JPS6236626B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing an electrolytic capacitor, and more particularly to a method for manufacturing a sintered body of a tantalum or niobium valve metal (hereinafter referred to as valve metal) powder compact.

In general, electrolytic capacitors are made by vacuum sintering a molded body of valve metal powder, and then a dielectric layer of oxide film is provided on the surface of the sintered body. It is a capacitor with a cathode formed by depositing a silver paste layer, etc. To explain in more detail, first, the average particle diameter is several μ.
After pressing and molding the valve metal powder into a predetermined shape and size while planting a metal wire of the same type as the valve metal powder that will become the lead wire of the anode,
1600℃ to 1600℃ under a vacuum of 1×10 ^-4 mmHg or less
A sintered body is produced by sintering at 2000°C for 30 to 60 minutes. Next, an oxide film that will become a dielectric layer is formed on the surface of the sintered body by anodization in an electrolyte solution such as an aqueous phosphoric acid solution, using the sintered body as an anode and the same type of metal as the anode as a cathode. Next, a manganese dioxide layer is formed by baking on the surface of the anodized element at the same time as the manganese nitrate solution with which the anodized element (anodized element) is impregnated is thermally decomposed. After that, a graphite layer is sequentially formed on the outer surface of the anodized element.
A silver paste layer is formed, a suitable metal wire serving as a cathode lead wire is connected by means such as soldering, and the capacitor is completed by reinforcing it with resin or the like.

As is well known, electrolytic capacitors are desired to have high breakdown voltage and low leakage current, and in order to achieve this purpose, elements other than valve metals (referred to as impurities, metals and non-metals) are used in sintered bodies. It is important to reduce the content of hydrogen (generic term for hydrogen). That is, since metals and non-metals other than valve metals are difficult to be anodized, when a valve metal containing these metals is anodized, the impurities prevent the growth of an oxide film. That is, the maximum formation voltage becomes low, making it impossible to obtain an electrolytic capacitor with a high breakdown voltage. On the other hand, hydrogen is tantalum,
It is often absorbed by niobium and has the property of embrittling these metals. For this reason, in a sintered body that stores hydrogen, hydrogen moves to the cathode while breaking through the oxide film formed during anodization, suppressing the growth of the oxide film, making it impossible to obtain an electrolytic capacitor with a high breakdown voltage. Furthermore, even if an oxide film is formed, during the subsequent electrolytic capacitor manufacturing process, pressure applied to the anodized element, such as thermal shock during baking of manganese dioxide, may cause cracks in the oxide film, increasing leakage current. become.

As a means to remove impurities, the molded body of valve metal powder is sintered at as high a temperature as possible in a high vacuum to the extent that the surface film of the powder can be effectively utilized; Oil diffusion pumps have been known for a long time as pumps for obtaining this, and in recent years, cryopumps have become well known.

However, according to the findings of the inventors, when the oil diffusion pump is in a high vacuum state, the oil vapor in the oil diffusion pump is sent into the vacuum furnace, making it impossible to obtain a capacitor with a high breakdown voltage due to oil contamination. be.

On the other hand, cryopumps do not use oil; they generally use helium gas as a refrigerant to cool the opposite side of the exhaust gas surface to over 10 degrees K, condense the exhaust gas on that surface (cooling surface), and create a high vacuum. There is no oil contamination, but gases with boiling points close to the cooling surface temperature, such as hydrogen
〓), the exhaust capacity for neon gas (27〓) is
It is lower than the exhaust capacity for gases with high boiling points. Furthermore, if hydrogen or neon gas condenses on the cooling surface, the exhaust capacity will be reduced. Therefore, if a molded body of valve metal powder containing several PPm to several tens of PPm of hydrogen is sintered in a high vacuum maintained by a cryopump, the sintered body will not have enough hydrogen removed. It has the disadvantage of accelerating the deterioration of the cryopump's pumping capacity.

The purpose of the present invention, as a result of studies in view of these drawbacks, is to provide a method for manufacturing a sintered body that has a high voltage at which an oxide film does not grow during anodization of the sintered body, that is, a so-called breakdown voltage. The company provides high quality electrolytic capacitors.

That is, the present invention focuses on the fact that hydrogen contained in valve metal powder is released in large quantities around 800°C in a high vacuum, and almost disappears around 1000°C, and as a result of detailed study, we found that tantalum, A process of sintering a niobium valve metal powder molded body in a vacuum furnace at a temperature ranging from room temperature to 1000°C using an oil diffusion pump while maintaining a desired high vacuum state;
Sintering is carried out while maintaining the desired high vacuum state using a cryopump that uses helium gas as a refrigerant for the heating process from 1000℃ to the specified sintering temperature, the holding time process at the specified sintering temperature, and the cooling process. The breakdown voltage of the sintered body obtained by sintering according to the present invention is lower than that of the sintered body obtained by sintering while maintaining a high vacuum using only a conventional oil diffusion pump and a cryopump. This is based on the fact that the breakdown voltage is higher than any other sintered body obtained by sintering while being held in a high vacuum. Furthermore, the present invention can be applied from room temperature to 1000℃.
In order to exhaust the hydrogen contained in the valve metal powder using an oil diffusion pump within the range of The cryopump process is significantly improved compared to when the cryopump is used alone. In other words, it is based on improving the sintering processing ability of the sintered body.

Next, embodiments will be described with reference to the drawings.

First, the relationship between the amount of released hydrogen contained in the valve metal powder and the temperature will be explained.

10g of tantalum powder with an average particle size of 3μm was collected in a tantalum container, placed in a vacuum furnace, and heated at a rising rate of 20℃ per minute while maintaining the degree of vacuum around 1×10 ^-5 mmHg using an oil diffusion pump. The amount of hydrogen released when the temperature was raised to 2000℃ was investigated using a gas analyzer. From Figure 1, which shows the relationship between hydrogen gas release amount and temperature, it can be seen that hydrogen gas gradually increases from around 500°C, reaches a maximum around 800°C, and almost disappears at 1000°C.

Example 1 1gr of tantalum powder with an average particle size of about 3μm was collected, and while a tantalum wire that would become the lead wire of the anode was planted on this powder, the powder was made to have a diameter of 6mm and a molding density of 6gr/cm ²
A press molded body was manufactured. Next, there are two conventional methods: (a) where the high vacuum state during the entire sintering process is maintained using an oil diffusion pump, and (b) where the high vacuum state during the entire sintering process is maintained using a cryopump using helium gas as a coolant. In the case of holding, and in the method of the present invention (c), a high vacuum state in the range from room temperature to 1000 ° C. is maintained using an oil diffusion pump, and a high vacuum state in the subsequent temperature rising and cooling process is maintained using helium gas as a refrigerant. We set three conditions for the evacuation system when held with a cryopump, and for each, heated the 10 previously obtained molded bodies using the usual method while maintaining the vacuum level below 1 × 10 ^-5 mmHg. Then, when the temperature reached 1700℃
After holding for 30 minutes, the temperature was lowered in the usual manner, and after the inside of the furnace had cooled, it was taken out into the atmosphere to obtain a sintered body.

Thereafter, these sintered bodies were immersed in a commonly used anodization method, that is, immersed in a 0.1% by volume phosphoric acid solution, and subjected to constant current anodization at a current density of 30 mA/gr to form a tantalum oxide film. After growing, we measured the voltage at which it stopped growing, the so-called breakdown voltage during chemical formation. The second part shows the relationship between breakdown voltage and vacuum pumping system conditions (a), (b), and (c).
It can be seen from the figure that the breakdown voltage is higher in the embodiment (c) of the present invention in which an oil diffusion pump and a cryopump are used in combination than in the conventional examples (a) and (b) in which each pump is pumped independently.

Example 2 100 press molded bodies similar to those in Example 1 were prepared for each sintering process, and sintering was carried out under the conventional example (b) and the present invention example (c) of the vacuum evacuation system conditions of example 1. , we investigated how many times each could be sintered. Comparing the number of times it takes for the cryopump's evacuation capacity to drop to a vacuum level of 2 x 10 ^-4 mmHg, it was 6 times in the conventional example, but in the present invention.
It was found that the processing capacity was increased to 16 times, and the processing capacity for sintered bodies was approximately tripled.

As is clear from the examples above, in sintering tantalum powder compacts, the present invention, which uses a high vacuum pumping system in combination with an oil diffusion pump and a cryopump, is more effective than the conventional method in which each pump is used individually.
A sintered body with a high breakdown voltage can be manufactured, and as a result, a high quality tantalum electrolytic capacitor can be provided.

Furthermore, the present invention has improved the processing capacity of sintered bodies by a cryopump, and has been able to significantly improve the productivity of sintered bodies.

In addition, in the example, an example was shown in which tantalum powder was used as the valve action metal powder, but a sintering process for a niobium powder molded body, which is sintered at a sintering temperature close to the sintering temperature of a tantalum powder molded body, may be applied. That is clear.

[Brief explanation of the drawing]

Figure 1 shows a tantalum powder compact of 1×10 ^-5 mm.
While maintaining the degree of vacuum near Hg, from room temperature to 2000
FIG. 2 is a diagram showing the relationship between the cumulative ratio of the amount of hydrogen gas released from the tantalum powder molded body and the temperature when the temperature is raised to ℃. Figure 2 shows the breakdown voltage during sintering of a sintered body sintered using conventional vacuum evacuation systems (a) and (b) and the vacuum evacuation system of the present invention (c) during sintering of a tantalum powder compact. Diagram showing.

Claims (1)

[Claims] 1. In a method for manufacturing an electrolytic capacitor in which a sintered body obtained by sintering a valve action metal powder molded body at high temperature in a high vacuum is used as an anode body, the molded body is heated from room temperature to
A process of sintering while maintaining the desired high vacuum state using an oil diffusion pump at temperatures up to 1000℃;
Using a cryopump with helium gas as a refrigerant, the heating process from °C to a predetermined sintering temperature, the holding time process at the predetermined sintering temperature, and the cooling process are carried out using a cryopump that uses helium gas as a refrigerant.
A method for manufacturing an electrolytic capacitor, comprising the step of sintering while maintaining a desired high vacuum state.