Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Forming nonmetal coating
Reexamination Certificate
2000-10-23
2002-02-12
Wong, Edna (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Forming nonmetal coating
C205S332000, C205S333000, C205S324000, C252S062200, C148S277000, C148S284000, C148S285000
Reexamination Certificate
active
06346185
ABSTRACT:
FIELD OF THE INVENTION
The invention is directed to a method and electrolyte for anodizing valve metals to high voltage.
BACKGROUND OF THE INVENTION
The dawn of radio broadcasting in the first decades of the twentieth century, and the growing demand for fractional horsepower motors requiring phase-splitting capacitors during this same period, combined to give impetus to the development and sale of high voltage aluminum electrolytic capacitors. These devices, which employ a thin film of anodic aluminum oxide as the capacitor dielectric, have found increasing use throughout the decades of the twentieth century and are now used in electric welders, electric vehicles, and other industrial applications. The relatively high capacitance per unit volume and high voltage capability at relatively low cost also make these devices useful for other applications, such as weaponry (so-called “rail guns”, etc.) and medical applications where they are used in implantable defibrillators which are often incorporated into heart pacemaker designs.
The performance and cost effectiveness of aluminum electrolytic capacitors depends upon the production of high quality anodic oxide films on the capacitors anode foils. The production process and electrolytes which have been used to produce these films since the 1930's are described in the chapters on foil anodizing in the books by Deeley and Georgiev: “Electrolytic Capacitors—The Theory, Construction, Characteristics, and Application of All Types” by Paul McKnight Deeley, published by The Cornell-Dubilier Electric Corp., South PlainField, N.J., 1938; and “The Electrolytic Capacitor” by Alexander M. Georgiev published by The Technical Division of Murray Hill Books, Inc., New York, 1945.
These works describe the use of a series of electrolyte solutions through which the electrified anode foil is passed to produce the anodic oxide film. The solutions contain boric acid and borax (hydrated sodium tetraborate) in varying concentrations so as to produce a series of electrolyte baths of increasingly higher resistivity. The foil passing through these electrolyte solution baths is biased positive and the baths are biased at progressively higher negative potential such that the anodic oxide film grows progressively thicker as the foil passes through baths containing electrolytes of progressively higher resistivity and biased to progressively higher voltages. The last electrolyte solution may contain only boric acid and have a resistivity of several thousand ohm-cm at 90° C. Deeley describes the voltage capability of this method/electrolyte combination as about 800 volts.
The above-cited volumes describe many ionogens which may be employed for anodizing capacitor foil to relatively low voltages (i.e., up to 300 or 400 volts), such as phosphate salts, the salts of organic acids, etc., but high voltage (above about 500 volts) anodizing has been carried-out almost exclusively in aqueous boric acid solutions.
Relatively recently, a method and aqueous electrolyte suitable for anodizing aluminum to approximately 900 to 1000 volts has appeared in the technical literature (“The Use of Synthetic Hydrotalcite as a Chloride Ion Getter for a Barrier Aluminum Anodization Process”, by J. K. G. Panitz, D. J. Sharp, and Brian Melody,
Plating and Surface Finishing
, December, 1996, pages 52-56). This method basically consists of anodizing aluminum at constant current in a room temperature dilute (high resistivity) solution of ammonium tartrate which has been depleted of chlorine ions down to approximately the 10 ppb level through the use of synthetic hydrotalcite scavenging. While this method of anodizing avoids the use of concentrated boric acid solutions (100 to 200 grams/liter) for the highest voltages, it has proven difficult to apply to reel-to-reel anodizing and is, by its nature, better suited to “batch anodizing”, in which pre-cut foils are anodized while suspended in the anodizing tank (instead of continuously advancing, as in the reel-to-reel anodizing).
It has been realized for many years that high-voltage anodic oxide films of high quality can be readily produced in organic solvent solutions of relatively low free water content. In U.S. Pat. No. 1,710,073, Samuel Ruben describes the use of glycerine solutions of boric acid and borax as a “fill” or “working” electrolyte for electrolytic capacitors. This electrolyte may be used to anodize aluminum to a few hundred volts with good results.
By 1930, Ruben had realized that the inherent limitations of glycerine/borate electrolytes, i.e., the tendency of these electrolytes to oxidize with the production of brown deposits on the anode foil surface during aluminum anodizing above about 150 to 200 volts, and obtained U.S. Pat. No. 1,891,207, which covers the use of ethylene glycol, boric acid, and ammonium borate electrolyte solutions (claim
7
and others). As indicated in the Georgiev book, cited above, (on page 72) properly formulated glycol-borate electrolytes are capable of anodizing aluminum to several hundred volts, with a maximum voltage capability of approximately 600 volts.
The ethylene glycol-based electrolytes must be almost totally esterified to the glycol-borate polyester for use as anodizing electrolytes above about 400 volts. If a significant amount of free ethylene glycol is present, the anodic oxidation products (oxalic acid, formic acid, etc.) of the glycol will attack the aluminum anode material. This corrosive attack by ethylene glycol oxidation products is described in “The Potential for Positive Tab Corrosion in High Voltage Aluminum Electrolytic Capacitors Caused By Electrolytic Decomposition Products”, by Brian Melody, presented at the 1993 Capacitor and Resistor Technology Symposium, pages 199-205
, Symposium Proceedings.
The anodizing voltage range of organic solvent-based electrolyte solutions was greatly extended by Julius Edgar Lilienfeld, who described the preparation and use of the esterification (condensation) product of boric acid with triethylene glycol in U.S. Pat. No. 2,013,564. Lilienfeld later extended this work to include other glycols, such as diethylene glycol, and other acids in addition to boric acid, such as citric acid and tartaric acid, in U.S. Pat. No. 1,986,779. The highest anodizing voltage capability of these materials is associated with the borate esters of glycols having at least 2 ethylene oxide groups (i.e., diethylene glycol, triethylene glycol, etc.)
The present inventors duplicated the borate polyester formed by the reaction of triethylene glycol with an equivalent amount of boric acid. This polyester forms upon heating the two ingredients to 130 to 160° C. to drive off the water produced by esterification. It was found that triethylene glycol borate polyester solution may be used to anodize aluminum foil to 1,500 volts, at a temperature of 150° C. to 160° C. (1 mA/cm
2
current density). This oxide thickness is equivalent to over 1,790 volts at 85° C. or approximately 1,980 volts at 50° C.
Thus, polyglycol borate esters, although well suited for the high voltage anodizing of aluminum for high voltage electrolytic capacitor use, are not suitable as anodizing media. Borate polyesters of ethylene glycol, glycerine, diethylene glycol, triethylene glycol, and the like are all solid at room temperature. These borate polyesters must be heated to temperatures substantially above the boiling point of water to render them sufficiently fluid for anodizing use. For example, the borate polyester of triethylene glycol must be heated to 150 to 160° C. for sufficient fluidity for anodizing. Moreover, these materials tend to become increasingly more viscous with time at elevated temperatures, limiting their service life.
SUMMARY OF THE INVENTION
It is an object of the invention to produce a borate polyester product that remains liquid at temperatures as low as 25° C. or even lower.
It is another object of the invention to produce a high resistivity liquid (in excess of 100,000 ohm-cm) capable of producing a very high voltage anodic oxide on valve metals, particularly aluminum.
I
Kinard John Tony
Lessner Philip Michael
Melody Brian John
Wheeler David Alexander
Banner & Witcoff , Ltd.
Kemet Electronics Corporation
Wong Edna
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