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Electrode compositions containing carbon nanotubes for solid electrolyte capacitorsElectrode compositions containing carbon nanotubes for solid electrolyte capacitors description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070171596, Electrode compositions containing carbon nanotubes for solid electrolyte capacitors. Brief Patent Description - Full Patent Description - Patent Application Claims
BACKGROUND [0001] The present invention is related to an improved method of forming a solid electrolyte capacitor and an improved capacitor formed thereby. More specifically, the present invention is related to conductive layers comprising carbon nanotubes and an improved capacitor comprising the improved conductive structure. [0002] The construction and manufacture of solid electrolyte capacitors is well documented. In the construction of a solid electrolytic capacitor a valve metal serves as the anode. The anode body can be either a porous pellet, formed by pressing and sintering a high purity powder, or a foil which is etched to provide an increased anode surface area. An oxide of the valve metal is electrolytically formed to cover all surfaces of the anode and serves as the dielectric of the capacitor. The solid cathode electrolyte is typically chosen from a very limited class of materials, to include manganese dioxide or electrically conductive organic materials such as 7,7',8,8'-tetracyanoquinonedimethane (TCNQ) complex salt, or intrinsically conductive polymers, such as polyaniline, polypyrol, polyethylenedioxythiophene and their derivatives. The solid cathode electrolyte is applied so that it covers all dielectric surfaces. An important feature of the solid cathode electrolyte is that it can be made more resistive by exposure to high temperatures. This feature allows the capacitor to heal leakage sites by Joule heating. In addition to the solid electrolyte the cathode of a solid electrolyte capacitor typically consists of several layers which are external to the anode body. In the case of surface mount constructions these layers typically include: a carbon layer; a layer containing a highly conductive metal, typically silver, bound in a polymer or resin matrix; a conductive adhesive layer such silver filled adhesive; and a highly conductive metal lead frame. The various layers connect the solid electrolyte to the outside circuit and also serve to protect the dielectric from thermo-mechanical damage that may occur during subsequent processing, board mounting, or customer use. [0003] In the case of conductive polymer cathodes the conductive polymer is typically applied by either chemical oxidation polymerization, electrochemical oxidation polymerization or spray techniques with other less desirable techniques being reported. [0004] The carbon layer serves as a chemical barrier between the solid electrolyte and the silver layer. Critical properties of the layer include adhesion to the underlying layer, wetting of the underlying layer, uniform coverage, penetration into the underlying layer, bulk conductivity, interfacial resistance, compatibility with silver layer, buildup, and mechanical properties. There has been a constant conflict in the art to optimize these various characteristics. For example, a higher concentration of resin is preferred for adhesion. As the resin concentration increases the adhesion of the carbon layer improves. Conductivity on the other hand occurs through the carbon particles and therefore it is preferred to minimize the resin to insure adequate conductivity. Those of skill in the art have heretofore been forced to optimize the conflicting parameters of adhesion with conductivity. It has long been considered important to avoid decreasing the carbon content due to the loss of conductivity. [0005] U.S. Pat. No. 6,556,427 attempts to circumvent the conflict between adhesion and conductivity of the carbon layer by allowing the binder of the carbon paste to infiltrate into the solid electrolyte layer. Controlling the degree of infiltration is difficult and variability in the infiltration will alter the composition of the carbon layer thereby resulting in variability in conduction and in adhesion with a subsequent layer. [0006] The resistance across the carbon layer increases as the carbon buildup increases since the electrical path length across the layer is increased. However, thin layers provide less thermo-mechanical protection to the dielectric. Therefore, the carbon layer has long been considered necessary and yet a limiting factor in the further advancement of solid electrolytic capacitors. [0007] The silver layer serves to conduct current from the lead frame to the cathode and around the cathode to the sides not directly connected to the lead frame. The critical characteristics of this layer are high conductivity, adhesive strength to the carbon layer, wetting of the carbon layer, and the mechanical properties. Compatibility with the subsequent layers employed in the assembly and encapsulation of the capacitor are also critical. In the case where a silver adhesive is used to attach to a lead frame compatibility with the silver adhesive is an issue. In capacitors which utilize solder to connect to the external lead solderability and thermal stability are important factors. In order for the solder to wet the silver layer, the resin in the silver must degrade below the temperature at which the solder is applied. However, excessive degradation of the resin creates an effect termed "silver leeching" resulting in a poor connection between the external cathode layers and the external cathode lead. The traditional approach to applying a silver layer requires a delicate compromise in thermal stability of the resin in order to simultaneously achieve solder wetting and to avoid silver leeching. [0008] Through diligent research the present inventors have developed a carbon layer which circumvents the problems encountered in the prior art. SUMMARY [0009] It is an object of the present invention to provide an improved capacitor with lower equivalent series resistance (ESR). [0010] It is another object of the present invention to provide a capacitor with improved conduction between conductive layers without detriment to the adhesion between these layers. [0011] A particular feature of the present invention is the ability to provide the improvements with minor changes in the manufacturing and with improved yields due to the decrease in the amount of unusable material which typically results from either poor adhesion or poor conductivity between layers. The increased mechanical strength of the carbon coating of this invention provides better tolerance to thermal mechanical stress which the capacitors are exposed to during the manufacturing process. This also provides improved yield. [0012] These, and other advantages, are provided in an improved capacitor. The capacitor has an anode with an anode wire and an oxide layer on the surface of the anode. A cathode layer is exterior to the oxide layer. A carbon conductive layer is exterior to the cathode layer wherein the cathode layer comprises 5-75 wt % resin and 25-95 wt % conductor. The conductor has carbon nanotubes. An anode lead is in electrical contact with the anode wire and a cathode lead is in electrical contact with the carbon conductive layer. [0013] Yet another embodiment is provided in a method for forming a capacitor with the steps of: [0014] forming an anode from a valve metal with an anode wire extending therefrom; [0015] exposing a surface of the anode to an electrolyte solution to form an oxide layer through anodization thereon; [0016] forming a cathode layer on at least a portion of the oxide layer; [0017] applying a carbon coating layer on at least a portion of the cathode layer wherein the carbon coating layer comprises solvent, resin and carbon nanotubes; [0018] removing the solvent from said carbon coating layer to form a carbon conductive layer; [0019] attaching an anode lead into electrical contact with the anode wire; and [0020] attaching a cathode lead into electrical contact with the cathode layer. BRIEF DESCRIPTION OF THE DRAWINGS [0021] FIG. 1 is a schematic representation of a capacitor of the present invention. [0022] FIG. 2 is a flow chart representation of the process of the present invention. DETAILED DESCRIPTION [0023] The present invention mitigates the deficiencies of the prior art by providing improved conduction at a given layer thickness thereby allowing for lower ESR. This was previously considered contradictory in a single layer. The present invention will be described with reference to the various figures which illustrate, without limiting, the invention. [0024] In FIG. 1, a cross-sectional view of a capacitor is shown as represented at 10. The capacitor comprises an anode, 11, comprising a valve metal as described herein. A dielectric layer, 12, is provided on the surface of the anode, 11. The dielectric layer is preferably formed as an oxide of the valve metal as further described herein. Coated on the surface of the dielectric layer, 12, is a conductive layer, 13. The conductive layer preferably comprises conductive polymer, such as polyethylenedioxythiophene (PEDT), polyaniline or polypyrrole or their derivatives; manganese dioxide, lead oxide or combinations thereof. A carbon layer, 14, comprising carbon nanotubes, 15, is provided as a chemical barrier between the conductive layer and subsequent layers. A silver layer, 16, forms a direct electrical contact with the cathode terminal, 17, such that current flows from the cathode terminal through the successive layers sequentially. The carbon layer together with the silver layer provides a strongly adhered conductive path between the conductive layer, 13, and the cathode terminal, 17. An anode wire, 18, provides electrical contact between the anode, 11, and an anode terminal, 19. The entire element, except for the terminus of the terminals, is then preferably encased in a non-conducting material, 20, such as an epoxy resin. [0025] The carbon layer comprises a conductive composition comprising resin; conductive particles and carbon nanotubes. The carbon layer may also comprise adjuvants such as crosslinking additives, surfactants and dispersing agents. The resin, conductive carbon particles, carbon nanotubes and adjuvants are preferably dispersed in an organic solvent or water to form a coating solution. [0026] It is most preferable that the dried conductive composition comprises about 5-75 wt % polymer resin and about 25-95 wt % conductor. More preferably, the conductive composition comprises about 5-25 wt % polymer resin and most preferably the conductive composition comprises about 15-20 wt % polymer resin. The conductor comprises carbon nanotubes. In a particularly preferred embodiment the conductor comprises about 7-99.975 wt % conductive particles and 0.025-93 wt % carbon nanotubes. More preferably the conductor comprises 15-99 wt % conductive carbon particles and even more preferably 20-99 wt % conductive carbon particles. Most preferably the conductor comprises 70-93 wt % conductive carbon particles and most preferably 1-30 wt % carbon nanotubes. [0027] For the purposes of the present invention conductive particles refer to discrete particles of conductive materials, excluding nanotubes, which are selected from the group consisting of carbon black, graphite and carbon fibers. Carbon black is the most preferred as commercially available from various commercial sources such as Degussa, Cabot or Columbian Chemicals. The conductive particles have a preferred particle size range of 5 nm-30 microns. More preferably the conductive carbon black particles have a preferred particle size range of 10-200 nm. Continue reading about Electrode compositions containing carbon nanotubes for solid electrolyte capacitors... 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