Chemistry of inorganic compounds – Silicon or compound thereof – Oxygen containing
Reexamination Certificate
2000-03-13
2002-05-14
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Oxygen containing
C423S609000, C423S625000, C423S635000, C423S592100, C423S608000, C065S017200
Reexamination Certificate
active
06387341
ABSTRACT:
DESCRIPTION
TECHNOLOGICAL FIELD OF THE INVENTION
This invention relates to a method of manufacturing a product based on a simple or mixed metal oxide, or silicon oxide, from one or more organic precursors using supercritical CO
2
as a reaction medium.
These metal oxides can be, for example, oxides of titanium, aluminum, magnesium, thorium, barium, beryllium, zirconium etc.
This method, depending on the operating conditions leads to products in the form of liquids, gels, powders, fibers etc being obtained.
One of the objectives of the invention is the formation of a powder, with a particle size and a particle geometry that is controllable as a function of the operating conditions used for its manufacture and the method of separation of the product obtained. Furthermore, the method of the invention permits the manufacture of a powder composed of nanometric particles.
These powders find application, for example, in the manufacture of ceramic materials, this manufacture requiring specially developed raw materials.
In effect, by doping a Si
3
N
4
ceramic with 5% SiC, in the form of nanometric particles, a nanocomposite ceramic is obtained five times more resistant to the propagation of cracks than the initial Si
3
N
4
ceramic. The nanoparticles are used here as dampers, absorbing applied mechanical stresses and allowing atomic planes to slide.
In addition, by using a nanocomposite ceramic formed only from nanometric particles, a nanocomposite is obtained with properties of toughness and ductility comparable to steel. Prospective applications are, for example, for engines, turbines and in space, for example as a refractory coating for space craft.
From the point of view of medical research, nanocomposite ceramics are being studied for the production of prostheses, for example hips and vertebrae, which are mechanically strong and compatible with the human body which is not the case for steel.
Furthermore, in the manufacture of these ceramic materials, an amorphous structure of the initial powder permits, in certain cases, sintering at a lower temperature.
Another example of a use of these compounded powders is the doping of ferromagnetic materials. In effect, when doped by nanoparticles, the ferromagnetic materials can acquire strong magnetization under a very small energizing field. This phenomenon, known by the name “super-paramagnetism” has a direct application in the improvement of reading heads for magnetic tapes and disks.
The method of manufacture according to the invention, also allows the manufacture of metal oxide powders which could find application in optics, for example, as surface coatings to improve the absorbing power in the visible spectrum, while at the same time reducing the losses of infra-red radiation.
Numerous other examples of uses of these powders based on oxides can be mentioned such as the manufacture of micro-porous solids used as catalysts, the manufacture of stationary phases for solid phase chromatography, the manufacture of selective membranes for nanofiltration, the separation of gases etc.
PRIOR ART
The production of an oxide powder can be provided from four manufacturing routes, the solid, liquid, gaseous and supercritical routes.
The solid routes very often require the application of a mechanical step, for example, grinding, abrasion etc. in order to obtain a desired particle size distribution for the powder. These methods, economically cost-effective, do not allow one to control precisely the size of the final particles and in particular to develop sub-micron particles.
The major problem with production of powders by a liquid route is the agglomeration of particles. In effect, the removal of the solvent, the seat of the reactions, generally brings about the partial agglomeration of the particles, sometimes making it difficult to use them industrially.
The sol-gel method also permits the manufacture of a fine metal oxide powder. This method consists of preparing a stable suspension of condensed species, in a liquid, from precursors (mineral salts or organo-metallic compounds). This suspension forms, from these condensed species, an amorphous three dimensional network in the sol that imprisons a fraction of the liquid, leading to the formation of a gel. The powder is obtained by the total removal of the liquid from this gel.
The precursors used in the sol-gel route for the preparation of oxide powders are organo-metallic precursors such as alkoxides or mineral precursors such as metal salts or hydroxides.
When the precursors are alkoxides, their activity can be modified notably by using complexing agents such as acetyl acetone which blocks the alkoxy groups. One then obtains modified alkoxides.
The gaseous routes do not allow one to obtain amorphous oxide powders because of the high temperatures generally used.
As for the supercritical routes, they are used in various techniques for the preparation of powders, for example, the hydrothermal synthesis technique, supercritical drying and reactions in a supercritical medium.
Hydrothermal synthesis is carried out under supercritical conditions, that is to say at a pressure greater than 2.2×10
7
Pa, and at a temperature greater than 374° C. Water is used for the development of large crystals by slow crystal growth.
Supercritical drying consists of removing a solvent while circumventing its critical point, that is to say by passing in a continuous way from the liquid to the gaseous state.
As for reactions in a supercritical medium, the document The Journal of Supercritical Fluids 4, p.55, 1991, describes a study of the solubility and the thermal resistance of organo-metallic compounds in supercritical CO
2
at 150 and 170° C. for a range of pressures between 1.2×10
7
and 2.2×10
7
Pa.
The document The Journal of Material Science 27, 1992, 2 187-2 192, describes the synthesis of sub-micron MgAl
2
O
4
powders in a supercritical ethanol medium from the double alkoxide Mg[Al(O-SecBu)
4
]
2
. This synthesis is carried out at about 360° C.
The document Materials Chemistry and Physics 32, 1992, pages 249 to 254, describes the synthesis of sub-micron powders of titanium oxides, in the vapor, liquid and supercritical phase. The synthesis of titanium oxide in supercritical phase is carried out at about 350° C. in a supercritical ethanol medium.
The document Silicates Industriels, 1994, 3-4, pages 141 to 143, describes the use of supercritical fluids as reaction media for the synthesis of ceramic powders. The powders formed are powders of titanium oxides and of the spinel MgAl
2
O
4
. The supercritical fluid used is a supercritical ethanol medium and the reaction temperature is about 360° C.
DESCRIPTION OF THE INVENTION
The invention relates to a method of manufacturing a product based on a simple or mixed metal oxide, or silicon oxide, from a charge of one or more precursors comprising one or more organo-metallic precursors, said method comprising bringing the charge of precursor(s) into contact with a reaction medium comprising supercritical CO
2
, at a temperature of from 31 to 50° C. and a supercritical pressure of from 10
7
to 5×10
7
Pa in order to form, from said precursor, a product based on a simple or mixed metal oxide or silicon oxide; and the separation of said product based on a simple or mixed metal oxide or silicon oxide, or organo-metallic product(s), from the reaction medium by reducing the pressure of the supercritical CO
2
to a pressure lower than the supercritical pressure.
The precursor charge may comprise one or more organo-metallic precursors only, but may also include, in addition to the organo-metallic precursor(s), organic compounds, such as, for example, iso-propanol, acetyl acetone etc.
The precursor or precursors are, for example, alkoxide precursors, identical to those used for the sol-gel route.
These precursors can be modified, notably by complexing agents such as acetyl acetone which blocks the alkoxy groups and thereby reduces the reactivity of the alkoxide with respect to hydrolysis.
These alkoxide precursors are, for example, tetra-ethoxy
Guizard Christian
Julbe Anne
Sarrade Stephane
Schrive Luc
Commissariat a l'Energie Atomique
Griffin Steven P.
Johnson Edward M.
Pearne & Gordon LLP
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