Compositions: coating or plastic – Materials or ingredients – Pigment – filler – or aggregate compositions – e.g. – stone,...
Reexamination Certificate
2000-06-13
2002-03-19
Wood, Elizabeth D. (Department: 1755)
Compositions: coating or plastic
Materials or ingredients
Pigment, filler, or aggregate compositions, e.g., stone,...
C423S593100, C423S626000
Reexamination Certificate
active
06358307
ABSTRACT:
The object of the present invention is vanadium dioxide microparticles, a method for preparing said microparticles and their applications, notably for surface coatings in which they are incorporated.
In a first aspect, the invention relates to microparticles of vanadium dioxide of formula V
1−x
M
x
O
2
in which 0≦x≦0.05 and M is a doping metal, said microparticles having a particle size of less than 10 &mgr;m, notably less than 5 &mgr;m, preferably in the order of 0.1 to 0.5 &mgr;m.
The doping metal may be selected from transition elements which offer an ionic beam greater than that of vanadium such as for example Nb 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 are constituted of doped vanadium dioxide of formula V
1−x
W
x
O
2
in which x is between 0 and 0.02.
The vanadium dioxide microparticles according to the invention may notably be used in the technical sector of coating compositions intended to be essentially deposited in thin layers in the form of a film or a leaf, such as paints, varnishes and any other type of coating that may be deposited in successive layers.
The aim of the invention is therefore to use the vanadium dioxide microparticles described above for carrying out an <<intelligent>> material which automatically reduces the transmission of solar rays in the domain of infra-red rays, when the material reaches a given temperature level. It is thus possible to benefit from the energy of the infra-red rays below the fixed temperature and to eliminate the excessive heating above this temperature.
One of the principal applications of the vanadium dioxide microparticles according to the invention is their use in coatings intended to be affixed on the facades of buildings exposed to bad weather. The dark coloured coatings exposed to the sun's rays heat up much more than those of light colour. They therefore undergo expansion-contraction cycles of very high amplitude which cause a premature degradation of the coating sheet. It is therefore not possible at the present time to guarantee a dark paint whose luminous luminance is lower than 35%.
This phenomenon may be limited by the addition of a vanadium dioxide pigment to the paint whose fixed transition temperature should be in the order of 25° C. for example.
Another application is that of the protection of transparent or translucent surfaces which must allow visible rays to pass through them, such as in greenhouses, verandas, housing glazings, but whose internal temperature needs to be controllable, such a use may also be envisaged within the context of glazings and coachwork of cars and all other transport vehicles.
In summer, by reducing the entry of incident solar energy into buildings, the coating enables reducing the needs for air-conditioning and, on the other hand, in winter, the coating limits the dissipation of heat towards the exterior. Thus the coating advantageously allows an economy in energy.
One of the objects of the present invention is in fact the controllability of the transfer and the absorption of calorific energy at the surface of a wall without necessitating specifically transforming or treating the material thereof, but by depositing a coating following any known method, such as is practised with paints, it being possible for said coating according to the invention to be itself such a paint, enabling an economic implementation and manufacture.
Now, various ionic or molecular compounds are known which, under the effect of a variation of temperature, can change the optical properties, principally the colour, linked to a change of electronic structure: such compounds are called <<thermochromic>> compounds. By extension, a compound may also be called <<thermochromic>> which has the property of absorbing and/or reflecting different types of rays according to temperature due to a change in electronic structure. Vanadium dioxide has thus been studied for several years which has a structural transition at a temperature T
t
=341 K or 68° C.: below T
t
the crystalline structure is monoclinic, whereas above T, the structure is rutile. This transition is associated with a sudden change in the electronic properties: the compound thus passes into the insulating state when the temperature is lower than T
t
and into the metallic state when the temperature is greater than T
t
; optically, this change manifests itself as deep modifications of the near and far infra-red absorbance and reflection properties.
In the rest of the description, the designation <<vanadium dioxide>> shall comprise vanadium dioxide commonly named VO
2
or V
2
O
4
.
Various studies have recently been carried out on this compound, such as those that may be picked out in the publications S. M. Babulanam, Mat. Opt. Sol. Light Techn. 692 (1986) 8 and J. C. Valmalette, Sol. Energy Mater 33 (1994) 135. Studies have therefore been conducted on thin layers of vanadium dioxide deposited on various substrates: they have notably revealed the practical interest of the development of a material which is transparent to light but which only allows the infra-red part of the solar spectrum to pass through at low temperature. From this, the vanadium dioxide seems at the present time to be the only compound for which the transition is situated in a range of temperature and wavelengths suitable to the thermal regulation of the housing.
Moreover, this compound has the additional advantage of being able to undergo chemical substitutions with appropriate atoms such as defined further on and enabling a displacement of the temperature T
t
towards lower temperatures.
Thus, many tests and researches have been developed to create thin layers of vanadium dioxide deposited on substrates, notably with the view to studying the optical transmittance in the visible and the near infra red; for this, various depositing techniques have been envisaged, such as cathodic spraying under vacuum, evaporation under beam, vapour phase chemical deposits and the <<sol-gel>> process.
According to the <<sol-gel>> process, vanadium dioxide is prepared from tetravalent vanadium by dissolution in a solvent, hydrolysis and condensation in order to gradually form a sol, then, by evaporating the solvent, forming a gel which is then submitted to a thermal treatment to give VO
2
, under a finely controlled atmosphere.
It is possible to directly form a VO
2
film on a substrate, by soaking an appropriate substrate in the sol. The gel is thus formed directly on the substrate. Such a process of moistening or <<dip-coating>> is notably described in the U.S. Pat. No. 4,957,725.
It is however difficult to control the quality of the final film deposited, i. e. to place the complete piece or even its surface at high temperature in a uniform way and to control the interactions between the support and the gel thus deposited, etc . . . Thus, on the one hand, such methods which do not apply to the materials already constituted do not really enable an application on very large surfaces such as can be done with a surface coating composition such as paint and, on the other hand, the results obtained are neither repetitive nor reliable. Moreover, it is a very costly process when it comes to large surfaces.
Processes via the dry route generally exist which are very long (in the order of fifteen days) and are therefore very costly, which only enable obtaining molecules-grains in the order of 30 microns and more, which is not compatible with an incorporation into a paint without modifying the colour of it, which does not allow a homogeneous mixture and which does not bring about the property of optical transmission.
The problem posed is therefore one of being able to obtain a powder of low particle size which essentially comprises vanadium dioxide which is doped or not notably with tungsten which may notably be able to be incorporated in a liquid or viscous support with the view to obta
Gavarri Jean-Raymond
Lefevre Daniel
Legrand Pierre
Vacquier Gilbert
Valmalette Jean-Christophe
Les Peintures Jefco
Wolf Greenfield & Sacks P.C.
Wood Elizabeth D.
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