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Electrolytic Capacitors

Part III: Chapter 10

NON-AQUEOUS ELECTROLYTES and THEIR
CHARACTERISTICS

Page Index

Overview

Organic
Acid Types of Electrolytes

Solvent
Types of Electrolytes

Plastic and
Resinous Type Electrolytes

Book ToC

Book Index

Overview

THE non-aqueous electrolytes, employed in dry electrolytic
capacitor structures, are so termed because the percentage of allowable water
present is relatively small.

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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Organic Acid Types of Electrolytes

Straight organic acids of the water soluble types or organic acids
associated with a salt may be employed. For example, acetic acid alone or in
combination with such salts as ammonium acid borate, sodium borate, sodium
potassium tartarate, ammonium phosphate, sodium acetate or ammonium acetate,
is frequently employed.

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:

Formula No. 1

Glacial Acetic Acid

Water content 10 per cent maximum by weight.

Formula No. 2

100 grams boric acid

175 grams glacial acetic acid

Ammonia gas (NH3) passed through mixture to pH value of from
6.5 to 7.

Formula No. 3

Glacial Acetic Acid

Ammonium borate

One gram molecule of commercial borate added to 3.6 gram molecules of
the acid.

Formula No. 4

100 grams boric acid

90 grams glacial acetic acid

90 grams lactic acid

Ammonia gas (NH3) passed through mixture to pH value of from
6.5 to 7.

Formula No. 5

100 grams boric acid

90 grams lactic acid

90 grams propionic acid

Ammonia gas (NH3) passed through mixture to pH value of trom
6.5 to 7.

Formula No. 6

100 grams boric acid

90 grams butyric acid

90 grams lactic acid

Ammonia gas (NH3) passed through mixture to pH value of from
6.5 to 7.

Formula No. 7

100 grams boric acid

90 grams phenol

90 grams lactic acid

Ammonia gas (NH3) passed through mixture to pH value of from
6.5 to 7.

Formula No. 8

100 grams boric acid

90 grams phenol

90 grams glacial acetic acid

Ammonia gas (NH3) passed through mixture to pH value of from
6.5 to 7.

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
75ƒC to 110ƒC. Temperatures above 110ƒC 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 115ƒC. 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:

Formula No. 9

100 grams ammonium acetate

33 grams disodium hydrogen phosphate.

Formula No. 10

100 grams ammonium acetate

25 grams ammonium citrate.

Formula No. 11

100 grams ammonium acetate

100 grams aluminum acetate

100 grams trisodium phosphate.

Formula No. 12

100 grams ammonium acetate

43 grams calcium lactate.

Formula No. 13

100 grams ammonium acetate

25 grams ammonium oxalate.

Formula No. 14

100 grams ammonium acetate

33 grams sodium perborate.

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:

Formula No. 15

100 grams lactic acid

70 cm3 ammonium hydroxide (28%).

Formula No. 16

100 grams lactic acid

80 cm3 sodium hydroxide (37%).

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 150ƒC to 190ƒC. 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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Solvent Type of Electrolytes

Electrolytes may be compounded from the hydroxy-alkylamines, such as
monoethanolamine, in combination with a weak acid of either the organic type,
inorganic type or both.

The hydroxy-alkylamine employed may be one of the following forms:

Monoethanolamine

H2N-CH2CH2OH

or Diethanolamine

HN(CH2CH2OH)2

or Triethanolamine

N(CH2CH2OH)3

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:

Formula No. 17

100 grams boric acid

65 grams triethanolamine.

Formula No. 18

100 grams boric acid

55 grams triethanolamine

25 grams sucrose.

Formula No. 19

100 grams boric acid

25 grams monoethanolamine

30 grams oleic acid.

Formula No. 20

100 grams malic acid

350 grams triethanolamine

200 grams stearic acid.

Formula No. 21

100 grams boric acid

100 grams diethanolamine.

Formula No. 22

100 grams tartaric acid

125 grams monoethanolamine

150 grams palmitic acid.

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.

*** Figure p132 ***

Formula No. 23

60 grams ethylene glycol

20 cm3 ammonium hydroxide (28%)

Variable quantity boric acid

Boiling point 120ƒC.

*** Figure p133 ***

Formula No. 24

60 grams ethylene glycol

10 grams boric acid

Variable quantity ammonium hydroxide (28%)

Boiling point 120ƒC.

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.

*** Figure p134 ***

Formula No. 25

60 grams ethylene glycol

10 grams boric acid

15 cm3 ammonium hydroxide (28%)

Variable boiling point.

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.

*** Figure p135 ***

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:

*** Figure p136 ***

Formula No. 26

100 grams ethlyene glycol

140 grams boric acid

20 cm3 ammonium hydroxide (28%)

Variable boiling point

600 volt dielectric film.

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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Plastic and Resinous Type Electrolytes

Certain applications have been encountered for an electrolyte of such a
physical structure that it may serve not only as the electrolyte but as the
separator also. By this is meant that no absorbent separator material is used
but that the electrolyte is of such a nature that it may be prepared in the
form of thin rubber-like sheets.

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.

Formula No. 27

Diethylene glycol 100 grams

Boric acid 46 grams

Sodium tetraborate 56 grams

Boiling point 180ƒC.

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.

*** Figure p138 ***

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.

Formula No. 28

100 grams lactic acid

75 cm3 ammonium hydroxide (28%).

Formula No. 29

100 grams lactic acid

80 cm3 sodium hydroxide (37%).

The boiling point temperature is usually carried to l90ƒC to 200ƒC and
specific resistivities range from 50,000 to 100,000 ohms per centimeter cubed.

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Updated: 03
April 2000

Publisher: Tyra Buczkowski
FaradNet URL: http://www.FaradNet.com
E-mail: [email-masked]
First edition copyright © 1938 Paul McKnight
Deeley.
This edition copyright © 1996-2000 Tyra T. Buczkowski. All rights
reserved.