Chemistry of inorganic compounds – Treating mixture to obtain metal containing compound
Reexamination Certificate
1998-10-05
2001-03-20
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Treating mixture to obtain metal containing compound
C423S263000, C423S592100, C423S607000, C423S608000, C423S610000, C423S625000, C423S632000, C423S633000, C075S352000, C075S354000
Reexamination Certificate
active
06203768
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the production of ultrafine particles and relates particularly, though not exclusively, to the production of ultrafine metallic and non-metallic powders consisting of individual particles with sizes in the range of 1 nm to 200 nm.
BACKGROUND TO THE INVENTION
Ultrafine powders have significant potential for a wide range of applications including catalysts, magnetic recording media, optoelectronic materials, magnetic fluids and composite materials. Ultrafine metallic powders have been prepared by physical methods, such as vapour deposition and sputtering, which have high quality, i.e. clean surfaces and uniform particle size distribution. However, industrial applications for such powders are limited by low yield rates and high cost. Alternative chemical production methods, such as thermal decomposition and precipitation are currently being studied for the preparation of a wide range of powders. Chemical methods can provide large quantities of ceramic powders for industrial applications. However, except for precious metals, chemical methods are generally not applied to the production of metallic powders.
Mechanical activation has been used for the production of fine powders with particle sizes typically in the range of 0.2 to 2 microns. One method for the production of powders by mechanical activation is the process of mechanical alloying described in U.S. Pat. No. 3,591,362, by which alloys are formed from pure starting materials by milling a mixture of the powders in a high energy ball mill. During milling the constituent particles undergo repeated collisions with the grinding balls causing deformation, welding and fracture of the particles which result in microstructural refinement and composition changes leading to the formation of nanocrystalline or amorphous alloys.
Another example of the use of mechanical activation to form fine powders, as described in U.S. Pat. No. 5,328,501, is concerned with a mechanochemical reduction process. This process involves the mechanically activated chemical reduction of reducible metal compounds with a reductant during milling in a high energy ball mill, to refine and manufacture metals, alloys and composite powders. During milling the energy imparted to the reactants through ball/reactant collision events causes repeated welding and fracture of the reactant particles. Consequently oxidation/reduction reactions occur at welded interfaces and reaction kinetics are enhanced without the need for high temperatures or melting to increase intrinsic reaction rates.
With both the above described prior art mechanically activated processes micron sized particles are formed which contain a nanometre scale mixture of phases, crystallites or amorphous regions. These processes have not previously been known to result in powders containing a significant fraction of particles with sizes less than 50 nm or interconnected networks of ultrafine particles exhibiting high values of specific surface area.
SUMMARY OF THE INVENTION
The present invention is concerned with a new process for the manufacture of ultrafine particles which is based on mechanically activated chemical reaction of a metal compound with a suitable reagent.
The process of the invention is based on the discovery that mechanical activation can be used to provide an improved, cost effective process for the production of ultrafine particles.
According to one aspect of the present invention there is provided a process for the production of ultrafine particles, the process comprising:
subjecting a mixture of a metal compound and a suitable reagent to mechanical activation to increase the chemical reactivity of the reactants and/or reaction kinetics such that a chemical reaction can occur which produces a solid nanophase substance and concomitantly forms a by-product phase, wherein said solid nanophase substance includes nano particles in the size range of 5 nm to 50 nm embedded in the by-product phase; and,
removing the by-product phase such that the solid nanophase substance is left behind in the form of ultrafine particles.
The term “ultrafine particles” as used above and throughout the remainder of the specification refers to individual particles in powder form as well as to particles interconnected or embedded in a porous matrix, and typically includes nano particles in the size range of 1 nm to 200 nm, or more typically in the range 5 nm to 50 nm.
During mechanical activation a composite structure is typically formed which consists of an intimate mixture of nano-sized grains of the nanophase substance and the reaction by-product phase. The step of removing the by-product phase, following mechanical activation, may involve subjecting the composite structure to a suitable solvent which dissolves the by-product phase, while not reacting with the solid nanophase substance. Removal of the by-product leaves behind ultrafine particles of the solid nanophase substance. Alternatively, the composite structure may be subjected to heating to remove the by-product phase by evaporation. If necessary, the surfaces of the ultrafine particles may be further processed (e.g. by gaseous reduction) to remove oxide or passivating films formed during removal of the by-product phase.
In one form of the process of the invention the metal compound is an unreduced metal compound and the reagent is a suitable reductant so that when the mixture is subjected to mechanical activation a chemical reaction occurs which reduces the metal compound to a metal phase, so that subsequent removal of the by-product phase leaves behind the metal phase in the form of ultrafine particles.
Preferably the unreduced metal compound is selected from the group containing metals of low electro-negativity, including but not limited to iron, nickel, cobalt, copper, gold and platinum. Typically the unreduced metal compound is a metal oxide, a metal chloride or a metal sulphide.
Preferably the reductant is a reducing agent which forms a soluble by-product phase. Examples of suitable reductants include sodium, calcium, magnesium and aluminium.
In one embodiment of the process the mixture formed during mechanical activation consists of nanocrystallites of the metal phase embedded within the reaction by-product phase, such that the metal phase does not percolate through the particles. Removal of the by-product phase leaves a powder consisting of nanoparticles of the metal phase.
In a second embodiment of the process, the nanocrystallites of the metal phase formed by mechanical activation are percolated or interconnected through the by-product phase. Removal of the reaction by-product phase then results in micron sized particles of the metal phase interconnected in a sponge-like structure.
In another form of the process of the invention mechanical activation results in an increase in the chemical reactivity of the reactants and/or reaction kinetics such that a displacement reaction can occur which produces a non-metallic phase such as a ceramic compound, so that subsequent removal of the by-product phase leaves behind the non-metallic phase in the form of ultrafine particles.
With some reactants the displacement reaction may not occur until the mixture is subject to thermal treatment, such as by annealing, either simultaneous with or subsequent to mechanical activation.
The process may be applied to the formation of single phase alloy particles, including solid solutions and intermetallics, metal oxide or sulphide particles.
The process may be applied to the formation of nano-sized powders consisting of a mixture of two types of particles each of a different phase.
The process may be applied to the manufacture of nano particles of oxides, carbides or other compounds by subsequent reaction of the metal particles with gaseous or liquid reactants to form the required phase.
The process may also utilise heat treatment of the powder following mechanical activation and prior to removal of the reaction by-product to form a desired phase or optimise morphology for particular applications.
Th
Ding Jun
McCormick Paul Gerard
Miao Wie-Fang
Street Robert
Advanced Nano Technologies Pty Ltd
Griffin Steven P.
Ildebrando Christina
Michael & Best & Friedrich LLP
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