US2775553A - Electrolytic etching process for electrolytic capacitors - Google Patents

Description

etching tantalum electrode surfaces.

United States Patent ELECTROLYTIC ETCHING PROCESS FOR ELECTROLYTIC CAPACITORS George J. Kahan, Williamstown, Mass, assignor to Sprague Electric Company, North Adams, Mass., a corporation of Massachusetts No Drawing. Application December 31, 1952, Serial No. 329,150

2 Claims. (Cl. 204-141) This invention relates to a process for etching electrolytic type capacitor electrodes and more particularly relates to a process for etching tantalum electrodes prior to oxidizing the surface thereof in order that the same may form one element in an electrolytic type capacitor.

The use of an etching process for preparing metallic surfaces for use as electrodes in electrolytic type capacitors has long been well-known in the capacitor art. Up until the time of the instant invention, however, it has been virtually impossible to successfully etch tantalum by any known etching process or solution. Moreover, the known etching processes which were attempted to be used on tantalum involved tedious, laborious, multiple steps, rendering the same impractical for most commercial applications.

It is therefore a primary object of the instant invention to avoid and/or eliminate the above-noted deficiencies and to provide a relatively simple process for A further object of the invention is to provide such type process which will complete the desired etching in a relatively short time, comparable to like processes for easily etched metals. Other distinct objects will become apparent from the description and claims which follow.

The invention may be briefly described as comprising a process in which the tantalum is immersed in an aqueous solution of hydrofluoric acid containing a slight amount of a film-forming ion which is insuflicient to compensate for the complex forming tendency of the hydrofluoric acid, and then impressing a voltage across the bath with the tantalum as anode until the etching is completed. This process may also be carried out with pure hydrofluoric acid. While the addition of a film forming anion will modify the action of the bath, the essential function of the bath is carried by the fluoride ions or complex fluoride ions in solution.

The concentration of hydrofluoric acid in aqueous solution may vary from low concentrations to the maximum possible wtihout giving rise to volatilization of hydrofluoric vapors at room temperature. The preferred concentration which has been found to be most efiicient in operation, however, is the 48% concentration. As a general rule, the concentration may be increased as the ambient temperature is lowered, or conversely, must be reduced as the ambient temperature is increased, due to the fact that the volatility of the acid concentration gives rise to noxious, dangerous fumes.

The film-forming anion may constitute any anion which is insufficient to compensate for the complex forming action of the hydrofluoric acid, the amount of anion used decreasing in relationship to the greater film-forming action inherent in it. Suitable anions are soluble chloride, soluble sulfates, soluble nitrates or bromides. The amount of film-forming anion used will vary according to the strength of its film-forming action up to a concentration of approximately 20% in the manner described.

2,775,553 Patented Dec. 25, 1956 A preferred etching solution contains a 38.4% concentration of hydrofluoric acid in an aqueous solution with a 7% concentration of hydrochloric acid. Preferably this electrolyte is used with low ambient temperatures varying between 40 and 50 F., although it may be carried out at higher temperatures with suitable provision for the removal of any noxious fumes given off by the electrolyte bath.

The following is listed as one example of the invention.

Example A tantalum electrode surface is immersed in an electrolytic etching bath comprising an aqueous solution containing 38.4% hydrofluoric acid and 7% hydrochloric acid. A potential of 8-15 volts is then impressed across the electrolyte bath and tantalum electrode with the tantalum as the anode (the current density being maintained to be at least 2 amps. per square inch or higher) and the etching allowed to proceed for a period of 1 to 2 minutes. After removal from the bath the exposed surface of the electrode is shown by inspection to have a satisfactory etch ratio. After etching the tantalum electrode is cleansed, and then oxidized to produce the desired oxide film on the etched surface. Etch ratios of 3 to 5 resulted. The completed electrode is then combined with a suitable electrolyte in usual manner of electrolytic capacitors.

The particular action by which the etching proceeds is not at present wholly understood but is definitely indicated to be critically limited by the amount of film-forming anion in the aqueous hydrofluoric acid solution, the critical limit being reached when the additive becomes sufiicient to compensate for the complex forming action of the hydrofluoric acid.

It has been further found that the electrolytic etching means is not restricted to direct current but that an alternating potential is equally satisfactory. The use of the alternating potential is not limited to the etching of tantalum but is highly satisfactory also for aluminum.

The great advantages of this discovery is quite apparent in that no longer will rectification of the electrical poten-. tial be necessary.

As many apparently widely different embodiments of this invention may be made Without departing from the spirit and scope hereof, it is to be understood that the of from 38.4 to about 48% hydrofluoric acid and up to u about 20% .of an anion selected from the group consisting of chloride, bromide, nitrate and sulfate.

2. The process of claim 1 in which the aqueous solution contains 38.4% hydrofluoric acid and 7% hydrochloric acid.

References Cited in the file of this patent UNITED STATES PATENTS 2,481,306 Gall et a1. Sept. 6, 1949 FOREIGN PATENTS 512,764 Belgium July 31, 1952 OTHER REFERENCES Treatise on Inorganic Chemistry, by Mellor, vol. 9 (published in 1929), page 891.

Claims (1)

1. A PROCESS FOR INCREASING THE EFFECTIVE SURFACE OF METALLIC TANTALUM COMPRISING ANODICALLY TREATING IT AT AN ANODIC CURRENT DENSITY OF AT LEAST 2 AMPERES PER SQUARE INCH IN AN AQUEOUS SOLUTION CONSISING ESSENTIALLY OF FROM 38.4 TO ABOUT 48% HYDROFLUORIC ACID AND UP TO ABOUT 20% OF AN ANION SELECTED FROM THE GROUP CONSIST ING OF CHLORIDE, BROMIDE, NITRATE AND SULFATE.