JP2005325417A - Diamond electrode and production method therefor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To inexpensively provide a diamond electrode having high reliability and suitable for an electrode for electrolyzing substance in an aqueous solution. <P>SOLUTION: A diamond polycrystal body composed of electrically conductive diamond particles 1 and an insulating protective phase 2 composed of any one kind selected from among silicon carbide, silicon oxide, silicon nitride, aluminum oxide, aluminum nitride and resin is joined to at least one face of an electrically conductive substrate 4 with an electrically conductive brazing filler metal 3, so that a diamond electrode can be obtained. The thickness of the diamond polycrystal body is controlled to the average particle diameter in the particle size distribution of the raw material diamond particles or smaller, and it is preferable that the area of 50 to 95% of the surface of the diamond polycrystal body is occupied by the surface of electrically conductive diamond. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a diamond electrode used for performing a process utilizing an electrochemical reaction such as electrolysis, detection of a substance in a solution, or generation of a substance in a solution. We propose a high-performance electrode that uses the wide potential window of conductive diamond at low cost.

  When two electrodes are put in water and a potential is applied between the electrodes, hydrogen is generated from the negative electrode side, oxygen is generated from the anode side, and water electrolysis occurs. At this time, the difference in potential necessary for generating hydrogen or oxygen from each electrode is called a potential window and is known to vary depending on the electrode material. Since an electrode having a wide potential window has a high potential for electrolyzing water, electrolysis of water does not occur unless a high voltage is applied.

  When various substances are dissolved in the solution, each substance decomposes at a specific voltage when an electric potential is applied by inserting an electrode into the solution. If an electrode having a wide potential window is used, a large voltage can be applied to the solution, so that many substances can be decomposed. Diamond is known to have a very wide potential window. For this reason, if electrolysis is performed using diamond as an electrode, many kinds of substances can be decomposed. That is, diamond is expected to have an excellent decomposing ability as an electrode material for electrolysis treatment of substances in a solution.

In general, when an electrolysis treatment of a solution is performed, the electrode material reacts with various substances dissolved in the solution and deteriorates due to electrolytic corrosion. For example, when removing solutes from wastewater by electrolytic oxidation, the anode material is gradually corroded during use in its harsh chemical environment if the solution being treated has a very strong corrosive action. However, there is a problem that the electrode is consumed or the electrode material flows into the environment and causes environmental pollution.
On the other hand, since diamond is extremely physically and chemically stable, the above problem does not occur when diamond is used as an electrode.

  Therefore, in recent years, attempts have been actively made to use diamond imparted with conductivity by adding impurities such as boron as an electrode for performing an electrochemical treatment of a solution. Since electrode materials used for such applications require a large area, the conventional technology has been manufactured by a chemical vapor synthesis (CVD) method using a carbon-containing gas such as methane as a main raw material. In the CVD method, when diamond is synthesized, diamond is usually deposited in a film on a substrate material. As the substrate material, Si, SiC, Mo or the like is used, and the obtained diamond film is generally polycrystalline.

Further, although diamond is usually an insulator, B can be added to the synthesized diamond crystal by introducing a gas containing a slight amount of boron (B) into the raw material gas. The film can be made conductive.
Diamond added with conductivity by adding impurities has higher corrosion resistance than platinum, which is a commonly used electrode material, and has a wide potential window. It is known that can be stably decomposed with high capacity.

Patent Document 1 describes a method in which a solution is electrolyzed using an anode containing conductive crystalline doped diamond, thereby oxidizing a solute in the solution.
As a conventional technique relating to a diamond electrode material, Patent Document 2 discloses that an electrode is composed of a semiconductor diamond film, and the surface of the semiconductor diamond film is chemically modified to control the characteristics of diamond. It is disclosed that it can be obtained.
Patent Document 3 discloses a diamond synthesis method in which a diamond film is substantially free of pinholes and does not cause pinholes. Even if the diamond film is minute, if pinholes are present, the solution reaches the substrate during electrolysis of the aqueous solution, and the substrate is corroded.

Japanese Patent No. 3442888 Japanese Patent Laid-Open No. 9-13188 JP 2000-313982 A

As described above, diamond has been found to have very excellent characteristics as an electrode for electrolyzing a substance in an aqueous solution, but on the other hand, its use is not widespread industrially.
The first reason is that it is extremely expensive compared to the metal electrode in terms of cost. In the diamond synthesis method by the CVD method, the growth rate is as low as several μm / hour, and a synthesis time of several hours to several tens of hours is required to coat the electrode with a required film thickness. Therefore, a significant improvement in productivity cannot be expected by the manufacturing method using the CVD method.

  The second reason is that anxiety has not been solved yet in terms of the reliability of the electrode during electrolysis. That is, a diamond film synthesized on a metal substrate such as Si, Nb, or Ti by a conventional CVD method may cause a problem that the diamond film peels off during electrolysis due to insufficient adhesion between the diamond film and the substrate. . Such peeling of the diamond film occurs suddenly and greatly impairs the stability of the electrolytic apparatus incorporating the diamond electrode.

  The present invention has been made to solve the above problems. That is, the present invention relates to a diamond electrode used for electrolyzing a substance in an aqueous solution, and has high reliability by using conductive diamond particles as a raw material and adhering it to a substrate with a conductive brazing material. The object is to provide a diamond electrode at low cost.

(1) A diamond electrode characterized in that a diamond polycrystal composed of conductive diamond particles and an insulating protective phase is bonded to at least one surface of a conductive substrate by a conductive brazing material.
(2) The diamond electrode according to (1), wherein the surface of the conductive diamond occupies an area of 50% to 95% of the surface of the polycrystalline diamond.
(3) The diamond electrode according to (1) or (2) above, wherein the thickness of the polycrystalline diamond is not more than the average particle size of the particle size distribution of the raw diamond particles.
(4) The diamond electrode according to any one of (1) to (3) above, wherein the diamond particles in the polycrystalline diamond have a particle size of 10 μm to 1 mm.
(5) The diamond electrode according to any one of (1) to (4) above, wherein the diamond particles in the polycrystalline diamond contain 10 ppm or more of boron.
(6) The material for forming the insulating protective phase of the polycrystalline diamond is any one of silicon carbide, silicon oxide, silicon nitride, aluminum oxide, aluminum nitride, and resin (1) The diamond electrode of (5)-(5).
(7) The above (1) to (1), wherein the substrate is any one of aluminum, tantalum, niobium, titanium, hafnium, zirconium, zinc, tungsten, bismuth, antimony, gold, platinum, and graphite. (6) Diamond electrode.

(8) a step of solidifying the conductive diamond particles after covering them with an insulating protective phase; a step of processing the solidified diamond polycrystalline body into a flat plate having a thickness equal to or less than the particle size of the diamond particles by grinding or polishing; The method for producing a diamond electrode according to any one of (1) to (7) above, comprising the step of heat-bonding the flat diamond polycrystalline body and the substrate with a conductive brazing material in a vacuum.
(9) The method for producing a diamond electrode according to (8), wherein the conductive diamond particles are obtained by crushing polycrystalline diamond synthesized by a gas phase synthesis method.
(10) The method for producing a diamond electrode according to (8), wherein the conductive diamond particles are granular single crystal diamond synthesized under an ultrahigh pressure and high temperature.
(11) The conductive diamond particles are obtained by acidifying a binder from a diamond sintered body that is sintered using one or more binders selected from iron, nickel, cobalt, and alloys thereof under ultra-high pressure and high temperature. (8) The method for producing a diamond electrode according to (8) above, wherein the diamond electrode is obtained by dissolution and removal after crushing.

  The diamond electrode of the present invention manufactured by a method of bonding conductive diamond particles to a metal substrate using a brazing material is superior in stability compared to a diamond electrode manufactured by a conventional CVD method. There is an effect that it can be provided at low cost.

  The diamond electrode of the present invention is characterized in that a polycrystalline diamond composed of conductive diamond particles and an insulating protective phase is bonded to a conductive substrate using a conductive brazing material. The diamond polycrystalline body electrically bonded to the conductive substrate and the conductive brazing material acts as an electrode for decomposing the substance in the solution. In order to fully exhibit the electrode performance as diamond, % To 95%, preferably 70% to 95%, more preferably 80% to 95% of the diamond surface.

  In addition, since power is supplied to the diamond electrode through the conductive substrate, the conductive substrate and the electrode surface must be electrically connected. In order for the conductive substrate and the electrode surface to be electrically connected, the presence of an insulator layer between the conductive brazing material layer and the diamond particles must be avoided. By making the thickness of the polycrystalline diamond layer below the average particle diameter of the diamond particles by polishing or the like, most of the surfaces of the diamond particles bonded to the conductive brazing material layer can form the electrode surface. Therefore, it is desirable that the thickness of the diamond polycrystalline layer be equal to or less than the average particle diameter of the diamond particles.

  The smaller the particle size of the diamond particles, the more uniform the electrode surface, so it is preferable to use fine particles. However, if the diamond particle diameter is too small, it is difficult to achieve electrical continuity between the substrate and the electrode surface. Therefore, the diamond particle diameter is 10 μm or more and 1 mm or less, preferably 30 μm or more and 600 μm or less, more preferably 50 μm or more and 300 μm or less is desirable.

  When conductive diamond particles are synthesized by the CVD method, a boron-added diamond film can be synthesized by mixing a boron-containing gas such as diborane, boric acid, or trimethylborate into the source gas. By crushing this film, boron-added diamond particles can be produced.

  When synthesizing diamond particles under ultra-high pressure and high temperature, a mixture of powdered raw graphite and solvent metal such as powdered iron, nickel, cobalt, etc., or laminating plate raw graphite and plate solvent metal In general, a method of synthesizing these materials by processing them under ultrahigh pressure and high temperature where diamond is thermodynamically stable is generally employed. When synthesizing conductive diamond particles, there is a method of adding boride powder such as boron, iron boride, nickel boride or the like to raw graphite powder or solvent metal powder. In either case, the diamond particles can contain 10 ppm or more of boron, and the diamond particles can be made conductive.

  If the conductive diamond particles are simply adhered to the conductive substrate, the solution may enter from the gaps between the conductive diamond particles, and the substrate or brazing material and the solution may be in direct contact with each other. In this case, a part of the current is supplied to the solution through the substrate or the brazing material, so that the electrolysis performance of the conductive diamond cannot be sufficiently exhibited, and the brazing material is corroded and the diamond particles fall off. May also occur. In order to prevent electrical contact between the substrate or brazing material and the solution, it is effective to protect the gaps between the conductive diamond particles with an insulating material.

  Any insulating material may be used as long as it has a melting point equal to or higher than the brazing material bonding temperature because it is heat-bonded to the substrate with the brazing material. Among them, silicon carbide, silicon oxide, silicon nitride, aluminum oxide, aluminum nitride, and various types of resin cages are readily available and excellent in stability, but from the viewpoint of adhesion and stability with diamond and brazing material. It is desirable to use silicon carbide, silicon oxide, or aluminum oxide.

  The substrate holds diamond particles while supplying electricity to the diamond particles from a power source. The substrate material may be any material that is conductive and has sufficient strength to hold diamond particles. When using metallic materials such as aluminum, tantalum, niobium, titanium, hafnium, zirconium, zinc, tungsten, bismuth, and antimony, the insulating protective phase should be broken and the substrate and solution become electrically Even if it contacts the surface, it is passivated by covering the surface with the oxide film, and the progress of corrosion can be prevented. Also, gold, platinum and graphite are very stable without being oxidized even when used at the anode. Among these conductive materials, niobium, titanium, and zirconium are preferably used from the viewpoint of compatibility with the brazing material.

  A schematic diagram of the diamond electrode of the present invention is shown in FIG. An embodiment of the present invention will be described with reference to FIG. In the figure, reference numeral 1 schematically depicts conductive diamond particles, and the particles may be polycrystalline lump. The surface of the conductive diamond particles is exposed, and the surface facing the surface is bonded to the substrate 4 by the brazing material 3. Current is passed from the substrate through the brazing material and diamond particles to the treated water. In the figure, reference numeral 2 denotes an insulating protective phase, which fills the gaps between the conductive diamond particles, and prevents direct contact between the treated water and the substrate or brazing material.

  As a method of protecting with the insulating protective phase, first, the conductive diamond particles are covered with the insulating protective phase so that the insulating protective material covers the whole, and the insulating protective phase is solidified by a method such as sintering. When the insulating protective phase is sintered, a slight amount of a sintering aid such as yttrium oxide may be added and sintered. Then, by polishing using an alumina grindstone or a diamond grindstone, the soft insulating protective material is preferentially removed compared to diamond, and the diamond surface is exposed on the surface.

  When the insulating protective phase and the conductive diamond particles are bonded by such a method, as can be seen from FIG. 1, the thickness of the conductive diamond layer is equal to or less than the particle size of the diamond particles. Diamond particles used as a raw material necessarily have a particle size distribution, but when a polishing process is performed to remove the insulating protective phase, the polycrystalline particles are polished to a thickness equal to or less than the average particle size of the particle size distribution. An area of 50% or more of the surface can be exposed to the surface.

The diamond electrode of the present invention can be manufactured by bonding the diamond polycrystalline plate made of the conductive diamond particles 1 and the insulating protective phase 2 thus manufactured to the substrate 4 using the conductive brazing material 3.
In addition, after joining the plate of the diamond polycrystal body composed of the conductive diamond particles 1 and the insulating protective phase 2 to the substrate 4 using the conductive brazing material 3, polishing with an alumina grindstone or a diamond grindstone, The same form of diamond electrode can be produced.

  EXAMPLES Next, although the detail of this invention is demonstrated in detail by an Example, this invention is not limited to what was actualized by these Examples.

[Example 1]
As the conductive diamond particles, boron-added diamond particles having the average particle size distribution shown in Table 1 synthesized under ultra-high pressure and high temperature are used. Insulating material was used. A powder of several microns was used as a raw material for the insulating material. The diamond particles and the insulating material were mixed at a mixing ratio shown in Table 1 and thinly spread into a flat plate shape, and then baked and hardened in a vacuum furnace. Both surfaces of this sintered body were polished with a diamond grindstone to a thickness of 10 μm to expose the diamond surface on the surface, and at the same time, were formed into a 100 mm square by cutting. Table 1 shows the approximate area ratio (area ratio) at which the diamond surface was exposed. A silver-steel-titanium brazing material was applied to the joint surface between this sintered body and a 100 mm square niobium substrate having a thickness of 3 mm, and was heated to 850 ° C. in a vacuum heating furnace to be brazed. The polycrystalline body using the conductive diamond particles produced in this way was used as an anode, and a long-term electrolytic test was conducted using a 1M sodium sulfate electrolyte solution to examine the stability of the diamond electrode.
As a comparative example, an electrolysis test was performed on a boron-added conductive diamond film synthesized on an Nb substrate by a hot filament CVD method. These results are summarized in Table 1.

[Example 2]
A boron-doped diamond polycrystalline film having a film thickness of 30 μm synthesized by the CVD method was crushed to produce polycrystalline particles having a particle size distribution with an average particle diameter of 30 μm. The powder obtained by mixing the conductive diamond polycrystalline particles and the silicon carbide particles was adhered to a 500 mm square titanium substrate on which a silver-copper-titanium brazing material was thinly applied on one side. This was heated to 850 ° C. in a vacuum heating furnace and bonded, and then polished with a diamond grindstone until the diamond layer had a thickness of 25 μm to expose the diamond surface. When a polycrystalline body using conductive diamond particles thus prepared was used as an anode, a current of 1 kA was passed and a 1 M sodium sulfate solution was subjected to a long-term electrolytic test. There was no dropout and no change from the initial state was observed.

  Since the diamond electrode of the present invention has high corrosion resistance and a wide potential window, the substance in the aqueous solution can be decomposed stably and with high ability, and the substance in the aqueous solution is reliable and inexpensive. It can be utilized as an electrode used for electrolyzing.

It is the figure which showed an example of the structure of the diamond electrode of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Conductive diamond particle 2 Insulation protective phase 3 Conductive brazing material 4 Board | substrate

Claims (11)

  A diamond electrode comprising: a diamond polycrystal composed of conductive diamond particles and an insulating protective phase bonded to at least one surface of a conductive substrate by a conductive brazing material.   The diamond electrode according to claim 1, wherein the surface of the conductive diamond occupies an area of 50% to 95% of the surface of the polycrystalline diamond.   The diamond electrode according to claim 1 or 2, wherein a thickness of the polycrystalline diamond is equal to or less than an average particle size of a particle size distribution of the raw material diamond particles. The diamond electrode according to any one of claims 1 to 3, wherein a particle diameter of diamond particles in the polycrystalline diamond is not less than 10 µm and not more than 1 mm.   The diamond electrode according to any one of claims 1 to 4, wherein the diamond particles in the polycrystalline diamond contain 10 ppm or more of boron.   The material for forming the insulating protective phase of the polycrystalline diamond is any one of silicon carbide, silicon oxide, silicon nitride, aluminum oxide, aluminum nitride, and resin. Diamond electrode in any one.   7. The substrate according to claim 1, wherein the substrate is any one of aluminum, tantalum, niobium, titanium, hafnium, zirconium, zinc, tungsten, bismuth, antimony, gold, platinum, and graphite. Diamond electrode as described in 2.   A step of solidifying the conductive diamond particles after covering them with an insulating protective phase; a step of processing the solidified diamond polycrystalline body into a flat plate having a thickness equal to or smaller than the particle size of the diamond particles by grinding or polishing; The method for producing a diamond electrode according to claim 1, comprising a step of heat-bonding the polycrystalline diamond and the substrate with a conductive brazing material in a vacuum.   The method for producing a diamond electrode according to claim 8, wherein the conductive diamond particles are obtained by crushing polycrystalline diamond synthesized by a gas phase synthesis method.   The method for producing a diamond electrode according to claim 8, wherein the conductive diamond particles are granular single crystal diamond synthesized under an ultrahigh pressure and a high temperature.   The conductive diamond particles are dissolved and removed with an acid from a diamond sintered body sintered with one or more binders selected from iron, nickel, cobalt and alloys thereof under an ultra-high pressure and high temperature. The method for producing a diamond electrode according to claim 8, wherein the diamond electrode is obtained by crushing.

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