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Electrolytic Capacitors

Part III: Chapter 9

FORMATION of the ANODIC FILM


Page Index

Still and Continuous Formation Methods
Relationship Between Anode Foil Surface Area and Capacity

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Still and Continuous Formation Methods

THE still formation methods of anodic film formation described as being used in the fabrication of wet electrolytic capacitors, apply equally to anodic film formations for dry electrolytic capacitors.

Although such methods were in common use a number of years ago they have since been discarded for the improvements obtained with continuous anodic film formation procedures.

In the still formation method, the anode foil was usually wound "zig-zag" fashion between aluminum pins which were in turn mounted in an aluminum frame but generally insulated from the frame by porcelain bushings. The aluminum frame with its load of anode foil was then placed into a copper or aluminum lined tank of electrolyte. The frame was also insulated from the tank. The aluminum foil was made the anode and the tank the cathode and the formation procedure carried out exactly in accord with that outlined as applying to the formation of the wet electrolytic anode assemblies with the exception that finished formation current densities did not have to be of the same low order of magnitude.

After formation, the frames were removed from the formation tanks and the foil had to be unwound from the frame pins by hand. Such a procedure presented many undesirable mechanical features as well as many undesirable problems of chemical contamination of the formed anode foil surface.

Although not now in use to any extent the still formation method is illustrated below:

The above illustration shows the position of the frame, with foil in place, in the formation tank. Frame and pins were made of high purity aluminum and as a rule one or more foil supporting pins extended from the surface of the electrolyte for purposes of external electrical connection to the source of formation potential.

In the continuous method of anodic film formation, the anode foil is passed through one or more tanks of electrolyte, at a fixed rate of travel and with a fixed value of formation potential applied at all times. Not only is the applied potential held at a constant value but the current also is maintained at a constant value.

As the anode foil progresses through the formation tank the actual potential applied at the foil surface and electrolyte juncture increases, in increments from a low value, in proportion to the potential drop through the electrolyte. The net effective result is that as the anodic film is progressively formed and the leakage cur rent decreases, the actual applied potential increases to a mean value approximately equal to the voltage ap plied to the system. As a rule, this potential is reached when the anode foil has passed one third of the distance through the formation tank. As the foil progresses from this point, the potential remains constant and the leakage current continues to decrease until it has reached a predetermined low value by the time the foil exits from the tank. It can be readily seen that an automatic accomplishment is actually obtained of the same effective method employed in the previously described still formation procedure.

The size of the formation tank and the rate of travel of the foil through the tank governs the time period of immersion and formation.

The continuous method of anodic film formation possesses many inherent advantages over the still formation method in addition to the primary requirement of being able to anodically form foil in continuous strips of unlimited length. Voltage and current values can more readily be kept at desired constant values. Temperatures of electrolytes can be readily maintained at constant values and the anode foil is not subject to contamination from handling. Also, any desired voltage of film formation can be instantly obtained by simply adjusting the applied potential to desired values.

The following illustration shows a typical continuous formation system.

From the first illustration it may be seen that the anode foil first passes over a series of metal rolls which serve to make contact between the foil and the positive terminal of the electrical power source. The foil enters and travels through the electrolyte by passing over a series of insulating rolls generally made of glass, porcelain, isolantite, bakelite or similarly satisfactory material.

The foil is generally drawn or pulled through the system by a pair of power driven rubber hced rolls in the form of a typical wringer mechanism such as is commonly used for laundry work. These rubber faced rolls also serve the purpose of squeezing surplus electrolyte from the surfaces of the foil. In most cases the foil then dries before being rewound into roll form. If not, a blast of warm air is used to provide the necessary drying action.

The tank is usually a wooden one lined with copper or aluminum depending on the chemical composition of the electrolyte employed.

Baffle plates are also frequently employed to reduce the resistive path, through the electrolyte, from the negative terminal of the power source to the anode foil surface. This is particularly true at the entrance part of the tank where the current drain is maximum. The baffle plates also serve to reduce electrolytic action on the tank walls.

Coils for either cooling or heating the electrolyte are also usually provided.

The insulating rollers are generally supported on an aluminum framework which fits inside the tank and this framework may also in turn support the baffle or cathode plates.

Where a single tank is used for continuous anodic film formation the system is termed a single formation.

The second figure shows in graphic form the voltage gradient or potential distribution on the foil surfaces as it progresses through the formation tank. The voltage gradient will obviously be shifted from right or left of the position illustrated in accord with changes in electrolyte resistivity and speed of anode foil travel through the system.

Generator equipment and forming tanks for
continuous anodic film formation (ca. 1938).


(Courtesy Cornell-Dubilier Electric Corp.)

Frequently more than one formation tank is employed for continuous anodic film formation and where this is the case the system is termed a multiple continuous formation method.

In the multiple system two or more tanks are used and the anode foil travels first through one formation tank then through a second or even third tank.

Such a system has been found to be very advantageous for anodic film formations of the order of 600 volts and higher for the reason that electrolytes having sufficiently high scintillation values do not efficiently produce initial anodic film formations.

In the multiple system it is therefore customary to form anodic films in the first tank to some intermediate voltage value, in an electrolyte of comparatively high ion concentration, and then form a higher voltage film in a second tank, using a second electrolyte of a much lower ion concentration.

Using such a multiple system, it has been found possible to form anodic films at potentials as high as 800 volts or even higher.

The following block diagram illustrates such a multiple continuous formation system:

This illustration shows a triple formation arrangement of tanks for the formation of anodic films at 600 volts. As it is noted that the anode foil is the common positive terminal of all three direct current generators it follows that the respective negative terminals must be isolated from each other. For this reason each tank, with its electrolyte content, must be electrically insulated from every other tank.

In this illustration it is also noted that voltage is applied in progressively increased steps, to each tank in the system.

This system of multiple step formation is not always utilized merely to form anodic films at the higher voltages but sometimes to economize on or take advantage of the use of smaller current generating units.

Other advantages to the use of the continuous method of anodic film formation are found in the fact that current generating equipment can be operated at constant and often full load thus realizing the maximum overall efficiency.

In the continuous formation system, the total current drain will be found to be a function of the applied voltage, type and concentration of electrolyte, speed of anode foil travel, total anode foil surface immersed in the electrolyte and temperature of the electrolyte.

The lower voltage formations in sulphuric and oxalic acid electrolytes may be applied to the continuous formation method but at the present stage of the art these electrolytes are rarely used.

Of all the possible electrolytes which might be used for anodic film formations only two have proved to be entirely satisfactory and these two are aqueous solutions of boric acid and either ammonium or sodium borate. Of these two the sodium borate type is the more satisfactory.

A general idea of the concentrations of boric acid and sodium borate required for electrolytes, to be used in the continuous formation of anodic films, can be gained from the following tabulation:

Water
(cm3)
Boric
Acid
(g)
Sodium
Borate
(g)
Formation
Voltage
(volt)
100 15 None 600 to 800
100 10 0.12 525 to 600
100 10 0.25 450 to 525
100 10 0.30 350 to 450
100 10 1.00 150 to 350
100 10 1.50 50 to 150
100 10 2.00 10 to 50

In the selection of the most suitable concentration of electrolyte for any particular formation voltage there are three basic factors to consider, these factors being: scintillation voltage, pH value and resistivity of the electrolyte.

In the formation of anodic films at the higher voltages, the selection of concentration is limited primarily by the scintillating voltage of the electrolyte. For lower values of formation voltage, however, the selection is not so limited but it must be pointed out that the higher the pH value of the electrolyte the greater will be the tendency to form the less active type of aluminum oxide film. This would naturally indicate the use of the higher voltage electrolytes for the lower voltages of anodic film formation. On the other hand, the electrolytes of lower pH values and smaller concentrations of sodium borate, possess relatively high specific resistivities; consequently they may present serious difficulties in the form of excessive p0tential drops in the electrolyte. For these reasons it is frequently necessary to make a judicious balance between these various factors, in the selection of a particular electrolyte, for a specific voltage of formation.

This phase of the problem is considerably aggravated in the case of etched anode foil where it is necessary to take excessive precautionary measures to obtain the most active type of aluminum oxide film.

The plain aluminum foil surface is somewhat protected by its natural oxide coating so there is a reduced tendency towards the formation of an inactive film or layer of aluminum hydroxide. The etched surface does not always have this protection and the tendency towards the formation of an inactive outer layer of aluminum hydroxide is very marked.

To illustrate this point, reference is made to the following assumed cross-sectional view of etched foil.

Assuming for the sake of illustration that the etch has a sort of saw toothed pattern, the dark areas serve to illustrate the coating of aluminum hydroxide which partially fills the bottom portions of the craters or apertures of the etch. Such a condition tends to be produced when the pH value of the formation electrolyte is higher than a certain range.

The presence of such coatings or layers of the inactive film of aluminum hydrate obviously causes a material increase in contact resistance between the active dielectric film and the electrolyte, in a completed dry electrolytic capacitor structure, with the result that effective capacities are reduced and equivalent series resistance values increased.


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Relationship Between Anode Foil Surface Area and Capacity

The relationship between effective anode surface area and capacity which exists in wet electrolytic capacitor structures does not hold true for dry electrolytic capacitors.

For a given capacity at a given voltage, the dry electrolytic capacitor generally requires an anode surface area some fifteen per cent greater than that required for a wet electrolytic capacitor of the same capacity and voltage ratings. This is a difference inherent in the two structures.

In determining the anode foil area required, for a given capacity at a given voltage of formation, the following formula applies to plain foil and gives a fairly accurate determination:

A = 0.015EC

Where

A = area of one side of foil in square inches
E = Formation potential in volts
C = Capacity in microfarads.

The relationship between capacity, anode foil area and formation voltage, is fairly uniform over the entire useful voltage range where plain anode foil is used. With the use of etched foil, however, this relationship is subject to a number of variables of considerable magnitude.

With any given procedure of etching, the ratio of increase in physical surface area may be fairly constant but the ratio of gain in capacity is by no means proportional or constant.

At the instant of the beginning of the formation of the anodic film, a certain amount of the aluminum surface must dissolve and go into solution in the electrolyte. This quantity of aluminum is of a minimum value with plain foil but even assuming that not to be the case the total surface area of plain foil would not be reduced by such dissolution.

With etched surface foil the case is entirely different because this dissolution may cause some of the peaks to be removed altogether. In addition an increasing amount of aluminum may be lost from the peaks by the actual combination of the aluminum with oxygen to form the aluminum oxide film. As these peaks or projections are very small, it is readily seen that some reduction in the original physical surface area will occur during formation of the anodic film.

As both dielectric film thickness and amount of aluminum dissolved from the surface, are a function of the magnitude of the forming potential it can also be seen that the reduction in anode surface area becomes a function of the formation voltage.

Reference is made to the two following cross-sectional illustrations to more clearly point out this characteristic of etched surface foil.

Here again, for sake of illustration, a saw toothed etch pattern is assumed but the rounding off of the peaks has resulted in a material decrease in the foil surface area.

Actual observed measurements have shown that the effective gain in anode surface area may vary from ten times that of plain foil to five times when the same etched surface is anodically formed at 10 and 500 volts respectively.

Under the circumstances it would be extremely difficult to set up any formula for calculating required anode areas of etched surface foil, even though such a calculation was based on a certain type of etching system.

The following graph will, however, serve to indicate the average values of foil areas required per microfarad at various values of formation potential:

Both plain and etched surface anode foils are formed by the same continuous formation procedures but it is customary to adjust the speed of travel of the etched foil through the formation tanks in more or less direct ratio to the increase in surface area obtained in the etching operation. This is obviously necessary to maintain the same formation voltage gradient with etched anode foil as is obtained with plain foil.

Also, in actual practice, it is found that it is necessary to form anodic films at slightly higher voltages than those values at which the completed electrolytic capacitor is rated. These higher voltage values are subject to considerable variation, depending on desired characteristics, type of electrolyte employed and potential drops in the formation system.

In general, it is common practice to form anodic films, for use in dry electrolytic capacitors, at voltages in excess of the rated operating voltages to the extent indicated by the following graphic illustration:


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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.