Metal foil, electrode foil, and electrolytic capacitors for electrolytic capacitors

Abstract

To provide a metal foil for electrode foils that has excellent winding characteristics and bending strength. [Solution] A metal foil for an electrolytic capacitor extends in the longitudinal direction and has a width direction perpendicular to the longitudinal direction, and a plurality of rows of recesses are formed on the surface of the metal foil and are spaced apart from each other in the longitudinal direction, each of the plurality of rows of recesses has a first straight section, a second straight section, and a connecting section that connects the first straight section and the second straight section, and extends in the width direction or in a direction oblique to the width direction, and in each of the plurality of rows of recesses, the first straight section and the second straight section are formed alternately via the connecting section, two adjacent connecting sections have a width distance of 50 μm or more and 5000 μm or less, the first straight section and the width direction form a first angle, the second straight section and the width direction form a second angle, and the magnitude of at least one of the first angle and the second angle is 5° or more and 45° or less.

Description

Technical Field

[0001] The present invention relates to a metal foil for electrolytic capacitors, an electrode foil, and an electrolytic capacitor.

Background Art

[0002] In recent years, with the increase in capacity and reduction in thickness and weight of electrolytic capacitors, various processings and deformation treatments have been performed on electrode foils in addition to surface enlargement treatments such as etching treatment, in order to increase the capacitance of capacitors.

[0003] For example, there is known a configuration of an electrode foil in which a plurality of dividing portions that extend in the width direction of the strip and divide the enlarged surface portion are provided on the enlarged surface portion (etching layer, etc.) formed on the surface of a strip-shaped electrode foil (Patent Document 1). By forming such dividing portions in the enlarged surface portion, bending stress during winding is dispersed, so cracks that even destroy the core portion during winding are less likely to occur, and good smoothly curved winding is enabled.

[0004] There is also known a configuration of a metal foil for electrolytic capacitors, in which at least one main surface region of the metal foil for electrolytic capacitors has a plurality of linearly extending recesses, and the depth of the recesses is 4 µm or more and 58 µm or less (Patent Document 2). This metal foil suppresses foil breakage and cracking of the metal foil for electrolytic capacitors.

Prior Art Literature

Patent Literature

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problem to be Solved by the Invention

[0006] After forming the enlarged surface and undergoing the chemical conversion process, the rigidity of the electrode foil itself increases as the film grows, resulting in a hard and brittle state. When stress such as winding is applied in this state, unexpected and irregular cracks are likely to occur in the electrode foil. The occurrence of uncontrollable cracks causes uneven bending of the electrode foil during the winding process. As a result, the capacitor element becomes jagged, the roundness of the capacitor element after winding decreases, and the element diameter (maximum outer diameter) of the capacitor element increases.

[0007] Furthermore, when various processes are performed on the metal foil of the electrode foil, uneven residual stress is generated within the metal foil, and deformation and work hardening can cause wrinkles, distortions, or undulations.

[0008] This invention has been made in view of the above circumstances, and one object of this invention is to provide a metal foil for electrode foil that has excellent winding characteristics and bending strength while suppressing the amount of deformation and processing of the metal foil. Another object of this invention is to provide a metal foil for electrode foil that can control the location and direction of crack generation in the enlarged surface portion of the electrode foil and form a desired crack. Yet another object of this invention is to provide an electrode foil or electrolytic capacitor containing the above metal foil. [Means for solving the problem]

[0009] [Aspect 1] An example of a means for solving the problems of the present invention is a metal foil for an electrolytic capacitor, wherein the metal foil extends in a longitudinal direction and has a width direction perpendicular to the longitudinal direction, a plurality of rows of recesses are formed on the surface of the metal foil and are spaced apart from each other in the longitudinal direction, and each of the plurality of rows of recesses has a first straight portion, a second straight portion, and a connecting portion connecting the first straight portion and the second straight portion, and in the width direction or relative to the width direction A metal foil that extends in an oblique direction, wherein in each of the plurality of recesses, the first straight portion and the second straight portion are alternately formed via the connecting portion, two adjacent connecting portions have a distance in the width direction of 50 μm to 5000 μm, the first straight portion and the width direction form a first angle, the second straight portion and the width direction form a second angle, and the magnitude of at least one of the first angle and the second angle is 5° to 45°.

[0010] [Aspect 2] In this case, as in aspect 1, each of the recesses in the plurality of rows extends in a zigzag pattern.

[0011] [Aspect 3] In either aspect 1 or aspect 2, the magnitude of both the first angle and the second angle is 5° or more and 45° or less.

[0012] [Aspect 4] In either aspect 1 or aspect 2, the magnitude of one of the first angle and the second angle is 5° or more and 45° or less, and the other is 0° or more and less than 5°.

[0013] [Aspect 5] In either aspect 1 or aspect 2, the magnitude of one of the first angle and the second angle is 5° or more and 45° or less, and the other is greater than 45° and less than 90°.

[0014] [Aspect 6] In any one of aspects 1 to 5, each of the recesses in the plurality of rows has a depth of 1 μm or more and 20 μm or less.

[0015] [Aspect 7] In any one of aspects 1 to 6, each of the recesses in the plurality of rows has a width of 1 μm or more and 20 μm or less.

[0016] [Aspect 8] Another example of a means for solving the problems of the present invention is an electrode foil comprising a metal foil according to any one of aspects 1 to 7, wherein an expanded surface portion is formed on the surface of the metal foil.

[0017] [Aspect 9] In this case, as in aspect 8, the enlarged surface portion is formed on the surface of the metal foil having the plurality of rows of recesses.

[0018] [Aspect 10] In aspect 8 or aspect 9, the crack is formed in the enlarged surface along a portion of the recesses of the plurality of rows.

[0019] [Aspect 11] In aspect 8 or aspect 9, the crack is formed in the enlarged surface along a portion of the recesses of two adjacent rows of the plurality of recesses and across the recesses of the two adjacent rows.

[0020] [Aspect 12] In any one of aspects 8 to 11, a conductive layer is provided on the surface of the enlarged portion.

[0021] [Aspect 13] Yet another example of means for solving the problems of the present invention is an electrolytic capacitor comprising an electrode foil as described in any one of aspects 8 to 12. [Effects of the Invention]

[0022] According to the present invention, it is possible to provide a metal foil for electrode foil having excellent winding characteristics and bending strength. According to the present invention, processing can be performed at the stage of high-strength metal foil, making it possible to reliably perform the necessary processing.

[0023] According to the present invention, when winding an electrode foil, the occurrence of unevenness is suppressed, so that particularly the element diameter (maximum value) of a wound electrolytic capacitor can be reduced, and the size of the wound electrolytic capacitor can be reduced.

[0024] The effects of the present invention also apply to electrode foils containing metal foil, so the present invention is useful for electrode foils. The effects of the present invention apply to electrolytic capacitors including electrode foils, so the present invention is useful for electrolytic capacitors. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] [Figure 1] (a) is a schematic diagram showing a wound state of a metal foil 10 according to an embodiment of the present invention. (b) is an enlarged schematic diagram of the 1b portion in (a). (c) is a surface schematic diagram showing a part of an electrode foil 12 in which an expanded portion is formed on a metal foil according to an embodiment of the present invention, and a crack Q is formed in the expanded portion. [Figure 2] (a) is a surface schematic diagram showing a part of a metal foil 10a according to another embodiment of the present invention. (b) is a surface schematic diagram showing a part of a metal foil 10b according to still another embodiment of the present invention. [Figure 3] (a) to (d) are SEM photographs of the surface of metal foils having dents of various shapes. [Figure 4] (a) and (b) are SEM photographs of cross sections of anodized foils of Comparative Example 3 (no dent) and Example 19 (dent depth 4 µm), respectively. [Figure 5] Figure 5 shows measurement results of bending strength. [Figure 6] Figure 6 shows measurement results of bending strength. [Figure 7] Figure 7 shows the relationship between the distance between connection portions and bending strength. [Figure 8] (a) and (b) are SEM photographs of fracture surfaces of anodized foils of Comparative Example 3 (no dent) and Example 19 (dent depth 4 µm), respectively. [Figure 9](a), (b), and (c) are SEM images of the surface after fracture of the anodized foils of Comparative Example 3 (no indentation), Example 15 (indentation depth 4 μm), and Example 19 (indentation depth 4 μm), respectively. [Figure 10] (a) and (b) are SEM images of cross-sections of capacitor elements formed using anodic deposition foil in Comparative Example 3 (no recess) and Example 19 (recess depth of 4 μm), respectively. [Modes for carrying out the invention]

[0026] The present invention will be described below with reference to the drawings. The drawings are used for illustrative purposes only, and the present invention is not limited to the contents shown in the drawings.

[0027] <Metal foil> Figure 1(a) is a schematic diagram showing a metal foil 10 wound up according to one embodiment of the present invention. Figure 1(b) is an enlarged schematic diagram of part 1b of (a). The width direction of the metal foil 10 is the X-axis direction in the drawing. The longitudinal direction of the metal foil 10 is the Y-axis direction in the drawing.

[0028] The metal foil 10 is a metal foil that extends in the longitudinal direction and has a width direction perpendicular to the longitudinal direction. A metal foil that extends in the longitudinal direction is typically a long metal foil, and more typically a rectangular metal foil. In the case of a long metal foil, the longitudinal direction can be the direction of the side that extends in the longitudinal direction. Alternatively, in the case of a rectangular metal foil, the longitudinal direction can be the direction perpendicular to the direction of the long side or the direction of the short side of the rectangle. The width direction is the direction perpendicular to the longitudinal direction. The metal foil can be a rectangle, a rectangle, a trapezoid, or any other shape from which a longitudinal direction can be set. Note that the metal foil of the present invention includes a square, in which case the longitudinal direction can be either the direction parallel to two opposing sides of the metal foil or the direction perpendicular thereto.

[0029] The metal foil 10 may be formed from a valve metal material such as aluminum, niobium, or tantalum. In particular, it is preferable that the metal foil 10 be formed from aluminum. Typically, aluminum foil of a purity suitable for aluminum electrolytic capacitors is used as the metal foil 10. The metal foil 10 is formed from metal foil, but may also be formed from surface-treated metal foil. A native oxide film may be formed on the metal foil. In this specification, a metal foil that has undergone surface widening treatment by etching may be referred to as etched foil. In this specification, a metal foil on which a dielectric film (also called a chemical conversion film) has been formed may be referred to as chemical conversion foil. In this specification, a metal foil that has undergone surface widening treatment or both surface widening treatment and chemical conversion treatment may be referred to as electrode foil. It is known that the bending strength of the materials decreases in the order of metal foil > etched foil > chemical conversion foil.

[0030] The thickness of the metal foil 10 is typically between 15 μm and 200 μm. The thickness of the metal foil 10 can be set appropriately depending on its application. For example, in the case of metal foil used as anode foil, the thickness may be between 70 μm and 200 μm. In the case of metal foil used as cathode foil, the thickness may be between 15 μm and 70 μm.

[0031] <dent> Multiple rows of depressions (also called crack-guiding depressions) are formed in the metal foil 10 to form desired cracks in the enlarged surface described later. The multiple rows of depressions are formed on the surface of the metal foil 10 and are spaced apart from each other in the longitudinal direction Y. Although only a portion of the multiple rows of depressions is shown in Figure 1, similar rows of depressions are repeatedly formed in the longitudinal direction Y and the width direction X.

[0032] Each recess P in the multiple rows of recesses can be formed by applying pressure to the metal foil 10, but can also be formed by a physical process such as blasting. It is preferable to form the recesses P by applying pressure to the metal foil 10.

[0033] A dent can be distinguished from a groove (hereinafter simply referred to as a groove) formed by processing such as laser or drilling. Laser or drilling processes remove material from the surface, thus changing the overall weight of the material. A dent does not remove material from the surface, so the overall weight of the material does not change. Dents and grooves can also be distinguished by visual inspection of images observed under a microscope. Alternatively, dents and grooves can be distinguished by the difference in the method of formation.

[0034] The depth D of the recess P is preferably 1 μm to 20 μm, more preferably 1 μm to 15 μm, even more preferably 1 μm to 10 μm, even more preferably 1 μm to 8 μm, even more preferably 1 μm to 6 μm, even more preferably 1 μm to 5 μm, even more preferably 1 μm to 4 μm, and even more preferably 1 μm to 3 μm. Preferably, if the depth D of the recess P is greater than or equal to each lower limit, cracks are more likely to form in the expanded surface along a part of the recess P when winding the metal foil with the expanded surface formed thereon. Preferably, if the depth D of the recess P is less than or equal to each upper limit, excessive physical processing of the metal foil can be suppressed, making it less likely for uneven residual stress to occur inside the metal foil, and making it less likely for wrinkles and distortions to occur. Alternatively, the ratio of the depth D to the thickness of the metal foil may preferably be 0.5% to 25%, more preferably 0.5% to 20%. However, preferably, regardless of the lower and upper limits of the ratio, the recess depth D is 1 μm or more and 20 μm or less. Preferably, if the ratio of depth D to the thickness of the metal foil is less than or equal to each upper limit, it is possible to suppress excessive physical processing of the metal foil.

[0035] The width W of the recess P is preferably 1 μm or more and 20 μm or less, more preferably 2 μm or more and 15 μm or less, even more preferably 2 μm or more and 12 μm or less, and even more preferably 3 μm or more and 9 μm or less. Preferably, if the width W of the recess P is greater than or equal to each lower limit, when winding the metal foil with the expanded surface formed thereon, cracks tend to form in the expanded surface along a part of the recess. Preferably, if the width W of the recess P is less than or equal to each upper limit, excessive physical processing of the metal foil can be suppressed, making it less likely for uneven residual stress to occur inside the metal foil, and making it less likely for wrinkles and distortions to occur in the metal foil.

[0036] The longitudinal spacing I between adjacent recesses P in the longitudinal direction is preferably 10 μm to 300 μm, more preferably 20 μm to 200 μm, and even more preferably 20 μm to 150 μm. Preferably, if the spacing I is above each lower limit, the amount of processing required for the recess P is reduced, making it less likely for wrinkles and distortions to occur. Preferably, if the spacing I is below each upper limit, cracks are more likely to form across the gap G between a recess P and an adjacent recess P. In this specification, when a recess P is formed from two edges and the wall surface between them when viewed from above, the spacing I is the longitudinal distance between the upper edge of a recess P and the lower edge of the recess P adjacent to it in Figure 1(b). In this case, the spacing I may be the longitudinal distance between the lower edge of a recess P and the upper edge of the recess P adjacent to it in Figure 1(b).

[0037] <Straight section> The recess P has a first straight section S1, a second straight section S2, and a connecting section that connects the first straight section S1 and the second straight section S2. In the recess P, the first straight section S1 and the second straight section S2 are formed alternately via the connecting section. In Figure 1(b), the recess P extends in a zigzag pattern in the width direction X. Also in Figure 1(b), a certain first straight section S1 and a second straight section S2 are connected via a connecting section C2, and this second straight section S2 is further connected to a further first straight section S1' via a further connecting section C1', and this structure is repeated. In this specification, the connecting sections C1, C2, C1', and C2' may simply be referred to as connecting section C. Note that in Figure 1(b), the connecting parts C1' and C2' are distinguished by reference numerals as connecting parts C1 and C2, respectively, but in this specification, connecting parts C1' and C2' may be referred to as connecting parts C1 and C2, respectively. Similarly, in Figure 1(b), the straight sections S1' and S2' are distinguished by reference numerals as straight sections S1 and S2, respectively, but in this specification, straight sections S1' and S2' may be referred to as straight sections S1 and S2, respectively. Multiple rows of recesses are configured to form cracks, as described later, along a portion of the multiple rows of recesses. Multiple rows of recesses are configured to form cracks, as described later, along a portion of two adjacent rows of recesses and across these two adjacent rows of recesses. In other words, the recesses in multiple rows are configured to form cracks, as described later, along at least a portion of one straight section S1 in a recess P in one row, along at least a portion of another straight section S1 in a recess P in a row adjacent to the first recess P in one row, and across the gap G between the one straight section S1 and the other straight section S1, and the same applies to the straight section S2.

[0038] Two adjacent connection portions C1 and C2 in the width direction have a distance in the width direction X between them of 50 μm to 5000 μm, preferably 100 μm to 4000 μm, more preferably 200 μm to 3000 μm, and even more preferably 300 μm to 2500 μm. In Figure 1(b), the distance L1 in the width direction X between connection portion C1 and connection portion C2 means the distance in the width direction X between the longitudinal axis containing connection portion C1 and the longitudinal axis containing connection portion C2. Similarly, in Figure 1(b), the distance L2 in the width direction X between connection portion C2 and yet another connection portion C1' means the distance in the width direction X between the longitudinal axis containing connection portion C2 and the longitudinal axis containing yet another connection portion C1'. Preferably, if the distances L1 and L2 are greater than or equal to the lower limits of the above, cracks are more likely to form in the widened surface portion across the two adjacent recesses P. Alternatively, preferably, if distances L1 and L2 are greater than or equal to their respective lower limits, cracks are more likely to form across the gap G between the straight portion of a recess in one row and the straight portion of a recess in an adjacent row. Preferably, if distances L1 and L2 are less than or equal to their respective upper limits, by appropriately selecting distances L1 and L2 when slitting (cutting) the metal foil, connection portions C are formed on each electrode foil after slitting, making it easier to form starting points for crack formation. This means, for example, that if the metal foil is slit to a width of 12 mm, connection portions C can be formed on each electrode foil after slitting without any problems even if distances L1 and L2 are 5000 μm, and if distances L1 and L2 are, for example, 2500 μm or less, connection portions C can be formed even if the slit is 3 mm wide. In other words, the shorter the distance L, the more connection portions that serve as starting points for crack formation can be made within the same width, and the more cracks can be formed along a part of the recess, but it becomes more difficult to form cracks that cross the recess. Conversely, the longer the distance L, the easier it is to form cracks that traverse the depression, but the number of starting points for the cracks decreases, resulting in fewer cracks along parts of the depression. To form the desired cracks described later, it is preferable that both cracks along parts of the depression and cracks that traverse the depression are formed. In this specification, distances L1 and L2 may be simply referred to as distance L. Also in this specification, the distance in the width direction between two adjacent connecting parts may be simply referred to as distance or distance between connecting parts.

[0039] The first straight section S1 and the width direction form a first angle θ1. The second straight section S2 and the width direction form a second angle θ2. The magnitude of at least one of the first angle θ1 and the second angle θ2 is 5° or more and 45° or less, preferably 10° or more and 40° or less, and more preferably 15° or more and 35° or less. In this specification, angles θ1 and θ2 refer to the acute angle of the two angles formed between the first straight section S1, the second straight section S2 and the width direction X, respectively. That is, the magnitude of both the first angle θ1 and the second angle θ2 is 0° or more and less than 90°. Preferably, if the magnitude of at least one of the angles θ1 and θ2 is greater than or equal to the respective lower limit, cracks are more likely to form in the enlarged surface along a part of the recess P. Preferably, if the magnitude of at least one of the angles θ1 and θ2 is less than or equal to the respective upper limit, cracks are more likely to form in the enlarged surface across two adjacent recesses. Alternatively, preferably, if the magnitude of at least one of the angles θ1 and θ2 is less than or equal to each upper limit, cracks are more likely to form across the gap G between the straight portion of a recess in one row and the straight portion of a recess in an adjacent row. Note that when angle θ1 is 5° or more and 45° or less, angle θ2 only needs to be 0° or more and less than 90°, and the reverse is also true. Furthermore, in this specification, angles θ1 and θ2 may be referred to as recess angles θ1 and θ2, respectively.

[0040] In the drawings of this specification, the first straight section S1 and the second straight section S2 extend at the same angle θ1=θ2 with respect to the width direction, but θ1 and θ2 may be different from each other. For example, the first straight section S1 may extend at an angle θ1 with respect to the width direction, and the second straight section S2 may extend at an angle θ2 that is smaller or larger than the angle θ1 with respect to the width direction. In this case, for example, the magnitudes of both the first angle θ1 and the second angle θ2 may be 5° or more and 45° or less. Alternatively, one of the first angle θ1 and the second angle θ2 may be 5° or more and 45° or less, and the other may be 0° or more and less than 5°. Also, one of the first angle θ1 and the second angle θ2 may be 5° or more and 45° or less, and the other may be greater than 45° and less than 90°. Furthermore, in Figure 1(b), the recess P extends in the width direction, but the recess P may extend in an oblique direction (not shown). The diagonal direction can be arbitrarily set by the direction in which the first straight section S1 and the second straight section S2 extend. Preferably, the angle in the direction in which the recess P extends is at least 0° (width direction) and at most the larger of the first angle θ1 or the second angle θ2. A preferred angle is 0° or more and less than 45°. Note that this angle refers to the angle with respect to the width direction of the straight line connecting C1 and C1' when the connecting sections are formed in the order C1, C2, and C1'. Also, in Figure 1(b), the straight sections S1 and S2 extend at the same angle θ1=θ2 with respect to the longitudinal direction Y. However, the straight sections S1 and S2 may extend at alternately different angles with respect to the longitudinal direction Y. For example, the straight section S1 may extend at an angle θ1 with respect to the longitudinal direction Y, and the straight section S2 may extend at an angle smaller or larger than θ1 with respect to the longitudinal direction Y (not shown).

[0041] <Connection part> The connecting parts C1 and C2 can be the vertices of a V-shape (see Figure 1(b)). The connecting parts C1 and C2 may also be called bent or folded parts. The connecting parts C1 and C2 may consist of parts that include the vertices. As shown in Figure 1(b), the recess P may extend with connecting parts C1 and C1' positioned on both sides of one connecting part C2. Alternatively, the recess P may extend with connecting parts C1 and C1' positioned on one side of one connecting part C2 (not shown). Or, if the origin (0,0) is set on the surface of the metal foil and divided in the width direction and longitudinal direction to form four quadrants, and connecting part C1 is positioned in the second quadrant (minus, plus), and another adjacent connecting part C2 is positioned at the origin, then another further adjacent connecting part C1' is preferably positioned in the first quadrant (plus, plus) or the fourth quadrant (plus, minus), and more preferably in the first quadrant (plus, plus).

[0042] Figure 2(a) is a schematic surface diagram showing a part of the metal foil 10a in another embodiment of the present invention. As shown in Figure 2(a), the connecting portions C1 and C2 may be straight sections having lengths a1 and a2 in the width direction, respectively. The connecting portions C1 and C2 may each be configured as straight sections as a whole. In this case, the length a in the width direction of the connecting portions C1 and C2 may be 1 μm or more and 50 μm or less. The length a1 of connecting portion C1 and the length a2 of connecting portion C2 may be the same or different. When an expanded surface is formed in the recess P1, the connecting portions C1 and C2 may include the starting point or initiation point of crack formation in the expanded surface. In Figure 2(a), the distance L1 in the width direction between connecting portion C1 and the adjacent connecting portion C2 in the width direction X can be the distance in the width direction between adjacent ends of connecting portions C1 and C2. Furthermore, in Figure 2(a), the widthwise distance L2 between connection C2 and the adjacent connection C1' in the width direction X can be the widthwise distance between the adjacent ends of connection C2 and C1'. In other words, in Figure 2(a), the widthwise distance L1 between connection C1 and the adjacent connection C2 in the width direction X can be the widthwise distance between the longitudinal axis including the right end of connection C1 in the figure and the longitudinal axis including the left end of connection C2 in the figure that is adjacent to the width direction X. Also, in Figure 2(a), the widthwise distance L2 between connection C2 and the adjacent connection C1' in the width direction X can be the widthwise distance between the longitudinal axis including the right end of connection C2 in the figure and the longitudinal axis including the left end of connection C1' in the figure that is adjacent to the width direction X. In Figure 2(a), the recess P1 extends in the width direction. Furthermore, in Figure 2(a), all straight sections S1 and S2 extend at the same angle θ1 = θ2 with respect to the longitudinal direction Y. However, each of the straight sections S1 and S2 may extend at alternately different angles with respect to the longitudinal direction Y. For example, straight section S1 may extend at an angle θ1 with respect to the longitudinal direction, while straight section S2 may extend at an angle smaller or larger than θ1 with respect to the longitudinal direction. In this specification, recess P1 may be referred to as recess P.

[0043] Figure 2(b) is a schematic surface diagram showing a part of the metal foil 10b in yet another embodiment of the present invention. As shown in Figure 2(b), the distance L1 in the width direction between connection part C1 and connection part C2 adjacent in the width direction X, and the distance L2 in the width direction between connection part C2 and connection part C1' adjacent in the width direction X, may be different. However, both distance L1 and distance L2 are within the numerical range of the distance between connection parts in the present invention (50 μm or more and 5000 μm or less). If distance L2 is shorter than distance L1, compared to when distance L1 is the same (for example, when distance L1 in Figure 2(a) is distance L1 in Figure 2(b)), the number of connection parts that serve as the starting point for crack formation within the same width can be increased, and therefore the number of cracks formed along a part of the recess can be increased. In Figure 2(b), the recess P2 extends in a direction oblique to the width direction. The oblique direction can be arbitrarily set by the direction in which the first straight section S1 and the second straight section S2 extend. Preferably, the angle in the direction in which the recess P2 extends is at least 0° (width direction) and at most the larger of the first angle θ1 or the second angle θ2. A preferred angle is between 0° and less than 45°. Note that this angle refers to the angle with respect to the width direction of the straight line connecting one end of C1 in the diagram (e.g., the right end) and the same side end of the next C1' in the diagram (e.g., the right end), when the connecting parts are formed in the order C1, C2, and C1'. Also, in Figure 2(b), the straight sections S1 and S2 extend at the same angle θ1=θ2 with respect to the longitudinal direction Y. However, the straight sections S1 and S2 may extend at alternately different angles with respect to the longitudinal direction Y. For example, the straight section S1 may extend at an angle θ1 with respect to the longitudinal direction Y, and the straight section S2 may extend at an angle smaller or larger than θ1 with respect to the longitudinal direction Y. In this specification, recess P2 may be referred to as recess P.

[0044] It is preferable that the multiple rows of indentations are formed on both sides of the metal foil 10. If the multiple rows of indentations are formed on one side of the metal foil 10, it is preferable that the side with the multiple rows of indentations be positioned on the outside when winding.

[0045] <Electrode foil> Electrode foils can be obtained by performing a surface expansion treatment on a metal foil in which multiple rows of indentations have been formed, or by performing a surface expansion treatment and a chemical conversion treatment. Electrode foils are suitably used as cathode foils and anode foils of capacitors. Electrode foils may be used as cathode foils without forming a dielectric film (also called a chemical conversion film). Electrode foils may also be used as cathode foils with a dielectric film formed on them. Electrode foils may also include those in which an etching treatment is performed after indentation formation, and a conductive layer is further provided. The conductive layer is either an inorganic conductive layer or an organic conductive layer. Examples of inorganic conductive layers include aluminum, tantalum, niobium, titanium, zirconium, carbon, copper, platinum, gold, silver, cobalt, nickel, and iron. Examples of methods for forming the inorganic conductive layer include CVD (chemical vapor deposition), ALD (atomic layer deposition), PVD (physical vapor deposition), coating, electroplating, and electroless plating. Examples of organic conductive layers that can be used include polypyrrole, polythiophene, polyaniline, and derivatives thereof. Methods for forming the organic conductive layer include, for example, chemical polymerization or electrolytic polymerization of raw material monomers, or contact with a solution or dispersion of a conductive polymer. The electrode foil may be used as an anode foil with a dielectric film formed on it. An anode foil before the formation of the dielectric film is also included in the electrode foil of the present invention.

[0046] <Enlarged surface section> The expanded portion is a layered portion formed by performing an expanded surface treatment on the metal foil 10. As described above, the expanded portion can be formed by performing an expanded surface treatment on a metal foil in which multiple rows of depressions have been formed. The expanded portion is usually formed by electrochemically or chemically etching the metal foil 10 in an electrolyte solution containing chloride ions. In this case, the pits formed by the etching process proceed from the surface of the metal foil in which multiple rows of depressions have been formed, so the expanded portion extends continuously from the surface of the multiple rows of depressions. Alternatively, the expanded portion may be formed by vapor deposition or sintering after the formation of multiple rows of depressions. After etching, post-treatment such as acid cleaning may be performed to remove any adhering chloride ions, etc. The expanded portion can have a structure with spongy pits, a structure with tunnel-shaped pits, or a structure in which tunnel-shaped pits and spongy pits are combined. When the electrode foil is used as a cathode foil or anode foil, a dielectric film is formed on the expanded portion in which multiple rows of depressions have been formed.

[0047] The thickness of the expanded portion is not particularly limited, but is preferably 1 μm to 150 μm, more preferably 2 μm to 125 μm, and even more preferably 4 μm to 100 μm. The thickness of the expanded portion can be arbitrarily designed depending on the required capacitance and strength. Note that if the expanded portion is formed on both sides of the metal foil, the thickness of the expanded portion refers to the total thickness.

[0048] <Core part> The core is the portion of the electrode foil where the expanded surface portion has not been formed. Alternatively, the core is a solid layered portion. The thickness of the core is not particularly limited, but is preferably 2 μm to 50 μm. The thickness of the core is arbitrarily designed depending on the required capacitance and strength.

[0049] <crack> In this specification, a crack refers to a crack formed on the enlarged surface of the electrode foil. In the case of chemical conversion treatment, cracks formed on the enlarged surface and the chemical conversion coating may also be referred to as cracks. Cracks may have a depth that does not reach the core, a depth that reaches the core, or a depth that penetrates into the interior of the core. Each crack may have a variety of depths. However, typically, cracks penetrate into the interior of the core but do not go further through to the core. In this specification, a desired crack is a crack formed over a wide area, including cracks formed on the enlarged surface along a portion of the recesses of multiple rows, and cracks formed on the enlarged surface across the recesses of two adjacent rows. Such cracks can provide the best anti-chipping effect, high bending strength, or excellent winding characteristics.

[0050] When manufacturing a capacitor element, cracks are formed in the enlarged surface area by applying stress to the electrode foil. Typically, cracks are formed when winding a laminate of electrode foil and separator. Cracks are formed in the enlarged surface area along a portion of the recesses of multiple rows. Cracks are also formed in the enlarged surface area along a portion of the recesses of two adjacent rows and across the recesses of two adjacent rows. For example, a crack is formed along at least a portion of one straight section S1 of a recess P in one row, along at least a portion of another straight section S1 of a recess P in another row adjacent to the recess P in one row, and across the gap G between the one straight section S1 and the other straight section S1, and this also applies to the straight section S2. Figure 1(c) is a schematic surface view showing an electrode foil 12 in which an enlarged surface area is formed on a metal foil according to one embodiment of the present invention, and a crack Q is formed in the enlarged surface area. In Figure 1(c), crack Q is shown by a solid line, and the locations of multiple rows of indentations formed on the surface of the metal foil are shown by dotted lines.

[0051] <Electrolytic Capacitor> The metal foil is metal foil for electrolytic capacitors. Preferably, the electrolytic capacitor is a wound-type electrolytic capacitor. The capacitor element of a wound-type electrolytic capacitor can be formed by sandwiching a separator between the anode foil and the cathode foil and winding them around a winding core. A wound-type electrolytic capacitor can be formed by impregnating the capacitor element with an electrolyte, a conductive polymer, or both the electrolyte and the conductive polymer, housing it in a bottomed cylindrical outer case, drawing out the anode and cathode terminals and sealing them with a sealing body, and then performing an aging treatment.

[0052] The following describes experiments verifying the properties of the metal foil in the present invention using examples and comparative examples. Each experiment and evaluation method is an example for illustrating the present invention and does not limit the present invention in any way.

[0053] <Preparation> Recesses were formed on metal foil by shape transfer. Specifically, aluminum foil was used as the metal foil. Figures 3(a) to 3(d) are SEM images of the surface of metal foil with recesses of various shapes. As shown in Figures 3(a) to 3(d), it was confirmed that recesses of various shapes can be formed on metal foil by pressing a mold onto aluminum foil.

[0054] Figures 4(a) and 4(b) are SEM images of cross-sections of the anodic deposition foils of Comparative Example 3 (no recess) and Example 19 (recess depth of 4 μm), respectively, which will be described later.

[0055] <Evaluation Method> (1) Distance between connection points The distance between connection points can be determined by taking a surface photograph of the metal foil using an optical microscope (microscope). For example, the distance in the width direction X between the longitudinal axis containing a connection point C1 and the longitudinal axis containing the adjacent connection point C2 can be measured at any 10 locations, and the average of these measurements is taken as L1. The observation magnification should be the maximum magnification that allows the two connection points C1 and C2 to be measured to be observed in a single field of view. Furthermore, the distance between connection points is defined as L1 when the straight section between the two connection points C1 and C2 is the first straight section S1, and as L2 when it is the second straight section S2. In the case of electrode foils with expanded surfaces formed on the metal foil, the distances between connection points L1 and L2 can be determined by similar measurements. For example, the distance between connection points of the recess in the metal foil, estimated from the recess in the expanded surface of the electrode foil, can be defined as the distance between connection points of the metal foil.

[0056] (2) angle of indentation The indentation angle can be determined by measuring the angle (acute angle) between the width direction X and the edge of a straight section at 10 arbitrary points in a surface photograph of the metal foil taken at 50x magnification using an optical microscope (microscope), and averaging these values. The indentation angle when the straight section being measured is the first straight section is denoted as θ1, and the indentation angle when it is the second straight section is denoted as θ2. In the case of electrode foil with an expanded surface formed on the metal foil, the indentation angles θ1 and θ2 can be determined by similar measurements. For example, the indentation angle estimated from the indentation of the expanded surface on the electrode foil can be used as the indentation angle of the metal foil.

[0057] (3) Depth of indentation The recess depth D can be determined by measuring the distance between the unrecessed foil surface and the deepest part of the recess P at 10 arbitrary points in a cross-sectional image of the metal foil taken at 1000x magnification using a scanning electron microscope (SEM), and averaging these distances. Even if an enlarged surface is formed on the metal foil, the recess depth D can be determined using a similar measurement method. For example, the recess depth estimated from the recess of the enlarged surface on an electrode foil can be used as the recess depth of the metal foil.

[0058] (4) width of indentation The recess width W can be determined by measuring the shortest distance between the upper and lower edges of the same recess P at any 10 locations in a surface photograph of the metal foil taken at 150x magnification using an optical microscope (microscope), and averaging these measurements. Even if an enlarged surface is formed on the metal foil, the recess width W can be determined using a similar measurement method. For example, the recess width estimated from the recess of the enlarged surface on an electrode foil can be used as the recess width of the metal foil.

[0059] (5) spacing of indentations The indentation spacing I can be determined by measuring the longitudinal distance at any 10 points in a surface photograph of the metal foil taken using an optical microscope (microscope), between one of the two edges of an indentation P and the edge of another longitudinally adjacent indentation P located on the opposite side from the aforementioned edge, and then averaging these measurements. The observation magnification should be the maximum magnification that allows both indentations P being measured to be observed in a single field of view. The indentation spacing I can also be determined by a similar measurement method when an expanded surface is formed on the metal foil. For example, the indentation spacing estimated from the indentation of the expanded surface on an electrode foil can be used as the indentation spacing of the metal foil.

[0060] (6)Bending strength Based on the Japan Electronics and Information Technology Industries Association (JEITA) standard EIAJ RC-2364A, the electrode foil is slit into 10mm width strips, and then a bending test is performed to measure the bending strength.

[0061] (7) Stuttering A microscope is used to observe the wound element. A cross-section of a capacitor element is observed in which a chemical foil is used as the anode foil, and the chemical foil and a conventionally used cathode foil are wound together via a conventionally used separator.

[0062] <Experiment 1>: Confirmation of the indentation angle θ and its effect The effect of the recess angle θ was investigated.

[0063] [Example 1] Using a 120 μm aluminum metal foil, recesses with a distance L = 2500 μm, angles θ1 and θ2 = 15°, and depth D = 3 μm were formed on both sides of the metal foil by shape transfer using a die press. The first and second straight sections S1 and S2 were positioned so that they were symmetrical with respect to the longitudinal straight line passing through the connection sections C1 and C2 (see Figure 1(b)). AC etching for low-pressure foil was performed so that the thickness of the enlarged surface formed on the surface of the metal foil with the recesses was approximately the same on both sides of the metal foil, forming an enlarged surface with a total thickness of 80 μm. Next, post-treatment for low-pressure foil was performed. Specifically, the foil was immersed in a post-treatment solution containing nitric acid.

[0064] [Examples 2-19, Comparative Examples 1-2] The anodic foils for Examples 2-19 and Comparative Examples 1-2 were prepared in the same manner as in Example 1, except that the conditions were as shown in Table 1 below. Comparative Examples 1-2 had a recess depth of 0 μm (no recess). In the table, E represents an example and C represents a comparative example.

[0065] [Table 1]

[0066] The bending strength of the electrode foils of Examples 1-14 and Comparative Examples 1-2 was measured. Figures 5 and 6 and Tables 2 and 3 show the measurement results of the bending strength. The bending strength of each example is expressed as a relative value with the bending strength of the corresponding comparative example set to 100 (the same applies hereafter). As shown in Figures 5 and 6 and Tables 1-3, a favorable effect was obtained at an angle of 15°. A slight effect was obtained at an angle of 45° and a distance of 50 μm. It could be inferred that no effect was obtained at angles greater than 45° or distances less than 50 μm.

[0067] [Table 2]

[0068] [Table 3]

[0069] Although the above study was conducted using anodic conversion foil as a representative example of chemical conversion electrode foil, similar effects can be obtained with cathode conversion foil.

[0070] <Experiment 2>: Confirmation of distance L and effect [Examples 15-19, Comparative Example 3] In Experiment 2, the properties of each compound foil were verified using the compound foils of Examples 15-19 and Comparative Example 3. The compound foils of Examples 15-19 and Comparative Example 3 were prepared in the same manner as in Example 1, except that the conditions in Table 4 below were used. In Comparative Example 3, the indentation depth was set to 0 μm (no indentation).

[0071] [Table 4]

[0072] Figure 7 and Table 5 show the relationship between distance L and bending strength. As shown in Figure 7 and Table 5, a slightly better effect than Comparative Example 3 was observed at 50 μm. A clearly better effect was observed at distances of 300 μm or more. The effect was confirmed to be significant at distances L of 1000 μm or more.

[0073] [Table 5]

[0074] Figures 8(a) and 8(b) are SEM images of the fracture surfaces of anodized foils with cracks formed in Comparative Example 3 (no indentation) and Example 19 (indentation depth 4 μm), respectively. Table 6 shows an example of the relationship between indentation depth D and bending strength. As shown in Figure 8(b), it can be seen that the strength increases as the stress during bending is distributed and many cracks are formed.

[0075] [Table 6]

[0076] Figures 9(a), (b), and (c) are SEM images of the surface of the anodized foil of Comparative Example 3, Example 19, and Example 15, respectively. In Figure 9(b), it can be seen that cracks are formed along a portion of the depression and cracks cross adjacent depressions, and that the strength improves as the number of cracks increases. Comparing Figures 9(b) and (c), cracks along a portion of the depression were confirmed in both. However, in Example 19 shown in Figure 9(b), there was a tendency for many cracks to cross the depressions. In Example 19 in Figure 9(b), the cracks cross the depressions and the number of cracks increases, so it is thought that many cracks are formed over a wide area, resulting in increased bending strength.

[0077] As described above, it was confirmed that foils with high bending strength develop more cracks when bent. Therefore, it can be evaluated that the higher the bending strength, the better the jerkiness can be suppressed.

[0078] <Experiment 3>: Confirmation of the effect of suppressing stuttering Winding observations were performed on the capacitor elements of Example 19 and Comparative Example 3. Figures 10(a) and (b) are SEM images of cross-sections of capacitor elements formed using anodized foil for Comparative Example 3 (no recess) and Example 19 (recess depth 4 μm), respectively. As shown in Figure 10(b), it was confirmed that Example 19 had an effect of suppressing jerking.

[0079] When forming capacitor elements, cracks are almost never formed in the foil at the end of the winding. This is because the stress at the end of the winding is not strong enough to cause cracks in the foil. Although not bound by any particular theory, it is thought that the optimal value of the recess shape at the time of maximum stress on the foil will be the optimal value at all stresses. In other words, if a recess shape that is effective at the time of maximum stress, when the radius is smallest at the beginning of the winding, is formed over the entire foil in advance, it is thought that the same shape will be effective from the beginning to the end of the winding. Furthermore, a correlation was confirmed between a large number of bends and the effectiveness of suppressing jerking. In this specification, we mainly focus on the number of bends and calculate the optimal values ​​of the distance and recess angle that best suppress jerking.

[0080] As a result of diligent research by the inventors, it was found that when bending of the electrode foil begins, stress concentration occurs at the connection points between the straight sections of the indentations, which can become the starting point for cracks. At this time, the crack propagates from the connection points of the indentations along at least a portion of the straight sections. If the distance between connection points is long, the crack will further cross adjacent indentations, forming numerous cracks. It was found that when numerous cracks are formed over a wide area, the stress during bending is distributed, and the strength is improved. It was also found that if the distance between connection points is short, it is difficult for cracks to cross adjacent indentations to form. In the case of grid-like or mesh-like indentations, it is difficult for cracks to cross the indentations to form.

[0081] This invention is not bound by any particular theory, but is based on the following findings obtained by the inventors: [1] A large number of cracks formed in the enlarged surface improves the bending strength of the electrode foil, which helps to suppress the jerking of the capacitor element. [2] In particular, a large number of cracks crossing the depressions improves the bending strength of the electrode foil, which helps to suppress the jerking of the capacitor element. [3] If the angle of the indentation is too large, the crack will not be able to follow the indentation well, resulting in the same outcome as if there were no indentation. [4] If the indentation angle is too small, it becomes difficult for cracks to form across the indentation, resulting in fewer cracks and a loss of the anti-chipping effect. [5] If the distance between the connection points is too short, it becomes difficult for cracks to form across the recess, resulting in fewer cracks and a loss of the anti-chipping effect. [6] If the distance between the connection points is too long, it becomes easier for cracks to form across the depression, but since the number of connection points that can serve as the starting point decreases, the number of cracks along parts of the depression decreases, and the effect of suppressing jerking is not achieved. [7] The synergistic effect of both cracks along the depression and cracks transverse the depression is thought to lead to an increase in bending strength. Therefore, the balance between cracks along a portion of the depression and cracks transverse the depression is expected to be one of the important factors. In this case, when the distance L is particularly in the range of 300 μm to 2500 μm, it is thought that cracks along the depression and cracks transverse the depression are formed in a balanced manner, and the bending strength can be further increased.

[0082] According to the present invention, the location and direction of crack formation during the manufacturing of capacitor elements (especially during winding) can be controlled, and desired cracks can be formed in the electrode foil in an intended manner. According to the present invention, numerous cracks can be formed in the enlarged surface while suppressing the amount of deformation and processing of the metal foil. Many cracks increase bending strength and suppress jaggedness. According to the present invention, the possibility of cracks penetrating the core during crack formation, which is undesirable for the electrode foil itself, can be greatly reduced.

[0083] In this specification, the upper or lower limit of a given numerical range may be arbitrarily combined with or interchanged with the lower or upper limit of another related numerical range. For example, if two numerical ranges are described as X or greater and Y or less, preferably Z or greater and W or less, this means that they may also be X or greater and W or Z or greater and Y or less. The aforementioned combinations or interchanges are also applicable to three or more numerical ranges in this specification.

[0084] The embodiments and examples disclosed herein should be considered illustrative and not restrictive. The scope of the present invention is not limited to the above description but is indicated by the claims, and all modifications within the scope of the claims and equivalents are intended. [Explanation of Symbols]

[0085] 10 Metal foil 12 Electrode foil P, P1, P2 indentation Q Crack S1, S2, S1', S2', Straight section C, C1, C2, C1', C2' connection part L, L1, L2 distance θ1, θ2 angle D Depth W width I interval G Gap a1, a2 Length of the connection part in the width direction

Claims

1. Metal foil for electrolytic capacitors, The metal foil extends in the longitudinal direction and has a width direction perpendicular to the longitudinal direction, Multiple rows of indentations are formed on the surface of the metal foil and are spaced apart from each other in the longitudinal direction. Each of the aforementioned rows of recesses has a first straight section, a second straight section, and a connecting section that connects the first straight section and the second straight section, and extends in the width direction or in a direction oblique to the width direction. In each of the aforementioned recesses in multiple rows, The first straight section and the second straight section are formed alternately via the connecting section. Two adjacent connection portions of the aforementioned connection portion have a distance in the width direction of 50 μm or more and 5000 μm or less. The first straight section and the width direction form a first angle, The second straight section and the width direction form a second angle, A metal foil in which the magnitude of at least one of the first angle and the second angle is 5° or more and 45° or less.

2. The metal foil according to claim 1, wherein each of the recesses in the plurality of rows extends in a zigzag pattern.

3. The metal foil according to claim 1, wherein the magnitude of both the first angle and the second angle is 5° or more and 45° or less.

4. The metal foil according to claim 1, wherein the magnitude of one of the first angle and the second angle is 5° or more and 45° or less, and the other is 0° or more and less than 5°.

5. The metal foil according to claim 1, wherein the magnitude of one of the first angle and the second angle is 5° or more and 45° or less, and the other is greater than 45° and less than 90°.

6. The metal foil according to claim 1, wherein each of the recesses in the plurality of rows has a depth of 1 μm or more and 20 μm or less.

7. The metal foil according to claim 1, wherein each of the recesses in the plurality of rows has a width of 1 μm or more and 20 μm or less.

8. An electrode foil comprising a metal foil according to any one of claims 1 to 7, wherein an expanded portion is formed on the surface of the metal foil.

9. The electrode foil according to claim 8, wherein the enlarged portion is formed on the surface of the metal foil having the plurality of rows of recesses.

10. The electrode foil according to claim 8, wherein cracks are formed in the enlarged surface portion along a portion of the recesses of the plurality of rows.

11. The electrode foil according to claim 8, wherein the crack is formed in the enlarged portion along a portion of the recesses of two adjacent rows among the plurality of recesses and across the recesses of the two adjacent rows.

12. The electrode foil according to claim 8, wherein a conductive layer is provided on the surface of the enlarged portion.

13. An electrolytic capacitor comprising the electrode foil described in claim 8.

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