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As a general rule, the non-aqueous electrolytes consist of a weak acid, a salt of a weak acid and a solvent. The solvent is generally one of the polyhydroxyl alcohol group such as a glycerol or glycol although in some cases it may be replaced with the use of a hydroxy alkylamine. The salt of the weak acid is generally a salt of the weak acid employed although this is not necessarily always true.
In some cases the electrolyte may not employ a solvent but may consist of two salts only, in its composition.
In most cases, the electrolyte is contained in a saturated separator medium but under certain circumstances the separator medium is replaced altogether, by the use of an electrolyte of such physical characteristics necessary to make such a replacement.
Non-aqueous electrolytes may also contain inert filler materials, for the purpose of increasing viscosity, such as bentonite, diatomaceous earth, silica gel, aluminum oxide, agar-agar, gum tragacanth and starch. In some instances, inert substances are added to the electrolyte for the purpose of increasing electrical conductivity. Such substances may be magnetite, graphite, colloidal graphite, carbon, colloidal silver or powdered metals such as aluminum and copper.
Physically, the non-aqueous electrolytes may, and do range from slightly viscous fluids to semi-hard or states.
As the stability of the dielectric film, both as to useful life and electrical breakdown, is determined to a large extent by the ion concentration of the electrolyte, a very narrow range of operatational limits is more or less automatically established for both pH value and conductivity of any spedfic electrolyte composition. Water being the main ionizing medium, it becomes imperative that the water content of any electrolyte composition be held within specifically determined bounds for any particular electrolyte and electrolyte application.
The electrical characteristics of any dry electrolytic capacitor structure are, in the main, determined by the type of non-aqueous electrolyte employed. It is for that reason that considerable space will be devoted to a general description of various elecrolyte compositions which have, by experimental determinations, been found to produce satisfactory results.
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In place of acetic acid, other liquid mono-carboxylic organic acids, capable of being associated with water, may be used.
These acids may be of the alipilatic series or the aromatic or cyclic type.
Illustrative of the aliphatic acids which may be employed are: Propionic acid, acrylic acid and butyric acid. Derivatives of the mono-carboxylic acids may also be employed, these being represented by such compounds as the following: lactic acid, hydroxy-acrylic acid, crotonic acid, ethylene lactic acid, dihydroxy propionic acid, isobutyric acid, diethyl acetic acid, iso-amyl acetic acid and iso-butyl acetic acid.
Creyulic acid (cresol) and carbolic add (phenol) are examples of suitable aromatic type acids.
Inorganic acids are also employed, in combination with organic acids of either or both the alipilatic and aromatic types.
The following tabulated list will serve to illustrate various electrolyte compositions which have either been comercially employed or have shown satisfactory experimental results:
As the specific resistivity of most of the acids mentioned is relatively high, ammonium borate or other ammonium salts are formed to effectively act as resistance reducing agents. The alkali reagents employed are not limited to ammonia but experimental determinations have shown ammonia to be the most desirable for reasons which will be mentioned in later paragraphs. Ammonium hydroxide may be used to replace ammonia gas. Its substitution would entail the additional procedure of water content adjustment of the prepared electrolyte.
Of the mentioned formulae, formula number two has been found most satisfactory in actual practice.
In the application of these electrolytes to dry electrolytic capacitor structures, the separator medium is saturated at electrolyte temperatures best suited for satisfactory impregnation results and temperatures range from 75°C to 110°C. Temperatures above 110°C cannot normally be employed or excess reduction in desirable water content will result. Also in the application or use of these types of electrolytes specific resistivities and pH values as well as final water contents must be adjusted to conform to the characteristics desired in the completed dry electrolytic capacitors.
Substantially the same types of electrolytes are frequently compounded by the mixing together of two or more salts, each of which is substantially dry in its normal separate state. Such salts are usually blended together at elevated temperatures not as a rule exceeding 115°C. A characteristic of such a mixture of salts, normally solids at room temperature, is that when blended together at elevated temperatures, they prevent recrystallization of one another upon cooling. Upon cooling to room temperature and depending on the salts employed, the mixture will vary in physical state from that of a viscous liquid to a plastic or semi-solid mass.
Illustrative of such electrolytes are the following formulae:
There are many other combinations which would produce substantially the same results but space would not permit their complete listing.
Still another electrolyte which employs no solvent in its composition is a type which employs also only a single salt. Illustrative of this single salt type are the following formulae:
This type of electrolyte is characterized by the unusual circumstance of producing satisfactory results in dry electrolytic capacitors despite the fact that the specific resistivity of the electrolyte, at room temperature, may be very high. The water content is normally extremely low and to obtain this low water content the equivalent boiling point of the electrolyte generally ranges from 150°C to 190°C. With this type of electrolyte, specific resistivities frequently are above values of 1 x 105 ohms per centimeter cubed. Such a value of resistivity is approximately 300 times greater than normally found most desirable in other types of electrolytes.
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The hydroxy-alkylamine employed may be one of the following forms:
A mixture of two or more of the above forms may also be employed.
In this type of electrolyte, although the amines are above designated by the trivalent nitrogen, it is found that they are analogous to ammonia as a gas (NH3) and in solution as ammonium hydroxide (NH4OH).
Thus, if a triethanolamine is combined with an acid, such as boric acid, a triethanolamine salt will result or if combined with a fatty acid such as oleic acid a triethanolamine soap will result. It is obvious that the mono and diethanolamines react in the same manner to produce end products of the same character.
The specific characteristics of an electrolyte of this type, such as pH value and conductivity, are largely determined by the character of the acid component of the salt formed. Acids which have been found satisfactory are boric acid, malic acid, citric acid, acetic acid, lactic acid and tartaric acid. The hydroxl-alkylamine salts formed in combination with these acids as well as with the fatty acids such as oleic, are gels in structure. This fact lends this type of electrolyte to advantageous physical structures for use in dry electrolytic capacitors.
In general, this type of electrolyte is prepared by first combining approximately equal molecular weights of mono, di or tri ethanolamine or a mixture of two or more of these and a fatty acid to produce a resultant soap like structure. To this is added enough boric acid to reduce the pH value to some point below 7. Water content and pH value are adjusted to conform to desired characteristics of the electrolytic capacitors in which the electrolyte is to be used.
Illustrative of electrolytes of this type are the following formulae:
The electrolytes so far described are not generally employed in the fabrication of dry electrolytic capacitors, primarily, because more consistently satisfactory results are obtained with the type which will now be described.
This type of electrolyte consists of a mixture of a polyhydroxy alcohol such as glycerol or ethylene glycol, boric acid and an alkali salt of boric acid such as sodium or ammonium borate.
Some six years ago, glycerol was in more common use than ethylene glycol but since that time the latter has almost entirely replaced the use of the former. Also, the use of sodium borate as phe alkali salt ingredient has been discontinued in general use.
With these changes, the electrolyte in almost universal use, in dry electrolytic capacitors, has evolved into a mixture of ethylene glycol, boric acid and ammonium borate.
Because of the almost universal use of this electrolyte, detailed data will be presented to show the relationship between electrolyte composition and characteristics of dry electrolytic capacitors incorporating the electrolytes in question.
In preference to the use of ammonium borate as a salt, ammonium hydroxide will be indicated in most cases.
The choice of an electrolyte for use in any type of dry electrolytic capacitor structure is not only a matter of compromise but is, in general, determined by factors such as, operating and breakdown voltages of the capacitor, permissible equivalent series resistance and leakage current values and stability of behavior with time and conditions. The physical state of the electrolyte may also be important, depending upon the type of container used to house the capacitor, in its finished state. The physical state of the electrolyte both at room and elevated temperatures may also be important in consideration of the type and corresponding absorbency of the separator material used in the capacitor structure. Meaning, of course, that electrolytes, at impregnation or saturation temperatures, may be required to be in a highly liquid state to ensure thorough penetration into separator materials. Meaning, also, that electrolytes may or may not be required to be semi-solids at room or normal operating temperatures, as circumstances dictate.
If boric acid is added to ethylene glycol, a chemical reaction will take place, even at room temperature. An increase in temperature will increase the rate of reaction. The result of this reaction is the formation of a glyco-borate and water. The end products of such a reaction may also include an excess of either boric acid or ethylene glycol depending on quantitative values of the original mixture.
The electrical conductivity of such a mixture will vary in proportion to the water present, as one of the end products.
If heat is applied to such a mixture, part of the water of reaction will be lost, resulting in an increase in the boiling temperature. Thus boiling point temperature and electrical conductivity becomes a measure of water content.
If the temperature of such a mixture is raised to the boiling point and boiling is continued, all the water may be driven off, with the formation of a glass hard polymerization product or resin. With the formation of the resin the boiling point will have increased to a very elevated temperature and the electrical conductivity will be reduced to substantially zero.
If ethylene glycol, boric acid and ammonium hy droxide are mixed together a similar chemical reaction takes place and the end products formed are water plus a complex compound thought to be an ammoniumglyco-borate. The exact structure of this compound has not been definitely determined but considerable knowledge has been gained, relative to its behavior as an electrolyte in dry electrolytic capacitor structures.
The addition of the ammonium hydroxide to the mixture to form a borate, materially increases the electrical conductivity. The specific resistivity of such a mixture is therefore determined by the amount of the borate formed and the water present. If this mixture is heated, water is again lost and if boiled to a sufficiently high temperature all water and ammonia will be finally driven off with the resultant formation of the glyco-borate resin. If boric acid is added to ethylene glycol in quantities not exceeding total solubility at room temperature, a liquid solution will result. If, however, an excess amount of boric acid is added, the balance will precipitate to form a turbid solution and if the excess is sufficient a semi-solid mass will result.
An increase in temperature will increase the total solubility. Thus, with a certain quantity of boric acid, the mixture may be a clear liquid solution at elevated temperatures and a semi-hard or even hard solid at room temperatures.
If ammonium hydroxide is added to the mixture the total solubility is increased at elevated as well as room temperatures.
A loss of water, by the application of heat, results in a decrease in boric acid solubility and therefore elevates the temperature at which precipitation will begin, on cooling of the mixture to room temperature.
A loss of water also increases the viscosity of the mixture thus producing a highly viscous liquid when the total water loss is sufficient and provided no excess of boric acid is present to form a semi-solid instead.
From the foregoing data it can be seen that a mixture can readily be produced, which by proper quantitative selection of ingredients and adjustment of water content, will possess physical characteristics ranging from liquids to solids and electrical conductivities of varying values.
Of equal importance is the relative acidity or alkalinity of such electrolyte mixtures because of the necessity of maintaining the pH value within predetermined boundaries in order to maintain anodic film stability.
The following graphs will serve to illustrate these various factors.
From a study of the graph curves, a number of important observations may be made. Among these are: that for a given quantity of ethylene glycol and ammonium hydroxide, the specific resistivity of the electrolyte varies with the quantity of boric acid and furthermore the effect of this variation is more pronounced at temperatures near room temperatures than at more elevated values of temperature. It is also quite evident that the lower the boric acid content, in this case, the lower the specific resistivity and obviously the more liquid the physical state of the electrolyte as well.
The above curves indicate an important fact: that is that for a given quantity of ethylene glycol and boric acid there is an optimum value of ammonium hydroxide which results in the highest electrical conductivity.
As has been mentioned in previous paragraphs, the continued elevation of electrolyte temperature results in continued loss of water with resultant increase in boiling point of the mixture and corresponding increase in resistivity.
A typical illustrative example of this relationship is shown by the following graph.
It must be noted that the increase in resistivity with boiling point temperature is not only caused by a reduction in the water content but in an appreciable loss of ammonia as well which in turn causes a proportional change in pH value. Illustrative of this change in relative acidity or alkalinity with elevation in boiling point temperature is the following graph.
It must be mentioned at this point that formulae numbers 23 to 25, inclusive, are not represented as being extremely suitable for use in dry electrolytic capacitor structures. They have merely been used for the purpose of showing the various relationships under examination. As a general rule the boric acid content of electrolytes actually employed ranges from 100 grams to 200 grams per 100 grams of ethylene glycol.
From the data thus far presented it can be seen that there are certain factors which determine the pH value and electrical conductivity of electrolytes of the ammonium-glyco-borate type.
In previous chapters it has been mentioned that the scintillating voltage or breakdown potential of an electrolytic capacitor is a log function of the specific resistivity and the temperature of the electrolyte and thickness of the dielectric oxide film. It must therefore hold that the breakdown voltage of a dry electrolytic capacitor is determined to a large extent by the pH value and water content of the electrolyte, which is in turn determined, to a considerable degree, by the boiling point temperature. That is, of course, provided that boiling point temperature variation is used to control the water content during the preparation of the electrolyte. To illustrate the relationship between water content or in other words electrical conductivity, and scintillating potentials at different electrolyte temperatures, the following graph is referred to:
From this illustration it may be seen that the breakdown voltage of a dry electrolytic capacitor is materially lowered by an increase in temperature and this is obviously an important factor in determining the operating limitations of dry electrolytic capacitor structures.
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Such electrolytes are generally compounded of a polyhydroxy alcohol, boric acid and an alkali salt of boric acid. The mixture is heated and boiled to a temperature where practically all water is removed and a polymerization product or resin is formed. Such a resinous compound has a relatively high resistivity. In fact, specific resistivities frequently reach values of 175,000 ohms per centimeter cubed and in order to render the compound sufficiently conductive, for use in dry electrolytic capacitors, inert substances such as carbon or graphite are added.
A typical electrolyte of this type is sometimes compounded in the following manner.
Such an electrolyte cannot, however, be used in connection with anode foils which are filmed with the thin active type of anodic film, otherwise the resulting capacitor would have no asymmetric characteristics. This would obviously be due to the fact that cold electronic emission would take place from the carbon or other conducting particles with which the electrolyte is loaded, where they contacted the active dielectric film. Under these conditions such a structure would be inoperative as a capacitor.
To overcome this drawback, the anode foil surface is first filmed or coated with a relatively thick film of aluminum hydrate by anodically forming it in an electrolyte such as: for example, sulphuric, oxalic or phosphoric acid. After this film has been formed, the thin active dielectric film is formed beneath the inactive film. The outer, inactive film now serves as a protective coating or barrier between the carbon particles of the electrolyte and the dielectric film proper. The resulting structure becomes operative as an electrolytic capacitor. A cross-sectional view of such an arrangement is shown below.
With this type electrolyte it may also be possible, under certain conditions, to eliminate the cathode coil entirely, making electrical contact to the electrolyte at only one point. Another type of electrolyte which reaches an almost semi-resinous state, in its preparation, is one which may be used with the more generally employed type of anodic film. This electrolyte is characterized by a relatively high specific resistivity which in turn is not fully reflected into the characteristics of the capacitor structures in which it may be utilized. Such an electrolyte may be compounded of the following ingredients.
The boiling point temperature is usually carried to l90°C to 200°C and specific resistivities range from 50,000 to 100,000 ohms per centimeter cubed.
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