JPS58157126A - Solid electrolytic condenser - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to solid electrolytic capacitors, and particularly to improving leakage current characteristics by thickening the oxide layer on the surface layer of a capacitor element.

In general, this type of solid electrolytic capacitor is shown in Figures 1 to 2, for example.
As shown in the figure, a capacitor is made by press-molding metal powder such as tantalum, niobium, aluminum, etc. into a cylindrical shape and sintering it. At the same time, the first external lead member C is welded to the electrode lead-out layer formed on the circumferential surface of the capacitor element A via an oxide layer E, a semiconductor layer F, and a graphite layer G. Solder to H
The main portion including the capacitor (I) and capacitor element (A) is covered with a resin material K.

At C filter, capacitor element A (7) fi conversion ME is capacitor element) A is immersed in a chemical solution such as phosphoric acid aqueous solution, capacitor element A) A is positive, and a specified direct current is set so that the chemical failure is negative. Since the oxide layer E is formed by applying a voltage for a long time, an oxide layer E having a substantially uniform thickness is formed in the surface layer and the deep layer of the capacitor element A. Since the semiconductor layer F is formed on the oxide layer, a capacitor with excellent breakdown voltage characteristics can be obtained due to the effect of supplying oxygen to the oxide layer E.

However, this capacitor can be equivalently represented as a series circuit consisting of the capacitor component formed by the oxide layer E and the resistance components formed by the semiconductor layer F and graphite layer G, as shown in FIG. , capacitor element) A
Because it has an extremely fine porous structure, the path to the electrode lead layer H of graphite NG, which serves as one electrode of the capacitor, is formed by the oxide layer E in the deep layer of the capacitor. If the length is long, the series resistance will increase accordingly, and in particular, if the graphite layer G is not sufficiently formed, it will further increase. On the other hand, in the surface layer, the path of the graphite layer G to the il & extraction layer HK is about the thickness of the n layer, or approximately that, so the series resistance due to the graphite layer G is much smaller than in the deep layer. Become.

Therefore, if a DC voltage is not applied to the capacitor element A through the anode lead B and the electrode lead layer H, the capacitor part in the surface layer where the series resistance is small rather than the deep part where the series resistance is large. , capacitor element) A
If a defect exists in the KFi, deterioration or destruction is more likely to occur in the surface layer where the voltage is shared rather than in the deep layer.
Leakage current characteristics and breakdown voltage characteristics are also likely to be impaired due to this t'L.

Therefore, the present inventors believe that if a thicker oxide layer can be formed on the surface layer of the capacitor element than in the deeper layer, the voltage sharing per unit film thickness can be reduced, and even if there are defects on the surface layer, deterioration will occur. After thorough investigation, we found that the purpose could be achieved by using a basic solution as the chemical conversion solution.

That is, focusing on the fact that basic solutions as chemical conversion liquids are rich in hydroxide ions (OH), which play a major role in the formation of oxidized layers, we investigated the relationship between the formation time and the oxidation layer when various basic solutions were used as chemical conversion liquids. After considering the relationship between the thickness of the produced film,
The results shown in FIG. 4 were obtained. In addition, the capacitor element FH tantalum powder was pressure-formed into a cylindrical shape of 3.5φ x 4 mm and sintered, and the chemical liquid had a concentration of 0.01.
Molar ammonium carbonate solution ((NH4) t COs
2H snow 0), ammonium oxalate solution ((NH4)t
・○-3BtOs -8HtO), a sodium aluminate solution (1JaAlot), and a phosphoric acid aqueous solution (reference) were used, and the current density was set to 40IIllA/9.

According to the same figure, when ammonium carbonate solution, #111m ammonium solution, and sodium aluminate solution were used as chemical conversion liquids, the formation rate of the oxide layer (TILtOs) was fast relative to the formation time, reaching about 1150A in 10 to 20 minutes. On the other hand, it is shown that a phosphoric acid aqueous solution (acidic solution) requires as much as 60 minutes of formation time to obtain the same film thickness. When a capacitor element that had been chemically treated was divided into two halves, the chemical color in the surface layer was almost the same for all chemical liquids, and in the deep layer, it was almost the same in the case where a basic solution was used. While no color formation was observed in the phosphoric acid aqueous solution, the color formation was the same in both the surface and deep layers.In addition, when using a basic solution, increasing the current density and formation voltage during formation It was also confirmed that the formation of an oxidized layer on the surface layer of the capacitor element could be done more quickly and intensively. From this point of view, by using the basic solution as a chemical liquid, it was possible to form an oxide layer on the surface layer of the capacitor element. It can be seen that a thicker oxide layer can be selectively formed in the deeper portions than in the deeper portions. Therefore, we investigated how the selective thickening of the surface layer affects capacitor characteristics, such as leakage current characteristics, and the results shown in FIG. 5 were obtained.

According to the figure, the thickness ratio of the oxide layer to the deep layer in the surface layer (thickness of the oxide layer in the surface layer/each oxidation in the deep layer) is shown. It is presumed that this reduces the voltage share per unit film thickness and suppresses deterioration or destruction.

The present invention has been realized based on these facts,
In a capacitor element formed of a metal powder having a valve action and from which a metal member having a valve action is derived as an anode IJ-v, an oxide layer and a semiconductor layer are formed on the surface layer of the capacitor element. The film thickness of the oxide layer in the deep part is set to be at least 1.5 times the film thickness of the oxide layer in the deep part.

One embodiment of the present invention will be described with reference to FIGS. 6 and 7. Reference numeral 1 denotes a capacitor element made of metal powder having a valve action. It is formed by pressure forming into a columnar shape and sintering it. From this capacitor element 1, a metal member having a valve function, such as a metal wire, is connected to the anode lead 2. It is derived as follows. In addition, the anode lead 2 can be planted at the center of the capacitor element 1 and led out prior to pressure forming of the capacitor element 1, or it can be welded to the surface of the capacitor element 1 and led out. In the deep layer 1a, there is a silica layer 3. However, on the surface layer 1b, oxide layers 3 and 3 are formed, each having a thickness 1.5 times or more that of the oxide layer 3, and on the oxide layers 3+ and 3m, a semiconductor layer 4. A graphite layer 5 is formed. Further, on the graphite layer on the curved surface of the capacitor element 1, an electrode lead layer 6 is formed of a conductive material such as silver paste. On the other hand, an L-shaped first external lead member 7, for example, is welded to the lead-out portion of the anode lead 2 from the capacitor element 7 so as to intersect with the bent portion 7a. A second external lead member 8 extending in the same direction as this first external lead member 7
has one end connected to the electrode lead layer 6 of the capacitor element 1.
It is soldered to the The main portion including the capacitor element 1 is covered with a resin material 9. still,
In addition to the molding method, the exterior packaging can also be done by dipping, powder coating, etc. Depending on the application, etc. Second
The oxide layer 3 has a thickness ratio of 15 times or more to the deep layer 1a.
．． As a result, even if the voltage sharing is large, the incidence of leakage current defects caused by deterioration or destruction can be significantly reduced, and the quality of the capacitor can be improved.

In addition, since the deterioration factors such as the leakage current characteristics of the capacitor are almost concentrated in the surface layer 1b of the capacitor element 1, the oxidized layer 3. The film thickness can be further increased. For this purpose, the capacitance can be increased. For example, if the capacitance is kept constant, the amount of dispersible metal powder used can be reduced in proportion to the increase in capacitance, making it possible not only to downsize the capacitor element but also to reduce costs.

Next, specific examples will be described.

Example 1 Tantalum powder was press-molded into a cylindrical shape of 3.5φ×4m and sintered. A capacitor element is immersed in a phosphorus WI permanent solution with a concentration of 0.1 capacity and a pH of 2.48, and the 05φ scale current density derived from the capacitor element is determined by the unit weight of the capacitor element (lχ oxide layer (Ta
gOs) was obtained. Next, this capacitor element was boiled and washed with pure water, dried, and then immersed in an ammonium monotate solution having a concentration of 1 volume and a pH of 8.45, and a DC voltage of 210 V was applied using a voltmeter.

The current density was set at 90 m tv'q. and,
After the terminal voltage of the capacitor element reached 210 V K, chemical conversion treatment was performed for another 5 minutes, and 330 V was applied only to the surface layer.
An 0X oxide layer (Ta, O,) was formed. Incidentally, the chemical conversion treatment time for the crazy body was set to 15 minutes. Hereinafter, a tantalum solid electrolyte starch f1 was prepared using a conventional method.

Next, this capacitor is heated to a temperature of 650. relative humidity is 95%
Left unloaded in an atmosphere of 500 hours, 100
After 0 hours, the battery was charged at 46V for 3 minutes, and the electrostatic capacity and leakage current were measured, and the results shown in the table below were obtained.

In addition, for the conventional product, the capacitor element is immersed in a phosphoric acid aqueous solution with a concentration of 0.1% by volume and a pH of 2.48.
The chemical conversion treatment was performed by applying a DC voltage of V, and the leakage current shows the initial value.

As is clear from the table above, the product of the present invention can significantly improve the failure rate of leakage current and leakage current during no-load humidity resistance compared to the conventional Content 1α product.

In addition, we applied the rated voltage (35V) to these capacitors, maintained the ambient temperature at 125°C, and measured the leakage current failure rate after 500 hours, which was 5% for conventional products. , the invention product Fio%.

The oxidation layer on the surface of the capacitor element is 4000
It was confirmed that the limit is about χ, and it is difficult to form a layer larger than that due to the phenomenon that the oxide layer is destroyed by the formation voltage. Therefore, in this example, the thickness ratio of the oxide layer to the deep layer in the surface layer is u a H''-5
twice ('ooo/"O")".

Example 2 In Example 1, a sodium aluminate solution with a concentration of 0.1% by volume and a pH of 11.05 was used in place of the ammonium borate solution, and after the terminal voltage of the capacitor element reached 210V, a further 12 The defectiveness rate was 0%.

Example 3 In Example 1, an ammonium carbonate solution with a concentration of 0.1 volume and a PK of 9.17 was used instead of the tlIilliII ammonium solution, a DC voltage of 190 V was applied, and the terminal voltage of the capacitor element was 190 Vπ. After reaching this point, chemical conversion treatment was performed for another 15 minutes, and the film thickness ratio between the surface layer and the deep layer was 1.88 (3020/1600A')
Met.

The failure rate during no-load humidity resistance (after 1000 hours) was 1.2%.

Example 4 In Example 1, the order of the chemical conversion treatment using the phosphoric acid aqueous solution and the ammonium borate solution was changed, and a thick oxide film was first formed on the surface layer of the capacitor element, but almost the same effect as in Example 1 was obtained. Obtained.

In the present invention, the chemical solution for forming a thick oxide layer is not limited to a basic solution, and an acidic solution may also be used depending on settings such as current density. or,
By forming an oxide layer with an appropriate film thickness ratio, the voltage distribution in the surface layer can be reduced.
Leakage current characteristics caused by deterioration, destruction, etc. can be effectively improved.

[Brief explanation of drawings]

Figure 1 is a side sectional view of the conventional example, Figure 2 is an enlarged view of the main parts, and Figure 3 is a side sectional view of the conventional example.
FA is an equivalent circuit diagram, Figure 4 is a diagram showing the relationship between the formation time and the thickness of the oxide layer, Figure 5 is a diagram showing the relationship between the thickness of the oxide layer and the failure rate of leakage current. FIG. 6 is a side sectional view showing one embodiment of the present invention, and FIG. 7 is an enlarged view of the main part of FIG. 6. In the figure, 1 is the capacitor element, la is the deep part, and 1b
is the surface layer, 2 is the anode lead, 3+, 3g is the oxide layer,
4 is a semiconductor layer.

Claims (1)

[Claims] In a capacitor element formed of a monolithic powder having a valve action and a metal member having a valve action derived from it as an anode lead, an oxide layer and a semiconductor layer are formed on the capacitor element. A solid electrolytic capacitor characterized in that the thickness of the oxide layer is set to be at least 1.5 times the thickness of the oxide layer in the deep layer.