Multilayer coating for optical solar reflectors

Abstract

The present invention relates to a product comprising a substrate (9) and a multilayer coating (1) for thermal control of a surface (6), comprising a first inner layer (2) intended to be deposited on said surface, a second intermediate layer (3) applied on said first inner layer (2), and a third outer layer (4) applied on said second intermediate layer (3): said first inner layer (2) comprises a co-dispersion of conductive nanoparticles and dielectric nanoparticles, the volume fraction of said conductive nanoparticles being greater than or equal to the volume fraction of said second intermediate layer (4) along the thickness of said first inner layer. (3) increases as it moves away from the visible region; said second intermediate layer comprises a plurality of layers, at least one layer (3') of a dielectric material transparent in the visible region and having a high refractive index in the visible region is alternately arranged with at least one layer (3") of a dielectric material transparent in the visible region and having a low refractive index in the visible region, and the refractive index of each layer (3') of the dielectric material transparent in the visible region and having a high refractive index in the visible region is higher than the refractive index of each adjacent layer (3") of the dielectric material transparent in the visible region and having a low refractive index in the visible region; and said third outer layer (4) has a refractive index of 1×10 -3 The present invention relates to an article made of a conductive oxide that is transparent in the visible range and has a resistivity of less than Ω·cm, and also to an optical solar reflector that includes the article.

Description

[Technical Field]

[0001] Cross-reference of related applications This patent application claims priority to Italian Patent Application No. 102020000022435, filed on 23 September 2020, the entire disclosure thereof is incorporated herein by reference.

[0002] Technical field of inventions The present invention generally relates to multilayer coatings having thermo-optical properties.

[0003] The present invention has proven to have advantageous applications in the space sector, for example, in the manufacture of coatings for spacecraft components including radiator panels or antennas, or for other external structures that require mitigation of temperature changes.

[0004] In particular, although not limited thereto, the present invention has proven advantageous for applications in the manufacture of optical sunlight reflectors. [Background technology]

[0005] A satellite's thermal control system includes one or more radiator panels with surfaces facing space, designed to regulate the heat exchanged between the spacecraft or satellite and the external environment. More specifically, since the thermal control system is primarily designed to prevent the spacecraft from overheating in high-temperature phases, the surfaces of the radiator panels need to reflect solar radiation and radiate heat generated on board.

[0006] These requirements are defined based on two thermo-optic parameters: solar absorptivity α and hemispherical emissivity ε.

[0007] On the radiator panel surface, the solar thermal absorptivity α should be as low as possible (generally ≤0.20), and the hemispherical emissivity ε should be as high as possible (generally ≥0.80). Both values ​​must remain constant throughout the satellite's lifespan (15 years for geostationary communication satellites).

[0008] In addition, the surface of the radiator panel must be able to dissipate a large amount of charge generated by interactions with electrons, protons, and ionized particles. Otherwise, it could cause electrostatic discharge and damage the onboard equipment.

[0009] An economical method for controlling the thermo-optic properties of a surface is to apply a white coating to the panel consisting of inorganic particles embedded in an organic matrix. However, white coatings have poor electrostatic dissipation properties and typically degrade over time. In particular, the interaction between the organic matrix, ultraviolet light, and particles causes a gradual increase in α. Furthermore, prolonged exposure to radiation or temperature changes can reduce adhesion to the substrate.

[0010] A better approach to controlling surface properties is to cover the radiator panel with an optical sun reflective (OSR). Two types of OSRs are currently commercially available: quartz OSRs and a second type of flexible surface mirror (SSM).

[0011] The quartz OSR is a small quartz tile, approximately 40 x 40 mm in size and about 100-200 microns thick, covered with a silver metal mirror protected by Inconel on the side facing the radiator. The quartz substrate provides a high ε value, while the metal layer provides a low α value. The outer surface is covered with a thin layer of transparent, conductive oxide to dissipate charge.

[0012] Conductive resins, applied manually or robotically, are used to bond the tiles to the panel. Quartz OSR exhibits excellent thermo-optic properties and durability in the space environment, but it is expensive to procure, apply, and launch, is fragile, and can only be used for flat radiators.

[0013] Flexible SSMs are based on the same structure and operating principle as quartz OSRs, but the quartz substrate is replaced with a transparent sheet of fluorinated ethylene polymer (FEP). Flexible SSMs are relatively inexpensive and easy to use and apply to both flat and curved panels. However, FEP films tend to degrade rapidly due to their interaction with the space environment, particularly ultraviolet light (which makes the film brittle and opaque) and atomic oxygen (which corrodes the film).

[0014] Attempts to improve durability by attaching UV filters to the outer surface of the film were only partially successful. Poor adhesion of the inorganic layer on the FEP is thought to be one of the causes. In general, flexible SSMs are not recommended for space missions exceeding 5-6 years. [Overview of the project]

[0015] Objective and Overview of the Invention Therefore, there is a need in this field for a new class of coatings that combines the thermo-optic properties and space durability of quartz OSR with the ease of use and low cost of flexible SSM.

[0016] Therefore, the object of the present invention is to provide a novel coating that has improved thermo-optical properties and durability in space, and is free from the drawbacks of known coatings.

[0017] This objective is achieved by the present invention relating to the multilayer coating described in claim 1, the product described in claim 13, and the optical sunlight reflecting device described in claim 14.

[0018] In particular, according to a first aspect of the present invention, a multilayer coating for thermal control of a surface is provided, comprising a first inner layer intended to be deposited on the surface, a second intermediate layer applied on the first inner layer, and a third outer layer applied on the second intermediate layer. The multilayer coating is as follows:

[0019] - The first inner layer contains a co-dispersion of conductive nanoparticles and dielectric nanoparticles, and the volume fraction of the conductive nanoparticles increases as it moves away from the second layer along the thickness of the first inner layer;

[0020] - The second intermediate layer includes a plurality of layers, at least one layer of a dielectric material with a high refractive index that is transparent in the visible region is alternately arranged with at least one layer of a dielectric material with a low refractive index that is transparent in the visible region, and the refractive index of each layer of the dielectric material with a high refractive index that is transparent in the visible region is higher than the refractive index of each adjacent layer of the dielectric material with a low refractive index that is transparent in the visible region; and

[0021] - The third outer layer is made of a transparent conductive oxide having a resistivity of less than 1 × 10 ,

[0024] , -3 , ,

[0026] , , , , ,

[0022] ,

[0025] , , -4 , [Figure 2] , , [Figure 1] ,

[0023] , , , , Ω·cm.

[0022] [[ID=十三]] The term "transparent in the visible region" means a material having an absorption coefficient of less than 1 × 10 -3 less, preferably less than 1 × 10 -4 less.

[0023] Advantageously, the coating of the present invention is a completely inorganic coating. This coating is resistant to stress from space and meets all the requirements of the thermo-optical and charge dissipation properties required for this application. Furthermore, it protects the substrate from direct interaction with ultraviolet rays, atomic oxygen, and low-energy charged particles.

[0024] According to a second aspect of the present invention, a product including the above multi-layer coating is provided.

[0025] [[ID=2九]] According to a third aspect of the present invention, an optical solar reflector including the above multi-layer coating is provided.

Brief Description of the Drawings

[0026] [Figure 1] Figure 1 shows a schematic view of a multi-layer coating according to the present invention. [Figure 2]Figure 2 shows a schematic diagram of a radiator panel including an optical solar radiator according to the present invention. [Modes for carrying out the invention]

[0027] Detailed description of preferred embodiments of the present invention Herein, the present invention will be described in detail with reference to the accompanying drawings so that those skilled in the art may manufacture and use it. Various modifications of the described embodiments will be immediately apparent to those skilled in the art, and the disclosed general principles can be applied to other embodiments and uses without departing from the scope of protection of the present invention as defined in the accompanying drawings.

[0028] Therefore, the present invention should not be considered to be limited to the embodiments described and shown, but rather the broadest scope of protection should be recognized according to the described and claimed features.

[0029] Unless otherwise defined, all technical and scientific terms used herein have the same meanings as those commonly used by those skilled in the art in the field of the present invention. In case of any conflict, the present invention, including the definitions provided, shall be binding. Furthermore, the examples are provided for illustrative purposes only and should not be considered limiting.

[0030] To facilitate understanding of the embodiments described herein, several specific embodiments are referenced and specific terminology is used to describe them. The terminology used herein is intended to describe only specific embodiments and is not intended to limit the scope of the invention.

[0031] Multilayer coating 1 Refer to Figure 1; the multilayer coating 1 of the present invention is a coating having thermo-optical properties that can be manufactured by sputtering or other physical vapor deposition techniques, and consists of three functionally and structurally distinct layers: - A first inner layer 2, which functions as an infrared radiator, is deposited on the surface of the substrate to be covered; - A second intermediate layer 3 positioned on top of the first inner layer 2, which functions as a sun reflector; and - A third outer layer 4 that faces outward, i.e., towards space, and functions as an electrostatic dissipator.

[0032] First inner layer 2 - Infrared radiator The first inner layer 2 is made of a codispersion of conductive nanoparticles and dielectric nanoparticles. These materials are ceramic-metal composites, also known as CERMET.

[0033] The first inner layer 2 has a gradient. That is, the volume fraction of conductive nanoparticles increases along the thickness of the first inner layer 2, moving away from the interface with the second intermediate layer 3. In one embodiment, the volume fraction of conductive nanoparticles increases along the thickness of the inner layer 2, from a maximum value of ≥70% at the interface with the substrate to a minimum value of ≤30% at the interface with the intermediate layer 3.

[0034] In one embodiment, the volume fraction of conductive nanoparticles increases continuously along the thickness of the first inner layer 2, away from the interface with the second intermediate layer 3. In another embodiment, the volume fraction of conductive nanoparticles increases in small, discontinuous steps.

[0035] The infrared absorption coefficient shows a similar trend: it is low at the interface between the inner layer 2 and the second intermediate layer 3, and high at the interface of the inner layer 2 facing the substrate side. This configuration creates an electromagnetic trap, where radiation from space is absorbed in the gradually denser medium without being reflected at the interface. In one embodiment, the first inner layer 2 effectively absorbs infrared radiation of 3 to 20 microns or more.

[0036] The following points will be considered: - Absorption rate and emissivity are equivalent by Kirchoff's law; - The operating temperature of the spacecraft's radiator panel is approximately 300K; and - The peak of blackbody emissivity at 300K is at a wavelength of 10 microns. Therefore, the first inner layer 2 is characterized by a high hemispherical emissivity ε.

[0037] The second relevant feature of the first inner layer 2 is of a mechanical type. Unlike many black coatings that have a porous or columnar structure, the first inner layer 2 is compact and rigid and exhibits excellent adhesion to various substrate materials. Furthermore, thanks to its gradient composition, it is effective in compensating for the difference in coefficient of thermal expansion (CTE) between the substrate (higher CTE) and the second intermediate layer 3 (lower CTE).

[0038] The first inner layer 2 may be deposited directly onto a bare substrate, or it may be deposited with an intermediate layer of alumina or other material interposed to promote thermomechanical bonding and adhesion to the substrate.

[0039] In one embodiment, the first inner layer 2 has a thickness of 1 to 4 microns and a hemispherical emissivity of 0.5 to 0.8.

[0040] In one embodiment, the first inner layer 2 is made of aluminum, Al x It is made using a conductive material selected from the group consisting of O (i.e., aluminum monoxide), TiN, indium tin oxide, zinc aluminum oxide, steel, Ti, Mo, rare earth elements, transition metals, and combinations thereof, preferably aluminum and AlxO.

[0041] In one embodiment, the first inner layer 2 is made using a dielectric material selected from the group consisting of Al2O3, Al, SiO2, Ta2O5, ZrO2, Nb2O5, Y2O3, TiO2, and combinations thereof, preferably Al2O3.

[0042] The first inner layer 2 can be produced by reactive sputtering from a single source, by gradually changing the oxidation state of the material during deposition. In another embodiment, the layer is produced by co-deposition of different materials from two or more different sources.

[0043] Second intermediate layer 3 - Sunlight reflector The second intermediate layer 3 is positioned on top of the first inner layer 2 and functions as a sunlight reflector. It consists of a series of layers of dielectric transparent material in which high refractive index material layers 3' and low refractive index material layers 3'' are arranged alternately.

[0044] The high refractive index material layer 3' can be manufactured using the same material or different materials. The term "high refractive index" means that the refractive index in the visible range is between 1.6 and 2.5.

[0045] Furthermore, the low refractive index material layer 3" can be manufactured using the same material or different materials. The term "low refractive index" means that the refractive index in the visible range is between 1.2 and 1.7.

[0046] For the intermediate layer 3 to perform its function, each layer 3' having a high refractive index must have a higher refractive index than the adjacent layer 3'' having a low refractive index. In one embodiment, this difference between the refractive index of layer 3' and the refractive index of layer 3'' is at least 0.5, and preferably 0.7.

[0047] The term "transparent" refers to a size of 1 × 10⁻¹⁶ in the visible spectrum. -3 Less than 1 × 10 -4 This refers to materials having an absorption coefficient of less than a certain value.

[0048] By utilizing constructive and destructive interference of electromagnetic waves at the interface between the high refractive index layer 3' and the low refractive index layer 3'', high reflectivity in the solar spectrum is generated.

[0049] The set of materials for the second intermediate layer 3 can be designed according to one of several approaches known to optical designers. For example, using a quarter-wavelength sequence, a quasi-periodic Fibonacci sequence, or a Gate filter and / or hybrid solution.

[0050] In the quarter-wavelength approach, the second intermediate layer 3 is created by the superposition of several filters that reflect different parts of the solar spectrum. Each filter is based on a 3-layer unit (U) that can be repeated several times to improve performance. The following standard notation is used: - 1H indicates a layer of high refractive index material with an optical thickness equal to 1 / 4 of the central wavelength; and - 1L represents a layer of low refractive index material with an optical thickness equal to 1 / 4 of the filter's central operating wavelength. The basic unit of a filter, U, is constructed using the following relationship:

[0051]

number

[0052] Each filter is obtained by repeating the basic unit 2 to 5 times. Various filters are used to reflect different regions of the solar spectrum. Overall, the second intermediate layer 3 is derived from a superposition of 15 to 50 filters centered on various wavelengths to uniformly cover the entire UV-VIS-NIR spectrum from 100 nm to 1500 nm.

[0053] The second intermediate layer 3 may be deposited directly onto the first inner layer 2, or it may be deposited with an intermediate layer of alumina or other material interposed to improve the thermo-optical and / or mechanical bonding between the two layers.

[0054] In one embodiment, the material of the second intermediate layer 3 is a material commonly used to construct UV-VIS-NIR optical coatings and includes SiO2, MgF2, Ta2O5, ZrO2, TiO2, Nb2O5, Y2O3, YF3, and mixtures thereof.

[0055] In one embodiment, the second intermediate layer 3 is deposited by reactive sputtering.

[0056] In one embodiment, the second intermediate layer 3 consists of 45 to 150 layers, has a total thickness of 5 to 15 microns, and guarantees α in the range of 0.20 ÷ 0.05 or less on a smooth surface.

[0057] The second intermediate layer 3 has the effect of constantly increasing the emissivity of the multilayer coating of the present invention. However, depending on the materials used and the total thickness, there may be a difference in degree, and the effect may be strong or weak.

[0058] The third outer layer 4 - Electrostatic dissipator The third outer layer 4 has a resistivity of less than 1 × 10 -3 Ω·cm and is made of a transparent conductive oxide having an absorption coefficient of less than 1 × 10 -3 in the visible region, preferably less than 1 × 10 -4 in the visible region.

[0059] In a preferred embodiment, the transparent conductive layer is made of a transparent conductive oxide selected from the group consisting of indium tin oxide, alumina-doped zinc oxide, cadmium oxide and its alloys, zinc oxide and its alloys, tin oxide and its alloys, antimony and fluorine-doped tin oxide.

[0060] The outer layer 4 may be directly deposited on the second intermediate layer 3, or, in particular, an intermediate layer made of SiO2, alumina, or other materials transparent in the UV-VIS-NIR spectrum, which can be used to protect the upper layer of the multilayer coating of the present invention from corrosion by atomic oxygen, may be interposed and deposited.

[0061] In one embodiment, the third outer layer 4 can be deposited on the second intermediate layer by sputtering, and can have a thickness of 10 ÷ 50 nm and a surface resistivity of 10 3 ÷10 6 Ω / sq.

[0062] The uses of the multilayer coating of the present invention The multilayer coating 1 of the present invention can be used to manufacture products such as radiator panels, or components of spacecraft including antennas, or other external structures that require mitigation of temperature changes, and the multilayer coating is applied to the substrate to be coated.

[0063] The multilayer coating 1 can be directly applied to surfaces with altered thermo-optical properties, such as aluminum or carbon fiber reinforced polymer outer shells of radiator panels or antennas. The coating can also be applied before the final assembly of the product, i.e., before the bonding of the product's outer shell to the honeycomb structure. To improve the coating's performance α, the surface to be coated can be polished before application.

[0064] In another embodiment, the multilayer coating of the present invention can be used to manufacture a flexible optical sun-reflecting device.

[0065] For this purpose, a multilayer coating is deposited onto a flexible substrate, preferably a metal film, polymer film, composite material (particularly CFRP), or flexible glass. Subsequently, the side of the film opposite to the side on which the coating was deposited is bonded to a radiator panel or another outer surface of the spacecraft, for example, using a conductive resin or pressure-sensitive adhesive (PSA).

[0066] In one embodiment, the flexible substrate is made of polyimide or polyetherketone (PEEK), and the film thickness is 25 to 100 microns.

[0067] In another embodiment, the flexible substrate is made of fluorinated ethylene polymer (FEP), polycarbonate, polyethylene, or other polymers suitable for space use, titanium, aluminum or other metallic materials, graphite, CFPR or other composite materials, or ultrathin flexible glass.

[0068] The substrate now functions solely as a flexible support for coating and no longer needs to be optically transparent. This allows for the replacement of FEP, which was used in known flexible SSMs, with other types of polymers that are cheaper and easier to coat.

[0069] In one embodiment, the surface of a flexible substrate not covered with the multilayer coating of the present invention is metallized, and electrical contact between the outward-facing surface of the optical sun reflective device and the surface facing the coated surface is determined via small through-hole paths (so-called perforated interconnects).

[0070] Figure 2 shows an exemplary embodiment of a solar radiator 100 incorporating the flexible optical solar light reflector 5 according to the present invention.

[0071] In particular, the flexible optical solar reflector 5 is applied to the outer surface (skin) 6 of the solar radiator 100 by a layer 7 of pressure-sensitive adhesive.

[0072] The flexible optical sun reflective device 5 comprises a first outer portion consisting of a multilayer coating 1 deposited on a polymer sheet 9. The surface of the polymer film facing the outer shell 6 of the radiator 100 is covered with a metal layer 8, which is in contact with the adhesive layer 7.

[0073] The optical solar reflector according to the present invention combines the ease of use and applicability of flexible SSMs to either planar or curved surfaces with the thermo-optic properties and spatial durability of quartz OSRs, making it suitable for applications in devices involved in 15-year on-orbit missions. Furthermore, the total cost of the optical solar reflector according to the present invention is similar to that of flexible OSRs known in the art, but significantly lower than that of quartz OSRs.

[0074] The multilayer coating of the present invention preferably has a hemispherical emissivity of 0.1 to 0.8.

Claims

1. A product comprising a substrate (9) and a multilayer coating (1) for thermal control of the surface (6) of the substrate (9), The material comprises a first inner layer (2) intended to be deposited on the surface, a second intermediate layer (3) applied on the first inner layer (2), and a third outer layer (4) applied on the second intermediate layer (3): - The first inner layer (2) comprises a codispersion of conductive nanoparticles and dielectric nanoparticles, wherein the volume fraction of the conductive nanoparticles increases along the thickness of the first inner layer as it moves away from the second intermediate layer (3), and the first inner layer (2) has a hemispherical emissivity of 0.5 to 0.8; - The second intermediate layer comprises a plurality of layers, wherein at least one layer (3') of dielectric material having an absorption coefficient in the visible spectrum less than 1 × 10⁻³ and a high refractive index of 1.6 to 2.5 in the visible region is alternately arranged with at least one layer (3'') of dielectric material having an absorption coefficient in the visible spectrum less than 1 × 10⁻³ and a low refractive index of 1.2 to 1.7 in the visible region, and the refractive index of each layer (3') of dielectric material having a high refractive index in the visible region is higher than the refractive index of each adjacent layer (3'') of dielectric material having a low refractive index in the visible region; and - The third outer layer (4) has an absorption coefficient in the visible spectrum of less than 1 × 10⁻³, and 1 × 10⁻³ －３ A product made of a transparent conductive oxide in the visible range with a resistivity of less than Ω·cm.

2. The product according to claim 1, characterized in that the conductive nanoparticles are made of a material selected from the group consisting of aluminum, aluminum monoxide, TiN, indium tin oxide, zinc aluminum oxide, steel, Ti, Mo, rare earth elements, transition metals, and combinations thereof.

3. The dielectric nanoparticles are Al ２ O ３ , Al, SiO ２ , Ta ２ O ５ , ZrO ２ , Nb ２ O ５ , Y ２ O ３ , TiO ２ , AlN, and combinations thereof, and the product according to claim 1, characterized in that it is made of a material selected from the group consisting of

4. The dielectric nanoparticles are Al ２ O ３ The product according to claim 1, characterized in that it is made of aluminum oxide or Al, and the conductive nanoparticles are made of aluminum oxide or Al.

5. The product according to claim 1, characterized in that the first inner layer (2) has a thickness of 1 to 4 micrometers.

6. The product according to claim 1, characterized in that the second intermediate layer (3) has a thickness of 5 to 15 micrometers.

7. The product according to claim 1, characterized in that the second intermediate layer (3) includes 45 to 150 layers of dielectric material that is transparent in the visible range.

8. The second intermediate layer (3) is SiO ２ MgF ２ Ta ２ O ５ , ZrO ２ , TiO ２ Nb ２ O ５ , Y ２ O ３ YF ３ The product according to claim 1, characterized in that it is made of a material selected from the group consisting of and mixtures thereof.

9. The product according to claim 1, characterized in that the difference between the refractive index of the dielectric material layer (3') which is transparent in the visible range and has a high refractive index in the visible range, and the refractive index of the dielectric material layer (3'') which is transparent in the visible range and has a low refractive index in the visible range, is at least 0.

5.

10. The product according to claim 1, characterized in that the layer (3') of dielectric material that is transparent in the visible range and has a high refractive index in the visible range has a refractive index of 1.6 to 2.5 in the visible range.

11. The product according to claim 1, characterized in that the layer (3") of dielectric material that is transparent in the visible range and has a low refractive index in the visible range has a refractive index of 1.2 to 1.7 in the visible range.

12. The product according to claim 1, characterized in that the third outer layer (4) is made of a material selected from the group consisting of indium tin oxide, alumina-doped zinc oxide, cadmium oxide and its alloys, zinc oxide and its alloys, tin oxide and its alloys, antimony, and fluorine-doped tin oxide.

13. The third outer layer (4) is 10 ３ ~10 ６ The product according to claim 1, characterized in that it has a surface resistivity of Ω / sq.

14. An optical sunlight reflecting device (5) comprising the product described in any one of claims 1 to 13.

15. The optical sunlight reflecting device (5) according to claim 14, characterized in that the substrate (9) is a flexible substrate (9).

16. The optical sunlight reflecting device according to claim 15, characterized in that the flexible substrate (9) is a metal film, a polymer film, a composite material, or flexible glass.

Images