US2146029A - Electrical condenser and process of making same - Google Patents

Description

Feb. 7, 1939. K. E. SCHIMKUS ,1

ELECTRICAL CONDENSER AND PROCESS OF MAKING SAME Filed Dec. 8, 1934 Patented Feb. 7, 1939 PATENT OFFICE ELECTRICAL CONDENSER AND PROCESS OF MAKING SAME Kurt E.'Schimkus, Chicago, Ill., assignor toUtah Radio Products Company, Chicago, 111., a corporation of Illinois Application December 8, 1934, Serial No. 756,595

9 Claims.

The invention relates to electrical condensers and more particularly to electrolytic condensers of the type which are characterized by the use of a semi-solid electrolyte.

An object of the invention is to provide a new and improved condenser of this nature which has high capacity, long life, and is capable of withstanding high break-down voltages.

Another object is to provide a novel process for producing such a condenser, the steps of which may be quickly and easily performed, is economical and by which accurate control may be had whereby uniformity of the product results.

Another object of the invention is to provide a novel condenser of the type embodying a semisolid electrolyte wherein the electrolyte is the sole means for separating the electrode elements and the use of spacing means is eliminated.

A further object resides in the provision of a novel process which includes the step of forming the electrolytic substance on the face of an electrode element, the formation being controlled so that an even, uniform coating of said face is produced.

Another object is to provide a novel process which includes the steps of electrically forming, either separately or simultaneously, on the anode and cathode elements of a condenser before assembly a coating of a compound; that on the anode element having high electrical resistivity, and that on the cathode element being capable of chemical conversion into a substance having electrolytic properties.

Further objects of the invention reside in the provision of a condenser of this nature wherein the moisture content of the electrolyte is accurately controlled to obtain a proper quantity, and in which the condenser assembly is sealed in a simple and economical manner to maintain the moisture content against subsequent variation.

Other objects and advantages will become apparent in the following description and from the accompanying drawing, in which:

Figure 1 is a side elevation of a completed condenser produced according to and embodying the features of the present invention, said view being partially in central section.v

Fig. 2 is a somewhatdiagrammatic cross sectional view on a greatly enlarged scale showing the relationship of the anode and cathode elements and'the coatings thereon at assembly.

Fig. 3 is a fragmentary face view of a metallic member from which the electrode elements are fashioned.

While the invention is susceptible of various modifications and alternative constructions, I have shown in the drawing and will herein describe in detail the preferred embodiment, but.

it is to be understood that I do not thereby intend to limit the invention to the specific form disclosed, but intend to cover all modifications and alternative constructions falling within the spirit has customarily been employed between the elec-.

trode elements. Among serious disadvantages resulting from the use of a spacing member is the possibility that a short circuit may be set up through the member. Moreover, the spacing of the electrode elements may be uneven and the quantity of electrolyte between the electrode elements not uniform. According to the present invention the spacing member is eliminated, along with the disadvantages inherent in its use, and the electrode elements are spaced apart only by a thin coating of electrolyte which preferably is formed directly on the face of one of the electrode elements. Furthermore, the size of the condenser unit is reduced and the cost of manufacture lowered.

To attain the foregoing result, a suitable metal, in the form of a thin sheet or foil, is selected as the base material for the electrode elements. One such sheet is processed to produce thereon a thin film or coat of a compound having high electrical resistance properties. This sheet is usually the anode element of the condenser. Another metallic sheet is processed to produce thereon a surface coating of a compound characterized byits capability for subsequent conversion into an electrolytic substance which is, in effect, anintegral part of the sheet. The sheet thus produced is usually the cathode element of the condenser. After the anode and cathode elements have been completed, they are placed together with the coatings thereon in contact and wound into a helical assembly which is placed in a suitable con-- tainer, sealed in a novel manner to insure against the entrance or egress of moisture, and formed by controllably subjecting the unit to the maximum electrical energy for which it is intended.

Preferably the anode coating and the initial cathode coating are electrically produced, the

anode being coated by electrolysis and the oathode by an action which closely simulates, if it is not actually, an electroplating one. In one embodiment of the invention, aluminum in the form of foil or thin sheets, is preferred as the metallic material from which the anode is produced.

Aluminum oxide, which has high electrical resistivity, is an eminently satisfactory anode coating and may be produced on the surface of the foil in any suitable manner. It is preferred, however, to subject the aluminum foil to the action of an electrolytic cell in which the foil is the proper electrode to receive an oxide coating. As an example of such a cell, the foil is connected as the anode of the cell, using an aluminum plate, rod or other suitable electrode as the cathode.

The electrolyte is a solution of a magnesium salt of a weak acid, boric acid being preferred, since the borate condenser apparently is capable of operating under the highest working voltage, but tartaric, citric or like acids being suitable according to the characteristics desired in the ultimate unit. Considering the solution as being one of magnesium borate, it is found that more satisfactory results are obtained if the magnesium borate is chemically produced in the preparation of the electrolyte since more accurate control may be had. In illustration of one method of preparing the electrolyte 4 to 5 parts of boric acid and 1 part of magnesium carbonate or other compound of magnesium, such as the oxide, which will react with the acid, are taken into solution with sufficient water to produce about a 10% concentration, heat being applied if required. The proportions are approximate, the aim being to convert the carbonate to the borate with an excess of acid which will produce a slight but definite acidity.

If a direct current is passed through such a cell, an aluminum oxide coat is produced on both surfaces of the aluminum anode foil and the action is continued until the current flow is substantially zero and the potential impressed is above the intended working voltage. To expedite the action, the electrolyte is preferably initially heated to about 135 F., this temperature being subsequently self-maintained.

As an illustration of the operation of the cell, if the condenser is intended for a working voltage of 475 volts, an initial direct current of onetenth ampere per square inch of anode foil area at to volts may be used. The action is completed when the current falls to approximately zero and the potential is approximately 500 volts.

As a final step in the preparation of the anode element, the oxide coated foil is removed from the electrolyte and dried.

The formation of the initial coating on the cathode element is quite similar except that the coat is produced by what appears to be an electroplating action. Substantially the same electrolyte may be employed with this difference; the proportions of magnesium carbonate and boric acid are varied so that a carbonate excess is present to provide an alkaline solution. Approximately 4 parts of boric acid to one plus part of magnesium carbonate give satisfactory results. As the anode of the cell, an electrode of magnesium is preferred since an oxide coating is thereby avoided and a renewal (which may be controlled by the current employed) of the magnesium concentration in the solution is obtained. The cathode is preferably an aluminum foil similar to that to "which the oxide coat was applied. Magnesium maybe used in place of aluminum. Across a cell of this nature a direct current is impressed to produce a relatively heavy film on both sides of the cathode foil, which film appears to' consist of the hydroxides or hydrates of magnesium (principally) and aluminum ,(from the cathode foil). Where a magnesium cathodeis used the same process may be employed or other magnesium salts, for example the carbonate, may be suitably produced as by the conversion of the surface of the cathode to the salt.

If the action .is continued a suflicient length of time, a crystalline deposit on the cathode is produced. This deposit is undesirable because it interferes with subsequent steps. Hence the action is continued as long as possible to obtain a heavy coat but is discontinued prior to crystal deposition or formation. It has been found that a direct current of approximately three-tenths amperes per square inch of cathode area at a potential of from 10 to 15 volts will require about fifteen minutes to form the desired coat without crystals.

Upon completion of the initial cathode forming step, the cathode and coating thereon are dried.

The final cathode forming step is the conversion, preferably by chemical reaction, of the hydroxide or hydrate coating to one which has the required properties and characteristics of an electrolyte for a condenser of this'nature. Thus, the coated cathode foil is subjected to the action of a suitable acid which preferably is the one used originally in forming the electroplating solution. Where, as in the particular illustration herein given, the cathode coat is obtained from a magnesium borate solution, the cathode is finally treated with boric acid.

The conversion by the acid is carried outin the presence of a suitable hygroscopic agent such as glycerine or any of the equivalent glycerol substances having hygroscopic properties. This agent provides in the electrolyte a certain amount. of moisture necessary to the operation of the condenser. Whether the glycerine or other hygroscopic agent enters into the reaction is not definitely known but the result is a coating on the cathode which is greyish, gelatinous, tacky and firmly adheres to. the cathode. The coating is of uniform thickness and consistency and is relatively heavier than the original hydrate coat.

Exemplary of an. acid bath which has been found satisfactory is one comprising the approximate proportions of boric acid, from 15% to 25% concentration, and glycerine, about 15% by volume. This solution is preferably used at a temperature of around F. The reaction is allowed to proceed approximately to completion and the resulting electrolyte apparently contains magnesium and aluminum borates, some free boric acid and glycerine.

When the electrolytic coating has been formed, the cathode is removed from the acid bath and the excess solution permitted to drain therefrom. The hygroscopic substance in the coating, whether in chemical combination or mechanically held, is present in the amount necessary to supply the required moisture for electrolytic action. The introduction of the hygroscopic agent is, of course, controlled primarily by the concentration thereof in the acid solution, and this control is not disturbed since both theoxide and hydrate coats are dry.

To complete the condenser the oxide coated anode and the electrolyte coated cathode are placed in surface abutment, as shown in Fig. 2, wherein the anode element is indicated at ID, the oxide coat thereon at H, while I2 designates the cathode element on which is an inside unconverted layer of hydroxide or hydrate I 3 and an outside layer of electrolyte M. The elements are then wound into helical form without the use of any spacing means other than the coatings on the elements. Since both sides of the anode and cathode foils are coated, the winding produces a unit, generally indicated at l5 (Fig. 1), in which cathode and anode alternate with intermediate properly disposed spacing coatings. It is preferred that the elements be of unequal widths and herein the anode is slightly the narrower as may be seen in Fig. 1. Moreover, the cathode terminates in an extra length which is uncoated and upon winding forms an outer shell i6. Suitable terminals H, H! extend from the anode and cathode respectively beyond one end of the unit for connection with leads iii, the anode terminal being usually protected from corrosion by a wax or like coat.

The unit is, in this instance, housed within a cardboard, fiber or similar closed case 20 which is longer than the unit to provide end spaces for a plug 2! of pitch, wax or other suitable sealing material.

In the present embodiment, the entire unit is sealed by an outside protective means which positively prevents variation of the moisture content of the electrolyte. This means comprises a rubber or rubber-like sheath 22 which is plated, deposited or otherwise applied over the exterior of the case 2| or over the unit if the case is not used. The leads l9, having the usual insulation 23 thereon, extend from the unit, and adjacent thereto the insulation is cut away, as at 24, to bare the leads so that the sheath may extend along the insulation and contact the leads. In this manner a moisture impervious seal is provided which is absolute assurance against the ingress or egress of moisture.

When the condenser unit has been completed it is ready for the so-called forming process. It ispreferred that the condensers be formed in the following manner. A completed but unformed condenser is connected in a circuit and current under a potential which is substantially lower than the intended working voltage is applied. Initially a relatively high current loss is noted due to leakage within the condenser, but this loss gradually decreases as current blocking films are formed. When an allowable minimum leakage is obtained, the potential is increased by a fraction of the difference between the applied and working voltages and the leakage again allowed to reach said minimum. This procedure is repeated until at the final stage the working voltage is applied and minimum leakage obtained. This forming method has the advantage that the newly assembled condenser unit is not subjected at once to the full strength of the electrical energy under which it is intended to operate.

The process which has been previously described relates primarily to the separate formation of the anode and cathode elements. However, if the solution of the oxide forming or electroplating baths are brought to a neutral condition, instead of one of acidity or alkalinity, both the anode oxidizing, and the initial cathode coating processes may be simultaneously performed, using the foils or sheets as the anode and cathode of the electrolytic cell. This process is convenient where space is at a premium but is somewhat slower since the increasing resistance of the oxide coat delays the action. After the cathode is initially coated, the remaining steps of the process are performed as previously disclosed.

The preferred physical form of the foil which is used is shown diagrammatically in Fig. 3.

The foil is pressed in an operation somewhat similar to embossing, in order to produce many small peaks and indentations across the foil surface. While these surface configurations are quite small, the surface area of the foil is ma terially increased and better adherence of the respective coatings thereto is obtained. By mechanically producing these configurations uniformity is obtained.

It will be evident from the foregoing that a new and improved condenser has been provided which possesses many advantages not heretofore obtainable. Due to the absence of a separate spacing member, the possibility of a short circuit is minimized and the condenser is self-healing since there is no impedance to the electrolytic action in closing pin holes therein. The novel process is simple, easy to control and is readily adapted for quantity production of condensers having a high degree of uniformity. The condenser in its final form embodies the proper quantity of moisture for efficient operation and the impervious seal thereon insures the constancy thereof. All of these factors result in a unit which is of high capacity and long life, will withstand a high break-down voltage and may be economically manufactured for sale at a relatively low cost.

I claim as my inventlonr 1. The process of making a condenser which includes the steps of electrically producing a metallic hydroxide or hydrate coating on the surface of an aluminum foil, drying said coating, and subjecting the coated foil to the action of a solution containing boric acid and a hygroscopic medium to convert the initially produced coating to a borate film which includes a quantity of said medium.

2. The process of making a condenser which includes thesteps of producing a coating on the surface of an aluminum foil by connecting said foil as the cathode in a direct current cell using an alkaline magnesium borate solution as an electrolyte, and subsequently chemically converting said coating by the action of boric acid in a solution containing glycerlne.

3. The process of making a condenser which includes the step of providing on the surface of a metallic foil electrode an electrolytic compound embodying a conversion product of said electrode.

4. The process of producing a condenser which includes the steps of electrolytically forming a high resistant coating on a metallic electrode, drying said coating, electrically forming a coating on a coacting metallic electrode characterized by having-inherent reaction properties with an electrolyte producing reagent, drying said coating, subjecting said last mentioned coating to such a reagent in the presence of a hygroscopic medium to produce an electrolyte layer containing such medium, and assembling said electrodes with said resistant coating and layer in contact.

5. The process of producing a condenser which includes the steps of depositing an oxide coat on an aluminum foil electrode by electrolysis from a.

cell which includes a solution of magnesium borate obtained from the reaction in said solution of a magnesium salt and boric acid, electrically depositing a hydroxide or hydrate coat on a similar electrode from a similar solution, converting includes the steps of producing an oxide coat on an aluminum foil electrode by electrolysis from a cell which includes an acidic solution of magnesium horate obtained from the reaction in said solution of a magnesium salt and boric acid, electrically depositing a hydroxide or hydratecoat on a similar electrode from a similarly produced but basic solution, converting the hydrate coat into a borate substance by the action oi! boric acid thereon, performing said conversion in the presence of a hygroscopic medium, and cooperatively assembling said electrodes.

7.In the process of making a condenser, the steps of providing on the surface of a metallic electrode an integral coating formed at least partly by a reaction of the metal of said electrode, and converting said coating into an electrolyte.

8. The process of forming a condenser which includes the steps of electrolytically forming a high resistant coating on a metallic electrode,

electrically forming a coating on a coacting metallic electrode characterized by having inherent reaction properties with an electrolyte producing reagent, subjecting said last mentioned coating to such a reagent to produce an electrolyte layer integral with the associated electrode, and assembling said electrodes with said resistant coating and layer in contact.

9. The process of producing a condenser which includes the steps of forming a high resistant coating on a metallic electrode, electrically forming a coating on a coacting metallic electrode characterized by having inherent reaction properties with an electrolyte producing reagent, drying said coating, subjecting said last mentioned coating to such a reagent to produce an electrolyte layer, and assembling said electrodes with said resistant coating and layer in contact.

KURT n SCI-HMKUS.

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