RU2162057C2 - Vanadium dioxide microparticles, method of preparation thereof, more particularly surface coatings - Google Patents

Abstract

FIELD: chemical industry, more particularly preparation of particles having thermochromic properties. SUBSTANCE: particles of vanadium dioxide of formula: V_1-xM_xO₂ wherein 0 ≅ x ≅ 0,05, M = Nb,Ta,Mo or W having particles size of less than 5 microns have thermochromic properties. Alloyed or nonalloyed ammonium hexavanadate is pyrolized. pyrolysis temperature ranges from 400 to 650 C. Temperature rises at rate of at least 100 C/min. Gases resulting from pyrolysis are maintained at volume where process occurs is contact with reaction medium for at least 0.5 hour to prepare particles having thermochromic properties. Surface coating compositions are prepared from said particles. EFFECT: material which automatically decreases penetration of solar rays when material attains desired temperature level. 14 cl, 12 dwg, 5 tbl

Description

 The subject of this invention is vanadium dioxide in the form of microparticles, a method for producing said microparticles and their use, especially for applying surface protective coatings into which they are introduced.

According to a first aspect, the invention relates to vanadium dioxide microparticles of the formula V _1-x M _x O ₂ , where 0≅x≅0.05 and M is an alloying metal, said microparticles having a particle size of less than 10 μm, in particular less than 5 μm, preferably of the order of 0.1 to 0.5 μm.

 The alloying metal may be selected from transition elements that have an ionic radius larger than the ionic radius of vanadium, such as, for example, Nb or Ta, or an electronic contribution, such as, for example, Mo or W, with W and Mo being preferred.

In a preferred aspect, the microparticles in accordance with this invention consist of doped vanadium dioxide of the formula V _1-X W _x O _x , where x takes values from 0 to 0.02.

 Vanadium dioxide microparticles in accordance with this invention can be particularly used in the technical field of coating compositions intended to be substantially deposited in thin layers in the form of a film or sheet, such as paints, varnishes and any other type of coating that may be precipitated in successive layers.

 Thus, the aim of the present invention is to use the vanadium dioxide microparticles described above to obtain a “reasonable” material that automatically reduces the transmission of sunlight into the infrared region when the material reaches a predetermined temperature level. Thus, it is possible to benefit from the energy of infrared radiation below a set temperature and to prevent excessive heating above this temperature.

 One of the main applications of vanadium dioxide microparticles in accordance with this invention is their use in coatings intended for application to facades of buildings exposed to bad weather. Dark-colored coatings exposed to sunlight are heated to a much greater extent than painted in bright colors. Therefore, they undergo expansion-contraction cycles with a very large amplitude that cause premature destruction of the protective coating. Therefore, it is not possible at present to guarantee dark paint whose light brightness is lower than 35%.

This phenomenon can be limited by the addition of a vanadium dioxide pigment to the paint, in which the fixed temperature of the (phase) transition should be of the order of, for example, 25 ^° C.

 Another application is to protect transparent or translucent surfaces that should allow visible rays to pass through them, such as in greenhouses, verandas, when glazing, but whose internal temperature must be controlled, this application can also be intended for use inside greenhouses and car cabs and all other vehicles.

 In summer, by reducing the penetration of incident solar energy into buildings, the protective coating reduces the need for air conditioning, and on the other hand, in winter, the coating limits heat dissipation into the environment. Thus, the protective coating advantageously provides an opportunity to save energy.

 One of the objects of this invention is actually the ability to control the transfer and absorption of thermal energy on the wall surface without the necessary specific conversion or processing of the material from which it is made, but by depositing the coating after any known method, such as the use of paints, and this is possible for the specified coatings in accordance with this invention, which may be such a paint, enabling economical use and production.

Currently, various ionic or molecular compounds are known which, under the influence of temperature changes, can change optical properties, mainly color, which is associated with a change in electronic structure: such compounds are called “thermocromic” compounds. In addition, compounds that possess the property of absorption and / or reflection of various types of rays in accordance with temperature, due to changes in the electronic structure, can also be called thermochromic. Thus, for several years, vanadium dioxide was studied, which undergoes a structural transformation at a temperature T _t = 341 K or 68 ^o C; at temperatures below T _t, its crystalline structure is monoclinic, while at temperatures above T _{t, the} structure is rutile. This transformation is associated with a sudden change in electronic properties: in this way, the compound goes into an insulating state when the temperature is lower than T _t and into a metallic state when the temperature is higher than T _t ; optically, this change itself proves how deep the modifications of the properties of optical density and reflection of the near and far infrared spectral region are.

In the text of the description, the definition of "vanadium dioxide" will include vanadium dioxide, usually denoted by VO ₂ or V ₂ O ₄ .

 Various studies have previously been performed on this compound, such as those described in S. M. Babulanam, Mat.Opt. Sol. Light Techn. 692 (1986) 8 and J. C. Valmalette, Sol. Energy Mater 33 (1994) 135. Thus, thin layers of vanadium dioxide deposited on various substrates were investigated; they showed particularly practical interest in the development of a material that is transparent to light, but which only transmits the infrared part of the spectrum of sunlight at low temperature. It turned out that vanadium dioxide is currently the only compound for which the transition (transformation) is in the range of temperature and wavelengths suitable for the thermal regulation of residential buildings.

In addition, this compound has the additional advantage that it is capable of undergoing chemical substitutions with suitable atoms, such as those indicated below, and allows the value of T _{t to} shift toward lower temperatures.

 Thus, to create thin layers of vanadium dioxide deposited on substrates (substrates), a large number of experiments and studies were carried out, especially from the point of view of studying optical transparency in the region of visible light and in the near infrared region of the spectrum; For this, various deposition methods were studied, such as cathodic sputtering under vacuum, evaporation under beam, chemical vapor deposition, and the sol-gel process.

According to the sol-gel process, vanadium dioxide is obtained from tetravalent vanadium by dissolving in a solvent, hydrolysis and condensation to gradually form a sol, followed by evaporation of the solvent and obtaining a gel, which is then subjected to heat treatment to form VO ₂ under a precisely controlled atmosphere .

You can get a film of VO ₂ directly on the substrate by soaking the corresponding substrate in the ash. Thus, the gel is formed directly on the substrate. Such a wetting method or dipping coating method is described in detail in US Pat. No. 4,957,725.

 However, it is difficult to control the quality of the final deposited film, i.e. place the entire sample or even its surface at high temperature in the same way and control the interaction between the carrier and the gel deposited in this way, etc. Therefore, on the one hand, such methods that are not applicable to materials that already exist are not applicable on very large surfaces, such as those made with a surface coating composition such as paint, and, on the other hand, the results obtained are not reproducible and not reliable. In addition, when used on large surfaces, it is a very expensive method.

 In the general case, there are “dry” methods that are very long (of the order of fourteen days) and therefore very expensive, which make it possible to obtain only grain molecules of the order of 30 μm or more, which are incompatible with the introduction of color without changing its color, which are not allow you to get a homogeneous mixture and which do not cause the properties of optical transmission (transparency).

 Therefore, there is a problem of obtaining a powder with small particles, which essentially includes vanadium dioxide, which is doped or not, especially tungsten and especially which can be introduced into a liquid or viscous substrate in order to obtain a protective coating.

In accordance with a second aspect, the present invention relates to a method for producing vanadium dioxide microparticles of formula V _1-x M _x O ₂ , where M is a dopant metal and 0≅x≅0.05, by pyrolysis of doped or undoped ammonium hexavanadate, characterized in that the specified pyrolysis is carried out at a temperature in the range from about 400 ^o C to about 650 ^o C at a rate of temperature increase of at least 100 ^o C / min and the fact that the gases formed during the specified pyrolysis are supported in a limited volume and directly contact with the reaction medium for a period of at least 1/2 hour, preferably 1 hour

The use of ammonium hexavanadate of the formula (NH ₄ ) ₂ V ₆ O _{16 is} known in the industry to produce V ₂ O ₅ , commonly used as a catalyst, but in which tetravalent vanadium is considered as a non-catalytic impurity to be removed. The experiments that were carried out with this precursor in order to also obtain vanadium dioxide alone did not lead to anything, since V ₂ O _{3 was} obtained, and in every publication until now it is stated that it is impossible to obtain pure vanadium dioxide.

The fulfillment of the characteristic conditions of the pyrolysis method according to the invention, namely:
- the rate of temperature increase is at least 100 ^o C / min, preferably 200 ^o C / min or 300 ^o C,
- the absence of evacuation of gases resulting from the thermal decomposition of ammonium hexavanadate, especially NH ₃ , which are maintained in a limited volume and in contact with the reaction medium for at least 5 minutes, preferably from 0.5 to 2 hours, and at most throughout the duration of the synthesis allows obtaining a complete reaction without any formation of residual V ₂ O ₅ in accordance with the following reaction scheme:
(NH ₄ ) ₂ V ₆ O ₁₆ ---> NH ₃ + V ₂ O ₅ ---> VO ₂ + H ₂ O + N ₂
In fact, it can be noted that, at a high pyrolysis rate, a “flash” occurs - a reaction resulting in the formation of N ₂ O, which is slowly decomposed upon interaction with an excess of NH ₃ to form H ₂ O and N ₂ .

In addition, to date, in most existing furnaces, the reaction site is either freed up by removing the obtained gaseous ammonia NH ₃ and thus stopping the recovery, as a result of which only V ₆ O _{13 is} obtained and does not allow further interaction to obtain vanadium oxide, VO ₂ , V ₂ O ₄ , or vice versa, in other methods in which NH ₃ is added by circulation, which causes too much reduction and leads to a reaction to produce V ₂ O ₃ and a mixture of various vanadium oxides.

 Advantageously, the gases resulting from the thermal decomposition of ammonium hexavanadate are collected in a gas bag (storage) under slight pressure, for example about 0.5 bar, preferably placed at a level higher than the level of the reactor.

The pyrolysis temperature should be in the range of from about 400 ^° C. to about 650 ^° C., preferably 635 ^° C. If the temperature is above about 650 ^° C., V ₂ O ₅ present in the reaction medium may melt before reaction. On the other hand, a reaction temperature below about 400 ^° C. leads to non-thermochromic VO ₂ (B).

 In the case of obtaining particles of doped vanadium dioxide, this duration must also be fixed so as to obtain doping homogeneity while preventing grain growth by optimizing the temperature-time conditions.

 For example, for a doping degree of 5% W, the condition (see table. 1 at the end of the description).

If it is necessary to obtain vanadium dioxide microparticles with a structural transition temperature other than 68 ^° C. (corresponding to pure vanadium dioxide), a substitute product must be added to it, the stable valence of which must be greater than 4.

 In a preferred aspect, a metal selected from Nb, Ta, Mo and W will be used as a substitute product, with W and Mo being preferred.

 Substitution with tungsten (W) allows you to get the final product with a significant gradient of temperature change, as a function of the percentage of substitution: a significant gradient allows you to actually cover to some extent a large temperature range. Thus, for examples of the x value indicated above, the transition temperature given in Table 1 is obtained. 2 (see the end of the description).

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

 In the next part of the description, the expression "metal alloying" a metal is as defined above, means that alloying is carried out using the metal in pure form or in the form of a compound containing this metal, such as, especially, tungstate or molybdate.

 Ammonium hexavanadate used in the process of this invention is commercially available. It can also be obtained in a known manner from ammonium metavanadate.

 In the case when it is necessary to obtain microparticles of doped vanadium dioxide, either ammonium hexavanadate can be doped or the alloyed metal can be introduced during the synthesis of hexavanadate from ammonium metavanadate.

 The use of tungsten as a substitute product is also advantageous, since ammonium tungstate is very soluble in water.

 In the case when it is necessary to introduce tungsten into the already synthesized ammonium hexavanadate, ammonium tungstate can be easily placed in solution in water with ammonium hexavanadate with a minimum moisture content of 20 wt.% To obtain a homogeneous soil (main) paste.

Chemical substitution or doping is carried out by pyrolyzing a mixture of ammonium tungstate and hexavanadate precursors in accordance with the following reduction and substitution reaction (1-x) (NH ₄ ) ₂ V ₆ O ₁₆ + x / 2 (NH ₄ ) ₂ H ₂ W ₁₂ O ₄₀ , yH ₂ O ---> 6 (V _1-x W _x ) O ₂ + nN ₂ + mH ₂ O
The choice of x for a homogeneous result is an accurate stoichiometric calculation that allows you to obtain the required temperature change, as for the transition temperature of 68 ^o C vanadium dioxide, it can be noted for this that the gradient δ _t / dx = -28 · 10 ² K / mol, but actually for x = 0.01 or 1%, δ _t = -28 ^o C.

In accordance with an advantageous aspect of the method of the present invention, ammonium hexavanadate is degassed prior to pyrolysis at a temperature lower than the decomposition temperature of ammonium hexavanadate, especially lower than about 230 ^° C, and preferably of the order of 200 ^° C, and by first pumping under vacuum for at least at least one minute, for example 15 minutes.

At the end of the pyrolysis, the resulting vanadium dioxide can advantageously be calcined in an inert gas atmosphere at a temperature of at least 600 ^° C. for a period of at least 1 hour, for example, for 5 hours.

For example, the calcination step can be carried out at 600 ^° C. for 14 hours. At a temperature of 800 ^° C. for 5 hours, the grain growth is more than 5 μm.

In a preferred aspect of the method of the invention and optionally after the above calcination step, vanadium dioxide needs to be cooled in an inert gas atmosphere to a temperature of about 120 ^° C. The cooling rate may be, for example, from about 150 ^° C / min to 250 ^° C / min.

 As an inert gas, for example, nitrogen or argon can be used.

When vanadium dioxide is released into the open air, the temperature should be at most about 100 ^° C. to prevent it from absorbing water and, therefore, the risk of re-oxidizing the surface.

An example of a furnace that allows the implementation of the method in accordance with this invention is shown in FIG. 1, which shows a movable tube furnace (2) on rails (3), relative to the processing chamber (1), consisting of a quartz tube, which allows to obtain a high rate of reaching the desired temperature, and a gas bag (4), which allows you to support gases, formed as a result of thermal decomposition of ammonium hexavanadate, especially NH ₃ in a limited volume over grains that are to be processed in the chamber (1). This allows you to maintain the partial pressure of ammonia at a level sufficient so that the reduction reaction to VO _{2 is} complete. Heavier gaseous N ₂ O is discharged to the bottom of the gas bag (4), which allows to increase and optimize the purity of the resulting compounds.

 In FIG. 1 also shows a valve (5), which allows the release of gases, cylinders, from which argon (6) and nitrogen (7), a vacuum pump (8) and a pressure gauge (9) are supplied.

 Thus, in accordance with the method of the present invention, vanadium dioxide microparticles having a size of less than 10 μm, especially less than 5 μm, preferably of the order of 0.1 to 0.5 μm, can be obtained.

 Some grains after the reaction can have a size of 2 to 10 microns, but they can be easily crushed by grinding; all of them, upon receipt by the method of the invention, are in the form of “pre-cut” plates, as shown in FIG. 2 in a photograph taken with a scanning electron microscope (SEM), while in prior art methods such as those listed above, even when pure vanadium dioxide was obtained, it was essentially a substance in the form of massive single crystals about 30 in size microns, which are very difficult to grind.

 Thus, in a preferred aspect of the process, vanadium dioxide obtained after pyrolysis, wherein said pyrolysis optionally follows a calcination and / or cooling process, such as those described above, is wet milled. The specified grinding can be carried out, for example, in a rotating mill with zircon balls at a speed of more than 3000 rpm for a period of time less than two hours or equal to two hours.

 Thus, after processing by grinding, immediately or later, to introduce it into the composition of the surface coating of vanadium dioxide obtained in accordance with this invention, it can all be crushed to obtain particles ranging in size from 0.1 to 0.5 microns: the particles then make it possible to obtain a transparent film that meets the target requirements and satisfies the applications of the invention, either by mixing with paint dyed with any pigment without a significant color change, or by introducing it into the varnish, preserving its transparency.

 According to a further aspect, the invention relates to the use of vanadium dioxide microparticles according to the invention for the preparation of surface coating compositions.

 The introduction of vanadium dioxide in the composition of the surface coating, especially in paint or varnish, can be carried out by any known method, such as kneading with the formation of a paste, introducing with mixing doped or undoped vanadium dioxide and a dispersing agent to facilitate its dispersion and to stabilize it in this form. Milling is optionally carried out to reduce particle size, such as, for example, in a rotary mill with zircon balls at a speed of more than 3000 rpm for 2 hours, which allows the grinding of microparticles of vanadium dioxide, which ultimately would have a size of more than 0, 1-0.5 microns, and the specified grinding of vanadium dioxide is carried out, as described above, before introducing into the composition of the surface coating.

 In a further aspect, the invention also relates to surface coating compositions that contain vanadium dioxide microparticles described herein.

 The invention is illustrated using the Examples below, which are not limiting the scope of the invention.

EXAMPLE
1: Synthesis of non-ligated vanadium dioxide
1. Obtaining the precursor of ammonium hexavanadate (GVA)
20 g of ammonium metavanadate (MBA) (Aldrich Ref. 20.555 9, purity: 99%, M: 116.78) was introduced into a 250 ml beaker. The glass is placed on a heating plate. A few drops of water are added with stirring so as to obtain a liquid paste to initiate the dissolution of MBA. The beaker is heated to 55 ^° C ± 5 ^° C while maintaining stirring. Then, a 1N hydrochloric acid solution (initially) is added dropwise with stirring and the temperature is kept constant.

 To control the rate of addition of acid and prevent unexpected surges, the pH is monitored. The total duration of this stage is more than half an hour.

With a volume of added hydrochloric acid of approximately 110 cm ³ (approximately 2 moles of acid per 3 moles of GVA), the pH suddenly drops without going back. After this, the pH reaches the limit value, and the resulting product is a yellow-orange paste, the addition of 1N hydrochloric acid can be stopped after 120 cm ³ .

The resulting yellow-orange precipitate was then washed with water (slightly acidified to prevent re-dissolution) in a filter crucible with porosity No. 5 by vacuum suction, then dried in air at 200 ^° C in an oven for 24 hours. 17 g of GVA was obtained.

 Mass yield (expressed relative to the initial mass of MBA) = molar yield (expressed in moles of vanadium): more than 99%.

11. Pyrolysis of the predecessor
a. Preliminary degassing of GVA:
2 g of GVA are precipitated in an aluminum basket in zone A (T = 200 ^o C) of the furnace. The first pumping under vacuum is carried out for 15 minutes

b. Thermal decomposition of GVA and the formation of VO ₂ .

Then the basket is placed directly in the second zone (B) of the furnace, where the temperature is 600 ± 5 ^o C (the rate of temperature increase is approximately 250 ^o C / min).

 All released gases are extracted into a gas bag under a slight pressure of 0.5 bar in direct contact with the reactor and placed at a height lower than the height of the reactor for 1 h.

c. Cooling and leaving the furnace:
Cooling in zone C of the furnace to a temperature of 120 ^° C. is carried out in an atmosphere resulting from decomposition at a rate of approximately 200 ^° C./min.

Mass of the obtained bluish-black powder (VO ₂ ): 1.6680 ± 0.0005 g.

 An X-ray spectrum, an IR spectrum, and a photograph taken with a scanning electron microscope (SEM) of the product of Example 1 are shown in FIG. 3, 4 and 5, respectively.

In FIG. 4a shows an IR spectrum recorded at a temperature above 68 ^° C, and in FIG. 4b presents the IR spectrum recorded at a temperature below 68 ^o C.

EXAMPLE 2: Synthesis of non-ligated vanadium dioxide from a precursor of GVA, obtained industrially
a. Preliminary degassing of GVA:
2 g of GVA (Treibacher (Austria), Ref. Trocken GVA 99%) is precipitated in an aluminum basket in a furnace in zone A (T = 200 ^o C). The first pumping under vacuum is carried out for 15 minutes

b. Thermal decomposition of GVA and the formation of VO ₂ .

Then the basket is placed directly in the second zone (B) of the furnace, where the temperature is 600 ± 5 ^o C (the rate of temperature increase is approximately 250 ^o C / min).

 All released gases are extracted into the gas bag under slight pressure and in direct contact with the reactor and placed at a height lower than the height of the reactor for 1 h.

c. Vanadium dioxide calcination:
The sample is left for 14 hours at a temperature of 600 ± 5 ^o C in the reactor in the presence of decomposition gases.

d. Cooling and leaving the furnace:
Cooling is carried out to a temperature of 120 ^° C. in the atmosphere resulting from decomposition at a rate of approximately 200 ^° C./min.

Mass of the resulting bluish-black powder (VO ₂ ): 1.6689 ± 0.0005 g.

EXAMPLE 3: Synthesis of non-ligated vanadium dioxide from a precursor of GVA, obtained industrially
Use industrially produced GVA (Treibacher (Austria), Ref. GVA Trocken 99%).

 a and b. The degassing and thermal decomposition of the GVA is carried out as shown in Example 2.

c. Vanadium dioxide calcination:
The sample is taken out and then returned to the reactor in zone A (T = 200 ^° C.). Then it is first placed under vacuum for 15 minutes. After that, the sample is placed in zone B of the furnace at a temperature of 800 ^o C for five hours.

d. Cooling and leaving the furnace:
Cooling in zone C of the furnace to 120 ^° C. is carried out in an atmosphere resulting from decomposition at a rate of approximately 200 ^° C./min.

The mass of the obtained bluish-black powder (VO ₂ ):
1.6682 ± 0.0005 g.

EXAMPLE 4: Synthesis of doped vanadium dioxide of the formula V _1-x M _x O ₂ , x = 0.01 and M = W
1. Obtaining a precursor of GVA with an alloying agent:
The precursor of GVA is prepared according to the procedure described in Example 1, but adding 0.459 ammonium tungstate (Aldrich Ref. 32,238.5, 99% purity, M = 265.88) before adding 1N hydrochloric acid.

 The mass of the obtained product is 19.424 g (including ammonium chloride). The product is characterized by an X-ray diffraction pattern and FTIR: in addition to the bands due to GWA, there are very weak bands due to the presence of tungstate.

II. Precursor Pyrolysis
a. Preliminary degassing of doped GVA.

2,000 g of doped GVA obtained as described above are precipitated in an aluminum basket in a furnace in zone A (200 ^° C.). The first evacuation under vacuum is performed within 15 minutes.

b. Thermal decomposition of doped GVA and the formation of VO ₂ doped with tungsten.

The basket is then placed directly in the second zone (B) of the furnace, where the temperature is 600 ± 5 ^° C (the rate of temperature increase is approximately 250 ^° C / min).

 All released gases are extracted into the gas bag under slight pressure and in direct contact with the reactor and placed at a height below the height of the reactor.

 c. Vanadium dioxide calcination.

The sample is kept at a temperature of 600 ^o C for 14 hours

 d. Cooling and leaving the furnace.

Cooling in zone C of the furnace to 120 ^° C. at a rate of approximately 200 ^° C./min is carried out under the atmosphere resulting from decomposition.

The mass of the obtained bluish-black powder (VO ₂ ): 1,669 ± 0,001 g

The IR spectrum and SEM micrograph of the product of Example 4 are shown in FIG. 6 and 7, respectively. In FIG. 6a shows an IR spectrum recorded at a temperature above 68 ^° C, and FIG. 6b - IR spectrum recorded at a temperature below 68 ^o C.

EXAMPLE 5: Synthesis of doped vanadium dioxide of the formula V _1-x M _x O ₂ , where x = 0.01 and M = W, from a precursor of GVA obtained by industrial methods
1. The introduction of an alloying agent
20 g of HBA are introduced into the mill in 25 ml of water in order to obtain a viscous paste. The paste is then subjected to first grinding, the purpose of which is to homogenize the dispersion of GWA in an aqueous medium.

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

The mixture thus obtained is dried under vacuum or in an oven at a temperature of 200 ^o C.

II. Pyrolysis of the doped precursor of GVA
Pyrolysis is carried out under the conditions described in detail in Example 2, performing calcination at a temperature of 800 ^o C for 5 hours

Mass of the obtained bluish-black powder (VO ₂ ): 1.670 ± 0.001 g.

EXAMPLE 6: Synthesis of doped vanadium dioxide of the formula V _1-x M _x O ₂ , where x = 0.02 and M = W
1. The introduction of an alloying agent
Conducted according to the method of example 5 of 20 g of GWA (Treibacher, Aistria, Ref. GWA, Troken 99%). The weight of the introduced ammonium tungstate is 1.089 g.

II. Pyrolysis of the doped precursor of GVA
Pyrolysis is carried out under the conditions described above in Example 5. Mass of the obtained bluish-black powder (VO ₂ ): 1,671 ± 0,001 g.

EXAMPLE 7: Film Research and Optical Measurements
Dry films are obtained which contain VO ₂ , whether or not doped, 1% tungsten (V _1-x M ₂ O ₂ , where x = 0.01 and M = W) (solvent phase), as follows.

1) Getting varnish
The empirical formula of varnish (see table. 3 at the end of the description).

 Solvantar S: 340 and White Spirit 17% are weighed and placed in a glass, then Rlexigum P 675 is added with stirring, after which the mixture is left to mix until complete homogenization.

2) Introduction of VO ₂ .

 100 g of varnish are weighed and placed in a glass.

1 g of VO ₂ (doped or non-ligated) is added with stirring. Stirring is continued at a speed of 1500 rpm for at least 15 minutes until complete homogenization. Grinding is carried out using a mill with glass microspheres.

 3) Application.

 The coating thus obtained is applied to the glass plate using a manual applicator, which allows deposition of a thickness of 50 μm in the wet state.

 Drying is carried out at room temperature.

Such films are characterized by the following methods:
FTIR spectroscopy (Fourier transform infrared spectroscopy,
- optical measurements using light dosing (measurement of luminous flux).

4) Results
a. FTIR spectroscopy conversion study.

 Free films are prepared by glass coating, drying and separation from the substrate.

Free films are examined using FTIR for transmission and for the determination of attenuated total reflection (ATR). The insulating-metal transition of the dioxide is clearly demonstrated during heating and cooling by the disappearance and reappearance of absorption lines due to VO ₂ at T _t = 66 ± 2 ^o C for a film that contains undoped VO ₂ and at T _t = ± 2 ^o C for a film that contains VO ₂ doped with 1% tungsten.

In FIG. 8a and 8b show a three-dimensional change in the absorption lines during heating of the film (FIG. 8a), then cooling (FIG. 8b) for a film that contains undoped VO ₂ . It can be seen that only the polymer lines remain unchanged.

 b. Scanning electron microscope.

- Using an electron microscope, on the one hand, it is possible to characterize the distribution of VO ₂ grains in a dry film,
- on the other hand, to carry out an accurate measurement of thickness.

 c. Optical measurement using light dosing.

 The device shown in FIG. 9, for measuring solar flux, it was developed specifically to demonstrate the thermochromic transformation of films in the near-infrared region. The principle of operation was described in a publication in an international review (J.C. Valmalette et al. Solar Energy Materials, 1994).

 An artificial light source (11) consists of a 50 W halogen lamp, the maximum radiation of which is centered at 1 μm. The samples are complex films or coatings (13) with a diameter of 58 mm deposited on a glass substrate (16) and placed opposite the source (11) and detector (10), which measures the light flux with a wavelength of 0.3 to 2.8 μm . The multimeter (15) shows the magnitude of the voltage produced 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 a thermometer (14).

 The experimental results make it possible to obtain three optical scales directly related to the method of manufacturing the film and the quality of the transition.

Examination of each of the films includes
- direct measurement of the radiation of the source (without sample),
- standardization from a translucent film, which includes a non-thermochromic black pigment, using low-temperature and high-temperature measurement,
- measurement on a single glass plate I ',
- measurement of a cold film (T <T _t ): I _cold ',
- measurement of a hot film (T> T _t ): I _hot .

Using the calculation, three scales are obtained (in the region of the solar spectrum of the detector):
- opacity
- relative efficiency (1- (I _hot / I _cold ), expressed in%,
- absolute efficiency (I _hot -I _cold ) in standard units: W m ^-2 .

The following results were obtained (the values of the flux transmitted in a cold film are expressed as a function of random radiation of 1,000 W m ^-2 ).

A dry film with a thickness of 10 μm, which contains a mass fraction of undoped VO ₂ equal to MF = 0.01.

- opacity = 34 ± 2%
- The stream passed in a cold film (T <T _t = 66 ^o C) = 662 W m ^-2 .

- The stream passed in the hot film (T> T _t = 66 ^o C) = 606W m ^-2 .

 - Relative efficiency = 8.5%.

- Amplification of absolute efficiency = 56W m ^-2 .

A dry film with a thickness of 10 μm, which contains a mass fraction of undoped VO ₂ equal to MF = 0.025.

- opacity = 40 ± 2%
- The stream passed in a cold film (T <T _t = 66 ^o C) = 631W m ^-2 .

- The stream passed in the hot film (T> T _t = 66 ^o C) = 527W m ^-2 .

- Relative efficiency = 16.5%
- Amplification of absolute efficiency = 104W m ^-2 .

A dry film with a thickness of 10 μm, which contains a mass fraction of undoped VO ₂ equal to MF = 0.05.

- opacity = 63%
- The stream passed in a cold film (T <T _t = 66 ^o C) = 270W m ^-2 .

- The stream passed in the hot film (T> T _t = 66 ^o C) = 186W m ^-2 .

- Relative efficiency = 31.1%
- Effort of absolute efficiency = 84W m ^-2 .

A dry film with a thickness of 10 μm, which contains a mass fraction of doped 1% tungsten VO ₂ equal to MF 0.005 ± 0.001.

- opacity = 68 ± 2%
- The stream passed in a cold film (T <T _t = 66 ^o C) = 708W m ^-2 .

- The stream passed in the hot film (T> T _t = 66 ^o C) = 635W m ^-2 .

- Relative efficiency = 31.1%
- Strengthening absolute efficiency = 73 W m ^-2 .

These results show that the volume of the pigment fraction has a direct effect on
- thermal strengthening in the process of its transformation,
- the opacity of the lacquer sheet.

EXAMPLE 8: Study of particle size
This study is carried out by counting on an electron microscope in some samples of films made with undoped VO ₂ obtained in accordance with Examples 1 and 2. The films were obtained in accordance with the method of Example 7.

 The results are shown in table. 4 and 5 (see the end of the description).

Claims (13)

1. Microparticles of vanadium dioxide of the formula
V _1-x M _x O ₂ ,
where 0 ≅ x ≅ 0.05;
M is an alloy metal
characterized in that said microparticles have a size of less than 5 microns, preferably 0.1-0.5 microns and exhibit thermochromic properties as a result of a structural change. 2. Microparticles according to claim 1 of the formula
V _1-x M _x O ₂ ,
where M is a metal selected from transition elements having 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 claim 1 or 2 of the formula
V _1-x M _x O ₂ ,
where x takes values in the range from 0 to 0.02.  4. Microparticles according to any one of claims 1 to 3, characterized in that they are used to obtain surface coating compositions. 5. A method of obtaining particles of vanadium dioxide of the formula
V _1-x M _x O ₂ ,
where M is an alloying metal;
0 ≅ x ≅ 0.05,
pyrolysis of doped or non-ligated ammonium hexavanadate, characterized in that said pyrolysis is carried out at a temperature in the range of about 400 - 650 ^° C at a rate of temperature increase of at least 100 ^° C / min and the gases resulting from said pyrolysis are kept in a limited volume and in direct contact with the reaction medium for a period of at least 0.5 hours, preferably 1 hour  6. The method according to claim 5, characterized in that the method is carried out with ammonium hexavanadate doped with a metal selected from the metals defined in claim 2. 7. The method according to claim 5 or 6, characterized in that the rate of temperature increase is at least 200 ^o C / min, preferably at least 300 ^o C / min  8. The method according to any one of paragraphs.5 to 7, characterized in that the retention time of the gases generated during the pyrolysis is at least 5 minutes, preferably 0.5 to 2 hours 9. The method according to any one of paragraphs.5 to 8, characterized in that before the pyrolysis of ammonium hexavanadate is subjected to degassing at a temperature below 230 ^o C and the implementation of the primary pumping under vacuum for at least 1 minute 10. The method according to any one of paragraphs.5 to 9, characterized in that after pyrolysis the resulting vanadium dioxide is subjected to the stage of calcination in an atmosphere of inert gas at a temperature of at least 600 ^o C for a period of time of at least 1 hour 11. The method according to any one of paragraphs.5 to 10, characterized in that after pyrolysis and after the calcination step, vanadium dioxide is cooled in an inert gas atmosphere to a temperature of about 120 ^o C.  12. The method according to any one of paragraphs.5 to 11, characterized in that the obtained vanadium dioxide after pyrolysis, moreover, after the specified pyrolysis optionally follows a calcination step and / or a cooling step, is subjected to grinding.  13. Compositions of surface coatings that contain microparticles according to any one of claims 1 to 3.

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