DE1151604B - Anodic-electrolytic etching process to enlarge the surface of electrode starting material for electrolytic capacitors - Google Patents

Description

GERMAN

PATENT OFFICE

B 20819 Vnic / 21g

REGISTRATION DATE: JUNE 16, 1952

NOTICE THE REGISTRATION AND ISSUE OF THE EDITORIAL: JULY 18, 1963

The invention relates to an anodic-electrolytic etching process for increasing the surface area of aluminum or aluminum alloys as electrode base material for electrolytic capacitors, to such an extent and in such a way that the etched metal when it is formed anodically at a voltage of 500 V, has a high capacity increase.

In order to enlarge the surface of such an electrode, electrolytic etching processes are proposed have been brought in which a smooth or substantially smoothed direct current is used and etching methods using alternating current have also been proposed.

It is also already known in the case of methods for bringing about a cathodic or anodic Surface treatment of metals alternating current using superimposed direct current too use. With this method, however, the polarity is changed with each pulsation of the treatment current vice versa.

By using one or the other of these methods, it is possible to achieve a considerable amount To achieve an increase in capacity. This increase can be expressed as the ratio between the capacitance at a certain or given forming voltage, the one with a coating has provided electrode, which has an etched surface, to the capacitance that at the same Forming tension a smooth surface provided in the same way with a coating Electrode of the same nominal size and shape received. This ratio should increase with capacity " are designated. By some of the proposed methods of electrolytic By etching aluminum electrodes it is possible to make capacitance increases of about 10 in cases to achieve in which the forming voltage does not exceed 100 V; however, it is not yet possible if forming voltages in the range of 500 V were used, by any of the known electrolytic etching process an increase in capacity of more than 4 in foils of 0.1 mm, this information being based on capacitance measurements carried out on electrodes were immersed in an aqueous electrolyte composed of a solution of boric acid Ammonium or boric acid of the kind which is usually used for forming purposes will.

If, however, the capacitance values were measured with the electrodes in a viscous working electro-

Anodic-electrolytic etching process

to enlarge the surface

of electrode starting material

for electrolytic capacitors

Applicant: British Dielectric Research Limited, London

Representative: Dipl.-Ing. E. Schubert, patent attorney, Siegen, Oranienstr. 14th

Claimed priority: Great Britain June 19, 1951 (No. 14 559)

Francis Joseph Peter Julius Burger, London, and Victor Frederick George TuIl,

South Harrow, Middlesex (Great Britain) have been named as inventors

immerse lytes, for example from a solution of boric acid ammonium or boric acid in ethylene glycol exists and is widely used in so-called electrolytic dry capacitors finds, significantly lower capacitance values were found for electrodes that were operated by means of the known method were electrolytically etched, with the result that the capacity increase is a much lower one.

It is the object of the invention to provide a method for anodic-electrolytic etching of foils made of aluminum or to create aluminum compounds as electrodes for electrolytic capacitors which an increase in capacity is achieved, which is significantly higher than the aforementioned, whereby a multiplication of the surface through the formation of as deep as possible and as many as possible etchings per unit area is achieved without the formation of a perforation in the film.

This object is achieved according to the invention in that the etching current is a unipolar above a Minimum pulsating current is used, which has a pulsation extent of 50 to 250 ■% and has a pulsation frequency below 2000 Hz, and that the amount of pulsation is within said range is chosen so that with a subsequent formation with a voltage of 500VoIt a

309 647/185

more than five times the capacity increase compared to one built with unetched metal and in the same way formed capacitor enters.

The etching current can pulsate sinusoidally or rectangularly.

Another embodiment of the method according to the invention is that the etching current is a Direct current of rectangular or approximately rectangular shape with current peaks is essential, the duration of which is shorter than the duration of the intervals between the successive current peaks, the degree of pulsation being selected to be no smaller than 80%.

The etching current can also be one with a rounded apex shape, which is caused by rectification and partial smoothing of mains alternating current or alternating current with sine wave form or approximate Sinusoidal waveform is obtained, then the amount of pulsation is chosen in the range of 80 to 150%.

Under a unipolar, pulsating direct current above a minimum value, a in a Direction of flowing uninterrupted current can be understood, the current value of which changes periodically changes, and the term "degree of pulsation" should be understood to mean the total change in current, expressed as a percentage of the mean current value. It is already an alternating current electrolytic etching process has been proposed by means of which the effective surface of one with a coating film provided electrode of an electrolytic capacitor is to be enlarged. This procedure yields an increase in capacity which is only slightly greater than that obtained by an etching process can be achieved with smoothed direct current, but is much less than that which is achieved by a unipolar, pulsating direct current above a minimum value is obtained, if the others Conditions are the same. This well-known procedure is accompanied by undesirable side effects, namely from an exceptionally high loss factor and surface contamination.

It has been found that when other factors are held constant, a particular one mean current value, the etching depth increases with the degree of pulsation, and a large amount of pulsation favors the formation of deep etching. The etchings are, if none high average current density is used, although low, but only achievable in small numbers. By using a unipolar current pulsating above a minimum value with a high average Density and a high degree of pulsation, deep etchings are produced in large numbers per Unit area of the electrode achieved. Since every deep erosion is a strong local enlargement of the brings effective surface area of the electrode with it, results in the combination of a large amount of pulsation with a high current density, a large increase in capacity. The extent of pulsation of the Current that can be used around an etched and essentially formed electrode to achieve, which has a magnification factor at 500VoIt forming voltage, which is essential is higher than that in the known electrolytic processes in which a substantially Smoothed direct current is used depends on the waveform, frequency and mean Density of the pulsating and uninterrupted direct current used from which Composition and concentration of the electrolyte and the etching depth, which is determined by the thickness of the electrode and the condition that perforation must be avoided.

It has already been mentioned that pulsating below a unipolar, above a minimum value Direct current is to be understood as a current whose magnitude changes periodically. The kind or the form of the pulsation can be quite different. Examples of such currents which turn out to be useful have shown are shown in Fig. 1, in which the curve (α) is a time-current curve of a reproduces pulsating direct current, in which the pulsation is sinusoidal. For example, he can can be generated by superimposing an alternating current having a sinusoidal waveform on a direct current. This can be done, as can be seen from Fig. 2, by means of a transformer 1, which of a AC power source 2 is fed via a voltage regulator 3 and its secondary winding in series with a direct current source 4, the circuit of which is via the aluminum anode 5 and the cathode 6 Etching bath 7 runs, expediently a rule. resistor 8 in the circuit and, if necessary, a choke 9 and a shunt capacitor 10 can be provided.

The curve (Jb) in Fig. 1 represents a pulsating uninterrupted direct current of rectangular shape, in which the duration of the individual current peaks is equal to the duration of the intermediate intervals, while the curve (c) represents a pulsating uninterrupted direct current of rectangular shape, in which the Current peaks are significantly narrower than the spaces between them. Curve (d) , on the other hand, represents a pulsating uninterrupted direct current, which also has a rectangular shape, but in which the duration of the individual current peaks is greater than the duration between them. A pulsating uninterrupted direct current of these three waveforms can, as shown in Fig. 3, be generated by means of a variable resistor 11 in series with a circuit which runs from a direct current source 4 to the anode 5 and the cathode 6 of the etching bath 7 and a power-driven rotary switch 12 has, the contacts of which have a corresponding arc length for the purpose of periodic short-circuiting of the resistor 11. A voltage regulator is indicated at 13. Instead of a rotary switch 12, the short-circuiting of the resistor 11 can also be effected by means of a thyratron timer, which is triggered periodically by a capacitor control. Curve (e) is a time-current curve of a unipolar, uninterrupted direct current pulsing above a minimum value with rounded peak values, which can be generated by a circuit arrangement as shown in FIG.
This circuit arrangement has a full-wave rectifier 14 which is fed from an alternating current source 2 via a regulator 3 and a transformer 1. The rectifier 14 supplies the etching bath with the electrodes 5 and 6 via a controllable damping or smoothing device, which is formed from a variable capacitor 15 and / or a variable choke 16, so that the current peak profile of the output

current can be regulated accordingly. The curve (/) in Fig. 1 is a time-current curve of a pulsating uninterrupted direct current with a waveform which has more pronounced current peaks than the curve (e) . The waveform is approximately rectangular, and it can be obtained by two-way rectification of an alternating current whose waveform is significantly different from the sinusoidal waveform, and smoothing is performed to the same extent. Other useful waveforms for the pulsating uninterrupted direct current can be determined by experiment, it must be assumed that a rapid pulsation combined with a relatively small but still noticeable uninterrupted current flow must occur during the intervals between the pulses. It should be noted here that unsmoothed rectified current from a three-phase alternation

The following conditions apply to the composition and concentration of the electrolyte:

1. The purely chemical attack power which the electrolyte has in relation to the electrode to be etched has to be small so that the peculiarities of the etching produced by electrical regulation can be effectively controlled.

2. The electrolyte must not be changed or changed either directly or indirectly by the current passing through it can be disturbed in its structure.

3. The electrolyte must not cause the formation of a high resistance anode layer on the cause electrode to be etched.

Examples of electrolytes which meet these conditions and which have been found to be useful

electricity network or a three-phase network with sine waves are chlorides, chlorates, bromides and iodides form is not usable because of the extent of the powder of sodium, potassium and ammonium. Under these ization (15.3 for half-wave rectification) too low and that unsmoothed rectified current is preferred. Electrolytes, one of which Single-phase alternating current network with sine waves is known that it is used for etching processes with direct current form is also useless, since even with two 25 are not usable, should for the inventive way rectification the degree of pulsation 157 ° / »according to the method are not used. The amount is which turns out to be too high for this wave centering of the electrolyte should be as small as possible shape turned out to be etched. In practice, the lower which is characterized by a strong increase in capacity limit due to the low electrical conductivity or in other words: the minimum of highly dilute solutions determines which value of the current between the pulses is too small. results in excessive heat generation. The extent of pulsation, which concentrations can be used for the application, are between 0.1 and

can come to achieve commercially viable results is than in the range of 50 to 2OO ^fl / o

0.2 n, while concentrations which are significantly higher than 0.2 η have proven to be unusable

for a pulsating uninterrupted direct current have 35,

with sine wave form, as indicated by the curve (α) in The etching depth naturally depends on the thickness of the

Fig. 1 has been determined lying down, while back slide or the other body, which the to

against for currents with square wave form according to corrosive electrode emits. It becomes functional

Curve {b) the useful range is chosen to be somewhat smaller and as large as possible without affecting the

and is between 80 and 195%. For currents with 40 risk of perforations occurring

the curve shape (c) is in the range between 80. The duration of the etching process is therefore of the

and 250 ⁰ Zo, depending on the duration of the one thickness of the film when the other

individual impulses compared to the duration of the intervals search conditions are constant, and best times

(the shorter the impulses, the higher the permissible can easily be done through experiments for each elec-

Amount of pulsation). For partially smoothed two- 45 trode be set.

rectified single-phase alternating current [curve The amount of pulsation required to achieve the highest

(e)] the range is even smaller, namely if it or any other value of a capacitance

between 80 and 150 ° / o. The usable areas for the degree of pulsation of pulsating uninterrupted

capacity increase depends on the aforementioned others Factors and can also be done through trials

Broken direct currents of deviating waveforms 50 can be determined. A useful experimental procedure is

can easily be determined through experiments. The frequency of the pulsation can vary in proportion wide limits from a few oscillations per second up to 2000 Hz following:

A cheap and commercially available electrolyte is chosen, for example 0.2N NaCl solution. A pulsating continuous direct current with changing. The preferred range is between 50 and 55 suitable waveform and frequency is de- and 250 Hz. speaking of the available electrical

It has already been mentioned that the mean current density of the pulsating uninterrupted direct current should be chosen high. In practice it will

Plant used. The current density is chosen as high as this is permissible for the particular etching device is which one to carry out this procedure

however, this current density is used by the violence of the 60th. The etching time is chosen in such a way that the gas and heat development during the etching process can be limited to the largest amount of metal. It is convenient to be close to the top without creating a perforation. After all the limits of working, which of course have been determined by the structure of these factors, only depends on the system used, in particular on it, a series of experiments with postponed and what precautions to be taken for the derivation of the pulsation levels in order to get stuck in the etching bath generated heat. Provide which results in the highest capacity increase play of current densities within this preferred one. These empirical or experimental detection ranges are current densities of 0.5 to 0.6 A / cm ² . lung can be determined by visual examination of the etched

Foil (or other body) are significantly promoted. When the amount of pulsation is too small is, then the etched film has a dark gray appearance, with the entire surface attacked. However, if the amount of pulsation is too high, the etched foil will appear quite bright, with a considerable one Part of the surface remains unaffected. The film is perforated in many places where an attack has taken place. If the right amount of pulsation is used, this is it Total area of the film attacked, but no perforations have occurred, and the film has a light gray appearance. A more precise determination of the magnification factor can thereby be carried out that a cross section of the etched foil is examined microscopically.

5, 6 and 7 show microscopic photographs of such cross-sections, with the presence von FoKe is visible in white on a black background. The film shown in Fig. 5 is smoothed Direct current has been etched, while the film shown in Fig. 6 with a pulsating Direct current has been produced in which the amount of pulsation is too high for the current density, waveform, Frequency and type of electrolyte used was. In the exemplary embodiment shown in FIG. 7, however, there is an etching with a pulsating one uninterrupted direct current takes place, in which the degree of pulsation for the current density used, Waveform, frequency and the chosen electrolyte was correct, given the working conditions for this purpose are given in the table below under Examples. When it turned out has that the amount of pulsation is too high or too low, it can either be reduced or be increased accordingly, but it can also on the other hand, since the value for the optimal pulsation extent with increasing frequency of the pulsation decreases, with increasing current density, the value of the optimum pulsation amount increases, and with increasing Film thickness, the size of the pulsation and the etching time can increase, the amount of pulsation be kept at the same value during one or more of the other aforementioned Factors can be changed in the appropriate direction until the selected degree of pulsation results in an etched foil with the desired high capacity increase. When parts of the surface the foil does not remain attacked, so it may be necessary to determine the mean current density of the pulsating To increase the current. The following examples of etching processes according to the invention, namely, Examples 2-15, have been given to express the excellent results to bring what can be achieved, and to carry out the invention in practice, without that it is necessary to determine, through experimentation, the various factors which are necessary, in order to achieve etched aluminum foils that increase capacity when using a forming voltage of 500 volts show which are significantly higher than those produced by known electrolytic Etching processes are achievable in which essentially smoothed direct current, alternating current or only the positive half-waves of alternating current are used. The examples la to Ic should therefore, insofar as a comparison is considered, be excluded.

example

Power source

Waveform frequency
(Hertz)

Amount of pulsation

Aqueous electrolyte

Capacity expansion

at 500 volts forming voltage

la

Ib

Ic

Smoothed direct current
Alternating current

Only positive half-waves of
Alternating current

DC + transformer
(Fig. 2)

DC + transformer
(Fig. 2)

DC + transformer
(Fig. 2)

DC + switch
(Fig. 3)

DC + switch
(Fig. 3)

DC + switch
(Fig. 3)

DC + switch
(Fig. 3)

DC + switch
(Fig. 3)

Sine 50 Sine 50 Sine 50 Sine 1140 Sine 2000 rectangle 10 rectangle 130 rectangle 130 rectangle 130 rectangle 130

160

80

65

140

170

160

180

180

0.21 n-NaCl 0.01 n-HCl

0.21 n-NaCl 0.01 n-HCl

0.21 n-NaCl 0.01 n-HCl

0.21 n-NaCl 0.01 n-HCl

0.21 n-NaCl 0.01 n-HCl

0.21 n-NaCl 0.01 n-HCl

f 0.21 n-NaCl \ 0.01 n-HCl

0.11 n-NaCl 0.005 n-HCl

J 0.21 n-NaCl \ \ 0.0 In-HCl j

0.68 n-NaCl 0.2 n-NH, Cl

3.7 4.6 2.9 8.2 5.2 4.8 5.5 12.4 8.7 6.9 8.0

9 Waveform frequency Pulsation
extent 10 Capacity (Hertz) enlargement
at 500 volts
Formier example Power source rectangle 130 180 Waterier
electrolyte tension rectangle 130 140 7.8 10 DC + switch
(Fig. 3) rectangle 130 180 0.2 n-KBr 7.8 11 DC + switch
(Fig. 3) rectangle 200 140 0.2 n-KI 8.2 12th DC + switch
(Fig. 3) Curve 100 140 0.2 n-KClO " 9.7 13th DC + switch
(Fig. 3) rectangle 130 250 f 0.21 n-NaCl]
I 0.01 n-HCl j 8.3 14th Single phase rectifier
+ Throttle (Fig. 4) J 0.21 n-NaCl 1
\ 0.01 n-HCl J. 7th 15th Fig. 3 arrangement, but with
Pulse period 24% of
vibration [0.21 N NaCl 1
\ 0.01 n-HCl J.

The above-mentioned results were obtained with aluminum foils with a degree of purity of 99.8%, except in the case of Example 7, where an "over-pure" foil was used, but foils with other degrees of purity can also be used. In all cases the original film thickness was 0.1 mm; however, of course, the invention is not limited in this regard. In all cases, the current density of the etching current was 0.5 A / cm ² and the duration of the etching process was 2 minutes and 24 seconds. The etching temperature was close to the boiling point of the electrolyte.

In general, in the manufacture of electrolytic capacitors having electrodes made of etched aluminum foil, it is customary to prepare the aluminum foil for etching by heat treatment and / or degreasing and that oxidation or other contamination of the surface by treatment with a solution of caustic soda or removed with another suitable chemical reagent. The same practice can generally be used in the process of etching according to the invention insofar as foils made of aluminum or aluminum compounds are concerned, unless the condition of the foil is such that one or the other of the pretreatment measures is unnecessary. An indication of whether a pretreatment is required can be obtained by subjecting a sample of a commercially available film to a direct current etching process according to Example 1a. If this does not result in a feather-like etching of the entire surface of the type shown in FIG. 5, then the remaining foils should be subjected to a pretreatment before they are subjected to an etching process with pulsating uninterrupted direct current according to the invention. It is evident that the exact course of the pretreatment, if such is necessary, depends on the condition of the film as it is supplied on the market. A pretreatment, which has been found to be useful in many cases and to which all films of Examples 1 to 15 were subjected, initially consists of a heat treatment of the film at 600 ^° C. for two hours, whereupon the film is immersed in a 0 for 2 minutes .02n solution of caustic soda is pre-cleaned at 90 ° C.

The capacity increases listed above are achieved when the etched and subsequently anodically formed foils are immersed in an aqueous electrolyte. If the Etched and shaped foils are used for viscous working electrolytes, so the enlargements reduced by about 10 to 25%. It can be seen that in all examples according to the invention the increases in capacity achieved are much higher than those achieved by etching with smoothed Direct current can be achieved, and in most cases they are more than twice as high. It deserves It should be emphasized that in all examples the listed capacity increases are those obtained when the etched foils are anodized at a forming voltage of 500VoIt should be formed, and it should be noted that that when lower forming voltages are used, higher magnifications are measured can be. The forming voltage of 500 volts, which is referred to here and in the claims is, has only been chosen as an expedient basic quantity, as it is for the experimental determination the capacity increase of an anodically formed etched body made of aluminum or an aluminum alloy can be used, which results when the body is an electrolytic Etching process is subjected. As a result, reference should be made to the particular forming voltage of 500 volts in the claims should not be construed as applying the invention to manufacture is limited by etched bodies made of aluminum or aluminum alloys, which is anodic have been or will be formed at a forming voltage of 500 volts.

Claims (5)

PATENT CLAIMS: ί. Anodic-electrolytic etching process to enlarge the surface of aluminum or aluminum alloys as electrode starting material for electrolytic capacitors by means of an alternating current combined with a direct current. 309 647/185 current, characterized in that a unipolar current pulsing above a minimum value is used as the etching current, which has a pulsation extent of 50 to 250 ¹¹ Vo and a pulsation frequency below 2000 Hz, and that the pulsation extent is selected within the stated range that with a subsequent formation with a voltage of 500VoIt a more than five-fold increase in capacitance occurs compared to a capacitor constructed with unetched metal and formed in the same way. 2. The method according to claim I _3, characterized in that the etching current pulsates sinusoidally. 3. The method according to claim 1, characterized in that the etching current is rectangular pulsates. 4. The method according to claim 1, characterized in that the current peaks of the pulsating Currents are shorter in time than the intervals between them. 5. The method according spoke 1, characterized in that the etching current with a pulsation in the range of 80 to 150% by rectifying and partially smoothing alternating current of sine or approximately sine wave form is won. Considered publications:
German Patent No. 514 157;
Swiss Patent No. 216 684;
French Patent Nos. 815 002, 822 057; British Patent Nos. 393 565, 467 024, 511590, 647 799;
U.S. Patent No. 2163,416. For this purpose, 1 sheet of drawings