CARBON-NANOTUBE-BASED COMPOSITE COATING AND PRODUCTION METHOD THEREOF

Abstract

A first aspect of the invention relates to a carbon-nanotube-based composite coating, comprising a layer of carbon nanotubes (CNTs) that comprise metal oxide claddings sheathing them. Another aspect of the invention relates to a method for producing such CNT-based composite coatings using chemical vapour deposition (CVD).

Claims

1. A carbon-nanotube-based composite coating on a substrate, the carbon-nanotube-based composite coating comprising a layer of non-aligned carbon nanotubes, the non-aligned carbon nanotubes obtained by CVD-growth on said substrate, the carbon nanotubes comprising metal oxide claddings that sheathe the carbon nanotubes.

2. The carbon-nanotube-based composite coating as claimed in claim 1, wherein the carbon nanotubes have an average diameter in the range from 0.3 to 150 nm.

3. The carbon-nanotube-based composite coating as claimed in claim 1, wherein the metal oxide claddings comprise or consist of MgO.

4. The carbon-nanotube-based composite coating as claimed in claim 1, comprising a ceramic cap as a layer atop said sheathed carbon nanotubes and/or as an infiltration in said layer of carbon nanotubes.

5. The carbon-nanotube-based composite coating as claimed in claim 4, wherein the ceramic cap layer consists of a different material than said metal oxide claddings.

6. The carbon-nanotube-based composite coating as claimed in claim 4, wherein said ceramic cap layer consists of a material selected from the group comprising Al.sub.2O.sub.3, Si.sub.2O, Si.sub.3N.sub.4, MgF.sub.2, SiO.sub.xN.sub.x, AlN, AlNO, MgO, ZnO, SnO.sub.2, NiO, ZrO.sub.2, Cr.sub.2O.sub.3, MoO.sub.2, RuO.sub.2, CoO.sub.x, CuO.sub.x, VO.sub.x, FeO.sub.x, MnO.sub.x, TiO.sub.2, CaF.sub.2, BaF.sub.2, ternary and complex oxides involving one or more elemental species of the foregoing, and mixtures thereof.

7. The carbon-nanotube-based composite coating as claimed in claim 1, wherein the carbon-nanotube-based composite coating is an optical black coating.

8. The carbon-nanotube-based composite coating as claimed in claim 1, wherein the carbon-nanotube-based composite coating is an optical black coating having a total hemispherical reflectivity of no more than 5% over the wavelength range from 400 nm to 1 μm for any incidence angle greater than 20°.

9. The carbon-nanotube-based composite coating as claimed in claim 1, wherein the metal oxide claddings sheathe the carbon nanotubes on their full lengths or along sections thereof.

10. A substrate comprising a carbon-nanotube-based composite coating thereon, wherein the carbon-nanotube-based composite coating comprises a layer of non-aligned carbon nanotubes CVD-grown on said substrate and wherein the carbon nanotubes comprise metal oxide claddings that sheathe the carbon nanotubes.

11. The substrate as claimed in claim 10, wherein the metal oxide claddings sheathe the carbon nanotubes on their full lengths or along sections thereof.

12. The substrate as claimed in claim 10, wherein the carbon nanotubes have an average diameter in the range from 0.3 to 150 nm.

13. The substrate as claimed in claim 1, comprising a ceramic cap as a layer atop said sheathed carbon nanotubes and/or as an infiltration in said layer of carbon nanotubes.

14. The substrate as claimed in claim 13, wherein the ceramic cap layer consists of a different material than said metal oxide claddings and wherein said ceramic cap layer consists of a material selected from the group comprising Al.sub.2O.sub.3, Si.sub.2O, Si.sub.3N.sub.4, MgF.sub.2, SiO.sub.xN.sub.x, AlN, AlNO, MgO, ZnO, SnO.sub.2, NiO, ZrO.sub.2, Cr.sub.2O.sub.3, MoO.sub.2, RuO.sub.2, CoO.sub.x, CuO.sub.x, VO.sub.x, FeO.sub.x, MnO.sub.x, TiO.sub.2, CaF.sub.2, BaF.sub.2, ternary and complex oxides involving one or more elemental species of the foregoing, and mixtures thereof.

15. The substrate as claimed in claim 10, wherein the carbon-nanotube-based composite coating is an optical black coating having a total hemispherical reflectivity of no more than 5% over the wavelength range from 400 nm to 1 μm for any incidence angle greater than 20°.

16. A carbon-nanotube-based composite coating on a substrate, the carbon-nanotube-based composite coating comprising a layer of non-aligned carbon nanotubes, the non-aligned carbon nanotubes attached to said substrate with an end thereof, the carbon nanotubes comprising metal oxide claddings that sheathe the carbon nanotubes on their full lengths or along sections thereof.

17. The carbon-nanotube-based composite coating as claimed in claim 16, wherein the carbon nanotubes have an average diameter in the range from 0.3 to 150 nm, wherein the metal oxide claddings comprise or consist of MgO.

18. The carbon-nanotube-based composite coating as claimed in claim 16, comprising a ceramic cap as a layer atop said sheathed carbon nanotubes and/or as an infiltration in said layer of carbon nanotubes, and wherein the ceramic cap layer consists of a different material than said metal oxide claddings.

19. The carbon-nanotube-based composite coating as claimed in claim 4, wherein said ceramic cap layer consists of a material selected from the group comprising Al.sub.2O.sub.3, Si.sub.2O, Si.sub.3N.sub.4, MgF.sub.2, SiO.sub.xN.sub.x, AlN, AlNO, MgO, ZnO, SnO.sub.2, NiO, ZrO.sub.2, Cr.sub.2O.sub.3, MoO.sub.2, RuO.sub.2, CoO.sub.x, CuO.sub.x, VO.sub.x, FeO.sub.x, MnO.sub.x, TiO.sub.2, CaF.sub.2, BaF.sub.2, ternary and complex oxides involving one or more elemental species of the foregoing, and mixtures thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] 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:

[0039] FIG. 1: is a cross-sectional schematic of a CNT-based coating comprising metal-oxide-sheathed CNTs,

[0040] FIG. 2: is a cross-sectional schematic of a CNT-based coating as in FIG. 1 with a ceramic cap layer on top;

[0041] FIG. 3: is a cross-sectional schematic of a CNT-based coating as in FIG. 2 with a thicker ceramic cap layer on top;

[0042] FIG. 4: is a schematic illustration of the so-called base-growth mechanism;

[0043] FIG. 5: is a schematic illustration of the so-called tip-growth mechanism;

[0044] FIG. 6: is a schematic drawing of a CVD reactor equipped for pulsed-spray evaporation CVD;

[0045] FIG. 7: is flow chart of an example of a pulsed-spray evaporation CVD process in accordance with a preferred embodiment of a method for growing oxide-coated CNTs;

[0046] FIG. 8: is a diagram comparing the total hemispherical reflectivity of the coatings described as examples 1 to 3;

[0047] FIG. 9: is a scanning electron micrograph (SEM) of a CNT-based coating according to a preferred embodiment of the invention;

[0048] FIG. 10: is a top view SEM of a CNT-based composite coating capped with an Al.sub.2O.sub.3 layer;

[0049] FIG. 11: is a cross sectional view SEM of a CNT-based composite coating capped with an Al.sub.2O.sub.3 layer;

[0050] FIG. 12: is a close up of the capped tips of the CNTs of FIG. 11;

[0051] FIG. 13: is a cross-sectional schematic of a CNT-based coating with a porous ceramic cap infiltration of the layer of CNTs;

[0052] FIG. 14: is a cross-sectional schematic of a CNT-based coating with a relatively dense ceramic cap infiltration of the layer of CNTs.

DETAILED DESCRIPTION OF ONE OR MORE PREFERRED EMBODIMENTS

[0053] A carbon-nanotube-based composite coating 10 according to a first preferred embodiment of the invention is schematically depicted in FIG. 1. The CNT-based composite coating 10 is applied as a black coating on substrate 12. It comprises a layer 14 of strongly entangled, non-aligned CNTs 16 that are individually covered with metal oxide claddings 18. The non-aligned coated CNTs 20 form a CNT thicket on the surface of the substrate 12. It is worthwhile noting that the CNT thicket may be much denser than shown in FIG. 1.

[0054] FIGS. 2 and 3 shows a CNT-based composite coating 10′ comprising a ceramic cap layer 22 atop the layer 14 of CNTs. The ceramic cap layer 22 is preferably deposited by CVD or ALD after the CNT growth has been terminated. When the ceramic cap layer 22 is very thin (i.e. of approximately the same thickness as the coated CNTs 20 or thinner), the ceramic cap layer may still be discontinuous or conformal to the CNTs. When the ceramic cap layer 22 grows thicker (see in particular FIG. 3) the islands of ceramic cap material on the tips of the CNTs join and begin to form continuous but still porous layer. Eventually, as the CVD or ALD growth of the ceramic cap layer goes on, more and more pores are filled with ceramic material, leading to a continuous overgrowth of the ceramic cap layer with little or no apparent pores. Depending on the deposition conditions, the species involved, etc., the ceramic capping material may substantially (e.g. completely) infiltrate the layer of carbon nanotubes, substantially filling up the interstices between the coated CNTs 20, thereby leading to the formation of a dense film (consisting of the thicket of coated CNTs surrounded by the capping material). A CNT-based composite coating 10″ comprising a ceramic cap layer 22 infiltrating the layer 14 of CNTs is shown in FIGS. 13 and 14.

[0055] FIGS. 4 and 5 illustrate two CNT growth mechanisms that may lead to the formation of metal-oxide-coated CNTs.

[0056] FIG. 4 illustrates the so-called base base-growth mechanism. A metal nanoparticle 24 on the surface of the substrate 12 dissociates the hydrocarbon molecules that serve as the precursors of the CNTs into a carbon and a hydrogen fraction. The hydrogen gas leaves the reaction zone, while the carbon is dissolved in the metal until the solubility limit is reached. As from that point, the carbon crystallises out in the form of a CNT on the face of the nanoparticle turned away from the substrate. While the CNT grows, the nanoparticle remains in contact with the substrate.

[0057] FIG. 5 illustrates the so-called tip-growth mechanism. In that case, the carbon crystallised out as a CNT on the interface between the nanoparticle 24 and the substrate, causing the nanoparticle 24 to lift off and to remain at the tip of the CNT as growth thereof goes on.

[0058] According to the method proposed in the context of the present invention, the metal and/or metal carbide nanoparticles 24 comprise CNT-growth-catalysing metal and/or metal carbide involving elements selected from Fe, Co and Ni (or mixtures thereof). A second metal species, preferably Mg, is provided from a metalorganic or organometallic precursor, in parallel with the hydrocarbon (preferably alcohol) molecules that serve as the feedstock for the CNT growth. In both FIGS. 4 and 5, the precursors of the CNTs and the second metal are represented as precursor vapour 26.

[0059] It is currently believed that the exposure of the metal nanoparticles to the precursor for the second metal and the CNT feedstock molecule drives the following scenarios: [0060] The metal species from the inorganic, metalorganic or organometallic precursor for the second metal forms an alloys or carbide compound with the Fe, Co and/or Ni containing phases already present as nanoparticles on the growth surface. In case of cobalt as the first metal and Mg as the second metal, CoMg.sub.2, MgCo.sub.2, or MgCo.sub.3C.sub.0.5 are likely to be obtained. [0061] The CNT feedstock molecule is decomposed on the surface of the nanoparticles modified by the second metal and the carbon crystallises out in the form of a single- or multi-walled CNT. [0062] The inorganic, metalorganic or organometallic precursor for the second metal decomposes on the surface of the nanoparticle to yield metal oxide deposit.

[0063] It is observed that the presence of the second metal facilitates the formation of the CNTs at lower temperatures. Furthermore, a metal oxide cladding is formed that coats the CNT but does not hinder CNT growth. Regarding the reaction mechanisms, they have to be investigated further. Accordingly, the above scenarios shall be regarded as hypotheses to which the inventor does not intend to be bound. After further investigations, it may turn out that they do not accurately or not completely describe the enhanced CNT growth and/or the formation of metal oxide claddings thereon.

[0064] Notwithstanding that, the experimental results (see in examples 1 to 6 hereinafter) suggest that the three scenarios occur but in a given logic. The modification of the first metal phases (Fe, Co, Ni) by the precursor of the second metal (Mg(acac).sub.2 in the examples) seems to be the most favourable scenario from the kinetic point of view. The formed phases feature a stronger catalytic activity towards the growth of CNTs. As a consequence, the formation of CNTs is substantially enhanced. The growth of the metal oxide claddings occurs with more modest kinetics leading to coverage of the CNTs without hindering their growth.

[0065] The fraction of each phase in the CNT-based composite coating (i.e. the CNT phase, the metal oxide cladding phase and the alloy phase of the modified nanoparticles) can be adjusted by controlling the delivery recipe.

[0066] FIG. 6 illustrates a CVD reactor 30 for carrying out pulsed-spray evaporation CVD of a carbon-nanotube-based composite coating. The CVD reactor 30 comprises a main chamber 32 having arranged therein a substrate holder 34 with a substrate heater 36. The substrate (not shown in FIG. 4) may be placed on the substrate heater 36 by a manipulator (not shown). A vacuum pump 38 is connected to the main chamber 32, as well as a trap for the carrier gas (not shown). The precursors and the carrier gas may be introduced into the main chamber 32 through a tubular evaporation and transport chamber 40. The evaporation and transport chamber 40 is equipped with heaters (not shown) allowing it to be brought to sufficiently high temperatures for the evaporation of the precursor solutions and the transport of the resulting vapour. The liquid precursor solutions are injected by respective injectors 42 into an evaporation zone 44 of the evaporation and transport chamber 40. In the evaporation zone 44, precursor vapours are then formed, which are transported by the carrier gas through a so-called transport zone 46 leading into the main chamber 32. The carrier gas (represented by arrow 48) may be introduced at a controlled flow rate through carrier gas inlet 50.

[0067] FIG. 7 shows a flow chart illustrating pulsed-spray evaporation CVD (PSE-CVD) according to a preferred embodiment of the invention. The sequence used and the numerical values in FIG. 7 are illustrative only and may be varied depending on the composition of the coating to deposit. In a first step (S51), the various parts of the reactor that is used to produce a black CNT-based composite coating are heated to the desired temperatures. A first deposition phase of metal nanoparticles from a first precursors or a first group of precursors is then started. In the illustrated case, a first precursor solution containing the first precursor(s) is injected into the evaporation zone (step S52) at a predefined rate (4 Hz in the example) and with predefined opening times of the injector (2 ms for each injection in the example). The precursor vapour formed in the evaporation zone is transported by the carried gas into the main chamber of the reactor, where the ceramic matrix is deposited on the substrate. The injection regime of the first precursor solution is maintained for a predefined amount of time (10 minutes in the example), before the injections of the first precursor solution are stopped and the reactor is purged using the flux of carrier gas (step S53) during a first purge time (30 s with nitrogen in the illustrated example). A second precursor solution containing the precursors for the second metal (and thus for the metal oxide cladding) and for the CNTs is then injected into the evaporation zone (step S52) at a predefined rate (4 Hz in the example) and with predefined opening times of the injector (2 ms for each injection in the example). The vapour formed from the second precursors is transported into the main chamber, where coated CNTs are formed as illustrated in FIG. 4 or 5. The injection regime of the second precursor solution is maintained for a predefined amount of time (20 minutes in the illustrated example), whereupon the reactor is again purged. The deposition steps are repeated a certain number of times. The injection parameters may be varied in accordance with the desired deposition profile. It should also be noted that the last deposition step is not necessarily a coated CNT growth step but could be one of depositing a ceramic cap (using a third precursor solution). When the growth of the carbon-nanotube-based composite coating has completed, the reactor is cooled down and the grown samples are taken out of the reactor.

[0068] The growth process of FIG. 7 may be greatly varied depending on the desired end product and the chemistry involved. For instance, while it may be preferable for practical reasons to have all first precursors necessary for the deposition of the nanoparticles in one solution, separate precursor sources could also be used. The same holds for the second precursors. It is also possible to mix the precursors 1 and precursors 2 in the same solution feedstock.

[0069] The flux of the carrier gas may be held constant throughout the entire growth process. Alternatively, the flux of carrier gas could be varied so as to adjust or optimize the deposition conditions.

[0070] The temperature of the evaporation and transport chamber is also preferably held constant while the deposition goes on. If necessary or deemed advantageous, however, that parameter could also be changed over time. The same is true for the temperature of the substrate.

Example 1

[0071] A carbon-nanotube-based composite coating with CNTs coated with MgO claddings was grown using a CVD reactor of the Luxembourg Institute of Science and Technology (LIST), equipped with a PSE unit for the controlled injection of liquid feedstock. The precursors selected were metal acetylacetonates that are easy to handle, store and implement. These precursors are soluble in ethanol and a large number of other solvents. In this example, the first precursor solution (for the deposition of the CNT-growth catalysing nanoparticle) was a 5 mM (5 mmol/I) solution of cobalt acetylacetonate (Co(acac).sub.2) in ethanol. The second precursor solution (serving as feedstock for the coated CNTs) was a 5 mM solution of magnesium acetylacetonate (Mg(acac).sub.2) in ethanol. The precursor solutions were injected as pulsed sprays into an evaporation tube maintained at 220° C. under vacuum. The precursors' delivery was performed by 2 ms opening of the injector with a frequency of 4 Hz, which yielding respective feeding rates of 2.5 ml/min. The carrier gas was nitrogen introduced with a flow rate of 40 sccm (standard cubic centimetres per minute) maintained constant through the entire deposition process.

[0072] The deposition conditions are summarised in the following table:

TABLE-US-00001 Pressure during deposition 5 mbar Carrier gas: nitrogen 40 sccm Precursor 1: Co(acac).sub.2 in ethanol 0.005 mol/l Delivery of the precursor 1 4 Hz, 2 ms opening time Precursor 2: Mg(acac).sub.2 in ethanol 0.005 mol/l Delivery of the precursor 2 4 Hz, 2 ms opening time Temperature of evaporation and transport 220° C. Deposition temperature 350° C.

[0073] The deposition cycle was the following: [0074] 1. Precursor solution 1 during 10 minutes [0075] 2. Purge with nitrogen during 30 s, [0076] 3. Precursor solution 2 during 20 minutes, [0077] 4. Purge with nitrogen during 30 s.

[0078] The deposition cycle was carried out 5 times, followed by a final deposition of precursor solution 2. After the total deposition time of 175 minutes, a film thickness of 1.2 μm was reached, which corresponded to an average growth rate of 6.8 nm/min.

[0079] The CNT-based composite coating according to example 1 had an Mg/Co atomic ratio of 0.62, which was measured by EDX (Energy-dispersive X-ray analysis).

[0080] In a comparative experiment, it was shown that the cobalt nanoparticles deposited in the same conditions as in example 1 catalysed the growth of carbon nanotubes after a reducing heat treatment and exposure to acetylene-hydrogen at temperatures far exceeding 500° C. It was found that the growth rate of CNTs in the absence of magnesium was marginal below 600° C., which is clear evidence for the enhancement of the CNT-growth catalysing activity by the addition of Mg.

Example 2

[0081] A carbon-nanotube-based composite coating with CNTs coated with MgO claddings was grown using the same deposition conditions as in example 1, with the sole exception that the substrate temperature was set to 400° C. After the total deposition time of 175 minutes, a film thickness of 7.03 μm was reached, which corresponded to an average growth rate of 40.2 nm/min.

Example 3

[0082] A carbon-nanotube-based composite coating with CNTs coated with MgO claddings was grown using the same deposition conditions as in examples 1 and 2, with the sole exception that the substrate temperature was set to 450° C. After the total deposition time of 175 minutes, a film thickness of 11.5 μm was reached, which corresponded to an average growth rate of 69.7 nm/min.

[0083] Examples 1 to 3 suggest that the growth rate in this limited range (350-450° C.) using separate precursors delivery (cobalt and magnesium acetylacetonate) linearly depends on the deposition temperature. The total hemispherical reflectivity (THR) of the three obtained coatings was evaluated in the spectral range from 250 nm to 2300 nm for an incidence angle of 8° (diffuse and specular reflections were integrated). The results are displayed in FIG. 8. All obtained CNT films feature a very low reflection in the UV-Vis-NIR spectral range. At the first glance, the film obtained at 350° C. exhibits lower optical performance since a somehow higher reflectance is measured. Nevertheless, it should be highlighted that the film of example 1, obtained at 350° C. is 10 times thinner than the film of example 3, which was grown at 450° C.

[0084] Integration of the THR over the 250-2300 nm spectral range yields a reflectance of 2.47% for the film obtained at 350° C., a reflectance of 0.6% for the film obtained at 400° C. and a reflectance of 0.55% for the film obtained at 450° C.

[0085] FIG. 9 shows a scanning electron micrograph of the coating obtained in example 1. The thicket of CNTs is clearly visible. The CNTs appear rough and of irregular diameter, which is due to the cladding of MgO sheathing the CNTs.

Example 4

[0086] A carbon-nanotube-based composite coating with CNTs coated with MgO claddings was grown using the same deposition conditions as in example 3, with the exception that the first precursor was Ni(acac).sub.2. After a total deposition time of 206 minutes, a film thickness of 5 μm was reached, which corresponded to an average growth rate of 24.27 nm/min. The integrated total hemispherical reflection over the 300-2300 nm spectral range was measured at 0.54%. The CNT-based composite coating according to example 4 had an Mg/Ni atomic ratio of 0.38, which was measured by EDX.

Example 5

[0087] A carbon-nanotube-based composite coating with CNTs coated with MgO claddings was grown using the same deposition conditions as in example 3, with the exception that the precursor 1 and precursor 2 were physically mixed (feedstock was ethanol with 2.5 mM of Mg(acac).sub.2 and 2.5 mM of Co(acac).sub.2). After a total deposition time of 120 minutes, the films featured an integrated total hemispherical reflection over the 300-2300 nm spectral range as low as 0.35%.

Example 6

[0088] A carbon-nanotube-based composite coating according to example 3 above was capped with a 20 nm thick conformal Al.sub.2O.sub.3 layer. This layer was applied at 120° C. (substrate temperature) and 3 mbar pressure using the following deposition cycle: [0089] 1. Exposure of the CNTs to trimethylaluminum (TMA) during 200 ms [0090] 2. Purge with nitrogen (flow rate of 350 sccm) during 2 s, [0091] 3. Exposure of the CNTs to water vapour during 200 ms [0092] 4. Purge with nitrogen (flow rate of 350 sccm) during 2 s,

[0093] Steps 1 to 4 were carried out 125 times. The purges with nitrogen were implemented to prevent parasitic CVD reactions.

[0094] FIG. 10 shows a SEM of the surface obtained (top view) in example 4. FIG. 11 is a cross sectional SEM of the CNT-based composite coating capped with the Al.sub.2O.sub.3 layer. FIG. 12 is a close up of the tips of the CNTs coated with the Al.sub.2O.sub.3 layer.

[0095] Capping the CNT-based composite coating inherently changes the optical performance. The deposition of the alumina layer (20 nm) yielded an increase of the reflectance in the UV-Vis-NIR from 0.55% (example 3) to 1.1% (example 4). It is worthwhile noting that this reflectance is still exceptionally low despite use of an oxide with relatively high refractive index as the cap layer. Further improvements in terms of low reflectance are expected if SiO.sub.2 or MgF.sub.2 is used for capping the coated CNTs. It was observed that the deposition of up to 80 nm SiO.sub.2 induced a marginal change of the THR. The integrated total hemispherical reflection over the 300-2300 nm spectral range was measured at 0.69%, 0.66%, 0.79% and 0.89% for SiO.sub.2 capping layer thicknesses of 20 nm, 37 nm, 80 nm and 200 nm, respectively.

[0096] 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.