Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Reexamination Certificate
1999-03-18
2001-01-30
Dunn, Tom (Department: 1754)
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
C075S594000, C075S595000, C075S596000, C075S597000, C075S598000
Reexamination Certificate
active
06179897
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the production of variable density metal vapors, including magnesium vapor, that can be used to form uniform powders of metal oxides. More specifically, the invention relates to a method for the production of metal vapors formed from the decomposition of metal carbides which bypass the liquidus phase.
Powders formed from metal oxides are widely used in a variety of industries as ceramic materials, catalysts, pigments, coatings and fillers. Some metal oxides, such as magnesium oxide and calcium oxide, have excellent heat resistance and electrical insulating properties and are especially useful in the electronics industry.
There are several standard ways to produce metal oxide powders. The most obvious approach is direct elemental synthesis. For example, MgO can be formed either by burning metal Mg ribbon or entraining Mg vapor in a carrier gas stream. The formula for this reaction is:
2Mg(vapor)+O
2
(vapor)→2MgO(solid) (1)
While this method is simple in principle, it can be difficult in practice since it requires processing at high temperature which is not easily controlled. Also, this method can leave unreacted Mg in the final product and it is difficult to control the particle size of the metal oxide that is formed by such a method.
Another commonly used method for forming a metal oxide is the thermal decomposition of either the hydroxide or carbonate form of the metal. For example, MgO can be synthesized via the thermal decomposition of either magnesium hydroxide, magnesium carbonate or magnesium formate. For the hydroxide form, the formula for the reaction process is:
Mg(OH)2→MgO+H
2
O (2)
This approach has the advantage of being simple, but irregular surface morphology, particle size distributions and surfaces terminated with hydroxyl groups are serious draw-backs.
It has been found that the particle size of metal oxide powders effects their characteristics and that very fine metal oxide powders exhibit unique properties that are not found in larger size particles. In particular, very fine metal oxide particles exhibit different magnetic and optical properties from those of larger metal oxide particles. This size dependant variation of physical properties is often desirable and so there is a need in the industry for a process that can produce small-size metal oxide particles.
It is known that fine metal oxide particles can be produced by various methods including liquid phase reaction methods and gas phase reaction methods. The gas phase reaction methods have been found to offer significant advantages because the oxidation of the metal vapors allows the particle size of the metal oxides to be controlled. The present invention provides a method for producing high purity metal vapors that are useful in forming metal oxide powders.
SUMMARY OF THE INVENTION
The present invention provides a method for producing a metal vapor that includes the steps of forming a metal carbide from a solid metal and a source of carbon in an oxygen-free and water-free atmosphere; and heating the metal carbide to form a vapor of the metal; wherein the solid metal is transformed to a vapor without transforming the metal to a liquid phase. Preferably, the source of carbon is graphite and the oxygen-free and water-free atmosphere is a dry, rare gas atmosphere, wherein the dry, rare gas is preferably argon gas.
The metal carbide is formed by heating the metal and the source of carbon to a first temperature which is preferably above the melting point of the metal. After, a period of time at the first temperature, the metal carbide is heated to a second temperature which is above the boiling point of the metal.
The molar ratio of the source of carbon to the metal is at least 2 to 1 and can vary depending on the metal that is being vaporized and the carbon source. The form and size of the metal and carbon materials that are used is also important. To provide rapid initial reaction time, the materials are preferably in the form of chips having a size of less than 5 cm.
In another embodiment, the present invention provides a method for producing a metal vapor that includes the steps of combining a metal and a carbon source, preferably graphite, in a vessel to form a mixture; heating the mixture to a first temperature in a dry, rare gas atmosphere to form a compound of the metal and carbon; maintaining the first temperature for a period of time; heating, the compound to a second temperature to form a metal vapor; and withdrawing the metal vapor from the vessel.
The pieces of metal and graphite are small in size so that they can be intimately mixed tog,ether into a uniform mixture, preferably the metal and graphite are in tile form of chips having a size of less than 5 cm. Graniular particles of metals and graphite are particularly well suited for the process. The molar ratio of the graphite to the metal is at least 2 to 1 and may be as high is 10 to 1 for some metals. The metal and graphite mixture is placed in a vessel, preferably a graphite crucible, and slowly heated to a first temperature which is greater than the melting point of the metal. As the metal begins to melt, a metal carbide compound is formed from the metal and the carbon in the graphite. The vessel has a dry, rare gas atmosphere preferably argon gas, which is substantially inert and does not react with the metal.
When the temperature of the mixture of metal and graphite reaches the melting point of the metal, the heat is maintained constant so that the temperature remains at approximately the melting point for a short period of time. Approximately 2 to 3 minutes is sufficient for most metals but additional heating, does not adversely effect the process. At the end of this short period of time, the metal carbide compound is heated to a second temperature which is greater than the boiling point of the metal to vaporize the metal portion of the metal carbide compound.
The metal vapors formed by the method of the present invention can be oxidized to produce metal oxide powders. A dopant can be combined with the metal vapors, during or just prior to oxidation, to produce modified metal oxides. In a preferred embodiment, the metal carbide is contacted with a dopant either before the metal carbide is formed or prior to heating the metal carbide to form the metal vapor. The dopant can be any metal powder, with iron, copper, nickel and zinc being the preferred metal powders.
A preferred embodiment of the present invention is a method for the production of magnesium vapor comprising: combining magnnesium and graphite in a vessel to form a mixture; heating the mixture at a first temperature in a dry, rare gas atmosphere, preferably argon gas, to form a compound of magnesium and carbon; maintaining the first temperature for a period of time; heating the compound to a second temperature to form a magnesium vapor; withdrawing the magnesium vapor and rare gas from the vessel; and optionally separating the magnesium vapor from the rare gas.
The molar ratio of graphite to magnesium is at least 2 to 1 and preferably at least 3 to 1. The magnesium and graphite are small in size so that they can be easily mixed together and provide the maximum surface contact between the two materials. Preferably, the magnesium and graphite are in the form of chips having a size of less than 5 cm. The mixture of magnesium and graphite is heated to a first temperature of between 600 C and 750 C, preferably between 650 C and 700 C, and is kept at this temperature for several minutes, preferably 2 to 3 minutes, before being heated to a second temperature of between 1050 C and 1150 C, preferably about 1100 C.
The magnesium vapors formed by the method of the present invention can be oxidized to produce magnesium oxide powders. A dopant can be combined with the magnesium vapors, during or just prior to oxidation, to produce modified magnesium oxides. In a preferred embodiment, the magnesium carbide is contacted with a dopant either before the magnesium carbide is formed or prior to
Kunnmann Walter
Larese John Z.
Bogosian Margaret C.
Brookhaven Science Associates
Dunn Tom
Nguyen Cam N.
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