Radar absorbing materials and method of manufacturing such materials

Abstract

A material having radar absorbing properties comprises a matrix material enclosing an amount of fiber. The fiber is coated by graphene and / or graphene oxide. Also disclosed is a laminated material, a sheet material, A 3D structure, a vehicle, a tubular structure, a use of a material, as well as a method of forming at least part of a product having radar absorbing properties.

Description

[0001]

[0002] RADAR ABSORBING MATERIALS AND METHOD OF MANUFACTURING SUCH MATERIALS

[0003] Technical field

[0004] The present disclosure relates generally to radar absorbing materials and to structures incorporating such materials or being formed of such materials.

[0005] Background

[0006] Stealth technology, known as low observable technology, finds extensive use in aircraft, ships, submarines, and missiles to reduce their visibility and detectability by radar systems, infrared, sonar, and other detection methods. Conventionally, materials can transmit, absorb, or reflect electromagnetic waves, making absorbing electromagnetic energy a key strategy for reducing radar observability. Various radar-absorbing materials (“RAM”) have been developed for such low observable applications, absorbing electromagnetic frequencies within the radar range.

[0007] In the past, the predominant use of metal in aerial structures directed RAM development towards coating metallic surfaces with paints, such as iron ball paint containing tiny spheres coated with carbonyl iron or ferrite. However, these stealth paints have drawbacks, being prone to wear and damage due to their surface placement atop the structure, particularly problematic under high temperatures and at high speeds, leading to substantial maintenance costs for aerial vehicle users.

[0008] Another type of RAM involves foams impregnated with conductive fillers like carbon, but their softness necessitates impractically thick layers, making them unsuitable for aerial vehicle applications. Alternatively, separate tiles of RAMs made of polymers mixed with conductive or magnetic fillers can be manufactured and glued onto aerial vehicles, but they add significant weight and risk delamination in operative environments. An alternative approach to achieve stealth functionality in aerial vehicles involves integrating RAMs into the vehicle structure. Modern and future aerial objects predominantly employ composite materials, primarily carbon or glass fiber-based composites. Carbon fiber reflects efficiently radar waves, making objects made of it easily visible to radar systems. While glass fiber is nearly transparent to radio waves, it results in low reflection, but it does not provide stealth function to the aerial vehicle, since it does not shield other objects behind it, like metal frames, or batteries and electronics.

[0009] One approach for integrated stealth functionality involves adding conductive fillers to the composite matrix (thermoplastic or thermoset resin). However, this method limits the design of complex internal structures and provide risks such as reducing the mechanical properties of the whole composite.

[0010] An alternative solution would be engineering of the reflectivity of the reinforcing fabrics, comprising a composite, providing them stealth functionality. Such fabrics would use complex design of the composite materials, providing fine control over the electromagnetic properties within in different directions within the composite material. This approach is presented in the current disclosure.

[0011] There is a need for further improvements in RAMs.

[0012] Summary

[0013] An objective of the present disclosure is to provide further improved RAMs. Particular objectives include the provision of RAMs which can be used to manufacture enclosures, hulls, fuselages, bodies, covers, etc. of products having low visibility to radar radiation.

[0014] The invention is defined by the appended independent claims, with embodiments being set forth in the dependent claims, in the following description and in the attached drawings.

[0015] According to a first aspect, there is provided a material having radar absorbing properties, comprising a matrix material enclosing an amount of fiber. The fiber is coated by graphene and / or graphene oxide. An amount of fiber may be formed by continuous fibers or yarn of fibers, that make up a fabric, such as a woven, knitted, non-woven, or the like, fabric that may be immersed or impregnated by the resin material.

[0016] An amount of fiber may be formed by short, such as chopped, fibers that may be mixed in the matrix material.

[0017] The fiber may be formed as an individual fiber, or as a bundle of fibers, effectively forming a yam.

[0018] The matrix material may be any type of polymer material used for forming composite materials, including thermosetting and thermoformable polymers.

[0019] The coating of graphene and / or graphene oxide is generally referred to herein as a graphene coating (“GC”).

[0020] A material as described can be tuned to absorb different wavelengths of radar radiation, while being formable into sheets or 3D shapes using existing forming technology, such as is used in e.g. forming of automotive parts from carbon fiber composite materials, or in injection molding.

[0021] The graphene and / or graphene oxide may be in the form of 2D flakes.

[0022] The fiber may be present in an amount of 10-90 wt.% of the material.

[0023] The fiber may be formed of at least one material selected from the group consisting of glass fiber, carbon fiber, aramid fiber, natural fiber and polyester fiber.

[0024] The graphene and / or graphene oxide may cover at least 90 % of a free fiber surface, preferably at least 95 %, at least 99 % or at least 99.9 %.

[0025] A “free fiber surface” is the surface of the fiber that is available for coating by the graphene and / or graphene oxide. Hence, in a chopped fiber, the entire fiber envelope and base surfaces will be available to form the free fiber surface, while in a bonded fiber forming a fabric or a non-woven, surface portions of adjacent fibers forming a bonding surface will not be part of the free fiber surface.

[0026] In the material, at least some of the fiber may form a fabric.

[0027] It is possible to combine a resin with dispersed short GC fibers used as a matrix enclosing a textile of CG fibers. At least some of the fiber may be formed by chopped fibers.

[0028] The fiber may have a fiber value in the range of about 1 to about 300 dtex.

[0029] The graphene or graphene oxide may be present as a plurality of flakes which are adhered to the surface of the fiber.

[0030] The matrix material may be present in an amount of 10-90 wt.%.

[0031] The matrix material may be formed of at least one material selected from the group consisting of an epoxy, a polyester, a vinylester, a polyetherimide, a polyetherketoneketone, a polyphthalamide, a polyetherketone, a polytheretherketone, a polyimide, a phenol-formaldehyde, and a bismaleimide. Matrix materials more generally can include resins (polymers), both thermosetting and thermoplastic, metals, ceramics, and cements. Thermosetting resins useful as matrix materials include phthalic / maelic type polyesters, vinyl esters, epoxies, phenolics, cyanates, bismaleimides, and nadic end-capped polyimides (e.g., PMR-15). Thermoplastic resins include polysulfones, polyamides, polycarbonates, polyphenylene oxides, polysulfides, polyether ether ketones, polyether sulfones, polyamide-imides, polyetherimides, polyimides, polyarylates, and liquid crystalline polyester.

[0032] The material may further comprise particles formed of a magnetic material, from a transition metal, from a transition metal compound, such as a salt thereof, from a nanoparticle, or from nanoparticles of other 2-dimensional materials such as hexagonal boron nitride, wherein the particles formed of the magnetic material are arranged between the fiber and the graphene and / or graphene oxide.

[0033] Hence the particles formed of a magnetic material are held to the fibers by the graphene and / or graphene oxide.

[0034] It may in this context be noted that the various embodiments of the material disclosed above may be combined one or more in any permuation (unless any of the embodiments above express mutually excluding alternatives). According to a second aspect, there is provided a laminated material, comprising at least two layers, which are laminated to each other, a first one of which being formed from a material as described above.

[0035] The laminated material may further comprise at least one second layer which is formed from a material as described above.

[0036] In the laminated material, the first layer may be different from the second layer with respect to sheet resistance.

[0037] In the laminated material, the first and second layers may be separated by at least one dielectric layer.

[0038] In the laminated material, the first and second layers may be separated by at least one spacer layer providing a space between the first and second layers.

[0039] The laminated material may further comprise a metallic layer.

[0040] The metallic layer may be formed from a sheet metal, from a metal foil or by deposition on a surface of the laminated material, such as by PVD or CVD,

[0041] The laminated material may further comprise at least one layer comprising further particles formed of a magnetic material, from a transition metal, from a transition metal compound, such as a salt thereof, from a nanoparticle, or from nanoparticles of other 2-dimensional materials such as hexagonal boron nitride wherein the further particles formed of the magnetic material are arranged between the fiber and the graphene and / or graphene oxide.

[0042] The layer may be formed from one of the graphene-containing layers or from a separate layer comprising the particles and a matrix or other binder. In particular, the particles may be dispersed in a binder used to bind two layers forming the laminated material.

[0043] It may in this context be noted that the various embodiments of the laminated material disclosed above may be combined one or more in any permuation (unless any of the embodiments above express mutually excluding alternatives). Also the various embodiments of the material disclosed above may be combined one or more in any permuation (unless any of the embodiments above express mutually excluding alternatives) in respect of the first material and the various embodiments of the material disclosed above may be combined one or more in any permuation (unless any of the embodiments above express mutually excluding alternatives) in respect of the second material.

[0044] According to a third aspect, there is provided a sheet material formed of a material as described above or from a laminated material described above.

[0045] According to a fourth aspect, there is provided a 3D structure formed of a material as described above or from a laminated material as described above.

[0046] The 3D structure may in particular be formed of a thin material having generally constant thickness and being formed to a three dimensional shape.

[0047] According to a fifth aspect, there is provided a vehicle comprising a vehicle body or fuselage part which is formed from at least one sheet material as described above or from a 3D structure shell as described above.

[0048] The vehicle may be an automobile, an aircraft, or a water craft. Such vehicle may be manned or unmanned.

[0049] According to a sixth aspect, there is provided a tubular structure formed at least partially from a sheet material as described above or from a 3D formed shell described above.

[0050] A tubular structure may be a pipe, a rod, a beam or similar hollow structure having an arbitrary cross section, including circular, oval or polygonal. The tubular material may form part of a frame or fuselage of a vehicle or part of a vehicle, or of a fixed installation, such as a tower, mast or building.

[0051] It may in this context be reiterated that the various embodiments of the material or laminated material disclosed above may be combined one or more in any permuation (unless any of the embodiments above express mutually excluding alternatives). Also the various embodiments of the material disclosed above may be combined one or more in any permuation (unless any of the embodiments above express mutually excluding alternatives) in respect of the first material and the various embodiments of the material disclosed above may be combined one or more in any permuation (unless any of the embodiments above express mutually excluding alternatives) in respect of the second material. All the above permutations are equally applicable to the sheet material, the 3D structure, the vehicle, and the tubular structure disclosed above.

[0052] According to a seventh aspect, there is provided the use of a material as described above, a laminated material as described above, a sheet material as described above, a 3D structure as described above or a tubular structure as described above for reducing radar reflection from an object formed at least partially thereof.

[0053] According to an eighth aspect, there is provided a method of forming at least part of a product having radar absorbing properties, comprising providing a fiber, coating the fiber with graphene or graphene oxide, immersing the fiber in a hardenable matrix material to form a structural material, and applying said structural material to form an outwardly exposed surface of said product, which surface is likely to be subjected to external radar radiation.

[0054] The above remark concerning that the various embodiments of the material or laminated material disclosed above may be combined one or more in any permuation (unless any of the embodiments above express mutually excluding alternatives) is equally applicable to the use and the method.

[0055] Drawings

[0056] Figs 1 a-1 j schematically illustrate various fiber cross sections.

[0057] Fig. 2 is a scanning electron microscopy image of a graphene coated fiber (“GCF”).

[0058] Fig. 3 is a schematic cross sectional view of a GCF, taken in a plane perpendicular to a fiber length direction.

[0059] Fig. 4 is a schematic cross sectional view of a GCF, taken along in a plane parallel with the fiber length direction.

[0060] Figs 5a-5f schematically illustrate various types of textiles. Fig. 6 schematically illustrates an embodiment of a radar absorbing composite material.

[0061] Fig. 7 schematically illustrates an embodiment of a radar absorbing composite material.

[0062] Fig. 8 schematically illustrates an embodiment of a radar absorbing composite material.

[0063] Fig. 9 schematically illustrates an embodiment of a radar absorbing composite material.

[0064] Fig. 10 schematically illustrates an embodiment of a radar absorbing composite material.

[0065] Fig. 11 schematically illustrates an embodiment of a radar absorbing composite material.

[0066] Fig. 12 schematically illustrates an embodiment of a radar absorbing composite material.

[0067] Fig. 13 schematically illustrates an embodiment of a radar absorbing composite material.

[0068] Fig. 14 schematically illustrates an embodiment of a radar absorbing composite material.

[0069] Fig. 15 schematically illustrates an embodiment of a radar absorbing composite material.

[0070] Fig. 16 schematically illustrates dampening of the reflection as a function of frequency for a composite material 601 designed in accordance with Fig. 6.

[0071] Fig. 17 schematically illustrates dampening of the reflection as a function of frequency for a composite material 1101 designed in accordance with Fig. 11.

[0072] Fig. 18 schematically illustrates dampening of the reflection as a function of frequency for a composite material 1301 designed in accordance with Fig. 13.

[0073] Description The present disclosure is directed, in part, to composite materials with radio waves absorptive function. The radar absorbing composite materials disclosed herein have graphene-coated fiber materials (“GCF”) disposed in a portion of a plastic matrix material. GCF materials are made by the method described in EP4204623A1 and provide unique design on the nanoscale, where graphene nanocrystals are covering and surrounding the fibers of the textiles, creating continuous electrically conductive layer over the whole surface of the fibers. Graphene as a two-dimensional nanomaterial has advanced electromagnetic absorption properties due to its high aspect ratio and electrical conductivity. Thickness, weight and density of the graphene coating can be precisely controlled during coating process, allowing fine tuning of the as electrical and thermal conductivity along each fiber and along the whole layer of the fabric too.

[0074] Components for composite materials are reinforcing fibers / fabrics, coated with electrically conductive graphene 2-dimensional nanocrystals on the fiber individual level, where each fiber is coated entirely by thin layer, consisting of multiple graphene flakes. Graphene-coated fibers comprise exclusively graphene flakes only, bonded chemically, mechanically, and electrically between each other without binding additives. Graphene-coated fabrics are electrically conductive, consisting of numerous graphene-coated fibers, having sheet resistance in the range between 10 MQ / n and 1 £) / □. The electrical conductivity of every fiber and respectively fabric sheet resistance can be controlled and is defined by average thickness of graphene coating layer applied on the surface of individual fiber. The average loading of graphene coating per one individual fiber may be at least 0.01 % of fiber’s own weight and at least 90 % of fiber's surface coverage to reach 10 M £) / □, and at least 5 % of fiber’s own weight and at least 99.9 % of fiber's surface coverage to reach 1 £) / □. Reinforcing fabrics that are used for graphene coating are well-known reinforcing textiles for composites manufacturing, particularly those electrically insulating (glass fiber, meta-aramid, paraaramid, etc.). Graphene coating of individual fiber is a dedicated part of the fiber and doesn’t have chemical bonding with the neighboring fibers; graphene coating in terms of the yarn doesn’t form an entire film. Graphene- coated fabrics demonstrate efficient electro-magnetic waves absorption in a wide frequency range - 0.1 - 1000 GHz; whereas absorption, transparency and reflection of the fabric can be controlled by change of its electrical resistance, due to controllable graphene coating thickness, being said 10 MO / n to 1 Q / n.

[0075] Referring to figs 1 a-1 j, there are illustrated various fiber materials that can be used for making graphene-coated fiber (“GCF”).

[0076] Fig. 1 a illustrates a fiber 101 which has a circular cross section and uniform in diameter, which can be formed of materials such as nylon, polyester, glass fiber, carbon fiber, lyocell.

[0077] Fig. 1 b illustrates a fiber 102, which has a polygonal cross section, such as by lumen (flax).

[0078] Fig. 1 c illustrates a fiber 103, which has an oval to round cross section, and which may have overlapping scales, as in the case of wool.

[0079] Fig. 1 d illustrates a fiber 104 having a flat, oval lumen cross section with one or more convolutions, as is the case with cotton.

[0080] Fig. 1 e illustrates a fiber 105 having a circular, serrated cross section, as is the case with ryon.

[0081] Fig. 1f illustrates a fiber 106 having a dog-bone-shaped cross section, as is the case with acrylic, spandex.

[0082] Fig. 1 g illustrates a fiber 107 having a squared cross section with voids, as is the case with modified nylon, modified polyester.

[0083] Fig. 1 h illustrates a fiber 108 having a triangular cross section with rounded edges, as is the case with silk.

[0084] Fig. 1 i illustrates a fiber 109 having a trilobal cross section, as is the case with modified nylon, modified polyester.

[0085] Fig. 1j illustrates a fiber 110 having a lobular cross section with lengthwise striations, as is the case with acetate.

[0086] A GCF consists of fiber material (101-110 - shown on figs 1 a-1 j) and graphene coating. Fiber material is a smallest fiber / filament manufactured (for example extruded) from precursor, which can be based on but not limited to inorganic (for example glass 101 , silica, metal), organic (105, 109 polyester, 107, 109 polyamide, 105, 109 polyethylene, 104 cellulose, etc.), or hybrid inorganic- organic blend structure. It can have natural (104 cotton, 102 flax, 103 wool, 108 silk) or synthetic origin (106 acrylic). Fiber’s surface can be flat or texturized. Fiber’s cross section can be 101 circular, 103 oval, 102, 110 multi- angular, 108 triangular, 105 serrated, kidney bean, 106 dog-bone, 107 hollow and rectangular. Fiber’s surface can be additionally covered with special layer - size - if manufacturing process requires it.

[0087] Graphene ink (“Gl”) - is a liquid dispersion comprised of solvent (water, organic solvents, alcohols, etc.) and nanoparticles. Nanoparticles are represented by carbon nanoparticles (graphene, graphite, graphene oxide, reduced graphene oxide, graphite oxide, reduced graphite oxide, functionalized graphene, functionalized graphite, nanotubes) and in some embodiments by mix of carbon nanoparticles and another nanoparticles (metal nanoparticles, magnetic nanoparticles, MXene nanoparticles, another 2-dimensional nanoparticles such as hBN). Gl can be utilized for making a graphene coating using the method described in EP4204623A1 .

[0088] Graphene coating (GC) - is a structure composed of multiple individual flakes of graphene with a lateral size of between 10 nm to 50 pm and thickness of between 1 to 10 atomic layers. Graphene flakes - may compose of pristine graphene, oxidized graphene (with degree of oxidation from 0.1 to 99 %), graphene functionalized with organic or inorganic molecules, hybrid structures of graphene and other two-dimensional materials (for example hexagonal boron nitride). In GC graphene flakes are bonded between each other via chemical bonding (for example hydrogen bonds, carboxylic bonds, or via additional functional groups), van der waals force, surface charge. Graphene flakes in GC are interconnected electrically and form the electrically conductive network in in-plane direction along the fiber; and out-of- plane direction. GC may comprise between 1 atomic layer and tens of thousands atomic layers which corresponds to 0.3 nm to 10e-4 m. That is, the graphene flakes are arranged with their lateral size along the surface of the fibres. The first layer of graphene flakes (the one that is in direct contact with the surface of the fiber) is bonded to the fiber material via chemical bonding (for example covalent bonding, hydrogen bonds, carboxylic bonds, or via additional functional groups), van der waals force, surface charge; or indirect bonding via additional adhesive substance. The detailed description of the GC is in EP4204623A1 and the SEM image of GCF is presented in fig. 2.

[0089] Figs 3 and 4 schematically illustrate cross-sectional view of the CGF, with fig. 3 illustrating a cross-sectional view taken in a plane perpendicular to a fiber longitudinal direction and with fig. 4 illustrating a cross-sectional view taken in a plane parallel with the fiber longitudinal direction.

[0090] In some cases, GC may comprise organic additives or polymer matrix.

[0091] In some cases GC may comprise mix of carbon and non-carbon particles (GCM).

[0092] GCF - is a complex structure that combines mechanical and physical properties of fiber material and physical, mechanical, and chemical properties of GC. GC covers the entire outer surface of the fiber and it can be confirmed by scanning electron microscopy (SEM), as shown on fig. 2. Pristine fiber surface 301 can be accessible only on the cross section (figs 3, 4) when the fiber is cut. The thickness of the 302, 403 GC may be uniform (with 1 to 50 % deviation) or non-uniform according to the application requirement.

[0093] GCF - conducts electricity, however, possesses an electrical resistance and is defined as resistive material. Electrical resistivity of GCF is defined by the content of the graphene flakes, degree of oxidation of graphene flakes, thickness of graphene flakes, thickness of GCF, presence of additives in GCF (for example additional adhesives), humidity of the environment.

[0094] Due to electrical conductivity of graphene flakes in 302 GC, GCF interacts with electromagnetic fields and electromagnetic waves (“EMW’). When the incident EMW collides with the 302 GC surface - three events happen. A part of EMWs energy passes through the entire thickness of 302 GC; a part of the EMWs energy reflects; and a part of EMWs energy is absorbed within the 302 GC. The percentage of each event is defined by the frequency range of EMW and electrical resistance of 302 GC.

[0095] GCF is capable of absorbing radio waves in a frequency range from between about 1 MHz to about 1 THz.

[0096] GCF with certain electrical sheet resistance possesses low reflectance against EMWs in certain frequency range. For example, for EMWs in 8-12 GHz frequency:

[0097] GCF with the sheet resistance of 1000 Q / n reflects 10 % (- 10 dB reflection loss) of the incident EMWs power; 60 % (2.7 dB attenuation) of EMWs power is passing through, and 30 % is absorbed and dissipated as a heat;

[0098] GCF with sheet resistance of 500 Q / n reflects 25 % (- 6.5 dB reflection loss) of the incident EMWs power; 25 % (6.56 dB attenuation) of EMWs power is passing through, and 50 % is absorbed and dissipated as a heat;

[0099] GCF with sheet resistance of 300 Q / n reflects 40 % (- 6.5 dB) of the incident EMWs power; 5 % (13 dB attenuation) of EMWs power is passing through, and 55 % is absorbed and dissipated as a heat;

[0100] GCF can be manufactured using a fiber material and method described in EP4204623A1 .

[0101] The GCFs can be used for manufacturing of graphene coated textiles (“GCT”) and graphene-coated fiber reinforced composites (“GFRC”).

[0102] In some embodiments, the GCFs can be applied as a coating, for example as a chopped fiber mixed in coating matrices, on an already existing surface of another composite or other article.

[0103] Combinaton of Graphene with magnetic particles for Stealth Composites.

[0104] Graphene coating is performed using the mixture of the combined with other nano or microsized particles, creating a mix of electroconductive and magnetic properties in the same layer. In some cases, GC 403 may comprise additional particles 404 of different from graphene substances (such as metal oxides (Fe3O4), metal alloys (FePt, FePd), Ni-based, Co-based, metal carbides (Fe5C2, Fe3C, and Fe2C)), in particular inorganic magnetic particles; or transition metal (any element or alloy of elements in the d-block of the periodic table. The size of this magnetic nanoparticles can vary between 2 nm and 5 mkm; the loading in the coating can vary between 0.1 wt% and 20 wt% of the fiber’s own weight. The term “transition metal” also includes salt forms of the base transition metal element such as oxides, carbides, nitrides, and the like.) nanoparticle; nanoparticles of other 2- dimensional materials such as hexagonal boron nitride (hBN).

[0105] The particles / nanoparticles 404 are incorporated into a coating via preparation of the respective ink - a mixture, dispersion of graphene flakes with particles / nanoparticles 404 in a certain content; the ink is then used for coating the fibers. The fibers are then coated by graphene flakes, encapsulating the particles / nanoparticles, making them part of the shell / skin of the fiber. The nanoparticles are completely encapsulated by graphene flakes and attached to a fiber. The particles are inserted in fact in between the flakes, being attached to the fibers surface (fig. 4). Some particles may have direct contact with the fiber surface, while others don’t have any. But all the particles are mechanically bonded and encapsulated, being also electrically interconnected via graphene flakes.

[0106] Graphene coated fibers can be arranged in a form of a fabric or textiles (“GOT”).

[0107] Referring to figs 5a-5e, GCT is a textile material that is partially or completely composed of GCFs. GCT can have 501 woven, 502 net, or 503 - 506 non-woven structures.

[0108] Fig. 5a schematically illustrates a woven textile 501 .

[0109] Fig. 5b schematically illustrates a net textile 502.

[0110] Fig. 5c schematically illustrates a knitted textile 503.

[0111] Fig. 5d schematically illustrates a braided textile 504.

[0112] Fig. 5e schematically illustrates a nonwoven textile 505.

[0113] Fig. 5f schematically illustrates a tufted textile 506.

[0114] GCT possesses all the properties of GCFs.

[0115] GCT which conducts electricity, however, possesses an electrical resistance and is defined as resistive material. Electrical resistivity of GCT is defined by the content of the graphene flakes, degree of oxidation of graphene flakes, thickness of graphene flakes, thickness of GCFs, presence of additives in GCFs (for example additional adhesives), humidity of the environment.

[0116] Due to electrical conductivity of graphene flakes in GC, GCT interacts with electromagnetic fields and electromagnetic waves (EMW). When the incident EMW collides with the GC surface - three events happen. A part of EMWs energy passes through the entire thickness of GCT; a part of the EMWs energy reflects; and a part of EMWs energy is absorbed within the GCT. The percentage of each event is defined by the frequency range of EMW and electrical resistance of GCT.

[0117] GCT can be manufactured using textile materials of the same origin that described fiber materials and the method described in EP4204623A1 ; assembling the GCFs for non-woven structures and / or weaving the GCFs in woven structures.

[0118] GCT can be used for composite manufacturing to produce GFRCs, in which GCTs are used as a reinforcement ply alone impregnated with plastic matrix (thermoplastic, thermoset, etc.); or in combination with regular reinforcement plies (glass fiber, carbon, Kevlar, natural fibers, etc) and respective plastic matrix.

[0119] Some exemplary and non-limiting simple design cases of stealth composites will now be briefly described.

[0120] Using GCF fabrics, a composite material can be created, comprising at least one top layer of graphene-coated reinforcing fabric and providing controllable absorption and reflection of incident electro-magnetic waves in the frequency range from MHz to THz.

[0121] Referring to figs 6-15, such composite structures can be in a form of plane sandwich 601 , comprising one GCF 602 (see figs 6, 7, 8) or several (see fig. 9) fabrics and other textiles 604 as well as matrixes materials 603.

[0122] It is understood that several GCFs may be immersed in the same matrix or that different sheets of matrix-immersed GCF may be bonded to each other. GCF fabrics can be a top layer or being inserted inside of such a composite structure. In the most extreme cases GCF can be all the layers / fabrics in the composite. The GCF layers in all abovementioned structures possess certain electrical sheet resistance, being in the range of 10 MQ / n - 1 M £) / □; 1 MQ / n - 1 kQ / n; 1 kQ / n - 100 Q / n, 100 Q / n - 1 Q / n. Due to the electrical conductivity, radar waves can be efficiently absorbed by the GCF layers, transforming the EM energy into a heat and spreading it equally laterally, thermally radiating it to the environment. The GCF may have a layer of electrically insulating textile reinforcers in-between them, or are stacked with natural and spontaneous electrical contact between adjacent GCF layers. The above described GCF layers can be assembled in one entire composite with other materials 701 , 801 (figs 7, 8), such as carbon fibers 705 or metallic surfaces 806.

[0123] Multilayers with descending order of resistance (Multigram). Alternatively, composite material can be in a form of flat sandwich, comprising stack 1401 of diverse textiles and matrixes (fig. 14), with at least two layers of graphene-coated fabrics 602 on top or inside of sandwich. These GCF layers are related to each other, having different electrical resistance, which is in descending order, e.g. a first fabric 602 with 1000 Ohm / n, followed by a second fabric 607 with 500 Ohm / n, followed by a third fabric 608 with 100 Ohm / n. The GCF may have a layer of electrically insulating textile reinforcers in-between them, or are stacked with natural and spontaneous electrical contact between adjacent GCF layers. The above described GCF layers constructions can be assembled in one entire composite with other materials, such as carbon fibers or metallic surfaces.

[0124] In the following, there will be discussed the design of a GRAM with a separator.

[0125] Using GCF fabrics, a composite material can be assembled in a form of 1001 , 1101 , 1201 , 1301 flat sandwich, comprising at least one layer of graphene-coated fabrics 602 and electrically insulating (dialectic) separator 1008 with specific thickness in the range of 0.1 - 1 mm; or 1 - 5 mm; or 5 - 10 mm; wherein graphene-coated fabric has sheet resistance in the range of 10 MQ / n - 1 M Q / n; 1 MQ / n - 1 kQ / n; 1 kQ / n - 100 Q / n, 100 Q / n - 1 Q / n (FIG 10-13). Separator material is attached to the outer layers via adhesive 1007 or matrix 603. Such structures can be used solely, as 1001 , 1101 , 1201 , 1301 one or several cascades 1501 (fig. 15), with different separators thickness. Furthermore, such structures, solely or in cascades, can be combined with other materials, forming one entire composite, with materials such as carbon fiber, metal, glass fibers etc.

[0126] GCF as a media for being added to other methods producing composite materials (vacuum injection, lamination etc.)

[0127] In some cases, GCF can be processed further, chopped and mixed with the polymers in liquid phase and injected and formed into a composite material. Typical ideal ratio between the weight of the fibers and the weight of the polymer matrix is 1 : 1 , however it can vary from part to part and have different values between 1 : 9 (fiber : matrix) to 9 : 1 (fiber : matrix). Such a composite materials are comprising graphene-coated fibers of glass, aramid, cellulose, etc., which are chopped and dispersed within the polymer, with or without electrical connection between each other. Furthermore, GCF can be pressed in one layer between neighboring other reinforcing fabrics / layers, as one discontinuous layer of any thickness, or be distributed inside the entire composite thickness.

[0128] As used herein, the term “radar” refers to any of the common bands of radar frequencies ranging from about 0.10 MHz to about 100 GGHz. Radar absorbing composite materials of the present disclosure are particularly effective, for example, in the L- through K-band as described herein further below.

[0129] As used herein, the term “radar absorption capacity” refers to the ability of the radar absorbing composite materials of the present disclosure to absorb electromagnetic radiation of any radar band.

[0130] In the following, some application and compatibility cases will be briefly discussed.

[0131] Our present concept can be used for different cases for improving materials with already low radar signature - such as radio waves transparent materials, like glass fiber and other electrically non-conductive mateials; or being in combination with the other textiles and composite reinforcers, which may have been previously limited in application due to their inherently poor signature control capabilities, such as carbon fibers.

[0132] Using the presently disclosed concepts, diverse composites designed for most efficient absorption and lowest reflection of EM waves can be designed. In particular, near the surface of a composite, textile with high electrical resistance can be used, creating a material that has a dielectric constant similar to air or a refractive index close to air creating a black bodylike structure where radar reflectance is substantially minimized. That is, in order to suppress reflection, the refractive index of the object can be close to that of air. Later on, every next layer in the composite can be more and more electrically conductive, creating more efficient absorption, while the reflection will be lost in the bulk of the composite due to internal reflection.

[0133] According to the literature, this solution to minimize reflectance is confirmed by Fresnel's law:

[0134] R=(n-no)2 / (n+no)2, where R is reflectance, n is the refractive index of the object, and no is the refractive index of air.

[0135] The electrical resistance of the fibers material can be tailored in the manufacturing process, described in EP4204623A1 , such that the graphene coated fiber material can have refractive index similar to that the refractive index of air, no.

[0136] As used herein, the term “radar absorbing composite material” refers to any composite material that has at least a GCF material disposed in a matrix material. The radar absorbing composite materials of the disclosure have three components: graphene, a fiber material - fabric, and a matrix material. Within this composite, graphene is coating fibers, and the fibers are arranged, creating fabric or textile. The fabrics are immersed into a matrix and can be arranged as a layers in different geometry and order, creating sandwich structures, with insulators or spacers in between the layers. Such a composites can prevent radar reflectance and / or absorb electromagnetic (EM) radiation associated with a radar transmitting source or reflected EM from an object in detection applications. The absorbed radar can be converted to heat and / or an electrical signal.

[0137] As used herein, the term “fiber material” refers to any material which has fiber as its elementary structural component. The term encompasses fibers, filaments, yarns, tows, tows, tapes, woven and non-woven fabrics, plies, mats, 3D woven structures and the like.

[0138] The present disclosure is directed, in part, to composite materials with radio waves absorptive function. The radar absorbing composite materials disclosed herein have graphene-coated fiber materials (GCF) disposed in a portion of a plastic matrix material. GCF materials are made according to the method described in EP4204623A1 . Graphene coating (GC) has desirable electromagnetic absorption properties due to its electrical conductivity, and when applied to a fiber material can be tailored for specific surface coverage and thicknesses. GC thickness therefore electrical conductivity, and dielectric constant are the parameters that can be adjusted in the manufacturing (method described in EP4204623A1 ). Thus, the dielectric constant of GC can be adjusted close to air value, and a refractive index of GC close to air creates a black body-like structure where radio wave reflectance is attenuated. The GC in the overall composite is capable of absorbing radio waves and dissipating the absorbed energy as heat, for example.

[0139] Graphene-coated fiber reinforced composite (GFRC).

[0140] GFRC is a complex structure composed of GCFs or GCTs, or both and plastic matrix, and, but not mandatory, other reinforcing fibers or textiles.

[0141] Due to the presence of GCFs or / and GCTs in the structure GFRC possesses electrical conductivity of the same level.

[0142] GCT can lay up in the GFRC to form a first layer that reduces radar reflectance and transmits incident wave to the inner layers. Part of the wave’s energy will be absorbed and dissipated as a heat.

[0143] GFRC can be composed of several layers of GCTs (where each GCT possesses certain sheet resistance, different from other GCTs), and resin media, creating a complex multilayer composite structure. In this structure GCT plies may be placed at certain positions (GCFs angle orientation - for example if the first ply is placed at 0 / 90 degree angle, the next ply should be placed at 45 degree angle). Each GCT ply possesses specific sheet resistance and is absorbing preferably radio waves of specific frequency. For the other frequency bands this GCT ply is nearly transparent or reflective.

[0144] GFRC can be composed of several layers of GCTs (where each GCT possesses certain sheet resistance, different from other GCTs), and uncoated glass fiber (UGF) plies, and resin media, creating a complex multilayer composite structure. In this structure GCT plies may be placed at certain positions interlayered with UGF. Each GCT ply possesses specific sheet resistance and is affecting preferably radio waves of specific frequency. For the other frequency bands the ply is nearly transparent or reflective. Each UGF ply is electrically insulative and near transparent for radio-waves. UGF plies have a dielectric separator function.

[0145] Possible compositions of the GFRC that is reinforced with GCTs and UGF:

[0146] The GFRC can be constructed to absorb one or more radar bands. Such multilayer composite structures may comprise at least, but not limited to: one layer of GCT (which is absorbing waves in S - Ka bands), UGF multilayer separator with the thickness of at least 4 mm (which is average A / 4 value); one layer of GCT (absorbing S - X bands), UGF multilayer separator 4 mm, one layer of GCT absorbing Ka-W bands range, UGF multilayer separator 2 mm; one layer of GCT (absorbing S - X bands), UGF multilayer separator 4 mm, one layer of GCT absorbing Ka band, UGF multilayer separator 2 mm, one layer of GCT absorbing W band, UGF multilayer separator 1 mm;

[0147] UGF material can be substituted with another dielectric composite reinforcing fibers for example Kevlar.

[0148] Dielectric separation layer can be substituted with another composite compatible materials, for example Nomex honeycomb paper, plastic foam, etc. In some embodiments, GFRC can include a plurality of additional layers between the GCT layers. These intermediate layers can be provided as a reinforcement layers (for example glass fiber, kevlar), dielectric separation layers (glass fiber, nomex, foam, polystyrene, etc.), layers with impregnated magnetic particles.

[0149] GFRC includes a matrix material to form the composite with the GCTs or / and GCFs. Such matrix materials can include, for example, an epoxy, a polyester, a vinylester, a polyetherimide, a polyetherketoneketone, a polyphthalamide, a polyetherketone, a polytheretherketone, a polyimide, a phenol-formaldehyde, and a bismaleimide. Matrix materials more generally can include resins (polymers), both thermosetting and thermoplastic, metals, ceramics, and cements. Thermosetting resins useful as matrix materials include phthalic / maelic type polyesters, vinyl esters, epoxies, phenolics, cyanates, bismaleimides, and nadic end-capped polyimides (e.g., PMR-15). Thermoplastic resins include polysulfones, polyamides, polycarbonates, polyphenylene oxides, polysulfides, polyether ether ketones, polyether sulfones, polyamide-imides, polyetherimides, polyimides, polyarylates, and liquid crystalline polyester.

[0150] GFRC can be manufactured using most widely used techniques for composite production - lay-up, vacuum infusion, autoclaving, etc.;

[0151] GFRC can be composed using carbon fiber (CF) as a main structural backbone. In this case CF layers may by placed under the GCT or GCF layers. CF is highly reflective for radio waves and should be covered by GCT or GCF layers.

[0152] GFRC can be manufactured using widely used prepreg material. In this case GCT may be additionally preliminary treated with resin (GCT-prepreg); or additional layer of epoxy film may be added on top of the GCT layer; or GCT may be wet with epoxy resin / another plastic matrix before the lay-up and curing process; or additional changes in recipe should be made considering presence of dry GCT ply.

[0153] GFRC can be used as cover that hides core carbon fiber structure; and / or metal-based frame / structure; In some embodiments, the GFRC is provided as integral part of an entire article or structure used in stealth applications. In other embodiments, the GFRC can be provided in a portion of the overall composite structure.

[0154] GFRC can be used as the structural element by itself (in cases where there is no carbon or metal structure underneath).

[0155] GFRC can be used as a structural element of flying vessels (for example UAV or missile) with the aim to decrease the radar visibility in certain frequency bands.

[0156] GFRC can absorb radar across the entire spectrum of radar frequency bands. In some embodiments, the GFRC can absorb high frequency radar waves (HF) in a range from between about 3 to about 30 MHz (10-100 m). This radar band is useful in coastal radar and over-the-horizon radar (OTH) radar applications. In some embodiments, the GFRC can absorb radar in the very high frequency band (VHF) in a range from between about 30 to about 330 MHz. The VHF band is useful in applications that are very long range, including, ground penetrating applications. In some embodiments, the GFRC can absorb radar in the ultra high frequency (UHF) band in a frequency range from between about 300 to about 1000 MHz. Applications of the UHF band include very long-range applications, such as ballistic missile early warning systems, ground penetrating and foliage penetrating applications. In some embodiments, the GFRC can absorb radar in the long (L) band in a frequency range from between about 1 to about 2 GHz. The L-band can be useful in long range applications including, for example, air traffic control and surveillance. In some embodiments, the GFRC can absorb radar in the short (S)-band in a frequency range from between about 2 to about 4 GHz. The S- band can be useful in applications such as terminal air traffic control, long- range weather, and marine radar. In some embodiments, the GFRC can absorb radar in the C-band which has frequencies in a range from between about 4 to about 8 GHz. The C-band has been used in satellite transponders and in weather applications. In some embodiments, GFRC can absorb radar in the X-band which has frequencies that range from between about 8 to about 12 GHz 2. The X-band is useful in applications such as missile guidance, marine radar, weather, medium-resolution mapping and ground surveillance. In some embodiments, GFRC can absorb radar in the K-band which includes frequencies between about 12 to about 18 GHz. The K-band can be used for detecting clouds by meteorologists, and used by police for detecting speeding motorists employing K-band radar guns. In some embodiments, the GFRC absorbs radar in the Ka-band which includes frequencies from between about 24 to about 40 GHz. The Ka-band can be used in photo radar, such as those used to trigger cameras at traffic signals. In some embodiments, the GFRC absorbs radar in the millimeter (mm) band which is broadly between about 40 to about 300 GHz. The mm-band includes the Q-band from between about 40 to about 60 GHz which is used in military communication, the V-band from between about 50 to about 75 GHz, which is strongly absorbed by atmospheric oxygen, the E-band from between about 60 to about 90 GHz, the W-band from between about 75 to about 110 GHz, which is used as a visual sensor for experimental autonomous vehicles, high- resolution meteorological observation, and imaging, and the UWB-band from between about 1 .6 to about 10.5 GHz, which is used for through-the-wall radar and imaging systems.

[0157] The structures, designed with graphene coated fabrics demonstrates efficient dampening of the reflection in the frequency range 2 to 60 GHz. Reflection loss curves demonstrate resonant-like behavior, with clear peak at certain frequency range as well as certain peak width. Typical reflection loss curves is disclosed in Figs. 16-18. In certain cases, with specific combination and sandwich design, the position of the peak can be moved and located at the required frequency range. Such a stealth tun ing / tailoring is possible at the composite design moment, via control of the components specific characteristics. Furthermore, the composites design system enables potentially dynamic control of the reflection loss peak, via change of the dielectric layer properties (permittivity and permeability of the spacer). This enables creating on the dynamic stealth materials.

[0158] In the following, some specific examples of application for the presently disclosed materials will be briefly discussed. In some embodiments, GFRC can be used for manufacturing of composite plate or screen. Such a composite screen may consist of one or multiple layers of GCT and may be used against radio waves. It can be used separately or as a part of bigger objects such as vehicle body part / panel, part of the room wall, window, etc.

[0159] In some embodiments, GFRC can be used for manufacturing of composite containers / boxes / capsules. In this case the outer surface of walls and doors of the container consists of GFRC. Main function of such a box is hiding of the objects (metal parts, carbon parts, electronics) which are inside the container from the electromagnetic waves and field outside the container. Such a container can used separately as protective container (for example for sensitive electronics in the regions where there is high electromagnetic pollution; or objects that are to be hidden from enemy radar detection). Such a container may be a structural part of the for-example UAV. In this case all the sensitive electronics parts of the UAV as well as the metal parts such as engine, wires, etc. may be placed in such type of containers, which will provide high protective effect against electromagnetic field and decrease the visibility against radar systems.

[0160] In some embodiments, GFRC can be used for manufacturing of composite parts in the shape of pipe / tube. Such a part can be used as a structural part of the for-example UAV. In this case all the sensitive electronics parts of the UAV as well as the metal parts such as engine, wires, etc. may be placed in such type of containers, which will provide high protective effect against electromagnetic field and decrease the visibility against radar systems.

[0161] In some embodiments, GFRC can be used as a structural component of the flying vessels. In this case GFRC may be used as a outer layer for the parts which possess high reflectance against radar and is intended to be hidden. It may concern metal parts of the fuselage, engine part of the vessel, electronics, radar systems, radoms.

[0162] In some embodiments, an entire aircraft body may be manufactured using GCTs and resin matrix resulting in GFRC-based composite aircraft body. Such a composition of the fuselage will grant low visibility of such an aircraft for the radar systems in main frequency bandwidths: S-band, X-band, Ka-band, W-band.

[0163] In some embodiments, the GFRC can be used as a part of wind turbine blade. Wind turbine blades are made of composite and GCT can be easily integrated in the blade structure without any additional treatment or studying for the personnel. Presence of GCT in the structure of wind turbine blade can decrease the radar visibility of the wind turbines in certain band width. Nowadays wind turbine blades are often recognised as helicopter blades by radar systems. In case when GFRC is integrated in the structure of wind turbine blade - the radar signature will differ from the one for helicopters and it will help to avoid many mistakes for the intelligence service.

[0164] In some embodiments, the GFRC can be used as a part of rotor blades, in particular for helicopters, rotor planes, or any other copters or flying vehicles that have a rotor blade propelling system. Rotor blades are made of composite, mostly of carbon fiber composite, and GCT can be easily integrated in the blade structure without any additional treatment or studying for the personnel. Presence of GCT in the structure of rotor blade can decrease the radar visibility of the vehicle in certain frequencies band width, in particular between 1 and 90 GHz, via adding of negligible weight and volume and preserving the mechanical strength and blade design. Currently rotor blades are the most radar reflective part of the rotor blade vehicles. In case when GFRC is integrated in the structure of rotor blade - the radar reflectance will be lower and it will help to the vehicle low observable.

[0165] In some embodiments, the entire marine uav body may be manufactured using GCTs and resin matrix resulting in GFRC-based composite vessel. Such a composition of the vessel will grant low visibility of such an marine uav for the radar systems in main frequency bandwidths: S- band, X-band, Ka-band, W-band.

Claims

CLAIMS1 . A material having radar absorbing properties, comprising: a matrix material enclosing an amount of fiber, characterised in that the fiber is coated by graphene and / or graphene oxide.

2. The material as claimed in claim 1 , wherein the fiber is present in an amount of 10-90 wt.% of the material.

3. The material as claimed in claim 1 or 2, wherein the fiber is formed of at least one material selected from the group consisting of glass fiber, carbon fiber, aramid fiber, natural fiber and polyester fiber.

4. The material as claimed in any one of the preceding claims, wherein the graphene and / or graphene oxide covers at least 90 % of a free fiber surface, preferably at least 95 %, at least 99 % or at least 99.9 %.

5. The material as claimed in any one of the preceding claims, wherein at least some of the fiber forms a fabric.

6. The material as claimed in any one of the preceding claims, wherein at least some of the fiber is formed by chopped fibers.

7. The material as claimed in any one of the preceding claims, wherein the fiber has a fiber value in the range of about 1 to about 300 dtex.

8. The material as claimed in any one of the preceding claims, wherein the graphene or graphene oxide is present as a plurality of flakes which are adhered to the surface of the fiber.

9. The material as claimed in any one of the preceding claims, wherein the matrix material is present in an amount of 10-90 wt.%.

10. The material as claimed in any one of the preceding claims, wherein the matrix material is formed of at least one material selected from the group consisting of an epoxy, a polyester, a vinylester, a polyetherimide, a polyetherketoneketone, a polyphthalamide, a polyetherketone, a polytheretherketone, a polyimide, a phenol-formaldehyde, and a bismaleimide. Matrix materials more generally can include resins (polymers), both thermosetting and thermoplastic, metals, ceramics, and cements. Thermosetting resins useful as matrix materials include phthalic / maelic type polyesters, vinyl esters, epoxies, phenolics, cyanates, bismaleimides, and nadic end-capped polyimides (e.g., PMR-15). Thermoplastic resins include polysulfones, polyamides, polycarbonates, polyphenylene oxides, polysulfides, polyether ether ketones, polyether sulfones, polyamide-imides, polyetherimides, polyimides, polyarylates, and liquid crystalline polyester.11 . The material as claimed in any one of the preceding claims, further comprising particles formed of a magnetic material, from a transition metal, from a transition metal compound, such as a salt thereof, from a nanoparticle, or from nanoparticles of other 2-dimensional materials such as hexagonal boron nitride, wherein the particles formed of the magnetic material are arranged between the fiber and the graphene and / or graphene oxide.

12. A laminated material, comprising at least two layers, which are laminated to each other, a first one of which being formed from a material as claimed in any one of the preceding claims.

13. The laminated material, as claimed in claim 12, further comprising a second layer which is formed from a material as claimed in any one of the preceding claims.

14. The laminated material as claimed in claim 13, wherein the first layer is different from the second layer with respect to sheet resistance.

15. The laminated material as claimed in claim 13 or 14, wherein the first and second layers are separated by at least one dielectric layer.

16. The laminated material as claimed in any one of claims 13-15, wherein the first and second layers are separated by at least one spacer layer providing a space between the first and second layers.

17. The laminated material as claimed in any one of claims 12-16, further comprising a metallic layer.

18. The laminated material as claimed in any one of claims 12-17, further comprising at least one layer comprising further particles formed of a magnetic material, from a transition metal, from a transition metal compound, such as a salt thereof, from a nanoparticle, or from nanoparticles of other 2- dimensional materials such as hexagonal boron nitride wherein the further particles formed of the magnetic material are arranged between the fiber and the graphene and / or graphene oxide.

19. A sheet material formed of a material as claimed in any one of claims 1-11 or from a laminated material as claimed in any one of claims 12- 18.

20. A 3D structure formed of a material as claimed in any one of claims 1-11 or from a laminated material as claimed in any one of claims 12- 18.21 . A vehicle comprising a vehicle body or fuselage part which is formed from at least one sheet material as claimed in claim 19 or from a 3D structure shell as claimed in claim 20.

22. A tubular structure formed at least partially from a sheet material as claimed in claim 19 or from a 3D formed shell as claimed in claim 20.

23. Use of a material as claimed in any one of claims 1 -11 , a laminated material as claimed in any one of claims 12-18, a sheet material as claimed in claim 19, a 3D structure as claimed in claim 20 or a tubular structure as claimed in claim 22 for reducing radar reflection from an object formed at least partially thereof.

24. A method of forming at least part of a product having radar absorbing properties, comprising: providing a fiber, coating the fiber with graphene or graphene oxide, immersing the fiber in a hardenable matrix material to form a structural material, and applying said structural material to form an outwardly exposed surface of said product, which surface is likely to be subjected to external radar radiation.

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