Coating processes – Particles – flakes – or granules coated or encapsulated
Reexamination Certificate
2001-10-01
2003-07-15
Beck, Shrive P. (Department: 1762)
Coating processes
Particles, flakes, or granules coated or encapsulated
C427S215000, C427S217000, C427S222000, C427S250000, C427S430100, C427S435000, C427S436000, C427S437000, C427S443100
Reexamination Certificate
active
06592938
ABSTRACT:
The present invention relates to a process for coating particles and to the coated particles obtained.
Particles of the core-shell type provide two benefits. On the one hand, they make it possible to increase the specific surface area of a material by dispersing it in the form of nanoparticles, thus causing a significant increase in its activity, or to isolate a particle from other particles by a protective layer and thus to modify the properties of the medium. On the other hand, in the case of the production of organic, mineral or hybrid composites, the coating of the particles makes it possible for the particles to be made compatible with the matrix. Mention may be made, for example, of the use of nanometric magnetic particles for recording data in the data processing field. Mention may also be made of the use of particles as solder binder in the electronics field. In the medical field, magnetic particles coated with organic substances are used.
Various processes for depositing a thin layer on a substrate are known. Particularly effective processes use a fluid raised to a pressure and to a temperature which are above the normal conditions, and especially a fluid placed under conditions very close to the critical pressure and critical temperature. These processes consist in depositing a film on a plane substrate, generally heated, placed in a reactor, by means of a supercritical fluid containing a precursor of the compound constituting the film, said precursor being converted before being deposited on the substrate, and the solvent for the fluid being removed by reducing the pressure in the reactor.
For example, “Oleg A. Louchev, et al., J. of Crystal Growth 155 (1995), 276-285” describes a process consisting in depositing copper on a heated substrate consisting of a silicon grid placed in a reactor under high pressure, by means of a supercritical fluid containing copper hexafluoroacetylacetonate as copper precursor. Conversion of the precursor is obtained by heating to a temperature of around 600 to 800° C.; this results in pyrolysis of the organic part of the precursor, which contaminates the substrate with carbon and with oxygen.
“J. F. Bocquet, et al., Surface and Coatings Technology, 70 (1994), 73-78” describes a process for depositing a film of metal oxide (TiO
2
) on a heated substrate placed in a reactor, using a supercritical solution of a TiO
2
precursor introduced into a pressurized reactor.
U.S. Pat. No. 5,789,027 (1996) describes a process for depositing a material on the surface of a substrate or within a porous solid. The process consists in dissolving a precursor of the material in a solvent under supercritical conditions, in bringing the substrate or the porous solid into contact with the supercritical solution, in adding a reactant which converts the precursor, thus causing the material to be deposited on the surface of the substrate or in the porous solid, and then in reducing the pressure in order to remove the solvent.
“Ya-Ping Sun, et al., Chemical Physics Letters 288 (1998), 585-588” describes the preparation of CdS nanoparticles coated with a film of polyvinylpyrrolidone. A solution of Cd(NO
3
)
2
in ammonia, brought under supercritical temperature and pressure conditions, is subjected to rapid expansion at room temperature in a solution of Na
2
S which also contains polyvinylpyrrolidone (PVP). The expansion causes precipitation of the Cd(NO
3
)
2
and makes the Cd(NO
3
)
2
react with the Na
2
S, thereby allowing CdS nanoparticles to form. Because the Na
2
S solution contains PVP, the CdS particles obtained are coated with PVP. This process makes it possible to prepare the particles in situ and at the same time to coat them. However, rapid expansion for the formation of particles to be coated is not very simple to implement as it involves passing a solution of particle precursors through a nozzle. A very small amount of material can be treated at each pass through the nozzle and the risks of blockage are not negligible. Furthermore, the rapid expansion is limited to particle precursors which may be dissolved in a supercritical solvent before the rapid expansion. Finally, the rapid expansion is obtained by a sudden drop in the pressure, which requires precise control of the nozzle temperature since the pressure reduction causes significant cooling.
It is an object of the present invention to provide a process allowing porous or nonporous particles to be simply and reliably coated with the aid of a precursor of the coating compound.
This is why the subject of the present invention is a process for depositing a film of a coating material on the surface of particles, or in the pores of porous particles, said process being characterized in that it consists in:
a) bringing, on the one hand, the particles to be coated and, on the other hand, an organometallic complex precursor of the coating material, optionally combined with one or more additional precursors which are organometallic complex or not, into contact in a fluid containing one or more solvents, said particles being kept dispersed in the fluid subjected to supercritical or slightly subcritical temperature and pressure conditions;
b) causing, within the fluid, the precursor of the coating material to be converted so that it is deposited on the particles;
c) bringing the fluid into temperature and pressure conditions such that the fluid is in the gaseous state in order to remove the solvent.
Within the context of the present invention, the term “particle” is understood to mean any object which has a mean size of less than one millimeter, whatever its shape. The process of the present invention is particularly suitable for coating particles of very small size, and especially for nanometric particles and micrometric particles, in particular for particles having a mean size of between 1 nm and 100 &mgr;m. The process is also very suited for coating particles having a complex shape. The particles may consist of a single chemical compound or by a mixture of compounds. The compounds may be mineral compounds, organic compounds or a mixture of organic or mineral compounds. The particles consisting of a mixture of compounds may be substantially homogeneous particles. However, they may also be heterogeneous particles in which the compound forming the core is different from the compound forming the external layer.
Within the context of the present invention, the fluid containing the particles to be coated and the precursor of the coating material is placed under supercritical or slightly subcritical temperature and pressure conditions. The term “supercritical conditions” is understood to mean conditions under which the temperature is above the critical temperature T
c
and the pressure is above the critical pressure P
c
. The term “slightly subcritical conditions” is understood to mean temperature T and pressure P conditions such that all the gases of the reaction mixture are dissolved in the liquid phase. The supercritical or slightly subcritical conditions are defined with respect to the pressure and to the temperature at the critical point P
c
and T
c
of the entire fluid constituting the reaction mixture. They generally lie within the range 0.5<T
c
/T<2, 0.5<P
c
/P<3. The reaction mixture consists of one or more solvents and various compounds in solution or in suspension. To a first approximation, the critical temperature and the critical pressure of such a fluid may be considered to be very close to those of the predominant solvent present in the fluid, and the supercritical or slightly subcritical conditions are defined with respect to the critical temperature and pressure of said predominant solvent. In general, the temperature of the fluid will be between 50° C. and 600° C., preferably between 100° C. and 300° C., and the pressure of the fluid will be between 0.2 MPa and 60 MPa, preferably between 0.5 MPa and 30 MPa. The particular values are chosen according to the precursor of the coating material.
The particles to be coated are kept dispersed in the reaction mixture by mechanical stir
Cansell François
Chevalier Bernard
Etourneau Jean
Pessey Vincent
Weill François
Beck Shrive P.
Burns Doane Swecker & Mathis L.L.P.
Centre National de la Recherche Scientifique
Tsoy Elena
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