Process for preparing nanosize metal oxide powders

Details Process for preparing nanosize metal oxide powders Process for preparing nanosize metal oxide powders

2001-03-28

2003-03-04

Wyszomierski, George (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Processes

Producing or purifying free metal powder or producing or...

C264S005000, C423S592100, C423S598000, C423S600000, C423S608000, C423S612000, C423S628000

Reexamination Certificate

active

06527825

ABSTRACT:

This invention relates to a process for producing nanoscale metals or metal based powders. In particular, this invention relates to a process for preparing nanoscale powders from a metal salt solution and an amphiphilic material.
Metal or metal oxide particles of nanoscale and submicron size are a valuable industrial commodity finding use in many applications, including the manufacture of industrial catalysts such as might be employed in the chemical industry, in the manufacture of ceramics, of electronic components, coatings, manufacture of catalysts, capacitors, mechanical-chemical polishing slurries, magnetic tapes and as fillers, for example, for plastics, paints or cosmetics.
A large variety of techniques are available for the manufacture of metal or metal oxide powders having a very fine particle size. Such techniques include solution processes and high temperature gas phase and condensed phase synthesis. For a comprehensive review of the general techniques available for producing nanosize particles, see for example, “Chemical Engineering Aspects of Advanced Ceramic materials” by V. Hlavacek and J. A. Puszynski published in the Journal of Industrial Engineering and Chemical Research, pages 349-377, Volume 35, 1996. A general overview of sol-gel processing is given in Controlled Particle, Droplet and Bubble Formation, edited by David J. Wedlock, Butterworth-Heinemann Ltd., 1994, pages 1-38.
Despite the numerous procedures available, nanoscale powders are generally expensive and difficult to prepare in large quantities, thus limiting their applications, for example, to high technology ceramics.
A simplified procedure for producing nanometer size particles is described in U.S. Pat. No. 5,240,493. The described process requires the calcination of a polyurethane foam containing a metal cation. In a related procedure U.S. Pat. 5,698,483 describes mixing an aqueous continuous phase containing a metal salt with a hydrophilic organic polymer, forming a gel, and then heat treating the gel to drive off water and organics, leaving as a residue a nanoscale powder. The yield of metal oxide produced from the polymer solution by the disclosed method is very low and is improved by using an intermediate drying step.
It would therefore be desirable to develop a cost effective procedure leading to the production of metal or metal oxide powders having a consistently fine particle size. Itis also desirable to have a procedure that could be operated using a high level of metal in proportion to a polymer. It would also be advantageous if such a procedure were able to provide for the production of metal powders in a high yield.
The present invention is a process for preparing nanoscale metal or metal-based powder by calcining at a temperature sufficient to drive off organics from a composition that comprises (a) a solution containing at least one metal salt (b) an amphiphilic ethylene oxide-containing copolymer wherein the copolymer has an average molecular weight of greater than 400, an ethylene oxide content of 1 to 90 percent and an hydrophilic-lipophilic balance (HLB) of between −15 and 15 and (c) optionally a coagulating agent, with the proviso that when aluminum is the sole metal, a coagulating agent is present.
The process produces metal-based powders of high purity and uniform size. The paste formed in the present process by the mixing of the metal salt and copolymer contains a high concentration of metal as compared to other known processes. The formation of a paste with a high metal concentration is advantageous as this reduces the amount of water that needs to be removed from the paste prior to or during calcination and decreases cost versus existing technologies.
In accordance with the process of the present invention, it has been unexpectedly found that by mixing at least one metal salt with an amphiphilic copolymer containing ethylene oxide, higher concentrations of metal salt and higher salt to copolymer ratios can be used as compared to similar known processes using hydrophilic polymers. The use of a high salt to copolymer ratio minimizes the decrease in activity of the salt solution upon addition of the copolymer. Activity is defined in the present context as grams of metal oxide obtained after calcination of 100 grams of metal salt solution or metal salt/copolymer paste. Additionally, the present process gives a substantial increase in the surface area of the nanoscale size particles compared to the surface area of particles prepared in the absence of the copolymer. Nanoscale particle means the primary particle or crystalline size is about 200 nanometers or less, preferably in the range of from 5 to 100 nanometers.
Mixing a copolymer with a metal salt according to the present invention produces a metal salt/copolymer paste. The term “paste” as used herein means a soft, smooth solid or semisolid. When the copolymer is added to the metal solution, a paste is formed.
The copolymers suitable for use in the present invention are amphiphilic copolymers containing ethylene oxide wherein the ethylene oxide content is between 1 and 90 percent. The percent ethylene oxide is the weight percent of ethylene oxide units in the total weight of the copolymer. Preferably the ethylene oxide content is greater than about 5 percent of the copolymer. More preferred are copolymers where the ethylene oxide content is about 8 percent or greater. Most preferred are copolymers where the ethylene oxide content is about 10 percent or greater. Preferably the ethylene oxide is less than about 80 percent of the copolymer. More preferred are copolymers where the ethylene oxide content is less than about 75 percent. In a preferred embodiment of the present process, the copolymers are block copolymers containing ethylene oxide.
The term amphiphilic as used herein means a compound which has a HLB between −15 and 15 as calculated per Davis,
Proc. Intern. Congr. Surface Activity,
Vol. 1 London 1957, p. 426. The procedure assigns numeric values to various groups, for example, hydrophilic groups —SO
4
−
Na
+
, —COO
−
K
+
, and —COOH are assigned values of +38.7, +21.1 and +2.1 respectively; the hydrophobic groups>CH—, —CH
2
—, and —CH
3
are all assigned a value of −0.475. For a given structure, the HLB number is calculated by substituting the group numbers into the following equation:
HLB=&Sgr;
(hydrophilic group numbers)+&Sgr;(lipophilic group numbers)+7.
Preferably the copolymers used in the present process have an HLB of greater than −10 and preferably less than 13. More preferred are copolymers having an HLB balance of −5 to 10.
Hydrophilic compounds as defined by the above HLB, show a tendency to be fully miscible with water in all proportions under ambient conditions, or in the case of solid materials, at some elevated temperature slightly above their melting point, (for example, about 60° C. for high molecular weight linear polyethylene oxide polymers). In contrast, lipophilic compounds show a tendency to be totally immiscible with water, even at elevated temperatures. The range of HLB values for copolymers of the present invention represent an intermediate case comprising materials which form liquid two-phase systems upon mixing with water (or for solids, after mild heating) such that at least one of the two phases contains more than trace amounts of the opposing phase. For the purpose of the present invention, this intermediate class is designated as amphiphilic, in contrast with hydrophilic and lipophilic classes representing, respectively the upper and lower segment of the HLB value range. In summary, as used herein an HLB>15 represents hydrophilic compounds; an HLB of −15 to 15 represents amphiphilic compounds; and an HLB of<−15 represents lipophilic compounds.
In addition to the ethylene oxide content of the copolymer, to obtain the desired yield of metal oxide, the copolymers for use in the present invention have an average molecular weight of greater than 400. Preferably the copolymers have an

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