JPH0360767B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a diamond synthesis method, and more particularly to a method for synthesizing diamond with few inclusions and good crystal grain shape. Industrially, diamonds are mainly polished, ground,
It is used for cutting, etc., but in this case, the grain shape of the diamond is a problem, and diamonds with good performance in grinding, etc. are said to have a polyhedral grain shape that is as close to a spherical shape as possible (so-called euhedral grains). It is also desirable to obtain diamonds with large particle sizes in as high a yield as possible. The object of the present invention is to produce large diamonds with excellent self-shape in a high yield. Conventionally, the most well-known method for this purpose is to carry out synthesis in the diamond stability region near the phase equilibrium line of diamond and non-diamond carbon (hereinafter the latter will simply be referred to as carbon). In order to obtain high-quality diamond crystals, it is necessary to suppress and reduce the generation of diamond crystal nuclei, and to gradually grow crystals based on the few nuclei. This is also the reason why the above synthesis is performed near the phase equilibrium line. However, since diamond synthesis is carried out at high temperatures and high pressures, it is not possible to directly control the temperature and pressure, and the only way to estimate the temperature and pressure within the synthesis system is through indirect methods. Therefore, it is difficult to maintain the temperature and pressure strictly in the vicinity of the phase equilibrium line, and in actual industrial methods, it is impossible to obtain a product with a good crystalline form in a high yield. The present invention is based on the discovery that by using a specific raw material carbon, the generation of synthetic nuclei can be controlled and crystal growth can be improved. That is, the present invention is characterized by using a carbon material that is easily graphitizable but has not yet been sufficiently graphitized. If this is expressed as the lattice constant C ₀ (002), which is an index of the degree of graphitization of carbon, the above raw material has a lattice constant C ₀ (002) at the time of use.
6.720 to 6.740, but if it is heated to 3000°C and graphitized, the lattice constant C ₀ (002) becomes 6.715 or less. A lot of research has been done on carbon as a raw material for diamonds, such as amorphous carbon and graphite, but as mentioned above, carbon that has the property of being easily graphitized but has not yet completely graphitized is used as a starting material. The idea had never been seen before. In order to obtain high quality and large diamonds with a good yield, the generation of nuclei during the heating process is small, but
Once the nucleus is generated, it is desirable that the crystal growth be relatively rapid. Since the carbon raw material in the present invention does not have a high degree of graphitization at the time of starting, the solubility of carbon in the solvent metal is low at the initial stage of diamond synthesis. Therefore, it is thought that the number of nuclei generated is small.
On the other hand, diamond synthesis is carried out at several hundred degrees Celsius and tens of thousands of atmospheric pressures, so if the temperature is maintained within this range, graphitization of carbon will proceed. In particular, since graphitizable carbon is used in the present invention, graphitization progresses quickly. As graphitization progresses, the solubility in the solvent metal increases, which contributes to the growth of the already generated nuclei. Therefore, during crystal growth, dissolved carbon is consumed in the growth of nuclei even under conditions of temperature and pressure that are far away from the phase equilibrium line and the diamond stability region, and the generation of new nuclei is suppressed. You can get something good. If a material with a high degree of graphitization is used as a starting material, the solubility in the solvent metal will be high, so too many nuclei will be generated, making it impossible to obtain a good quality product. On the other hand, when using a raw material with poor graphitizability, the generation of fewer nuclei is the same as in the present invention, but the solubility of carbon in the solvent metal is low even during diamond crystal growth.
Not enough diamond growth occurs. Carbon having a lattice constant C ₀ (002) in the above range in the present invention can be obtained, for example, by forming petroleum coke as it is or by adding petroleum pitch or the like to this coke powder and calcining it at about 2000 to 2500°C. . The temperature is too low and the lattice constant C ₀ (002)
If it is larger than 6.800, good results may not be obtained, either because the initial nucleation itself is not sufficient, or because the diamond synthesis conditions are not at a temperature sufficient for carbon to graphitize, so there is a problem with the solubility of carbon during crystal growth. I couldn't get it. In the present invention, the conditions for synthesizing diamonds may be the same as those for ordinary diamond synthesis, except that the carbon raw material mentioned above is selected. Solvent metals include Ni, Fe, C ₀ , Cr, Mn,
Ta, Pt, and metals containing these can be used.
The method for assembling the solvent metal and the carbon raw material may be such that each of them is formed into a thin plate and these are alternately stacked, or the powders of both may be simply mixed. The metal to carbon ratio is 100 parts metal to 30 to 500 parts carbon by weight. The temperature is 1300~2000℃, the pressure is
A range of 50-70Kbar is suitable. The method of the present invention can also be applied to the well-known seed method for growing diamond crystals, that is, a method in which diamond seeds are mixed in advance in a diamond system and crystals from these seeds are grown. If the carbon of the present invention is used as a carbon raw material in a synthesis system, diamonds will grow on the seeds with almost no generation of nuclei, resulting in a diamond with a good crystalline system. Next, the present invention will be specifically explained with reference to Examples and Comparative Examples. [Example 1] As a carbon raw material, petroleum coke was pulverized, petroleum pitch was added thereto, molded, and fired at about 2400°C. The lattice constant C ₀ (002) of this product was 6.730 as determined by powder X-ray diffraction. The graphitized version of this same product at 3000℃ has a lattice constant C ₀ (002)
was 6.713. A thin plate with a diameter of 28.6 mm and a thickness of 1.6 mm was cut from this carbon compact and used as a carbon raw material. Thin plates of 30Ni-70Fe alloy with a diameter of 28.6 mm and a thickness of 0.25 mm were used as the solvent metal, and these were cut out alternately. Stacking 24 sheets at a time and loading them into a belt-type ultra-high pressure device, it is estimated that 1450
℃ and 57 Kbar for 15 minutes, and diamond synthesis was performed twice each. [Example 2] As a carbon raw material, a mixture of pulverized coal pitch coke and coal pitch was molded and heated to approximately 2500°C.
I used the one that was fired. This lattice constant C ₀
(002) was 6.740. Using this graphitic carbon, diamond synthesis was performed twice in the same manner as in Example 1. Furthermore, when this carbon is further fired to 3000℃, C ₀ (002)
was 6.714. [Example 3] A polyvinyl chloride mass was fired at about 2800°C.
The lattice constant C ₀ (002) at this time was 6.725. From this, cut out a round bar with a diameter of 28.6 mm and a thickness of 1
It was made into a thin plate of mm. Cobalt plates with a thickness of 0.2 mm were used as the solvent metal, and they were alternately laminated in the same manner as in Example 1, and diamond synthesis was performed twice at 1500° C. and 58 Kbar. C ₀ (002) when this carbon is fired to 3000℃
was 6.715. [Comparative Example 1] The same molded body as in Example 1 was used, which was fired at 1000°C. The lattice constant C ₀ (002) of this product was found to be 6.810 by powder X-ray diffraction. [Comparative Example 2] The same molded body as in Example 1, which was fired at 3000°C, was used. [Comparative Example 3] Furan resin treated at about 2400°C was used. The lattice constant C ₀ (002) of this product is 6.840, and the lattice constant C ₀ (002) of the graphitized product treated at 3000℃
was 6.740. The results of Examples 1 to 3 and Comparative Examples 1 to 3 are shown in Table 1.

[Table] However, the yield in the table is the conversion rate of carbon to diamond.
Even for coal pitch coke and polyvinyl chloride, diamonds with regular self-shapes can be synthesized when the lattice constant of the present invention is within the range of C ₀ (002). As described above, it can be seen that the diamond synthesized by the method of the present invention has large crystals and is well-shaped, and although the yield is slightly lower than that of Comparative Example 2, it is extremely superior overall.

Claims (1)

[Claims] 1. In the method of synthesizing diamond made of non-diamond carbon and solvent metal, the non-diamond carbon has a lattice constant Co(002) of 6.720 to 6.740.
A diamond synthesis method characterized by using easily graphitizable carbon whose lattice constant Co(002) is 6.715 or less when graphitized at 3000°C, and treating at high temperature and high pressure.