US2094048A - Method of making electrolytic cells - Google Patents

Description

Sept. 28, 1937. s, slEGEL 2,094,048

METHOD OF MAKING ELECTROLYTIC CELLS Filed May 5, 193] I .5. I INVENTSR I Z 1 B garrzae Jaye 8-&M %M 4? ATTORNEY5 Q-sv Patented Sept. 28, 1937 UNITED STATES PATENT OFFICE Samuel Siegel, Brooklyn,

vox Corporation,

N. Y., assignor to Ama corporation of New York Application May 5, 1931, Serial No. 535,180

3Clalms.

My present invention, considered from one aspect, relates to electrolytic cells useful, for instance, as rectifiers and lightning arr-esters, but peculiarly applicable to condensers of the electrolytic type in which the dielectric is formed in a thin film upon the surface of the anode by electrochemical action.

The invention considered from another aspect is concerned generally with pretreatment of aluin minum, particularly as applied to the anodes of an electrolytic cell.

The invention will be discussed primarily in its relation to the problem of condenser manufacture, it being understood that both the process and the product of the invention, however, are

susceptible of use in other relations.

The prior Patent No. 1,789,949, to Alexander Georgiev issued January 20th, 1931, discloses and claims an essentially dry electrolytic condenser of a dependable type, affording high capacitance in small bulk, of low electric leakage, self-healing in character and especially adapted for use with out likelihood of breakdown in filtering circuits and power supply units of radio receiving sets, in telephony and in other relations. This patent discloses an especially effective process for manufacturing such condensers, particularly for impregnating the condensers and forming the desired dielectric film on the anode.

Reference is therefore had to said prior patent for a more complete understanding of one type of electrolytic condenser and one method of condenser manufacture with which the present invention is of particular utility, it being understood however, that the invention may be advantageously employed in connection with condensers and other electrolytic cells of various kinds.

An object of the present invention is to provide a condenser in which there will be no material increase in leakage current even when relatively high voltages are imposed upon the condenser after long periods of idleness.

Another object is to provide a condenser hav- 45 ing peculiarly rapid self-healing characteristics so that increased leakage due to exceptional conditions of use would promptly diminish to normal when regular operating conditions are restored.

Another object is to provide in a construction of this character, a specially prepared anode, highly receptive to the electro-chemical process which builds up the dielectric film, whereby the time required for accomplishing the action is very materially reduced.

Another object is to provide a condenser of the above type of less expensive construction and 01' less bulk than prior devices of the same general structure and having the same capacitance; and withstanding higher breakdown voltage.

Another object is to pretreat the anode plate of the condenser in such manner that the total 5 time required for manufacture of the condenser is greatly reduced despite the time required for pretreatment, and the cost of the condenser is reduced despite the materials used and the energy consumed in such pretreatment.

In accordance with a preferred embodiment of the invention, the anode plate or foil of the condenser is subjected to an electro-chemical process in a bath of sulphuric acid or equivalent material, This treatment effectively cleans the surface of the aluminum, the acid dissolving impurities from the surface.

A coating is built up by the electro-chemical reaction in pretreatment, materially increasing the receptivity of the anode to the subsequent electro-chemical process which is used to apply the dielectric film according to the prior patent.

The electro-chemical reaction causes release of oxygen from the sulphuric acid bath with consequent oxidation of the foil or plate. Within the range of equivalency above alluded to and embraced within the scope of the claims, is any suitable ionizing compound which, substantially like sulphuric acid, is capable of releasing oxygen during the electro-chemical process.

The receptivity due to the treatment above set forth seems to characterize the condenser throughout its life in that condensers having their anodes so treated, have exceptionally low initial electrical leakage properties even after long periods of idleness and exceptionally low operating leakage properties as well.

The anode plates after pretreatment in the acid bath are washed in ordinary water and dried and the subsequent manufacture of the condenser may proceed in the manner set forth in said prior patent, a thin electrolyte impregnated gauze interposed between the cathode and anode plates or foils being feasible and preferred to a thicker one of two or more plies.

The invention may be more fully understood from the following description in connection with the accompanying drawing, wherein Fig. 1 is a perspective view showing the constituent elements of the condenser before winding 0 and with sections broken away for the sake of clearness.

Fig. 2 is a perspective view showing the condenser completely wound.

Fig. 3 is a diagrammatic view showing anode foils being pretreated by the use of alternating current.

Fig. 4 is a similar view showing diagrammatically the apparatus for pretreatment using direct current, and

Fig. 5 is a diagrammatic view showing a condenser roll being formed.

For the purposes of my present invention, the structure of a condenser made by the process here disclosed may be briefly described as follows: The condenser includes a pair of armature foils, comprising an anode i0 and a cathode ll, of suitable film-forming material, preferably aluminum, with an absorbent separator between them. This separator may comprise an interposed sheet if of gauze, paper or the'like, of width greater than that of the foils, and covering the total area of the anode, in order to insure maximum capacity and best forming.

The cathode foil ii is provided with a terminal tab iiipreferably formed by slitting the foil transversely near one end thereof for nearly its entire width, to provide a tongue l5, which is reversely bent as at E6, so that the tab may protrude beyond one end of the condenser roll. The anode foil 50 is similarly provided with a similar terminal'tab it, preferably at its opposite end and protruding from the opposite edge of the condenser roll.

The superposed layers of foil and absorbent medium are wound together to provide a roll with the terminal tab l4 of the anode foil preferably projecting from the center of the roll and the cathode foil terminal I3 projecting from the surface of the roll. The gauze or other absorbent electrolytic retainer should extend at least to the free protruding edge of the anode foil and preferably to the edge of the cathode foil and the roll is held against unwinding by an encircling band 23. 40

The interlay of the condenser roll is now impregnated with a suitable electrolyte. This may be accomplished by immersion of the complete dry roll in a tank containing the hot electrolyte.

' Various electrolytes commonly'employed for the purpose, may be used, such for instance, as the glycerol and carbonate of soda disclosed in the prior patent to Hickley, No. 900,278 of October 6th, 1908, but the voltage withstood by the condenser, and also its capacitance, will necessarily depend to some extent upon the choice of electro- 157 8, and I prefer the electrolyte disclosed in the copending application of Alexander Georgiev, since issued on July 21, 1931, as Patent No. 1,815,768. The best results are obtained as set forth in said patent when the electrolyte includes a polyhydrlc alcohol, a. weak acid such as boric acid and ammonia.

The condenser elements are then subjected to the forming operation, (1. e. dielectric film formation) by passing a current through the condensers, preferably by laying them in trays 20 of glass (Fig. 5) or other material unaffected by the electrolyte solution and preferably filled with liquid identical with the batch in the impregnation tank. During this process the terminal tabs.

.may follow that disclosed in said prior patent,

and need not be discussed here. It will be understood however, that during the 'forming process, the voltage impressed upon the convoltage slightly exceeding the peak voltage on which the condenser is to be used, in a wide field of application at a tension of 600 volts more or less.

The foregoing description of the condenser and its process of manufacture, are introduced here, primarily because they are conducive to a better understanding of one type of construction and one process in conjunction and in combination with which the present invention may be used in especially advantageous fashion. In accordance with the present invention, the anode plates are pretreated before they are mechanically correlated with their cathodes and formed. If desired, the anodes may be cleaned before pretreating, in some alkaline solution, such for instance, as a combination of sodium carbonate and trisodium phosphate. If the plates are so cleaned, cleaning must be followed by a thorough rinsing in water. In practice, however. I have discovered that cleaning is not essential since the pretreatment itself effectively dissolves practically all the undesirable foreign matter from the surface of the aluminum.

The pretreating bath preferably consists of sulphuric acid diluted in water which need not' be distilled; in fact ordinary tap water has been found suitable. In practice, a solution of 7% to 35% of sulphuric acid is suitable for the purpose and a good solution is obtained at 10%. The tank in which the pretreatment takes place, may be of any conventional type. If direct current is used, the tank 24 as is shown in Fig.4, could be of, or lined with a suitable metal, like lead in order to serve as the cathode and resist the corrosive action of the acid. The aluminum foils to be coated are immersed in the bath and constitute the anodes. One side of the line 25 is connected to tank 24 and the other side of the line 26 is connected in parallel to all of the foils. If alternating curent is used, the current is passed between two foils or two sets of foils connected to opposite sides of the electric line, and both sets of foils become treated as anodes. When using alternating current for the pretreatment, the tank 21 (Fig. 3) should preferably be made of some non-conductor, such as stone, wood, hard rubber or the like. Here alternate foils are connected to opposite sides 28, 29 of the line.

When using direct current, about twenty volts is found satisfactory for the process and when using alternating currents, about twenty volts R. M. 5. may be applied although lower and higher voltages also are practicable. When using direct current, at the above voltage the anodes are preferably treated electrically for about fifteen minutes and when using alternating current at the above voltage the time of treatment is approximatelydoubled. A current density of 15 to 30 amperes per square foot has proved suitable in practice. The temperature of the sulphuric acid during treatment should preferably be maintained below degrees F. A temperature of about 55 degrees is preferable.

The current density, the concentration of the solution and the duration of the electrolytic pretreatment determine the characteristics of the Afterthe electrolytic coating in the acid soluj tion has been completed, the anode foils are thoroughly rinsed with tap water and dried preferably at a temperature around 170 degrees F. If the anodeandits formed coating are not first adequately dried, corrosion is apt toensue during thefilm forming step or in service, and the same difficulty arises toa degree, if the drying-,is'performedslowly. Atthis stageofthe process, the foil presents athincoating which cracks in an audible andevisible manner when the foil is bent. Apparently, the coating is pervious, absorbentor porous in structure and is so closely adherent tothevfoil that the cracking does not dislodge it or interfere with the subsequent formingoi the foil on high voltage orotherwise impair the good properties of the condenser.

While an anode of foil or' other material thus treated, apparently possesses highly desirable characteristics for subsequent use-in electrolytic cells of many types, ithas noteworthy advantages in connection with the manufacture of condensers such asthat described above. The surv face of the anode after coating in the diluted sulphuric acid, becomes freed'of impurities and is in a condition where it promotes the main electro-chernical film forming process under high voltage reducing the forrning time by 66% or more. The pretreatment process on the-foil, followed by the forming of the main dielectric film results in a composite covering composed of the coating andthe film, both chemically combined with the foil and intimately associated therewith and with each other.

An important advantage is the minimizing of the so-called deforming of the condenser which may manifest itself as relatively high initial leakage currents when voltage is applied to the condenser after long periods of disuse. This leakage current in an electrolytic condenser, gradually drops until it reaches its regular operating value, which in a good condenser is very low, say a small fraction of one milliampere per microfarad. While in many cases, this deformation" is not a serious defect, it does, however, present a problem when the condensers are used in certain circuits, where only very low initial leakage currents are permissible.

By the above described pretreatment of the anode, such deforming is practically prevented, and the condenser preserves its good leakage characteristics over long periods of idleness. In a remarkably short time, after voltage application, a condenser pretreated in accordance with the present invention, registers a very low leakage current, in fact, a leakage current which is substantially at its regular operating value and negligible for all ordinary purposes.

Due to the aptitude of a condenser pretreated in accordance with the present invention, to form rapidly, its self healing characteristics are materially enhanced. In other words, if the film should be injured, due to the application of an excessive voltage, it reforms quicklyand regains its good operating characteristics with great alacrity. A condenser pretreated in accordance with the present invention, may be used with far greater safety on high voltages.

The phenomena of quicker forming and rehealing and less liability to leakage currents attained by the use of the present invention, have been thoroughly demonstrated and verified both in the laboratory and in actual commercial service.

' 'I'he forming operati manentlyiinjuring 'even the small amount of elec- :trolyte atthe center of the condenser, is substantially negligible.

g In resuming operation after a period of idleness the lower leakage currents compared with initial "coating designates the result of the pretreatment step,

on the foil due to the successive coating and filming operations.

The capacitance of the conthe operating voltage of the mechanism not understood idleness of the condenser.

It will thus be seen that scribed ent of the United States is:

1. A method which consists in electro-chemical action at a considerably higher voltage in the presence of an electrolyte differing from the sulphuric acid bath of character to produce a film or higher dielectric constant than that produced in the sulphuric acid bath.

2. A method of forming aluminum anodes which consists in producing a coating thereon by an electro-chemical reaction, performed in a sulphuric acid bath, at a tension in the order of 20 volts, rinsing the coated anode in water, drying the same rapidly at a. temperature in the neighborhood of 170 degrees F. and finally forming a dielectric film by subjecting the previously coated anode to an electro-chemical action at a voltage progressively rising to above the working voltage of the finished condenser in the order of 500 volts or more, said latter electro-chemical action being conducted in an electrolyte differing from the sulphuric acid bath.

3. A method of forming aluminum foil which consists in producing an oxide coating thereon by an electro-chemical reaction performed in a sulphuric acid ionizing bath at a relatively low voltage rapidly drying the foil after the sulphuric acid treatment and then forming a thin film of high dielectric properties intimately associated with said coating by subjecting the previously coated anode to an electro-chemical action at a considerably higher voltage in the presence of an electrolyte composed of polyhydric alcohol, boric acid and ammonia.

SAMUEL SIEGEL.

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