Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Reexamination Certificate
1999-08-19
2001-07-17
Jenkins, Daniel (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
C075S364000, C075S369000
Reexamination Certificate
active
06261337
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
The present invention relates to powders and products of tantalum, niobium, and their alloys having low oxygen contents, and processes for producing the same.
One common method of producing powder metal products of tantalum, niobium or alloys of such metals with each other, and either or both with other metals, is to first cold isostatically press the powder into a preform, such as a bar or rod. The preform is resistance sintered at a relatively high temperature to produce a formed product of tantalum, niobium or their alloys. Generally, for resistance sintering, the ends of the preform are clamped between water-cooled copper terminals in a high vacuum chamber and then the preform is heated by passing an electrical current through the preform. The resistance sintering simultaneously lowers the oxygen content and densifies the preform.
However, there are many disadvantages in utilizing resistance sintering to densify and remove oxygen. First, resistance sintering may only be utilized to produce products of certain limited shapes, generally bars or rods. For resistance sintering, the cross-section of the preform must be uniform along the path of electrical current in order to prevent localized overheating and hot shorting. Additionally, the cross section must be small enough so that the oxygen reduction in the center of the preform occurs before the disappearance of the interconnected porosity. For effective oxygen removal, preforms greater than about 1.5 inches in their shortest dimension are not resistance-sintered. Still further the preform must be small enough to prevent sagging associated with creep and hot pressing during unsupported resistance sintering. Thus, the preforms generally do not weigh greater than about 35 lbs.
The present invention relates to tantalum and/or niobium powder and more particularly to tantalum powders usable in making powder metallurgy components that can be further processed into rod, wire, sheet, foil and other mill or fabricated parts or simply made in net-shape by conventional powder consolidation methods followed by surface finishing and/or small dimensional modifications. Such products are also usable as fully dense coatings to modify the surface chemistry of other mill products or fabricated parts.
It is well known to make tantalum powders for use as sintered anodes for electrolytic capacitors, by hydriding an ingot or chips from an ingot of tantalum, comminuting (taking advantage of the embrittlement that results from this massive hydriding) to powder form and then dehydriding to form tantalum powder. Such powders can be used to produce capacitors with low electrical leakage. In principle such process is also applicable to niobium but it is not very practical.
It is also known to deoxidize tantalum or niobium capacitor powders (however made) in primary or secondary (agglomerated) forms by contact with vapors of alkaline earn, metals to effectively getter oxygen at the powder surface and remove it as an alkaline earth, metal oxide by acid leaching and/or volatilization.
SUMMARY OF THE INVENTION
The invention comprises new powders of tantalum, niobium or alloys of tantalum or niobium having an oxygen content of less than about 300 ppm, preferrably below 200 ppm and more preferrably below 100 ppm. The invention also comprises a method for producing these powders wherein hydrides of tantalum, niobium or alloy powders are heated in the presence of an oxygen-active metal, such as magnesium.
The invention also comprises formed powder metal products having oxygen contents less than about 300 ppm, preferrably below 200 ppm and more preferrably below 100 ppm, formed from tantalum, niobium, and their alloys. I have still further discovered a new process for producing formed powder metal products of tantalum, niobium and their alloys, having very low oxygen contents without resistance sintering.
The present invention utilizes a combination and variation of the two lines of very old prior art development outlined above, taken together with the further realization that this is a way to achieve a powder of very fine size with low oxygen usable in mill products/fabricated parts manufacture. Normally the achievement of fine size (and related high surface area) of powder is associated with high oxygen pick-up deleterious to subsequent processing and use.
It is a principal object of the present invention to provide a method of achieving fine tantalum and/or niobium powder with low oxygen, preferably averaging under 150 micrometer (micron) size and below 300 ppm of oxygen, preferrably below 200 ppm and more preferrably below 100 ppm.
This is accomplished by providing a fine size of tantalum hydride powder of minus 150 microns and mixing it with a small amount of magnesium or calcium, less than ½% of the hydride weight. A precursor of the alkaline earth metal such as a hydride thereof can also be employed. The mixture is heated in a ramping up heating schedule to vaporize the alkaline earth metal and to start reduction of oxygen by the vapor, holding to complete the reaction of oxygen, then cooling, and acid and water washing to leach off residual alkaline earth metal and drying to yield a tantalum powder of low oxygen (typically under 150 ppm) and particle size averaging under 150 microns FAPD (Fisher Average particle Diameter).
An advantage of the powder of the present invention is that it comprises relatively non-spherical particles suited for unidirectional mechanical pressing.
A further advantage of the powder of the present invention is that it comprises relatively small particles well suited for cold isostatic pressing.
An advantage of the formed products of tantalum, niobium or their alloys, of the present invention, is that the products can be of any shape, cross-section or size.
An advantage of the process for producing formed products of the present invention is that the process allows for the production of tantalum, niobium, or alloy formed products having low oxygen content as described above and being of any shape cross-section or size.
In addition to application for Ta, Nb and alloys (Ta—Nb), the invention can also be applied to other refractory metals, e.g., Mo, W, Ti, Zr, Hf, Re and alloys of the same with each other and/or Nb or Ta.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION
The metal powders of low oxygen content of the present invention are produced via the following procedure.
A first metal (tantalum, niobium or alloy) hydride powder is placed into a vacuum chamber, which also contains a metal having a higher affinity for oxygen than the first metal, such as calcium or magnesium, preferably the latter. Preferably, the starting hydride powder has an oxygen content less than about 1000 ppm. The chamber is then heated to the deoxidation temperature to produce a powder of tantalum, niobium or alloy of tantalum or niobium having a target reduced oxygen content of less than about 300 ppm preferrably below 200 ppm and more preferrably below 100 ppm. The magnesium, containing the oxygen, is then removed from the metal powder by evaporation and subsequently by selective chemical leaching or dissolution of the powder.
The alloys of tantalum or niobium of the present invention include alloys of tantalum and/or niobium, either or both with other metals, and further includes incorporation of an oxide, which has a higher free energy of formation than Ta and/or Nb oxide, such as for example yttrium oxide, thorium oxide, or aluminum oxide. The oxide is blended into the tantalum and/or niobium powder having oxygen content of less than about 300 ppm. The alloys of the present invention also include alloys of tantalum and/or niobium and a further alloying element with a low oxygen content blended into the tantalum or niobium powder, provided that the oxygen content of the blend is less than about 300 ppm. The alloys of the present invention further include alloys of tantalum and/or niobium hydride and a further alloying element wherein the alloying element and the tantalum and/or niobium hydri
Cohen Jerry
Jenkins Daniel
Perkins Smith & Cohen LLP
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