Multilayer coating for optical solar reflectors

Abstract

The invention relates to a product comprising a substrate (9) and a multilayer coating (1) for thermal control of a surface (6), the multilayer coating comprising: a first inner layer (2) 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), wherein said first inner layer (2) comprises a co-dispersion of electrically conductive nanoparticles and dielectric nanoparticles, wherein the volume fraction of electrically conductive nanoparticles increases away from said second intermediate layer (3) along the thickness of said first inner layer; said second intermediate layer comprises a plurality of layers, wherein at least one layer of dielectric material transparent in the visible light and having a high refractive index in the visible light alternates with at least one layer of dielectric material transparent in the visible light and having a low refractive index in the visible light, and wherein the refractive index of each layer of dielectric material transparent in the visible light and having a high refractive index in the visible light is higher than the refractive index of each adjacent layer of dielectric material transparent in the visible light and having a low refractive index in the visible light; and said third outer layer (4) is made of an electrically conductive oxide transparent in the visible light having an electrical resistivity less than 1 x 10 ‑3 ohm x cm. An optical solar reflector comprising the product is also provided.

Description

[0001] Cross-references to related applications

[0002] This patent application claims priority to Italian Patent Application No. 102020000022435, filed on September 23, 2020, the entire disclosure of which is incorporated herein by reference. Technical Field

[0003] This invention relates in general to a multilayer coating with thermo-optical properties.

[0004] This invention has discovered advantageous applications (though not the only ones) in the space field, such as in the manufacture of coatings for radiator panels or parts of spacecraft that include antennas or other external structures that require mitigation of rapid temperature increases.

[0005] In particular, the present invention has discovered advantageous applications in the manufacture of optical solar reflectors, although not the only ones. Background Technology

[0006] A satellite's thermal control system includes one or more radiator panels with surfaces facing space, designed to regulate the heat exchange 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 during its thermal phase, the surfaces of the radiator panels must reflect solar radiation and radiatively dissipate the heat generated on the spacecraft.

[0007] These requirements are based on two thermo-optical parameters: solar absorptivity α and hemispherical emissivity ε.

[0008] On the surface of the radiator panel, the solar absorptivity α must be as low as possible (typically ≤0.20), and the hemispherical emissivity ε must be as high as possible (typically ≥0.80), and both values ​​must remain constant throughout the satellite's lifespan (15 years for telecommunications satellites in geostationary orbit).

[0009] In addition, the surface of the radiator panel must be able to dissipate to accumulate the charge generated by the interaction with electrons, protons and ionized particles, otherwise these charges will cause electrostatic discharge and damage the instruments on the spacecraft.

[0010] An economical method for controlling surface thermo-optical properties is to coat the panel with white paint composed of inorganic particles incorporated into an organic matrix. However, white paint has poor electrostatic dissipation properties and typically ages. In particular, α gradually increases due to the interaction between the organic matrix and UV radiation and the particles. Furthermore, adhesion to the substrate decreases with prolonged exposure to radiation or a sudden increase in temperature.

[0011] A better way to control surface properties is to cover the radiator panel with an optical solar reflector (OSR). Two types of OSRs are available on the market today: quartz OSRs and second flexible surface mirrors (SSMs).

[0012] Quartz OSRs are small quartz tiles, approximately 40 × 40 mm in size, with a thickness of about 100 to 200 micrometers. A silver metallic mirror protected by Inconel is coated on the radiator-facing side. The quartz substrate provides a high ε value, while the metallic layer provides a low α value. A thin layer of transparent and conductive oxide is coated on the outward-facing surface to allow charge dissipation. The tiles are bonded to the panel using a conductive resin applied manually or robotically. Quartz OSRs exhibit excellent thermo-optical properties and durability in space environments; however, they have high procurement, application, and emission costs, are prone to breakage, and can only be used in planar radiators.

[0013] Flexible SSMs are based on the same architecture and operating principles as quartz OSRs, but the quartz substrate is replaced by a transparent sheet of fluorinated ethylene polymer (FEP). Flexible SSMs are relatively inexpensive and easy to handle and apply to flat or curved panels; however, they tend to age quite rapidly due to the interaction of the FEP film with the space environment, particularly with UV radiation (which makes the film brittle and opaque) and with atomic oxygen (which corrodes the film). Attempts to improve durability by adding UV filters to the outer surface of the film have only been partially successful. Poor adhesion of the inorganic layers on the FEP is one possible cause. Generally, flexible SSMs are not recommended for space missions longer than 5 to 6 years. Summary of the Invention

[0014] Therefore, there is a need in the field for a new class of coatings that combine the thermo-optical properties and space durability of quartz OSR with the ease of use and low cost of flexible SSM.

[0015] Therefore, the object of the present invention is to provide a new coating that has improved thermo-optical properties and space durability, and does not have the disadvantages of known coatings.

[0016] This objective is achieved by the present invention, as the present invention relates to a multilayer coating according to claim 1, a product according to claim 13, and an optical solar reflector according to claim 14.

[0017] In particular, according to a first aspect of the invention, a multilayer coating for surface thermal control is provided, comprising: a first inner layer for deposition on the surface; a second intermediate layer applied to the first inner layer; and a third outer layer applied to the second intermediate layer, wherein:

[0018] - The first inner layer comprises a co-dispersion of conductive nanoparticles and dielectric nanoparticles, wherein the volume fraction of the conductive nanoparticles increases away from the second layer along the thickness of the first inner layer.

[0019] - The second intermediate layer comprises a plurality of layers, wherein at least one layer of a dielectric transparent material having a high refractive index in visible light alternates with at least one layer of a dielectric transparent material having a low refractive index in visible light, and wherein the refractive index of each layer of the dielectric transparent material having a high refractive index in visible light is higher than the refractive index of each adjacent layer of the dielectric transparent material having a low refractive index in visible light; and

[0020] -The third outer layer has a resistivity of less than 1×10⁻⁶. -3 It is made of transparent conductive oxide with an ohm×cm.

[0021] The term "transparent in visible light" means that the extinction coefficient in the visible spectrum is less than 1 × 10⁻⁶. -3 , preferably 1×10 -4 Materials.

[0022] Advantageously, the coating of the present invention is a completely inorganic coating. This coating resists stress from space and meets all the heat, light, and charge dissipation requirements of the application. Furthermore, the coating protects the substrate from direct interaction with UV rays, atomic oxygen, and low-energy charged particles.

[0023] According to a second aspect of the invention, a product comprising a multilayer coating as described above is provided.

[0024] According to a third aspect of the invention, an optical solar reflector comprising the multilayer coating described above is also provided. Attached Figure Description

[0025] Figure 1 A schematic diagram of a multilayer coating according to the present invention is shown.

[0026] Figure 2 A schematic diagram of a radiator panel including an optical solar radiator according to the present invention is shown. Detailed Implementation

[0027] The invention will now be described in detail with reference to the accompanying drawings to enable those skilled in the art to make and use it. Various modifications to the described embodiments will immediately become apparent to those skilled in the art, and the general principles disclosed can be applied to other embodiments and applications without departing from the scope of protection of the invention as defined in the drawings. Therefore, the invention should not be considered limited to the described and illustrated embodiments, but should be granted the widest scope of protection according to the described and claimed features.

[0028] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly used by one of ordinary skill in the art. In the event of conflict, this invention (including the definitions provided) shall be binding. Furthermore, the examples provided are for illustrative purposes only and should not be construed as limiting.

[0029] To facilitate understanding of the embodiments described herein, reference will be made to specific embodiments, and specific language will be used to describe them. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of the invention.

[0030] Multi-layer coating 1

[0031] Reference Figure 1 The multilayer coating 1 of the present invention is a coating with thermo-optical properties, which can be produced by sputtering or using other physical vapor deposition techniques, and consists of three functionally and structurally different layers:

[0032] - The first inner layer 2, which acts as an infrared radiation emitter, is deposited on the surface of the substrate to be coated;

[0033] - A second intermediate layer 3, which acts as a solar reflector, is arranged above the first inner layer 2; and

[0034] - The third outer layer 4, which acts as an electrostatic dissipator, faces outward, i.e., towards space.

[0035] First Inner Layer 2 - Infrared Emitter

[0036] The first inner layer 2 is made of a co-dispersion of conductive and dielectric nanoparticles. These materials are ceramic-metal composites, also known as CERMET.

[0037] The first inner layer 2 is gradient, meaning that the volume fraction of conductive nanoparticles increases along the thickness of the first inner layer 2 away from its 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.

[0038] In one embodiment, the volume fraction of the conductive nanoparticles increases continuously along the thickness of the first inner layer 2 away from its interface with the second intermediate layer 3. In an alternative embodiment, the volume fraction of the conductive nanoparticles increases in small, discontinuous steps.

[0039] The infrared extinction coefficient follows the same trend: 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. This configuration creates an electromagnetic trap, in which radiation from space is absorbed in a medium with gradually increasing density, with no reflection at the interface. In one embodiment, the first inner layer 2 effectively absorbs infrared radiation of 3 to 20 micrometers and above.

[0040] Considering:

[0041] According to Kirchoff's laws, absorptivity and emissivity are equivalent;

[0042] - The spacecraft's radiator panels operate at a temperature of approximately 300K; and

[0043] - The emissivity of a blackbody peaks at a wavelength of 10 micrometers at 300K.

[0044] Therefore, the first inner layer 2 is characterized by a high hemispherical emissivity ε.

[0045] The second relevant feature of the first inner layer 2 is mechanical. Unlike many black coatings with porous or columnar structures, the first inner layer 2 is dense and rigid, and exhibits excellent adhesion to a variety of substrate materials. Furthermore, due to its gradient composition, it is effective in compensating for misalignment of the coefficient of thermal expansion (CTE) between the substrate (higher CTE) and the second intermediate layer 3 (lower CTE).

[0046] The first inner layer 2 can be deposited directly on the bare substrate or deposited by inserting an intermediate layer of alumina or other materials, which promotes thermomechanical coupling and adhesion to the substrate.

[0047] In one embodiment, the first inner layer 2 has a thickness between 1 micrometer and 4 micrometers and a hemispherical emissivity between 0.5 and 0.8.

[0048] In one embodiment, the first inner layer 2 uses materials made of aluminum, Al x Conductive materials selected from the group consisting of O (i.e., low-value oxides of aluminum), TiN, indium tin oxide, zinc aluminum oxide, steel, Ti, Mo, rare earth elements, transition metals, and combinations thereof (preferably aluminum and Al). x Made from O).

[0049] In one embodiment, the first inner layer 2 is made of a dielectric material (preferably Al2O3) selected from the group consisting of Al2O3, Al, SiO2, Ta2O5, ZrO2, Nb2O5, Y2O3, TiO2 and combinations thereof.

[0050] The first inner layer 2 can be generated by reactive sputtering from a single source by gradually changing the oxidation state of the material during deposition. In an alternative embodiment, this layer is generated by co-depositing different materials from two or more different sources.

[0051] Second intermediate layer 3 - Solar reflector

[0052] The second intermediate layer 3 is arranged above the first inner layer 2 and acts as a solar reflector. It consists of a series of dielectric transparent material layers, wherein a material layer 3' with a high refractive index is alternated with a material layer 3” with a low refractive index.

[0053] The high-refractive-index material layer 3' can be made entirely of the same material or different materials. The term "high-refractive-index" refers to the refractive index in visible light, which is between 1.6 and 2.5.

[0054] The material layer 3 with a low refractive index can also be made entirely of the same material or different materials. The term "low refractive index" refers to the refractive index in visible light between 1.2 and 1.7.

[0055] For the intermediate layer 3 to perform its function, it is necessary that the refractive index of each layer 3' having a high refractive index is higher than that of the adjacent layer 3" having a low refractive index. In one embodiment, the difference between the refractive index of layer 3' and the refractive index of layer 3" is at least 0.5, preferably 0.7.

[0056] The term "transparent" refers to an extinction coefficient of less than 1 × 10⁻⁶ in the visible spectrum. -3 , preferably 1×10 -4 Materials.

[0057] The constructive and destructive interference of electromagnetic waves at the interface between layer 3' with a high refractive index and layer 3” with a low refractive index is used to generate high reflectivity in the solar spectrum.

[0058] The second intermediate layer 3 can be designed using one of several methods known to optical designers, such as using quarter-wavelength sequences, quasi-periodic Fibonacci sequences, or pleated filters and / or hybrid solutions. In the quarter-wavelength method, the second intermediate layer 3 is generated by stacking several filters reflecting different portions of the solar spectrum. Each filter is based on a three-layer unit (U), which can be repeated several times to improve performance. Using standard notation, where:

[0059] -1H indicates a material layer with a high refractive index, whose optical thickness is equal to one-quarter of the center wavelength; and

[0060] -1L indicates a layer with a low refractive index material, whose optical thickness is equal to one-quarter of the center working wavelength of the filter.

[0061] The basic unit U of a filter is constructed using the following relationship:

[0062] 1U = 1 / 2L 1H = 1 / 2L

[0063] or

[0064] 1U = 1 / 2H 1L = 1 / 2H.

[0065] Each filter is obtained by repeating the basic unit 2 to 5 times. Different filters are used to reflect different regions of the solar spectrum. Overall, the second intermediate layer 3 is derived from the stacking of 15-50 filters with different center wavelengths to uniformly cover the entire UV-VIS-NIR spectrum from 100 nm to 1500 nm.

[0066] The second intermediate layer 3 can be deposited directly on the first inner layer 2, or deposited by inserting an intermediate layer of alumina or other materials that improves the thermal, optical and / or mechanical coupling between the two layers.

[0067] In one embodiment, the material of the second intermediate layer 3 is a material commonly used to construct optical coatings in UV-VIS-NIR spectra, and includes SiO2, MgF2, Ta2O5, ZrO2, TiO2, Nb2O5, Y2O3, YF3 and mixtures thereof.

[0068] In one embodiment, the second intermediate layer 3 is deposited by reactive sputtering.

[0069] In one embodiment, the second intermediate layer 3 consists of 45-150 layers with a total thickness of 5 to 15 micrometers, and ensures that α is in the range of 0.20 to 0.05 or less on a smooth surface.

[0070] The second intermediate layer 3 always has a positive effect on the emissivity of the multilayer coating of the present invention, which tends to increase the emissivity. However, this effect varies in strength depending on the materials used and the total thickness.

[0071] Third outer layer 4 - Static dissipation device

[0072] The third outer layer 4 is made of transparent conductive oxide with a resistivity of less than 1×10⁻⁶. -3 The extinction coefficient in the visible light is less than 1 × 10⁻⁶ ohm×cm. -3 1×10 is preferred -4 .

[0073] In a preferred embodiment, the transparent conductive layer is made of a transparent conductive oxide selected from the group consisting of indium tin oxide, aluminum oxide-doped zinc oxide, cadmium oxide and its alloys, zinc oxide and its alloys, tin oxide and its alloys, antimony and fluorine-doped tin oxide.

[0074] The outer layer 4 can be deposited directly on the second intermediate layer 3, or it can be intermediated by an intermediate layer made of SiO2, alumina, or other materials that are transparent in the UV-VIS-NIR spectrum, wherein the other materials can be used to protect the upper layer of the multilayer coating of the present invention from atomic oxygen erosion.

[0075] In one embodiment, the third outer layer 4 can be deposited over the second intermediate layer by sputtering, and can have a thickness of 10 to 50 nm and 10 3 Up to 10 6 Surface resistivity in ohms per square.

[0076] Application of the multilayer coating of the present invention

[0077] The multilayer coating 1 of the present invention can be used to manufacture products, such as radiator panels or parts of spacecraft that require mitigation of rapidly increasing temperatures for antennas or other external structures, wherein the multilayer coating is applied to a substrate to be coated.

[0078] The multilayer coating 1 can be applied directly to the surface whose thermo-optical properties need to be modified, such as the aluminum or carbon fiber reinforced polymer surface of a radiator panel or antenna. The coating can also be applied before the final assembly of the product, i.e., before the bonding between the product's surface and the honeycomb support structure. Before applying the coating, the surface to be coated can be polished to improve the coating's performance α.

[0079] In an alternative embodiment, the multilayer coating of the present invention can be used to manufacture flexible optical solar reflectors.

[0080] For this purpose, a multilayer coating is deposited on a flexible substrate, preferably a metal film, a polymer film, a composite material (especially CFRP), or flexible glass. Subsequently, the surface of the film opposite to the surface on which the coating is deposited is bonded to the radiator panel or another outer surface of the spacecraft using, for example, a conductive resin or a pressure-sensitive adhesive (PSA).

[0081] In one embodiment, the flexible substrate is made of polyimide or polyetherketone (PEEK), and the film thickness is between 25 and 100 micrometers.

[0082] In alternative embodiments, the flexible substrate is made of fluorinated ethylene polymer (FEP), polycarbonate, polyethylene or other polymers suitable for use in space, titanium, aluminum or other metallic materials, graphite, CFPR or other composite materials, or ultrathin flexible glass.

[0083] The substrate serves only as a flexible support for the coating and no longer needs to be optically transparent. This allows FEP, which is used in known flexible SSMs, to be replaced by other types of polymers that are cheaper and easier to coat.

[0084] In one embodiment, the surface of a flexible substrate not coated with the multilayer coating of the present invention is metallized, and electrical contact between the surface of the outward-facing optical solar reflector and the surface facing the coating is determined by a path of small vias (so-called through-hole interconnects).

[0085] Figure 2 An exemplary embodiment of a solar radiator 100 according to the present invention is shown, which incorporates a flexible optical solar reflector 5.

[0086] Specifically, the flexible optical solar reflector 5 is applied to the outer surface (surface layer) 6 of the solar radiator 100 by means of a pressure-sensitive adhesive layer 7.

[0087] The flexible optical solar reflector 5 includes a first exterior consisting of a multilayer coating 1 deposited on a polymer sheet 9. The surface of the polymer film facing the radiator 100 is coated with a metal layer 8, which is then in contact with an adhesive layer 7.

[0088] The flexible optical solar reflector according to the invention combines the ease of use and applicability of a flexible SSM (Surface-Mounted Solar Surface) on a planar or curved surface with the thermo-optical properties and space durability of a quartz OSR (Optical Sealant Reflector), making it suitable for use in devices involved in 15-year missions in orbit. Furthermore, the total cost of the optical solar reflector according to the invention is similar to, but significantly lower than, that of a known flexible OSR.

[0089] The multilayer coating of the present invention preferably has a hemispherical emissivity between 0.1 and 0.8.

Claims

1. An optical solar reflector (5) comprising a substrate (9) and a multilayer coating (1) for thermally controlling the surface (6) of the substrate (9), the multilayer coating (1) comprising: A first inner layer (2) is used to be deposited on the surface; The second intermediate layer (3) is applied to the first inner layer (2); and a third outer layer (4), which is applied to the second intermediate layer (3), wherein, - The first inner layer (2) comprises a co-dispersion of conductive nanoparticles and dielectric nanoparticles, wherein the volume fraction of conductive nanoparticles increases with the thickness of the first inner layer away from the second intermediate layer (3), and wherein the first inner layer (2) has a hemispherical emissivity between 0.5 and 0.

8. - The second intermediate layer comprises a plurality of layers, wherein at least one dielectric material layer (3') that is transparent in visible light and has a high refractive index in visible light alternates with at least one dielectric material layer (3') that is transparent in visible light and has a low refractive index in visible light, and wherein the refractive index of each dielectric material layer (3') that is transparent in visible light and has a high refractive index in visible light is higher than the refractive index of each adjacent dielectric material layer (3') that is transparent in visible light and has a low refractive index in visible light; and - The third outer layer (4) is made of a material that is transparent in visible light and has a resistivity of less than 1×10⁻⁶. -3 It is made of conductive oxide with an ohm×cm.

2. The optical solar reflector (5) according to claim 1, characterized in that, The conductive nanoparticles are selected from aluminum and Al. x Materials consisting of the group consisting of O, TiN, indium tin oxide, zinc aluminum oxide, steel, Ti, Mo, rare earth elements, transition metals, and combinations thereof.

3. The optical solar reflector (5) according to claim 1, characterized in that, The dielectric nanoparticles are selected from materials consisting of Al2O3, Al, SiO2, Ta2O5, ZrO2, Nb2O5, Y2O3, TiO2, AlN, and combinations thereof.

4. The optical solar reflector (5) according to claim 1, characterized in that, The dielectric nanoparticles are made of Al₂O₃, and the conductive nanoparticles are made of Al₂O₃. x Made of O or Al.

5. The optical solar reflector (5) according to claim 1, characterized in that, The first inner layer (2) has a thickness between 1 micrometer and 4 micrometers.

6. The optical solar reflector (5) according to claim 1, characterized in that, The second intermediate layer (3) has a thickness between 5 micrometers and 15 micrometers.

7. The optical solar reflector (5) according to claim 1, characterized in that, The second intermediate layer (3) comprises 45 to 150 dielectric layers that are transparent in visible light.

8. The optical solar reflector (5) according to claim 1, characterized in that, The second intermediate layer (3) is made of a material selected from the group consisting of SiO2, MgF2, Ta2O5, ZrO2, TiO2, Nb2O5, Y2O3, YF3 and mixtures thereof.

9. The optical solar reflector (5) according to claim 1, characterized in that, The difference between the refractive index of the dielectric material layer (3') that is transparent in visible light and has a high refractive index in visible light and the refractive index of the dielectric material layer (3") that is transparent in visible light and has a low refractive index in visible light is at least 0.

5.

10. The optical solar reflector (5) according to claim 1, characterized in that, The dielectric material layer (3'), which is transparent in visible light and has a high refractive index in visible light, has a refractive index of 1.6 to 2.5 in visible light.

11. The optical solar reflector (5) according to claim 1, characterized in that, The dielectric material layer (3), which is transparent in visible light and has a low refractive index in visible light, has a refractive index of 1.2 to 1.7 in visible light.

12. The optical solar reflector (5) 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, aluminum oxide-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 optical solar reflector (5) according to claim 1, characterized in that, The third outer layer (4) has 10 3 Up to 10 6 Surface resistivity between ohms and square.

14. The optical solar reflector (5) according to claim 1, characterized in that, The substrate (9) is a flexible substrate (9).

15. The optical solar reflector (5) according to claim 14, characterized in that, The flexible substrate (9) is a metal film, polymer film, composite material or flexible glass.

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