Electricity: electrical systems and devices – Electrolytic systems or devices – Solid electrolytic capacitor
Reexamination Certificate
2003-10-20
2004-10-12
Dinkins, Anthony (Department: 2831)
Electricity: electrical systems and devices
Electrolytic systems or devices
Solid electrolytic capacitor
C361S525000, C029S025030
Reexamination Certificate
active
06804109
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to methods for improving the adherence of intrinsically conductive organic polymers to anodic valve metal dielectric films and capacitors prepared by use of the method which exhibit reduced frequency of short failures and reduced equivalent series resistance (ESR).
BACKGROUND AND PRIOR ART
Electrolytic capacitors having aluminum anode and cathode foils separated by layers of absorbent paper, wet with an liquid electrolyte, have been an item of commerce since the 1920's, as have electrolytic capacitors having a powder metallurgy tantalum anode impregnated with a highly conductive liquid electrolyte (such as an aqueous solution of sulfuric acid or lithium chloride). There are many liquid electrolyte solutions which have been proposed and/or adopted for use in electrolytic capacitors. Some of these electrolyte solutions contain materials which render them solids or semisolids at room temperature. Mannitol, various gums, poly-vinyl-pyrrolidone, etc. have been employed in capacitor electrolyte fabrication for the purpose of increasing the sparking voltage, increasing the viscosity (to the point of solidity at room temperature) or reducing the vapor pressure of electrolyte solutions. Despite the addition of materials to liquid electrolyte solutions which tend to thicken or solidify these solutions, they remain essentially liquids and conduct current by an ionic conduction mechanism.
It was recognized fairly early in the development of electrolytic capacitors that true solid-state conductors would have to be found which were compatible with the anodic oxide films covering the valve metal anodes used in electrolytic capacitors to substantially reduce the resistance of the cathode layer and overall resistance of these devices. In 1933, J. E. Lilienfeld obtained a patent (U.S. Pat. No. 1,906,691) covering the use of true, solid-state cathode materials, such as cuprous sulfide, cupric oxide, copper/cupric oxide combinations, lead oxide, etc., covered by a layer of silver, nickel, aluminum, copper, etc., for the purpose of reducing device resistance.
Lilienfeld's solid capacitors apparently never went into production due to the onset of the Great Depression.
A successful solid-state electrolytic capacitor was patented by Haring et. al., in 1963 (filed on Apr. 2, 1953), U.S. Pat. No. 3,093,883. This device employs pyrolytic manganese dioxide (produced via the pyrolysis of aqueous manganese nitrate solutions) as the cathode material.
In a 1967 patent (U.S. Pat. No. 3,345,544), Metcalfe extended the manganese dioxide cathode technology to include anodized aluminum foil anodes. Metcalfe's major finding was that, in order to resist the corrosive effects of manganese nitrate pyrolysis, the aluminum anode foil required phosphate and/or chromate anodizing electrolyte solutions (it has since been determined that anodic aluminum films formed in phosphate solutions consist largely (up to 90%) of aluminum phosphate, a very insoluble and non-reactive aluminum compound). Such films are better referred to as anodic layers than oxide layers.
The equivalent series resistance (E.S.R.) and dissipation factor (d.f.) tends to be much lower for capacitors fabricated with manganese dioxide cathode material than for equivalent capacitance capacitors constructed with liquid electrolyte solutions due to the lower resistivity of manganese dioxide (1-3 orders of magnitude lower resistivity than liquid electrolyte solutions).
With the continuing development of ever-faster microprocessors and lower-voltage logic circuits, the demand for lower E.S.R. capacitors for use in conjunction with faster micro-processors has motivated capacitor manufacturers to develop solid state cathode materials which are more conductive (less electrically resistive) than manganese dioxide.
In the early 1980's, aluminum electrolytic capacitors were introduced which were fabricated having a T.C.N.Q.-amine complex acting as the cathode. These capacitors established the stability and high conductivity achievable with solid-state organic cathode materials. The ongoing effort to increase the maximum temperature capability of organic cathode electrolytic capacitors has led to the development of methods of capacitor fabrication employing intrinsically conductive organic polymers, such as polypyrrole, polythiophenes such as polyethylene dioxythiophene, polyanilines and polyacetylenes. Numerous substituted monomers (derivatives) are useful as are mixtures of two or more monomers from different types, i.e., mixtures. These are relatively high molecular weight materials which possess electrical conductivity along a poly-acetylene-like backbone contained within the polymer and as such there is minimal interfacial resistance such as is encountered by electrons traveling through the myriad of layers of charge-transfer complexes, such as the T.C.N.Q. or T.C.N.E. charge transfer complexes. As a result of the higher molecular weight, intrinsically conductive organic polymers not only tend to possess higher conductivity than charge-transfer complexes, but also tend to exhibit higher thermal stability. While charge-transfer complexes tend to become unstable at temperatures above about 105° C., intrinsically conductive polymers, such as polyethylene dioxythiophene, exhibit high temperature stability to 150° C. or even 175° C. in the absence of atmospheric oxygen.
There are two routes used to deposit intrinsically conductive organic polymers on electrolytic capacitor anode bodies:
1) Chemical
2) Electrochemical
The first of these approaches is well described in U.S. Pat. No. 4,910,645, to Jonas et. al., and consists of immersing the anodized substrate sequentially in an aqueous solution of an oxidizing agent such as, but not necessarily limited to, ammonium persulfate, and a dopant acid and/or a salt of a dopant acid, such as p-toluene sulfonic acid or sodium p-toluene sulfonate [Intrinsically conductive organic polymers generally contain one dopant acid anion for each 3 to 4 monomer units which have been joined to form the polymer. The presence of a strong dopant acid anion is thought to result in a delocalization of electric charge which provides electrical conductivity]. After a drying step, conducted to free the pores of the anode bodies from the water portion of the oxidizer/dopant solution, the anode bodies are then immersed in a solution of the monomer, usually dissolved in at least a partially organic solvent such as a low molecular weight alcohol, etc. Once the solution of monomer, which may consist entirely of monomer, is introduced into the capacitor anode bodies, the anodes are allowed to stand, generally in an oven above room temperature, to facilitate production of the intrinsically conductive polymer material. Repeat dipping sequences may be employed to more completely fill the pore structure of the anode bodies. In practice, rinsing cycles are generally employed to remove reaction by-products, such as ammonium sulfate, sulfuric acid, iron salts (when an iron (III) oxidizer is employed), or other by-products depending on the system employed. Chemical production of intrinsically conductive organic polymers may also be carried-out with capacitor anode bodies by first introducing the monomer to the capacitor bodies, followed by introduction of the oxidizer and dopant (the reverse order of polymer pre-cursor introduction described above). It is also possible to mix the dopant acid(s) with the monomer solution rather than with the oxidizer solution if this is found to be advantageous. U.S. Pat. Nos. 6,001,282 and 6,056,899 describe a chemical means of producing an intrinsically conductive organic polymer through the use of a single solution which contains both the monomer and the oxidizing agent, which has been rendered temporarily inactive via complexing with a high vapor pressure solvent. As the solution is warmed and the inhibiting solvent evaporates, the oxidative production of conductive polymers ensues. The dopant acid anion is also contained in the stabilized poly-precu
Hahn Randolph S.
Key Elisabeth Crittendon
Kinard John T.
Melody Brian J.
Pritchard Kimberly L.
Dinkins Anthony
Kemet Electronics Corporation
Nexsen Pruet , LLC
O'Toole J. Herbert
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