US2111993A - Electrolytic device - Google Patents

Description

March 22, 1938.

P. ROBINSON ELECTROLYTIC DEVICE Filed July 2, v1936 INVENTOR. PRESTON ROBINSON BY 2 gel/6V ATTORNEYS Patented Mar. 22, 1938 UNITED STATES PATENT OFFICE ELECTROLYTIC DEVICE Mass., a corporation of Massachusetts Application July 2, 1936, Serial No. 88,654

11 Claims.

The present invention relates to electrolytic devices, and more particularly to electrolytic condensers, and is applicable both to so-called wet" and dry types of electrolytic condensers.

My invention may be applied, for example, to D. C. wet electrolytic condensers as used, for example, in filter circuits of radio sets. In such condensers the container usually constitutes the cathode and may be of either filming metal, as aluminum, or of non-filming metal, as copper.

As the price of such electrolytic condensers is a highly important factor, and as their cathodecontainer represents a considerable portion of their cost, various attempts have been made to reduce the cost, for example, by using for the container, metals which are less expensive than those presently used.

As zinc is one of the least expensive metals and can be extruded in comparatively simple operations into the shapes commonly used for such cathode-containers, various attempts have been made to make such containers out of zinc. Such attempts, however, have been unsuccessful so far.

I have found that these failures were primarily due to the fact that the electrolytes which gave the best results when used with cathodes made of other metals, were either entirely unsuited to be used with zinc cathodes or at least gave condensers the initial and operating characteristics of which compared very unfavorably with condensers having. for example, alum num cathodes.

More particularly I have found that when using zinc cathodes with ammonium-borate electrolytes (with or without the addition of free boric acid)--which electrolytes are the most commonly used for wet electrolytic condensers and generally give the best results with chromeplated as well as unplated aluminum cathodethe results were entirely unsatisfactory, and that this was due to the fact that the zinc of the oathode goes into solution and forms a zinc-ammonia complex ion which ion is quite avid in its attack on the aluminum anode. As a result of this attack the anode corrodes, causing a gradual and permanent mechanical and electrochemical deterioration of the condenser.

Similarly, when using with a zinc cathode, borax electrolytes (with or without free boric acid)which electrolytes give excellent results when used with copper or nickel cathodes-the results were also unsatisfactory. In this case I have found that this is due to the fact that the zinc again goes into solution at the cathode, this time in the form of a zincate ion (ZnOr) which also attacks the anode, although to somewhat lesser extent than occurs in the case of ammonium-borate electrolytes.

I have found, however, that zinc cathode condensers can be made successfully and having substantially as good initial and operating characteristics as similar condensers using successful cathode metals, by using electrolytes which either as main constituents or preferably as comparatively small additions, comprise substances which prevent or at least minimize the attack of zinccontaining ions (hereafter briefly referred to as zinc ions) on the anode. I have furthermore found that by using such additions the electrolytes which have been found the most successful with other cathode metals can be used with the same success with zinc cathodes.

In general I have found that to obtain satisfactory results zinc-cathode condensers should have electrolytes in which the formation of a zinc ion which would attack the anode, is prevented, and/or the zinc ions formed at the cathode are removed from the solution. This I obtain by providing electrolytes which consist of or comprise solutions which with the zinc ion forms insoluble salts and/or in which the solubility of the zinc ion is suppressed.

As a rule the electrolyte used may have as major constituents the commonly employed film-maintaining solutions, for example, aqueous solutions of ammonium, sodium or potassium salts of weak ac ds with or without free acid, to which are added substances which bring about the above desired results.

I have found that generally two types of such additions can be successfully used and that these can also be used in combination. As one type of addition I may use acids which form with the zinc ion an insoluble salt. Such acids are, for example. oxalic acid, phosphoric acid, and ferrocyanic acid, the zinc salts of which, namely zinc oxalate, zinc phosphate and zinc ferrocyanate, are insoluble in the electrolyte. Of these apparently oxalic acid gives the best results.

As the second type of addition I may use soluble compounds of alkaline earth metals, for example, their oxides, hydroxides, or their salts from dibasic or polybasic acids, for example calcium oxide, magnesium hydroxide, etc.

The action of such alkaline earth metal compounds in this connection is quite singular and not fully understood. It seems that the alkaline earth metal of these compounds reacts with the zinc ions (and particularly with zincate ions) without, however, forming a true chemical compound therewith. Apparently the alkaline earth metal precipitates as a hydroxide and by a process of selective adsorption carries with it the zinc ions.

Thus, for example, if calcium is the earth metal, instead of a true calcium-zincate being formed, a mixture is precipitated which contains calcium hydroxide and zinc hydroxide. Thereby the ratio of calcium to zinc may vary in the precipitate over a fairly wide range. In its effect the calcium suppresses the solubility of the zinc ion and the zinc ion suppresses the solubility of the calcium ion.

In practice I usually prefer to combine these two types of additions, by adding to the electrolyte a small amount of an alkaline earth salt of an acid as oxalic, phosphoric or ferrocyanic acid. Thereby the zinc ions form insoluble salts with the acid, whereas the alkaline earth metal ions and the zinc ions tend to suppress each others solubility. A very suitable addition of this type is, for example, magnesium oxalate.

I shall describe my invention in greater detail in connection with specific examples, both for wet and dry electrolytic condensers.

In the drawing forming part of this specification:

Figure l is a partly sectionized side elevation of a wet electrolytic condenser embodying my invention;

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

Referring to Fig. 1, the cylindrical container I is made of zinc which is substantially pure zinc, although a small amount of impurities such as copper may be advantageous for mechanical reasons.

The container l0 constitutes the cathode of the condenser, and is provided at one end with a reduced neck or tubular extension Ii. A seal provided in the tubular extension consists of a plug l2 of rubber or other resilient material, which fills the extension I l and slightly extends beyond the two ends thereof.

The plug 12 is provided with a central bore through which protrudes ananode-stem l5, which may be a round or a rectangular bar of aluminum. The neck II is crimped or squeezed around the plug l2 to form a liquidand gastight seal between the neck I i, plug l2, and about the stem IS.

The other end of the container is provided with a circular cap 20 of metal having crowned portion 2! and fitting into the container ID at 22. A vent gasket in the form of a wax-impregnated cloth 25 is interposed between the condenser l0 and the cap 20, whereby the free edge of the cap is preferably spun over a rim provided on the container.

The anode l6 consists of an aluminum sheet or foil folded back and forth upon itself in the manner of accordion pleating, and is riveted or otherwise secured to the stem IS.

A perforated spacer ll of hard rubber or similar material acts as a separator between the cathode and the anode.

The container is almost completely filled with the electrolyte i1, due allowance being made for expansion of the electrolyte on heating, or on freezing.

The electrolyte, in accordance with my invention, comprises preferably an aqueous solution of a good film-forming salt, for instance an ammonium or alkaline metal salt of a weak acid, to which may be added a free weak acid. The electrolyte furthermore contains additions of the types previously stated.

The exact composition of the electrolyte depends upon various factors, for example, on the voltage at which the condenser is to be used; for instance, for a 450-volt D. C. condenser a very suitable electrolyte is one comprising an aqueous solution of 3 grams borax, 50 grams boric acid per liter of water, to which are added about 1 to 2 grams of a substance preventing deleterious action by zinc ions. Such addition may be either an acid of the group of oxalic, phosphoric or ferrocyanic acid, for example 1 gram of oxalic acid, or a compound of an alkaline earth metal, for example 1 grams of calcium hydroxide or 2 grams of magnesium oxide, or again, an alkaline earth metal salt of one of the above referred to acids, for example 2 grams of magnesium oxalate. The addition, as stated, is usually of the order of 1 to 2 grams per liter of water, although somewhat larger or smaller amounts of such additions may be successfully used.

For low-voltage condensersup to about 200 to 250 volts-I usually prefer to use electrolytes having higher salt concentrations; for example, when using an electrolyte of the above type a higher concentration of borax; and I also prefer as a rule to use somewhat larger quantities of additions, for example, for the above type of electrolytes 2 to 3 grams per liter of water.

If I use as addition a soluble salt of an acid of the above type, for example, sodium oxalate, there is a,tendency on the part of the acid ion, in this case on the part of the oxalate ion, to

somewhat affect the film. While the quantities of the additions are sufllciently small that such action of the oxalate ion is not serious, nevertheless, in order to obtain an absolutely stable condenser, I prefer when using the soluble salts of such acids, to also form the film on the anode in a film-forming solution of such acids. For example, in case of using oxalate salts I prefer to have the film formed in a solution containing oxalate in accordance with the process described in my copending application Ser. No. 78,700, filed May 8, 1936.

Figure 2 illustrates a dry electrolytic condenser made in accordance with my invention. The aluminum electrode 3! provided with an electrolytically-formed film, is wound together with an electrode 32 of zinc, with the interposition of suitable absorbent spacers 33 of gauze, paper, cellophane" or the like. The electrodes 3| and 32 are provided with leads 34 and 35, respectively, which may form integral parts of the electrode foils.

The condenser roll is impregnated in known manner with a suitable electrolyte, hereinafter specified, and the whole assembly may be enclosed in a suitable container 36, which may be of cardboard or of metal in the latter case, for example in accordance with the invention, of zinc.

The container 36 is provided on its top with a cover 38, which may be of insulating material,

or may be formed of a layer of compound. If-

desirable a vent hole 37 may be provided on the container for the escape of the generated gases.

The electrolyte is one made in accordance with the invention, and can have the same major constituents as have the present-day dry condensers using aluminum cathodes; however there is added to the electrolyte a substance to prevent the action of the zinc on the anode. For example, the electrolyte may comprise as ionogens ammonium or alkaline metal salts of a weak acid, with or without the addition of tree weak acid, and as a solventa viscous polyhydric alcohol, also containing a definite amount of water. In general, to obtain a suitable viscosity-tern perature characteristic of the condenser, I prefer to use ammonium and amine salts rather than alkaline salts, and as additions .I prefer to use acids like oxalic acid, phosphoric acid or ferrocyanic acid, or soluble salts of such acids, rather than using the type of additions consisting or. other compounds of alkaline earth metals.

For example, for a 450-ivolt dry electrolytic condenser a suitable electrolyte is one containing 1 liter of glycol, 1 kilogram of ammonium pentaoorate, and 5 grams of sodium oxalate. The above mixture is so treated as to bring its water content between 5 to 30% or" the liquid portion or" the electrolyte, and to have a specific electrolyte resistance of between 100 to Still ohms per centimeter cube when measured at lild 6'3.

In case there is precipitate formation during the manufacture of this or similar electrolytes, such precipitate is removed either by filtration, decantation, or by other well-known methods, and only the clear solution is used to impregnate the condensers. The impregnation then takes place by any well-known method.

While I have described my invention in connection with specific examples and in specific applications, 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. In an electrolytic condenser comprising a filming electrode provided with a film, an electrolyte and a zinc cathode, said electrolyte comprising a film-maintaining solution and an addition which prevents zinc ions from attacking the film ing electrode.

2. In an electrolytic condenser, a filmed aluminum electrode, anelectrolyte and a zinc cathode, said electrolyte comprising a film-maintaining solution and an addition which prevents the formation of a soluble zinc salt.

3. In an electrolytic condenser, a filmed aluminum electrode and a zinc cathode, anelectrolyte comprlsing a film-maintaining solution and an addition consisting of an alkaline earth metal salt which forms with the zinc ions an insoluble salt.

4. in an electrolytic condenser, a filmed electrode, a zinc cathode and an electrolyte, said electrolye consisting of a film-maintaining solution to which is added an acid which forms insoluble salts with zinc.

5. in an electrolytic condenser, a filmed electrode, a zinc cathode and an electrolyte, said electrolyte consisting of a film-maintaining solution to which is added an alkaline earth metal salt of an acid, which acid forms an insoluble salt with zinc.

A wet electrolytic condenser comprising a filmed aluminum anode and a zinc cathode, and an electrolyte comprising an aqueous solution of a borate salt and of boric acid, and having an addition of about 1 to 3 grams per liter of Water of a substance which prevents attack of the aluminum anode by zinc ions.

l. A dry electrolytic condenser comprising a filmed aluminum electrode, a viscous electrolyte and a zinc cathode, said electrolyte comprising an ionogen which provides good film-maintaining properties to the electrolyte, and an added substance Which prevents zinc ions from attacking the filmed electrode.

S. In an electrolytic condenser, a filmed anode, a zinc cathode and an electrolyte, said electrolyte comprising a film-maintaining solution to which added a compound of oxalic acid to prevent the zinc ions from attacking the anode.

9. An electrolytic condenser" comprising a filmed anode, a zinc cathode and an electrolyte, said electrolyte consisting of a film-maintaining solution and of an addition of about 1 to 3 grams per liter oi water of a salt of an acid, which acid forms an insoluble salt with zinc.

10. In an electrolytic condenser, a filmed anode, an electrolyte and a zinc cathode, said electrolyte comprising a film-maintaining solution and as an addition an alkaline earth metal compound which suppresses the solubility of the zinc ions.

minum electrode, a weakly acid film-maintaining electrolyte and a zinc cathode, said electrolyte comprising an addition which prevents the formation of a soluble zinc salt.

PRESTON ROBINSON.

ll. In an electrolytic condenser, a, filmed alu- 4

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