Patent Application
20080010797
Provided herein is an improved capacitor and method for making the improved capacitor. More particularly, provided herein is a capacitor comprising a conducting polymeric cathode with a break down voltage of over 60 V and ESR of no more than 150 mohms. This was previously considered unavailable to those of skill in the art.
The Invention will be described with reference to the various figures forming an integral part of the instant specification.
After analyzing the BDV behavior of a series of polymer coated anodes at various process stages, it was found that the BDV degrades as the polymer coating process progresses. This is contrary to the expectations of those skilled in the art. As shown in
The generally accepted mechanism of in situ polymerization is shown in the following scheme:
It is generally agreed that M, the monomers, are to be oxidized to form charged radicals, M*, which then couple with each other to form dimers. The same process will lead to the formation of trimers, tetramers, oligomers, and eventually polymers. It is known that the radicals are of high energy and very reactive. It is now hypothesized that the radicals generated during the in situ reaction not only react with each other, but also react with Ta2O5, or the dielectric, by injecting electrons into it, thereby significantly degrading the dielectric performance. This hypothesis is illustrated in the following scheme:
Based on the now hypothesized reaction mechanism the dielectric degradation is caused by the interactions between high energy radicals and the dielectric. Dielectric degradation can't be avoided if an in-situ polymerization process is employed to make the polymer since the radicals are the intermediates of the polymer. In order to minimize the dielectric degradation, or to increase BDV, in situ polymerization processing should be minimized, or totally eliminated if possible. By the use of conductive polymer slurries, which are pre-made conductive polymers dispersed or partially dissolved in either aqueous or organic liquid media in prior to the coating process, the problems can be overcome
Based on above new understanding, the technical challenge for capacitor manufacturers is how to impregnate the anodes with polymer to achieve expected performances, such as capacitance and ESR, while still forming a robust external polymer layer for anode protection with minimum involvement of in situ reactions. This has led to the unexpected realization that applying slurry containing pre-made intrinsically conducting polymer provides advantages not previously considered. It is most preferred that the polymer have a molecular weight of at least about 500 to no more than about 10,000,000. Below about 500 the polymer chains are of insufficient length to offer high conductivity and to form a coating with sufficient physical integrity. Above about 10,000,000 the polymeric chain is too large to form an adequate slurry.
The invention will be described with reference to the
In
The anode is typically prepared by pressing tantalum powder and sintering to form a compact. For convenience in handling, the valve metal is typically attached to a carrier thereby allowing large numbers of elements to be processed at the same time. Other valve metals and metal oxides such as aluminium, titanium, niobium, and niobium oxide may be employed as the anode material.
It is most desirable that the dielectric of the anode be an oxide of tantalum or of other valve metals. The oxide is preferably formed by dipping the valve metal into an electrolyte solution and applying a positive voltage to the valve metal.
Preferred electrolytes for formation of the oxide on the valve metal include diluted inorganic acids such as sulphuric acid, nitric acid, phosphoric acids, aqueous solutions of dicarboxylic acids, such as ammonium adipate. Other materials may be incorporated into the oxide such as phosphates, citrates, etc. to impart thermal stability or chemical or hydration resistance to the oxide layer.
The conductive polymer layer is preferably formed by dipping the anodized valve metal anodes into a slurry of intrinsically conductive polymer. It is preferred that the anode be dipped into the slurry from 1 to 15 times to insure formation of an adequate coating. The anode should remain in the slurry for a period about 0.5 minute to 2 minutes to allow complete slurry coverage of its surface.
In a less preferred embodiment the anode is initially impregnated with polymer formed in situ, then overcoated with polymer slurry. In the in-situ polymerization process, the anodized valve metal anode is dipped into an oxidant solution followed by dipping in liquid monomer or a solution of monomer. It is preferred that the anode be processed in the multiple process steps no more than 6 times. Above 6 process steps the BDV degrades significantly. Most preferably the anode is processed by the in situ polymerization process as few times as possible but not at the expense of insuring adequate polymer coverage and achieving low ESR. In practice two dips are typically sufficient and less than 6, is preferred.
A particularly preferred conducting polymer is illustrated in Formula I:
R1 and R2 of Formula 1 are chosen to prohibit polymerization at the β-site of the ring. It is most preferred that only α-site polymerization be allowed to proceed. Therefore, it is preferred that R1 and R2 are not hydrogen. More preferably R1 and R2 are α-directors. Therefore, ether linkages are preferable over alkyl linkages. It is most preferred that the groups are small to avoid steric interferences. For these reasons R1 and R2 taken together as —O—(CH2)2—O— is most preferred.
In Formula 1, X is S or N most preferable X is S.
R1 and R2 independently represent linear or branched C1-C16 alkyl or C2-C18 alkoxyalkyl; or are C3-C8 cycloalkyl, phenyl or benzyl which are unsubstituted or substituted by C1-C6 alkyl, C1-C6 alkoxy, halogen or OR3; or R1 and R2, taken together, are linear C1-C6 alkylene which is unsubstituted or substituted by C1-C6 alkyl, C1-C6 alkoxy, halogen, C3-C8 cycloalkyl, phenyl, benzyl, C1-C4 alkylphenyl, C1-C4 alkoxyphenyl, halophenyl, C1-C4 alkylbenzyl, C1-C4 alkoxybenzyl or halobenzyl, 5-, 6-, or 7-membered heterocyclic structure containing two oxygen elements. R3 preferably represents hydrogen, linear or branched C1-C16 alkyl or C2-C18 alkoxyalkyl; or are C3-C8 cycloalkyl, phenyl or benzyl which are unsubstituted or substituted by C1-C6 alkyl.
Both pure monomer and monomer solution in various solvents can be employed. Common oxidants including iron (III) toluenesulfonate, hydrogen peroxide, and ammonium persulfate are preferred to be used in the polymerization process.
As typically employed in the art, various dopants can be incorporated into the polymer during the polymerization process. Dopants can be derived from various acids or salts, including aromatic sulfonic acids, aromatic polysulfonic acids, organic sulfonic acids with hydroxy group, organic sulfonic acids with carboxylhydroxyl group, alicyclic sulfonic acids and benzoquinone sulfonic acids, benzene disulfonic acid, sulfosalicylic acid, sulfoisophthalic acid, camphorsulfonic acid, benzoquinone sulfonic acid, dodecylbenzenesulfonic acid, toluenesulfonic acid. Other suitable dopants include sulfoquinone, anthracenemonosulfonic acid, substituted naphthalenemonosulfonic acid, substituted benzenesulfonic acid or heterocyclic sulfonic acids as exemplified in U.S. Pat. No. 6,381,121 which is included herein by reference thereto.
Binders and cross-linkers can be also incorporated into the conductive polymer layer if desired. Suitable materials include poly(vinyl acetate), polycarbonate, poly(vinyl butyrate), polyacrylates, polymethacrylates, polystyrene, polyacrylonitrile, poly(vinyl chloride), polybutadiene, polyisoprene, polyethers, polyesters, silicones, and pyrrole/acrylate, vinylacetate/acrylate and ethylene/vinyl acetate copolymers.
Carbon paste layers and silver paste layers are formed for attaching electrode leads as known in the art. The device is then sealed in a housing.
Other adjuvants, coatings, and related elements can be incorporated into a capacitor, as known in the art, without diverting from the present invention. Mentioned, as a non-limiting summary include, protective layers, multiple capacitive levels, terminals, leads, etc.
One group of 15 uF-25V anodes was processed through various in situ polymerization cycles and the other group from the same batch was coated with polymer slurry. As shown in
Our recent discovery that the use of conductive polymer slurries, including polyaniline (PANI) and polyethyldioxythiophene (PEDOT), leads to a significant increase in BDV. It is believed that the current in-situ polymerization process degrades the dielectric insulating properties by injecting electrons via free radicals, which are parts of the intermediates of the polymerization reactions. The dielectric layer becomes less stable by accepting these electrons. Applying polymer slurry onto the dielectric instead of forming a polymer coating in situ eliminates the exposure of the dielectric to the high energy radicals generated during the polymerization reaction, thereby minimizing dielectric degradation during multiple polymer coating process steps.
A 15 uF tantalum anode with a size of (4.90 mm×3.25 mm×1.70 mm) was dipped into a solution of iron (III) toluenesulfonate (oxidant) for 1 minute and sequentially dipped into ethyldioxythiophene (monomer) for 1 minute. The anodes were washed to remove excess monomer and by products of the reactions after the completion of 60 minutes polymerization, which formed a thin layer of conductive polymer (PEDOT) on the dielectric of the anodes. This process was repeated 10 times. The anodes were tested for BDV and other electrical properties after the anodes were coated with graphite and silver. The test results are listed in the following Table 3.
A 15 uF tantalum anode with a size of (4.90 mm×3.25 mm×1.70 mm) was dipped into a solution of ion (III) toluenesulfonate (oxidant) for 1 minute and sequentially dipped into ethyldioxythiophene (monomer) for 1 minute. The anodes were washed to remove excess monomer and by products of the reactions after the completion of 60 minutes polymerization, which formed a thin layer of conductive polymer (PEDOT) on the dielectric of the anodes. This process was repeated 6 times. The anodes were then dip-coated twice using a PEDOT slurry to form a thick external polymer layer. Graphite and Ag coating was applied onto the anodes after the conductive polymer slurry on the anodes was dried. The anodes were tested for BDV and other electrical properties. The test results are listed in the following Table 3.
A 15 uF tantalum anode with a size of (4.90 mm×3.25 mm×1.70 mm) was dipped into a solution of ion (III) toluenesulfonate (oxidant) for 1 minute and sequentially dipped into ethyldioxythiophene (monomer) for 1 minute. The anodes were washed to remove excess monomer and by products of the reactions after the completion of 60 minutes polymerization, which formed a thin layer of conductive polymer (PEDOT) on the dielectric of the anodes. This process was repeated 2 times. The anodes were then dip-coated twice using a polyaniline (PANI) slurry to form a thick external polymer layer. Graphite and Ag coating was applied onto the anodes after the conductive polymer slurry on the anodes was dried. The anodes were tested for BDV and other electrical properties. The test results are listed in the following Table 3.
A 15 uF tantalum anode with a size of (4.90 mm×3.25 mm×1.70 mm) was dipped into a diluted PEDOT slurry for 1 minute and dried at 120° C. for 20 minutes. This process was repeated 5 times. The anodes were then dip-coated three times using a PEDOT slurry to form a thick external polymer layer. Graphite and Ag coating was applied onto the anodes after the conductive polymer slurry on the anodes was dried. The anodes were tested for BDV and other electrical properties. The test results are listed in the following Table 3.
This invention has been described with particular reference to the preferred embodiments without limit thereto. One of skill in the art would realize additional embodiments and alterations without deviating from the scope of the invention which is more particularly set forth in the claims appended hereto.
