Patent Publication Number: US-9406445-B2

Title: Solid electrolytic capacitor with enhanced wet-to-dry capacitance

Description:
RELATED APPLICATIONS 
     The present application claims priority to U.S. Provisional Application Ser. No. 61/673,409, filed on Jul. 19, 2012, which is incorporated herein in its entirety by reference thereto. 
    
    
     BACKGROUND OF THE INVENTION 
     Solid electrolytic capacitors (e.g., tantalum capacitors) are typically made by pressing a metal powder (e.g., tantalum) around a metal lead wire, sintering the pressed part, anodizing the sintered anode, and thereafter applying a solid electrolyte. Intrinsically conductive polymers are often employed as the solid electrolyte due to their advantageous low equivalent series resistance (“ESR”) and “non-burning/non-ignition” failure mode. Such electrolytes can be formed through in situ chemical polymerization of the monomer in the presence of a catalyst and dopant. One of the problems with conventional capacitors that employ in situ polymerized polymers is that they tend to fail at high voltages, such as experienced during a fast switch on or operational current spike. In an attempt to overcome some of these issues, premade conductive polymer slurries have also been employed in certain applications as an alternative solid electrolyte material. While some benefits have been achieved with these capacitors in high voltage environments, problems nevertheless remain. For instance, one problem with polymer slurry-based capacitors is that they can achieve only a relatively low percentage of their wet capacitance, which means that they have a relatively large capacitance loss and/or fluctuation in the presence of atmosphere humidity. 
     As such, a need currently exists for a solid electrolytic capacitor having improved performance in high voltage environments. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the present invention, a solid electrolytic capacitor is disclosed that comprises a sintered porous anode having an external surface and an interior surface. A dielectric layer overlies the anode, wherein a first portion of the dielectric layer overlies the external surface of the anode and a second portion of the dielectric layer overlies the interior surface of the anode. The first portion has a greater thickness than the second portion. A solid electrolyte overlies the dielectric layer, wherein the solid electrolyte comprises a plurality of pre-polymerized conductive polymer particles and a hydroxy-functional nonionic polymer. 
     In accordance with another embodiment, a method for forming a solid electrolytic capacitor is disclosed. The method comprises forming a dielectric layer on a sintered porous anode by a multi-stage process that includes a first stage in which the anode is anodically oxidized at a first forming voltage and a second stage in which the anode is anodically oxidized at a second forming voltage that is greater than the first forming voltage. A solid electrolyte is formed over the dielectric layer by a process that includes coating the dielectric layer with a dispersion that contains a plurality of pre-polymerized conductive polymer particles, and thereafter, applying a hydroxy-functional nonionic polymer. 
     Other features and aspects of the present invention are set forth in greater detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A full and enabling disclosure of the present invention, including the best mode thereof, directed to one of ordinary skill in the art, is set forth more particularly in the remainder of the specification, which makes reference to the appended figures in which: 
         FIG. 1  is a schematic illustration of one embodiment of a capacitor that may be formed in accordance with the present invention; and 
         FIG. 2  is a schematic illustration of a dielectric layer with a differential thickness in accordance with one embodiment of the present invention. 
     
    
    
     Repeat use of references characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention. 
     DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS 
     It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only, and is not intended as limiting the broader aspects of the present invention, which broader aspects are embodied in the exemplary construction. 
     Generally speaking, the present invention is directed to a capacitor for use in relatively high voltage environments. The present inventors have discovered that the ability to achieve such voltages can be achieved through a unique and controlled combination of features relating to the formation of the anode, dielectric, and solid electrolyte. For example, anodization may be carried out in a manner so that the dielectric layer possesses a relatively thick portion that overlies an external surface of the anode and a relatively thin portion that overlies an interior surface of the anode. Without intending to be limited by theory, it is believed that such a thickness distribution can help protect the anode at those external locations where oxide flaws are more vulnerable to current surges experienced at high voltages. In addition to employing a dielectric layer with a differential thickness, the solid electrolyte is also formed from the combination of pre-polymerized conductive polymer particles and a hydroxy-functional nonionic polymer. The pre-polymerized particles can minimize the use of high energy radicals (e.g., Fe 2+  or Fe 3+  ions) that can otherwise lead to dielectric degradation, particularly at the high voltages noted above. Further, it is believed that hydroxy-functional nonionic polymers can improve the degree of contact between the polymer particles and the surface of the internal dielectric, which is typically relatively smooth in nature as a result of higher forming voltages. This unexpectedly increases the breakdown voltage and wet-to-dry capacitance of the resulting capacitor. 
     Thus, as a result of the present invention, the resulting capacitor is able to be employed in high voltage applications. The capacitor may, for example, exhibit a relatively high “breakdown voltage” (voltage at which the capacitor fails), such as about 60 volts or more, in some embodiments about 70 volts or more, and in some embodiments about 80 volts or more. Likewise, the capacitor may also be able to withstand relatively high surge currents, which is also common in high voltage applications. The peak surge current may be, for example, about 100 Amps or more, in some embodiments about 200 Amps or more, and in some embodiments, and in some embodiments, from about 300 Amps to about 800 Amps. The capacitor can also exhibit a relatively high percentage of its wet capacitance, which enables it to have only a small capacitance loss and/or fluctuation in the presence of atmosphere humidity. This performance characteristic is quantified by the “wet-to-dry capacitance percentage”, which is determined by the equation:
 
Wet-to-Dry Capacitance=(Dry Capacitance/Wet Capacitance)×100
 
     The capacitor of the present invention, for instance, may exhibit a wet-to-dry capacitance percentage of about 50% or more, in some embodiments about 60% or more, in some embodiments about 70% or more, and in some embodiments, from about 80% to 100%. 
     Various embodiments of the present invention will now be described in more detail. 
     I. Anode 
     The anode is formed from a valve metal composition. The specific charge of the composition may vary, such as from about 2,000 μF*V/g to about 150,000 μF*V/g, in some embodiments from about 3,000 μF*V/g to about 70,000 μF*V/g or more, and in some embodiments, from about 4,000 to about 50,000 μF*V/g. As is known in the art, the specific charge may be determined by multiplying capacitance by the anodizing voltage employed, and then dividing this product by the weight of the anodized electrode body. 
     The valve metal composition generally contains a valve metal (i.e., metal that is capable of oxidation) or valve metal-based compound, such as tantalum, niobium, aluminum, hafnium, titanium, alloys thereof, oxides thereof, nitrides thereof, and so forth. For example, the valve metal composition may contain an electrically conductive oxide of niobium, such as niobium oxide having an atomic ratio of niobium to oxygen of 1:1.0±1.0, in some embodiments 1:1.0±0.3, in some embodiments 1:1.0±0.1, and in some embodiments, 1:1.0±0.05. The niobium oxide may be NbO 0.7 , NbO 1.0 , NbO 1.1 , and NbO 2 . Examples of such valve metal oxides are described in U.S. Pat. No. 6,322,912 to Fife; U.S. Pat. No. 6,391,275 to Fife et al.; U.S. Pat. No. 6,416,730 to Fife et al.; U.S. Pat. No. 6,527,937 to Fife; U.S. Pat. No. 6,576,099 to Kimmel, et al.; U.S. Pat. No. 6,592,740 to Fife, et al.; and U.S. Pat. No. 6,639,787 to Kimmel, et al.; and U.S. Pat. No. 7,220,397 to Kimmel, et al., as well as U.S. Patent Application Publication Nos. 2005/0019581 to Schnitter; 2005/0103638 to Schnitter, et al.; 2005/0013765 to Thomas, et al. 
     To form the anode, a powder of the valve metal composition is generally employed. The powder may contain particles any of a variety of shapes, such as nodular, angular, flake, etc., as well as mixtures thereof. In particular embodiments, the particles can have a flake-like morphology in that they possess a relatively flat or platelet shape. Such particles can provide a short transmission line between the outer surface and interior of the anode and also provide a highly continuous and dense wire-to-anode connection with high conductivity. Among other things, this may help increase the breakdown voltage (voltage at which the capacitor fails) and help lower equivalent series resistance (“ESR”). The particles may also increase the specific charge of the anode when anodized at higher voltages, thereby increasing energy density. 
     When employed, the flake particles are generally flat. The degree of flatness is generally defined by the “aspect ratio”, i.e., the average diameter or width of the particles divided by the average thickness (“Da”). For example, the aspect ratio of the particles may be from about 2 to about 100, in some embodiments from about 3 to about 50, in some embodiments, from about 4 to about 30. The particles may also have a specific surface area of from about 0.5 to about 10.0 m 2 /g, in some embodiments from about 0.7 to about 5.0 m 2 /g, and in some embodiments, from about 1.0 to about 4.0 m 2 /g. The term “specific surface area” generally refers to surface area as determined by the physical gas adsorption (B.E.T.) method of Bruanauer, Emmet, and Teller, Journal of American Chemical Society, Vol. 60, 1938, p. 309, with nitrogen as the adsorption gas. The test may be conducted with a MONOSORB® Specific Surface Area Analyzer available from QUANTACHROME Corporation, Syosset, N.Y., which measures the quantity of adsorbate nitrogen gas adsorbed on a solid surface by sensing the change in thermal conductivity of a flowing mixture of adsorbate and inert carrier gas (e.g., helium). 
     The bulk density (also known as Scott density) is also typically from about 0.1 to about 2 grams per cubic centimeter (g/cm 3 ), in some embodiments from about 0.2 g/cm 3  to about 1.5 g/cm 3 , and in some embodiments, from about 0.4 g/cm 3  to about 1 g/cm 3 . “Bulk density” may be determined using a flow meter funnel and density cup. More specifically, the powder sample may be poured through the funnel into the cup until the sample completely fills and overflows the periphery of the cup, and thereafter sample may be leveled-off by a spatula, without jarring, so that it is flush with the top of the cup. The leveled sample is transferred to a balance and weighed to the nearest 0.1 gram to determine the density value. Such an apparatus is commercially available from Alcan Aluminum Corp. of Elizabeth, N.J. The particles may also have an average size (e.g., width) of from about 0.1 to about 100 micrometers, in some embodiments from about 0.5 to about 70 micrometers, and in some embodiments, from about 1 to about 50 micrometers. 
     To facilitate the construction of the anode, certain additional components may also be included in the powder. For example, the powder may be optionally mixed with a binder and/or lubricant to ensure that the particles adequately adhere to each other when pressed to form the anode body. Suitable binders may include, for instance, poly(vinyl butyral); poly(vinyl acetate); poly(vinyl alcohol); poly(vinyl pyrrolidone); cellulosic polymers, such as carboxymethylcellulose, methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, and methylhydroxyethyl cellulose; atactic polypropylene, polyethylene; polyethylene glycol (e.g., Carbowax from Dow Chemical Co.); polystyrene, poly(butadiene/styrene); polyamides, polyimides, and polyacrylamides, high molecular weight polyethers; copolymers of ethylene oxide and propylene oxide; fluoropolymers, such as polytetrafluoroethylene, polyvinylidene fluoride, and fluoro-olefin copolymers; acrylic polymers, such as sodium polyacrylate, poly(lower alkyl acrylates), poly(lower alkyl methacrylates) and copolymers of lower alkyl acrylates and methacrylates; and fatty acids and waxes, such as stearic and other soapy fatty acids, vegetable wax, microwaxes (purified paraffins), etc. The binder may be dissolved and dispersed in a solvent. Exemplary solvents may include water, alcohols, and so forth. When utilized, the percentage of binders and/or lubricants may vary from about 0.1% to about 8% by weight of the total mass. It should be understood, however, that binders and/or lubricants are not necessarily required in the present invention. 
     The resulting powder may then be compacted to form a pellet using any conventional powder press device. For example, a press mold may be employed that is a single station compaction press containing a die and one or multiple punches. Alternatively, anvil-type compaction press molds may be used that use only a die and single lower punch. Single station compaction press molds are available in several basic types, such as cam, toggle/knuckle and eccentric/crank presses with varying capabilities, such as single action, double action, floating die, movable platen, opposed ram, screw, impact, hot pressing, coining or sizing. The powder may be compacted around an anode lead wire. The wire may be formed from any electrically conductive material, such as tantalum, niobium, aluminum, hafnium, titanium, etc., as well as electrically conductive oxides and/or nitrides of thereof. 
     After compaction, the resulting anode body may then be diced into any desired shape, such as square, rectangle, circle, oval, triangle, hexagon, octagon, heptagon, pentagon, etc. The anode body may also have a “fluted” shape in that it contains one or more furrows, grooves, depressions, or indentations to increase the surface to volume ratio to minimize ESR and extend the frequency response of the capacitance. The anode body may then be subjected to a heating step in which most, if not all, of any binder/lubricant are removed. For example, the anode body is typically heated by an oven that operates at a temperature of from about 150° C. to about 500° C. Alternatively, the binder/lubricant may also be removed by contacting the pellet with an aqueous solution, such as described in U.S. Pat. No. 6,197,252 to Bishop, et al. 
     Thereafter, the pellet is sintered to form a porous, integral mass. The temperature, atmosphere, and time of the sintering may depend on a variety of factors, such as the type of anode, the size of the anode, etc. Typically, sintering occurs at a temperature of from about from about 800° C. to about 1900° C., in some embodiments from about 1000° C. to about 1500° C., and in some embodiments, from about 1100° C. to about 1400° C., for a time of from about 5 minutes to about 100 minutes, and in some embodiments, from about 30 minutes to about 60 minutes. If desired, sintering may occur in an atmosphere that limits the transfer of oxygen atoms to the anode. For example, sintering may occur in a reducing atmosphere, such as in a vacuum, inert gas, hydrogen, etc. The reducing atmosphere may be at a pressure of from about 10 Torr to about 2000 Torr, in some embodiments from about 100 Torr to about 1000 Torr, and in some embodiments, from about 100 Torr to about 930 Torr. Mixtures of hydrogen and other gases (e.g., argon or nitrogen) may also be employed. 
     The anode may also have a relatively low carbon and oxygen content. For example, the anode may have no more than about 50 ppm carbon, and in some embodiments, no more than about 10 ppm carbon. Likewise, the anode may have no more than about 3500 ppm oxygen, in some embodiments no more than about 3000 ppm oxygen, and in some embodiments, from about 500 to about 2500 ppm oxygen. Oxygen content may be measured by LECO Oxygen Analyzer and includes oxygen in natural oxide on the tantalum surface and bulk oxygen in the tantalum particles. Bulk oxygen content is controlled by period of crystalline lattice of tantalum, which is increasing linearly with increasing oxygen content in tantalum until the solubility limit is achieved. This method was described in “Critical Oxygen Content In Porous Anodes Of Solid Tantalum Capacitors”, Pozdeev-Freeman et al., Journal of Materials Science: Materials In Electronics 9, (1998) 309-311 wherein X-ray diffraction analysis (XRDA) was employed to measure period of crystalline lattice of tantalum. Oxygen in sintered tantalum anodes may be limited to thin natural surface oxide, while the bulk of tantalum is practically free of oxygen. 
     As noted above, an anode lead may also be connected to the anode body that extends in a longitudinal direction therefrom. The anode lead may be in the form of a wire, sheet, etc., and may be formed from a valve metal compound, such as tantalum, niobium, niobium oxide, etc. Connection of the lead may be accomplished using known techniques, such as by welding the lead to the body or embedding it within the anode body during formation (e.g., prior to compaction and/or sintering). 
     II. Dielectric 
     The dielectric layer of the capacitor possesses a differential thickness throughout the anode in that it possesses a first portion that overlies an external surface of the anode and a second portion that overlies an interior surface of the anode. As indicated above, the first portion is selectively formed so that its thickness is greater than that of the second portion. Without intending to be limited by theory, it is believed that such a thickness distribution can help protect the anode at those external locations where oxide flaws are more vulnerable to current surges experienced at high voltages. Referring to  FIG. 2 , for instance, a schematic illustration of a section of an anodicially oxidized, porous anode body  200  is shown. The oxidized body  200  contains a dielectric layer  210  that overlies a sintered anode  202  with pores  209 . The anode  202  has an outer region  205  defined by an external surface  209  and an inner region  207  defined by an interior surface  211 . A dielectric layer  210  overlies both the outer region  205  and the inner region  207  of the anode  200 , with a first portion  220  being located adjacent to the external surface  209  and defining a first thickness “t 1 ”, and a second portion  230  being located adjacent to the interior surface  211  and defining a second thickness “t 2 .” As indicated, the first thickness “t 1 ” is greater than the second thickness “t 2 .” It should be understood, however, that the thickness of the dielectric layer need not be uniform within the inner or outer regions. Certain portions of the dielectric layer adjacent to the external surface may, for example, actually be thinner than certain portions of the layer at the interior surface, and vice versa. Nevertheless, the dielectric layer is formed such that at least a portion of the layer at the external surface has a greater thickness than at least a portion at the interior surface. Although the exact difference in these thicknesses may vary depending on the particular application, the ratio of the thickness of the first portion to the thickness of the second portion is typically from about 1.2 to about 40, in some embodiments from about 1.5 to about 25, and in some embodiments, from about 2 to about 20. 
     To form a dielectric layer having a differential thickness, a multi-stage process is generally employed. In each stage of the process, the sintered anode is anodically oxidized (“anodized”) to form a dielectric layer (e.g., tantalum pentoxide). Anodization is performed by initially applying an electrolyte to the anode, such as by dipping the anode into an electrolyte. The electrolyte is generally in the form of a liquid, such as a solution (e.g., aqueous or non-aqueous), dispersion, melt, etc. A solvent is generally employed in the electrolyte, such as water (e.g., deionized water); ethers (e.g., diethyl ether and tetrahydrofuran); alcohols (e.g., methanol, ethanol, n-propanol, isopropanol, and butanol); triglycerides; ketones (e.g., acetone, methyl ethyl ketone, and methyl isobutyl ketone); esters (e.g., ethyl acetate, butyl acetate, diethylene glycol ether acetate, and methoxypropyl acetate); amides (e.g., dimethylformamide, dimethylacetamide, dimethylcaprylic/capric fatty acid amide and N-alkylpyrrolidones); nitriles (e.g., acetonitrile, propionitrile, butyronitrile and benzonitrile); sulfoxides or sulfones (e.g., dimethyl sulfoxide (DMSO) and sulfolane); and so forth. The solvent may constitute from about 50 wt. % to about 99.9 wt. %, in some embodiments from about 75 wt. % to about 99 wt. %, and in some embodiments, from about 80 wt. % to about 95 wt. % of the electrolyte. Although not necessarily required, the use of an aqueous solvent (e.g., water) is often desired to facilitate formation of an oxide. In fact, water may constitute about 1 wt. % or more, in some embodiments about 10 wt. % or more, in some embodiments about 50 wt. % or more, in some embodiments about 70 wt. % or more, and in some embodiments, about 90 wt. % to 100 wt. % of the solvent(s) used in the electrolyte. 
     A current is passed through the electrolyte to form the dielectric, and the value of voltage manages the charge (current multiplied by time) and thereby the thickness of the dielectric layer. During the first stage of anodization, a relatively small forming voltage is typically employed to ensure that the desired dielectric thickness is achieved for the interior surface, such as forming voltages ranging from about 1 to about 90 volts, in some embodiments from about 2 to about 50 volts, and in some embodiments, from about 5 to about 20 volts. Thereafter, the sintered body may then be anodically oxidized in a second stage of the process to increase the thickness of the dielectric to the desired level. This is generally accomplished by anodizing in an electrolyte at a higher voltage than employed during the first stage, such as at forming voltages ranging from about 50 to about 350 volts, in some embodiments from about 60 to about 300 volts, and in some embodiments, from about 90 to about 200 volts. During the first and/or second stages, the electrolyte may be kept at a temperature within the range of from about 15° C. to about 95° C., in some embodiments from about 20° C. to about 90° C., and in some embodiments, from about 25° C. to about 85° C. 
     The electrolytes employed during the first and second stages of the anodization process may be the same or different. Typically, however, it is desired to employ different solutions to help better facilitate the attainment of a higher thickness at the outer portions of the dielectric layer. For example, it may be desired that the electrolyte employed in the second stage has a lower ionic conductivity than the electrolyte employed in the first stage to prevent a significant amount of oxide film from forming on the internal surface of anode. In this regard, the electrolyte employed during the first stage may contain an acidic compound, such as hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, polyphosphoric acid, boric acid, boronic acid, etc. Such an electrolyte may have an electrical conductivity of from about 0.1 to about 100 mS/cm, in some embodiments from about 0.2 to about 20 mS/cm, and in some embodiments, from about 1 to about 10 mS/cm, determined at a temperature of 25° C. The electrolyte employed during the second stage typically contains a salt of a weak acid so that the hydronium ion concentration increases in the pores as a result of charge passage therein. Ion transport or diffusion is such that the weak acid anion moves into the pores as necessary to balance the electrical charges. As a result, the concentration of the principal conducting species (hydronium ion) is reduced in the establishment of equilibrium between the hydronium ion, acid anion, and undissociated acid, thus forms a poorer-conducting species. The reduction in the concentration of the conducting species results in a relatively high voltage drop in the electrolyte, which hinders further anodization in the interior while a thicker oxide layer is being built up on the outside to a higher formation voltage in the region of continued high conductivity. Suitable weak acid salts may include, for instance, ammonium or alkali metal salts (e.g., sodium, potassium, etc.) of boric acid, boronic acid, acetic acid, oxalic acid, lactic acid, adipic acid, etc. Particularly suitable salts include sodium tetraborate and ammonium pentaborate. Such electrolytes typically have an electrical conductivity of from about 0.1 to about 20 mS/cm, in some embodiments from about 0.5 to about 10 mS/cm, and in some embodiments, from about 1 to about 5 mS/cm, determined at a temperature of 25° C. 
     If desired, each stage of anodization may be repeated for one or more cycles to achieve the desired dielectric thickness. Furthermore, the anode may also be rinsed or washed with another solvent (e.g., water) after the first and/or second stages to remove the electrolyte. 
     III. Solid Electrolyte 
     A solid electrolyte overlies the dielectric that generally functions as the cathode for the capacitor. The solid electrolyte contains a conductive polymer, which is typically π-conjugated and has electrical conductivity after oxidation or reduction, such as an electrical conductivity of at least about 1 μS/cm. Examples of such π-conjugated conductive polymers include, for instance, polyheterocycles (e.g., polypyrroles, polythiophenes, polyanilines, etc.), polyacetylenes, poly-p-phenylenes, polyphenolates, and so forth. In one embodiment, for example, the polymer is a substituted polythiophene, such as those having the following general structure: 
     
       
         
         
             
             
         
       
     
     wherein, 
     T is O or S; 
     D is an optionally substituted C 1  to C 5  alkylene radical (e.g., methylene, ethylene, n-propylene, n-butylene, n-pentylene, etc.); 
     R 7  is a linear or branched, optionally substituted C 1  to C 18  alkyl radical (e.g., methyl, ethyl, n- or iso-propyl, n-, iso-, sec- or tert-butyl, n-pentyl, 1-methylbutyl, 2-methylbutyl, 3-methylbutyl, 1-ethylpropyl, 1,1-dimethylpropyl, 1,2-dimethylpropyl, 2,2-dimethylpropyl, n-hexyl, n-heptyl, n-octyl, 2-ethylhexyl, n-nonyl, n-decyl, n-undecyl, n-dodecyl, n-tridecyl, n-tetradecyl, n-hexadecyl, n-octadecyl, etc.); optionally substituted C 5  to C 12  cycloalkyl radical (e.g., cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl cyclodecyl, etc.); optionally substituted C 6  to C 14  aryl radical (e.g., phenyl, naphthyl, etc.); optionally substituted C 7  to C 18  aralkyl radical (e.g., benzyl, o-, m-, p-tolyl, 2,3-, 2,4-, 2,5-, 2-6, 3-4-, 3,5-xylyl, mesityl, etc.); optionally substituted C 1  to C 4  hydroxyalkyl radical, or hydroxyl radical; and 
     q is an integer from 0 to 8, in some embodiments, from 0 to 2, and in one embodiment, 0; and 
     n is from 2 to 5,000, in some embodiments from 4 to 2,000, and in some embodiments, from 5 to 1,000. Example of substituents for the radicals “D” or “R 7 ” include, for instance, alkyl, cycloalkyl, aryl, aralkyl, alkoxy, halogen, ether, thioether, disulphide, sulfoxide, sulfone, sulfonate, amino, aldehyde, keto, carboxylic acid ester, carboxylic acid, carbonate, carboxylate, cyano, alkylsilane and alkoxysilane groups, carboxylamide groups, and so forth. 
     Particularly suitable thiophene polymers are those in which “D” is an optionally substituted C 2  to C 3  alkylene radical. For instance, the polymer may be optionally substituted poly(3,4-ethylenedioxythiophene), which has the following general structure: 
     
       
         
         
             
             
         
       
     
     Methods for forming conductive polymers, such as described above, are well known in the art. For instance, U.S. Pat. No. 6,987,663 to Merker, et al., which is incorporated herein in its entirety by reference thereto for all purposes, describes various techniques for forming substituted polythiophenes from a monomeric precursor. The monomeric precursor may, for instance, have the following structure: 
                         
wherein,
 
     T, D, R 7 , and q are defined above. Particularly suitable thiophene monomers are those in which “D” is an optionally substituted C 2  to C 3  alkylene radical. For instance, optionally substituted 3,4-alkylenedioxythiophenes may be employed that have the general structure: 
     
       
         
         
             
             
         
       
     
     wherein, R 7  and q are as defined above. In one particular embodiment, “q” is 0. One commercially suitable example of 3,4-ethylenedioxthiophene is available from H. C. Starck GmbH under the designation Clevios™ M. Other suitable monomers are also described in U.S. Pat. No. 5,111,327 to Blohm, et al. and U.S. Pat. No. 6,635,729 to Groenendaal, et al., which are incorporated herein in their entirety by reference thereto for all purposes. Derivatives of these monomers may also be employed that are, for example, dimers or trimers of the above monomers. Higher molecular derivatives, i.e., tetramers, pentamers, etc. of the monomers are suitable for use in the present invention. The derivatives may be made up of identical or different monomer units and used in pure form and in a mixture with one another and/or with the monomers. Oxidized or reduced forms of these precursors may also be employed. 
     A. Pre-Polymerized Particles 
     Regardless of the particular type of polymer, the solid electrolyte generally includes a plurality of pre-polymerized particles applied in the form of a dispersion. One benefit of employing a dispersion is that it may minimize the presence of ionic species (e.g., Fe 2+  or Fe 3+ ) produced during in situ polymerization, which can cause dielectric breakdown under high electric field due to ionic migration. Thus, by applying the conductive polymer as a dispersion rather through in situ polymerization, the resulting capacitor may exhibit a relatively high “breakdown voltage.” To enable good impregnation of the anode, the particles employed in the dispersion typically have a small size, such as an average size (e.g., diameter) of from about 1 to about 150 nanometers, in some embodiments from about 2 to about 50 nanometers, and in some embodiments, from about 5 to about 40 nanometers. The diameter of the particles may be determined using known techniques, such as by ultracentrifuge, laser diffraction, etc. The shape of the particles may likewise vary. In one particular embodiment, for instance, the particles are spherical in shape. However, it should be understood that other shapes are also contemplated by the present invention, such as plates, rods, discs, bars, tubes, irregular shapes, etc. The concentration of the particles in the dispersion may vary depending on the desired viscosity of the dispersion and the particular manner in which the dispersion is to be applied to the capacitor. Typically, however, the particles constitute from about 0.1 to about 10 wt. %, in some embodiments from about 0.4 to about 5 wt. %, and in some embodiments, from about 0.5 to about 4 wt. % of the dispersion. 
     The dispersion also generally contains a counterion that enhances the stability of the particles. That is, the conductive polymer (e.g., polythiophene or derivative thereof) typically has a charge on the main polymer chain that is neutral or positive (cationic). Polythiophene derivatives, for instance, typically carry a positive charge in the main polymer chain. In some cases, the polymer may possess positive and negative charges in the structural unit, with the positive charge being located on the main chain and the negative charge optionally on the substituents of the radical “R”, such as sulfonate or carboxylate groups. The positive charges of the main chain may be partially or wholly saturated with the optionally present anionic groups on the radicals “R.” Viewed overall, the polythiophenes may, in these cases, be cationic, neutral or even anionic. Nevertheless, they are all regarded as cationic polythiophenes as the polythiophene main chain has a positive charge. 
     The counterion may be a monomeric or polymeric anion that counteracts the charge of the conductive polymer. Polymeric anions can, for example, be anions of polymeric carboxylic acids (e.g., polyacrylic acids, polymethacrylic acid, polymaleic acids, etc.); polymeric sulfonic acids (e.g., polystyrene sulfonic acids (“PSS”), polyvinyl sulfonic acids, etc.); and so forth. The acids may also be copolymers, such as copolymers of vinyl carboxylic and vinyl sulfonic acids with other polymerizable monomers, such as acrylic acid esters and styrene. Likewise, suitable monomeric anions include, for example, anions of C 1  to C 20  alkane sulfonic acids (e.g., dodecane sulfonic acid); aliphatic perfluorosulfonic acids (e.g., trifluoromethane sulfonic acid, perfluorobutane sulfonic acid or perfluorooctane sulfonic acid); aliphatic C 1  to C 20  carboxylic acids (e.g., 2-ethyl-hexylcarboxylic acid); aliphatic perfluorocarboxylic acids (e.g., trifluoroacetic acid or perfluorooctanoic acid); aromatic sulfonic acids optionally substituted by C 1  to C 20  alkyl groups (e.g., benzene sulfonic acid, o-toluene sulfonic acid, p-toluene sulfonic acid or dodecylbenzene sulfonic acid); cycloalkane sulfonic acids (e.g., camphor sulfonic acid or tetrafluoroborates, hexafluorophosphates, perchlorates, hexafluoroantimonates, hexafluoroarsenates or hexachloroantimonates); and so forth. Particularly suitable counteranions are polymeric anions, such as a polymeric carboxylic or sulfonic acid (e.g., polystyrene sulfonic acid (“PSS”)). The molecular weight of such polymeric anions typically ranges from about 1,000 to about 2,000,000, and in some embodiments, from about 2,000 to about 500,000. 
     When employed, the weight ratio of such counterions to conductive polymers in the dispersion and in the resulting layer is typically from about 0.5:1 to about 50:1, in some embodiments from about 1:1 to about 30:1, and in some embodiments, from about 2:1 to about 20:1. The weight of the electrically conductive polymers corresponds referred to the above-referenced weight ratios refers to the weighed-in portion of the monomers used, assuming that a complete conversion occurs during polymerization. 
     In addition to conductive polymer(s) and counterion(s), the dispersion may also contain one or more binders to further enhance the adhesive nature of the polymeric layer and also increase the stability of the particles within the dispersion. The binders may be organic in nature, such as polyvinyl alcohols, polyvinyl pyrrolidones, polyvinyl chlorides, polyvinyl acetates, polyvinyl butyrates, polyacrylic acid esters, polyacrylic acid amides, polymethacrylic acid esters, polymethacrylic acid amides, polyacrylonitriles, styrene/acrylic acid ester, vinyl acetate/acrylic acid ester and ethylene/vinyl acetate copolymers, polybutadienes, polyisoprenes, polystyrenes, polyethers, polyesters, polycarbonates, polyurethanes, polyamides, polyimides, polysulfones, melamine formaldehyde resins, epoxide resins, silicone resins or celluloses. Crosslinking agents may also be employed to enhance the adhesion capacity of the binders. Such crosslinking agents may include, for instance, melamine compounds, masked isocyanates or functional silanes, such as 3-glycidoxypropyltrialkoxysilane, tetraethoxysilane and tetraethoxysilane hydrolysate or crosslinkable polymers, such as polyurethanes, polyacrylates or polyolefins, and subsequent crosslinking. 
     Dispersion agents may also be employed to facilitate the formation of the solid electrolyte and the ability to apply it to the anode part. Suitable dispersion agents include solvents, such as aliphatic alcohols (e.g., methanol, ethanol, i-propanol and butanol), aliphatic ketones (e.g., acetone and methyl ethyl ketones), aliphatic carboxylic acid esters (e.g., ethyl acetate and butyl acetate), aromatic hydrocarbons (e.g., toluene and xylene), aliphatic hydrocarbons (e.g., hexane, heptane and cyclohexane), chlorinated hydrocarbons (e.g., dichloromethane and dichloroethane), aliphatic nitriles (e.g., acetonitrile), aliphatic sulfoxides and sulfones (e.g., dimethyl sulfoxide and sulfolane), aliphatic carboxylic acid amides (e.g., methylacetamide, dimethylacetamide and dimethylformamide), aliphatic and araliphatic ethers (e.g., diethylether and anisole), water, and mixtures of any of the foregoing solvents. A particularly suitable dispersion agent is water. 
     In addition to those mentioned above, still other ingredients may also be used in the dispersion. For example, conventional fillers may be used that have a size of from about 10 nanometers to about 100 micrometers, in some embodiments from about 50 nanometers to about 50 micrometers, and in some embodiments, from about 100 nanometers to about 30 micrometers. Examples of such fillers include calcium carbonate, silicates, silica, calcium or barium sulfate, aluminum hydroxide, glass fibers or bulbs, wood flour, cellulose powder carbon black, electrically conductive polymers, etc. The fillers may be introduced into the dispersion in powder form, but may also be present in another form, such as fibers. 
     Surface-active substances may also be employed in the dispersion, such as ionic or non-ionic surfactants. Furthermore, adhesives may be employed, such as organofunctional silanes or their hydrolysates, for example 3-glycidoxypropyltrialkoxysilane, 3-aminopropyl-triethoxysilane, 3-mercaptopropyl-trimethoxysilane, 3-metacryloxypropyltrimethoxysilane, vinyltrimethoxysilane or octyltriethoxysilane. The dispersion may also contain additives that increase conductivity, such as ether group-containing compounds (e.g., tetrahydrofuran), lactone group-containing compounds (e.g., γ-butyrolactone or γ-valerolactone), amide or lactam group-containing compounds (e.g., caprolactam, N-methylcaprolactam, N,N-dimethylacetamide, N-methylacetamide, N,N-dimethylformamide (DMF), N-methylformamide, N-methylformanilide, N-methylpyrrolidone (NMP), N-octylpyrrolidone, or pyrrolidone), sulfones and sulfoxides (e.g., sulfolane (tetramethylenesulfone) or dimethylsulfoxide (DMSO)), sugar or sugar derivatives (e.g., saccharose, glucose, fructose, or lactose), sugar alcohols (e.g., sorbitol or mannitol), furan derivatives (e.g., 2-furancarboxylic acid or 3-furancarboxylic acid), an alcohols (e.g., ethylene glycol, glycerol, di- or triethylene glycol). 
     The polymeric dispersion may be applied using a variety of known techniques, such as by spin coating, impregnation, pouring, dropwise application, injection, spraying, doctor blading, brushing, printing (e.g., ink-jet, screen, or pad printing), or dipping. Although it may vary depending on the application technique employed, the viscosity of the dispersion is typically from about 0.1 to about 100,000 mPas (measured at a shear rate of 100 s −1 ), in some embodiments from about 1 to about 10,000 mPas, in some embodiments from about 10 to about 1,600 mPas, and in some embodiments, from about 100 to about 1000 mPas. Once applied, the layer may be dried and/or washed. One or more additional layers may also be formed in this manner to achieve the desired thickness. Typically, the total thickness of the layer(s) formed by this particle dispersion is from about 1 to about 50 μm, and in some embodiments, from about 5 to about 20 μm. The weight ratio of counterions to conductive polymers is likewise from about 0.5:1 to about 50:1, in some embodiments from about 1:1 to about 30:1, and in some embodiments, from about 2:1 to about 20:1. 
     B. Hydroxy-Functional Nonionic Polymer 
     In addition to pre-polymerized particles, the solid electrolyte also contains a hydroxy-functional nonionic polymer. The term “hydroxy-functional” generally means that the compound contains at least one hydroxyl functional group or is capable of possessing such a functional group in the presence of a solvent. Without intending to be limited by theory, it is believed that hydroxy-functional nonionic polymers can improve the degree of contact between the polymer particles and the surface of the internal dielectric, which is typically relatively smooth in nature as a result of higher forming voltages. This unexpectedly increases the breakdown voltage and wet-to-dry capacitance of the resulting capacitor. Furthermore, it is believed that the use of a hydroxy-functional polymer with a certain molecular weight can also minimize the likelihood of chemical decomposition at high voltages. For instance, the molecular weight of the hydroxy-functional polymer may be from about 100 to 10,000 grams per mole, in some embodiments from about 200 to 2,000, in some embodiments from about 300 to about 1,200, and in some embodiments, from about 400 to about 800. 
     Any of a variety of hydroxy-functional nonionic polymers may generally be employed for this purpose. In one embodiment, for example, the hydroxy-functional polymer is a polyalkylene ether. Polyalkylene ethers may include polyalkylene glycols (e.g., polyethylene glycols, polypropylene glycols polytetramethylene glycols, polyepichlorohydrins, etc.), polyoxetanes, polyphenylene ethers, polyether ketones, and so forth. Polyalkylene ethers are typically predominantly linear, nonionic polymers with terminal hydroxy groups. Particularly suitable are polyethylene glycols, polypropylene glycols and polytetramethylene glycols (polytetrahydrofurans), which are produced by polyaddition of ethylene oxide, propylene oxide or tetrahydrofuran onto water. The polyalkylene ethers may be prepared by polycondensation reactions from diols or polyols. The diol component may be selected, in particular, from saturated or unsaturated, branched or unbranched, aliphatic dihydroxy compounds containing 5 to 36 carbon atoms or aromatic dihydroxy compounds, such as, for example, pentane-1,5-diol, hexane-1,6-diol, neopentyl glycol, bis-(hydroxymethyl)-cyclohexanes, bisphenol A, dimer diols, hydrogenated dimer diols or even mixtures of the diols mentioned. In addition, polyhydric alcohols may also be used in the polymerization reaction, including for example glycerol, di- and polyglycerol, trimethylolpropane, pentaerythritol or sorbitol. 
     In addition to those noted above, other hydroxy-functional nonionic polymers may also be employed in the present invention. Some examples of such polymers include, for instance, ethoxylated alkylphenols; ethoxylated or propoxylated C 6 -C 24  fatty alcohols; polyoxyethylene glycol alkyl ethers having the general formula: CH 3 —(CH 2 ) 10-16 —(O—C 2 H 4 ) 1-25 —OH (e.g., octaethylene glycol monododecyl ether and pentaethylene glycol monododecyl ether); polyoxypropylene glycol alkyl ethers having the general formula: CH 3 —(CH 2 ) 10-16 —(O—C 3 H 6 ) 1-25 —OH; polyoxyethylene glycol octylphenol ethers having the following general formula: C 8 —H 17 —(C 6 H 4 )—(O—C 2 H 4 ) 1-25 —OH (e.g., Triton™ X-100); polyoxyethylene glycol alkylphenol ethers having the following general formula: C 9 —H 19 —(C 6 H 4 )—(O—C 2 H 4 ) 1-25 —OH (e.g., nonoxynol-9); polyoxyethylene glycol esters of C 8 -C 24  fatty acids, such as polyoxyethylene glycol sorbitan alkyl esters (e.g., polyoxyethylene (20) sorbitan monolaurate, polyoxyethylene (20) sorbitan monopalmitate, polyoxyethylene (20) sorbitan monostearate, polyoxyethylene (20) sorbitan monooleate, PEG-20 methyl glucose distearate, PEG-20 methyl glucose sesquistearate, PEG-80 castor oil, and PEG-20 castor oil, PEG-3 castor oil, PEG 600 dioleate, and PEG 400 dioleate) and polyoxyethylene glycerol alkyl esters (e.g., polyoxyethylene-23 glycerol laurate and polyoxyethylene-20 glycerol stearate); polyoxyethylene glycol ethers of C 8 -C 24  fatty acids (e.g., polyoxyethylene-10 cetyl ether, polyoxyethylene-10 stearyl ether, polyoxyethylene-20 cetyl ether, polyoxyethylene-10 oleyl ether, polyoxyethylene-20 oleyl ether, polyoxyethylene-20 isohexadecyl ether, polyoxyethylene-15 tridecyl ether, and polyoxyethylene-6 tridecyl ether); block copolymers of polyethylene glycol and polypropylene glycol (e.g., Poloxamers); and so forth, as well as mixtures thereof. 
     The hydroxy-functional nonionic polymer may be incorporated into the solid electrolyte in a variety of different ways. In certain embodiments, for instance, the hydroxy-functional polymer may simply be incorporated into any layer(s) formed by the initial dispersion described above. In such embodiments, the concentration of the hydroxy-functional polymer in the dispersion is typically from about 1 wt. % to about 50 wt. %, in some embodiments from about 5 wt. % to about 40 wt. %, and in some embodiments, from about 10 wt. % to about 30 wt. %. 
     In other embodiments, however, the hydroxy-functional polymer may be applied after the initial polymer dispersion is applied to the anode body. In such embodiments, the technique used to apply the hydroxy-functional polymer may vary. For example, the polymer may be applied in the form of a liquid solution using various methods, such as immersion, dipping, pouring, dripping, injection, spraying, spreading, painting or printing, for example, inkjet, screen printing or tampon printing. Solvents known to the person skilled in the art can be employed in the solution, such as water, alcohols, or a mixture thereof. The concentration of the hydroxy-functional polymer in such a solution typically ranges from about 5 wt. % to about 95 wt. %, in some embodiments from about 10 wt. % to about 70 wt. %, and in some embodiments, from about 15 wt. % to about 50 wt. % of the solution. If desired, such solutions may be generally free of conductive polymers. For example, conductive polymers may constitute about 2 wt. % or less, in some embodiments about 1 wt. % or less, and in some embodiments, about 0.5 wt. % or less of the solution. 
     Alternatively, however, it may also be desired to employ a conductive polymer in combination with the hydroxy-functional polymer. For example, in certain embodiments, a second polymer dispersion that contains conductive polymer particles and a hydroxy-functional polymer is applied to the anode after the initial layer(s) formed from the first polymer dispersion are applied to the anode body. The conductive particles of the second polymer dispersion are generally described above, although they need not be identical to those employed in the first polymer dispersion. For instance, the first and second polymer dispersion may employ particles of a different size and/or chemical composition. Regardless, the concentration of the hydroxy-functional polymer in the second polymer dispersion is typically from about 1 wt. % to about 50 wt. %, in some embodiments from about 5 wt. % to about 40 wt. %, and in some embodiments, from about 10 wt. % to about 30 wt. %. Likewise, in those embodiments in which the hydroxy-functional polymer is employed in a second dispersion, it may also be desirable that the first dispersion is generally free of such hydroxy-functional nonionic polymers. For example, hydroxy-functional polymers may constitute about 2 wt. % or less, in some embodiments about 1 wt. % or less, and in some embodiments, about 0.5 wt. % or less of the first polymer dispersion. Once applied, the layer formed by the second polymer dispersion may be dried and/or washed. One or more additional layers may also be formed in this manner to achieve the desired thickness. Typically, the total thickness of the layers formed by the second polymer dispersion is from about 0.1 to about 5 μm, in some embodiments from about 0.1 to about 3 μm, and in some embodiments, from about 0.2 to about 1 μm. 
     IV. External Polymer Coating 
     Although not required, an external polymer coating may also be applied to the anode body and overlie the solid electrolyte. The external polymer coating generally contains one or more layers formed from a dispersion of pre-polymerized conductive particles, such as described in more detail above. The external coating may be able to penetrate into the edge region of the capacitor body to increase the adhesion to the dielectric. This results in a more mechanically robust part, which may reduce equivalent series resistance and leakage current. Because it is generally intended to improve the degree of edge coverage rather to impregnate the interior of the anode, the particles used in the external coating typically have a larger size than those employed in the initial dispersion of the solid electrolyte. For example, the ratio of the average size of the particles employed in the external polymer coating to the average size of the particles employed in the initial dispersion of the solid electrolyte is typically from about 1.5 to about 30, in some embodiments from about 2 to about 20, and in some embodiments, from about 5 to about 15. For example, the particles employed in the dispersion of the external coating may have an average size of from about 50 to about 500 nanometers, in some embodiments from about 80 to about 250 nanometers, and in some embodiments, from about 100 to about 200 nanometers. 
     If desired, a crosslinking agent may also be employed in the external polymer coating to enhance the degree of adhesion to the solid electrolyte. Typically, the crosslinking agent is applied prior to application of the dispersion used in the external coating. Suitable crosslinking agents are described, for instance, in U.S. Patent Publication No. 2007/0064376 to Merker, et al. and include, for instance, amines (e.g., diamines, triamines, oligomer amines, polyamines, etc.); polyvalent metal cations, such as salts or compounds of Mg, Al, Ca, Fe, Cr, Mn, Ba, Ti, Co, Ni, Cu, Ru, Ce or Zn, phosphonium compounds, sulfonium compounds, etc. Particularly suitable examples include, for instance, 1,4-diaminocyclohexane, 1,4-bis(amino-methyl)cyclohexane, ethylenediamine, 1,6-hexanediamine, 1,7-heptanediamine, 1,8-octanediamine, 1,9-nonanediamine, 1,10-decanediamine, 1,12-dodecanediamine, N,N-dimethylethylenediamine, N,N,N′,N′-tetramethylethylenediamine, N,N,N′,N′-tetramethyl-1,4-butanediamine, etc., as well as mixtures thereof. 
     The crosslinking agent is typically applied from a solution or dispersion whose pH is from 1 to 10, in some embodiments from 2 to 7, in some embodiments, from 3 to 6, as determined at 25° C. Acidic compounds may be employed to help achieve the desired pH level. Examples of solvents or dispersants for the crosslinking agent include water or organic solvents, such as alcohols, ketones, carboxylic esters, etc. The crosslinking agent may be applied to the capacitor body by any known process, such as spin-coating, impregnation, casting, dropwise application, spray application, vapor deposition, sputtering, sublimation, knife-coating, painting or printing, for example inkjet, screen or pad printing. Once applied, the crosslinking agent may be dried prior to application of the polymer dispersion. This process may then be repeated until the desired thickness is achieved. For example, the total thickness of the entire external polymer coating, including the crosslinking agent and dispersion layers, may range from about 1 to about 50 μm, in some embodiments from about 2 to about 40 μm, and in some embodiments, from about 5 to about 20 μm. 
     V. Other Components of the Capacitor 
     If desired, the capacitor may also contain other layers as is known in the art. For example, a protective coating may optionally be formed between the dielectric and solid electrolyte, such as one made of a relatively insulative resinous material (natural or synthetic). Such materials may have a specific resistivity of greater than about 10 Ω·cm, in some embodiments greater than about 100, in some embodiments greater than about 1,000 Ω·cm, in some embodiments greater than about 1×10 5  Ω·cm, and in some embodiments, greater than about 1×10 10  Ω·cm. Some resinous materials that may be utilized in the present invention include, but are not limited to, polyurethane, polystyrene, esters of unsaturated or saturated fatty acids (e.g., glycerides), and so forth. For instance, suitable esters of fatty acids include, but are not limited to, esters of lauric acid, myristic acid, palmitic acid, stearic acid, eleostearic acid, oleic acid, linoleic acid, linolenic acid, aleuritic acid, shellolic acid, and so forth. These esters of fatty acids have been found particularly useful when used in relatively complex combinations to form a “drying oil”, which allows the resulting film to rapidly polymerize into a stable layer. Such drying oils may include mono-, di-, and/or tri-glycerides, which have a glycerol backbone with one, two, and three, respectively, fatty acyl residues that are esterified. For instance, some suitable drying oils that may be used include, but are not limited to, olive oil, linseed oil, castor oil, tung oil, soybean oil, and shellac. These and other protective coating materials are described in more detail U.S. Pat. No. 6,674,635 to Fife, et al., which is incorporated herein in its entirety by reference thereto for all purposes. 
     If desired, the part may also be applied with a carbon layer (e.g., graphite) and silver layer, respectively. The silver coating may, for instance, act as a solderable conductor, contact layer, and/or charge collector for the capacitor and the carbon coating may limit contact of the silver coating with the solid electrolyte. Such coatings may cover some or all of the solid electrolyte. 
     The capacitor may also be provided with terminations, particularly when employed in surface mounting applications. For example, the capacitor may contain an anode termination to which the anode lead of the capacitor element is electrically connected and a cathode termination to which the cathode of the capacitor element is electrically connected. Any conductive material may be employed to form the terminations, such as a conductive metal (e.g., copper, nickel, silver, nickel, zinc, tin, palladium, lead, copper, aluminum, molybdenum, titanium, iron, zirconium, magnesium, and alloys thereof). Particularly suitable conductive metals include, for instance, copper, copper alloys (e.g., copper-zirconium, copper-magnesium, copper-zinc, or copper-iron), nickel, and nickel alloys (e.g., nickel-iron). The thickness of the terminations is generally selected to minimize the thickness of the capacitor. For instance, the thickness of the terminations may range from about 0.05 to about 1 millimeter, in some embodiments from about 0.05 to about 0.5 millimeters, and from about 0.07 to about 0.2 millimeters. One exemplary conductive material is a copper-iron alloy metal plate available from Wieland (Germany). If desired, the surface of the terminations may be electroplated with nickel, silver, gold, tin, etc. as is known in the art to ensure that the final part is mountable to the circuit board. In one particular embodiment, both surfaces of the terminations are plated with nickel and silver flashes, respectively, while the mounting surface is also plated with a tin solder layer. 
     Referring to  FIG. 1 , one embodiment of an electrolytic capacitor  30  is shown that includes an anode termination  62  and a cathode termination  72  in electrical connection with a capacitor element  33 . The capacitor element  33  has an upper surface  37 , lower surface  39 , front surface  36 , and rear surface  38 . Although it may be in electrical contact with any of the surfaces of the capacitor element  33 , the cathode termination  72  in the illustrated embodiment is in electrical contact with the lower surface  39  and rear surface  38 . More specifically, the cathode termination  72  contains a first component  73  positioned substantially perpendicular to a second component  74 . The first component  73  is in electrical contact and generally parallel with the lower surface  39  of the capacitor element  33 . The second component  74  is in electrical contact and generally parallel to the rear surface  38  of the capacitor element  33 . Although depicted as being integral, it should be understood that these portions may alternatively be separate pieces that are connected together, either directly or via an additional conductive element (e.g., metal). 
     The anode termination  62  likewise contains a first component  63  positioned substantially perpendicular to a second component  64 . The first component  63  is in electrical contact and generally parallel with the lower surface  39  of the capacitor element  33 . The second component  64  contains a region  51  that carries an anode lead  16 . In the illustrated embodiment, the region  51  possesses a “U-shape” for further enhancing surface contact and mechanical stability of the lead  16 . 
     The terminations may be connected to the capacitor element using any technique known in the art. In one embodiment, for example, a lead frame may be provided that defines the cathode termination  72  and anode termination  62 . To attach the electrolytic capacitor element  33  to the lead frame, a conductive adhesive may initially be applied to a surface of the cathode termination  72 . The conductive adhesive may include, for instance, conductive metal particles contained with a resin composition. The metal particles may be silver, copper, gold, platinum, nickel, zinc, bismuth, etc. The resin composition may include a thermoset resin (e.g., epoxy resin), curing agent (e.g., acid anhydride), and coupling agent (e.g., silane coupling agents). Suitable conductive adhesives may be described in U.S. Patent Application Publication No. 2006/0038304 to Osako, et al., which is incorporated herein in its entirety by reference thereto for all purposes. Any of a variety of techniques may be used to apply the conductive adhesive to the cathode termination  72 . Printing techniques, for instance, may be employed due to their practical and cost-saving benefits. 
     A variety of methods may generally be employed to attach the terminations to the capacitor. In one embodiment, for example, the second component  64  of the anode termination  62  and the second component  74  of the cathode termination  72  are initially bent upward to the position shown in  FIG. 1 . Thereafter, the capacitor element  33  is positioned on the cathode termination  72  so that its lower surface  39  contacts the adhesive and the anode lead  16  is received by the upper U-shaped region  51 . If desired, an insulating material (not shown), such as a plastic pad or tape, may be positioned between the lower surface  39  of the capacitor element  33  and the first component  63  of the anode termination  62  to electrically isolate the anode and cathode terminations. 
     The anode lead  16  is then electrically connected to the region  51  using any technique known in the art, such as mechanical welding, laser welding, conductive adhesives, etc. For example, the anode lead  16  may be welded to the anode termination  62  using a laser. Lasers generally contain resonators that include a laser medium capable of releasing photons by stimulated emission and an energy source that excites the elements of the laser medium. One type of suitable laser is one in which the laser medium consist of an aluminum and yttrium garnet (YAG), doped with neodymium (Nd). The excited particles are neodymium ions Nd 3+ . The energy source may provide continuous energy to the laser medium to emit a continuous laser beam or energy discharges to emit a pulsed laser beam. Upon electrically connecting the anode lead  16  to the anode termination  62 , the conductive adhesive may then be cured. For example, a heat press may be used to apply heat and pressure to ensure that the electrolytic capacitor element  33  is adequately adhered to the cathode termination  72  by the adhesive. 
     Once the capacitor element is attached, the lead frame is enclosed within a resin casing, which may then be filled with silica or any other known encapsulating material. The width and length of the case may vary depending on the intended application. Suitable casings may include, for instance, “A”, “B”, “C”, “D”, “E”, “F”, “G”, “H”, “J”, “K”, “L”, “M”, “N”, “P”, “R”, “S”, “T”, “V”, “W”, “Y”, “X”, or “Z” (AVX Corporation). Regardless of the case size employed, the capacitor element is encapsulated so that at least a portion of the anode and cathode terminations are exposed for mounting onto a circuit board. As shown in  FIG. 1 , for instance, the capacitor element  33  is encapsulated in a case  28  so that a portion of the anode termination  62  and a portion of the cathode termination  72  are exposed. 
     As a result of the present invention, the capacitor assembly may exhibit excellent electrical properties even when exposed to high temperature environments. For example, the capacitor assembly may have an equivalence series resistance (“ESR”) of less than about 100 mohms, in some embodiments less than about 75 mohms, in some embodiments from about 0.01 to about 60 mohms, and in some embodiments, from about 0.05 to about 50 mohms, measured at an operating frequency of 100 kHz. In addition, the leakage current, which generally refers to the current flowing from one conductor to an adjacent conductor through an insulator, can be maintained at relatively low levels. For example, the numerical value of the normalized leakage current of a capacitor of the present invention is, in some embodiments, less than about 1 μA/μF*V, in some embodiments less than about 0.5 μA/μF*V, and in some embodiments, less than about 0.1 μA/μF*V, where μA is microamps and uF*V is the product of the capacitance and the rated voltage. Such ESR and normalized leakage current values may even be maintained after aging for a substantial amount of time at high temperatures. For example, the values may be maintained for about 100 hours or more, in some embodiments from about 300 hours to about 3000 hours, and in some embodiments, from about 400 hours to about 2500 hours (e.g., 500 hours, 600 hours, 700 hours, 800 hours, 900 hours, 1000 hours, 1100 hours, 1200 hours, or 2000 hours) at temperatures ranging from about 50° C. to about 250° C., and, in some embodiments from about 70° C. to about 225° C., and in some embodiments, from about 100° C. to about 225° C. (e.g., 100° C., 125° C., 150° C., 175° C., or 200° C.). 
     The capacitor may also exhibit a high energy density that enables it suitable for use in high pulse applications. Energy density is generally determined according to the equation E=½*CV 2 , where C is the capacitance in farads (F) and V is the working voltage of capacitor in volts (V). The capacitance may, for instance, be measured using a capacitance meter (e.g., Keithley 3330 Precision LCZ meter with Kelvin Leads, 2 volts bias and 1 volt signal) at an operating frequency of 120 Hz and a temperature of 25° C. For example, the capacitor may exhibit an energy density of about 2.0 joules per cubic centimeter (J/cm 3 ) or more, in some embodiments about 3.0 J/cm 3 , in some embodiments from about 4.0 J/cm 3  to about 10.0 J/cm 3 , and in some embodiments, from about 4.5 to about 8.0 J/cm 3 . The capacitance may likewise be about 1 milliFarad per square centimeter (“mF/cm 2 ”) or more, in some embodiments about 2 mF/cm 2  or more, in some embodiments from about 5 to about 50 mF/cm 2 , and in some embodiments, from about 8 to about 20 mF/cm 2 . 
     The present invention may be better understood by reference to the following examples. 
     Test Procedures 
     Equivalent Series Resistance (ESR) 
     Equivalence series resistance may be measured using a Keithley 3330 Precision LCZ meter with Kelvin Leads 2.2 volt DC bias and a 0.5 volt peak to peak sinusoidal signal. The operating frequency was 100 kHz and the temperature was room temperature. 
     Dry and Wet Capacitance 
     The capacitance was measured using a Keithley 3330 Precision LCZ meter with Kelvin Leads with 2.2 volt DC bias and a 0.5 volt peak to peak sinusoidal signal. The operating frequency was 120 Hz and the temperature was room temperature. The “dry capacitance” refers to the capacitance of the part after application of the solid electrolyte, graphite, and silver layers, while the “wet capacitance” refers to the capacitance of the part after formation of the dielectric, measured in 17% sulfuric acid in reference to 1 mF tantalum cathode. 
     V/I Characteristics: 
     V/I testing was conducted at the temperature. The test was performed by charging an individual capacitor through the 100 Ohm resistor (charging slope dU/dt=100). The starting voltage was 5V and the ending voltage was 30V (the step of voltage was 1V each 1 min). The values of the voltage and the current were recorded. 
     Example 1 
     70,000 μFV/g tantalum powder was used to form anode samples. Each anode sample was embedded with a tantalum wire, sintered at 1280° C., and pressed to a density of 6.8 g/cm 3 . The resulting pellets had a size of 1.80×1.20×2.40 mm. The pellets were anodized to 14.4V in water/phosphoric acid electrolyte with conductivity 8.6 mS at temperature of 85° C. to form the dielectric layer. A conductive polymer coating was then formed by dipping the anodes into a dispersed poly(3,4-ethylenedioxythiophene) having a solids content 1.1% and viscosity 20 mPa·s (Clevios™ K, H. C. Starck). Upon coating, the parts were dried at 125° C. for 20 minutes. This process was repeated 10 times. Thereafter, the parts were dipped into a dispersed poly(3,4-ethylenedioxythiophene) having a solids content 2% and viscosity 20 mPa·s (Clevios™ K, H. C. Starck). Upon coating, the parts were dried at 125° C. for 20 minutes. This process was not repeated. Thereafter, the parts were dipped into a dispersed poly(3,4-ethylenedioxythiophene) having a solids content 2% and viscosity 160 mPa·s (Clevios™ K, H. C. Starck). Upon coating, the parts were dried at 125° C. for 20 minutes. This process was repeated 8 times. The parts were then dipped into a graphite dispersion and dried. Finally, the parts were dipped into a silver dispersion and dried. Multiple parts (200) of 150 μF/6.3V capacitors were made in this manner. 
     Example 2 
     70,000 pFV/g tantalum powder was used to form anode samples. Each anode sample was embedded with a tantalum wire, sintered at 1280° C., and pressed to a density of 6.8 g/cm 3 . The resulting pellets had a size of 1.80×1.20×2.40 mm. The pellets were anodized to 14.4V in water/phosphoric acid electrolyte with conductivity 8.6 mS at temperature of 85° C. to form the dielectric layer. The pellets were anodized again to 60V in water/boric acid/disodium tetraborate with conductivity 2.0 mS at temperature of 30° C. for 25 seconds to form a thicker oxide layer built up on the outside. A conductive polymer coating, graphite coating, silver coating was then formed in the same manner described in Example 1. Multiple parts (200) of 150 μF/6.3V capacitors were made in this manner. 
     Example 3 
     Capacitors were formed in the manner described in Example 2, except using a different conductive polymer coating. A conductive polymer coating was formed by dipping the anodes into a dispersed poly(3,4-ethylenedioxythiophene) having a solids content 1.1% and viscosity 20 mPa·s (Clevios™ K, H. C. Starck). Upon coating, the parts were dried at 125° C. for 20 minutes. This process was repeated 10 times. Thereafter, the parts were dipped into a dispersed poly(3,4-ethylenedioxythiophene) having a solids content 2% and viscosity 20 mPa·s (Clevios™ K, H. C. Starck) and additional 20% solids content of poly(ethylene glycol) with molecular weight 600 (Sigma Aldrich®). Upon coating, the parts were dried at 125° C. for 20 minutes. This process was not repeated. Thereafter, the parts were dipped into a dispersed poly(3,4-ethylenedioxythiophene) having a solids content 2% and viscosity 160 mPa·s (Clevios™ K, H. C. Starck). Upon coating, the parts were dried at 125° C. for 20 minutes. This process was repeated 8 times. The parts were then dipped into a graphite dispersion and dried. Finally, the parts were dipped into a silver dispersion and dried. Multiple parts (200) of 150 μF/6.3V capacitors were made in this manner. 
     Example 4 
     Capacitors were formed in the manner described in Example 2, except using a different conductive polymer coating. A conductive polymer coating was formed by dipping the anodes into a dispersed poly(3,4-ethylenedioxythiophene) having a solids content 1.1% and viscosity 20 mPa·s (Clevios™ K, H. C. Starck). Upon coating, the parts were dried at 125° C. for 20 minutes. This process was repeated 10 times. Thereafter, the parts were dipped into a aqueous solution having a solids content 20% of poly(ethylene glycol) with molecular weight 600 (Sigma Aldrich®). Upon coating, the parts were dried at 125° C. for 20 minutes. This process was not repeated. Thereafter, the parts were dipped into a dispersed poly(3,4-ethylenedioxythiophene) having a solids content 2% and viscosity 20 mPa·s (Clevios™ K, H. C. Starck). Upon coating, the parts were dried at 125° C. for 20 minutes. This process was not repeated. Thereafter, the parts were dipped into a dispersed poly(3,4-ethylenedioxythiophene) having a solids content 2% and viscosity 160 mPa·s (Clevios™ K, H. C. Starck). Upon coating, the parts were dried at 125° C. for 20 minutes. This process was repeated 8 times. The parts were then dipped into a graphite dispersion and dried. Finally, the parts were dipped into a silver dispersion and dried. Multiple parts (200) of 150 μF/6.3V capacitors were made in this manner. 
     The finished capacitors of Examples 1-4 were then tested for electrical performance before an assembly process. The median results of capacitance, dissipation factor (Df), and ESR are set forth below in Table 1. The wet capacitance was 145.0 μF in all examples. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Electrical Properties 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Cap 
                 Dry/Wet 
                   
                 ESR 
               
               
                   
                 [μF] 
                 Cap [%] 
                 Df 
                 [mΩ] 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 Example 1 
                 116.33 
                 80.2 
                 7.08 
                 46.8 
               
               
                   
                 Example 2 
                 90.27 
                 62.3 
                 4.90 
                 57.6 
               
               
                   
                 Example 3 
                 110.90 
                 76.5 
                 5.39 
                 54.2 
               
               
                   
                 Example 4 
                 115.63 
                 79.7 
                 5.50 
                 52.4 
               
               
                   
                   
               
            
           
         
       
     
     As indicated, the shell-formed parts of Example 3 and 4, which also contained polyethylene glycol, had a higher dry/wet capacitance value than the shell-formed part of Example 2, which did not contain polyethylene glycol. 
     The finished capacitors of Examples 1-4 were also tested for V/I characteristics before an assembly process. The median results of current in mAmps at given voltage are set forth below in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 V/I Characteristics 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 5 V 
                 10 V 
                 15 V 
                 20 V 
                 25 V 
                 30 V 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Example 1 
                 0.0012 
                 0.0113 
                 0.1202 
                 3.0927 
                 5.9234 
                 5.4772 
               
               
                 Example 2 
                 0.0004 
                 0.0070 
                 0.2003 
                 1.1496 
                 5.7773 
                 3.8219 
               
               
                 Example 3 
                 0.0129 
                 0.0014 
                 0.0128 
                 0.0286 
                 0.1133 
                 0.1805 
               
               
                 Example 4 
                 0.0006 
                 0.0012 
                 0.0029 
                 0.0230 
                 0.1379 
                 0.2868 
               
               
                   
               
            
           
         
       
     
     As indicated, the parts with poly(ethylene glycol) (Examples 3 and 4) had the lowest current at 30V. 
     These and other modifications and variations of the present invention may be practiced by those of ordinary skill in the art, without departing from the spirit and scope of the present invention. In addition, it should be understood that aspects of the various embodiments may be interchanged both in whole or in part. Furthermore, those of ordinary skill in the art will appreciate that the foregoing description is by way of example only, and is not intended to limit the invention so further described in such appended claims.