JPS6026288B2 - Sintered electrodes for solid electrolytic capacitors - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an electrode for solid electrolytic capacitors, and is an object of the present invention to provide an electrode that generates less reverse current.

The major difference between electrolytic capacitors and other types of capacitors such as ceramic capacitors and film capacitors is that the electrode terminals have polarity.

In other words, when a positive voltage is applied to the metal anode with respect to the counter electrode (manganese dioxide cathode), the leakage current (referred to as forward current) is extremely small, but when applied in the opposite direction, The drawback is that an excessive amount of current flows (we call it reverse current), which can lead to damage to the capacitor. Such directionality of current is a property that depends on the dielectric film formed by anodic oxidation, that is, the anodic oxide film. An object of the present invention is to reduce reverse current in a dawn-fed tantalum solid electrolytic capacitor.

A further object is to reduce reverse current without increasing forward current. The inventors conducted research with the aim of reducing the reverse current of a solid electrolytic capacitor by alloying the dawn tantalum electrode with a different metal, without increasing the forward current of the solid electrolytic capacitor. We released it after seeing that it was suitable for the purpose.

This is because the vanadium contained in the tantalum electrode is incorporated into the dielectric film during anodic oxidation.
It is thought that the properties of the dielectric film have been changed to a state where the reverse current is decreased without increasing the forward current. The content of vanadium added to tantalum metal was 0.003 to 3 atomic % based on the total amount of both.
Furthermore, the inventors have discovered and developed an electrode configuration that is more effective in reducing forward and reverse currents. That is,
Tantalum metal is alloyed so that the concentration of vanadium gradually decreases from the surface to the inside. This was particularly effective in reducing forward current. Next, the effect of adding vanadium to tantalum will be explained.

The directionality of current observed in the anodic oxide film used as a transfer body for electrolytic capacitors is thought to be caused by the following reasons.

The oxide film part close to the metal side is formed of an n-type semiconductor layer that contains an excess of metal ions, and the outermost layer is a p-type semiconductor layer that contains an excess of oxygen ions injected from the electrolyte during anodic oxidation. It is believed that the i-layer is formed as an insulating layer of stoichiometric tantalum oxide between the two layers. Therefore, if a voltage is applied in the same direction as during anodic oxidation, the growth of the i-layer is promoted and the withstand voltage becomes higher. When a voltage is applied in the opposite direction to the solid electrolytic capacitor, the oxygen ions move outward and further to the manganese diacid counter electrode. As a result, there is a shortage of oxygen ions in the i-layer, so the i-layer becomes relatively metal-rich and changes into an n-type semiconductor. It is considered that for this reason, the electrical conductivity in the reverse direction becomes high. Vanadium in the present invention is thought to have the effect of blocking the movement of oxygen ions as described above when a reverse voltage is applied to the anodic oxide film, resulting in a decrease in reverse current. Conceivable. Vanadium is a metal with the same pentavalent valence as tantalum, so even if it is replaced with tantalum in the i-layer of the tantalum anodic oxide film, which is stoichiometrically considered to be an insulating layer, for example, tantalum Semiconducting such as n-type semiconducting when tungsten or molybdenum having a higher valence of hexavalence is substituted should not occur.

However, since the insulating properties of vanadium oxide are inferior to those of tantalum oxide, adding too much vanadium to tantalum is thought to result in the formation of vanadium oxide, which is particularly preferable in order to increase the forward current. do not have. The limit is up to 3 atomic % based on the total amount of tantalum and vanadium. Further, adding vanadium in an amount of 0.003 atomic % or less has almost no effect on improving reverse turtle flow. By the way, as mentioned above, it is assumed that vanadium prevents oxygen ions in the tantalum anodic oxide film from moving to the manganese dioxide counter electrode of the solid capacitor by applying a reverse voltage.

It is thought that the movement of this oxygen ion is more pronounced in the part of the anodic oxide film closer to the manganese dioxide counter electrode. Therefore, as in the second aspect of the present invention, if the concentration gradient is such that the vanadium content is higher in the outer part of the anodic oxide film near the manganese dioxide counter electrode, the movement of oxygen ions will be reduced when a voltage is applied in the opposite direction. The blocking is more effective, and on the other hand, the deterioration of the insulation properties of the i-layer due to the application of a positive voltage is avoided. A concentration gradient of vanadium in which the concentration becomes higher toward the counter electrode of manganese dioxide in the anodic oxide film can be obtained as follows. That is, this is a method of anodizing an electrode made of a tantalum-vanadium alloy in which the vanadium concentration is higher closer to the surface.

During anodic oxidation, the above electrode follows the chemical formula: +5 plate--1 Mr. 1 M24 (M represents Ta-V alloy).OH- ions pass through the oxide film and reach the interface between the oxide and the electrode metal. and reacts with the electrode metal to form an oxide.

Therefore, initially, the metal part with high vanadium concentration is converted to oxide, and as anodic oxidation progresses, the part with low vanadium concentration inside the electrode is converted to oxide at the already formed oxide-metal interface. As a result, an anodic oxide film with a higher vanadium concentration is formed on the outer side of the oxide (on the side opposite to the manganese dioxide electrode when the capacitor is formed). In addition, in the above chemical formula, hydrogen ions generated at the oxide-metal interface pass through the oxide and are released into the electrolytic solution, following a process similar to 15 sails - 1 wind 0.

Next, the electrode of the present invention will be specifically explained again.

There are several methods for producing tantalum electrodes containing vanadium. In either method,
It is necessary that vanadium be contained homogeneously in the two-dimensional direction of the tantalum metal surface. The electrode of the present invention is made as follows.

That is, a vanadium film is coated on the surface of tantalum powder for electrolytic capacitors, which is commonly used, and then heated in a vacuum or an inert atmosphere to thermally diffuse vanadium. Either the alloy powder thus obtained is compression molded and crystallized in vacuum or an inert atmosphere, or the vanadium-coated tantalum powder is formed into an electrode shape and then heated at a temperature below the crystallization temperature. It can be obtained by heating to sufficiently progress the diffusion of vanadium into tantalum, and then increasing the heating temperature to cause competitive bonding.
Furthermore, after the vanadium-coated tantalum powder is formed into an electrode shape, the heating for diffusing the vanadium may be omitted, and the diffusion and crystallization may be performed simultaneously. in this case,
The gradually decreasing concentration gradient of vanadium from the tantalum surface to the interior can be made larger. As a method for depositing vanadium on the surface of tantalum powder, physical methods such as sputtering and hollywood deposition, and chemical methods such as gas phase decomposition reaction can be used. A tantalum powder having a particle size of 100 mesh or less is suitable for properties and coating processing. A method using sputtering will be described as a method for depositing a vanadium film on tantalum powder. In a commonly used sputtering device, tantalum powder is sprinkled into a shallow dish-shaped container placed opposite the vanadium target, and while the powder is sufficiently concentrated, vanadium is sputtered onto the surface of the tantalum powder. A vanadium film is formed on the surface. The anode lead wire embedded in the powder during electrode molding can be a tantalum-vanadium alloy wire, a vanadium-coated tantalum wire, or a wire whose surface is alloyed by thermally diffusing vanadium. Of course, tantalum wire that does not contain vanadium may also be used. Even for tantalum wires that do not contain vanadium, the vanadium contained in the powder is diffused during sintering, and the surface of the tantalum wire is alloyed with vanadium. In order to more uniformly distribute vanadium inside the powder particles, tantalum alloyed with vanadium can be crushed using a melting method, and the resulting powder can be used to create a sintered electrode, or tantalum coated with vanadium can be used to create a sintered electrode. This is accomplished by heating the powder for a long time to fully diffuse vanadium into the tantalum powder, and then making a sintered electrode using tantalum powder.

Next, examples of the method of the present invention will be described. A shallow dish-shaped container with a diameter of 10 mm was placed opposite to a vanadium target with a diameter of 10 mm placed as a coating metal in the sputtering device, and 30 mm of tantalum powder finer than 100 mesh was sprinkled into this container, and the container was rotated by power. Let the tantalum powder be worshiped with the fly. Next, after reducing the pressure inside the sputtering equipment, an argon atmosphere of 0.05 Hg was further introduced, and 1
A high frequency input of 50 W is applied between the electrodes. The amount of vanadium deposited is proportional to the sputtering time.

Weigh out 9 of the tantalum powder 160 that has been coated with vanadium, mold it together with the tantalum lead wire, and then
A porous sintered electrode is obtained by sintering at 1800 oo for 20 minutes in a vacuum of 10-5 tons or less. Then, this electrode was anodized in a 1% phosphoric acid aqueous solution by applying a voltage of 50 V for 2 hours, and then a manganese dioxide cathode was attached by a manganese nitrate thermal decomposition method. A solid electrolytic capacitor was then fabricated by forming a counter electrode terminal on the manganese dioxide by attaching colloidal carbon and silver bayonet. Figure 1 shows the current (
This figure shows how the reverse current changes depending on the amount of vanadium. In addition, Figure 2 shows that the tantalum anode has 11
This figure shows how the current (forward current) when a voltage of 0 V is applied changes depending on the amount of vanadium.
As is clear from this, in the electrode of the present invention, since vanadium is contained in tantalum in a range of 0.003 to 3 atomic percent, the reverse current and positive current are higher than when the electrode is composed only of tantalum. Both directional currents are decreasing.

In addition, tantalum powder coated with vanadium was heated in vacuum for 1
A capacitor was fabricated in the same manner as the electrode fabrication process and solid capacitor fabrication process described above, using powder that had been heat-treated at 600 qo for 3 hours to fully diffuse vanadium into the tantalum.

In this case, the reverse current was at the same level as the former electrode obtained by molding and sintering the vanadium-coated powder and simultaneously performing vanadium diffusion and powder sintering, but the forward current was at a similar level. A slight increase was observed in comparison.

Of course, the effects of the present invention can also be obtained with this electrode.

[Brief explanation of the drawing]

FIGS. 1 and 2 respectively show the relationship between the amount of vanadium contained in tantalum and reverse current and forward current in the solid electrolytic capacitor electrode of the present invention. Figure 1 Figure 2

Claims (1)

[Scope of Claims] 1. A solid electrolytic capacitor made of an alloy of tantalum and vanadium, characterized in that the vanadium is contained in an amount of 0.003 to 3 atomic % based on the total amount of the vanadium and the tantalum. Sintered electrode. 2. The sintered electrode for a solid electrolytic capacitor according to claim 1, wherein the vanadium concentration is high at the metal surface and gradually decreases toward the inside.