Patent Publication Number: US-11387047-B2

Title: Solid electrolytic capacitor with improved performance at high temperatures and voltages

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims filing benefit of U.S. Provisional Patent Application Ser. No. 62/409,425 having a filing date of Oct. 18, 2016, and which is incorporated herein by reference in its entirety. 
    
    
     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. For example, such electrolytes can be formed through in situ chemical polymerization of a 3,4-dioxythiophene monomer (“EDOT”) in the presence of a catalyst and dopant. However, conventional capacitors that employ in situ polymerized polymers tend to have a relatively high leakage current (“DCL”) and fail at high voltages, such as experienced during a fast switch on or operational current spike. In an attempt to overcome these issues, dispersions have also been employed that are formed from a complex of poly(3,4-ethylenedioxythiophene) and poly(styrene sulfonate (“PEDOT:PSS”). While the PEDOT:PSS dispersions can result in some improved properties, the capacitance of such materials tends to significantly degrade at high temperatures, particularly when subjected to relatively high voltages. 
     As such, a need currently exists for a solid electrolytic capacitor having an improved performance. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the present invention, a capacitor assembly is disclosed that comprises a capacitor element. The capacitor element comprises a porous anode body that contains a valve metal compound, a dielectric that overlies the anode body and includes an oxide of the valve metal compound, and a solid electrolyte that overlies the dielectric. The solid electrolyte includes at least one conductive polymer layer that contains a sulfonyl ion. Further, the capacitor element comprises an organofunctional silane that is bonded to the oxide of the dielectric and is capable of bonding to the sulfonyl ion. 
     In accordance with another embodiment of the present invention, a method for forming a capacitor element is disclosed. The method comprises anodically oxidizing a sintered porous anode body to form a dielectric that includes an oxide of a valve metal compound; applying a solution to the anode that includes an organofunctional silane compound; applying a dispersion that contains conductive polymer particles and a sulfonyl counterion to form a conductive polymer layer; and bonding the organofunctional silane compound to the oxide of the dielectric. 
     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 cross-sectional view of one embodiment of a capacitor assembly of the assembly of the present invention; 
         FIG. 2  is a cross-sectional view of another embodiment of a capacitor assembly of the assembly of the present invention; 
         FIG. 3  is a cross-sectional view of yet another embodiment of a capacitor assembly of the assembly of the present invention; and 
         FIG. 4  is a top view of still another embodiment of a capacitor assembly of the assembly of the present 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 assembly that is capable of exhibiting good electrical properties even under a wide variety of different conditions. More particularly, the capacitor contains a capacitor element that includes a porous anode body that contains a valve metal compound (e.g., tantalum), a dielectric that overlies the anode body and includes an oxide of the valve metal compound (e.g., tantalum pentoxide), and a solid electrolyte that overlies the dielectric. The solid electrolyte includes at least one conductive polymer layer that contains sulfonyl ions. The layer may, for instance, be “extrinsically” conductive in that it contains a complex of a conductive polymer doped with a sulfonyl ion (e.g., polystyrene sulfonate). In other embodiments, the layer may be “intrinsically” conductive in that the conductive polymer itself includes the sulfonyl ion as part of the polymer backbone or as a functional group that is attached to the polymer backbone. Regardless of the nature of the conductive polymer, an organofunctional silane compound is also employed that bonds with the oxide of the dielectric and is capable of bonding to sulfonyl ions contained in the conductive polymer layer. Without intending to be limited by theory, it is believed that this may result in multiple benefits to the capacitor. For example, when exposed to a voltage at high temperatures, it is believed that the sulfonyl ions can migrate to the dielectric, which reduces its strength and thus results in a higher degree of leakage current. In the capacitor assembly of the present invention, however, all or a portion of the sulfonyl ions can become bonded to the organofunctional silane compound, which may reduce the tendency for such ions to migrate into the dielectric. Furthermore, it is also believed that the bonding of the organofunctional silane compound to the oxide of the dielectric and the sulfonyl ions can improve the interfacial adhesion of these layers, which can improve capacitance stability, particularly when subjected to high temperature and voltages. 
     Thus, as a result of the present invention, the resulting capacitor assembly may exhibit a variety of unique and beneficial properties. For instance, the capacitor assembly may exhibit excellent electrical properties even when exposed to high temperatures. For example, the capacitor assembly may be placed into contact with a hot atmosphere having a temperature of from about 80° C. or more, in some embodiments from about 100° C. to about 150° C., and in some embodiments, from about 105° C. to about 130° C. (e.g., 105° C. or 125° C.). Even at such high temperatures, the capacitor assembly can exhibit a 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 assembly 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%. The dry capacitance may be about 30 nanoFarads per square centimeter (“nF/cm 2 ”) or more, in some embodiments about 100 nF/cm 2  or more, in some embodiments from about 200 to about 3,000 nF/cm 2 , and in some embodiments, from about 400 to about 2,000 nF/cm 2 , measured at a frequency of 120 Hz. The capacitance can also remain stable at such temperatures for a substantial period of time, such as 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., about 500 hours). In one embodiment, for example, the ratio of the capacitance value of the assembly after being exposed to the hot atmosphere (e.g., 125° C.) for about 500 hours to the capacitance value of the assembly when initially exposed to the hot atmosphere is from about 0.7 to 1.0, in some embodiments from about 0.75 to 1.0, and in some embodiments, from about 0.80 to 1.0. 
     Notably, such high capacitance values can be maintained when applied with a high voltage. For example, the ratio of the capacitance value of the assembly after being exposed to the hot atmosphere (e.g., 125° C.) and rated voltage (maximum operating voltage) to the initial capacitance value of the assembly when exposed to the hot atmosphere but prior to being applied with the rated voltage is about 0.7 to 1.0, in some embodiments from about 0.75 to 1.0, and in some embodiments, from about 0.80 to 1.0. The rated voltage may, for instance, be about 15 volts or more, in some embodiments about 18 volts or more, and in some embodiments, from about 20 to about 150 volts. In one embodiment, for example, the ratio of the capacitance value of the assembly after being exposed to the hot atmosphere (e.g., 125° C.) for about 500 hours at a voltage of 25 volts to the initial capacitance value of the assembly in the hot atmosphere but prior to being applied with the voltage may be within the ranges noted above. The present inventors have discovered that the capacitance is stable and maintained within the ranges noted above even when the rated voltage begins to approximate the forming voltage used to form the dielectric—i.e., when the ratio of the forming voltage to the rated voltage (“forming ratio”) is about 2.6 or less, in some embodiments about 2.5 or less, and in some embodiments, from about 1.5 to about 2.3 (e.g., 1.7 or 2.1). The high capacitance values can also be maintained when applied with multiple voltage pulses. For example, the ratio of the capacitance value of the assembly after being subjected to 1000 pulses (30 second charge) at the rated voltage (e.g., 16 volts) in the hot atmosphere (e.g., 105° C.) and the initial capacitance of the assembly prior to applying such pulses is about 0.7 to 1.0, in some embodiments from about 0.75 to 1.0, and in some embodiments, from about 0.80 to 1.0. 
     In addition to exhibiting good capacitance stability, the capacitor assembly may also exhibit a relatively low leakage current (“DCL”) after being exposed to a hot atmosphere (e.g., 105° C. or 125° C.) and high voltage (e.g., 20 or 35 volts) as described above. For example, after being subjected to a hot atmosphere (e.g., 125° C. and rated voltage (e.g., 20 or 35 volts) for 90 seconds, the capacitor assembly may exhibit a DCL of only about 50 microamps (“pA”) or less, in some embodiments about 40 pA or less, in some embodiments about 20 pA or less, and in some embodiments, from about 0.1 to about 10 pA. The leakage current can also remain within the ranges noted above, or even lower, after being exposed to the hot atmosphere and rated voltage for a substantial period of time, such as 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., about 500 hours). 
     Various embodiments of the invention will now be described in more detail. 
     I. Capacitor Element 
     A. Anode Body 
     The capacitor element includes an anode that contains a dielectric formed on a sintered porous body. The porous anode body may be formed from a powder that contains a valve metal compound (i.e., metal that is capable of oxidation), such as tantalum, niobium, aluminum, hafnium, titanium, alloys thereof, oxides thereof, nitrides thereof, and so forth. Tantalum is particular suitable. For example, the powder may be formed from a reduction process in which a tantalum salt (e.g., potassium fluotantalate (K 2 TaF 7 ), sodium fluotantalate (Na 2 TaF 7 ), tantalum pentachloride (TaCl 5 ), etc.) is reacted with a reducing agent. The reducing agent may be provided in the form of a liquid, gas (e.g., hydrogen), or solid, such as a metal (e.g., sodium), metal alloy, or metal salt. In one embodiment, for instance, a tantalum salt (e.g., TaCl 5 ) may be heated at a temperature of from about 900° C. to about 2,000° C., in some embodiments from about 1,000° C. to about 1,800° C., and in some embodiments, from about 1,100° C. to about 1,600° C., to form a vapor that can be reduced in the presence of a gaseous reducing agent (e.g., hydrogen). Additional details of such a reduction reaction may be described in WO 2014/199480 to Maeshima, et al. After the reduction, the product may be cooled, crushed, and washed to form a powder. 
     The specific charge of the powder typically varies from about 2,000 to about 800,000 microFarads*Volts per gram (“μF*V/g”) depending on the desired application. For instance, in certain embodiments, a high charge powder may be employed that has a specific charge of from about 100,000 to about 800,000 μF*V/g, in some embodiments from about 120,000 to about 700,000 μF*V/g, and in some embodiments, from about 150,000 to about 600,000 μF*V/g. In other embodiments, a low charge powder may be employed that has a specific charge of from about 2,000 to about 100,000 μF*V/g, in some embodiments from about 5,000 to about 80,000 μF*V/g, and in some embodiments, from about 10,000 to about 70,000 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 powder may be a free-flowing, finely divided powder that contains primary particles. The primary particles of the powder generally have a median size (D50) of from about 5 to about 250 nanometers, in some embodiments from about 10 to about 200 nanometers, and in some embodiments, from about 20 to about 150 nanometers, such as determined using a laser particle size distribution analyzer made by BECKMAN COULTER Corporation (e.g., LS-230), optionally after subjecting the particles to an ultrasonic wave vibration of 70 seconds. The primary particles typically have a three-dimensional granular shape (e.g., nodular or angular). Such particles typically have a relatively low “aspect ratio”, which is the average diameter or width of the particles divided by the average thickness (“D/T”). For example, the aspect ratio of the particles may be about 4 or less, in some embodiments about 3 or less, and in some embodiments, from about 1 to about 2. In addition to primary particles, the powder may also contain other types of particles, such as secondary particles formed by aggregating (or agglomerating) the primary particles. Such secondary particles may have a median size (D50) of from about 1 to about 500 micrometers, and in some embodiments, from about 10 to about 250 micrometers. 
     Agglomeration of the particles may occur by heating the particles and/or through the use of a binder. For example, agglomeration may occur at a temperature of from about 0° C. to about 40° C., in some embodiments from about 5° C. to about 35° C., and in some embodiments, from about 15° C. to about 30° C. Suitable binders may likewise include, for instance, poly(vinyl butyral); poly(vinyl acetate); poly(vinyl alcohol); poly(vinyl pyrollidone); 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 resulting powder may 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, which may be in the form of a wire, sheet, etc. The lead may extend in a longitudinal direction from the anode body and 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. Connection of the lead may also be accomplished using other 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). 
     Any binder may be removed after pressing by heating the pellet under vacuum at a certain temperature (e.g., from about 150° C. to about 500° C.) for several minutes. Alternatively, the binder 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 pellet is typically sintered at a temperature of from about 700° C. to about 1600° C., in some embodiments from about 800° C. to about 1500° C., and in some embodiments, from about 900° C. to about 1200° C., for a time of from about 5 minutes to about 100 minutes, and in some embodiments, from about 8 minutes to about 15 minutes. This may occur in one or more steps. 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. 
     B. Dielectric 
     The anode is also coated with a dielectric. The dielectric may be formed by anodically oxidizing (“anodizing”) the sintered anode so that a dielectric layer is formed over and/or within the anode. For example, a tantalum (Ta) anode may be anodized to tantalum pentoxide (Ta 2 O 5 ). Typically, anodization is performed by initially applying a solution to the anode, such as by dipping anode into the electrolyte. A solvent is generally employed, such as water (e.g., deionized water). To enhance ionic conductivity, a compound may be employed that is capable of dissociating in the solvent to form ions. Examples of such compounds include, for instance, acids, such as described below with respect to the electrolyte. For example, an acid (e.g., phosphoric acid) may constitute from about 0.01 wt. % to about 5 wt. %, in some embodiments from about 0.05 wt. % to about 0.8 wt. %, and in some embodiments, from about 0.1 wt. % to about 0.5 wt. % of the anodizing solution. If desired, blends of acids may also be employed. 
     A current is passed through the anodizing solution to form the dielectric layer. The value of the formation voltage manages the thickness of the dielectric layer. For example, the power supply may be initially set up at a galvanostatic mode until the required voltage is reached. Thereafter, the power supply may be switched to a potentiostatic mode to ensure that the desired dielectric thickness is formed over the entire surface of the anode. Of course, other known methods may also be employed, such as pulse or step potentiostatic methods. The voltage at which anodic oxidation occurs typically ranges from about 4 to about 250 V, and in some embodiments, from about 9 to about 200 V, and in some embodiments, from about 20 to about 150 V. During oxidation, the anodizing solution can be kept at an elevated temperature, such as about 30° C. or more, in some embodiments from about 40° C. to about 200° C., and in some embodiments, from about 50° C. to about 100° C. Anodic oxidation can also be done at ambient temperature or lower. The resulting dielectric layer may be formed on a surface of the anode and within its pores. 
     Although not required, in certain embodiments, the dielectric layer may possess 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. In such embodiments, the first portion is selectively formed so that its thickness is greater than that of the second portion. It should be understood, however, that the thickness of the dielectric layer need not be uniform within a particular region. 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 may be 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). 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 inner region, 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 70 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. 
     C. Organofunctional Silane Compound 
     As indicated above, an organofunctional silane compound is also applied to the dielectric that can form a bond with the oxide of the dielectric (e.g., tantalum pentoxide) and sulfonyl ions present in a conductive polymer layer. Typically, the organofunctional silane compound has the following general formula (I): 
                         
wherein,
 
     R 1 , R 2 , and R 3  are independently alkyl (e.g., methyl, ethyl, propyl, etc.), alkenyl (e.g., ethenyl, propyenyl, etc.), aryl (e.g., phenyl), heteroaryl (e.g., pyridyl), cycloalkyl (e.g., cyclopropyl, cyclobutyl, etc.), heterocyclyl (e.g., piperidinyl), halo (e.g., fluoro), haloalkyl (e.g., fluoroalkyl), or hydroxyalkyl (e.g., hydroxymethyl, hydroxyethyl, hydroxypropyl, etc.); and 
     X is a functional group that is capable of bonding to a sulfonyl ion, such as an amino functional group, sulfur functional group, etc. Typically, such functional groups are organic functional groups. 
     In certain embodiments, at least one of R 1 , R 2 , and R 3  may a hydroxyalkyl (e.g., OCH 3 ). For example, each of R 1 , R 2 , and R 3  may be a hydroxyalkyl. In other embodiments, however, R 1  may be an alkyl (e.g., CH 3 ) and R 2  and R 3  may a hydroxyalkyl (e.g., OCH 3 ). Suitable aminofunctional organosilane compounds may include, for instance, monoamine functional silanes having the following general formula (II): 
                         
wherein,
 
     R 1 , R 2 , and R 3  are as defined above; 
     R 4  and R 5  are independently hydrogen, alkyl, independently alkyl, alkenyl, aryl, heteroaryl, cycloalkyl, heterocyclyl, halo, haloalkyl, hydroxyalkyl, or alternatively N, R 4 , and R 5  together with one or more additional atoms form a ring structure (e.g., heteroaryl or heterocyclyl); and 
     Z is an organic group that links the nitrogen atom to the silicon atom, such as alkyl (e.g., ethyl or propyl), aryl (e.g., phenyl), etc. 
     Examples of monoamino functional organosilane compounds may include, for instance, primary amine compounds (e.g., 3-aminopropyltriethoxysilane, 3-aminopropyltrimethoxysilane, 4-aminobutyltriethoxysilane, m-aminophenyltrimethoxysilane, p-aminophenyltrimethoxysilane, aminophenyltrimethoxysilane, 3-aminopropyltris(methoxy-ethoxy)silane, 11-aminoundecyltriethoxysilane, 2(4-pyridylethyl)triethoxysilane, 2-(trimethoxysilylethyl)pyridine, N-(3-trimethoxysilylpropyl)pyrrole, 3-(m-aminophenoxypropyltrimethoxysilane, aminopropylsilanetriol, 3-aminopropylmethyldiethoxysilane, 3-aminopropyldiisopropylethoxysilane, 3-aminopropyldimethylethoxysilane, etc.); secondary amine compounds (e.g., N-butylaminopropyltrimethoxy silane, N-ethylaminoisobutyltrimethoxysilane, n-methylaminopropyltrimethoxysilane, N-phenylaminopropyltrimethoxy silane, 3-(N-allylamino)propyltrimethoxysilane, cyclohexylaminomethyl)triethoxysilane, N-cyclohexylaminopropyltrimethoxysilane, N-ethylaminoisobutylmethyldiethoxysilane, (phenylaminoethyl)methyl-diethoxysilane, N-phenylaminomethytrimethoxysilane, N-methylaminopropylmethyl-dimethoxysilane, etc.); tertiary amine compounds (e.g., bis(2-hydroxyethyl)3-aminopropyltriethoxysilane, diethylaminomethyltriethoxysilane, (N, N-diethyl-3-aminopropyl)trimethoxysilane, etc.); as well as combinations thereof. Further, if desired, an additional group may be bonded to the nitrogen atom so that the compound is a quaternary amine functional silane compound. 
     Diaminofunctional silane compounds may also be employed, such as those having the following general formula (III): 
                         
wherein,
 
     R 1 , R 2 , R 3 , R 4 , and R 5  are as defined above; and 
     Z 1  is an organic group that links the nitrogen atom to the silicon atom and Z 2  is an organic group that links the nitrogen atoms, such as alkyl (e.g., ethyl or propyl), aryl (e.g., phenyl), etc. Examples of such diaminofunctional silane compounds may include, for instance, N-(2-aminoethyl)-aminopropyltrimethoxysilane, N-(2-aminoethyl)-3-aminopropyltrimethoxysilane, N-(6-aminohexyl)aminomethyl-triethoxysilane, N-(6-aminohexyl)aminopropyltrimethoxysilane, N-(2-aminoethyl)-11-aminoundecyltrimethoxysilane), (aminoethylaminomethyl)-phenethyltrimethoxysilane, N-3-[(amino(polypropylenoxy)]-aminopropyltrimethoxysilane, N-(2-aminoethyl)-3-aminopropylsilanetriol, N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane, N-(2-aminoethyl)-3-aminoisobutylmethyldimethoxysilane, (aminoethylamino)-3-isobutyldimethylmethoxysilane, etc., as well as combinations thereof. Triaminonfunctional compounds, such as (3-trimethoxysilylpropyl)-diethylenetrimamine, may also be employed. 
     Of course, as indicated above, other functional groups may also be employed that are capable of forming a bond with the sulfonyl ion. For example, sulfur-functional silane compounds may be employed in certain embodiments, such as 3-mercaptopropyltrimethoxysilane, 3-mercaptopropyltriethoxysilane, 2,2-dimethoxy-1-thia-2-silacyclopentane, 11-mercaptoundecyltrimethoxysilane, S-(octanoyl)mercaptopropyltriethoxysilane, 2-(2-pyridylethyl)thiopropyl-trimethoxysilane, 2-(4-pyridethyl)thiopropyltrimethoxysilane, 3-thiocyantopropyltrimethoxysilane, 2-(3-trimethoxysilylpropylthio)-thiophene, mercaptomethylmethyldiethoxysilane, 3-mercaptopropylmethyldimethoxysilane, bis[3-(triethoxysilyl)propyl]tetrasulfide, bis[3-(triethoxysilyl)propyl]disulfide, bis[m-(2-triethoxysilylethyl)tolyl]polysulfide, bis[3-(triethoxysilyl)propyl]thiourea, etc., as well as combinations thereof. 
     Regardless of the particular compound employed, the organofunctional silane compound may be bonded to the oxide of the dielectric using techniques known in the art. For example, the organosilane compound may be subjected to a hydrolysis reaction in which one or more of groups (e.g., R 1 , R 2 , and/or R 3  in formula (I)) are converted into hydroxyl groups. These hydroxyl groups, in turn, react with the dielectric oxide through a condensation reaction so that the organofunctional silane compound becomes grafted or otherwise bonded to the dielectric. 
     To initiate the reaction, the organofunctional silane compound may initially be dissolved in a solvent to form a solution. Particularly suitable are organic solvents, such as hydrocarbons (e.g., benzene, toluene, and xylene); ethers (e.g., tetrahydrofuran, 1,4-dioxane, and diethyl ether); ketones (e.g., methyl ethyl ketone); halogen-based solvents (e.g., chloroform, methylene chloride, and 1,2-dichloroethane); alcohols (e.g., methanol, ethanol, isopropyl alcohol, and isobutyl alcohol); and so forth, as well as combinations of any of the foregoing. Alcohols are particularly suitable for use in the present invention. The concentration of the organic solvent within the solution may vary, but is typically employed in an amount of from about 70 wt. % to about 99 wt. %, in some embodiments from about 80 wt. % to about 98 wt. %, and in some embodiments, from about 85 wt. % to about 97 wt. % of the solution. Organofunctional silane compounds may likewise constitute from about 1 wt. % to about 30 wt. %, in some embodiments from about 2 wt. % to about 20 wt. %, and in some embodiments, from about 3 wt. % to about 15 wt. % of the solution. Once formed, the solution is then coated onto the dielectric using any of a variety of known techniques, such as by dipping, drop coating, spin coating, spraying, etc. The solution may then be heated to remove the solvent and bond the organofunctional silane compound to the dielectric. Heating may occur at a temperature of 25° C. to about 350° C., in some embodiments from about 30° C. to about 325° C., and in some embodiments, from about 50° C. to about 300° C. The time period for heating may likewise vary, but is typically from about 1 to about 100 minutes, in some embodiments from about 2 to about 60 minutes, and in some embodiments, from about 4 to about 50 minutes. Once the solvent is removed, a silane network can be created in which oxygen atoms link adjacent silicon atoms and also covalently bond silicon atoms to the dielectric. At the interface, the silane reacts with itself and also with the dielectric, crosslinking and interlocking mechanically with the layer. 
     As discussed above, the functional group of the silane compound is also available for bonding to the sulfonyl ions, which are present in the conductive polymer layer that is applied subsequent to the organofunctional silane compound. The nature of the bond may vary depending on the particular compounds employed, such as covalent bonding, ionic bonding, hydrogen bonding, etc. For example, an amino functional group (NH 2 ) of a silane compound can become hydrogenated so that the functional group becomes positively charged (NH 3   + ). This hydrogenated silane compound can, in turn, form an ionic bond with a negatively charged sulfonyl counterion in the conductive polymer layer, such as a sulfonate ion (SO 3   − ). Bonding between the organofunctional silane compound can occur during formation of the capacitor, or even after it is formed depending on the particular conditions of use or storage. In certain embodiments, for example, bonding between the silane compound and the sulfonyl group may occur only during certain conditions, such as at high temperatures and/or voltages. 
     D. Solid Electrolyte 
     As indicated above, a solid electrolyte overlies the dielectric and organofunctional silane compound and generally functions as the cathode for the capacitor assembly. The solid electrolyte contains one or more layers containing extrinsically and/or intrinsically conductive polymers. In certain embodiments, an “extrinsically” conductive polymer may be employed in the solid electrolyte that has repeating units of the following formula (IV): 
                         
wherein,
 
     R 7  is a linear or branched, 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.); C 5  to C 12  cycloalkyl radical (e.g., cyclopentyl, cyclohexyl, cycloheptyl, cyclooctyl, cyclononyl, cyclodecyl, etc.); C 6  to C 14  aryl radical (e.g., phenyl, naphthyl, etc.); 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.); and q is an integer from 0 to 8, in some embodiments, from 0 to 2, and in one embodiment, 0. In one particular embodiment, “q” is 0 and the polymer is poly(3,4-ethylenedioxythiophene). One commercially suitable example of a monomer suitable for forming such a polymer is 3,4-ethylenedioxthiophene, which is available from Heraeus under the designation Clevios™ M. 
     The polymers of formula (IV) are generally considered to be “extrinsically” conductive to the extent that they typically require the presence of a separate counterion that is not covalently bound to the polymer. As indicated above, the counterion is generally a sulfonyl ion, which may be provided in the form of a monomeric or polymeric anion. Polymeric anions can, for example, polymeric sulfonic acids (including salts thereof), such as polystyrene sulfonic acids or sulfonates (“PSS”), polyvinyl sulfonic acids, etc. 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 or salts thereof (e.g., dodecane sulfonic acid); aliphatic perfluorosulfonic acids or salts thereof (e.g., trifluoromethane sulfonic acid, perfluorobutane sulfonic acid or perfluorooctane sulfonic acid); aromatic sulfonic acids or salts thereof 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 or salts thereof (e.g., camphor sulfonic acid or tetrafluoroborates, hexafluorophosphates, perchlorates, hexafluoroantimonates, hexafluoroarsenates or hexachloroantimonates); and so forth. PSS is a particularly suitable polymeric anion. 
     Intrinsically conductive polymers may also be employed that have a positive charge located on the main chain that is at least partially compensated by sulfonate covalently bound to the polymer. For example, one example of a suitable intrinsically conductive polymer may have repeating units of the following formula (V): 
                         
wherein,
 
     R is (CH 2 ) a —O—(CH 2 ) b ; 
     a is from 0 to 10, in some embodiments from 0 to 6, and in some embodiments, from 1 to 4 (e.g., 1); 
     b is from 1 to 18, in some embodiments from 1 to 10, and in some embodiments, from 2 to 6 (e.g., 2, 3, 4, or 5); 
     Z is a sulfonyl anion, such as SO 3   − , CF 3 SO 3   − , etc.; 
     X is a cation, such as hydrogen, an alkali metal (e.g., lithium, sodium, rubidium, cesium or potassium), ammonium, etc. 
     In one particular embodiment, Z in formula (V) is SO 3  such that the intrinsically conductive polymer contains repeating units of the following formula (VI): 
     
       
         
         
             
             
         
       
     
     wherein, R and X are defined above. In formula (V) or (VI), a is preferably 1 and b is preferably 3 or 4. Likewise, X is preferably sodium or potassium. 
     If desired, the polymer may be a copolymer that contains other types of repeating units. In such embodiments, the repeating units of formula (VI) typically constitute about 50 mol. % or more, in some embodiments from about 75 mol. % to about 99 mol. %, and in some embodiments, from about 85 mol. % to about 95 mol. % of the total amount of repeating units in the copolymer. Of course, the polymer may also be a homopolymer to the extent that it contains 100 mol. % of the repeating units of formula (II). Specific examples of such homopolymers include poly(4-(2,3-dihydrothieno-[3,4-b][1,4]dioxin-2-ylmethoxy)-1-butane-sulphonic acid, salt) and poly(4-(2,3-dihydrothieno-[3,4-b][1,4]dioxin-2-ylmethoxy)-l-propanesulphonic acid, salt). 
     Various methods may be utilized to form a conductive polymer layer. For example, an in situ polymerized layer may be formed by chemically polymerizing monomers in the presence of an oxidative catalyst. The oxidative catalyst typically includes a transition metal cation, such as iron(III), copper(II), chromium(VI), cerium(IV), manganese(IV), manganese(VII), or ruthenium(III) cations, and etc. A dopant may also be employed to provide excess charge to the conductive polymer and stabilize the conductivity of the polymer. The dopant typically includes a sulfonyl anion. In certain embodiments, the oxidative catalyst has both a catalytic and doping functionality in that it includes a cation (e.g., transition metal) and the sulfonyl anion. For example, the oxidative catalyst may be a transition metal salt that includes iron(III) cations, such as iron(III) halides (e.g., FeCl 3 ) or iron(III) salts of other inorganic acids, such as Fe(ClO 4 ) 3  or Fe 2 (SO 4 ) 3  and the iron(III) salts of organic acids and inorganic sulfonic acids. Examples of iron (III) salts of inorganic sulfonic acids include, for instance, iron(III) salts of sulfuric acid monoesters of C 1  to C 20  alkanols (e.g., iron(III) salt of lauryl sulfate). Likewise, examples of iron(III) salts of organic sulfonic acids include, for instance, iron(III) salts of C 1  to C 20  alkane sulfonic acids (e.g., methane, ethane, propane, butane, or dodecane sulfonic acid); iron (III) salts of aliphatic perfluorosulfonic acids (e.g., trifluoromethane sulfonic acid, perfluorobutane sulfonic acid, or perfluorooctane sulfonic acid); iron (III) salts of 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); iron (III) salts of cycloalkane sulfonic acids (e.g., camphor sulfonic acid); and so forth. Mixtures of these above-mentioned iron(III) salts may also be used. Iron(III)-p-toluene sulfonate, iron(III)-o-toluene sulfonate, and mixtures thereof, are particularly suitable. One commercially suitable example of iron(III)-p-toluene sulfonate is available from Heraeus Clevios under the designation Clevios™ C. 
     The oxidative catalyst and monomer may be applied either sequentially or together to initiate the polymerization reaction. Suitable application techniques for applying these components include screen-printing, dipping, electrophoretic coating, and spraying. As an example, the monomer may initially be mixed with the oxidative catalyst to form a precursor solution. Once the mixture is formed, it may be applied to the anode part and then allowed to polymerize so that a conductive coating is formed on the surface. Alternatively, the oxidative catalyst and monomer may be applied sequentially. In one embodiment, for example, the oxidative catalyst is dissolved in an organic solvent (e.g., butanol) and then applied as a dipping solution. The anode part may then be dried to remove the solvent therefrom. Thereafter, the part may be dipped into a solution containing the monomer. Regardless, polymerization is typically performed at temperatures of from about −10° C. to about 250° C., and in some embodiments, from about 0° C. to about 200° C., depending on the oxidizing agent used and desired reaction time. Suitable polymerization techniques, such as described above, may be described in more detail in U.S. Pat. No. 7,515,396 to Biler. Still other methods for applying such conductive coating(s) may be described in U.S. Pat. No. 5,457,862 to Sakata, et al., U.S. Pat. No. 5,473,503 to Sakata, et al., U.S. Pat. No. 5,729,428 to Sakata, et al., and U.S. Pat. No. 5,812,367 to Kudoh, et al. 
     In addition to in situ application, the conductive polymer may also be applied in the form of a dispersion of conductive polymer particles. 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 may also contain one or more components to enhance the overall properties of the resulting solid electrolyte. For example, the dispersion may contain a binder to further enhance the adhesive nature of the polymeric layer and also increase the stability of the particles within the dispersion. The binder 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 crosslinkable polymers, such as polyurethanes, polyacrylates or polyolefins, and subsequent crosslinking. Dispersion agents may also be employed to facilitate the ability to apply the layer to the anode. 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). 
     If desired, a hydroxyl-functional nonionic polymer may also be employed in the solid electrolyte. 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 the use of a hydroxy-functional polymer with a certain molecular weight can 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 nonionic polymer may simply be incorporated into a dispersion of conductive polymers. In such embodiments, the concentration of the nonionic polymer in the layer may be 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 nonionic polymer may be applied after the initial outer layer(s) are formed. In such embodiments, the technique used to apply the nonionic polymer may vary. For example, the nonionic 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, or screen 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 nonionic 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. 
     The 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. 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 0.5 to about 10,000 mPas, in some embodiments from about 1 to about 1,500 mPas, and in some embodiments, from about 5 to about 1000 mPas. 
     D. External Polymer Coating 
     An external polymer coating may also be applied that overlies the solid electrolyte. The external polymer coating generally contains one or more layers formed from pre-polymerized conductive polymer particles such as described above (e.g., dispersion of extrinsically conductive polymer particles). The external coating may be able to further penetrate into the edge region of the capacitor body to increase the adhesion to the dielectric and result 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 body, the particles used in the external coating typically have a larger size than those employed in 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 any 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 80 to about 500 nanometers, in some embodiments from about 90 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. 
     E. Other Optional Components 
     If desired, the capacitor element may also contain other layers as is known in the art. For example, an adhesive layer may optionally be formed between the dielectric and solid electrolyte, such as one made of a relatively insulative resinous material (natural or synthetic). The layer may be applied before, after, or in combination with the organofunctional silane compound. The resinous materials may have a specific resistivity of greater than about 10 ohms-cm, in some embodiments greater than about 100, in some embodiments greater than about 1,000 ohms-cm, in some embodiments greater than about 1×10 5  ohms-cm, and in some embodiments, greater than about 1×10 10  ohms-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 adhesive layer materials are described in more detail U.S. Pat. No. 6,674,635 to Fife, et at, 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. 
     II. Terminations 
     Once formed, the capacitor element may be provided with terminations, particularly when employed in surface mounting applications. For example, the capacitor assembly may contain an anode termination to which an 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. 
     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 and anode termination. To attach the electrolytic capacitor element to the lead frame, a conductive adhesive may initially be applied to a surface of the cathode termination. 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 compound (e.g., silane compounds). Suitable conductive adhesives may be described in U.S. Patent Application Publication No. 2006/0038304 to Osako, et al. Any of a variety of techniques may be used to apply the conductive adhesive to the cathode termination. Printing techniques, for instance, may be employed due to their practical and cost-saving benefits. The anode lead may also be electrically connected to the anode termination using any technique known in the art, such as mechanical welding, laser welding, conductive adhesives, etc. Upon electrically connecting the anode lead to the anode termination, the conductive adhesive may then be cured to ensure that the electrolytic capacitor element is adequately adhered to the cathode termination. 
     III. Housing 
     Due to the ability of the capacitor assembly to exhibit good electrical performance in various environments, it is not necessary for the capacitor element to be hermetically sealed within a housing. Nevertheless, in certain embodiments, it may be desired to hermetically seal the capacitor element within a housing. In one embodiment, for example, the capacitor element may be hermetically sealed within a housing in the presence of a gaseous atmosphere that contains an inert gas, thereby further limiting the amount of moisture supplied to the solid electrolyte of the capacitor element. 
     The capacitor element may be sealed within a housing in various ways. In certain embodiments, for instance, the capacitor element may be enclosed within a case, which may then be filled with a resinous material, such as a thermoset resin (e.g., epoxy resin) that can be cured to form a hardened housing. Examples of such resins include, for instance, epoxy resins, polyimide resins, melamine resins, urea-formaldehyde resins, polyurethane resins, phenolic resins, polyester resins, etc. Epoxy resins are also particularly suitable. Still other additives may also be employed, such as photoinitiators, viscosity modifiers, suspension aiding agents, pigments, stress reducing agents, non-conductive fillers, stabilizers, etc. For example, the non-conductive fillers may include inorganic oxide particles, such as silica, alumina, zirconia, magnesium oxide, iron oxide, copper oxide, zeolites, silicates, clays (e.g., smectite clay), etc., as well as composites (e.g., alumina-coated silica particles) and mixtures thereof. Regardless, the resinous material may surround and encapsulate the capacitor element so that at least a portion of the anode and cathode terminations are exposed for mounting onto a circuit board. When encapsulated in this manner, the capacitor element and resinous material form an integral capacitor assembly. 
     Of course, in alternative embodiments, it may be desirable to enclose the capacitor element within a housing that remains separate and distinct. In this manner, the atmosphere of the housing may be gaseous and contain at least one inert gas, such as nitrogen, helium, argon, xenon, neon, krypton, radon, and so forth, as well as mixtures thereof. Typically, inert gases constitute the majority of the atmosphere within the housing, such as from about 50 wt. % to 100 wt. %, in some embodiments from about 75 wt. % to 100 wt. %, and in some embodiments, from about 90 wt. % to about 99 wt. % of the atmosphere. If desired, a relatively small amount of non-inert gases may also be employed, such as carbon dioxide, oxygen, water vapor, etc. In such cases, however, the non-inert gases typically constitute 15 wt. % or less, in some embodiments 10 wt. % or less, in some embodiments about 5 wt. % or less, in some embodiments about 1 wt. % or less, and in some embodiments, from about 0.01 wt. % to about 1 wt. % of the atmosphere within the housing. 
     Any of a variety of different materials may be used to form the housing, such as metals, plastics, ceramics, and so forth. In one embodiment, for example, the housing includes one or more layers of a metal, such as tantalum, niobium, aluminum, nickel, hafnium, titanium, copper, silver, steel (e.g., stainless), alloys thereof (e.g., electrically conductive oxides), composites thereof (e.g., metal coated with electrically conductive oxide), and so forth. In another embodiment, the housing may include one or more layers of a ceramic material, such as aluminum nitride, aluminum oxide, silicon oxide, magnesium oxide, calcium oxide, glass, etc., as well as combinations thereof. 
     The housing may have any desired shape, such as cylindrical, D-shaped, rectangular, triangular, prismatic, etc. Referring to  FIG. 1 , for example, one embodiment of a capacitor assembly  100  is shown that contains a housing  122  and a capacitor element  120 . In this particular embodiment, the housing  122  is generally rectangular. Typically, the housing and the capacitor element have the same or similar shape so that the capacitor element can be readily accommodated within the interior cavity. In the illustrated embodiment, for example, both the capacitor element  120  and the housing  122  have a generally rectangular shape. 
     If desired, the capacitor assembly of the present invention may exhibit a relatively high volumetric efficiency. To facilitate such high efficiency, the capacitor element typically occupies a substantial portion of the volume of an interior cavity of the housing. For example, the capacitor element may occupy about 30 vol. % or more, in some embodiments about 50 vol. % or more, in some embodiments about 60 vol. % or more, in some embodiments about 70 vol. % or more, in some embodiments from about 80 vol. % to about 98 vol. %, and in some embodiments, from about 85 vol. % to 97 vol. % of the interior cavity of the housing. To this end, the difference between the dimensions of the capacitor element and those of the interior cavity defined by the housing are typically relatively small. 
     Referring to  FIG. 1 , for example, the capacitor element  120  may have a length (excluding the length of the anode lead  6 ) that is relatively similar to the length of an interior cavity  126  defined by the housing  122 . For example, the ratio of the length of the anode to the length of the interior cavity ranges from about 0.40 to 1.00, in some embodiments from about 0.50 to about 0.99, in some embodiments from about 0.60 to about 0.99, and in some embodiments, from about 0.70 to about 0.98. The capacitor element  120  may have a length of from about 5 to about 10 millimeters, and the interior cavity  126  may have a length of from about 6 to about 15 millimeters. Similarly, the ratio of the height of the capacitor element  120  (in the −z direction) to the height of the interior cavity  126  may range from about 0.40 to 1.00, in some embodiments from about 0.50 to about 0.99, in some embodiments from about 0.60 to about 0.99, and in some embodiments, from about 0.70 to about 0.98. The ratio of the width of the capacitor element  120  (in the −x direction) to the width of the interior cavity  126  may also range from about 0.50 to 1.00, in some embodiments from about 0.60 to about 0.99, in some embodiments from about 0.70 to about 0.99, in some embodiments from about 0.80 to about 0.98, and in some embodiments, from about 0.85 to about 0.95. For example, the width of the capacitor element  120  may be from about 2 to about 7 millimeters and the width of the interior cavity  126  may be from about 3 to about 10 millimeters, and the height of the capacitor element  120  may be from about 0.5 to about 2 millimeters and the width of the interior cavity  126  may be from about 0.7 to about 6 millimeters. 
     Although by no means required, the capacitor element may be attached to the housing in such a manner that an anode termination and cathode termination are formed external to the housing for subsequent integration into a circuit. The particular configuration of the terminations may depend on the intended application. In one embodiment, for example, the capacitor assembly may be formed so that it is surface mountable, and yet still mechanically robust. For example, the anode lead may be electrically connected to external, surface mountable anode and cathode terminations (e.g., pads, sheets, plates, frames, etc.). Such terminations may extend through the housing to connect with the capacitor. The thickness or height of the terminations is generally selected to minimize the thickness of the capacitor assembly. 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 0.1 to about 0.2 millimeters. 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, the termination(s) are deposited with nickel and silver flashes, respectively, and the mounting surface is also plated with a tin solder layer. In another embodiment, the termination(s) are deposited with thin outer metal layers (e.g., gold) onto a base metal layer (e.g., copper alloy) to further increase conductivity. 
     In certain embodiments, connective members may be employed within the interior cavity of the housing to facilitate connection to the terminations in a mechanically stable manner. For example, referring again to  FIG. 1 , the capacitor assembly  100  may include a connection member  162  that is formed from a first portion  167  and a second portion  165 . The connection member  162  may be formed from conductive materials similar to the external terminations. The first portion  167  and second portion  165  may be integral or separate pieces that are connected together, either directly or via an additional conductive element (e.g., metal). In the illustrated embodiment, the second portion  165  is provided in a plane that is generally parallel to a lateral direction in which the lead  6  extends (e.g., −y direction). The first portion  167  is “upstanding” in the sense that it is provided in a plane that is generally perpendicular the lateral direction in which the lead  6  extends. In this manner, the first portion  167  can limit movement of the lead  6  in the horizontal direction to enhance surface contact and mechanical stability during use. If desired, an insulative material  7  (e.g., Teflon™ washer) may be employed around the lead  6 . 
     The first portion  167  may possess a mounting region (not shown) that is connected to the anode lead  6 . The region may have a “U-shape” for further enhancing surface contact and mechanical stability of the lead  6 . Connection of the region to the lead  6  may be accomplished using any of a variety of known techniques, such as welding, laser welding, conductive adhesives, etc. In one particular embodiment, for example, the region is laser welded to the anode lead  6 . Regardless of the technique chosen, however, the first portion  167  can hold the anode lead  6  in substantial horizontal alignment to further enhance the dimensional stability of the capacitor assembly  100 . 
     Referring again to  FIG. 1 , one embodiment of the present invention is shown in which the connective member  162  and capacitor element  120  are connected to the housing  122  through anode and cathode terminations  127  and  129 , respectively. More specifically, the housing  122  of this embodiment includes an outer wall  123  and two opposing sidewalls  124  between which a cavity  126  is formed that includes the capacitor element  120 . The outer wall  123  and sidewalls  124  may be formed from one or more layers of a metal, plastic, or ceramic material such as described above. In this particular embodiment, the anode termination  127  contains a first region  127   a  that is positioned within the housing  122  and electrically connected to the connection member  162  and a second region  127   b  that is positioned external to the housing  122  and provides a mounting surface  201 . Likewise, the cathode termination  129  contains a first region  129   a  that is positioned within the housing  122  and electrically connected to the solid electrolyte of the capacitor element  120  and a second region  129   b  that is positioned external to the housing  122  and provides a mounting surface  203 . It should be understood that the entire portion of such regions need not be positioned within or external to the housing. 
     In the illustrated embodiment, a conductive trace  127   c  extends in the outer wall  123  of the housing to connect the first region  127   a  and second region  127   b . Similarly, a conductive trace  129   c  extends in the outer wall  123  of the housing to connect the first region  127   a  and second region  127   b . The conductive traces and/or regions of the terminations may be separate or integral. In addition to extending through the outer wall of the housing, the traces may also be positioned at other locations, such as external to the outer wall. Of course, the present invention is by no means limited to the use of conductive traces for forming the desired terminations. 
     Regardless of the particular configuration employed, connection of the terminations  127  and  129  to the capacitor element  120  may be made using any known technique, such as welding, laser welding, conductive adhesives, etc. In one particular embodiment, for example, a conductive adhesive  131  is used to connect the second portion  165  of the connection member  162  to the anode termination  127 . Likewise, a conductive adhesive  133  is used to connect the cathode of the capacitor element  120  to the cathode termination  129 . 
     Optionally, a polymeric restraint may also be disposed in contact with one or more surfaces of the capacitor element, such as the rear surface, front surface, upper surface, lower surface, side surface(s), or any combination thereof. The polymeric restraint can reduce the likelihood of delamination by the capacitor element from the housing. In this regard, the polymeric restraint may possesses a certain degree of strength that allows it to retain the capacitor element in a relatively fixed positioned even when it is subjected to vibrational forces, yet is not so strong that it cracks. For example, the restraint may possess a tensile strength of from about 1 to about 150 Megapascals (“MPa”), in some embodiments from about 2 to about 100 MPa, in some embodiments from about 10 to about 80 MPa, and in some embodiments, from about 20 to about 70 MPa, measured at a temperature of about 25° C. It is normally desired that the restraint is not electrically conductive. Referring again to  FIG. 1 , for instance, one embodiment is shown in which a single polymeric restraint  197  is disposed in contact with an upper surface  181  and rear surface  177  of the capacitor element  120 . While a single restraint is shown in  FIG. 1 , it should be understood that separate restraints may be employed to accomplish the same function. In fact, more generally, any number of polymeric restraints may be employed to contact any desired surface of the capacitor element. When multiple restraints are employed, they may be in contact with each other or remain physically separated. For example, in one embodiment, a second polymeric restraint (not shown) may be employed that contacts the upper surface  181  and front surface  179  of the capacitor element  120 . The first polymeric restraint  197  and the second polymeric restraint (not shown) may or may not be in contact with each other. In yet another embodiment, a polymeric restraint may also contact a lower surface  183  and/or side surface(s) of the capacitor element  120 , either in conjunction with or in lieu of other surfaces. 
     Regardless of how it is applied, it is typically desired that the polymeric restraint is also in contact with at least one surface of the housing to help further mechanically stabilize the capacitor element against possible delamination. For example, the restraint may be in contact with an interior surface of one or more sidewall(s), outer wall, lid, etc. In  FIG. 1 , for example, the polymeric restraint  197  is in contact with an interior surface  107  of sidewall  124  and an interior surface  109  of outer wall  123 . While in contact with the housing, it is nevertheless desired that at least a portion of the cavity defined by the housing remains unoccupied to allow for the inert gas to flow through the cavity and limit contact of the solid electrolyte with oxygen. For example, at least about 5% of the cavity volume typically remains unoccupied by the capacitor element and polymer restraint, and in some embodiments, from about 10% to about 50% of the cavity volume. 
     Once connected in the desired manner, the resulting package is hermetically sealed as described above. Referring again to  FIG. 1 , for instance, the housing  122  may also include a lid  125  that is placed on an upper surface of side walls  124  after the capacitor element  120  and the polymer restraint  197  are positioned within the housing  122 . The lid  125  may be formed from a ceramic, metal (e.g., iron, copper, nickel, cobalt, etc., as well as alloys thereof), plastic, and so forth. If desired, a sealing member  187  may be disposed between the lid  125  and the side walls  124  to help provide a good seal. In one embodiment, for example, the sealing member may include a glass-to-metal seal, Kovar® ring (Goodfellow Camridge, Ltd.), etc. The height of the side walls  124  is generally such that the lid  125  does not contact any surface of the capacitor element  120  so that it is not contaminated. The polymeric restraint  197  may or may not contact the lid  125 . When placed in the desired position, the lid  125  is hermetically sealed to the sidewalls  124  using known techniques, such as welding (e.g., resistance welding, laser welding, etc.), soldering, etc. Hermetic sealing generally occurs in the presence of inert gases as described above so that the resulting assembly is substantially free of reactive gases, such as oxygen or water vapor. 
     It should be understood that the embodiments described are only exemplary, and that various other configurations may be employed in the present invention for hermetically sealing a capacitor element within a housing. Referring to  FIG. 2 , for instance, another embodiment of a capacitor assembly  200  is shown that employs a housing  222  that includes an outer wall  123  and a lid  225  between which a cavity  126  is formed that includes the capacitor element  120  and polymeric restraint  197 . The lid  225  includes an outer wall  223  that is integral with at least one sidewall  224 . In the illustrated embodiment, for example, two opposing sidewalls  224  are shown in cross-section. The outer walls  223  and  123  both extend in a lateral direction (−y direction) and are generally parallel with each other and to the lateral direction of the anode lead  6 . The sidewall  224  extends from the outer wall  223  in a longitudinal direction that is generally perpendicular to the outer wall  123 . A distal end  500  of the lid  225  is defined by the outer wall  223  and a proximal end  501  is defined by a lip  253  of the sidewall  224 . 
     The lip  253  extends from the sidewall  224  in the lateral direction, which may be generally parallel to the lateral direction of the outer wall  123 . The angle between the sidewall  224  and the lip  253  may vary, but is typically from about 60° to about 120°, in some embodiments from about 70° to about 110°, and in some embodiments, from about 80° to about 100° (e.g., about 90°). The lip  253  also defines a peripheral edge  251 , which may be generally perpendicular to the lateral direction in which the lip  253  and outer wall  123  extend. The peripheral edge  251  is located beyond the outer periphery of the sidewall  224  and may be generally coplanar with an edge  151  of the outer wall  123 . The lip  253  may be sealed to the outer wall  123  using any known technique, such as welding (e.g., resistance or laser), soldering, glue, etc. For example, in the illustrated embodiment, a sealing member  287  is employed (e.g., glass-to-metal seal, Kovar® ring, etc.) between the components to facilitate their attachment. Regardless, the use of a lip described above can enable a more stable connection between the components and improve the seal and mechanical stability of the capacitor assembly. 
     Still other possible housing configurations may be employed in the present invention. For example,  FIG. 3  shows a capacitor assembly  300  having a housing configuration similar to that of  FIG. 2 , except that terminal pins  327   b  and  329   b  are employed as the external terminations for the anode and cathode, respectively. More particularly, the terminal pin  327   b  extends through a trace  327   c  formed in the outer wall  323  and is connected to the anode lead  6  using known techniques (e.g., welding). An additional section  327   a  may be employed to secure the pin  327   b . Likewise, the terminal pin  329   b  extends through a trace  329   c  formed in the outer wall  323  and is connected to the cathode via a conductive adhesive  133  as described above. Similarly, an additional section  329   a  may be employed to secure the pin  329   b.    
     The embodiments shown in  FIGS. 1-3  are discussed herein in terms of only a single capacitor element. It should also be understood, however, that multiple capacitor elements may also be hermetically sealed within a housing. The multiple capacitor elements may be attached to the housing using any of a variety of different techniques. Referring to  FIG. 4 , for example one particular embodiment of a capacitor assembly  400  that contains two capacitor elements is shown and will now be described in more detail. More particularly, the capacitor assembly  400  includes a first capacitor element  420   a  in electrical communication with a second capacitor element  420   b . In this embodiment, the capacitor elements are aligned so that their major surfaces are in a horizontal configuration. That is, a major surface of the capacitor element  420   a  defined by its width (−x direction) and length (−y direction) is positioned adjacent to a corresponding major surface of the capacitor element  420   b . Thus, the major surfaces are generally coplanar. Alternatively, the capacitor elements may be arranged so that their major surfaces are not coplanar, but perpendicular to each other in a certain direction, such as the −z direction or the −x direction. Of course, the capacitor elements need not extend in the same direction. 
     The capacitor elements  420   a  and  420   b  are positioned within a housing  422  that contains an outer wall  423  and sidewalls  424  and  425  that together define a cavity  426 . Although not shown, a lid may be employed that covers the upper surfaces of the sidewalls  424  and  425  and seals the assembly  400  as described above. Optionally, a polymeric restraint may also be employed to help limit the vibration of the capacitor elements. In  FIG. 4 , for example, separate polymer restraints  497   a  and  497   b  are positioned adjacent to and in contact with the capacitor elements  420   a  and  420   b , respectively. The polymer restraints  497   a  and  497   b  may be positioned in a variety of different locations. Further, one of the restraints may be eliminated, or additional restraints may be employed. In certain embodiments, for example, it may be desired to employ a polymeric restraint between the capacitor elements to further improve mechanical stability. 
     In addition to the capacitor elements, the capacitor assembly also contains an anode termination to which anode leads of respective capacitor elements are electrically connected and a cathode termination to which the cathodes of respective capacitor elements are electrically connected. Referring again to  FIG. 4 , for example, the capacitor elements are shown connected in parallel to a common cathode termination  429 . In this particular embodiment, the cathode termination  429  is initially provided in a plane that is generally parallel to the bottom surface of the capacitor elements and may be in electrical contact with conductive traces (not shown). The capacitor assembly  400  also includes connective members  427  and  527  that are connected to anode leads  407   a  and  407   b , respectively, of the capacitor elements  420   a  and  420   b . More particularly, the connective member  427  contains an upstanding portion  465  and a planar portion  463  that is in connection with an anode termination (not shown). Likewise, the connective  527  contains an upstanding portion  565  and a planar portion  563  that is in connection with an anode termination (not shown). Of course, it should be understood that a wide variety of other types of connection mechanisms may also be employed. 
     Regardless of its particular construction, the resulting capacitor assembly can exhibit a variety of beneficial properties as indicated above. For example, the capacitor can exhibit a relatively high “breakdown voltage” (voltage at which the capacitor fails), such as about 50 volts or more, in some embodiments about 100 volts or more, in some embodiments about 150 volts or more, and in some embodiments, from about 200 volts to about 300 volts. The capacitor may also exhibit a relatively low equivalence series resistance (“ESR”), such as about 200 mohms, in some embodiments less than about 150 mohms, and in some embodiments, from about 0.1 to about 100 mohms, measured at an operating frequency of 100 kHz. It is also believed that the dissipation factor of the capacitor assembly may be maintained at relatively low levels. The dissipation factor generally refers to losses that occur in the capacitor and is usually expressed as a percentage of the ideal capacitor performance. For example, the dissipation factor of the capacitor of the present invention is typically from about 0.5% to about 25%, in some embodiments from about 0.8% to about 10%, and in some embodiments, from about 1% to about 5%, as determined at a frequency of 120 Hz. The capacitor assembly may also be able to be employed in high voltage applications, such as at rated voltages of about 35 volts or more, in some embodiments about 50 volts or more, and in some embodiments, from about 60 volts to about 200 volts. The capacitor assembly 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 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 may 100 kHz and the temperature may be 23° C.±2° C. 
     Dissipation Factor 
     The dissipation factor may be 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 may be 120 Hz and the temperature may be 23° C.±2° C. 
     Capacitance 
     The capacitance may be 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 may be 120 Hz and the temperature may be 23° C.±2° C. In some cases, the “wet-to-dry” capacitance can be determined. The “dry capacitance” refers to the capacitance of the part before 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 14% nitric acid in reference to 1 mF tantalum cathode with 10 volt DC bias and a 0.5 volt peak to peak sinusoidal signal after 30 seconds of electrolyte soaking. 
     Leakage Current 
     Leakage current may be measured using a leakage test meter at a temperature of 23° C.±2° C. and at the rated voltage (e.g., 16 volts) after a minimum of 60 seconds (e.g., 90 seconds, 180 seconds, or 300 seconds). 
     Temperature Life Testing 
     Temperature life testing may be conducted (e.g., 10 parts) at a temperature of 125° C. with different voltages (e.g., 0 volts, 20 volts, or 35 volts). The recovery time after the test conditions may be 30 minutes. The capacitance and ESR may be measured after the recovery time. 
     Temperature Pulse Testing 
     Temperature pulse testing may be conducted (e.g., 25 parts) at a temperature of 125° C. with different voltages. The resistance may be 33 ohms and 30 seconds of charge/discharge. The capacitance and ESR may be measured at 1000, 2000 and 3000 hours after the recovery time. 
     Example 1 
     40,000 μFV/g tantalum powder was used to form anode samples. Each anode sample was embedded with a tantalum wire, sintered at 1410° C., and pressed to a density of 5.3 g/cm 3 . The resulting pellets had a size of 5.20×3.70×0.65 mm. The pellets were anodized to 49.0 volts in water/phosphoric acid electrolyte with a conductivity of 8.6 mS at a temperature of 85° C. to form the dielectric layer. The pellets were anodized again to 130 volts in a water/boric acid/disodium tetraborate with a conductivity of 2.0 mS at a temperature of 30° C. for 25 seconds to form a thicker oxide layer built up on the outside. 
     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, Heraeus). 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 2.0% and viscosity 20 mPa·s (Clevios™ K, Heraeus). Upon coating, the parts were dried at 125° C. for 20 minutes. This process was repeated 3 times. Thereafter, the parts were dipped into a dispersed poly(3,4-ethylenedioxythiophene) having a solids content of 2% and viscosity 160 mPa·s (Clevios™ K, Heraeus). 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. 
     Example 2 
     Capacitors were formed in the manner described in Example 1, except that three (3) pre-coat layers were formed using a solution of (3-aminopropyl)trimethoxysilane in ethanol (1.0%). Multiple parts (450) of 47 μF/20V capacitors were formed and encapsulated in a silica resin. 
     Example 3 
     Capacitors were formed in the manner described in Example 1, except that three (3) pre-coat layers were formed using a solution of [3-(2-aminoethylamino)-propyl]trimethoxysilane in ethanol (1.0%). 
     Multiple parts (450) of 47 μF/20V capacitors were formed as described in Examples 1-3 and encapsulated in a silica resin. The parts were tested for capacitance (CAP), “wet-to-dry” capacitance (W/D), dissipation factor (Df), ESR, and leakage current (DCL). The median results are set forth below in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Electrical Properties 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 1 st  DCL @ 
                 2 nd  DCL @ 
               
               
                   
                 CAP 
                 W/D 
                 Df 
                 ESR 
                 90 sec 
                 300 sec 
               
               
                   
                 [μF] 
                 [%] 
                 [%] 
                 [mohms] 
                 [μA] 
                 [μA] 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Example 1 
                 44.29 
                 78.95 
                 3.0 
                 37.3 
                 82.75 
                 5.92 
               
               
                 Example 2 
                 44.43 
                 79.20 
                 1.3 
                 48.4 
                 77.25 
                 74.04 
               
               
                 Example 3 
                 44.35 
                 79.06 
                 1.2 
                 56.3 
                 1.16 
                 0.38 
               
               
                   
               
            
           
         
       
     
     Multiple parts (10) of 47 μF/20V capacitors (before encapsulation) were also formed as described in Examples 1-3 and dried at 150° C. for 4 hours. The resulting parts were then placed into a ceramic housing purged with nitrogen gas, and then hermetically sealed by seam welding. The parts were tested for capacitance and leakage current using “life testing” and “pulse testing” as described above. The median results are set forth below in Tables 2 and 3. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 125° Life Testing Results 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Time 
                 CAP 
                 Ratio of Cap. 
                 DCL @ 300 sec 
               
               
                   
                 [h] 
                 [μF] 
                 to Initial Cap. 
                 [μA] 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Example 1 
                 0 
                 44.43 
                 1.00 
                 5.57 
               
               
                   
                 500 
                 42.64 
                 0.86 
                 159.27 
               
               
                   
                 1000 
                 39.82 
                 0.74 
                 247.47 
               
               
                 Example 2 
                 0 
                 44.38 
                 1.00 
                 270.86 
               
               
                   
                 500 
                 43.13 
                 0.96 
                 2.77 
               
               
                   
                 1000 
                 41.39 
                 0.90 
                 3.58 
               
               
                 Example 3 
                 0 
                 44.23 
                 1.00 
                 0.37 
               
               
                   
                 500 
                 38.07 
                 0.97 
                 1.46 
               
               
                   
                 1000 
                 32.69 
                 0.93 
                 1.29 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 125° C. Pulse Testing Results 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Time 
                 CAP 
                 Ratio of Cap. 
                 DCL @ 300 sec 
               
               
                   
                 [h] 
                 [μF] 
                 to Initial Cap. 
                 [μA] 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Example 1 
                 0 
                 43.95 
                 1.00 
                 10.24 
               
               
                   
                 1000 
                 37.83 
                 0.86 
                 146.76 
               
               
                   
                 2000 
                 35.01 
                 0.80 
                 69.31 
               
               
                   
                 3000 
                 32.89 
                 0.75 
                 75.51 
               
               
                 Example 2 
                 0 
                 44.33 
                 1.00 
                 30.62 
               
               
                   
                 1000 
                 41.18 
                 0.93 
                 4.44 
               
               
                   
                 2000 
                 39.20 
                 0.88 
                 1.79 
               
               
                   
                 3000 
                 37.48 
                 0.85 
                 0.45 
               
               
                 Example 3 
                 0 
                 44.33 
                 1.00 
                 0.46 
               
               
                   
                 1000 
                 41.40 
                 0.93 
                 0.54 
               
               
                   
                 2000 
                 39.54 
                 0.89 
                 0.59 
               
               
                   
                 3000 
                 38.08 
                 0.86 
                 0.42 
               
               
                   
               
            
           
         
       
     
     Example 4 
     70,000 μFV/g tantalum powder was used to form anode samples. Each anode sample was embedded with a tantalum wire, sintered at 1440° C., and pressed to a density of 5.1 g/cm 3 . The resulting pellets had a size of 5.60×3.65×0.90 mm. The pellets were anodized to 71.0 volts in water/phosphoric acid electrolyte with a conductivity of 8.6 mS at a temperature of 85° C. to form the dielectric layer. The pellets were anodized again to 150 volts in a water/boric acid/disodium tetraborate with a conductivity of 2.0 mS at a temperature of 30° C. for 25 seconds to form a thicker oxide layer built up on the outside. 
     The conductive polymer coating was formed by dipping the anodes into a solution of poly(4-(2,3-dihydrothieno-[3,4-b][1,4]dioxin-2-ylmethoxy)-1-butane-sulphonic acid having a solids content of 1.9% (Clevios™ K, Heraeus). Upon coating, the parts were dried at 125° C. for 20 minutes. This process was repeated 2 times. Thereafter, the parts were dipped into a dispersed poly(3,4-ethylenedioxythiophene) having a solids content 1.1% and viscosity 20 mPa·s (Clevios™ K, Heraeus). Upon coating, the parts were dried at 125° C. for 20 minutes. This process was repeated 8 times. Thereafter, the parts were dipped into a dispersed poly(3,4-ethylenedioxythiophene) having a solids content 2.0% and viscosity 20 mPa·s (Clevios™ K, Heraeus). Upon coating, the parts were dried at 125° C. for 20 minutes. This process was repeated 3 times. Thereafter, the parts were dipped into a dispersed poly(3,4-ethylenedioxythiophene) having a solids content of 2% and viscosity 160 mPa·s (Clevios™ K, Heraeus). 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. 
     Example 5 
     Capacitors were formed in the manner described in Example 4, except that three (3) pre-coat layers were formed using a solution of (3-aminopropyl)-trimethoxysilane in ethanol (1.0%). Furthermore, the 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, Heraeus). 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 2.0% and viscosity 20 mPa·s (Clevios™ K, Heraeus). Upon coating, the parts were dried at 125° C. for 20 minutes. This process was repeated 3 times. Thereafter, the parts were dipped into a dispersed poly(3,4-ethylenedioxythiophene) having a solids content of 2% and viscosity 160 mPa·s (Clevios™ K, Heraeus). 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. 
     Example 6 
     Capacitors were formed in the manner described in Example 4, except that three (3) pre-coat layers were formed using a solution of [3-(2-aminoethylamino)-propyl]trimethoxysilane in ethanol (1.0%). 
     Multiple parts (1300) of 47 μF/35V capacitors were formed as described in Examples 1-3 and encapsulated in a silica resin. The parts were tested for capacitance (CAP), “wet-to-dry” capacitance (W/D), dissipation factor (Df), ESR, and leakage current (DCL). The median results are set forth below in Table 4. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Electrical Properties 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                   
                 1 st  DCL @ 
                 2 nd  DCL @ 
               
               
                   
                 CAP 
                 W/D 
                 Df 
                 ESR 
                 90 sec 
                 180 sec 
               
               
                   
                 [μF] 
                 [%] 
                 [%] 
                 [mohms] 
                 [μA] 
                 [μA] 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
            
               
                 Example 4 
                 50.28 
                 90.09 
                 2.9 
                 129.7 
                 2.16 
                 1.11 
               
               
                 Example 5 
                 43.70 
                 78.29 
                 3.3 
                 121.8 
                 0.07 
                 0.06 
               
               
                 Example 6 
                 47.34 
                 84.82 
                 3.2 
                 151.0 
                 0.55 
                 0.20 
               
               
                   
               
            
           
         
       
     
     Multiple parts (10) of 47 μF/35V capacitors (before encapsulation) were also formed as described in Examples 4-6 and dried at 150° C. for 4 hours. The resulting parts were then placed into a ceramic housing purged with nitrogen gas, and then hermetically sealed by seam welding. The parts were tested for capacitance and leakage current using “life testing” and “pulse testing” as described above. The median results are set forth below in Table 5. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 125° C. Pulse Testing Results 
               
            
           
           
               
               
               
               
               
            
               
                   
                 Time 
                 CAP 
                 Ratio of Cap. 
                 DCL @ 300 sec 
               
               
                   
                 [h] 
                 [μF] 
                 to Initial Cap. 
                 [μA] 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Example 4 
                 0 
                 51.37 
                 1.00 
                 22.87 
               
               
                   
                 1000 
                 48.92 
                 0.95 
                 14.85 
               
               
                   
                 2000 
                 48.10 
                 0.94 
                 3.95 
               
               
                   
                 3000 
                 46.99 
                 0.91 
                 8.86 
               
               
                 Example 5 
                 0 
                 41.62 
                 1.00 
                 0.33 
               
               
                   
                 1000 
                 37.14 
                 0.89 
                 0.24 
               
               
                   
                 2000 
                 34.89 
                 0.84 
                 0.20 
               
               
                   
                 3000 
                 32.48 
                 0.78 
                 0.23 
               
               
                 Example 6 
                 0 
                 46.64 
                 1.00 
                 0.50 
               
               
                   
                 1000 
                 45.65 
                 0.98 
                 0.22 
               
               
                   
                 2000 
                 45.78 
                 0.98 
                 0.18 
               
               
                   
                 3000 
                 45.42 
                 0.97 
                 0.19 
               
               
                   
               
            
           
         
       
     
     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.