METHOD FOR PRODUCING ELECTRODE FOILS FOR CAPACITORS, ELECTRODE FOILS, AND CAPACITORS COMPRISING SAID ELECTRODE FOILS

Abstract

The invention relates to a method for producing electrode foils (1) for capacitors (10), comprising the method steps of: A) providing a metal foil (1), B) transferring microstructures (2) located on a stamping die onto a main surface of the metal foil by a reforming process.

Claims

1. A method for producing electrode foils for capacitors, comprising the method steps of: A) providing a metal foil, B) transferring microstructures located on a stamping die to a main surface of the metal foil by a reforming process.

2. The method according to claim 1, a metal foil comprising or consisting of a valve metal being used in method step A).

3. The method according to claim 1, with a method step C) following the method step B) C) producing a metal oxide layer on the metal foil.

4. The method according to claim 1, in method step B) the metal foil being positioned between two stamping dies and the microstructures located on the two stamping dies being transferred onto the main surfaces of the metal foil by pressing.

5. The method according to claim 1, designed as a continuous process, in method step B) the metal foil being passed between two rotating rolls as stamping dies and the microstructures thereby transferred onto at least one main surface of the metal foil.

6. The method according to claim 1, in method step B) regular microstructures being produced.

7. The method according to claim 1, in method step B) a microstructure comprising trenches with an aspect ratio of at least 4:1 being produced.

8. The method according to claim 7, in method step B) a microstructure comprising trenches with a depth of at least 20 μm and a width of at most 5 μm being produced.

9. The method according to claim 1, in method step B) the stamping dies being pressed against the metal foil at a temperature of below 400° C., preferably below 200° C., more preferably less than 100° C., it also being possible for the temperature to be room temperature.

10. The method according to claim 1, in method step B) the stamping dies and the metal foil being pressed against one another with a pressure of 10 to 100 MPa.

11. The method according to claim 1, in method step A) a metal foil with a crystalline texture, for example a cubic texture, of <90%, preferably <50%, more preferably without a texture, being provided.

12. The method according to claim 1, in method step A) a metal foil with a purity of >95%, preferably >98%, being provided.

13. The method according to claim 1, in method step A) a metal foil with a thickness of at least 120 μm, preferably at least 100 μm, most preferably at least 80 μm, being provided.

14. The method according to claim 1, in method step B) microstructures being transferred to the main surface of the metal foil as a multiplicity of depressions.

15. An electrode foil for capacitors, comprising: a metal foil with regular microstructures on at least one main surface.

16. The electrode foil for capacitors according to claim 15, the metal foil comprising a valve metal or consisting thereof.

17. The electrode foil for electrolytic capacitors according to claim 15, the regular microstructures comprising trenches with an aspect ratio of at least 4:1.

18. The electrode foil according to claim 15, the trenches having a depth of at least 20 μm and a width of at most 5 μm.

19. The electrode foil according to claim 15, the metal foil comprising a crystalline (cubic) texture of <90%, preferably <80%, more preferably no texture.

20. The electrode foil according to claim 15, a metal oxide layer being arranged over the microstructures.

21. The electrode foil according to claim 15, the microstructures taking the form of a multiplicity of depressions.

22. An electrolytic capacitor, comprising: an electrode foil according to claim 15 as a first electrode, a further metal foil as a current collector for a second electrode and an electrolyte solution, which is arranged between the first electrode and the current collector of the second electrode

Description

[0054] Exemplary embodiments of the invention are to be explained in more detail below on the basis of figures, in which:

[0055] FIG. 1a shows in cross section an electrode foil produced by means of a method according to the invention and comprising microstructures,

[0056] FIG. 1b shows the corresponding microstructures of FIG. 1a in a perspective view, and

[0057] FIG. 1c shows method step B), during which a stamping die is pressed as a rotating roll onto a metal foil, and accordingly the trenches shown in FIGS. 1a and 1b are produced as microstructures,

[0058] FIG. 2a shows scanning electron micrographs of the trenches,

[0059] FIG. 2b shows by contrast with FIG. 2a an aluminum foil etched by conventional methods and comprising irregular surface structures,

[0060] FIG. 3 shows in a perspective view a hot embossing process for producing the structured metal foils,

[0061] FIG. 4a shows a variant of a method according to the invention as a continuous method, two stamping dies being used as rotating rolls,

[0062] FIG. 4b shows scanning electron micrographs of microstructures which have been produced by means of the embossing process, and

[0063] FIG. 5 shows in cross section an aluminum electrolytic capacitor with an electrode foil according to the invention.

[0064] FIG. 1a shows in cross section an electrode foil 1 embossed by means of a reforming process according to the invention, the microstructures 2 having been produced on both main surfaces 1a, 1b. It can be seen here that trenches 2 have been formed on both main surfaces, the width of the trenches 2b being of a size similar to the distance 2a of two adjacent trenches from one another. The width of the trenches 2b and the distance 2a between two adjacent trenches may also have different dimensions, the walls of the channels not necessarily having to be perpendicular to the plane of the foil, but may also have a different angle to the plane of the foil. In first experiments, trenches with a depth 2c of at least 20 μm and a width 2b of at most 5 μm and also a distance from one another 2a of at most 5 μm have already been produced by means of the method according to the invention. It can also be seen from FIG. 1a that the thickness of the metal foil is greater than the depth of the microstructures, so that, in the cross section of the foil, there is in the interior 1c of the foil an unembossed region 1c without microstructures, which gives the foil great mechanical stability.

[0065] FIG. 1b shows in a perspective view the profile of the trenches in relation to one another. It can be seen from both FIGS. 1a and 1b that the width of the trenches remains approximately the same toward the interior of the electrode foil.

[0066] FIG. 1c schematically shows how the microstructures 2 are transferred to the electrode foil 1 by means of a roll as a stamping die 3.

[0067] FIG. 2a shows typical scanning electron micrographs of the trenches 2 of a 120 μm thick aluminum electrode foil, both in plan view and in a perspective view. The regular arrangement of the microstructures can be clearly seen, it also being possible for webs 2d that can in particular provide an increase in the mechanical stability of the microstructures to run between individual trenches. The depth of the trenches is 20 μm and their width is 5 μm. The distance between adjacent trenches is also approximately of the order of magnitude of 5 μm.

[0068] FIG. 2b shows by contrast with FIG. 2a tunnels 2e, which can be formed in aluminum electrode foils by means of a pre-etching step in the course of conventional etching processes. The irregular arrangement of the tunnels 2e can be clearly seen, in distinct contrast to the regular, defined microstructures of the embossed foils in FIG. 2a.

[0069] FIG. 3 shows in a perspective, schematic view a hot embossing process in which microstructures 2 are produced on an electrode foil 1. The hot embossing process may be subdivided into four sub-steps during a reforming process according to the invention for method step B). These method steps are:

[0070] B1) heating up the metal foil 1 in particular to the embossing temperature, the embossing temperature preferably being below 400° C., as described further above;

[0071] B2) isothermal embossing by pressing the stamping die 3 onto the metal foil 1. During this method step, in particular the temperature of the metal foil 1 is not changed;

[0072] B3) cooling down the arrangement comprising the stamping die 3 and the embossed metal foil 1, the pressure being maintained, and

[0073] B4) demolding the arrangement by opening the stamping die and detaching the embossed metal foil from the stamping die.

[0074] In the case of embossing on only one main surface of the metal foil (aluminum electrode foil), a piece of an aluminum foil 1 with a thickness that is greater than the height of the structures to be produced may be positioned on what is known as a substrate plate 3b, the dimensions of the metal foil 1 corresponding approximately to the dimensions of the substrate plate 3b. Subsequently, both the substrate plate and the stamping die 3a, in which the microstructures to be transferred are present, can be heated up to the embossing temperature, for example a temperature of <320° C. As soon as the embossing temperature has been reached, the embossing begins, the stamping die 3a and the substrate plate 3b being moved toward one another at a constant embossing rate until the preset maximum embossing force is reached. The relative movement between the stamping die 3a and the substrate plate 3b is determined by this constant embossing force. During this time, the metal foil 1 flows under the constant pressure. On account of this flowing, the thickness of the metal foil ..creases the longer it is left in the embossing device. During the embossing process, the temperature is kept constant. It is also possible to apply a vacuum during the isothermal embossing process in order to allow complete filling of the cavities of the stamping die, which is of advantage particularly in the case of static embossing operations with a flat stamping die in order to reduce or avoid the formation of air inclusions between the stamping die and the embossed structures. In the case of dynamic embossing, for example in the case of roll-to-roll embossing processes with rolls as stamping dies, the application of a vacuum is not absolutely necessary because, especially when producing open microstructures at the edge of the metal foils, the air can escape there. After the embossing time has elapsed, the cooling down of the stamping die and the substrate plate begins, while in particular still maintaining the embossing force. After the cooling down, the embossed metal foil is demolded by a relative movement between the substrate plate and the embossed metal foil. During this step, the adhesion of the residual layer of the metal foil on the substrate plate plays an important part. The residual layer is understood here as meaning the thickness of the metal foil that it has after the reforming process. A greater adhesion of the remaining layer on the substrate plate ensures that the microstructures can be demolded in the vertical direction, which reduces the risk of damage. The demolding is the most critical process step of hot embossing. The demolding is particularly important when reducing the structure size in the case of microstructures because of the increasing influence of the shrinkage of the material. Shrinkage may take place in particular in the size range of the structure size of the microstructures, and therefore increases the risk of damage to freestanding microstructures. The effect of the shrinkage is also a function of the process parameters during the embossing process, in particular the embossing force and the embossing temperature. If under hot embossing conditions, that is to say in the embossed state, the metal foil, in particular the aluminum foil, is regarded as a Newtonian fluid, the embossing force can be defined by the following equation for a simple embossing model between parallel plates:

[00001]$F=\frac{3}{2}.Math.\frac{\mathrm{\eta \pi }.Math..Math.R^4}{h_0^3}.Math.\frac{\mathrm{dz}}{\mathrm{dt}}$

[0075] where F is the embossing force, η is the viscosity of the material, R is the diameter of the plate, h.sub.0 is the thickness of the material to be embossed and dz/dt represents the reforming rate, the rate at which the material of the metal foil flows into the cavities or microstructures of the stamping die. It is clear from these relationships that the embossing force increases both with decreasing thickness and with increasing area of the material to be embossed.

[0076] For simple, recurring regular structures in electrode foils, a roll-to-roll production process in particular is very cost-effective as a continuous process in which the electrode foils, the metal foils, are passed between two rotating rolls under pressure, and possibly while being heated to an embossing temperature, so that a continuous embossing operation can be carried out.

[0077] Specific drilling through the metal foils can be used to create multi-layered electrodes, for example anode foils, in which for example a central foil is flanked by two further foils on its respective main surfaces. Through the drilled holes, all of the layers of the foils can be wetted with the electrolyte.

[0078] Such a continuous roll-to-roll process is schematically shown in cross section in FIG. 4a, two rolls 3 with microstructures 2 rotating against one another and a metal foil 1 being passed through under pressure, the structures 2 being produced particularly easily on the metal foil as an electrode foil. The arrows schematically indicate here the rotation of the roll stamping dies and the movement of the metal foil 1.

[0079] FIG. 4b shows scanning electron micrographs of regular microstructures in aluminum electrode foils with a thickness of 120 μm in plan view (illustration on the left) and in cross section (illustration on the right). By contrast with the regular trenches shown in FIG. 2a, here regular depressions with a square cross section are produced in the plan view of the main surface of the metal foil, the cross sections of the microstructures narrowing toward the interior of the electrode foil.

[0080] FIG. 5 schematically shows in cross section a capacitor, for example an aluminum electrolytic capacitor, in which an electrode foil 1 according to the invention is used. This electrode foil 1 has the already described microstructures 2 as regular trenches and also an oxide layer 5 applied on them as a dielectric. The electrode foil 1 may be used for example as an anode foil. Also provided is a further metal foil 4, which may serve as a current collector. Between this further metal foil 4 and the electrode foil 1 there may be in particular a spacer 7, for example a sheet of plastic or a sheet of paper that is impregnated with an electrolyte 6. With the further electrode foil 4 as a current collector, the electrolyte may act as a counterelectrode to the electrode foil 1, that is to say in particular as a cathode. In this case, the further electrode foil 4 may be designed differently than the electrode foil 1 or else be of the same construction. In particular, the further electrode foil 4 may also be an aluminum foil with a great surface area, although the forming layer does not have to be present.

[0081] On account of the increased regular surface area of the electrode foil 1, such electrolytic capacitors have an increased capacitance and increased stability. With the existing technology and using Al.sub.2O.sub.3 as the metal oxide layer (dielectric), capacitance values of >0.2 μF/cm.sup.2 at a forming voltage of 900 V can be achieved in the case of metal foils, in particular aluminum foils, while the thickness of the metal foils should be <80 μm. In particular, such electrolytic capacitors can also be created particularly easily as electrode coils, in the case of which the electrode foils are unwound from rollers and are subject to higher mechanical stress. On account of their increased mechanical stability, electrode foils according to the invention can be subjected to these roll-to-roll production processes without mechanical damage having to be expected. As an alternative to this roll-to-roll production process, the electrode foils according to the invention may also be used with preference in the case of capacitors that are produced by stacking electrode foils one on top of the other.

[0082] Any desired conventional electrolyte solutions may be used as electrolyte solutions for the electrolytic capacitors, in particular aluminum electrolytic capacitors, for example electrolyte solutions that contain ethylene glycol as a solvent and ammonium pentaborate NH.sub.4B.sub.5O.sub.8 as a conducting salt. These electrolyte solutions may contain still further additives. Alternatively, electrolyte solutions that at least partially contain water as a solvent may also be used. Electrode foils according to the invention may also be used in the case of capacitors that comprise anhydrous ionic liquids or else, particularly favorably, also solids as electrolytes.

[0083] The invention is not restricted by the description on the basis of the exemplary embodiments. Rather, the invention comprises every novel feature and every combination of features, which includes in particular every combination of features in the patent claims, even if this feature or this combination itself is not explicitly specified in the patent claims or exemplary embodiments.