JPS6253594B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for anodizing tantalum metal. Tantalum solid electrolytic capacitors and tantalum thin film capacitors are capacitors that use as a dielectric an oxide film formed by anodizing the tantalum metal surface in an electrolyte. An electrolytic solution with water as a solute and a solvent is used. Anodic oxidation is performed by placing tantalum metal facing a counter electrode made of a conductive material such as metal or carbon, immersing both in an electrolytic solution, and applying current between the tantalum metal and the counter electrode using the tantalum metal as an anode.

For example, the voltage between the electrodes increases with time due to so-called constant current formation in which current is applied at a constant current. After a desired voltage is reached, the current is reduced by anodic oxidation, so-called constant voltage anodization, in which the voltage is maintained at this voltage, and the generation of the oxide film tends to stop. When the tantalum metal with the anodic oxide film formed is immersed in an electrolytic solution with as good conductivity as possible, facing the cathode,
A capacitor action is exhibited between these two poles. That is, a capacitor is constructed in which the oxide film is used as a dielectric, and the tantalum metal and electrolyte on both sides thereof are used as electrode bodies. The product of the formation voltage V mentioned earlier and the capacitance C per unit area of the oxide film dielectric is called the CV value (μF・V/cm ² ), and it is approximately the same regardless of the formation voltage. constant. In the case of an ordinary electrolytic solution for anodizing using water as a solvent, the CV value is 12.5 μF·V/cm ² after constant voltage formation at 25° C. for 2 hours.

On the other hand, the operating voltage of a capacitor is regulated by the formation voltage, and in so-called wet electrolytic capacitors in which the electrolyte is the counter electrode, the voltage is 60 to 70% of the formation voltage.
For solid electrolytic capacitors with manganese dioxide as the counter electrode, an operating voltage of 25 to 35% is selected, and in thin film capacitors with a metal thin film as the counter electrode, an even lower operating voltage is selected to avoid dielectric breakdown of the oxide film.

Traditionally, efforts to save tantalum metal and downsize capacitors have focused on increasing the surface area by making the tantalum powder used for sintered tantalum electrodes finer and lowering the sintering temperature, and by improving the baking conditions for manganese dioxide in solid capacitors. Progress has been made by improving the operating voltage by avoiding insulation deterioration of the film.

The present invention aims to reduce the amount of tantalum material and downsize the capacitor by increasing the CV value of the oxide film. In the present invention
The increase in the CV value is characterized by using a phosphoric acid solution with low water content as the electrolyte for anodizing. The preferred concentration range of the phosphoric acid solution in the anodizing method of the present invention is one consisting of 90 to 99% H ₃ PO ₄ and the balance water; at H ₃ PO ₄ concentrations below this, almost no increase in the CV value is observed. First, if the concentration is higher than this, that is, if the water content is further reduced, H ₃ PO ₄ will shift to a dehydrated component, and an insulating oxide film will not be formed in such a composition range. In the preferred range
The CV value can be increased by up to about 8% compared to a normal dilute phosphoric acid aqueous electrolyte, and tan δ and leakage current do not deteriorate.

Next, the method of the present invention will be explained in more detail based on examples.

Commercially available phosphoric acid consists of approximately 85% H ₃ PO ₄ concentration and the balance is water, and a higher concentration phosphoric acid solution can be prepared by adding phosphorus pentoxide to this and reacting with water. can. A tantalum foil with a sample area of 8 cm ² was chemically polished with a mixed acid of sulfuric acid, nitric acid, and hydrofluoric acid to obtain a clean surface, and then electrolytes of various concentrations prepared by the preparation method described above were used. hand,
Constant current formation was performed at 25°C with a current density of 2 mA/cm ² to 50 V with respect to the counter electrode, and then constant voltage formation was performed at this voltage for 2 hours. After this, capacitance and loss were measured using a capacitance bridge at 120 Hz in a 23% sulfuric acid aqueous solution with a silver plate as the counter electrode, and a positive voltage of 30 V was applied to the counter electrode for 3 minutes. The leakage current was measured. The figure shows H ₃ PO ₄ in the electrolyte for anodizing.
This figure shows the relationship between the concentration and the electrical properties of the above-mentioned anodic oxidation sample.

As is clear from the figure, according to the anodic oxidation method of the present invention, the leakage current and loss tan δ are at the same level as in the case of the conventional dilute phosphoric acid aqueous electrolyte, and an anodic oxide film with a larger CV value can be obtained. Can be done.
Therefore, it is possible to downsize the tantalum capacitor and save tantalum metal.

[Brief explanation of the drawing]

The figure shows the relationship between the electrical properties of the tantalum anodic oxide film and the H ₃ PO ₄ concentration of the electrolyte used for anodization.

Claims (1)

[Claims] 1. A method for anodizing tantalum metal, characterized by using an electrolytic solution consisting of 90 to 99% by weight of H ₃ PO ₄ and the balance water.