Device for controlling the infrared reflectivity

Abstract

The invention relates to an infrared reflectivity control device comprising at least one cell (1) which forms a housing comprising a top cover (2) that is transparent in the visible range and a bottom plate (3) bearing at least two electrodes (4a, 4b) which can be connected to an electrical generator (5), the housing enclosing a fluid (6) that is transparent in the visible range and which is charged with particles (7). This device is characterised in that: - the top cover (2) and the fluid (6) are transparent in the wavelength range from 2.5 micrometres to 20 micrometres; - the electrodes (4a, 4b) are reflective in the infrared range; - the electrodes (4a, 4b) are asymmetric, a first electrode (4a) having a first surface area and a second electrode (4b) having a second surface area, the first surface area being greater than the second surface area; - the particles (7) are of nanometric particle size and comprise at least one material capable of being electrically charged (non-zero zeta potential).

Description

Infrared reflectivity control device

[0001] The technical field of the invention is that of devices enabling the control of the infrared reflectivity of a medium.

[0002] The problem of controlling the infrared reflectivity of a material arises in many fields. Examples include temperature control of structures exposed to solar radiation, such as windows or satellites orbiting the Earth. This reflectivity control is also encountered in the military field for the design of camouflage for vehicles, fixed structures, or even personnel.

[0003] In the field of camouflage, we know of patent US2018 / 0267382 which proposes a structure comprising two networks of crossed electrodes arranged on either side of cells in which are housed beads containing a fluid charged with particles of various sizes and natures.

[0004] The electrode array allows for pixel-by-pixel addressing and selective cell control. This device, combined with a suitable control system, enables the creation of camouflage patterns in various colors and also infrared masking by using particles that reflect or absorb near-infrared light (wavelength less than 2 micrometers).

[0005] A lower layer of the device blocks infrared radiation, particularly from the structure being masked. Variations in the electric field between the electrodes distribute the particles to achieve the desired camouflage in the visible or near-infrared range.

[0006] This device structure is particularly complex and does not allow for effective infrared masking of a substrate, especially in the mid-infrared range (generally covering wavelengths from 2.5 micrometers to 20 micrometers). The particles, having varying masses and electrical properties, impair the masking efficiency, particularly in the infrared range.

[0007] It is particularly difficult to ensure either the absorption or the reflection of infrared radiation, because each type of behavior is associated with particles of different natures which are mixed in the same fluid and which must then be moved in a differentiated way.

[0008] US patent 2008 / 211764 also describes a display that allows for reflectivity control. This display uses a fluid transparent in the visible spectrum and charged with particles. It employs electrophoresis to create various types of displays. However, this display is limited to controlling the color between white and black, or between two or more colors. Such a display does not allow for control of the reflectivity or emissivity of a cell in the mid-infrared range as defined previously. Furthermore, it provides no details on the absorption or transparency properties of the fluid in this wavelength range.

[0009] Also known from US patent 7034987 is an electrophoretic display capable of generating a colored image. This display comprises a visible volume to which is attached at least one reservoir for electrophoretic particles. The reservoir contains at least two types of particles, each transparent in one part of the optical spectrum and absorbing or reflecting in the other part. This display increases the contrast and brightness of the image but does not allow control of infrared reflectivity or emissivity in the mid-infrared range.

[0010] The aim of the invention is to provide a simple design infrared reflectivity control device that continuously ensures performance with respect to infrared radiation, ranging from radiation absorption to radiation reflection.

[0011] The device according to the invention is thus a variable emissivity device that is more particularly intended to control reflectivity in the wavelength range of 2.5 micrometers to 20 micrometers (the range corresponding to what is generally called the mid-infrared). This range includes, in particular, bands II and III. Band II: wavelengths from 3 micrometers to 5 micrometers. Band III: wavelengths from 8 micrometers to 12 micrometers.

[0012] Thus, the device according to the invention constitutes a device enabling the effective control of the infrared emissivity of the support on which it is applied, which greatly increases the masking capabilities in the mid-infrared range.

[0013] Such performance ensures excellent infrared masking regardless of the nature of the environment of the structure to be masked.

[0014] They also allow for good temperature control of the substrate, which enables the production of glazing or insulating walls.

[0015] Thus, the invention relates to an infrared reflectivity control device comprising at least one cell forming a housing including a transparent upper cover in the visible range and a base plate carrying at least two electrodes that can be connected to an electrical generator, the housing containing a fluid transparent in the visible range and which is loaded with particles, the device characterized in that: - the upper cover (2) and the fluid (6) are transparent in the range of wavelengths from 2.5 micrometers to 20 micrometers corresponding to the mid-infrared; - the electrodes are reflective in the infrared range; - the electrodes are asymmetrical, a first electrode having a first surface and a second electrode having a second surface, the first surface being larger than the second surface;- the particles are of nanometric size and include at least one material capable of being electrically charged (non-zero zeta potential), the size being chosen so as to ensure the absorption of infrared in the wavelength range of 2.5 micrometers to 20 micrometers.;

[0016] Advantageously, the electrodes will be able to cover at least 90% of the surface of the base plate.

[0017] Advantageously, the first surface will be much larger than the second surface; for example, the first surface will be at least ten times larger than the second surface.

[0018] According to a particular embodiment, the electrodes may be transparent in the visible range.

[0019] According to a particular embodiment, the electrodes may be formed of parallel bands arranged alternately, the first electrode comprising wide bands and the second electrode comprising narrow bands, two consecutive wide bands being separated from each other by a narrow band.

[0020] According to a particular embodiment, the strips may include an alternation of crenellations and tongues.

[0021] According to a first variant, the crenellations and tongues could be rectangular in shape.

[0022] According to a second variant, the crenellations and tongues could be triangular in shape.

[0023] Advantageously, the material that can be loaded may be a metal oxide doped with a metal, this material having a particle size between 40 and 80 nanometers.

[0024] In particular, the material to be loaded may be chosen from the following: aluminum-doped zinc oxide (Al:ZnO), gallium-doped zinc oxide (Ga:ZnO), indium-doped zinc oxide (In:ZnO), niobium-doped titanium dioxide (Nb:TiO2), and tin-doped indium oxide (ITO). ITO is essentially a mixture of indium oxide (In2O3) and tin oxide (SnO2).

[0025] Advantageously, the material can be present in the fluid with a concentration of 10 to 60 milligrams per milliliter.

[0026] The invention will be better understood upon reading a detailed description made with reference to the attached drawings, in which:

[0027] schematically shows an example of the implementation of a cell of a reflectivity control device according to the invention;

[0028] represents this cell in emissive mode;

[0029] represents this cell in reflective mode;

[0030] represents, in front view, a first example of the realization of the cell electrodes;

[0031] represents a cell comprising electrodes according to this first example of embodiment, and seen in cross-section along the plane whose trace AA is located at the;

[0032] represents, in front view, a second example of the realization of the cell electrodes;

[0033] represents, in front view, a third example of the realization of the cell electrodes;

[0034] is a diagram of an infrared reflectivity control device using multiple cells;

[0035] is a diagram showing an infrared reflectivity control device according to the invention associated with masking cells in the visible range.

[0036] Referring to the diagram, a cell 1 of a device for controlling the infrared reflectivity of a structure, for example a vehicle (structure not shown), has been schematically represented.

[0037] The cell 1 forms a housing 1 comprising an upper cover 2 that is transparent in the visible range and in the range of wavelengths from 2.5 micrometers to 20 micrometers (mid-infrared), and a base plate 3 that carries at least two electrodes 4a and 4b that can be connected to an electrical generator 5. The housing 1 contains a fluid 6 that is transparent in the visible range and in the range of wavelengths from 2.5 micrometers to 20 micrometers (mid-infrared) and that is charged with particles 7. As an example, an isoparaffinic hydrocarbon of the type sold by Exxon Mobil under the Isopar brand (type L, M, or G) could be used as the fluid. An alkane, such as octane, nonane, decane, undecane, dodecane, tridecane, tetradecane, hexadecane, etc., could also be used.

[0038] The base plate 3 will for example be a glass plate and the electrodes 4a and 4b will be deposited on this plate by conventional vapor phase deposition techniques or by screen printing.

[0039] The cover 2 will, for example, be made of a polyethylene film. Its sole function is to ensure the airtightness of the housing 1 containing the fluid 6 laden with particles 7. Other materials transparent in the visible range and in the range of wavelengths from 2.5 micrometers to 20 micrometers (mid-infrared) would be possible, for example parylene or poly(p-xylylene).

[0040] According to one feature of the invention, the electrodes are made of a material that reflects infrared radiation. Electrodes 4a and 4b could, for example, be metallic (chromium deposits on plate 3, for example).

[0041] According to a particular embodiment, electrodes 4a and 4b can be made of a material that is reflective in the infrared range and also transparent in the visible range, for example ITO or PEDOT-PSS. As is well known to those skilled in the art, ITO (or Indium Tin Oxide) refers to an alloy of indium oxide (In2O3) and tin oxide (SnO2).

[0042] PDOT-PSS refers to a mixture of two polymers, poly(3,4-ethylenedioxythiophene) (or PEDOT) and sodium polystyrene sulfonate (or PSS).

[0043] The choice of transparent electrodes in the visible spectrum ensures the transparency of cell 1, allowing it to be used in conjunction with any other effective camouflage technology in the visible spectrum. Visible camouflage could be a simple static camouflage fabric or an adaptive camouflage method, such as electronically controlled camouflage. Such a variant will be described in detail later.

[0044] According to another feature of the invention, electrodes 4a and 4b are asymmetrical. Thus, a first electrode 4a has a first surface area, and a second electrode 4b has a second surface area, with the first surface area being larger than the second. The difference between the values ​​of each surface area allows for the characterization of the extreme positions, from a reflection or absorption standpoint, that the cell can adopt. Advantageously, a relatively large difference can be provided between the values ​​of the two surfaces; for example, a first surface area that is at least ten times larger than the second surface area.

[0045] On laon was shown schematically a first electrode 4a positioned in the middle of the base plate 3 and a second electrode 4b formed of two small bars arranged on either side of the first electrode 4a and at a distance from it.

[0046] The two 4b strips are electrically connected to each other, either by the external connector as shown, or by engraved tracks (not shown) carried by the base plate 3.

[0047] According to another feature of the invention, the particles 7 are of nanometric size and comprise at least one material capable of being electrically charged. Such a chargeable material is often referred to as having a non-zero zeta potential.

[0048] A nanometric particle size for a powdered material is generally less than 100 nanometers. For the particles used here, a particle size between 40 and 80 nanometers will be chosen. This range corresponds to the optimum for ensuring the absorption of infrared radiation in the wavelength range of 2.5 micrometers to 20 micrometers (mid-infrared), and in particular in bands II and III, which are the observation ranges of the main infrared detectors.

[0049] Choosing a material with a non-zero zeta potential allows for a stable suspension of particles 7 in the fluid 6. The choice of a nanometric particle size, coupled with the choice of a material with a non-zero zeta potential (conductive), allows the particles of the material to acquire plasmonic properties. The plasmonic effect results in very intense absorption within a well-defined wavelength band. The nanometric particle size also prevents light scattering in the visible range.

[0050] The plasmonic properties of a granular material have the particular advantage of increasing the infrared absorption capacities of the material.

[0051] As an example, we could choose as a material: a metal oxide doped with a metal, for example aluminum-doped zinc oxide (Al:ZnO).

[0052] This material is readily available. The particles are doped with approximately 2% aluminum atoms and 98% zinc atoms. The crystal lattice of these particles is of the würtzite type.

[0053] Preferably the material present in the fluid will have a concentration of 10 to 60 milligrams per milliliter.

[0054] This concentration range allows for significant modulation of reflectivity and also allows for a stable suspension.

[0055] The tests confirmed that, in the infrared frequency bands of 3 to 5 micrometers and 8 to 12 micrometers, infrared absorption was approximately 70% higher with a concentration of 40 milligrams per milliliter of aluminum-doped zinc oxide compared to a concentration of 10 milligrams per milliliter.

[0056] It should be noted that, in the same infrared bands, the absorption of a fluid loaded with aluminum-doped zinc oxide with a concentration of 10 milligrams per milliliter is 25% to 50% greater than that of the fluid alone.

[0057] These figures demonstrate the significant absorption performance of the proposed granular material.

[0058] Lamontre shows the cell 1 when the generator 5 applies a potential difference between the two electrodes 4a and 4b. The charge on the first electrode 4a is greater than that on the second electrode 4b. This results in the electrophoretic displacement of particles 7 in the fluid 6 (these particles are naturally negatively charged). This displacement occurs mainly laterally with respect to the cell, that is, parallel to the cover 2 and the base plate 3.

[0059] Approximately all the particles 7 are concentrated here on the first electrode 4a (large electrode) and the second electrode 4b is completely clear.

[0060] Thus, particles 7 mask the first electrode 4a, whose reflective face is covered. This results in an emissive behavior for cell 1, which no longer reflects the infrared radiation it receives from the outside but absorbs it at the level of particles 7.

[0061] Conversely, when generator 5 reverses the potentials between the two electrodes 4a and 4b, all the particles 7 concentrate on the second electrode 4b, leaving the first electrode 4a completely unobstructed. Since the first electrode 4a is reflective of infrared radiation, cell 1 is then in a reflective mode. It reflects virtually all the infrared radiation reaching it from the outside.

[0062] It should be noted that, for a given cell, the most reflective state of infrared radiation (electrode 4a completely uncovered) and the most absorbing state of infrared radiation – called the emissive state – (electrode 4a completely covered) will depend on the relative surface areas of the two electrodes. It is therefore particularly advantageous to have a large difference in surface areas between the two electrodes so that the reflective or absorbing states of the cell are clearly defined.

[0063] In practice, a direct current (DC) voltage is applied to move the particles from one electrode to the other. The voltage is then switched off to stop the particle movement. Depending on the duration of DC voltage application, a greater or lesser quantity of particles will be grouped on the wide or narrow electrodes. The cell's state can therefore be controlled almost continuously between its emissive and reflective modes. Advantageously, a low-frequency alternating current (10 Hz) is applied to disperse the particles homogeneously within the cell. This dispersion is achieved before the application of a new DC voltage in either direction.

[0064] Advantageously, electrodes 4a and 4b (and more specifically the first electrode 4a) cover at least 90% of the surface of the base plate. The aim is to give the cell significant reflective capacity when it is in its reflective state. Therefore, most of the surface of the base plate 3 must be infrared reflective (thanks to the first electrode 4a).

[0065] Figures 1 to 3 are schematic figures that allow for a clearer demonstration of the operation of the cell according to the invention.

[0066] Specifically, electrodes 4a and 4b have a particular shape that ensures maximum infrared reflectivity in the reflecting mode and also a homogeneous distribution of particles, guaranteeing maximum infrared absorption, in the emissive mode.

[0067] Lamontre front view of the base plate 3 of a cell 1 including a first example of the realization of electrodes 4a and 4b. This figure can be studied in parallel with the one which shows the associated cell 1, in section along the plane whose trace AA is represented in the one.

[0068] As can be seen in the figure, electrodes 4a and 4b are formed of parallel bands which are arranged alternately. The first electrode 4a has broad bands Bi and the second electrode 4b has narrow bands bi, two consecutive broad bands B1 and B2 being separated from each other by a narrow band b1.

[0069] All the Bi widebands are electrically connected to each other by a first 8a strip.

[0070] All the narrow bi strips are electrically connected to each other by a second 8b strip.

[0071] The first strip 8a is electrically connected to a first connection track 9a.

[0072] The second 8b strip is electrically connected to a second connection track 9b.

[0073] Thus, when cell 1 is in the reflective state, all the particles 7 are grouped on the narrow bands bi. The wide bands Bi are then clear and the infrared reflecting surface is at its maximum.

[0074] Conversely, when cell 1 is in the emissive state, all the particles 7 are grouped on the broad bands Bi. The surface area of ​​the narrow bands bi is much smaller than that of the broad bands Bi and the absorption of infrared by the particles is maximal.

[0075] Of course, the widths of the wide and narrow bands, as well as the width of the spaces between them, can be modified. The important thing is to always maintain a significant surface area difference between the wide and narrow bands within the cell, while covering as much of the base plate 3 surface area as possible.

[0076] This allowed us to test wide bands with a width varying between 200 micrometers and 500 micrometers combined with narrow bands 20 micrometers wide.

[0077] The space between the wide and narrow bands could be varied between 110 micrometers and 20 micrometers.

[0078] Other shapes are possible for the first electrode 4a and the second electrode 4b.

[0079] Lamontre thus created wide bands Bi and narrow bands bi, each with an alternating pattern of notches Ci / ci and tabs Li / li. Each tab li of a narrow band bi fits into a notch Ci of a wide band Bi. Similarly, each tab Li of a wide band Bi fits into a notch ci of a narrow band bi.

[0080] In the method of implementation, the crenellations and tongues are also rectangular in shape.

[0081] This embodiment results in greater coverage of the base plate surface. This variant also reduces particle accumulation.

[0082] Lamontre another embodiment which is analogous to that of lamais for which the crenellations Ci / ci and tongues Li / li are of triangular shapes.

[0083] Lamontre an infrared reflectivity control device 10 which uses several cells 1 assembled on a common support 11, for example a canvas or a rigid plate supported by a structure.

[0084] All the cells 1 have a structure of the type described previously. But each cell 1 can be individually controlled by a control means 12 equipped with means enabling it, by addressing, to apply a particular voltage to each cell 1 to give it a particular emissive or reflective state.

[0085] This state will be defined by a calculation method incorporated into the control means 12 and associated, for example, with a camera means enabling it to determine the infrared emission or absorption characteristics of the medium surrounding the structure carrying the device. See, for example, patent EP2992292, which describes such a device.

[0086] This makes it possible to give the device 10 a particular infrared signature that varies from cell to cell.

[0087] As an example, cells 1a, 1b, 1c have been represented with different emissive or reflective states.

[0088] Lamontre has a cell 1 whose base plate 3 is transparent in the visible range. It is positioned on three cells K1, K2, and K3 containing cholesteric liquid crystals, producing the following colors: blue for K1, green for K2, and red for K3. Each cell K1, K2, and K3 can be individually controlled by a control device, allowing for a greater or lesser intensity of each color.

[0089] Cells that allow for the production of colors with varying levels of intensity are known, for example from patent EP3213146.

[0090] The stack of cells 1, K1, K2 and K3 is fixed on a camouflaged S-structure.

[0091] Thanks to the invention it becomes possible to combine masking in the visible range provided by cells K1, K2 and K3 with infrared masking provided by cell 1.

[0092] It is of course also possible to associate cells 1 with a support whose camouflage pattern is fixed.

Claims

Infrared reflectivity control device comprising at least one cell (1) forming a housing including an upper cover (2) transparent in the visible range and a base plate (3) carrying at least two electrodes (4a,4b) that can be connected to an electrical generator (5), the housing containing a fluid (6) transparent in the visible range and which is loaded with particles (7), device characterized in that:- the upper cover (2) and the fluid (6) are transparent in the range of wavelengths from 2.5 micrometers to 20 micrometers;- the electrodes (4a,4b) are reflective in the infrared range;- the electrodes (4a,4b) are asymmetrical, a first electrode (4a) having a first surface and a second electrode (4b) having a second surface, the first surface being larger than the second surface;- the particles (7) are of nanometric size and comprise at least one material capable of being electrically charged (non-zero zeta potential), the size being chosen so as to ensure the absorption of infrared in the wavelength range of 2.5 micrometers to 20 micrometers.; Infrared reflectivity control device according to claim 1, characterized in that the electrodes (4a,4b) cover at least 90% of the surface of the base plate (3). Infrared reflectivity control device according to claim 1 or 2, characterized in that the first surface is at least ten times larger than the second surface Infrared reflectivity control device according to any one of claims 1 to 3, characterized in that the electrodes (4a,4b) are transparent in the visible range. Infrared reflectivity control device according to any one of claims 1 to 4, characterized in that the electrodes (4a,4b) are formed of parallel bands (Bi,bi) arranged alternately, the first electrode (4a) comprising wide bands (Bi) and the second electrode (4b) comprising narrow bands (bi), two consecutive wide bands being separated from each other by a narrow band. Infrared reflectivity control device according to claim 5, characterized in that the bands (Bi,bi) have an alternation of slots and tabs. Infrared reflectivity control device according to claim 6, characterized in that the slots and tabs are rectangular in shape. Infrared reflectivity control device according to claim 6, characterized in that the slots and tabs are triangular in shape. Infrared reflectivity control device according to any one of claims 1 to 8, characterized in that the material capable of being charged is a metal-doped metal oxide, this material having a particle size between 40 and 80 nanometers. Infrared reflectivity control device according to claim 9, characterized in that the material capable of being loaded is chosen from the following materials: aluminum-doped zinc oxide (Al:ZnO), gallium-doped zinc oxide (Ga:ZnO), indium-doped zinc oxide (In:ZnO), niobium-doped titanium dioxide (Nb:TiO2), tin-doped indium oxide (ITO). Infrared reflectivity control device according to claim 10, characterized in that the material is present in the fluid (6) with a concentration of 10 to 60 milligrams per milliliter.

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