US2206050A - Electrolytic device - Google Patents

Description

July 2, 1940. P. RoBzNsoN ELECTROLYTIC DEVICE Original Filed March 21, 1934 INVENTOR. PRESTON ROBINSON BY W Km ATTORNEYS Patented July 2, 1940 UNITED STATES PATENT OFFICE ELECTROLYTIC DEVICE Original application March 21, 1934, Serial No. 716,714. Divided and this application June 12,

1935, Serial No. 26,291

11 Claims.

This invention relates to a novel process of manufacture of electrolytic devices and more particularly to the manufacture of electrolytic condensers.

The present invention is a divisional application of my copending application Serial No. 716,714 filed March 21, 1934.

Electrolytic condensers in their usual form comprise two electrodes, at least one .of which is I. of so-called filming metal, for instance, of aluminum, tantalum, zirconium, etc. The condenser is provided with a suitable electrolyte, which may be highly fluid, as in the case in the so-called wet electrolytic condensers, or may be more I. or less viscous, as is the case in the so-called "dry electrolytic condensers. My invention applies to both wet and dry types of electrolytic condensers and irrespective of whether one or both of the electrodes of the condenser are filmed.

In electrolytic condensers the capacity effect is due mainly tothe dielectric properties of the film formed on the filmed electrode or electrodes. This film consists as a rule of an electrolyticallyformed, partly hydrated oxide of the metal of the filming electrode.

As is known, the capacity of any type of condenser is proportionate with the effective area of the electrode and inversely proportionate with the thickness of the dielectric layer.

In the electrolytic condensers the thickness of the film depends to a considerable extent upon the forming voltage, respectively operating voltage of the condenser, this thickness increasing 3; and the capacity of the condenser decreasing with increasing voltages. Under substantially identical conditions a square inch of filmed electrode area, at a given voltage, will thus produce in the condenser a capacity of a substantial fixed a value. For instance, in the wet electrolytic condensers now widely used, having electrolytes consisting of boric acid and a salt of boric acid dissolved in water, at room temperature and low frequencies, one square inch of superficial filmed electrode area at 450 volts gives about .1 mid.

As aluminum is the most commonly used filmforming metal, in the further descriptionI shall assume aluminum as the film-forming metal. It should, however, be well understood that certain aspects of my invention apply also to condensers having electrodes made of other film'- forming metals, for instance, tantalum, zirconium, etc.

In present-day electrolytic condensers the aluminum electrode subjected to film formation is usually a foil or thin sheet having a microscopically smooth surface. While it has been realized long since that by roughening or etching the surface of the aluminum, its effective area could be increased, and thus with a given 5 aluminum electrode a condenser of a larger capacity obtained, past endeavors in this direction have generally been unsuccessful and did not result in satisfactory condensers.

In the copending application of Preston Robinson and Joseph L Collins, Serial No. 526,118, filed March 28, 1931, now U. S. Patent No. 2,067,703, issued January 12, 1937 a treatment of the aluminum foil has been described, by means of which the effective area of a filmed electrode can be substantially increased, whereby a considerable saving in aluminum is obtained without affecting the quality of the condensers. More specifically, in said application, there is described a method of treating an aluminum electrode, prior to film. formation, in a strong alkaline solution in the presence of a proper inhibiting agent. By means of this treatment the aluminum surface is evenly etched and the effective area of the electrode and the speciflc capacity of the condenser considerably increased.

I have found that the successful etching of the film-forming electrode, for instance, of aluminum, to obtain thereby an increased capacity for a given electrode area, depends primarily upon the grain structure of the aluminum.

The grain structure of aluminum not only depends on its purity but also on the character and amount of the impurities contained therein, and 35 I have found that the finer grained and the denser the structure of the aluminum, the better results can be obtained by etching the aluminum surface prior to film formation, and the larger thereby the gain in the capacity of the condenser for a given aluminum electrode.

In present day practice a commercially available, high-purity aluminum is used for the fihning electrode. Such aluminum may comprise about 99.6%99.8% of aluminum, the balance being made up of various impurities, as iron,

' silicon, with traces of copper and manganese.

Isl-5.2

oi the alurhlimm is of coz cleralole coarseness.

I have found by to aluminum, whether having a urity of oo.s%-oo.o%, or even 3 less, certain ingredients, hereafter referred to as motlhlers, much liner and. denser grain strucof aluminum Toe ohtaihccl. A particularly cooll modifier tor instance, sodiuhi, either as a or in the form of an easfly reducible salt, for instance a sodium halide. the? alkaline or eaith metals and salts thereof, as cotassium, calcium and the halides thereof, can he successfully used as such mooi reduction of gr in structure of a silicoualloy oy si or modifiers has alreacly fles riheol Boy race in J. S. Fateut No. 3 8%. But the fact that such mod rs also vorahly influence the grain structure of. h-omity alum-Mouse, and especially that the a so modified gives for better results an high-purity as a material for etchecl electrode of electrolytic condensers, is a most surprising resul The advantage in this use of aluminum savio very grain structure, seems to he following:

au eut day condensers, using commercial high-malty alumimuu electrodes, the film is termed on smooth aituuiouui surface, for instance, on roller]. ioils. the rolling ooeratioh aluminum crystals on the surface the foil are compressed. When forming the film on such a smooth surfaced aluminum electrocle, it is immaterial whether the grain structure below the surface is finer or coarser.

However, when an aluminum electrode is eulojectecl to etching, the character of the etched suriace and its adaptability to film formation --zgreatly depends upon the grain structure of the aluminum. This is partly clue to the "fact that the area and character oi the resulting etched surface is to a great extent dependent on the grain structure of the aluminum, whereby a grain structure permits a greater increase of the effective area through etching than does a coarser grain stiucture, and partly because or the oxide film formed on the electrode a very zine structure, a better film coverage and a hotter adhering film can he obtained on a line-grained aluminum surface than on a coarse-grained suaface.

This can be explained as follows: When aluminum is attacked by etching reagents, one crystal dissolves at a time. with large crystals in the aluminum, the dissolution of a given amouut oi aluminum results in a surface deeply pitted where large crystals have dissolved out. With the hue grain aluminum, such as is the modified aluminum used according to my invention, the dissolution of an equal amount of aluminum will result in o. microscopicahy smooth surface, which, however,

under the microscope reveals that a great number of small crystals have dissolved out. "in consequence the surface area left on the modiiieol aluminum is much greater than on the unmodifiecl aluminum.

The better film characteristics and easier formation are due primarily to the smoother cohtours of the surface of the etched mocliiiecl alumisum, as compared with the etched unmodified aluminum. The film on the aluminum, in contraclistlnction to the parent metal itself, is brittle. Consequently when in case of largealumiuum crystals, it is stretched. over a sharply that when etching modified the solu-* tion of the aluminum does not take place preier= ehtially, e., crystal by crystal, Bout place uuitormly over the exposed surface. assue-option seems to warranted Toy the fact the beneficial results ohtaiuecl the modifies"; aluminum are even greater than could he pectecl merely because of the line crystalli structure of such aluminum. other words, the smoothness of the contour moclirlecl he greater than it it were merely clue to the smallness of the crystals.

irrespective of what the reason may he, alialiiuum tree, with a modifying agent provides for a larger increase the efiective area through etching for thou coco commercial high-purity aluminum. Further more, the modified; aluminum is easier to etch, can he etchec in a shorter time with less etching solution, or in a less strong etching agent.

The amount of sodium or oi other modifying; agent to he aclcleol to the aluirlihum neeol not be large aucl about .1 to 2% usually sumces. During the casting of the aluminum a great portion of this sodium evaporates alter the agent has formed its action in modifying the grain structure. The remaining sodium alter the casting oi the aluminum may he as small as 191%.

But even it sodium or another alkaline or earth metal forming the modifying agent remains larger quantities in the aluminum, its presence does not cleleteriously influence the properties oi the aluminum in the film formation or its actiou as a filmed electrode of the condenser, as l have fouucl that in the etching process practically all of the sodium disappears from the aluminum surface.

In this regal-oi these modifying agents behave 1 quite differently from other impurities normally present in aluminum, as iron, silicon, etc. The latter impurities in the etching solution are clec tro-negative with respect to the aluminum, out therefore do not dissolve during the etching in preference to aluminum. Sodium and the allca line metals and some alkali earth metals, as calcium, are, however, electro-posltive with respect to and dissolve in the etc solution in preference to aluminum. Thus these modifying agents disappear from the surface the aluminum during the etching, leaving a pa and rice-grained aluminum surface for film Iormauoa.

While a so modified aluminum gives much better results when subjected to etching and subscquest film formation, than does commercial high purity aluminum, nevertheless the electrodes so etched may still exhibit some drawbacks usuall present in the case of etched aluminum elec trodes.

These drawbacks involve difficulties in formation and corrosion in the operation of condenser.

l" have loans. that these difilculties have common came and can. be overcome simultaneously, will he shows-i J.

til

In the electrolytic formation of the film on an aluminum surface. the best results are obtained when the aluminum is freely suspended in the electrolyte; those portions of the aluminum electrode which contact with foreign material and which contact with adjacent portions of the electrode, are difficult to form and in the operation of the condenser, corrosion is likely to occur at such portions.

In the past it has been assumed that poor film formation and corrosion of such portions was because the proper access of the electrolyte to such portions was prevented.

I have found that such difficulties are not only present in case of direct contact between such portions, but exist as long as the distance between adjacent portions of the filmed electrode is below certain limits, the values of which depend primarily on the voltage applied to the filmed electrode.

I believe that this is due to high-speed positive ions being emitted from the filmed electrade-which constitutes the anode in formation as well as in operation of the condenserand which ions impinge upon and damage the film of closely-spaced adjacent electrode portions.

For instance, in condensers formed or operated at 450 volts, these difficulties will exist to a more or less marked extent, unless the mean free distance between adjacent portions of the filmed electrode is at least .02 to .03 inch.

In etched electrodes adjacent portions of the filmed electrodes are necessarily quite close to each other, and this causes difliculties in filmformation and corrosion in operation.

I have found that such difiicu-lties can be overcome, or at least can be greatly reduced, both in the case of smooth surfaced aluminum electrodes having closely spaced portion (for instance closely pleated aluminum foils) as well as in the case of etched electrodes, by the additions to the electrolyte of certain substances which I shall refer to as film protective agents.

Such agents are, for instance, certain carbohydrates, which have the property of forming insoluble compounds with the aluminum oxide film. Thus the high-speed positive ions which are emitted from the film, instead of impinging on adjacent portions of the film, are caught or absorbed in the carbohydrate compound layer protecting the film.

A suitable carbohydrate for this purpose is, for instance sucrose, and similarly levulose, lactose, maltose, etc.

I have also found that besides carbohydrates other chemicals may be used as such film protecting agents. For instance, monobasic acids of the long chain type, as palmitic, stearlc, oleic, abietic acids, can be used for this purpose, since they form compounds with the film, which compounds are insoluble in the electrolyte.

Carbohydrates are soluble in both aqueous solutions and polyhydric alcohol solutions of the usual electrolytes, for instance of electrolytes comprising boric acid, phosphoric acid and citric acid, and/or the salts of such weak acids, and can therefore be added directly to the electrolyte.

In the case of monobasic, fatty, long-chain acids, as above enumerated, which are not soluble in the electrolyte usually employed, these acids are preferably emulsified in the electrolyte by the addition of salts of these insoluble acids, for example of sodium or ammonium salts of such acids, and by mechanically agitating the electrolyte.

I have found that a very small amount of such a protective agent, for instance, 1% of sucrose, has a very marked effect on the film formation and prevention of corrosion on such critical points. The amount added, however, depends on the agent, on the specific properties of the electrolyte, on the thickness of the aluminum, and on the voltage for which the condenser is designed.

The amount to be added is usually smaller in the case of wet condensers than in the case of dry condensers.

,In the case of wet condensers I may add about .5% to 5% of the agent, and in the case of dry condensers I may add about 5 to 40%, although :nore or less this amount may be used at discreion.

The addition of such agents does not injure the electrolyte as long as it is otherwise free of impurities.

I shall describe more fully some embodiments of my invention in connection with the attached drawing, in which:

Figure l is a partly sectionized side view of a wet electrolytic condenser embodying my invention.

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

Fig. 3 is an enlarged schematic section through an etched aluminum electrode made in accordance with my invention.

In the manufacture of condensers, according to my invention, aluminum of high purity, preferably of 99.6-99.8% purity, is molten and cast together with the modifying agent. As far as the etching results are concerned, the purity of the aluminum is not critical. However, the filmforming properties of the aluminum are improved with higher purity, and therefore I prefer to use high-purity aluminum as above stated.

The modifying agent is preferably an alkaline metal either in metallic form, or in the form of an easily reducible salt, for instance, a halide of an alkaline metal. Alkaline earth metals which are electropositive with regard to aluminium, for instance calcium, can also be used. I prefer to use sodium and add about .1 to .2% sodium to the aluminum before casting it.

During the casting, most of the sodium evaporates, the remaining sodium being of the order of about .01%.

The above treatment gives an aluminum of iine-grained crystalline and high-density strucare.

The aluminum is then subjected to the mechanical operations to bring it in proper shape, for instace, to rolling to obtain aluminum sheets or foils, and to corrugation or pleating, etc. The so-formed aluminum is then subjected to etching.

As to the exact schedule of the etching process I have found in general any of the methods used in preparing aluminum for electro-plating such as are described in The Aluminum Industry by Edwards, Frary and Jeffries, McGraw-Hill, New

York 1930, page 492, et seq., may be used. For example, the metal may be first cleaned with an alkaline cleaner or with a solvent such as kerosene. The surface of the metal may then be made uniformly active by a dip of from five to thirty seconds in a solution of one part hydrofluoric acid to nine parts of water, or in a solution containing one part 50% hydrofluoric acid and three parts of nitric acid. The main chlorlzle may be added.

The main difference between the technique developed for preparing aluminum for electroplating and preparing or film iormation so as to bring about a high capacity, is that in general a longer schedule of ogcerations is necessary. For example, where for electro-plating, hydrochloric acid with or without the addition of other chlorides need only he applied for times of the order of one or more minutes, "for the purpose of the present application a schedule of the order of magnitude of one or more hours yields better results.

also, other processes etching aluminum well in the art may lee adjusted to apply r to the pre ent object of increasing the surface area of tne as an e ample oi the comparative effects, I have treaterso-callefl commercially cure aluminum and modified aluminum plates according to the schedule:

flipped hydrofluoric acid and nitric acid mixture as described above for live seconds, immersed in 5% hydrochloric aciol for one hour. The commercially pure aluminum (99.8%) has approximately the following impurities: iron 32%; silicon 597%; copper .lll% or less; manganese .Dl% or less. The modified aluminum harl substantially the same composition except that it had been modified with sodium and conta'lnerl a remnant 31% or less sodium. Plates so treated were rinsed in alkali and. in boric acid solution and were formed in an electrolyte of boron anti borlc acid to ice volts and the following was found: The capacity of the commerciol aluminum was 0.13 mid. per square inch;

the capacity of the modified. aluminmn was 0.2 mid. per square inch.

Ordinary aluminum of the same composition, but not subjected to etching, will upon similar film formation give a capacity oi about it]. mid. per square inch.

Other schedules of etching than the example cited produce difierent capacities on the aluminum plate, rating as high for the modified aluminum as 9.5 mid. per square inch, and, somewhat erratically, even higher.

Modifiezi'cluminmn has the lurther advantage of requiring much shorter etching or less strong solutions than does commercial aluminum.

During the etching, as has been stated, the remaining sodium or other modifying agent, dissolves in the etching solution, which leaves a clean and pure aluminum surface.

The etching greatly increases the aluminum surface compared to smooth surface, a two or threefold increase lacing obtain-eel as a rule, although under certain conditions a five-fold and even higher increase may be obtained.

The etched aluminum, alter suitable rinsing is, in known manner, subjected to film-formation which may take place with the application of a suitable voltage, which for present-day condensore is normally 30 to 600 volts depending on the use of the condenser. The electrolyte used in the formation process contains as a rule an aqueous solution of an acid, for instance, horic acid, phosphoric acid, citric acid, etc, and preferably also contains a salt of weal: ecirl, for instance, a sodium or ammonium salt of looric acid, phosphonic acid, citric acid, eta, whereby the salt aeoaoso oi the acid oioes not need to be the acid used. in the electrolyte.

To improve the film formation, in overcoming the dlmculties caused by the closeness of adjacent portions of the etched electrodes, I preferably add to the forming electrolyte a. film-protectlve agent in the form of a. carbohydrate soluble in the electrolyte, for instance, sucrose, lactose, etc., or a monobasio fatty, long-chain acid, as palmitic, stearic, oleic and abietic acids. Such acids as a rule are insoluble in the electrolyte and l emulsify these acids in the electrolyte by adding to the electrolyte salts of these acids, for example, their sodium or ammonium salts, and by mechanically agitating the electrolyte.

The addition of such film-protective agents, of which it normally use .5 to 5%, obviates the dificulties encountered in the filming of etched electrodes.

llhe filmed electrodes are then assembled into condensers, for instance, into wet condensers of the type as shown in Figure 1. This figure lllustrates a wet condenser as used in filter circuits; it has a corrugated aluminum electrode N which has been modified, etched and filmed according to the above-described method and has a capacity several times as large as has a corrugated electrode of exactly the same dimensions, but having a smooth surface. The container in forms the other electrode of the condenser and is preferably a chromium-plated aluminum can, the advantages of which are described in my Patent No. 1,938,464.

The container H) is closed by a. cover l2 of insulating material, through which projects a threaded extension I? of the electrode l3, said extension being provided with nuts (ls-l5 to form one of the outside terminals of the condenser, the container 10 forming the other terminal. The cover is provided with a vent l8 and a. gasket 19, around which is crimpecl the free end or the container l9. rreierably sealing means for example a gasket 3t and a resilient washer 3! are also provided between the protruding end. of the electrode 33 and the cover [2.

The mechanical construction of the condenser is given merely as an illustration and various other known constructions may be used.

The electrolyte I6 is preferably an aqueous solution of a weak acid, for instance of boric, phosphoric, citric acid, etc., to which may be added a salt of a weak acid, for instance, an ammonium or alkaline salt of a. weak acid, whereby the salt added need not be that of the acid used in the electrolyte.

The electrolyte preferably also comprises a. film-protective agent to prevent corrosion of the filmed electrode in the operation of the condenser, for the reasons previously set forth. One of the protective agents previously discussed in connection with the formation of the electrode, may be used, and preferably about .5 to 5% of such an agent is added. to the electrolyte.

Fig. 2 illustrates my invention in a dry condenser. If this condenser is used for alternating current, as for example, for the starting of capacitor motors, both of the electrodes 20-20 are of film-forming metal, for instance of aluminum. The electrodes are treated as previously described and thus are of a fine-grained and dense modified aluminum and etched and filmed in accordance with the invention.

The filmed electrodes are assembled in rolls or stacks either with or without the interposition of spacers. In case spacers 2l-2I are used, as shown in the drawing, these consist preferably of a fabric, as gauze or of other absorbent material, for instance of Cellophane, paper, etc., and also serve as carrier for the electrolyte.

The electrolyte is more or less viscous and comprises preferably a weak acid, to which the salt of a weak acid may be added. These electrolytes have in general the same type of ionogens as enumerated in connection with the wet electrolytic condensers. However, the solvent of such dry condensers as a rule comprises a polyhydrio alcohol, as glycerol, ethylene glycol, etc.; also a substance which increases the viscosity and/or conductivity of the electrolyte may be added.

Furthermore, to prevent corrosion of the condenser in operation, I prefer to add to the electrolyte a film-protective agent, as previously described, namely, a soluble carbohydrate or a monobasic, fatty, long-chain acid. The amount of such protective agent may greatly vary and may be between 5 and 40%. This agent may partly replace the polyhydric alcohol.

As stated above, the use of a protective agent of this type is also useful in the forming and/r final electrolyte of both wet and dry condensers, the electrodes of which are smooth, especially when the electrodes of such condensers have closely-spaced corrugations, or have electrodes in which closely adjacent portions may adversely influence each other.

In general, I prefer (both in the case of wet and dry condensers), to have a final electrolyte the pH of which is lower than the pH of the forming electrolyte, the advantage of this being fully described in the U. S. patent to Preston Robinson and Joseph L. Collins No. 1,916,586.

In the case of dry condensers, I prefer to use the forming processing methods described in my copending application Ser. No. 548,270, filed July 1, 1931, now U. S. Patent No. 2,057,314, issued October 13, 1936, and my Patent No. 1,935,860.

The novel articles obtained with the method of the present invention are claimed in my above application Ser. No. 716,714.

While I have described my invention on hand of specific embodiments and in specific applications, I do not wish to be limited to same, but desired the appended claims to be construed as broadly as permissible in view of the prior art.

What I now claim as new and desire to secure by Letters Patent, is:

1. In the manufacture of electrolytic condensers, the process which comprises the steps, treating molten aluminum with a modifying agent which comprises a metal of the group of alkali and alkali earth metals which is electro-positive with regard to aluminum, casting said modified aluminum, and subjecting the aluminum so obtained to chemical etching to increase the effective surface of said aluminum, and removing by said etching the modifying agent from the aluminum surface.

2. In the manufacture of an electrolytic device, the process which comprises the steps, forming an etched electrode of filming material in an electrolyte comprising a weak acid which is soluble in water and a monobasic fatty, long-chain acid.

3. In the manufacture of electrolytic condensers, the process which comprises the steps, forming an electrode of filming material in an elect o y comprising a Weak acid which is soluble in water and a compound of a mono-basic fatty long-chain acid radical.

4. In the manufacture of electrolytic condensers, the process which comprises the steps, adding to an electrolyte comprising as a major constituent a weak acid which is soluble in water, salts of an insoluble monobasic fatty long-chain acid, emulsifying the long-chain acid in the electrolyte and forming an aluminum electrode therein.

5. In the manufacture of electrolytic condensers, the process of reducing the crystalline structure of the aluminum which comprises the steps, treating molten aluminum with a modifying agent of the group consisting of the alkali and alkali earth metals which are electropositiv'e with regard to the aluminum and of the easily re-' ducible salts of these metals.

6. In the manufacture'of electrolytic condensers, the process of providing a fine crystalline structure for the aluminum which comprises the treatment of molten aluminum with .1% to .2% sodium.

7. In the manufacture of electrolytic condensers, the process which comprises the steps, forming an aluminum electrode having closely adjacent protruding portions, in. an electrolyte having as major constituent a weak acid which is soluble in water and comprising an agent which forms a protective layer for the film during its formation in said electrolyte.

8. In the process of providing film-forming electrodes for electrolytic condensers and the like, the steps of combining with a film-forming metal such as pure aluminum, a relatively smaller but appreciable quantity of another metal not normally associated with said film-forming metal as an impurity, thereafter exposing said metal composition to the action of a chemical agent adapted to etch out said added metal without readily affecting said film-forming metal.

9. In the process of treating film forming electrodes for electrolytic condensers and the like, the step of exposing an electrode composed substantially of pure aluminum but containing an appreciable though smaller quantity of at least one other element intentionally added to the aluminum, said added element being normally not present as an impurity in aluminum to the action 01' a solution adapted to readily etch out said added element without substantially etching out the aluminum, whereby the effective film forming surface of said electrode is materially increased.

10. In the process of providing film forming electrodes for electrolytic condensers and the like, the steps of combining with a pure film forming metal such as aluminum, a relatively smaller but appreciable quantity of another metal not normally associated with said film forming metal as animpurity, thereafter exposing said metal composition to the action of an etching solution to remove said added metal from the surface 01' said composition.

11. In the process of providing film forming electrodes for electrolytic condensers and the like, the steps of combining with aluminum of a purity of at least 99.2% a relatively smaller but appreciable quantity oi'another metal not normally associated with aluminum as an impurity, thereafter exposing said metal composition to the action of a solution adapted to etch out said added metal from the surface of said metal composition.

PRESTON ROBINSON.

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