JPH0512440B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for electrolytically etching aluminum foil used for electrodes of low-voltage aluminum electrolytic capacitors. In low-voltage aluminum electrolytic capacitors that use aluminum foil as one electrode, in order to increase the capacitance of the capacitor, the aluminum foil used as one electrode is electrochemically etched to enlarge its surface area. things are being done. This enlarged foil is actually assembled into a capacitor after further chemical treatment, but the condition of the foil surface enlarged by this surface area enlargement treatment is such that even if the etched layer becomes thicker, there will be no surface defects. If uniform dissolution does not occur and the foil is dense, the surface area expansion rate per unit volume is large, and as a result, a capacitor with a large capacitance can be obtained. Therefore, the surface area expansion treatment of the foil is important for low voltage capacitors. It has a significant effect on capacitance, which is performance, and its quality is one of the important points for low-voltage capacitor components. That is, the following methods have been proposed as typical electrochemical etching methods, so-called electrolytic etching methods, for the aluminum foil that have been conventionally performed for this purpose. In other words, the first method is to electrolytically etch aluminum foil by applying alternating current or direct current in an aqueous solution of hydrochloric acid or a mixed acid solution in which an acid such as phosphoric acid or formic acid is added to the aqueous solution. Electrolytic etching has the drawback that the etch pits drilled on the surface of the aluminum foil connect with each other and lose density, and these etch pits do not appear to grow and the surface area of the aluminum foil does not increase beyond a certain value. Furthermore, in the electrolytic etching process using direct current, the current concentrates only on the etch pits generated in the early stage of etching, and although the etch pits are etched deep, the side walls of the etch pits are also etched, and on the other hand, the etch pits that occur late are already etched. The foil surface is etched non-uniformly due to the connection with the etched pits which are largely etched, and as a result, a sufficiently high surface area expansion rate cannot be obtained, and this is combined with the fact that the etched pits are deeply etched. However, this method has the drawback of reducing the strength of the foil. As a second method, an AC/DC superposition method has been proposed that aims to achieve a state intermediate between AC and DC, but due to the relationship with the electrolyte composition used, it has not yet been possible to find combination conditions that provide sufficient effects. Not yet. As another method, a so-called two-stage etching method has been proposed, in which the electrolytic etching process is divided into a first stage and a second stage, and each stage combines alternating current and direct current.
According to the research conducted by the present inventors, when direct current is used for etching processing, the non-uniformity of etching due to direct current remains as described above, there is a limit to the surface area expansion rate per unit volume, and there is a limit to the foil strength. Furthermore, since different types of electrolytes are used in the front and rear stages, processing operations are complicated, such as interposing an electrolyte cleaning step between the front and rear stages. Conventionally, the electrolytic solution used for electrolytic etching of aluminum foil has been an aqueous solution of hydrochloric acid or its salts to which nitric acid, phosphoric acid, sulfuric acid, oxalic acid, or salts thereof have been added; Even when applying various currents that have already been proposed, a sufficiently high surface area expansion rate could not be obtained. In view of these circumstances, the inventors have conducted extensive studies and found that when performing electrolytic etching treatment on aluminum foil for electrolytic capacitors using a hydrochloric acid-based electrolyte, a voltage is applied to the aluminum foil using an alternating waveform power source. is applied, the ratio of the anode time to the voltage application time of one cycle of the voltage and the ratio of the anode average voltage to the total average voltage (the sum of the anode average voltage and the cathode average voltage) are the pit density. We found that the shape of the pit, such as size and depth, greatly affects the shape of the pit, and based on this, we can precisely control the ratio of the anode time and the average voltage during the anode to the applied voltage. Aluminum electrolytic foil is suitable for applications where the anodization voltage applied in the process is low, approximately 100V or less, i.e., for use in low-voltage electrolytic capacitors, etc.In other words, even if the etching layer becomes thick, uneven dissolution on the surface will hardly occur. As a result, we have completed a method for obtaining aluminum foil with a large surface area expansion rate per unit volume and high capacitance without any problems occurring. That is, the product according to the present invention has a weight of 5 to 25
When aluminum foil is electrolytically etched using an alternating waveform power source in a hydrochloric acid-based electrolytic solution of The ratio of the average voltage at the anode to the total average voltage of The ratio of the average voltage at the anode to the total average voltage in the subsequent stage is more than 0.5 and 1.0 or less, and the ratio of the time at the anode to the voltage application time of one cycle of the waveform in the previous stage is the waveform 1 in the latter stage. A voltage that is smaller than the ratio of the anode time to the voltage application time of the cycle, and the ratio of the anode average voltage to the total average voltage in the preceding stage is smaller than the ratio of the anode average voltage to the total average voltage in the latter stage. This is an electrolytic etching method for aluminum foil, characterized by applying an electric current to the aluminum foil. To further explain the present invention, the electrolytic etching solution used in the method of the present invention generates a high density of initial electrolytic etch pits where etching starts in the subsequent electrolytic treatment, and etches the pits into a desired shape in the subsequent electrolytic treatment. In addition to a simple aqueous solution of hydrochloric acid, it is also possible to use a mixed acid aqueous solution containing an acid that forms a protective film on the surface of aluminum, such as phosphoric acid, formic acid, acetic acid, etc. .
When the hydrochloric acid concentration in this electrolytic treatment solution becomes 5% or less by weight, it is not possible to obtain a high-density electrolytic initial etching in the first stage, so even if the current is continuously applied in the second stage and the pits are etched deeply, the pits are not etched sufficiently. It is not possible to obtain a high surface area expansion rate per unit volume, and in the subsequent electrolytic treatment, it is not possible to perform an etching process that prevents the pit diameter from increasing and increases the pit depth. In addition, if the hydrochloric acid concentration exceeds 25% by weight, the chemical dissolution of the foil surface will be severe, and the formation of the initial electrolytic etching pit will become unstable in the first stage of electrolytic treatment, which will cause the subsequent stage of electrolytic treatment to become unstable. In the electrolytic treatment, the surface of the foil is dissolved, and a sufficiently high surface area expansion rate cannot be obtained. Further, the amount of acid added to form the above-mentioned protective film is not particularly limited, but in terms of weight, sulfuric acid is 0.1 to 2.5%, phosphoric acid is 0.1 to 3%, and formic acid is 0.1 to 2.5% by weight.
Addition of 0.2 to 4% and acetic acid of 0.5 to 4.5% alone or in combination shows a more preferable expansion ratio. However, the current applied to the aluminum foil using the electrolytic solution containing a high concentration of hydrochloric acid as described above can be obtained by using a known alternating waveform power source. The alternating waveform power source referred to herein is a power source that produces a waveform in which positive and negative polarities are alternately exchanged, that is, an alternating waveform, and the voltage waveform is illustrated in FIG. In Figure 1, A is a sine wave, B is a rectangular wave, and C is a trapezoidal wave, where a is the anode time, b is the voltage application time of one cycle, c is the anode voltage, and d is the cathode voltage. Although shown, the alternating waveform used in the present invention is not limited to the waveform described above. Thus, the alternating waveform voltage applied in the previous stage using the alternating waveform power source is for generating electrolytic initial pits in the foil at a high density, and the conditions of the applied voltage that can obtain this high density of initial pits are as follows. A voltage at which the ratio of the anode time to the voltage application time of one cycle of the voltage is more than 0.3 and 0.8 or less, and the ratio of the anode time average voltage to the total average voltage of the current is more than 0.3 and 0.8 or less, If a voltage outside these upper and lower limits is applied, even if the conditions of the alternating waveform voltage applied in the subsequent stage are optimized,
It is not possible to obtain a sufficiently high surface area expansion rate. In addition, the voltage applied in the first stage is a general voltage used in this type of electrolytic treatment, and if this voltage is lower than 1V as the average voltage at the anode, the pit density tends to decrease, and if it is higher than 10V, the pit density tends to decrease. Then, the pits begin to connect, and even if the electrolytic treatment conditions in the latter stage are selected, the desired effect may not be obtained in some cases. In addition, the alternating waveform voltage applied in the subsequent stage is used to etch the electrolytic initial etching pit, which has been pre-drilled into the foil in the previous stage, to a smaller diameter and more deeply, and the voltage applied for this purpose is , the ratio of the anode time to the voltage application time of one cycle of the voltage exceeds 0.5.
1.0 or less, and the ratio of the average voltage at the anode to the total average voltage exceeds 0.5 and is 1.0 or less, and if a voltage outside the upper and lower limits is applied, high density Even if the initial electrolytic etching pits are drilled, these initial pits do not undergo favorable etching and therefore a sufficiently high surface area expansion ratio cannot be obtained. In addition, the voltage applied in the latter stage is in the same range as in the previous stage; if it is lower than 1V, it tends to be difficult to drill the pits that occurred in the previous stage sufficiently, and if it is higher than 10V If this happens, the diameter of the pit will begin to increase, and in any case, the desired effect may not be obtained. Also, using an alternating waveform power supply, the voltage applied at the front stage and the voltage applied at the rear stage are equal to the voltage at the front stage.
The ratio of the anode time to the voltage application time of a cycle is smaller than the ratio of the anode time to the voltage application time of one cycle in the subsequent stage, and the ratio of the anode average voltage to the total average voltage in the previous stage is smaller than the ratio of the anode time to the total voltage application time in the latter stage. It is necessary to apply a voltage smaller than the ratio of the average voltage at the anode to the average voltage. Further, the frequency of the alternating waveform used in the method of the present invention is not particularly limited, but the preferable frequency range in the first stage is 1 to 200 Hz. If it is smaller than 1 Hz, the pit density will be small, and if it is larger than 200 Hz, the pits will be connected. This tends to make it difficult to obtain the desired effect. In addition, the preferred frequency range for the latter stage is 10 to 200 Hz; if it is lower than 10 Hz, the pit formed in the earlier stage cannot be drilled deeply, and if it is higher than 200 Hz, the diameter of the pit will tend to be increased. It is difficult to obtain the effect of When carrying out the method of the present invention, if two electrolytic treatment tanks are provided corresponding to the first and second stages, the aluminum foil can be processed continuously, or if only one tank is used, the tank After the aluminum foil is treated under the first-stage treatment conditions in the tank, it can be treated under the second-stage treatment conditions in the same tank. In addition, if the composition of the electrolytic treatment solution is within the range of the method of the present invention,
Even if the latter stages are the same or have different compositions, the desired effect can be obtained. Specific examples according to the present invention and comparative examples thereof are described below. Using hard aluminum foil with a purity of 99.98% and a thickness of 70 μm, 13 wt% hydrochloric acid and 0.5 wt sulfuric acid were used.
% electrolyte and the temperature of the electrolyte was 55℃ at 60Hz.
Then, a voltage was applied using a 120 Hz alternating waveform power source for etching. The conditions for this applied voltage are as shown in Table 1 below when the voltage is 60Hz.
Both the front and rear stages are controlled by square waves, and in the case of 120 Hz, as shown in Table 2, both the front and rear stages are controlled by sine wave alternating current; one processed under the first stage conditions and the other processed under the latter conditions, respectively. and those treated in the first and second stages were obtained.

【table】

【table】

[Table] Note: In both Tables 1 and 2, total pressure is the total average voltage, and positive pressure is the anode.
It is the hourly average voltage.
In addition, as a comparative example, purity 99.99%
An aluminum foil with a thickness of 70 μm made of a soft material was etched using an electrolytic solution containing 2 wt% hydrochloric acid and 0.5 wt% phosphoric acid at an electrolyte temperature of 55°C by applying a superimposition of 3 V AC and 2 V DC at 60 Hz. However, each of the aluminum foils electrolytically treated as described above has a 15V chemical conversion, and the capacitance of the chemically treated foils is summarized in relation to the etching time as shown in Figures 2 and 3 of the attached drawings. It is as follows. That is, Fig. 2 shows the case of 60Hz, and Fig. 3 shows the case of 60Hz.
In the case of 120Hz, the dotted line indicates processing under the first stage condition, the broken line indicates processing under the second stage condition, the relatively thick solid line indicates processing in the preceding and subsequent stages, and the thin line indicates processing in the comparative example. However,
As shown in each figure, results superior to those of the comparative example can be obtained even with post-processing alone, and even better processing results can be obtained in any case by performing both pre- and post-processing. be able to. According to the present invention as explained above, the surface area is uniform and dense, and the surface area expansion rate per unit volume is large.
In other words, it is possible to accurately perform the etching treatment of aluminum foil for low-voltage electrolytic capacitors, which has a large capacitance, and the treatment operation is simple because the same electrolytic solution can be used for both the front and rear stages. In addition, the strength of the treated aluminum foil is high and it is advantageous for use, and the etching process is also suitable for relatively thin layer material aluminum foil, doubling the excellent surface area expansion rate as mentioned above. This invention has the effect of making it possible to obtain a product with even higher capacitance, and is an invention that has great industrial effects.

[Brief explanation of the drawing]

The drawings show the technical contents of the present invention, and FIG. 1 shows a typical example showing the time-voltage relationship of an alternating waveform current, and FIGS. 2 and 3 show examples of the present invention, respectively. 2 is a diagram summarizing the relationship between etching time and capacitance of etched aluminum foils according to comparative examples.

Claims (1)

[Claims] 1. When electrolytically etching aluminum foil in a hydrochloric acid electrolyte containing 5 to 25% by weight using an alternating waveform power supply, the ratio of the anode time to the voltage application time of one cycle of the first waveform in the first stage exceeds 0.3.
0.8 or less, and the ratio of the average voltage at the anode to the total average voltage in the previous stage is more than 0.3 and 0.8 or less, and in the latter stage, the ratio of the time at the anode to the voltage application time of one cycle of the waveform in the latter stage is 0.5. 1.0 or less, and the ratio of the average voltage at the anode to the total average voltage in the subsequent stage is more than 0.5 and 1.0 or less, and the ratio of the time at the anode to the voltage application time of one cycle of the waveform in the preceding stage. is smaller than the ratio of the anode time to the voltage application time of one cycle of the waveform in the latter stage, and the ratio of the anode average voltage to the total average voltage in the previous stage is smaller than the ratio of the anode average voltage to the total average voltage in the latter stage. A method for electrolytic etching of aluminum foil, characterized by applying a voltage smaller than the ratio.