Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Post sintering operation
Reexamination Certificate
1999-10-18
2001-11-20
Mills, Gregory (Department: 1763)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Post sintering operation
C419S028000, C419S036000, C419S037000, C148S513000, C075S343000
Reexamination Certificate
active
06319459
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the removal of organic acid based binders from powder metallurgy compacts.
BACKGROUND OF THE INVENTION
Sintered powder metallurgy compacts having a density significantly below the theoretical density, so as to give rise to porosity throughout the bodies of the compacts, find use in many fields. Powder metallurgy compacts are used as filters for liquids and gases, as catalytic and heat exchange surfaces in chemical reaction chambers, and in the electrical industry as electrodes in flash tubes (strobe lights), as electrodes in wet-cell rectifiers and at least one type of lighting arrestor. Perhaps the most demanding use for porous powder metallurgy compacts is as capacitor anode bodies in the fabrication of electrolytic capacitors.
Powder metallurgy compacts fabricated from valve metals, particularly tantalum are employed as the anodes in electrolytic capacitors and are manufactured on a huge scale, amounting to several billion powder metallurgy anode bodies world-wide annually. The mass production of capacitor anode bodies on such a scale requires the use of automatic anode body compacting presses which must be capable of running many tens of thousands of anode bodies without attention in order to carry-out the anode compact fabrication in a cost-effective fashion.
Experience has demonstrated that compacting press die life may be significantly extended by the incorporation of a low percentage of a binder/lubricant in the valve metal powder used in the fabrication of porous capacitor anode bodies. Typical materials used as binders/lubricants in the fabrication of powder metallurgy capacitor anode bodies include higher molecular weight carboxylic acids, such as stearic acid, amide waxes, such as ethylene diamine bis di-stearamide (sold by Lonza Chemical Company under the tradename “Acrawax”), and polyethylene glycol 8000 (sold by The Union Carbide Corporation under the tradename “Carbowax 8000”). These materials are employed at a concentration of from about 0.1 wt. % to about 5 wt. %, based upon the weight of the metal powder. For tantalum powders, the weight of binder/lubricant is typically 1 wt. % to 2 wt. %.
The binder/lubricant may be mixed with the metal powder either by dry-blending the solids together or by dissolving the binder/lubricant in a suitable solvent, then mixing the binder/lubricant solution with the metal powder and evaporating the solvent (so-called “wet blending”). Following wet-blending, the binder-coated metal powder is typically screened to give a powder composed of agglomerates of the metal powder/binder combination having the flow properties required for mass-production anode fabrication on automatic anode pressing equipment.
As the demand for capacitor miniaturization, ever-increasing capacitance per package size, and reduced valve metal cost per device continues, capacitor powder producers have introduced tantalum powders having surface areas of 0.5 square meter per gram and greater. The use of these finer capacitor powders has led to two major problems in the fabrication of capacitor anode bodies on mass-production equipment. The first of these problems is the generation of a relatively large amount of fine dust during processing through anode compacting presses. This dust, while of a relatively low toxicity, is a fire and explosion hazard, necessitating the use of high air flow ventilation systems in anode pressing areas. The airborne dust from high surface area capacitor powders also finds its way into press bearings, gears, etc., resulting in greatly accelerated wear of these surfaces. It has been found that the so called “wet blending” of the binder/lubricant greatly reduces dust generation and machine wear by anchoring the fine dust in the capacitor powder/binder agglomerates.
The second major problem associated with the high surface area capacitor powders is the removal of the binder/lubricant from the capacitor anode compacts prior to the sintering operation. The binder has traditionally been removed by heating the anode compacts to temperatures ranging from 300° C. to 600° C. under vacuum. With the use of increasingly finer capacitor powders, it has become increasingly difficult to remove the binder before it reacts with the valve metal capacitor powder. The carbon and oxygen remaining after binder removal process tend to react with the valve metal during the sintering operation. Residual carbon and oxygen remaining after the sintering operation tend to cause the anodic oxide film grown on the anode surfaces to be flawed. Flawed anodic oxide is more electrically leaky and less stable than more flaw-free oxide.
One approach to the reduction of residual carbon, etc., in sintered anodes which have been fabricated from binder-containing capacitor powder is the use of polypropylene carbonate as the binder. This material, sold under the tradename of “Q-Pac” by PAC-Polymers, Inc., may be wet-blended with the capacitor powder using an aggressive solvent (e.g., acetone, hot toluene, chlorinated organics, etc.). Polypropylene carbonate is thermally decomposed at 250° C. to yield propylene carbonate, propylene oxide, and carbon dioxide, all of which may be removed under vacuum. As capacitor powder surface areas exceed about 0.5 square meter per gram, especially with materials more active than tantalum (for example, niobium), the removal of the last few hundred ppm of carbon from anode body compacts containing polypropylene carbonate becomes increasingly difficult.
Another approach to the problem of removing binders from anode compacts prior to sintering is described in U.S. Pat. No. 5,470,525 (Tripp, et al.) The inventors employ a water or water and detergent wash in combination with a water-soluble binder (or a binder that can be rendered water-soluble through the use of a detergent) to remove the binder from the anode compacts prior to the sintering step. This method works quite well for anode bodies containing water-soluble binders, such as polyethylene glycol 8000 (Carbowax 8000).
It is pointed-out, however, in PCT International Publication Number WO 98/30348, Title: “Binder Removal”, Bishop et al., that the method of Tripp, et. al. (U.S. Pat. No. 5,470,525) requires a number of hours for both the detergent wash stage and the water rinse stage when water-insoluble binders, such as stearic acid, are employed. Bishop, et. al. employ dilute (i.e., 0.5%) 80° C. solutions of alkali metal hydroxide, such as sodium hydroxide, or ammonium hydroxide, to convert the fatty acid binder, such as stearic acid, to a water soluble salt which is then readily removed via water rinsing.
Unfortunately, with very high surface area capacitor powders, it is difficult to establish circulation of the dilute hydroxide solutions through the pores of the capacitor anode bodies. This lengthens the time required to leach the fatty acid binder from the anode bodies due to the increased time required for complete reaction to form water soluble salt species. The problem is further aggravated by anode size; fatty acid binders are more difficult to remove from larger anodes than from smaller anodes due to the increasing pore path length into the internal portions of the larger anodes. The problem is yet further aggravated by large anode load size and still further aggravated by low rates of leach solution and rinse water flow.
In addition, concentrated solutions of ammonium hydroxide give off noxious fumes at 80° C. and solutions of alkali metal hydroxides which are significantly more concentrated than 0.5% tend to attack even tantalum capacitor powders. 5 wt. % potassium hydroxide gives a visible reaction with tantalum powder at 80° C., resulting in the production of a thick potassium tantalum oxide coating having a purple color and containing over 10% oxygen.
There is a need, therefore, for an efficient method of leaching the higher molecular weight or “fatty” carboxylic acids binders or other organic binders from pressed anode bodies without the need to circulate large quantities of detergent or dilute hydroxide solutions through the
Kinard John Tony
Melody Brian John
Moore Keith Lee
Wheeler David Alexander
Banner & Witcoff , Ltd.
Kemet Electronics Corporation
Mills Gregory
Zervigon Rudy
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