Powder metallurgy processes – Powder metallurgy processes with heating or sintering – Controlled cooling after sintering
Reexamination Certificate
2000-11-03
2002-02-26
Mai, Ngoclan (Department: 1742)
Powder metallurgy processes
Powder metallurgy processes with heating or sintering
Controlled cooling after sintering
C419S026000, C419S023000, C419S031000, C075S228000, C266S250000, C266S259000, C148S513000
Reexamination Certificate
active
06350406
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing an anode unit for a solid electrolytic capacitor and an anode unit. The invention relates, more particularly, to a method of manufacturing a sintered anode unit and a sintered anode unit capable of extremely decreasing a content of oxygen that is included in the sintered pellet and also capable of extremely lowering a leakage current of the capacitor, by using a fine metal powder having a valve action. This is for reducing the size of the capacitor and for improving the capacity of the capacitor.
The present invention also relates to a continuous sintering apparatus capable of taking out a sintered unit into the atmosphere by decreasing the content of oxygen included after sintering at a high temperature and in a high vacuum.
Further, the present invention also relates to a method of manufacturing secondary particles of a valve-action metal powder including a small content of oxygen that is excellent in flowability, compactibility or fillability and handleability at the time of a compacting, by using a metal powder of fine primary particles having a valve action.
2. Description of the Related Art
Conventionally, a sintered anode unit of an electrolytic capacitor has been prepared by press-molding or compacting a fine metal powder having a valve action, such as tantalum or aluminum, into a pellet and then by sintering this pellet at a high temperature and in a high vacuum to form a porous sintered anode unit.
In recent years, along with the trend that semiconductor integrated circuits are manufactured in increasingly fine and highly integrated small structures, personal computers and telephones are also provided as portable units in small and light-weigh structures. Under such circumstances, capacitors that are used in these units have also been required to be in small structures with a larger capacity.
In order to meet these requirements, metal powders of finer primary particle sizes have come to be used as a starting raw material having a valve action. This is for achieving a small structure with a larger capacity, by increasing the porosity of the sintered unit and increasing the surface area of the anode unit. For example, particularly in recent years, there have been used raw metal powders having primary particle sizes of 1.5 to 0.7 &mgr;m as an average particle size.
Further, in molding such a fine powder, the following practice is general from the viewpoint of the fillability, flowability, compactibility and handleability of the powder. at the time of the molding or compacting. It is general to use a metal powder that has been granulated to have secondary particles by a thermal cohesion of the powder under a heat treatment at a high temperature and in a high vacuum.
Japanese Patent Application Laid-open No. 2-310301 discloses a method of manufacturing a tantalum powder. According to this method, fine primary particles obtained at a high temperature and in a high vacuum are heat treated for 30 minutes at a high temperature of 1,400 to 1,550° C. in a vacuum furnace at a degree of vacuum of 10
−3
Pa. Then, the thermally cohered powder is crushed lightly, and is deoxidized under the heating at 850° C. after adding of magnesium.
However, in general, when a metal powder becomes fine, the surface activity becomes high in proportion to the fineness. Particularly, a metal powder such as tantalum that has a valve action, for example, has its surface activity becoming higher along with the degree of fineness. Among metal powders having a valve action that are used as anode units for solid electrolytic capacitors, the metal powder like tantalum has a strong affinity with oxygen. It is general that when the metal powder of tantalum is used as a sintered unit, it is extremely difficult to decrease the content of oxygen included.
When this sintered unit is used as the anode unit for a solid electrolytic capacitor, for example, the increase in the content of oxygen in this sintered unit leads to a deterioration of the leakage current characteristics of the capacitor. Therefore, it is extremely important to lower the content of oxygen included in the sintered unit in the manufacture of the sintered unit.
Japanese Patent Application Laid-open No. 2-39417 discloses a method of manufacturing an electrolytic capacitor that uses a sintered unit as the anode unit. This sintered unit is prepared by sintering a metal powder having a valve action, at a high temperature and in a high vacuum, by paying attention to the content of oxygen included.
According to the proposal in the above publication, the metal powder as a starting raw material before the sintering has an oxygen density of 2,000 ppm or below. However, this publication does not describe a content of oxygen included after the metal powder has been sintered.
As described above, under the above situation, it is important that a sintered anode unit for a solid electrolytic capacitor is prepared as follows. A tantalum powder having a valve action (secondary powdered particles) is press-molded together with a tantalum line. Then, this molded unit is sintered in a high vacuum at a high temperature to form a porous pellet. This porous pellet must be the one that is not adsorbed physically and chemically by the oxygen in the atmosphere even when the sintered unit is taken into the atmosphere.
According to a conventional manufacturing practice, it is general that after a metal powder has been sintered in a high vacuum at a high temperature, the sintered unit is taken out into the atmosphere while lowering the high temperature of the sintered unit in an inert gas atmosphere. Therefore, according to this practice, a leakage to the vacuum occurs that the atmospheric gas is introduced during this process. Further, the surface of the sintered unit is also brought into contact with oxygen in the atmosphere after the sintered unit is taken out from the vacuum chamber.
As explained above, according to the conventional metal powder of a valve action like tantalum having a high affinity with oxygen, it can be understood that it is extremely difficult to lower the content of oxygen included in the sintered unit because of the contact of the sintered unit with oxygen in the atmosphere. This is against the trend of requirement for finer particles that are to be used for capacitors of a compact structure having a higher capacity.
Therefore, even if a known method of a forced cooling is employed under the inert gas atmosphere, there is the following difficulty. The conventional methods of manufacturing a sintered unit, including the methods described in the above publications, cannot sufficiently achieve a reduction in the content of oxygen included due to the contact with oxygen in the atmosphere and/or a reduction in the affinity of the metal powder and the sintered unit with oxygen. There is no sintering apparatus that can sufficiently meet the above requirements, either.
From the above, it can be said that according to the conventional manufacturing method, there is a trend of increasing the content of oxygen when the metal powder has finer primary particle sizes.
In order to decrease the leakage current by decreasing the content of oxygen, as one of the characteristics of the capacitors, the following manufacturing method is necessary. That is, for a metal powder having a high affinity with oxygen, it. is necessary to provide a manufacturing method that does not increase the content of oxygen included in the metal powder after the sintering as compared with the content of oxygen included before the sintering.
Thus, it is the current situation that there is no sintered anode unit for a solid electrolytic capacitor and there is no manufacturing method that can sufficiently satisfy the above-described requirements. In other words, it has not yet been possible to further decrease particle sizes of the starting raw material to meet the requirement for a more compact and a larger capacity of the capacitors in recent years. Further, it has not
Ida Yoshio
Satou Hideaki
Mai Ngoclan
McGinn & Gibb PLLC
NEC Corporation
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