US20100065434A1

US20100065434A1 - Method of manufacturing a solid electrolytic capacitor with a sufficiently low impedance in a high frequency range

Info

Publication number: US20100065434A1
Application number: US12/557,612
Authority: US
Inventors: Yasuhisa Sugawara; Kazuyuki Katoh; Masako Ohya
Original assignee: NEC Tokin Corp
Current assignee: Tokin Corp
Priority date: 2008-09-12
Filing date: 2009-09-11
Publication date: 2010-03-18
Also published as: CN101673623A; TW201017699A; JP2010067875A; KR20100031459A

Abstract

In a method of manufacturing a solid electrolytic capacitor in which an electrolyte layer containing a conductive polymer is formed by chemical oxidative polymerization on a dielectric oxide film layer formed by anodizing surfaces of a valve metal, an ammonium peroxodisulfate solution adjusted to pH 6 to 8 is used as an oxidant in forming the conductive polymer layer.

Description

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2008-234354, filed on Sep. 12, 2008, the disclosure of which is incorporated herein in its entirety by reference,

TECHNICAL FIELD

This invention relates to a method of manufacturing a solid electrolytic capacitor using a conductive polymer as an electrolyte layer.

BACKGROUND ART

In recent years, following a reduction in size, an increase in speed, and digitization of electronic devices, there has also been an increasing demand in the solid electrolytic capacitor field for small-size and large-capacitance capacitors with low impedances in a high frequency range.
As capacitors for use in a high frequency range of 100 kHz to several tens of MHz, there are a mica capacitor and a multilayer ceramic capacitor. It is difficult to increase the capacitances of these capacitors because the sizes thereof become large.
On the other hand, as large-capacitance capacitors, there are electrolytic capacitors such as an aluminum electrolytic capacitor and a tantalum solid electrolytic capacitor. Generally, an electrolyte layer of the aluminum electrolytic capacitor is in the form of an electrolyte solution, while, an electrolyte layer of the tantalum solid electrolytic capacitor is made of manganese dioxide. Since the conductivities of these electrolyte layers are low, it is difficult to design the capacitors to have sufficiently low impedances in a high frequency range.
JP-B-H4-56445 (Patent Document 1) discloses a solid electrolytic capacitor using a conductive polymer compound with high conductivity as an electrolyte layer. Since the conductivity of the electrolyte layer is high, it is possible to reduce the impedance of this solid electrolytic capacitor in a high frequency range.
JP-A-H9-320900 (Patent Document 2) discloses a solid electrolytic capacitor using polyethylenedioxythiophene with high conductivity as an electrolyte layer. Since the conductivity of the electrolyte layer is high, it is also possible to reduce the impedance of this solid electrolytic capacitor in a high frequency range.

SUMMARY OF THE INVENTION

However, in the solid electrolytic capacitor disclosed in either of Patent Documents 1 and 2, a problem tends to arise in the state of formation of the electrolyte layer and therefore there is a possibility of causing failure in dielectric dissipation factor (tans) or equivalent series resistance (ESR). This makes it difficult to provide a solid electrolytic capacitor with a low impedance in a high frequency range.
It is therefore an exemplary object of this invention to provide a method of manufacturing a solid electrolytic capacitor with a sufficiently low impedance in a high frequency range by forming an excellent electrolyte layer.
Other objects of the present invention will become clear as the description proceeds.
According to an exemplary aspect of the present invention, there is provided a method of manufacturing a solid electrolytic capacitor which comprises a valve metal, a dielectric oxide film layer formed by anodizing a surface of the valve metal, and an electrolyte layer containing a conductive polymer, the electrolyte layer being formed on the dielectric oxide film layer by chemical oxidative polymerization, wherein a solution containing ammonium peroxodisulfate and adjusted to pH 6 to 8 is used as an oxidant for causing the chemical oxidative polymerization.
According to an exemplary aspect of the present invention, there is provided a method of manufacturing a solid electrolytic capacitor, the method comprising preparing a valve metal, forming a dielectric oxide film layer by anodizing a surface of the valve metal, and forming an electrolyte layer containing a conductive polymer on the dielectric oxide film layer by chemical oxidative polymerization using an oxidant, wherein a solution containing ammonium peroxodisulfate and adjusted to pH 6 to 8 is used as the oxidant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exemplary sectional view for explaining a solid electrolytic capacitor manufacturing method according to an exemplary embodiment of this invention; and

FIG. 2 is a partial enlarged view showing only a portion a of FIG. 1 in an enlarged manner.

DESCRIPTION OF THE EMBODIMENTS

First, referring to FIGS. 1 and 2, a solid electrolytic capacitor manufacturing method according to an exemplary embodiment of this invention will be described.
This manufacturing method is basically nearly the same as a conventional solid electrolytic capacitor manufacturing method except an oxidant for use in forming a conductive polymer layer of a solid electrolyte. That is, materials of other than the oxidant, shapes, and so on can be known ones and are not particularly limited.
A solid electrolytic capacitor shown in FIGS. 1 and 2 can be manufactured by a manufacturing method which will be described hereinbelow.
First, a valve metal 1 is prepared and a dielectric oxide film layer 2 is formed by anodizing surfaces of the valve metal 1. Then, on the dielectric oxide film layer 2, an electrolyte layer containing a conductive polymer (hereinafter referred to as a “conductive polymer layer”) 3 is formed by chemical oxidative polymerization of a monomer such as pyrrole using as an oxidant an ammonium peroxodisulfate solution adjusted to pH 6 to 8. Thereafter, a cathode layer 4 made of a conductive paste is formed on the conductive polymer layer 3 Further, predetermined packaging is carried out, thereby obtaining a solid electrolytic capacitor.
Using as the oxidant the ammonium peroxodisulfate solution adjusted to pH 6 to 8 as described above, the reaction rate is suppressed as compared with a case of using, as an oxidant, iron ions or an ammonium peroxodisulfate solution with a pH of less than 6, Therefore, the conductive polymer layer 3 is densely formed on the dielectric oxide film layer 2. Thus, there is obtained a low-impedance solid electrolytic capacitor excellent in tanδ and ESR characteristics.
A dopant contained in the oxidant is preferably one or more selected from p-toluenesulfonic acid, phenolsulfonic acid, naphthalenesulfonic acid, and aromatic derivatives each having sulfonic groups. Particularly, the dopant is preferably a derivative of naphthalene having sulfonic groups.
A conductive polymer excellent in conductivity is obtained by selectively using p-toluenesulfonic acid, phenolsulfonic acid, and naphthalenesulfonic acid as the dopant. Using this conductive polymer as the conductive polymer layer 3, it is possible to obtain a low-impedance solid electrolytic capacitor excellent in tanδ and ESR characteristics.
Using as the dopant an aromatic derivative having sulfonic groups, particularly a derivative of naphthalene having sulfonic groups, a conductive polymer more excellent in conductivity is obtained. Using this conductive polymer as the conductive polymer layer 3, it is possible to obtain a low-impedance solid electrolytic capacitor excellent in tans and ESR characteristics.
The concentration of ammonium peroxodisulfate in the ammonium peroxodisulfate solution used as the oxidant is preferably 1 to 50 mass %. If the concentration of ammonium peroxodisulfate is greater than 50 mass %, the ammonium peroxodisulfate becomes saturated in the aqueous solution at room temperature and thus crystals thereof remain in the solution. Therefore, there is a problem that when filling the oxidant into fine pores of the valve metal, the fine pores are plugged so that the coverage of a conductive polymer layer decreases. On the other hand, if the concentration of ammonium peroxodisulfate is smaller than 1 mass %, since the amount of a conductive polymer formed at a time is small, the number of times of polymerization for forming the conductive polymer layer 3 becomes very large.
It is preferable that the oxidant ammonium peroxodisulfate solution be adjusted to pH 6 to 8. If the pH is smaller than 6 the polymerization rate becomes high and therefore there is a problem that it is difficult to densely form a conductive polymer on the dielectric oxide film layer 2 and thus a capacitor with excellent tanδ and ESR is not obtained. On the other hand, if the pH is greater than 8, the amount of cations used for pH adjustment increases in the oxidant so that the cations are entrapped as a dopant of a conductive polymer, and therefore, the conductivity decreases as compared with a conductive polymer using, as a dopant, one or more selected from p-toluenesulfonic acid, phenolsulfonic acid, naphthalenesulfonic acid, and aromatic derivatives each having sulfonic groups. Thus, there is a problem that a capacitor with excellent tanδ and ESR is not obtained.
Next, a description will be given of manufacturing methods of Examples 1 to 6 of this invention and manufacturing methods of Comparative Examples 1 and 2.

Example 1

A solid electrolytic capacitor manufactured by the manufacturing method of Example 1 has the same structure as that of the solid electrolytic capacitor described in the embodiment and, therefore, a description will be given with reference to FIGS. 1 and 2 again.
A solid electrolytic capacitor shown in FIGS. 1 and 2 comprises a valve metal 1 as an anode-side electrode, a dielectric oxide film layer 2 formed by anodizing surfaces of the valve metal 1, a conductive polymer layer 3 as a solid electrolyte, a cathode layer 4 made of a conductive paste, external electrodes 61 and 62, and a packaging resin 8.
When manufacturing this solid electrolytic capacitor, the valve metal 1 in the form of a sintered body of tantalum powder having a size of 3.5 mm (length)×3.0 mm (width)×1.5 mm (thickness) was first produced. A voltage of 30V was applied to the valve metal 1 in a phosphoric acid aqueous solution to anodize it, thereby obtaining a pellet covered with the dielectric oxide film layer 2 over its entire tantalum powder surfaces.
Then, there was prepared an aqueous solution, as an oxidant, containing 20 mass % ammonium peroxodisulfate and 20 mass % 1,3,6-naphthalenetrisulfonic acid and adjusted to pH 7 using imidazole. In this aqueous solution, the pellet covered with the dielectric oxide film layer 2 was immersed for 10 minutes to fill the oxidant therein. Then, after drying the pellet at room temperature for 30 minutes, the pellet was immersed in pyrrole for 10 minutes and then maintained at room temperature for 30 minutes, thereby polymerizing the pyrrole. The sequence of operations (polymerization operations) of filling the oxidant and filling the pyrrole was repeated five times to form the conductive polymer layer 3 in the form of a conductive polypyrrole layer.
Subsequently, the pellet formed with the conductive polymer layer 3 was washed with ethanol and, after drying, a silver paste was coated on surfaces of the conductive polypyrrole layer and then heated to be cured, thereby forming the cathode layer 4 with a thickness of 10 to 50 μm to obtain a capacitor element.
Thereafter, using an adhesive layer 5 made of a silver paste, the cathode layer 4 of the capacitor element was connected to the external electrode 61. On the anode side of the capacitor element, a valve metal wire 7 drawn out of the tantalum sintered body in advance was welded to the external electrode 62. Further, the capacitor element was covered from the outside with an epoxy resin to form the packaging resin 8, thereby completing the solid electrolytic capacitor having the structure shown in FIG. 1.
In Example 1, polypyrrole was used for forming the conductive polymer layer 3. However, an equivalent operation can be obtained using polythiophene, polyaniline, a derivative of such a conductive polymer, or the like

Example 2

In the manufacturing method of Example 2, the pH of an oxidant aqueous solution containing a dopant was adjusted to 6. The others were the same as in Example 1.
First, a valve metal 1 being the same as in Example 1 was prepared and a dielectric oxide film layer 2 was formed thereon by the same method as in Example 1, thereby obtaining a pellet.
Then, there was prepared an aqueous solution, as an oxidant, containing 20 mass % ammonium peroxodisulfate and 20 mass % 1,3,6-naphthalenetrisulfonic acid and adjusted to pH 6 using imidazole. In this aqueous solution, the pellet covered with the dielectric oxide film layer 2 was immersed for 10 minutes to fill the oxidant therein. Then, after drying the pellet at room temperature for 30 minutes, the pellet was immersed in pyrrole for 10 minutes and then maintained at room temperature for 30 minutes, thereby polymerizing the pyrrole. The sequence of polymerization operations of filling the oxidant and filling the pyrrole was repeated five times to form a conductive polymer layer 3 in the form of a conductive polypyrrole layer.
Also in Example 2, polypyrrole was used for forming the conductive polymer layer 3. However, an equivalent operation can be obtained using polythiophene, polyaniline, a derivative of such a conductive polymer, or the like.

Example 3

In the manufacturing method of Example 3, the pH of an oxidant aqueous solution containing a dopant was adjusted to 8. The others were the same as in Example 1.
First, a valve metal 1 being the same as in Example 1 was prepared and a dielectric oxide film layer 2 was formed thereon by the same method as in Example 1, thereby obtaining a pellet.
Then, there was prepared an aqueous solution, as an oxidant, containing 20 mass % ammonium peroxodisulfate and 20 mass % 1,3,6-naphthalenetrisulfonic acid and adjusted to pH 8 using imidazole. In this aqueous solution, the pellet covered with the dielectric oxide film layer 2 was immersed for 10 minutes to fill the oxidant therein. Then, after drying the pellet at room temperature for 30 minutes, the pellet was immersed in pyrrole for 10 minutes and then maintained at room temperature for 30 minutes, thereby polymerizing the pyrrole. The sequence of polymerization operations of filling the oxidant and filling the pyrrole was repeated five times to form a conductive polymer layer 3 in the form of a conductive polypyrrole layer.
Also in Example 3, polypyrrole was used for forming the conductive polymer layer 3. However, an equivalent operation can be obtained using polythiophene, polyaniline, a derivative of such a conductive polymer, or the like.

Example 4

In the manufacturing method of Example 4, use was made of an oxidant containing p-toluenesulfonic acid as a dopant The others were the same as in Example 1.
First, a valve metal 1 being the same as in Example 1 was prepared and a dielectric oxide film layer 2 was formed thereon by the same method as in Example 1, thereby obtaining a pellet.
Then, there was prepared an aqueous solution, as an oxidant, containing 20 mass % ammonium peroxodisulfate and 20 mass % p-toluenesulfonic acid and adjusted to pH 7 using imidazole. In this aqueous solution, the pellet covered with the dielectric oxide film layer 2 was immersed for 10 minutes to fill the oxidant therein. Then, after drying the pellet at room temperature for 30 minutes, the pellet was immersed in pyrrole for 10 minutes and then maintained at room temperature for 30 minutes, thereby polymerizing the pyrrole. The sequence of polymerization operations of filling the oxidant and filling the pyrrole was repeated five times to form a conductive polymer layer 3 in the form of a conductive polypyrrole layer.
Also in Example 4, polypyrrole was used for forming the conductive polymer layer 3. However, an equivalent operation can be obtained using polythiophene, polyaniline, a derivative of such a conductive polymer, or the like.

Example 5

In the manufacturing method of Example 5, the concentration of ammonium peroxodisulfate in an oxidant aqueous solution containing a dopant was set to 1 mass %. The others were the same as in Example 1.
First, a valve metal 1 being the same as in Example 1 was prepared and a dielectric oxide film layer 2 was formed thereon by the same method as in Example 1, thereby obtaining a pellet.
Then, there was prepared an aqueous solution, as an oxidant containing 1 mass % ammonium peroxodisulfate and 20 mass % 1,3,6-naphthalenetrisulfonic acid and adjusted to pH 7 using imidazole, In this aqueous solution, the pellet covered with the dielectric oxide film layer 2 was immersed for 10 minutes to fill the oxidant therein. Then, after drying the pellet at room temperature for 30 minutes, the pellet was immersed in pyrrole for 10 minutes and then maintained at room temperature for 30 minutes, thereby polymerizing the pyrrole. The sequence of polymerization operations of filling the oxidant and filling the pyrrole was repeated five times to form a conductive polymer layer 3 in the form of a conductive polypyrrole layer.
Also in Example 5, polypyrrole was used for forming the conductive polymer layer 3. However, an equivalent operation can be obtained using polythiophene, polyaniline, a derivative of such a conductive polymer, or the like.

Example 6

In the manufacturing method of Example 6, the concentration of ammonium peroxodisulfate in an oxidant aqueous solution containing a dopant was set to 50 mass %. The others were the same as in Example 1.
First, a valve metal 1 being the same as in Example 1 was prepared and a dielectric oxide film layer 2 was formed thereon by the same method as in Example 1, thereby obtaining a pellet.
Then, there was prepared an aqueous solution, as an oxidant, containing 50 mass % ammonium peroxodisulfate and 20 mass % 1,3,6-naphthalenetrisulfonic acid and adjusted to pH 7 using imidazole, In this aqueous solution, the pellet covered with the dielectric oxide film layer 2 was immersed for 10 minutes to fill the oxidant therein. Then, after drying the pellet at room temperature for 30 minutes, the pellet was immersed in pyrrole for 10 minutes and then maintained at room temperature for 30 minutes, thereby polymerizing the pyrrole. The sequence of polymerization operations of filling the oxidant and filling the pyrrole was repeated five times to form a conductive polymer layer 3 in the form of a conductive polypyrrole layer.
Also in Example 6, polypyrrole was used for forming the conductive polymer layer 3. However, an equivalent operation can be obtained using polythiophene, polyaniline, a derivative of such a conductive polyme, or the like.

Comparative Example 1

In the manufacturing method of Comparative Example 1, a 20 mass % ferric methanol p-toluenesulfonate solution was used as an oxidant. The others were the same as in Example 1.
First, a valve metal 1 being the same as in Example 1 was prepared and a dielectric oxide film layer 2 was formed thereon by the same method as in Example 1 thereby obtaining a pellet. This pellet was immersed in a ferric methanol p-toluenesulfonate solution being an oxidant for 10 minutes to fill the oxidant therein. Then, after drying the pellet at room temperature for 30 minutes, the pellet was immersed in pyrrole for 10 minutes and then maintained at room temperature for 30 minutes, thereby polymerizing the pyrrole. The sequence of polymerization operations of filling the oxidant and filling the pyrrole was repeated five times to form a conductive polymer layer 3 in the form of a conductive polypyrrole layer.

Comparative Example 2

In the manufacturing method of Comparative Example 2, the pH of an oxidant aqueous solution containing a dopant was adjusted to 1. The others were the same as in Example 1.
First, a valve metal 1 being the same as in Example 1 was prepared and a dielectric oxide film layer 2 was formed thereon by the same method as in Example 1, thereby obtaining a pellet. Then, there was prepared an aqueous solution, as an oxidant, containing 20 mass % ammonium peroxodisulfate and 20 mass % 1,3,6-naphthalenetrisulfonic acid and having a pH of 1 In this aqueous solution, the pellet covered with the dielectric oxide film layer 2 was immersed for 10 minutes to fill the oxidant therein. Then, after drying the pellet at room temperature for 30 minutes, the pellet was immersed in pyrrole for 10 minutes and then maintained at room temperature for 30 minutes, thereby polymerizing the pyrrole. The sequence of polymerization operations of filling the oxidant and filling the pyrrole was repeated five times to form a conductive polymer layer 3 in the form of a conductive polypyrrole layer.
Next, a comparison between Examples 1 to 6 and Comparative Examples 1 and 2 will be described.
Table 1 shows capacitance, tanδ, and ESR characteristics of the solid electrolytic capacitors obtained by the manufacturing methods of Examples 1 to 6 and the manufacturing methods of Comparative Examples 1 and 2. The number of samples was 20 per Example or Comparative Example and a value shown for each of the characteristics is an average of 20 samples.

TABLE 1

Capacitance (μF)	tanδ (%)	ESR (Ω)
(120 Hz)	(120 Hz)	(100 kHz)

Example 1	68.4	2.0	0.031
Example 2	68.5	2.0	0.030
Example 3	68.4	2.1	0.033
Example 4	68.3	2.2	0.037
Example 5	68.0	2.4	0.055
Example 6	67.8	2.4	0.054
Comparative	67.8	2.5	0.058
Example 1
Comparative	68.0	2.4	0.056
Example 2

Referring to Table 1, the results will be evaluated about the solid electrolytic capacitors obtained by the manufacturing methods of Examples 1 to 6 and the manufacturing methods of Comparative Examples 1 and 2. With respect to tanδ and ESR, the characteristics are excellent in the solid electrolytic capacitors of Examples 1 to 6 as compared with Comparative Examples 1 and 2. As compared with Comparative Example 1 using the ferric methanol p-toluenesulfonate solution as the oxidant and Comparative Example 2 using the ammonium peroxodisulfate aqueous solution of pH 1, the values of the characteristics are suppressed to be low in Examples 1 to 6 where the pH of the oxidant was adjusted between 6 and 8.
In a solid electrolytic capacitor using a conductive polymer as an electrolyte, if the state of formation of the conductive polymer formed on a dielectric oxide film layer is bad, the coverage of particularly the inner conductive polymer is lowered and thus the capacitor with excellent tanδ and ESR cannot be obtained. In order to solve this, by improving a polymerization method (oxidant and dopant) for forming the conductive polymer on the dielectric oxide film layer so as to reduce the reaction rate, the conductive polymer is densely formed on the dielectric oxide film layer so that the solid electrolytic capacitor with a low impedance in a high frequency range is obtained.
In Examples 1 to 6, since the ammonium peroxodisulfate aqueous solution of pH 6 to 8 was used as the oxidant, as compared with the oxidant ferric methanol p-toluenesulfonate solution of Comparative Example 1 and the ammonium peroxodisulfate aqueous solution of pH 1 of Comparative Example 2, polymerization of pyrrole slowly proceeds to achieve an optimal polymerization reaction time and thus the conductive polymer layer is densely formed over the entire inner surfaces of the pellet. Because of this effect, the solid electrolytic capacitor with suppressed tanδ and ESR is obtained in each of Examples 1 to 6.
As described above, according to Examples 1 to 6, since the film of the conductive polymer is densely formed on the dielectric oxide film layer by chemical polymerization, the low-impedance solid electrolytic capacitor excellent in tanδ and ESR characteristics is obtained. Particularly, by setting the pH of the oxidant ammonium peroxodisulfate solution to 6 to 8, the polymerization rate of the conductive polymer becomes moderate so that the film of the conductive polymer is more densely formed on the dielectric oxide film layer, and thus the low-impedance solid electrolytic capacitor excellent in tanδ and ESR characteristics is obtained.
While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the claims.

Claims

1. A method of manufacturing a solid electrolytic capacitor which comprises:

a valve metal;

a dielectric oxide film layer formed by anodizing a surface of the valve metal; and

an electrolyte layer containing a conductive polymer and being formed on the dielectric oxide film layer by chemical oxidative polymerization,

wherein a solution containing ammonium peroxodisulfate and adjusted to pH 6 to 8 is used as an oxidant for causing the chemical oxidative polymerization.

2. The method according to claim 1, wherein the solution contains, as a dopant, at least one selected from p-toluenesulfonic acid, phenolsulfonic acid, naphthalenesulfonic acid, and aromatic derivatives each having sulfonic groups.

3. The method according to claim 2, wherein a concentration of the ammonium peroxodisulfate in the solution is 1 to 50 mass %.

4. The method according to claim 1, wherein a concentration of the ammonium peroxodisulfate in the solution is 1 to 50 mass %.

5. A method of manufacturing a solid electrolytic capacitor, the method comprising:

preparing a valve metal;

forming a dielectric oxide film layer by anodizing a surface of the valve metal; and

forming an electrolyte layer containing a conductive polymer on the dielectric oxide film layer by chemical oxidative polymerization using an oxidant,

wherein a solution containing ammonium peroxodisulfate and adjusted to pH 6 to 8 is used as the oxidant.

6. The method according to claim 5, wherein the solution contains, as a dopant, at least one selected from p-toluenesulfonic acid, phenolsulfonic acid, naphthalenesulfonic acid, and aromatic derivatives each having sulfonic groups.

7. The method according to claim 5, wherein a concentration of the ammonium peroxodisulfate in the solution is 1 to 50 mass %.

8. The method according to claim 5, further comprising:

forming a cathode layer on the electrolyte layer; and

connecting external electrodes to the valve metal and the cathode layer, respectively.

9. The method according to claim 8, further comprising covering, with a packaging resin, all portions except a part of each of the external electrodes.

10. The method according to claim 9, further comprising extending a part of each of the external electrodes along a surface of the packaging resin.