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Electrolytic Capacitors

Part II: Chapter 4

The Fabrication of Wet Electrolytic Capacitors


Index

Anodes--Plain and Etched Types
Chemical Etching of Anodes
Straight Hydrochloric Acid Etching
Copper Chloride--Hydrochloric Acid Etching
Copper Chloride Etching

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Anodes--Plain and Etched Types

AS HAS been previously mentioned, the capacity of an electrolytic capacitor is determined by the area of the anode surface and the thickness of the dielectric film covering this surface. That being the case it becomes apparent that an increase in capacity can be obtained if the surface of a given size of anode is increased by roughening the plain surface. It is also apparent that the increase in capacity thus obtained will be in direct proportion to the increase in effective surface area by such a roughening procedure.

Capacitors fabricated with a plain surface anode structure are called plain foil capacitors and capacitors fabricated with a roughened surface anode structure are called etched foil capacitors.

The majority of present day wet electrolytic capacitors employ the use of etched surface anode structures.

A number of methods have been developed for the roughening of the anode surface and such methods include sand blasting, mechanical embossing, scratching with rotating brushes, use of abrasive materials, forming in rotary dies and chemical etching with acids and salts of acids.

Most of the mechanical roughening methods present certain difficulties and disadvantages and some of these will be briefly described.

Sand blasting of the anode surface produces only a comparatively small increase in surface area. Sand particles are embedded in the surface which are difficult of subsequent removal. Also, if thin foil or sheets of aluminum are sand blasted, the hammering action of the sand particles tends to make the aluminum very hard and brittle.

Mechanical embossing also has the disadvantage of a comparatively small increase in surface area and in addition, the increase in surface area is obtained at the expense of an increase in thickness of the embossed material.

Scratching of the anode surface with rotating brushes with metallic bristles will increase the surface area but the increase thus gained will be nonuniform, of small magnitude and small metallic particles from the brush bristles will be embedded in the anode surface. These embedded particles are very difficult of subsequent removal.

A fairly satisfactory method, of mechanically roughening the anode surface, has been developed whereby the anode material is passed through a series of rotating copper rollers. An abrasive such as aluminum oxide is fed to these copper rollers. The abrasive particles become embedded into the surface of the comparatively soft copper and in turn cut deeply into the surface of the anode material. By this method, increases in the anode surface area of from two to two and a half times have been obtained.

Increases in anode surface areas of the same order of two and one half times have also been obtained by passing the aluminum, in the form of thin sheets or plates, through massive steel rollers which have been finely grooved with concentric rings. In this process the aluminum is actually drawn or formed to fit the groove pattern with the result that it assumes a shape similar to that illustrated below, obviously in magnified form of a cross-sectional view:

Of all proposed and known methods of roughening of the anode surface, the chemical etching processes have been found to be most satisfactory and are therefore in general use, in one form or another.


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Chemical Etching of Anodes

The characteristic change in surface structure, of aluminum, effected by etching is not readily visible to the naked eye, and it requires a highly perfected technique to show these changes. For that reason a special effort will be made to show just what happens when aluminum surfaces are etched for use in electrolytic capacitors.

The manner in which various reagents etch aluminum is usually characteristic of the specific etching agent; in general, however, alkalies tend to attack aluminum surfaces uniformly, whereas certain of the acid reagents have a definite roughening action. Etching with alkalies, therefore, reduces the thickness of the anode material uniformly with only a minor increase in effective surface area. Hydrochloric acid singly or in combination with metallic salts of hydrochloric acid, however, has a pronounced roughening action on aluminum, and the result of this roughening action is to increase the anode surface area.

Aluminum exposed to the air always has a thin film of oxide on its surface. In the event that annealed foil is used for anode structures, this oxide film may be increased slightly in thickness by the heating incident to annealing. The full roughening action of the hydrochloric acid reagents cannot become effective until this oxide film is penetrated. As a result, the most effective way of etching aluminum foil is to first remove the oxide film by the uniform attack of an alkali, such as sodium hydroxide. Sodium hydroxide is best used hot and in concentrations of from 4 to 10 per cent. The time of immersion can best be determined by experiment, but should only be long enough to remove the surface film without any material loss of metal.

Following etching with sodium hydroxide and washing, the foil can be immersed in a hydrochloric acid reagent of a number of types. Here the importanr variables are found to be the concentration of the reagent, the temperature of the reagent and the time of immersion. The characteristics of the aluminum sheet or foil are also important in determining the results obtained by etching. For best results, the foil should be especially fabricated for etching. There is a definite relation between the crystal structure of the metal and the effectiveness of the etching. It has been found, for example, that aluminum, annealed to a high temperature, has a more coarse grain structure than aluminum annealed for a long period of time at a lower temperature. Hard drawn aluminum does not etch as satisfactorily as dead soft aluminum and the coarser grain structures produce the best results.

The purity of the aluminum also has a marked effect on the etching results; aluminum of a purity of 99.99% will be hardly affected by the same procedure which produces full effective etching of aluminum of a purity of 99.8%. It might be mentioned in passing, that full advantage is taken of this fact in obtaining an accurate method of quickly checking for total aluminum content of aluminum foil.

Etching of aluminum foil brings with it the important problem of removing all traces of chlorides and other contaminating substances, such as heavy metals, from the surface. Metals such as iron and copper plate out electrolytically on the aluminum and must be later removed. This is invariably accomplished with treatment with nitric acid. More detailed data on the subject of removal of chlorides and heavy metals will be mentioned in connection with specific etching procedures.


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Straight Hydrochloric Acid Etching

Illustrative of the results and effects of variations in acid concentration, acid temperature and time of immersion, in connection with the use of straight hydrochloric acid for etching, the following observations are made:

HCl Conc.
(%)
Temperature
(°C)
Etching Time
(minutes)
Surface Area
Increase
(times)
5 30 5 1.3
5 65 1.5 2.9
8 30 5 1.8
8 65 1.5 5.0
11 30 5 2.8
11 65 1.5 6.0

In obtaining the above data three etching solutions were used; namely, 5, 8 and 11% of hydrochloric acid. The concentrated acid contained 37 per cent HCl and the solutions contained approximately 11.4, 18.2 and 25.1 per cent, by volume of the concentrated acid.

It will be noted that temperature plays an important part in determining the amount and rate of attack. It has been determined, by microscopic examination of cross-sectional areas, that temperature also has an important effect in controlling the character of the attack.

In comparing the above figures, there is obviously more roughening of the surface when the acid solution is used at a temperature of 65°C than at a temperature of 30°C. There is, however, a deeper penetration in the longer time employed with the lower temperature solution. It has been observed that this deeper penetration takes the form of minutely small, hair like, capillary pits which penetrate deeply beneath the foil surface. The chlorides contained in these capillary pits are almost impossible of removal.

For the above reasons, the more uniform attack, with the least amount of the extra deep penetrations, forms the most desirable character of etch. This limits the procedure very definitely to the use of hot solutions of sufficient concentration to obtain the desired increases in surface area.

The effect of variation in time of immersion is shown graphically in the following illustration:

This graphically illustrated data is based on the results obtained with an anode plate 0.005" thick by 2" wide by 5" long. Prior to immersion in the etching solution, the anode plate was cleaned for three minutes at 90°C in a 3 per cent solution of sodium hydroxide then washed in cold tap water. The etching solution consisted of 10.8 per cent of HCl maintained at an etching temperature of 65°C.

Photomicrograph of hydrochloric acid etched anode foil.
Magnification 50 times.


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Copper Chloride--Hydrochloric Acid Etching

It has been observed that comparatively small additions of copper chloride, iron chloride or aluminum chloride to the hydrochloric acid solutions produce an accelerating action in the attack of the aluminum surface.

A typical example of the use of such a fact is contained in the following tabulated data which shows the variation in surface area increase with time of immersion in etching solution.

Prior to etching, anodes were cleaned for 3 minutes at 90°C in a 3 per cent solution of sodium hydroxide then washed in cold tap water.

Etching solution consisted of the following:

HCl 100 cm3
CuCl2 0.05 g
H2O 400 cm3
Etching Temperature 75°C

Etching
Time
(seconds)
Surface Area
Increase
(times)
5 2.4
30 2.5
60 2.7
90 3.3
120 3.7
150 3.8
180 4.0


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Copper Chloride Etching

If a piece of aluminum foil is immersed in a solution of copper chloride, consisting of approximately 5% of CuCl2 dissolved in an aqueous solution, immediate action at room temperatures will be obtained. It will be noted that the surface of the aluminum will be rapidly coated with a fairly heavy layer of copper.

If, after a short immersion, the aluminum is removed, rinsed with tap water and treated with concentrated nitric acid to remove the copper coating, the surface will be found to have been very effectively etched. This type of etch readily produces increases in surface areas of five times or more.

The amount of copper plated out of the copper chloride etching solution onto the surface of the aluminum is rather large and must be replaced for subsequent etching operations.

A complete step by step, diagrammatic presentation of this process follows.

While there are other detailed methods of etching anodes intended for use in wet electrolytic capacitors, these bther methods are all variations or combinations of the basic methods already described and for that reason it is not believed to be necessary to go into the multitudinous ramifications of these basic chemical etching methods.


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Updated: 03 April 2000

Publisher: Tyra Buczkowski
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First edition copyright © 1938 Paul McKnight Deeley.
This edition copyright © 1996-2002 Tyra T. Buczkowski. All rights reserved.