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Mechanical methods of etching or roughening of the surface, having proven less satisfactory than chemical methods, will not be the subject of further description. In order to facilitate the handling of the material in subsequent formation and winding operations, it is very desirable, if not absolutely necessary, that the anode foil be etched continuously.
By continuous etching is meant the continuous passage of the anode foil through the various steps which constitute the etching process and its rewinding into a roll.
Basically, the methods of chemically etching or roughening the anode foil surface, already described, remain the same. Certain other factors however must be considered, the most important being the control of the actual degree of etching.
The following diagram illustrates the form of a typical etching system:
It can be seen that in a system of continuous etchirjg, the anode foil passes through a series of tanks or vats which contain the different materials needed for the corresponding steps in the process.
As the effectiveness of the etching procedure is subject to certain variables, means must be provided to control these variables. The effective variables have been shown to be as follows:
Cleaning operations are subject to the same set of variables and therefore the sodium hydroxide and nitric acid washes are affected by the following factors:
If known and desired results are to be obtained these three basic variables must be controlled to within very narrow limits of variation.
Once the correct time of immersion, in any given solution of any given concentration and temperature, has been determined, it can easily be fixed by fixing the rate of linear travel of the foil through the system of tanks, adjusting the physical size of each tank so that the foil remains immersed in each tank the correct proportionate time.
Temperature is more difficult to control, especially the temperature of the more common acid etching solutions, because the chemical action of the etching operation usually produces a large amount of heat. Very elaborate systems of thermostatically controlled cooling coils or chambers are usually employed to dissipate this heat energy.
Concentration of solutions is us~lly maintained by allowing fresh solution to run into the solution being used at such a rate as to exactly replace or balance those chemicals being used up, overflowing the surplus. In continuous types of etching set-ups, the wash tanks are rapidly contaminated or "loaded" with chemicals and the usual procedure is to have the wash tanks continuously overflow, fresh water being continually flowed into the tanks. To further prevent carry over of contaminating chemicals from wash tanks into solution tanks, wash tanks are usually equipped with spray washes of fresh water at the point of exit by the foil from the wash tanks.
The etched foil is usually dried thoroughly, with a blast of warm air, as it emerges from the last wash tank prior to being rewound into a tight, smooth roll.
As the etched foil generally goes directly into the process of anodic film formation without additional cleaning, it is important that the washing process, after etching, be sufficiently effective as to remove the last trace of etching or cleaning chemicals.
In order to minimize the formation of aluminum hydroxide on the etched surface, the last distilled water washes are frequently acidified slightly with boric acid. This procedure has been found very effective in reducing the amount of hydrolysis, especially when the distilled water wash is heated.
The three basic methods of chem]cal etching; namely, hydrochloric acid, hydrochloric acid and copper chloride, and copper chloride solutions are all readily adaptable to the continuous etching procedure. Furthermore they are in general use.
It has been found that if the anode foil, while immersed in the etching solution, is made the positive electrode with respect to another electrode immersed in the same etching solution, an accelerated attack is obtained. This accelerated action is the result of the chlorine ions (in the case of hydrochloric acid) being attracted or driven to the anode foil surface by the electrical potential applied. This principle is frequently employed in a system of etching, termed electrochemical etching. In the employment of such a system, it has been observed that a wide variation in the pattern and depth of attack can be obtained with variations of solution concentration, current density and applied potential. This method has one drawback and that is the anode foil being etched, must be of a certain minimum cross section in order that the high values of current required, can be carried without actually melting the foil in question.
In still other methods of etching, the same principle of accelerated action is obtained without the application of an electrical potential from external sources. This is accomplished by a simultaneous generation of potential during the actual etching.
One illustration of such electrochemical action is the method of etching previously described wherein an aqueous solution of copper chloride is employed. In this case the aluminum tends to dissolve into solution and copper in turn plates out of the solution onto the aluminum surface. This coating of copper is not firmly bonded to the aluminum surface and the result is a galvanic cell structure. The electrical potentials resulting from such a galvanic couple attract the chlorine ions to the aluminum surface with the result that the overall etching action or rate of attack is accelerated to violent proportions.
Another system of continuously etching anode foil is to pass the foil through an etching solution with its surfaces held in close contact with two metallic screens. These screens are of comparatively fine mesh and the metal forming the screens is so selected as to promote galvanic action. Also, the metal forming the screens is selected for its position in the electrochemical series where it is best suited for minimum attack by the chemicals of the etching solution. In general, it has been observed that the most satisfactory characteristics of etch are obtained where the galvanic couple formed by the anode foil, the screen and the etching solution generates a potential difference of approximately 0.5 volts.
Copper screens and an etching solution of hydrochloric acid provide a fairly satisfactory combination.
As a general rule, the screens are used in the form of two endless belts which pass slowly through the etching solution, carrying the anode foil between them. Such a system is illustrated in the following illustration.
With this system of etching, and using a hydrochloric acid solution of approximately 25% (1 part acid to 3 parts water by volume) at or near a temperature of 40 centigrade, the etching time has been observed to be approximately one minute.
The more or less conventional method of cleaning with a hot solution of sodium hydroxide with tap water rinse precedes the acttial passage of the foil through the etching solution. After etching, the also conventional treatment, of washing in nitric acid and succeeding washes of tap and distilled waters, is necessary. This type of etch is readily recognized by the outline pattern of the screen which appears on the etched surface of the foil.
Utilizing the various systems of chemical etching which have been described, increases in anode foil surface area from four to ten times that of plain foil are obtained with foil thicknesses varying from 0.003" to 0.005".
In describing the various etching methods and procedures no mention has been made in regard to the actual mechanics of moving foils through etching and washing or cleaning operations. Neither has any mention been made of the problems involved in the selection of proper construction materials. Such details are comparatively simple problems in chemical and mechanical engineering and are therefore not believed to be within the intended scope of this book.
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By this process a very large increase in effective anode surface area is obtained. This increase in anode surface area is proportional to the mesh of the gauze and the diameter or gauge of thread employed. In actual practice, two thicknesses of gauze are frequently employed and the mesh of the gauze ranges from 60 by 60 to 80 by 80.
The general method of coating the gauze with aluminum is to pass the gauze continuously through streams of aluminum being emitted from a number of spray guns. These spray guns are of the oxygen-acetylene burning type in which the aluminum is heated to a molten state and sprayed out with high pressure air.
After the gauze has been coated with aluminum, the material is anodically filmed by the same general methods used for filming or forming the more standard aluminum foil.
So far, this type of anode structure has been limited to the construction of dry electrolytic capacitors due to inherent difficulties in the form of small percentages of impurities occluded in the gauze. While such impurities do not affect a dry electrolytic capacitor structure they would render a wet electrolytic capacitor inoperative.
With the metal coated gauze anode structure it has been observed that the effective gain in anode surface area, over plain aluminum surface foil, is ten or more times at even the higher voltage ranges. The employment of such an effective gain in anode surface area results in a completed capacitor of very small physical size.
In commercial practice, however, a material reduction in physical size is not always desirable as such a reduction may frequently limit the range of application of the capacitor in question. This is especially true when a capacitor is used in a circuit network where an appreciable alternating current potential is applied. Under such conditions the small physical size of the capacitor may be such as to be incapable of radiating the heat resulting from both alternating and direct current wattage losses.
One interesting phenomenon has been observed in connection with the use of anode structures of very high gain surface area increases and that is the effect of the corresponding reduction in the surface areas of the cathode foil.
As any electrolyte has resistance there is a voltage gradient established between anodic film, electrolyte path and cathode foil surface upon the passage of alternating current through the capacitor. The cathode foil does not become positive with respect to the anode polarizing potential but it may become positive with respect to the electrolyte immediately adjacent to cathode foil surface without affecting its negative value of polarization with respect to the anode. The net result is that the cathode becomes anodically filmed to the value of positive potential applied to it. The value of this potential is proportional to the resistivity of the electrolyte and the magnitude of the alternating current passing through the capacitor.
The effective voltage of formation of the cathode is always of the order of only a few volts and in plain foil structures does not measurably affect the overall capacity of the capacitor due to the fact the effective capacity of the cathode foil surface is very high.
Where high gain anode surface areas are employed this effect of cathode formation is entirely different. In such structures, the cathode foil surface is materially reduced and the result of even a comparatively low voltage of film formation produces a relatively low capacity. This capacity is effectively in series with the capacity of the anode; so if the value of capacity of the cathode formation is low the rated capacity of the unit will be reduced.
It has been observed that reductions in original capacity values of as much as fifty per cent occur under normal filter circuit applications where very high gain area, coated gauze anode structures are employed.
This effect is more noted with capacitor structures designed for the lower voltage applications.
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