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Method of making composite particles with tailored surface characteristicsRelated Patent Categories: Specialized Metallurgical Processes, Compositions For Use Therein, Consolidated Metal Powder Compositions, And Loose Metal Particulate Mixtures, Processes, Producing Or Purifying Free Metal Powder Or Producing Or Purifying Alloys In Powder Form (i.e., Named Or Of Size Up To 1,000 Microns In Its Largest Dimension)Method of making composite particles with tailored surface characteristics description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060150770, Method of making composite particles with tailored surface characteristics. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND [0002] Materials have properties that can be ascribed to either surface or bulk characteristics. These parameters include catalytic activity, electrical and thermal conductivity, optical, electronic, chemical and mechanical properties. Particles with metallic exteriors of tailored compositions and thickness can exploit these properties while providing control over other properties such and density, material cost and particle behavior. [0003] Such metals have application in a wide variety of fields, including but not limited to water treatment, remediation, diagnostic medicine, drug delivery, and cosmetics. For example, zero valent metals may be useful as fillers, such as those for thermal and electrical conductivity, capacitance, charge dissipation and electromagnetic interference and absorbance. Such metals also may be used as magnetic and/or luminescent materials, taggants, pigments, conductive inks and coatings, sensors, dopants, alloys, sintering aids, and catalysis supports such as in fuel cells and batteries, for example. Additionally, zero valent metals may be useful for infrared missile decoys, for example as chaff or pyrophoric tracers. [0004] Zero valent metals are known to supply electrons that enable and sustain electrochemical reactions. Electrochemical reactions include those that convert toxic chemical compounds into innocuous products. For example, halogenated chemical compounds, such as trichloroethylene and carbon tetrachloride, can be converted to hydrocarbons such as ethane and methane, for example. Similarly, toxic compounds such as chromium (VI) (Cr.sup.+6), lead (II) (Pb.sup.+2) and arsenic (V) (As.sup.+5), for example, can be converted to compounds having less toxic oxidation states or aqueous solubility. Therefore, zero valent metals have a wide variety of uses in the field of remediation, such as in ex-situ and in-situ electrochemical reduction, and in ex-situ and in-situ immobilization. The foregoing examples provide only an indication of the wide variety of uses for zero valent metals, such uses being clearly understood and appreciated by any person having ordinary skill within arts utilizing metal particles. [0005] In the field of remediation, the targeted substances are often located underground. A promising in-situ remediation protocol uses zero valent metals that are injected into the subsurface where they react with and destroy the targeted contaminants. It is difficult or impossible to transport coarse microscale particles through the subsoil as the particles settle rapidly and are strained or filtered before they reach the contaminant phase. Smaller sub-micrometer and nanoscale particles can more easily fit within the interstices of soil particles and travel greater distances through the sub-surface to reach the contaminant phase. [0006] In the field of remediation and catalysis, the targeted reactions take place on the particle surface. Therefore, it is desirable for the particles to have a high surface area. The relationship between particle size and specific surface area is shown in Equation (1) where d is the diameter of a spherical particle, SSA is the specific surface area (area/mass) and .rho. is particle density (mass/volume). d=6(.rho.SSA).sup.-1 (1) This expression reveals an inverse relationship between specific surface area and particle size. Smaller particles offer more surface area per unit mass and are often desirable for remediation applications. [0007] Commercially available zero valent metal particles are typically microscale with dimensions greater than one micrometer and these large particles offer a low specific surface area. Using iron as an example, spray atomized and electrolytic iron particles typically offer dimensions greater than 10 micrometers and have an active surface area of only about 0.1 to 0.2 m.sup.2/g. Carbonyl iron particles that are produced by condensation from the vapor phase typically offer dimensions of 1 to 10 micrometers and have an active surface area of only about 0.5 to 1 m.sup.2/g. The large particle size makes this product unsuitable for many applications including in-situ remediation, for example. [0008] Making nanocrystalline and sub-micrometer metal particles can alleviate problems associated with microscale product. Nanoscale zero valent metal particles having a surface area in excess of 20 m.sup.2/g has been produced by various methods. One method involves the electrochemical reduction of a metal salt solution in water. Equation (2) shows the electrochemical reduction of a trivalent iron ion with a borohydride ion. 4Fe.sup.+3+3BH.sub.4.sup.-+9H.sub.2O.fwdarw.4Fe.sup.0+3H.sub.2BO.sub.3.su- p.-+12H.sup.++6H.sub.2 (2) This is a slow and expensive process that results in material having very limited commercial use. The particles also have residual boron that is undesirable for some applications. These particles also tend to form aggregates of smaller particles. Aggregated particles still provide a high surface area but behave like coarse particles when injected underground and offer less than ideal sub-surface mobility. [0009] Another method that is employed to make high surface area zero valent metal involves the thermal reduction of a metal oxide. The metal oxide particles are heated in a reducing atmosphere (typically hydrogen or carbon monoxide) to produce zero valent metal. An example is provided in equation (3) and involves the reaction of hematite (Fe.sub.2O.sub.3) with hydrogen gas. Fe.sub.2O.sub.3+3H.sub.2.fwdarw.3H.sub.2O+2Fe.sup.0 (3) Although less expensive than chemically precipitated product, this product is also prohibitively expensive for most applications. This product is also aggregated with less than ideal underground mobility. [0010] Another method for producing zero valent metal particles involves the chemical precipitation of zero valent iron onto the surface of an inert carrier particle, as described in U.S. Pat. No. 6,689,485. Using iron salt and borohydride precursors in water and a reaction similar to that shown in equation (2) produces nanocrystalline iron deposits on the carrier surface. This procedure also offers the disadvantages of expensive chemical precursors and difficulties in scaling the process. [0011] Moreover, chemically precipitated particles are synthesized in water and all of the materials, including thermally reduced particles, are stored and shipped in water. Metal reacts with and electrochemically reduces water as depicted in equation (4). This consumes the iron prematurely and can also lead to the formation of solid surface deposits of water insoluble metal hydroxides (show in equation (5)), oxides, and oxyhydroxides. These non-metallic substances can prematurely passivate the particle surface and inhibit reactivity. Fe.sup.0+H.sub.2O.fwdarw.Fe.sup.+2+2OH.sup.-+H.sub.2 (4) Fe.sup.+2+2OH.sup.-.fwdarw.Fe(OH).sub.2 (5) [0012] Other efforts have focused on the mechanical attrition or communition of coarse iron particles into material having smaller particles. This methodology is ineffective for producing discreet, sub-micrometer or nanoscale particles, since iron is ductile at room temperature and does not become brittle unless cooled to cryogenic temperatures. When electrolytic iron is processed in a stirred media mill with spherical steel media, instead of breaking down into smaller particles the particles are mechanically deformed and flattened into particles with a flake morphology and a surface area of 1-2 m.sup.2/g. FIG. 2 is a SEM micrograph showing the particles resulting from this process. The particles are very large, typically 10 micrometer to 100 micrometer. These are poorly suited for remediation applications. [0013] To date, there is no cost-effective and easy way to manufacture large quantities of the high surface area zero valent metal particles of the type described herein, such particles having a wide variety of uses in a large number of fields. SUMMARY OF THE INVENTION [0014] The present invention is a cost-effective and scalable technique for producing sub-micrometer and nanocrystalline zero valent metal particles and particle suspensions by depositing metal onto the surface of a host particle. The particles typically offer a surface area of greater than 5 m.sup.2/g and can be inexpensively produced on a large scale. Moreover the particles have reactivity superior to currently available microscale metal particles. BRIEF DESCRIPTION OF THE DRAWINGS [0015] FIG. 1(a) depicts a host particle (010) prior to mixing with metal. [0016] FIG. 1(b) depicts a host particle (010) with a metal surface layer (011). [0017] FIG. 2 is a scanning electron microscope (SEM) picture of particles produced by mechanical attrition of coarse iron particles. [0018] FIG. 3 is a scanning electron microscope (SEM) picture of particles produced according to the methods disclosed herein. [0019] FIG. 4 is a scanning electron microscope (SEM) picture of particles produced by thermal reduction. DETAILED DESCRIPTION [0020] Throughout this specification, the terms "a" and "an" and variations thereof represent the phrase "at least one." In all cases, the terms "comprising", "comprises" and any variations thereof should not be interpreted as being limitative to the elements listed thereafter. Unless otherwise specified in the description, all words used herein carry their common meaning as understood by a person having ordinary skill in the art. In cases where examples are listed, it is to be understood that combinations of any of the alternative examples are also envisioned. The scope of the invention is not to be limited to the particular embodiments disclosed herein, which serve merely as examples representative of the limitations recited in the issued claims resulting from this application, and the equivalents of those limitations. [0021] Zero valent metals supply electrons that enable and sustain electrochemical reactions (reduction-oxidation, redox). Equation (6) depicts the corrosion of a zero valent metal (M) to produce a metal ion (M.sup.+, valence of x) and x electrons (e.sup.-). M.sup.0.fwdarw.M.sup.+x+xe.sup.- (6) Continue reading about Method of making composite particles with tailored surface characteristics... Full patent description for Method of making composite particles with tailored surface characteristics Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Method of making composite particles with tailored surface characteristics patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. 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