Nanosturctured Coating and Coating Method

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

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified diagram of part of the device for implementing the method of the invention, allowing the colloidal sol of nanoparticles to be injected into a plasma jet.

FIG. 2 shows a simplified diagram of a way of injecting the colloidal sol of nanoparticles into a plasma jet with a schematic representation of the plasma torch.

FIG. 3 shows an X-ray powder diffraction diagram of a zirconia.

FIG. 4 shows two micrographs obtained by transmission electron microscopy on a zirconia sol.

FIG. 5 is a graph comparing by X-ray diffraction the crystalline structure of a coating deposited by the method of the present invention with that of the initial sol of ZrO₂ nanoparticles.

FIGS. 6 a and 6b show micrographs taken in transmission electron microscope of the zirconia coating: a) at the surface of the zirconia coating; and b) in cross section.

EXAMPLES

Example 1

Method of the Invention and Coating Obtained from a Zirconia Sol

An aqueous 10% zirconia (ZrO₂) sol was injected into an argon/hydrogen (75 vol % Ar) transferred (blown)-arc plasma.

The experimental set-up used for producing the nanostructured zirconia coatings is shown in FIGS. 1 and 2. It consisted of:

• • a Sulzer-Metco F4 VB (trade mark) DC plasma torch (3) fitted with an anode of 6 mm inside diameter;
  • the device for injecting the liquid, described in FIG. 1; and
  • a device (9) for fixing and for moving the substrate to be coated relative to the torch at a given distance (FIG. 2).

With regard to the injection device, this comprised a container (R) containing the colloidal sol (7) and a cleaning container (N) containing a cleaning liquid (L) for cleaning the injector and the pipework (v). It also included pipes (v) for conveying the liquids from the containers to the injector (I), pressure-reducing valves (m) for adjusting the pressure in the containers (pressure>2×10⁶ Pa). The assembly was connected to a compression gas (G), here air, allowing a compressed-air supply to be created in the pipes. Under the effect of the pressure, the liquid was conveyed to the injector.

As regards the liquid injection, the diameter of the outlet orifice (t) of the injector (I) was 150 μm and the pressure in the container (R) containing the sol was 0.4 MPa. This implied a liquid flow rate of 20 ml/min and a speed of 16 m/s. The sol was expelled from the injector in the form of a liquid jet that fragmented mechanically into the form of coarse drops having a calibrated diameter ranging from 2 μm to 1 mm, on average twice as large as the diameter of the circular outlet hole. The injector (FIG. 2) could be inclined to the axis of the plasma jet at an angle ranging from 20 to 160°. In the trials, the angle of inclination used was 90°.

The initial sol was obtained according to the method described in document [8]. In this sol, the zirconia particles were crystallized in two phases, one monoclinic (m.ZrO₂) and the other, less significant tetragonal (t.ZrO₂) as the X-ray diffraction diagram given in FIG. 3 shows (I=intensity).

The mean diameter of the crystallites, observed in TEM (transmission electron microscopy) was about 9 nm as the micrographs in FIG. 4 show (see Example 2 below).

The zirconia coatings obtained from plasma spraying were obtained at 70 mm from the intersection between the liquid jet and the plasma jet. Various types of substrates to be coated were tested: aluminium wafers, silicon wafers and glass plates.

The deposition rate was 0.3 μm for each pass of the torch in front of the substrate.

Depending on the spray time, the thickness of the coatings obtained were between 4 μm and 100 μm.

FIG. 5 is a graph comparing by X-ray diffraction (I=intensity) the crystalline structure of a coating deposited by the method of the present invention (dep) and of the initial sol of ZrO₂ nanoparticles (sol).

Usually in plasma spraying, the zirconia sprayed is in the tetragonal form in the coating, with a small amount of monoclinic corresponding to unmelted or partially melted particles, whatever the initial phase. Here, the structure and the proportion of the crystalline phases present in the coating were practically the same as those of the initial sol:

• • 61% monoclinic and 39% tetragonal initially; and
  • 65% monoclinic and 35% tetragonal in the coating obtained.

The size of the crystals in the coating was between 10 and 20 nm, and was very close to that of the particles of the initial sol.

The TEM observations of the interface between the silicon substrate and the coating (cross section) showed good adhesion of the zirconia particles to the mirror-polished surface.

Furthermore, the surface finish of the substrate had no effect on the adhesion of the plasma coating.

Example 2

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

This zirconia sol consisted of nanoparticles crystallized in monoclinic phase and in tetragonal phase. The size distribution was obtained from TEM micrographs of the zirconia sol. The mean diameter of the zirconia particles was 9 nm. The micrograph on the right in appended FIG. 4 is a TEM micrograph taken on this zirconia sol used. The bar at the bottom left indicates the scale of the micrograph, here representing 10 nm in the micrograph.

The coating produced by plasma spraying said sol according to the method of the invention consisted, using TEM surface and thickness analysis, of zirconia nanoparticles having a morphology similar to those of the initial sol and with a mean diameter of 10 nm. These measurements can be deduced from the appended FIGS. 6a and 6b. The bar at the bottom right of these micrographs indicates the scale of the micrograph, here 100 nm in the upper micrograph (FIG. 6a) and 50 nm in the lower micrograph (FIG. 6b).

The particles sprayed by the method of the present invention were therefore not chemically modified.

X-ray diffraction analysis of the initial zirconia sol particles (sol) (broken line) was compared with that of the coating obtained by plasma spraying the same zirconia sol (dep) (continuous line). This analysis is shown in appended FIG. 5 (y-axis: intensity; x-axis: 2θ). The crystallite size and the distribution of the crystalline phases were determined by resolving the X-ray diagrams using the Rietveld method.

The zirconia sol as the zirconia coating obtained from this sol had crystallites of the same diameter and were crystallized in the same two, monoclinic and tetragonal, phases. The table below gives the distribution in % of these crystalline phases present in the zirconia sol and the zirconia coating, and also their size.

	Distribution of the
	crystalline phases	Crystallite sizes
Materials	Monoclinic	Tetragonal	Monoclinic	Tetragonal
ZrO2 Sol	65%	35%	11.8 nm	8.9 nm
ZrO2	61%	39%	12 nm	8.9 nm
coating

These results clearly show that the size and the proportion of nanoparticles crystallized in the monoclinic phase and in the tetragonal phase are typically the same in the initial sol and the sprayed coating. This innovative specific feature in which the intrinsic properties of the sol are maintained in the plasma coating is the result of using, according to the method of the present invention, a dispersed and stabilized colloidal suspension that does not change during thermal spraying.

Example 3

Preparation of a Nanoparticle Sol

This example illustrates one of many ways of preparing a nanoparticle sol that can be used for implementing the present invention.

A colloidal solution of titanium oxide TiO₂ was prepared by adding, drop by drop, a titanium tetraisopropoxide solution (0.5 g) dissolved in 7.85 g of isopropanol to 100 ml of a dilute hydrochloric acid solution (pH=1.5) with vigorous stirring. The mixture obtained was kept magnetically stirred for 12 hours.

Transmission electron microscopy observations showed a mean diameter of the colloids of about 10 nm. The X-ray diagram was characteristic of that of titanium oxide in anatase form.

The pH of this sol was about 2 and the mass concentration of TiO₂ was brought to 10% by distillation (100° C./10⁵ Pa).

Before being used in the method of the invention, the colloidal nanoparticle solution could be filtered, for example to 0.45 μm.

LITERATURE REFERENCES

• [1] U.S. Pat. No. 5,032,568, Lau et al, 1991.
• [2] U.S. Pat. No. 4,982,067, Marantz et al, 1991.
• [3] U.S. Pat. No. 5,413,821, Ellis et al, 1995.
• [4] U.S. Pat. No. 5,609,921, Gitzhofer et al, 1997.
• [5] U.S. Pat. No. 6,447,848, Chow et al, 2002.
• [6] WO-A-97/18341, Kear et al, 1997.
• [7] N. P. Rao, H. J. Lee, D. J. Hansen, J. V. R. Heberlein, P. H. McMurry and S. L. Girshick, “Nanostructured Materials production by Hypersonic Plasma Particle Deposition”, Nanostructured Materials, 1997, 9, pp. 129-132.
• [8] FR-A-2 707 763 (CEA), H. Floch and P. Belleville, “Matériau composite á indice de réfraction élevé, procédé de fabrication de ce matériau composite et matériau optiquement actif comprenant ce matériau composite [High-refractive-index composite material, process for manufacturing this composite material, and optically active material comprising this composite material]”.
• [9] FR-A-2 682 486 (CEA), H. Floch and M. Berger, “Miroir diélectrique interférentail et procédé de fabrication d'un tel miroir [Interference dielectric mirror and process for manufacturing such a mirror]”.
• [10] FR-A-2 703 791 (CEA), P. Belleville and H. Floch, “Procédé de fabrication de couches minces preésentant des propriétés optiques et de résistance á l'abrasion [Process for manufacturing thin films having optical and abrasion resistance properties]”.
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• [13] “Functional Hybrid Materials”, P. Gomez-Romero and C. Sanchez, Wiley-VCH Publishers, 2004.
• [14]“Plasmas thermiques: Production et applications [Thermal plasma: Production and Applications]”, P. Fauchais, Techniques de l'ingénieur, Traite Génie Électrique [Electrical Engineering Treaty], D2820-1 to D2820-25.
• [15] “Characterizations of LPPS processes under various spray conditions for potential applications”, A. Refke, G. Barbezat, J L. Dorier, M. Gindrat and C. Hollenstein, Proc Of International Thermal Spray, Conference 2003, Orlando, Fla., USA, 5-8 May 2003.
• [16] “New applications and new product qualities by radiofrequency plasma spraying”, R. Henne, V. Borck, M. Müller, R. Ruckdäsch and G. Schiller, Proc. Of Tagunsband Proceedings, United Thermal Spray Conference, Düseldorf, 17-19 Mar. 1999.
• [17] S. Somiya, M. Yoshimura, Z. Nakai, K. Hishinuma and T. Kumati, “Hydrothermal processing of ultrafine single crystal zirconia and hafnia powders with homogenous dopants”, Advances in ceramics, 21, 1987, pp. 43-55.

Claims

1. A method of coating a surface of a substrate with nanoparticles, wherein said method comprises injecting a colloidal sol of said nanoparticles, into a thermal plasma jet that sprays the colloidal sol of said nanoparticles onto said surface.

2. The method according to claim 1, in which the nanoparticles are dispersed and stabilized in the colloidal gel.

3. The method according to claim 1, in which the nanoparticles have a size from 1 to 100 nm.

4. The 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.

5. The method according to claim 4, in which the nanoparticles precursor is chosen from the group comprising a metalloid salt, a metal salt, a metal alkoxide, or a mixture thereof.

6. The method according to claim 5, in which the metal or metalloid of the salt or of the alkoxide of the nanoparticles precursor is chosen 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.

7. The method according to claim 1, in which the sol is prepared by synthesizing a solution of metal nanoparticles from a metal nanoparticles precursor using an organic or mineral reducing agent in solution, by a method chosen from the group comprising a reduction of metal slats in an emulsion medium and chemical reduction of organometallic or metallic precursors or of metal oxides.

8. The method according to claim 7, in which the reducing agent is chosen from the group comprising polyols, hydrazine and its derivatives, quinone and its derivatives, hydrides, alkali metals, cysteine and its derivatives, and ascorbate and its derivatives.

9. The method according to claim 7, in which the metal nanoparticles precursor is chosen from the group comprising salts of metalloids or metals such as gold, silver, platinum, palladium, nickel, copper, cobalt, aluminum, ruthenium and rhodium, or the various metal alkoxides of these metals.

10. The method according to claim 1, in which the sol is a mixed sol.

11. The method according to claim 1, in which the sol comprises nanoparticles 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.

12. The method according to claim 11, 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.

13. The method according to claim 1, in which the sol further includes organic molecules.

14. The method according to claim 13, in which the organic molecules are molecules for stabilizing the nanoparticles in the sol and/or molecules that functionalize the nanoparticles.

15. The method according to claim 1, in which the colloidal sol is injected into the plasma jet in the form of drops.

16. The method according to claim 1, in which the plasma jet is an arc-plasma jet.

17. The method according to claim 1, in which the plasma jet is such that it causes partial melting of the injected nanoparticles.

18. The method according to claim 1, in which the plasma constituting the jet has a temperature ranging from 5000 K to 15000 K.

19. The 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.

20. The method according to claim 1, in which the plasma jet is generated from a plasma-forming gas chosen from the group consisting Ar, H2, He and N2.

21. A nanostructured coating obtainable by a method according to claim 1.

22. The nanostructured coating according to claim 21 having a thickness ranging from 0.1 to 50 μm.

23. The nanostructured coating according to claim 21, consisting of grains with a size of less than or of the order of 1 micron.

24. A substrate having at least one surface coated with the nanostructured coating according to claim 21.

25. The substrate according to claim 24, said substrate consisting of an organic, inorganic or hybrid material.

26. A device comprising the nanostructured coating according to claim 21.

27. A fuel cell comprising the nanostructured coating according to claim 21.

28. A thermal barrier comprising the nanostructured coating according to claim 21.

29. A device for implementing the method of claim 1, said device comprising: a thermal plasma torch capable of producing a plasma jet;a container containing a plasma-forming gas;a container containing a colloidal sol of dispersed stabilized nanoparticles;a means for fixing and for positioning the substrate relative to the plasma torch;an injection system connecting the colloidal sol container and, an injector whose end is microperforated with a hole for injecting the colloidal sol into the plasma jet generated by the plasma torch; anda pressure-reducing valve for adjusting the pressure inside the container.

30. The device according to claim 29, in which the plasma torch is an arc-plasma torch.

31. The device according to claim 29, in which the plasma torch is capable of producing a plasma jet having a temperature ranging from 5000 K to 15000 K.

32. The device according to claim 29, in which the plasma torch is capable of producing a plasma jet having a viscosity ranging from 10−4 to 5×10−4 kg/m.s.

33. The device according to claim 29, in which the inclination of the injector to the longitudinal axis of the plasma jet may vary from 20 to 160°.

34. The device according to claim 29, in which the injector makes it possible to form drops of the colloidal sol, which drops into the plasma jet when the plasma torch is actuated.

35. The device according to claim 29, in which the hole of the injector is circular.

36. The device according to claim 29, in which the hole of the injector has a diameter ranging from 10 to 500 μm.

37. The device according to claim 29, in which the plasma-forming gas is chosen from the group comprising Ar, H2, He and N2.

38. The device according to claim 29, which further includes a container containing a cleaning solution, said container being connected via an injection system to the injector.

39. The device according to claim 26, wherein said device is an optical device.

40. The device according to claim 26, wherein said device is an electronic device.

41. A device comprising the substrate according to claim 24.

42. The device according to claim 41, wherein said device is an optical device.

43. The device according to claim 41, wherein said device is an electronic device.

44. A fuel cell comprising the substrate according to claim 24.

45. A thermal barrier comprising the substrate according to claim 24.