Composite material

Abstract

An aspect of the present invention relates to a composite material comprising a foamed substrate having pores and having a three-dimensional structured surface with a void ratio of more than 0.4 and a coating on the three-dimensional structured surface, so that the composite material has a total hemispherical reflectivity of no more than 5% over the entire wavelength range from 400 nm to 2.5 µm and for any angle of incidence lower than 60°, preferably at an angle of incidence lower than 80°, more preferably an angle of incidence lower than 85°.

Description

DESCRIPTIONCOMPOSITE MATERIALField of the Invention[oooi] The invention generally refers to a novel composite material having low reflectivity and high emissivity.Background of the Invention

[0002] The performance of certain optical devices strongly depends on the ability of avoiding and / or eliminating stray light. Stray light reduction is an important issue especially in space-borne astronomical instruments, where it may notably affect both the geometric and the radiometric image quality. To prevent light originating from outside the desired field of view of the instrument, baffles are typically arranged around the optical axis. Such baffles are usually cylindrical or conical and may comprise vanes on their interior walls in order to trap as much stray light as possible. In such systems, the absorptance of undesired off-axis radiation in the spectral range of the detector should be as close as possible to 100% irrespective of the angle of incidence. Furthermore, the reflectance of the surfaces should ideally be Lambertian. Black coatings have been developed to cover all mechanical surfaces close to the optical beam. An overview can be found in: M. J. Persky, « Review of black surfaces for space-borne infrared systems », Review of scientific instruments, vol. 70, no 5, p. 2193-2217, 1999. Historically, in most spatial missions, black surfaces were obtained using paints (e.g. Aeroglaze™ from Lord Corporation, DeSoto™ Flat Black from Pacific Western Paints, etc.) or anodizations (e.g. Martin Black™, Enhanced Martin Black™, or Infrablack™ from Martin Marietta Corporation, etc.). Other advanced optically black diffuse surfaces such as plasma sprayed boron-on-beryllium, plasma sprayed boron carbide-on-silicon carbide and plasma sprayed beryllium-on-beryllium have been developed. These are diffuse absorptive surfaces that employ microscopic structures to absorb, scatter or trap light. Other black surfaces can be obtained by electrodeposition (e.g. black chrome, black cobalt) or by electroless nickel coating. In the 21st century, several advanced coatings have been developed by various companies. Acktar’s inorganic coatings (Nano black™, Magic black™, Vacuum black™, Fractal black™ and Ultra black™)are fabricated using vacuum deposition technology and feature very low reflectance, high thermal stability, excellent adhesion, and low outgassing. Surrey NanoSystems have developed a super-black material (called Vantablack™) that absorbs 99.96% of surface light. Vantablack™ is produced using a low-temperature carbon nanotube (CNT) growth process. When light strikes the layer of CNTs, instead of bouncing off it is trapped between the tubes before eventually becoming heat. Titanium and silicon substrates have been used to demonstrate the efficiency.

[0003] Apart from optical instruments, black materials, in particular black coatings, have applications in passive thermal management (requiring high emissivity), solar energy harvesting (e.g. solar water heating, concentrated solar power generation, etc.), infrared sensing (e.g. in MEMS IR sensors), thermal actuation (e.g. in MEMS thermal actuators), etc.

[0004] It is an object of an aspect of the present invention to provide a composite material which may serve as a black or superblack coating with improved properties, in particular in terms of absorptance of undesired off-axis radiation in the spectral range of the detector.General Description

[0005] An aspect of the present invention relates to a composite material comprising a foamed substrate having pores (i.e. open cells) and having a three- dimensional structured surface with a void ratio of more than 0.4 and a coating on the three-dimensional structured surface, so that the composite material has a total hemispherical reflectivity of no more than 5% over the entire wavelength range from 400 nm to 2.5 pm and for any angle of incidence lower than 6o°, preferably at an angle of incidence lower than 8o°, more preferably an angle of incidence lower than 85°.

[0006] The void ratio is the ratio between the volume of voids and the volume of solids. The ratio can be determined by comparing the density of the porous material to the bulk-density of the constituting material. The void radio may be determined by according to ASTM D792.

[0007] The composite material may be at least one of a black material or superblack material, the term “black” qualifies a surface with a total hemispherical reflectivity (THR) of no more than 5% over the entire wavelength range from400 nm to 2.5 m and for any angle of incidence (AOI -angle between the normal to the surface and the incoming beam at the point of incidence) smaller than 6o°. As used herein, a “superblack” surface is a matte black surface having, within the wavelength range from 400 nm to 2.5 pm, a total hemispherical reflectivity (THR) of no more than 1.5% around normal incidence: AOI < 10°, no more than 3% for 10° < AOI < 50°, no more than 5 % for 50 < AOI < 70°, and no more than 10% for 70 < AOI < 85°. In a preferred embodiment, the composite material has an integrated total hemispherical reflectivity of no more than 1.5% over the entire wavelength range from 400 nm to 2.5 pm and for AOI < 85°.

[0008] The three-dimensional structured surface may have a void ratio of more than 0.4, more preferably more than 0.6, even more preferably more than 0.9, most preferably more than 0.95. The void ratio maybe not more than 0.99.

[0009] The three-dimensional structured surface with the coating may have a void ratio of more than 0.35, preferably more than 0.55, even more preferably more than 0.85, most preferably more than 0.9.

[0010] In an embodiment, the void ratio of the substrate and the void ratio of the substrate with the coating do not differ by more than 50%, preferably by more than 25%, even more preferably by more than 10%, most preferably by more than 2.5%.

[0011] The substrate may comprise open pores. A size of the pore being preferably comprised in the range from 0.2 to 4 mm, preferably from 0.5 to 2 mm, even more preferably from 0.75 to 0.1 mm.

[0012] Optionally, the foam is a metal foam.

[0013] The substrate may comprise at least one of: Ti, Ni, Cu, Al, Ag, Fe-Ni, Ni-Cr, Fe, Co, C (e.g. carbon nanotubes), stainless steel, a ceramic.

[0014] The coating may comprise a ceramic matrix not being a carbide matrix, wherein said ceramic matrix has carbide nanoparticles and / or metal-carbon composite nanoparticles embedded therein, wherein said carbide nanoparticles are metastable and wherein said metal-carbon composite nanoparticles are decay products of the metastable carbide nanoparticles. The pores may be more or less filled with the ceramic matrix.

[0015] The carbide nanoparticles embedded within the ceramic matrix are metastable and the metal-carbon composite nanoparticles are the decay products of the metastable carbide nanoparticles. It is worth noting that the term ’’metastable” is used herein with its ordinary meaning, i.e. designating a state in which a system may remain for an extended time (when no energy is introduced into the system from the outside), which state is not, however, the system’s state of least energy. In the system under consideration, the state of least energy corresponds to the configuration in which a carbon phase and a metal phase coexist as a nanoparticular inclusion within the ceramic matrix. The metastable state (the carbide phase) corresponds to a local minimum of the internal energy of the system, whereas the stable state (the carbon and metal phases) corresponds to the global minimum of the internal energy at room temperature (20°C) and atmospheric pressure (1013.25 hPa).

[0016] The ceramic composite may comprise the metal carbide nanoparticles and the metal-carbon composite nanoparticles in any proportion relative to each other, ranging from 100% carbide nanoparticles and 0% metal-carbon composite nanoparticles to 0% carbide nanoparticles and 100% metal-carbon composite nanoparticles. A ceramic composite with only metal-carbon composite nanoparticles may be obtained by annealing.

[0017] The nanoparticles may have an average size (greatest diameter) in the range from 5 to 500 nm, more preferably in the range from 10 to 400 nm, even more preferably in the range from 20 to 300 nm, still more preferably in the range from 20 to 200 nm and most preferably in the range from 20 to 100 nm.

[0018] The ceramic matrix may have metal-carbon composite nanoparticles embedded therein, said metal-carbon composite nanoparticles comprising metal cores with carbon shells.

[0019] The ceramic matrix may be a metal oxide matrix. The metal oxide matrix may consist of an oxide selected from the group consisting of: V02, A12O3, Si02, MgO, Ti02, Zr02, Mn3O4, Sn02, ZnO, spinel having the general formula AB2O4with A and B being metal cations having different valences, perovskite having the general formula A’B’O3with A’ and B’ being differently sized metal cations, and mixtures thereof.

[0020] The density of the carbide nanoparticles and / or metal-carbon composite nanoparticles in said matrix may be non-uniform across the thickness of the ceramic composite coating.

[0021] The carbide nanoparticles may consist of carbides of metals selected from the group consisting of: Ni, Co, Fe, Cr, Mo, Pt, Pd and mixtures thereof.

[0022] The coating may comprise a layer of non-aligned carbon nanotubes, the carbon nanotubes comprising metal oxide claddings that sheathe the carbon nanotubes.

[0023] The CNTs of the composite material are forming an (isotropic) CNT thicket rather than a CNT forest (aligned CNTs). The fact that the CNTs are not aligned ensures that no directional scattering of the incident light is privileged, yielding Lambertian or nearly Lambertian reflection behaviour.

[0024] The metal oxide claddings are preferably transparent and conformal with the CNTs. They may sheathe the carbon nanotubes on their full lengths or along sections thereof. The pores or interstices between the metal-oxide-coated CNTs may be more or less filled with metal oxide or other material(s).

[0025] As will be appreciated, the metal oxide coating leads to a passivation of the CNTs and may reduce the ability of the CNTs to trap organic molecules or to react with atomic oxygen or aggressive chemicals. Furthermore, the metal oxide claddings increase the mechanical stability of the CNT layer and make it more resistant against mechanical stress. The claddings may be adjusted (in terms of chemical composition and thickness) to achieve a compromise between light absorption, mechanical stability, and protection against reaction with oxygen or aggressive chemicals.

[0026] Preferably, the CNTs have an average diameter in the range from 0.3 to 150 nm, more preferably in the range from 0.3 to 20 nm.

[0027] The coating may comprise a ceramic cap as a layer atop said sheathed carbon nanotubes and / or as an infiltration in said layer of carbon nanotubes. The ceramic cap layer may be conformal with the carbon nanotubes, leaving the individual capped sheathed CNTs visible by (scanning electron) microscopy.

[0028] The ceramic cap layer may consist of a different material than said metal oxide claddings. The ceramic cap layer may e.g. consist of a material selected from the group comprising or consisting of A12O3, Si20, Si3N4, SiOxNx, AIN, AINO, MgO, ZnO, Sn02, NiO, Zr02, Cr2O3, MoO2, Ru02, CoOx, CuOx, VOX, FeOx, Mn0x, Ti02, CaF2, BaF2, MgF2, ternary and / or complex oxides involving one or more of the elemental species of the mentioned compounds and mixtures thereof.

[0029] The composite material may be flexible. As used herein, the term “flexible” designates a composite material that can be bent to a radius of curvature of 50 cm, preferably to a radius of curvature of 25 cm, even more preferably to a radius of curvature of 10 cm, yet more preferably to a radius of curvature of 1 cm, most preferably to a radius of curvature of 0.1 cm, without visible deterioration.Brief Description of the Drawings

[0030] By way of example, preferred, non-limiting embodiments of the invention will now be described in detail with reference to the accompanying drawings, in which:Fig. 1: is a cross-sectional schematic view of a composite material according to an aspect of the present invention;Fig. 2: is a cross-sectional schematic view of an advanced cermet applied as a coating on the substrate so as to form a composite material according to an aspect of the present invention;Fig. 3: is a cross-sectional schematic view of a carbon-nanotube-based composite coating applied as a coating on the substrate so as to form a composite material according to an aspect of the present invention;Fig. 4: is a cross-sectional schematic view of a carbon-nanotube-based composite coating applied as a coating on the substrate so as to form a composite material according to an aspect of the present invention;Fig. 5: shows two optical microscopic images of the substrate (Al foam) coated with a carbon-nanotube-based composite coating;Fig. 6: shows a plot illustrating the THR of a carbon-nanotube-based composite coating applied on a flat aluminium substrate or on a textured surface (Al foam);Fig. 7: shows two optical microscopic images of the substrate (Cu foam substrate) coated with a carbon-nanotube-based composite layer;Fig. 8: shows the measured in-plane diffusion of polychromatic light for aCNT composite coating as a function of the diffusion angle, for an angle of incidence of 450(BRDF at 450);Fig. 9: shows the measured in-plane diffusion of polychromatic light for an advanced cermet as a function of the diffusion angle, for an angle of incidence of 450(BRDF at 450);Fig. 10: shows directional spectral emissivity of CNT-based composite coating (flat metallic surface substrate) in the 5-100 m spectral range;Fig. 11: shows directional spectral emissivity of CNT-based composite coating(Cu foam substrate) in the 5-100 pm spectral range;Fig. 12: shows directional spectral emissivity of advanced cermet layer (flat metallic surface substrate), in the 5-100 pm spectral range; andFig. 13: shows directional spectral emissivity of advanced cermet layer (Cu foam substrate), in the 5-100 pm spectral range.

[0031] The reader’s attention is drawn to the fact that the drawings are not to scale. Furthermore, for the sake of clarity, proportions between height, length and / or width may not have been represented correctly.Detailed Description of Preferred Embodiments of the Invention

[0032] Fig. 1 shows a composite material 10 comprising a substrate 12 having a three-dimensional structured surface 14 and a (black or superblack) coating 16 on the three-dimensional structured surface 14. The substrate 12 has a void ratio of more than 0.4. The coating 16 in combination with the three-dimensional structured surface 14 allows for having a total hemispherical reflectivity of no more than 5% over the entire wavelength range from 400 nm to 2.5 pm and for any angle of incidence lower than 6o°, preferably any angle of incidence lower than 8o°, more preferably any angle of incidence lower than 85°. In a preferred embodiment, the composite material 10 has an integrated total hemispherical reflectivity of no more than 1.5% over the entire wavelength range from 400 nm to 2.5 pm and for AOI < 85. In other words, the synergy of the black or superblack coating 16 with thethree-dimensional structured surface 14 allows for achieving a highly absorbing material in terms of THR. The composite material 10 maybe flexible.

[0033] The three-dimensional structured surface 14 may have a void ratio of more than 0.4, more preferably more than 0.6, even more preferably more than 0.9, most preferably more than 0.95. The three-dimensional structured surface 14 with the coating 16 may have a void ratio of more than 0.35, preferably more than 0.55, even more preferably more than 0.85, most preferably more than 0.9. The void ratio maybe not more than 0.99. In an embodiment, the void ratio of the substrate and the void ratio of the substrate with the coating 16 do not differ by more than 50%, preferably by more than 25%, even more preferably by more than 10%, most preferably by more than 2.5%. The substrate may comprise open pores. A size of the pore being preferably comprised in the range from 0.2 to 4 mm, preferably from 0.5 to 2 mm, even more preferably from 0.75 to 0.1 mm. The substrate is a foamed substrate comprising pores. Optionally, the foam is a metal foam.

[0034] In an embodiment, the substrate 12 has a density that is lower than 30%, preferably lower than 20%, even more preferably lower than 10%, most preferably lower than 5%, of the density of a bulk substrate made of the same material than substrate 12, without voids.

[0035] The coating 16 will be described in greater details in the following. Figs. 2-4 are enlargement of Fig. 1 (see ref sign 18).

[0036] In a first embodiment as depicted in Fig. 2, an advanced cermet 20 is applied as the coating 16 on the substrate 12. For the sake of clarity, the surface of substrate is depicted as plane. Of course, this may not be the case and the surface may be curved. The advanced cermet 20 comprises a metal oxide matrix 22 with embedded nanoparticles 24-1, 24-2 (together referred to as 24). The nanoparticles 24-1 are metastable metal carbide nanoparticles. The nanoparticles 24-2 are the stable decay products of the metastable carbide nanoparticles 24-1. Any proportion of embedded nanoparticles 24-1 with respect to embedded nanoparticles 24-2 may be achieved, by starting with a state where 100% of the embedded nanoparticles are in the metastable carbide state and by incompletely annealing the advanced cermet 20. By completely annealing the advanced cermet 10, all carbide nanoparticles have been transformed into the metal-carbon composite nanoparticles 24-2 with separate metal and carbon phases. The metal-carboncomposite nanoparticles 24-2 have a core-shell configuration, the core 26 being formed by the metal phase whereas the shell 28 is formed by the carbon phase.

[0037] The advanced cermet 20 may be applied as the coating 16 on the substrate 12 by chemical vapor deposition (CVD). CVD involves deposition at elevated temperatures and possibly at low pressures. These conditions are unfavourable for the incorporation of volatile organic molecules or water into the coating, thereby mitigating the risk of outgassing. That point is especially advantageous for space and high-vacuum applications. Another advantage of advanced cermets is their ability to withstand direct sun illumination or, more generally speaking, intense radiation without alteration. Last but not least, the chemical inertness of the advanced cermets is an advantage (e.g. low sensitivity to atomic oxygen) appreciated in many applications.

[0038] Two precursors may be used for producing the advanced cermet 20: a first precursor for depositing the ceramic matrix and a second precursors for depositing the carbide nanoparticles, the second precursors comprising an inorganic, metalorganic or organometallic precursor and at least one volatile carbon source such as alcohols.

[0039] CVD offers many degrees of freedom for applying the coating, including: thickness, composition of the matrix (choice of the ceramic), nature of the involved metal, size, density and density profile of the embedded nanoparticles, fraction of carbon to metal.

[0040] In a second embodiment as depicted in Figs. 3 and 4, a carbon- nanotube-based composite coating 30 is applied as the coating 16 on the substrate 12. Here also, for the sake of clarity, the surface of substrate is depicted as plane. Of course, this may not be the case and the surface maybe curved. The CNT- based composite coating 30 comprises a layer 32 of strongly entangled, non-aligned CNTs 34 that are individually covered with metal oxide claddings 36. The non- aligned coated CNTs 34 form a CNT thicket on the surface of the substrate 12. A ceramic cap layer 38 atop the layer 32 of CNTs is also present. As depicted in Fig. 3, the ceramic cap layer may lie atop the CNTs 34, In other embodiments, the ceramic cap layer 38 may infiltrate layer 32 (see Fig. 4). The ceramic cap layer 38 may be deposited by CVD. The CNTs 34 maybe grown by CVD. Of course, the CNTs 4 and the ceramic cap layer 38 maybe grown / deposited in a two-stage process.

[0041] Example 1: CNTs on an Al foam

[0042] The substrate 12 is an Al foam having pores size comprised in the range from 0.63 to 1 mm, a density of 1.41 g / cm3 and a void fraction of 65%.

[0043] Alternatively, the substrate 12 is an Al foam having pores size comprised in the range from 0.15 to 0.315 mm, a density of 0.52 g / cm3 and a void fraction of 44%.

[0044] CNTs are deposited by CVD (as described herein) on the Al foam so as to form a carbon-nanotube-based composite coating. The CNTs infiltrate the pores of the Al foam, as shown in Fig. 5. Fig. 5 shows two optical microscopic images of the substrate 12 coated with CNTs. The bright spots correspond to a deep noncoated porosity. The other locations are coated with CNTs. As can be readily seen, the pores of the foam are maintained even after deposition of CNTs. In other words, the CNTs are not obstructing the pores of the substrate 12, the three-dimensional structured surface 14 remains substantially the same.

[0045] Fig. 6 shows the THR of the coated Al foam, as well as a planar Al substrate coated with a CNT-based composite layer, integrated over incident wavelength in the range from 250 to 2500 nm. The black continuous line shows the requirement for achieving a superblack coating for space applications. The planar Al substrate, coated with a CNT-based composite layer, features absolute THR values that are significantly below the upper acceptable limit of reflection. The THR remains substantially unaffected in the range of AOI from o to 50°. For AOIs exceeding 6o°, the THR starts increasing although it remains far below the acceptable upper limit of reflection. This increase of the THR with the AOI is substantially reduced when the planar Al substrate is replaced by Al foam. The decreased AOI-dependence of the THR is of high relevance for straylight mitigation as even light at grazing incidences will be more efficiently absorbed.

[0046] Example 2 : Comparison between CNT -based composite coatings on anAl planar, on a Al foam or on a Cu foam substrate; as well as between advanced cermets on planar Al substrate or Cu foam substrate.

[0047] The first substrate 12 is an Al foam having pores size comprised in the range from 0.15 mm to. 0.315 mm, a density of 1.5 g / cm3 and a void fraction of 44%.

[0048] The second substrate 12 is a Cu foam having pores size of approx. 0.5 mm, a density of 0.15 g / cm3 and a void fraction of 98%.

[0049] Also, a reference Al substrate is provided. By contrast the reference substrate has a flat surface.

[0050] CNT-based composite layers are deposited by CVD (as described herein) on the Cu foam (or Al foam) so as to form a carbon-nanotube-based composite coating. The coatings infiltrate the pores of the Cu foam, as shown in Fig. 7 (or Al foam, Fig. 5). Fig. 7 shows two optical microscopic images of the Cu foam substrate 12 coated with a CNT-based composite layer. The bright spots correspond to surface defects. As can be readily seen, the pores of the foam remain even after deposition of CNTs. In other words, the CNTs are not obstructing the pores of the substrate 12, the three-dimensional structured surface 14 remains substantially the same.Table 1: Measured THR in the UV-visible-NIR spectrum for CNT-based composite coating and advanced cermet coating.

[0051] Table 1 shows the THR for a CNT-based composite layer coated on a flat metallic substrate (Al) and on Al and Cu foam. The THR is integrated over incident wavelength in the range from 250 to 2500 nm. The acceptable reflection level of a superblack coating for space applications is also provided (see row “Acceptable reflection”). The coated metallic foam with the highest void fraction (Cu foam) features substantially lower THR values relative to the flat metallic surface and relative to metallic foams with lower void fraction (Al foam). No substantial variation of the THR can be observed for coated Cu foam (void fraction of 98%) for an AOI in the range from o to 75%. In the same range, the THR is lower or equal to 0.33%.

[0052] Table 1 also shows the THR for the advanced cermet coating both on the flat metallic substrate (Al) and for the Cu foam surface integrated over incident wavelength in the range from 250 to 2500 nm. The Cu-foam coated with the advanced cermet exhibits THR values that are far lower than that of the flat reference. It is also worth noting that the AOI-dependence is nearly suppressed. Indeed, no significant variation of the THR can be observed for an AOI in the range from o to 75%. In the same range, the THR is lower or equal to 0.5%.

[0053] The strongly reduced fraction of reflected light in the case of coated Cu-foam, with CNT-based composite coating or with the advanced cermet coating, shows no preferential direction as confirmed by bidirectional reflectance distribution function (BRDF) measurements, see Figs. 8-9 showing an example of BRDF measurements with an incidence angle of 450for a CNT-base composite and for an advanced cermet on a Cu foam substrate. Indeed, for example no specular reflection can be seen at an angle of +450. The very small dip at -450corresponds to the angle of incidence and is expected, due to technical limitation of the method.

[0054] The CNT-based composite coating applied on a Cu foam substrate and advanced cermet coated Cu foam show a high emissivity in the spectral range from 5 pm to 100 pm for opaque samples. A high emissivity of a material comes together with a high absorptance, i.e. low light reflection for opaque samples. The results displayed in Table 2 show that the coated Cu-foam exhibits a low emissivitydependence on the direction of observation relative to the coated flat substrates. The directional emissivity of CNT coated flat metallic substrate (Al) is 13.5% lower at 70° than at io°, whereas it is only at 2.2% for the same coating applied on Cu foam.Table 2: Measured total directional emissivity in the far infrared spectral range for the CNT coating and the advanced cermet coating on the flat or Cu foam substrate.

[0055] The spectral emissivity of the CNT-based composite coating on the flat surface for different AOIs is displayed in Fig. 10. A significant wavelength-dependence emissivity is observed for the coated flat surface with values between nearly i to 0.3. This spectral dependence of the emissivity is substantially reduced when the coating is applied on the Cu foam, as shown in Fig. 11. A nearly constant emissivity of -0.93 is exhibited in the 5-iopm spectral range irrespective of the angle of observation. A nearly constant emissivity of ~o.8 is recorded in the spectrum 35-100 pm.

[0056] The spectral emissivity of the advanced cermet layer on the flat surface for different AOIs is shown in Fig. 12. The applied coating on the flat metallic substrate features a significant wavelength-dependence emissivity with values ranging from nearly 1 to 0.6. This spectral dependence of the emissivity is substantially reduced when the advanced cermet coating is applied on the Cu foam, as shown in Fig. 13. An emissivity of ~o.9 is observed in the entire spectral range 5- 100pm.

[0057] For Figs. 10-13, the emissivity is recorded under angles of observation of 10-70° at a temperature of 8o°C.

[0058] While specific embodiments have been described herein in detail, those skilled in the art will appreciate that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention, which is to be given the full breadth of the appended claims and any and all equivalents thereof.

Claims

Claims1. A composite material comprising a foamed substrate having pores and having a three-dimensional structured surface with a void ratio of more than 0.4 and a coating on the three-dimensional structured surface, so that the composite material has a total hemispherical reflectivity of no more than 5% over the entire wavelength range from 400 nm to 2.5 pm and for any angle of incidence lower than 60 °.

2. The composite material according to claim 1, wherein the three-dimensional structured surface with the coating has a void ratio of more than 0.35.

3. The composite material according to any one of claims 1 to 2, wherein the substrate comprises open pores, a size of the pores being preferably comprised in the range from 0.2 to 4 mm.

4. The composite material according to any one of claims 1 to 3, wherein the substrate comprises at least one of: Ti, Ni, Cu, Al, Ag, Fe-Ni, Ni-Cr, Fe, Co, C stainless steel, a ceramic.

5. The composite material according to any one of claims 1 to 4, wherein the coating comprises a ceramic matrix not being a carbide matrix, wherein said ceramic matrix has carbide nanoparticles and / or metal-carbon composite nanoparticles embedded therein, wherein said carbide nanoparticles are metastable and wherein said metal-carbon composite nanoparticles are decay products of the metastable carbide nanoparticles.

6. The composite material according to claim 5, wherein said ceramic matrix has metal-carbon composite nanoparticles embedded therein, said metal-carbon composite nanoparticles comprising metal cores with carbon shells.

7. The composite material according to any one of claims 5 to 6, wherein said ceramic matrix is a metal oxide matrix.

8. The composite material according to claim 7, wherein said metal oxide matrix consists of an oxide selected from the group consisting of: V02, A12O3, Si02, MgO, Ti02, Zr02, Mn3O4, Sn02, ZnO, spinel having the general formula AB2O4with A and B being metal cations having different valences, perovskite havingthe general formula A’B’O3with A’ and B’ being differently sized metal cations, and mixtures thereof.

9. The composite material according to any one of claims 5 to 8, wherein the density of said carbide nanoparticles and / or metal-carbon composite nanoparticles in said matrix is non-uniform across the thickness of the ceramic composite coating.

10. The composite material according to any one of claims 5 to 9, wherein said carbide nanoparticles consist of carbides of metals selected from the group consisting of: Ni, Co, Fe, Cr, Mo, Pt, Pd and mixtures thereof.

11. The composite material according to any one of claims 1 to 4, the coating comprising a layer of non-aligned carbon nanotubes, the carbon nanotubes comprising metal oxide claddings that sheathe the carbon nanotubes.

12. The composite material according to claim 11, the coating comprising a ceramic cap as a layer atop said sheathed carbon nanotubes and / or as an infiltration in said layer of carbon nanotubes.

13. The composite material according to claim 12, wherein the ceramic cap layer consists of a different material than said metal oxide claddings.

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