Diamond electrode and method for production thereof

The invention relates to a diamond electrode with synthetically produced and conductive (doped) diamonds and a method for their production.

Diamond electrodes are characterized by their high overvoltage for oxygen and hydrogen and are therefore particularly suitable for a large number of oxidation processes in aqueous solution. Possible and particularly interesting applications would therefore be in the field of drinking water treatment (disinfection) and water treatment through anodic oxidation.

At present, diamond electrodes are produced by directly producing boron-doped diamond layers on substrate materials, in particular by CVD (Chemical Vapor Deposition) processes. The known techniques differ from one another primarily in the type of energy input. In the so-called hot filament technology, tungsten wires are heated in a gas mixture of hydrogen, a carbon and a boron source over a substrate. The use of different substrates is possible, which differ in the CVD process mainly due to their different carbon solubility.

Practically only copper and gold are inert. Titanium, zirconium, hafnium, niobium, tantalum, chromium, molybdenum, tungsten and silicon form carbide layers during the coating, which, depending on the material, have to reach a certain thickness (Si a few nm, Ti a few pm) until nucleation and thus diamond deposition occurs. Even longer lead times for CVD deposition occur with carbon-dissolving substrates such as iron, cobalt, nickel, platinum and palladium. At the same time, reactions with atomic hydrogen and carbon occur during the coating process, creating intermediate carbide layers (SiO₂, ZrO₂). For these reasons, not all metals are amenable to coating to the same extent.

When used as electrodes, it has also been found that various carrier materials which, when connected as anode, have a low oxygen overvoltage, have too short a service life. If fine cracks appear in the diamond layer, they form immediately

Electrolytic oxygen, which causes the diamond layer to peel off.

Boron-doped diamond layers produced with microwave plasma (MPCVD) can also be produced without a substrate, so that pure diamond electrodes can be produced in a very high quality. In order to ensure a certain mechanical stability, such electrodes must be produced in a relatively large layer thickness, which causes very high production costs and prevents this technology from being used economically.

DE 100 25 167 A1 deals with a method for producing an electrode in which, in a first step, perforations are made in an active electrode body by means of eroding with an erosion tool and the active side is coated with an electrically conductive diamond before and after the erosion the PVD process is generated.

In this PVD process, in addition to the production of the diamond, the introduction of the foreign atoms, which give the diamond parts produced electrical conductivity, can also take place. This method does not make it possible to produce diamond electrodes with an economically justifiable expense.

DE 198 42 396 A1 describes the production of a boron-doped diamond electrode using the CVD process. A base body is coated with a diamond layer using the CVD process and a dopant is added in the gas phase. As already mentioned, this method allows only a very cost-intensive and therefore uneconomical production of diamond electrodes. DE 199 48 184 A1 deals with the electrochemical production of peroxo-sulfuric acid using diamond-coated electrodes. It is mentioned in this publication that the diamond electrodes can be produced according to the known CVD technique.

JP 2000-045097 A2 discloses forming a layer on the cathode from an electroplating bath with dispersed glass, diamond and graphite particles, which layer is then sintered. The English-language abstract does not give any indication of whether more than 50 mainly conductive, doped diamonds are used here.

It is known from US Pat. No. 6,267,866 to coat conductive metal meshes with a boron-doped diamond layer, the coating preferably taking place in accordance with the FACVD (Filament Assisted Chemical Vapor Deposition) process. The uneconomical CVD method for coating is therefore proposed here as well. US Pat. No. 6,306,270 describes bipolar electrodes, the manufacture of which is based on the HFCVD process. Here han2

AT 41 2 002 B is the same method as with FACVD, in which elements are used for plasma formation, which is necessary for CVD deposition. US patent application 2002/0029977 A1 relates to an electrode for electrochemical uses made of a conductive metal mesh which is diamond-coated. The electrodes are preferably manufactured using filament technology, and there is particular talk of “diamond like carbon” (DLC). This carbon configuration is not as resistant to oxidation as diamond, so that the anodes produced in this way do not have great durability.

The invention is based on the object of providing or developing diamond electrodes and a method for producing diamond electrodes which make it possible to produce diamond electrodes with an economically justifiable outlay and therefore in a cost-effective manner. Thus, for the first time, it should be possible to use diamond electrodes economically in a large number of oxidation processes in aqueous solution. In particular, it should be possible to produce correspondingly large-area electrodes that are particularly suitable for drinking water treatment or wastewater treatment.

This object is achieved by a diamond electrode designed according to the invention, which has diamond particles embedded in the surface of a metal or metal alloy layer, which create a conductive connection to the metal or the metal alloy.

According to the method according to the invention for the production of diamond electrodes, a powder of doped, synthetically produced diamonds is produced at least in the connection to the metal or the metal alloy.

Surface of a metal or a metal alloy embedded in such a way that a conductive connection is created between the metal or the metal alloy and the diamond particles.

The invention is therefore based on using industrially and thus comparatively inexpensive produced diamond powder as a starting material and connecting it to a metal or a metal alloy to form a diamond electrode with the formation of a conductive connection between the metal and the diamond particles. It is known to use industrially produced diamond powder for various purposes, for example for grinding pastes. This diamond powder can also be produced in doped form by one of the customary processes, for example by high-pressure, high-temperature processes with metal catalysts. With these manufacturing techniques, the diamond powder can be made conductive by introducing boron in the production process.

In the case of an electrode designed according to the invention, the locations remaining on the surface of the electrode between the diamond particles are provided with a non-conductive oxide layer and are passivated in this way. This ensures that the conductivity of the diamond particles is higher than that of the embedding material and that no bare metal surfaces can react with the electrolyte solution. In addition, these points can be provided with a sealing layer, for example a silicate layer.

In a preferred embodiment of the invention, the embedding layer is applied to a layer of carrier material which, in particular, consists of a layer passivated by oxides

Metal consists or has such as an alloy component or coating. Titanium or aluminum, for example, and alloys made from these metals are therefore suitable. If a mechanically stable material is used for the carrier material, this can also be used to make contact with the electrode. If the electrode is designed with a double-sided coating of the carrier layer, or if the back is insulated, other materials (iron, steel, etc.), but also non-conductors, can be used.

The embedding layer consists at least partially of elements that are capable of forming non-conductive oxides. Metals or metal alloys from the group consisting of magnesium, aluminum, titanium, yttrium, zirconium, hafnium, tantalum, vanadium and zinc are therefore suitable.

The diamond powder embedded in the surface of the embedding layer is in particular with

Boron or nitrogen doped and has a grain size in the order of 1 to 500 μm, in particular up to 200 μm. The use of a diamond powder with at least essentially matching grain sizes is advantageous.

According to the method according to the invention, the metal surfaces or locations that remain free between the diamond particles are passivated. This ensures that the

Conductivity of the diamond particles is greater than that of the embedding material.

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It is therefore advantageous if metal alloys are used for the embedding layer which at least partially consist of at least one element capable of forming non-conductive oxides, such as magnesium, aluminum, titanium, yttrium, zirconium, hafnium, tantalum, vanadium or zinc.

The passivation takes place in particular by generating an oxide layer by means of anodic or chemical oxidation.

The anodic oxidation can be carried out by direct current, pulsed direct current or alternating current with a predominant anodic phase duration. The easiest way is to use direct current. Pulsed direct current enables an improved delivery of peroxide formers (sulfate, borate), io and thus complete oxidation, due to the pauses in which diffusion but no reaction takes place. When using alternating current, the anodic phase is interrupted by a short cathodic phase, which destroys the oxidizing agent. These are newly formed in the next anodic phase. This can prevent oxidizing agents from penetrating deep into pores and oxidation that is too deep. A defined layer thickness can therefore be oxidized without the layers underneath

To draw pity.

To carry out the anodic oxidation, particularly aqueous solutions are used which contain a combination of borate, sulfate, phosphate and fluoride ions. In this way, particularly wear-resistant and corrosion-resistant protective layers or oxidation layers can be produced.

In a further preferred embodiment of the method, the oxidation solutions are buffered. This prevents the solutions from becoming basic and, instead of oxides, also hydroxides, which are water-soluble, are formed.

The pores of the oxide layer can also be sealed. This increases the mechanical strength and improves the insulation properties. In particular, the

Treat the surface of the oxide layer with an aqueous silicate which is cured in air that is rich in carbon dioxide.

Alternatively, sealing can also be achieved by converting the surface into a layer with ceramic properties through the penetration of dissolved metal salts with or without applied potential. Such a treatment of the surface can also take place before a silicate treatment. The ceramic properties can be improved even further in a subsequent sintering process. The term ceramic properties refers to the resulting stable crystal structures. The sintering process is carried out after the additional metals have been introduced into the oxide layer, with the material being held for several hours at a temperature around the melting point of the alloy used. The increased temperature causes a mobility of the molecules, which enables the formation of more stable modifications. The density, the mechanical strength and the thermal stability of the layer can be improved in this way.

In a method according to the invention, diamond powders of various grain sizes and from various production processes can be processed into electrodes. Of special

It is advantageous to use conductive or semiconducting diamond powder which has a grain size of 1 to 500 μm, in particular up to 200 μm. The doping is preferably carried out with boron or nitrogen.

The diamond powder can be introduced into the conductive embedding material in various ways. A cost-effective and technically less complex process consists in melting low-melting materials, for example magnesium or a magnesium alloy, as a conductive material on a metallic carrier material with a higher melting point, then applying the diamond powder to the liquid metal and finally cooling it. In a simplified technique, a mixture of a pulverized magnesium alloy and the diamond powder can also be melted directly.

Another method that is well suited for the production of electrodes designed according to the invention is the galvanic co-cutting of a metal or a metal alloy with diamond powder from aqueous solution or from a molten salt, the diamond powder being kept in suspension by stirring or the like and being stored in the deposited metal.

Further features, advantages and details of the invention will now be explained with reference to the drawing, the single figure of which is a schematic cross-section through an embodiment according to the invention

AT 412 002 B

Represents electrode, described in more detail.

The schematic embodiment shown in the drawing figure of an electrode 1 embodied according to the invention comprises a layer 2 made of a carrier material, a metallic embedding layer 3 for diamond particles 5 which protrude outwardly beyond the layer 3, as well as a die

Embedding layer 3 between the individual diamond particles 5 passivating and sealing oxide layer 4.

The material used for the carrier layer 2 is, in particular, a mechanically correspondingly stable material which can preferably also be used to make contact with the electrode. In the case of the one-sided coating of the carrier material shown, a material is preferably used which can form a non-conductive oxide layer. A typical and particularly suitable material is, for example, titanium sheet. The embedding layer 3 consists of an oxide-forming metal or an oxide-forming metal alloy. Magnesium alloys are particularly suitable for the embedding layer, but also alloys which at least partially consist of elements that are capable of forming non-conductive oxides, such as

Aluminum, titanium, yttrium, zirconium, hafnium, tantalum, vanadium or zinc. The oxide layer 4 provided between the partially embedded diamond particles 5 and required can be produced by means of anodic or chemical oxidation of the embedding layer 3. In the event of any mechanical damage to the electrode, this oxide layer, anodically polarized, is self-regenerating.

To produce an electrode 1 constructed according to the invention, a conductive diamond powder, for example a diamond powder doped with boron, is used. The conductive diamond powder is not the subject of the invention; industrially produced diamond powder can be used here. Various methods can be used to embed the diamond powder in the conductive material and to form the embedding layer 3.

Low-melting starting materials for the embedding layer 3, for example magnesium and magnesium alloys, can be melted directly, in particular onto a metallic carrier material with a higher melting point, for example titanium sheet. This takes place under a protective argon atmosphere and, if necessary, under reduced pressure. The diamond powder is applied to the liquefied metal, for example sprinkled on and cooled.

Another possibility for embedding the diamond powder and forming the embedding layer 3 consists in the galvanic co-cutting of a metal or a metal alloy with diamond powder from an aqueous solution, for example a zinc solution or from a molten salt, for example titanium from an alkali salt melt. The diamond powder is kept in suspension by stirring and thus embedded almost randomly in the deposited metal.

The places remaining between the individual diamond particles 5 are places with bare metal surfaces of the embedding layer 3, which would react with the solution. They are therefore passivated so that the current is transported exclusively or predominantly through the diamond particles 5. An aqueous electrolyte bath containing appropriate oxidizing agents or peroxide formers can be used for the oxidation process. If the embedding layer 3 consists or contains magnesium, an electrolyte bath can be used to produce a particularly corrosion and wear-resistant protective layer by anodic oxidation, which according to EP-B1-0 333 048 10 to 80 g per liter of borate or sulfate ions, 10 to 70 g per liter of phosphate ions and 5 to 35 g per liter of fluoride ions and less than 100 g per liter

Contains alkali ions and which is set to a pH of 5 to 11, preferably 7 to 9. It works with direct current with increasing voltage up to 400 volts and the direct current is briefly interrupted or polarized opposite. For further details on the implementation of this known method, reference is made to the patent mentioned.

Further anions and / or cations that promote the formation of dense ceramic layers, such as aluminum salts, can be incorporated into the oxide layer that is being formed during the oxidation process. In particular, such salts are used, through which the mechanical strength and the insulation properties are improved. As an alternative to anodic oxidation, chemical oxidation is also possible.

The mechanical strength and / or the insulation properties can be achieved by sealing the oxide layer, for example by means of aqueous alkali silicate solutions containing carbon dioxide as a

AT 41 2 002 B weak acid precipitates in the pores, can be improved. Curing can take place in air that is rich in carbon dioxide. Before or as an alternative, the entire surface of the electrode can be converted into a layer with ceramic properties by introducing dissolved metal salts with or without applied potential. The ceramic properties can be further improved by a subsequent sintering process.

A typical shape of the insulating layer corresponds to a technical ceramic such as cordierite (Mg2AI₄Si₂0i2) Cordierite is an aluminum-magnesium silicate, on the basis of which the material groups C 410 as well as C 511, C 512 and C 520 are built. They are characterized by particularly low thermal expansion and, associated with this, by a very high resistance to temperature changes. These properties are used for the most well-known applications as automotive catalytic converter supports and as high-quality, fire-resistant household utensils. Other shapes are based on ceramic techniques, which are based on the use of sintered corundum (Al2O3) or yttrium oxide (Y₂O₃) are based on. In terms of the chemical stability of the electrodes in different media, the ceramics can be tailored to the respective application by means of special element compositions.

Since the materials used for the embedding layer are at least partially

Metals which are capable of forming a non-conductive oxide layer are involved, a certain self-healing effect is possible in the event of mechanical damage to the electrode surface. This is particularly favorable when the electrode is connected as an anode in use, as would be the case with anodic oxidation or drinking water disinfection.

Finally, it should also be mentioned that no carrier layer has to be provided. If one is provided, it can also be provided with an embedding layer with diamond particles on both sides.