Metal working – Barrier layer or semiconductor device making – Barrier layer device making
Reexamination Certificate
2000-01-06
2002-10-22
Nguyen, Ha Tran (Department: 2812)
Metal working
Barrier layer or semiconductor device making
Barrier layer device making
C361S523000, C361S525000
Reexamination Certificate
active
06468317
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an improved method of impregnating electrolytic capacitor stacks or wound rolls with a polymer based electrolyte, such as a hydroxyethylmethacrylate (HEMA) or hydroxyethylacrylate (HEA) based electrolyte, to render them suitable for use in electrolytic capacitors, and to such electrolytic capacitors.
2. Related Art
Conventionally, an electrolytic capacitor includes an etched aluminum foil anode, an aluminum foil or film cathode, and an interposed kraft paper or fabric gauze separator impregnated with a solvent-based liquid electrolyte. Typically, the electrolytic or ion-producing component of the electrolyte is a salt that is dissolved in the solvent. The entire laminate is rolled up into the form of a substantially cylindrical body, or wound roll, that is held together with adhesive tape and is encased, with the aid of suitable insulation, in an aluminum tube or canister. Connections to the anode and the cathode are made via tabs. Alternative flat constructions for aluminum electrolytic capacitors are also known, composing a planar, layered, stack structure of electrode materials with separators interposed therebetween. While aluminum electrolytic capacitors having anodes and cathodes comprised of aluminum foil are most common, foils of other conventional valve metals such as titanium, tantalum, magnesium, niobium, zirconium and zinc are also used.
In known processes for impregnating electrolytic capacitor stacks or wound rolls with a polymer based electrolyte, a polymerization initiator, such as a persulfate salt of the alkyl or ammonium families, is mixed with the polymer based electrolyte prior to impregnation. For example, U.S. Pat. No. 5,628,801 to MacFarlane et al. discloses an electrolytic capacitor where a separator impregnated with an elastomeric solid electrolyte is utilized in the dual capacity of electrolyte and adhesive material to hold together the anode and cathode plates of the capacitor. The preferred electrolyte consists of: 17.5 parts by weight of hydroxyethylmethacrylate, 32.5 parts by weight ethylene glycol, 7.0 parts by weight ammonium adipate, 6.7 parts by weight ammonium glutarate, 0.45 parts by weight tetraethyleneglycoldiacrylate, and 2.2 parts by weight of initiator solution. The capacitor assembly is impregnated with this polymerizable liquid electrolyte/adhesive and then heated to approximately 55° C. for at least 2 hours, but preferably 24 hours to cure the electrolyte/adhesive.
Similarly, U.S. Pat. No. 5,748,439 to MacFarlane et al. discloses an electrolytic capacitor having interposed between the electrically conductive anode and cathode layers thereof a spacer comprised of a mechanical separator means such as kraft paper impregnated with a crosslinked elastomeric electrolyte. The polymer based electrolyte is preferably made up as a liquid prepolymer electrolyte mixture prior to impregnation into the capacitor element and the polymer is preferably formed in situ thereafter from the prepolymer mixture. The mixture is preferably made up by first dissolving a salt into a liquid plasticizer component by stirring at elevated temperatures, cooling the mixture to room temperature, and then adding to the mixture a monomer corresponding to the desired polymer and a crosslinking agent, as well as a polymerization initiator.
The problem with the previous processes for impregnating electrolytic capacitors with a polymer based electrolyte is the incomplete filling of the microscopic tunnels in the etched anodes. Polymerization begins as soon as the polymerization initiator is mixed with the polymer based electrolyte, increasing the viscosity of the initiator/polymer based electrolyte solution and reducing the working pot life to approximately 15 to 30 minutes. Because of the increased viscosity and the reduced working time, the polymer based electrolyte has insufficient time to fully incorporate itself into the microscopic features of the etched anode. Capacitance is lost due to the incomplete usage of the increased area of the etched foil.
SUMMARY OF THE INVENTION
The present invention provides an improved method of impregnating electrolytic capacitor stacks or wound rolls with a polymer based electrolyte, such as a HEMA or HEA based electrolyte, allowing complete filling of the microscopic tunnels in the etched anodes. According to the present invention, the electrolytic capacitor is first impregnated with a polymerization initiator and a surfactant or surface active wetting agent prior to impregnating the capacitor with the polymer based electrolyte. Polymerization does not begin until impregnation of the capacitor with the polymer based electrolyte. The surfactant allows the polymer based electrolyte to more fully incorporate itself into the microscopic features of the anode foil.
The process according to the present invention results in a fully impregnated capacitor stack or wound roll. Accordingly, the present invention provides improved methods and compositions for impregnating electrolytic capacitor stacks or wound rolls, as well as electrolytic capacitors comprising such stacks or wound rolls.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides for an improved method of impregnating electrolytic capacitor stacks or wound rolls with a polymer based electrolyte, such as a HEMA or HEA based electrolyte. This process improves the incorporation of the polymer based electrolyte into the anode foil, thereby increasing capacitance.
Prior to impregnating the electrolytic capacitor with a polymer based electrolyte, the etched and formed anode foil is pre-loaded with a polymerization initiator and a surfactant or surface active wetting agent, including, but not limited to, Dioctyl sodium sulfosuccinate (docusate sodium). Other surfactants that can be used include Lauric acid and Stearic acid. In the case of a liquid surfactant, the surfactant can be added directly to the fill electrolyte and just pre-impregnate the capacitor with the polymerization initiator alone. One example of a liquid surfactant is N-octyl alcohol added to the fill electrolyte in a proportion of 1 to 2% by weight. The polymerization initiator is preferably an aqueous solution containing a persulfate (S
2
O
8
−2
) salt, typically an alkyl or ammonium salt, such as potassium persulfate, ammonium persulfate or sodium persulfate. Other free radical initiators are suitable as well, such as azoxyisobutyronitrile(AIBN) or benzoyl peroxide. The polymerization initiator and surface active wetting agent may be incorporated into the anode foil by means of soaking the anode foil or capacitor stack in a dilute aqueous solution of a persulfate salt and a surface active wetting agent, or by other means know to those skilled in the art. The preferred initiator/surfactant solution is 0.75% docusate sodium, approximately 10% K
2
S
2
O
8
in an aqueous solution. Some of the water may be replaced with a polar organic solvent with a high vapor pressure such as ethanol to make it easier to dry. However, solubility can be a problem in the lower water solutions. The present invention improves the incorporation of the polymer into the anode foil, by locating the polymerization initiator and surface active wetting agent in intimate contact with the areas where polymerization is desired (as in the anode foil tunnels, paper, or cathode structure).
Separating the persulfate salt from the polymer based electrolyte solution allows the polymer based electrolyte solution to be heated without causing premature polymerization. Polymerization does not begin to occur until contact is made with the polymerization initiator, which occurs upon impregnation of the capacitor with the polymer based electrolyte. Heating the polymer based electrolyte reduces the viscosity of the polymer based electrolyte solution and lessens resistance when the polymer based electrolyte solution is filling the voids of the anode foil. Additionally, the surfactant allows the polymer based electrolyte solution to more thoroughly fill the micr
Carson Dean F.
Marshall Timothy R.
Strange Thomas F.
Mitchell Steven M.
Pacesetter Inc.
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