i.e., carbonyl iron powder, ferrite, and iron silicon aluminum; for the electromagnetic wave frequency band absorbed above 8 GHz, the absorbent selects at least one of dielectric absorbents, i.e., carbon black, graphite flakes, carbon fiber, and silica; for ultra-wide or full-band absorption of electromagnetic waves, the absorbent simultaneously selects at least one of the magnetic absorbents and at least one of the dielectric absorbents.

8. The preparation method according to claim 4, characterized in that, the resin selects one of phenolic resin and polyimide resin.

9. The preparation method according to claim 4, characterized in that, the impregnation time of the microwave-absorbing aramid paper is 5-30 min; after vibrating to remove the excess resin, the weight gain ratio is 50%-300%.

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

Images