WO2011157946A1 - Method for preparing carbon-nanotube conductive transparent films - Google Patents

Method for preparing carbon-nanotube conductive transparent films Download PDF

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Publication number
WO2011157946A1
WO2011157946A1 PCT/FR2011/051352 FR2011051352W WO2011157946A1 WO 2011157946 A1 WO2011157946 A1 WO 2011157946A1 FR 2011051352 W FR2011051352 W FR 2011051352W WO 2011157946 A1 WO2011157946 A1 WO 2011157946A1
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film
films
carbon nanotubes
salts
nanotubes
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PCT/FR2011/051352
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French (fr)
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Alain Penicaud
Amélie CATHELINE
Patrice Gaillard
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Arkema France
C. N. R. S.
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Publication of WO2011157946A1 publication Critical patent/WO2011157946A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01B—CABLES; CONDUCTORS; INSULATORS; SELECTION OF MATERIALS FOR THEIR CONDUCTIVE, INSULATING OR DIELECTRIC PROPERTIES
    • H01B1/00—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors
    • H01B1/04—Conductors or conductive bodies characterised by the conductive materials; Selection of materials as conductors mainly consisting of carbon-silicon compounds, carbon or silicon
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y10/00—Nanotechnology for information processing, storage or transmission, e.g. quantum computing or single electron logic
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B82—NANOTECHNOLOGY
    • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
    • B82Y40/00—Manufacture or treatment of nanostructures
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B32/00—Carbon; Compounds thereof
    • C01B32/15—Nano-sized carbon materials
    • C01B32/158—Carbon nanotubes
    • C01B32/168—After-treatment
    • C01B32/174—Derivatisation; Solubilisation; Dispersion in solvents
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K85/00—Organic materials used in the body or electrodes of devices covered by this subclass
    • H10K85/20—Carbon compounds, e.g. carbon nanotubes or fullerenes
    • H10K85/221—Carbon nanotubes
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2202/00—Structure or properties of carbon nanotubes
    • C01B2202/02—Single-walled nanotubes
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2202/00—Structure or properties of carbon nanotubes
    • C01B2202/06—Multi-walled nanotubes
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2202/00—Structure or properties of carbon nanotubes
    • C01B2202/20—Nanotubes characterized by their properties
    • C01B2202/34—Length
    • C—CHEMISTRY; METALLURGY
    • C01—INORGANIC CHEMISTRY
    • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2202/00—Structure or properties of carbon nanotubes
    • C01B2202/20—Nanotubes characterized by their properties
    • C01B2202/36—Diameter
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K30/00—Organic devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation
    • H10K30/80—Constructional details
    • H10K30/81—Electrodes
    • H10K30/82—Transparent electrodes, e.g. indium tin oxide [ITO] electrodes
    • H10K30/821—Transparent electrodes, e.g. indium tin oxide [ITO] electrodes comprising carbon nanotubes
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/549—Organic PV cells

Definitions

  • the present invention relates to the field of transparent conductive films.
  • the present invention relates to the preparation of conductive transparent films from solutions of salts of carbon nanotubes, the transparent conductive films thus obtained and their various uses.
  • Indium-tin oxide films (hereinafter referred to as ITO) represent current commercial solutions in the field of transparent conductive applications, such as, for example, electrodes.
  • the main characteristic of indium and tin oxide is its combination of electrical conductivity and optical transparency.
  • the electrical properties expressed as surface resistivity are of the order of 30 ohms / sq.
  • ITO-based electrodes For the best ITO-based electrodes, and generally of the order of 250 ohm / sq.
  • a difficulty in producing these electrodes lies in the compromise that must be achieved during the deposition of ITO films, the increase in the ITO concentration inducing an increase in the conductivity of the material, but at the same time a loss. of its transparency.
  • the ITO-based conductive transparent films have numerous drawbacks related in particular to the high cost and the availability of the indium oxide raw material, to the lack of flexibility of the ITO thin layers leading to rigid films, at a limited chemical stability, and a method of deposition difficult to achieve on plastic substrates because of its high temperature.
  • carbon nanotubes are potential candidates for transparent conductive films. Indeed, the carbon nanotubes (here after NTC designated) have excellent electrical and mechanical properties, even in the form of thin films of a few nanometers thick. Since robust NTC films can be obtained with extremely small thicknesses, the resulting films are not only conductive, but also transparent. In addition, NTCs can be applied to flexible substrates without the disadvantages of ⁇ , so NTC-based transparent conductive films can cover a wide range of applications.
  • a transparent electrode consists of a transparent substrate on which is deposited a thin film formed from a composition comprising CNTs and a possibly conductive dispersant.
  • the preparation of the electrode requires a post-treatment, acidic or basic, to "dope" the dispersant contained in the film, and improve the conductivity of the electrode.
  • Document US 2008/0118634 describes a process for preparing a transparent conductive film on a glass structure comprising the following steps: preparation of a "slurry" of CNTs, application of this slurry on the glass, drying of the formed layer and heating at a temperature of 300 ° C to 500 ° C under an inert atmosphere to solidify the surface layer comprising CNTs.
  • this method has the disadvantage of implementing the CNT in the form of slurry whose preparation is complex and uses an organic support resulting from a mixture of a solvent, a plasticizer and a stabilizer.
  • a conductive transparent film on a polyethylene terephthalate (PET) plastic film having a transmittance of 86.1% is obtained from a dispersion comprising functionalized mono-wall CNTs and whose viscosity is controlled by the presence of a carbamate additive prepared beforehand from an amino derivative.
  • PET polyethylene terephthalate
  • the surface resistivity of this film is very high (10 8 ohms / sq).
  • the process for preparing a transparent conductive film described in WO 2009/048269 is based on spraying at a pressure ranging from 0.05 to 60 kg / cm 2 of an aqueous dispersion comprising CNTs and a dispersing agent. on a support which may be glass, a polymer film or a ceramic film. Surface resistivities of the order of 700 to 800 ohms / sq are obtained for films having a transmittance of 84-85%.
  • a conductive transparent film having both CNTs and ⁇ is prepared by sequentially and alternately dipping a PET substrate in an ink containing CNTs and in an ink containing indium and tin, a water rinse, air drying and the deposition of a layer of polymer serving as a binder between the CNT and ⁇ being performed between the deposition of each layer.
  • the film thus prepared is characterized by a transmittance of 85% and a surface resistance of 1500 ohms / sq.
  • the document FR 2 923 823 describes a process for the preparation of conductive carbon nanotube aerogels that can be used for the preparation of electrochemical components. This procedure may involve an intermediate step of filtration of the solid CNT salts before the individualization step in a solvent medium. following steps consisting of a freezing of the solution and sublimation of the solvent. The aerosol can then be formed into membranes by compression.
  • the present invention therefore aims to provide a process for the preparation of transparent conductive films based on carbon nanotubes, which is simple, fast (with the fewest possible steps), and easy to implement.
  • NTCs are difficult to disperse because of their entangled structure that can generate strong Van der Waals interactions between their molecules according to their method of manufacture. Without a good dispersion of the CNTs, it is difficult to obtain films having a good homogeneity, which is indispensable for the application envisaged.
  • the dispersions of CNTs in water are generally carried out using surfactants as dispersants, and ultrasound.
  • the surfactant having a lower conductivity than the CNTs, plays an insulating role between the CNTs and can lead to a decrease in the conductivity of the final film. Sonication can damage CNTs by shortening them.
  • the present invention therefore relates to a method for preparing a transparent conductive film comprising at least the following steps:
  • step a The preparation of the solution of salts of carbon nanotubes (step a) is based on a method called "soft dissolution”, developed by Penicaud et al. ("Spontaneous dissolution of a single wall carbon nanotube"), J. Am., Chem., Soc., 2005, 127, 8-9, "Dissolution Douce of single walled carbon nanotubes", published by Kirchberg, AIP conference proceedings, vol786, 2005, p266-270), which was the subject of the patent application WO 2005/073127.
  • This method makes it possible to disperse the nanotubes in organic solvents without functionalization or prior strongly acid treatment, and without using surfactants or ultrasound.
  • a nanotube salt is formed by exposing the nanotubes to a solution of tetrahydrofuran (THF) of potassium naphthalenide. Then, it is possible to recover in the solid state by filtration the salts of CNT thus obtained in order to keep them several months at room temperature. It should be noted that at this stage, the nanotubes have not yet been dissolved and the filtration makes it possible to recover the salt in the form of aggregates. Subsequent exposure of these salts to a polar organic solvent such as DMSO leads to a solution of individualized nanotubes.
  • THF tetrahydrofuran
  • the CNTs thus solubilized in the form of salts after reduction in an organic solvent retain all their characteristics and their integrity. Their size or surface is not damaged by chemical treatments or the supply of mechanical energy. Measurements made by light scattering on the solutions thus obtained showed that the average length of the CNTs in the organic solution is approximately 3 times greater than that of the NTCs dispersed in the aqueous phase. In in particular, the length of the CNT varies between ⁇ and 2 ⁇ in organic solution, while their length varies between 300 nm and 700 nm when they are dispersed in aqueous phase.
  • the filming from the solution of carbon nanotube salts prepared in step a) of the process according to the invention is based on the work already carried out on aqueous dispersions of carbon nanotubes (Transparent, conductive carbon nanotubes films, Wu et al, Science, 2004, 305, 1273), and on the work done on graphene films (Electrical Conductivity of Graphene Films with Poly (allylamine hydrochloride) Supporting Layer, Kong et al, Langmuir 2009, 25 (18). ), 11008-11013).
  • the CNT salt films are obtained in the process of the invention, by implementing both steps b) and c).
  • a first film is obtained by vacuum filtration of the solution of NTC salts on a solvent-resistant filter membrane of the solution.
  • step c) of the process according to the invention the film deposited on the filtering membrane is transferred onto a transparent substrate.
  • This support transfer for the CNT salt film is carried out by dissolving the filter membrane in a suitable medium according to the nature of the membrane, then introducing a transparent substrate into this medium in order to deposit the film on this substrate.
  • the transparent conductive film thus obtained is then separated from the medium in which it was formed, and may be optionally rinsed after drying.
  • the conductive transparent film comprises carbon nanotubes of average length greater than 1 ⁇ , and up to a few hundred ⁇ , preferably of average length ranging from ⁇ to 10 ⁇ , on a transparent substrate.
  • the transparent conductive film according to the invention advantageously has a transmittance greater than 20% up to 99%, preferably between 40% and 99%.
  • the surface resistivity of the transparent conductive film of the invention is less than 2 10 5 ohms / sq and can go up to 10 ohms / sq, preferably it is comprised between 2 10 4 ohms / sq and 10 ohm / sq.
  • the invention also relates to the various uses of said transparent conductive film, in particular for producing transparent electrodes in the fields of light-emitting diodes, image sensors, solar cells, touch screens, liquid crystal screens. The process according to the present invention will now be described in more detail.
  • Carbon nanotubes have particular, tubular, hollow and closed structures, composed of atoms regularly arranged in pentagons, hexagons and / or heptagons, obtained from carbon. CNTs generally consist of one or more sheets of graphene rolled up. Thus, single-wall nanotubes (Single Wall Nanotubes or SWNTs) and multiwall nanotubes (Multi Wall Nanotubes or MWNTs) are distinguished.
  • the double-walled nanotubes can in particular be prepared as described by FLAHAUT et al in Chem. Com. (2003), 1442.
  • the multi-walled nanotubes may themselves be prepared as described in WO 03/02456.
  • the carbon nanotubes used in the process according to the invention usually have a mean diameter ranging from 0.1 to 200 nm, preferably from 0.1 to 100 nm, more preferably from 0.4 to 50 nm, and better still , from 5 to 30 nm and advantageously a length of more than 0.1 ⁇ and advantageously from 0.1 to 20 ⁇ , for example about 6 ⁇ . Their length / diameter ratio is advantageously greater than 10 and most often greater than 100.
  • These carbon nanotubes therefore comprise in particular nanotubes known as "VGCF" (carbon fibers obtained by chemical vapor deposition or Vapor Grown Carbon Fibers).
  • the carbon nanotubes may be single-walled (SWNT), or may comprise some walls (FWNT) or they may be multi-walled, for example comprising from 5 to 15 sheets and more preferably from 7 to 10 sheets.
  • An example of raw multi-walled carbon nanotubes is grade
  • the carbon nanotubes used in the process according to the invention are single-walled carbon nanotubes having a metallic character, that is to say they contain a high content, at least 70% and preferably at least 90% of metal tubes.
  • Metallic character means nanotubes whose state density diagram has accessible electronic states without thermal activation, unlike semiconductor nanotubes which have an energy barrier between the last occupied states (valence band) and the first ones. vacant states (conduction band).
  • SWNT carbon nanotubes can be made especially according to the CoMoCat ® process, and are available for example under the name CoMoCat ® CG200 the South West Technologies.
  • the carbon nanotubes may be purified and / or treated (in particular oxidized or functionalized) and / or milled before being used in the process according to the invention.
  • the grinding of the carbon nanotubes may in particular be carried out cold or hot and be carried out according to known techniques used in devices such as ball mills, hammers, grinders, knives, jet gas or any other system grinding capable of reducing the size of the entangled network of nanotubes. It is preferred that this grinding step is performed according to a gas jet grinding technique and in particular in an air jet mill.
  • the purification of the nanotubes can be carried out by washing with a sulfuric acid solution, or another acid, so as to rid them any residual mineral and metallic impurities arising from their preparation process.
  • the weight ratio of the nanotubes to the sulfuric acid may especially be between 1: 2 and 1: 3.
  • the purification operation may also be carried out at a temperature ranging from 90 to 120 ° C, for example for a period of 5 to 10 hours. This operation may advantageously be followed by rinsing steps with water and drying the purified nanotubes.
  • Another way of purifying nanotubes, intended in particular to remove iron and / or magnesium, or any other metal they contain, is to subject them to a heat treatment at more than 1,000 ° C.
  • the oxidation of the nanotubes is advantageously carried out by putting them in contact with a solution of sodium hypochlorite containing from 0.5 to 15% by weight of NaOCl and preferably from 1 to 10% by weight of NaOCl, for example in a weight ratio of nanotubes to sodium hypochlorite ranging from 1: 0.1 to 1: 1.
  • the oxidation is advantageously carried out at a temperature below 60 ° C. and preferably at room temperature, for a period ranging from a few minutes to 24 hours. This oxidation operation may advantageously be followed by filtration and / or centrifugation, washing and drying steps of the oxidized nanotubes.
  • the carbon nanotubes be used in the process according to the invention in the raw state, or simply free of residual impurities from their preparation process.
  • single-walled carbon nanotubes having a metallic character are used.
  • the invention also relates to a conductive transparent film comprising single-walled carbon nanotubes with a metallic character of average length greater than 1 ⁇ , and which can be up to a few hundred ⁇ , preferably of average length ranging from ⁇ to 10. ⁇ , on a transparent substrate.
  • Step a) of the process according to the invention consists in preparing a solution in an organic solvent of a salt of carbon nanotubes.
  • a salt of carbon nanotubes is first prepared by reduction of the nanotubes, which leads to negatively charged nanotubes with positive counter ions.
  • the reduction of the CNTs is carried out by addition, under anaerobic conditions, of a salt of formula: A + B "
  • a + represents a salt of an alkaline ion, such as, for example, lithium, sodium or potassium;
  • B " represents an anion of a polyaromatic compound, so as to electrically charge the carbon nanotubes, ⁇ anion of the polyaromatic compound being a reducing agent for these CNTs.
  • the polyaromatic compound may be chosen from naphthalene, benzophenone, fluorenone and anthraquinone. Naphthalene is preferred.
  • the salt A + B can be obtained by reaction of a polyaromatic compound B with an alkali metal A in suspension in an organic solvent such as THF.
  • an organic solvent such as THF.
  • the Li + salt Naph is prepared by reacting naphthalene with an excess of lithium in THF to development of a dark green color close to black. One can proceed in the same manner using the potassium place of lithium.
  • the salt of carbon nanotubes is obtained by mixing the CNT powder and the organic solution of the salt A + B " , generally with stirring for a few hours, at ambient temperature, which is carried out under a controlled atmosphere, for example under argon.
  • the carbon nanotube salt is separated from the medium by filtration, and may be subjected to several rinses with the aid of the organic solvent used for the preparation of the salt A + B " .
  • the solid is then dried, generally under vacuum at room temperature. .
  • This solid, a negatively charged NTC powder, has a good storage stability of at least several months in a controlled atmosphere.
  • the carbon nanotube salt is prepared directly by mixing an alkali metal A and NTC (binary mixture), said mixture being heated to a high temperature, typically at a temperature that may range from 150 ° C. at 400 ° C, for a period of the order of a few minutes to a few days, the metal vapors leading to a salified form of CNTs.
  • NTC binary mixture
  • the carbon nanotube salt is dissolved in an organic solvent.
  • the previously obtained solid is introduced into an organic solvent, resulting in an organic phase comprising the dissolved NTC salt, and an undissolved solid phase generally comprising the impurities and aggregates of nanotubes and other undissolved species.
  • the CNT salt solution is then separated, for example, by centrifugation of the solid residue.
  • Suitable solvents include polar organic solvents such as sulfolane, dimethylsulfoxide (DMSO), dimethylformamide, N-methylpyrrolidone, or N-methylformamide.
  • DMSO dimethylsulfoxide
  • This dissolution has the advantage of inducing no denaturation of carbon nanotubes.
  • the concentration of the organic solution of CNT salts prepared during stage a) is generally between 0.001 and 10 g / l, preferably between 0.01 and 5 g / l.
  • the organic solution of CNT salts can be diluted before being subjected to the next step b) of filtration, to optimize this step in terms of feasibility.
  • a concentration of NTC salts ranging from 0.0001 to 0.01 g / l will then be targeted.
  • Step b) of the process according to the invention consists in filtering under vacuum the organic solution of salts of carbon nanotubes prepared in step a), on a filtering membrane.
  • the filtration membrane must resist the solvent and allow the formation of a film in the form of a network of random NTC and homogeneous in thickness. It must also be eliminated without damaging the network of nanotubes thus formed.
  • alumina alumina, nylon or Teflon membranes which make it possible to obtain a very smooth film without apparent inhomogeneity.
  • alumina membrane is used.
  • the thickness of the film on the membrane is controlled by the amount of filtered solution.
  • the thickness of the film is between 0.7 nm and 1000 nm, preferably between 1 nm and 500 nm, more particularly between 1 nm and 200 nm.
  • the content of CNT in the film can range from 0.2 to 50 ⁇ s / ⁇ 2 , preferably it ranges from 1 to 10 ⁇ s / ⁇ 2 , representing a good compromise between the properties concerned.
  • Filtration of the solution can be carried out under an inert atmosphere, for example in a glove box, under an argon atmosphere.
  • the membrane film is then re-oxidized ("neutralized") in dry air for a period of one to a few hours.
  • the CNT salt solution is exposed to air prior to the filtration step then carried out in the open air, in order to restore the neutral state of the CNTs.
  • step c) of the process according to the invention the film deposited on the filter membrane is transferred onto a transparent substrate.
  • the membrane is first dissolved in a medium chosen according to the nature of the membrane to be dissolved.
  • a medium chosen according to the nature of the membrane to be dissolved.
  • a strongly alkaline medium will be selected.
  • the film supported on the membrane is introduced into a bath containing a 1.5 M sodium hydroxide solution for a period of time ranging from a few minutes to one hour.
  • a transparent substrate is introduced into the medium under the supernatant NTC film.
  • the medium may be subject to prior neutralization, in the case of the implementation of a basic medium or an acid medium.
  • the CNT film is held on the substrate by capillarity, eliminating the liquid medium.
  • the assembly is then dried to remove any residual solvent layer between the substrate and the film, thus leading to good adhesion of the film on the support.
  • the drying temperature depends on the nature of the substrate and it is chosen not to deteriorate it. Generally, the temperature may range from 40 ° C to 500 ° C, preferably from 40 ° C to 100 ° C.
  • the transparent substrate according to the invention may be an inorganic substrate such as glass, quartz, mica or a ceramic, or a rigid or flexible plastic substrate.
  • a flexible substrate may be chosen for example from the following materials: polyethylene terephthalate, polyethylene naphthalate, polyethylene sulphone, polycarbonate, polystyrene, polypropylene, polyester, polyimide, polyether ether ketone, polyetherimide, acrylic resins, olefin-maleimide copolymers, norbornene-based resins, without this list being limiting.
  • the transparent substrate can be composed of several layers of different materials.
  • the substrate advantageously has a thickness ranging from 0.1 micron to 10 mm which will be adapted to the application of the film envisaged.
  • the transparent conductive film obtained according to the process of the invention has better electrical properties (vs transmittance) than those conventionally obtained with films derived from aqueous dispersions of carbon nanotubes. According to the invention, it is possible to achieve performance equivalent to those of conventional..
  • the percolation threshold of the film according to the invention that is to say the level of CNT charge from which a measurement of the conductivity of the film is possible, in other words from which a conductive path is formed from one end to the other of the film, is lower than for the films obtained from aqueous dispersions of CNTs.
  • the percolation threshold can be determined from the evolution of the surface conductance on the one hand, and the evolution of the transmittance on the other hand, as a function of the amount of CNT present in the film.
  • the transparent conductive film obtained according to the method of the invention can be used in all applications that require both good light transmission and conductive properties, especially in the fields of light-emitting diodes, image sensors, cells solar panels, touch screens, LCD screens.
  • the invention also relates to the use of a transparent conductive film comprising single-walled carbon nanotubes with a metallic character of average length greater than 1 ⁇ , and being able to go up to a few hundred ⁇ , on a transparent substrate, in the fields of light-emitting diodes, image sensors, solar cells, touch screens, liquid crystal displays.
  • SWeNT SouthWest NanoTechnologies
  • Carbon nanotubes salts are synthesized by mixing CNT powder and an organic solution of tetrahydrofuran (THF) containing a naphthalene salt, Naph "K +.
  • THF tetrahydrofuran
  • the naphthalene salt was prepared by reacting naphthalene with an excess of potassium in THF.
  • This solution is poured onto a CNT powder and left stirring at room temperature for four hours. Then this solution is filtered on a Millipore type membrane (0.45 microns porosity). The solid is rinsed several times with THF until colorless THF is obtained after passing through the filter. The drying is then carried out under vacuum at room temperature. The solid obtained is a negatively charged NTC powder.
  • FWNTs length 1.80 ⁇ ⁇ 0.1 ⁇ for a diameter of 1 nm
  • MWNTs length 1.65 ⁇ ⁇ 0.1 ⁇ for a diameter of 15 nm
  • SWNTs length 3,30 ⁇ ⁇ 0,1 ⁇ for a diameter of 0,7 nm
  • Aqueous dispersions containing 0.3% of CNTs are obtained as follows:
  • FWNTs are dispersed using sodium cholate as surfactant, in the proportions 1: 1 to 0.3% in water. Their dispersion is obtained by sonication for 1 hour at 20W.
  • the MWNTS are dispersed in the same proportions for 30 minutes using ultrasound at a power of 20W.
  • SWNTS are dispersed with sodium cholate in the ratio of 1: 1 to 0.1% in water. Their dispersion is obtained by sonication for 1 hour at 20W, followed by a slight centrifugation at 2000 rpm for 30 min to remove the remaining large aggregates.
  • the average lengths of the CNTs, determined by light scattering are:
  • FWNTs length 630 nm for a diameter of 1 nm
  • MWNTs length 312 nm for a diameter of 15 nm
  • SWNTs length of 2.8 ⁇ for a diameter of 1 nm
  • Filtration of the NTC salt solution is carried out on an alumina membrane (Whatman part number 3802Z, porosity 0.02 ⁇ m, diameter 47 mm).
  • the filtration is carried out in glove box.
  • the membrane film is then left in dry air for 2 hours before being transferred to a PET substrate (ref ES301400 from Goodfellow, thickness ⁇ ).
  • the transfer is carried out by removing the alumina membrane in a 1.5M sodium hydroxide bath for about 10 minutes. By successive washings, the bath is brought back to neutral pH and the PET substrate is then introduced into the bath under the film of salts of CNT floating on the surface of the water. The film is gently deposited on the PET by emptying the bath. The film is held on the PET by capillarity, it is then dried in an oven at 50 ° C for 24 hours.
  • Filtration of the aqueous dispersion is carried out on a cellulose membrane (Millipore GSWP04700, porosity 0.22 ⁇ , diameter 47 mm) as described in Transparent, conductive carbon nanotube films, Wu et al, Science, 2004, 305, 1273.
  • the film thickness on cellulose is controlled by the amount of the filtered dispersion.
  • the filtration makes it possible to obtain a network of NTC that is random and homogeneous in thickness.
  • the second step involves removing the cellulose membrane and depositing the NTC film on a PET substrate (Goodfellow ES301400, thickness ⁇ ).
  • the membrane is impregnated with 1, 2 dichlorobenzene and then "stuck" on the PET substrate of the same diameter as the membrane, the face consisting of CNTs is placed on the substrate.
  • the assembly is then immersed in an acetone bath for 20 min in order to dissolve the cellulose while retaining the NTC film on the PET.
  • the film is rinsed with acetone and isopropanol and then dried with nitrogen.
  • the residual cellulose is removed by re-dipping the film in an acetone bath for a few hours without the CNT network peeling off the PET.
  • Transmittance measured at 550 nm by absorption spectroscopy on films (device: Unicam UV4-100 absorption spectrometer, wavelength range 400 - 900 nm).
  • PET transparent films obtained from a solution of CNT salts, characterized respectively by a quantity of MWNTs of 0.95 ⁇ g / cm 2 and 4.74 ⁇ g / cm 2 , were observed in SEM.
  • Table 1 collates the values of surface resistivity (Rsq, expressed in kohms / sq) and transmittance (T in%) at 550 nm measured on the films obtained from the solutions of salts of CNT, according to the amount of filtered solution on alumina filter.
  • the films FWNTs and MWNTs respectively have a resistivity of the order of 6 and 29 kohms / sq, and the SWNT films have a resistivity of the order of 200 ohms / sq.
  • the films FWNTs and MWNTs respectively have a resistivity of 2 and 5.5 kohms / sq, and the films of SWNTs a resistivity of the order of 100 ohms / sq.
  • Table 2 collates the values of surface resistivity (Rsq, expressed in kohms / sq) and transmittance (T in%) at 550 nm measured on the films obtained from the aqueous dispersions of CNTs, as a function of the quantity filtered dispersion on membrane.
  • the films FWNTs and MWNTs respectively have a resistivity of 53 and 2125 kohms / sq, and the SWNTs films have a resistivity of the order of 290 ohms / sq.
  • the films FWNTs and MWNTs respectively have a resistivity of 14.8 and 141.7 kohms / sq, and the SWNTs films have a resistivity of the order of 600 ohms / sq.
  • FIG. 2 illustrates the evolution of the transmittance of the films as a function of their surface resistivity.
  • the resistivity of the films is improved by one to two orders of magnitude when they are made from solutions of CNT salts, compared to the films obtained from the aqueous dispersions of CNTs.
  • metal-walled single-wall NTCs since they make it possible to achieve performances close to those of "current" (100 ohms / sq. At 80% transmittance) and those of ⁇ high performance (20 ohms / sq. at 80% transmittance).

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Abstract

The present invention relates to the field of conductive transparent films. Specifically, the present invention relates to the preparation of conductive transparent films from solutions of carbon-nanotube salts, to the resulting conductive transparent films, and to the various uses thereof. The use of solutions of carbon-nanotube salts obtained by reducing carbon nanotubes enables the preparation of films having better electrical properties (vs. transmittance) than those obtained conventionally using films from aqueous dispersions of carbon nanotubes.

Description

PROCEDE DE PREPARATION DE FILMS TRANSPARENTS  PROCESS FOR PREPARING TRANSPARENT FILMS
CONDUCTEURS A BASE DE NANOTUBES DE CARBONE  CONDUCTORS BASED ON CARBON NANOTUBES
Domaine de l'invention Field of the invention
La présente invention concerne le domaine des films transparents conducteurs. The present invention relates to the field of transparent conductive films.
Plus particulièrement, la présente invention a pour objet la préparation de films transparents conducteurs à partir de solutions de sels de nanotubes de carbone, les films transparents conducteurs ainsi obtenus et leurs différentes utilisations. Les films à base d'oxyde d'indium et d'étain (ci-après désigné ITO) représentent les solutions actuelles commerciales dans le domaine des applications conductrices transparentes, telles que par exemple les électrodes. En effet, la caractéristique principale de l'oxyde d'indium et d'étain est sa combinaison de conductivité électrique et sa transparence optique. Ainsi, les propriétés électriques exprimées en résistivité de surface, sont de l'ordre de 30 ohms/sq pour les meilleures électrodes à base d'ITO, et généralement de l'ordre de 250 ohm/sq pour les électrodes commerciales à base d'ITO. Cependant, une difficulté dans la réalisation de ces électrodes réside dans le compromis qui doit être atteint lors du dépôt de films d'ITO, l'augmentation de la concentration en ITO induisant une augmentation de la conductivité du matériau, mais en même temps une perte de sa transparence. More particularly, the present invention relates to the preparation of conductive transparent films from solutions of salts of carbon nanotubes, the transparent conductive films thus obtained and their various uses. Indium-tin oxide films (hereinafter referred to as ITO) represent current commercial solutions in the field of transparent conductive applications, such as, for example, electrodes. Indeed, the main characteristic of indium and tin oxide is its combination of electrical conductivity and optical transparency. Thus, the electrical properties expressed as surface resistivity are of the order of 30 ohms / sq. For the best ITO-based electrodes, and generally of the order of 250 ohm / sq. For the commercial electrodes based on ITO. However, a difficulty in producing these electrodes lies in the compromise that must be achieved during the deposition of ITO films, the increase in the ITO concentration inducing an increase in the conductivity of the material, but at the same time a loss. of its transparency.
Par ailleurs, les films transparents conducteurs à base d'ITO présentent de nombreux inconvénients liés notamment au coût élevé et à la disponibilité de la matière première oxyde d'indium, au manque de flexibilité des couches minces d'ITO conduisant à des films rigides, à une stabilité chimique limitée, et à une méthode de dépôt difficilement réalisable sur des substrats plastiques en raison de sa température élevée.  In addition, the ITO-based conductive transparent films have numerous drawbacks related in particular to the high cost and the availability of the indium oxide raw material, to the lack of flexibility of the ITO thin layers leading to rigid films, at a limited chemical stability, and a method of deposition difficult to achieve on plastic substrates because of its high temperature.
Il apparaît donc nécessaire de disposer d'alternatives aux films d'ITO permettant de pallier les inconvénients précités. Technique antérieure  It therefore appears necessary to have alternatives to ITO films to overcome the aforementioned drawbacks. Prior art
Dans ce contexte, les nanotubes de carbone s'avèrent des candidats potentiels pour l'obtention de films transparents conducteurs. En effet, les nanotubes de carbone (ci- après désignés par NTC) présentent d'excellentes propriétés électriques et mécaniques, même sous la forme de films minces de quelques nanomètres d'épaisseur. Comme des films robustes de NTC peuvent être obtenus avec des épaisseurs extrêmement petites, les films résultants sont non seulement conducteurs, mais aussi transparents. De plus, les NTC peuvent être appliqués sur des substrats flexibles sans les inconvénients de ΓΙΤΟ, les films transparents conducteurs à base de NTC peuvent couvrir ainsi un large éventail d'applications. In this context, carbon nanotubes are potential candidates for transparent conductive films. Indeed, the carbon nanotubes (here after NTC designated) have excellent electrical and mechanical properties, even in the form of thin films of a few nanometers thick. Since robust NTC films can be obtained with extremely small thicknesses, the resulting films are not only conductive, but also transparent. In addition, NTCs can be applied to flexible substrates without the disadvantages of ΓΙΤΟ, so NTC-based transparent conductive films can cover a wide range of applications.
Il a été mis en évidence dans de précédentes études que l'utilisation de dispersions aqueuses de NTC et de tensio-actifs permettent l'obtention de films transparents conducteurs ayant une transmittance moyenne de 80% pour une résistivité surfacique de 30D/sq. (Transparent, conductive carbon nanotubes films, Wu et al, Science, 2004, 305, 1273). Aguirre et al. montrent qu'il est possible d'utiliser ces films en tant qu'électrode transparente et conductrice dans les diodes organiques électroluminescentes. (Carbon nanotubes sheets as électrodes in organic light-emitting diodes, Appl. Phys. Let., 2006, 88, 183104).  It has been demonstrated in previous studies that the use of aqueous dispersions of CNTs and surfactants makes it possible to obtain conductive transparent films having an average transmittance of 80% for a surface resistivity of 30D / sq. (Transparent, conductive carbon nanotubes films, Wu et al, Science, 2004, 305, 1273). Aguirre et al. show that it is possible to use these films as a transparent and conductive electrode in organic electroluminescent diodes. (Carbon nanotube sheets as electrodes in organic light-emitting diodes, Appl Phys Let, 2006, 88, 183104).
Différents procédés de préparation de films conducteurs transparents à base de NTC sont par ailleurs décrits.  Various methods for preparing transparent conductive films based on CNTs are also described.
Par exemple, dans le document EP 1 993 106, il est proposé de préparer une dispersion de nanotubes de carbone soumis préalablement à un traitement acide, dans un solvant organique en présence d'un liant tel que par exemple une résine, d'appliquer cette dispersion sur un substrat, puis de mettre en œuvre une étape de séchage pour éliminer le solvant et solidifier le liant, conduisant ainsi à un film transparent conducteur. Ce procédé nécessite un traitement chimique préalable des NTC pour améliorer leur dispersibilité dans le solvant, et la mise en œuvre d'un liant qui reste présent dans le film final. Pour une transmittance de 80%>, des résistivités de surface de l'ordre de 100 - 300 ohms/sq sont obtenues avec des NTC mono-parois (SWNT) et de l'ordre de 250 - 500 ohms/sq avec des NTC multi-parois (MWNTs).  For example, in the document EP 1 993 106, it is proposed to prepare a dispersion of carbon nanotubes subjected before an acid treatment, in an organic solvent in the presence of a binder such as for example a resin, to apply this dispersion on a substrate, then to implement a drying step to remove the solvent and solidify the binder, thus leading to a transparent conductive film. This process requires a prior chemical treatment of the CNTs to improve their dispersibility in the solvent, and the implementation of a binder which remains present in the final film. For a transmittance of 80%>, surface resistivities of the order of 100 - 300 ohms / sq are obtained with single-walled NTCs (SWNT) and of the order of 250 - 500 ohms / sq with multi NTCs. -Woods (MWNTs).
Dans le document US 2008/0088219, une électrode transparente est constituée d'un substrat transparent sur lequel est déposé un film mince formé à partir d'une composition comprenant des NTC et un dispersant éventuellement conducteur. La préparation de l'électrode nécessite un post-traitement, acide ou basique, pour « doper » le dispersant contenu dans le film, et améliorer la conductivité de l'électrode. Le document US 2008/0118634 décrit un procédé de préparation d'un film conducteur transparent sur une structure en verre comprenant les étapes suivantes : préparation d'un « slurry » de NTC, application de ce slurry sur le verre, séchage de la couche formée, et chauffage à une température allant de 300°C à 500°C sous atmosphère inerte pour solidifier la couche superficielle comprenant les NTC. Outre l'étape finale réalisée à une température élevée, ce procédé présente l'inconvénient de mettre en œuvre les NTC sous forme de slurry dont la préparation est complexe et utilise un support organique résultant d'un mélange d'un solvant, d'un plastifiant et d'un stabilisant. In US 2008/0088219, a transparent electrode consists of a transparent substrate on which is deposited a thin film formed from a composition comprising CNTs and a possibly conductive dispersant. The preparation of the electrode requires a post-treatment, acidic or basic, to "dope" the dispersant contained in the film, and improve the conductivity of the electrode. Document US 2008/0118634 describes a process for preparing a transparent conductive film on a glass structure comprising the following steps: preparation of a "slurry" of CNTs, application of this slurry on the glass, drying of the formed layer and heating at a temperature of 300 ° C to 500 ° C under an inert atmosphere to solidify the surface layer comprising CNTs. In addition to the final step carried out at a high temperature, this method has the disadvantage of implementing the CNT in the form of slurry whose preparation is complex and uses an organic support resulting from a mixture of a solvent, a plasticizer and a stabilizer.
Dans le document WO 2009/018261, un film transparent conducteur sur un film plastique en polyéthylène téréphtalate (PET) présentant une tranmittance de 86,1% est obtenu à partir d'une dispersion comprenant des NTC mono-parois fonctionnalisés et dont la viscosité est contrôlée par la présence d'un additif de type carbamate préparé préalablement à partir d'un dérivé aminé. Cependant, la résistivité de surface de ce film est très élevée (108 ohms/sq). In WO 2009/018261, a conductive transparent film on a polyethylene terephthalate (PET) plastic film having a transmittance of 86.1% is obtained from a dispersion comprising functionalized mono-wall CNTs and whose viscosity is controlled by the presence of a carbamate additive prepared beforehand from an amino derivative. However, the surface resistivity of this film is very high (10 8 ohms / sq).
Le procédé de préparation d'un film transparent conducteur décrit dans le document WO 2009/048269 est basé sur la pulvérisation à une pression allant de 0,05 à 60 kg/cm2 d'une dispersion aqueuse comprenant des NTC et un agent dispersant, sur un support pouvant être du verre, un film de polymère ou un film de céramique. Des résistivités surfaciques de l'ordre de 700 à 800 ohms/sq sont obtenues pour des films présentant une transmittance de 84-85%. The process for preparing a transparent conductive film described in WO 2009/048269 is based on spraying at a pressure ranging from 0.05 to 60 kg / cm 2 of an aqueous dispersion comprising CNTs and a dispersing agent. on a support which may be glass, a polymer film or a ceramic film. Surface resistivities of the order of 700 to 800 ohms / sq are obtained for films having a transmittance of 84-85%.
Dans le document WO 2009/154830, un film transparent conducteur comportant à la fois des NTC et de ΓΙΤΟ est préparé en plongeant de façon successive et alternée un substrat en PET dans une encre contenant des NTC et dans une encre contenant de l'oxyde d'indium et d'étain, un rinçage à l'eau, un séchage à l'air et le dépôt d'une couche de polymère servant de liant entre les NTC et ΓΙΤΟ étant effectués entre le dépôt de chaque couche. Le film ainsi préparé est caractérisé par une transmittance de 85%o et une résistance surfacique de 1500 ohms/sq.  In WO 2009/154830, a conductive transparent film having both CNTs and ΓΙΤΟ is prepared by sequentially and alternately dipping a PET substrate in an ink containing CNTs and in an ink containing indium and tin, a water rinse, air drying and the deposition of a layer of polymer serving as a binder between the CNT and ΓΙΤΟ being performed between the deposition of each layer. The film thus prepared is characterized by a transmittance of 85% and a surface resistance of 1500 ohms / sq.
Le document FR 2 923 823 décrit un procédé de préparation d' aérogels de nanotubes de carbone conducteurs utilisables pour la préparation de composants électrochimiques. Ce mode opératoire peut impliquer une étape intermédiaire de fïltration des sels de NTC solides avant l'étape d'individualisation en milieu solvant, les étapes suivantes consistant en une congélation de la solution puis sublimation du solvant. L'aérosol peut ensuite être mis sous forme de membranes par compression. The document FR 2 923 823 describes a process for the preparation of conductive carbon nanotube aerogels that can be used for the preparation of electrochemical components. This procedure may involve an intermediate step of filtration of the solid CNT salts before the individualization step in a solvent medium. following steps consisting of a freezing of the solution and sublimation of the solvent. The aerosol can then be formed into membranes by compression.
L'ensemble de ces procédés présente de nombreux inconvénients et sont difficiles à mettre en œuvre étant donnée leur complexité.  All of these methods have many disadvantages and are difficult to implement because of their complexity.
L'élaboration de films transparents conducteurs à l'aide de nanotubes de carbone soulève donc encore de nombreux points négatifs qui nécessitent d'être améliorés.  The development of transparent conductive films using carbon nanotubes therefore still raises many negative points that need to be improved.
La présente invention a donc pour but de fournir un procédé de préparation de films transparents conducteurs à base de nanotubes de carbone, qui soit simple, rapide (comportant le moins d'étapes possibles), et facile à mettre en œuvre.  The present invention therefore aims to provide a process for the preparation of transparent conductive films based on carbon nanotubes, which is simple, fast (with the fewest possible steps), and easy to implement.
La plupart des méthodes actuelles ont en commun la mise en œuvre des nanotubes de carbone sous forme de dispersions, généralement aqueuses. Les NTC sont difficiles à disperser en raison de leur structure enchevêtrée pouvant générer de fortes interactions de Van der Waals entre leurs molécules selon leur mode de fabrication. Sans une bonne dispersion des NTC, il est difficile d'obtenir des films présentant une bonne homogénéité, indispensable pour l'application envisagée.  Most current methods have in common the implementation of carbon nanotubes in the form of dispersions, generally aqueous. NTCs are difficult to disperse because of their entangled structure that can generate strong Van der Waals interactions between their molecules according to their method of manufacture. Without a good dispersion of the CNTs, it is difficult to obtain films having a good homogeneity, which is indispensable for the application envisaged.
Ainsi, les dispersions de NTC dans l'eau sont généralement réalisées en utilisant des tensio-actifs comme dispersants, et des ultrasons. Cependant, le tensio-actif, présentant une conductivité plus faible que les NTC, joue un rôle d'isolant entre les NTC et peut conduire à une diminution de la conductivité du film final. La sonication peut, elle, endommager les NTC en les raccourcissant.  Thus, the dispersions of CNTs in water are generally carried out using surfactants as dispersants, and ultrasound. However, the surfactant, having a lower conductivity than the CNTs, plays an insulating role between the CNTs and can lead to a decrease in the conductivity of the final film. Sonication can damage CNTs by shortening them.
C'est pourquoi, il subsiste le besoin de disposer d'une méthode de mise en œuvre des NTC qui ne présente pas les inconvénients précités, pour réaliser des films présentant si possible une transparence et une conductivité électrique supérieure à celles des films obtenus à partir de dispersions aqueuses de NTC.  This is why there remains the need to have a method of implementation of CNTs that does not have the aforementioned drawbacks, to make films with possible greater transparency and electrical conductivity than films obtained from aqueous dispersions of CNTs.
Or, la demanderesse a découvert que ce besoin pouvait être satisfait en utilisant des solutions organiques de sels de nanotubes de carbone, appelées également solutions polyélectrolytes de NTC. De façon surprenante, l'utilisation de ces solutions permet, pour un même type de nanotubes de carbone et pour une transmission donnée, d'obtenir une conductance surfacique 10 fois plus élevée qu'à partir des dispersions aqueuses classiques de NTC avec tensio-actifs et sonication. Résumé de l'invention However, the Applicant has discovered that this need could be satisfied by using organic solutions of carbon nanotube salts, also called polyelectrolyte solutions of CNTs. Surprisingly, the use of these solutions makes it possible, for the same type of carbon nanotubes and for a given transmission, to obtain a surface conductance 10 times higher than from conventional aqueous dispersions of CNTs with surfactants. and sonication. Summary of the invention
La présente invention a donc pour objet un procédé de préparation d'un film transparent conducteur comprenant au moins les étapes suivantes :  The present invention therefore relates to a method for preparing a transparent conductive film comprising at least the following steps:
a) la préparation d'une solution organique de sels de nanotubes de carbone ; b) la filtration sous vide de ladite solution sur une membrane conduisant à un film sur la membrane filtrante ;  a) preparing an organic solution of carbon nanotube salts; b) vacuum filtration of said solution on a membrane leading to a film on the filter membrane;
c) le transfert du film, de la membrane filtrante sur un substrat transparent.  c) the transfer of the film, the filter membrane on a transparent substrate.
Il est bien entendu que ce procédé peut comprendre d'autres étapes préliminaires, intermédiaires ou subséquentes à celles ci-dessus. It is understood that this process may include other preliminary steps, intermediate or subsequent to those above.
La préparation de la solution de sels de nanotubes de carbone (étape a) est basée sur une méthode appelée « dissolution douce », mise au point par Pénicaud et al. (« Spontaneous dissolution of a single wall carbon nanotube sait » , J. Am. Chem. Soc. 2005, 127, 8-9 ; « Dissolution Douce of single walled carbon nanotubes », publi Kirchberg, AIP conférence proceedings, vol786, 2005, p266-270), qui a fait l'objet de la demande de brevet WO 2005/073127.  The preparation of the solution of salts of carbon nanotubes (step a) is based on a method called "soft dissolution", developed by Penicaud et al. ("Spontaneous dissolution of a single wall carbon nanotube"), J. Am., Chem., Soc., 2005, 127, 8-9, "Dissolution Douce of single walled carbon nanotubes", published by Kirchberg, AIP conference proceedings, vol786, 2005, p266-270), which was the subject of the patent application WO 2005/073127.
Cette méthode permet de disperser les nanotubes dans les solvants organiques sans fonctionnalisation ni traitement fortement acide préalable, et sans utiliser de tensio- actifs ni d'ultrasons.  This method makes it possible to disperse the nanotubes in organic solvents without functionalization or prior strongly acid treatment, and without using surfactants or ultrasound.
Dans cette méthode, un sel de nanotube est formé par exposition des nanotubes à une solution dans le tétrahydrofurane (THF) de naphtalénure de potassium. Ensuite, il est possible de récupérer à l'état solide par filtration les sels de NTC ainsi obtenus afin de les conserver plusieurs mois à température ambiante. A noter qu'à ce stade, les nanotubes n'ont pas encore été dissous et la filtration permet de récupérer le sel sous forme d'agrégats. L'exposition ultérieure de ces sels à un solvant organique polaire tel que le DMSO conduit à une solution de nanotubes individualisés.  In this method, a nanotube salt is formed by exposing the nanotubes to a solution of tetrahydrofuran (THF) of potassium naphthalenide. Then, it is possible to recover in the solid state by filtration the salts of CNT thus obtained in order to keep them several months at room temperature. It should be noted that at this stage, the nanotubes have not yet been dissolved and the filtration makes it possible to recover the salt in the form of aggregates. Subsequent exposure of these salts to a polar organic solvent such as DMSO leads to a solution of individualized nanotubes.
Les NTC ainsi solubilisés sous forme de sels après réduction dans un solvant organique conservent toutes leurs caractéristiques et leur intégrité. Leur taille ou leur surface n'est pas endommagée par des traitements chimiques ni par l'apport d'énergie mécanique. Des mesures réalisées par diffusion de la lumière sur les solutions ainsi obtenues ont montré que la longueur moyenne des NTC au sein de la solution organique est environ 3 fois plus grande que celle des NTC dispersés en phase aqueuse. En particulier, la longueur des NTC varie entre Ιμηι et 2μηι en solution organique, alors que leur longueur varie entre 300 nm et 700 nm quand ils sont dispersés en phase aqueuse. The CNTs thus solubilized in the form of salts after reduction in an organic solvent retain all their characteristics and their integrity. Their size or surface is not damaged by chemical treatments or the supply of mechanical energy. Measurements made by light scattering on the solutions thus obtained showed that the average length of the CNTs in the organic solution is approximately 3 times greater than that of the NTCs dispersed in the aqueous phase. In in particular, the length of the CNT varies between Ιμηι and 2μηι in organic solution, while their length varies between 300 nm and 700 nm when they are dispersed in aqueous phase.
La filmification à partir de la solution de sels de nanotubes de carbone préparée à l'étape a) du procédé selon l'invention, est basée sur les travaux déjà réalisés sur les dispersions aqueuses de nanotubes de carbone (Transparent, conductive carbon nanotubes films, Wu et al, Science, 2004, 305, 1273), et sur les travaux réalisés sur les films de graphène (Electrical Conductivity of Graphene Films with a Poly(ally lamine hydrochloride) Supporting Layer, Kong et al, Langmuir 2009, 25(18), 11008-11013).  The filming from the solution of carbon nanotube salts prepared in step a) of the process according to the invention is based on the work already carried out on aqueous dispersions of carbon nanotubes (Transparent, conductive carbon nanotubes films, Wu et al, Science, 2004, 305, 1273), and on the work done on graphene films (Electrical Conductivity of Graphene Films with Poly (allylamine hydrochloride) Supporting Layer, Kong et al, Langmuir 2009, 25 (18). ), 11008-11013).
Les films de sels de NTC sont obtenus dans le procédé de l'invention, en mettant en œuvre les deux étapes b) et c).  The CNT salt films are obtained in the process of the invention, by implementing both steps b) and c).
Selon l'étape b) du procédé selon l'invention, un premier film est obtenu par filtration sous vide de la solution de sels de NTC sur une membrane filtrante résistante au solvant de la solution.  According to step b) of the process according to the invention, a first film is obtained by vacuum filtration of the solution of NTC salts on a solvent-resistant filter membrane of the solution.
Selon l'étape c) du procédé selon l'invention, le film déposé sur la membrane filtrante est transféré sur un substrat transparent. Ce transfert de support pour le film de sels de NTC est réalisé par dissolution de la membrane filtrante dans un milieu adéquat selon la nature de la membrane, puis introduction d'un substrat transparent dans ce milieu afin de déposer le film sur ce substrat.  According to step c) of the process according to the invention, the film deposited on the filtering membrane is transferred onto a transparent substrate. This support transfer for the CNT salt film is carried out by dissolving the filter membrane in a suitable medium according to the nature of the membrane, then introducing a transparent substrate into this medium in order to deposit the film on this substrate.
Le film transparent conducteur ainsi obtenu est ensuite séparé du milieu dans lequel il s'est formé, et peut être soumis éventuellement à un rinçage après séchage.  The transparent conductive film thus obtained is then separated from the medium in which it was formed, and may be optionally rinsed after drying.
Avantageusement, le film transparent conducteur comporte des nanotubes de carbone de longueur moyenne supérieure à 1 μιη, et pouvant aller jusque quelques centaines de μιη, de préférence de longueur moyenne allant de Ιμιη à 10 μιη, sur un substrat transparent. Advantageously, the conductive transparent film comprises carbon nanotubes of average length greater than 1 μιη, and up to a few hundred μιη, preferably of average length ranging from Ιμιη to 10 μιη, on a transparent substrate.
Le film transparent conducteur selon l'invention présente avantageusement une transmittance supérieure à 20 % jusqu'à 99%, de préférence comprise entre 40 % et 99 %. La résistivité surfacique du film transparent conducteur selon l'invention est inférieure à 2 105 ohms/sq et peut aller jusque 10 ohms/sq, de préférence elle est comprise entre 2 104 ohms/sq et 10 ohms/sq. L'invention a aussi pour objet les différentes utilisations dudit film transparent conducteur, notamment pour fabriquer des électrodes transparentes dans les domaines des diodes électroluminescentes, les capteurs d'image, les cellules solaires, les écrans tactiles, les écrans à cristaux liquides. Le procédé selon la présente invention sera à présent décrit plus en détails. The transparent conductive film according to the invention advantageously has a transmittance greater than 20% up to 99%, preferably between 40% and 99%. The surface resistivity of the transparent conductive film of the invention is less than 2 10 5 ohms / sq and can go up to 10 ohms / sq, preferably it is comprised between 2 10 4 ohms / sq and 10 ohm / sq. The invention also relates to the various uses of said transparent conductive film, in particular for producing transparent electrodes in the fields of light-emitting diodes, image sensors, solar cells, touch screens, liquid crystal screens. The process according to the present invention will now be described in more detail.
Description détaillée detailed description
Les nanotubes de carbone (NTC) possèdent des structures particulières, de forme tubulaire, creuses et closes, composées d'atomes disposés régulièrement en pentagones, hexagones et/ou heptagones, obtenues à partir de carbone. Les NTC sont en général constitués d'un ou plusieurs feuillets de graphène enroulés. On distingue ainsi les nanotubes mono-parois (Single Wall Nanotubes ou SWNT) et les nanotubes multi- parois (Multi Wall Nanotubes ou MWNTs). Les nanotubes à double paroi peuvent notamment être préparés comme décrit par FLAHAUT et al dans Chem. Com. (2003), 1442. Les nanotubes à parois multiples peuvent de leur côté être préparés comme décrit dans le document WO 03/02456.  Carbon nanotubes (CNTs) have particular, tubular, hollow and closed structures, composed of atoms regularly arranged in pentagons, hexagons and / or heptagons, obtained from carbon. CNTs generally consist of one or more sheets of graphene rolled up. Thus, single-wall nanotubes (Single Wall Nanotubes or SWNTs) and multiwall nanotubes (Multi Wall Nanotubes or MWNTs) are distinguished. The double-walled nanotubes can in particular be prepared as described by FLAHAUT et al in Chem. Com. (2003), 1442. The multi-walled nanotubes may themselves be prepared as described in WO 03/02456.
Les nanotubes de carbone mis en œuvre dans le procédé selon l'invention ont habituellement un diamètre moyen allant de 0,1 à 200 nm, de préférence de 0,1 à 100 nm, plus préférentiellement de 0,4 à 50 nm et, mieux, de 5 à 30 nm et avantageusement une longueur de plus de 0,1 μιη et avantageusement de 0,1 à 20 μιη, par exemple d'environ 6 μιη. Leur rapport longueur/diamètre est avantageusement supérieur à 10 et le plus souvent supérieur à 100. Ces nanotubes de carbone comprennent donc notamment les nanotubes dits "VGCF" (fibres de carbone obtenues par dépôt chimique en phase vapeur, ou Vapor Grown Carbon Fibers). Leur surface spécifique est par exemple comprise entre 100 et 300 m2/g, de préférence entre 200 et 250 m2/g, et leur densité apparente peut notamment être comprise entre 0,01 et 0,5 g/cm3 et plus préférentiellement entre 0,07 et 0,2 g/cm3. Les nanotubes de carbone peuvent être mono-parois (SWNT), ou peuvent comprendre quelques parois (FWNT) ou encore ils peuvent être multi-parois pouvant par exemple comprendre de 5 à 15 feuillets et plus préférentiellement de 7 à 10 feuillets. Un exemple de nanotubes de carbone multi-parois bruts est le gradeThe carbon nanotubes used in the process according to the invention usually have a mean diameter ranging from 0.1 to 200 nm, preferably from 0.1 to 100 nm, more preferably from 0.4 to 50 nm, and better still , from 5 to 30 nm and advantageously a length of more than 0.1 μιη and advantageously from 0.1 to 20 μιη, for example about 6 μιη. Their length / diameter ratio is advantageously greater than 10 and most often greater than 100. These carbon nanotubes therefore comprise in particular nanotubes known as "VGCF" (carbon fibers obtained by chemical vapor deposition or Vapor Grown Carbon Fibers). Their specific surface area is for example between 100 and 300 m 2 / g, preferably between 200 and 250 m 2 / g, and their apparent density may especially be between 0.01 and 0.5 g / cm 3 and more preferably between 0.07 and 0.2 g / cm 3 . The carbon nanotubes may be single-walled (SWNT), or may comprise some walls (FWNT) or they may be multi-walled, for example comprising from 5 to 15 sheets and more preferably from 7 to 10 sheets. An example of raw multi-walled carbon nanotubes is grade
Graphistrength® Cl 00, fabriqué par la société ARKEMA. Graphistrength ® Cl 00, manufactured by the company ARKEMA.
Avantageusement, les nanotubes de carbone mis en œuvre dans le procédé selon l'invention sont des nanotubes de carbone mono-parois présentant un caractère métallique, c'est-à-dire qu'ils contiennent une forte teneur, au moins 70% et de préférence au moins 90%, de tubes de caractère métallique. On entend par caractère métallique, des nanotubes dont le diagramme de densité d'état présente des états électroniques accessibles sans activation thermique, au contraire des nanotubes semiconducteurs qui présentent une barrière d'énergie entre les derniers états occupés (bande de valence) et les premiers états vacants (bande de conduction). Ce sont notamment des tubes dont les indices (n,m) sont tels que n-m est un multiple de 3 (les indices n et m décrivent l'enroulement des tubes et sont expliqués dans la littérature sur le sujet, par exemple par Saito, Dresselhaus, & Dresselhaus dans Physical properties of carbon nanotubes, Impérial Collège Press 2001). De tels nanotubes de carbone SWNT peuvent être fabriqués notamment selon le procédé CoMoCat®, et sont disponibles par exemple sous la dénomination CoMoCat® CG200 de la société South West Technologies. Advantageously, the carbon nanotubes used in the process according to the invention are single-walled carbon nanotubes having a metallic character, that is to say they contain a high content, at least 70% and preferably at least 90% of metal tubes. Metallic character means nanotubes whose state density diagram has accessible electronic states without thermal activation, unlike semiconductor nanotubes which have an energy barrier between the last occupied states (valence band) and the first ones. vacant states (conduction band). These are especially tubes whose indices (n, m) are such that nm is a multiple of 3 (the indices n and m describe the winding of the tubes and are explained in the literature on the subject, for example by Saito, Dresselhaus , & Dresselhaus in Physical Properties of Carbon Nanotubes, Imperial College Press 2001). Such SWNT carbon nanotubes can be made especially according to the CoMoCat ® process, and are available for example under the name CoMoCat ® CG200 the South West Technologies.
Les nanotubes de carbone peuvent être purifiés et/ou traités (en particulier oxydés ou fonctionnalisés) et/ou broyés, avant leur mise en œuvre dans le procédé selon l'invention.  The carbon nanotubes may be purified and / or treated (in particular oxidized or functionalized) and / or milled before being used in the process according to the invention.
Le broyage des nanotubes de carbone peut être notamment effectué à froid ou à chaud et être réalisé selon les techniques connues mises en oeuvre dans des appareils tels que broyeurs à boulets, à marteaux, à meules, à couteaux, jet de gaz ou tout autre système de broyage susceptible de réduire la taille du réseau enchevêtré de nanotubes. On préfère que cette étape de broyage soit pratiquée selon une technique de broyage par jet de gaz et en particulier dans un broyeur à jet d'air.  The grinding of the carbon nanotubes may in particular be carried out cold or hot and be carried out according to known techniques used in devices such as ball mills, hammers, grinders, knives, jet gas or any other system grinding capable of reducing the size of the entangled network of nanotubes. It is preferred that this grinding step is performed according to a gas jet grinding technique and in particular in an air jet mill.
La purification des nanotubes peut être réalisée par lavage à l'aide d'une solution d'acide sulfurique, ou d'un autre acide, de manière à les débarrasser d'éventuelles impuretés minérales et métalliques résiduelles, provenant de leur procédé de préparation. Le rapport pondéral des nanotubes à l'acide sulfurique peut notamment être compris entre 1 :2 et 1 :3. L'opération de purification peut par ailleurs être effectuée à une température allant de 90 à 120°C, par exemple pendant une durée de 5 à 10 heures. Cette opération peut avantageusement être suivie d'étapes de rinçage à l'eau et de séchage des nanotubes purifiés. Une autre voie de purification des nanotubes, destinée en particulier à éliminer le fer et/ou le magnésium, ou tout autre métal qu'ils renferment, consiste à les soumettre à un traitement thermique à plus de 1.000°C. The purification of the nanotubes can be carried out by washing with a sulfuric acid solution, or another acid, so as to rid them any residual mineral and metallic impurities arising from their preparation process. The weight ratio of the nanotubes to the sulfuric acid may especially be between 1: 2 and 1: 3. The purification operation may also be carried out at a temperature ranging from 90 to 120 ° C, for example for a period of 5 to 10 hours. This operation may advantageously be followed by rinsing steps with water and drying the purified nanotubes. Another way of purifying nanotubes, intended in particular to remove iron and / or magnesium, or any other metal they contain, is to subject them to a heat treatment at more than 1,000 ° C.
L'oxydation des nanotubes est avantageusement réalisée en mettant ceux-ci en contact avec une solution d'hypochlorite de sodium renfermant de 0,5 à 15% en poids de NaOCl et de préférence de 1 à 10% en poids de NaOCl, par exemple dans un rapport pondéral des nanotubes à l'hypochlorite de sodium allant de 1 :0,1 à 1 : 1. L'oxydation est avantageusement réalisée à une température inférieure à 60°C et de préférence à température ambiante, pendant une durée allant de quelques minutes à 24 heures. Cette opération d'oxydation peut avantageusement être suivie d'étapes de fïltration et/ou centrifugation, lavage et séchage des nanotubes oxydés.  The oxidation of the nanotubes is advantageously carried out by putting them in contact with a solution of sodium hypochlorite containing from 0.5 to 15% by weight of NaOCl and preferably from 1 to 10% by weight of NaOCl, for example in a weight ratio of nanotubes to sodium hypochlorite ranging from 1: 0.1 to 1: 1. The oxidation is advantageously carried out at a temperature below 60 ° C. and preferably at room temperature, for a period ranging from a few minutes to 24 hours. This oxidation operation may advantageously be followed by filtration and / or centrifugation, washing and drying steps of the oxidized nanotubes.
On préfère toutefois que les nanotubes de carbone soient utilisés dans le procédé selon l'invention à l'état brut, ou simplement débarrassés des impuretés résiduelles provenant de leur procédé de préparation.  However, it is preferred that the carbon nanotubes be used in the process according to the invention in the raw state, or simply free of residual impurities from their preparation process.
Par ailleurs, on préfère selon l'invention utiliser des nanotubes de carbone obtenus à partir de matières premières d'origine renouvelable, en particulier d'origine végétale, comme décrit dans le document FR 2 914 634.  Furthermore, it is preferred according to the invention to use carbon nanotubes obtained from raw materials of renewable origin, in particular of plant origin, as described in document FR 2 914 634.
Selon un mode préféré de l'invention, on utilise des nanotubes de carbone monoparois présentant un caractère métallique.  According to a preferred embodiment of the invention, single-walled carbon nanotubes having a metallic character are used.
Ainsi, l'invention porte également sur un film transparent conducteur comportant des nanotubes de carbone mono-parois à caractère métallique de longueur moyenne supérieure à 1 μιη, et pouvant aller jusque quelques centaines de μιη, de préférence de longueur moyenne allant de Ιμιη à 10 μιη, sur un substrat transparent. L'étape a) du procédé selon l'invention, consiste à préparer une solution dans un solvant organique d'un sel de nanotubes de carbone. Pour cela, on prépare dans un premier temps un sel de nanotubes de carbone par réduction des nanotubes, ce qui conduit à des nanotubes chargés négativement avec des contre-ions positifs. Thus, the invention also relates to a conductive transparent film comprising single-walled carbon nanotubes with a metallic character of average length greater than 1 μιη, and which can be up to a few hundred μιη, preferably of average length ranging from Ιμιη to 10. μιη, on a transparent substrate. Step a) of the process according to the invention consists in preparing a solution in an organic solvent of a salt of carbon nanotubes. For this purpose, a salt of carbon nanotubes is first prepared by reduction of the nanotubes, which leads to negatively charged nanotubes with positive counter ions.
Selon un premier mode de réalisation, la réduction des NTC est réalisée par addition, dans des conditions anaérobies, d'un sel de formule : A+ B" According to a first embodiment, the reduction of the CNTs is carried out by addition, under anaerobic conditions, of a salt of formula: A + B "
dans laquelle  in which
A+ représente un sel d'un ion alcalin, tel que par exemple le lithium, le sodium ou le potassium ; A + represents a salt of an alkaline ion, such as, for example, lithium, sodium or potassium;
B" représente un anion d'un composé polyaromatique ; de manière à charger électriquement les nanotubes de carbone, Γ anion du composé polyaromatique étant un réducteur pour ces NTC. B " represents an anion of a polyaromatic compound, so as to electrically charge the carbon nanotubes, Γ anion of the polyaromatic compound being a reducing agent for these CNTs.
De manière avantageuse, le composé polyaromatique peut être choisi parmi le naphtalène, la benzophénone, la fluorénone et l'anthraquinone. Le naphtalène est préféré.  Advantageously, the polyaromatic compound may be chosen from naphthalene, benzophenone, fluorenone and anthraquinone. Naphthalene is preferred.
Le sel A+ B peut être obtenu par réaction d'un composé polyaromatique B avec un métal alcalin A en suspension dans un solvant organique tel que le THF. Par exemple, le sel Li+ Naph" est préparé par réaction de naphtalène avec un excès de lithium dans du THF jusqu'à développement d'une couleur vert sombre, proche du noir. On peut procéder de la même manière en utilisant du potassium au lieu de lithium. The salt A + B can be obtained by reaction of a polyaromatic compound B with an alkali metal A in suspension in an organic solvent such as THF. For example, the Li + salt Naph "is prepared by reacting naphthalene with an excess of lithium in THF to development of a dark green color close to black. One can proceed in the same manner using the potassium place of lithium.
Le sel de nanotubes de carbone est obtenu en mélangeant la poudre de NTC et la solution organique du sel A+ B", généralement sous agitation pendant quelques heures, à température ambiante. On opère sous atmosphère contrôlée, par exemple sous argon. The salt of carbon nanotubes is obtained by mixing the CNT powder and the organic solution of the salt A + B " , generally with stirring for a few hours, at ambient temperature, which is carried out under a controlled atmosphere, for example under argon.
Le sel de nanotube de carbone est séparé du milieu par fïltration, et peut être soumis à plusieurs rinçages à l'aide du solvant organique utilisé pour la préparation du sel A+ B". Puis le solide est séché, généralement sous vide à température ambiante. The carbon nanotube salt is separated from the medium by filtration, and may be subjected to several rinses with the aid of the organic solvent used for the preparation of the salt A + B " . The solid is then dried, generally under vacuum at room temperature. .
Ce solide, une poudre de NTC chargés négativement, présente une bonne stabilité au stockage d'au moins plusieurs mois sous atmosphère contrôlée.  This solid, a negatively charged NTC powder, has a good storage stability of at least several months in a controlled atmosphere.
Selon un second mode de réalisation, le sel de nanotubes de carbone est préparé directement par mélange d'un métal alcalin A et de NTC (mélange binaire), ledit mélange étant porté à haute température, typiquement à une température pouvant aller de 150°C à 400°C, pendant une durée de l'ordre de quelques minutes à quelques jours, les vapeurs de métal conduisant à une forme salifiée des NTC. Dans un second temps, le sel de nanotubes de carbone est mis en solution dans un solvant organique. According to a second embodiment, the carbon nanotube salt is prepared directly by mixing an alkali metal A and NTC (binary mixture), said mixture being heated to a high temperature, typically at a temperature that may range from 150 ° C. at 400 ° C, for a period of the order of a few minutes to a few days, the metal vapors leading to a salified form of CNTs. In a second step, the carbon nanotube salt is dissolved in an organic solvent.
Pour cela, on introduit le solide obtenu précédemment dans un solvant organique, conduisant à une phase organique comprenant le sel de NTC dissous, et une phase solide non dissoute comprenant généralement les impuretés et aggrégats de nanotubes et autres espèces non dissoutes. On sépare alors la solution de sel de NTC, par exemple, par centrifugation du résidu solide.  For this, the previously obtained solid is introduced into an organic solvent, resulting in an organic phase comprising the dissolved NTC salt, and an undissolved solid phase generally comprising the impurities and aggregates of nanotubes and other undissolved species. The CNT salt solution is then separated, for example, by centrifugation of the solid residue.
Comme solvants utilisables, on peut citer les solvants organiques polaires tels que le sulfolane, le diméthylsulfoxyde (DMSO), le diméthylformamide, la N- méthylpyrrolidone, ou le N-méthyl formamide. De préférence, on utilise le DMSO.  Suitable solvents include polar organic solvents such as sulfolane, dimethylsulfoxide (DMSO), dimethylformamide, N-methylpyrrolidone, or N-methylformamide. Preferably, DMSO is used.
Cette mise en solution a l'avantage de n'induire aucune dénaturation des nanotubes de carbone.  This dissolution has the advantage of inducing no denaturation of carbon nanotubes.
La concentration de la solution organique de sels de NTC préparée lors de l'étape a) est comprise généralement entre 0,001 et 10 g/ 1, de préférence entre 0,01 et 5g/L  The concentration of the organic solution of CNT salts prepared during stage a) is generally between 0.001 and 10 g / l, preferably between 0.01 and 5 g / l.
Avantageusement, la solution organique de sels de NTC peut être diluée avant d'être soumise à l'étape suivante b) de fîltration, pour optimiser cette étape en termes de faisabilité. On visera alors une concentration en sels de NTC allant de 0,0001 à 0,01 g/1.  Advantageously, the organic solution of CNT salts can be diluted before being subjected to the next step b) of filtration, to optimize this step in terms of feasibility. A concentration of NTC salts ranging from 0.0001 to 0.01 g / l will then be targeted.
L'étape b) du procédé selon l'invention consiste à filtrer sous vide la solution organique de sels de nanotubes de carbone préparée à l'étape a), sur une membrane filtrante. Step b) of the process according to the invention consists in filtering under vacuum the organic solution of salts of carbon nanotubes prepared in step a), on a filtering membrane.
La membrane de fîltration doit résister au solvant et permettre la formation d'un film sous forme d'un réseau de NTC aléatoire et homogène en épaisseur. Elle doit de plus pouvoir être éliminée sans endommager le réseau de nanotubes ainsi formé.  The filtration membrane must resist the solvent and allow the formation of a film in the form of a network of random NTC and homogeneous in thickness. It must also be eliminated without damaging the network of nanotubes thus formed.
Parmi les membranes pouvant convenir, on peut citer les membranes en alumine, Nylon ou Téflon qui permettent d'obtenir un film bien lisse sans inhomogénéité apparente. De préférence, on utilise une membrane en alumine.  Among the membranes that may be suitable, mention may be made of alumina, nylon or Teflon membranes which make it possible to obtain a very smooth film without apparent inhomogeneity. Preferably, an alumina membrane is used.
L'épaisseur du film sur la membrane est contrôlée par la quantité de solution filtrée. Avantageusement, l'épaisseur du film est comprise entre 0,7 nm et 1000 nm, de préférence entre 1 nm et 500 nm, plus particulièrement entre 1 nm et 200 nm. La teneur en NTC dans le film peut aller de 0,2 à 50 μ§/α 2, de préférence elle va de 1 à 10 μ§/α 2, représentant un bon compromis entre les propriétés visées. The thickness of the film on the membrane is controlled by the amount of filtered solution. Advantageously, the thickness of the film is between 0.7 nm and 1000 nm, preferably between 1 nm and 500 nm, more particularly between 1 nm and 200 nm. The content of CNT in the film can range from 0.2 to 50 μs / α 2 , preferably it ranges from 1 to 10 μs / α 2 , representing a good compromise between the properties concerned.
La filtration de la solution peut être réalisée sous atmosphère inerte, par exemple en boite à gants, sous atmosphère d'argon. Dans ce cas, le film sur membrane est ensuite ré-oxydé (« neutralisé ») sous air sec pendant une durée de une à quelques heures.  Filtration of the solution can be carried out under an inert atmosphere, for example in a glove box, under an argon atmosphere. In this case, the membrane film is then re-oxidized ("neutralized") in dry air for a period of one to a few hours.
Dans un autre mode de réalisation de l'invention, la solution de sels de NTC est exposée à l'air préalablement à l'étape de filtration réalisée alors à l'air libre, afin de restaurer l'état neutre des NTC.  In another embodiment of the invention, the CNT salt solution is exposed to air prior to the filtration step then carried out in the open air, in order to restore the neutral state of the CNTs.
Selon l'étape c) du procédé selon l'invention le film déposé sur la membrane filtrante est transféré sur un substrat transparent. According to step c) of the process according to the invention, the film deposited on the filter membrane is transferred onto a transparent substrate.
On procède d'abord à la dissolution de la membrane filtrante dans un milieu choisi en fonction de la nature de la membrane à dissoudre. Par exemple, dans le cas d'une membrane en alumine, on choisira un milieu fortement alcalin. En particulier, on introduit le film supporté sur la membrane dans un bain contenant une solution de soude à 1,5 M, pendant une durée pouvant aller de quelques minutes à une heure.  The membrane is first dissolved in a medium chosen according to the nature of the membrane to be dissolved. For example, in the case of an alumina membrane, a strongly alkaline medium will be selected. In particular, the film supported on the membrane is introduced into a bath containing a 1.5 M sodium hydroxide solution for a period of time ranging from a few minutes to one hour.
Après dissolution complète de la membrane, un substrat transparent est introduit dans le milieu sous le film de NTC surnageant. Le milieu peut être soumis au préalable à une neutralisation, dans le cas de la mise en œuvre d'un milieu basique ou d'un mileu acide. Le film de NTC est maintenu sur le substrat par capillarité, en éliminant le milieu liquide. L'ensemble est ensuite séché pour éliminer toute couche résiduelle de solvant entre le substrat et le film, conduisant ainsi à une bonne adhésion du film sur le support. After complete dissolution of the membrane, a transparent substrate is introduced into the medium under the supernatant NTC film. The medium may be subject to prior neutralization, in the case of the implementation of a basic medium or an acid medium. The CNT film is held on the substrate by capillarity, eliminating the liquid medium. The assembly is then dried to remove any residual solvent layer between the substrate and the film, thus leading to good adhesion of the film on the support.
Puis il peut être rincé sans risque de détérioration mécanique. Then it can be rinsed without risk of mechanical deterioration.
La température de séchage dépend de la nature du substrat et elle est choisie pour ne pas le détériorer. Généralement, la température peut aller de 40°C à 500°C, de préférence de 40°C à 100°C.  The drying temperature depends on the nature of the substrate and it is chosen not to deteriorate it. Generally, the temperature may range from 40 ° C to 500 ° C, preferably from 40 ° C to 100 ° C.
Le substrat transparent selon l'invention peut être un substrat inorganique tel que le verre, le quartz, le mica ou une céramique, ou un substrat plastique rigide ou flexible. Un substrat flexible peut être choisi par exemple parmi les matériaux suivants : polyéthylène téréphtalate, polyéthylène naphtalate, polyéthylène sulphone, polycarbonate, polystyrène, polypropylène, polyester, polyimide, polyéher éther cétone, polyétherimide, résines acryliques, copolymères oléfîne-maléimide, résines à base de norbornène, sans que cette liste soit limitative. The transparent substrate according to the invention may be an inorganic substrate such as glass, quartz, mica or a ceramic, or a rigid or flexible plastic substrate. A flexible substrate may be chosen for example from the following materials: polyethylene terephthalate, polyethylene naphthalate, polyethylene sulphone, polycarbonate, polystyrene, polypropylene, polyester, polyimide, polyether ether ketone, polyetherimide, acrylic resins, olefin-maleimide copolymers, norbornene-based resins, without this list being limiting.
Le substrat transparent peut être composé de plusieurs couches de matériaux différents.  The transparent substrate can be composed of several layers of different materials.
Le substrat présente avantageusement une épaisseur pouvant aller de 0,1 micron à 10 mm qui sera adaptée à l'application du film envisagée.  The substrate advantageously has a thickness ranging from 0.1 micron to 10 mm which will be adapted to the application of the film envisaged.
Le film transparent conducteur obtenu selon le procédé de l'invention possède des propriétés électriques (vs transmittance) meilleures que celles obtenues classiquement avec des films issus de dispersions aqueuses de nanotubes de carbone. Selon l'invention, il est possible d'atteindre des performances équivalentes à celles de ΓΙΤΟ classique. De plus, le seuil de percolation du film selon l'invention, c'est-à-dire le niveau de charge de NTC à partir duquel une mesure de la conductivité du film est possible, autrement dit à partir duquel un chemin conducteur se forme d'un bout à l'autre du film, est plus bas que pour les films obtenus à partir de dispersions aqueuses de NTC. Le seuil de percolation peut être déterminé à partir de l'évolution de la conductance surfacique d'une part, et l'évolution de la transmittance d'autre part, en fonction de la quantité de NTC présente dans le film. Le film transparent conducteur obtenu selon le procédé de l'invention peut être utilisé dans toutes les applications qui nécessitent à la fois une bonne transmission de lumière et des propriétés conductrices, notamment dans les domaines des diodes électroluminescentes, les capteurs d'image, les cellules solaires, les écrans tactiles, les écrans à cristaux liquides. The transparent conductive film obtained according to the process of the invention has better electrical properties (vs transmittance) than those conventionally obtained with films derived from aqueous dispersions of carbon nanotubes. According to the invention, it is possible to achieve performance equivalent to those of conventional.. In addition, the percolation threshold of the film according to the invention, that is to say the level of CNT charge from which a measurement of the conductivity of the film is possible, in other words from which a conductive path is formed from one end to the other of the film, is lower than for the films obtained from aqueous dispersions of CNTs. The percolation threshold can be determined from the evolution of the surface conductance on the one hand, and the evolution of the transmittance on the other hand, as a function of the amount of CNT present in the film. The transparent conductive film obtained according to the method of the invention can be used in all applications that require both good light transmission and conductive properties, especially in the fields of light-emitting diodes, image sensors, cells solar panels, touch screens, LCD screens.
Ainsi, l'invention porte également sur l'utilisation d'un film transparent conducteur comportant des nanotubes de carbone mono-parois à caractère métallique de longueur moyenne supérieure à 1 μιη, et pouvant aller jusque quelques centaines de μιη, sur un substrat transparent, dans les domaines des diodes électroluminescentes, les capteurs d'image, les cellules solaires, les écrans tactiles, les écrans à cristaux liquides.  Thus, the invention also relates to the use of a transparent conductive film comprising single-walled carbon nanotubes with a metallic character of average length greater than 1 μιη, and being able to go up to a few hundred μιη, on a transparent substrate, in the fields of light-emitting diodes, image sensors, solar cells, touch screens, liquid crystal displays.
D'autres particularités et avantages du procédé selon l'invention apparaîtront à la lecture de la partie expérimentale ci-après, donnée à simple titre d'illustration. PARTIE EXPERIMENTALE Other features and advantages of the method according to the invention will appear on reading the experimental part below, given for illustrative purposes only. EXPERIMENTAL PART
Produits utilisés  Products used
- NTC multi-parois Graphistrength® C100, lot NME 0004 de NTC purifiés à l'acide, de la société Arkema, désignés dans les exemples par MWNTs. - multiwall CNTs Graphistrength ® C100, lot 0004 NME NTC purified acid, from Arkema, designated in the examples by MWNTs.
- NTC Elicarb® lot 3772, fourni par Thomas Swan, désignés dans les exemples par FWNTs. - NTC Elicarb ® lot 3772, supplied by Thomas Swan, designated in the examples by FWNTs.
- NTC CoMoCat® CG200 lot 000-0004 fourni par SouthWest NanoTechnologies (SWeNT), désignés par SWNTs. - NTC CoMoCat ® CG200 lot 000-0004 supplied by SouthWest NanoTechnologies (SWeNT), designated by SWNTs.
Préparation des solutions de sels de nanotubes de carbone Preparation of carbon nanotube salt solutions
La solubilisât ion des NTC sous forme de sels de NTC est réalisée en voie solvant et sous atmosphère inerte (boîte à gants sous atmosphère d'argon sec), comme décrit dans le document WO 2005/73127. The solubilization of CNTs in the form of CNT salts is carried out in a solvent and under an inert atmosphere (glove box under dry argon atmosphere), as described in document WO 2005/73127.
Les sels de nanotubes de carbone sont synthétisés en mélangeant la poudre de NTC et une solution organique de tétrahydrofurane (THF) contenant un sel de naphtalène, Naph"K+ . Ce sel de naphtalène a été préparé par réaction du naphtalène avec un excès de potassium dans le THF. Carbon nanotubes salts are synthesized by mixing CNT powder and an organic solution of tetrahydrofuran (THF) containing a naphthalene salt, Naph "K +. The naphthalene salt was prepared by reacting naphthalene with an excess of potassium in THF.
Cette solution est versée sur une poudre de NTC, et laissée sous agitation à température ambiante pendant quatre heures. Puis cette solution est filtrée sur membrane de type Millipore (0,45 microns de porosité). Le solide est rincé plusieurs fois avec du THF jusqu'à l'obtention de THF incolore après passage à travers le filtre. On procède ensuite au séchage sous vide à température ambiante. Le solide obtenu est une poudre de NTC chargés négativement.  This solution is poured onto a CNT powder and left stirring at room temperature for four hours. Then this solution is filtered on a Millipore type membrane (0.45 microns porosity). The solid is rinsed several times with THF until colorless THF is obtained after passing through the filter. The drying is then carried out under vacuum at room temperature. The solid obtained is a negatively charged NTC powder.
Ce solide se solubilise spontanément dans le diméthylsulfoxyde (DMSO) sous agitation pendant quelques heures à température ambiante. Une centrifugation à 4000 t/min pendant 1 heure est nécessaire afin d'éliminer les impuretés carbonées et autres non solubilisées. This solid solubilized spontaneously in dimethylsulfoxide (DMSO) with stirring for a few hours at room temperature. Centrifugation at 4000 rpm for 1 hour is necessary to remove carbon and other non-solubilized impurities.
Trois solutions de sels de nanotubes de carbone dans le DMSO ont ainsi été préparées, respectivement de concentration 2,1 mg/ml en sel de FWNTs, 1,7 mg/ml en sel de MWNTs, et 0,032 mg/ml en sel de SWNTs. La longueur moyenne des NTC en solution a été déterminée par diffusion de la lumière (Mesures de diffusion dynamique de la lumière, de 30 à 130 degrés, effectuées avec un laser de longueur d'onde 644 nm) : Three solutions of carbon nanotube salts in DMSO were thus prepared, respectively 2.1 mg / ml concentration of FWNTs salt, 1.7 mg / ml of MWNTs salt, and 0.032 mg / ml of SWNTs salt. . The average length of the NTC in solution was determined by light scattering (dynamic light scattering measurements, from 30 to 130 degrees, performed with a laser of wavelength 644 nm):
FWNTs : longueur 1,80 μιη ± 0,1 μιη pour un diamètre de 1 nm  FWNTs: length 1.80 μιη ± 0.1 μιη for a diameter of 1 nm
longueur 1 ,58 μιη ± 0, 1 μιη pour un diamètre de 2 nm  length 1, 58 μιη ± 0, 1 μιη for a diameter of 2 nm
MWNTs : longueur 1,65 μιη ± 0,1 μιη pour un diamètre de 15 nm  MWNTs: length 1.65 μιη ± 0.1 μιη for a diameter of 15 nm
longueur 1,88 μιη ± 0,1 μιη pour un diamètre de 10 nm  length 1.88 μιη ± 0.1 μιη for a diameter of 10 nm
SWNTs : longueur 3,30 μιη ± 0,1 μιη pour un diamètre de 0,7 nm SWNTs: length 3,30 μιη ± 0,1 μιη for a diameter of 0,7 nm
Longueur 3,05 μιη ± 0,1 μιη pour un diamètre de 1,3 nm  Length 3.05 μιη ± 0.1 μιη for a diameter of 1.3 nm
Préparation des dispersions aqueuses de nanotubes de carbone Preparation of aqueous dispersions of carbon nanotubes
Des dispersions aqueuses contenant 0,3% de NTC sont obtenues comme suit : Aqueous dispersions containing 0.3% of CNTs are obtained as follows:
Les FWNTs sont dispersés en utilisant le cholate de sodium comme tensio-actif, dans les proportions 1 : 1 à 0,3% dans l'eau. Leur dispersion est obtenue par sonication pendant 1 heure à 20W. FWNTs are dispersed using sodium cholate as surfactant, in the proportions 1: 1 to 0.3% in water. Their dispersion is obtained by sonication for 1 hour at 20W.
Les MWNTS sont dispersés dans les mêmes proportions pendant 30 minutes à l'aide d'ultra-sons à une puissance de 20W.  The MWNTS are dispersed in the same proportions for 30 minutes using ultrasound at a power of 20W.
Les SWNTS sont dispersés à l'aide de cholate de sodium dans les proportions 1 : 1 à 0,1%) dans l'eau. Leur dispersion est obtenue par sonication pendant 1 heure à 20W, suivie d'une légère centrifugation à 2000 rpm pendant 30 min pour éliminer les gros agrégats restants.  SWNTS are dispersed with sodium cholate in the ratio of 1: 1 to 0.1% in water. Their dispersion is obtained by sonication for 1 hour at 20W, followed by a slight centrifugation at 2000 rpm for 30 min to remove the remaining large aggregates.
Les longueurs moyennes des NTC, déterminées par diffusion de la lumière (longueur d'onde du laser: 532 nm) sont :  The average lengths of the CNTs, determined by light scattering (wavelength of the laser: 532 nm) are:
FWNTs : longueur 630 nm pour un diamètre de 1 nm  FWNTs: length 630 nm for a diameter of 1 nm
longueur 520 nm pour un diamètre de 3 nm  length 520 nm for a diameter of 3 nm
soit une longueur moyenne 550 nm ± 50 nm  an average length of 550 nm ± 50 nm
MWNTs : longueur 312 nm pour un diamètre de 15 nm MWNTs: length 312 nm for a diameter of 15 nm
longueur 412 nm pour un diamètre de 10 nm  length 412 nm for a diameter of 10 nm
soit une longueur moyenne 360 nm ± 50 nm  an average length 360 nm ± 50 nm
SWNTs : longueur de 2,8 μιη pour un diamètre de 1 nm SWNTs: length of 2.8 μιη for a diameter of 1 nm
longueur de 2,15 μιη pour un diamètre de 4 nm  length of 2.15 μιη for a diameter of 4 nm
soit un longueur moyenne 2,5 μιη ± 0,1 μιη Réalisation des films de nanotubes de carbone sur un substrat transparentan average length of 2.5 μιη ± 0.1 μιη Production of carbon nanotube films on a transparent substrate
- à partir des solutions (voie solvant) - from solutions (solvent route)
On procède à une filtration de la solution de sels de NTC sur une membrane en alumine (réf. 3802Z de Whatman, porosité 0,02 μιη, diamètre 47 mm).  Filtration of the NTC salt solution is carried out on an alumina membrane (Whatman part number 3802Z, porosity 0.02 μm, diameter 47 mm).
La filtration est réalisée en boite à gants. Le film sur membrane est ensuite laissé sous air sec pendant 2h avant d'être transférés sur un substrat en PET (réf. ES301400 de Goodfellow, d'épaisseur ΙΟΟμιη).  The filtration is carried out in glove box. The membrane film is then left in dry air for 2 hours before being transferred to a PET substrate (ref ES301400 from Goodfellow, thickness ΙΟΟμιη).
Le transfert est effectué en éliminant la membrane en alumine dans un bain de soude à 1,5M, pendant environ 10 min. Par lavages successifs, le bain est ramené à pH neutre et le substrat en PET est ensuite introduit dans le bain sous le film de sels de NTC qui flotte à la surface de l'eau. Le film est déposé délicatement sur le PET en vidant le bain. Le film est maintenu sur le PET par capillarité, il est ensuite séché à l'étuve à 50°C pendant 24h.  The transfer is carried out by removing the alumina membrane in a 1.5M sodium hydroxide bath for about 10 minutes. By successive washings, the bath is brought back to neutral pH and the PET substrate is then introduced into the bath under the film of salts of CNT floating on the surface of the water. The film is gently deposited on the PET by emptying the bath. The film is held on the PET by capillarity, it is then dried in an oven at 50 ° C for 24 hours.
- à partir des dispersions (voie aqueuse) from the dispersions (aqueous route)
On procède à une filtration de la dispersion aqueuse sur une membrane en cellulose (réf. GSWP04700 de Millipore, porosité 0,22 μιη, diamètre 47 mm) tel que décrit dans Transparent, conductive carbon nanotubes films, Wu et al, Science, 2004, 305, 1273. L'épaisseur du film sur cellulose est contrôlée par la quantité de dispersion filtrée. La filtration permet d'obtenir un réseau de NTC aléatoire et homogène en épaisseur.  Filtration of the aqueous dispersion is carried out on a cellulose membrane (Millipore GSWP04700, porosity 0.22 μιη, diameter 47 mm) as described in Transparent, conductive carbon nanotube films, Wu et al, Science, 2004, 305, 1273. The film thickness on cellulose is controlled by the amount of the filtered dispersion. The filtration makes it possible to obtain a network of NTC that is random and homogeneous in thickness.
La seconde étape consiste à éliminer la membrane en cellulose et à déposer le film de NTC sur un substrat en PET (réf. ES301400 de Goodfellow, d'épaisseur ΙΟΟμιη). The second step involves removing the cellulose membrane and depositing the NTC film on a PET substrate (Goodfellow ES301400, thickness ΙΟΟμιη).
Pour cela, la membrane est imbibée de 1 ,2 dichlorobenzène puis « collée » sur le substrat PET de même diamètre que la membrane, La face constituée des NTC est posée sur le substrat. L'ensemble est ensuite plongé dans un bain d'acétone pendant 20 min afin de dissoudre la cellulose tout en conservant le film de NTC sur le PET. Le film est rincé à l'acétone et à l'isopropanol puis séché à l'azote. La cellulose résiduelle est éliminée en plongeant de nouveau le film dans un bain d'acétone pendant quelques heures sans que le réseau de NTC ne se décolle du PET. Propriétés des films For this, the membrane is impregnated with 1, 2 dichlorobenzene and then "stuck" on the PET substrate of the same diameter as the membrane, the face consisting of CNTs is placed on the substrate. The assembly is then immersed in an acetone bath for 20 min in order to dissolve the cellulose while retaining the NTC film on the PET. The film is rinsed with acetone and isopropanol and then dried with nitrogen. The residual cellulose is removed by re-dipping the film in an acetone bath for a few hours without the CNT network peeling off the PET. Movie properties
Les films sont caractérisés par des mesures de transmittance et de conductivité selon les méthodes suivantes :  The films are characterized by transmittance and conductivity measurements according to the following methods:
Transmittance : mesurée à 550 nm par spectroscopie d'absorption sur les films (appareil : spectromètre d'absorption Unicam UV4-100, gamme de longueurs d'onde 400 - 900 nm).  Transmittance: measured at 550 nm by absorption spectroscopy on films (device: Unicam UV4-100 absorption spectrometer, wavelength range 400 - 900 nm).
Résistivité surfacique : selon la méthode "Measurement of sheet resistivity with the four-point probe, Smits F. M." Bell System Technical Journal. 1958;37(3):711-718.  Surface resistivity: according to the method "Measurement of sheet resistivity with the four-point probe, Smits F. M." Bell System Technical Journal. 1958; 37 (3): 711-718.
RESULTATS RESULTS
Caractérisation des films par microscopie électronique à balayage (MEB)  Characterization of films by scanning electron microscopy (SEM)
Deux films transparents sur PET (films a et b) obtenus à partir d'une solution de sels de NTC, caractérisés respectivement par une quantité de MWNTs de 0,95 μg/cm2 et 4,74 μg/cm2 , ont été observés en MEB. Two PET transparent films (films a and b) obtained from a solution of CNT salts, characterized respectively by a quantity of MWNTs of 0.95 μg / cm 2 and 4.74 μg / cm 2 , were observed in SEM.
Les clichés représentés sur la figure 1 annexée montrent une bonne homogénéité des films. De plus, selon le cliché MEB du film (a), la longueur des NTC varie entre Ιμιη et 2μιη. Etude de la conductivité/transmittance  The pictures shown in Figure 1 attached show good homogeneity of the films. In addition, according to the SEM image of film (a), the length of the NTC varies between Ιμιη and 2μιη. Conductivity / transmittance study
Films obtenus par voie solvant (selon l'invention)  Films obtained by a solvent route (according to the invention)
Le tableau 1 ci-après rassemble les valeurs de résistivité surfacique (Rsq, exprimé en kohms/sq) et de transmittance (T en %) à 550 nm mesurées sur les films obtenus à partir des solutions de sels de NTC, en fonction de la quantité de solution filtrée sur filtre alumine.  Table 1 below collates the values of surface resistivity (Rsq, expressed in kohms / sq) and transmittance (T in%) at 550 nm measured on the films obtained from the solutions of salts of CNT, according to the amount of filtered solution on alumina filter.
Tableau 1  Table 1
Figure imgf000018_0001
Pour une transmittance d'environ 90%, les films FWNTs et MWNTs présentent respectivement une résistivité de l'ordre de 6 et 29 kohms/sq, et les films de SWNTs une résistivité de l'ordre de 200 ohms/sq.
Figure imgf000018_0001
For a transmittance of about 90%, the films FWNTs and MWNTs respectively have a resistivity of the order of 6 and 29 kohms / sq, and the SWNT films have a resistivity of the order of 200 ohms / sq.
Pour une transmittance d'environ 77%, les films FWNTs et MWNTs présentent respectivement une résistivité de 2 et 5,5 kohms/sq, et les films de SWNTs une résistivité de l'ordre de 100 ohms/sq.  For a transmittance of about 77%, the films FWNTs and MWNTs respectively have a resistivity of 2 and 5.5 kohms / sq, and the films of SWNTs a resistivity of the order of 100 ohms / sq.
- Films obtenus par voie aqueuse (comparatif)  - Films obtained by aqueous route (comparative)
Le tableau 2 ci-après rassemble les valeurs de résistivité surfacique (Rsq, exprimé en kohms/sq) et de transmittance (T en %) à 550 nm mesurées sur les films obtenus à partir des dispersions aqueuses de NTC, en fonction de la quantité de dispersion filtrée sur membrane.  Table 2 below collates the values of surface resistivity (Rsq, expressed in kohms / sq) and transmittance (T in%) at 550 nm measured on the films obtained from the aqueous dispersions of CNTs, as a function of the quantity filtered dispersion on membrane.
Tableau 2  Table 2
Figure imgf000019_0001
Pour une transmittance d'environ 90%, les films FWNTs et MWNTs présentent respectivement une résistivité de 53 et 2125 kohms/sq, et les films de SWNTs une résistivité de l'ordre de 290 ohms/sq.
Figure imgf000019_0001
For a transmittance of approximately 90%, the films FWNTs and MWNTs respectively have a resistivity of 53 and 2125 kohms / sq, and the SWNTs films have a resistivity of the order of 290 ohms / sq.
Pour une transmittance d'environ 80%, les films FWNTs et MWNTs présentent respectivement une résistivité de 14,8 et 141,7 kohms/sq, et les films de SWNTs une résistivité de l'ordre de 600 ohms/sq.  For a transmittance of about 80%, the films FWNTs and MWNTs respectively have a resistivity of 14.8 and 141.7 kohms / sq, and the SWNTs films have a resistivity of the order of 600 ohms / sq.
- Comparaison entre les films obtenus par voie solvant et par voie aqueuse La figure 2 annexée illustre l'évolution de la transmittance des films en fonction de leur résistivité surfacique. - Comparison between the films obtained by solvent and aqueous route The appended FIG. 2 illustrates the evolution of the transmittance of the films as a function of their surface resistivity.
On observe que la résistivité des films est améliorée de un à deux ordres de grandeur lorsqu'ils sont réalisés à partir de solutions de sels de NTC, comparativement aux films obtenus à partir des dispersions aqueuses de NTC.  It is observed that the resistivity of the films is improved by one to two orders of magnitude when they are made from solutions of CNT salts, compared to the films obtained from the aqueous dispersions of CNTs.
Les solutions de sels de NTC permettent ainsi d'obtenir des films transparents conducteurs aux propriétés électriques (vs transmittance) meilleures que celles obtenues par des dispersions aqueuses de mêmes types de NTC, et cela quelle que soit la nature des NTC  The solutions of CNT salts thus make it possible to obtain conductive transparent films with electrical properties (vs transmittance) that are better than those obtained by aqueous dispersions of the same types of CNT, and this, whatever the nature of the CNTs.
On observe qu'il est particulièrement avantageux d'utiliser des NTC mono-parois à caractère métallique puisqu'ils permettent d'atteindre des performances avoisinant celles de ΓΙΤΟ « courant » ( 100 ohms/sq à 80% de transmittance) et celles de ΓΙΤΟ de haute performance (20 ohms/sq à 80% de transmittance). It is observed that it is particularly advantageous to use metal-walled single-wall NTCs since they make it possible to achieve performances close to those of "current" (100 ohms / sq. At 80% transmittance) and those of ΓΙΤΟ high performance (20 ohms / sq. at 80% transmittance).

Claims

REVENDICATIONS
1. Procédé de préparation d'un film transparent conducteur comprenant au moins les étapes suivantes : A method for preparing a conductive transparent film comprising at least the following steps:
a) la préparation d'une solution organique de sels de nanotubes de carbone ; b) la filtration sous vide de ladite solution sur une membrane conduisant à un film sur la membrane filtrante ;  a) preparing an organic solution of carbon nanotube salts; b) vacuum filtration of said solution on a membrane leading to a film on the filter membrane;
c) le transfert du film, de la membrane filtrante sur un substrat transparent.  c) the transfer of the film, the filter membrane on a transparent substrate.
2. Procédé selon la revendication 1 caractérisé en ce que les nanotubes de carbone sont mono-parois ou multi-parois. 2. Method according to claim 1 characterized in that the carbon nanotubes are single-walled or multi-walled.
3. Procédé selon la revendication 1 ou 2 caractérisé en ce que les nanotubes de carbone sont des nanotubes de carbone mono-parois à caractère métallique. 3. Method according to claim 1 or 2 characterized in that the carbon nanotubes are single-walled carbon nanotubes with a metallic character.
4. Procédé selon l'une quelconque des revendications précédentes caractérisé en ce que les sels de nanotubes de carbone sont obtenus par réduction des nanotubes conduisant à des nanotubes chargés négativement avec des contre-ions positifs. 4. Method according to any one of the preceding claims, characterized in that the carbon nanotube salts are obtained by reducing the nanotubes leading to negatively charged nanotubes with positive counter-ions.
5. Procédé selon l'une quelconque des revendications précédentes caractérisé en ce que les sels de nanotubes de carbone sont obtenus par addition, dans des conditions anaérobies, d'un sel de formule : A+ B" 5. Method according to any one of the preceding claims, characterized in that the salts of carbon nanotubes are obtained by adding, under anaerobic conditions, a salt of formula: A + B "
dans laquelle  in which
- A+ représente un sel d'un ion alcalin - A + represents a salt of an alkaline ion
B" représente un anion d'un composé polyaromatique. B " represents an anion of a polyaromatic compound.
6. Procédé selon l'une quelconque des revendications 1 à 4 caractérisé en ce que les sels de nanotubes de carbone sont obtenus par mélange d'un métal alcalin A et de NTC, ledit mélange étant porté à une température allant de 150°C à 400°C. 6. Method according to any one of claims 1 to 4 characterized in that the carbon nanotube salts are obtained by mixing an alkali metal A and CNT, said mixture being heated to a temperature ranging from 150 ° C to 400 ° C.
7. Procédé selon l'une quelconque des revendications précédentes caractérisé en ce que, selon l'étape a), les sels de nanotubes de carbone sont mis en solution dans un solvant organique polaire choisi parmi le sulfolane, le diméthylsulfoxyde (DMSO), le diméthylformamide, la N-méthylpyrrolidone, ou le N- méthyl formamide. 7. Method according to any one of the preceding claims, characterized in that, according to step a), the carbon nanotube salts are dissolved in a polar organic solvent selected from sulfolane, dimethylsulfoxide (DMSO), dimethylformamide, N-methylpyrrolidone, or N-methylformamide.
8. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce que la concentration de la solution organique de sels de NTC est comprise entre 0,001 et 10 g/ 1. 8. Process according to any one of the preceding claims, characterized in that the concentration of the organic solution of CNT salts is between 0.001 and 10 g / l.
9. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce que l'épaisseur du film obtenu à l'étape b) sur la membrane filtrante est comprise entre 0,7 nm et 1000 nm, de préférence entre 1 nm et 500 nm. 9. Method according to any one of the preceding claims, characterized in that the thickness of the film obtained in step b) on the filter membrane is between 0.7 nm and 1000 nm, preferably between 1 nm and 500 nm. nm.
10. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce que la membrane filtrante est en alumine. 10. Method according to any one of the preceding claims, characterized in that the filter membrane is alumina.
11. Procédé selon l'une quelconque des revendications précédentes, caractérisé en ce que l'étape c) est réalisée par dissolution de la membrane filtrante dans un milieu choisi en fonction de la nature de la membrane à dissoudre, puis intoduction d'un substrat transparent dans ledit milieu sous le film de NTC surnageant. 11. Process according to any one of the preceding claims, characterized in that step c) is carried out by dissolving the filtering membrane in a medium chosen according to the nature of the membrane to be dissolved, and then introducing a substrate. transparent in said medium under the NTC film supernatant.
12. Procédé selon l'une quelconque des revendications précédentes caractérisé en ce que le substrat transparent est un substrat inorganique tel que le verre, le quartz, le mica ou une céramique, ou un substrat plastique rigide, un substrat plastique flexible tel que par exemple les matériaux suivants : polyéthylène téréphtalate, polyéthylène naphtalate, polyéthylène sulphone, polycarbonate, polystyrène, polypropylène, polyester, polyimide, polyéher éther cétone, polyétherimide, résines acryliques, copolymères oléfine-maléimide, résines à base de norbornène. 12. Method according to any one of the preceding claims, characterized in that the transparent substrate is an inorganic substrate such as glass, quartz, mica or a ceramic, or a rigid plastic substrate, a flexible plastic substrate such as for example the following materials: polyethylene terephthalate, polyethylene naphthalate, polyethylene sulphone, polycarbonate, polystyrene, polypropylene, polyester, polyimide, polyether ether ketone, polyetherimide, acrylic resins, olefin-maleimide copolymers, norbornene-based resins.
13. Film transparent conducteur comportant des nanotubes de carbone mono-parois à caractère métallique de longueur moyenne supérieure à 1 μιη, et pouvant aller jusque quelques centaines de μηι, sur un substrat transparent, susceptible d'être obtenu selon le procédé selon l'une quelconque des revendications 1 à 12. 13. Conductive transparent film comprising single-walled carbon nanotubes with a metallic character of average length greater than 1 μιη, and capable of to go up to a few hundred μηι, on a transparent substrate, obtainable by the method according to any one of claims 1 to 12.
14. Utilisation du film transparent conducteur selon la revendication 13 dans les domaines des diodes électroluminescentes, les capteurs d'image, les cellules solaires, les écrans tactiles, les écrans à cristaux liquides. 14. Use of the transparent conductive film according to claim 13 in the fields of light-emitting diodes, image sensors, solar cells, touch screens, liquid crystal screens.
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