JP3995406B2 - Aluminum foil for electrolytic capacitors - Google Patents

Description

【００２８】
【表２】

【００２９】
【表３】

【００３０】
【発明の効果】
上述した如く、本発明の電解コンデンサ用アルミニウム箔は、化成処理を施こしても全般的に静電容量高く、しかもコイル内位置による静電容量のばらつきが小さいから、信頼性の高いコンデンサが製作できる。
【図面の簡単な説明】
【図１】図１は、コイル内サンプル採取位置を示す部分断面図かつ斜視図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an aluminum foil for electrolytic capacitors that forms an anodic oxide film on the surface of a foil that has been increased by forming pits by etching, and has a high capacitance and a small variation depending on the part.
[0002]
[Prior art]
Aluminum can easily increase the surface area by forming fine pits by chemical or electrochemical etching, and a high-quality anodic oxide film is formed by subjecting the surface to anodization called chemical conversion treatment. it can. Moreover, since this film becomes a dielectric, the thin rolled aluminum foil is subjected to etching treatment, and the surface is subjected to chemical conversion treatment with various conversion voltages according to the use voltage to form an anodized film. A variety of compatible capacitors are manufactured.
[0003]
The minute pits formed by the etching process are drilled into a shape suitable for the level of the formation voltage according to the operating voltage. In other words, when used for a medium- and high-voltage capacitor with a high working voltage, it is necessary to form a thick chemical film by increasing the chemical voltage. Therefore, the pit is formed by direct current so that the pit is not filled with the thick chemical film. It is performed by electrochemical etching, and the pit shape is a tunnel type. At that time, the etching process is performed in two stages, primary and secondary. In the primary etching, for example, direct current is applied to form a thin tunnel-shaped initial pit, and in the secondary etching, chemical or electrochemical etching is performed. The diameter of the initial pit is expanded.
[0004]
An aluminum foil for a medium to high voltage capacitor is generally manufactured as follows. That is, cast aluminum molten metal containing various elements such as Si, Fe, Cu, Mn, etc., and homogenize the obtained ingot, and after hot rolling and cold rolling, a thickness of about 0.3 mm Use foil. Next, this foil is cold-rolled to obtain an aluminum foil having a target thickness of about 0.1 mm and wound in a coil shape with a foil width of about 400 to 600 mm. Further, for adjusting the crystal orientation and the like, the coil is subjected to a final heat treatment in a vacuum or an inert gas atmosphere to obtain an aluminum foil for a medium-high voltage capacitor. This final heat treatment is performed at a heating temperature of about 470 to 600 ° C.
[0005]
[Problems to be solved by the invention]
The aluminum foil manufactured as described above is adjusted in dimensions or the like as necessary, and then formed by etching as described above to form pits to increase the surface area, followed by chemical conversion to obtain an electrolytic capacitor foil. However, there was a large difference in the capacitance of the foil at the position in the coil of the foil, that is, the position in the width direction and thickness direction (radial direction) of the coil. Further, since the capacitance varies depending on the portion of the foil, there is a problem that a high capacitance cannot be obtained stably.
[0006]
An object of the present invention is to provide an aluminum foil for an electrolytic capacitor that reduces variation in electrostatic capacitance depending on a portion and has a high overall capacitance.
[0007]
[Means for Solving the Problems]
That is, the present invention is for an electrolytic capacitor used for etching, characterized in that carbon is mixed from the surface to a depth of 200 nm or more and 500 nm or less , and carbon cannot be detected by Auger electron spectroscopy at a position exceeding the depth . Aluminum foil.
The present inventor found that the foil mixed with carbon up to a deeper position than the conventional foil forms tunnel-like pits by etching, and the overall capacitance increases even when chemical conversion is applied, and the final heat treatment The present inventors have found a novel fact that the variation in capacitance due to the position in the coil at the time becomes small, and based on this fact, the present invention has been completed.
[0008]
In the present invention, the analysis of carbon along the depth direction from the surface of the aluminum foil was performed by Auger electron spectroscopy, and the mixing depth of carbon was determined based on the detection limit value of carbon at that time.
In the present invention, the definition that carbon is mixed from the surface to a position having a depth of 200 nm or more defines the state after the final heat treatment of the foil coil.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
The composition of aluminum that can be used in the present invention will be described below. However, the aluminum used in the present invention need not be limited to the following composition, and various aluminum alloys can be used in addition to high-purity aluminum. For example, in the case of an aluminum foil for medium- and high-voltage capacitors that is processed at a formation voltage of about 200 V or more, aluminum containing Si 5 to 50 ppm, Fe 5 to 50 ppm, Cu 25 to 70 ppm or a method described in JIS H2111 is preferable. In addition to aluminum having an Al purity of 99.98 wt% or more measured in accordance with the above, an aluminum alloy may be used that contains an element such as Zn and Ga arbitrarily selected.
[0010]
In addition, Pb is an element normally added to the aluminum foil for medium- and high-voltage capacitors, and it is preferable to add Pb also to the aluminum foil of the present invention. That is, Pb has an effect of increasing the initial number of tunnel pits by promoting the reaction with the electrolyte used in the etching process for increasing the surface area of the foil. In combination with the effect of the presence of aluminum, it is advantageous in obtaining an aluminum foil having a high capacitance and a small variation.
[0011]
In particular, the Pb content is preferably 40 ppm to 2000 ppm in a depth portion of 0.1 μm from the surface of the foil. When the amount of Pb contained in the depth portion of 0.1 μm from the surface of the foil is less than the lower limit, the above effect is small. On the other hand, when the upper limit is exceeded, excessive dissolution of the foil surface is induced and the capacitance is high. It becomes difficult to obtain foil.
In order to achieve the above Pb amount, for example, 4 ppm or less of Pb is added to the molten aluminum, and the final heat treatment of the foil coil is performed at a heating temperature of about 470 ° C. or more.
[0012]
The analysis of carbon along the depth direction from the aluminum foil surface is performed by Auger electron spectroscopy as described above, and as the depth increases from the foil surface, the carbon content decreases sequentially, and as a reliable analytical value The depth at which carbon can no longer be detected is defined as the carbon mixing depth.
The reason why the above-described effect can be obtained by mixing carbon from the surface of the foil to a depth of 200 nm or more has not been clarified yet, but is presumed as follows.
[0013]
That is, as described above, the aluminum foil used for the medium- and high-voltage capacitors is subjected to final heat treatment in a state where, for example, a foil having a width of 400 to 600 mm is wound backward in a coil shape. The thickness of the rolled foil and the tension applied backward when it is wound around the coil are not necessarily uniform in the width direction. Further, in the thickness direction (radial direction) of the coil, the contact pressure of the foil in contact with the upper and lower sides is large in the vicinity of the coil core corresponding to the start of winding of the foil and in the vicinity of the outer periphery of the coil corresponding to the end of winding. There is a difference. In such a state, there is a variation in the amount of oxidizing substances such as rolling oil residue and air trapped between the foils at each position in the width and thickness direction of the coil. As a result, even if the final heat treatment of the coil is performed in a vacuum or in an inert gas atmosphere, Al on the foil surface reacts with the oxidizing substance during the heat treatment, resulting in a large difference in the amount of reaction products, depending on the position of the foil It is considered that the variation in capacitance increases.
[0014]
In contrast, when carbon is present from the foil surface to a deep position as in the present invention, the foil surface has corrosion resistance to the etching solution, and the amount of reaction products generated during the heat treatment varies as in the past. Even if there is, there is an increase in the corrosion resistance of the foil surface due to the presence of carbon from the foil surface to a deep position, thereby preventing excessive dissolution of the foil surface during the etching process for bit formation, and finally the coil It is considered that variation due to the inner position is reduced, and the capacitance is increased on average. In order to obtain the effect of improving the electrostatic capacity and reducing variations therefor, it is preferable that the carbon mixing depth is large, and it is more preferable that the carbon is mixed up to a position of 250 nm or more or 300 nm or more. However, as the carbon mixing depth increases, operations such as hot rolling and cold rolling become difficult. Moreover, when the carbon mixing depth exceeds 500 nm, the improvement in the effect of reducing the variation in capacitance tends to be saturated.
[0015]
The aluminum foil for medium and high voltage capacitors is typically etched as follows to increase the surface area due to pit formation. For example, the surface area is increased by drilling tunnel-like pits by continuous etching using direct current. This etching process is divided into two stages of primary etching and secondary etching. In the primary etching process, a large number of initial tunnel pits are formed on the foil surface, and in the secondary etching process, the processing conditions are changed to change the diameter of the initial tunnel pits. Expanding.
[0016]
The electrolytic solution used in the primary etching process may be a known solution such as an acid aqueous solution containing hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid and the like, and is not particularly limited. When the temperature of the electrolytic solution is high, the reaction is promoted, which is preferable. However, when the temperature is too high, the reaction is too fast and the foil surface is so melted that it becomes difficult to form uniform initial tunnel pits. A preferable liquid temperature is 60 to 95 ° C. Moreover, a preferable processing time is about 2 to 4 minutes. The amount of electricity is preferably 15 to 30 coulomb / cm ² and the current density is preferably 100 to 300 mA / cm ² .
[0017]
The initial tunnel pit diameter enlargement process by the secondary etching process increases the surface area by increasing the diameter of the initial pit formed by the primary etching by using a direct current electrolytic process, a chemical process, or a combination of both. The length of the tunnel-like pit varies depending on the foil thickness and the use of the etched foil, but is generally several tens of μm.
The conditions of the secondary etching treatment do not limit the present invention. For example, the electrolyte solution is preferably an acid aqueous solution obtained by adding a small amount of sulfuric acid, phosphoric acid, oxalic acid or the like to hydrochloric acid, or an acid aqueous solution obtained by adding nitric acid. The temperature of the electrolytic solution is preferably 60 to 95 ° C., and the current density is preferably 60 to ²⁰⁰ mA / cm ² . The treatment time is appropriately about 2 to 20 minutes although it depends on the diameter of the tunnel pit.
[0018]
The presence of carbon from the surface of the aluminum foil to the deep position as in the present invention improves the corrosion resistance of the foil surface, and even if the final annealing is performed and the state of the foil surface is not uniform, It is considered that the excessive dissolution of the portion where the excessive dissolution is likely to occur is prevented, the deformation of the tunnel pit is suppressed, the overall capacitance is improved, and the variation in the capacitance depending on the position in the coil is reduced.
[0019]
An aluminum foil having a surface area increased by forming pits by etching treatment is subjected to chemical conversion treatment using this foil as an anode. The chemical conversion treatment may be performed under known conditions. For example, as an electrolytic solution, a buffer solution such as ammonium borate, ammonium phosphate, or organic acid ammonium is used, and a voltage of about 200 V or more is applied in one or more stages depending on the application of the capacitor. A chemical conversion film, that is, a dielectric film is formed by application.
[0020]
Prior to the etching treatment, the surface of the aluminum foil for medium- and high-voltage capacitors may be treated with an acid or an alkali solution for degreasing and surface adjustment. In this treatment, for example, 0.05 to 1 mol / liter of nitric acid or caustic soda aqueous solution is used as a treatment solution, and the treatment is carried out at a temperature of 40 to 60 ° C.
The foil of the present invention is an aluminum foil in which carbon is mixed from the surface to a depth of 200 nm or more as a state after the final heat treatment. In order to obtain a foil in which carbon is mixed to a deep position as described above, for example, various conditions of cold rolling for obtaining a foil or a foil are adjusted. That is, among the conditions used in the cold rolling process, for example, cold rolling is performed by combining cold rolling oil, rolling speed, work roll diameter, reduction ratio, and the like. However, it is not necessary to limit to the above method.
[0021]
As mentioned above, the present invention has been described in detail for the aluminum foil for medium- and high-voltage capacitors. However, the present invention is not limited to this, and can be applied to the aluminum foil for low-voltage capacitors and the aluminum foil for cathodes as long as the final heat treatment is performed. it can.
[0022]
【Example】
[Sample preparation]
Semi-continuous casting of an Al purity 99.99wt% aluminum alloy containing 0.8ppm of Pb, 15ppm of Si, 10ppm of Fe and 50ppm of Cu gives a 530mm-thick ingot, and the ingot is homogenized at 600 ° C for 10 hours. The both sides were cut by 15 mm at room temperature. It was reheated and hot rolling was started at an ingot temperature of 520 ° C. to obtain a hot rolled sheet having a thickness of 6 to 10 mm. Hot rolling was performed while cleaning the roll with a brush, and the end temperature was 300 ° C. Subsequently, cold rolling was performed by changing the roughness, rolling oil viscosity, work roll diameter, and rolling reduction of the work roll of the cold rolling mill to obtain a foil having a thickness of 0.25 mm. Table 1 shows the conditions at this time. Subsequently, the foil obtained under each condition shown in Table 1 was rolled and wound into a coil having a thickness of 106 μm. The width of the foil is 500 mm. The wound coil was subjected to a final heat treatment at 530 ° C. for 6 hours in an argon gas atmosphere, and the (100) plane was aligned with the rolled surface. When the amount of Pb in the portion from the foil surface to a depth of 0.1 μm after the final heat treatment was chemically analyzed, it was 800 ppm.
[0023]
[Measurement of carbon in foil surface layer]
As shown in FIG. 1, about three positions of the coil core part, the intermediate part, and the outer peripheral part, a total of nine foils including a central part in the width direction and both end parts (positions of 80 mm from the foil end toward the central part). Collected as a sample.
This sample was measured by Auger electron spectroscopy until carbon reached the detection limit, and the depth position where carbon was present was determined. Argon ions were used for etching the surface. The measurement conditions are shown below. The measurement results are shown in Table 2.
[0024]
[Measurement of carbon content of foil surface layer by Auger electron spectroscopy]
<Electron beam system>
Measuring device: MICROLAB310D manufactured by VG
Electron beam source: LaB ₆ filament acceleration voltage: 10 kV
Filament current: 0.1 mA
Measurement area: about 100 μm × 100 μm
Irradiation current: About 20nA-30nA (Faraday cup measurement)
<Ion gun system>
Ion gun: VG EX050 ion gun acceleration voltage: 3kV
Filament current: 5mA
Irradiation current: Approximately 500nA (Faraday cup measurement)
The Ar ion etching rate was about 4 nm / min in terms of Al ₂ O ₃ , and 5 points per sample were measured to obtain the average value.
[0025]
[Etching treatment]
The sample at each position was etched after the pretreatment to increase the foil surface area. The processing conditions are shown below.
<Processing conditions>
-Pretreatment liquid: 0.1 mol / liter sodium hydroxide, 50 ° C
Conditions: immersion 60 seconds, primary etching solution: 1 mol / liter hydrochloric acid, 3 mol / liter sulfuric acid mixed solution, 85 ° C.
Electrolysis: DC, 200 mA / cm ² × 120 sec. Secondary etching solution: Same as above Liquid condition: After immersion for 15 minutes, 200 V chemical conversion treatment was performed with a boric acid aqueous solution.
[0026]
[Measurement of foil capacitance]
The capacitance was measured for each sample in an aqueous ammonium borate solution. The capacitance of the other sample was displayed with the electrostatic capacity at the center in the width direction of the intermediate portion in the radial direction of the coil of sample number 3 being 100. The results are shown in Table 3.
From the results shown in Tables 2 and 3, the present invention example (numbers 3, 4, 6, and 7) in which carbon exists from the surface to a deep position of 200 nm or more at the measurement point has a variation of 1.0 to It can be seen that it is as small as 1.9 and that the overall capacitance is high. On the other hand, in the comparative examples (numbers 1, 2, and 5) where the position where carbon is present is less than 200 nm, there are places where the electrostatic capacity is extremely low, and the variation in capacity depending on the foil position in the coil is 5.0-5. It can be seen that it is as large as 9, and that the capacitance is generally low.
[0027]
[Table 1]

[0028]
[Table 2]

[0029]
[Table 3]

[0030]
【The invention's effect】
As described above, the aluminum foil for electrolytic capacitors of the present invention is generally high in capacitance even when subjected to chemical conversion treatment, and the variation in capacitance due to the position in the coil is small, so a highly reliable capacitor can be manufactured. it can.
[Brief description of the drawings]
FIG. 1 is a partial cross-sectional view and a perspective view showing a sampling position in a coil.

Claims (1)

An aluminum foil for electrolytic capacitors to be used for etching, characterized in that carbon is mixed from the surface to a depth of 200 nm or more and 500 nm or less , and carbon cannot be detected by Auger electron spectroscopy at a position exceeding the depth .

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