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FIELD OF THE INVENTION
The present invention relates to a solid electrolytic capacitor using an
electrically conductive organic polymer as a solid electrolyte and a
process for the producing the same.
BACKGROUND OF THE INVENTION
An electrolytic capacitor is conventionally constructed by applying the
electrolytic oxidation treatment to the surface of a metal foil or a
sintered metal composed of aluminum or tantalum to form an insulating film
and using the insulating film as a dielectric film.
As the recent main technical requirements for an electrolytic capacitor,
there are the reduction of the size, lowering of the impedance with the
increase of the frequency of a circuit, a high reliability, the reduction
of cost, etc. For these requirements, in the field of an aluminum
electrolytic capacitor, an investigation for using a solid electrolyte for
an electrolytic capacitor in place of a liquid electrolyte conventionally
used for an electrolytic capacitor has been made and as such a solid
electrolyte, various electrically conductive polymers such as polypyrrole,
polythiophene, polyfuran, and polyaniline are proposed.
More in detail, for example, JP-A-63-173313 (the term "JP-A" as used herein
means an "unexamined published Japanese patent application") describes
that an oxidized dielectric film is formed on a film-forming metal,
polypyrrole is deposited thereon by the chemically oxidative
polymerization of pyrrole to form an electrically conductive layer, the
pyrrole is further electrolytically polymerized utilizing the conductive
layer, and the electrically conductive polymer comprising the polypyrrole
is deposited on the chemically oxidative polymerized polypyrrole as a
solid electrolyte. Also, JP-A-1-253226 similarly describes that an
electrically conductive layer comprising manganese dioxide is formed on a
dielectric film and polypyrrole or polythiophene is deposited thereon by
an electrolytic polymerization to form a solid electrolyte.
However, in these methods as described above, it is necessary to deposite
polypyrrole, etc., by an electrolytic reaction on a dielectric film or
coating, which is not essentially an electric conductor, and there is a
problem in this point. That is, in these methods, the chemically oxidative
polymerized film layer or the manganese dioxide layer must be formed on
the dielectric film as an electrically conductive layer which is used as
an electrode for the electrolytic polymerization and by forming the layer,
the electrolytic polymerization can be first practiced.
Thus, JP-A-3-35516 proposes a method of preparing a solution of polyaniline
soluble in a solvent, coating the solution on a dielectric film to form a
polyaniline film, and immersing the polyaniline film in a solution of a
protonic acid to apply a doping treatment. According to the method, an
electrically conductive polyaniline film can be formed on the dielectric
film by a simple means without need of forming an electrode and hence the
method is advantageous in the points of the production efficiency and the
cost as compared with the above-described methods.
Furthermore, in the case of using the electrically conductive film
comprising the soluble polyaniline as described above, there is sometimes
a problem in the adhesion between the dielectric film and polyaniline and
it is proposed in JP-A-5-3138 that the problem is solved by mixing the
soluble polyaniline with an aromatic polyamic acid, a soluble aromatic
polyimide, a polyalkylene glycol, a polymer of a vinyl compound, etc., in
an amount of from 1 to 25% by weight, preferably from 2 to 15% by weight,
and more preferably from 3 to 10% by weight, based on the weight of the
polyaniline.
However, the solid electrolytic capacitor obtained by such a method may
show an excellent performance in a capacitance efficiency, tan .delta., a
high frequency impedance, etc., but a leak current is considerably large
and hence there is a problem that the solid electrolytic capacitor does
not have sufficient characteristics as an electrolytic capacitor.
In this case, the leak current is a very important factor in the
characteristics of an electrolytic capacitor and for obtaining a
practically usable electrolytic capacitor, it is necessary to reduce the
value of the leak current as less as possible.
SUMMARY OF THE INVENTION
As a result of various investigations to overcome the above-described
problems in the solid electrolytic capacitor utilizing the electrically
conductive film obtained from the soluble polyaniline as the solid
elecrolyte, it has been found that by mixing a polymer having a specific
chemical structure with a soluble polyaniline, forming a film of the
composite polymer, and subjecting the film to a doping treatment to form
an electrically conductive film, the leak current which is the serious
property of an electrolytic capacitor can be sufficiently reduced to a
practically usable level. The present invention has been attained based on
this finding.
That is, according to the present invention, there is provided a solid
electrolytic capacitor comprising a film-forming metal, a dielectric oxide
film formed thereon, and an electrically conductive composite polymer film
formed on the dielectric oxide film as a solid electroyte,
the electrically conductive composite polymer film comprising
(a) a polyaniline as a first polymer soluble in an organic solvent in an
undoped state, which is a polymer having a quinonediimine structural unit
and a phenylenediamine structural unit represented by the following
formula (I) as the main repeating unit
##STR2##
wherein m and n show the molar fractions of the quinonediimine structural
unit and the phenylenediamine structural unit, respectively, in the
repeating unit, and 0<m<1, 0<n<1, and m+1=1,
(b) a second polymer selected from a polymer having a structure containing
an ester group or an amido group at the main chain or a side chain as a
main repeating unit and a cellulose derivative, and
(c) a protonic acid having a pKa value of 4.8 or less.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is described in detail below.
In the present invention, as the film-forming metal, aluminum or tantalum
is generally used but, if necessary, other metals or composite metals such
as alloys, etc., can be used. A dielectric film is formed on the
film-forming metal to form an anode body of the electrolytic capacitor.
In the present invention, the polyaniline used as the first polymer is a
polyaniline soluble in a solvent in an undoped state and is shown by the
formula (I) described above. The production of the polyaniline, the
undoping method, the solubility thereof in solvents, etc., are described
in detail in JP-A-3-28229. In particular, it is preferred that the
intrinsic viscosity [.eta.] of the polyaniline measured in
N-methylpyrrolidone at 30.degree. C. is at least 0.40 dl/g. Thus, the
polyaniline used in the present invention is distinguished in the points
that the molecular weight thereof is high and the polymer is soluble in a
solvent, from the conventional polyanilines as described in JP-A-3-28229
and is further structurally distinguished from the conventional
polyanilines.
The polyaniline used in the present invention, which is the polymer having
the quinonediimine structural unit and the phenylenediamine structural
unit shown by the formula (I) described above as the main repeating unit
and is soluble in an organic solvent in an undoped state (hereinafter, the
polyaniline is referred to as undoped polyaniline) can be obtained by
gradually adding an aqueous solution of an oxidizing agent having a
standard electrode potential, which is determined as the
electromotive-force in a reduction half cell reaction using a standard
hydrogen electrode as the standard, of at least 0.6 volt to aniline in a
solvent in the presence of a protonic acid having the acid dissociation
constant pKa value of 3.0 or less while keeping the temperature below
5.degree. C., and preferably below 0.degree. C., in an amount of at least
2 equivalents, and preferably from 2 to 2.5 equivalents, the equivalent
being defined as the amount that 1 mole of the oxidizing agent is divided
by the number of electrons necessary for reducing one molecule of the
oxidizing agent, per mole of aniline to form the oxidized polymer of
aniline doped with the protonic acid (hereinafter referred to as a "doped
polyaniline") and then un-doping the doped polyaniline with a basic
material.
The polyaniline obtained by oxidative polymerizing aniline in the presence
of the protonic acid to obtain a polyaniline and undoping the polyaniline
as described above has a high molecular weight and can be dissolved in
various organic solvents.
Examples of the organic solvent are N-methyl-2-pyrrolidone,
N,N-dimethylacetamide, N,N-dimethylformamide, dimethyl sulfoxide,
1,3-dimethyl-2-imidazolidinone, sulforan, etc.
The solubility of the undoped polyaniline depends upon the average
molecular weight of the undoped polyaniline and the kind of the solvent
used, but usually from 0.5% to 100% of the polymer is dissolved and a
solution of from 1 to 30% by weight of the undoped polyaniline can be
obtained. In particular, the undoped polyaniline shows a high solubility
in N-methyl-2-pyrrolidone, usually from 20% to 100% of the polymer is
dissolved in the solvent, and the solution of from 3 to 30% by weight of
the undoped polyaniline in N-methyl-2-pyrrolidone can be obtained.
However, the undoped polyaniline is not dissolved in tetrahydrofuran, an
aqueous 80% acetic acid solution, an aqueous 60% formic acid solution,
acetonitrile, etc.
Furthermore, in the present invention, the polyaniline in the undoped state
described above can be partially or completely reduced using a reducing
agent. The extent of the reduction of the polyaniline can be controlled by
selecting the equivalent ratio of the reducing agent to the polyaniline.
By reducing the polyaniline in the undoped state as described above, the
solubility of the polyaniline in an organic solvent can be increased.
The extent of the reduction of the polyaniline can be evaluated by the
electronic spectrum of the solution of the polyaniline dissolved in
N-methyl-2-pyrrolidone. The electronic spectrum of the
N-methyl-2-pyrrolidone solution of the solvent-soluble polyaniline has the
maximum absorptions at 340 nm and 640 nm. When the polyaniline shown by
the formula (I) described above is completely reduced, the maximum
absorption of 640 nm is vanished and the intensity of the absorption of
340 nm is increased. Therefore, it is considered that the absorption of
640 nm is originated in the quinonediimine structure and the absorption of
340 nm is originated in the phenylenediamine structure.
For the reduction of the polyaniline, hydrazines such as hydrazine hydrate,
phenylhydrazine, etc.; metal hydrides such as lithium aluminum hydride,
lithium borohydride, etc.; hydrogen, etc., can be suitably used. From the
points that phenylhydrazine is dissolved in an organic solvent, in
particular, in N-methyl-2-pyrrolidone but does not reduce
N-methyl-2-pyrrolidone, phenylhydrazine is most peferably used.
In the present invention, as the solvent-soluble polyaniline, in the
polyaniline shown by the formula (I) described above, the polyaniline
wherein the molar fraction n of the phenylenediamine structural unit is
larger than the molar fraction m of the quinonediimine structural unit is
preferably used. Accordingly, the partially or completely reduced
polyaniline is preferably used.
The reduction of the leak current in the solid electrolytic capacitor,
which is the object of the present invention, is described below.
The leak current is an electric current passing through a defect portion of
the oxide film which is the dielectric material of the electrolytic
capacitor at applying a voltage and as the surface area of the dielectric
is larger and as the voltage is higher, the leak current becomes larger.
Thus, the amount of the leak current is frequently expressed by the
product of the value of the capacitance in proportion to the dielectric
surface area and the value of the applied voltage as the standard.
Accordingly, when the capacitance is shown by C (.mu.F) and the applied
voltage is V (volt), the leak current (.mu.A) is shown to the value of CV,
for example, when the leak current is 1/100 of the CV value, the leak
current is shown by 0.01 CV. Usually, in the electrolytic capacitor as a
commercial product, the leak current must be 0.01 CV or less.
In a solid electrolytic capacitor using an electrically conductive film
formed by only doping a soluble polyaniline as the solid electrolyte, the
leak current is high as from about 0.1 to 0.5 CV but in the electrolytic
capacitor using the electrically conductive composite polymer film
obtained by mixing the polymer having the specific chemical structure as
the second polymer with the soluble polyaniline as the first polymer and
doping the composite polymer film obtained as the solid electrolyte
according to the present invention, the leak current is blow 0.01 CV.
However, from the standpoint of obtaining a practically usable solid
electrolytic capacitor, it is required that not only the solid
electrolytic capacitor has less leak current but also the electrolytic
capacitor has other excellent characteristis as electrolytic capacitor,
such as the excellent capacitance efficiency and the excellent tan .delta.
value. That is, the capacitance efficiency is the ratio of the capacitance
obtained by using the electrically conductive film comprising the
polyaniline in place of a conventional electrolyte (the electrolyte is an
electrolyte solution when the electrolytic capacitor is an aluminum
electrolytic capacitor and is manganese dioxide when the electrolytic
capacitor is a tantalum electrolytic capacitor) to the capacitance
obtained by using the conventional electrolyte shown by percentage. If the
capacitance obtained in the case of using the polyaniline electrically
conductive film is same as the capacitance as the case of using the
conventional electrolyte, the capacitance efficiency is 100% and if the
capacitance in the former case is only a half of the latter case, the
capacitance efficiency is 50%. Since as the case may be, the leak current
is low owing to the low capacitance efficiency, it is necessary to attain
the less leak current by other factors of the solid electrolytic capacitor
than the low capacitance efficiency. In the present invention, the aimed
value of the capacitance efficiency is generally at least 95%.
As to the value of tan .delta., the lower, the better, and the aimed value
is 20% or lower in the present invention. The value of tan .delta. is few
% or lower in the case of commercial product. It frequently happens that
when the electric conductivity of the electrolyte does not become higher,
the value of tan .delta. become larger and since in this case, the leak
current becomes, as the case may be, less, it is necessary to attain the
less leak current by other factors than the increase of the tan .delta.
value.
Accordingly, in the present invention, the less leak current means that the
leak current is less in addition to attaining the aimed values of the
capacitance efficiency and the tan .delta. value.
In the present invention, the second polymer is usually largely compounded
with the polyaniline as the first polymer in the range of from 10 to 300%
by weight, and preferably from 25 to 150% by weight, based on the weight
of the polyaniline. By compounding such a large amount of the second
polymer, the amount of the second polymer which is an insulating material
becomes relatively larger than the amount of the polyaniline and hence
from the standpoint of common sence, it is considered that such a
composite polymer will not show a high electric conductivity and hence it
is considered that when such a composite polymer is used as a solid
electrolyte for an electrolytic capacitor, the electric conductivity is
not increased, whereby the capacitance efficiency becomes lower and the
tan .delta. value becomes larger. In fact, JP-A-5-3138 described above
described that the amount of the polymer added as a polymeric binder is
from 1 to 25% by weight, and preferably from 2 to 15% by weight to the
polyaniline and if the amount thereof added is too much, it gives
undesirably bad influences on the induction characteristics of the
electrolytic capacitor and also it is described therein that in the
example, the polymeric binder is added to the polyaniline in an amount of
5% by weight to the amount of the polyaniline and in the comparative
example, when polyamic acid was added to the polyaniline as the second
polymer in an amount of 30% by weight to the polyaniline, tan .delta.
became more than several tens percents.
However, in the present invention, when the second polymer having the
specific chemical structure is selectively admixed with the polyaniline
and film is formed using such a polyaniline composite polymer followed by
applying a doping treatment, in spite of that the second polymer exists in
the composite polymer in a large amount of from 10 to 300% by weight based
the weight of the polyaniline, the electric conductivity of the
electrically conductive film obtained from such a composite polymer is not
reduced and when the electrically conductive film is used as an
electrolyte for the electrolytic capacitor, the capacitance efficiency and
tan .delta. of the electrolytic capacitor sufficiently attain the aimed
values. Further, a large leak current which is a problem about a
conventional solid electrolyte composed of the polyaniline only is greatly
reduced by using the electrically conductive film comprising the composite
polymer and the leak current can be reduced below 0.1 CV according to the
present invention.
Moreover, according to the present invention, in the electrolytic capacitor
using the electrically conductive film formed by applying a doping
treatment to the polyaniline composite polymer comprising the polyaniline
and the second polymer as the solid electrolyte, by applying a D.C.
voltage under a high humidity atmosphere of a relative humidity of at
least 80%, the value of the leak current can be greatly lowered to 0.1 CV
or lower. The applying time of the D.C. voltage is from about 1 to 30
minutes, and preferably from about 1 to 15 minutes.
As described above, by using the electrically conductive film of the
polyaniline composite polymer as the solid electrolyte of the solid
electrolytic capacitor according to the present invention and applying
thereto a D.C. voltage under a high humidity atmosphere, the value of the
leak current can be greatly lowered.
The reason that the leak current in the solid electrolytic capacitor of the
present invention is greatly decreased has not yet been clarified, but the
function that the composite polymer film restrains the damage of giving to
the dielectric oxide film which is coated thereon at film forming thereof
and the function that water is supplied to the defect portions existing on
the dielectric oxide film at the application of a D.C. voltage under a
high humidity atmosphere to restore the defect portions by the anodic
oxidation in the state, which results in reducing the leak current, are
considered to simultaneous take part in the reduction of the leak current
in the present invention.
The second polymer having the specific chemical structure used in the
present invention is at least one polymer selected from a polymer having a
structure containing an ester group or an amido group in the main chain or
the side chain as the main repeating unit and a cellulose derivative. In
the present invention, the polymer is used by adding to a polyaniline
solution and hence it is necessary that the polymer is soluble in
N-methyl-2-pyrrolidone which is the solvent for the polyaniline.
Specific examples of the second polymer suitably used in the present
invention are described below.
As the polymer having an ester group in the main chain, there are a linear
saturated polyester resin being on the market as a trade name of "VYLON"
(made by Toyobo Co., Ltd.), a linear saturated polyester resin being on
the market as a trade name of "elitel" (made by Unitika, Ltd.), etc. These
polymers are commercially available as various types of goods and they can
be all suitably used in the present invention and show a remarkable effect
in the reduction of the leak current. The linear saturated polyester
resins illustrated above are aliphatic polyesters, but aromatic polyesters
can be used in the present invention if they are soluble in a solvent. The
polymer called "polyarylate" corresponds to the aromatic polyester and is
on the market from Unitika Ltd., and Kanegafuchi Chemical Industry Co.,
Ltd.
As the polymer having an ester group in the side chain, there are at least
one kind of a (co)polymer of (meth)acrylic acid esters such as an acrylic
acid ester, a methacrylic acid ester, etc., and vinyl ester compounds such
as vinyl acetate, vinyl propionate, vinyl butyrate, vinyl stearate, etc.,
and further copolymers of these monomers and other monomers.
Specific examples of the polymer having an ester group at the side chain
are polyacrylic acid esters, polymethacrylic acid esters, polyvinyl
acetate, polyvinyl propionate, polyvinyl butyrate, polyvinyl stearate,
etc.
The polymer having an amido group at the main chain is the polymer called a
polyamide resin or a nylon resin, and very many kinds of polymers are on
the market but the polymers which can be used in the present invention are
those soluble in an organic solvent.
Since in polyamide resins, a strong intermolecular hydrogen bond is formed
by the amido groups themselves, many polyamide resins are generally
insoluble in solvents. However, a transparent nylon resin CX-3000 (trade
name) being on the market from Unitika Ltd., and soluble nylons CM4000 and
CM8000 being on the market from Toray Industries, Inc. are soluble in
N-methyl-2-pyrrolidone and are excellent as the polymers used in the
present invention. In addition to these polymers, there is further a
polymer called methoxymethylated polyamide, which is the polymer obtained
by methoxymethylating a part of the amido groups of a polyamide resin, and
the polymer is excellent in the solubility in an organic solvent and can
be suitably used in the present invention.
As the polymer having an amido group at the side chain, there are
polyvinylpyrrolidone having pyrrolidone at the side chain, which is a
cyclic amide, and polyacrylamides.
As the cellulose derivatives, there are a cellulose ester obtained by
suitably esterifying the hydroxyl group of cellulose with an acid, a
cellulose ester obtained by etherifying the hydroxyl group of cellulose,
etc.
Specific examples of the cellulose ester are cellulose acetate, cellulose
acetate butyrate, nitrocellulose, etc. Specific examples of the cellulose
ether are methyl cellulose, ethyl cellulose, benzyl cellulose,
ethylhydroxyethyl cellulose, etc.
In the present invention, the second polymer is used in an amount of
usually from 10 to 400% by weight, and preferably from 20 to 300% by
weight, based on the weight of the polyaniline. If the amount of the
second polymer added to the polyaniline is too small, the effect of
reducing the leak current in the solid electrolytic capacitor prepared
cannot be obtained, while if the amount added is over the above range, the
electric conductvity of the electrically conductive film is lowered, which
gives a bad influence on the characteristics of the solid electrolytic
capacitor.
There is no particular restriction on the protonic acid having a pKa value
of 4.8 or less used in the present invention but the organic acids shown
below can be preferably used. Examples of the organic acids are aliphatic,
aromatic, aroaliphatic, or alicyclic monobasic or polybasic acids. These
organic acids may have a hydroxyl group, a halogen atom, a nitro group, a
cyano group, an amino group, etc.
Specific examples of these organic acids are acetic acid, n-butyric acid,
pentadecafluorooctanic acid, pentafluoroacetic acid, trifluoroacetic acid,
trichloroacetic acid, dichloroacetic acid, monofluoroacetic acid,
monobromoacetic acid, monochloroacetic acid, cyanoacetic acid,
acetylacetic acid, nitroacetic acid, triphenylacetic acid, formic acid,
oxalic acid, benzoic acid, m-bromobenzoic acid, p-chlorobenzoic acid,
m-chlorobenzoic acid, p-chlorobenzoic acid, o-nitrobenzoic acid,
p-nitrobenzoic acid, m-nitrobenzoic acid, trimethylbenzoic acid,
p-cyanobenzoic acid, m-cyanobenzoic acid, Thymol Blue, salicylic acid,
5-aminisalicylic acid, o-methoxybenzoic acid, 1,5-dinitro-4-chlorophenol,
2,6-dinitrophenol, p-oxybenzoic acid, bromophenol blue, mandelic acid,
phthalic acid, isophthalic acid, maleic acid, fumaric acid, malonic acid,
tartaric acid, citric acid, lactic acid, succinic acid, .alpha.-alanine,
.beta.-alanine, glycine, glycolic acid, thioglycolic acid,
ethylenediamine-N,N'-diacetic acid, ethylenediamine-N,N,N',N'-tetraacetic
acid, etc.
The organic acid may have a sulfonic acid group or a sulfuric acid group.
Specific examples of the organic acid are aminonaphtholsulfonic acid,
metanilic acid, sulfanilic acid, allylsulfonic acid, laurylsulfuric acid,
xylenesulfonic acid, chlorobenzenesulfonic acid, 1-propanesulfonic acid,
1-butanesulfonic acid, 1-hexanesulfonic acid, 1-heptanesulfonic acid,
1-octanesulfonic acid, 1-nonanesulfonic acid, 1-decanesulfonic acid,
1-dodecanesulfonic acid, benzenesulfonic acid, styrenesulfonic acid,
p-toluenesulfonic acid, naphthalenesulfonic acid, ethylbenzenesulfonic
acid, propylbenzenesulfonic acid, butylbenzenesulfonic acid,
pentylbenzenesulfonic acid, hexylbenzenesulfonic acid,
heptylbenzenesulfonic acid, octylbenzenesulfonic acid,
nonylbenzenesulfonic acid, decylbenzenesulfonic acid,
undecylbenzenesulfonic acid, dodecylbenzenesulfonic acid,
pentadecylbenzenesulfonic acid, octadecylbenzenesulfonic acid,
diethylbenzenesulfonic acid, dipropylbenzenesulfonic acid,
dibutylbenzenesulfonic acid, methylnaphthalenesulfonic acid,
ethylnaphthalenesulfonic acid, propylnaphthalenesulfonic acid,
butylnaphthalenesulfonic acid, pentylnaphthalenesulfonic acid,
hexylnaphthalenesulfonic acid, heptylnaphthalenesulfonic acid,
octylnaphthalenesulfonic acid, nonylnaphthalenesulfonic acid,
decylnaphthalenesulfonic acid, undecylnaphthalenesulfonic acid,
dodecylnaphthalenesulfonic acid, pentadecylnaphthalenesulfonic acid,
octadecylnaphthalenesulfonic acid, dimethylnaphthalenesulfonic acid,
diethylnaphthalenesulfonic acid, dipropylnaphthalenesulfonic acid,
dibutylnaphthalenesulfonic acid, dipentylnaphthalenesulfonic acid,
dihexylnaphthalenesulfonic acid, diheptylnaphthalenesulfonic acid,
dioctylnaphthalenesulfonic acid, dinonylnaphthalenesulfonic acid,
trimehylnaphthalenesulfonic acid, triethylnaphthalenesulfonic acid,
tripropylnaphthalenesulfonic acid, tributylnaphthalenesulfonic acid,
camphasulfonic acid, acrylamido-t-butylsulfonic acid, etc.
In particular, the protonic acids which can be preferably used in the
present invention are polyfunctional organic sulfonic acids having at
least 2 sulfonic acid groups in the molecule.
Examples of the polyfunctional organic sulfonic acid are ethanedisulfonic
acid, propanedisulfonic acid, butanedisulfonic acid, pentanedisulfonic
acid, hexanedisulfonic acid, heptanedisulfonic acid, octanedisulfonic
acid, nonanedisulfonic acid, decanedisulfonic acid, benzenedisulfonic
acid, naphthalenedisulfonic acid, toluenedisulfonic acid,
ethylbenzenedisulfonic acid, propylbenzenedisulfonic acid,
butylbenzenedisulfonic acid, dimethylbenzenedisulfonic acid,
diethylbenzenedisulfonic acid, dipropylbenzenedisulfonic acid,
dibutylbenzenedisulfonic acid, methylnaphthalenedisulfonic acid,
ethylnaphthalenedisulfonic acid, propylnaphthalenedisulfonic acid,
butylnaphthalenedisulfonic acid, pentylnaphthalenedisulfonic acid,
hexylnaphthalenedisulfonic acid, heptylnaphthalenedisulfonic acid,
octylnaphthalenedisulfonic acid, nonylnaphthalenedisulfonic acid,
dimethylnaphthalenedisulfonic acid, diethylnaphthalenedisulfonic acid,
dipropylnaphthalenedisulfonic acid, dibutylnaphthalenedisulfonic acid,
naphthalenetrisulfonic acid, naphthalenetetrasulfonic acid,
anthracenedisulfonic acid, anthraquinonedisulfonic acid,
phenanthrenedisulfonic acid, fluorenonedisulfonic acid,
carbazoledisulfonic acid, diphenylmethanedisulfonic acid,
biphenyldisulfonic acid, terphenyldisulfonic acid, terphenyltrisulfonic
acid, a naphthalenesulfonic acid-formalin condensate, a
phenanthrenesulfonic acid-formalin condensate, an anthracenesulfonic
acid-formalin condensate, a fluorenesulfonic acid-formalin condensate, and
a carbazolesulfonic acid-formalin condensate. In these polyfunctional
organic sulfonic acids, the position of the sulfonic acid group at the
aromatic ring is optional.
Moreover, the organic acid used in the present invention may be a polymer
acid. Examples of the polymer acid are polyvinylsulfonic acid,
polyvinylsulfuric acid, polystyrenesulfonic acid, a sulfonated
styrene-butadiene copolymer, polyallylsulfonic acid, polymetallylsulfonic
acid, poly-2-acrylamido-2-methylpropanesulfonic acid, halogenated
polyacrylic acid, polyisoprenesulfonic acid, N-sulfoalkylated polyaniline,
and nuclear sulfonated polyaniline. Also, a fluorine-containing polymer
known as Nafion (trade name, made by E. I. Du Pont de Nemours and Company)
can be suitably used as the polymer acid in the present invention.
In the present invention, aromatic polyvalent sulfonic acids such as
1,5-naphthalenedisulfonic acid and also polymer acids are particularly
preferably used since these acids give electrically conductive
polyanilines excellent in heat resistance and water resistance.
The solid electrolytic capacitor of the present invention can be obtained
by forming a dielectric film by an anodic oxidation on the film-forming
metal which is in a porous form for increasing the surfce area, immersing
the film-forming metal having formed thereon the dielectric film in a
solution of a mixture of the soluble polyaniline and the second polymer,
evaporating off the solvent in a dryer to form a composite polymer film
comprising the polyaniline and the second polymer on the dielectric film,
and then subjecting the composite polymer film to a doping treatment to
form an electrically conductive composite film.
As the case may be, the solid electrolytic capacitor of the present
invention can be obtained by dissolving the protonic acid having a pKa
value of 4.8 or less or the salt thereof in a solution of a mixture of the
soluble polyaniline and the second polymer to prepare a mixed solution
containing the soluble polyaniline, the second polymer, and the protonic
acid or the salt thereof, coating the solution on the dielectric film on
the film-forming metal followed by drying to form a composite polymer
film, and then subjecting the composite polymer film to an oxidation
treatment to form an electrically conductive composite film.
For forming the composite polymer film on the dielectric film, as other
method of immersing the dielectic film formed on the film-forming metal in
the solution of the composite polymer followed by drying as described
above, the the solution of the composite polymer may be coated on the
dielectric film and dried. Also, as other method, the solution of the
composite polymer may be spilt onto the dielectric film by a means such as
a dispenser, etc., or the dielectric film may be impregnated with the
solution of the composite polymer in vacuo.
As the drying method, the film may be dried in an inert gas at normal
pressure, may be dried at a low temperature under a reduced pressure, or
may be dried by heating under a reduced pressure. That is, as described
above, for drying the film, various methods can be used without particular
restrictions.
The drying temperature is in the range of usually from 30.degree. C. to
200.degree. C., preferably from 60.degree. C. to 180.degree. C., and more
preferably from 80.degree. C. to 160.degree. C. The drying time is in the
range of usually from 20 minutes to 3 hours, preferably from 20 minutes to
2 hours, and more preferably from 30 minutes to 1 hours.
In the present invention, by doping the polyaniline in the composite
polymer film thus obtained, the electrically conductive composite film is
formed.
In the present invention, for doping the composite polymer film, there are
following three methods. That is, the first method is a protonic acid
doping method as described in JP-A-3-35516, the second method is an
oxidative doping method as described in Japanese Patent Application No.
4-279675, and the third method is an oxidative ion-exchange doping method
as described in Japanese Patent Application No. 5-175739.
The first protonic acid doping method is a method of contacting the
polyaniline (oxidation-type polyaniline) having the quinonediamine
structural unit and the phenylenediamine structural unit as the main
repeating unit with a solution of the protonic acid having a pKa value of
4.8 or less or with the protonic acid in a non-solvent state and
maintaining them as they are until the electric conductivity of the
composite polymer film is increased. In the first method, however, it
takes a considerably long time to increase the electric conductivity to a
desired value and hence the productivity is low for industrially
practicing the first method.
The second oxidative doping method is a method for improving the delay of
the doping speed in the first protonic acid doping method, that is, a
method of once reducing the oxidation-type polyaniline with a reducing
agent to form a polyaniline (reduction-type polyaniline) having the
imino-p-phenylene structural unit as the main repeating unit and then
doping the polyaniline by immersing the polyaniline-containing film in a
mixed solution of an oxidizing agent and a protonic acid. The second
method shows a very high doping speed as compared with the first protonic
acid doping method and is suitable for the industrial practice.
The third method is a method of previously adding the protonic acid as a
state of the salt thereof to a solution of the polyaniline in a reduced
state, admixing the second polymer with the mixture obtained above,
forming a composite polymer film from the mixture, and then doping the
film by immersing it in a mixed solution of an oxidizing agent and the
protonic acid. The third method is a method which can be suitably used in
the case of doping a bulky protonic acid dopant which is reluctant to dope
by the second oxidative doping method.
According to the present invention, by employing any of the methods
described above, a solid electrolytic capacitor having formed thereon the
electrically conductive composite film comprising the polyaniline, the
second polymer, and the protonic acid and having an electric conductivity
of from 1 to 80 S/cm as the solid electrolyte can be produced.
After applying the doping treatment, the electrically conductive composite
film is washed with a proper solvent such as ethanol, acetone, etc.,
followed by drying. Thereafter, terminals are fitted onto the electrically
conductive composite film using an electrically conductive paste such as a
carbon paste, a silver paste, etc., the assembly is molded with an epoxy
resin, etc., and preferably the molded assembly is subjected to an aging
treatment to obtain a solid electrolytic capacitor.
In the present invention, as the film-forming metal, aluminum or tantalum
is preferably used and hence as the dielectric film, the film of aluminum
oxide or tantalum oxide is preferably used.
As described above, since in the solid electrolytic capacitor of the
present invention, the electrically conductive composite polymer film
comprising a soluble polyaniline as the first polymer, a second polymer
selected from a polymer having the structure having an ester group or an
amido group at the main chain or the side chain as the main repeating unit
and a cellulose derivative, and a protonic acid having a pKa value of 4.8
or less is formed on the oxidized dielectric film as the solid
electrolyte, the solid electrolytic capacitor shows a high capacitance
efficiency and low tan .delta. and has the excellent characteristic that
the leak current is very low.
That is, according to the present invention, by forming a composite polymer
of the definite second polymer together with the polyaniline, the leak
current which is from about 0.1 to 0.5 CV in the case of using the
polyaniline alone can be reduced below 0.1 CV and also by compounding the
second polymer, when a D.C. voltage is applied under a high humidity
atmosphere, the leak current can be further reduced below 0.01 CV. Also,
by forming the composite polymer of the polyaniline and the second
polyemr, the polyaniline is protected from the outside environment by the
second polymer to give a good effect to the durability of the solid
electrolytic capacitor, whereby the solid electrolytic capacitor having a
high durability can be obtained.
The present invention is described in more detail by the following
referrence examples and the examples but the invention should not be
limited by these examples. All percents, parts, ratios and the like are by
weight unless otherwise indicated.
REFERENCE EXAMPLE 1
Production of quinonediimine-phenylenedime type electrically conductive
polyaniline in doped state by the oxidative polymerization of aniline:
In a 10 liter separable flask equipped with a stirrer, a thermometer, and a
straight tube adapter were placed 6,000 g of distilled water, 360 ml of
36% hydrochloric acid, and 400 g (4.295 mols) of aniline in this order and
aniline was dissolved. Apart from this, 434 g (4.295 mols) of 97%
concentrated sulfuric acid was added to 1,493 g of distilled water in a
beaker while cooling with ice-water followed by mixing to obtain an
aqueous sulfuric acid solution. The aqueous sulfuric acid solution was
added to the solution in the separable flask and the whole flask was
cooled to -4.degree. C. in a low-temperature bath kept at constant
temperature.
980 g (4.295 mols) of ammonium peroxodisulfate was dissolved in 2,293 g of
distilled water in a beaker to obtain an aqueous oxidizing agent solution.
While keeping the temperature of the reaction mixture in the flask below
-3.degree. C. by cooling the whole flask in a low-temperature bath, the
aqueous solution of ammonium peroxodisulfate was gradually added dropwise
to the acidic aqueous solution of the aniline salt in the flask with
stirring from the straight tube adapter using a tubing pump at a rate of 1
ml/minute or less. The solution which was colorless and transparent at
first became from greenish blue to blackish green with the progress of the
polymerization and a blackish green powder was precipitated.
At the precipitation of the powder, the temperature of the reaction mixture
was raised and thus the reaction mixture was cooled to restrain the
temperature in the reaction system below -3.degree. C. After the
precipitation of the powder, the dropping rate of the aqueous solution of
ammonium peroxodisulfate might be increased to, for example, about 8
ml/minutes. However, in this case, it was necessary to control the
dropping rate such that the temperature of the reaction mixture was kept
below -3.degree. C. while monitering the temperature of the reaction
mixture. After completion | | |
