A microwave-transparent and infrared-compatible low-emissivity coating

Abstract

The present invention provides a microwave-transparent and infrared-compatible low-emissivity coating, which belongs to the technical field of camouflage materials. The microwave-transparent and infrared-compatible low-emissivity coating consists of a double-layer structure consisting of a bottom microwave-transparent layer and a surface infrared functional layer; the materials of the microwave-transparent layer are hollow dielectric microbeads and polyvinylidene fluoride; the materials of the infrared functional layer are modified flaky aluminum powder, modified hexagonal boron nitride nanosheets and polyvinylidene fluoride. The microwave-transparent and infrared-compatible low-emissivity coating provided by the present invention has an infrared radiation suppression effect in the two atmospheric window bands of 3‑5μm and 8‑14μm, and has high transmittance in the 2‑18 GHz electromagnetic band. It is particularly suitable for scenes such as stealth drones, ship decks and 5G base station casings that need to suppress infrared characteristics and maintain microwave signal transmission at the same time.

Description

Technical Field

[0001] The present invention relates to the technical field of camouflage materials, and in particular to a microwave-transparent and infrared-compatible low-emissivity coating. Background Art

[0002] Low-emissivity infrared materials are not only used for radiative temperature control but are also attracting widespread attention in the field of thermal infrared camouflage. Thermal infrared camouflage primarily reduces the target object's radiant energy or temperature in the 3-5 μm and 8-14 μm bands, thereby concealing the object's true temperature and preventing detection by infrared detection equipment.

[0003] Infrared stealth requires coatings to have low infrared absorption and high reflectivity. Metal powders (such as aluminum powder) are often used as low-emissivity fillers in coatings. However, the addition of metal powder conflicts with the high absorption and low reflectivity required for microwave (radar) stealth. When the addition level of micron-sized flake aluminum powder is low (<20wt%), the aluminum powder is dispersed and isolated, failing to form a continuous reflective layer, resulting in a limited decrease in emissivity. In the critical range (20-40wt%), the aluminum powder forms a conductive network, significantly reducing emissivity to 0.3-0.5. When the addition level continues to increase (>50wt%), the emissivity may rebound or stabilize due to agglomeration, increased coating porosity, or deterioration in mechanical properties. However, increasing the aluminum powder content increases the coating's microwave reflectivity and decreases its microwave transmittance. Generally, the addition of 5wt% aluminum powder will reduce the transmittance of epoxy resin materials in the X-band (8-12 GHz) by about 10-15%. At a 10wt% addition, the scattering effect of metal particles is significantly enhanced, causing the microwave transmittance to further decrease by 20-30%.

[0004] It can be seen that the electromagnetic wave reflection characteristics of infrared and radar stealth are contradictory to each other, and infrared radar compatible stealth cannot be achieved. The existing multi-band stealth composite coatings using a metal powder system are difficult to overcome this inherent contradiction. Summary of the Invention

[0005] In view of this, an object of the present invention is to provide a microwave-transparent and infrared-compatible low-emissivity coating.

[0006] In order to achieve the above object, the present invention provides the following technical solutions:

[0007] One of the technical solutions of the present invention is a microwave-transparent and infrared-compatible low-emissivity coating, which has a double-layer structure consisting of a bottom microwave-transparent layer and a surface infrared functional layer; the materials of the microwave-transparent layer are hollow dielectric microbeads and polyvinylidene fluoride; the materials of the infrared functional layer are modified flaky aluminum powder, modified hexagonal boron nitride nanosheets and polyvinylidene fluoride.

[0008] The second technical solution of the present invention is a method for preparing the above-mentioned microwave-transparent and infrared-compatible low-emissivity coating, which comprises dissolving polyvinylidene fluoride in an organic solvent, then adding hollow dielectric microbeads, mixing evenly, and then coating the coating on the surface of the substrate to obtain an underlying microwave-transparent layer; dissolving polyvinylidene fluoride in an organic solvent, then adding modified flaky aluminum powder and modified hexagonal boron nitride nanosheets, mixing evenly, and then coating the coating on the surface of the underlying microwave-transparent layer to obtain the microwave-transparent and infrared-compatible low-emissivity coating.

[0009] The present invention discloses the following technical effects:

[0010] The microwave-transparent and infrared-compatible low-emissivity coating provided by the present invention has an infrared radiation suppression effect in the two atmospheric window bands of 3-5 μm and 8-14 μm, and has high transmittance in the 2-18 GHz electromagnetic band. It is particularly suitable for scenarios such as stealth drones, ship decks, and 5G base station casings that require simultaneous suppression of infrared characteristics and maintenance of microwave signal transmission. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0012] Figure 1 Schematic diagram of the double-layer structure of the microwave-transparent and infrared-compatible low-emissivity coating of the present invention. DETAILED DESCRIPTION

[0013] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0014] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. The intermediate value within any stated value or stated range, and each smaller range between any other stated value or intermediate value within the stated range, is also encompassed within the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0015] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0016] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments described herein without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the description of the invention. The description and examples are intended to be exemplary only.

[0017] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0018] The first aspect of the present invention provides a microwave-transparent and infrared-compatible low-emissivity coating, which comprises a double-layer structure consisting of a bottom microwave-transparent layer and a surface infrared functional layer; the materials of the microwave-transparent layer are hollow dielectric microbeads and polyvinylidene fluoride; the materials of the infrared functional layer are modified flaky aluminum powder, modified hexagonal boron nitride nanosheets and polyvinylidene fluoride.

[0019] In a preferred embodiment of the present invention, the microwave transparent layer comprises 10-25% hollow dielectric microbeads and the balance polyvinylidene fluoride, and the infrared functional layer comprises 5-15% modified flaky aluminum powder, 3-10% modified hexagonal boron nitride nanosheets and the balance polyvinylidene fluoride, calculated by mass percentage.

[0020] In a preferred embodiment of the present invention, the microwave transparent layer comprises 15-20% hollow dielectric microbeads and the balance polyvinylidene fluoride, and the infrared functional layer comprises 5-8% modified flaky aluminum powder, 5-8% modified hexagonal boron nitride nanosheets and the balance polyvinylidene fluoride, calculated by mass percentage.

[0021] In the present invention, if the bottom layer remains unchanged and the ratio of aluminum powder to hexagonal boron nitride in the surface layer is not within the above range, the following effects will occur:

[0022] A high aluminum powder ratio (e.g., >15%) can lead to agglomeration, reduced dispersibility, increased dielectric constant (e.g., ε = 3.5 in Group 2 of Example 2), and decreased microwave transmittance (68%). A low aluminum powder ratio results in high infrared emissivity. A low h-BN ratio (e.g., <3%) fails to effectively fill gaps in the aluminum powder, increasing surface roughness (Ra > 0.1 μm) and microwave loss (e.g., 70% transmittance in Comparative Example B). A high h-BN ratio (>10%) hinders the aluminum powder's reflective network, slightly increasing infrared emissivity (e.g., 0.50 in Comparative Example A).

[0023] Comparative Example A: 3% aluminum powder + 12% h-BN on the surface → infrared emissivity 0.50, microwave transmittance 75%.

[0024] Comparative Example B: 20% aluminum powder + 2% h-BN on the surface → infrared emissivity 0.48, transmittance 70%.

[0025] In the present invention, if the surface layer remains unchanged and the ratio of hollow dielectric microspheres in the bottom layer is higher or lower than the above parameters, the following effects will occur:

[0026] A high microbead ratio (e.g., >25%) can lead to increased coating brittleness (decreased adhesion), increased porosity, and a low dielectric constant (ε <2.0), but microwave transmittance can be improved to a certain extent (e.g., 90% transmittance in Example 3). A low microbead ratio (e.g., <10%) can increase the dielectric constant (ε >3.0) and significantly decrease microwave transmittance (e.g., 68% transmittance in Group 2 in Example 2).

[0027] In a preferred embodiment of the present invention, the modified flaky aluminum powder is prepared by wet ball milling the flaky aluminum powder and phosphoric acid, and then passivating the flaky aluminum powder with phosphoric acid to form an AlPO4 layer; the mass ratio of the flaky aluminum powder to the phosphoric acid is 1:1.5 to 1:2; the ball milling medium is ethanol, the speed is 400 to 500 rpm, the time is 6 hours, and the ball-to-material ratio is 10:1;

[0028] The modified hexagonal boron nitride nanosheets are prepared by mixing hexagonal boron nitride nanosheets with a hydroxyl-containing compound and then ball milling the mixture; the mass ratio of the hexagonal boron nitride nanosheets to the hydroxyl-containing compound is 1:6 to 1:9; the ball milling speed is 400 to 500 rpm, the ball-to-material ratio is 10:1, and the ball-to-material ratio is 10:1, and the ball milling time is 2 to 10 hours.

[0029] In the present invention, during the preparation of the modified flaky aluminum powder, the ball milling time will have the following effects on the properties of the final coating:

[0030] If the modified aluminum powder is ball-milled for too short a time (e.g., <4 h), the AlPO4 passivation layer is incomplete, and the aluminum powder is easily oxidized, resulting in a decrease in infrared reflectivity. If the ball-milling time is too long (e.g., >8 h), the flaky aluminum powder structure may be destroyed (reducing the aspect ratio), affecting the formation of a dense reflective layer (see Example 3, where the aluminum powder had an aspect ratio >20 and an emissivity of 0.42 after 8 h of ball-milling).

[0031] Flake aluminum powder provides low infrared emissivity, hollow microspheres reduce the dielectric constant, and h-BN supplements the low emissivity and suppresses microwave loss. This invention achieves both dispersion stability and interfacial compatibility among the flake aluminum powder, hollow microspheres, and h-BN through surface modification processes such as -OH functionalization of the h-BN and phosphoric acid passivation of the aluminum powder. This ensures low infrared emissivity while maintaining high microwave transmittance.

[0032] In a preferred embodiment of the present invention, the aspect ratio of the flaky aluminum powder is greater than 15, and the particle size D50 is 10-14 μm; the thickness of the hexagonal boron nitride nanosheets is less than 10 nm (preferably less than 5 nm); and the hydroxyl-containing compound is polyvinyl alcohol or urea.

[0033] In a preferred embodiment of the present invention, the particle size of the hollow dielectric microspheres is 10-50 μm (preferably 20-40 μm); the hollow dielectric microspheres are hollow glass microspheres or hollow ceramic microspheres.

[0034] In the present invention, the aspect ratio and particle size of the flaky aluminum powder, the thickness of the hexagonal boron nitride nanosheets, and the particle size of the hollow dielectric microspheres will have the following effects:

[0035] (1) Influence of different parameters of flake aluminum powder:

[0036] The diameter-to-thickness ratio of flake aluminum powder is less than 15 or the particle size is too small (D 50 <10 μm) will reduce the reflection efficiency; aluminum powder with a high aspect ratio (such as >20) can enhance the parallel arrangement and form a dense reflection layer (see Example 3, emissivity 0.42).

[0037] Particle size D50 range: 10-14 μm (too small will increase scattering, too large will easily settle).

[0038] For example: Comparative Example G: Aluminum powder diameter-to-thickness ratio = 10 → emissivity 0.55, transmittance 70%.

[0039] (2) Influence of different parameters of hexagonal boron nitride nanosheets:

[0040] If the thickness of h-BN nanosheets is greater than 10 nm, the efficiency of surface grafting of hydroxyl groups will be reduced, affecting the dispersibility. The thickness of h-BN should be controlled to be less than 5 nm (e.g., Ra = 0.08 μm in Example 1).

[0041] For example: Comparative Example H: h-BN thickness = 50 nm → emissivity 0.45, transmittance 75%.

[0042] (3) Influence of different parameters of hollow dielectric microspheres:

[0043] Hollow microbeads with a particle size greater than 50 μm will introduce interface defects, and those with a particle size less than 10 μm will reduce the dielectric control effect; 20-40 μm microbeads are selected (such as in Example 1, ε=2.5, transmittance 87%).

[0044] For example: Comparative Example I: Bead size = 80 μm → ε = 3.0, transmittance 70%.

[0045] In a preferred embodiment of the present invention, the thickness of the bottom microwave transparent layer is 50-150 μm; the thickness of the surface infrared functional layer is 10-30 μm.

[0046] In a preferred embodiment of the present invention, the thickness of the bottom microwave transparent layer is 120 μm; the thickness of the surface infrared functional layer is 30 μm.

[0047] In a preferred embodiment of the present invention, the thickness of the bottom microwave transparent layer is 50 μm; the thickness of the surface infrared functional layer is 10 μm.

[0048] In a preferred embodiment of the present invention, the thickness of the bottom microwave transparent layer is 80-120 μm; the thickness of the surface infrared functional layer is 20-25 μm.

[0049] In the present invention, the thickness of the bottom microwave transparent layer and the surface infrared functional layer have a significant impact on the performance of the final coating:

[0050] A bottom layer thickness of <50 μm will lead to unstable dielectric constant, and a bottom layer thickness of >150 μm will increase coating weight; the optimized bottom layer thickness is 80-120 μm, which can balance dielectric performance and lightweight (for example, in Example 3, the transmittance of a 150 μm bottom layer is 90%).

[0051] A surface thickness of <10 μm results in a discontinuous reflection layer, and a surface thickness of >30 μm increases microwave loss. The surface thickness is selected to be 20-25 μm (as in Example 4, the transmittance of the 10 μm surface layer is 88%, but the emissivity is slightly higher at 0.45).

[0052] In a preferred embodiment of the present invention, the surface of the surface infrared functional layer further includes an optical regulation layer in the visible light band.

[0053] In a preferred embodiment of the present invention, the preparation method of the optical regulation layer in the visible light band is: premixing Cr2O3 nanoparticles (particle size 50 nm, 3 wt%) with a PVDF matrix, ultrasonically dispersing them (power 400 W, 20 min), and then electrostatically spraying them on the surface of the infrared functional layer.

[0054] In a preferred embodiment of the present invention, the thickness of the optical regulation layer in the visible light band is 5 μm.

[0055] The second aspect of the present invention provides a method for preparing the above-mentioned microwave-transparent and infrared-compatible low-emissivity coating, which comprises dissolving polyvinylidene fluoride in an organic solvent, adding hollow dielectric microbeads, mixing evenly, and then coating the coating on the surface of the substrate to obtain a bottom microwave-transparent layer; dissolving polyvinylidene fluoride in an organic solvent, adding modified flaky aluminum powder and modified hexagonal boron nitride nanosheets, mixing evenly, and then coating the coating on the surface of the bottom microwave-transparent layer to obtain the microwave-transparent and infrared-compatible low-emissivity coating.

[0056] In a preferred embodiment of the present invention, the coating method is high-pressure spraying or electrostatic spraying.

[0057] In the present invention, the choice of the organic solvent is not particularly limited. Any organic solvent known to those skilled in the art that can dissolve polyvinylidene fluoride, such as N-methylpyrrolidone (NMP), can be used.

[0058] In the present invention, when preparing the bottom microwave transparent layer, the ratio of the polyvinylidene fluoride to the organic solvent is 1:5 to 1:7; when preparing the surface infrared functional layer, the ratio of the polyvinylidene fluoride to the organic solvent is 1:6 to 1:8.

[0059] Unless otherwise specified, the technical solutions described in the present invention are all conventional solutions in the field, and the reagents or raw materials used, unless otherwise specified, are purchased from commercial channels or have been disclosed.

[0060] In order to better understand the present invention, the content of the present invention is further illustrated below in conjunction with the examples, but the content of the present invention is not limited to the following examples.

[0061] Example 1

[0062] 1. Structure and function of coating:

[0063] The coating has a double-layer structure, with the top layer being an infrared functional layer and the bottom layer being a microwave transparent layer:

[0064] The surface layer is composed of 8 wt% modified flake aluminum powder (aspect ratio > 15, particle size D50 10-14 μm) + 5 wt% modified hexagonal boron nitride (h-BN) nanosheets (thickness < 10 nm) + the remainder of polyvinylidene fluoride (PVDF) matrix, with a thickness of 30 μm; the bottom layer is composed of 15 wt% hollow glass microspheres (particle size 30 μm, wall thickness 1.5 μm) + the remainder of PVDF matrix, with a thickness of 120 μm.

[0065] The surface layer achieves an infrared emissivity of 0.35 (8-14 μm), a surface roughness of Ra = 0.08 μm, and a bottom layer dielectric constant of ε = 2.5.

[0066] 2. Preparation process:

[0067] Modified aluminum flake powder: Aluminum flake powder and phosphoric acid (mass ratio of aluminum flake powder to phosphoric acid is 1:1.8) were mixed and wet-milled for 6 hours (planetary ball mill, speed 400 rpm, zirconia balls, ball-to-powder ratio 10:1, ethanol medium) to passivate the mixture with phosphoric acid to form an AlPO4 layer.

[0068] Modification of h-BN nanosheets: h-BN nanosheets were mixed with polyvinyl alcohol (PVA) (mass ratio of h-BN nanosheets to PVA was 1:8) and then ball-milled (planetary ball mill, 400 rpm, steel balls, ball-to-material ratio of 10:1, time 2 h) to promote -OH grafting through mechanochemical action.

[0069] Coating, coating and curing: After PVDF was dissolved in N-methylpyrrolidone (NMP) (the mass ratio of PVDF to NMP was 1:5), hollow glass microspheres were added, ultrasonically dispersed for 30 minutes, and high-pressure airless spraying (pressure 20 MPa) was applied to the substrate surface, and the temperature was kept at 80°C → 120°C for 1 hour (evenly heated within 0.5 hours), and then the temperature was evenly increased to 160°C within 0.5 hours and kept for 1 hour to promote full volatilization of the solvent and increase the crystallinity to obtain a microwave transparent layer; after PVDF was dissolved in N-methylpyrrolidone (NMP) (the mass ratio of PVDF to NMP was 1:6), modified flaky aluminum powder and modified hexagonal boron nitride nanosheets were added, ultrasonically dispersed for 30 minutes, and high-pressure airless spraying (pressure 20 MPa) on the surface of the microwave transparent layer, and keep the temperature at 80℃→120℃ for 0.5h (evenly increase the temperature within 0.5h), then evenly increase the temperature to 160℃ within 0.5h and keep the temperature for 1h to obtain the infrared functional layer. The microwave transparent layer and the infrared functional layer constitute a microwave-transparent and infrared-compatible low-emissivity coating.

[0070] 3. Technical effects:

[0071] The microwave-transparent and infrared-compatible low-emissivity coating has an infrared emissivity of <0.4, a microwave transmittance of 87% from 2 to 18 GHz, and an adhesion of 5.2 MPa, meeting the temperature cycle resistance requirements of the military standard MIL-STD-810G.

[0072] Example 2

[0073] With reference to Example 1, a control experiment was designed. The structure of the microwave-transparent and infrared-compatible low-emissivity coating used is shown in Table 1. Example 1 was used as control group 1, and the material properties, surface layer and bottom layer preparation methods of the remaining groups were the same as those of Example 1.

[0074] Table 1 Control experiment

[0075] ,

[0076] The performance parameters of the obtained coating, such as infrared emissivity and microwave transmittance, are shown in Table 2.

[0077] Table 2 Coating properties of control experiment

[0078] ,

[0079] The following conclusions can be drawn from Tables 1 and 2:

[0080] First, by comparing Group 2 and Group 4, it can be seen that the introduction of modified h-BN reduced the infrared emissivity (Group 2: 0.50 → Group 4: 0.41) and increased the microwave transmittance of the coating (Group 2: 68% → Group 4: 73%), but the adhesion was almost unchanged;

[0081] Secondly, comparing Group 2 and Group 3, the introduction of a base layer with 20 wt% hollow glass microspheres significantly reduced the dielectric constant (Group 3: ε = 2.7), and the microwave transmittance increased to 82%, but the improvement in infrared emissivity was limited (Group 2: 0.50 → Group 3: 0.48), and the adhesion of the coating was improved.

[0082] Comparing Group 1 and Group 4, it can be seen that the introduction of the base layer with the addition of 15wt% hollow glass microspheres further improved the microwave transmittance on the basis of Group 4 (Group 4: 73% → Group 1: 87%), significantly reduced the infrared emissivity (Group 4: 0.41 → Group 1: 0.35), and significantly improved the adhesion, indicating that the three have a synergistic effect and achieved unexpected results.

[0083] Example 3

[0084] With reference to Example 1, a control experiment was designed, in which the material properties, surface layer and bottom layer preparation methods used were the same as those in Example 1.

[0085] The surface layer formulation is the same as that in Example 1, using 8 wt% flake aluminum powder (aspect ratio > 15, particle size D50 of 10-14 μm) + 5 wt% hexagonal boron nitride (h-BN) nanosheets (thickness < 10 nm) + 87 wt% polyvinylidene fluoride (PVDF) matrix, with a thickness of 30 μm;

[0086] Two microwave transparent layers with different formulations were designed, namely Group 5 and Group 6. Among them:

[0087] Group 5: 10 wt% hollow glass microspheres + 90 wt% PVDF, thickness 120 μm;

[0088] Group 6: 25 wt% hollow glass microspheres + 75 wt% PVDF, thickness 120 μm.

[0089] The infrared emissivity of the obtained coating is almost the same as that of Group 1 (Example 1). Its performance parameters such as microwave transmittance and adhesion are shown in Table 3. It can be seen that when the microbead content is reduced to 10%, the dielectric constant increases (ε=3.1), the microwave transmittance decreases to 78%, and the adhesion decreases slightly; when the microbead content is increased to 25%, the dielectric constant further decreases (ε=2.1), but the excessive amount of microbeads causes the coating structure to become loose, and the adhesion decreases significantly (4.3 MPa).

[0090] Table 3 Coating properties of control experiment

[0091] .

[0092] Example 4

[0093] With reference to Example 1, a control experiment was designed. The material properties, surface layer and bottom layer preparation methods used were the same as those in Example 1. The microwave transparent layer formulation was also the same as that in Example 1. Two infrared functional layers with different formulations were designed, namely Group 7 and Group 8.

[0094] Group 7: 3 wt% modified flake aluminum powder + 3 wt% modified h-BN + 94 wt% PVDF;

[0095] Group 8: 10 wt% modified flake aluminum powder + 10 wt% modified h-BN + 80 wt% PVDF.

[0096] The adhesion of the obtained coating is consistent with that of Group 1 (Example 1). Its performance parameters such as infrared emissivity, microwave transmittance and surface roughness are shown in Table 4. It can be seen that when the content of aluminum powder and h-BN is too low (3%), the infrared emissivity increases to 0.55 due to the incomplete reflection network, but the microwave transmittance is slightly improved (89%); when the content of aluminum powder and h-BN is too high (10%), the infrared emissivity is further reduced (0.33), but the excessive filler leads to increased microwave loss (transmittance 71%), and the reduced surface roughness may affect the mechanical properties.

[0097] Table 4 Coating properties of relevant control experiments

[0098] .

[0099] Example 5

[0100] To investigate the effects of aluminum powder modification, a control experiment, Group 9, was designed. Group 9 used unmodified flake aluminum powder. Other material properties, surface layer, and base layer preparation methods were identical to those in Example 1. The microwave-transparent layer formulation was also identical to that in Example 1 (15 wt% hollow microspheres + 85 wt% PVDF, 120 μm thickness). The infrared functional layer formulation was:

[0101] Group 9: 8 wt% unmodified flake aluminum powder + 5 wt% modified h-BN + 87 wt% PVDF.

[0102] The performance parameters of the obtained coating, such as infrared emissivity, microwave transmittance, and surface roughness, corresponding to Group 1 (Example 1), are shown in Table 5. It can be seen that the unmodified aluminum powder has insufficient surface passivation and poor dispersion, resulting in a reduced density of the reflective layer and an increased infrared emissivity (0.45); the agglomeration of the aluminum powder exacerbates the dielectric loss, the microwave transmittance decreases significantly (65%), and the adhesion is also reduced due to weak interfacial bonding.

[0103] Table 5 Coating properties of relevant control experiments

[0104] .

[0105] Example 6

[0106] To investigate the effects of modified hexagonal boron nitride (h-BN) nanosheets, a control experiment, Group 10, was designed. Group 10 used unmodified h-BN nanosheets. Other material properties, surface layer, and bottom layer preparation methods were identical to those in Example 1. The microwave-transparent layer formulation was also identical to that in Example 1 (15 wt% hollow microspheres + 85 wt% PVDF, 120 μm thickness). However, the infrared functional layer was modified by omitting the polyvinyl alcohol (PVA) modification and ball milling steps of the h-BN nanosheets and using unmodified h-BN nanosheets directly. The infrared functional layer formulation was:

[0107] Group 10: 8 wt% modified flake aluminum powder + 5 wt% unmodified h-BN + 87 wt% PVDF, thickness 30 μm.

[0108] The performance parameters of the obtained coating, such as infrared emissivity, microwave transmittance, and surface roughness, corresponding to Group 1 (Example 1), are shown in Table 6. It can be seen that the unmodified h-BN cannot effectively fill the gaps in the aluminum powder due to its poor dispersibility, resulting in a significantly increased surface roughness (Ra = 0.18 μm), a decrease in the density of the reflective layer, and a significant increase in infrared emissivity to 0.52 (compared to 0.35 in Example 1); the agglomeration of the unmodified h-BN leads to an increase in dielectric loss (ε = 3.8, tanδ = 0.05), and the microwave transmittance drops sharply to 63% (compared to 87%); the interface between h-BN and the PVDF matrix is weak, and the adhesion is reduced to 3.2 MPa (compared to 5.2 MPa), which cannot meet the military standard requirements; the -OH group of the modified h-BN can originally improve electrical insulation and reduce dielectric loss, but this advantage is lost when it is unmodified, and the overall performance is significantly inferior to that of Example 1.

[0109] Table 6 Coating properties of relevant control experiments

[0110] ,

[0111] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A microwave-transparent and infrared-compatible low-emissivity coating, characterized in that: The coating has a double-layer structure consisting of a bottom microwave-transparent layer and a surface infrared functional layer; the microwave-transparent layer is made of hollow dielectric microbeads and polyvinylidene fluoride; the infrared functional layer is made of modified flaky aluminum powder, modified hexagonal boron nitride nanosheets, and polyvinylidene fluoride; In terms of mass percentage, the microwave transparent layer comprises 10-25% hollow dielectric microbeads and the balance polyvinylidene fluoride; the infrared functional layer comprises 5-15% modified flaky aluminum powder, 3-10% modified hexagonal boron nitride nanosheets and the balance polyvinylidene fluoride; The modified flaky aluminum powder is prepared by mixing flaky aluminum powder with phosphoric acid and then wet ball milling the mixture; the mass ratio of the flaky aluminum powder to the phosphoric acid is 1:1.5 to 1:2; the ball milling medium is ethanol, the speed is 400 to 500 rpm, the time is 6 hours, and the ball-to-material ratio is 10:1; The modified hexagonal boron nitride nanosheets are prepared by ball milling the hexagonal boron nitride nanosheets and the hydroxyl-containing compound; the mass ratio of the hexagonal boron nitride nanosheets to the hydroxyl-containing compound is 1:6 to 1:9; the ball milling speed is 400 to 500 rpm, the ball-to-material ratio is 10:1, and the ball-to-material ratio is 10:1, and the ball milling time is 2 to 10 hours. The thickness of the bottom microwave transparent layer is 120 μm; the thickness of the surface infrared functional layer is 30 μm.

2. The microwave-transparent and infrared-compatible low-emissivity coating according to claim 1, characterized in that: The aspect ratio of the flaky aluminum powder is greater than 15, and the particle size D50 is 10-14 μm; the thickness of the hexagonal boron nitride nanosheet is less than 10 nm; and the hydroxyl-containing compound is polyvinyl alcohol.

3. The microwave-transparent and infrared-compatible low-emissivity coating according to claim 1, characterized in that: The particle size of the hollow dielectric microspheres is 10-50 μm; the hollow dielectric microspheres are hollow glass microspheres or hollow ceramic microspheres.

4. The microwave-transparent and infrared-compatible low-emissivity coating according to claim 1, characterized in that: The surface of the surface infrared functional layer also includes an optical regulation layer in the visible light band.

5. A method for preparing the microwave-transparent and infrared-compatible low-emissivity coating according to claim 1, characterized in that: The polyvinylidene fluoride is dissolved in an organic solvent, and then hollow dielectric microbeads are added, mixed evenly, and then coated on the surface of the substrate to obtain a bottom microwave transparent layer; the polyvinylidene fluoride is dissolved in an organic solvent, and then modified flaky aluminum powder and modified hexagonal boron nitride nanosheets are added, mixed evenly, and then coated on the surface of the bottom microwave transparent layer to obtain the microwave-transparent and infrared-compatible low-emissivity coating.

6. The preparation method according to claim 5, characterized in that The coating method is high-pressure spraying or electrostatic spraying.

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