METHOD FOR PRODUCING SURFACE DISCHARGE ELECTRODES AND SEMIFINISHED PRODUCT FOR CARRYING OUT THE METHOD

Abstract

Method for producing metallic surface discharge electrodes on nonmetallic substrates comprising the following steps: a) producing a metallic seed layer on a substrate; b) electrically contacting the seed layer with a metal wire network and an electrolyte containing metal ions; c) electrodepositing a metal film from the electrolyte at least on the seed layer, with the metal wire network being embedded into the metal film, wherein d) metal wire filaments that are movable relative to one another are arranged to form an electrically percolating metal wire network, e) the arrangement of the metal wire filaments is cast into a gel and the gel is dried thereafter to the gel matrix, and f) the dried gel matrix with the metal wire network embedded therein is applied to the substrate and is wetted with a solvent of the gel matrix. Furthermore, the invention relates to a semifinished product for carrying out the method.

Claims

1. A method for the production of metallic surface discharge electrodes on non-metallic substrates with the steps a. producing a metallic seed layer on a substrate; b. electrically contacting the seed layer with a metal wire network and an electrolyte containing metal ions; c. electrodepositing a metal film from the electrolyte at least on the seed layer, with the metal wire network being embedded into the metal film, wherein d. metal wire filaments that are movable relative to one another are arranged to form an electrically percolating metal wire network; e. the arrangement of the metal wire filaments is cast into a gel and the gel is dried thereafter to the gel matrix, and f. the dried gel matrix with the metal wire network embedded therein is applied to the substrate and is wetted with a solvent of the gel matrix.

2. The method of claim 1, wherein the arrangement of metal wire filaments is randomized by pouring onto a flat surface.

3. The method according to claim 1, wherein the metal wire filaments have lengths in the range of from 2 mm to 1 cm.

4. The method according to claim 1, wherein the diameter of the metal wire filaments is in the range of from 50 to 500 microns.

5. The method according to claim 1, wherein the electrical network resistance of the metal wire network formed of metal wire filaments is determined along different directions between pairs of oppositely lying edge points of contact, and are compared with each other, wherein a metal wire network is only used when the minimum and the maximum resistance value differ by a factor of no more than two.

6. The method according to claim 1, characterized by a storage or transport step between the steps e. and f.

7. The method according to claim 1, wherein chemical substances are mixed into the gel matrix/the gel, which upon contact with the substrate and in the presence of a reaction activator carry out a chemical deposition of the seed layer on the substrate.

8. The method according to claim 1, wherein the substrate is made of silicon exhibiting an array of parallel, upright nanowires, wherein the dried gel matrix with the metal wire filaments is applied on the free ends of the nanowires.

9. The method according to claim 8, wherein said method is used for producing electrodes for lithium-ion batteries.

10. A semi-finished product for carrying out the method according to claim 1, characterized by an arrangement of metal wire filaments in a dried gel matrix, wherein the gel matrix consists of polyacrylamides having additions of copper sulphate and hydrofluoric acid.

11. The method according to claim 1, wherein the metal wire filaments have lengths in the range of from 5-9 mm.

12. The method according to claim 1, wherein the diameter of the metal wire filaments is in the range of from 100 to 200 micrometers.

Description

[0041] Below an embodiment of the invention and two very advantageous embodiments are presented. Therein, the figures show:

[0042] FIG. 1 photograph of a semi-finished product usable for the production of surface discharge electrodes;

[0043] FIG. 2 photograph of two surface discharge electrodes.

[0044] The semi-finished product shown in FIG. 1 is produced by sprinkling or pouring copper wire filaments with diameters between 100 and 200 microns and a length of 9 mm onto a dry substrate. The thus formed wire network is cast into a matrix of polyacrylamide (pure, powder, Roth), wherein an amount of 0.2 g PAA powder in 6 ml of deionized water is used. The PAA powder dissolves very well in water below 50° C. If a higher proportion by weight of PAA used a “food of the gods” (ambrosia)—like gel forms, which is useless for this purpose. Also, too high a proportion of water is not appropriate.

[0045] Subsequently, the gel is dried for 24 hours in room air, whereby it transitions into a film-like plastic state. The semi-finished product shown is then complete. It can be cut into pieces and distributed for example onto two substrates.

[0046] Surface discharge electrodes as shown in FIG. 2 can be produced from the semi-finished product shown in FIG. 1. Prior to electroplating, pieces of the semi-finished product are each arranged on substrates and placed together with the substrates for 2 minutes in 30° C. warm deionized water. It should be ensured that the gel matrix does not dissolve completely.

[0047] A particularly advantageous embodiment of the invention is the fact that in the hydrogel already during casting of wire network—i.e. the manufacture of semi-finished product—the electrolyte and other chemical substances for the wet-chemical deposition of a seed layer may be added. However, this deposition is to be initiated only upon contact of the semi-finished product with the substrate to be plated, that is, the semi-finished product is deprived of an activating component, which is introduced only upon contact with the substrate. This component will be referred to herein as reaction activator. The method follows the proposal of the aforementioned U.S. Pat. No. 6,194,032 B1, and further improves this in a manner which already leads directly to the embedding of the wire filaments in the then produced seed layer.

[0048] It should here be understood that in the context of the invention, the embedding of the filaments in the seed layer represents a construction according to the preamble of contacting of the seed layer, even if this is only formed in the presence of the filaments.

[0049] Specifically, for electroplating of silicon with copper, it has been found that the hydrogel is preferably mixed—gelled—with a solution of copper sulphate (CuSO.sub.4) and hydrofluoric acid (HF). In particular, PAA appears resistant also to HF. For example, 0.2 g PAA powder is dissolved in 6 ml of hydrofluoric acid. The solution consists of 4 wt. % hydrofluoric acid and 1.9 g CuSO.sub.4 dissolved in 98 ml of deionized water.

[0050] The gel dries as described above, and there results a semi-finished product with predominantly green color due to the copper salt. If the semi-finished then placed on silicon and wetted with water, so immediately a chemical reaction initiates, in which dissolution occurs at the surface of the silicon and a thin copper film is deposited between the filaments. In this particular case, the substrate itself is the reaction activator, wherein the hydrofluoric acid from the semi-finished product attacks the silicon.

[0051] The chemical attack on the silicon can also have undesirable effects on the substrate. In some applications, however small losses of silicon are completely unproblematic. This is especially true when making contact with free-standing silicon nanowire arrays as are known for example from published document US 2012/164529 A1 or from WO 2011/066818 A1. Published document WO 2011/066818 A1 teaches on page 3 that, in the production of battery electrodes, it is highly desirable to continue processing those arrays on the side of the freestanding nanowire ends, especially with a metal film, which partially envelopes the nanowire.

[0052] But the documents do not specify how this could be quickly and inexpensively accomplished, preferably by electrodeposition. The basic problem is the creation of a seed layer on the outside lying, free nanowire ends, which also have a lateral connectivity, with simple means.

[0053] With the help of the inventive method and the semi-finished product the production of a wire-enveloping metal film is now quite simple. The semi-finished product, preferably loaded as described above with CuSO.sub.4 and HF, is placed on the freestanding silicon tips. It is advantageous to use nanowire arrays according to the description of WO 2011/066818 A1 as a substrate because the free ends then have largely symmetrical arrangement. When wetting the semi-finished product with water, there forms, beginning at the tips of the silicon tips, of which the upper tips are being dissolved, a laterally together-growing, thin copper film. Once the chemical reaction comes to a completion, the copper film lies on the remaining nanowire peaks, which at this time did not need to have been considerably shortened. In the course of the subsequently possible electrochemical deposition the copper film is then thickened, whereby partial sections of the silicon nanowires gradually continue to be enclosed, since the electrolyte can reach both sides of the copper film.