US2206430A - Electrolytic condenser - Google Patents

Description

July 2, 1940.

P. ROBINSON ELECTROLYTIC Coiubmzsmg Filed Marbh 21, 1934 INVENTOR ATTORNEYS.

Patented July 2, 1940 UNITED STATES PATENT OFFICE ELECTROLYTIC CONDENSER Application March 21, 1934, Serial No. 716,714

6 Claims.

This invention relates to electrolytic devices and more particularly to electrolytic condensers and to the manufacture of such condensers.

Electrolytic condensers in their usual form comprise two electrodes, at least one of which is 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 is the case in the so-called wet electrolytic condensers, or may be more or less viscous, as is the case in the socalled 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 to the 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 eifective area of the electrode and inversely proportionate with the thickness of the dielectric layer.

In electrolytic condensers the thickness of the film depends to a considerable extent on the forming, respectively operating voltage of the condenser, this thickness increasing 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 med 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.

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 filmforming 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 since long that by roughening or etching the surface of the aluminum, its effective area could be increased, and thus with a given 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. 1 :3

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 ,Ml 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 methocljla 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 electrodeand the specific 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, do ends 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 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 con 5 denser for a given aluminum electrode.

In present day practice a commercially availaisle, high-purity aluminum is used for the filming electrode. Such aluminium 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.

l have found that such high-purity aluminum is not the best material to hesubjected to such etching (and to thereby obtain a large increase in capacity) and this not only because of the deleterious influence of the remaining impurities upon its grain structure, but because even if these impurities are entirely removed, the grain structure of the pure aluminum itself is of con- 59 siderable coarseness.

I have found that by adding to the aluminum, whether having a purity of 99.6%-99.8%, or even less, certain ingredients, hereafter referred to as modifiers, a much finer and denser grain strucw ture' of aluminum can be obtained. A particu-- larly good modifier is, for instance, sodium, either as a metal, or in the form of an easily reducible 1,387,900. But the fact that such modifiers also favorably influence the grain structure of highpurity aluminum, and especially that the aluminum so modified gives far better results than high-purity aluminum as a material for an etched electrode of elctrolytic'condensers, is a most surprising result. I

The advantage in this use of an aluminum having a very fine grain structure, seems to be due to the following:

In present-day condensers, using commercial high-purity aluminum electrodes, the film is formed on a smooth aluminum surface, for instance, on rolled aluminum foils. In the rolling operation the aluminum crystals on the surface of the foil are compressed. When forming the film on such a smooth surfaced aluminum electrode, it is immaterial whether the grain structure below the surface is fine or coarse.

However, when an aluminum electrode is subjected to etching, the character of the etched surface and its adaptabflty to film formation 1 greatly depends upon the grain structure of the,

aluminum. This is partly due to the fact that the area and character of the resulting etched surface is to a great extent dependent on' the grain structure of the aluminum, whereby a finer grain structure permits a greater increase of the effective area through etohingthan does a coarser grain structure, and partly because of the oxide film formed on the electrode having a very fine structure, a better film coverage and a better adhering film can be obtained on a fine-grained aluminum surface than on a coarse-grained surface.

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 amount of aluminum results in a. surface deeply pitted where large crystals have dissolved out. With the fine grain aluminum, such as is the modified aluminum used according to my invention, the

dissolution of an equal amount of aluminum will result in a microscopically smooth surface, which,

however, under the microscope reveals that agreat number of small crystals have dissolved out. In consequence the surface area left on the modified aluminum is much greater than on the unmodified aluminum.

The better film characteristics and easier formation are due primarily to the smoother contours of the surface of the etched modified aluminum, as compared to the etched unmodified aluminum. The film on the aluminum, in contradistinction to the parent metal itself, is hard and brittle. Consequently when in case of large aluminum crystals, it is stretched over a sharply angled surface, it may fracture, which will result in a poorly formed film or at least necessitate the reformation of the film at such points.

Such treatment of the aluminum with the modifying agent takes place preferably while the aluminum is in a molten state. The aluminum so treated, upon fracture, showsa silky appearpresent in aluminum, as iron, silicon, etc.

aaoaaso ance due to'the aluminum crystals being broken through by the fracturing process.

From this, by analogy, it seems not unlikely that when etching modified aluminum the solution of the aluminum does not take place preferentially, i. e., crystal by crystal, but takes place uniformly over the exposed surface. This assumption seems to be warranted by the fact that the beneficial results obtained with the modified aluminum are even greater than could be expected merely because of the fine crystalline "structure of such alumium. Or in other words,

the smoothness of the contour of the etched modified aluminum may be greater than if it were merely due to the smallness of the crystals.

Irrespective of what the reason may be, the

' aluminum treated with a modified agent provides for a larger increase in the effective area through .etching and for a better film than does commercial high-purity aluminum. Furthermore, the modified aluminum is easier to etch, can be etched in a shorter time with less etching solution, or in a less strong etching agent.

The amount of sodium or of othermodifying agent to be added to the aluminum need not be large and about .1 to .2% usually suffices. During the casting of the aluminum a great portion of this sodium evaporates after the agent has performed its action in modifying the grain structure. The remaining sodium after the casting of thealuminum may be as small as .01%.

But even if sodium or another alkaline or earth metal forming the modifying agent remains in larger quantities in the aluminum, its

presence does not deleteriously influence the properties of the aluminum in the film formation or its action as a filmed electrode of the condenser, as I have found that in the etching process practically all of the sodium disappears from the aluminum surface.

In this'regard these modifying agents behave quite differently from'other impurities normally The latter impurities in the etching solution are electro -negative with respect to the aluminum, and therefore do not dissolve during the etching in preference to aluminum. Sodium and the alkaline metals and some alkali earth metals, as calcium, are, however, electro-positive with respect to aluminum, and dissolve in the etching solution in preference to aluminum. Thus these modifying agents disappear from the surface of the aluminum during the etching, leaving a pure and fine-grained aluminum surface for film for mation.

While a s0 modified aluminum gives much better results when subjected to etching and subsequent film formation, than does commercial high-purity aluminum, nevertheless the electrodes so etched may still exhibit some drawbacks 5 iii 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 betweenadjacent 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 electrodewhich constitutes the anode in formation as well as in operation of the condenser-and which ions impinge upon and damage the film of closelyspaced adjacent electrode portions.

For instance, in condensers formed or operated at 450 volts, these difliculties 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".

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

I have found that such difficulties can be overcome, or at least can be greatly reduced, both in the case of smooth surfaced aluminum electrodes having closely spaced portions (for instance closely pleated aluminum foils), as well as in the case of etched electrodes, by the addition 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 beused as such film protecting agents. For instance, monobasic acids of the long chain type, as palmitic, stearic, .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 soint-ions and polyhydric alcohol solutions or the usual electrolytes, for instance of electrolytes comprising boric acid, phosphoric acid and citric acid, and/or the salts of such weal: 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 electro lyte.

I have found that a very small amount or such a protective agent, for instance, 1% or" 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. v

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

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

To more fully illustrate my invention on hand of specific examples, I shall describe some embodiments thereof with regard to the attached drawing in which:

Figure l is a partly sectionalized side view of a Wet" electrolytic condenser embodying my invention.

Fig. 2 is a partly sectionalized 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 or an easily reducible salt, for instance, a halide of an alkaline metal. Alkaline earth metals which are electropositive' with regard to aluminum, 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 fine-grained crystalline and high-density structure.

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

As to the exact schedule of the etching process It have found in general any ofthe methods used in preparing aluminum for electro-plating such as are described in The Aluminum Industry by Edwards, Frary and Jefiries, lvlc'Graw-Hill, New York 1930, page 4:92, 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 50% 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 etching action may then be carried out in a dilute solution of hydrochloric acid to which, however, other reagents such as sodium chloride, ferrous chloride, nickelous chloride or manganous chloride may be added.

The main difference, between the technique developed for preparing aluminum for electroplating and preparing it for film formation so as to bring about a high capacity, is that in general a longer schedule of operations is necessary. For example, where for .electro-plating, hydrochloric acid with or without the addition of other chlorides need only be 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 for etching aluminum well known in the art may be adjusted to apply to the present object of increasing the surface area of the aluminum.

As an example of the comparative effects, I have treated so-called commercially pure aluminum and modified aluminum plates according to the following schedule:

Dipped in hydrofluoric acid and nitric acid mixture as described above for five seconds, immersed in 5% hydrochloric acid for one hour. The commercially pure aluminum (99.8%) had approximately the following impurities-z iron 12%; silicon .07%; copper .01% or less; manganese .01% or less. The modified aluminum had substantially the same composition except that it had been modified with sodium and contained a remnant of .01% or less sodium. Plates so treated were rinsed in alkali and in boric acid solution. and were then formed in an electrolyte of -borax and boric acid to 450 volts and the following was found: The capacity of the commercial aluminum was 0.13 mfd. per square inch; the capacity of the modified aluminum was 0.2 mfd. per square inch.

Ordinary aluminum of the same composition, but not subjected to etching, will upon similar film formation give a capacity of about 0.1 mfd. per square inch.

' Other schedules of etching than the example cited will produce different capacities on the aluminum plate, rating as high for the modified aluminum as 0.5 mfd. per square inch, and,

somewhat erratically, even higher.

Modified aluminum has the further 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 a smooth surface, a two or threefold increase being obtained as a rule, although under certain conditions a five-fold and even higher increase may be obtained.

The etched aluminum, after 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 condensers 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, boric acid, phosphoric acid, citric acid, etc., and preferably also contains a salt of a weak acid, for instance, a sodium or ammonium salt of boric acid, phosphoric acid, citric acid, etc.,

whereby the salt of the acid does not need to be the acid used in the electrolyte.

To improve the film formation, in overcoming the difficulties caused by the closeness of adjacent portions of the etched electrodes, I preferably add to the forming electrolyte a filmlactose, etc., or a monobasic fatty, long-chain acid, as palmitic, stearic, oleic and abietic acids. Such acids as a rule are insoluble in the electrolyte and I 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 agitatingthe electrolyte.

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

The filmed electrodes are then assembled into condensers, for instance, into wet condensersof the type as shown in Figure 1. This figure illustrates a wet condenser as used in filter circuits; it has a corrugated aluminum electrode I3 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,

threaded extension I! of the electrode l3, said extension being provided with nuts I5-l5 to form one of the outside terminals of the condenser, the container l0 forming the other terminal. The cover is provided with a vent l8 and a gasket l9, around which is crimped the free end of the container I0. Preferably sealing means (not shown) are also provided between the protruding end of the electrode l3- and the cover l2.

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 aweak acid, for instance, an ammonium or alkaline salt of a weak acid, whereby the acid of the salt does not need to be 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 con trode, may be used, and preferably about .5% to 5% of such an agent is added to the electrolyte.

Fig. 2 illustrates my invention on a dry condenser. If this condenser is used for alternating current, as for example, for the starting of capacitor motors, both of the electrodes 2ll20 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 Zl-Zl 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 a 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. Theseelectrolytes have in general the same type of ionogens as enumerated in connection with the we electrolytic condensers. However, the solvent of such dry condensers as a rule comprises a poly hydric 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/or 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 Patent No. 2,057,314, and my Patent No. 1,935,860.

Claims regarding the novel process of manufacturing the condensers of the present invention are made in my copending application Ser. No. 26,291, filed June 12, 1935.

' 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 desire 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. An electrolytic condenser comprising an etched aluminum electrode having a fine-grained crystalline structure, and an electrolyte comprising a weak acid and the salt of a weak acid, and a monobasic fatty, long-chain acid said longchain acid forming less than 10% of the electrolyte.

2. An electrolytic condenser of the dry type comprising an etched electrode of aluminum provided with a film, and a viscous electrolyte comprising a weak acid and 5 to 40% of a monobasic fatty, long-chain acid emulsified in the electrolyte.

3. A film forming electrode for electrolytic condensers comprising aluminum in substantially pure form and an added element, said added element being present in an appreciable but relatively small proportion and in an amount greater than normally associated with the aluminum as an impurity, said electrode having an etched roughened surface resulting from the removal of the added element from the surface.

4. An electrode for electrolytic condensers and the like comprising a metal composition of a film forming element in substantially pure form and an added element, said added element being one not normally associated with said film forming element as an impurity, said added element being present in an appreciable but relatively small proportion, said electrode having an etched roughened surface resulting from the removal of the added metal from said surface.

5. An electrode for electrolytic condensers and the like comprising pure aluminum having added thereto an appreciable but smaller proportion of another element not normally present as an impurity in aluminum, said electrode having an etched roughened surface resulting from the removal of the added metal from said surface.

6. An electrode for electrolytic condensers and the like composed of aluminum of a purity of at least 99.2% having intentionally alloyed therewith another element not normally associated with aluminum as an impurity, said added eleeflective film forming surface of said electrode is PRESTON Ronmson. l0

greatly increased.

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