U.S. patent application number 09/829303 was filed with the patent office on 2001-10-18 for solid electrolytic capacitor and method of manufacturing the same.
This patent application is currently assigned to Matsushita Electric Industrial Co., Ltd.. Invention is credited to Igaki, Emiko, Shimada, Mikinari, Shiraishi, Seigo, Shoji, Masashi, Tanahashi, Masakazu.
Application Number | 20010030848 09/829303 |
Document ID | / |
Family ID | 18625648 |
Filed Date | 2001-10-18 |
United States Patent
Application |
20010030848 |
Kind Code |
A1 |
Shoji, Masashi ; et
al. |
October 18, 2001 |
Solid electrolytic capacitor and method of manufacturing the
same
Abstract
There is provided a solid electrolytic capacitor including an
anode body formed of a valve-action metal with a dielectric oxide
coating layer formed on its surface, a cathode body, and an
electroconductive polymer layer disposed between the anode body and
the cathode body. The electroconductive polymer layer contains a
softener for softening the electroconductive polymer layer, so that
a solid electrolytic capacitor is provided that has small
variations in characteristics, a lower equivalent series resistance
(ESR), and an excellent high frequency property. In addition, when
the anode and cathode of a solid electrolytic capacitor including
an electroconductive polymer as a solid electrolyte are joined to
each other, a lower ESR can be obtained under a lower pressure, and
the electroconductive polymer layer can be prevented from being
peeled off.
Inventors: |
Shoji, Masashi; (Osaka,
JP) ; Shiraishi, Seigo; (Osaka, JP) ; Igaki,
Emiko; (Hyogo, JP) ; Tanahashi, Masakazu;
(Osaka, JP) ; Shimada, Mikinari; (Kyoto,
JP) |
Correspondence
Address: |
MERCHANT & GOULD PC
P.O. BOX 2903
MINNEAPOLIS
MN
55402-0903
US
|
Assignee: |
Matsushita Electric Industrial Co.,
Ltd.
|
Family ID: |
18625648 |
Appl. No.: |
09/829303 |
Filed: |
April 9, 2001 |
Current U.S.
Class: |
361/523 |
Current CPC
Class: |
H01G 9/025 20130101 |
Class at
Publication: |
361/523 |
International
Class: |
H01G 009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 14, 2000 |
JP |
2000-113731 |
Claims
What is claimed is:
1. A solid electrolytic capacitor, comprising: an anode body formed
of a valve-action metal with a dielectric oxide coating layer
formed on its surface; a cathode body; and an electroconductive
polymer layer disposed between the anode body and the cathode body,
wherein the electroconductive polymer layer contains a softener for
softening the electroconductive polymer layer.
2. The solid electrolytic capacitor according to claim 1, wherein
the softener has a melting point not exceeding 30.degree. C. and a
boiling point of at least 200.degree. C., under a normal
atmospheric pressure.
3. The solid electrolytic capacitor according to claim 2, wherein
the softener has a boiling point of at least 240.degree. C. under
the normal atmospheric pressure.
4. The solid electrolytic capacitor according to claim 1, wherein
the softener is at least one selected from polyhydric alcohols,
fatty alcohols, aromatic alcohols, phenols, and ethers.
5. The solid electrolytic capacitor according to claim 4, wherein
the softener is at least one selected from glycerin, diethylene
glycol, 2-anilinoethanol, m-methoxyphenol, and ethylene glycol
monobenzyl ether.
6. The solid electrolytic capacitor according to claim 1, wherein
the softener is blended in a ratio of 5.0 wt. % in the
electroconductive polymer layer.
7. The solid electrolytic capacitor according to claim 1, wherein
the electroconductive polymer layer contains no binder resin.
8. A method of manufacturing a solid electrolytic capacitor
comprising an anode body formed of a valve-action metal with a
dielectric oxide coating layer formed on its surface, a cathode
body, and an electroconductive polymer layer disposed between the
anode body and the cathode body, the method comprising: forming an
electroconductive polymer layer on at least one electrode body
selected from the anode body and the cathode body; impregnating the
electroconductive polymer layer with a softener to soften the
electroconductive polymer layer, the softer being diluted with a
low-boiling-point solvent; and evaporating the low-boiling-point
solvent.
9. The method of manufacturing a solid electrolytic capacitor
according to claim 8, wherein the low-boiling-point solvent has a
boiling point not exceeding 150.degree. C. under a normal
atmospheric pressure.
10. The method of manufacturing a solid electrolytic capacitor
according to claim 8, wherein the low-boiling-point solvent is at
least one selected from methyl alcohol, ethanol, isopropyl alcohol,
acetone, toluene, and xylene.
11. The method of manufacturing a solid electrolytic capacitor
according to claim 8, wherein the low-boiling-point solvent has a
drying temperature not exceeding 150.degree. C. under a normal
atmospheric pressure.
12. The method of manufacturing a solid electrolytic capacitor
according to claim 8, wherein the softener has a boiling point of
at least 200.degree. C. under a normal atmospheric pressure.
13. The method of manufacturing a solid electrolytic capacitor
according to claim 12, wherein the softener has a boiling point of
at least 240.degree. C. under the normal atmospheric pressure.
14. The method of manufacturing a solid electrolytic capacitor
according to claim 8, wherein the softener is at least one selected
from polyhydric alcohols, fatty alcohols, aromatic alcohols,
phenols, and ethers.
15. The method of manufacturing a solid electrolytic capacitor
according to claim 14, wherein the softener is at least one
selected from glycerin, diethylene glycol, 2-anilinoethanol,
m-methoxyphenol, and ethylene glycol monobenzyl ether.
16. The method of manufacturing a solid electrolytic capacitor
according to claim 8, wherein the softener is blended in a ratio of
5.0 wt. % in the electroconductive polymer layer.
17. The method of manufacturing a solid electrolytic capacitor
according to claim 8, wherein the electroconductive polymer layer
contains no binder resin.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to a solid
electrolytic capacitor with a valve-action metal such as aluminum,
tantalum, or the like used for an anode and an electroconductive
polymer used for a solid electrolyte, and to a method of
manufacturing the same.
[0003] 2. Related Background Art
[0004] Conventionally, solid electrolytic capacitors have been used
widely in computers, portable telephones, and the like.
[0005] FIG. 5 shows a configuration of a conventional solid
electrolytic capacitor with a valve-action metal used as an anode.
In FIG. 5, numeral 5 indicates a valve-action metal porous body,
numeral 6 a dielectric oxide coating, numeral 7 a solid electrolyte
layer, numeral 8 a carbon layer, numeral 9 a silver (Ag) paste
layer, numeral 10 an anode leading-out terminal, and numeral 11 a
cathode leading-out terminal. Initially, the dielectric oxide
coating 6 is formed on a surface of the valve-action metal porous
body 5 such as aluminum with a roughened surface, powder-sintered
tantalum, or the like. Next, an electroconductive polymer such as
polypyrrole, manganese dioxide, or the like is formed on the
surface of the dielectric oxide coating 6 as the solid electrolyte
layer 7. Subsequently, a cathode layer including the carbon layer
8, the silver paste layer 9, and the like is formed on the solid
electrolyte layer 7. Thus, a capacitor element is produced.
Afterward, the anode leading-out terminal 10 is attached to an
anode lead part by welding or the like and the cathode leading-out
terminal 11 is attached to the cathode layer with a conductive
adhesive. Finally, a package (not shown in the figure) is formed to
cover the whole capacitor element except for parts of the anode and
cathode leading-out terminals. Thus, a solid electrolytic capacitor
is obtained. The package serves for maintaining airtightness from
the outside. Generally, the package is a tip type package formed
with a mold using an epoxy-based thermosetting resin containing a
silicon oxide filler or the like, or a lead wire type package
formed by dipping. With this configuration, in order to obtain a
lower equivalent series resistance (ESR), it is necessary to
increase the conductivity of the solid electrolyte layer and to
give consideration to characteristics of the materials of the
carbon layer and the silver paste layer and methods of forming
them.
[0006] There is a solid electrolytic capacitor with a configuration
in which an anode body and a cathode body are laminated so that an
increased capacitance of the product and a lower ESR are obtained
(JP 11(1999)-219861 A). In JP 11(1999)-219861 A, a method of
manufacturing the solid electrolytic capacitor is proposed in which
a solid electrolyte layer is connected directly to a cathode
leading-out electrode. In a conventional solid electrolytic
capacitor with an electroconductive polymer used for a solid
electrolyte, in order to form the electroconductive polymer inside
pores uniformly by increasing reaction resistance in a
polymerization reaction and in order to improve the adhesive
strength between an electroconductive polymer layer and a
polymerized body when the electroconductive polymer is formed using
an electrolyte (a polymeric monomer solution) by electrolytic
polymerization, generally a binder resin is contained in the
electrolyte (a polymeric monomer solution) as proposed in JP
1949637 (1995). Usually, such a binder resin also is contained in
the electroconductive polymer layer.
[0007] Since a binder resin is contained in the electroconductive
polymer layer of a conventional solid electrolytic capacitor, there
has been a problem in that the resistance of the electroconductive
polymer increases. On the other hand, when the binder resin is
omitted to reduce the resistance, the electroconductive polymer
layer is peeled off easily. In addition, even when the binder resin
is present, the electroconductive polymer may be peeled off
partially from a polymerized body during a drying step.
[0008] When a solid electrolytic capacitor is formed using a metal
foil for a cathode body with an anode body and the cathode body
connected directly with each other through an electroconductive
polymer formed on the anode body, it is necessary to apply a
sufficiently high pressure to secure sufficient adhesion of the
electroconductive polymer so as to increase the connection area.
Even when a pressure of 100 kgf/cm.sup.2 is applied, an ESR of 10
m.OMEGA. or lower cannot be obtained (for an element effective area
of 3.times.5 mm.sup.2). In this case, when the pressure to be
applied is low, the area where the metal foil as the cathode body
and the electroconductive polymer are in contact with each other
also is reduced and thus the contact resistance tends to
increase.
[0009] Hence, in order to increase the contact area to lower the
ESR, the package must be formed so that a high pressure is applied
to the contact surface. However, it is difficult to form such a
package, and there is a possibility that the dielectric oxide
coating may be broken by the pressure.
SUMMARY OF THE INVENTION
[0010] In order to solve the conventional problems described above,
it is an object of the present invention to provide a solid
electrolytic capacitor with small variations in characteristics, a
lower ESR, and an excellent high frequency property and to provide
a method of manufacturing the same.
[0011] In order to achieve the above-mentioned object, a solid
electrolytic capacitor of the present invention includes an anode
body formed of a valve-action metal with a dielectric oxide coating
layer formed on its surface; a cathode body; and an
electroconductive polymer layer disposed between the anode body and
the cathode body. The electroconductive polymer layer contains a
softener for softening the electroconductive polymer layer.
[0012] A method of manufacturing a solid electrolytic capacitor,
which includes an anode body formed of a valve-action metal with a
dielectric oxide coating layer formed on its surface, a cathode
body, and an electroconductive polymer layer disposed between the
anode body and the cathode body, of the present invention includes:
forming an electroconductive polymer layer on at least one
electrode body selected from the anode body and the cathode body;
impregnating the electroconductive polymer layer with a softener
diluted with a low-boiling-point solvent to soften the
electroconductive polymer layer; and evaporating the
low-boiling-point solvent.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a flowchart showing a schematic method of
manufacturing a solid electrolytic capacitor according to an
example of the present invention.
[0014] FIG. 2 is a schematic sectional view showing a configuration
of a capacitor element part of a solid electrolytic capacitor
according to an embodiment of the present invention.
[0015] FIG. 3 is a graph showing the relationship between
equivalent series resistance (ESR) and a pressure applied to an
anode-cathode connection portion in a capacitor element for
evaluation according to an example of the present invention.
[0016] FIGS. 4A and 4B are graphs showing examples of frequency
characteristics with respect to the impedance and capacitance of
the solid electrolytic capacitor according to Example 3 of the
present invention, respectively.
[0017] FIG. 5 is a sectional view showing an example of a
configuration of a conventional solid electrolytic capacitor with a
valve-action metal used for an anode.
DETAILED DESCRIPTION OF THE INVENTION
[0018] In the solid electrolytic capacitor of the present
invention, an electroconductive polymer layer formed on both or one
of the anode body and the cathode body is impregnated with a
softener, so that the electroconductive polymer is softened and
thus the connection resistance can be reduced when the anode body
and the cathode body are connected with each other with the
electroconductive polymer interposed therebetween. This is because
the electroconductive polymer and the anode body or the cathode
body are brought easily into contact by their surfaces with each
other due to the softening of the electroconductive polymer.
Accordingly, the electroconductive polymer can be prevented from
being peeled off.
[0019] In the present invention, preferably, the softener has a
melting point not exceeding 30.degree. C. and a boiling point of at
least 200.degree. C., further preferably at least 240.degree. C.,
under a normal atmospheric pressure. The softener is, for example,
at least one selected from polyhydric alcohols, fatty alcohols,
aromatic alcohols, phenols, and ethers. More specifically, it is
preferable that the softener is at least one selected from glycerin
(with a boiling point of 290.5.degree. C.), diethylene glycol (with
a boiling point of 245.8.degree. C.), 2-anilinoethanol,
m-methoxyphenol, and ethylene glycol monobenzyl ether.
[0020] Preferably, the softener is blended in a ratio of 5.0 wt. %
in the electroconductive polymer layer.
[0021] In the present invention, it is preferable that the
electroconductive polymer layer contains no binder resin.
[0022] In the method of the present invention, preferably, the
low-boiling-point solvent has a boiling point not exceeding
150.degree. C. under normal atmospheric pressure. Preferably, the
low-boiling-point solvent is at least one selected from methyl
alcohol, ethanol, isopropyl alcohol, acetone, toluene, and xylene.
It also is preferable that the low-boiling-point solvent has a
drying temperature not exceeding 150.degree. C. under normal
atmospheric pressure. Preferably, the softener has a boiling point
of at least 200.degree. C. under the normal atmospheric pressure.
Further preferably, the softener has a boiling point of at least
240.degree. C. under normal atmospheric pressure. It is preferable
that the softener is at least one selected from polyhydric
alcohols, fatty alcohols, aromatic alcohols, phenols, and ethers.
In addition, preferably, the softener is at least one selected from
glycerin, diethylene glycol, 2-anilinoethanol, m-methoxyphenol, and
ethylene glycol monobenzyl ether. Moreover, it is preferable that
the softener is blended in a ratio of 5.0 wt. % in the
electroconductive polymer layer.
[0023] In the method of the present invention, when the
electroconductive polymer layer is formed with an electrolyte (a
monomer solution for forming a polymer) containing no binder resin,
there is a high possibility that the electroconductive polymer
layer may be peeled off at a drying step after the formation.
Therefore, it is desirable to carry out a step of impregnating the
electroconductive polymer layer with the softener before the drying
step.
[0024] Furthermore, in the solid electrolytic capacitor of the
present invention, the softener may be any materials as long as
they soften the polymer layer through the impregnation of the
polymer layer with them, but typically, it is preferable that the
softener is glycerin.
[0025] In the solid electrolytic capacitor of the present
invention, preferably, an electroconductive polymer contains no
binder resin, so that the resistance of the electroconductive
polymer itself and the interface resistance decrease and thus the
ESR of the capacitor decreases.
[0026] In the solid electrolytic capacitor of the present
invention, the cathode body may be formed of a carbon layer or a
silver paste layer, or may be formed of a metal foil.
[0027] The solid electrolytic capacitor and the method of
manufacturing a solid electrolytic capacitor according to the
present invention allow a solid electrolytic capacitor with a lower
ESR and an excellent high frequency property to be obtained.
[0028] Embodiments of the present invention are described in detail
as follows.
[0029] In a solid electrolytic capacitor of the present invention,
an anode is formed of a valve-action metal. Preferably, aluminum,
tantalum, or niobium is used as the valve-action metal. The
valve-action metal is a porous body having a number of minute holes
or pores leading to the exterior.
[0030] When being made of, for example, aluminum, the anode is a
porous body formed of an aluminum foil roughened by electrolytic
etching or the like to be provided with a number of small holes.
When being made of, for example, tantalum, the anode is a porous
body formed of tantalum powder that is press-formed and then
sintered, or is a porous body formed of a tantalum sheet with
tantalum powder applied thereto and then sintered.
[0031] A dielectric oxide coating layer is formed as an oxide
coating on the surface of the valve-action metal porous body as an
anode by anodic oxidation, so that an anodic body is obtained. This
dielectric oxide coating layer also is formed on the surfaces of
many minute holes in the anode porous body.
[0032] In the present invention, an electroconductive polymer
material such as, for instance, polypyrrole, polythiophene, or
polyaniline may be used as a solid electrolyte. This solid
electrolyte layer is formed on the anode body and also is formed
inside the minute holes of the porous body. This electroconductive
polymer is not particularly limited in the present invention. Any
electroconductive polymers may be used as long as they generally
can be used for solid electrolytic capacitors.
[0033] In the solid electrolytic capacitor of the present
invention, a cathode body is used for collecting electric charges
extracted by the solid electrolyte layer. When a metal foil is used
for the cathode body as an example, a Ni foil or an aluminum foil
with carbon implanted in its surface is used with an
electroconductive polymer formed on its surface by electrolytic
polymerization or is used without requiring any further
processing.
[0034] An electroconductive polymer layer is formed on the anode
body and the anode body is connected to the cathode body with the
electroconductive polymer layer interposed therebetween. Thus, a
capacitor element for evaluation is configured. In this state, its
characteristics as a capacitor can be evaluated with an impedance
analyzer.
[0035] Using a metal foil for the cathode body, an arbitrary number
of anode bodies with electroconductive polymer layers formed
thereon and cathode bodies are laminated alternately under pressure
to be connected with one another. Portions of the respective anode
bodies where the electroconductive polymer layers are not formed
are joined to each other by welding or the like. Portions of the
cathode bodies that are not in contact with the anode bodies are
joined to each other by welding or the like. A package is formed by
molding using thermosetting resin such as epoxy so as not to
contain parts of the anode body and the cathode body. Thus, a solid
electrolytic capacitor is obtained.
[0036] In the present invention, the electroconductive polymer
layers disposed between anode bodies and cathode bodies in the
solid electrolytic capacitor are impregnated with a softener to be
softened. Therefore, an electroconductive polymer layer located in
a connection portion between an anode body and a cathode body
becomes soft. Accordingly, the contact area between the anode and
cathode bodies can be increased under a lower pressure in
connecting the anode body and the cathode body and thus contact
resistance can be reduced.
[0037] In the solid electrolytic capacitor of the present
invention, the electroconductive polymer layer is formed by
electrolytic polymerization using an electrolyte (a polymeric
monomer solution) containing no binder resin. Accordingly, the
electroconductive polymer layer contains no binder resin.
Consequently, the resistance of the electroconductive polymer
itself and connection resistance can be reduced and thus a solid
electrolytic capacitor with a lower ESR can be obtained. In
addition, the electroconductive polymer is moistened due to the
softener and therefore is not peeled off. Thus, an effect of
peeling prevention can be obtained in addition to the
softening.
[0038] In this case, it is desirable that the softener used for
softening the electroconductive polymer has a boiling point of at
least 200.degree. C. For example, when transfer molding or dip
molding with resin is employed for the package formation, a heating
process at 150.degree. C. to 200.degree. C. is carried out in a
step of curing the resin. It is necessary for the softener not to
be boiled off during such a step. Hence, when such a process is
carried out, it is desirable that the softener has a boiling point
of at least 200.degree. C.
[0039] Furthermore, when the solid electrolytic capacitor of the
present invention is mounted on a board by soldering or the like,
the solid electrolytic capacitor is heat-treated at about
240.degree. C. to 260.degree. C. When any material that is gasified
quickly in such a step is added to and is contained in the solid
electrolytic capacitor, the air pressure inside the capacitor
increases. Particularly, when the transfer molding or dip molding
with resin is employed for the package formation, cracks may occur
in the package so that the gas pressure is released. Consequently,
it is desirable that the softener to be contained in the solid
electrolytic capacitor has a boiling point of at least 260.degree.
C.
[0040] FIG. 1 shows an embodiment of a method of manufacturing the
solid electrolytic capacitor according to the present invention
described above.
[0041] Initially, an Al foil is etched electrolytically in an acid
solution or a powder sintered body is formed with Ta powder as the
step of forming the anode body described above. Next, a step of
forming a dielectric oxide coating by anodic oxidation is carried
out. Then, an electroconductive polymer is formed on both the anode
and cathode bodies or on the anode alone. Afterward, a washing step
is carried out using pure water and then the electroconductive
polymer is softened with a softener (a softening step).
Subsequently, a drying step is carried out and the anode and
cathode bodies are laminated alternately under pressure, so that a
capacitor part is formed. Then, an anode extraction lead and a
cathode extraction lead may be connected in some cases. Finally,
the capacitor part is sealed and thus a solid electrolytic
capacitor is obtained. In this case, the number of alternately
laminated anodes and cathodes is changed as required.
[0042] The process of softening the electroconductive polymer
includes: a softening step in which a softener is diluted with a
low-boiling-point solvent and the electroconductive polymer layer
is impregnated with the dilution to be softened; and a drying step
for evaporating the low-boiling-point solvent by heating. For
peeling prevention, it is necessary to carry out the softening step
before the electroconductive polymer is dried (before the drying
step) directly after the electroconductive polymer is formed and is
washed with pure water in the washing step. This allows the
electroconductive polymer layer to be impregnated with the softener
efficiently to be softened. In addition, in joining the anode and
cathode bodies to each other with the electroconductive polymer
layer interposed therebetween, the contact area between them can be
increased under a lower pressure and this facilitates the step of
forming the package. Since the electroconductive polymer layer can
be prevented from being peeled off, the variations in
characteristics of the solid electrolytic capacitor caused by the
peeling off also can be avoided. As the method of softening the
electroconductive polymer with a softener, the following methods
also can be used: for example, a method in which vapor of a
softener is produced and the electroconductive polymer portion is
exposed to the vapor atmosphere, and a method in which a softener
is dissolved in a solution for forming an electroconductive
polymer.
[0043] In the step of forming an electroconductive polymer, the
electroconductive polymer is formed by electrolytic polymerization
with an electrolyte (a polymeric monomer solution) containing no
binder resin, which generally is contained in an electrolyte.
[0044] According to the present invention, the electroconductive
polymer to be formed between an anode body and a cathode body is
formed by electrolytic polymerization with an electrolyte
containing no binder resin, so that a lower ESR can be obtained as
compared to that in a conventional solid electrolytic capacitor. In
addition, a solid electrolyte layer formed of an electroconductive
polymer is softened with higher alcohol or a softener, so that the
electroconductive polymer can be prevented from being peeled off
from electrode foils. According to the present invention, it is
possible to produce a solid electrolytic capacitor with small
variations in characteristics, a lower ESR, and an excellent high
frequency property.
EXAMPLE
[0045] The following description is directed to specific examples.
However, the embodiments of the present invention are not limited
to the following examples.
Example 1
[0046] In the present example, aluminum was used as a valve-action
metal for an anode, an aluminum foil with carbon implanted in its
surface as a cathode metal foil, and polythiophene as an
electroconductive polymer.
[0047] Both surfaces of a part (3 mm.times.3.5 mm) of a roughened
surface of the aluminium foil except for a lead part were anodized
at a formation voltage of 8V, so that a dielectric oxide coating
layer was formed. Afterward, a solid electrolyte pre-coat layer was
formed with polythiophene on the whole surface of the dielectric
oxide coating layer, including the surfaces inside pores, by
chemical polymerization through immersion in a solution containing
a thiophene monomer and a dopant and immersion in an oxidizing
solution. Furthermore, polythiophene was formed on the pre-coat
layer by electrolytic polymerization and thus a solid electrolyte
layer was formed. Next, a part (3.times.5 mm.sup.2) of the aluminum
foil with the surface (3.times.10 mm.sup.2) including carbon
implanted therein was used as a lead part, and electroconductive
polymer polythiophene was formed on the aluminum foil surface
(3.times.5 mm.sup.2) except for the lead part by electrolytic
polymerization, so that a cathode was formed. The electrolyte used
in this case contained binder resin.
[0048] The polythiophene formed on the dielectric oxide coating
layer and on the aluminum foil surface with carbon implanted
therein was softened with a softener. Thus, a capacitor element for
evaluation was obtained. Glycerin with a melting point of
17.degree. C. and a boiling point of 290.degree. C. was used as the
softener. The impregnation process was carried out as follows:
anode and cathode foils with polythiophene formed thereon were
immersed for 20 minutes in a solution prepared by dissolution of 10
wt. % glycerin in ethanol, and then the anode and cathode foils
were removed and were heated to 60.degree. C., so that the ethanol
was evaporated.
[0049] Afterward, a capacitor element for evaluation was assembled.
FIG. 2 shows a schematic view of the capacitor element for
evaluation. In FIG. 2, numeral 1 indicates an aluminum anode foil
as an anode body, numeral 2 a dielectric oxide coating, numeral 3
an electroconductive polymer, and numeral 4 a cathode metal foil.
Terminals of two cathode metal foils serving for extracting a
cathode were subjected to resistance welding. The contact area
between the anode and cathode was 21 mm.sup.2.
[0050] The evaluation results are described later.
Example 2
[0051] Polythiophene was formed by electrolytic polymerization on a
dielectric oxide coating layer and on an aluminum foil surface with
carbon implanted therein. In this example, for the formation of the
polythiophene, an electrolyte containing no binder resin was used
in the electrolytic polymerization and in addition, an
electroconductive polymer was not softened with glycerin. Thus, a
capacitor element for evaluation like the one in Example 1 was
produced. The evaluation results are described later.
Example 3
[0052] A capacitor element for evaluation was obtained by the same
method as in Example 1 except that the polythiophene formed on the
dielectric oxide coating layer and on the aluminum foil surface
with carbon implanted therein by electrolytic polymerization was
formed using an electrolyte containing no binder resin in the
electrolytic polymerization, and the polythiophene thus formed was
softened with glycerin. As the impregnation process, the same
process as in Example 1 was carried out. The evaluation results are
described later.
Comparative Example 1
[0053] A conventional capacitor element was obtained by the same
method as in Example 1 without using the glycerin as the softener.
The evaluation results are described later.
[0054] Evaluation Results
[0055] Each ESR of the capacitor elements for evaluation according
to Examples 1 to 3 and Comparative Example 1 was measured by a
measuring instrument, IMPEDANCE/GAIN-PHASE ANALYZER (4191A,
manufactured by Yokogawa Hewlett-Packard) with the connection
portion in each capacitor element pressurized.
[0056] FIG. 3 shows the measurement results. The ESR values shown
in FIG. 3 were measured at 400 kHz. The ESR of the capacitor
element for evaluation according to Example 1 that included an
electroconductive polymer layer softened with glycerin is lower
than that of the capacitor element for evaluation according to
Comparative Example 1. Particularly, this tendency is found clearly
in the low applied pressure range. In addition, the capacitor
elements for evaluation according to Examples 2 and 3 including
polythiophene formed using the electrolyte containing no binder
resin by electrolytic polymerization have a significantly lower ESR
than those of the capacitor elements for evaluation according to
Example 1 and Comparative Example 1.
[0057] In other words, as compared to the case of Comparative
Example 1, Example 1 shows an effect of lowering the ESR by an
addition of glycerin and furthermore, Examples 2 and 3 show an
effect of lowering the ESR by using an electrolyte free from the
binder resin.
[0058] In the capacitor elements for evaluation of Example 2 and
Comparative Example 1, partial peeling off of polythiophene was
observed. However, no peeling off was observed in the solid
electrolytic capacitors according to Examples 1 and 3 including
polythiophene softened with glycerin. FIGS. 4A and 4B show
frequency characteristics with respect to impedance and capacitance
of the capacitor element for evaluation according to example 3
under a pressure of about 100 kgf/cm.sup.2 under which the lowest
ESR was obtained, respectively. Similarly from FIGS. 4A and 4B, it
can be seen that the capacitor element according to Example 3 has a
very low impedance and is excellent in high frequency property.
[0059] As is apparent from the experimental results described
above, it was confirmed that as in Example 3, a solid electrolytic
capacitor with a very low ESR and an excellent high frequency
property in which the electroconductive polymer was not peeled off
from electrode foils was obtained through the combination of the
characteristics of Examples 1 and 2, i.e. the combination of the
softening with glycerin and the use of an electrolyte free from the
binder resin.
[0060] As described above, the present invention can provide an
excellent solid electrolytic capacitor having a very low ESR and
causing very small variations in ESR among samples. According to
the present invention, the pressure applied to the connection
portion during the formation of a package can be reduced. The
above-mentioned examples were described with the aluminum solid
electrolytic capacitor, but the present invention also can be
applied to a tantalum solid electrolytic capacitor.
[0061] Furthermore, polythiophene was used as the solid electrolyte
in the examples. However, the same effect as in the present
examples also can be obtained using other electroconductive
polymers.
[0062] Moreover, an aluminum foil with a surface including carbon
implanted therein was used as the cathode metal foil in the
above-mentioned examples. However, the same effect as in the
present examples also can be obtained using other metal foils.
[0063] The invention may be embodied in other forms without
departing from the spirit or essential characteristics thereof. The
embodiments disclosed in this application are to be considered in
all respects as illustrative and not limiting. The scope of the
invention is indicated by the appended claims rather than by the
foregoing description, and all changes which come within the
meaning and range of equivalency of the claims are intended to be
embraced therein.
* * * * *