Electricity: electrical systems and devices – Electrolytic systems or devices – Liquid electrolytic capacitor
Reexamination Certificate
2000-02-01
2002-11-12
Dinkins, Anthony (Department: 2831)
Electricity: electrical systems and devices
Electrolytic systems or devices
Liquid electrolytic capacitor
C361S504000, C361S503000, C361S527000, C252S062200
Reexamination Certificate
active
06480371
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is directed to electrolytic solutions suitable for anodizing valve metal anodes prepared from fine powders, to methods of preparing capacitors, and to capacitors prepared with fine powder anodes and suitable electrolytes
2. Description of Related Art
The enormous growth experienced by the electronics industry since WWII is due, at least in part, to the relative reduction in the cost of electronics circuits over time. This reduction in the relative cost of electronics makes possible the introduction of an ever larger number of consumer electronic devices at prices which the general public finds sufficiently attractive for mass consumption. A portion of the relative reduction in the cost of consumer electronics goods is a function of the economies of large scale mass production, but a greater portion of the reduction in cost is due to the ever increasing scale of circuit integration which has made possible the fabrication of an increasingly larger number of circuit components of progressively smaller size on circuit chips of silicon and other materials.
Some components have proven difficult to integrate into the integrated circuit structure and are still mounted separately on circuit boards, either separately or in modular groups. In order to reduce the cost of electronics as a whole, manufacturers of the individual components have been forced to reduce the cost of these components in the same general fashion as the integrated circuit manufacturers.
One circuit component which is found in most advanced circuits is the solid tantalum capacitor. The high capacitance per unit volume, low leakage current, and high reliability of solid tantalum capacitors have made them the capacitor of choice for many circuit applications and these devices are now manufactured at a rate exceeding 1,000,000,000 devices per month worldwide.
Solid tantalum capacitors are fabricated by pressing tantalum powder to produce compacts at ¼ to ¾ of the theoretical density, followed by high temperature (~1200° C. to 2200° C.) vacuum sintering to produce the capacitor anodes. Electrical connection to the individual anodes is made through valve metal wire, typically tantalum wire. Tantalum wires are partially imbedded in the compacts during the pressing step or are welded to the compacts after sintering.
The sintered, powder metallurgy tantalum anodes are suspended by the tantalum wires in an electrolytic solution and are anodized to produce the dielectric layer on all surfaces and within all of the interstices of the anodes. The tantalum anode acts as the anode capacitor plate and the anodic oxide acts as the capacitor dielectric.
After anodizing, the counter electrode or cathode capacitor plate is applied by filling the pores or interstices of the anodes with manganese dioxide, conductive salts, or an intrinsically conductive polymer. Layers of conductive carbon and conductive paint are generally applied to produce the finished capacitor body ready for lead attachment and encapsulation.
Efforts to reduce the cost of solid tantalum capacitors have tended to concentrate on the reduction in the particle size of the tantalum powder so as to produce a larger amount of useful tantalum surface area upon which the dielectric oxide may be anodically grown per gram of tantalum powder. A lower weight of tantalum is required per capacitor with higher surface area tantalum powders, which makes possible a reduction in the cost of the capacitors produced (due to the reduced amount of tantalum consumed per device produced.) Tantalum powders having higher surface area per unit weight also make possible a reduction in the physical size of capacitors having a given rating since smaller anode bodies may be used for the same capacitance and oxide thickness.
The use of high surface area tantalum powders (i.e., tantalum powders having more than about 0.4 square meters per gram after sintering and having approximately 40,000 microcoulombs per gram, or more) has been found to have some disadvantages as well as the obvious advantages of smaller anode size and lower anode cost. The major disadvantage of high CV tantalum powders observed with the anodes produced with these powders is that the smaller size of the tantalum particles which make up these powders is accompanied by smaller pores between the particles in the interstices of the anodes. During the anodizing process used to produce the dielectric oxide, the current must travel through the electrolyte solution contained within the pores of the anodes. As the surface area of the tantalum powders increases, the amount of current flow per unit of anode volume increases (at least with the commonly employed industry practices of applying a fixed amount of current per unit of surface area of the powder contained within the sintered anodes or the application of a fixed rate of voltage rise per unit time with the current allowed to increase until a pre-set voltage is achieved.) This increase in current flow per unit of anode volume and the increasingly difficulty of diffusion of electrolyte species within the interstices of the anodes with increasingly high surface area tantalum powders leads to a deposit of relatively insoluble phosphate species within the interstices of the anode bodies when the anodizing process is carried out in traditional aqueous phosphoric acid or aqueous phosphoric acid/ethylene glycol electrolytes (as described in co-pending application Ser. No. 09/143,373, hereby incorporated by reference in its entirety.
The presence of relatively insoluble phosphate species within the interstices of anodized tantalum (or other valve metal) powder metallurgy anodes produced from high surface area metal powders and anodized in traditional aqueous phosphoric acid solutions, with or without ethylene glycol present in the solution, gives rise to less complete anode impregnation with manganese dioxide, higher dissipation factors, and higher d.c. leakage values per unit of capacitance than are obtained with anodes which do not contain these insoluble phosphate deposits.
Co-pending application Ser. No. 09/143,373 describes the use of electrolyte solutions for anodizing anodes fabricated from high surface area valve metal powders which avoid or greatly minimize the deposition of insoluble phosphate species while maintaining phosphate ions in the solution through the use of alkali metal phosphate salts in combination with aqueous solutions of polyethylene glycol, polyethylene glycol mono methyl ether, and/or polyethylene glycol di methyl ether. The presence of phosphate in the anodizing solution, and hence in the anodic oxide, is associated with increased thermal stability. The near neutral pH of these electrolyte solutions (i.e., pH=5 to 9) renders these solutions much less corrosive to stainless steel tank components and thus reduces the metal contamination of the electrolytes to much lower levels than is commonly found in phosphoric acid electrolytes (which may have a pH of 2 or less, and are therefore much more acidic and corrosive.)
The lower concentrations of metal ion contamination (i.e., chromium, nickel, iron) generally observed with the electrolyte solutions of co-pending application Ser. No. 09/143,373 prevents or greatly reduces the deposition of metallic phosphates within the interstices of the anode bodies anodized in these solutions.
It was discovered that even low pH phosphoric acid-containing electrolyte solutions containing little metallic contamination, deposit a sparingly soluble phosphate species, probably polyphosphoric acid, within the interstices of anodes fabricated from high surface area valve metal powders. This deposition of insoluble phosphate species from relatively metal contamination-free phosphoric acid electrolytes eliminates the relatively simple solution to the problem of phosphate species deposition via constant circulation of the electrolytes through a cation exchange resin (such as polystyrene/poly divinyl benzene sulfonate) to remove metal ion contamin
Kinard John Tony
Melody Brian John
Wheeler David Alexander
Banner & Witcoff LLP
Dinkins Anthony
Ha Nguyen
Kemet Electronics Corporation
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