WO1996015068A1

WO1996015068A1 - Vanadium dioxide microparticles, method for preparing same, and use thereof, in particular for surface coating

Info

Publication number: WO1996015068A1
Application number: PCT/FR1995/001450
Authority: WO
Inventors: Pierre Legrand; Jean-Raymond Gavarri; Jean-Christophe Valmalette; Gilbert Vacquier; Daniel Lefevre
Original assignee: Les Peintures Jefco
Priority date: 1994-11-09
Filing date: 1995-11-03
Publication date: 1996-05-23
Also published as: FR2726545A1; FR2726545B1; CN1058469C; RU2162057C2; CA2204910A1; AU4119596A; JPH10508573A; KR970707045A; EP0790959A1; CN1162949A

Abstract

Vanadium dioxide microparticles having formula V1-xMxO2, wherein 0 « x « 0.05 and M is a doping metal, and being characterised in that they have a particle size of less than 10 νm, a method for preparing same, and the use of said microparticles, in particular for surface coating, are disclosed.

Description

Vanadium dioxid microparticles, process for obtaining said microparticles and their use, in particular for surface coatings.

The subject of the present invention is microparticles of vanadium dioxide, a process for obtaining said microparticles and their applications, in particular for surface coatings in which they are incorporated.

In a first aspect, the invention relates to vanadium dioxide microparticles of formula N ₁ _ _x M _x O2 in which 0 ≤ x s. 0.05 and M is a doping metal, said microparticles having a particle size less than 10 μm, in particular less than 5 μm, preferably of the order of 0.1 to 0.5 μm.

The doping metal can be chosen from transition elements offering an ionic radius greater than that of vanadium such as for example Νb or Ta or an electronic contribution such as for example Mo or W, W and Mo being preferred.

In a preferred aspect, the microparticles according to the invention consist of doped vanadium dioxide of formula Nι_ _x W _x O2 in which x is between 0 and 0.02. The vanadium dioxide microparticles according to the invention can in particular be used in the technical sector of coating compositions intended to be deposited essentially in thin layers in the form of film or film, such as paints, varnishes and any type of coating that one can deposit in successive layers. The object of the invention is therefore in particular to use the microparticles of vanadium dioxide described above to produce an "intelligent" material which automatically reduces the transmission of solar radiation in the field of infrared rays, when the material reaches a given temperature level. It is thus possible to benefit from the energy of the infrared below the set temperature and to eliminate excessive heating above this temperature.

One of the main applications of the vanadium dioxide microparticles according to the invention is their use in coatings intended to be affixed to the facades of buildings exposed to the weather. Dark-colored coatings exposed to the sun's rays heat up much more than light-colored coatings. They therefore undergo very high amplitude expansion-shrinkage cycles which cause premature degradation of the coating film. To date, it is therefore not possible to guarantee a dark paint whose luminance is less than 35. This can be limited by adding a vanadium dioxide pigment in the paint which the transition temperature is set in the range of 25 ^° C for example.

Another application is that of protecting transparent or translucent surfaces which must allow visible radiation to pass through, such as in greenhouses, verandas, residential glazing, but for which the interior temperature is to be controlled; such use can also be envisaged in the context of glazing and the bodywork of cars or any transport vehicle. In summer, by reducing the entry of incident solar energy into buildings, the coating reduces the need for air conditioning and, on the other hand, in winter, the coating limits heat dissipation to the outside. Thus the coating advantageously allows energy savings.

Indeed, one of the objectives of the present invention is to be able to control the transfer and absorption of heat energy on the surface of a wall, without the need to transform or treat in a specific way the material of that - Here, but by depositing according to any known process a coating, as is done with paints, said coating according to the invention can itself be such a paint, which allows an implementation and an economical manufacture.

However, various molecular or ionic compounds are known which, under the effect of a temperature variation, can change optical properties, mainly color, linked to a change in electronic structure: such compounds are said to be thermochromic. By extension, thermochromium is also called a compound which has the property of absorbing and and reflecting differently depending on the temperature certain types of radiation due to a change in electronic structure. Vanadium dioxide has thus been studied for a few years, which has a structural transition at a temperature T {= 341 * Kelvin or 68 * C: below T _t , the crystal structure is monoclinic, while above from T _t , the structure is of rutile type. This transition is associated with a sudden change in electronic properties: the compound thus passes from the insulating state when the temperature is lower than T to the metallic state when the temperature is higher than T _t ; optically, this This change results in profound changes in the absorption and reflectance properties in the near and far infrared.

In the following description, the term "vanadium dioxide" will include vanadium dioxide commonly known as VO2 or V2O4. Various studies have recently been carried out on this compound such as those which can be noted in the publications SM Babulanam, Mat. Opt. Ground. Iight Tcchn. 692 (1986) 8 and JC Nalmalette, Sol. Energy Mater 33 (1994) 135. Studies have thus been carried out on thin layers of vanadium dioxide deposited on various substrates: they have in particular revealed the practical interest of the development of a material transparent to light but do not letting the infrared part of the solar spectrum pass only at low temperature. In fact, vanadium dioxide currently seems to be the only compound for which the transition is in a temperature and wavelength range specific to the thermal regulation of the habitat. In addition, this compound has the additional advantage of being able to undergo chemical substitutions by suitable atoms as defined below and allowing a displacement of the temperature T { ^veτs • ^cs bass- ⁶ - ⁵ temperatures.

Thus, numerous tests and research have been developed to produce thin layers of vanadium dioxide deposited on substrates, with a view in particular to the study of optical transmittance in the visible and near infrared; for this, various deposition techniques have been envisaged, such as sputtering under vacuum, evaporation under beam, chemical vapor deposition and the "sol-gel" process. According to the "sol-gel" process, the vanadium dioxide is prepared from tetravalent vanadium by dissolution in a solvent, hydrolysis and condensation to gradually form a sol, then, by evaporation of the solvent, formation of a gel which is then subjected to a heat treatment to give NO2, under a finely controlled atmosphere. It is possible to form a NO2 film directly on a substrate, by dipping an appropriate substrate in the soil. The gel thus forms directly on the substrate. Such a process by wetting or "dip coating" is described in particular in US Pat. No. 4,957,725. It is however difficult to control the quality of the final film deposited, to place the complete part or even its surface at high temperature in a uniform manner and to control the interactions between the support and the gel thus deposited, etc. Thus, d On the one hand, such methods which do not apply to already made materials do not really allow application on very large surfaces as can be done with a surface coating composition such as a paint and, d On the other hand, the results obtained are not repetitive or reliable. In addition, it is a very expensive process when it comes to large areas.

There are also dry processes, very long (of the order of fifteen days) and therefore very expensive, which only allow to obtain grain molecules of the order of 30 microns and more, which does not is not compatible with incorporation into a paint without modifying the color, does not allow a homogeneous mixing and does not provide the property of optical transition.

The problem is therefore to be able to obtain a powder of small particle size comprising essentially vanadium dioxide undoped or doped, in particular with tungsten, which can in particular be incorporated into a liquid or viscous support with a view to obtaining a coating of area.

In a second aspect, the invention therefore relates to a process for obtaining vanadium dioxide microparticles of formula V} _ _x M _x O2 in which M is a doping metal and 0 ≤ x ≤ 0.02, by pyrolysis of doped or undoped ammonium hexavanadate, characterized in that said pyrolysis is carried out at a temperature between approximately 400 * C and approximately 650 ^* C, with a rate of temperature rise of at least 100'C / min , and in that the gases from said pyrolysis are kept in confinement and in direct communication with the reaction medium for a period of at least 1/2 h, preferably 1 h.

The use of ammonium hexavanadate (NH ^ N6 χ6 is known in the industry for the manufacture of N2O5 commonly used as a catalyst, but in which tetravalent vanadium is considered as a non-catalytic impurity which is sought l The trials which could have been carried out with this precursor to also obtain vanadium dioxide alone were unsuccessful since V2O3 was then obtained and all the publications known to date affirm that it was not possible to obtain pure vanadium dioxide. The implementation of the conditions characteristic of the pyrolysis process according to the invention, namely:

- a temperature rise rate of at least 100 * C min, preferably 200 * C min or 300 * C / min, and - the non-evacuation of the gases resulting from the thermal decomposition of ammonium hexavanadate, in particular NH3, kept in confinement and in communication with the reaction medium for at least 5 minutes, preferably between 1 2 h and 2 h, and at most during the entire duration of the synthesis, makes it possible to obtain a complete reaction without formation of Residual N2 5 according to the following reaction scheme:

(ΝH4) 2 ₆ 0 ₁₆ → NH3 + V ₂ 0 ₅ - NO2 + H ₂ O + Ν ₂ . It has indeed been observed that at a rapid pyrolysis rate, a "flash" reaction is produced, producing N2O which decomposes slowly by reacting on the excess NH3 to form H2O and N2- Furthermore, to date, in most existing ovens, either sweep the reaction chamber, which takes the NH3 gas released and then stops the reduction which produces only VgOi _j and does not allow to go to the vanadium oxide VO2, N2O4 _> or on the contrary, other processes add ΝH3 by circulation, which creates an excessive reduction and brings about the reaction until N2O3 and a mixture of several vanadium oxides are obtained.

Advantageously, the gases resulting from the thermal decomposition of ammonium hexavanadate are collected in a gas bag under slight overpressure, for example of about 0.5 bar, preferably placed at a height lower than that of the reactor. The pyrolysis temperature must be between approximately 400 ° C. and approximately 650 ° C., preferably 635 ° C. If the temperature is greater than about 650 ^* C, the V2O5 present in the reaction medium may melt before reacting. On the other hand, a reaction temperature below about 400 ° C leads to non-thermochromic VO2 (B). In the case of the preparation of doped vanadium dioxide microparticles, this duration must also be fixed so as to obtain doping homogeneity while preventing grain growth by optimizing the temperature-time compromise. For example, for a doping rate of 5% of W: temperature 600 * C 650 * C 700 * C minimum time 6 h 3 h 1 h maximum time 60 h 12 h 6 h If you want to prepare microparticles of vanadium dioxide having a different structural transition temperature of 68 ^° C (corresponding to pure vanadium oxide), it is necessary to dope it with a substitute product for which stable valency must be greater than 4.

In a preferred aspect, a metal chosen from Nb, Ta, Mo and W will be used as substitution product, W and Mo being preferred.

Substitution with tungsten (W) makes it possible to obtain a final product having a significant temperature variation slope as a function of the percentage of substitution: a significant slope in fact makes it possible to cover a fairly wide range of temperatures. Thus, for examples of the value of x below, we obtain a transition temperature of: x = 0% 1% 2% 3%

Tt = 68 * C 40 * C 12 * C - 16 * C

In a preferred aspect of the process, ammonium hexavanadate is therefore used, doped with a metal chosen from Nb, Ta, Mo and W, W and Mo being preferred.

In the following description, the term "doping with a metal" means said metal being as defined above, the doping carried out using the metal in pure form or in the form of a compound containing it, such as in particular a tungstate or a molybdate. The ammonium hexavanadate used in the process according to the invention is commercially available. It can also be prepared in a known manner from ammonium metavanadate.

When it is desired to prepare microparticles of doped vanadium dioxide, it is therefore possible either to dop ammonium hexavanadate, or to incorporate the doping metal during the synthesis of hexavanadate from ammonium metavanadate.

The use of tungsten as a substitute is also advantageous insofar as the ammonium tungstate is very soluble in water. In particular, when it is desired to incorporate the tungsten into ammonium hexavanadate already synthesized, the ammonium tungstate can be easily dissolved in water with the ammonium hexavanadate, with a minimum of humidification 20% by mass to obtain a homogeneous ground paste.

Chemical substitution or doping is thus carried out by pyrolysis of the mixture of hcxavanadate and ammonium tungstate precursors according to the following reduction and substitution reaction:

(1 - x) (NH4) 2 V ₆ 0i6 + * / 2 (NH ^ H ₂ W ₁₂ O ₄₀ , y H ₂ O → 6 (Vχ_ _x W, -) O2 + n N2 + m H ₂ 0.

The choice of x for a homogeneous result is an exact stoichiometric calculation which makes it possible to obtain the temperature variation which one wishes compared to the transition temperature of 68 * C of vanadium dioxide: one can notice for this one that the slope δ-j / dx = - 28 in 10 ² K / mole, either in fact for x = 0.01 or 1%, δ _j = -28 * C.

According to an advantageous aspect of the process according to the invention, the ammonium hexavanadate is subjected before degassing to degassing at a temperature below the decomposition temperature of the ammonium hexavanadate, in particular below about 230 ° C. and, preferably of the order of 200 ° C., and by pumping under primary vacuum for at least 1 min, for example 15 min.

At the end of the pyrolysis, the vanadium dioxide obtained can advantageously be subjected to an annealing step under inert gas, at a temperature of at least 600 ° C., for a period of at least 1 h, for example 5 h. For example, this annealing step could be carried out at 600 ° C. for

2 p.m. At 800 ° C. for 5 h the grain growth is greater than 5 μ.

In a preferred aspect of the process according to the invention, and optionally after the annealing step mentioned above, the vanadium dioxide will be cooled under inert gas to a temperature of about 120 ° C. The cooling rate can be, for example, around 150 ° C. min at 250 ° C./min.

As the inert gas, nitrogen or argon will be used, for example. When leaving vanadium dioxide in the open air, the temperature should be at most around 100 ° C in order to avoid water uptake and therefore a risk of surface reoxidation. An example of an oven for carrying out the method according to the invention is shown in FIG. 1 in which, in addition to mounting a mobile tubular oven (2) on a rail (3) relative to the treatment chamber (1) consisting of a quartz tube, which makes it possible to obtain the desired rapid temperature setting, a gas bag (4) is shown which allows the gases resulting from the thermal decomposition of hexavanadate d to be kept in confinement ammonium, in particular NH3, above the grains to be treated in the chamber (1). This maintains a partial ammonia pressure sufficient for the reduction reaction to VO2 to be complete. The heavier N2O gas is drawn downwards into said gas bag (4), which makes it possible to increase and optimize the purity of the compounds obtained.

In FIG. 1 are also represented in (5) the valve allowing the possible exit of the gases, in (6) and (7) the carboys supplying argon and nitrogen, in (8) a vacuum pump and in (9) the pressure indicator dial. Thus, according to the method of the invention, it is possible to obtain microparticles of vanadium dioxide having a particle size less than 10 μm, in particular less than 5 μm, preferably of the order of 0.1 to 0-5 μ.

Some grains can be 2 to 10 μ after reaction but can be easily broken by grinding; in fact, they are all advantageously in the form of "pre-cut" plates when they are prepared by the method of the present invention, as shown in the scanning electron microscopy (SEM) photography shown in FIG. 2, while in the processes of the prior art, as mentioned above, even when pure vanadium oxide is obtained, these are essentially solid single crystals of the order of 30 μm and which are therefore very difficult to break.

Thus, in a preferred aspect of the process, the vanadium dioxide obtained at the end of the pyrolysis, said pyrolysis possibly being followed by an annealing and / or cooling step as mentioned above, is subjected to wet grinding. . Said grinding can be carried out for example in a zirconia ball mill rotating at more than 3000 revolutions / min for a duration less than or equal to 2 h.

Thus, after treatment by grinding, immediately or subsequently in the context of its incorporation into a surface coating composition of the vanadium dioxide obtained according to the invention, it is possible to bring all of the the latter in a particle size range of 0.1 to 0.5 μm: such base particles then make it possible to obtain a transparent film which meets the desired objectives and satisfies the applications of the invention, either by mixing with a paint colored by any pigment, without significantly changing the color, either by incorporation into a varnish while retaining its transparency.

In a further aspect, the invention therefore relates to the use of the vanadium dioxide microparticles according to the invention for the preparation of surface coating compositions. The incorporation of vanadium dioxide into the surface coating composition, in particular a paint or a varnish, can be carried out by any known process such as the pasting, by stirring introduction, of doped or undoped vanadium dioxide and of a dispersing agent to aid its dispersion and stabilize it in this form. A grinding is optionally carried out to reduce the particle size, such as for example in a zirconia ball mill rotating at more than 3000 revolutions per minute for 2 hours, which makes it possible to break down the vanadium dioxide microparticles which would possibly be greater at 0.1 - 0.5 μm, said grinding can also be carried out, as mentioned above, on vanadium dioxide before incorporation into the surface coating composition.

In a further aspect, the invention also relates to surface coating compositions containing the vanadium dioxide microparticles described in the present application.

The invention is illustrated by the examples below which are in no way limiting.

EXAMPLE 1 Synthesis of undoped vanadium dioxide. I. Manufacture of the ammonium hexavanadate precursor (HVA.:

20 g of ammonium metavanadate (MVA) (ALDRICH ref. 20,555 9; purity: 99%; M: 116.78) are introduced into a 250 ml beaker. The beaker is placed on a hot plate. A few drops of water are added with stirring so as to form a fluid paste in order to initiate the dissolution of the MVA. The beaker is heated to 55 ° C. ± 5 ° C. while maintaining agitation. A solution of IN hydrochloric acid is then added dropwise (at the start) while stirring and keeping the temperature constant. The pH is controlled so as to regulate the rate of addition of the acid and prevent sudden drops in pH. The total duration of this stage is more than half an hour.

For a volume of acid poured of approximately 110 cm 3 (approximately 2 moles of acid for 3 moles of HNA), the pH drops abruptly without rising. Beyond this, the pH reaches a limit value, the product obtained is an orange-yellow paste, the addition of 1Ν hydrochloric acid can cease from 120 cm-- '.

The orange-yellow precipitate obtained is then rinsed with water (slightly acidic to avoid re-dissolution) in a filter crucible of porosity n * 5 by drawing under vacuum, then air-dried at 200 ° C in an oven for 24 hours. 17 g of HNA are obtained. mass yield (expressed relative to the initial mass of MVA) = molar yield (expressed in moles of vanadium): greater than 99% l. Pvrolvse of the precursor a. Prior degassing of HNA:

2 g of HNA are deposited in an alumina nacelle in zone A (T = 200 * from the oven. A pumping under primary vacuum is carried out for 15 min. B. Thermal decomposition of the HNA and formation of NO2-

The basket is then placed directly in the second zone (B) of the oven where the temperature is 600 ± 5 * C (temperature rise rate of approximately 250 * C / min.).

All of the gases emitted are recovered in a gas bag at slight overpressure of about 0.5 bar in direct communication with the reactor and placed at a height below that of the reactor for 1 h. c Cooling and removal from the oven:

The cooling in zone C of the furnace is carried out in the atmosphere resulting from the decomposition to a temperature of 120 * C at a speed of 200 * C / min. about.

Mass of black-blue powder (θ_ formed: 1.6680 ± 0.0005 g. The X-ray spectrum, the IR spectrum and the scanning electron microscopy (SEM) photograph of the product of Example 1 are respectively represented on Figures 3, 4 and 5. In Figure 4a, the IR spectrum is measured at a temperature above 68 * C and in Figure 4b the IR spectrum is measured at a temperature below 68 * C. EXAMPLE 2 Synthesis of undoped vanadium dioxide from an industrial HNA precursor. at. Prior degassing of HNA:

We have 2 g of HVA (TREIBACHER (Austria), ref. AHV Trocken 99%) in an alumina basket in zone A of the oven (T = 200 ^* Q. A primary vacuum pumping is carried out for 15 min. b) Thermal decomposition of HNA and formation of VO2:

The basket is then placed directly in the second zone (B) of the oven where the temperature is 600 ° C. (temperature rise rate of approximately 250 ° C./min.).

All of the gases emitted are recovered in a gas bag at slight overpressure and in direct communication with the reactor and placed at a height below that of the reactor for 1 h. c Annealing of vanadium dioxide: The sample was left for 14 hours at 600 ± 5 ° C in the reactor in the presence of the decomposition gases. d. Cooling and leaving the oven:

The cooling was carried out in the atmosphere resulting from the decomposition to a temperature of 120 ° C. at a speed of approximately 200 ° C./min.

Mass of black-blue powder VO_) formed: 1, 6689 ± 0.0005 g. EXAMPLE 3 Synthesis of undoped vanadium dioxide from an industrial HVA precursor. An industrial HVA precursor is used (TREIBACHER (Austria) ref. AHV Trocken 99%). a and b. The degassing and thermal decomposition of the HVA are carried out as indicated above in Example 2. c. Annealing of vanadium dioxide:

The sample was taken out then put back in the reactor in zone A (T = 200 ^* . The whole was then put under primary vacuum for 15 minutes. Then the sample was moved in zone B of the furnace to the temperature of 800 * C for 5 hours. d. Cooling and leaving the oven:

The cooling in the zone C of the furnace was carried out in the atmosphere resulting from the decomposition to a temperature of 120 * C at a speed of 200 * C / min. about. Mass of black-blue powder (VO2) formed: 1.6682 ± 0.0005 g.

EXAMPLE 4 Synthesis of doped vanadium dioxide of formula V} _ _x M _x O2, with x = 0.01 and M = WT Manufacture of the precursor HVA with dopant:

The HVA precursor is prepared as indicated above in Example 1, but by adding 0.459 g of ammonium tungstate (ALDRICH ref. 32,238.5; purity: 99%; M = 265.88) before the addition of acid hydrochloric acid, IN.

The mass of product obtained is 19.424 g (including ammonium chloride). The product is characterized by an X-ray diffraction diagram and an IRTF spectrum: very weak bands due to tungstate are added to that of HVA.

Il.Pyrolvse of the precursor. at. Prior degassing of doped HVA.

2,000 g of doped HVA prepared above are deposited in an alumina nacelle in zone A (T = 200 * from the oven. A primary vacuum pumping is carried out for 15 min. B. Thermal decomposition of doped HVA and formation VO2 doped with tungsten.

The basket is then placed directly in the second zone (B) of the oven where the temperature is 600 ± 5 * C (temperature rise rate of approximately 250 * C / min.). All of the gases emitted are recovered in a gas bag in slight su etression and in direct communication with the reactor and placed at a height lower than that of the reactor. c Annealed vanadium dioxide.

The sample is left at 600 ° C for 14 hours. d. Cooling and removal from the oven.

The cooling in the zone C of the furnace was carried out in the atmosphere resulting from the decomposition to a temperature of 120 * C at a speed of 200 * C / min. about.

Mass of black-blue powder (VO) formed: 1.669 ± 0.001 g, The IR spectrum and the SEM photograph of the product of Example 4 are shown in Figures 6 and 7 respectively. In Figure 6a, the IR spectrum is measured at a temperature above 68 ° C and in Figure 6b the IR spectrum is measured at a temperature below 68 ° C. EXAMPLE 5 Synthesis of doped vanadium dioxide of formula Vj_ _x M _x O2 with x = 0.01 and M = W, from an industrial HVA precursor. T. Incorporation of the dopant.

20 g of HVA are introduced into a grinder in 25 ml of water so as to form a viscous paste. The paste is then subjected to a first grinding which aims to homogenize the dispersion of the HVA in the aqueous medium.

Ammonium tungstate is a white powder soluble in water. 0.539 g is added to the grinding paste and the dispersion is continued for several minutes.

The mixture thus produced was dried under vacuum or in an oven at 200 ^° C. π. Pvrolvse of the doped HVA precursor.

The pyrolysis is carried out under the same operating conditions as in Example 2 above, by conducting the annealing step for 5 hours at 800 ^* C. Mass bluish-black powder (VO2 doped) formed: 1.670 ± 0.001 g . EXAMPLE 6 Synthesis of doped vanadium dioxide of formula Vj_ _x M _x O2 with x = 0.02 and M = W.

I. Incorporation of the dopant.

The procedure is as indicated above in Example 5 starting from 20 g of HVA (Treibacher, Austria, ref. AHV Trocken 99%). The mass of incorporated ammonium tungstate is 1.089 g. yi.Pyrolvse of the doped HVA precursor.

The pyrolysis is carried out under the same operating conditions as in Example 5 above.

Mass of black-blue powder (doped VO2) formed: 1.671 ± 0.001 g. EXAMPLE 7: Characterization of the films and optical measurements. Dry films were prepared containing VO2 undoped or doped with 1% tungsten (V _j _ _x M _x O2 with x = 0.01 and M = W) (solvent phase) as follows: 1) Preparation of the varnish. Coarse varnish formula Plexigum P675 (HULS, Germany) 34 (Acrylic copolyraere) Solvantar S340 (ELF, France) 28 (100% aromatic hydrocarbon solvent)

White Spirit 17% (ELF, France) 38 (Hydrocarbon solvent 17% aromatic)

100.00 The Solvantar S340 and the White Spirit 17% are weighed in a beaker, then the Plexigum P675 is added with stirring and the mixture is left to rotate until perfectly homogenized. 2) Incorporation of VO2

Weigh 100 g of the varnish into a beaker. Then poured with stirring 1 g of VO2 (doped or not). Stirring is maintained at 1500 rpm for at least 15 min until perfect homogenization. The grinding is carried out using a glass microbead mill. 3) Application.

The coating thus obtained is applied to a glass plate using a manual applicator allowing the deposition of a thickness of 50 μm wet. Drying takes place at room temperature.

These films were characterized by the following techniques: - IRTF spectroscopy

- scanning electron microscopy (SEM)

- optical metric flow measurements (solar flux measurement). 4) Results a. Characterization of the transition by IRTF spectroscopy. The free films are prepared by application on glass, drying and detachment of the substrate.

The free films were characterized by IRTF transmission spectroscopy and RTA (Attenuated Total Reflection). The insulator-metal transition of the dioxide was clearly demonstrated during heating and cooling by the disappearance and reappearance of the absorption bands from to VO2 at T _t = 66 ± 2 * C for the film containing non-VO2 doped and at T _t = 43 ± 2 ^* C for a film containing VO2 doped with 1% tungsten.

The attached Figures 8a and 8b show in 3 dimensions the evolution of the absorption bands during heating of the film (Figure 8a) then cooling (FIG. 8b) for a film containing undoped VO2. It can be observed that only the bands of the polymer remain unchanged. b. Scanning electron microscopy. SEM allowed - on the one hand to characterize the distribution of VO2 grains in the dry film,

- on the other hand, to perform an accurate thickness measurement. c Optical measurements by flux-metrics.

A device represented in FIG. 9 for measuring solar flux has been specially designed in order to highlight the thermochromic transition of films in the near infrared domain. The principle has been published in an international journal (J.C. VALMALETTE et al., Solar

Energy Materials, 1994).

The artificial solar source (11) consists of a 50 W halogen lamp, the maximum emission of which is centered on 1 μm. The samples are composite films or coatings (13) 58 mm in diameter deposited on a glass substrate (16) placed between the source (11) and the detector (10) measuring the light flux for wavelengths included between 0 and 2.8 μm. The multimeter (15) measures the voltage delivered by the detector (10). Each sample can be heated or cooled by air flow (12) and the film temperature is measured using a thermocouple connected to the thermometer (14).

The exploitation of the experimental results provides access to three optical quantities directly linked to the film manufacturing process and to the quality of the transition.

The characterization of each of the films includes:

- direct measurement of the radiation from the source (without samples),

- a calibration from a semi-opaque film comprising a non-thermochromic black pigment by a high and low temperature measurement, - a measurement on a glass plate alone I *

- a measurement of the cold film (T <T): Ifroid

- a measurement of the hot film (T> T {): Ichaud.

We defined, by calculation, three quantities (in the solar spectral range of the detector): - opacity

- the relative efficiency (l- (Ichaud / Ifroid) expressed in%

- and absolute efficiency (Ichaud ~ Ifroid) ^{cn un} ^te s standard: W. m ~ ² .

The results obtained are as follows (the flux values transmitted when cold are expressed as a function of an incident radiation of 1000 W. m ~ ² ).

Dry film 10 μm thick containing a mass fraction of undoped VO2 equal to F.M. = 0.01.

- Opacity = 34 ± 2%

- Flow transmitted cold (T <T _t = 66 * = 662 W. m ^-2 - flow transmitted hot (T> T _t = 66 * = 606 W. m ^-2

- Relative efficiency = 8.5%

- Absolute efficiency gain = 56 W. m_2

Dry film 10 μm thick containing a mass fraction of undoped VO2 equal to F.M. = 0.025. - Opacity = 40 ± 2%

- Flux transmitted cold (T <T _t = 66 *) = 631 W. m ~ ²

- flux transmitted hot (T> T _t = 66 * C) = 527 W. m ^-2

- Relative efficiency = 16.5%

- Gain in absolute efficiency = 104 W. m " ² Dry film 10 μm thick containing a mass fraction of undoped VO2 equal to FM = 0.05.

- Opacity = 63%

- Flux transmitted cold (T <T _t = 66 * Q = 270 W. m ^-2

- flux transmitted hot (T> T _t = 66 * C) = 186 W. m ^-2 - Relative efficiency = 31.1%

- Gain in absolute efficiency = 84 W. m " ²

Dry films 100 μm thick containing a mass fraction of VO2 doped with 1% tungsten equal to F.M. = 0.005 ± 0.001.

- Opacity = 68 ± 2% - Flow transmitted cold (T <T _t = 66 * Q = 708 W. m ^-2

- flux transmitted hot (T> T _t = 66 * C) = 635 W. m ^-2

- Relative efficiency = 31.1%

- Absolute efficiency gain = 73 W. m ^-2 These results show that the volume fraction of pigment has a direct impact on:

- the thermal gain during the transition,

- the opacity of the varnish film. EXAMPLE 8: Grain size study.

This study was carried out by counting with an electron microscope on several samples of films produced with undoped VO2 obtained according to Examples 1 and 2. The films were prepared according to the method of Example 7.

The results are reported in Tables 1 and 2 below.

TABLE 1

Films containing undoped VO2 according to Example 1 log-normal distribution centered on D * = 0.3 μm with a standard deviation Ln σ = 60. film thickness = 10 μ volume fraction (pigment volume / total film volume) = 0.02

D = particle diameter; D * = average particle diameter

TABLE 2

Films containing undoped VO2 according to Example 2 lognormal distribution centered on D * = 1 μm with a standard deviation Ln σ = 0.80 film thickness = 10 μm volume fraction (pigment volume / total film volume) = 0.02

D = particle diameter; D * = average particle diameter.

Claims

1. Microparticles of vanadium dioxide of formula Vι_ _x M _x O2 in which 0 ≤ x ≤ 0.05 and M is a doping metal, characterized in that they have a particle size less than 5 μm, preferably of the order from 0.1 to 0.5 μm.

2. Microparticles according to claim 1 of formula Vj. _x M _x O2 in which M is a metal chosen from transition elements offering an ionic radius greater than that of vanadium such as Nb or Ta or an electronic contribution such as Mo and W.

3. Microparticles according to any one of claims 1 or 2 of formula Vj. _x W _x O2 in which x is between 0 and 0.02.

4. Process for obtaining vanadium dioxide microparticles of formula Vι_ _x M _x O2 in which M is a doping metal and 0 <x <0.05, by pyrolysis of doped or undoped ammonium hexavanadate, characterized in that said pyrolysis is carried out at a temperature between about 400 ° C and about 650 ° C, with a temperature rise rate of at least 100 ° C / min, and in that the gases from said pyrolysis are kept in confinement and in direct communication with the reaction medium for a period of at least 1/2 h, preferably 1 h.

5. Method according to claim 4, said method being carried out with ammonium hexavanadate doped with a metal chosen from those defined in claim 2.

6. Method according to one of claims 4 or 5, characterized in that the rate of temperature rise is at least 200 ° C / min, preferably at least 300 ° C / min.

7. Method according to any one of claims 4 to 6, characterized in that the duration of confinement of the gases resulting from the pyrolysis is at least 5 minutes, preferably between 1/2 h and 2 h.

8. Method according to any one of claims 4 to 7, characterized in that, before pyrolysis, the ammonium hexavanadate is subjected to degassing at a temperature below 230 ° C and by pumping under primary vacuum for at least 1 min.

9. Method according to any one of claims 4 to 8, characterized in that, at the end of the pyrolysis, the vanadium dioxide obtained is subjected to an optional step of annealing under inert gas, at a temperature of at least 600 ° C. for a period of at least 1 h.

10. Method according to any one of claims 4 to 9, characterized in that, at the end of the pyrolysis, and optionally after the optional annealing step as defined in claim 8, the vanadium dioxide is cooled obtained under an inert gas to a temperature of about 120 ^° C.

11. Method according to any one of claims 4 to 10, characterized in that one optionally submits the vanadium dioxide obtained at the end of the pyrolysis, said pyrolysis possibly being followed by an annealing step and / or from a cooling step as defined in claims 9 and 10, to grinding.

12. Use of the microparticles according to any one of claims 1 to 3 for the preparation of surface coating compositions.

13. Surface coating compositions containing microparticles according to any one of claims 1 to 3.