Electrical performance gradient graded wave-absorbing material and preparation method thereof

Abstract

This invention proposes a gradient-gradient electromagnetic absorbing material and its preparation method. The material is composed of an organic polymer and a solid radar wave absorber. The absorber concentration gradient varies gradually at different thicknesses within the material, with the absorber concentration increasing progressively along the electromagnetic wave incident direction. This invention achieves a stratified gradient of absorber concentration within the electromagnetic absorbing material through precise design and control of process parameters.

Description

Technical Field

[0001] This invention relates to an electrically graded absorbing material and its preparation method, belonging to the field of radar wave absorbing material technology. It can be applied to prepare various radar absorbing materials with wideband high absorption performance, and is especially suitable for radar stealth design of aircraft outer surfaces. Background Technology

[0002] Currently, publicly available domestic and international methods for radar stealth materials for military aircraft mainly fall into two categories. One involves first processing the radar-absorbing material into a uniform thickness and then attaching it to the location where radar echo attenuation occurs. The other involves preparing the radar-absorbing material into a coating and applying it to the non-developable surface of the aircraft to achieve radar echo attenuation.

[0003] Due to limitations in preparation methods, the aforementioned disclosed absorbing material application technologies are mainly limited to single-layer homogeneous dielectric absorbing materials, where the radar wave absorber concentration remains consistent at different locations within the material. To achieve a gradient change in radar wave absorber concentration, it is generally necessary to composite multiple layers of absorbing materials with different radar wave absorber concentrations. This approach suffers from significantly increased costs associated with adhesive / coating applications, unstable electrical properties, and difficulties in controlling the precision of the shape.

[0004] Porous matrices can achieve a gradient in radar wave absorber concentration within the material by adsorbing absorbers of varying concentrations at different thicknesses. However, for materials such as rubber that require compounding, it is difficult to control the internal absorber concentration to achieve a gradient change along the thickness direction. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a microwave absorbing material with a gradually changing internal absorber concentration gradient, stable electrical properties, and high shape accuracy, as well as its preparation method.

[0006] The technical solution of this invention: a method for preparing an electrically graded microwave absorbing material, comprising the following steps:

[0007] The first step is to prepare a homogeneous radar-absorbing raw material containing a solid radar absorber;

[0008] The second step involves calendering the homogeneous microwave-absorbing raw rubber material prepared in the first step into a sheet, placing it into a mold cavity composed of upper and lower molds at room temperature. After the mold is closed, the temperature is raised to the vulcanization temperature for vulcanization. Once the set vulcanization time is reached, heating is stopped, and the material is allowed to cool naturally to room temperature. After the mold is opened, the microwave-absorbing material is obtained. Temperature control is implemented throughout the heating and vulcanization process. During the heating process, the heating rate and temperature of the upper mold are always higher than those of the lower mold. After the upper mold reaches the vulcanization temperature until vulcanization is complete, the temperature of the lower mold is 15℃~30℃ lower than that of the upper mold.

[0009] A gradient-gradient electromagnetic wave absorbing material is composed of organic polymer materials and solid radar wave absorber materials. The concentration gradient of the absorber changes gradually at different thicknesses within the material, and the concentration of the absorber gradually increases along the incident direction of the electromagnetic wave.

[0010] The beneficial effects of this invention compared to the prior art are as follows:

[0011] (1) This invention achieves a gradual gradient of absorber concentration in the internal layer of the absorbing material through precise design and control of process parameters;

[0012] (2) The concentration of the absorbent in the inner layer of the present invention gradually changes in a stratified gradient, and the concentration of the absorbent in the outer layer is low, which makes it easier to reduce the reflection of radar waves on the outer surface. The concentration of the absorbent in the inner layer gradually increases, making it easier for radar waves to enter the interior of the material and be absorbed.

[0013] (3) The present invention achieves better results than current homogeneous absorbing materials or absorbing coating materials, and the preparation process is simple and easy to implement. It can be conveniently applied to various targets such as military aircraft that need to reduce radar echoes. Attached Figure Description

[0014] Figure 1 This is a schematic diagram showing the heat source temperature, upper mold monitoring temperature, and lower mold detection temperature during a typical production process of this invention;

[0015] Figure 2 This is a cross-sectional view of the preparation apparatus according to an embodiment of the present invention;

[0016] 1-Upper mold, 2-Lower mold, 3-Asbestos felt, 4-Wave-absorbing material to be heated and vulcanized. Detailed Implementation

[0017] The present invention will now be described in detail with reference to specific examples and accompanying drawings.

[0018] This invention provides an electrically graded microwave absorbing material, which is composed of organic polymer materials and solid radar wave absorber materials. The concentration gradient of the absorber changes gradually at different thicknesses within the material, and the concentration of the absorber gradually increases along the incident direction of the electromagnetic wave.

[0019] Furthermore, the thickness of the patch body is 1mm to 3mm; along the electromagnetic wave incident direction, the patch body is divided into two surfaces, an inner surface and an outer surface, with the lowest concentration of absorbent layering on the outer surface and the highest concentration of absorbent layering on the inner surface.

[0020] Furthermore, the solid radar wave absorber of this invention employs one or a mixture of materials such as carbon black, carbon nanotubes, and graphene, as well as various commercially available ferromagnetic radar wave absorbers (such as commercially available MZ-1, MZ-2, etc.), with the specific type determined according to the actual stealth performance design. Preferably, if the radar wave absorber is a ferromagnetic radar wave absorber, the absorber particle size is between 1 micrometer and 100 micrometers, the mass fraction is 65% to 85%, and the remainder is polymer materials; if carbon black, carbon nanotubes, and / or graphene are used, the mass fraction is 10% to 55%, and the remainder is polymer materials.

[0021] Furthermore, the polymer material of this invention is an organic rubber material, and the matrix material of conventional microwave absorbing materials in the art can be selected, such as polyurethane elastomer, nitrile rubber, ABS rubber, etc.

[0022] Furthermore, the present invention also provides a method for preparing an electrically graded microwave absorbing material, comprising the following steps:

[0023] The first step is to prepare a homogeneous microwave absorbing raw material.

[0024] This step is a well-known technique in the field, and specifically includes:

[0025] A1.1. Based on the requirements for radar absorption performance, select the type of radar absorber, raw rubber material, and mass fraction of radar absorber used in the radar absorption material.

[0026] A1.2 Add the radar absorber determined in step A1.1 to the raw rubber material and mix evenly to obtain a raw rubber material with a uniform distribution of radar wave absorber.

[0027] This step can be achieved by physically crushing or other methods to ensure even mixing.

[0028] The second step involves calendering the homogeneous microwave-absorbing raw rubber material prepared in the first step into a sheet. This sheet is then placed into a mold cavity composed of upper and lower molds at room temperature. After mold closing, the temperature is raised to the vulcanization temperature for vulcanization. Once the set vulcanization time is reached, heating is stopped, and the material is allowed to cool naturally to room temperature. After mold opening, the microwave-absorbing material is obtained. Temperature control is implemented throughout the entire heating and vulcanization process. During heating, the heating rate and temperature of the upper mold are consistently higher than those of the lower mold. From the time the upper mold reaches the vulcanization temperature until vulcanization is complete, the temperature of the lower mold is 15℃ to 30℃ lower than that of the upper mold. Figure 2 As shown.

[0029] Furthermore, the upper mold in this step can be designed based on existing molds, and includes a heating system and a temperature measuring system, which can meet the requirements of heating, heat preservation and temperature measurement of the upper mold.

[0030] Furthermore, the lower mold in this step of the forming mold includes at least a temperature measuring system. The heating path can be through heat conduction from the upper mold, or a heating system can be installed. By controlling the heating system of the lower mold, its temperature can be made to meet the aforementioned requirements. Further optimization involves incorporating cooling measures into the lower mold, which allows for more convenient control of the lower mold temperature and maintenance of the temperature difference between the upper and lower molds.

[0031] Furthermore, in this step, heat insulation materials such as glass felt are added between the upper and lower molds to reduce the heat transfer efficiency between the two parts.

[0032] The vulcanization temperature and vulcanization time in this step are determined according to the type of raw rubber material required.

[0033] A major drawback of existing radar-absorbing materials and coatings is that these materials are all homogeneous. When the concentration of absorbent inside the material is too low, the absorption capacity of incident radar waves is limited. However, if the concentration of absorbent is increased indiscriminately, the conductivity of the material will increase dramatically, making it difficult for incident radar waves to enter the material. Instead, they will be strongly reflected at the outer interface of the material, i.e., the air-absorbing material interface, making it difficult to achieve the goal of strong absorption.

[0034] The outer surface of the absorbing material of the present invention can reduce the concentration of the absorber, making it easier for incident radar waves to enter the interior of the structure. The concentration of the absorber inside the structure increases layer by layer, which significantly enhances the absorption effect of incident radar waves. Its absorption performance is significantly better than that of current homogeneous absorbing coating materials or absorbing patch materials.

[0035] The preparation method of this invention utilizes the temperature difference and gravity within the liquid. When a temperature difference exists within the liquid, the average rate of Brownian motion of the molecules differs, causing the solid absorbent particles within the liquid to accumulate in the lower temperature region. Simultaneously, gravity is another significant factor leading to the gradual change in the absorbent concentration gradient within the rubber material. Under gravity, absorbent powder with higher bulk density accumulates from the upper surface to the lower surface. This process occurs when the rubber material is in a liquefied state and gradually ceases before the vulcanization process is complete.

[0036] Therefore, when the rubber material is in a near-liquefied state, by controlling the temperature difference between the upper and lower molds, making the temperature of the upper mold significantly higher than that of the lower mold, it is possible to control the solid particles inside the liquid material to accumulate on the lower surface where the temperature is lower, and to complete the vulcanization of the rubber material while maintaining this temperature difference, ultimately obtaining a sheet-like microwave absorbing material with gradually changing electrical properties.

[0037] Furthermore, in the preparation method of the present invention, the structural temperature difference can be controlled by the following preferred method:

[0038] 1. The heating rod is only arranged in the upper mold. Through heat conduction, the upper mold is placed in the high temperature zone, which makes the upper surface of the cavity where the rubber material is arranged in the high temperature zone. Since there is no heat source on the lower surface, the temperature of the lower surface of the cavity is significantly lower than that of the upper surface.

[0039] 2. Minimize the contact surface at the mold closing position of the upper and lower molds, and apply heat insulation materials such as glass felt and aerogel to the contact surface to reduce heat conduction between the upper and lower surfaces without passing through the internal cavity and increase the internal temperature difference of the structure.

[0040] 3. The upper mold is a high-temperature zone, and the lower mold is a low-temperature zone. In the airflow field, local convection is reduced, and the thermal interaction between the upper and lower molds is reduced.

[0041] The above measures significantly control the temperature difference between the upper and lower surfaces of the cavity, which can be detected by the built-in temperature sensor in the mold.

[0042] Under the combined effects of temperature difference and gravity gradient in the mold, structural layering can be achieved. Actual testing shows that, under the same conditions, the radar echo energy of the absorbing material controlled by temperature difference and gravity gradient is significantly reduced compared to the absorbing material without such measures.

[0043] Example 1

[0044] One batch of 100 microwave absorbing materials was prepared using carbon black and nitrile rubber, with the carbon black comprising 10% by mass. The specific preparation process is as follows:

[0045] 1. Add 10% by mass of carbon black to the nitrile rubber raw material and mix evenly in a mixer to obtain a homogeneous carbon black / nitrile rubber raw material.

[0046] 2. Calender the homogeneous carbon black / nitrile rubber raw material into 1mm sheets, and cut them to the required size.

[0047] 3. Place the calendered sheet carbon black / nitrile rubber raw material into the mold cavity composed of upper and lower molds at room temperature. After the mold is closed, heat it to 140℃ and vulcanize it. After reaching the set vulcanization time of 2 hours, stop heating and let it cool naturally to room temperature. After opening the mold, the microwave absorbing material is obtained. Temperature is controlled throughout the heating and vulcanization process. During the heating process, the heating rate and temperature of the upper mold are always higher than those of the lower mold. After the upper mold reaches the vulcanization temperature until vulcanization is completed, the temperature of the lower mold is 15℃~30℃ lower than that of the upper mold.

[0048] In this example, the mold is used as follows: Figure 2As shown, the inner surface of the upper mold is bonded to the outer surface of the sheet carbon black / nitrile rubber raw material, and the inner surface of the lower mold is bonded to the inner surface of the sheet carbon black / nitrile rubber raw material. Holes are made in the metal material of the upper mold to insert heating rods or temperature sensors. Holes are also made in the lower mold to insert temperature sensors. The two mold parts can be fastened together with metal bolts. The heating rods heat the upper mold, and the temperature is fed back to the system via the temperature sensor. Once the upper mold reaches 140℃, the temperature of the upper mold structure is controlled at approximately 140℃, while the temperature of the lower mold structure is controlled to be 15℃ to 30℃ lower than that of the upper mold structure.

[0049] The obtained absorbing material was randomly sampled and local on-site reflectivity tests were conducted. Each sample was tested from the outside to the inside and from the inside to the outside. When the radar wave was incident vertically from the outside of the low concentration (i.e. the side where the patch contacts the upper surface of the mold cavity), its reflectivity was lower than that when it was incident from the inside of the high concentration. The difference in working frequency points could reach more than 3dB.

[0050] Examples 2-8

[0051] Using a process similar to that in Example 1, the vulcanization temperature and vulcanization time were adjusted accordingly based on the selected polymer material. Other variable parameters and reflectivity are shown in Table 1.

[0052] Comparative Examples 1 and 2

[0053] Comparative Example 1 uses existing conventional processes to prepare homogeneous microwave absorbing materials. Comparative Example 2 uses the same process as Example 1, except that the temperature difference between the upper and lower molds is controlled at 50°C.

[0054] Table 1

[0055]

[0056] As can be seen from the RCS data in Example 1 and Table 1, the present invention can make the concentration gradient inside the prepared radar absorbing material gradually change, providing an important technical means for better matching the incident radar wave. Under the same conditions of weight gain and construction cost, a lower radar echo can be achieved.

[0057] The parts of this invention not described in detail are techniques known to those skilled in the art.

Claims

1. A method for preparing a microwave absorbing material with graded electrical properties, characterized in that, Includes the following steps: The first step is to prepare a homogeneous radar-absorbing raw material containing a solid radar absorber; The second step involves calendering the homogeneous microwave-absorbing raw rubber material prepared in the first step into a sheet, placing it into a mold cavity composed of upper and lower molds at room temperature. After the mold is closed, the temperature is raised to the vulcanization temperature for vulcanization. Once the set vulcanization time is reached, heating is stopped, and the material is allowed to cool naturally to room temperature. After the mold is opened, the microwave-absorbing material is obtained. Temperature control is implemented throughout the heating and vulcanization process. During the heating process, the heating rate and temperature of the upper mold are always higher than those of the lower mold. After the upper mold reaches the vulcanization temperature until vulcanization is complete, the temperature of the lower mold is 15℃~30℃ lower than that of the upper mold.

2. The method for preparing a graded-gradient electrical performance absorbing material according to claim 1, characterized in that: In the second step, the upper mold in the forming mold includes a heating system and a temperature measuring system, and the lower mold includes at least a temperature measuring system. The lower mold is heated by heat conduction from the upper mold or by setting up a heating system.

3. The method for preparing a gradient-effect absorbing material according to claim 2, characterized in that: Cooling measures are installed on the lower mold to control its temperature and maintain the temperature difference between the upper and lower molds.

4. A method for preparing a graded-gradient electrical performance absorbing material according to any one of claims 1-3, characterized in that: In the second step, heat insulation material is added between the upper and lower molds.

5. The method for preparing a graded-gradient electrical performance absorbing material according to claim 1, characterized in that: The first step includes, A1.

1. Based on the requirements of radar absorption performance, select the type of solid radar absorber, raw rubber material, and mass fraction of solid radar absorber used in the radar absorption material. A1.2 Add the solid radar absorber determined in step A1.1 to the raw rubber material and mix evenly to obtain a raw rubber material with a uniform distribution of solid radar wave absorber.

6. The method for preparing a gradient-effect absorbing material according to claim 5, characterized in that: The solid radar wave absorber is one or a mixture of carbon black, carbon nanotubes, graphene, and ferromagnetic radar wave absorber.

7. The method for preparing a graded-gradient electrical performance absorbing material according to claim 6, characterized in that: The solid radar wave absorber is a ferromagnetic radar wave absorber with a particle size between 1 micrometer and 100 micrometers and a mass fraction of 65% to 85%, the remainder being raw rubber material.

8. The method for preparing a graded-gradient absorbing material with varying electrical properties according to claim 6, characterized in that: The solid radar wave absorber is made of carbon black, carbon nanotubes and / or graphene, with a mass fraction of 10% to 55%, and the remainder is raw rubber material.

9. The electrically graded microwave absorbing material obtained by any one of the preparation methods described in claims 1-8 is composed of organic polymer materials and solid radar wave absorber materials, wherein the concentration gradient of the absorber varies at different thicknesses within the material, and the absorber concentration gradually increases along the incident direction of the electromagnetic wave.

10. The electrically graded microwave absorbing material according to claim 9, characterized in that: The absorbing material is a sheet with a thickness of 1mm to 3mm. Along the incident direction of the electromagnetic wave, the sheet is divided into two surfaces: an inner surface and an outer surface. The stratification concentration of the absorber is lowest on the outer surface and highest on the inner surface.

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