The present invention is related to an electrolyte solution for anodizing aluminum anode foil for use in electrolytic capacitors and the capacitors containing this anode foil.
We have found that low water content variations of the glycerine and orthophosphate-containing electrolytes described in U.S. Pat. No. 6,409,905, which is incorporated herein by reference thereto, may be used for the anodization of aluminum foil to voltages sufficiently high to facilitate the use of the aforementioned foil in intermediate and high voltage electrolytic capacitors.
Previously, the maximum anodizing voltage obtainable from the aqueous phosphate solutions traditionally used to anodize aluminum capacitor foil for applications requiring extreme foil stability and oxide hydration resistance was about 220 volts, as stated in U.S. Pat. No. 3,733,291. The corrosion of the foil being anodized in aqueous phosphate solutions increases with the anodizing voltage and is sufficiently severe to result in dielectric failure above about 220 volts. The corrosion by-products formed during aluminum foil anodizing in aqueous phosphate solutions must be removed from the solution via filtering, etc., or they will deposit upon the foil and anodizing tank components in amounts sufficient to interfere with the anodizing process.
The difficulties encountered with aqueous phosphate anodizing of aluminum foil for use in relatively low voltage capacitors are such that, in spite of the superior electrical stability of foil anodized in phosphate solutions nearly all of the low voltage foil produced today is anodized in non-phosphate solutions with the exception of a relatively small amount of phosphate which may be present to help impart hydration resistance. Due to the voltage limitations of aqueous phosphate anodizing solutions mentioned above, intermediate and high voltage capacitor foils have not traditionally been anodized in aqueous phosphate solutions.
Aluminum electrolytic capacitors for use at intermediate voltages typically contain anode foil hydrated by passing the foil through a hot water bath prior to anodizing, as defined in U.S. Pat. No. 4,582,574. These capacitors are typically for use at voltages from 150 to 250 volts and contain anode foil anodized to about 200 to 350 volts. This pre-anodizing hydration step is carried out in order to reduce the amount of electric current required to form the anodic oxide dielectric layer and is normally applied to foils to be anodized to 200 volts and above, as described in U.S. Pat. No. 4,481,073. By carefully adjusting the parameters of the pre-anodizing hydration process, as described in U.S. Pat. No. 4,242,575, the hydration process may be successfully employed with foils which are anodized to voltages significantly less than 200 volts. The energy savings associated with the pre-anodizing hydration process is sufficiently great that the vast majority of aluminum foil manufactured today is processed in this manner.
The crystallinity of the anodic oxide present on aluminum anode foil is another factor directly determining the cost of the foil for a given rating of capacitor. Crystalline anodic aluminum oxide has a higher withstanding voltage per unit thickness than does amorphous anodic aluminum oxide. As a result of the higher withstanding voltage of crystalline oxide, only about 10 angstroms of crystalline oxide is required to support each volt of applied field during anodizing as compared with approximately 14 angstroms for each volt of applied field for amorphous oxide. As a result of the higher withstanding voltage of crystalline anodic aluminum oxide, the capacitance of anode foil coated with crystalline oxide may be as much as about 40% higher than anode foil anodized to the same voltage but coated with amorphous oxide.
Crystalline anodic aluminum oxide may be readily produced by anodizing aluminum anode foil in solutions containing salts of dicarboxylic acids as the primary ionogen, as described in U.S. Pat. No. 4,481,084. Anodic oxide formation in solutions of dicarboxylic acid salts (generally at 70–95° C.) may be combined with a pre-anodizing foil hydration step to achieve a very significant savings in both energy and foil consumed per unit capacitance at a given anodizing voltage.
Hydration resistance, which is an important consideration for foil used in electrolytic capacitors, may be enhanced by the inclusion of a small amount of an alpha-hydroxy carboxylic acid (such as tartaric acid or citric acid) in the anodizing electrolyte solution, as described in U.S. Pat. No. 4,481,084. The tendency of anodic aluminum oxide to absorb water, forming a variety of hydrated species having impaired dielectric properties appears to be, at least in part, a function of the hydration status of the outermost portion of the anodic oxide at the end of the anodizing process. Lilienfeld, in U.S. Pat. No. 2,826,724 states that “it is the hydration stratum of the oxide film, adjacent the film-electrolyte interface, which causes most of the power loss; and that the progressive development of hydration at the interface causes the aforesaid instability.”
Alwitt, in U.S. Pat. No. 3,733,291, describes a method of removing the residual hydration layer from the outer surface of anodized aluminum capacitor foil which has been exposed to a pre-anodizing hydration step (Alwitt refers to this as a “preboil”) prior to anodizing in order to conserve electrical energy during anodizing. Alwitt employs a dilute phosphoric acid solution, generally with a small chromate content (to inhibit corrosion), to dissolve the outer, hydration layer.
In addition to the problems associated with the residual hydration layer on anodized foil, which has been processed through a pre-anodizing hydration or preboil step prior to anodizing, there exists another potential problem with the stability of the anodic oxide grown on preboiled aluminum foil. The formation of the anodic oxide on preboiled foil takes place via a dehydration reaction in which the layer of pseudoboehmite (i.e. hydration product) is progressively dehydrated from the foil-oxide interface outward. Apparently, the dehydration does not take place through the ejection of water molecules but rather through the ejection of hydrogen ions and the liberation of oxygen gas within the body of the oxide. The liberated oxygen gas may become trapped within the anodic oxide, rendering the oxide susceptible to cracking and dielectric failure in service. This topic is treated well in the article, entitled: “Trapped Oxygen in Aluminum Oxide Films and Its Effect on Dielectric Stability”, by Walter J. Bernard and Philip G. Russell (Journal of the Electrochemical Society, Volume 127, number 6, June 1980, pages 1256–1261).
Stevens and Shaffer describe a method of determining the concentration of oxide flaws as a function of distance from the metal-oxide interface for trapped-oxygen flaws which are exposed via thermal relaxation steps followed by re-anodizing under carefully controlled and monitored conditions (“Defects in Crystalline Anodic Aluminum”, by J. L. Stevens and J. S. Shaffer, Journal of the Electrochemical Society, volume 133, number 6, June 1986, pages 1160–1162).
Stabilization processes have been developed which tend to expose and repair trapped oxygen flaws (in anodic oxide films on preboiled foils) as well as impart hydration resistance to the oxide film. Examples of these processes are described in U.S. Pat. Nos. 4,113,579 and 4,437,946.
For maximum anodic oxide film stability on aluminum foil, it is desirable to form the anodic film in a phosphate solution and, again, for maximum stability (i.e., freedom from trapped oxygen flaws) the foil should not be preboiled prior to the anodizing process.
The skilled artisan has therefore been limited in the ability to form oxides on the anode at high voltage, particularly with phosphate incorporation into the oxide layer.