JP2003151860A - Method of forming composite titanium oxide film and titanium electrolytic capacitor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a small size titanium electrolytic capacitor assuring a large capacity and a low level leakage current through a method of forming, on the surface of titanium, a stable composite titanium oxide film having a large dielectric constant. SOLUTION: In the method of forming a composite titanium oxide film on a metal titanium basic substance, a composite titanium oxide film including at least a kind of metal ion selected from among titanium, lithium, magnesium, calcium, strontium, barium and lanthanum is formed on the surface of the metal titanium basic substance by forming a potassium salt of titanic acid on the surface of the metal titanium basic substance, and then treating the film with an aqueous solution including at least a kind of metal ion selected from among lithium, magnesium, calcium, strontium, barium and lanthanum.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for forming a composite titanium oxide film and a capacitor using the same,
Specifically, a method for forming a dense composite titanium oxide film having excellent insulation and a large electrostatic capacity on the surface of a metal titanium substrate and a metal titanium substrate having a dense oxide film using the method. The present invention relates to a titanium electrolytic capacitor used as an anode.

[0002]

2. Description of the Related Art Tantalum electrolytic capacitors and aluminum electrolytic capacitors have been put into practical use as small-sized large-capacity electrolytic capacitors represented by conventional solid electrolytic capacitors.

A tantalum electrolytic capacitor is manufactured by using a porous sintered body of metal tantalum as an electrode and anodizing this to form a dielectric oxide film. The tantalum oxide film thus formed is very stable and therefore has the characteristics of good dielectric properties and long life. An aluminum electrolytic capacitor is also manufactured by similarly forming aluminum oxide as a dielectric on a metal aluminum foil or a sintered body by anodic oxidation.

FIG. 3 is a schematic view showing an example of a tantalum electrolytic capacitor and an aluminum electrolytic capacitor together for convenience. Here, as the anode, a Ta powder porous sintered body having tantalum oxide (Ta ₂ O ₅ ) as an insulator layer or an aluminum oxide (Al ₂
Fig. ₃ shows an Al powder porous sintered body containing O3), in which Ta wire or Al wire is embedded in the sintered body as an anode lead wire. As the cathode, manganese dioxide (Mn
O ₂ ) Solid electrolyte and carbon + Ag cathode (cathode conductive layer)
And are illustrated.

In the case of a tantalum electrolytic capacitor, for example,
Tantalum powder having a particle size of 10 to 20 μm is compression-molded by a press and sintered to form a porous sintered body. This is anodized to obtain an oxide film. Since the surface area of this porous sintered body is extremely large, a large capacitance can be obtained. Then, a manganese compound such as manganese sulfate is heat-treated on the oxide film to use manganese oxide as a cathode, or the porous sintered body is immersed in an aqueous solution of manganese nitrate and pyrolyzed in an electric furnace. The process of making manganese dioxide is repeated to grow a manganese dioxide layer to form a sufficient electrolyte layer. Manganese dioxide is filled and coated in every corner of the pores of the porous sintered body. Alternatively, a capacitor can be prepared by using a conductive polymer compound as a cathode. A carbon layer is attached on top of this to reduce the conductive resistance, and then a silver paste is applied and an external lead wire (not shown) is soldered.
Conductive polymer formed after manganese dioxide formation 2
It can also have a heavy structure. The use of liquid electrolytes is also possible. This also applies to aluminum electrolytic capacitors.

However, tantalum electrolytic capacitors have a problem that tantalum is expensive. On the other hand, in the aluminum electrolytic capacitor, although aluminum is inexpensive, as shown in a partially enlarged view of the aluminum electrolytic capacitor in FIG. 3, when the capacitor is formed, oxygen defects occur in the aluminum oxide film, There is a problem in that the life is short because the leakage current generated by the semiconductor formation is large, and aluminum has a dielectric constant per unit area smaller than tantalum, and it is difficult to make a small-sized and large-capacity capacitor.

In order to solve the above conventional problems,
Many attempts have been made to develop a titanium electrolytic capacitor in which metallic titanium is used for the anode and an oxide film such as titanium oxide or composite titanium oxide is formed thereon. In other words, titanium is cheaper than tantalum, and titanium oxide has a much higher dielectric constant than tantalum oxide or aluminum oxide. It has great potential for development.

FIG. 1 shows a conceptual diagram of a titanium electrolytic capacitor (in FIG. 3, Ta powder and Al powder are replaced with Ti powder and T
a, Al oxide to Ti oxide and Ta, Al wire to T
i line respectively). A titanium oxide film or a composite titanium oxide film 2 is formed on the titanium substrate anode 1 to form an anode. A titanium wire is attached to the anode. Similar to FIG. 3, MnO ₂ is shown as an example of the solid electrolyte 3, a carbon layer 4 is attached thereon to reduce the conductive resistance, and a silver paste 5 is further applied to solder an external lead wire (not shown). The structure is illustrated. The finished element is case 6 to protect it from external moisture and contamination.
Is enclosed in. In order to develop such a titanium electrolytic capacitor, various attempts have been made with a focus on improving the dielectric constant of a titanium oxide film or a composite titanium oxide film as a dielectric film.

For example, in Japanese Unexamined Patent Publication (Kokai) No. Hei 5-121275, anodization of a titanium metal plate is completed before an electric current starts to rise during anodization at a constant voltage in an aqueous solution containing an electrolyte, and then Anodization was performed at a temperature of 60 ° C. or less using an electrolytic solution composed of an organic solvent having a water content of 60% by weight or less to form an oxide film on the titanium plate.
Heat treatment is performed at a temperature of 0 to 350 ° C., the obtained titanium is used as an anode, and a solid electrode (manganese dioxide or the like) or an electrolyte solution (ammonium phosphate 4% by weight-water 36% by weight) -ethylene glycol is used as a cathode on the oxide film. 60% by weight)
Disclosed is a method of manufacturing a titanium electrolytic capacitor by forming an electrode (platinum foil) via the electrode. A titanium plate having an oxide film obtained by anodic oxidation in an aqueous solution is anodized again using an electrolytic solution containing an organic solvent having a water content of 60% by weight or less to form an oxide film on titanium. And

In Japanese Unexamined Patent Publication No. 9-17684, a porous sintered body made of a metal containing titanium as a main component and a perovskite type complex oxide such as strontium titanate formed on the surface of the sintered body are disclosed. A dielectric film as a main component, an electrode made of a conductor or a semiconductor formed on the surface of the dielectric film, and a counter electrode (graphite layer, silver) which is electrically connected to the dielectric or semiconductor electrode and faces the sintered body. An electrode layer), wherein the conductor or semiconductor has a two-layer structure of a metal oxide such as manganese or nickel and a conductive polymer compound (polypyrrole). Since the conductor or semiconductor electrode is formed on the dielectric film, a large capacitance can be realized without increasing the size of the entire capacitor.

Further, in Japanese Patent Laid-Open No. 2000-77274, a porous sintered body made of a metal containing titanium as a main component is used as A ion (A is at least one of Ba, Sr or Pb) and B ion. (B is at least one of Zr and Ti) is heat-treated in an alkaline aqueous solution containing
ABO ₃ coating is formed on the surface of porous sintered body,
The porous sintered body on which the ABO ₃ coating is formed is converted into C ions (C
Is at least one of Ba or Sr) and heat-treated in an alkaline aqueous solution containing Pb ions to obtain conductive CPb.
A capacitor obtained by forming an O ₃ thin film as a counter electrode and then forming a graphite layer and a silver electrode layer and a method for manufacturing the same are disclosed. It is described to manufacture a capacitor that is small, has a large capacitance, and is easy to manufacture.

[0012]

SUMMARY OF THE INVENTION The above-mentioned conventional technique improves the dielectric constant of a dielectric film to produce a capacitor having a large capacitance. However, the titanium-based oxide film on the surface of metallic titanium prepared by the above-mentioned conventional technique has a high dielectric constant, but lacks its denseness and stability, and has a very large leakage current when used as a capacitor, and is put to practical use. Was still insufficient.

Therefore, an object of the present invention is to develop a method for forming a stable composite titanium oxide film having a large dielectric constant on the surface of a metal titanium substrate, and by using such an oxide film, a small size, a large capacity and An object of the present invention is to provide a titanium electrolytic capacitor having a small leakage current and a long life.

[0014]

In view of the above problems of the prior art, the inventors of the present invention have earnestly studied a method of forming a composite titanium oxide film on the surface of a metallic titanium substrate, and as a result, first of all, titanium was first formed on the surface of the metallic titanium substrate. The present inventors have found that a stable composite titanium oxide film having a high dielectric constant and a small leakage current can be formed by forming a film of a potassium salt of an acid and then treating it with an aqueous solution containing metal ions, and completed the present invention.

That is, in the method for forming a composite titanium oxide film of the present invention, after forming a film of a potassium salt of titanic acid on the surface of a metallic titanium substrate, at least one selected from lithium, magnesium, calcium, strontium, barium and lanthanum. Coating of a composite titanium oxide containing titanium and at least one metal ion selected from lithium, magnesium, calcium, strontium, barium and lanthanum on the surface of the metal titanium substrate by treating with an aqueous solution containing one metal ion. Is formed. Further, the composite titanium oxide is preferably at least one selected from barium titanate, strontium titanate and barium strontium titanate. Further, the composite titanium oxide film formed by the method of the present invention has a thickness of 5 μm or less and an average particle size of 5 to 1
It is characterized by being composed of particles of 000 nm.

Another aspect of the present invention provides a titanium electrolytic capacitor characterized by using a metal titanium substrate having a composite titanium oxide film produced by the above method as an anode.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION The composite titanium oxide film formed by the method of the present invention contains titanium and at least one metal ion selected from lithium, magnesium, calcium, strontium, barium and lanthanum, and has the following general formula: The compound represented by (1) is included. (M _{2 / x} O) · y (TiO ₂ ) (1) (where M is at least one metal ion selected from lithium, magnesium, calcium, strontium, barium and lanthanum, and x is a metal ion M Which is equal to the valence of y and y is 0.1 to 10), and among these, strontium and barium are preferable as the metal ion M. Specifically, they are barium titanate, strontium titanate, and barium strontium titanate, and are represented by the following general formula (2) Ba _n Sr _1-n TiO ₃ (2) (n is a real number represented by 0 ≦ n ≦ 1). ). In the general formula (2), (Ba + Sr) and Ti
Does not necessarily have to be 1, and 0.90 ≦ (B
It may be a + Sr) /Ti≦1.10. Further, the composite titanium oxide film of the present invention comprises, in addition to the composite titanium oxide represented by the general formula (1), a titanium oxide film between the surface of the metallic titanium substrate and the composite titanium oxide, or the composite titanium oxide film. May be present inside or on the surface of.

The metal titanium substrate used in the present invention is a metal titanium plate or a porous sintered body obtained by sintering metal titanium powder. Usually, when a titanium electrolytic capacitor is prepared, the latter metal is used as an anode. A porous sintered body obtained by sintering titanium powder is used. When making this porous sintered body, metal titanium powder may be used as a raw material,
Embrittled titanium hydride powder can also be used.
When titanium hydride powder is used, it is dehydrogenated by heat treatment under reduced pressure before, during or after sintering.

The metal titanium powder used when preparing the above-mentioned porous sintered body of metal titanium powder usually has a particle size of 0.1.
˜150 μm, preferably 0.1 to 50 μm, and the average particle size is 0.5 to 100 μm, preferably 0.5 to 10 μm. A known method can be adopted as a method for producing the porous sintered body. For example, first, titanium powder is pressure-molded by a press molding machine. At this time, if necessary, a binder such as styrene resin, acrylic resin, camphor, or a mixture of these binders dissolved in an organic solvent such as toluene or xylene is mixed with the titanium powder. The thus-formed product is fired in vacuum at 600 to 1100 ° C.
When the thus-prepared porous sintered body is used for a capacitor, a titanium wire is attached during pressure molding or after firing. In order to increase the capacitance when the capacitor is produced, it is necessary to make the porous sintered body so that the specific surface area is as large as possible. Specifically, the sintered density of the porous sintered body (ratio of the density of the porous sintered body to the true specific gravity of metallic titanium) is preferably 30 to 70%. The specific surface area decreases as the sintered density increases. If the sintering density is too low, the specific surface area will be large, but the strength of the porous sintered body will be small and it will not be possible to use it as a capacitor.

The method for forming the composite titanium oxide film of the present invention will be specifically described below. First, a film of potassium titanate salt is formed on the surface of the metal titanium substrate. Specifically, the film of potassium titanate salt is formed by treating the metal titanium substrate with an aqueous solution of potassium hydroxide. Here, the potassium salt of titanic acid is represented by a compound having a composition represented by KTiO ₂ (OH) or K ₂ O · n (TiO ₂ ) (where n is an integer of 2 to 8). It is potassium titanate.

The thickness of the composite titanium oxide film obtained by the treatment conditions in the first treatment step and the particle size of the particles constituting the film are influenced, and first, the concentration of the alkali metal aqueous solution is
It is 0.1 to 100 M, preferably 3 to 20 M. The treatment temperature is 200 ° C or lower, preferably 60 to 150 ° C. The treatment time varies depending on the concentration of the alkali metal aqueous solution and the treatment temperature, but is usually 10 minutes or longer, preferably 30 minutes to 10 hours.

During the treatment with the alkali metal aqueous solution as described above, a surface for removing the dirt adhering to the surface of the metal titanium substrate and also improving the specific surface area of the substrate surface before the treatment operation. It is desirable to perform processing. Specifically, it is treated with an acid such as hydrogen fluoride or another oxidizing agent.

Since the film of potassium titanate formed on the surface of the metallic titanium substrate in this way is in the form of dense fibers or whiskers, it is preferable for forming the composite titanium oxide film of the present invention. The potassium titanate coating has a thickness of 0.1 to 10 μm.

After forming a film of potassium titanate on the surface of the metal titanium substrate, the metal titanium substrate having the film formed thereon is dried if necessary. Drying at this time is from room temperature to 2
It is 00 ° C.

Then, the metal titanium substrate having the alkali metal titanate coating film formed on the surface is placed in an aqueous solution containing at least one metal ion selected from lithium, magnesium, calcium, strontium, barium and lanthanum. Turn on and process. As these metal compounds, hydroxides, nitrates, sulfates, chlorides and the like are used. Specific compounds include lithium hydroxide, lithium chloride, lithium nitrate, magnesium hydroxide, magnesium chloride, magnesium nitrate, calcium hydroxide, calcium nitrate, calcium sulfate, calcium chloride, strontium hydroxide, strontium nitrate, strontium sulfate, Strontium chloride, barium hydroxide, barium nitrate, barium sulfate, barium chloride, lanthanum hydroxide,
Lanthanum nitrate, lanthanum sulfide, and lanthanum chloride, among which lithium hydroxide, strontium hydroxide, barium hydroxide, strontium chloride, and barium chloride. These compounds are used alone or in combination of two or more.

The thickness of the obtained composite titanium oxide film and the particle size of the particles constituting the film also depend on the processing conditions in the second processing step. First, lithium, magnesium, calcium, strontium, barium and lanthanum are used. The concentration of the metal ion in the aqueous solution containing at least one metal ion selected from 0.1 to 100M,
It is preferably 0.3 to 10 M. The processing temperature is 20
It is 0 ° C. or lower, preferably 40 to 150 ° C. The treatment time varies depending on the concentration of the alkali metal aqueous solution and the treatment temperature, but is usually 1 minute or longer, preferably 30 minutes to 10 hours. Further, the alkali metal such as potassium hydroxide or sodium hydroxide used in the first treatment step may be allowed to coexist in the above metal ion aqueous solution.

In the method of the present invention, the potassium titanate coating film formed first is treated with another metal ion as described above, whereby the potassium ion is replaced with another metal. A composite titanium oxide film is formed. The preferred method of forming the composite titanium oxide film of the present invention is as follows: 1) First, the metallic titanium substrate is treated with an aqueous potassium hydroxide solution at 50 to 100 ° C. (first treatment step), and then dried at room temperature, A fibrous potassium titanate coating is formed on the surface of the metallic titanium substrate. 2) The metal titanium substrate on which the potassium titanate coating is formed is treated with an aqueous barium hydroxide or strontium hydroxide (or mixture thereof) solution at 50 to 100 ° C. (second treatment step). This is dried to obtain barium titanate, strontium titanate or barium strontium titanate coating, which is the composite titanium oxide coating of the present invention.

The thickness of the composite titanium oxide film formed by the method of the present invention is 5 μm or less, and the thickness of the film is 5 μm.
When it is larger than m, the capacitance of the dielectric becomes low when it is used as a capacitor. When the thickness of the oxide film is extremely small, the electrostatic capacity is high, but the strength of the film is weak, and the uniformity of the film is poor, and leakage current may occur when the capacitor is formed. Therefore, the thickness of the composite titanium oxide film is preferably 0.5 to 5 μm.

The greatest feature of this composite titanium oxide coating is that the coating has a fine average particle size of 5 to 1000 nm, preferably 10 to 50 nm, and is represented by the above general formula (1) or general formula (2). It is composed of particles of titanium oxide. An SEM photograph of the surface of the composite titanium oxide film of the present invention is shown in FIG. In the present invention, the particles forming the composite titanium oxide film are secondary particles formed by aggregating primary particles of several nm, and in the SEM photograph of FIG. One section. By forming a coating by superposing a plurality of layers of fine particles as described above, a dense coating is formed, and when a titanium electrolytic capacitor is formed, leakage current due to cracking or peeling of the oxide coating is small, and the capacity is high. A stable capacitor is formed. On the contrary, when the average particle size of the particles forming the coating film is larger than 1 μm, the number of particles to be laminated is extremely small and a gap is generated between the particles, and as a result, the leakage current is extremely increased when the capacitor is formed.

A solid electrolytic capacitor can be prepared by using the metal titanium substrate having the composite titanium oxide film obtained as described above as an anode. At this time, as the cathode,
As with tantalum electrolytic capacitors, manganese compounds such as manganese sulfate are heat-treated on the oxide film to make manganese oxide the cathode, or the porous sintered body is immersed in an aqueous solution of manganese nitrate and heated in an electric furnace. The process of decomposing to manganese dioxide can be repeated to grow a manganese dioxide layer, or a capacitor can be prepared using a conductive polymer compound as a cathode. A carbon layer is attached on top of this to reduce the conductive resistance, and then a silver paste is applied and an external lead wire (not shown) is soldered. It is also possible to form a double structure in which a conductive polymer is formed after the formation of manganese dioxide. The use of liquid electrolytes is also possible. Any known cathode structure as exemplified above in the prior art can be employed, including solid electrolytes and liquid electrolytes.

[0031]

EXAMPLES The effects of the present invention will be specifically described below with reference to Examples and Comparative Examples. Here, the evaluation of the insulating property of the formed oxide film and the measurement of the electric capacity were carried out by the following methods. 1) Evaluation of insulating property (measurement of leak current) A test sample of a titanium plate was masked with an insulating tape to leave an electrode area of about 1 cm ² . A counter electrode is formed on this electrode surface with a conductive paste, a conductive tape or the like to form a negative electrode. Applied voltage 5V, 10V, 15V, 20V, 30V
The current value (leakage current) flowing between the positive electrode and the counter electrode was measured 1 minute after each of the above. At this time, since the measured value of the leakage current changes depending on the order in which the voltage is applied, the measurement was performed from the low voltage side. 2) Capacitance measurement Using a test sample with leak current measured as the positive electrode and a titanium plate (20 mm x 100 mm) as the counter electrode, directly measure the electric capacity of the membrane with an LCR (inductance, capacitance, resistance) meter under the following conditions: did.・ Measurement conditions Electrolyte: 150g / L ammonium adipate aqueous solution Frequency: 120Hz Amplitude: 1V (Note) In this measurement method, the capacitance of the counter electrode surface is added in series, but the capacitance of the Ti plate is sufficient compared to the test electrode. It can be ignored because it is large (1 / total volume = 1 / sample volume + 1 /
It is expressed in Ti plate capacity, so the Ti plate capacity is negligible if it is sufficiently large compared to the sample capacity).

Example 1 A metallic titanium plate was placed in an 8 M aqueous potassium hydroxide solution and treated at 80 ° C. for 240 minutes (first treatment step). This titanium metal plate was dried at room temperature and then charged in a 1M aqueous barium hydroxide solution.
Treatment was performed at 0 ° C. for 360 minutes (second treatment step). The treated titanium metal plate was dried at room temperature to form a barium titanate coating film on the titanium metal surface. With respect to this oxide film, the thickness of the film, the particle size of the particles, the evaluation of the insulating property and the electric capacity were measured. The obtained results are shown in Table 1. A SEM photograph of the obtained composite titanium oxide film is shown in FIG. 2, and an X-ray spectrum of the composite oxide film is shown in FIG.

Example 2 A titanium metal plate was placed in a 10 M aqueous potassium hydroxide solution and treated at 80 ° C. for 240 minutes (first treatment step). This metal titanium plate was dried at room temperature, then charged into a 0.5 M strontium hydroxide aqueous solution and treated at 80 ° C. for 420 minutes (second treatment step). The treated metal titanium plate was dried at room temperature to form a strontium titanate coating film on the metal titanium surface. With respect to this oxide film, the thickness of the film, the particle size of the particles, the evaluation of the insulating property and the electric capacity were measured. The obtained results are shown in Table 1.

Example 3 A metallic titanium plate was placed in an 8 M aqueous potassium hydroxide solution and treated at 80 ° C. for 240 minutes (first treatment step). This metal titanium plate was dried at room temperature, and then charged into a mixed aqueous solution of barium hydroxide / strontium hydroxide having a total ion concentration of 1M containing equimolar amounts of barium ions and strontium ions.
The treatment was performed at 360 ° C. for 360 minutes (second treatment step). The treated titanium metal plate was dried at room temperature to form a barium titanate coating film on the titanium metal surface. With respect to this oxide film, the thickness of the film, the particle size of the particles, the evaluation of the insulating property and the electric capacity were measured. The obtained results are shown in Table 1.

Comparative Example 1 A 10 M potassium hydroxide aqueous solution and a 0.5 M barium hydroxide aqueous solution were mixed to prepare a mixed water of potassium hydroxide / barium hydroxide. A metallic titanium plate was charged into this mixed aqueous solution and treated at 80 ° C. for 240 minutes. The treated titanium metal plate was dried at room temperature to form a barium titanate coating film on the titanium metal surface. With respect to this oxide film, the thickness of the film, the particle size of the particles, the evaluation of the insulating property and the electric capacity were measured. The obtained results are shown in Table 1.

[0036]

[Table 1]

From Table 1, it can be seen that the composite titanium oxide film of the present invention is stable, and by using this, a titanium electrolytic capacitor having extremely small leakage current and large electric capacity can be obtained.

[0038]

The present invention provides a method for forming a stable and dense composite titanium oxide film having a large dielectric constant on the surface of a metallic titanium substrate, and by using the oxide film formed by such a method, the device is small in size, has a large capacity, and leaks. We have succeeded in developing a titanium electrolytic capacitor with low current and long life.

[Brief description of drawings]

FIG. 1 shows a conceptual diagram of a titanium electrolytic capacitor.

FIG. 2 is an SEM photograph of a composite titanium oxide film formed by the method of the present invention.

FIG. 3 is a schematic view showing a tantalum electrolytic capacitor and an aluminum electrolytic capacitor together.

FIG. 4 is an X-ray spectrum of the composite titanium oxide film of the present invention.

[Explanation of symbols]

1 Titanium-based anode 2 Titanium oxide film 3 Solid electrolyte 4 carbon layer 5 silver paste 6 cases

Front page continuation (51) Int.Cl. ⁷ Identification code FI theme code (reference) C23C 22/83 H01G 9/00 501 H01G 9/00 9/24 B 9/07 9/05 H

Claims (4)

[Claims] 1. Forming a film of a potassium salt of titanic acid on the surface of a metallic titanium substrate, and then treating with an aqueous solution containing at least one metal ion selected from lithium, magnesium, calcium, strontium, barium and lanthanum. To the surface of the metallic titanium substrate with titanium and lithium, magnesium, calcium, strontium,
A method for forming a composite titanium oxide film on a metal titanium substrate, which comprises forming a film of a composite titanium oxide containing at least one metal ion selected from barium and lanthanum. 2. The method for forming a composite titanium oxide film according to claim 1, wherein the composite oxide is at least one selected from barium titanate, strontium titanate and barium strontium titanate. 3. The composite titanium oxide film has a thickness of 5 μm.
The method for forming a composite titanium oxide film according to claim 1, wherein the composite titanium oxide film is composed of particles having the following average particle diameter of 5 to 1000 nm. 4. A titanium electrolytic capacitor using the metal titanium substrate having the composite titanium oxide film according to claim 1 as an anode.