A multifunctional metamaterial

Abstract

This invention discloses a multifunctional metamaterial comprising a radiative cooling layer and a plurality of electromagnetic wave absorption units arrayed on the surface of the radiative cooling layer. The electromagnetic wave absorption units are made of a material with a dielectric constant greater than 90 and a loss tangent of 0.008 to 0.01. The arrayed electromagnetic wave absorption units in this multifunctional metamaterial achieve electromagnetic wave absorption in a compact size, ensuring that the radiative cooling function of the radiative cooling layer is fully utilized, thereby effectively combining electromagnetic stealth and radiative cooling. Furthermore, the metamaterial has a simple structure, requiring no external active devices, reducing energy consumption and weight.

Description

Technical Field

[0001] The present invention relates to the technical field of metamaterials, and in particular to a multifunctional metamaterial. Background Art

[0002] At present, communications and transportation vehicles need to meet higher performance requirements in order to adapt to the working environment, such as stabilizing the internal temperature environment of the transportation vehicle, making multiple detection equipment invisible at the same time, and cooling stealth high-speed flying objects. The development of related high-performance materials has become a hot issue.

[0003] The absorbing coatings commonly used to achieve electromagnetic stealth are thick, and composite absorbing materials like Salisbury screens also have drawbacks such as difficulty adapting to the form factors of communications and transportation vehicles and inconvenient for large-scale assembly and application. Implementation of cooling functions often requires the addition of active devices, such as a blackbody radiation source requiring a cooling medium, which increases energy consumption and overall weight, reduces maneuverability, and leaves room for improvement in cooling effectiveness. Electromagnetic stealth requires covering a large area of ​​the equipment surface with materials that possess the corresponding functions. The effectiveness of passive radiative cooling is proportional to the assembly area. Simply combining these two structures often compromises stealth or cooling effectiveness while increasing process complexity. Summary of the Invention

[0004] The present invention aims to solve at least one of the technical problems existing in the prior art. To this end, the present invention proposes a multifunctional metamaterial that can simultaneously achieve stealth and radiative cooling functions without significantly increasing energy consumption and weight, which is of great significance for reducing the weight and energy of equipment.

[0005] In a first aspect of the present invention, a multifunctional metamaterial is proposed, comprising a radiative cooling layer and a plurality of electromagnetic wave absorption units arrayed on the surface of the radiative cooling layer; the electromagnetic wave absorption units are made of a material with a dielectric constant greater than 90 and a loss tangent value of 0.008 to 0.01.

[0006] The multifunctional metamaterial according to the embodiment of the present invention has at least the following beneficial effects: the multifunctional metamaterial can realize the combination of electromagnetic stealth and radiation cooling functions by arranging a number of electromagnetic wave absorption units made of materials with high dielectric constant and high loss tangent value on the surface of the radiation cooling layer. Among them, the radiation cooling layer can be used as the base layer for arranging the electromagnetic wave absorption units on the one hand, and can realize the radiation cooling function on the other hand; and the electromagnetic wave absorption units arranged in the array can couple with the incident electromagnetic wave to produce magnetic resonance and absorb electromagnetic waves, thereby realizing electromagnetic stealth. Specifically, when the electromagnetic wave is transmitted to the electromagnetic wave absorption units arranged in the array, Mie resonance occurs inside the electromagnetic wave absorption unit, producing the required regional electric field and magnetic field, absorbing electromagnetic waves near the resonant frequency, and realizing the stealth effect on the electromagnetic waves in this band; the electromagnetic wave absorption unit specifically adopts materials with high dielectric constant and high loss tangent value, and the material The high loss tangent value can improve the electromagnetic wave absorption rate, and its high dielectric constant enables the electromagnetic wave absorption unit to achieve electromagnetic stealth absorption function in a smaller size, which can reduce the area ratio of the electromagnetic wave absorption unit on the surface of the radiation cooling layer under the same resonance conditions, provide a larger effective area for the radiation cooling layer, reduce the mutual influence between the electromagnetic stealth and the cooling function structure, so as to fully exert the function of the radiation cooling layer, and at the same time reduce the overall structural cross-section thickness to achieve weight reduction; and its structure is simple. Compared with conventional electromagnetic stealth and radiation cooling combination devices, it does not require external active devices and has a high degree of integration.

[0007] In some embodiments of the present invention, the electromagnetic wave absorption unit has a surface coverage of 1-10% on the radiation cooling layer, wherein the surface refers to the side of the radiation cooling layer that contacts the electromagnetic wave absorption unit.

[0008] The electromagnetic wave absorption units in this array operate within the microwave frequency range. Therefore, the material selection and design of the electromagnetic wave absorption units must meet the requirements of electromagnetic resonance and possess high dielectric constant and loss tangent within this operating band. In some embodiments of the present invention, the electromagnetic wave absorption units are arranged in a periodic, evenly spaced array on the surface of the radiative cooling layer to ensure accurate resonant frequency within the operating band. Furthermore, the electromagnetic wave absorption units can be designed to have identical shapes and uniform spacing. The specific spacing can be determined through simulation calculations, and the specific shape can be a cube, sphere, or other shape.

[0009] In some embodiments of the present invention, the material of the electromagnetic wave absorption unit includes at least one absorbing material selected from barium titanate, calcium titanate, barium strontium titanate, barium strontium niobate, or bismuth magnesium niobate, or such absorbing material containing a dopant. The dopant is selected from at least one of magnesium oxide, silicon oxide, and rare earth oxides. The rare earth oxide may be lanthanum oxide, cerium oxide, or the like. For example, the electromagnetic wave absorption unit may be made of barium strontium titanate doped with magnesium oxide. This application does not specify the shape of the electromagnetic wave absorption unit; any shape capable of generating Mie resonance that meets the above-mentioned electromagnetic resonance and electromagnetic enhancement requirements may be employed. For example, the electromagnetic wave absorption unit may be designed as a cubic structure, which may absorb electromagnetic waves near the resonant frequency through magnetic dipole electromagnetic resonance.

[0010] According to Kirchhoff's law of thermal radiation, under thermal equilibrium conditions, an object's absorptivity to thermal radiation is always equal to its emissivity at the same temperature. Emissivity is the ratio of the radiant flux emitted per unit area of ​​an object's surface to the radiant flux emitted by a blackbody at the same temperature. It measures an object's ability to release energy through radiation. The higher the emissivity, the greater the object's ability to radiate for cooling. Therefore, increasing a structure's absorptivity can improve its radiative cooling capacity. Electromagnetic waves in the thermal infrared window are rarely reflected, absorbed, or scattered when passing through the atmosphere. If a multifunctional metamaterial has a high emissivity in the thermal infrared window, it can achieve cooling by establishing a stable heat exchange with outer space, which acts as a constant cooling source. In some embodiments of the present invention, the radiative cooling layer can be designed to have an emissivity of no less than 80% in the 8-14 μm band. Furthermore, the radiative cooling layer can be designed to have an absorptivity of no more than 30% in the 0.3-2.5 μm wavelength band. This low absorptivity in this wavelength band further reduces the metamaterial's absorption of solar radiation, preventing heat buildup caused by sunlight and enhancing the cooling effect. Furthermore, the thickness of the radiative cooling layer can be controlled within 100 μm to avoid affecting the transmittance of the solar radiation band.

[0011] In some embodiments of the present invention, the radiation cooling layer includes a substrate and radiation cooling particles dispersed in the substrate. The radiation cooling particles dispersed in the substrate are doped to produce high-order phonon polarization resonance in the action band (mainly the thermal infrared window or part of the thermal infrared window band), thereby reducing the dispersion characteristics of the material and achieving broadband impedance matching with the free space to reduce reflection and improve absorption.

[0012] The radiative cooling layer's effective wavelength band can encompass the infrared band (e.g., the 8-14 μm band). Specifically, the radiative cooling layer can be designed as an infrared radiative cooling layer. In some embodiments of the present invention, the substrate has a transmittance of no less than 80% in the infrared band; the radiative cooling particles are selected from at least one of silica, alumina, and silicon carbide. The shape, size, and doping level of the radiative cooling particles can be arbitrarily selected, provided that the following basic requirements are met: the size corresponds to the wavelength of electromagnetic waves at which high-order phonon polariton resonance occurs. Under specific particle size and volume fraction conditions, high-order phonon polariton resonance can occur in a band encompassing or partially encompassing the thermal infrared window, reducing the material's dispersion characteristics and achieving broadband impedance matching with free space, thereby achieving high absorptivity and emissivity. Specifically, during the preparation of the radiative cooling layer, the particle size and doping level of the radiative cooling particles can be adjusted to achieve high-order phonon polariton resonance, reduce the dispersion characteristics of the radiative cooling particle material itself, achieve broadband impedance matching with free space in the infrared band, reduce reflection, and achieve high emissivity. The particle size can be adjusted based on the material parameters of the radiative cooling particles and the operating wavelength of the application. A balance must be found in the doping level. Too little does not provide a good cooling effect, while too much affects transmittance. Simulations can be used to determine the appropriate doping level. For example, the particle size of the radiative cooling particles can be controlled at approximately 4 μm, and the doping level at approximately 6%. The shape of the radiative cooling particles can be spherical, ellipsoidal, cubic, or other shapes.

[0013] In some embodiments of the present invention, the material of the substrate is selected from a polymer material or an inorganic material, specifically a polymer material or an inorganic material having a transmittance of not less than 80% in the infrared band. For example, the polymer material can be selected from at least one of poly (4-methylpentene-1), poly (methyl methacrylate), and polydimethylsiloxane, and the inorganic material can be selected from at least one of alumina, zirconium oxide, zirconate, and ceria.

[0014] In some embodiments of the present invention, the multifunctional metamaterial further comprises a metal base layer, which is disposed on the surface of the radiation cooling layer away from the electromagnetic wave absorption unit. The above metal base layer is provided to block the transmission of electromagnetic waves and support the overall metamaterial structure, and can serve as a common part of the electromagnetic stealth and infrared radiation cooling structures. In other embodiments of the present invention, the metal base layer can be eliminated, and the metamaterial can be used in conjunction with other devices having a surface similar to the above metal base layer. Specifically, the multifunctional metamaterial is coated on the metal surface of the device during use, and the radiation cooling layer on the multifunctional metamaterial is adhered to the metal surface of the device. The metal surface of the device can be regarded as the metal base layer of the multifunctional metamaterial.

[0015] In some embodiments of the present invention, the metal base layer is made of a good conductor material or a semiconductor material. For example, good conductor materials such as gold, silver, copper, and aluminum can be used, and semiconductor materials such as germanium, gallium arsenide, and gallium nitride can also be used.

[0016] In some embodiments of the present invention, the thickness of the metal substrate is greater than or equal to the skin depth of the electromagnetic waves acting on the multifunctional metamaterial. For example, if the electromagnetic waves acting on the multifunctional metamaterial are infrared waves, the thickness of the metal substrate is set to be greater than or equal to the skin depth of the infrared waves to prevent the infrared waves from transmitting through the metal substrate. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention will be further described below with reference to the accompanying drawings and embodiments, in which:

[0018] Figure 1 This is a schematic structural diagram of the multifunctional metamaterial prepared in Example 1;

[0019] Figure 2 For the Figure 1 Cross-sectional view along line AA;

[0020] Figure 3 This is a working principle diagram of the multifunctional metamaterial in Example 1;

[0021] Figure 4 This is a graph showing the test results of the absorption rate of the multifunctional metamaterial of Example 1 to electromagnetic waves at different frequencies;

[0022] Figure 5 The equivalent refractive index test results of PMMA films made with SiO2 glass microspheres of different particle sizes are shown below.

[0023] Figure 6 The infrared band emissivity spectrum of the PMMA film prepared in Example 1;

[0024] Figure 7 The graph shows the test results of the absorption rate of electromagnetic waves at different frequencies by multifunctional metamaterials with electromagnetic wave absorption units of different sizes;

[0025] Figure 8 The test results of the absorption rate of electromagnetic waves at different frequencies of the multifunctional metamaterial with electromagnetic wave absorption units arranged in arrays with different periods. DETAILED DESCRIPTION

[0026] The following will clearly and completely describe the concept and technical effects of the present invention in conjunction with the embodiments to fully understand the purpose, features and effects of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative work are all within the scope of protection of the present invention.

[0027] Example 1

[0028] This embodiment prepares a multifunctional metamaterial, and the specific preparation process includes:

[0029] S1. Take a 200 μm thick Al metal plate as the metal base layer;

[0030] S2. Solid SiO2 glass microspheres with a diameter of 4 μm are mixed with polymethyl methacrylate (PMMA) slurry (the solvent is dimethylformamide), and ultrasonically vibrated uniformly. Then, a PMMA film is prepared on a metal substrate by casting, extrusion, casting, etc. to form a radiative cooling layer with a thickness of about 100 μm, wherein the volume proportion of SiO2 glass microspheres is 6%;

[0031] S3. A BST-15wt.% MgO (i.e., barium strontium titanate doped with 15wt.% MgO) ceramic sheet was sintered onto a PMMA film via solid-phase sintering or hot pressing. Wire cutting or laser cutting was then used to fabricate an array of cubic electromagnetic wave absorbing elements with a side length of 1.63 mm. The electromagnetic wave absorbing elements were arranged in a square matrix with a spacing of 12 mm, creating a multifunctional metamaterial. Testing revealed that the dielectric constant of this arrayed electromagnetic wave absorbing element was 106 + 0.92i, and the loss tangent was 0.0087. Furthermore, the surface area of ​​the electromagnetic wave absorbing elements on the radiative cooling layer accounted for approximately 1.8% of the total surface area of ​​the radiative cooling layer.

[0032] The structural diagram of the multifunctional metamaterial prepared above is as follows Figure 1 and Figure 2 As shown, the multifunctional metamaterial includes a metal base layer 11, a radiation cooling layer 12 and a plurality of electromagnetic wave absorption units 13. The radiation cooling layer 12 is provided on one surface of the metal base layer 11, and the electromagnetic wave absorption units 13 are arranged in an array on the other surface of the radiation cooling layer 12 away from the metal base layer 11. The metal base layer 11 is a metal Al plate; the radiation cooling layer 12 includes a PMMA substrate and radiation cooling particles SiO2 glass microspheres dispersed in the PMMA substrate. The material of the electromagnetic wave absorption unit 13 is BST-15wt.% MgO ceramic material.

[0033] The working principle diagram of the above multifunctional metamaterial is as follows Figure 3The arrayed electromagnetic wave absorption units 13 and the metal base layer work together in the microwave band, the particles undergo magnetic resonance to absorb electromagnetic waves, and the metal base layer blocks transmission; the part of the radiation cooling layer 12 that is not blocked by the electromagnetic wave absorption units 13 generates infrared band thermal radiation for passive cooling.

[0034] The absorption rate of electromagnetic waves at different frequencies of the multifunctional metamaterials prepared above was calculated by CST STUDIO SUITE electromagnetic field simulation software. Figure 4 As shown by Figure 4 It can be seen that the above multifunctional metamaterial can achieve perfect absorption of electromagnetic waves at the 14.5GHz electromagnetic stealth frequency band. In the above embodiment, the electromagnetic wave absorption unit is made of magnesium oxide-doped barium strontium titanate. The addition of magnesium oxide generally reduces the loss value of barium strontium titanate, and materials with lower loss values ​​are generally not conducive to wave absorption. However, according to the above wave absorption test results, the electromagnetic wave absorption unit array has excellent wave absorption properties. It can be seen that its wave absorption properties do not depend on the absorption capacity of the electromagnetic wave absorption unit material itself, but rather on the characteristics exhibited by the electromagnetic wave absorption unit after electromagnetic resonance.

[0035] In order to investigate the influence of the particle size of the radiative cooling particles in the radiative cooling layer on the emissivity of the radiative cooling layer, SiO2 glass microspheres with a radius of 0.5 μm were used instead of the SiO2 glass microspheres with a radius of 2 μm in Example 1 to prepare PMMA films as the radiative cooling layer. Then, the equivalent medium theory was used to calculate the equivalent refractive index of the PMMA films prepared using SiO2 glass microspheres with different particle sizes using MATLAB software.

[0036] The calculation method is as follows: dielectric microspheres are randomly and uniformly distributed in the matrix film material. The equivalent dielectric constant and equivalent magnetic permeability of the entire film material can be obtained through Mie scattering theory. If there are N particles in a unit volume, the volume ratio of the incorporated microspheres can be obtained as: Where a is the radius of the microsphere. The dielectric constant, magnetic permeability and refractive index of the microsphere particles are ε p 、μ p 、n p , the dielectric constant, magnetic permeability and refractive index of PMMA matrix are ε m 、μ m 、n m The Mie scattering coefficient is obtained as:

[0037]

[0038]

[0039] Where x = ωa / c, a is the particle radius, c is the speed of light in vacuum, ω is the angular frequency of the incident light; function ψ nWith ξ n is the Riccati-Bessel function. Then, according to the equivalent medium theory, the equivalent dielectric constant ε of the entire film material under this structure is obtained. eff and equivalent magnetic permeability μ eff for:

[0040]

[0041]

[0042] where K m is the wave vector in the matrix, μ0 is the vacuum magnetic permeability, a1 and b1 are the average first-order Mie scattering coefficients, and the equivalent refractive index of the thin film material is:

[0043]

[0044] The real and imaginary parts of the equivalent refractive index when the particle size is 0.5 μm and 2 μm are obtained by the above calculation formula. Figure 5 shown.

[0045] The results show that using SiO2 glass microspheres with a radius of 2μm and a controlled volume ratio of 6% to prepare a PMMA film as a radiative cooling layer achieves high emissivity. However, using 0.5μm SiO2 glass microspheres at the same volume ratio to prepare a PMMA film fails to achieve broadband impedance matching due to the inherent scattering properties of the material, resulting in reduced absorption. This suggests that by adjusting the particle size and volume ratio of the radiative cooling particles, high-order phonon polariton resonance can be achieved, reducing the dispersion characteristics of the radiative cooling particle material. Using equivalent medium theory, the equivalent refractive index of the film doped with radiative cooling particles can be obtained, achieving broadband impedance matching with free space in the infrared band, reducing reflection and achieving high emissivity. Furthermore, the operating frequency of the metamaterial can be tuned by adjusting the size or material composition of the radiative cooling particles in the radiative cooling layer.

[0046] In addition, the infrared band emissivity spectrum of the PMMA film in Example 1 was obtained by using the transfer matrix method through MATLAB software. Figure 6 As shown in FIG, under normal incidence, the absorptivity, i.e., the emissivity, of the material is calculated by the transfer matrix theory. From the results, it can be seen that the PMMA film prepared in Example 1 can achieve an absorptivity of nearly 1 in the atmospheric transparent window band and is highly transparent in the solar radiation band, indicating that the PMMA film can achieve the purpose of passive radiation cooling.

[0047] From the above, it can be seen that the arrayed electromagnetic wave absorption units in the multifunctional metamaterial of the present application can realize the electromagnetic wave absorption function in a smaller size, ensuring that the radiation cooling function of the radiation cooling layer can be fully utilized, thereby realizing the effective combination of electromagnetic stealth and radiation cooling functions. Moreover, its structure is simple, and no external active device is required, which reduces energy consumption and weight. It can be used in microwave stealth technology, antennas, security inspection, thermal radiation detection, thermal radiation imaging, non-destructive detection and other fields.

[0048] In addition, the size (i.e., side length) and period (i.e., spacing) of the electromagnetic wave absorption unit were adjusted respectively, and a multifunctional metamaterial was prepared in the same manner as in Example 1; wherein, the size adjustment specifically adjusted the side lengths of the cubic electromagnetic wave absorption unit to 1.4mm, 1.5mm, 1.6mm, 1.7mm, and 1.8mm, respectively, and the spacing between the electromagnetic wave absorption units was the same as in Example 1, which was 12mm; the period adjustment specifically adjusted the spacing between the electromagnetic wave absorption units to 11mm, 13mm, 14mm, and 15mm, respectively, and the size of the electromagnetic wave absorption unit was the same as in Example 1, which was 1.63mm. The CST STUDIO SUITE electromagnetic field simulation software was then used to calculate the absorption rates of electromagnetic waves at different frequencies of the multifunctional metamaterial prepared above and the functional metamaterial of Example 1, and the results were as follows: Figure 7 and Figure 8 As shown in the figure, the operating frequency of the metamaterial can be adjusted by adjusting the period and / or structural dimensions of the electromagnetic wave absorption units. For example, increasing the period of the electromagnetic wave absorption units and increasing the structural dimensions will shift the response wavelength toward longer wavelengths; decreasing the period and structural dimensions will shift the response wavelength toward shorter wavelengths, thus achieving universal operation over a wide frequency range. The structural dimensions of the electromagnetic wave absorption units have a significant impact on the wavelength.

[0049] Furthermore, for the radiative cooling structure, the thickness and material of the metal base layer can be adjusted to ensure that the metal base layer itself has basically no absorption in the solar radiation band.

[0050] The above-described embodiments merely illustrate several implementations of the present invention. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, and all such variations and improvements fall within the scope of protection of the present invention.

Claims

1. A multifunctional metamaterial, characterized in that: The invention comprises a radiation cooling layer and a plurality of electromagnetic wave absorption units arrayed on the surface of the radiation cooling layer; the emissivity of the radiation cooling layer in the 8-14 μm band is not less than 80%, and the absorptivity in the 0.3-2.5 μm band is not higher than 30%; the material of the electromagnetic wave absorption unit is a material with a dielectric constant of more than 90 and a loss tangent value of 0.008-0.01, and the surface coverage rate of the electromagnetic wave absorption unit on the radiation cooling layer is 1-10%.

2. The multifunctional metamaterial according to claim 1, characterized in that The electromagnetic wave absorbing units are arranged in a periodic and equidistant array on the surface of the radiation cooling layer.

3. The multifunctional metamaterial according to claim 1, characterized in that The material of the electromagnetic wave absorbing unit includes at least one absorbing material selected from barium titanate, calcium titanate, barium strontium titanate, barium strontium niobate, and bismuth magnesium niobate, or the absorbing material containing a dopant, and the dopant is selected from at least one of magnesium oxide, silicon oxide, and rare earth oxide.

4. The multifunctional metamaterial according to claim 1, characterized in that The radiation cooling layer includes a substrate and radiation cooling particles dispersed in the substrate; the transmittance of the substrate in the infrared band is not less than 80%; the radiation cooling particles are selected from at least one of silicon dioxide, aluminum oxide, and silicon carbide.

5. The multifunctional metamaterial according to claim 4, characterized in that: The material of the substrate is selected from a polymer material or an inorganic material; the polymer material is selected from at least one of poly-4-methylpentene-1, polymethyl methacrylate, and polydimethylsiloxane, and the inorganic material is selected from at least one of aluminum oxide, zirconium oxide, zirconate, and ceria.

6. The multifunctional metamaterial according to any one of claims 1 to 5, characterized in that It also includes a metal base layer, which is arranged on the surface of the radiation cooling layer away from the electromagnetic wave absorption unit.

7. The multifunctional metamaterial according to claim 6, characterized in that The material of the metal base layer is selected from a good conductor material or a semiconductor material.

8. The multifunctional metamaterial according to claim 6, characterized in that The thickness of the metal base layer is greater than or equal to the skin depth of the electromagnetic wave acting on the multifunctional metamaterial.

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