Specialized metallurgical processes – compositions for use therei – Compositions – Loose particulate mixture containing metal particles
Reexamination Certificate
2001-08-10
2004-02-10
Mai, Ngoclan (Department: 1742)
Specialized metallurgical processes, compositions for use therei
Compositions
Loose particulate mixture containing metal particles
C075S245000, C361S529000
Reexamination Certificate
active
06689187
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a tantalum powder suitable for the raw material of anodes of solid state electrolytic capacitors.
2. Description of Related Art
Tantalum sintered bodies having a porosity of 70% by volume or more are used for the anodes of tantalum solid state electrolytic capacitors (to be referred to as simply tantalum capacitors). The tantalum sintered body is anodized, a film of a solid state electrolyte is formed on its surface, and after connecting a cathode terminal on that film by soldering and so forth, a resin casing is formed to obtain the tantalum capacitor.
In order to produce the tantalum sintered body, primary powder of tantalum is first obtained by a known method such as sodium reduction of potassium tantalum fluoride or hydrogen reduction of tantalum pentachloride. Next, this primary powder is then subjected to an acid-water washing step to remove miscellaneous impurities in the powder as necessary. Next, water and so forth is added to the primary powder to perform a preliminary agglomeration step to weakly agglomerate the primary powder. The preliminary agglomeration step serves to control the bulk density of thermally agglomerated powder in the next heat treatment step. Following this preliminary agglomeration step, the pre-agglomerated primary powder is heat treated at a temperature of 1000° C. and above to agglomerate the primary powder by heat treatment and obtain agglomerated particles. A deoxygenating step is then performed to remove excess oxygen from the thermally agglomerated particles.
After press molding the hard sponge-like tantalum agglomerated particles having a diameter of several ten to several hundred &mgr;m obtained in this manner, a tantalum sintered body is obtained by sintering.
By the way, accompanying the reduced size and higher frequencies of electronic equipment and electronic circuits in recent years, there has been a growing need for tantalum capacitors offering higher capacitance and lower equivalent series resistance (ESR). Capacitor capacitance is proportional to the surface area of the anode. Consequently, it is necessary to use tantalum agglomerated particles for the anode raw material that have as large a surface area as possible.
For example, tantalum agglomerated particles having a specific surface area of about 1 m
2
/g (0.35 &mgr;m when converted as the spherical approximate diameter) as determined by BET are used for the raw material of an anode for which the CV value indicative of capacitor capacitance is 50,000 &mgr;F·V/g (standard formation conditions: 60° C., 20 V). After press molding these agglomerated particles, the anode is produced by sintering at a temperature of about 1400° C. Other agglomerated particles have been disclosed in Japanese Unexamined Patent Application, First Publication No. 58-27903 in which the particle size as measured with the air permeability method (Fischer Subsieve Sizer: FSS method) is 1.1 to 1.8 &mgr;m (0.33 to 0.20 m
2
/g in terms of surface area) and the BET specific surface area as measured with the nitrogen adsorption method is 0.27 to 0.75 m
2
/g. In addition, agglomerated particles are disclosed in Japanese Unexamined Patent Application, First Publication No.63-238203 in which the particle size as measured with the FSS method is 1.0 to 3.0 &mgr;m (0.36 to 0.12 m
2
/g in terms of surface area) and the BET specific surface area is about 0.4 m
2
/g. In addition, agglomerated particles are disclosed in Japanese Unexamined Patent Application, First Publication No. 2-38501 in which the particle size as measured with the FSS method is 0.3 to 0.7 &mgr;m (1.20 to 0.52 m2/g in terms of surface area) and the BET specific surface area is 1.75 to 3.50 m
2
/g. Furthermore, since the specific gravity of tantalum is 16.6, the relationship in which particle size as measured by the FSS method×6/(16.6×surface area as determined by FSS) exists between particle size as measured with the FSS method (&mgr;m) and surface area (m
2
/g).
On the other hand, the ESR value of a capacitor is characterized by being related to the magnitude of heat generation accompanying increased speeds of electronic circuits such that as ESR increases, heat generation also increases. Thus, tantalum capacitors used in the CPUs and power supply circuits of personal computers are required to have a low ESR. In order to decrease ESR, it is necessary to uniformly form a solid state electrolytic coating on the tantalum sintered body. Manganese oxide is typically used for the solid state electrolytic coating. In the case of forming a solid state electrolytic coating comprised of manganese oxide, a manganese nitrate solution and so forth is impregnated into the sintered body followed by heating and thermal decomposition of the manganese nitrate. In order to uniformly form a solid state electrolytic coating on the tantalum sintered body, it is necessary to use a tantalum sintered body having uniform porosity with minimal closed pores and micropores. In addition, electrically conductive polymers have recently come to be frequently used as solid state electrolytic coatings. Since these electrically conductive polymers are composed of large molecules, it is necessary to more precisely control the porosity of the tantalum sintered body.
In order to produce such a tantalum capacitor having high capacitance and low ESR, it is important to use for the anode a tantalum sintered body having uniform porosity with minimal closed pores and micropores. In order to produce such a tantalum sintered body, it is necessary to suitably control the particle size distribution, cohesive strength and porosity of the tantalum agglomerated particles used as the raw material. For example, in order to produce an anode for a tantalum capacitor having a CV value of 50,000 &mgr;F·V/g under standard formation conditions (60° C., 20 V), it is first necessary to select a tantalum primary powder of a suitable particle size, and then subject this to preliminary agglomeration and heat treatment under appropriate conditions to obtain tantalum agglomerated particles.
However, in the case of producing a tantalum capacitor having a high capacitance in excess of 50,000 &mgr;F·V/g, a primary powder is required that has a larger surface area. Since primary powders like this have extremely high reactivity, the primary powder ends up strongly agglomerating with itself making it susceptible to the formation of agglomerated particles having high cohesive strength. Agglomerated particles having high cohesive strength are not easily broken up even when subjected to press molding. Tantalum sintered bodies obtained by press molding and sintering such agglomerated particles have large pores between a plurality of agglomerated particles, and in addition to having fine pores within each of the agglomerated particles, also have closed pores. Namely, such tantalum sintered bodies do not have uniform porosity. It is therefore difficult to uniformly form a solid state electrolytic coating on such tantalum sintered bodies. Thus, when such a tantalum sintered body is used as an anode, the resulting tantalum capacitor ends up having a large ESR.
Consequently, in Japanese Unexamined Patent Application, Primary Publication No. 8-97096, for example, a method for inhibiting excessive agglomeration by primary powder having a large surface area is proposed in which the heat treatment step is omitted, and the primary powder is thermally agglomerated in a step in which the primary powder is deoxygenated. However, this method was not effective since it is difficult to simultaneously optimize the degree of deoxygenation and the degree of thermal agglomeration.
As has been explained above, in order to produce a high-capacitance tantalum capacitor, although it is necessary to use tantalum agglomerated particles having a large surface area, there were many cases in which such agglomerated particles did not have uniform porosity due to excessively large cohesive strength. Namely, the production of tantalum agglomerated particles
Cabot Supermetals K.K.
Kilyk & Bowersox
Mai Ngoclan
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