Chemistry of inorganic compounds – Silicon or compound thereof – Binary compound
Reexamination Certificate
1997-09-24
2001-04-10
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Silicon or compound thereof
Binary compound
C423S439000, C423S440000
Reexamination Certificate
active
06214309
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to carbide powders. In particular, this invention relates to a method for synthesizing micron- and submicron-sized, high purity, high surface area, nanostructured carbide powders from oxides using a high energy milling step.
2. Brief Description of the Prior Art
Micron-, submicron-, and nanosized carbide powders are important materials for engineering applications. As used herein, “micron-sized” powders refers to powder particles wherein the mean particle size is equal to or greater than about 1.0 micron. “Submicron-sized” powders refers to powder particles wherein the mean particle size is in the range from about 0.1 to about 1.0 microns. “Nanosized” powders refers to powder particles wherein the mean particle size is less than about 0.1 micron (about 100 nm). In certain instances, micron- and submicron-sized particles comprise many nanosized crystalline grains rather than a single, large grain. In these cases, the particles are referred to as “micron- and/or submicron-sized particles with nanostructures” or “micron- and/or submicron-sized, nanostructured particles”. Nanosized and micron- and/or submicron-sized, nanostructured particles have a high fraction of atoms located at the grain boundaries of the particle. Such materials accordingly have different, and often advantageous properties compared to conventional particles having the same chemical compositions.
Currently, the primary process for the production of micron- and submicron-sized carbide powders is high-temperature, carbothermic reduction of the corresponding oxides by carbon powders. For example, most industrial silicon carbide (SiC) powders are manufactured via the Acheson process through carbothermic reduction of silicon dioxide (SiO
2
) by carbon powder at temperatures up to 2000-2300° C. for 30 hours. A carbothermic method based on reduction of one or more metal oxides reacted with a binder material and a source of carbon is disclosed in U.S. Pat. No. 4,784,839 to Bachelard et al. Similarly, as described in UK Patent No. 811,906 (issued in 1959), industrial titanium carbide (TiC) powders are produced through reduction of titanium dioxide (TiO
2
) by carbon at temperatures ranging from 1700 to 2100° C. for 10-20 hours. U.S. Pat. No. 5,417,952 to Rasit et al. discloses carbothermic reduction of a titanium precursor after pyrolytic deposition of the titanium.
Advantages of using high-temperature carbothermic reduction for the production of carbides include low cost of the oxide raw materials, and ease in scale-up for tonnage-level production. However, the final products have a wide range of particle sizes, and moreover are normally larger than one micron, due to high reaction temperatures and long reaction times. Milling after carbothermic reduction is required. Undesirable inhomogeneities are also frequently found in the stationary reaction mix. These inhomogeneities are due to diffusion gradients established during the reduction reaction, and require extensive milling and purification procedures in order to convert the as-synthesized products into high quality, sinterable powders. The SiC powder produced by the Acheson process, for example, has a large grain size and is contaminated with oxygen. Accordingly, there remains a need for a method whereby homogenous carbide powders may be produced having a controlled and uniform size, without extensive milling and purification procedures.
SUMMARY OF THE INVENTION
The above-described drawbacks and deficiencies of the prior art are alleviated by the method of manufacture of the present invention, wherein carbide powders are produced from their corresponding oxides using high energy milling of selected oxide powders with carbon powder at ambient temperature, followed by heating the milled powder mixture to between 500 and 1500° C. Preferably, carbon monoxide is removed from the reaction chamber during heating in order to drive the reaction to completion at low temperatures and/or short times. The high energy milling step serves to mix the oxide and carbon on a nanosized scale and to increase the reactivity of the reactants by increasing surface area, introducing structural defects and internal strains, and transforming the crystalline materials to an amorphous state. The method in accordance with this invention produces high purity, high surface area, micron- or submicron-sized carbide powders having a narrow particle size distribution and internal nanostructures. The method is conducted at low temperatures, for short processing times, and thus significantly lower cost.
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Ren Ruiming
Shaw Leon L.
Yang Zhenguo
Cantor & Colburn LLP
Griffin Steven P.
Hendrickson Stuart L.
University of Connecticut
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