Microwave-transmitting and infrared-compatible low-emissivity coating

Abstract

The invention provides a microwave-transmitting and infrared-compatible low-emissivity coating, and belongs to the technical field of camouflage materials. The microwave-transmitting and infrared-compatible low-emissivity coating is of a double-layer structure composed 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. The microwave-transmitting compatible infrared low-emissivity coating provided by the invention has an infrared radiation inhibition effect in two atmospheric window wave bands of 3-5 microns and 8-14 microns, and has high transmissivity in an electromagnetic wave band of 2-18 GHz; the method is especially suitable for the scenes, such as stealth unmanned aerial vehicles, ship decks and 5G base station shells, where infrared characteristics need to be suppressed and microwave signal transmission needs to be kept at the same time.

Description

Technical Field

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

[0002] Infrared low-emissivity materials can not only be used for radiation temperature control, but also receive extensive attention in the field of thermal infrared camouflage. Thermal infrared camouflage mainly reduces the radiation energy or radiation temperature of the target object in the 3-5 μm and 8-14 μm bands to hide the true temperature of the object, thereby preventing the target from being detected by infrared detection equipment.

[0003] Infrared stealth requires that the coating has low absorption and high reflection characteristics for infrared rays. Metal powder (such as aluminum powder) is often used as a low-emissivity filler in the coating. However, the addition of metal powder conflicts with the microwave (radar) stealth requirement that the coating has high absorption and low reflection characteristics. When the addition amount of micron-sized flaky aluminum powder is low (<20wt%), the aluminum powder is dispersed and isolated, unable to form a continuous reflection layer, and the emissivity decreases only slightly; in the critical range (20-40wt%), the aluminum powder forms a conductive network, and the emissivity decreases significantly, reaching 0.3-0.5; when the addition amount continues to increase (>50wt%), the emissivity may rebound or stabilize due to agglomeration, increased coating porosity, or deterioration of mechanical properties. However, as the aluminum powder content increases, the microwave reflectivity of the coating also increases, and the microwave transmittance decreases. 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, 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: 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.

[0007] 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 a 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.

[0008] The present invention discloses the following technical effects: 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 a 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0009] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments will be briefly introduced below. 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 creative work.

[0010] 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

[0011] 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 should be understood as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0012] It should be understood that the terms described in the present invention are only for describing a particular embodiment and are not intended to limit the present invention. In addition, for the numerical range in the present invention, it should be understood that each intermediate value between the upper and lower limits of the scope is also specifically disclosed. The intermediate value in any stated value or stated range, and each smaller range between any other stated value or intermediate value in the described range is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded in the scope.

[0013] Unless otherwise indicated, all technical and scientific terms used herein have the same meanings as those generally understood by those skilled in the art. Although the present invention describes only preferred methods and materials, any methods and materials similar or equivalent to those described herein may also be used in the implementation 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 a conflict with any incorporated document, the content of this specification shall prevail.

[0014] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention description without departing from the scope or spirit of the present invention. Other embodiments derived from the present invention description will be apparent to the skilled artisan. The present invention description and examples are exemplary only.

[0015] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0016] The first aspect of the present invention provides 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.

[0017] 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, by mass percentage.

[0018] 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, by mass percentage.

[0019] 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: If the proportion of aluminum powder is too high (e.g. >15%), it will lead to agglomeration, reduce dispersibility, increase dielectric constant (e.g., Group 2 in Example 2, ε=3.5), and reduce microwave transmittance (68%); if the proportion of aluminum powder is too low, the infrared emissivity will be high. If the proportion of h-BN is too low (e.g., <3%), it will not be able to effectively fill the gaps between aluminum powders, the surface roughness will increase (Ra>0.1 μm), and the microwave loss will increase (e.g., Comparative Example B: transmittance 70%). If the proportion of h-BN is too high (>10%), it will hinder the aluminum powder from reflecting the network, and the infrared emissivity will increase slightly (e.g., Comparative Example A: 0.50).

[0020] Comparative Example A: 3% aluminum powder + 12% h-BN on the surface → infrared emissivity 0.50, microwave transmittance 75%.

[0021] Comparative Example B: 20% aluminum powder + 2% h-BN on the surface → infrared emissivity 0.48, transmittance 70%.

[0022] 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: If the proportion of microbeads is too high (e.g. >25%), the coating will become more brittle (decreased adhesion), the porosity will increase, and the dielectric constant will be too low (ε<2.0), but the microwave transmittance will be improved to a certain extent (e.g., Example 3, the transmittance is 90%). If the proportion of microbeads is too low (e.g. <10%), the dielectric constant will increase (ε>3.0), and the microwave transmittance will decrease significantly (e.g., Group 2 in Example 2, the transmittance is 68%).

[0023] In a preferred embodiment of the present invention, the preparation method of the modified flaky aluminum powder is: mixing the flaky aluminum powder with phosphoric acid and then performing wet ball milling, and phosphoric acid passivation to generate AlPO 4 layer; the mass ratio of the flaky aluminum powder to the phosphoric acid is 1:1.5~1:2; the ball milling medium is ethanol, the speed is 400~500 rpm, the time is 6 h, and the ball-to-material ratio is 10:1; The preparation method of the modified hexagonal boron nitride nanosheets is as follows: the hexagonal boron nitride nanosheets and the hydroxyl-containing compound are mixed and then ball milled; the mass ratio of the hexagonal boron nitride nanosheets to the hydroxyl-containing compound is 1:6-1:9; the rotation speed of the ball mill is 400-500 rpm, the ball-to-material ratio is 10:1, and the time is 2-10 h.

[0024] 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: The ball milling time of modified aluminum powder is too short (e.g. <4 h), AlPO 4The passivation layer is incomplete and the aluminum powder is easily oxidized, resulting in a decrease in infrared reflectivity. Ball milling for too long (e.g. > 8 h) may destroy the structure of the flaky aluminum powder (the diameter-to-thickness ratio decreases), affecting the formation of a dense reflective layer (reference Example 3, the aluminum powder diameter-to-thickness ratio after ball milling for 8 h is > 20, and the emissivity is 0.42).

[0025] The flaky aluminum powder provides low infrared emissivity, the hollow microspheres reduce the dielectric constant, and the h-BN supplements the low emissivity and suppresses microwave loss; the present invention solves the dispersion stability and interface compatibility of the flaky aluminum powder, the hollow microspheres and the h-BN through surface modification processes such as -OH functionalization of the h-BN and phosphoric acid passivation of the aluminum powder. Thus, low infrared emissivity is achieved under the premise of ensuring high microwave transmittance.

[0026] In a preferred embodiment of the present invention, the aspect ratio of the flaky aluminum powder is >15, and the particle size D50 is 10-14 μm; the thickness of the hexagonal boron nitride nanosheets is <10 nm (preferably <5 nm); and the hydroxyl-containing compound is polyvinyl alcohol or urea.

[0027] 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.

[0028] 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: (1) Influence of different parameters of flake aluminum powder: 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; high aspect ratio aluminum powder (such as >20) can enhance the parallel arrangement and form a dense reflection layer (reference Example 3, emissivity 0.42).

[0029] Particle size D50 range: 10-14 μm (too small will increase scattering, too large will easily settle).

[0030] For example: Comparative Example G: Aluminum powder diameter-to-thickness ratio = 10 → emissivity 0.55, transmittance 70%.

[0031] (2) Influence of different parameters of hexagonal boron nitride nanosheets: When the thickness of h-BN nanosheets is >10 nm, the efficiency of surface grafted hydroxyl groups will be reduced and the dispersibility will be affected. The thickness of h-BN should be controlled to be <5 nm (such as in Example 1, Ra=0.08 μm).

[0032] For example: Comparative Example H: h-BN thickness = 50 nm → emissivity 0.45, transmittance 75%.

[0033] (3) The influence of different parameters of hollow dielectric microspheres: Hollow microbeads with a particle size >50 μm will introduce interface defects, and those with a particle size <10 μm will reduce the dielectric control effect; 20-40 μm microbeads are selected (such as Example 1, ε=2.5, transmittance 87%).

[0034] For example: Comparative Example I: Bead size = 80 μm → ε = 3.0, transmittance 70%.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] In the present invention, there is a significant influence between the thickness of the bottom microwave transparent layer and the surface infrared functional layer and the performance of the final coating: 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 (such as in Example 3, the transmittance of a 150 μm bottom layer is 90%).

[0040] 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).

[0041] 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.

[0042] In a preferred embodiment of the present invention, the method for preparing the optical control layer in the visible light band is: 2 O 3 Nanoparticles (particle size 50 nm, 3 wt%) were premixed with the PVDF matrix, ultrasonically dispersed (power 400 W, 20 min), and then electrostatically sprayed on the surface of the infrared functional layer.

[0043] In a preferred embodiment of the present invention, the thickness of the optical regulation layer in the visible light band is 5 μm.

[0044] 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, then adding hollow dielectric microbeads, mixing evenly, and then coating the coating on the surface of a 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.

[0045] In a preferred embodiment of the present invention, the coating method is high-pressure spraying or electrostatic spraying.

[0046] In the present invention, the present invention does not impose any particular limitation on the selection of the organic solvent, and any organic solvent known to those skilled in the art that can dissolve polyvinylidene fluoride may be selected, such as N-methylpyrrolidone (NMP).

[0047] In the present invention, when preparing the bottom microwave transparent layer, the usage ratio of the polyvinylidene fluoride to the organic solvent is 1:5 to 1:7; when preparing the surface infrared functional layer, the usage ratio of the polyvinylidene fluoride to the organic solvent is 1:6 to 1:8.

[0048] The technical solutions described in the present invention, unless otherwise specified, are all conventional solutions in the art, and the reagents or raw materials used, unless otherwise specified, are purchased from commercial channels or have been disclosed.

[0049] In order to better understand the present invention, the content of the present invention is further explained below in conjunction with the embodiments, but the content of the present invention is not limited to the following embodiments.

[0050] Example 1 1. Structure and function of coating: 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: The surface layer is made of 8 wt% modified flaky aluminum powder (diameter-to-thickness ratio>15, particle size D50 is 10-14 μm) + 5 wt% modified hexagonal boron nitride (h-BN) nanosheets (thickness <10 nm) + the remaining polyvinylidene fluoride (PVDF) matrix, with a thickness of 30 μm; the bottom layer is made of 15 wt% hollow glass microspheres (particle size 30 μm, wall thickness 1.5 μm) + the remaining PVDF matrix, with a thickness of 120 μm; The surface layer has an infrared emissivity of 0.35 (8-14 μm) and a surface roughness of Ra = 0.08 μm. The bottom layer has a dielectric constant of ε = 2.5.

[0051] 2. Preparation process: Modified flaky aluminum powder: The flaky aluminum powder and phosphoric acid (the mass ratio of flaky aluminum powder to phosphoric acid is 1:1.8) are mixed and wet-milled for 6 hours (planetary ball mill, speed 400rpm, zirconia balls, ball-to-material ratio 10:1, ethanol medium), and phosphoric acid is passivated to generate AlPO 4 layer; Modified h-BN nanosheets: h-BN nanosheets were mixed with polyvinyl alcohol (the mass ratio of h-BN nanosheets to polyvinyl alcohol was 1:8) and then ball-milled together (planetary ball mill, speed 400 rpm, steel balls, ball-to-material ratio 10:1, time 2 hours) to promote -OH grafting by mechanochemical action; 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 evaporation 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), and 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.

[0052] 3. Technical effects: 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 resistance and cycling requirements of the military standard MIL-STD-810G.

[0053] Example 2

[0054] With reference to Example 1, a control experiment was designed. The structure of the microwave-transparent and infrared-compatible low-emissivity coating used was 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.

[0055] Table 1 Control experiment , The performance parameters of the obtained coating, such as infrared emissivity and microwave transmittance, are shown in Table 2.

[0056] Table 2 Coating properties of control experiment , The following conclusions can be drawn from Tables 1 and 2: 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; 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 was increased to 82%, but the improvement on the infrared emissivity was limited (Group 2: 0.50→Group 3: 0.48), and the adhesion of the coating was improved.

[0057] Comparing Group 1 and Group 4, it can be seen that the introduction of the bottom layer with 15wt% hollow glass microspheres further improved the microwave transmittance on the basis of Group 4 (Group 4: 73% → Group 1: 87%), and significantly reduced the infrared emissivity (Group 4: 0.41 → Group 1: 0.35). At the same time, the adhesion was significantly improved, indicating that the three have a synergistic effect and achieved unexpected results.

[0058] Example 3 With reference to Example 1, a control experiment was designed, and the material properties, surface layer and bottom layer preparation methods used were the same as those in Example 1.

[0059] The surface layer formulation is the same as that in Example 1, using 8 wt% flaky aluminum powder (diameter-to-thickness 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; Two microwave transparent layers with different formulations are designed, namely Group 5 and Group 6. Among them: Group 5: 10 wt% hollow glass microspheres + 90 wt% PVDF, thickness 120 μm; Group 6: 25 wt% hollow glass microspheres + 75 wt% PVDF, thickness 120 μm.

[0060] 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 microbeads lead to a loose coating structure and a significant decrease in adhesion (4.3 MPa).

[0061] Table 3 Coating properties of control experiment .

[0062] Example 4

[0063] 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 formula was also the same as that in Example 1. Two infrared functional layers with different formulas were designed, namely Group 7 and Group 8. Among them: Group 7: 3 wt% modified flake aluminum powder + 3 wt% modified h-BN + 94 wt% PVDF; Group 8: 10 wt% modified flake aluminum powder + 10 wt% modified h-BN + 80 wt% PVDF.

[0064] 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%), due to the incomplete reflection network, the infrared emissivity increases to 0.55, 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.

[0065] Table 4 Coating properties of relevant control experiments .

[0066] Example 5

[0067] In order to investigate the effect of aluminum powder modification, a control experiment group 9 was designed. Unmodified flaky aluminum powder was used in group 9, and other material properties, surface layer and bottom layer preparation methods were the same as those in Example 1. The microwave transparent layer formula was also the same as that in Example 1 (15 wt% hollow microspheres + 85 wt% PVDF, thickness 120 μm), and its infrared functional layer formula was: Group 9: 8 wt% unmodified flake aluminum powder + 5 wt% modified h-BN + 87 wt% PVDF.

[0068] The performance parameters of the obtained coatings corresponding to Group 1 (Example 1), such as infrared emissivity, microwave transmittance and surface roughness, are shown in Table 5. It can be seen that the unmodified aluminum powder has insufficient surface passivation and poor dispersion, resulting in a decrease in the density of the reflective layer and an increase in infrared emissivity (0.45); the agglomeration of aluminum powder aggravates the dielectric loss, the microwave transmittance decreases significantly (65%), and the adhesion is also reduced due to weak interface bonding.

[0069] Table 5 Coating properties of relevant control experiments .

[0070] Example 6

[0071] In order to investigate the effect of the modification of hexagonal boron nitride (h-BN) nanosheets, a control experiment group 10 was designed. Unmodified h-BN nanosheets were used in group 10, and the other material properties, surface layer and bottom layer preparation methods were the same as those in Example 1. The formula of the microwave transparent layer was also the same as that in Example 1 (15 wt% hollow microspheres + 85 wt% PVDF, thickness 120 μm). The infrared functional layer was adjusted by omitting the polyvinyl alcohol (PVA) modification and ball milling steps of the h-BN nanosheets and directly using unmodified h-BN nanosheets. The formula of the infrared functional layer is: Group 10: 8 wt% modified flake aluminum powder + 5 wt% unmodified h-BN + 87 wt% PVDF, thickness 30 μm.

[0072] The performance parameters of the obtained coatings corresponding to Group 1 (Example 1), such as infrared emissivity, microwave transmittance and surface roughness, 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 poor dispersibility, resulting in a significant increase in 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 with 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 a sudden drop in microwave transmittance to 63% (compared with 87%); the interface between h-BN and the PVDF matrix is ​​weak, and the adhesion is reduced to 3.2 MPa (compared with 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 not modified, and the overall performance is significantly inferior to that of Example 1.

[0073] Table 6 Coating properties of relevant control experiments , 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 principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A microwave-transparent and infrared-compatible low-emissivity coating, characterized in that: The coating is composed of a double-layer structure 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.

2. The microwave-transparent, infrared-compatible, low-emissivity coating according to claim 1, characterized in that: In terms of mass percentage, the microwave transparent layer includes 10-25% hollow dielectric microbeads and the balance polyvinylidene fluoride; the infrared functional layer includes 5-15% modified flaky aluminum powder, 3-10% modified hexagonal boron nitride nanosheets and the balance polyvinylidene fluoride.

3. The microwave-transparent, infrared-compatible, low-emissivity coating according to claim 1 or 2, characterized in that: The preparation method of the modified flaky aluminum powder is as follows: the flaky aluminum powder is mixed with phosphoric acid and then wet ball milled; the mass ratio of the flaky aluminum powder to the phosphoric acid is 1:1.5-1:2; the medium of the ball milling is ethanol, the rotation speed is 400-500 rpm, the time is 6 hours, and the ball-to-material ratio is 10:1; The preparation method of the modified hexagonal boron nitride nanosheets is as follows: the hexagonal boron nitride nanosheets and the hydroxyl-containing compound are mixed and then ball milled; the mass ratio of the hexagonal boron nitride nanosheets to the hydroxyl-containing compound is 1:6-1:9; the rotation speed of the ball mill is 400-500 rpm, the ball-to-material ratio is 10:1, and the time is 2-10 h.

4. The microwave-transparent, infrared-compatible, low-emissivity coating according to claim 3, characterized in that: The diameter-to-thickness 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 or urea.

5. The microwave-transparent, 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.

6. The microwave-transparent, infrared-compatible, low-emissivity coating according to claim 1, characterized in that: The thickness of the bottom microwave transparent layer is 50-150 μm; the thickness of the surface infrared functional layer is 10-30 μm.

7. The microwave-transparent, 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.

8. 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.

9. The preparation method according to claim 8, characterized in that: The coating method is high pressure spraying or electrostatic spraying.

Images