|
|
| FaradNet The Capacitor Resource |
| Home | Community | Companies | Catalogs | Knowledge | Search | Store |
It was as late as 1931 that the dry electrolytic capacitor was first developed to a state of commercial usefulness and even then the range of application was decidedly limited. Improvements have, however, been steadily made until today, the dry electrolytic capacitor has reached a comparatively high state of development.
The electrolytes employed in the dry electrolytic capacitor structures are not only non-aqueous but are more or less of low conductivity. This low conductivity of electrolyte necessitates certain basic physical changes in general structure.
In the wet electrolytic capacitor, the can or container serves to make electrical contact with the aqueous electrolyte but with the use of the non-aqueous electrolytes this type of construction will no longer function in a satisfactory manner. This obviously is due to the low conductivity of the non-aqueous electrolyte which would, in turn, result in too great an increase in the resistance equivalently in series with the capacitor. To overcome such a difficulty, it is necessary to make the first basic change in physical structure which exists between the wet and dry types of electrolytic capacitors.
This change in structure consists of providing a plate or foil which serves to make electrical contact with the electrolyte in such a manner as to reduce the mean resistive path to a minimum value.
An enlarged cross-sectional view of such an arrangement is shown in the following illustration:
An examination of this cross-sectional view discloses the fact that on each side of the anode plate there is - a metallic plate which lies parallel to it and that between these metallic plates and the anode plate there is a separating medium which is saturated with the non-aqueous electrolyte. Such a metallic contact plate is called the cathode plate because it serves to make electrical contact with the actual cathode member, the electrolyte.
The electrolyte proper will not normally serve as a physical separating medium between the dielectric film of the anode plate and the cathode plate so it has become common practice to use certain absorbent materials to provide the necessary space for the electrolyte. This absorbent material is normally in the form of thin sheets or layers which are called the separators. These separators are saturated with the electrolytes employed.
Dry electrolytic capacitors could be constructed by stacking, one layer on top of the other: anode, electrolyte saturated separator, cathode, another separator, another anode and so on. This procedure is seldom, if ever, employed because of economical reasons and other equally important factors.
The basic form of construction, which is today almost unitrersally followed, consists of rolling up or winding two separators, the anode plate and the cathode plate into a concentric roll. This basic form of construction is shown in the following illustrations:
From these illustrations it can be noted that anode and cathode plates are of the same width and that the separators are wider than the anode and cathode plates. It can also be noted that the cathode plate is so placed as to be on the outside and completely encircle the winding. Obviously this is done to provide complete coverage of both sides of the anode plate and its dielectric film. It can be further noted from the illustrations that a means has been provided for making external electrical connections to both anode and cathode plates in the form of projecting portions of the plates. These projections are called anode and cathode tabs, respectively.
The following sketches serve to show some of the more common methods of cutting and folding anode or cathode plates to form these tabs:
So far it can be seen that the dry electrolytic capacitor basically consists of the four following elements:
(Courtesy Cornell-Dublilier Electric Corp.)
Book ToC | Book Index | Page Index
The aluminum foil is usually supplied from the mill in 5" to 10" rolls specially packed to prevent mechanical injury or chemical contamination.
The aluminum foil is supplied dead soft (fully annealed) and a smooth, bright mirror-like surface is most desired. This surface, to be entirely satisfactory, must be entirely free from oil or grease, especially in the thinner sizes. For this reason, it has been found to be highly important that the lubricants used in the rolling mill operations be vegetable oils such as cocoanut or palm oil so that should any quantity remain on the foil surface it can easily saponify with alkali solutions in processing operations. Rolling mill lubricants should also be selected for the type which will most completely volatilize during annealing operations without leaving any carbon or oxidation products on the foil surface.
This is particularly important in connection with the thinner foils because they are generally anodically formed without any preliminary cleaning of the surfaces.
The purity of the anode foil is very important and the aluminum content should never be below 99.8%. Higher purities are very desirable and aluminum contents of 99.85% and 99.9% produce marked improvements in dry electrolytic capacitor characteristics. For reasons of economy, however, a purity of 99.8% aluminum content is normally employed.
In the construction of dry electrolytic capacitors both plain and etched surface anode foils are used, the thinner foils of from 0.0015" to 0.002" thick being used for plain surface structures and the thicker foils of from 0.003 to 0.005" thick being used for etched surface structures.
Book ToC | Book Index | Page Index
The cathode foil surface must also be free from oil, grease or oxidation products for two reasons: first, to keep electrical contact resistance with the electrolyte at minimum values and second, to prevent any chemical contamination of the electrolyte in finished dry electrolytic capacitor structures.
The thicknesses of cathode foil, generally employed, range from 0.0015" to 0.0025". Attempts to use thinner cathode foils have been frequently made but with varying degrees of success due to difficulties encountered in obtaining satisfactory tab connections. Certain advantages, on the other hand, have been claimed for the use of very thin cathode foils and this phase of the matter will be more fully mentioned in later paragraphs.
Book ToC | Book Index | Page Index
Ordinarily, one thickness of gauze was used between anode and cathode plates but in some instances a double thickness (2 layers) was employed in an attempt to obtain higher voltage dry electrolytic capacitor structures.
The average width of gauze separator was such as to produce a margin of from 1/8" to 3/16" on each side of the anode and cathode plates in order to minimize mechanical touching of the foils and at the same time allow for slight variations in foil alignment during the winding of capacitor assemblies.
Since 1934 the employment of the gauze type of separator has practically been abandoned and today the cellulose or paper type of separator is almost exclusively used. This type of separator consists of thin sheets of specially fabricated paper or cellulose of extremely high purity, from a chemical standpoint, and of high absorbent qualities from a physical standpoint.
The better grades of cellulose separators are fabricated from selected cotton rag stock although certain quantities of other materials such as kapok, jute or hemp may be added to the basic cotton rag pulp. Straight wood pulps, especially those of the sulphited processes such as kraft, are especially avoided.
Cellulose separator materials are especially fabricated for high chemical purity with particular efforts being made towards the complete elimination of soluble chlorides, sulphates, nitrates, resins, heavy metals and conducting particles such as carbon.
The total thickness of cellulose separator empfoyed between anode and cathode plates will vary with the voltage rating of the capacitor structure and the type of electrolyte employed but, on the average, this total thickness varies from 0.003" to 0.008" in connection with capacitors rated at from 6 volts to 600 volts respectively.
Some degree of success has been obtained with the use of separators consisting of thin sheets of regenerated cellulose such as cellophane but this type of material is generally employed in combination with a layer of cellulose due to the relatively low absorbency characteristics of the cellophane. Certain advantages are claimed for this type of material and further details will be covered in subsequent chapters.
Various combinations of all types of separators are sometimes employed in order to obtain certain results or characteristics in completed capacitor assemblies.
Book ToC | Book Index | Page Index
The more commonly used electrolytes are compounds or mixtures such as glyco-ammonium borates, glycerol-ammonium borates, ammonium acetates-ammonium borates, ammonium lactates, amine acetates, amine borates and literally hundreds of similar chemical combinations the more important of which will be described in detail later on.
Regardless of the specific chemical composition of the electrolyte employed there are certain fundamental factors which cannot ever be overlooked and these are that satisfactory performance can only be obtained from relatively narrow operating limits of pH value, water content and conductivity.
While it is true that a dry electrolytic capacitor structure is not as subject to corrosion difficulties as a wet electrolytic capacitor, nevertheless the same precautions are found necessary in regard to freedom from contaminating agents, particularly chlorides, in the electrolyte and chemicals used in preparing it.
In dry electrolytic capacitors the electrolytes are occluded into the separator materials and there are a number of ways of saturating this separator material with the electrolyte. Among these methods, soak impregnation, vacuum impregnation, and centrifugal impregnation are the most commonly used ones. For that reason, electrolytes must frequently, not only possess certain chemical and electrical characteristics but certain physical characteristics as well, in order that they be adapted to certain impregnation methods.
Book ToC | Book Index | Page Index