US1935860A - Electrolytic device - Google Patents

Description

Patented Nov. 21, 1933 UNITED STATES PATENT OFFICE ELECTROLYTIC DEVICE -No Drawing. Application August 2, 1932 Serial No. 627,305

9' Claims.

This invention relates to the manufacture of electrolytic condensers of the so-called semi-dry or dry type and more particularly to a method of treating the completed condensers to improve their electric characteristics.

Electrolytic condensers .of the above type consist of two metal foil electrodes between which is interposed a highly viscous or pasty electrolyte. One or both of the metal foils are of film-form- 10 ing metal, for instance of aluminum, tantalum, titanium etc., and at least one of the electrodes has a film formed thereupon.

In the case of condensers used with rectified or direct current as a rule only one electrode, namely,

the anode, is filmed although the cathode may be also of a film-forming metal as aluminum. In

case of condensers used with alternating current,

as a rule both electrodes are of film-forming metal which are mostly both formed.

This film is a partly hydrated oxide of the film-forming metal; for instance in the case of aluminum, it is a partly hydrated aluminum oxide film. The electrolyte comprises as a rule a weak acid and preferably also a salt of a weak acid, for instance, boric acid, phosphoric acid, citric acid, tartaric acid, etc., and salts of such acids whereby the salt does not need to be the salt of the acid used. The electrolyte also comprises a viscous ionizing solvent, for instance, a polyhydric alcohol, as glycerine orethyleneglycol and preferably also a small amount of water also acting as an ionizing solvent. In addition the electrolyte may comprise inert substances to increase the viscosity of the electrolyte. In most cases the 5 electrolyte is carried by a fibrous spacer, for instance a gauze.

As is well known, the condenser action of such devices is based on the peculiar properties of the electro-chemically formed film and the charac-- teristics of the condensers are primarily dependent on the quality of this film.

A method which is particularly well suited to .obtain semi-dry or dry condensers of excellent characteristics, is described in my co-pending application, Serial No. 548,270, filed July 1, 1931.

As has been set forth in said application, socalled dry or semi-dry electrolytic condensers having highly viscous electrolytes and closely spaced electrodes, present conditions not present in the case of so-called wet electrolytic condensers. In the condensers of the dry or semi-dry type it is desirable to keep the film as thin as possible and my prior application describes a method in which a thin film of excellent properties can be obtained by a short-time continuous formation process,

especially when a high purity aluminum and proper electrolytes are used.

Thereby, the film-forming electrode or electrodes are formed prior to the assembly of the condenser. Preferably endless sheets of the film- 0 ing metal, for instance, of aluminum, are formed in a continuous process and the foil then cut to the length required for the electrodes of the condensers. This causes, however, the presence of unformed or raw edges of the condenser. At the 5 same time the aluminum being a soft, ductile metal and the oxide film formed thereon being hard and brittle, the film cracks when the foil is wound into a roll or might be otherwise impaired during handling. The defects due to raw edges may be partially overcome by coating them with insulating material or by subjecting the aluminum strips to a second forming process after they have been cut to the proper length. Such treatments, while they may be beneficial, do not quite overcome the impairments of the film, and especially do not take care of the cracks occurring in the film due to the rolling and other handling of the condenser.

In the above-referred to prior application I have described an equalizing process to which the completed condensers are subjected arid which to a great extent overcomes the above differences. This process briefiy consists in applying to the completed condenser for a period of about ten minutes the maximum operating voltage and simultaneously heating the condensers to about 110 F.

I have found that while the above process considerably improves the characteristics of the condensers and reduces the number of rejects, a processing which comprises two different treatments and which will be described below, improves the characteristics of the condensers to a much greater extent and eliminates rejects altogether.

The two steps of my novel treating process are as follows: The first step consists in subjecting the assembled condensers in a cold state to a no voltage equal to the maximum peak voltage which the condenser has to stand in operation and connecting a resistance in series with the source of supply and the condenser. For instance, in case of condensers which are to be operated up to voltages of 550 volts peak value, I apply a D. C. voltage of 550 volts and insert a series resistance the value of which is inversely proportional to the-capacity of the condenser and is approximately 100,000 ohms for one micro-fared. Thus for 10 mfd. the resistance will have a value of 10,000 ohms.

No outside heat is applied to the condensers during this treatment, but some heat development takes place due to the electric current passing same. The condensers are subject to this treatment for a given time until the leakage current drops to a definite value, this time being 10 to 60 minutes depending upon various factors.

After the condensers have been thus subjected to this first treament they are placed in an oven having a temperature of between 120 to 180 degrees F. and are baked in this oven from 2 to 15 hours, the baking time depending upon the baking I temperature, the lower temperatures requiring a longer time of baking. As the most suitable temperature 140 F. has been found requiring a time of baking of about 6 hours. Higher temperatures, permissible and even advantageous in the case of certain. electrolytes, adversely affect others. On the other hand, temperatures below 140 degrees F. require too long a baking time, which is objectionabe in production. While shorter baking times already have a marked effect, the full benefit of the baking is obtained when the baking time is of the order as given above.

During this baking of the condensers it is not necessary to apply voltage. However, the maximum voltage to which the condensers are to be subjected in operation should be applied directly to the condensers after the baking has been completed. However, I prefer to apply the maximum voltage to the condensers also during part of or during the whole baking process.

While it has been already proposed in the manufacture of electrolytic condensers to use a heat treatment during the formation process, especially in the case of formation of wet electrolytic condensers, my present treatment greatly differs from such prior heat applications.

Heat treatment of wet electrolytic condensers during their formation was proposed to increase the heat resistance of the condensers. In such a process the condensers are heated by the formation current or otherwise during the forming proccess. The heating decreases the film resistance and permits the use of larger froming currents for given forming voltages, thus permitting formation 'with greater energy. The films so formed are of relatively great thickness.

In the present instance the heat treatment does not take place during the formation of the film, but after the film has been formed and the condenser assembled. This treament thus does not affect the energy at which the film is formed, neither the thicknes of the film, nor the heat resistivity of the condenser.

In my method of treatment the first step merely serves to repair the damaged or unfilmed portions of the condenser and this step can be regarded as a repairing process.

Due to the low resistance of the unfilmed portions of the condenser compared to the high resitsance of the filmed portions, the current during this treatment flows almost entirely through the unfilmed portions. Thereby it forms a film on the unfilmed portions and the well filmed portions remain substantially unaffected.

Thus when the condensers are subjected to the second or baking step of my process, they are covered with a substantially uniform thin film. Such a thin film is much less porous and more uniform than a thick film, and its advantage in case of semi-dry and dry condensers has been explained in my prior application, But even with very thin layers porosity and lack of 'uniformity are not entirely eliminated. The reasons therefore are as follows:

As the oxide film is formed on the metal it first forms on the outer surface of the metal and gradually penetrates inwardly. During this film formation oxygen is liberated at the metal, which oxygen is partly used up for film formation, the excess of it escaping through the film. As the film formation or oxidation of the metal progresses inwardly the oxygen liberated at the in ner layers passes through the outside films alreadyformed, thereby increasing their porosity. The thicker the film the more pronounced the difference or porosity of the different layers of the film.

By providing a rapid formation as set forth in my prior application a very thin film is ob tained, but as stated, even such a thin film shows to some extent the above property, namely, that the inside layers of the film are substantially nonporous whereas the outside layers still have a marked porosity or hydration. Such porosity or hydration causes a comparatively high electrolyte resistance adjacent to the film, which is responsible for a major portion of the total electrolyte resistance and of the power factor of the condenser.

When the condensers are subjected to my baking process, the hot electrolyte is readily absorbed by the hydrous outside layers of the film, which causes a very large reduction in the electrolyte to film resistance and in the power factor of the condenser. At the same time, as the effectiveness of the film increases the capacity of the condenser is also increased.

The baking also provides for a uniform mechanical setting of the condenser which also greatly improves its shelf-life, as it provides for more permanent characteristics.

To illustrate the advantage of my present treating process the following data might serve as an illustration Taking condensers of a certain design and dimensions, which condensers are adapted to operate at voltages of 500 volt peak, when such condensers are subjected to my forming process but not subsequently subjected to any equalizing process or further treatment, such condensers will exhibit at 500 volts and F. a leakage current of more than .2 milliamperes per microfarad, whereas the series resistance of the condenser at such temperature and at 120 cycles generally exceeds 50 ohms. Such condensers do not favorably compare with the best wet electroytic condensers now available, and at the same time there is a considerable percentage of rejects.

If similar condensers are subjected to the equalizing treatment, as described in my prior application, the leakage current is reduced well below .2 milliamperes per microfarad and the series resistance to about 15 to 20 ohms. The capacity of the condenser is increased by 1 to 2%.

If similar condensers are subjected to the treatment set forth in the present application the leakage current will be less than .05 milliamps per mfd. and the series resistance decreased to less than 6 ohms. The capacity is increased about 10% and rejects are practically eliminated. The characteristics of such semi-dry or dry condensers far surpass those of the best wet electrolytic condensers available.

It should be noted that in my present process the first step primarily affects the insulation resistance of the condensers and in fact the baking process after such first treatment, does not substantially increase their insulating resistance for normal voltages. However, the baking does increase the insulation resistance near the breakdown voltage thus giving the condensers a better overload characteristic. However, the principal advantage of the baking manifests itself in the very marked decrease in the electrolyteto-film contact resistance and 'thus in the decrease of the power factor. In many instances the baking reduces the power factor of the condensers to one-half to a third of their values before baking. Besides that it also provides for a mechanical setting of the condenser which'increases the permanency of its characteristics.

Instead of subjecting the completed condenser to my treating process it is also possible to provide the filmed electrodes with the electrolyte paste, subject same to my treatment, and subsequently assemble the condenser into a roll or into stacks.

While I have given specific examples of practicing my invention I do not wish to be limited to such examples, but desire the appended claims to be construed as broadly as permissible in view of the prior art.

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

1. In the manufacture of electrolytic condensers comprising a filming electrode and a low fluidity electrolyte, the process which comprises the steps, subjecting the filming electrode toelectrolytic formation, providing the formed electrode with a pasty electrolyte having a solvent which comprises a polyhydric alcohol and water and subjecting same to a baking treat- .ment for a time of two to fifteen hours and a temperature of 120 to 180 F.

2. In the manufacture of electrolytic condensers having a filmed electrode and a highly viscous electrolyte, the process comprising the steps of subjecting the filming electrode to electrolytic formation, providing the formedelectrode-with a pasty electrolyte comprising a weak acid, a salt of a weak acid, a polyhydric alcohol and water and subjecting sameto a baking treatment at a temperature of about 140 F. fora time of about six hours.

3. In the manufacture of electrolytic condensers having a filmed electrode and a highly viscous electrolyte comprising a weak acid, a salt of a weak acid, a polyhydric alcohol and water, the

process comprising the step of subjecting the completed and formed condenser to a baking 55 treatment at a temperature of about 140 1''. for

v a time of about six hours.

4. In the manufacture of electrolytic condensers having a filmed electrode and a highly viscous electrolyte comprising water, the process which comprises subjecting the completed and pre-. formed condenser to a baking treatment at a temperature of about 140 F. for about six hours and applying to the condenser during its baking a direct current voltage of a value substantially equal to the maximum peak voltage which the condenser may be subjected to in operation.

5. In the manufacture of electrolytic condensers having a filmed electrode and a highly viscous electrolyte comprising water, the process which '50 comprises subjecting the completed and preformed condenser to a baking treatment at a temperature of about 140 F. for about six hours and applyingto said condenser after said baking is terminated a direct current voltage of a value about equal to the peak voltage which the condenser has to stand in operation.

6. In the manufacture of electrolytic condensers having a-filmed electrode and an electrolyte of high viscosity and having a solvent comprising a polyhydric alcohol and water, the process which comprises subjecting the formed and completed condensers to a treatment consisting in applying to the condensers in their cold state,

a direct voltage substantially equal to the maximum peak voltage which the condensers have to stand in operation, limiting the current flow through said condensers by a resistance, proceeding with such treatment for a period of 10 to 60 minutes until the leakage current of the condensers has dropped to a desired value, and subjecting the condensers hereafter to a baking operation at a temperature of 120 to 180 F. for two to fifteen hours.

7. In the manufacture of electrolytic condensers having an anode of .film-forming metal and a highly viscous electrolyte comprising the steps of subjecting an endless foil of the film-forming metal to a continuous short-time electrolytic formation, cutting said formed metal into suitable lengths for the anode of the condenser, assembling said anode with a cathode foil and an interposed highly viscous electrolyte having a solvent comprising a polyhydric alcohol and-water into a condenser, subjecting the condenser to a treatment consisting in the application of directpairing formation to film the raw edges of the filming electrode and repair the cracks in the film and subjecting the repaired condenser to a baking process to absorb the electrolyte in the outer layers of the film, said baking taking place at a temperature of about 140 F. for about six hours. i

9. In the manufacture of electrolytic condensers having two electrodes both of film-forming metal and a highly viscous electrolyte comprising a weakacid, the steps which comprise subjecting endless foils of a film-forming metal to a continuous short-time electrolytic formation, cutting said formed foils into suitable lengths and assembling the same with an interposed highly viscous electrolyte having a solvent comprising a polyhydric alcohol and water into a condenser, subjecting the condenser to a treatment consisting in the application of direct current voltage substantially equal to the maximum peak voltage which the condenser has to stand in operation, and subsequently baking said condenser at a temperature of 120 to 180.1". for a time of I to 15 hours.

PREBTON'ROBINBON.