U.S. patent application number 11/032076 was filed with the patent office on 2005-10-06 for solid electrolytic capacitor and a fabrication method therefor.
This patent application is currently assigned to SANYO ELECTRIC CO., LTD.. Invention is credited to Kimoto, Mamoru, Takatani, Kazuhiro, Yano, Mutsumi.
Application Number | 20050219801 11/032076 |
Document ID | / |
Family ID | 35050022 |
Filed Date | 2005-10-06 |
United States Patent
Application |
20050219801 |
Kind Code |
A1 |
Yano, Mutsumi ; et
al. |
October 6, 2005 |
SOLID ELECTROLYTIC CAPACITOR AND A FABRICATION METHOD THEREFOR
Abstract
A solid electrolytic capacitor of the present invention is
provided with: an anode employing titanium; a dielectric layer
formed on said anode, which comprises titanium oxide containing
fluorine and at least one element selected from a group consisting
of phosphorus, boron, sulfur, and tungsten; and a cathode layer
formed on said dielectric layer.
Inventors: |
Yano, Mutsumi;
(Hirakata-City, JP) ; Takatani, Kazuhiro;
(Takatsuki-City, JP) ; Kimoto, Mamoru;
(Hirakata-City, JP) |
Correspondence
Address: |
MCDERMOTT WILL & EMERY LLP
600 13TH STREET, N.W.
WASHINGTON
DC
20005-3096
US
|
Assignee: |
SANYO ELECTRIC CO., LTD.
|
Family ID: |
35050022 |
Appl. No.: |
11/032076 |
Filed: |
January 11, 2005 |
Current U.S.
Class: |
361/523 |
Current CPC
Class: |
H01G 9/0032 20130101;
H01G 9/15 20130101; Y02E 60/13 20130101; H01G 9/042 20130101; H01G
11/56 20130101 |
Class at
Publication: |
361/523 |
International
Class: |
H01G 009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 2004 |
JP |
2004-104843 |
Claims
1. A solid electrolytic capacitor provided with: an anode employing
titanium; a dielectric layer formed on said anode, which comprises
titanium oxide containing fluorine and at least one element
selected from a group consisting of phosphorus, boron, sulfur, and
tungsten; and a cathode layer formed on said dielectric layer.
2. The solid electrolytic capacitor as claimed in claim 1, wherein
concentration of at least one element selected from the group
consisting of phosphorus, boron, sulfur, and tungsten is higher in
a cathode layer side of said dielectric layer than an anode side of
said dielectric layer.
3. The solid electrolytic capacitor as claimed in claim 1, wherein
thickness of said dielectric layer is in a range of 4 nm to 20
nm.
4. The solid electrolytic capacitor as claimed in claim 1, wherein
an electrolyte layer is formed between said dielectric layer and
the cathode layer.
5. A fabrication method of a solid electrolytic capacitor
comprising steps of: forming a dielectric layer by anodizing an
anode employing titanium; and forming a cathode layer on said
dielectric layer; wherein said step of forming the dielectric layer
comprises: a first step of anodizing said anode in an aqueous
solution containing fluoride ion; and a second step of anodizing
the anode after the first step in the aqueous solution containing
at least one ion selected from a group consisting of phosphate ion,
borate ion, sulfate ion, and tungstate ion.
6. A fabrication method of a solid electrolytic capacitor
comprising steps of: forming a dielectric layer by anodizing an
anode employing titanium; forming an electrolyte layer on said
dielectric layer; and forming a cathode layer on said electrolyte
layer; wherein said step of forming the dielectric layer comprises:
a first step of anodizing said anode in an aqueous solution
containing fluoride ion; and a second step of anodizing the anode
after the first step in the aqueous solution containing at least
one ion selected from a group consisting of phosphate ion, borate
ion, sulfate ion, and tungstate ion.
Description
[0001] RELATED APPLICATION
[0002] The priority application(s) Number(s) Japanese Patent
Application No. 2004-104843 upon which this patent application is
based is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] The present invention relates to a solid electrolytic
capacitor and a fabrication method therefor.
[0005] 2. Description of the Related Art
[0006] Solid electrolytic capacitors provided with a dielectric
layer of titanium oxide having a dielectric constant about 5 times
as large as aluminum oxide have been well-known (JP-A-5-121275 and
JP-A-11-312628).
[0007] In fabrication of the solid electrolytic capacitors provided
with the dielectric layer of titanium oxide, an anode employing
titanium metal plates or of sintered bodies of titanium powder is
anodized.
[0008] However, in the above-mentioned solid electrolytic
capacitors provided with the dielectric layer of titanium oxide
prepared by anodizing the anode of the titanium metal plates or of
the sintered bodies of titanium powder, there have remained
problems that titanium oxide is easily crystallized in a process of
the anodic oxidation, consequently, the electrical insulation of
the dielectric layer decreases and leakage current between an anode
and a cathode layer increases.
SUMMARY OF THE INVENTION
[0009] A first object of the present invention is to decrease
leakage current between an anode and a cathode layer in a solid
electrolytic capacitor provided with a dielectric layer of titanium
oxide prepared by anodizing the anode.
[0010] Another object of the invention is to provide a fabrication
method of the solid electrolytic capacitor with less leakage
current between the anode and the cathode layer.
[0011] A solid electrolytic capacitor according to the present
invention is provided with: an anode employing titanium; a
dielectric layer formed on said anode, which comprises titanium
oxide containing fluorine and at least one element selected from a
group consisting of phosphorus, boron, sulfur, and tungsten; and a
cathode layer formed on the dielectric layer.
[0012] When the dielectric layer of titanium oxide contains
fluorine and at least one element selected from the group
consisting of phosphorus, boron, sulfur, and tungsten as in the
solid electrolytic capacitor of the present invention,
crystallization of titanium oxide which occurs in a process of
anodic oxidation of the anode is well-controlled.
[0013] Consequently, in the solid electrolytic capacitor of the
present invention, decrease of the electrical insulation of the
dielectric layer is obviated and the leakage current between the
anode and the cathode layer decreases.
[0014] In the solid electrolytic capacitor of the present
invention, concentration of at least one element selected from the
group consisting of phosphorus, boron, sulfur, and tungsten is
preferably set to be higher in a cathode layer side of the
dielectric layer than an anode side of the dielectric layer. The
crystallization of titanium oxide on the cathode layer side which
is a surface side of the dielectric layer is further controlled by
such a composition, therefore, volume change caused by the
crystallization of titanium oxide on the cathode layer side of the
dielectric layer is controlled. Consequently, occurrence of cracks
on the surface of the dielectric layer is prevented and the
decrease of the electrical insulation of the dielectric layer is
further obviated.
[0015] In the solid electrolytic capacitor of the present
invention, an insufficient thickness of the dielectric layer
results in the insufficient electrical insulation whereas an
excessive thickness of the dielectric layer results in the decrease
of the electrical insulation of the dielectric layer because of the
necessity of long time for the anodic oxidation to form the
dielectric layer and the progress of the crystallization of
titanium oxide which occurs in the process of the anodic oxidation.
Therefore, the thickness of the dielectric layer is preferably in a
range of 4 nm to 20 nm and, more preferably, in the range of 6 nm
to 15 nm. When the thickness of the dielectric layer is within the
above-mentioned range, the electrical insulation of the dielectric
layer sufficiently increases.
[0016] A fabrication method of a solid electrolytic capacitor of
the present invention comprises steps of: forming a dielectric
layer by anodizing an anode employing titanium; and forming a
cathode layer on said dielectric layer; wherein said step of
forming the dielectric layer comprises: a first step of anodizing
the anode in an aqueous solution containing fluoride ion; and a
second step of anodizing the anode after the first step in the
aqueous solution containing at least one ion selected from a group
consisting of phosphate ion, borate ion, sulfate ion, and tungstate
ion.
[0017] In the fabrication method of the solid electrolytic
capacitor of the present invention, a first anodically oxidized
film of titanium oxide containing fluorine is formed on the anode
employing titanium as a result of the first step of forming the
dielectric layer. In addition, a second anodically oxidized film of
titanium oxide containing at least one element selected from the
group consisting of phosphorus, boron, sulfur, and tungsten is
formed as a result of the second step. Consequently, the dielectric
layer comprising the first anodically oxidized film and the second
anodically oxidized film, which are successively layered is formed
on the anode employing titanium.
[0018] Fluorine contained in the first anodically oxidized film
easily diffuses, therefore, the second anodically oxidized film
also contains fluorine by formation of the second anodically
oxidized film on the first anodically oxidized film.
[0019] On the other hand, at least one element selected from the
group consisting of phosphorus, boron, sulfur, and tungsten hardly
diffuses, therefore, concentration of said elements in the
dielectric layer comprising the first anodically oxidized film and
the second anodically oxidized film is higher in a cathode layer
side of the second anodically oxidized film than an anode side of
the first anodically oxidized film.
[0020] Consequently, the dielectric layer of titanium oxide
comprising the first anodically oxidized film and the second
anodically oxidized film contains fluorine and at least one element
selected from the group consisting of phosphorus, boron, sulfur,
and tungsten. As a result, the crystallization of titanium oxide is
controlled and the decrease of the electrical insulation of the
dielectric layer is obviated.
[0021] In addition, the concentration of at least one element
selected from the group consisting of phosphorus, boron, sulfur,
and tungsten is higher in the cathode layer side of the dielectric
layer, therefore, the crystallization of titanium oxide on the
cathode layer side which is the surface side of the dielectric
layer is further controlled. Consequently, volume change caused by
the crystallization of titanium oxide on the cathode layer side of
the dielectric layer is controlled. Therefore, occurrence of cracks
on the surface of the dielectric layer is prevented and the
decrease of the electrical insulation of the dielectric layer is
further obviated.
[0022] These and other objects, advantages and features of the
invention will become apparent from the following description
thereof taken in conjunction with the accompanying drawings which
illustrate specific embodiments of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a structural cross-sectional view of a solid
electrolytic capacitor of an embodiment of the present
invention.
[0024] FIG. 2 is an illustration showing a structure and an
evaluation method of a solid electrolytic capacitor of Example 1 of
the present invention.
[0025] FIG. 3 is an illustration showing measurement results of an
anode and a dielectric layer of the solid electrolytic capacitor of
Example 1 of the present invention by ESCA.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] Hereinafter, preferred embodiments of the present invention
will specifically be described based on the attached figures. It is
to be noted that the present invention should not be limited to the
following examples thereof and suitable changes and modifications
may be made thereto within the scope of the invention.
[0027] FIG. 1 is a structural cross-sectional view of a solid
electrolytic capacitor of an embodiment of the present
invention.
[0028] In a solid electrolytic capacitor 100 of the embodiment, as
shown in FIG. 1, a dielectric layer 2 of titanium oxide containing
fluorine is formed on a surface of a plate-shaped anode of a porous
sintered body prepared by sintering and molding titanium particles
around an anode lead 1a in a vacuum environment.
[0029] The dielectric layer 2 comprises a first anodically oxidized
film 2a at an anode 1 side and a second anodically oxidized film 2b
at a cathode layer 4 side. The second anodically oxidized film 2b
contains fluorine and at least one element selected from a group
consisting of phosphorus, boron, sulfur, and tungsten.
[0030] An electrolyte layer 3 is formed on the dielectric layer 2.
The electrolyte layer 3 comprises manganese dioxide or conducting
polymer including polypyrrole and polyaniline.
[0031] A cathode layer 4 is formed on the electrolyte layer 3. The
cathode layer 4 comprises a first conductive layer 4a of carbon
paste or the like formed on the electrolyte layer 3 and a second
conductive layer 4b of silver paste or the like formed on the first
conductive layer 4a.
[0032] A cathode terminal 6 is installed on the cathode layer 4 by
conductive adhesive 5 and an anode terminal 7 is installed to an
anode lead la of said anode 1.
[0033] One end of the anode terminal 7 and that of the cathode
terminal 6 are thrust outside a mold outer resin 8.
[0034] A fabrication method of the solid electrolytic capacitor
will be specified hereinafter.
[0035] Titanium particles are sintered and molded around the anode
lead 1a in the vacuum environment to obtain a plate-shaped anode 1
of a porous sintered body prepared by welding the titanium
particles.
[0036] Next, in formation of the dielectric layer 2 on the anode 1,
the anode 1 is anodized in an aqueous solution containing fluoride
ion including ammonium fluoride aqueous solution as a first step.
As a result, the first anodically oxidized film 2a of titanium
oxide containing fluorine is formed on the anode 1.
[0037] Further, the anode 1 is anodized in at east one aqueous
solution selected from the aqueous solution containing phosphate
ion including ammonium dihydrogen phosphate aqueous solution, the
aqueous solution containing borate ion including ammonium borate
aqueous solution, the aqueous solution containing sulfate ion
including sulfate aqueous solution, and the aqueous solution
containing tungstate ion including sodium tungstate aqueous
solution as a second step. As a result, the second anodically
oxidized film 2b of titanium oxide containing at least one element
selected from phosphorus, boron, sulfur, and tungsten is formed on
the first anodically oxidized film 2a.
[0038] Thus, the dielectric layer 2 comprising the first anodically
oxidized film 2a and the second anodically oxidized film 2b, which
are layered successively is formed on the surface of the anode
1.
[0039] The second anodically oxidized film 2b also contains
fluorine when the second anodically oxidized film 2b is formed by
the second step because fluorine contained in the first anodically
oxidized film 2a easily diffuses to the second anodically oxidized
film 2b. On the other hand, at least one element selected from
phosphorus, boron, sulfur, and tungsten contained in the second
anodically oxidized film 2b hardly diffuses to the first anodically
oxidized film 2a, therefore, concentration of said elements in the
dielectric layer 2 comprising the first anodically oxidized film 2a
and the second anodically oxidized film 2b is higher in the cathode
layer side of the second anodically oxidized film 2b than the anode
side of the first anodically oxidized film 2a.
[0040] In addition, an electrolyte layer 3 of manganese dioxide or
conducting polymer including polypyrrole and polyaniline is formed
on the dielectric layer 2. Various polymerization processes can be
applied to form the electrolyte layer 3 of the conducting polymer
whereas pyrolysis can be applied to form the electrolyte layer 3 of
manganese dioxide. When the electrolyte layer 3 is formed by the
above-mentioned manner, the electrolyte layer 3 fills up chinks of
the dielectric layer 2 on the surface of the porous sintered body
as the anode 1.
[0041] Moreover, the first conductive layer 4a is formed by
applying carbon paste or the like on the electrolyte layer 3 and
the second conductive layer 4b is formed by applying silver paste
or the like on the first conductive layer 4a so as to form the
cathode layer 4 comprising the first conductive layer 4a and the
second conductive layer 4b.
[0042] The cathode terminal 6 is installed on the cathode layer 4
by the conductive adhesive 5 and the anode terminal 7 is installed
to the anode lead la salient from the dielectric layer 2, the
electrolyte layer 3, and the cathode layer 4. Furthermore, one end
of the anode terminal 7 and that of the cathode terminal 6 are
thrust outside the mold outer resin 8.
[0043] In the solid electrolytic capacitor 100 of the embodiment,
the dielectric layer 2 of titanium oxide formed on the anode 1
contains fluorine and at least one element selected from the group
consisting of phosphorus, boron, sulfur, and tungsten, therefore,
crystallization of titanium oxide is well-controlled. Consequently,
decrease of the electrical insulation of the dielectric layer 2
caused by the crystallization of titanium oxide is obviated and
leakage current between the anode 1 and the cathode layer 4
decreases.
[0044] In the solid electrolytic capacitor 100 of the embodiment,
the concentration of the at least one element selected from the
group consisting of phosphorus, boron, sulfur, and tungsten is
higher in the cathode layer side of the second anodically oxidized
film 2b than the anode side of the first anodically oxidized film
2a. The crystallization of titanium oxide on the cathode layer 4
side which is a surface side of the dielectric layer is further
controlled by such a composition, therefore, volume change caused
by the crystallization of titanium oxide on the cathode layer side
of the dielectric layer is controlled. Consequently, occurrence of
cracks on the surface of the dielectric layer 2 is prevented and
the decrease of the electrical insulation of the dielectric layer 2
is further obviated.
[0045] In the embodiment, in the formation of the dielectric layer
2 on the anode 1, the first anodically oxidized film 2a of titanium
oxide containing fluorine is formed on the anode 1 by anodic
oxidation first and the second anodically oxidized film 2b of
titanium oxide containing at least one element selected from the
group consisting of phosphorus, boron, sulfur, and tungsten is
formed by the anodic oxidation secondly. Consequently, the
dielectric layer 2 containing fluorine and at least one element
selected from the group consisting of phosphorus, boron, sulfur,
and tungsten is formed and the concentration of at least one
element selected from the group consisting of phosphorus, boron,
sulfur, and tungsten is easily set to be higher in the cathode
layer 4 side of the dielectric layer 2.
[0046] Consequently, the crystallization of titanium oxide in the
dielectric layer 2 is controlled and the occurrence of the cracks
on the surface of the dielectric layer 2 is prevented so as to
easily fabricate the solid electrolytic capacitor 100 provided with
the dielectric layer 2 with an increased electrical insulation.
[0047] In the embodiment, there was used the plate-shaped anode 1
of the porous sintered body. The anode 1, however, is not limited
to said shape but may be a rod-shaped anode or a foil-shaped anode.
The anode 1 may also be the plate-shaped anode or the foil-shaped
anode respectively prepared by casting and rolling or the like.
[0048] In addition, in the embodiment, the electrolyte layer 3 is
formed between the dielectric layer 2 and the cathode layer 4. The
formation of the electrolyte layer 3, however, may be omitted and
the cathode layer 4 may directly be formed on the dielectric layer
2.
[0049] Hereinafter, a solid electrolytic capacitor of examples of
the present invention will specifically be described while
comparative examples will be cited to demonstrate that examples of
the inventive electrolytic capacitor decrease leakage current.
EXAMPLE 1
[0050] FIG. 2 is an illustration showing a structure and an
evaluation method of a solid electrolytic capacitor of Example 1 of
the present invention.
[0051] In fabrication of the solid electrolytic capacitor of
Example 1, an anode 11 of plate-shaped titanium substrate about 50
mm long, 10 mm wide, and about 100 .mu.m thick prepared by casting
and rolling or the like was formed.
[0052] Next, in formation of the dielectric layer 2 on the anode
11, the anode 11 was anodized at a constant voltage of about 5 V in
an aqueous solution containing about 0.1 wt % of ammonium fluoride
(concentration of fluoride ion: about 0.05 wt %) at about
50.degree. C. for about 30 minutes as the first step to form a
first anodically oxidized film 2a of titanium oxide containing
fluorine on the anode 11. The ammonium fluoride aqueous solution is
an example of the aqueous solution containing fluoride ion.
[0053] Further, the anode 11 was anodized at the constant voltage
of about 5 V in the aqueous solution containing about 0.1 wt % of
ammonium dihydrogen phosphate at about 50.degree. C. for about 30
minutes to form the second anodically oxidized film 2b on the first
anodically oxidized film 2a. The ammonium dihydrogen phosphate
aqueous solution is an example of the aqueous solution containing
phosphate ion.
[0054] Thus, the dielectric layer 2 comprising the first anodically
oxidized film 2a and the second anodically oxidized film 2b having
total thickness of about 10 nm was formed on the anode 11.
[0055] The dielectric layer 2 comprising the first anodically
oxidized film 2a and the second anodically oxidized film 2b formed
on the anode 11 was analyzed by ESCA (Electron Spectroscopy for
Chemical Analysis). The results are shown in FIG. 3. In FIG. 3,
vertical axes show content of elements in the capacitor and
horizontal axes show sputtering time. The sputtering time
corresponds to a position in thickness direction and sputtering
depth per minute of the sputtering time is about 1 nm.
[0056] As shown in FIG. 3, distribution of oxygen (O) confirms that
the dielectric layer 2 of titanium oxide containing titanium (Ti)
and oxygen (O), having the thickness of about 10 nm was formed on
the anode 11. In FIG. 3, the region on a right side of the
dielectric layer 2, in which oxygen (O) hardly exists is anode
11.
[0057] In addition, FIG. 3 shows that the dielectric layer 2
comprises the region (i) about 6 nm thick which contains fluorine
(F) and phosphorus (P) on a surface side (cathode layer 4 side) and
the region (ii) about 4 nm thick which contains fluorine (F) but
does not contain phosphorus (P) on an anode 11 side. Therefore, it
was confirmed that the region (i) was the second anodically
oxidized film 2b formed in the above-mentioned second step and the
region (ii) was the first anodically oxidized film 2a formed in the
above-mentioned first step.
[0058] Further, about 3% by atom of fluorine (F) was contained in
the interior of the region (ii), therefore, fluorine (F) expectedly
diffused in the region (i). It was also confirmed that the
concentration of phosphorus (P) of the region (i) was highest
(about 4% by atom) on the surface side (cathode layer 4 side) and
the concentration gradually decreased in the interior.
[0059] Moreover, an electrolyte layer 3 of polypyrrole was formed
on the dielectric layer 2 by electrolytic polymerization.
Furthermore, carbon paste was applied on the electrolyte layer 3 to
form a first conductive layer 4a and silver paste was applied on
the first conductive layer 4a to form a second conductive layer 4b
so as to form the cathode layer 4 comprising the first conductive
layer 4a and the second conductive layer 4b on the electrolyte
layer 3. Thus was fabricated the solid electrolytic capacitor A of
Example 1.
COMPARATIVE EXAMPLE 1
[0060] In comparative example 1, in the anodic oxidation of the
anode 11 of the plate-shaped titanium substrate, time for the
anodic oxidation according to the first step of Example 1 was
changed to 60 minutes and the second step of was not carried.
Except for the above, the same procedure as that in Example 1 was
taken to fabricate the solid electrolytic capacitor X of
comparative example 1, in which the dielectric layer comprising
only the first anodically oxidized film having the thickness of
about 10 nm was formed on the anode.
[0061] As a result of measurement of the dielectric layer
comprising the first anodically oxidized film formed on the anode
by ESCA, it was confirmed that fluorine was contained in the
dielectric layer whereas phosphorus was not contained in the
dielectric layer.
COMPARATIVE EXAMPLE 2
[0062] In comparative example 2, in the anodic oxidation of the
anode 11 of the plate-shaped titanium substrate, the first step of
Example 1 was not carried out and the time for the anodic oxidation
according to the second step was changed to 60 minutes. Except for
the above, the same procedure as that in Example 1 was taken to
fabricate the solid electrolytic capacitor Y of comparative example
2, in which the dielectric layer comprising only the second
anodically oxidized film having the thickness of about 10 nm was
formed on the anode.
[0063] As a result of the measurement of the dielectric layer
comprising the second anodically oxidized film formed on the anode
by ESCA, it was confirmed that phosphorus was contained in the
dielectric layer whereas fluorine was not contained in the
dielectric layer.
[0064] The constant voltage of about 2.5 V was applied between the
anode 11 and the cathode layer 4 of the resultant solid
electrolytic capacitors A, X, and Y of Example 1, comparative
examples 1, and 2 so as to determine the respective leakage current
after about 20 seconds. An index number of the leakage current of
each of the solid electrolytic capacitors was determined on a basis
of the leakage current of the solid electrolytic capacitor of
Example 1 defined as 100. The results are listed in Table 1 as
below.
1 TABLE 1 leakage current solid electrolytic capacitor A 100 solid
electrolytic capacitor X 700 solid electrolytic capacitor Y
1000
[0065] As is apparent from the results, the leakage current of the
solid electrolytic capacitor A of Example 1 provided with the
dielectric layer containing fluorine and phosphorous decreased so
notably as to about {fraction (1/7)} compared with the solid
electrolytic capacitor X of comparative example 1 provided with the
dielectric layer which contained fluorine but did not contain
phosphorus, and to about {fraction (1/10)} compared with the solid
electrolytic capacitor Y of comparative example 2 provided with the
dielectric layer which contained phosphorus but did not contain
fluorine.
EXAMPLE 2
[0066] In Example 2, in the anodic oxidation of the anode 11 of the
plate-shaped titanium substrate, the aqueous solution containing
about 0.1 wt % of ammonium borate was used in place of the aqueous
solution containing about 0.1 wt % of ammonium dihydrogen phosphate
used in the second step of Example 1. Except for the above, the
same procedure as that in Example 1 was used to fabricate the solid
electrolytic capacitor A1. The ammonium borate aqueous solution is
an example of the aqueous solution containing borate ion.
[0067] As a result of a composition analysis of the dielectric
layer of Example 2 by ESCA in the same manner as in Example 1, it
was confirmed that the dielectric layer having the thickness of
about 10 nm was formed on the anode employing titanium and that the
dielectric layer formed on the anode comprised the first anodically
oxidized film of titanium oxide containing fluorine and the second
anodically oxidized film of titanium oxide containing fluorine and
boron, which were successively layered.
EXAMPLE 3
[0068] In Example 3, in the anodic oxidation of the anode 11 of the
plate-shaped titanium substrate, the aqueous solution containing
about 0.1 wt % of ammonium tungstate was used in place of the
aqueous solution containing about 0.1 wt % of ammonium dihydrogen
phosphate used in the second step of Example 1. Except for the
above, the same procedure as that in Example 1 was used to
fabricate the solid electrolytic capacitor A2. The ammonium
tungstate aqueous solution is an example of the aqueous solution
containing tungstate ion.
[0069] As a result of the composition analysis of the dielectric
layer of Example 3 by ESCA in the same manner as in Example 1, it
was confirmed that the dielectric layer having the thickness of 10
nm was formed on the anode employing titanium and that the
dielectric layer formed on the anode comprised the first anodically
oxidized film of titanium oxide containing fluorine and the second
anodically oxidized film of titanium oxide containing fluorine and
tungsten, which were successively layered.
EXAMPLE 4
[0070] In Example 4, in the anodic oxidation of the anode 11 of the
plate-shaped titanium substrate, the aqueous solution containing
about 0.1 wt % of sulfate was used in place of the aqueous solution
containing about 0.1 wt % of ammonium dihydrogen phosphate used in
the second step of Example 1. Except for the above, the same
procedure as that in Example 1 was used to fabricate the solid
electrolytic capacitor A3. The sulfate aqueous solution is an
example of the aqueous solution containing sulfate ion.
[0071] As a result of the composition analysis of the dielectric
layer of Example 4 by ESCA in the same manner as in Example 1, it
was confirmed that the dielectric layer having the thickness of 10
nm was formed on the anode employing titanium and that the
dielectric layer formed on the anode comprised the first anodically
oxidized film of titanium oxide containing fluorine and the second
anodically oxidized film of titanium oxide containing fluorine and
sulfur, which were successively layered.
EXAMPLE 5
[0072] In Example 5, in the anodic oxidation of the anode 11 of the
plate-shaped titanium substrate, the aqueous solution containing
mixture of about 0.1 wt % of ammonium dihydrogen phosphate aqueous
solution and about 0.1 wt % of tungstate aqueous solution in a
ratio of 1:1 was used in place of the aqueous solution containing
about 0.1 wt % of ammonium dihydrogen phosphate used in the second
step of Example 1. Except for the above, the same procedure as that
in Example 1 was used to fabricate the solid electrolytic capacitor
A4.
[0073] As a result of the composition analysis of the dielectric
layer of Example 4 by ESCA in the same manner as in Example 1, it
was confirmed that the dielectric layer having the thickness of 10
nm was formed on the anode employing titanium and that the
dielectric layer formed on the anode comprised the first anodically
oxidized film of titanium oxide containing fluorine and the second
anodically oxidized film of titanium oxide containing fluorine,
phosphorus, and tungsten, which were successively layered.
[0074] In the same manner as in Example 1, the constant voltage of
about 2.5 V was applied between the anode 11 and the cathode layer
4 of the resultant solid electrolytic capacitors A1 to A4 of
Examples 2 to 5 so as to determine the respective leakage current
after about 20 seconds. The index number of leakage current of each
of the solid electrolytic capacitors was determined on the basis of
the leakage current of the solid electrolytic capacitor of Example
1 defined as 100. The results are listed in Table 2 as below.
2 TABLE 2 leakage current solid electrolytic capacitor A 100 solid
electrolytic capacitor A1 150 solid electrolytic capacitor A2 102
solid electrolytic capacitor A3 120 solid electrolytic capacitor A4
110
[0075] As is apparent from the results, the leakage current of the
solid electrolytic capacitors A1 to A4 of Examples 2 to 5 decreased
so notably compared with the solid electrolytic capacitors X and Y
of comparative examples 1 and 2.
[0076] As a result, it was confirmed that the leakage current
decreases so notably when the solid electrolytic capacitor contains
fluorine and at least one element selected from the group
consisting of phosphorus, boron, sulfur, and tungsten.
[0077] In addition, in order to decrease the leakage current, Table
2 also confirms that at least one of phosphorus and tungsten is
preferably contained in the second anodically oxidized film 2 of
the dielectric layer and, more preferably, one of phosphorus and
tungsten is contained.
EXAMPLE 6
[0078] In Example 6, relation between the thickness of the
dielectric layer and the leakage current was examined.
[0079] In Example 6, in the formation of the dielectric layer 2 on
the anode 11 in the fabrication of the solid electrolytic capacitor
A of Example 1, the voltage applied for the anodic oxidation in the
first step and the second step was changed respectively to about 1
V, about 2 V, about 3 V, about 7.5 V, about 10 V, and about 12.5 V.
Except for the above, the same procedure as that in Example 1 was
used to fabricate the solid electrolytic capacitors B1 to B6.
[0080] As a result of the composition analysis of the dielectric
layers of each of the solid electrolytic capacitors B1 to B6 by
ESCA in the same manner as in Example 1, it was confirmed that the
dielectric layer comprising the first anodically oxidized film of
titanium oxide containing fluorine and the second anodically
oxidized film of titanium oxide containing fluorine, which were
layered successively was formed on the anode employing titanium in
each of the solid electrolytic capacitors and that the thickness of
the dielectric layers of the solid electrolytic capacitors B1 to B6
was respectively about 2 nm, about 4 nm, about 6 nm, about 15 nm,
about 20 nm, and about 25 nm.
[0081] Then, about half constant voltages of the above-mentioned
constant voltages applied for the anodic oxidation, especially
about 0.5 V, about 1 V, about 1.5 V, about 3.2 V, about 5 V, and
about 6.3 V, was respectively applied between the anode 11 and the
cathode layer 4 of each of the resultant solid electrolytic
capacitors B1 to B6 so as to determine the respective leakage
current after about 20 seconds. The index number of leakage current
of each of the solid electrolytic capacitors was determined on the
basis of the leakage current of the solid electrolytic capacitor of
Example 1 defined as 100. The results are listed in Table 3 as
below.
3 TABLE 3 thickness of dielectric layer (nm) leakage current solid
electrolytic capacitor B1 2 550 solid electrolytic capacitor B2 4
130 solid electrolytic capacitor B3 6 102 solid electrolytic
capacitor A 10 100 solid electrolytic capacitor B4 15 104 solid
electrolytic capacitor B5 20 125 solid electrolytic capacitor B6 25
500
[0082] As is apparent from the results, the leakage current of the
solid electrolytic capacitors B1 to B6 of Example 6 decreased so
notably compared with the solid electrolytic capacitors X and Y of
comparative examples 1 and 2.
[0083] It was also confirmed that, in order to decrease the leakage
current, the thickness of the dielectric layer is preferably in the
range of 4 nm to 20 nm and, more preferably, in the range of 6 nm
to 15 nm.
[0084] Although the present invention has been fully described by
way of examples, it is to be noted that various changes and
modifications will be apparent to those skilled in the art.
[0085] Therefore, unless otherwise such changes and modifications
depart from the scope of the invention, they should be construed as
being included therein.
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