Patent Application
20050268747
1. Field of the Invention
The present invention relates to a solid electrolytic capacitor which uses an anode made by integrating metal powder and a lead section through sintering with a structure in which valve metal powder is accumulated in a layer on a valve metal base material, and a valve metal powder used for this.
2. Description of the Related Art
In the case of molding valve metal powder in a layer on a valve metal base material, for example, as described in Japanese Unexamined Patent Publication No. 2003-209028, by manufacturing a dispersion liquid (paste) using valve metal powder, and molding it through an industrial method such as metallic mask printing, and sintering it in vacuum, the valve metal base material and the layered powder molding part are integrated, and it is possible to use it as an anode for a solid electrolytic capacitor.
However, conventionally, with a type of capacitor using valve metal powder as its anode, such as a tantalum capacitor, the valve metal powder is provisionally molded through an industrial method called dry press, and an anode is then made by sintering. At this time, agglomerate powder of the valve metal powder is used. However, the particle size distribution of the agglomerate powder has a central particle size of around 100 μm and the distribution range is from around 10 μm to 200 μm.
If a powder layer is formed by dispersing such an agglomerate powder used in the dry press method, over the aforementioned paste, sufficient welding between the valve metal base material and the agglomerate powder does not occur at the time of sintering, and it becomes prone to flaking. Furthermore, voids (hole-like cavities) are likely to occur in the layer of the powder molding part, and the occurrence of such voids not only decreases the product yield, but also has been the cause of degradation in the properties of solid electrolytic capacitors.
The occurrence of the voids will be described with reference to the drawings.
When the valve metal powder is agglomerated, a method is used where the powder (primary particles) is weakly bonded through heating in order to obtain an agglomerate powder (secondary particles). However, in the case of large agglomerate powders such as the agglomerate powder for the dry press method where the central particle size is adjusted to around 100 μm, sintering of the powder is already degraded by heating in the agglomeration process. Therefore, there is a problem that, as described before, sufficient welding does not occur between the valve metal base material and the agglomerate powder at the time of sintering.
Moreover, since the voids originate from the gaps between the agglomerate powder particles, then in the case of using large agglomerate powder, the gaps become large and voids are likely to occur.
Cracking and warping due to sintering shrinkage will be described with reference to the drawings.
In the case of using non-agglomerate powder with a central particle size of around 0.3 μm and a distribution range of around 0.2 μm to 1 μm, it does not undergo a heating agglomeration process. Therefore, the degree of sintering is very high, and the welding with the valve metal base material can be sufficient. However, deformation due to shrinkage during the sintering becomes large, and cracking of the powder layer occurs, or warping of valve metal base material on which the powder layer is welded occurs. Therefore, the properties and yield of the capacitor are again degraded.
As described before, regarding the manufacture of an anode for a solid electrolytic capacitor with a structure in which valve metal powder is accumulated in a layer on a valve metal base material, the particle size distribution of the valve metal powder has a large effect on the properties and yield of the capacitor. Therefore, an object of the present invention is to provide a valve metal powder with optimized particle size distribution, and a solid electrolytic capacitor which uses this.
The valve metal powder for the solid electrolytic capacitor according to the present invention is an agglomerate powder to be used for manufacturing an anode with a structure in which valve metal powder is molded in a layer on a valve metal base material, and contains at least 90% of all the powder within a particle size range of 1 μm to 50 μm.
Also, it is desirable that at least 90% of all the powder is contained within a particle size range of 1 μm to 30 μm.
Moreover, preferably a product of BET specific surface area and specific gravity (d25) is greater than 17 m2/g.
Furthermore, it is desirable that the aforementioned valve metal is tantalum, tantalum alloy, niobium, or niobium alloy.
The solid electrolytic capacitor according to the present invention is made using a valve metal powder for a solid electrolytic capacitor according to the present invention.
Regarding the manufacture of the anode for a solid electrolytic capacitor with a structure in which a valve metal powder is accumulated in a layer on a valve metal base material, prior to the formation of the powder layer using a valve metal powder dispersion liquid by a process such as metallic mask printing, granulation and calcination are performed for the powder used in the valve metal powder dispersion liquid, using an appropriate method and conditions so as to produce agglomerate powder in which particle size distribution is restricted within a constant range. As a result, control of cracking, warping, voids, and flaking of the powder layer that occur during the sintering and molding of the anode becomes possible, so that it is possible to improve the properties and yield of the solid electrolytic capacitor.
The valve metal powder for a solid electrolytic capacitor according to the present invention is an agglomerate powder to be used for manufacturing an anode with a structure in which valve metal powder is molded in a layer on a valve metal base material, and contains at least 90% of all the powder within a particle size range of 1 μm to 50 μm.
Furthermore, it is desirable that aforementioned valve metal is tantalum, tantalum alloy, niobium, or niobium alloy. The valve metal to be used is selected in accordance with the required capacity characteristics, costs and so forth.
As a method for obtaining valve metal agglomerate powder, firstly there is a method of manufacturing agglomerate powder from primary powder of a valve metal which has been subjected to reduction beforehand; involving for example, reducing potassium tantalate fluoride using metallic sodium to make metallic tantalum primary particles, and after this, calcining the obtained metallic tantalum primary particles in a vacuum or inert gas atmosphere to obtain a metallic tantalum mass, and then crushing this to obtain agglomerate powder. Secondly, there is a method of manufacturing agglomerate powder of a valve metal by initially converting into agglomerate powder in an oxide form and reducing it; involving for example, granulating tantalum pentoxide primary particles, and baking in the atmosphere to convert into tantalum pentoxide agglomerate powder, and then reducing using metallic magnesium to obtain valve metal agglomerate powder.
Whether to granulate the valve metal primary particles as described in the first method, or to granulate the primary particles of the valve metallic oxide as described in the second method depends on the particle size of the base powder as well as its particle size distribution, and a proper method is to be selected and used.
As a granulation method, there is a dry granulation method and a wet granulation method.
Regarding the dry granulation method, there is: mesh granulation in which crushing is performed after agglomerating the powder and it is then sieved; agitated mixing granulation in which granulation is performed by agitatedly mixing the powder with an agitation blade while adding a binder; and rolling and fluidizing dry granulation in which granulation is performed by agitatedly mixing the powder using compressed air while adding a binder. Regarding the wet granulation method, there are methods such as: fluidized bed granulation in which granulation is performed by repeatedly spraying, drying and coating a suspension consisting of fine powder of niobium pentoxide and pure water; and spray dry granulation in which granulation is performed by similarly spraying and drying the suspension.
The dispersion liquid is made by including at least a solvent and a solvent-soluble binder into the valve metal agglomerate powder, which is obtained in the abovementioned manner where at least 90% of all the powder is contained within the particle size range of 1 μm to 50 μm, and the product of BET specific surface area and specific gravity (d25) is preferably greater than 17 m2/g.
Having over 90% of the powder within the particle size range of 1 μm to 50 μm is for preventing deformation, cracking, and warping during the sintering for obtaining the anode for a solid electrolytic capacitor by integrating the valve metal base material and a layer of the powder molding part as mentioned later.
Using the obtained dispersion liquid, a powder layer is formed on a valve metal base material through an industrial method such as metallic mask printing. Furthermore, after removing the solvent by drying, the product is sintered in a vacuum, the binder is removed through decomposition, and the valve metal base material and a layer of the powder molding part are integrated, and an anode for a solid electrolytic capacitor is thus obtained.
With respect to the anode for a solid electrolytic capacitor obtained in the above manner, using a method called anode oxidation in which a voltage is applied within an electrolytic solution, a chemical conversion coating serving as a dielectric is formed on the surface of the anode. Next a semi-conductor layer known as a solid electrolytic layer made of manganese dioxide, polypyrrole and so forth is formed on the chemical conversion coating. Then a conductive layer called a cathode layer made of graphite, silver and so forth is formed to obtain an element part of the solid electrolytic capacitor. On this element part, an external electrode terminal for mounting, and a resin jacket for element protection is attached, and high temperature superimposed voltage aging combined with initial defect removal is performed, to obtain a solid electrolytic capacitor according to the present invention.
Moreover, for the valve metal agglomerate powder to be used, it is desirable that the product of the BET specific surface area and the specific gravity (d25), is greater than 17 m2/g.
If the product of the BET specific surface area and the specific gravity (d25) is greater than 17 m2/g, the capacitance of the solid electrolytic capacitor to be obtained will be sufficiently manifested during the use. Therefore this is desirable.
The anode for the solid electrolytic capacitor according to the present invention is made using the valve metal powder for a solid electrolytic capacitor of the present invention.
The solid electrolytic capacitor of the present invention is made using the valve metal powder for a solid electrolytic capacitor of the present invention.
After charging 300 g of niobium pentoxide powder of specific surface area 3.2 m2/g, particle size distribution 0.3 μm to 5.0 μm, and D50=0.9 μm, and 400 g of pure water into a 2 liter ball mill made of zirconia, and then mixing and dispersing for 15 hours, a suspension of mixed and dispersed niobium pentoxide fine powder was obtained.
To the obtained suspension, PVA was added to make up 0.5 mass % in solid. Then spraying, drying and coating of the suspension was repeated inside a fluidized bed granulation apparatus, and after drying the obtained powder at 80° C., it was baked at a temperature of 1250° C. to make an agglomerate powder of niobium oxide powder. After reducing the obtained agglomerate powder using Mg vapor at a temperature of 1000° C., it was acid cleaned with hydrochloric acid, and further mixed with and reduced by magnesium (Mg), acid cleaned and washed in water, after which it was subjected to vacuum drying to obtain 100 g of agglomerate powder of niobium metal.
Characteristics such as the particle size distribution, of the obtained agglomerate powder of the niobium metal are shown in Table 1 and Table 2.
100 g of the agglomerate powder of niobium metal obtained in the above manner and 55 g of a mixed solvent of polyvinyl alcohol (Kuraray Co. Ltd., PVA205-C) 5% solution as a binder and methyl alcohol was blended, and then kneaded for two hours using a shaker, and a dispersion liquid thus obtained.
Next, using a print mask made by providing a 3.0×4.0 mm rectangular opening in a 200 μm thick plastic sheet, a niobium dispersion liquid layer was formed on a 3.1 mm×4.0 mm×50 μm thick niobium foil serving as a base material. Then after removing the solvent and moisture by drying at 105° C. for 10 minutes, baking at 100° C. was performed for 20 minutes inside a vacuum of 1×10−5 Torr to generate sintering among the niobium powders as well as sintering between the niobium foil and the niobium powder. As a result, an anode for a solid electrolytic capacitor where the mean thickness of the powder layer was 150 μm (of this, 50 μm was the thickness of the base material) was obtained.
An external appearance photograph of the surface of the obtained anode for a solid electrolytic capacitor is shown in
With respect to the aforementioned anode for a solid electrolytic capacitor, anodization was performed by using 0.1% phosphoric acid solution and applying a voltage of 20V, to thereby form on the surface a chemical conversion coating which serves as a dielectric. The anode in this condition was soaked in a 10 mass % iron dodecylbenzensulfonate methanol solution for 5 minutes and after taking it out, the solvent was air dried, and it was then soaked for 5 minutes in a pyrrole monomer liquid, and after taking it out it was left for one hour after which it was washed with methanol, to thereby form on the chemical conversion coating a conductive polypyrrole layer serving as a solid electrolyte. On this, a graphite layer and a silver layer was formed in this order by a paste immersion coating method. Then after attaching an external electrode terminal, a resin jacket was applied, and after this, aging was performed by applying a voltage of 6V for ten hours in an atmosphere of 85° C., to obtain a niobium solid electrolytic capacitor.
Characteristics and yield of the obtained niobium solid electrolytic capacitor are shown in Table 3.
100 g of an agglomerate powder of the niobium metal was obtained in the same way as for the first example, except that the PVA to be added to the suspension was 3.0 mass %.
The particle size distribution of the obtained agglomerate powder of niobium metal is shown in Table 1 and Table 2. Compared to the agglomerate powder of niobium metal obtained in the first example, both the central particle size (D50) and the distribution range were larger.
Furthermore, an anode for a solid electrolytic capacitor was obtained in the same way as for the first example.
An external appearance photograph of the surface of the obtained anode for a solid electrolytic capacitor is shown in
Using the obtained anode for a solid electrolytic capacitor, a niobium solid electrolytic capacitor was obtained in the same way as for the first example.
The characteristics and the yield of the obtained niobium solid electrolytic capacitor are shown in Table 3.
Compared to the first example, the ESR (equivalent series resistance) and leakage current properties were inferior, and a large difference in the yield was found. This is due to the fact that, because of the voids and flaking in the anode for the solid electrolytic capacitor, then at first the leakage current is increased, and this in turn deteriorates the solid electrolyte during aging of the product.
*1)Specific gravity (d25) is ratio of mass to water at 25° C.
*2)Product is product of specific surface area and specific gravity (d25)
*1)Capacitance measurement conditions: 120 Hz, 1 Vrms, Bias voltage 1.5 V
*2)Leakage current measurement conditions: Rated voltage 4 V applied, 1 min
*3)ESR measurement conditions: 100 Hz, 1 Vrms, Bias voltage 1.5 V
A reduction was performed on potassium tantalate fluoride 400 g using metallic sodium at 850° C. Then after acid cleaning, and water washing, this was passed through a vacuum heat treatment in a vacuum of 1×10−5 Torr at a temperature 1200° C. for 0.5 hour, to thereby obtain a sintered mass. Crushing was performed on the sintered mass using a hammer-type granulator at a rotational speed of 8000 rpm. Then, this was subjected to mixing with and reducing by magnesium (Mg), acid cleaning, water washing, and vacuum drying, to thereby obtain 10 g of agglomerate powder of tantalum metal.
Characteristics such as the particle size distribution of the obtained tantalum metal agglomerate powder are shown in Table 4 and Table 5.
Because of the change from niobium to tantalum, a temperature of 1300° C. was used during the sintering of the anode for a solid electrolytic capacitor. Apart from this, an anode for a solid electrolytic capacitor was obtained in the same way as for the first working example.
The appearance of the obtained anode for a solid electrolytic capacitor was almost the same as for the first example.
Using the obtained anode for a solid electrolytic capacitor, a tantalum solid electrolytic capacitor was obtained in the same way as for the first example.
Characteristics as well as yield of the obtained tantalum solid electrolytic capacitor are shown in Table 6.
Apart from changing the rotational speed of the hammer-type granulator to 3000 rpm, 100 g of tantalum metal agglomerate powder was obtained in the same way as for the second example.
Characteristics such as particle size distribution of the obtained tantalum metal agglomerate powder are shown in Table 4 and Table 5. Compared to the tantalum metal agglomerate powder obtained in the second example, both the central particle size (D50) and the range were greater.
Moreover, an anode for a solid electrolytic capacitor was obtained in the same way as for the second example. Its appearance and the degree of flaking were almost the same as for the first comparative example.
Using the obtained anode for a solid electrolytic capacitor, a tantalum solid electrolytic capacitor was obtained in the same way as for first example.
Characteristics and yield of the obtained tantalum solid electrolytic capacitor are shown in Table 6.
When the capacitor properties and yields of the second example and the second comparative example are compared, then in the same way as for first example and the first comparative example, capacitor properties and yields were inferior to those of the comparative examples using powder of larger particle size.
*1)Specific gravity (d25) is ratio of mass to water at 25° C.
*2)Product is product of specific surface area and specific gravity (d25)
*1)Capacitance measurement conditions: 120 Hz, 1 Vrms, Bias voltage 1.5 V
*2)Leakage current measurement conditions: Rated voltage 4 V applied, 1 min
*3)ESR measurement conditions: 100 Hz, 1 Vrms, Bias voltage 1.5 V
After charging 300 g of niobium pentoxide powder of specific surface area 5.1 m2/g, particle size distribution 0.2 μm to 3.0 μm and D50=0.6 μm, and 400 g of pure water into a 2 liter ball mill made of zirconia, and then mixing and dispersing for 15 hours, a suspension was obtained.
Niobium oxide agglomerate powder was then made by drying the obtained suspension by spraying it into a spray drier at a disk rotational speed of 14000 rpm and a temperature 200° C., then baking the obtained powder at a temperature of 1200° C. to make an agglomerate powder of niobium oxide powder. After reducing the obtained agglomerate powder using Mg vapor at a temperature of 1000° C., it was acid cleaned with hydrochloric acid, and further mixed with and reduced by magnesium (Mg), acid cleaned and washed in water, after which it was subjected to vacuum drying to obtain 100 g of agglomerate powder of niobium metal.
Characteristics such as the particle size distribution of the obtained niobium agglomerate powder are shown in Table 7 and Table 8.
Further, in the same way as for the first example, an anode for a solid electrolytic capacitor was obtained. The appearance of the surface of the obtained anode for a solid electrolytic capacitor was almost the same as for the first example.
Using the obtained anode for a solid electrolytic capacitor, a niobium solid electrolytic capacitor was obtained in the same way as for the first example.
Characteristics and yield of the obtained niobium solid electrolytic capacitor are shown in Table 9. The properties and yield of the obtained niobium solid electrolytic capacitor were almost the same as for the first example.
After performing Mg vapor reduction at a temperature of 1000° C. on 300 g of non-agglomerated niobium pentoxide powder used in the third example, the product was acid cleaned with hydrochloric acid. Furthermore, mixing with and reducing by magnesium (Mg), acid cleaning, water washing and vacuum drying were performed, and 100 g of niobium metal agglomerate powder was obtained.
The particle size distributions of the obtained agglomerate powder of niobium metal are shown in Table 7 and Table 8. The degree of agglomeration was markedly weak, and the central particle size (D50) as well as the range showed a particle size distribution close to that of a primary particle.
Furthermore, an anode for a solid electrolytic capacitor was obtained in the same way as for the first example.
An external appearance photograph of the surface of the obtained anode for a solid electrolytic capacitor is shown in
Using the obtained anode for a solid electrolytic capacitor, a niobium solid electrolytic capacitor was obtained in the same way as for the first example.
Characteristics and yield of the obtained niobium solid electrolytic capacitor are shown in Table 9.
Referring to Table 9, the characteristics and yield of the third comparative example were both inferior compared to the third example.
*1)Specific gravity (d25) is ratio of mass to water at 25° C.
*2)Product is product of specific surface area and specific gravity (d25)
*1)Capacitance measurement conditions: 120 Hz, 1 Vrms, Bias voltage 1.5 V
*2)Leakage current measurement conditions: Rated voltage 4 V applied, 1 min
*3)ESR measurement conditions: 100 Hz, 1 Vrms, Bias voltage 1.5 V
A comparison was carried out between; a situation (first example) in which optimization was performed regarding the particle size distribution of valve metal powder to be used for manufacturing the anode for a solid electrolytic capacitor of a structure with a valve metal powder layer accumulated on a valve metal base material; and a situation (first comparative example) which uses a valve metal powder having a particle size distribution suited for the dry press method, which is a conventional manufacturing method for anodes on solid electrolytic capacitors. Since the powder suited for the dry press method requires high fluidity, the particle size becomes larger than that of the powder suited for dispersion liquid (hereunder, the powder with particle size suited for the dry press method is referred to as press powder, and the powder with a particle size suited for dispersion is referred to as dispersion liquid powder).
In the first comparative example, in which a dispersion liquid (paste) is made using a press powder, and a powder layer is formed on the valve metal base material by an industrial method such as metal mask printing, at the time of sintering, sufficient welding does not occur between the valve metal base material and the powder, and flaking is likely to occur. Furthermore voids (hole-like cavities) are likely to form on the powder layer after sintering. For these reasons, as shown in Table 3, the capacitor properties and product yield of the first comparative example are degraded.
Contrary to the first comparative example, in the first example in which the powder layer is formed on the valve metal base material by making a dispersion liquid using the powder having a proper particle size distribution, there was no occurrence of the sintering-related problems that occurred in the first comparative example, and also regarding the capacitor properties and product yield of the first example, as shown in Table 3, favorable results were shown.
These results indicate similar results as in the case of using niobium as the valve metal as seen in the first example and the first comparative example, even in the case of using tantalum as the valve metal as seen in the second example and the second comparative example.
The third comparative example is the case where non-agglomerate primary particle powder was used for the valve metal powder, as illustrated in the particle size distributions in Table 7 and Table 8. In the third comparative example using primary particles, while the welding between the valve metal base material and the powder layer becomes sufficient, the deformation becomes large due to the shrinkage during the sintering, and this causes cracking of the powder layer and warping on the base material to which the powder layer is welded. For these reasons, as shown in Table 9, capacitor properties and product yield of the third comparative example are degraded.
Contrary to the third comparative example, in the third example using dispersion liquid powder sintered and agglomerated with proper particle size distribution as shown in Table 7 and Table 8, there was no occurrence of the sintering-related problems that occurred in the third comparative example, and also regarding the capacitor properties and product yield of the third example, as shown in Table 9, favorable results were shown.
While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, omissions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description, and is only limited by the scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2003-307608 | Aug 2003 | JP | national |
