WO2002037536A2

WO2002037536A2 - Solutions of conducting polyaniline

Info

Publication number: WO2002037536A2
Application number: PCT/IL2001/000998
Authority: WO
Inventors: Eli Harlev; Keren Regev; David A. Strawser
Original assignee: Al-Coat Ltd.
Priority date: 2000-10-30
Filing date: 2001-10-29
Publication date: 2002-05-10
Also published as: AU2002214217A1; WO2002037536A3; IL139351A0

Abstract

The present invention describes a polymer solution system for applying a conducting polymer solid electrolyte to solid electrolyte anodized capacitor anodes, said polymer solution system comprising:polyaniline, a dopant acid, and formic acid.

Description

SOLUTIONS OF CONDUCTING POLYANILINE

FIELD OF THE INVENTION The present invention describes solution systems of conducting polyaniline and solution systems of conductive polyaniline admixed with conductive, semi-conductive, or non-conductive components, that use formic acid as the solvent, and methods of preparing said solution systems. The invention also relates to a process for forming conducting polymer solid electrolyte (counter electrode) layers, particularly in solid-state electrolytic capacitors using said solution systems.

BACKGROUND OF THE INVENTION

A general solid electrolytic capacitor has a structure in which a molded porous body comprising a valve metal powder of Group III and V of the Periodic Table of the Elements, particularly of tantalum or aluminum, is used as an anode. The surface of the anode body is anodized to form an oxide dielectric layer. The anode's dielectric layer is then coated with a solid electrolyte, such as manganese dioxide, that serves as part of the cathode.

Aluminum-based electrolytic capacitors usually differ from their tantalum-based counterparts, the anode being constructed in the form of a foil, or an etched (roughened) foil, either rolled or stacked. Also, foil tantalum capacitors are described in prior art as being constructed by rolling two strips of thin foil, separated by a paper saturated with electrolyte, into a convolute roll. In solid electrolytic capacitors, the solid electrolyte electrically connects the entire surface of the dielectric in the molded porous pellet body to an electrode lead, and also repairs electrical short-circuiting caused by defects in the dielectric oxide coating. The latter function is a result of the self-healing ability of manganese dioxide.

However, the electrical conduction of manganese dioxide is too low to confer good high-frequency properties (low equivalent series resistance - ESR) when used as part of the cathode in solid electrolytic capacitors. This is due to its high impedance at high frequencies. Furthermore, manganese dioxide, being a powerful oxidizing agent, can ignite and destroy the capacitor when it comes in direct contact with the tantalum anode through a crack in the oxide layer. As a result, conducting polymers, such as polythiophene and polypyrrole, either non-substituted or substituted, have been suggested to replace manganese dioxide in electrolytic capacitors as described in prior art.

Conducting polymers are endowed with electrical conduction 10 to 100 times greater than that of manganese dioxide. They also cannot cause the capacitor to ignite upon failure of the dielectric oxide layer. Furthermore, like manganese dioxide, they display a 'self-healing' effect as described by Harada (NEC Technical Journal, 1996). A proposed mechanism of the self-healing is given by Prymak (J. D. Prymak, CARTS-EUROPE '99).

However, conducting polymers pose a major application problem due to the insolubility of their doped (conducting) form in conventional solvents. Their application to capacitor elements thus requires the use of methods that circumvent this obstacle.

One prior art approach has been to carry out an in situ chemical oxidative polymerization of the related monomers inside the pores of the anodized anode element. EP No. 0654805 and EP No. 0652576, for example, teach impregnating the pores of the anode body (porous tantalum pellet) with an aqueous solution of pyrrole (monomer). This is followed by a second impregnation of an oxidizer solution that initiates the in situ polymerization to polypyrrole. US Pat. No. 5,223,002 describes the same approach in which an aniline monomer, an oxidizing agent and a dopant are reacted inside the capacitor's porous pellet to form the conductive form of polyaniline.

US Pat. No. 6,001 ,281 uses the same idea with ethylenedioxythiophene as monomer. In the last patent, the in situ polymerization reaction is better controlled through the use of a more elaborate solvent system.

A modified method is taught in EP No. 0652576, where the electrolyte is made of two layers of a conducting polymer. The first layer is created by an in situ oxidative polymerization of a precursor monomer as described above. This is followed by the deposition of a second layer comprised of the non-conductive form of polyaniline (emeraldine base). Finally, the solvent is evaporated off, turning the non-conductive polyaniline layer into its conductive form through an in situ doping reaction.

The above in situ polymerization/doping procedures suffer from drawbacks, one being the many steps required for obtaining the solid electrolyte coating. US. Patent No. 5,223,002, for example, describes 11 steps for creating the conductive polymer electrolyte. Another disadvantage is the undesired by-products that are generated along with the polymer during in situ polymerization. These include excess reactants, the reduced form of the oxidizing agent, and unwanted reaction products other than the polymer. The latter must be removed via elaborate washing processes. Some of the polymerization by-products, such as benzidine, formed during the polymerization of aniline, are considered highly toxic, necessitating appropriate measures against environmental pollution and worker exposure during the washing process.

Another drawback, also inherent to in situ polymerizations, is the inability to control the exact stoichiometry between monomer and oxidizing agent. Control of stoichiometry is critical to achieve improved conduction levels.

In order to avoid the above-mentioned deficiencies, that, as said, are inherent to in situ polymerizations, alternative procedures have been devised in which the conducting polymer is applied from solutions of acid-doped polymers.

US Pat. No. 5,948,234, for example, describes an acid-doped, conducting polyaniline solution system that can be applied to form a solid electrolyte in tantalum capacitors. The polymer in its doped, conductive, state is applied to the capacitor elements when dissolved in one or more bicyclic terpenes, such as gum turpentine, cr-pinene and /3-pinene. This system is claimed to have lower drying temperatures (140°C) and lower toxicity than a previously practiced system containing a solvent mixture of xylene and ethyleneglycol monomethyl ether. However, the application process as described in US Pat. No. 5,948,234, includes a preliminary stage of dissolving the acid-doped polyaniline in the very same, or similar, xylene containing solvent mixture that is used to make a polyaniline film. Only later is it re-dissolved in the above-mentioned bi-cyclic terpenes. The above, therefore, makes a cumbersome multi-stage process for the preparation of the conducting polymer solution to be applied in the capacitor elements.

The above patent further teaches that subsequent to forming the conducting polymer film on the anodized anode, driving off the solvent (or solvent mixture), and reforming at an elevated temperature, there still may remain residual monomer and reaction by-products that must be washed out. Apart from the complex procedure needed for creating the doped polyaniline solution, its application in tantalum capacitors requires several deposition cycles in order to reach the desired thickness of the impregnated acid-doped conducting polymer. Each cycle comprises dipping, drying, heating and washing steps.

US Pat. No. 5,885,650 and JP Pat. No. 5152168 teach solvent systems containing, respectively, N-ethyl-pyrrolidone and N-methyl-pyrrolidone as solvents for sulfonic acid-doped polyaniline that can be applied to solid state electrolytic capacitor anodes. The former deals with molded tantalum capacitor anode bodies, while the latter describes the application of the conducting polymer to aluminum-based anodized anodes. In the latter case, the conducting polymer is applied to the etched and anodized (foil-like) aluminum substrates (anodes). The above two solution systems suffer from the deficiency of using solvents with boiling points exceeding 200°C, that, in turn, requires high drying temperatures and a prolonged drying process, even if carried out under vacuum conditions. Despite the high drying temperature, residual solvent and by-products are still entrapped inside the capacitor elements, necessitating complex, multi-stage washing procedures. This may also be true of the aforementioned bi-cyclic terpene-based solutions that have quite high boiling points; 156°C for cr-pinene and 166°C for /3-pinene.

Thus, a solution system for efficient, simple and inexpensive application of a doped conducting polymer in valve metal-based capacitors, especially tantalum-based or aluminum-based electrolytic capacitors, that is also environmentally friendly, is desirable.

SUMMARY OF THE INVENTION

The present invention describes solution systems of conducting polyaniline and solution systems of conductive polyaniline admixed with conductive, semi-conductive, or non-conductive components, that use formic acid as the solvent, and methods of preparing said solution systems. The invention also describes a process using the above said solution systems for forming a polyaniline-based electrolyte layer in solid-state electrolytic capacitors that incorporate valve metal anodes, especially anodes prepared from tantalum, aluminum, and niobium.

The polyaniline solution systems of the present invention are characterized by simple preparation, low drying temperature, and relatively low viscosity. This affords an improved system for creating a polyaniline-based electrolyte layer in solid-state electrolytic capacitors. In its simplest form, the solution system described herein is not a true solution by the strict definition of what constitutes a solution, but is a dispersion of micrometer or smaller sized particles of polyaniline in a liquid medium. The liquid medium may comprise a single component, or may include soluble materials (solutes), such as non-conductive polymers, and other liquid materials that function as co-solvents for said solutes.

It is an object of the present invention that the solution system used for applying the conducting polymer electrolyte to anodized anodes, such as within the pores of tantalum pentoxide dielectric (tantalum porous pellet) of a solid electrolytic tantalum capacitor, or within the pores of niobium oxide dielectric (niobium porous pellet) of a solid electrolytic niobium capacitor, or to the etched surface of an aluminum oxide-coated aluminum foil of a solid electrolytic aluminum capacitor, have adjustable viscosity.

It is an object of the present invention that the process of applying a solid conducting polymer-based electrolyte to capacitor elements be a facile, simple and inexpensive process. It should also render washing the product unnecessary.

It is yet another object of the present invention to provide conducting polymer-based solid electrolytic capacitors having improved high-frequency characteristics, such as lower ESR and larger capacitance per unit volume. It is still a further object of the present invention that solid valve metal capacitors made using the material and procedure of the present invention are ignition free.

Other purposes and advantages of the invention will become apparent as the description proceeds. The present invention teaches a method to prepare a polymer solution system and a method of applying a conducting polymer solid electrolyte to anodized valve metal substrates and subsequently forming solid electrolyte capacitors. The polymer solution system includes as a minimum polyaniline, a dopant acid, and formic acid. The polymer solution system may optionally further include any or all of the following materials: carbonaceous materials, conductive or semi-conductive, including but not limited to graphite, carbon black, carbon nanotubes, carbon nanoparticles and the like; non-conductive polymers including but not limited to poly(methylmethacrylate), polycarbonate, and the like; co-solvents for the non-conductive polymers, including but not limited to methylethyl ketone, ethyl acetate, methyl acetate, acetone and the

like. The metals that form the anodes of the solid electrolyte capacitors are

selected from the group of valve metals, including tantalum, aluminum, and

niobium.

In the polymer solution system, the dopant acid can be selected from a

group of protonic acids. Preferred protonic acids can be selected from the group

comprisng sulfonic acids and phosphoric acids.

The sulfonic acid can be selected from a group comprising methane

sulfonic acid, camphor sulfonic acid, dodecylbenzene sulfonic acid, 1-propanesulfonic acid, 1 -butane sulfonic acid, 1-hexane sulfonic acid,

naphthalene sulfonic acid, dinonyl naphthalene sulfonic acid, and toluene sulfonic acid.

The phosphoric acid can be selected from a group of phosphoric acids

incorporating ethyl-hexyl phosphate, di-phenyl phosphate and di-butyl phosphate anions.

In one embodiment, the solution includes polyaniline in a concentration

ranging between about 0.01 weight percent (wt %) and about 5 wt % with the

dopant acid in a molar ratio of between about 0.3 and about 1.0 with respect to

the polyaniline. In another embodiment, the polymer solution system includes polyaniline

in a concentration ranging between about 1 wt % and 3.5 wt %. The dopant acid

has a molar ratio of between about 0.4 and about 0.6 with respect to the

polyaniline. In one embodiment, the polymer solution system includes polyaniline in a concentration ranging between about 0.01 wt % and about 5 wt %. In yet another embodiment the polyaniline has a concentration ranging between about 1 wt % and about 3.5 wt %. In still another embodiment, the dopant acid has a molar ratio ranging between about 0.3 and about 1 with respect to polyaniline. In yet another embodiment, the dopant acid has a molar ratio ranging between about 0.4 and about 0.6 with respect to polyaniline.

In one embodiment, the formic acid used in the polymer solution system is commercial grade formic acid having a concentration ranging between about 90% and about 100%. In yet another embodiment, the formic acid used is commercial grade formic acid having a concentration ranging between about

98% and about 100%.

The present invention also teaches a method for preparing a polymer solution system comprising polyaniline, a dopant acid and formic acid, where the method includes the step of co-dissolving the emeraldine-base form of polyaniline and a dopant acid in formic acid.

The invention further teaches a solid electrolyte tantalum capacitor having an anodized porous tantalum pellet. The pellet is coated with a polyaniline solid electrolyte, where the solid electrolyte is produced from a polymer solution system comprising polyaniline, a dopant acid and formic acid.

The invention also recites a solid electrolyte aluminum capacitor having an anodized aluminum anode foil. The foil is coated with a polyaniline solid electrolyte where the solid electrolyte is produced from a polymer solution system comprising polyaniline, a dopant acid and formic acid. The invention further teaches a process for applying a polymer solid electrolyte to a capacitor element. The process comprises the stages of first

impregnating said element by immersing it in Solution A, and then drying the

element to remove the solvent. Subsequently, a first coating is applied to the

impregnated element by dipping the impregnated element in Solution B and

drying the element to remove the solvent. Solution A and Solution B are comprised according to the polymer solution system described above; that is

polyaniline, a dopant acid, and formic acid. The capacitor anode element is

selected from a group comprising an anodized porous tantalum pellet, an

anodized prous niobium pellet, and an anodized aluminum anode foil.

In another embodiment, the process further includes the stage of applying

a second coating to the impregnated element by dipping the element in Mixture

C and then drying the element to remove the solvent. Mixture C comprises

acid-doped polyaniline, a conducting powdered carbonaceous component such

as graphite powder, poly(methyl metacrylate) and formic acid.

In yet another embodiment, the process further comprises the stage of

applying a second coating layer to the impregnated element by dipping the

element in a commercially available mixture of tradename Aquadag followed by

drying the capacitor element to remove the solvent. In a still further embodiment, the process further comprises the stage of:

applying an Aquadag mixture subsequent to applying a coating of Mixture C.

Solution A includes: polyaniline at a concentration ranging between about

0.5 wt % and about 2 wt %; dopant acid at a molar ratio of between about 0.4

and about 0.6 with respect to the polyaniline. Solution B includes: polyaniline at a concentration ranging between about 2 wt % and about 3.5 wt %; dopant acid at a molar ratio of between about 0.4 and about 0.6 with respect to the polyaniline.

In one embodiment of the process, Mixture C includes: polyaniline at a concentration ranging between about 2 wt % and about 3.5 wt %; dopant acid at a molar ratio of between 0.4 and 0.6 with respect to the polyaniline.

In yet another embodiment of the process, Mixture C further includes: polyaniline having a weight percent ranging between about 5 wt % and about 50 wt % of the dry solids of Mixture C; powdered carbonaceous material such as graphite powder having a weight percent ranging between about 5 wt % and about 60 wt % of the dry solids of Mixture C; PMMA having a weight percent of between about 2 wt % and about 35 wt % of the dry solids of Mixture C.

In still another embodiment of the process, Mixture C further includes: polyaniline having a weight percent of between about 15 wt % and about 40 wt % of the dry solids of Mixture C; graphite powder having a weight percent of between about 15 wt % and about 50 wt % of the dry solids of Mixture C; PMMA having a weight percent ranging between about 5 wt % and about 15 wt % of the dry solids of Mixture C.

The impregnation stage of the process comprises a plurality of successive immersing and drying steps. Each of the immersing steps is followed by a drying step. Each immersing step followed by a drying step constitutes a cycle, and the number of the cycles ranges between 1 and 20. In a further embodiment of the process, the number of such cycles ranges between 4 and 10. In one embodiment of the process, the drying step of the impregnation stage comprises drying the coated capacitor elements in a vacuum oven at a temperature not exceeding 100°C. In yet another embodiment, this drying step is carried out at a temperature not exceeding 65°C. In one embodiment of the process, the drying step of the impregnation stage comprises drying the coated capacitor elements in a convection oven at a temperature not exceeding 100°C. In yet another embodiment, this drying step is carried out at a temperature not exceeding 65°C.

In one embodiment of the process, the first coating stage includes a plurality of successive dipping and drying steps, each dipping step followed by a drying step. Each dipping step followed by a drying step constitutes a cycle, and the number of cycles ranges between 1 and 10. In yet another embodiment of the process, the number of these cycles ranges between 1 and 5.

In one embodiment of the process, the drying step of the first coating stage includes drying the coated elements in a vacuum oven at a temperature not exceeding 100°C. In yet another embodiment, the drying temperature does not exceed 65°C.

In one embodiment of the process, the drying step of the first coating stage includes drying the coated elements in a convection oven at a temperature not exceeding 100°C. In yet another embodiment, the drying temperature does not exceed 65°C.

In one embodiment, the second coating stage includes a plurality of successive dipping and drying steps, each dipping step followed by a drying step. Each dipping step followed by a drying step constitutes a cycle, and the number of cycles ranges between 1 and 5. In yet another embodiment, the number of these cycles ranges between 1 and 2.

In an embodiment of the process, the drying step of the second coating stage includes drying the coated capacitor elements in a vacuum oven at a temperature not exceeding 100°C. In another embodiment the drying temperature does not exceed 65°C.

In an embodiment of the process, the drying step of the second coating stage includes drying the coated capacitor elements in a convection oven at a temperature not exceeding 100°C. In another embodiment the drying temperature does not exceed 65°C.

In an embodiment of the process, the second coating stage further includes the step of applying a silver-acrylic coating to the capacitor pellets subsequent to applying Mixture C. In yet another embodiment of the process, the second coating stage further comprises the step of applying a silver-acrylic coating to the capacitor pellets subsequent to applying Aquadag.

DETAILED DESCRIPTION OF THE INVENTION

The polyaniline used for the polymer solutions of the present invention is synthesized by a method that is a modification of procedures known in the art. One such procedure is described by Cao at al, Polymer, 30, 2305 (1989). In that publication, aniline and a protonic acid, such as hydrochloric acid, are co-dissolved in water and subsequently an aqueous solution of an oxidizing agent, such as ammonium peroxydisulfate, is introduced to initiate the polymerization reaction. In the present invention, the polymerization procedure is modified so that a protonic acid other than hydrochloric acid is used, such as a sulfonic acid, preferably methane sulfonic acid (MSA).

The polyaniline powder obtained is in the conducting (acid-doped) emeraldine salt form. It is separated from the reaction mixture, washed to remove residual monomer and other impurities, and vacuum dried. It is then converted into the semi-conducting, but dissolvable, emeraldine base form of polyaniline by reacting it with an aqueous base solution. It is separated from the base solution, washed to remove excess base solution, and vacuum dried.

The emeraldine base form of polyaniline is then co-dissolved together with a dopant acid in formic acid. Commercial grade formic acid can be used in which the formic acid concentration may range between 90% and 100%, but preferably between 98% and 100%.

The dopant acid may be selected from the group of protonic acids. Preferred protonic acids include the groups of sulfonic acids, selenic acids, phosphoric acids, boric acids and carboxylic acids. The preferred dopant acids are organic sulfonic acids. Examples of sulfonic acids are methane sulfonic acid, camphor sulfonic acid, dodecylbenzene sulfonic acid, 1 -propane sulfonic acid, 1 -butane sulfonic acid, 1-hexane sulfonic acid, naphthalene sulfonic acid, dinonyl naphthalene sulfonic acid, and toluene sulfonic acid. Of this group, most preferred are methane sulfonic acid and camphor sulfonic acid.

Examples of phosphoric acids that can be used include phosphoric acids containing the following anions: ethyl-hexyl phosphate, di-phenyl phosphate and di-butyl phosphate. The dissolution stage comprises introducing the emeraldine base and the dopant acid into formic acid while mixing. The mixture is subsequently heated and sonicated to deagglomerate and disperse the polyaniline and aid in the reaction of the dopant acid with the emeraldine base. Other means of dispersion as practiced in the art may be employed. Solutions having polyaniline weight percents ranging from 0.01 wt % to 5 wt %, and polyaniline/dopant acid molar ratios ranging between 0.3 and 1.0, can be prepared by the method of the present invention. The polymer solution is stable for at least two weeks in a closed vessel when kept at ambient conditions.

Upon spreading the solution on a substrate and removing the formic acid solvent, a stable solid coating of the conducting (acid-doped) form of polyaniline is formed having an average electrical conduction of 10 S/cm.

The application of the conducting polymer to a tantalum capacitor anode element is a three-stage process. In the first stage, the polyaniline is introduced into the capacitor's sintered (porous) tantalum pentoxide-coated anode body element via several impregnation cycles, each cycle comprising the steps of immersing the elements in the conducting polymer solution followed by drying, either with vacuum or without vacuum, at an elevated temperature.

To assure an efficient impregnation of the polyaniline, a low concentration of the acid-doped polyaniline solution (solution A) is used. In solution A, the polyaniline is preferably between about 0.5 weight percent (wt %) and about 2 wt % of the total solution weight, more preferably about 1.5 wt % of the total solution weight.

The number of immersing-drying cycles during the impregnation stage may vary between 1 and 20. More preferably between 4 and 10 cycles are required. Owing to the relatively low boiling point of formic acid (100°C) the total time required for drying i.e. removal of the formic acid solvent, may not exceed a few minutes. The temperature to which the capacitor elements are subjected during the drying process may vary between 30°C and 100°C. Preferably, drying occurs at a temperature not exceeding 65°C. The formic acid can also be totally removed under ambient conditions of temperature and pressure, but the drying process is then more time consuming.

Subsequent to completing the impregnation stage, an external coating (over-coating) of conducting polyaniline is applied to the capacitor's impregnated anode body element. However, in order to assure a robust and continuous conductive coating, this stage requires a more concentrated polyaniline solution than Solution A. For this purpose, a more concentrated solution (Solution B) is used in which the polyaniline concentration is preferably between about 2 wt % and about 4 wt %, and even more preferably about 3 wt %. This first coating (over-coating) stage also comprises several repetitive dipping-drying cycles. The

number of cycles ranges between 1 and 10, preferably between 2 and 5

dipping-drying cycles.

Typically, the already impregnated capacitor anode elements are dipped

in the more concentrated Solution B of polyaniline, followed by air drying at room

temperature and then vacuum drying at an elevated temperature that may range

between 30°C and 100°C, preferably not exceeding 65°C.

The polymer-coated capacitor element may then be "reformed" according

to known art by immersing the element in an acidic reforming solution.

The final steps for completing the cathode are those practiced in the

known art. These usually include covering the solid electrolyte (manganese

dioxide or conducting polymer) with a conductive layer of powdered graphite, or

other conducting and semi-conducting carbonaceous materials, such as carbon

black, carbon nanotubes, carbon nanofibers, carbon nanoparticles and the like.

These materials are applied as an aqueous mixture, suspension or dispersion. A

typical commercially available mixture is Aquadag E. In what is written herein,

whenever graphite powder alone is mentioned as the carbon source of this

layer, it is understood that other forms of conducting carbonaceous material are

deemed equivalents without being stated explicitly. After applying the conductive layer of carbon black or powdered graphite,

a top-coating of acrylic silver is applied. A typical acrylic silver suspension, such

as commercially available SILVER CONDUCTOR 2232 purchasable from METECH, can be used. A further embodiment of the present invention deals with optionally replacing the currently used Aquadag by a different, and potentially better, conducting carbonaceous mixture. This new mixture (Mixture C) has the same function in solid-state capacitors as that of the currently used Aquadag mixture. Mixture C comprises a conducting polymer, conducting powdered carbonaceous material, a non-conducting polymer such as poly(methyl methacrylate) (PMMA) and formic acid.

Upon applying Mixture C to the already impregnated and over-coated capacitor body and removing the formic acid, a highly conducting solid mixture results, comprising graphite powder, conducting polymer and non-conductive polymer. Because of the polyaniline contained in Mixture C, the mixture, when applied to solid-state capacitor elements, is expected to adhere strongly to both the conducting polymer coating below and to the silver/acrylic coating on top, resulting in a better integrated cathode endowed with improved electrical properties.

Surprisingly, it was found that the presence of PMMA, when dissolved in the formic acid of Mixture C, resulted in an increase in the electrical conduction of the solid mixture after the formic acid was removed. It is suggested that this improvement in electrical conduction is due to PMMA enhancing the electrical contact between the conducting graphite particles and the acid-doped polyaniline. Following removal of the formic acid, the resulting solid comprises a mixture of graphite powder, acid-doped polyaniline and PMMA..

Mixture C may contain concentrations of graphite powder varying between about 5 wt % and about 60 wt %, preferably between about 15 wt % and about 50 wt %, of the dry solids contained in Mixture C. The acid-doped polyaniline portion in the solid mixture may vary between about 5 wt % and about 50 wt %, preferably between 15 wt % and 40 wt % of the dry solids found in Mixture C. The PMMA content in the solid mixture may vary between about 2 wt % and about 35 wt % of the dry solids found in Mixture C, preferably between about 5 wt % and about 15 wt % of the dry solids found in Mixture C.

In yet another embodiment, an Aquadag mixture may be applied to the capacitor elements in addition and subsequent to the application of Mixture C.

The final stage comprises applying a state-of-the-art silver/acrylic layer over the layer formed by applying Aquadag, Mixture C, or both Mixture C and Aquadag.

EXAMPLES Procedure 1 : Synthesis of Polyaniline

Polyaniline is prepared via a chemical oxidative polymerization reaction according to the method described by Cao et-al, [Polymer, 30, (1989) 2305]. Instead of hydrochloric acid, methane sulfonic acid (MSA) can be used as the dopant acid. An aqueous solution of 10 wt % freshly distilled aniline and the dopant sulfonic acid (molar ratio 1 :1) is cooled to 0°C, and polymerization is initiated through the drop-wise introduction of a 30% aqueous solution of ammonium persulfate (APS). The aniline/APS molar ratio in the mixture is 1 :0.5. While adding the oxidant, the reaction mixture is further cooled to between -5°C and -10°C and the reaction mixture is stirred for an additional 4 hours at a low temperature. The reaction is then terminated by the addition of methanol to the reaction mixture. The precipitated polymer is recovered, filtered and washed with methanol and water. The polymer is then vacuum dried at room temperature for 12 hours to obtain the polyaniline-acid salt.

The polyaniline-acid salt is then converted to the non-conductive emeraldine-base state by reacting it with 3 wt % NH OH. The precipitated polyaniline base is then filtered and thoroughly washed with water and ethanol, and then vacuum dried at room temperature for 12 hours.

Procedure 2: Preparation of Acid-Doped Polyaniline Solutions 1. Preparation of Solution A (Impregnation Stage):

Polyaniline-base as prepared in Procedure 1 and methane sulfonic acid are introduced to formic acid while stirring. The mixture is heated to boiling while stirring, and then sonicated to yield a solution of acid-doped polyaniline. The polyaniline/dopant molar ratio in the solution is 1 :0.5, with the preferred concentration of polyaniline being about 1.5 wt %.

2. Preparation of Solution B (First Coating Stage): A solution of doped-polyaniline is prepared according to Procedure 1, but the polyaniline concentration is about 3 wt %.

3. Preparation of Mixture C (Second Coating Stage): To an acid-doped polyaniline solution where the dopant acid is camphor sulfonic acid prepared as described above, a graphite powder and PMMA are added while mixing. The polyaniline concentration is between 2 and 5 weight %. The graphite powder comprises between 15 wt % and 40 wt. % of the solids in Mixture C, and the PMMA comprises between 5 wt % and 15 wt % of the solids in Mixture C.

Procedure 3: Application of Acid-Doped Polyaniline to Solid Tantalum

Capacitor Anode Elements

Electrically oxidized (anodized) sintered tantalum anode pellets are impregnated with the acid-doped polyaniline through successive steps of immersing the capacitor anode pellets in Solution A for 10 seconds to 5 minutes, preferrably 1 to 5 minutes, more preferrably 1 to 3 minutes, followed by vacuum drying at 65°C for 5 minutes. This procedure is repeated between 4 and 10 times for obtaining efficient impregnation of the porous pellets.

Next, the impregnated pellets are top-coated with acid-doped polyaniline by repeated dipping/drying steps. The impregnated pellets are dipped in Solution B, after which they are air-dried at room temperature for 15 minutes and then vacuum dried at 65°C for 10 minutes. A continuous outer conductive coating results. The thickness of this layer is controlled through modifying the solution concentration and the number of dipping/drying cycles.

In the next stage, a conductive carbon black graphite powder layer is applied on top of the conducting polymer coating. A mixture containing carbon black/graphite powder, such as commercial Aquadag E, may be used, as is practiced in the art. Alternatively, Mixture C may be used instead of, or in addition to, Aquadag E, as described above. When Mixture C alone is used (replacing Aquadag E), the impregnated capacitor pellets, subsequent to being treated with Solution B, are further dipped in Mixture C. The pellets are first air-dried for 15 minutes at room temperature and then vacuum-dried at 65°C for 10 minutes. In an alternative embodiment, subsequent to applying Mixture C the capacitor elements are further dipped in the Aquadag E mixture. The Aquadag coated capacitor elements are then dried in air at room temperature followed by vacuum drying at an elevated temperature as described above.

To complete the cathode of the solid electrolytic capacitor, a conducting acrylic-silver top layer is applied on top of the Aquadag solid mixture or on top of the Mixture C solid mixture using a commercial acrylic-silver mixture according to procedures commonly practiced in the art.

It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described herein above. Rather, the scope of the present invention is defined only by the claims that follow:

Claims

1. A polymer solution system for applying a conducting polymer solid electrolyte to solid electrolyte anodized capacitor anodes, said polymer solution system comprising: polyaniline, a dopant acid, and formic acid.

2. A polymer solution according to claim 1 wherein said solid electrolyte capacitor anodes are selected from a group comprising valve metals.

3. A polymer solution according to claim 1 wherein said solid electrolyte capacitor anodes are selected from a group comprising valve metals tantalum, niobium and aluminum.

4. A polymer solution system according to claim 1 wherein said dopant acid is selected from a group consisting of sulfonic acids and phosphoric acids.

5. A polymer solution system according to claim 1 wherein said dopant acid is a sulfonic acid

6. A polymer solution system according to claim 5 wherein said sulfonic acid is selected from a group consisting of methane sulfonic acid, camphor sulfonic acid, dodecylbenzene sulfonic acid, 1-propanesulfonic acid, 1-butane sulfonic acid, 1-hexane sulfonic acid, naphthalene sulfonic acid, dinonyl naphthalene sulfonic acid, and toluene sulfonic acid.

7. A polymer solution system according to claim 5 wherein said sulfonic acid is selected from a group consisting of methane sulfonic acid and camphor sulfonic acid.

8. A polymer solution system according to claim 5 wherein said sulfonic acid is methane sulfonic acid.

9. A polymer solution according to claim 1 where said dopant acid is selected from a group of phosphoric acids consisting of phosphoric acids with anions chosen from a group consisting of ethyl-hexyl phosphate, di-phenyl phosphate and di-butyl phosphate.

10. A polymer solution system according to claim 1 wherein said polyaniline

has a concentration ranging between about 0.01 weight percent (wt %) and

about 5 wt %, and said dopant acid has a molar ratio of between about 0.3

and about 1.0 with respect to said polyaniline.

11. A polymer solution system according to claim 1 , wherein said polyaniline has a concentration ranging between about 1 wt % and 3.5 wt %, and said dopant acid has a molar ratio of between about 0.4 and about 0.6 with respect to said polyaniline.

12. A polymer solution system according to claim 1 wherein said polyaniline has a concentration ranging between about 0.01 wt % and about 5 wt %.

13. A polymer solution system according to claim 1 wherein said polyaniline has a concentration ranging between about 1 wt % and about 3.5 wt %.

14. A polymer solution system according to claim 1 wherein said dopant acid has a molar ratio ranging between about 0.3 and about 1 with respect to said polyaniline.

15. A polymer solution system according to claim 1 wherein said dopant acid has a molar ratio ranging between about 0.4 and about 0.6 with respect to said polyaniline,

16. A polymer solution system according to claim 1 wherein said formic acid used is a commercial grade formic acid having a concentration ranging between about 90% and about 100%.

17. A polymer solution system according to claim 1 wherein said formic acid used is a commercial grade formic acid having a concentration ranging between about 98% and about 100%.

18. Method for preparing a polymer solution system according to claim 1 , said method comprising the step of: co-dissolving the emeraldine-base form of said polyaniline and said dopant acid in formic acid.

19. Solid electrolyte tantalum capacitor wherein an anodized porous tantalum pellet is coated with a polyaniline solid electrolyte, said solid electrolyte produced from a polymer solution system comprised according to claim 1.

20. Solid electrolyte niobium capacitor wherein an anodized porous niobium pellet is coated with a polyaniline solid electrolyte, said solid electrolyte produced from a polymer solution system comprised according to claim 1.

21. Solid electrolyte aluminum capacitor wherein an anodized aluminum anode foil is coated with a polyaniline solid electrolyte, said solid electrolyte produced from a polymer solution system, said polymer solution system comprised according to claim 1.

22. Process for applying a polymer solid electrolyte to a capacitor element, said process comprising the stages of: impregnating said capacitor element by immersing it in Solution A, and thereafter drying said capacitor element to remove the solvent; and applying a first coating to said capacitor element through dipping said impregnated capacitor element in Solution B and drying said capacitor element, wherein Solution A and Solution B are comprised according to claim 1.

23. Process according to claim 22 wherein said capacitor element is selected from a group consisting of an anodized porous tantalum pellet, anodized porous niobium pellet and an anodized aluminum anode foil.

24. Process according to claim 22 further comprising the stage of; applying a second coating to said capacitor element by dipping said capacitor element in Mixture C and then drying said capacitor element to remove the solvent, wherein Mixture C comprises acid-doped polyaniline, conducting carbonaceous powder, poly(methyl metacrylate) and formic acid.

25. Process according to claim 22 further comprising the stage of: applying a second coating layer to said capacitor element by dipping said capacitor element in an Aquadag mixture and then drying said capacitor element.

26. Process according to claim 24 further comprising the stage of: applying an Aquadag mixture subsequent to applying Mixture C.

27. Process according to claim 22 wherein said Solution A includes said polyaniline at a concentration ranging between about 0.5 wt % and about 2 wt %, and said dopant acid at a molar ratio of between about 0.4 and about 0.6 with respect to said polyaniline.

28. Process according to claim 22 wherein said Solution B includes said polyaniline at a concentration ranging between about 2 wt % and about 3.5 wt %, and said dopant acid at a molar ratio of between about 0.4 and about 0.6 with respect to said polyaniline.

29. Process according to claim 24 wherein said Mixture C includes polyaniline at a concentration ranging between about 2 wt % and about 3.5 wt %, and said dopant acid at a molar ratio of between 0.4 and 0.6 with respect to said polyaniline.

30. Process according to claim 24 wherein said Mixture C further includes polyaniline having a weight percent ranging between about 5 wt % and about

50 wt % of the dry solids of Mixture C, graphite powder having a weight percent ranging between about 5 wt % and about 60 wt % of the dry solids of Mixture C, and PMMA having a weight percent of between about 2 wt % and about 35 wt % of the dry solids of Mixture C.

31. Process according to claim 24 wherein Mixture C further includes polyaniline having a weight percent of between about 15 wt % and about 40 wt % of the dry solids of Mixture C, graphite powder having a weight percent of between about 15 wt % and about 50 wt % of the dry solids of Mixture C, and PMMA having a weight percent ranging between about 5 wt % and about 15 wt % of the dry solids of Mixture C.

32. Process according to claim 22 wherein said impregnation stage comprises a plurality of successive immersing and drying steps, each said immersing step is followed by a drying step, wherein each immersing step followed by a drying step constitutes a cycle, and wherein the number of said cycles ranges between 1 and 20.

33. Process according to claim 22, wherein said impregnation stage comprises a plurality of successive immersing and drying steps, each said immersing step is followed by a drying step, wherein each immersing step followed by a drying step constitutes a cycle, and wherein the number of said cycles ranges between 4 and 10.

34. Process according to claim 22 wherein said drying step of said impregnation stage comprises drying the coated capacitor elements in a vacuum oven at a temperature not exceeding 100°C.

35. Process according to claim 22 wherein said drying step of said impregnation stage comprises drying the coated capacitor elements in a vacuum oven at a temperature not exceeding 65°C.

36. Process according to claim 22 wherein said first coating stage comprises a plurality of successive dipping and drying steps, each dipping step followed by a drying step, wherein each dipping step followed by a drying step constitutes a cycle, wherein the number of cycles ranges between 1 and 10.

37. Process according to claim 22, wherein said first coating stage comprises a plurality of successive dipping and drying steps, each dipping step followed by a drying step, wherein each dipping step followed by a drying step constitutes a cycle, wherein the number of cycles ranges between 1 and 5.

38. Process according to claim 22, wherein said drying step of said first

coating stage comprises drying the coated capacitor elements in a vacuum

oven at a temperature not exceeding 100°C.

39. Process according to claim 22, wherein said drying step of said first

coating stage comprises drying the coated capacitor elements in a vacuum

oven at a temperature not exceeding 65°C.

40. Process according to claim 24, wherein said second coating stage

comprises a plurality of successive dipping and drying steps, each dipping

step followed by a drying step, wherein each dipping step followed by a

drying step constitutes a cycle, wherein the number of cycles ranges between 1 and 5.

41. Process according to claim 24 wherein said second coating stage comprises a plurality of successive dipping and drying steps, each dipping step followed by a drying step, wherein each dipping step followed by a drying step constitutes a cycle, wherein the number of cycles ranges between 1 and 2.

42. Process according to claim 24, wherein said drying step of said second coating stage comprises drying the coated capacitor elements in a vacuum oven at a temperature not exceeding 100°C.

43. Process according to claim 24, wherein said drying step of said second coating stage comprises drying the coated capacitor elements in a vacuum oven at a temperature not exceeding 65°C.

44. Process according to claim 24, wherein said second coating stage further comprises the step of applying a silver-acrylic coating to said capacitor pellets subsequent to applying Mixture C.

45. Process according to claims 25 and 26 wherein said second coating stage further comprises the step of applying a silver-acrylic coating to said capacitor pellets subsequent to applying said Aquadag.

46. A solution system according to any of claims 1-17 substantially as described hereinabove.

47. A process according to any of claims 22-45 substantially as described herein above.

48. Mixture C according to claim 24, wherein the conducting carbonaceous powder is graphite powder.

49. Mixture C according to claim 24, wherein the conducting carbonaceous powder is nano-partical made.

50. Mixture C according to claim 24, wherein the conducting carbonaceous powder is carbon black powder.