Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Powder pretreatment
Reexamination Certificate
2000-09-11
2002-11-26
Jenkins, Daniel J. (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Powder pretreatment
Reexamination Certificate
active
06485676
ABSTRACT:
FIELD OF THE INVENTION
The invention is directed to a method of thermally removing binder from porous compacts pressed from metallic or ceramic materials using atmospheric pressure. The invention is further directed to binder-free compacts produced with the method.
BACKGROUND OF THE INVENTION
Porous, sintered powder metallurgy and ceramic compacts have been used for many years for a wide variety of applications, including fluid filters (for both liquids and gases), oil mist eliminators, catalyst beds, electrolytic lightning arresters, electrodes for gas lighting and strobe light tubes, for oil-impregnated bearing surfaces, and for electrolytic capacitor anodes.
While some metal powders and a few ceramic powders can be pressed to significantly less than the theoretical densities of the materials without the use of a binder to help hold the pressed compacts together, most materials cannot be so-compacted successfully. Because of the poor strength of powder metallurgy or ceramic material compacts pressed at sufficiently low densities to yield significant pressed-compact porosity, most compacts pressed from these materials contain a binder which aids in increasing the pressed-compact strength prior to a sintering operation. Binders also act to lubricate the punches and dies during the pressing operation, thereby extending tooling life and minimizing downtime for press repairs/tooling changes. The presence of a binder within the pores of a powder metallurgy or ceramic compact also helps to provide an open pore structure at a given as-pressed density, resulting in effectively increased compact porosity.
For many applications, such as filters, bearings, oil mist eliminators, and gas tube electrodes, a small amount of binder decomposition products retained within the bodies of the sintered compacts presents no problem with respect to the end use of the sintered compacts. For other applications, such as catalyst beds or electrolytic capacitor anodes, binder residues, usually present as carbon or metallic carbides, present significant problems with respect to the end use of the sintered compacts. The residual carbon contamination resulting from binder decomposition/reaction tends to give rise to flaws in the anodic oxide films grown on sintered valve metal compacts used as electrolytic capacitor anodes, for example, leading to increased leakage current and short-circuit failures of the finished devices containing flawed dielectric oxide films. Although there appears not to be a threshold value for carbon contamination for tantalum anodes, for example (i.e., there does not seem to be a carbon content from binder residuals below which there is no problem and above which the onset of problems is observed), it is generally agreed upon by those in the industry that carbon should be below 100 ppm in sintered tantalum anodes and preferably as low as possible.
The highly variable purity requirements for sintered compacts, depending on the end use of the parts, has led to the use of a wide variety of binder materials. Powder metallurgy or ceramic powder compacts for those applications not requiring a low residual post-sintering carbon content are frequently pressed with binders such as paraffin, polyethylene Glycol 8000, glyptal-brand glycerine polyester, etc. For those applications requiring low residual post-sintering carbon levels, but where the particle size of the material is relatively coarse (e.g., 10 microns and larger) and the material is of a relatively inert nature (bronze, stainless steel, etc.), the above binders are still found to have merit.
For applications involving reactive materials and having low post-sintering residual carbon requirements such as tantalum powder metallurgy electrolytic capacitor anodes, there has been an ongoing search for binders having lubricity during the pressing operation and sufficiently high vapor pressure for ready removal at elevated temperatures.
One binder material used for many years in the fabrication of tantalum electrolytic capacitor anodes is ethylene diamine bis-disteramide, sold under the brand name “Acrawax” (manufactured by the Lonza Corp.) With tantalum powders having low to medium surface area, e.g., 25,000 microfarad volts/gram or approx. 0.25 square meters/gram (sintered surface area), vacuum distillation at temperatures of up to 400° C. or more will reduce the residual binder content to the point that a post-sintering anode carbon content below 100 ppm is readily achievable.
As the demand for higher surface area tantalum powders (for purposes of economy and volumetric efficiency) has led to the introduction of powders having surface areas (in the sintered state) in excess of 0.5 square meter per gram and CV products in excess of 50,000 micro farad volts (micro coulombs) per gram, it has become increasingly more difficult to reduce the residual post sintering carbon content to acceptable levels. Traditional binders, such as Acrawax , stearic acid, camphor, etc., partially decompose during vacuum distillation from high surface area tantalum powders, resulting in post-sintering carbon levels in excess of 100 ppm.
In the production environment found in the powder metallurgy electrolytic capacitor industry, it has proven impractical to press high surface area tantalum and other valve metals and valve metal compounds (such as nitrides and suboxides, etc.) into capacitor anode compacts without binder due to the dust generation, abrasive wear, and inadequate pressed compact strength associated with binderless pressing. The problem of adequate removal of binders, necessary for the efficient pressing of capacitor anodes under production conditions, from high surface area anode compacts has been solved in an ingenious manner by Tripp et al. U.S. Pat. No. 5,470,525 describes a method of water leaching of water soluble binders from tantalum anode compacts. This water leaching method of binder removal was extended to water-insoluble acidic binders, such as stearic acid, with the method of using alkali metal hydroxide leach solutions described in PCT No. WO98/38348. This last method suffers from the disadvantage of corrosive attack of the anode bodies if hot and or even mildly concentrated (5%) hydroxide solutions are employed (binder removal is more efficient with hot, concentrated solutions.) In co-pending application, Ser. No. 09/419,893, the problem of anode compact attack by the leaching solution was addressed by the substitution of one or more alkanolamines for the alkali metal hydroxide.
Unfortunately, leaching of capacitor compacts results in partial or total disintegration of the anode bodies when these are pressed from unagglomerated valve material powders. The problem can be overcome through employment of the binder (dimethyl sulfone) and vacuum distillation binder removal method described in our co-pending application Ser. No. 09/397,032.
Although the binder (dimethyl sulfone) and methods of application in the copending application were found to yield minimal residual post-sintering carbon levels with valve metal powders with both water leaching and vacuum distillation binder removal methods, these methods are not ideal from the standpoint of process cost and throughput. Water leaching requires not only a relatively large volume of high purity water, but also a post-leaching drying step to remove the residual water prior to sintering to avoid excessive out gassing during the sintering operation. Vacuum distillation removal of dimethyl sulfone from pressed capacitor anode compacts, while efficient and thorough, requires a closed system capable of withstanding both heat and vacuum, as well a vacuum pump, etc.
SUMMARY OF THE INVENTION
The invention is directed to a method of removing a binder, in particular dimethyl sulfone, from pressed compacts, such as anode bodies, comprising heating, preferably to about 100° C. and about 350° C., the pressed compacts at about atmospheric pressure and circulating or passing a sweep gas over the pressed compacts for a time sufficient to evaporate the binder from the pressed compacts and remove the evap
Kinard John Tony
Melody Brian J.
Moore Keith Lee
Stenzinger Duane Earl
Wheeler David Alexander
Banner & Witcoff , Ltd.
Jenkins Daniel J.
Kemet Electronics Corporation
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