Compositions – Electrically conductive or emissive compositions
Reexamination Certificate
2000-11-06
2002-01-01
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
C427S080000, C427S318000, C427S337000, C427S435000
Reexamination Certificate
active
06334966
ABSTRACT:
FIELD OF THE INVENTION
The invention is directed to preparation of conductive polymers using chemical oxidation.
BACKGROUND OF THE INVENTION
A solid state electrolytic capacitor is comprised of three basic elements, the anode, the dielectric, and the cathode. Valve metals such as tantalum, aluminum, and niobium are particularly suited for the manufacture of high surface area solid electrolytic capacitors in which the valve metal serves as the anode, and an oxide of the valve metal formed by anodic oxidation of the valve metal surfaces serves as the dielectric. In order to maximize the dielectric surface area, and hence increase the volumetric efficiency of the capacitor, the valve metal substrates are porous bodies. Conductive polymers such as polythiophene, polypyrrole, polyaniline, and their derivatives are finding increased utilization as the cathode for these capacitors.
Conductive polymers offer several advantages over manganese dioxide, the traditional cathode material utilized for solid electrolytic capacitors manufactured from valve metals. The conductive polymer can be applied to the dielectric surfaces of the capacitors using process temperatures that do not cause harm to the dielectric. The polymers are non-oxidizing, eliminating the potential for ignitions. The conductive polymers are highly conductive, typically 10 to 100 times as conductive as manganese dioxide. The higher conductivity of the conductive polymer results in a substantial reduction in equivalent series resistance (ESR) for the finished capacitor relative to devices manufactured with manganese dioxide. Low ESR is an increasingly important requirement in several capacitor applications, including voltage regulation for microprocessors.
Chemical oxidative polymerization is an effective way to coat the dielectric surfaces of a valve metal solid electrolytic capacitor. In chemical oxidative polymerization, a monomer, oxidizing agent, and a dopant are reacted on the dielectric surfaces to form the conductive polymer. Several methods have been used to get the monomer, oxidizer, and dopant to react on the dielectric surface. In one method, all components are mixed together in a combined solution. However, the monomer and oxidizer can react in the dipping bath, causing premature polymerization, adding processing costs and degrading the quality of the capacitor. This is especially a problem with pyrrole monomer and Fe(III) oxidizing agents.
In an alternate dip approach, the dielectric surfaces are first immersed in a solution containing one reactant (either dilute monomer or oxidizer/dopant), drying, and then dipping in the second reactant. If a porous pellet is first dipped in a dilute monomer solution, the monomer tends to be drawn into the porous pellet on drying, making it difficult to coat the external surfaces of the valve metal substrate.
In a preferred method, the dielectric surfaces are coated with oxidizer solution by dipping in a solution containing the oxidizer and dopant. The oxidizer solution is dried on the dielectric surfaces. Monomer is applied by dipping in a solution containing the monomer. The monomer, oxidizer, and dopant react to form a conductive polymer film on the dielectric surface.
Suitable monomers include aniline, pyrrole, thiophene, and derivatives of these monomers. A derivative of thiophene, 3,4-ethylenedioxythiophene is particularly preferred because the polymer produced, poly(3,4-ethylenedioxythiophene) has a high stability in hot and humid environments.
After coating the dielectric surface with the conductive polymer, the excess reactants and reaction byproducts must be removed. Due to the moderate to low solubility of most suitable monomers, organic solvents must be used to wash away excess monomer from the dielectric surface. The preferred monomer, 3,4-ethylenedioxythiophene, has a low solubility in pure water (2.1 grams/liter at 25° C.). In the preferred case where Fe(III) salts are utilized as the oxidizer, the Fe(II) by products of the polymerization reaction are removed by an organic solvent because of the low solubility of Fe(II) salts in pure water. Failure to remove excess monomer and Fe(II) can cause ESR and/or leakage current of the finished device to increase. The use of organic solvents to remove excess monomer and Fe(II) from the dielectric surface substantially increases manufacturing costs due to the raw material cost of the organic solvent, the disposal cost of the used wash solution, and the capital costs associated with building equipment compatible with the use of flammable solvents.
In order to avoid using organic solvents to wash away excess monomer following the polymerization reaction, the concentrations of monomer and oxidizer must be matched to insure complete, or nearly complete, polymerization of the monomer present on the dielectric surfaces. It has been found that it is very difficult to completely coat external surfaces of a porous pellet using a single dip solution comprising oxidizer, monomer, and dopant unless the concentrations of monomer and oxidizer are very high. In such solutions premature polymerization occurs. Thus an alternate dip approach in which the oxidizer is applied followed by dipping in the monomer is the preferred approach. The monomer solution must be diluted with a solvent in order to stoichiometrically match the amount of oxidizer available to drive the polymerization reaction.
It is thus preferred to avoid the use of organic solvents when removing Fe(II) from the dielectric surfaces. There have been attempts to use water as the solvent. For example, Sakata et al. (EP 0 893 807 A2) disclose a method in which an Fe(III)-containing salt is applied to a capacitor, dried, and then the capacitor is dipped in a solution of pyrrole monomer in water followed by polymerization to polypyrrole. If the Fe(III)-containing salt has a low solubility in water, this prevents cross contamination of the pyrrole with the oxidizer. However, this method is not applicable to Fe(III)-containing salts that have significant solubility in water (such as Fe(III) tosylate), and it is also not applicable to monomers that are sparingly soluble in water such as 3,4-ethylenedioxythiophene. Fe(III) tosylate is the preferred oxidizer/dopant for converting 3,4-ethylenedioxythiophene into poly(3,4-ethylenedioxythiophene) the process of Sakata does not solve the problems.
Sakata is silent on the method of washing the excess pyrrole and Fe(II) reaction byproducts. Historically, this process would require the use of an organic solvent to remove the excess hydrophobic monomer and the Fe(II) reaction byproduct.
SUMMARY OF THE INVENTION
It is an object of the invention to coat or impregnate articles with highly conductive polymers made from hydrophobic monomers and Fe(III)-containing oxidizers using a chemical oxidative process which eliminates the need for washing with an organic solvent.
It is another object of this invention to control the amount of monomer applied to the substrate by dissolving the monomer in a solvent in which it is soluble and thereby limit the amount of hydrophobic monomer that must be removed during an aqueous washing step.
It is yet another object of this invention to develop an aqueous wash process which effectively removes Fe(II) residues formed as a by product of the reaction between a monomer and Fe(III) oxidizer. Preferably the capacitor is soaked in a solution which enhances the solubility of Fe(II). The capacitor may then be washed in water.
It is a further object of the present invention to substantially reduce cross contamination of dilute solutions of monomer by Fe(III) oxidizers by dissolving the monomer in a solvent in which the monomer is soluble but the Fe(III) oxidizer is substantially insoluble.
It is another object of this invention to apply this process to the production of low ESR and low leakage valve metal capacitors with conductive polymer cathodes.
The present invention is directed to a process of preparing conductive polymer layer on an anodized surface of a valve metal substrate comprising a) dipping the substrate
Hahn Randolph S.
Kinard John T.
Lessner Philip M.
Melody Brian J.
Banner & Witcoff , Ltd.
Kemet Electronics Corporation
Kopec Mark
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