Patent Publication Number: US-2021175022-A1

Title: Tantalum Capacitor With Increased Stability

Description:
RELATED APPLICATIONS 
     The present application claims priority to U.S. Provisional Application Ser. No. 62/945,913, filed on Dec. 10, 2019 and 62/947,014, filed on Dec. 12, 2019, which are incorporated herein in their entirety by reference thereto. 
    
    
     BACKGROUND OF THE INVENTION 
     Tantalum capacitors are typically made by pressing a tantalum powder around a metal lead wire, sintering the pressed part, anodizing the sintered anode, and thereafter applying a solid electrolyte. 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 sulfonic acid (“PEDOT:PSS”). While the PEDOT:PSS dispersions can result in improved leakage current values, other problems nevertheless remain. For example, one problem with polymer slurry-based capacitors is that they can exhibit relatively poor capacitance stability when subjected to repeated voltage charge cycles. 
     As such, a need exists for an improved solid electrolytic capacitor that exhibits relatively stable electrical properties. 
     SUMMARY OF THE INVENTION 
     In accordance with one embodiment of the present invention, a solid electrolytic capacitor is disclosed that contains a capacitor element. The capacitor element contains an anode body that contains tantalum, a dielectric that overlies the anode body, and a solid electrolyte that overlies the dielectric. The solid electrolyte includes an intrinsically conductive polymer containing repeating thiophene units. Further, the capacitor exhibits a dielectric strength of about 0.6 volts per nanometer or more. The capacitor also exhibits a charge-discharge capacitance after being subjected to 3,000 cycles of a surge voltage and an initial capacitance prior to being subjected to the surge voltage, wherein the ratio of the charge-discharge capacitance to the initial capacitance is from about 0.75 to 1. 
     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 of the assembly of the present invention; 
         FIG. 2  is a cross-sectional view of another embodiment of a capacitor of the assembly of the present invention; 
         FIG. 3  is a cross-sectional view of yet another embodiment of a capacitor of the assembly of the present invention; and 
         FIG. 4  is a top view of still another embodiment of a capacitor of the assembly of the present invention. 
     
    
    
     Repeat use of references characters in the present specification and drawings is intended to represent same or analogous features or elements of the invention. 
     DETAILED DESCRIPTION OF REPRESENTATIVE EMBODIMENTS 
     It is to be understood by one of ordinary skill in the art that the present discussion is a description of exemplary embodiments only, and is not intended as limiting the broader aspects of the present invention, which broader aspects are embodied in the exemplary construction. 
     Generally speaking, the present invention is directed to a solid electrolytic capacitor that contains a capacitor element including an anode body that includes tantalum, a dielectric overlying the anode body, and a solid electrolyte overlying the dielectric that includes a conductive polymer containing repeating thiophene units. By selectively controlling the particular nature of the capacitor element and the materials from which it is formed, the present inventors have discovered that a capacitor can be formed having a high degree of dielectric strength, which can improve capacitance stability. The “dielectric strength” generally refers to the ratio of the “breakdown voltage” of the capacitor (voltage at which the capacitor fails in volts, “V”) to the thickness of the dielectric (in nanometers, “nm”). The capacitor typically exhibits a dielectric strength of about 0.6 V/nm or more, in some embodiments about 0.65 V/nm or more, in some embodiments about 0.7 V/nm or more, in some embodiments from about 0.75 to about 1 V/nm, and in some embodiments, from about 0.8 to about 0.9 V/nm. The capacitor may, for example, exhibit a relatively high breakdown voltage, such as about 55 volts or more, in some embodiments about 65 volts or more, in some embodiments about 85 volts or more, in some embodiments about 90 volts or more, in some embodiments about 95 volts or more, and in some embodiments, from about 100 volts to about 300 volts, such as determined by increasing the applied voltage in increments of 3 volts until the leakage current reaches 1 mA. While its thickness can generally vary depending on the particular location of the anode body, the “dielectric thickness” for purposes of determining dielectric strength is generally considered as the greatest thickness of the dielectric, which typically is about 60 nm or more, in some embodiments from about 60 to about 500 nm, in some embodiments from about 80 to about 350 nm, and in some embodiments, from about 100 to about 300 nm. 
     The resulting capacitor may be able to maintain stable electrical properties (e.g., capacitance) under a wide variety of different conditions. For example, the ratio of the capacitance after being subjected to repeated cycles of a surge voltage (“charge-discharge capacitance”) to the initial capacitance value prior to such testing may be from about 0.75 to 1, in some embodiments from about 0.8 to 1, in some embodiments from 0.85 to 1, in some embodiments from about 0.9 to 1, in some embodiments from 0.91 to 0.99, and in some embodiments, from 0.92 to 0.99. The surge voltage may be applied for 1,000 to 16,000 cycles (e.g., 1,000, 2,000, 3,000, 4,000, 5,000, 8,000, 12,000, or 16,000 cycles). After 3,000 cycles, for instance, the capacitor may exhibit a charge-discharge capacitance of from 0.75 to 1, in some embodiments from about 0.95 to 1, in some embodiments from about 0.96 to 1, and in some embodiments, from about 0.97 to 1. 
     In addition, the capacitance may also remain stable even after being exposed to a high temperature, such as 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.) for a substantial period of time, such as for about 100 hours or more, and in some embodiments, from about 150 hours to about 3,000 hours (e.g., 3,000 hours). In one embodiment, for example, the ratio of the capacitance after being exposed to the high temperature (e.g., 105° C.) for 3,000 hours to the initial capacitance value (e.g., at 23° C.) is from about 0.7 to 1, in some embodiments from about 0.8 to 1, in some embodiments from about 0.9 to 1, and in some embodiments, from 0.91 to 0.99. The actual capacitance value (dry) may vary, but is typically about 1 milliFarad per square centimeter (“mF/cm 2 ”) or more, in some embodiments about 2 mF/cm 2  or more, in some embodiments from about 5 to about 50 mF/cm 2 , and in some embodiments, from about 8 to about 20 mF/cm 2 , measured at a frequency of 120 Hz. 
     In addition to those noted above, the capacitor may also exhibit other improved electrical properties. For instance, after being subjected to an applied voltage (e.g., 120 volts) for a period of time from about 30 minutes to about 20 hours, in some embodiments from about 1 hour to about 18 hours, and in some embodiments, from about 4 hours to about 16 hours, the capacitor may exhibit a leakage current (“DCL”) of only about 100 microamps (“μA”) or less, in some embodiments about 70 μA or less, and in some embodiments, from about 1 to about 50 μA. Notably, the capacitor may exhibit such low DCL values even under dry conditions, such as described above. 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, in some embodiments from about 0.01 to about 125 mohms, and in some embodiments, from about 0.1 to about 100 mohms, measured at an operating frequency of 100 kHz and temperature of 23° C. 
     The capacitor may also exhibit such ESR values even after being exposed to 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.) for a substantial period of time, such as for about 100 hours or more, and in some embodiments, from about 150 hours to about 3,000 hours (e.g., 3,000 hours). In one embodiment, for example, the ratio of the ESR of the capacitor after being exposed to the high temperature (e.g., 105° C.) for 3,000 hours to the initial ESR value of the capacitor (e.g., at 23° C.) is about 2.0 or less, in some embodiments about 1.5 or less, and in some embodiments, from 1.0 to about 1.3. 
     It is also believed that the dissipation factor of the capacitor 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 is typically about 250% or less, in some embodiments about 200% or less, and in some embodiments, from about 1% to about 180%, as determined at a frequency of 120 Hz. 
     Various embodiments of the capacitor 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 an anode body. The anode body may be in the form of a sheet, foil, mesh, pellet, etc. Regardless of its form, the anode body is typically formed from tantalum. In one embodiment, for instance, the anode body may be in the form of a pellet is formed from a tantalum powder. The tantalum powder may be formed from a reduction process in which a tantalum salt (e.g., potassium fluorotantalate (K 2 TaF 7 ), sodium fluorotantalate (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. 
     When employed, the specific charge of the powder typically varies from about 2,000 to about 600,000 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 600,000 μF*V/g, in some embodiments from about 120,000 to about 500,000 μF*V/g, and in some embodiments, from about 150,000 to about 400,000 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 500 nanometers, in some embodiments from about 10 to about 400 nanometers, and in some embodiments, from about 20 to about 250 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 to the anode body 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 1800° C., in some embodiments from about 800° C. to about 1700° C., and in some embodiments, from about 900° C. to about 1400° 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. As indicated above, the dielectric is formed by anodically oxidizing (“anodizing”) the 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 an electrolyte to the anode, such as by dipping anode into the electrolyte. The electrolyte is generally in the form of a liquid, such as a solution (e.g., aqueous or non-aqueous), dispersion, melt, etc. A solvent is generally employed in the electrolyte, such as water (e.g., deionized water); ethers (e.g., diethyl ether and tetrahydrofuran); glycols (e.g., ethylene glycol, propylene glycol, etc.); alcohols (e.g., methanol, ethanol, n-propanol, isopropanol, and butanol); triglycerides; ketones (e.g., acetone, methyl ethyl ketone, and methyl isobutyl ketone); esters (e.g., ethyl acetate, butyl acetate, diethylene glycol ether acetate, and methoxypropyl acetate); amides (e.g., dimethylformamide, dimethylacetamide, dimethylcaprylic/capric fatty acid amide and N-alkylpyrrolidones); nitriles (e.g., acetonitrile, propionitrile, butyronitrile and benzonitrile); sulfoxides or sulfones (e.g., dimethyl sulfoxide (DMSO) and sulfolane); and so forth. The solvent(s) may constitute from about 50 wt. % to about 99.9 wt. %, in some embodiments from about 75 wt. % to about 99 wt. %, and in some embodiments, from about 80 wt. % to about 95 wt. % of the electrolyte. Although not necessarily required, the use of an aqueous solvent (e.g., water) is often desired to facilitate formation of an oxide. In fact, water may constitute about 1 wt. % or more, in some embodiments about 10 wt. % or more, in some embodiments about 50 wt. % or more, in some embodiments about 70 wt. % or more, and in some embodiments, about 90 wt. % to 100 wt. % of the solvent(s) used in the electrolyte. 
     The electrolyte is electrically conductive and may have an electrical conductivity of about 1 milliSiemens per centimeter (“mS/cm”) or more, in some embodiments about 30 mS/cm or more, and in some embodiments, from about 40 mS/cm to about 100 mS/cm, determined at a temperature of 25° C. To enhance the electrical conductivity of the electrolyte, an ionic compound is generally employed that is capable of dissociating in the solvent to form ions. Suitable ionic compounds for this purpose may include, for instance, acids, such as nitric acid, sulfuric acid, phosphoric acid, polyphosphoric acid, boric acid, boronic acid, etc.; organic acids, including carboxylic acids, such as acrylic acid, methacrylic acid, malonic acid, succinic acid, salicylic acid, sulfosalicylic acid, adipic acid, maleic acid, malic acid, oleic acid, gallic acid, tartaric acid, citric acid, formic acid, acetic acid, glycolic acid, oxalic acid, propionic acid, phthalic acid, isophthalic acid, glutaric acid, gluconic acid, lactic acid, aspartic acid, glutaminic acid, itaconic acid, trifluoroacetic acid, barbituric acid, cinnamic acid, benzoic acid, 4-hydroxybenzoic acid, aminobenzoic acid, etc.; sulfonic acids, such as methanesulfonic acid, benzenesulfonic acid, toluenesulfonic acid, trifluoromethanesulfonic acid, styrenesulfonic acid, naphthalene disulfonic acid, hydroxybenzenesulfonic acid, dodecylsulfonic acid, dodecylbenzenesulfonic acid, etc.; polymeric acids, such as poly(acrylic) or poly(methacrylic) acid and copolymers thereof (e.g., maleic-acrylic, sulfonic-acrylic, and styrene-acrylic copolymers), carageenic acid, carboxymethyl cellulose, alginic acid, etc.; and so forth. The concentration of ionic compounds is selected to achieve the desired electrical conductivity. 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 electrolyte. If desired, blends of ionic compounds may also be employed in the electrolyte. 
     To form the dielectric, a current is typically passed through the electrolyte while it is in contact with the anode body. 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 5 to about 200 V, and in some embodiments, from about 10 to about 150 V. During oxidation, the electrolyte 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 may be 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, the electrolyte employed during at least one stage of the dielectric development process contains an ionic compound as explained above. In one particular embodiment, 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 ionic compound that is an acid, such as 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 may likewise contain an ionic compound that is 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. Pre-Coat 
     If desired, a pre-coat may be employed that overlies the dielectric and positioned between the dielectric and the solid electrolyte. The pre-coat may include an organometallic compound, such as a compound having the following general formula (V): 
     
       
         
         
             
             
         
       
     
     wherein, 
     Z is an organometallic atom, such as silicon, titanium, and so forth; 
     R 1 , R 2 , and R 3  are independently an alkyl (e.g., methyl, ethyl, propyl, etc.) or a hydroxyalkyl (e.g., hydroxymethyl, hydroxyethyl, hydroxypropyl, etc.), wherein at least one of R 1 , R 2 , and R 3  is a hydroxyalkyl; 
     n is an integer from 0 to 8, in some embodiments from 1 to 6, and in some embodiments, from 2 to 4 (e.g.,  3 ); and 
     X is an organic or inorganic functional group, such as glycidyl, glycidyloxy, mercapto, amino, vinyl, etc. 
     In certain embodiments, at least one of R 1 , R 2 , and R 3  in Formula (II) may be 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 ). 
     In certain embodiments, X may be an amino group. 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. 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. 
     The particular manner in which the pre-coat is applied to the capacitor body may vary as desired. In one particular embodiment, the compound is dissolved in an organic solvent and applied to the part as a solution, such as by screen-printing, dipping, electrophoretic coating, spraying, etc. The organic solvent may vary, but is typically an alcohol, such as methanol, ethanol, etc. Organometallic compounds may constitute from about 0.1 wt. % to about 10 wt. %, in some embodiments from about 0.2 wt. % to about 8 wt. %, and in some embodiments, from about 0.5 wt. % to about 5 wt. % of the solution. Solvents may likewise constitute from about 90 wt. % to about 99.9 wt. %, in some embodiments from about 92 wt. % to about 99.8 wt. %, and in some embodiments, from about 95 wt. % to about 99.5 wt. % of the solution. Once applied, the part may then be dried to remove the solvent therefrom and form a pre-coat layer containing the organometallic compound. 
     D. Solid Electrolyte 
     A solid electrolyte overlies the dielectric and optional pre-coat and generally functions as the cathode for the capacitor. Typically, the total thickness of the solid electrolyte is from about 1 to about 50 μm, and in some embodiments, from about 5 to about 20 μm. Typically, the solid electrolyte contains an intrinsically conductive polymer that has a positive charge located on the main chain that is at least partially compensated by anions covalently bound to the polymer. For example, one example of a suitable intrinsically conductive polymer may have repeating thiophene units of the following formula (I): 
     
       
         
         
             
             
         
       
     
     wherein, 
     R is (CH 2 ) a —O—(CH 2 ) b -L, where L is a bond or HC([CH 2 ] c H); 
     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); 
     c is from 0 to 10, in some embodiments from 0 to 6, and in some embodiments, from 1 to 4 (e.g., 1); 
     M is an anion, such as SO 3 , C(O)O, BF 4 , CF 3 SO 3 , SbF 6 , N(SO 2 CF 3 ) 2 , C 4 H 3 O 4 , ClO 4 , 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, M in formula (I) is a sulfonate ion such that the intrinsically conductive polymer contains repeating units of the following formula (II): 
     
       
         
         
             
             
         
       
     
     wherein, R and X are defined above. In formula (I) or (II), 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 (I) 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 (I). 
     In another embodiment, the intrinsically conductive polymer has repeating thiophene units of the following general formula (III): 
     
       
         
         
             
             
         
       
     
     wherein, 
     a and b are as defined above; 
     R 5  is an optionally substituted C 1 -C 6  linear or branched alkyl group (e.g., methyl) or a halogen atom (e.g., fluorine); 
     X is a hydrogen atom, an alkali metal (e.g., Li, Na, or K), NH(R 1 ) 3 , or HNC 5 H 5 , wherein R 1  is each independently a hydrogen atom or an optionally substituted C 1 -C 6  alkyl group. 
     Specific examples of thiophene compounds used to form such repeating are described in U.S. Pat. No. 9,718,905 and may include, for instance, sodium 3-[(2,3-dihydrothieno[3,4-b][1,4]dioxin-2-yl)methoxy]-1-methyl-1-propanesulfonate, sodium 3-[(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl)methoxy]-1-ethyl-1-propanesulfonate, sodium 3-[(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl)methoxy]-1-propyl-1-propane-sulfonate, sodium 3-[(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl)methoxy]-1-butyl-1-propanesulfonate, sodium 3-[(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl)methoxy]-1-pentyl-1-propane-sulfonate, sodium 3-[(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl)methoxy]-1-hexyl-1-propanesulfonate, sodium 3-[(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl)methoxy]-1-isopropyl-1-propanesulfonate, sodium 3-[(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl)methoxy]-1-isobutyl-1-propanesulfonate, sodium 3-[(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl)methoxy]-1-isopentyl-1-propanesulfonate, sodium 3-[(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl)methoxy]-1-fluoro-1-propanesulfonate, potassium 3-[(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl)methoxy]-1-methyl-1-propanesulfonate, 3-[(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl)methoxy]-1-methyl-1-propanesulfonic acid, ammonium 3-[(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl)methoxy]-1-methyl-1-propane-sulfonate, triethylammonium 3-[(2,3-dihydrothieno[3,4-b]-[1,4]dioxin-2-yl)methoxy]-1-methyl-1-propanesulfonate, etc., as well as combination thereof. Each of the above exemplified thiophene monomers may be prepared from thieno[3,4-b]-1,4-dioxin-2-methanol and a branched sultone compound in accordance with a known method (e.g., Journal of Electroanalytical Chemistry, 443, 217 to 226 (1998)). 
     The intrinsically conductive polymer may be formed through a variety of techniques as would be understood by those skilled in the art. In one particular embodiment, for example, a thiophene compound having the general formula (I) may be polymerized in the presence of an oxidative catalyst. Derivatives of these monomers may also be employed that are, for example, dimers or trimers of the above compounds. The derivatives may be made up of identical or different monomer units and used in pure form and in a mixture with one another and/or with the monomers. Oxidized or reduced forms of these precursors may also be employed. The amount of the oxidizing catalyst used in this polymerization reaction is not particularly limited, and may be within a range of from 1 to 50 molar times, more preferably from 1 to 20 molar times to the number of moles of the thiophene compound used as a material charged. The oxidative catalyst may be a transition metal salt, such as a salt of an inorganic or organic acid that contain ammonium, sodium, gold, iron(III), copper(II), chromium(VI), cerium(IV), manganese(IV), manganese(VII), or ruthenium(III) cations. Particularly suitable transition metal salts include halides (e.g., FeCl 3  or HAuCl 4 ); salts of other inorganic acids (e.g., Fe(ClO 4 ) 3 , Fe 2 (SO 4 ) 3 , (NH 4 ) 2 S 2 O 8 , or Na 3 Mo 12 PO 40 ); and salts of organic acids and inorganic acids comprising organic radicals. Examples of salts of inorganic acids with organic radicals include, for instance, iron(Ill) salts of sulfuric acid monoesters of C 1  to C 20  alkanols (e.g., iron(Ill) salt of lauryl sulfate). Likewise, examples of salts of organic acids include, for instance, iron(Ill) salts of C 1  to C 20  alkane sulfonic acids (e.g., methane, ethane, propane, butane, or dodecane sulfonic acid); iron (Ill) salts of aliphatic perfluorosulfonic acids (e.g., trifluoromethane sulfonic acid, perfluorobutane sulfonic acid, or perfluorooctane sulfonic acid); iron (Ill) salts of aliphatic C 1  to C 20  carboxylic acids (e.g., 2-ethylhexylcarboxylic acid); iron (Ill) salts of aliphatic perfluorocarboxylic acids (e.g., trifluoroacetic acid or perfluorooctane acid); iron (Ill) 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 (Ill) salts of cycloalkane sulfonic acids (e.g., camphor sulfonic acid); and so forth. Mixtures of these above-mentioned salts may also be used. 
     Oxidative polymerization generally occurs in the presence of one or more solvents. Suitable solvents may include, for instance, water, glycols (e.g., ethylene glycol, propylene glycol, butylene glycol, triethylene glycol, hexylene glycol, polyethylene glycols, ethoxydiglycol, dipropyleneglycol, etc.); glycol ethers (e.g., methyl glycol ether, ethyl glycol ether, isopropyl glycol ether, etc.); alcohols (e.g., methanol, ethanol, n-propanol, iso-propanol, and butanol); ketones (e.g., acetone, methyl ethyl ketone, and methyl isobutyl ketone); esters (e.g., ethyl acetate, butyl acetate, diethylene glycol ether acetate, methoxypropyl acetate, ethylene carbonate, propylene carbonate, etc.); amides (e.g., dimethylformamide, dimethylacetamide, dimethylcaprylic/capric fatty acid amide and N-alkylpyrrolidones); sulfoxides or sulfones (e.g., dimethyl sulfoxide (DMSO) and sulfolane); phenolic compounds (e.g., toluene, xylene, etc.), and so forth. Water is a particularly suitable solvent for the reaction. The amount of the solvent used in this polymerization reaction is not particularly limited so long as the thiophene compound used as a material is dissolved in the solvent, however, it is preferably from 0.1 to 100 times, more preferably from 0.1 to 50 times the weight of the thiophene compound charged. The temperature at which the reaction occurs typically varies from about −20° C. to about 140° C., and in some embodiments, from about 20° C. to about 100° C. Upon completion of the reaction, known purification techniques may be employed to remove any salt impurities, such by washing with a solvent, re-precipitation, centrifugal sedimentation, ultrafiltration, dialysis or ion exchange resin treatment, etc., as well as combination thereof. 
     Regardless of how it is formed, the polymer is considered “intrinsically” conductive to the extent that it has a positive charge located on the main chain that is at least partially compensated by anions covalently bound to the polymer. The polymer may, for example, have a relatively high specific conductivity, in the dry state, of about 1 Siemen per centimeter (“S/cm”) or more, in some embodiments about 10 S/cm or more, in some embodiments about 25 S/cm or more, in some embodiments about 40 S/cm or more, and in some embodiments, from about 50 to about 500 S/cm. As a result of its intrinsic conductivity, the solid electrolyte does not require the addition of conventional dopants, such as polystyrene sulfonic acid. In fact, the solid electrolyte may be substantially free of such dopants. Nevertheless, it should be understood that dopants may be employed in certain embodiments of the present invention. When utilized, however, dopants are typically present in the solid electrolyte in an amount of about 5 wt. % or less, in some embodiments about 2 wt. % or less, and in some embodiments, about 1 wt. % or less. 
     The polymer is also generally highly soluble in water, which enables it to be more easily and effectively applied to the anode. The soluble polymer is also able to more readily impregnate the small pores formed by the high specific charge powder, so that the resulting solid electrolyte has a “film-like” configuration and coats at least a portion of the anode in a substantially uniform manner. This improves the quality of the resulting oxide as well as its surface coverage, and thereby enhances the electrical properties of the capacitor assembly. 
     i. Inner Layers 
     The solid electrolyte is generally formed from one or more “inner” conductive polymer layers. The term “inner” in this context refers to one or more layers formed from the same material and that overly the dielectric, whether directly or via another layer (e.g., pre-coat). The inner layer(s), for example, typically contain an intrinsically conductive polymer such as described above. In one particular embodiment, the inner layer(s) are generally free of extrinsically conductive polymers and thus formed primarily from intrinsically conductive polymers. More particularly, intrinsically conductive polymers may constitute about 50 wt. % or more, in some embodiments about 70 wt. % or more, and in some embodiments, about 90 wt. % or more (e.g., 100 wt. %) of the inner layer(s). One or multiple inner layers may be employed. For example, the solid electrolyte typically contains from 2 to 30, in some embodiments from 4 to 20, and in some embodiments, from about 5 to 15 inner layers (e.g., 10 layers). 
     The inner layer(s) may be applied in the form of a solution containing a solvent. The concentration of the polymer may vary depending on the desired viscosity of and the particular manner in which the layer is to be applied to the anode. Typically, however, the polymer constitutes 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 solution. Solvent(s) may likewise constitute from about 90 wt. % to about 99.9 wt. %, in some embodiments from about 95 wt. % to about 99.6 wt. %, and in some embodiments, from about 96 wt. % to about 99.5 wt. % of the solution. While other solvents may certainly be employed, it is generally desired that water is the primary solvent such that the solution is considered an “aqueous” solution. In most embodiments, for example, water constitutes at least about 50 wt. %, in some embodiments at least about 75 wt. %, and in some embodiments, from about 90 wt. % to 100 wt. % of the solvent(s) employed. When employed, a solution may be applied to the anode using any known technique, such as dipping, casting (e.g., curtain coating, spin coating, etc.), printing (e.g., gravure printing, offset printing, screen printing, etc.), and so forth. The resulting conductive polymer layer may be dried and/or washed after it is applied to the anode. 
     ii. Outer Layers 
     The solid electrolyte may contain only “inner layers” so that it is essentially formed from the same material, i.e., intrinsically conductive polymers. Nevertheless, in other embodiments, the solid electrolyte may also contain one or more optional “outer” conductive polymer layers that are formed from a different material than the inner layer(s) and overly the inner layer(s). For example, the outer layer(s) may be formed from an extrinsically conductive polymer. In one particular embodiment, the outer layer(s) are formed primarily from such extrinsically conductive polymers in that they constitute about 50 wt. % or more, in some embodiments about 70 wt. % or more, and in some embodiments, about 90 wt. % or more (e.g., 100 wt. %) of a respective outer layer. One or multiple outer layers may be employed. For example, the solid electrolyte may contain from 2 to 30, in some embodiments from 4 to 20, and in some embodiments, from about 5 to 15 outer layers. 
     When employed, the extrinsically conductive polymer may, for instance, have 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.); C 1  to C 4  hydroxyalkyl radical, or hydroxyl radical; and 
     q is an integer from 0 to 8, in some embodiments, from 0 to 2, and in one embodiment, 0. 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 require the presence of a separate counterion that is not covalently bound to the polymer. The counterion may be a monomeric or polymeric anion that counteracts the charge of the conductive polymer. Polymeric anions can, for example, be anions of polymeric carboxylic acids (e.g., polyacrylic acids, polymethacrylic acid, polymaleic acids, etc.); polymeric sulfonic acids (e.g., polystyrene sulfonic acids (“PSS”), polyvinyl sulfonic acids, etc.); and so forth. The acids may also be copolymers, such as copolymers of vinyl carboxylic and vinyl sulfonic acids with other polymerizable monomers, such as acrylic acid esters and styrene. Likewise, suitable monomeric anions include, for example, anions of C 1  to C 20  alkane sulfonic acids (e.g., dodecane sulfonic acid); aliphatic perfluorosulfonic acids (e.g., trifluoromethane sulfonic acid, perfluorobutane sulfonic acid or perfluorooctane sulfonic acid); aliphatic C 1  to C 20  carboxylic acids (e.g., 2-ethyl-hexylcarboxylic acid); aliphatic perfluorocarboxylic acids (e.g., trifluoroacetic acid or perfluorooctanoic acid); aromatic sulfonic acids optionally substituted by C 1  to C 20  alkyl groups (e.g., benzene sulfonic acid, o-toluene sulfonic acid, p-toluene sulfonic acid or dodecylbenzene sulfonic acid); cycloalkane sulfonic acids (e.g., camphor sulfonic acid or tetrafluoroborates, hexafluorophosphates, perchlorates, hexafluoroantimonates, hexafluoroarsenates or hexachloroantimonates); and so forth. Particularly suitable counteranions are polymeric anions, such as a polymeric carboxylic or sulfonic acid (e.g., polystyrene sulfonic acid (“PSS”)). The molecular weight of such polymeric anions typically ranges from about 1,000 to about 2,000,000, and in some embodiments, from about 2,000 to about 500,000. 
     When employed, it may be desirable that the extrinsically conductive polymer is applied in the form of a dispersion of pre-polymerized conductive particles. Such particles typically have an average size (e.g., diameter) of from about 1 to about 100 nanometers, in some embodiments from about 2 to about 80 nanometers, and in some embodiments, from about 4 to about 50 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 element. 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 binders to further enhance the adhesive nature of the polymeric layer and also increase the stability of the particles within the dispersion. The binders may be organic in nature, such as polyvinyl alcohols, polyvinyl pyrrolidones, polyvinyl chlorides, polyvinyl acetates, polyvinyl butyrates, polyacrylic acid esters, polyacrylic acid amides, polymethacrylic acid esters, polymethacrylic acid amides, polyacrylonitriles, styrene/acrylic acid ester, vinyl acetate/acrylic acid ester and ethylene/vinyl acetate copolymers, polybutadienes, polyisoprenes, polystyrenes, polyethers, polyesters, polycarbonates, polyurethanes, polyamides, polyimides, polysulfones, melamine formaldehyde resins, epoxide resins, silicone resins or celluloses. Crosslinking agents may also be employed to enhance the adhesion capacity of the binders. Such crosslinking agents may include, for instance, melamine compounds, masked isocyanates or functional silanes, such as 3-glycidoxypropyltrialkoxysilane, tetraethoxysilane and tetraethoxysilane hydrolysate or crosslinkable polymers, such as polyurethanes, polyacrylates or polyolefins, and subsequent crosslinking. 
     Dispersion agents may also be employed to facilitate the 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-mercaptopropyltrimethoxysilane, 3-metacryloxypropyltrimethoxysilane, vinyltrimethoxysilane or octyltriethoxysilane. The dispersion may also contain additives that increase conductivity, such as ether group-containing compounds (e.g., tetrahydrofuran), lactone group-containing compounds (e.g., γ-butyrolactone or γ-valerolactone), amide or lactam group-containing compounds (e.g., caprolactam, N-methylcaprolactam, N,N-dimethylacetamide, N-methylacetamide, N,N-dimethylformamide (DMF), N-methylformamide, N-methylformanilide, N-methylpyrrolidone (NMP), N-octylpyrrolidone, or pyrrolidone), sulfones and sulfoxides (e.g., sulfolane (tetramethylenesulfone) or dimethylsulfoxide (DMSO)), sugar or sugar derivatives (e.g., saccharose, glucose, fructose, or lactose), sugar alcohols (e.g., sorbitol or mannitol), furan derivatives (e.g., 2-furancarboxylic acid or 3-furancarboxylic acid), an alcohols (e.g., ethylene glycol, glycerol, di- or triethylene glycol). 
     The 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 1 to about 10,000 mPas, in some embodiments from about 10 to about 1,500 mPas, and in some embodiments, from about 100 to about 1000 mPas. 
     If desired, a hydroxyl-functional nonionic polymer may also be employed in the outer layer(s) of 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 outer layers in a variety of different ways. In certain embodiments, for instance, the nonionic polymer may simply be incorporated into the dispersion of extrinsically 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, screen printing or tampon printing. Solvents known to the person skilled in the art can be employed in the solution, such as water, alcohols, or a mixture thereof. The concentration of the 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. 
     E. External Polymer Coating 
     If desired, an external polymer coating may also be applied to the anode that overlies the solid electrolyte. When employed, the external polymer coating generally contains one or more layers formed from conductive polymer particles, such as described above (e.g., formed from an extrinsically conductive polymer). 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 any optional particles employed in the solid electrolyte (e.g., in the outer layer(s)). For example, the ratio of the average size of the particles employed in the external polymer coating to the average size of any particles employed in 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 external coating may have an average size of from about 50 to about 800 nanometers, in some embodiments from about 80 to about 600 nanometers, and in some embodiments, from about 100 to about 500 nanometers. 
     A crosslinking agent may also be optionally employed in the external polymer coating to further 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. 
     F. Cathode Coating 
     If desired, the capacitor element may also employ a cathode coating that overlies the solid electrolyte and external polymer coating. The cathode coating may contain a metal particle layer includes a plurality of conductive metal particles dispersed within a polymer matrix. The particles typically constitute from about 50 wt. % to about 99 wt. %, in some embodiments from about 60 wt. % to about 98 wt. %, and in some embodiments, from about 70 wt. % to about 95 wt. % of the layer, while the polymer matrix typically constitutes from about 1 wt. % to about 50 wt. %, in some embodiments from about 2 wt. % to about 40 wt. %, and in some embodiments, from about 5 wt. % to about 30 wt. % of the layer. 
     The conductive metal particles may be formed from a variety of different metals, such as copper, nickel, silver, nickel, zinc, tin, lead, copper, aluminum, molybdenum, titanium, iron, zirconium, magnesium, etc., as well as alloys thereof. Silver is a particularly suitable conductive metal for use in the layer. The metal particles often have a relatively small size, such as an average size of from about 0.01 to about 50 micrometers, in some embodiments from about 0.1 to about 40 micrometers, and in some embodiments, from about 1 to about 30 micrometers. Typically, only one metal particle layer is employed, although it should be understood that multiple layers may be employed if so desired. The total thickness of such layer(s) is typically within the range of from about 1 μm to about 500 μm, in some embodiments from about 5 μm to about 200 μm, and in some embodiments, from about 10 μm to about 100 μm. 
     The polymer matrix typically includes a polymer, which may be thermoplastic or thermosetting in nature. Typically, however, the polymer is selected so that it can act as a barrier to electromigration of silver ions, and also so that it contains a relatively small amount of polar groups to minimize the degree of water adsorption in the cathode coating. In this regard, the present inventors have found that vinyl acetal polymers are particularly suitable for this purpose, such as polyvinyl butyral, polyvinyl formal, etc. Polyvinyl butyral, for instance, may be formed by reacting polyvinyl alcohol with an aldehyde (e.g., butyraldehyde). Because this reaction is not typically complete, polyvinyl butyral will generally have a residual hydroxyl content. By minimizing this content, however, the polymer can possess a lesser degree of strong polar groups, which would otherwise result in a high degree of moisture adsorption and result in silver ion migration. For instance, the residual hydroxyl content in polyvinyl acetal may be about 35 mol. % or less, in some embodiments about 30 mol. % or less, and in some embodiments, from about 10 mol. % to about 25 mol. %. One commercially available example of such a polymer is available from Sekisui Chemical Co., Ltd. under the designation “BH-S” (polyvinyl butyral). 
     To form the cathode coating, a conductive paste is typically applied to the capacitor that overlies the solid electrolyte. One or more organic solvents are generally employed in the paste. A variety of different organic solvents may generally be employed, such as glycols (e.g., propylene glycol, butylene glycol, triethylene glycol, hexylene glycol, polyethylene glycols, ethoxydiglycol, and dipropyleneglycol); glycol ethers (e.g., methyl glycol ether, ethyl glycol ether, and isopropyl glycol ether); ethers (e.g., diethyl ether and tetrahydrofuran); alcohols (e.g., benzyl alcohol, methanol, ethanol, n-propanol, iso-propanol, and butanol); triglycerides; ketones (e.g., acetone, methyl ethyl ketone, and methyl isobutyl ketone); esters (e.g., ethyl acetate, butyl acetate, diethylene glycol ether acetate, and methoxypropyl acetate); amides (e.g., dimethylformamide, dimethylacetamide, dimethylcaprylic/capric fatty acid amide and N-alkylpyrrolidones); nitriles (e.g., acetonitrile, propionitrile, butyronitrile and benzonitrile); sulfoxides or sulfones (e.g., dimethyl sulfoxide (DMSO) and sulfolane); etc., as well as mixtures thereof. The organic solvent(s) typically constitute from about 10 wt. % to about 70 wt. %, in some embodiments from about 20 wt. % to about 65 wt. %, and in some embodiments, from about 30 wt. % to about 60 wt. %. of the paste. Typically, the metal particles constitute from about 10 wt. % to about 60 wt. %, in some embodiments from about 20 wt. % to about 45 wt. %, and in some embodiments, from about 25 wt. % to about 40 wt. % of the paste, and the resinous matrix constitutes from about 0.1 wt. % to about 20 wt. %, in some embodiments from about 0.2 wt. % to about 10 wt. %, and in some embodiments, from about 0.5 wt. % to about 8 wt. % of the paste. 
     The paste may have a relatively low viscosity, allowing it to be readily handled and applied to a capacitor element. The viscosity may, for instance, range from about 50 to about 3,000 centipoise, in some embodiments from about 100 to about 2,000 centipoise, and in some embodiments, from about 200 to about 1,000 centipoise, such as measured with a Brookfield DV-1 viscometer (cone and plate) operating at a speed of 10 rpm and a temperature of 25° C. If desired, thickeners or other viscosity modifiers may be employed in the paste to increase or decrease viscosity. Further, the thickness of the applied paste may also be relatively thin and still achieve the desired properties. For example, the thickness of the paste may be from about 0.01 to about 50 micrometers, in some embodiments from about 0.5 to about 30 micrometers, and in some embodiments, from about 1 to about 25 micrometers. Once applied, the metal paste may be optionally dried to remove certain components, such as the organic solvents. For instance, drying may occur at a temperature of from about 20° C. to about 150° C., in some embodiments from about 50° C. to about 140° C., and in some embodiments, from about 80° C. to about 130° C. 
     G. Other Components 
     If desired, the capacitor may also contain other layers as is known in the art. In certain embodiments, for instance, a carbon layer (e.g., graphite) may be positioned between the solid electrolyte and the silver layer that can help further limit contact of the silver layer with the solid electrolyte. In addition, a pre-coat layer may also be employed that overlies the dielectric and includes an organometallic compound. 
     II. Terminations 
     Once formed, the capacitor element may be provided with terminations, particularly when employed in surface mounting applications. For example, the capacitor 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 coupling agent (e.g., silane coupling agents). Suitable conductive adhesives may be described in U.S. Patent Application Publication No. 2006/0038304 to Osako, et al. 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 
     The capacitor element may be incorporated 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. 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. 
     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 can be selectively controlled so that it is dry, which limits the degree of moisture that can contact the capacitor element. For example, the moisture content of the atmosphere (expressed in terms of relative humidity) may be about 10% or less, in some embodiments about 5% or less, in some embodiments about 3% or less, and in some embodiments, from about 0.001 to about 1%. For example, the atmosphere 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  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 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 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. 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  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  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 possess 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  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. 
     Still other possible housing configurations may be employed in the present invention. For example,  FIG. 3  shows a capacitor  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   a  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. 
     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  400  that contains two capacitor elements is shown and will now be described in more detail. More particularly, the capacitor  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 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  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. 
     The present invention may be better understood with reference to the following examples. 
     Test Procedures 
     Capacitance 
     The capacitance was measured using a Keithley 3330 Precision LCZ meter with Kelvin Leads with 2.2 volt DC bias and a 0.5 volt peak to peak sinusoidal signal. The operating frequency was 120 Hz and the temperature may be 23° C.±2° C. 
     Breakdown Voltage 
     The breakdown voltage was measured using Keithley 2400 SourceMeter at the temperature 23° C. 12° C. An individual capacitor is charged with constant current determined by the equation: 
       Current( A )=Nominal Capacitance( F )× dU/dt,  
 
     where dU/dt represents voltage slope typically set to 10 V/s. Voltage is measured during charging and when applied voltage decreases more than 10%, the maximum achieved voltage value is recorded as breakdown voltage. 
     Equivalent Series Resistance (ESR) 
     Equivalence series resistance may be measured using a Keithley 3330 Precision LCZ meter with Kelvin Leads 2.2 volt DC bias and a 0.5 volt peak to peak sinusoidal signal. The operating frequency was 100 kHz and the temperature was 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. 
     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 after a minimum of 60 seconds. 
     Surge Voltage Testing 
     Surge voltage testing may be conducted (10-25 parts) at a temperature of 85° C.±3° C. and the rated voltage multiple by 1.3 (e.g., 45.5V). The resistance used in test circuit may be 33 ohms. Each cycle consists of 30 seconds surge voltage application followed by a 30 second discharge period. Tested samples are dried before testing at 125° C. for at least 12 hours. The capacitance may be measured at each 1,000 pulse cycles up to 3,000 or 5,000 pulses after the recovery time. 
     Dielectric Thickness 
     The dielectric thickness may be measured using a Zeiss Sigma FESEM at 20,000× to 50,000× magnification. Samples may be prepared by cutting a finished part in plane perpendicular to the longest dimension of the finished part. A thickness measurement may be performed at sites where the cut was perpendicular direction through the dielectric layer. 
     Example 1 
     40,000 μFV/g tantalum powder was used to form anode samples. Each anode sample was embedded with a tantalum wire, pressed to a density of 5.3 g/cm 3  and sintered at 1380° C. The resulting pellets had a size of 5.60×3.65×0.72 mm. The pellets were anodized to 76.0 volts in water/phosphoric acid electrolyte with a conductivity of 8.6 mS at a temperature of 40° 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 10 seconds to form a thicker oxide layer built up on the outside. Upon anodization, four pre-coat layers of organometallic compound were used that contained a solution of (3-aminopropyl)triethoxysilane in ethanol (1.0%). A 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. Upon coating, the parts were dried at 125° C. for 15 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 15 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 15 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 15 minutes. This process was repeated 14 times. The parts were then dipped into a graphite dispersion and dried. Finally, the parts were dipped into a silver dispersion and dried. Multiple parts (450) of 47 μF/35V capacitors were made in this manner and encapsulated in a silica resin. 
     Example 2 
     Capacitors were formed in the manner described in Example 1, except that a different conductive polymer coating was used. Namely, 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. Upon coating, the parts were dried at 125° C. for 15 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 15 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 15 minutes. This process was repeated 14 times. The parts were then dipped into a graphite dispersion and dried. Finally, the parts were dipped into a silver dispersion and dried. Multiple parts (450) of 47 μF/35V capacitors were made in this manner and encapsulated in a silica resin. 
     Example 3 
     40,000 μFV/g tantalum powder was used to form anode samples. Each anode sample was embedded with a tantalum wire, pressed to a density of 5.3 g/cm 3  and sintered at 1410° C. The resulting pellets had a size of 5.60×3.65×0.80 mm. The pellets were anodized to 76.0 volts in water/phosphoric acid electrolyte with a conductivity of 8.6 mS at a temperature of 40° 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 5 seconds to form a thicker oxide layer built up on the outside. Upon anodization, four pre-coat layers of organometallic compound were used that contained a solution of (3-aminopropyl)trirnethoxysilane in ethanol (1.0%). A 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)-2-butane-sulphonic acid. Upon coating, the parts were dried at 125° C. for 15 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 15 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 15 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 15 minutes. This process was repeated 14 times. The parts were then dipped into a graphite dispersion and dried. Finally, the parts were dipped into a silver dispersion and dried. Multiple parts (450) of 47 μF/35V capacitors were made in this manner and encapsulated in a silica resin. 
     Example 4 
     Capacitors were formed in the manner described in Example 3, except that a different conductive polymer coating was used. Namely, 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)-2-butane-sulphonic acid. Upon coating, the parts were dried at 125° C. for 15 minutes. This process was repeated 6 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 15 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 15 minutes. This process was repeated 14 times. The parts were then dipped into a graphite dispersion and dried. Finally, the parts were dipped into a silver dispersion and dried. Multiple parts (450) of 47 μF/35V capacitors were made in this manner and encapsulated in a silica resin. 
     The minimum, mean and maximum measured BDV values and average dielectric thickness are set forth below in Table 1. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 BDV, Dielectric Thickness And Calculated Dielectric Strength 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                   
                   
                   
                 Mean 
                   
               
               
                   
                 Minimum 
                 Mean 
                 Maximum 
                 Dielectric 
                 Dielectric 
               
               
                   
                 BDV 
                 BDV 
                 BDV 
                 Thickness 
                 Strength 
               
               
                   
                 [V] 
                 [V] 
                 [V] 
                 [nm] 
                 [V/nm] 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 1 
                 68.4 
                 70.4 
                 72.5 
                 113.1 
                 0.60 
               
               
                 Example 2 
                 68.3 
                 69.8 
                 72.4 
                 112.9 
                 0.60 
               
               
                 Example 3 
                 69.9 
                 76.3 
                 85.7 
                 113.3 
                 0.62 
               
               
                 Example 4 
                 74.2 
                 77.7 
                 80.7 
                 113.2 
                 0.66 
               
               
                   
               
            
           
         
       
     
     The median results of capacitance within the surge voltage testing are set forth below in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Surge Voltage Testing Results 
               
            
           
           
               
               
               
               
            
               
                   
                   
                   
                 Ratio of Capacitance 
               
               
                   
                   
                 Capacitance median 
                 After Surge Testing 
               
               
                   
                 Cycles 
                 (μF) 
                 to Initial Capacitance 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                 Example 1 
                 0 
                 44.54 
                 — 
               
               
                   
                 1,000 
                 42.69 
                 0.96 
               
               
                   
                 2,000 
                 41.08 
                 0.92 
               
               
                   
                 3,000 
                 39.08 
                 0.88 
               
               
                 Example 2 
                 0 
                 45.81 
                 — 
               
               
                   
                 1,000 
                 45.22 
                 0.99 
               
               
                   
                 2,000 
                 44.58 
                 0.97 
               
               
                   
                 3,000 
                 43.54 
                 0.95 
               
               
                 Example 3 
                 0 
                 48.11 
                 — 
               
               
                   
                 1,000 
                 43.60 
                 0.91 
               
               
                   
                 2,000 
                 40.37 
                 0.84 
               
               
                   
                 3,000 
                 37.27 
                 0.77 
               
               
                 Example 4 
                 0 
                 48.17 
                 — 
               
               
                   
                 1,000 
                 47.81 
                 0.99 
               
               
                   
                 2,000 
                 47.03 
                 0.98 
               
               
                   
                 3,000 
                 47.10 
                 0.98 
               
               
                   
               
            
           
         
       
     
     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 byway of example only, and is not intended to limit the invention so further described in such appended claims.