EP1802783B1 - Coating method - Google Patents

Abstract

The present invention relates to a method of coating a surface with nanoparticles, to a nanostructured coating that can be obtained by this method, and also to a device for implementing the method of the invention. The method is characterized in that it comprises an injection of a colloidal sol of said nanoparticles into a plasma jet that sprays them onto said surface. The device ( 1 ) comprises: a plasma torch ( 3 ); at least one container ( 5 ) containing the colloidal sol ( 7 ) of nanoparticles; a device ( 9 ) for fixing and for moving the substrate(S); and a device ( 11 ) for injecting the colloidal sol into the plasma jet ( 13 ) of the plasma torch. The present invention has applications in optical, electronic and energy devices (cells, thermal barriers) comprising a nanostructured coating that can be obtained by the method of the invention.

Description

TECHNICAL AREA

The present invention relates to a method for coating a surface of a substrate with nanoparticles, a nanostructured coating obtainable by this method, and a device for implementing the process of the invention. invention.

The present invention also relates to optical, mechanical, chemical, electronic and energetic devices comprising a nanostructured coating obtainable by the method of the invention.

Nanostructured materials are defined as materials having nanoscale organization, i.e. on a scale ranging from a few nm to a few hundred nm. This size domain is where the characteristic lengths of the various physical, electronic, magnetic, optical, superconductivity, mechanical, and other processes are found. and where the surface plays a predominant role in these processes, which gives these "nanomaterials" specific and often exalted properties. Because of these characteristics, these materials offer real potential in the construction of new high-performance buildings with specific properties.

The possibility of manufacturing nanostructures makes it possible to develop innovative materials and offers the possibility of exploiting them in many different ways. areas such as optics, electronics, energy, etc. These nanomaterials offer undeniable fundamental benefits and important application and application potential in various future technologies such as fuel cells, "smart" coatings, resistant materials (thermal barrier).

The present invention makes it possible to develop new nanostructured coatings by a simple and easily industrializable process, and opens these technologies to manufacturers. The essence of the "nano" concept is self-assembly, which leads complex molecules to form larger heterogeneous aggregates, capable of performing a sophisticated function or of constituting a material with unprecedented properties.

References in square brackets ([]) refer to the list of bibliographic references presented following the examples.

Prior art

There is currently no simple technique to implement and to obtain nanoparticle coatings that meet the requirements of greater and greater homogeneity of structure and thickness, even at a scale of a few microns, and mechanical strength, due to the miniaturization of electromechanical microsystems and / or optical and / or electrochemical.

The inventors of the present are interested in plasma projection. It's about a a technique used in research laboratories and in industry to make deposits of ceramic, metallic or cermet materials, or polymers and combinations of these materials on different types of substrates (shape and nature). Its principle is as follows: the material to be deposited is injected dry in the plasma jet in the form of particles, average diameter generally greater than 5 microns, using a carrier gas. In this medium, the particles are melted totally or partially and accelerated to a substrate where they come to pile up.

However, the layer thus formed, of thickness generally greater than 100 microns, has a strongly anisotropic lamellar structure characteristic of deposits made by plasma spraying. These techniques therefore do not make it possible to form nanoparticle coatings or coatings with thicknesses of less than 100 μm, up to a few microns.

In addition, the coatings obtained have the disadvantage of being micro-cracked, especially in the case of ceramics deposits, fragile materials that relax the internal stresses.

In addition, it has been found that the coating obtained has a lamellar structure which strongly conditions its thermomechanical properties, which therefore clearly limits, a priori, the potential applications of the plasma projection.

In particular, the emergence of new applications, particularly in microelectronics and on-chip labs, requires of thickness less than 50 microns, consisting of sub-micron sized grains not necessarily having a lamellar structure, and using high deposition rates. However, it is not currently possible to penetrate particles smaller than one micron in a plasma jet using a conventional vector gas injector, without significantly disturbing it. Indeed, the high speed of the cold carrier gas, necessary for the acceleration of fine particles, causes a sharp decrease in the temperature and the flow velocity of the plasma, essential properties for melting and driving the particles.

Different solutions have been proposed. Thus, the document [1] of Lau et al. describes the use of an aqueous solution, consisting of at least three metal salts, atomized in a non-supersonic inductive plasma. This results in deposits of superconducting ceramics but which do not have a nanometric structure.

The document [2] by Marantz et al. describes an axial injection into a blown arc plasma of a colloidal solution. The production of nanostructured deposits is neither mentioned nor suggested. In addition, this process is difficult to industrialize because it requires the use of two to four plasma torches operating simultaneously.

The document [3] of Ellis et al. discloses a process in which an organometallic compound is introduced into a non-supersonic inductive plasma under gaseous or solid form. The deposit formed, however, does not have a nanometric structure.

In document [4], Gitzhofer et al. describe the use of a micron-sized particle-laden liquid. This liquid is injected into a plasma in the form of droplets by means of an atomizer. This technique is limited to radio-frequency type plasmas and the resulting deposits are not nanostructured.

In document [5], Chow et al. describe a method consisting of injecting several solutions into a plasma jet in order to obtain deposits having a nanometric structure. However, the final material comes from a chemical reaction in flight in the plasma, making the complex method to master. In addition, in this method (which involves a chemical reaction in the plasma) the particle sizes are 100 nm; the method nominally predicts a chemical conversion during the projection process and uses dispersants; and the projection conditions are explicitly chosen not to vaporize the solvent from the projected solution before reaching the substrate.

In document [6], Kear et al propose the injection of a solution containing agglomerates of nanostructured powders in the form of a spray in a plasma. The use of a spray imposes different stages so that the size of the particles to be injected is sufficiently large (of the order of one micron) to penetrate into the plasma: drying of the solution containing small particles, agglomeration of these particles with a binder and colloidal suspension agglomerates larger than one micron. This method requires ultrasonic assistance or the use of dispersants, for example surfactants, to maintain the dispersion of the particles in suspension in the liquid.

Document [7] by Rao N.P. et al. describes a method where gaseous precursors, injected radially in an arc plasma, give rise to the formation of solid particles in flight by nucleation-growth. However, the thickness of the deposits formed can not exceed ten microns and it is not possible to make any type of materials.

The problems related to the plasma technique are therefore very numerous, the solutions also proposed, but none of these solutions currently makes it possible to solve all of these problems.

The inventors have also been interested in existing sol-gel deposition processes, particularly in the field of optics. These processes usually use liquid deposition methods such as spin-coating, laminar coating, dip-coating, aerosol spray ("spray-coating"). These different techniques lead to thin layers whose thickness is generally less than one micron. Some of these deposition methods make it possible to coat large surfaces, for example from a few hundred cm ² to a few m ² , which is an advantage.

However, the coatings obtained by these processes crack beyond critical micron thicknesses. The main cause of this major defect lies in the stress of voltage applied by the substrate during heat treatments necessary for their development. Another disadvantage lies in the impossibility of depositing homogeneous coatings having a good adhesion, even for thicknesses greater than about 150 nm.

The problems associated with this other technique are therefore also very numerous, even if recent techniques have made it possible to solve some by acting on the chemical composition of the sol-gel.

In summary, none of these techniques of the prior art makes it possible to obtain a coating of nanoparticles of homogeneous thickness and structure, and none of these techniques indicates a promising way to achieve this simply.

In addition, the document EP-A1-1 134 302 discloses a method for preparing nanoparticle films by thermal spraying techniques such as plasma spraying wherein a nanocompartmentalized solution of a metallic material is used as a filler.

There is no mention in this document that said nanocompartmentalized solution is or may be a colloidal sol in which nanoparticles of a metal oxide are dispersed and stabilized.

STATEMENT OF THE INVENTION

The object of the present invention is precisely to provide a method for forming a nanostructured coating that meets the needs indicated above and provides a solution to all of the aforementioned drawbacks.

The object of the present invention is still to provide a coating of nanoparticles which does not have the drawbacks, defects and disadvantages of the coatings of the prior art, and which can be used in optical, mechanical, chemical, electronic and microsystem devices and microsystems. energy present and future with excellent performance.

The process of the invention is a process for coating a surface of a substrate with nanoparticles, characterized in that it comprises an injection of a colloidal sol of said nanoparticles into a thermal plasma jet which projects them onto said surface .

The inventors are the first to solve the aforementioned drawbacks of prior art techniques relating to plasma deposition by this method. Compared to the old techniques, it consists in particular to replace the injection gas in the dry process with a carrier liquid consisting of a colloidal sol. The projected particles are thus stabilized in a liquid medium before being accelerated in a plasma.

As stated above, more recent work has already been done on the injection of a material in a form other than powder in a plasma and especially in liquid form. However, none of these works uses or suggests a direct injection into a plasma jet of colloidal sol, or colloidal sol-gel solution, of nanoparticles, and the possibility of producing nanostructured deposits of any type of material possessing the same chemical and structural composition as the initial product.

The method of the present invention also allows, unexpectedly, the preservation of the nanostructural properties of the projected material, by thermal projection of a stabilized suspension (sol) of nanoscale particles. The method of the invention makes it possible to avoid the use of stabilizing additives such as dispersants or surfactants as in the processes of the prior art, and / or the essential use of additional dispersing means such as ultrasound atomization, mechanical agitation, etc. during the projection phase. The present invention therefore makes it possible at the same time to preserve the purity of the projected material and to simplify the method of implementation. It is also notably thanks to the use of a soil that the aggregation of the nanoparticles is limited, and that the process of the invention results in a homogeneous nanostructured coating.

In addition, by virtue of the process of the present invention, the inventors exploit the singular advantage of soils-gels to offer a very large number of physicochemical pathways for obtaining stable colloidal suspensions and nanoparticles. The soft chemistry of constitution of the soils-gels makes it possible in particular to synthesize, from very numerous inorganic or organometallic precursors, a plurality of different metal oxides.

In addition, the present invention also uses the advantageous property of soils-gels to allow the synthesis of inorganic particles of different crystalline phases, in the same soil, for example using the hydrothermal route or in milder conditions. In this chemistry, the nucleation of the particles takes place in a liquid medium. Access to mixed colloidal sols consisting of a mixture of metal oxide nanoparticles of different types, or a mixture of metal oxide nanoparticles and metal nanoparticles and / or doped metal oxide nanoparticles by another metal oxide or other metal element, also offers many variants.

Moreover, thanks to the method of the invention, one can further improve and refine the homogeneity and stability of the soil by judiciously selecting the particle size of the soil particles and the solvent used. Indeed, preferred conditions of the process of the invention make it possible to further limit or even avoid segregations of nanoparticles, concentration gradients or sedimentations.

Also, plasma projection conditions, as well as soil injection protocols allow to act on the quality of the nanoparticle coating formed, and, according to various examples presented below, can further improve the quality and to refine the conservation of the properties of the particles of the colloidal sol within the coating material.

The definitions, as well as the general and preferred operating conditions of the process of the invention are set forth below.

According to the invention, the substrate may be organic, inorganic or mixed (that is to say organic and inorganic on the same surface). Preferably it supports the operating conditions of the process of the invention. It may consist for example of a material chosen from semiconductors such as silicon; organic polymers such as poly (methyl methacrylate) (PMMA), polycarbonate (PC), polystyrene (PS), polypropylene (PP) and polyvinyl chloride (PVC); metals such as gold, aluminum and silver; the glasses ; inorganic oxides, for example layered, such as SiO ₂ , Al ₂ O ₃ , ZrO ₂ , TiO ₂ , Ta ₂ O ₅ , MgO, etc. ; and composite or mixed materials comprising several of these materials.

The surface of the substrate to be coated will optionally be cleaned in order to remove organic and / or inorganic contaminants which could prevent the deposition or even the attachment of the coating to the surface and improve the adhesion of the coating. The cleaning used depends on the nature of the substrate and can be selected from the physical, chemical or mechanical processes known to those skilled in the art. For example, and without limitation, the cleaning process may be chosen from immersion in an organic solvent and / or washing detergent and / or etching assisted by ultrasound; these cleanings being eventually followed by rinsing with tap water, then rinsing with deionized water; these rinses being optionally followed by drying by "lift-out", by a spray of alcohol, by a jet of compressed air, with hot air, or by infrared rays. Cleaning can also be a cleaning by ultraviolet rays.

By "nanoparticles" is meant particles of nanometric size, generally ranging from 1 nm to a few hundred nanometers. The term "particles" is also used.

A "sol-gel process" means a series of reactions where soluble metal species hydrolyze to form a metal hydroxide. The sol-gel process involves a hydrolysis-condensation of metal precursors (salts and / or alkoxides) allowing easy stabilization and dispersion of particles in a growth medium.

"Soil" is a colloidal system in which the dispersion medium is a liquid and the dispersed phase is a solid. Soil is also called "colloidal sol-gel solution" or "colloidal sol." The nanoparticles are dispersed and stabilized thanks to the colloidal sol.

According to the invention, the sol can be prepared by any method known to those skilled in the art. We will of course prefer the processes that make it possible to obtain a greater homogeneity of size of the nanoparticles, as well as a greater stabilization and dispersion of the nanoparticles. The methods for preparing the sol-gel colloidal solution described herein include the various conventional methods for synthesizing nanoparticles dispersed and stabilized in a liquid medium.

According to a first variant of the present invention, the sol can be prepared for example by precipitation in an aqueous medium or by sol-gel synthesis in an organic medium from a precursor of nanoparticles.

• étape 1 : synthèse hydrothermale des nanoparticules par utilisation d'un autoclave à partir de précurseurs métalliques ou synthèse des nanoparticules par co-précipitation à pression ordinaire ;
• étape 2 : traitement des nanoparticules (poudre), dispersion et stabilisation des nanoparticules en milieu aqueux (lavages, dialyses) ;
• étape 3 (facultative) : modification du solvant de stabilisation : dialyse, distillation, mélange de solvant ;
• étape 4 : (facultative) : dispersion des nanoparticules dans un milieu organique pour former un sol hybride organique-inorganique par dispersion des particules au sein d'un polymère ou oligomère organique et/ou par fonctionnalisation de la surface des particules par tout type de fonctions organiques réactives ou non.

When the sol is prepared by precipitation in an aqueous medium from a precursor of nanoparticles, the preparation may comprise, for example, the following steps:

• step 1: hydrothermal synthesis of the nanoparticles by using an autoclave from metal precursors or synthesis of the nanoparticles by co-precipitation at ordinary pressure;
• step 2: treatment of the nanoparticles (powder), dispersion and stabilization of the nanoparticles in an aqueous medium (washing, dialysis);
• step 3 (optional): modification of the stabilizing solvent: dialysis, distillation, solvent mixture;
• step 4: (optional): dispersion of the nanoparticles in an organic medium to form an organic-inorganic hybrid sol by dispersing the particles within an organic polymer or oligomer and / or by functionalizing the surface of the particles by any type of function organic reactive or not.

Documents [8], [9] and Example 2 below describe examples of this way of preparation by precipitation in an aqueous medium, with different precursors (metalloid salts, metal salts, metal alkoxides), usable for the implementation of the present invention.

• étape (a) : hydrolyse-condensation de précurseurs organométalliques ou de sels métalliques en milieu organique ou hydroalcoolique ;
• étape (b) : nucléation des nanoparticules stabilisées et dispersées en milieu organique ou hydroalcoolique par mûrissement, croissance ;
• étape (c) (facultative) : formation d'un sol hybride organique-inorganique par dispersion des particules au sein d'un polymère ou oligomère organique et/ou par fonctionnalisation de la surface des particules par tout type de fonctions organiques réactives ou non.

When the sol is prepared by sol-gel synthesis in an organic medium from a precursor of nanoparticles, the preparation may comprise, for example, the following succession of steps:

• step (a): hydrolysis-condensation of organometallic precursors or metal salts in an organic or aqueous-alcoholic medium;
• step (b): nucleation of the nanoparticles stabilized and dispersed in organic or hydroalcoholic medium by ripening, growth;
• step (c) (optional): formation of an organic-inorganic hybrid sol by dispersing the particles within an organic polymer or oligomer and / or by functionalizing the surface of the particles by any type of reactive or non-reactive organic functions.

The document [10] describes examples of this route of preparation by sol-gel synthesis in organic medium, with different precursors (metalloid salts, metal salts, metal alkoxides), usable in the present invention.

Thus, as explained above, the nanoparticles can directly be stabilized in the solvent used during the synthesis or subsequently peptized if they are synthesized by precipitation. In both cases the suspension obtained is a soil.

Regardless of the chosen preparation route, according to the invention, the nanoparticle precursor is typically selected from the group consisting of a metalloid salt, a metal salt, a metal alkoxide, or a mixture thereof. The aforementioned documents illustrate this technical aspect.

For example, the metal or metalloid of the salt or alkoxide precursor of nanoparticles may be chosen for example from the group comprising silicon, titanium, zirconium, hafnium, aluminum, tantalum, niobium, cerium , nickel, iron, zinc, chromium, magnesium, cobalt, vanadium, barium, strontium, tin, scandium, indium, lead, yttrium, tungsten, manganese, gold, silver, platinum, palladium, nickel, copper, cobalt, ruthenium, rhodium, europium and other rare earths, or a metal alkoxide of these metals.

• une réduction chimique de précurseurs organométalliques ou métalliques ou d'oxydes métalliques, par exemple de la manière décrite dans le document [11].

According to a second variant of the present invention, the sol may be prepared for example by synthesis of a solution of metal nanoparticles from a precursor of metal nanoparticles by using an organic or inorganic reducer in solution, for example by a chosen process. in the group comprising:

• a chemical reduction of organometallic or metal precursors or oxides metal, for example as described in document [11].

Whatever the method chosen in this second variant, according to the invention, the reducing agent may be chosen for example from those cited in the abovementioned documents, for example in the group comprising polyols, hydrazine and its derivatives, quinone and its derivatives, hydrides, alkali metals, cysteine and its derivatives, ascorbate and its derivatives.

Also, according to the invention, the precursor of metal nanoparticles can be chosen for example from those cited in the aforementioned documents, for example in the group comprising the salts of metalloids or metals such as gold, silver, platinum, palladium, nickel, copper, cobalt, aluminum, ruthenium or rhodium or the various metal alkoxides of these metals.

According to a third variant of the present invention, the sol may be prepared by preparing a mixture of nanoparticles dispersed in a solvent, each family may be derived from the preparations described in documents [8], [9], [10] and Example 2 below.

Whatever the variant for obtaining the soil used, in the process of the invention, it is of course possible to use a mixture of different sols which differ in their chemical nature and / or in their method of production.

Typically, the soil used in the process of the present invention may comprise, for example, nanoparticles of a metal oxide selected from the group consisting of SiO ₂ , ZrO ₂ , TiO ₂ , Ta ₂ O ₅ , HfO ₂ , ThO ₂ , SnO ₂ , VO ₂ , In ₂ O ₃ , CeO ₂ , ZnO, Nb ₂ O ₅ , V ₂ O ₅ , Al ₂ O ₃ , Sc ₂ O ₃ , Ce ₂ O ₃ , NiO, MgO, Y ₂ O ₃ , WO ₃ , BaTiO ₃ , Fe ₂ O ₃ , Fe ₃ O ₄ , Sr ₂ O ₃ , (PbZr) TiO ₃ , (BaSr) TiO ₃ , CO ₂ O ₃ , Cr ₂ O ₃ , Mn ₂ O ₃ , Mn ₃ O ₄ , Cr ₃ O ₄ , MnO ₂ , RuO ₂ or a combination of these oxides, for example by doping the particles or by mixing the particles. This list is of course not exhaustive since it includes all the metal oxides described in the aforementioned documents.

In addition, according to the invention, the sol may comprise, for example, metallic nanoparticles of a metal chosen from the group comprising gold, silver, platinum, palladium, nickel, ruthenium or rhodium, or a mixture of different metallic nanoparticles made of these metals. Here too, this list is not exhaustive since it includes all the metal oxides described in the aforementioned documents.

The size of the nanoparticles of the soil obtained is perfectly controlled by its synthesis conditions, in particular by the nature of the precursors used, the solvent (s), the pH, the temperature, etc. and can range from a few angstroms to a few microns. This control of the particle size in the preparation of the soil is described for example in document [12].

According to the invention, for example in the applications mentioned herein, the nanoparticles preferably have a size of 1 to 100 nm, this especially in order to be able to achieve thin layers or coatings, for example with a thickness ranging from 0.1 to 50 μm.

Besides the nanoparticles, the soil also comprises a carrier liquid, which comes from its manufacturing process, called growth medium. This carrier liquid is an organic or inorganic solvent such as those described in the aforementioned documents. It may be for example a liquid selected from water, alcohols, ethers, ketones, aromatics, alkanes, halogens and any mixture thereof. The pH of this carrier liquid depends on the soil manufacturing process and its chemical nature. It is usually from 1 to 14.

In the soils obtained, the nanoparticles are dispersed and stabilized in their growth medium, and this stabilization and / or dispersion can be promoted by the soil preparation process and the chemistry used (see above). The process of the present invention takes advantage of this property of soils.

According to the invention, the sol may further comprise organic molecules. It may be, for example, molecules for stabilizing the nanoparticles in the soil and / or molecules that functionalize the nanoparticles.

Indeed, an organic compound can be added to the nanoparticles to give them a particular property. For example, the stabilization of these nanoparticles in liquid medium by steric effect leads to materials called organic-inorganic hybrid materials of class I. The interactions which regulate the stabilization of these particles are weak of electrostatic nature of hydrogen bonds or Van Der Waals type. Such compounds usable in the present invention, and their effect on soils, are described for example in documents [13] and Example 2 below.

Also, according to the invention, the particles can be functionalized with an organic compound either during synthesis by introduction of suitable organomineral precursors, or by grafting on the surface of the colloids. Examples have been given above. These materials are then called organic-inorganic class II materials since the interactions between the organic component and the mineral particle are strong, of a covalent or ionocovalent nature. Such materials and their method of production are described in document [13].

The properties of the hybrid materials that can be used in the present invention depend not only on the chemical nature of the organic and inorganic components used to form the soil, but also on the synergy that can appear between these two chemistries. Document [13] describes the effects of the chemical nature of the organic and inorganic components used and of such synergies.

The method of the invention comprises injecting the colloidal sol in a jet or flow of thermal plasma. The injection of the ground into the plasma jet can be carried out by any appropriate means of injecting a liquid, for example by means of an injector, by example in the form of jet or drops, preferably with a momentum adapted to be substantially identical to that of the plasma flow. Examples of injectors are given below.

The temperature of the soil during its injection may range, for example, from room temperature (20 ° C.) to a temperature below its boiling point. Advantageously, it is possible to control and modify the temperature of the soil for its injection, for example to be from 0 ° C. to 100 ° C. The soil then has a different surface tension, depending on the temperature imposed, resulting in a more or less rapid and efficient fragmentation mechanism when it arrives in the plasma. The temperature can therefore have an effect on the quality of the coating obtained.

The injected soil, for example in the form of drops, enters the plasma jet, where it is exploded into a multitude of droplets under the effect of plasma shear forces. The size of these droplets can be adjusted according to the desired microstructure of the deposit, depending on the properties of the soil (liquid) and the plasma flow. Advantageously, the size of the droplets ranges from 0.1 to 10 μm.

The kinetic and thermal energies of the plasma jet serve respectively to disperse the drops in a multitude of droplets (fragmentation), then to vaporize the liquid. When the liquid soil reaches the jet core, which is a medium at high temperature and high speed, it is vaporized and the nanoparticles are accelerated to be collected on the substrate to form a nanostructured deposit (coating) having a crystalline structure identical to that of the particles initially present in the starting soil. The vaporization of the liquid brings about the bringing together of fine nanoparticles of material belonging to the same droplet and their agglomeration. The resulting agglomerates, generally less than 1 μm in size, are found in the heart of the plasma where they are melted, partially or totally, then accelerated before being collected on the substrate. If the agglomerate fusion is complete, the grain size in the deposit is a few hundred nanometers to a few microns. On the other hand, if the melting is only partial, the size of the grains in the deposit is close to that of the particles contained in the starting liquid and the crystalline properties of the particles are well preserved within the deposit.

Generally, thermal plasmas are plasmas producing a jet having a temperature of 5000 K to 15000 K. In the implementation of the method of the invention, this temperature range is preferred. Of course, the temperature of the plasma used for the projection of the ground on the surface to be coated may be different. It will be chosen according to the chemical nature of the soil and the desired coating. According to the invention, the temperature will be chosen so as to be preferentially in a configuration of partial or total melting of the particles of the soil, preferably partial melting to best preserve their starting properties within the layer.

The plasma may be for example an arc plasma, blown or not, or an inductive or radiofrequency plasma, for example in supersonic mode. It can operate at atmospheric pressure or at lower pressure. The documents [14], [15] and [16] describe plasmas that can be used in the present invention, and the plasma torches making it possible to generate them. Advantageously, the plasma torch used is an arc plasma torch.

According to the invention, the plasma jet may advantageously be generated from a plasmagenic gas chosen from the group comprising Ar, H ₂ , He and N ₂ . Advantageously, the jet of plasma constituting the jet has a viscosity of 10 ^-4 to 5 × 10 ^-4 kg / ms Advantageously, the plasma jet is an arc plasma jet.

The substrate to be coated is, for obvious reasons, preferentially positioned relative to the plasma jet so that the projection of the nanoparticles is directed on the surface to be coated. Different tests make it very easy to find an optimal position. The positioning is adjusted for each application, according to the selected projection conditions and the microstructure of the desired deposit.

The rate of growth of the deposits, high for a method of manufacturing finely structured layers, depends essentially on the mass percentage of material in the liquid and the flow of liquid. With the method of the invention, it is possible easily obtain a coating deposition rate of nanoparticles of 1 to 100 microns / min.

The thin layers or coatings which can be obtained by the process of the invention, easily ranging from 0.1 to 50 microns, may consist of grains of smaller size or of the order of one micron. They can be dense or porous. They can be pure and homogeneous. The synthesis of a stable and homogeneous sol-gel solution of nanoparticles of defined particle size associated with the liquid plasma spraying method of the invention makes it possible to preserve the intrinsic properties of the starting sol within the deposit and to obtain a nanostructured coating in advantageously controlling the properties following porosity / density; homogeneity in composition; "exotic" stoichiometry (mixed soils and mixtures); nanometric structure (size and crystalline phases); granulometry of the grains; thickness of the homogeneous deposit on object with a complex shape; possibility of deposit on all types of substrates, whatever their nature and their roughness.

The process of the invention may be carried out several times on the same substrate surface, with different soils - in composition and / or in concentration and / or in particle size - to produce successive layers of different materials or deposits with compositional gradients. These deposits of successive layers are useful for example in applications such as layers with electrical properties (electrode and electrolyte), layers with optical properties (low and high refractive index), thermal property layers (conductive and insulating), diffusion barrier layers and / or controlled porosity layers.

The projection method of the present invention is easily industrializable since its specificity and its innovative character reside in particular in the injection system which can adapt to all thermal spray machines already present in the industry; in the nature of the sol-gel solution; and in the choice of plasma conditions for obtaining a nanostructured coating having the properties of the projected particles.

• une torche à plasma thermique capable de produire un jet de plasma ;
• un réservoir de gaz plasmagène;
• un réservoir de sol colloïdal de nanoparticules ;
• un moyen de fixation et de déplacement du substrat par rapport à la torche à plasma ;
• un système d'injection reliant d'une part le réservoir de sol colloïdal et d'autre part un injecteur dont l'extrémité est microperforée d'un trou d'injection du sol colloïdal dans le jet de plasma généré par la torche à plasma ; et
• un mano-détendeur permettant d'ajuster la pression à l'intérieur du réservoir.

A device for coating a surface of a substrate that can be used for carrying out the method of the invention comprises:

• a thermal plasma torch capable of producing a plasma jet;
• a reservoir of plasma gas;
• a colloidal soil reservoir of nanoparticles;
• means for fixing and moving the substrate relative to the plasma torch;
• an injection system connecting on the one hand the colloidal solids reservoir and on the other hand an injector whose end is microperforated with an injection hole of the colloidal sol in the plasma jet generated by the plasma torch; and
• a pressure regulator for adjusting the pressure inside the tank.

Advantageously, the plasma torch is capable of producing a plasma jet having a temperature of 5000 K to 15000 K. Advantageously, the plasma torch is capable of producing a plasma jet having a viscosity of 10 ^-4 to 5 × 10 ^{- 4} kg / ms Advantageously, the plasma torch is an arc plasma torch. Examples of plasmagenic gases are given above, the reservoirs of these gases are commercially available. The reasons for these advantageous choices are outlined above.

Advantageously, the device that can be used for carrying out the process of the invention comprises several reservoirs respectively containing several sols loaded with nanoparticles, the sols being different from each other by their composition and / or diameter and / or concentration. The device may further comprise a cleaning tank containing a solution for cleaning the piping and the injector. Thus, the piping and the injector can be cleaned between each implementation of the process.

• de la pression dans le réservoir utilisé et/ou de la pompe,
• des caractéristiques des dimensions de l'orifice de sortie (diamètre de profondeur), et
• des propriétés rhéologiques du sol.

The tanks can be connected to a compressed air network by means of pipes and a source of compression gas, for example compressed air. One or more pressure reducer (s) allows (s) to adjust the pressure inside the tank (s), usually at a pressure of less than 2 × 10 ⁶ Pa (20 bar). In this case, under the effect of the pressure, the liquid is conveyed to the injector, or the injectors if there are several, by pipes and then exits the injector, for example in the form of a jet of liquid that fragments mechanically in the form of large drops, preferably of calibrated diameter, on average two times greater than the diameter of the circular exit hole. A pump is also usable. The flow rate and the amount of movement of the soil at the outlet of the injector depend in particular on:

• the pressure in the tank used and / or the pump,
• characteristics of the dimensions of the outlet orifice (depth diameter), and
• rheological properties of the soil.

The injector makes it possible to inject the ground into the plasma. It is preferably such that the injected soil mechanically fragments at the outlet of the injector in the form of drops as indicated above. The hole of the injector may be of any form for injecting the colloidal sol in the plasma jet, preferably under the aforementioned conditions. Advantageously, the hole is circular. Advantageously, the hole of the injector has a diameter of 10 to 500 μm. The device may be provided with several injectors, for example according to the quantities of soil to be injected.

According to a particular embodiment of the device that can be used for carrying out the method of the invention, the inclination of the injector relative to the longitudinal axis of the plasma jet can vary from 20 to 160 °. Advantageously also, the injector can be moved in the longitudinal direction of the plasma jet. These movements are indicated schematically on the figure 2 attached. Thus, the injection of the colloidal sol in the plasma jet can be oriented. This orientation makes it possible to optimize the injection of the colloidal sol, and thus the formation of the projected coating on the surface of the substrate.

The soil injection line may be thermostatically controlled to control and possibly modify the temperature of the injected soil. This temperature control and this modification can be carried out at the level of the pipes and / or at the level of the tanks.

The device may comprise means for fixing and moving the substrate relative to the plasma torch. This means may consist of clamps or equivalent system for gripping (securing) the substrate and maintaining it during the plasma projection at a selected position, and means for moving in rotation and in translation the surface of the substrate facing the plasma jet and in the longitudinal direction of the plasma jet. Thus, one can optimize the position of the surface to be coated, relative to the plasma jet, to obtain a homogeneous coating.

Direct injection can be achieved by means of a well-adapted injection system, for example using the device described above, of a stable suspension of nanoparticles, a solution called "sol" since it results from the synthesis of a colloid by sol-gel process involving the hydrolysis condensation of metal precursors (salts or alkoxides) allowing stabilization and easy dispersion of particles in their growth medium.

• la conservation de la taille et de la répartition granulométrique des nanoparticules ;
• la conservation de l'état cristallin du matériau projeté ;
• la conservation de la stoechiométrie initiale et de l'état d'homogénéité ;
• le contrôle de la porosité du film ;
• l'accès à des épaisseurs de revêtements submicroniques sans aucune difficulté, contrairement au procédé de projection thermique classique de l'art antérieur ;
• l'obtention d'un excellent et inhabituel rendement pondéral de projection thermique en limitant les pertes de matière, c'est-à-dire un rapport de masse déposée/masse projetée, supérieur à 80% en poids ;
• la réduction des températures auxquelles sont soumis les matériaux projetés, autorisant ainsi l'utilisation de compositions- sensibles thermiquement ;
• la possibilité, aujourd'hui inédite, de réaliser des dépôts sur des supports de toute nature et de toute rugosité comme du verre ou des wafers de silicium polis miroir (sur ces derniers la très faible rugosité de surface des substrats interdisait l'adhérence des revêtements) ;
• la capacité de réalisation par projection thermique de revêtements à composition SiO₂, composition jusqu'à présent inaccessible pour les procédés classiques ; et
• l'obtention de revêtements mécaniquement résistants et adhérents.

The main advantages of the present invention compared to the methods of the prior art are:

• preserving the size and particle size distribution of the nanoparticles;
• the preservation of the crystalline state of the projected material;
• the preservation of the initial stoichiometry and the state of homogeneity;
• the control of the porosity of the film;
• access to thicknesses of submicron coatings without any difficulty, unlike the conventional thermal spraying method of the prior art;
• obtaining an excellent and unusual weight efficiency of thermal spraying by limiting the losses of material, that is to say a mass ratio deposited / projected mass, greater than 80% by weight;
• reducing the temperatures to which the projected materials are subjected, thus allowing the use of thermally sensitive compositions;
• the possibility, today unpublished, of depositing on supports of any kind and roughness such as glass or mirror-polished silicon wafers (on the latter the very low surface roughness of the substrates prevented the adhesion of the coatings );
• the capacity for thermal projection of SiO ₂ composition coatings, composition hitherto inaccessible for conventional processes; and
• obtaining mechanically resistant and adherent coatings.

• Le revêtement de métaux et d'oxydes pour les rendre résistants à la corrosion. Pour cella, on peut utiliser par exemple un sol colloïdal tel que ceux décrits dans le document [8] pour mettre en oeuvre le procédé de l'invention.
• Le dépôt de revêtements composites résistants à l'abrasion. Pour cela, on peut utiliser par exemple un sol colloïdal tel que ceux décrits dans les documents [8], [9], [10] et l'exemple 2 ci-dessous pour mettre en oeuvre le procédé de l'invention.
• Le dépôt de revêtements résistants à haute température, tels que les dépôts de matériaux réfractaires et de revêtements composites. Pour cela, on peut utiliser par exemple un sol colloïdal tel que ceux décrits dans les documents [8], [9], [10] et l'exemple 2 ci-dessous pour mettre en oeuvre le procédé de l'invention.
• Le dépôt de revêtements qui interviennent dans des interactions de surfaces en mouvement relatif (tribologie), tels que les revêtements composites résistants à l'abrasion et/ou lubrifiants. Pour cela, on peut utiliser par exemple un sol colloïdal tel que ceux décrits dans le document [10] pour mettre en oeuvre le procédé de l'invention.
• Le dépôt de revêtements qui interviennent dans la conversion et le stockage d'énergie, tels que :
  • les revêtements qui interviennent dans la conversion photothermique de l'énergie solaire. Pour cela, on peut utiliser par exemple un sol colloïdal tel que ceux décrits dans l'exemple 2 ci-dessous pour mettre en oeuvre le procédé de l'invention.
  • les revêtements sous forme d'empilements de matériaux actifs, par exemple pour les électrodes et les électrolytes, par exemple pour pile à combustible à oxyde solide, les générateurs électrochimiques, par exemple les batteries au plomb, les batteries Li-ion, les supercondensateurs, etc. Pour cela, on peut utiliser par exemple un sol colloïdal tel que ceux décrits dans les documents [8] et [17] pour mettre en oeuvre le procédé de l'invention.
• Les revêtements qui interviennent dans des réactions de catalyse, par exemple pour la fabrication de catalyseurs supportés pour la dépollution des gaz, la combustion ou la synthèse. Pour cela, on peut utiliser par exemple un sol colloïdal tel que ceux décrits dans les documents [8] et l'exemple 2 ci-dessous pour mettre en oeuvre le procédé de l'invention.
• Le dépôt de revêtements qui interviennent en tant que microréacteurs chimiques ou biologiques. Pour cela, on peut utiliser par exemple un sol colloïdal tel que ceux décrits dans le document [10] pour mettre en oeuvre le procédé de l'invention.
• Le dépôt de revêtements sur des systèmes micro-électromécaniques (MEMS) ou micro-opto-électromécaniques (MOEMS), par exemple dans les domaines de L'automobile, des télécommunications, de l'astronomie, de l'avionique, et les dispositifs d'analyse biologique et médical.

The present invention has applications in all technical fields where it is necessary to obtain a nanostructured coating, because it allows the manufacture of such a coating of excellent quality in terms of fineness, homogeneity, thickness and particle size. By way of non-exhaustive examples, the present invention may be used in the following applications:

• The coating of metals and oxides to make them resistant to corrosion. For this purpose, it is possible to use, for example, a colloidal sol such as those described in document [8] for carrying out the process of the invention.
• The deposition of composite coatings resistant to abrasion. For this, one can use for example a colloidal sol such as those described in documents [8], [9], [10] and Example 2 below to implement the method of the invention.
• Deposition of high temperature resistant coatings, such as refractory deposits and composite coatings. For this purpose, it is possible to use, for example, a colloidal sol such as those described in documents [8], [9], [10] and Example 2 below to implement the method of the invention.
• The deposition of coatings that intervene in relative motion surface interactions (tribology), such as abrasion-resistant composite coatings and / or lubricants. For this, one can use for example a colloidal sol such as those described in document [10] to implement the method of the invention.
• The deposition of coatings involved in the conversion and storage of energy, such as:
  • coatings involved in the photothermal conversion of solar energy. For this purpose, it is possible to use for example a colloidal sol such as those described in Example 2 below to implement the process of the invention.
  • coatings in the form of stacks of active materials, for example for electrodes and electrolytes, for example for solid oxide fuel cells, electrochemical generators, for example lead-acid batteries, Li-ion batteries, supercapacitors, etc. For this, one can use for example a colloidal sol such as those described in documents [8] and [17] to implement the method of the invention.
• The coatings involved in catalysis reactions, for example for the manufacture of supported catalysts for gas depollution, combustion or synthesis. For this we can use for example a colloidal sol such as those described in documents [8] and Example 2 below to implement the method of the invention.
• The deposition of coatings that act as chemical or biological microreactors. For this, one can use for example a colloidal sol such as those described in document [10] to implement the method of the invention.
• The deposition of coatings on micro-electromechanical systems (MEMS) or micro-optoelectromechanical systems (MOEMS), for example in the fields of automobiles, telecommunications, astronomy, avionics, and biological and medical analysis.

The present invention thus also relates to an optical and / or electronic device comprising a nanostructured coating that can be obtained by the method of the invention, that is to say having the physical and chemical characteristics of the coatings obtained by the method of the invention.

The present invention thus also relates to a fuel cell comprising a nanostructured coating obtainable by the method of the invention, that is to say having the physical and chemical characteristics of the coatings obtained by the process of the invention. 'invention.

The present invention therefore also relates to a thermal barrier comprising a coating that can be obtained by the method of the invention, that is to say having the physical and chemical characteristics of the coatings obtained by the method of the invention.

Other features and advantages of the invention may appear on reading the following examples given for illustrative and non-limiting with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

• The figure 1 presents a simplified diagram of a part of the device for implementing the method of the invention for injecting the colloidal sol of nanoparticles in a plasma jet.
• The figure 2 presents a simplified diagram of a method of injecting the colloidal sol of nanoparticles into a plasma jet with a schematic representation of the plasma torch.
• The figure 3 presents an X-ray powder diffraction pattern of a zirconia.
• The figure 4 presents two photographs obtained by transmission electron microscopy on a zirconia sol.
• The figure 5 is an X-ray diffraction comparison graph of the crystalline structure of a coating deposited by the method of the present invention and the soil of ZrO ₂ nanoparticles of departure.
• The Figures 6a and 6b present photographs taken by transmission electron microscopy of the zirconia deposit: a. on the surface of the zirconia deposit, and b. in cross section.

EXAMPLES EXAMPLE 1 Process of the invention and coating obtained from a zirconia sol

An aqueous sol of zirconia (ZrO ₂ ) at 10% is injected into an argon-hydrogen blown arc plasma (75% by volume of Ar).

• d'une torche à plasma (3) à courant continu Sulzer-Metco F4 VB (marque de commerce), munie d'une anode de diamètre interne 6 mm,
• du système d'injection de liquide décrit sur la figure 1, et
• d'un dispositif (9) permettant de fixer et de déplacer le substrat à revêtir par rapport à la torche à une distance donnée (figure 2).

The experimental setup which made it possible to produce the nanostructured zirconia deposits is represented on the Figures 1 and 2 . It consists :

• a Sulzer-Metco F4 VB (trademark) continuous plasma (3) plasma torch with an anode of 6 mm internal diameter,
• of the liquid injection system described on the figure 1 , and
• a device (9) for fixing and moving the substrate to be coated with respect to the torch at a given distance ( figure 2 ).

Regarding the injection system, it comprises a reservoir (R) containing the colloidal sol (7) and a cleaning tank (N), containing a cleaning liquid (L) of the injector and the pipe (v). It also includes pipes (v) for conveying liquids from the tanks to the injector (I), pressure regulators (m) allowing to adjust the pressure in the tanks (pressure <2x10 ⁶ Pa). The assembly is connected to a compression gas (G), here air, to create in the pipes a network of compressed air. Under the effect of pressure, the liquid is conveyed to the injector.

With regard to the injection of liquid, the diameter of the outlet orifice (t) of the injector (I) is 150 μm and the pressure in the reservoir (R) containing the soil is 0.4 MPa, which involves a liquid flow of 20 ml / min and a speed of 16 m / s. The soil exits the injector in the form of a jet of liquid that mechanically fragments in the form of large drops of calibrated diameter ranging from 2 microns to 1 mm, on average two times greater than the diameter of the circular exit hole. The injector ( figure 2 ) can be inclined with respect to the axis of the plasma jet from 20 to 160 °. In the tests, a 90 ° inclination was used.

Regarding the initial soil, it is obtained according to the process described in document [8]. In this soil, the zirconia particles are crystallized in two phases, a monoclinic (ZrO ₂ m) and the other quadratic (ZrO ₂ q) more minority, as shown by the X-ray diffraction diagram presented on the figure 3 ("1" = intensity).

The average diameter of the crystallites, observed in transmission electron microscopy, is about 9 nm as shown by the photographs of the figure 4 (see example 2 below).

The zirconia deposits from the plasma projection are obtained at 70 mm from the intersection between the jet of liquid and the plasma jet. Different types of substrates have been tested to be coated: aluminum plates, silicon wafers or glass plates.

The deposition rate was 0.3 μm each time the torch passed the substrate.

Depending on the duration of the projection, the thickness of the deposits obtained was between 4 μm and 100 μm.

The figure 5 is an X-ray diffraction comparison graph ("I" = intensity) of the crystalline structure of a coating deposited by the process of the present invention (dp) and the soil of starting ZrO ₂ nanoparticles (Sol).

• 61% de monoclinique et 39% de quadratique au départ, et
• 65% de monoclinique et 35% de quadratique dans le revêtement obtenu.

Usually in plasma projection, the projected zirconia is in the deposit in the quadratic form, with a small amount of monoclinic corresponding to the unmelted or partially melted particles, irrespective of the initial phase. Here, the structure and the proportion of the crystalline phases present in the deposit are almost the same as those of the starting soil:

• 61% monoclinic and 39% quadratic at baseline, and
• 65% monoclinic and 35% quadratic in the coating obtained.

The size of the crystals in the coating (deposit) is between 10 and 20 nm; it is very close to that of particles of the starting soil.

Transmission electron microscopy (TEM) observations of the interface between the silicon substrate and the deposition (cross section) show a good adhesion of the zirconia particles to the mirror polished surface.

In addition, the surface condition of the substrate does not interfere with the adhesion of the plasma deposit.

Example 2

The zirconia sol of Example 1, having specific properties of the present invention (dispersion and stabilization), is projected into a plasma jet as described in Example 1.

This zirconia sol consists of crystalline nanoparticles in the monoclinic phase and in the quadratic phase. A size distribution was made from transmission electron microscopy (TEM) micrographs of the zirconia sol. The average diameter of the zirconia particles is 9 nm. The right photograph on the figure 4 annexed presents a photograph taken by transmission electron microscopy on this zirconia sol used. The line at the bottom left indicates the scale of the shot. This line represents 10 nm in the photograph.

The deposition carried out by plasma spraying of said sol according to the process of the invention consists, according to the surface and thickness transmission microscopy (TEM) analysis, of zirconia nanoparticles of morphology similar to those of starting and average diameter of 10 nm. These measures are deductible from Figures 6a and 6b attached. The line at the bottom right of these snapshots indicates the scale of the snapshot. This line represents 100 nm on the top photograph ( figure 6a ), and 50 nm on the bottom photograph ( figure 6b ).

There is therefore no chemical modification of the projected particles by the process of the present invention.

An X-ray diffraction analysis of the particles of the starting zirconia soil (soil) (discontinuous line) is compared with that of the deposit obtained by plasma projection of the same zirconia sol (dp) (solid line). This analysis is represented on the figure 5 appended (abscissa: intensity, ordinate: θ). The size of the crystallites and the distribution of the crystalline phases were determined by resolution of X-ray diagrams according to the Rietveld method.

The zirconia sol, like the zirconia deposit resulting from this sol, has crystallites of identical diameter and crystallized according to the same two monoclinic and quadratic phases. The following table shows the distribution in% of these crystalline phases present in the zirconia sol and the zirconia deposit, as well as their size. Materials Distribution of crystalline phases Sizes of crystallites monoclinic Quadratic monoclinic Quadratic Sol ZrO ₂ 65% 35% 11.8 nm 8.9 nm ZrO ₂ deposit 61% 39% 12 nm 8.9 nm

These results clearly demonstrate that the size and proportion of crystallized nanoparticles in the monoclinic phase and in the quadratic phase are typically the same in the starting soil and the projected deposit. This innovative specificity of conservation of the intrinsic properties of the sol in the plasma deposition is the result of the use, according to the process of the present invention, of a dispersed and stabilized colloidal suspension which does not evolve during thermal spraying.

Example 3 Preparation of Nanoparticle Soil

This example illustrates one of the many modes of preparation of a nanoparticle sol that can be used to implement the present invention.

A colloidal solution of titanium dioxide TiO ₂ is prepared by adding dropwise a solution of titanium tetraisopropoxide (0.5 g) dissolved in 7.85 g of isopropanol to 100 ml of an acid solution. dilute hydrochloric acid (pH = 1.5) with vigorous stirring. The mixture obtained is kept under magnetic stirring for 12 hours.

Transmission electron microscopy observations reveal an average colloid diameter of about 10 nm. The X-ray pattern is characteristic of that of titanium oxide in anatase form.

The pH of this sol is about 2 and the mass concentration of TiO ₂ is brought to 10% by distillation (100 ° C, 10 ⁵ Pa).

Before being used in the process of the invention, the colloidal solution of nanoparticles can be filtered, for example to 0.45 μm.

BIBLIOGRAPHIC REFERENCES

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Claims (22)

1. Method of coating a surface of a substrate with nanoparticles, characterized in that it comprises an injection of a colloidal sol wherein said nanoparticles are dispersed and stabilized, into a thermal plasma jet that sprays them onto said surface; in that said nanoparticles are of a metal oxide chosen from the group comprising SiO₂, ZrO₂, TiO₂, Ta₂O₅, HfO₂, ThO₂, SnO₂, VO₂, In₂O₃, CeO₂, ZnO, Nb₂O₅, V₂O₅, Al₂O₃, Sc₂O₃, Ce₂O₃, NiO, MgO, Y₂O₃, WO₃, BaTiO₃, Fe₂O₃, Fe₃O₄, Sr₂O₃, (PbZr)TiO₃, (BaSr)TiO₃, Co₂O₃, Cr₂O₃, Mn₂O₃, Mn₃O₄, Cr₃O₄, MnO₂, RuO₂ or a combination of these oxides by doping the particles or by mixing; and in that said nanoparticles have been stabilized directly in the solvent used during their synthesis or peptized subsequent to their synthesis.
2. Method according to Claim 1, in which the nanoparticles have a size from 1 to 100 nm.
3. Method according to Claim 1, in which the sol is prepared by precipitation in an aqueous medium or by sol-gel synthesis in an organic medium from a nanoparticles precursor.
4. Method according to Claim 3, in which the nanoparticles precursor is chosen from the group comprising a metalloid salt, a metal salt, a metal alkoxyde, or a mixture thereof.
5. Method according to Claim 4, in which the metal or metalloid of the salt or of the metal alkoxyde of the nanoparticles precursor is chosen from the group comprising silicon, titanium, zirconium, hafnium, aluminium, tantalum, niobium, cerium, nickel, iron, zinc, chromium, magnesium, cobalt, vanadium, barium, strontium, tin, scandium, indium, lead, yttrium, tungsten, manganese, gold, silver, platinum, palladium, nickel, copper, cobalt, ruthenium, rhodium, europium and other rare earths.
6. Method according to Claim 1, in which the sol is a mixed sol.
7. Method according to Claim 1, in which the sol further includes metal nanoparticles of a metal chosen from the group comprising gold, silver, platinum, palladium, nickel, ruthenium and rhodium, or a mixture of various metal nanoparticles consisting of these metals.
8. Method according to Claim 1, in which the sol further includes organic molecules.
9. Method according to Claim 8, in which the organic molecules are molecules for stabilizing the nanoparticles in the sol and/or molecules that functionalize the nanoparticles.
10. Method according to Claim 1, in which the colloidal sol is injected into the plasma jet in the form of drops.
11. Method according to Claim 1, in which the plasma jet is an arc-plasma jet.
12. Method according to Claim 1, in which the plasma jet is such that it causes partial melting of the injected nanoparticles.
13. Method according to Claim 1, in which the plasma constituting the jet has a temperature ranging from 5000 K to 15000 K.
14. Method according to Claim 1, in which the plasma constituting the jet has a viscosity ranging from 10^-4 to 5 × 10^-4 kg/m.s.
15. Method according to Claim 1, in which the plasma jet is generated from a plasma-forming gas chosen from the group comprising Ar, H₂, He and N₂.
16. Method according to any one of the preceding claims, wherein the coating is a nanostructured coating.
17. Method according to Claim 16, wherein the coating has a thickness ranging from 0.1 to 50 µm.
18. Method according to Claim 16 or 17, wherein the coating consists of grains with a size of less than or of the order of 1 micron.
19. Method according to any one of claims 16 to 18, wherein said substrate consists of an organic, inorganic or hybrid material.
20. Method according to any one of claims 16 to 19, wherein the nanostructured coating is part of an optical and/or electronic device.
21. Method according to any one of claims 16 to 19, wherein the nanostructured coating is part of a fuel cell.
22. Method according to any one of claims 16 to 19, wherein the nanostructured coating is part of a thermal barrier.

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