US2193711A - Electrolytic device - Google Patents

Description

March 12, 1940. J, BURNHAM 2,193,711

ELECTROLYTIC DEVICE Filed Dec. 14, 1957 INVENTOR. JOHN BURNHAM BY WIQQXW ATTORNEYS meme Mn. 12, 1940 UNITED STATES PATENT OFFICE r 2,193,111 mo'mopmc mrvrca John Bnrnham. North Adamsrlllasa, assignor to Sprague Specialties Company, North Adams, Mass, a corporation of Massachusetts Application December 14, 1937, Serial No. 179,778

' 4 Chims. (01. 148-8) The present invention relates to electrolytic devices, and more particularly to electrolytic condensers and their manufacture.

Electrolytic condensers in their usual form comprise two electrodes, at least one of which is of so-called filming metal, and is provided usually with an electrolytically-formed film, formed of the partly hydrated oxide of the metal of the electrode. The condenser is provided with a suitable film-forming electrolyte, which may be highly fluid, as in the case of so-called "we electrolytic condensers, or may be more or less viscous, as in the case of dry" electrolytic con-- densers. My invention applies to both wet and dry types of electrolytic condensers, and irrespective of whether one or both of the electrodes are filmed.

As a material for the filming electrode, aluminum is usually used and my invention particularly relates to aluminum electrodes.

To obtain a film of the most favorable properties the aluminum used for the filmed electrode should be of high purity. Impurities in the aluminum impair the quality of the film for several reasons, of which the following two are the most important.

First, as the impurities are generally nonfilming materials, no film is formed on the spots where they appear in or on the surface of the aluminum. This results in a porous film, which increases the leakage and power factor of the condenser.

Secondly, most of the impurities, under the action of the electrolyte of the condenser are subject to electro-chemical reaction, which causes local damage to the film, thereby reducing the useful life of the condenser.

Films formed on electrolytically refined aluminum having a purity greater than 99.9% and usually referred to as Hoopes aluminum. are found to be of much better quality than films formed on aluminum of commercial purity. In practice, however, with unetched electrodes, the

' difference is not of sufiicient-commercial importance to warrant the increased cost due to the higher price of Hoopes aluminum.

However, if the electrodes are to be chemically etched to increase their surface area and thus to correspondingly increase the capacity of the condenser, the situation changes considerably. First, the deleterious influence of impurities may be accentuated with etched surfaces, and secondly, the cost of the aluminum plays a much smaller part in the cost of the condenser.

Thus it would seem highly advantageous to use Hoopes aluminum for etched electrodes. This, however, could not be achieved so far because Hoopes aluminum did not lend itself to etching by known processes.

That Hoopes aluminum could not be etched in solutions and by processes which have been found satisfactory for the etching of aluminum of commercial purity, seems to be due to the following:

In aluminum of commercial purity the etching is produced primarily by two simultaneous reactions. One is due to the metallic impuritiespresent in the aluminum which set up minute voltaic cells which in the presence of the acid solution rapidly etches the surface of the aluminum. The second is due to actionof the hydrogen ions of the acid.. The voltaic action is the more important and accounts for the major part of the etching. The attack by the hydrogen ions in this reaction can be considered negligible.

In Hoopes aluminum, however, the absence of the metallic impurities, and the voltaic cells formed thereby, slows down the etching process to an almost negligible extent.

Furthermore, assuming the same heat treatment and working, the crystal size of the Hoopes aluminum is considerably larger than that of'the less pure aluminum, and as the etching takes place primarily at the inter-crystalline boundaries, and these are few in Hoopes aluminum, the attack by the etching solution of the Hoopes aluminum is practically nil. I

The object of the present invention is to provide a novel etching solution and etching process which permits the etching of the Hoopes aluminum, and in general of high purity aluminum, and achieves this in an economical way.

According to the invention I provide an etching solution which comprises an etching acid and a positive catalyst added thereto, which catalyst is effective in increasing the attack of the hydrogen ions of the acid solution.

Such a catalyst is a salt and should preferably be a salt which is highly soluble in the etching acid solution. As a rule I prefer to use an aluminum salt, for example, aluminum chloride or aluminum sulphate, aluminum nitrate, etc.

-Salts of other metals which are lower in the electromotive series than aluminum may also be used, for example, salts of copper or iron, such as their chlorides, are suitable. However, when using salts of other metals than aluminum there v num of lower purity has been subjected to etching in a hydrochloric acid solution, some aluminum chloride was also produced during the etching reaction. However, if Hoopes aluminum is placed in a similar hydrochloric acid solution, the etching reaction does not start and no aluminum chloride is formed, nor would the small amount of aluminum chloride incidentally formed in the aforementioned reaction be sufficient to help to any important extent the etching of Hoopes aluminum.

To perform the etching process of my invention in a reasonably short time, the etching solution should be maintained at elevated temperatures, preferably between 50 and 85 C.

I have also found that to obtain the best results, definite relations have to exist between the concentration of the etching acid, the amount of catalyst contained therein, the temperature of the solution, and the duration of the etching. While a wide range of concentrations of the etching acid have been found suitable, as a rule it is necessary that irrespective of its concentration, it should be saturated or substantially saturated with the catalyst.

In the drawing forming part of the specification:

Figure l is a partly sectionized side viewof a wet electrolytic condenser embodying the invention;

Fig. 2 is a partly sectionized side view of a dry electrolytic condenser embodying the invention;

Fig. 3 is an enlarged section of a portion of an aluminum electrode etched in accordance with the invention.

Referring to Figure l, the condenser there shown is a'D. C. condenser of the type used in the filter circuits of radio receivers. The condenser comprises a cylindrical metallic container I, which preferably constitutes the cathode of the condenser, and which may be either of filmforming or non-film-forming metal. The container I is provided with a reduced tubular extension 3, threaded at 4 and extending into a further reduced tubular portion 5.

Disposed within the container I is an anode assembly 6 comprising a corrugated thin 'foil 1 of Hoopes aluminum etched in accordance with the invention. The foil 1 is secured to an anode 'riser 8 which serves as a support and terminal therefor.

The top of the container is provided with a crowned metal cap 9 which fits into the container end 2. A vent gasket ID of suitable material, for example of wax-impregnated cloth, is interposed between the container l and the cap 9. The free edge of cap 9, with the gasket Ill interposed, is

crimped over a rim provided on the container I to form a liquid-tight seal.

To prevent the anode foil 1 from contacting the container I, there is interposed between same and the anode assembly 6 an insulator, for instance, a thin hard rubber cylinder II. To permit free circulation between cathode and anode, the insulator II is provided with perforations I2 throughout its area.

The threads 4 on the reduced neck portion 3 are adapted to receive a nut (not shown) by means of which the condenser may be secured to a suitable base.

The extension has preferably the same inside diameter as the extension 3, so that the two form a common bore II. The wall of the extension 5 is, however, preferably much thinner than the wall of extension 3.

Placed in the bore I3 is a tightly-fitting plug it preferably of semi-hard vulcanized rubber through which is inserted the anode riser 8.

The plug it is preferably of such length as to extend on both sides of the neck portion 5. Proper sealing between the plug it and the neck 5 may be obtained by fluting the neck portion.

, A suitable film-maintaining electrolyte I5 is provided in the container and fills the container close to its top.

The electrolyte i5 is preferably an aqueous solution of a weak acid, forinstance of boric, phosphoric, citric acid, etc., to which may be added a salt of a weak acid, which salt does not need to be that of the acid used in the electrolyte.

Fig. 2 illustrates a dry condenser embodying the invention. The condenser comprises two electrodes 20 and 2|, which are usually both of aluminum, although in case of D. C. condensers only one electrode (the anode) needs to be provided with a dielectric film. The filmed electrode consists of Hoopes aluminum and is etched in accordance with the invention. The electrodes are assembled into a roll with the interposition of spacers 22-22, which are preferably of an absorbent material, such as gauze, paper, Cellophane, or a combination of same, and which serve as carrier for an electrolyte 23.

The electrolyte 23 may be more or less viscous and its ionogens are usually of the same types as have been enumerated above in connection with the wet condensers; however, the solvent of the electrolyte usually comprises a polyhydric alcohol,

such as glycerol, ethylene glycol, etc., and the electrolyte may also contain substances which increase its viscosity and/or conductivity.

The assembled condenser roll is disposed in a container 25, which may be of metal or insulating material. A layer 26 of a suitable sealing compound serves to seal the container and protect the condenser from moisture. Terminal wires 21 and 28 of the electrodes 20 and 2| respectively,

pass through the sealing layer 26 to serve as outside connections for the condenser.

As has been stated before, the filmed electrode or electrodes are made of Hoopes aluminum and are etched in accordance with the invention.

For this purpose the aluminum prior to its etching is cleansed in known manner, for example by dipping it for about 15 seconds in a caustic solution comprising 5% sodium hydroxide heated to about C. This cleansing removes all traces of grit grease and other extraneous impurities that may adhere to the surface of the aluminum.

Following this, the electrode is rinsed with distilled water to remove all traces of the caustic solution.

The electrode is then subjected to the etching, whereby in accordance with the invention, a suitable catalyst is added to the etching acid solution prior to the etching and in an amount sufficient to saturate or substantially saturate the solution. Suitable catalysts are the salts of the type previously mentioned, which as stated, should be highly soluble in the etching solution so as to be present therein in large amounts.

As a rule I prefer to use hydrochloric acid solutions to which I add a suilicient amount of aluminum chloride to saturate the solution. The acid concentration may be selected within wide limits; it may be a concentration as low as 3%, or it may be a practically saturated solution of about 35%.

Too high concentration of the hydrochloric acid solution is, however, preferably avoided as with increasing concentration of the acid the solubility of the aluminum chloride decreases, so that a 35% hydrochloric acid solution becomes saturated with about .05% aluminum chloride, which is insunicient to get the full benefit of my invention.

Too low acid concentration (below 5%) of the etching solution is also objectionable as it reduces the etching ratio.

In practice I therefore prefer to use concentrations of 5% to 25% (acid by weight).

To a 5% hydrochloric acid solution I add about 25% aluminum chloride, whereas to a 25% acid solution I add about 6.5% aluminum chloride. In both cases I obtain substantial saturation.

I found that when using hydrochloric acid as etching acid and aluminum chloride as catalyst, the sum of the concentrations of the hydrochloric acid and of the aluminum chloride in the solution is to be between about 30-35% (by weight) to obtain the best etching results.

The etching time usually also depends on the temperature of the etching solution. For example, ,with an etching solution comprising 18% of hydrochloric acid and 12% of aluminum chloride, and maintained at a temperature of to C., the etching time will be about 5 to 7 minutes. By raising the temperature of the solution to about to C. this time can be reduced to 3 or 4 minutes.

Too high etching speeds are as a rule undesirable because they reduce the etching ratio.

After their etching the electrodes are washed in distilled water to remove all traces of the etchitng solution and then subjected to film formaion.

In forming the dielectric film on the electrode, various known film-forming electrolytes may be used. For instance, in case of wet condensers suitable for 550 volts peak voltage, I may use an electrolyte comprising grams of boric acid per liter of distilled water.

While I have described my invention in connection with specific embodiments and by specific examples, I do not wish to be limited thereto, but desire the appended claims to be construed as broadly as permissible in view of the prior art.

What I claim is: i

1. In the manufacture of electrodes for electrolytic condensers, the process which comprises the steps of adding aluminum chloride in a quantity sufllcient to saturate an aqueous solution of hydrochloric acid, and subjecting aluminum containing less than .1% impurities to etching in said solution.

2. In the manufacture of electrodes for electrolytic condensers, adding to an 18% solution of hydrochloric acid 12% by weight of aluminum chloride, and etching in said solution aluminum having less than .l%

impurities.

between 15 to 35%.

JOHN BURNHAM.

the process which comprises,-

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