JP7470291B2 - Carbide-bonded polycrystalline diamond electrode material - Google Patents

Description

The present invention relates to diamond electrodes, in particular to electrode materials using conductive diamond suitable for use in water treatment facilities for generating ozone water and purifying polluted water on various scales, and to methods for manufacturing the same.

A widely used method for obtaining conductive diamond electrodes that can be used in water treatment is to form a boron-doped diamond film on a metal substrate such as silicon or niobium or tantalum using the CVD method. The surface of the film formed is not necessarily flat; rather, unevenness increases the effective surface area and is considered advantageous as an electrode, so a method of forming a pattern on the surface of the substrate material in advance to obtain a large surface area is also widely used.

The formation of boron-doped diamond (BDD) using the CVD method has the advantage that it is possible to form high-purity diamond, and the amount of boron doped during the formation process can be changed as desired. However, the formation reaction takes a long time and requires a large amount of gas, so productivity is not high.

Boron-doped diamond is also produced in the form of abrasive grains by adding a boron compound to the starting material in a synthesis reaction using high pressure and high temperature. High pressure and high temperature technology has been used in Japan since around 1962 to produce cutting tool materials and wear-resistant materials, producing polycrystalline diamond (PCD), a hard material made by sintering diamond powder. (Patent Document 1)

Polycrystalline diamond, which is intended to be used as a tool material and wear-resistant material, is required to have not only outstanding hardness but also toughness, so there is a demand for dense products made from fine powder raw materials. For this reason, binderless nano-polycrystalline diamond, which has a hardness exceeding that of single-crystal diamond, has been developed as the ultimate polycrystalline diamond. This polycrystalline diamond has high heat resistance because it does not contain a binder, and the constituent particles are very fine, measuring several tens of nanometers, making it possible to form a high-strength, sharp cutting edge. (Non-Patent Document 1)

In the general production of polycrystalline diamond, cobalt-based metals are primarily used as sintering aids, and it is understood that the presence of molten metal at high temperatures and under ultra-high pressure promotes rearrangement of diamond particles, bonding between particles through a dissolution and precipitation mechanism, and densification.

Cobalt-based sintering aid metals remain between the diamond particles even after sintering, contributing to improved density and toughness of the sintered body, but at high temperatures they become an inverse catalyst that promotes graphitization of diamond. For applications exposed to high temperatures, a sintered body with 5 to 30% voids by volume has been proposed, in which the metal remaining between the particles is removed by acid dissolution treatment, while maintaining the direct bond between the diamond particles. (Patent Document 2)

On the other hand, polycrystalline diamond, which uses carbide as a binder, is also known as a highly heat-resistant sintered body, and the following examples are known as prior art that use transition metal carbides produced during the sintering reaction as a binder. All of these are intended to produce dense sintered products as materials for cutting tools, etc.

[1] This method involves mixing diamond powder with metal powders such as titanium and zirconium, melting the metal under high temperature and pressure conditions in the diamond stability range, and solidifying (sintering) the diamond powder through the metal carbide produced by the reaction with the diamond. A maximum sintering temperature of 1950°C is listed. (Patent Document 3)

[2] A known example of a highly heat-resistant sintered body is a sintered body made by exposing boron-containing diamond powder to high temperatures of over 1600°C using an alkaline earth metal carbonate as a binder. It has been shown that this sintered body can be used as a tool material that is heat-resistant and can be used for electric discharge machining. (Patent Document 4)

Patent Publication No. 39-20483 JP 53-114589 A Japanese Patent Publication No. 51-73512 JP 2008-133172 A

SEI Technical Review No. 165, 68-74 (2004)

The objective of this invention is to increase the productivity of conductive electrode materials by forming a porous, integrated product from pre-synthesized abrasive conductive diamond using pressure sintering technology in the manufacture of conductive electrode materials, while also enabling diversification of electrode shapes and characteristics.

The gist of the present invention is that when conducting sintering under pressure, a large amount of transition metal is placed between diamond particles as a sintering accelerator in advance, and the diamond particles are joined via the transition metal carbide produced by the reaction between diamond and the transition metal at high temperatures, and a sintered body with wide particle spacing is produced. In a subsequent process, the metal remaining between the particles is dissolved to obtain a porous sintered body.

The gist of the present invention is a diamond electrode material composed of a porous diamond aggregate in which conductive synthetic diamond particles are bonded together with voids between them, wherein adjacent diamond particles in the diamond aggregate are indirectly bonded together via transition metal carbides formed on the surfaces of the diamond particles, and the diamond aggregate has voids that account for more than 20% of the volume of the entire diamond aggregate, and the distributed voids are interconnected and open to the outer surface, providing spaces through which generated gas and surrounding water can pass during water electrolysis .

The electrode material is advantageously manufactured by a process which includes the steps of:
(1) Placing transition metal powder between conductive bulk diamond particles;
(2) subjecting the whole to a pressure and heat treatment to convert the transition metal in contact with the diamond particles into carbides and to bond the diamond particles together via the carbides;
(3) The transition metal powder remaining between the diamond particles is dissolved and removed to form conductive voids between the particles.

According to the present invention, in the production of polycrystalline diamond using high pressure and high temperature, the heating time of the sample is sufficient at 10 to 20 minutes, and even if the time for increasing and decreasing the pressure is included, the reaction cycle is about 30 minutes, making it possible to produce conductive electrode materials in less than one-tenth the time required for CVD reactions.

The transition metals used in the present invention are preferably Ti, Zr, Hf, Nb, Ta, Mo and W, and one or more of these metals can be used in a composition or blend. These metals have a strong affinity with carbon, so a carbide layer is formed at the boundary between the two materials by placing them adjacent to each other and heating them.

In the electrode material of the present invention, Ti and Zr are suitable as transition metals to be interposed between diamond particles from the viewpoint of dissolving them in the acid treatment in the subsequent process, and Ti is the most suitable from the viewpoint of cost.

Cobalt or nickel can also be added to these transition metals to promote rearrangement of diamond particles through the coexistence of a liquid phase, as well as direct bonding between diamond particles through a partial dissolution/precipitation mechanism.

In the manufacture of the electrode material of the present invention, it is convenient to use diamond particles that contain boron as a conductive material, i.e., so-called boron-doped diamond. Such diamonds are commercially available as special abrasive grains and are easy to obtain. There are also known conductive diamond varieties that are doped with nitrogen and phosphorus, which can be used in the same way as boron-doped diamond.

For sintered bodies intended as conductive electrode materials, it is sufficient if they are strong enough to maintain their shape, and they are required to have spaces between the diamond particles through which water and gas can pass, and a large surface area in contact with water. For this reason, it is desirable to use coarse mesh-sized abrasive grains as the raw diamond particles, and to create a porous sintered body in which the particles are bonded together via transition metal carbides formed in situ.

For this reason, in the present invention, boron-doped diamond abrasive grains of a size coarser than 400 mesh (35 μm) are used as the raw material, but it is particularly preferable to use a size of 200 mesh (75 μm) or more, and it is even more preferable to use abrasive grains of a size coarser than 100 mesh (150 μm) to form large pores.

The phenomenon of forming transition metal carbides by the reaction between diamond (carbon) and transition metals is observed even in the solid phase region, with the reaction clearly progressing at temperatures above 1000°C. Therefore, when joining diamond particles, it is not necessarily necessary to heat the material to a temperature that exceeds the melting point of the transition metal.

The sintering reaction is preferably carried out in a high-pressure region where diamond is thermodynamically stable, but the stable region is not a requirement, and techniques such as HIP, hot pressing, and spark plasma sintering can also be used by using a reducing atmosphere and shortening the reaction time.

To maintain the flatness of the sintered diamond layer, it is advantageous to use a transition metal plate or a ceramic plate as the substrate for the filled diamond. For example, when a transition metal, especially Ti or Nb, is used as the substrate material, a strong bond is formed at the contact point between the diamond particles and the substrate by chemical bonding via the carbide formed by the reaction between the two. In other words, the diamond particles are fixed together by titanium carbide formed by the reaction between titanium powder and diamond, for example, and at the contact point between the diamond particles and the substrate, a bond is formed by chemical bonding via the carbide formed in situ by the reaction between the two when heated.

When a transition metal plate is used as the substrate material, not only does it ensure the flatness of the electrode surface, it also functions as a reinforcing plate for the diamond sintered layer, making it possible to form the diamond layer as thin as a single layer.

Any metal remaining in the gaps between the diamond particles after the sintering reaction can be removed by a subsequent hydrochloric acid dissolution process. During this process, some of the substrate metal dissolves from the surface, but since the dissolution rate is slower than that of lightly sintered transition metal powder, it can be left in the form of a plate, which can be used as a substrate for conducting electricity.

Niobium and tantalum are almost insoluble in hydrochloric acid, making them the most suitable materials for the substrate. WC-Co-based cemented carbide can also be used instead of the transition metal plate, in which case the cobalt in the cemented carbide can be dissolved and removed together with the residual metal between the diamond particles during the hydrochloric acid dissolution process.

In place of hydrochloric acid dissolution, electrolytic extraction can also be used to remove residual metal between diamond particles. In this case, the sintered product is used as the anode in combination with an iron cathode in dilute sulfuric acid. For large-scale processing, it is effective to place the sintered product together in a titanium basket and perform the electrolytic operation.

Magnesia is a suitable ceramic substrate material. It has high thermal conductivity, a high melting point of 2800°C, and is soluble in hydrochloric acid. This means that it can be removed together with the residual metal between the diamond particles in a subsequent hydrochloric acid dissolution process, resulting in a self-supporting conductive thin plate made entirely of boron-doped diamond.

When using electric discharge machining to cut electrodes of a desired shape from electrode material, it is desirable to perform hydrochloric acid dissolution treatment after cutting into the desired shape.

In the method of the present invention, the sintering aid metal remaining between the diamond particles is dissolved and removed, forming conductive voids on the surface, making the electrode material porous and increasing the active surface area. This porous structure makes it possible to increase the active surface area, which results in the effects of increasing the gas generation efficiency by facilitating the movement of gas and water, and improving the electrode life by reducing the current density.

The proportion of voids in the diamond particle layer can be adjusted by the amount of sintering aid mixed with the diamond particles , but the relative amount of voids in the diamond particle layer (porosity) is preferably at least 5% of the total volume of the diamond particle layer, and more preferably 20% or more.

Next, the present invention will be explained using examples. In each of the following examples, a capsule made of thin niobium plate with a thickness of 0.15 mm and an outer diameter of 63 mm was used.

The reaction materials were filled into a Nb capsule in the following order to prepare a pressurized and heated sample having a thickness of approximately 5 mm.
Filling order (bottom to top)
Ti substrate, thickness 0.5mm
Mixture of 15g of boron-doped diamond (BDD) #80/100 (180μm) and 10g of Ti powder (45μm or less) Nb thin plate (thickness 0.15mm)

Ten of these capsules were stacked in layers and packed into a reaction chamber, which was then maintained under pressure and heating conditions of 5.5 GPa and 1350° C. for 10 minutes.
The reaction product was shot blasted to remove the surrounding niobium plate, and the titanium plate bonded with a sintered diamond layer approximately 2 mm thick was recovered.

Next, the titanium powder remaining between the diamond particles and lightly sintered was dissolved and removed using 6N hot hydrochloric acid.
Microscopic observation revealed that the diamond particles were bonded together to form a network structure, and X-ray diffraction confirmed that the sintered product was composed of diamond and titanium carbide.

The bonds between the diamond particles and between the diamond particle layer and the titanium substrate were both strong, and no peeling occurred during shot blasting or grinding.
From volume and mass measurements, the porosity of the diamond layer was estimated to be about 30%.

A 0.5 mm thick niobium plate was used as the substrate. A mixture of 20 g of boron-doped diamond #200/230 (approximately 75 μm) and 10 g of Ti powder (45 μm or less) was filled, and a 0.15 mm thick niobium plate was used as the lid.
The sintering conditions were 6 GPa and 1400° C., and these pressure and temperature conditions were maintained for 10 minutes.

The thickness of the diamond layer on the recovered sintered product was 2.3 mm. Since no dimensional change was observed in the niobium plate during post-processing, when it was dissolved in hydrochloric acid, the measurement accuracy of the porosity was high and it was estimated to be 22%.

A 2.5 mm thick WC-8%Co cemented carbide plate was used as the substrate. A mixture of 30 g of boron-doped diamond #200/230 (approximately 75 μm), 7 g of Ti powder (45 μm or less), and 3 g of Co powder (approximately 2 μm) was used as the filling mixture powder. The sintering conditions were 5.5 GPa and 1300°C, and the temperature was maintained for 10 minutes.

The thickness of the diamond layer of the sintered product was approximately 3 mm. After grinding to reduce the thickness of the cemented carbide to less than 0.5 mm, the metal components between the diamond particles were dissolved and removed by boiling in hydrochloric acid. The porosity in the diamond layer was estimated to be approximately 15%.

50 g of boron-doped diamond #270/325 (approximately 50 μm) was packed onto a 5 mm thick magnesia plate in combination with the same titanium-cobalt mixed powder as in Example 3. The sintering conditions were 5.5 GPa, 1300°C, and the temperature was maintained for 10 minutes. The sintered product was shot blasted to remove the niobium thin plate capsule material, and then dissolved in hot hydrochloric acid to remove the magnesia along with the residual metal between the diamond particles, resulting in a free-standing boron-doped diamond thin plate with a thickness of approximately 5 mm and a porosity of approximately 10%.

Claims (4)

This is a polycrystalline diamond electrode material comprising a porous diamond aggregate in which conductive synthetic diamond particles are bonded together with voids between them, wherein adjacent diamond particles in the diamond aggregate are indirectly bonded together via transition metal carbides formed on the surfaces of the diamond particles, and the diamond aggregate has voids that account for more than 20% of the volume of the entire diamond aggregate, and the distributed voids are interconnected and open to the outer surface, providing spaces through which generated gas and surrounding water can pass during water electrolysis . The polycrystalline diamond electrode material of claim 1, wherein the conductive synthetic diamond is boron-doped diamond (BDD) . The polycrystalline diamond electrode material described in claim 1, wherein the diamond particles are indirectly bonded to each other via transition metal carbides formed on the surfaces of the diamond particles, and the aggregate as a whole is fixed to the transition metal reinforcing material by chemical bonds. The polycrystalline diamond electrode material according to claim 1, for use in a water treatment electrolytic cell .