US2168135A - Electrolytic condenser - Google Patents

Description

Aug. 1, 1939. F. PAVELKA ELECTRGLYTIC CONDENSER Filed June 19, 1957 Patented Aug. l, 1939 UNITED STATES PATENT OFFICE ELECTROLYTIC CONDENSER Application June 19,

1937, Serial No. 149,266

In Austria July 1, 1936 9 Claims.

My invention relates to electrolytic condensers, and more particularly to electrodes for such condensers.

One of the main advantages of electrolytic condensers is that, for the same voltage and load capacity, they have a much greater capacity for a given volume than do other types of condensers, such as paper condensers. Numerous attempts have been made to increase this advantage, for

10 example byI roughening the surface of the aluminium body serving as the anode, prior to its electrolytic oxidation by chemical etching, in

` order to greatly increase its surface area, i. e.,

to obtain a large etching factor.

The term etching factor as used herein is to be understood to mean a ratio which is generally determined indirectly by measuring the capacity obtained by using the same forming method with etched foil and with unetched foil; the ratio of these capacities being approximately equal to the etching factor.

As the electrode bodies are generally in the form of foils, I shall refer to same hereinafter as foils for the purpose of simplicity; however this is not intended to limit'the bodies to aluminum foils, or to foils in general.' Attempts to obtain very high etching factors have been made by selecting particular chemical reagents for the etching processor by selecting definite concentrations or Workin temperatures. In such cases, however, it has been difiicult or impossible to obtain very high etching factors which could be reproduced in ymass production.

The main object of my invention is to obtain electrodes for electrolytic condensers which have very high etching factors.

A further object of my invention is to provide electrodes of high etching factors which can be produced with uniformity inl mass production.

Further objects of my invention will appear as the specification progresses. e

As the result of extended metallographic research, I have found that there is a very close relationship between the etching factor obtained and the grain size of the crystallites of the material of the foil, for instance of aluminum, and that the smaller the crystal grain size of the foil, the greater is the etching factor. While this etching factor is not inversely proportional to the grain size, it appears to be a logarithmic function thereof. In addition, however, thematerial of the foil should be such that the current loss, i. e., leakage current of the completed condenser, should be as low as possible.

In accordance with the invention I form such electrodes of a material having a very small grain size, i. e., a very large number of crystals per unit surface area.

Very good results have been obtained by. using aluminium in which the number of individual crystals at the active surface is of the order of and even more than 109 per sq. cm., which corresponds to a grain size of from 0.0001 to 0.001 mm. When making the velectrodes of a condenser of such material, I obtain an etching factor up to and exceeding 13.

By further decreasing the grain size of the aluminum foil of the anode I have found that etching factors up to 15 can be readily obtained. In forming such etched aluminum foils of for instance'0.2 millimeter in thickness in the usual electrolytic bath at a voltage of 650 volts, I secure a. capacity up to and exceeding' about 0.08 mfd. per sq. cm. of anode surface, for example 0.12 mfd. per sq. cm. of surface area. This increase in capacity .is of great advantage, as in prior constructions a value of only about 0.1 mfd. Was'obtained. Thus with anodes having a crystal construction according to the invention, the sur- 'face may be only about half that required with" anodes having a maximum surface area.

To obtain the required small grain size, it is desirable to add very small quantities of admixtures to the aluminum: However I have found that greatly improved results are obtained by using aluminumvof a very high degree of purity, especially when the foil is relatively thick, for yinrstance 0.2 mm. thick.

Electrolytic condensers using electrodes according to the invention and having identical capacity and identical voltage resistance, are considerably smaller than prior art condensers. For example, I can produce an ,electrolytic condenserv which, for instance, at a working voltage of 500 volts and a capacity ofglO mfd. with the casing yin a state of readiness for operation, occupies a space of only 15 cubic centimeters. Thus I obtain a specific capacity up to and above about 0.7 mfd. per cubic cm. and at lower operating voltages it is possible to obtain a correspondingly higher specific capacity. In general, the invention permits of increasing the specific capacity of electrolytic condensers up to and above 3159 mfd.

y l l U per cubic cm.; u designating the operating voltage. Thus the electrolytic condensers according to the invention permit a reduction in the size of various electrical apparatus, such as radio receivorder to give the latter a maximum surface area.

and thus to avoid the connecting eifect; this expression being understood to mean the phenomenon of the gradual decrease in capacity brought about by a sneaking oxide formationrv on the cathode resulting from numerous connecting operations, which are always accompanied by a reversed passage of current.

In order that my invention maybe clearly understood and readily carried into effect, I shall describe same in more detail with reference to the accompanying drawing, in which:y

Figure 1 is a partly-sectionized side-view of a dry electrolytic condenser using an electrode according to the invention.

Fig'. 2 is a microscopic view of a surface portion of the anode of Figure 1 and indicates the crystal structure; and

Fig. 3 ls a microscopic cross-sectional view of the anode of -Figure 1.

'I'h'e dry electrolytic condenser shown in Figure 1 comprises a cup-shaped container 5 having its open end closed with a disc 4 of insulating material. Within container 5 is a condenser unit of-the rolled type comprising two electrodes l and 2 of aluminum .foil separated by a gauze spacer 3 carrying an electrolyte. The electrolyte comprises, for instance, a solution of ammonium borate and boric acid, and water and fglycerlne as the ionizing agent, and if desired also an inert substance; the electrolyte having a low fluidity, for instance a pasty consistency.

Electrodes Iwand 2 -are provided with suitable terminals 6 and 1 respectively, extending through disc 4 in a liquid-tight manner.

In accordance with the invention, either one or both of the foils l and 2' is made'from an M aluminum foil of very small grain size, for in-4 stance smaller than 0.001 mm., and there are about 109 individual crystals per sq. cm. of active surface area. (see Fig. 2)

-The method of producing the material of the electrodes adcordiingy to the invention is described and claimed in my copending U. S. patent application Ser. No. 149,265, led June 19th, 1937; however I shall incorporate a description of the method in the present applicatlorf in order to complete the disclosure.

The small grain size of the aluminum foil'l, which will be considered as the anode, may be obtained by means known for this purpose in metal working. For example, satisfactory results are obtained by rolling in a single rolling operation a comparatively thick aluminum foil, for instance 2 mm. thick, to a nal thickness of about 0.2 mm. 'I'his 9.0% reduction in thickness produces the enormous increase in the number of crystal nuclei required by the invention. It is not essential that the rolling operationbe effected in la single step, but it' may be carried out in several steps, provided 'the usual Aheat treatment between the successive steps, which causes larger crystal individuals, is avoided. On the other hand, it is possible to use a heattreatment which4 is carriedout in such manner that the internal stresses of the material are decreased without materially increasing the size of the crystals.

I have found that after rolling and prior to etching, it is preferable that the foils be subjected to such a tempering operation, which may consist in transient heating. More particularly, after the foil has been reduced to its nal thickness it may be heated to above about 270 C. On the other hand, the reduction in thickness may be effected in such a manner that crystal structure is shattered by the extreme deformation and the crystals are split up into very small particles, in which case recrystallization is prevented by avoiding prolonged subsequent or intermediate heat treatment.4

To reduce the speed of recrystallization, the aluminum may have admxturesalloyed with it, care being taken that the admixtures be such that they do not increase the current of loss (leakage Acurrent) of the completed condenser. Such admixtures may be, for example, very small quantities, i. e., below 0.1%, of alumina, and also beryllium or tantalum, or theoxides of these elements.

I have found that greatly improved results are obtained when, instead of the admixture of slight quantities of impurities, the following procedure is followed.

As a result of systematic X-ray analysis research regarding the structu're of aluminum in connection with the other chemical and physical constants of this material, I have found that aluminum having a very high degree of purity, i. e., a purity greater than 99.8%, and preferably greater than 99.98% has highly characteristic and surprising properties, which are very advantageous when the aluminum has `crystal grains of the size of the present invention, i. e., smaller than' 0.001 mm. and preferably of the order of magnitude of 0.0001 mm. A material having this grain structure may be recognized in radiographs by the absence of individual reilexes and the occurrence of permanent interferences; similar to those with colloidal solutions. v It is known that the greater the purity of base metals, such as aluminum, the better they can, as a rule, resist chemical attack by etching vagents'. This is probably because local elements are formed between the occluded impurities, the metal and the etching liquid, and

' these elements by electrolysis dissolve the metal,

and the smaller amounts of impurities present, the slower this process proceeds.

I have found that in the case of aluminumthe manner in which the chemical attack is effected is also influenced to a large extent by the grain size of the crystallites, and 'that by using aluminum of a. particular purity, and of a particularly small grain structure of the material, a new product is obtained which is resistant to the etching, but which can be chemically etched to a great depthfthus increasing the etching factor.

For example, consider the case of an aluminum foil about 0.2 mm. thick and having a finegrained crystal structure, i. e., a prevailing grain size about 0.0003 mm., and a degree of purity of about 99.99%. `If this foil is subjected for about 16 hours at a temperature less than 20 C. and preferably vbetween and 15l C., to the action of concentrated commercial hydrochloric acid that may have 20% of water added to it, and it is subsequently washed and dried, the

foil will have lost about 45% in weight, whereas no substantial decrease in thickness can be determined by a micrometer. On the otherhand,

if a commercial aluminum foil having a purity size as the crystals.

In contradistinction to the unetched foil, which is absolutely opaque, it is observed upon intensively illuminating one surface of the etched foil, that there is on the opposite surface a diffused luminous glimmer which may obviously be accounted for by repeated reflection of light on -the irregular walls of the minute pores formed in the foil.

That a spongy structure of great flneness, for example such as indicated in Fig. 3, is produced, is shown by the marked capillary action of such foils. For example, if a dry strip of the etchedthrough aluminum foil is placed in pure water at ordinary temperatures, the water rises bycapillary action to a height of about 22 cms.` After electrolytic oxidation of the surface of the foil, which may be effected in the well known manner, the-height .to which the water will riseis decreased, for example to 10 cms., by reason of the altered capillary constant of the material. The marked reduction in vapor pressure of the sucked-up liquid, which is closely related to the capillary action, is also remarkable.

The electrolytic oxidation can be carried out in a bath containing boric acid and ammonia, dissolved in distilled water, the latter being as free from chlorine as possible. 'The formation tension is slowly increased till it reaches the value of about 650 volts.

An aluminiuml foil suitable as a starting material for the etching process described above, may be obtained in the following manner: A sheet of extremely pure aluminum, for instance a purity of about 99.99%, having the usual crystal size, i. e., grain-size about 0.1 mm. and a thickness of about 2 mms. or even larger, is used as the starting material. This sheet is rolled in one or more steps to a final thickness of about 0.2 mm. For example, 2 rolling steps may be used with a 45% reduction in thickness for each step. During the rolling operations the usual grain-increasing intermediate heating operations, which are usually used between the rolling operations, are not used. As has beenl set forth above, this extreme decrease in thickness shatters the crystal individuals so that the required small-grain structure is set up. Although with aluminum having a lower degree of purity, such extreme deformation produces an extremely strong strain on the rolling tools because a product of great hardness is obtained, this is not true to the same extent with very pure aluminum which remains comparatively soft in spite 'of the extreme change in form. This is also of importance in connectionwith the treatment subsequent to the rolling process.

yParticular care must, of course, be taken that the purity of the foil is not decreased during the process, for example by particles of the roller surface sticking to it.

After the final rolling operation the foil can be tempered, i. e., subjected to a heat treatment which, however, as above stated, should be so carried out that merely a relaxation but no increase in size of thecrystals takes place. Fbi' this purpose the heat treatment should not be effected above about 270? C. and should preferably be so carried out as to insure that no differences in temperature are set up between the individual parts of the foil, i. e., by slowly heating the foil to the desired temperature.

The crystal grains produced in the foil by theabove method -are not arranged in a completely irregular manner, but form a ilbrous structure similar, for example, to the brous structure of unfired drawn tungsten wire. The presence of this fibrous structure can be ascertained, particularly after the etching, by the fact that a drop of liquid dropped upon the etched foil does not flow out to form a circle but forms an .ellipse whose major axis coincides with the rolling direction. This structure should be taken into account 'during the further Working, i. e., during the folding, rolling, etc., of the foil.

After being rolled and tempered -in the above manner/ the foil has an oxide Ifilm which` is harmful for many purposes. For example, etching of the above-described type can only be effected after removal of this oxide lm, as otherwise even the concentrated hydrochloric acid vwould not attack the foil. It is characteristic of the chemical lresistance of pure aluminum foil that the oxide lm cannot be removed with a bath of lye, whereas this is directly possible in the case of commercial foils. It is therefore advisable that the oxide film vbe removed mechanically, for example by a rotary steel brush. This brushing operation should, however, be effected in a very cautious and delicate manner, not only to avoid introduction of impurities but also to prevent local increases in temperature and surface irregularities which would affect subsequent attack by etching acid.

The new material produced in the above manner is highly suitable for being used, according to the invention, as an electrode in electrolytic condensers whether the electrode is in the form of a foil, wires, profiled rods, tubes, etc.

Since it is possible for aluminum foil, according to the invention, to be etched through without otherwise losing its coherence, the total active surfacewhich canbe obtained-and thus the etching factoris also dependent on the thickness of the initial material. The etching factor is therefore a function not only of the material used, but also. of the shape of this material. Although the etching factor increases with the thickness of the foil, it does not increase linearly with the thickness but in a smaller degree -so that there is a maximum foil thickness-dependent on the etching process, the grain size, and the degree of purity above which there is no appreciable increase in the etching factor.

The degree of purity of the material is particularly important in connection with thedepth of etching, because with metal containing more than 0.2% of impurities, particularly heavy metals, the etching process proceeds in a different manner, i. e., the etching proceeds essentially parallel to the surface, and not in the perpendicular direction, and the etched pores are coarser and smoother. Although it would appear that the etching proceeds most rapidly along the grain surfaces, and that the new material because of its very fine structure assists the advance of the etching in the depth direction, this theory per se must be considered insumcient because the material would be split up during etching into a such, but is equally applicable to electrodes of4 powder. This, however, is not the case, .because the individual grains of the spongy material produced during etching have relatively large metallic coherence. On the other hand, the pores are interconnected to form the spongy-like material. t

The purer the material used, the more concentrated must be the etching liquid, for example hydrochloric acid, and the longer this liquid must act to produce the desired degree of etching. On 4the other hand, the optimum composition and acting time of the etching liquid is also dependent upon the thickness of the foil to be etched. In the case of excessive foilthicknesses, for example above 0.2 mm., the etching through is more difficult because, due to the prolonged acting l time required, the portions of the foil nearer the surface are attacked to an excessively high extent. Furthermore, if after etching through the etching liquid is allowed to continue its action, the factor of the increase in surface is again decreased.

It is advisabe that during the manufacture, samples be taken from time to time, for example after rolling, and the grain size tested by X-rays. Furthermore, the extent to which the grain size has advanced should be determined during the etching, since the etching time is dependent not only upon the initial degree of purity of the metal, but also upon the type of impurities present and upon the amount of reaction products in the etching liquid. For instance comparatively small quantities of aluminum chloride in the hydrochloric acid expedite the etching process.

While l have described my invention in connection with a foil electrcde and a dry electrolytic condenser of the rolled type it is not limited to other shapes and` to .electrolytic condensers of other types. Furthermore, the invention can be used in connection with either the anode or cathode or, both. Therefore, I desire the appended claims to be construed as broadly as permissible in view of the prior art.-

What I claim is- 1. An electrode for an electrolytic condenser comprising an etched body of aluminum containing less than 0.05% of impurities, the crystal grain size of the aluminum being less than 0.001 mm.

2. An electrode for an electrolytic condenser comprising an etched body of aluminum containing less than 0.02% of impurities, the crystal grain-size of the aluminum being less than 0.001 mm.

3. An electrode for an electrolytic condenser comprising an etched body of aluminum containing less than 0.02% of impurities and having a crystal grain-size less than 0.001 mm., the surface of. said electrode having a spongy structure with hollows about equal to the grain-size.

4. An electrode for an electrolytic condenser comprising a body of aluminum containing less than 0.02% of impurities and having a crystal grain-size less than 0.001 mm., said body being etched through and having throughout a spongy structure provided with hollows about equal to the grain-size.

5. An electrode for an electrolytic condenser comprising a body of aluminum containing less than 0.02% of impurities and having a crystal grain-size less than 0.001 mm., said body being etched through and having a spongy structure and a capillarity for pure water at C. of

at least 8 cms. ,Y

6. An electrode fr an electrolytic condenser comprising an etched body of aluminum containing less than 0.02% of impurities and having a crystal grain-size less than 0.001 mm., the system of crystallites of said body having a fibrous structure.

7. An electrode for an electrolytic condenser comprising an etched body of aluminum containing less than 0.05% of impurities and having a crystal grain-size less than 0.001 mm., said body having an etching factor greater than 13. 8. An electrode for an electrolytic condenser comprising an etched body of aluminum containing less than 0.05% of impurities and having a crystal grain-size less than 0.001 mm., said body having an etching factor greater than l5.

`electrolyte and electrodes including an etched aluminum body containing less than 0.05% of impurities and having more than 109 crystal individuals per sq. cm. of active surface, and a di- FRIEDRICH PAvELia li J4 0 9. An electrolytic condenser comprising an

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