Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Reexamination Certificate
2000-11-13
2003-01-07
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
C423S593100, C423S598000, C423S608000, C423S609000, C423S610000, C423S625000, C423S637000, C423S263000
Reexamination Certificate
active
06503475
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for the production of ultrafine powders and relates particularly, though not exclusively, to the production of ultrafine 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.
A method for the manufacture of ultrafine powders with particle sizes less than 50 nm is described in International Application No. PCT/AU96/00539. This process involves a mechanically activated chemical reaction between a metal compound and a suitable reagent which occurs either during mechanical milling or during subsequent heat treatment of the milled powder. During mechanical activation a composite structure is formed which consists of nano-sized grains of the nano-phase substance within the matrix of the by-product phase. Removal of the by-product phase yields nano particles of the desired material.
The above described prior art techniques require the occurrence of a mechanically activated chemical reaction between the starting powders to form nano-sized particles. Mechanical milling processes, which do not involve the occurrence of chemical reactions between the major constituents have not previously been known to result in powders containing a significant fraction of particles with sizes less than 50 nm. For example, ultrafine grinding processes such as attrition milling are known to be effective in producing powders with mean particle sizes down to about 500 nm. However, the achievement of smaller particle sizes generally requires long milling times and significant energy inputs and is therefore limited by economic considerations. Contamination of the product may also be a problem. In addition it is widely accepted that the existence of a so-called ‘limiting particle size’ limits the practical minimum particle size that can be attained by grinding to values greater than 100 nm, irrespective of the type of ball mill employed.
SUMMARY OF THE INVENTION
The present invention is concerned with a new process for the manufacture of ultrafine powders which is based on the mechanical milling of two or more nonreacting powders. The process of the invention is based on the discovery that mechanical milling of multi-phase systems can be used to provide an improved, lower cost process for the production of ultrafine powders.
Throughout this specification the term “comprising” is used inclusively, in the sense that there may be other features and/or steps included in the invention not expressly defined or comprehended in the features or the steps specifically defined or described. What such other features and/or steps may include will be apparent from the specification read as a whole.
According to one aspect of the present invention there is provided a process for the production of ultrafine powders, the process comprising:
subjecting a mixture of a suitable precursor metal compound and a non-reactant diluent phase to mechanical milling which through the process of mechanical activation reduces the microstructure of the mixture to the form of nano-sized grains of the metal compound uniformly dispersed in the diluent phase;
heat treating the milled powder to convert the nano-sized grains of the metal compound into a desired metal oxide phase; and,
removing the diluent phase such that said nano-sized grains of the metal oxide phase are left behind in the form of an ultrafine powder.
According to another aspect of the present invention there is provided a process for the production of ultrafine powders, the process comprising:
providing a suitable precursor metal compound heat treated to convert the metal compound into a desired metal oxide phase;
subjecting a mixture of the metal oxide phase and a non-reactant diluent phase to mechanical milling which through the process of mechanical activation reduces the microstructure of the mixture to the form of nano-sized grains of the desired metal oxide phase uniformly dispersed in the diluent phase; and,
removing the diluent phase such that said nano-sized grains of the desired metal oxide phase are left behind in the form of an ultrafine powder.
The term “ultrafine powder” as used above and throughout the remainder of the specification refers to individual dispersed nano-sized particles in powder form and includes powder particles in the size range of lnm to 200 nm, or more typically in the size range 10 nm to 100 nm.
In a preferred form of the invention, mechanical milling and activation is performed inside a mechanical mill, for example, a ball mill. Mechanical activation occurs in a ball mill when grinding media, typically steel or ceramic balls, are kept in a state of continuous relative motion with a feed material by the application of mechanical energy, such that the energy imparted to the feed material during ball-feed-ball and ball-feed-liner collisions is sufficient to cause mechanical activation.
Throughout the remainder of the specification reference will be made to mechanical activation being carried out inside a ball mill. Examples of this type of mill are attritor mills, nutating mills, tower mills, planetary mills, vibratory mills and gravity-dependent-type ball mills.
It will be appreciated that the mechanical activation may also be achieved by any suitable means other than ball milling. For example, mechanical activation may also be achieved using jet mills, rod mills, roller mills or crusher mills.
During mechanical activation the ball-powder collision events cause the powder particles to be deformed and fractured. Cold-welding of overlapping particles occurs between surfaces formed by prior fracture. The competing processes of deformation, fracture and welding continue during milling, and result in microstructural refinement. While the changes in microstructure that occur during mechanical
McCormick Paul Gerard
Tzuzuki Takuya
Advanced Nano Technologies Pty Ltd.
Bos Steven
Michael & Best & Friedrich LLP
LandOfFree
Process for the production of ultrafine powders of metal oxides does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Process for the production of ultrafine powders of metal oxides, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Process for the production of ultrafine powders of metal oxides will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3018685