Stock material or miscellaneous articles – All metal or with adjacent metals – Having metal particles
Reexamination Certificate
1999-09-23
2001-05-01
Jones, Deborah (Department: 1775)
Stock material or miscellaneous articles
All metal or with adjacent metals
Having metal particles
C419S023000, C419S035000, C419S064000, C419S065000, C419S066000, C427S212000, C427S216000, C428S558000, C428S539500, C428S403000, C428S407000, C428S660000, C428S662000
Reexamination Certificate
active
06224990
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to coating valve metal powder with polypropylene carbonate as a binder/lubricant and use of the coated valve metal powder in manufacturing powder metallurgy compacts such as capacitor anodes.
BACKGROUND OF THE INVENTION
During the past several decades, the use of capacitors formed using valve metal powders has grown exponentially. This increase is mainly due to a large growth in the use of solid tantalum capacitors. Solid tantalum capacitor use has increased due to their high reliability, high capacitance per unit volume, and wide variety of surface-mount configurations available.
Also contributing to the popularity of solid tantalum capacitors is the continuing decrease in cost per unit capacitance for these devices. The reduction in the cost per unit capacitance is, in part, the result of the increasing economy of scale. As ever greater numbers of the devices are manufactured ever more quickly, the fixed costs per capacitor are reduced, thereby fueling the market for these devices. Another very important factor in the continuation of cost reduction for solid tantalum capacitors is the availability of finer, higher surface area tantalum powders. The use of tantalum powders, having greater surface area per unit weight, allows the use of less tantalum powder per device, thereby facilitating a savings in the “contained tantalum” component of device cost.
Unfortunately, as tantalum capacitor powders having higher surface areas per unit weight have come into use, several disadvantages of these finer (i.e. smaller particle size) powders have become apparent. Finer powders exhibit less-than-ideal flow characteristics during the anode pressing process. The generally slower and less even flow characteristics of finer tantalum powders results in less uniform anode weights unless slower machine speeds are employed; this, in turn, makes the anode fabrication process less efficient as fewer parts are produced per unit of time.
The finest particles present in higher surface area capacitor powders tend to become airborne readily during processing on anode presses, necessitating expensive explosion-resistant high air flow rate exhaust systems to prevent injury to workers and to reduce the fire/explosion hazard from airborne dust. The dust from high surface area capacitor powders has also proven to be highly abrasive in contact with the dies, punches, sliding, and rotary bearing surfaces of anode pressing equipment. The presence of the fine dust from high surface area tantalum powders requires the use of more precise punch and die tolerances, cemented carbide tooling in place of hardened steel, and frequent bearing replacement, all of which add to the cost of capacitor anode fabrication with these powders.
The simple expedient of employing a powdered binder/lubricant material, such as ethylene diamine bis d-stearamide (sold under the tradename of “Acrawax”, by the Lonza Corporation) in mechanical mixture with the higher surface area capacitor powders imparts lubricity to these powders, minimizing anode press repairs due to wear, but results in very little improvement in flow properties or fine dust generation.
The coating of fine capacitor powders with binder/lubricant via tumbling the powders in a solution of the binder/lubricant (such as a solution of the binder, stearic acid, dissolved in one or more chlorinated solvents and/or acetone), followed by dynamic vacuum-drying of the binder-coated capacitor powder in a Patterson-Kelly V-shell type blender results in a reduction of fine powder dust generation, as well as improved pressing equipment lubrication, but does not address powder flow considerations.
An additional problem is observed with high surface area capacitor powders, such as tantalum having a surface area above about 0.3 square meter per gram, which is that traditional binder/lubricant materials become increasingly more difficult to remove completely. Tantalum powders having a surface area of 0.4-0.5 square meter per gram, mixed with 1% to 5% stearic acid or ACRAWAX (with or without the use of a solvent) and pressed into 0.1 gram anode pellets are frequently found to contain 300 to 400 ppm carbon after a thermal binder removal step in vacuum and 150 to 200+ ppm carbon following the vacuum sintering step used to produce the finished anode bodies prior to electroprocessing (anodizing, counter-electrode fabrication, and encapsulation). The level of carbon remaining in the anode bodies after vacuum-sintering is proportionally higher with progressively finer capacitor powders and larger anode size.
The presence of carbon on the valve metal surfaces within the interstices of the anodes after vacuum sintering leads to the production of anodic oxide having flaws or high electrical leakage regions. These flaws are thought to be due to the presence of spots of tantalum carbide on tantalum anode surfaces; the tantalum carbide is thought to give rise to holes or thin spots in the tantalum oxide film which conduct electricity under the application of voltage (this leakage current mechanism is discussed in Young's 1961 book,
Anodic Oxide Films,
in the chapters dealing with tantalum). Whatever the mechanism, the correlation between elevated levels of carbon in anodes after vacuum sintering and high finished device leakage currents has been empirically established for many years.
SUMMARY OF THE INVENTION
The present invention is directed to a high surface area valve metal powders coated with polypropylene carbonate by tumbling the valve metal powder in a solution of polypropylene carbonate in a suitable solvent, such as acetone, and then statically drying the coated powder. The polypropylene carbonate behaves as a binder/lubricant in the manufacture of powder metallurgy capacitor anodes.
The present invention is further directed to a method of preparing powder metallurgy anodes with compacted coated valve metal powder wherein the coated valve metal powder is prepared by tumbling the valve metal powder in a solution of polypropylene carbonate in a suitable solvent and statically drying the coated powder.
DETAILED DESCRIPTION OF THE INVENTION
It is known to coat valve metal powders such as tantalum powders with polypropylene carbonate and then dry the coated powder using dynamic means, e.g. tumbling in a drier. It was discovered that coating the valve metal powders, in particular tantalum powders, with polypropylene carbonate and then drying the coated powder using static means produces a coated product having better properties. Specifically, the method of drying the polypropylene carbonate-coated valve metal has an unexpected and profound effect upon the suitability of the powder for capacitor anode fabricator.
Valve metal powders coated with polypropylene carbonate using the static drying method of the present invention have higher flow rates and reduced fine dust generation than uncoated powder or powder coated and dynamically dried. This allows the use of higher valve metal capacitor body pressing rates, as well as a reduction in the rate of wear of press components, thereby reducing the cost of anode fabrication. Thus, the polypropylene carbonate-coated powder, prepared in accordance with the invention, is lubricious toward anode press components due to the lubricity of the polymer and the relative absence of “fines” in the powder.
In accordance with the invention, a high surface area valve metal powder is coated with polypropylene carbonate by tumbling the powder in a solution of polypropylene carbonate in a suitable solvent. Then, the coated is dried using static means.
The valve metal powder may be any suitable valve metal powder used in the preparation of powder metallurgy compacts. Such valve metal powders include, but are not limited to, tantalum and niobium powders. Preferably the valve metal powder is tantalum. There is not limit as to the particle size of the powders that can be used in this invention, however the method of the invention is particularly more effective than the prior art in lowering residual carbon
Bhimaraja Udaya Shankar
Fernstrom Peter James
Hahn Randolph S.
Melody Brian J.
Banner & Witcoff , Ltd.
Jones Deborah
Kemet Electronics Corporation
Koehler Robert R.
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