JPH042109A - Aluminum electrode for electrolytic capacitor - Google Patents

Abstract

PURPOSE:To obtain an electrode for an electrolytic capacitor having large electrostatic capacitance per a unit area and high reliability by forming a tungsten nitride layer onto the surface of aluminum having high purity. CONSTITUTION:A tungsten nitride layer is formed onto the surface of aluminum having high purity, and an aluminum electrode used as the cathode of an electrolytic capacitor is manufactured. Vacuum deposition, cathode arc evaporation, sputtering, ion plating, a plasma CVD method, etc., can be exampled as a means for forming a tungsten nitride thin film. Accordingly, the surface of metallic aluminum, on which a natural oxide film is hardly formed and which has high conductivity, is protected stably by tungsten nitride as it is, thus acquiring a high electrostatic capacitance value as a whole.

Description

[Detailed description of the invention] [Industrial application field]

The present invention relates to an aluminum electrode used in an electrolytic capacitor, and particularly to an aluminum electrode used as a cathode.

[Conventional technology]

Electrolytic capacitors are small, large-capacitance, inexpensive, and have excellent characteristics for applications such as rectifying and smoothing output, and are important components of various electrical and electronic devices. Electrolytic capacitors generally use a so-called valve metal such as aluminum on which an insulating oxide film can be formed as an anode, and the insulating oxide film is used as a dielectric layer and is held between a cathode electrode for current collection and a separator. A capacitor element is created by interposing an electrolytic solution, and the capacitor element is housed in a sealed container. As mentioned above, the anode materials used include aluminum, tantalum, niobium, and titanium. Further, the same type of metal as the anode material is used for the cathode electrode material for current collection. However, valve metals generally have an oxide film layer formed on their surfaces due to natural oxidation. This tendency is particularly noticeable in aluminum. Since this natural oxide film is an extremely thin insulating layer, capacitance is also formed on the cathode side, and an electrolytic capacitor is a composite capacitance in which the capacitance on the anode side and the capacitance on the cathode side are connected in series. , the desired capacitance cannot be obtained. In addition, in order to obtain the desired capacitance, it is necessary to make the capacitance on the anode side larger than necessary. If the capacitance value is made extremely high, the effect of the capacitance on the cathode side can be almost ignored, but the capacitance per unit area of the anode of low-voltage electrolytic capacitors is at a considerably high level. It is difficult to further increase the capacitance, and the capacitance value inevitably decreases due to the combined capacitance.Therefore, in order to increase the capacitance value on the cathode side, the surface area of the cathode electrode is expanded by etching. However, although this technique for increasing the surface area is now highly sophisticated, it is becoming increasingly difficult to dramatically increase the capacitance of electrolytic capacitors using this technique alone. In order to solve the problem of the decrease in capacitance due to the composite capacitance of It is possible to prevent the value from decreasing.As such, there are known methods, such as those disclosed in JP-A-60-1826, in which various conductive metals are vacuum-deposited. In addition to the above-mentioned vacuum evaporation method, there are various physical methods such as ion blating method, sputtering method, and plasma CVD method. Precious metals have almost no reaction with chemicals and can maintain good conductivity even when used as electrolytic capacitors for long periods of time.However, this type of precious metal is not suitable for electrolytic capacitors, which are required to be produced in large quantities at low cost. It has not yet been adopted for economic reasons. Other metals undergo chemical reactions in the presence of an electrolyte.
As the surface condition changes over time, corrosion accidents may occur,
The drawback was that the characteristics of electrolytic capacitors were unstable.

[Problem to be solved by the invention]

This invention forms a thin film on the surface of high-purity aluminum that is conductive and has high stability when used as an electrolytic capacitor. The purpose is to obtain electrodes for capacitors.

[Means to solve the problem]

This invention focuses on the fact that tungsten nitride forms a thin film that is suitable for the purpose of this invention, and the electrode for an electrolytic capacitor of this invention has
It is characterized by the formation of a tungsten nitride layer. That is, the present invention covers the surface of an aluminum electrode with a thin film of tungsten nitride. According to the present invention, as the material to be treated, high-purity foil-like or plate-like aluminum, which is used for the cathode of an ordinary electrolytic capacitor, can be used. This aluminum surface is cleaned in advance by degreasing or the like. Further, the aluminum surface may be etched or left plain. However, during etching, it may be necessary to select the range of fineness of the unevenness caused by etching depending on the means for forming the tungsten nitride layer. The thickness of the tungsten nitride layer formed must be such that at least the aluminum surface is uniformly covered. Moreover, if the thickness is more than necessary, the coating process will take time, so the thickness is preferably 0.02 to 5 μm, more preferably 0.1 to 2 μm. As a means to form a tungsten nitride thin film,
Although various methods can be applied, in general, since the film is thin, it is preferable to use a dry process using physical means, since the thickness and condition can be easily controlled. Such means include vacuum evaporation, cathodic arc evaporation,
Sputtering, ion blating, plasma CV
An example is method D.

[For production]

Tungsten nitride is W2 depending on the valence of tungsten.
It can take the form of N, Wt N:l , WNz, etc., but all of them are hard compounds that have properties similar to interstitial compounds or intermetallic compounds and have high electrical conductivity. Since it has good reactivity with aluminum, a dense thin film with high conductivity is formed on the aluminum surface. As a result, the aluminum electrode has an extremely thin natural oxide film with high capacity formed on its surface, or in certain minute areas, the highly conductive metal aluminum surface with almost no natural oxide film is stabilized by tungsten nitride. It is thought that the electrode will be protected by a high capacitance value as a whole. In addition, tungsten nitride does not react with the electrolyte.
, to maintain the surface condition of the electrode stably over a long period of time.

【Example】

The present invention will be described in more detail below based on Examples. A high-purity aluminum workpiece having a tungsten nitride thin film of the present invention formed on its surface was prepared as in Examples 1 and 2 below. Comparative Examples 1 to 3 were those in which a metal tungsten layer other than nitride was formed, and those in which only etching treatment was performed on the surface of conventionally used high-purity aluminum. (Example 1) High purity aluminum foil (purity 99.95%, thickness 10
0 μm) cut into 50 mm x 100 mm is used as the material to be treated, and the total pressure including nitrogen gas is 5 x 10-
Deposition was performed using cathodic arc evaporation in a 'Torr chamber. The deposition conditions were as follows: The material to be treated was heated to 300° C., and the deposition was carried out for 2 minutes at an arc discharge voltage of 100 μm, an arc current of 200 A, and a deposition rate of 1000 people/min. As a result, a 0.2 μm thick tungsten nitride layer was formed on the surface. (Example 2) A tungsten nitride thin film was formed on the same high-purity aluminum as in Example 1 by ion blasting. The formation conditions are such that the total pressure including nitrogen gas in the chamber is I
In an atmosphere of
Ion blating was performed for a minute. As a result, a 0.2 μm thick tungsten nitride layer was formed on the surface. (Comparative Example 1) The same material as in Example was used as the material to be treated, and a metal tungsten thin film was formed by the same cathodic arc evaporation method as in Example 1 in a chamber with an argon gas atmosphere of 2 x 1O-3 Torr at room temperature. was formed. The deposition conditions were an arc discharge voltage of 100cm, an arc current of 200A, and a deposition rate of 2.
The deposition was carried out for 10 minutes at 00 people/min. As a result, a metal tungsten vapor deposited film with a film thickness of 0.2 μm was formed. (Comparative Example 2) A thin film of metallic tungsten was formed by the same ion blating method as in Example 2. The same material as in Example 1 was used as the material to be treated. The thin film was formed under an argon gas atmosphere of 2 Xl0-2 Torr, and a voltage of 1200 V was applied between the material to be treated and the evaporation source for 60 minutes. As a result, a metallic tungsten film with a film thickness of 0.2 μm was formed. (Comparative Example 3) A high-purity aluminum material made of the same material as in the example was prepared, the surface of which was etched by alternating current electrolysis. When the capacitance value per unit area of each of the treated materials of the Examples and Comparative Examples was measured, the results shown in Table 1 were obtained. (Table 1) As is clear from the results, the capacitance values per unit area of the examples of the present invention are higher than those of the comparative examples. Next, in order to examine the stability of the formed thin films, electrolytic capacitors were created using each of these treated materials as cathodes of electrolytic capacitors, and life tests were conducted to examine changes in characteristics. The created electrolytic capacitor is an electrolytic capacitor with lead wires in the same direction.The element is made by winding a foil electrode with a separator and is impregnated with an electrolytic solution.The electrolytic capacitor is housed in a metal case, and the opening is sealed with a rubber seal. It is. All of the materials constituting the electrolytic capacitors were common, except that the cathode foil used in each of the above Examples and Comparative Examples was used. The assembly method is also the same. The rated voltage of the electrolytic capacitor is 6.3■, and the rated capacity is 47
μF, external dimensions are 5ff1m in diameter and 7mm in length. The composition of the electrolyte used was 78% ethylene glycol, 10% ammonium adipate, and 12% water.
It has a higher water content than commonly used electrolytes. This is to make the occurrence of hydration deterioration of the electrode foil due to water more noticeable. A rated voltage was applied to this electrolytic capacitor, and a life test was conducted at 110° C. for 500 hours, and the capacitance value and the rate of change between the initial capacitance value and the capacitance value were examined. The results are shown in Table 2. (Table 2) As can be seen from these results, the electrolytic capacitor using the aluminum electrode of the present invention has a high capacitance value even at the initial value, and even after performing the high temperature load life test. It can be seen that since no characteristic deterioration such as hydration deterioration occurs on the electrode surface, there is little variation in electrical characteristics and stable characteristics can be maintained over a long period of time.

【Effect of the invention】

As described above, according to the present invention, as an electrode for an electrolytic capacitor, the electrostatic capacitance per unit area can be increased, so that a small and large-capacity electrolytic capacitor can be obtained, especially in a low voltage region. Furthermore, since the electrode surface is protected by tungsten nitride and deterioration of the electrode surface such as hydration deterioration is prevented, stable characteristics can be maintained over a long period of time.

Claims (1)

[Claims] (1) An aluminum electrode for an electrolytic capacitor, characterized in that a tungsten nitride layer is formed on the surface of high-purity aluminum.