JPS5833688B2 - Manufacturing method of electrolytic capacitor - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention is applied to a so-called sintered electrolytic capacitor in which a metal powder having a valve action is sintered with an oxide film of tantalum, titanium, niobium, etc. The present invention relates to a method of manufacturing an electrolytic capacitor having a structure in which the thickness and composition of the internal and external oxide films are different.

In conventional electrolytic capacitors, when forming an oxide film as a dielectric, electrochemical anodic oxidation is performed using a non-metallic oxygen acid such as phosphoric acid, sulfuric acid, or nitric acid as an electrolyte. In order to make the thickness of the external film uniform, constant voltage aging was performed for several hours.

Thereafter, a solid or liquid electrolyte layer such as manganese dioxide or sulfuric acid is provided on this film, and a conductive layer such as graphite or silver paste is further provided on top of the solid or liquid electrolyte layer.

When a direct current voltage is applied to a structure like this, the cathode layer relaxes the instantaneous voltage, and the oxide film blocks the passage of current, but the leakage current is much larger than that of a structure immediately after formation, and the withstand voltage is also decreases significantly.

Therefore, in order to increase reliability, the coating must be made thicker, and the forming voltage is increased to 8 to 4 times the rated voltage.

Therefore, in order to obtain a given capacity, the surface area must be
In other words, a large-sized sintered body must be used.

However, the increase in leakage current and the drop in breakdown voltage are due to the thermal and chemical shocks that occur when manganese dioxide, lead dioxide, and other electrolytes are provided. , often near the surface of the sintered body itself.

Considering this phenomenon, for example, in the case of tantalum solid electrolytic capacitors, a semiconductor such as manganese dioxide is used as the cathode, but in the case of sintered bodies, as shown in Figure 1, A model can be considered in which an infinite number of resistors are connected in series with a capacitor in parallel.

It can be easily inferred from the figure that this series resistance is smaller near the surface of the sintered body and larger at the center.

Here, we actually connected various series resistors to a single 25V/4.7μF tantalum electrolytic capacitor, boosted the voltage at a rate of 5μ/sec, caused micro-destruction several times, and then measured the change in leakage current. Shown in Figure 2.

Furthermore, under the same conditions as above, the voltage at which the capacitor is completely destroyed and short-circuited is taken as the maximum breakdown voltage, and the voltage is shown in FIG. 3 for various series resistances.

The series resistance in the figure is the resistance per rated voltage, for example R820.5, and Q/V is z5Vxo, 5Q=1
This means that a 2.5ff resistor is connected in series.

As is clear from this figure, when a series resistor is included, the leakage current deteriorates less, and as the series resistor is increased, the maximum breakdown voltage (hereinafter simply referred to as breakdown voltage) also increases.

In other words, it is thought that when a breakdown occurs, the larger the series resistance is, the smaller the impact on the film is because it suppresses the power flowing in from the external circuit.

For this reason, in the case of a sintered capacitor, it is thought that the film deterioration near the surface is greater when a voltage is applied than in the inside.

The present invention focuses on this point and is a manufacturing method that aims to prevent the above-mentioned drawbacks by making the oxide film on the outside of the sintered body, that is, near the surface, thicker than that on the inside.

The thickness of the internal and external coatings of this sintered body itself can be controlled by a stepwise chemical conversion method.

That is, first, as a first chemical formation, an oxide film is formed on the inside and outside of the sintered body to a uniform thickness at a predetermined formation voltage for a long time.

Next, the second chemical formation to make the oxide film thicker on the outside than the inside of the sintered body is performed at a higher voltage and current density than the first formation voltage, and for a shorter time than the first formation time.

As the second chemical formation, it is desirable to use an electrolytic solution such as sodium aluminate or sodium stannate, which has low ion mobility in the electrode reaction.

Next, as an example of the present invention, a tantalum solid electrolytic capacitor will be described with reference to the drawings.

As a sample, tantalum powder was pressure-molded and 10-
Sintered at 2000°C for 30 minutes in a vacuum of 5 Hg or higher, external dimensions 6φ x 6.8mm, weight 1.6g.

Cylindrical pellets with a press density Dg of 9.0 were used.

Impurities in the sample greatly affect the chemical conversion coating, so
It was thoroughly washed with hydrofluoric acid, trichloride, etc.

First, the first chemical treatment was performed at a current density of 1 in a 0.02% H3PO4 bath.
Constant flow chemical formation was performed at mA/- to a chemical formation voltage of 95V.

After reaching the formation voltage, the voltage was kept constant for 2 hours to make the internal and external coatings of the sintered body uniform.

Thereafter, anodization was carried out in an electrolytic solution containing 210 M/l of aluminic acid at a high current density of 10 mA/- to various anodizing voltages (100 to 200 V).

In this case, the formation was terminated at the same time as each formation voltage was reached.

Next, manganese nitrate was thermally decomposed on the film at 200°C, manganese dioxide was attached, and graphite and silver paste were further provided as a conductive layer.

FIG. 4 shows an enlarged cross-sectional view of one hole in the porous sintered body of the electrolytic capacitor prototyped using the method of the present invention.

1 is a metal such as tantalum having a valve action, 2 is an oxide film of the metal, 3 is an electrolyte layer, and 4 is a conductive layer such as graphite or silver paste.

As is clear from the figure, the thickness of the oxide film 2 is thicker on the surface side of the sintered body, and thinner inside the sintered body, that is, deeper into the pores.

Note that in the figure, the holes are filled with an electrolyte layer 3.

Figure 5 shows the breakdown voltage when the first formation voltage is kept constant at 95V and the second formation voltage is varied from 120 to 200V. It can be seen that the results have improved.

Moreover, it is strong against pulses. Table 1 shows each formation voltage and capacitance value of the second formation.

In addition, in the table, Vf is the first formation voltage (internal formation voltage)
, and Vf' represents the second formation voltage (external formation voltage).

Since the chemical film formed by the second chemical formation is near the surface of the sintered body, the capacitance decreases little even if the chemical formation voltage is increased.

If the internal and external film thicknesses are made uniform, V f
When '/Vf is increased, the capacitance becomes a value multiplied by the reciprocal of the value of Vf'/Vf.

For example, Vf'/Vf= 2.0 (first chemical formation is 1oov
In the case of the present invention, when the second conversion command is 200V), it is 44.8
While it is 0 μF, it is 23.75 μF in the conventional method.

That is, it has the effect of reducing the capacitance reduction and significantly increasing the breakdown voltage.

As a result, the formation voltage ratio to the rated voltage can be lowered.

The rated voltage is usually designed to be 1/2 of the breakdown voltage.

In other words, if the breakdown voltage is 70V, the rated voltage is 3
Set it to 5V.

In the present invention, when Vf'/Vf = 2.0, the breakdown voltage is 90V and can be increased to the rated voltage of 45V.

If the capacitance and rated voltage are the same, the present invention allows the formation voltage to be about 35/45=77.8φ compared to the conventional method, making it a regenerative type.

The present invention can be applied to any metal having valve action, such as tantalum, titanium, siliconium, niobium, etc., and its effects are extremely significant.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing the equivalent circuit of a sintered electrolytic capacitor, Fig. 2 is a diagram showing the breakdown voltage when voltage is applied to the capacitor by changing various series resistances,
Fig. 3 is a diagram showing the series resistance dependence of LC change due to breakdown, Fig. 4 is a schematic cross-sectional view of an electrolytic capacitor according to the present invention, and Fig. 5 is a diagram showing the internal formation voltage (Vf) and external formation voltage (Vf). ') shows a diagram showing the relationship between breakdown voltage and ratio. DESCRIPTION OF SYMBOLS 1... Metal having valve action, 2... Oxide film, 3... Electrolyte layer, 4... Conductive layer.

Claims (1)

[Claims] 1. A method for manufacturing an electrolytic capacitor comprising a step of creating a porous sintered body of a valve metal, and a step of anodizing the sintered body to form an oxide film, wherein the anodizing step A first step of forming an oxide film with a substantially uniform thickness on the inside and outside of the body pore, and after the first step, the thickness of the oxide film on the outside of the pore is made larger than the inside of the pore. A method for manufacturing an electrolytic capacitor, comprising a second step of forming.