JPH02120219A - Synthesis of diamond powder - Google Patents

Abstract

PURPOSE:To synthesize high-purity diamond powder in high efficiency and in high productivity by bringing into contact with a cooling solid a reaction mixture containing active seeds formed by reaction of a carbon source in a plasma under reduced pressure. CONSTITUTION:A carbon source 6 is allowed to react under reduced pressure (an evacuation apparatus 9) in the presence of H2 gas in a plasma (a torch 1, direct current source 2, and gas 5 for plasma generation), and the resultant gaseous reaction mixture containing active seeds formed is then brought into contact with a solid cooling form 4 to effect quenching, thus separating the objective diamond powder. The material for cooling form 4 is pref. molybdenum, boron nitride or silicon. This cooling form 4 is pref. chilled with a coolant such as water, being at ca.400-1400 deg.C.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for synthesizing diamond powder.

[Prior Art] The following two methods are conventionally known as methods for synthesizing diamond powder using a gas phase method.

(1) As shown in JP-A No. 62-158195, a method in which an organic compound or carbon material containing hydrocarbons is introduced into high-temperature thermal plasma and decomposed or evaporated to precipitate diamond. In the method described in this publication, active species generated by injecting a cooling gas into a reactor are cooled in an undefined space to precipitate diamond powder.

(2) As shown in Japanese Patent Application Laid-Open No. 63-156009, a mixed gas of an organic compound containing carbon and hydrogen is introduced into high-frequency plasma or microwave plasma, and at the same time, a nucleus for diamond production is introduced into the plasma. method of depositing diamonds on it.

[Problem to be solved by the invention] However, these methods have the following drawbacks.

That is, in the method (1), a cooling gas is mixed with a reaction mixture containing active species in a non-limited, relatively free space to cool the mixture and precipitate diamond powder. The degree of supersaturation of active species cannot be made very high, and the amount of diamond precipitated is extremely small. In addition, method (2) has the disadvantage that the purity is poor because the nucleus for diamond generation is introduced from the beginning, and the generated powder is a composite powder or mixed powder of diamond and the foreign substance used as the nucleus. It had

SUMMARY OF THE INVENTION An object of the present invention is to provide a synthesis method that eliminates the above-mentioned drawbacks and allows diamond powder of good purity to be precipitated efficiently and with high productivity.

[Means for Solving the Problems] As a result of intensive research into a method for synthesizing diamond powder, the present inventors have developed a reaction mixture containing active species produced by reacting a carbon source in plasma under reduced pressure. They have discovered that the above object can be achieved by contacting with a cooling solid, and have completed the present invention.

That is, the present invention reacts a carbon source in a plasma in the presence of hydrogen gas under reduced pressure, and rapidly cools the gas phase reaction mixture containing the generated active species by bringing it into contact with a cooling body made of a solid. A method of synthesizing diamond powder is provided which comprises precipitating the powder.

The plasma used in the present invention is obtained by discharging the gas described below, but the power source used for the discharge is not particularly limited, and for example, any one of direct current, high frequency, microwave, low frequency alternating current, or a superposition of these may be used. It may also be one in which a magnetic field is applied to direct current.

Further, as the gas used for plasma generation in the present invention, at least one kind selected from hydrogen gas, an inert gas such as argon, neon, helium, and xenon, and a substance that is a carbon source can be used alone or in combination. It is used as a mixed gas of more than one species. Further, a gas such as nitrogen or ammonia may coexist with this. Typically, hydrogen gas, inert gas, or a mixed gas thereof is used.

The carbon source used in the present invention may be any gas, liquid, or solid as long as it dissociates in a plasma flame to generate active species such as carbon-containing ionic species and radical species.

Examples of this carbon source include saturated hydrocarbons such as methane, ethane, and propane; unsaturated aliphatics such as ethylene, propylene, and acetylene; aromatic hydrocarbons such as benzene; alicyclic hydrocarbons such as cyclohexane and cyclopropane; Alcohols such as ethanol, propatool, tert-butyl alcohol, ethers such as dimethyl ether and diethyl ether, aldehydes such as formaldehyde and acetaldehyde, ketones such as acetone, halides such as methyl chloride and acetyl bromide, formic acid , fatty acids such as acetic acid, carboxylic acids such as Shellacic acid and malonic acid, esters such as methyl formate and formic acid esters, acid amides such as acetic acid amide, amines such as methylamine and trimethylamine, and polymers such as polyethylene and polypropylene. Compounds, organic compounds containing sulfur (S) such as thiophine, organic compounds containing phosphorus (P) such as phosphine, carbon oxide, carbon dioxide, graphite, and the like.

One or more of these carbon source substances can be used. When the carbon source is liquid or solid, an inert gas such as argon or helium or hydrogen gas may be used as the carrier gas.

In the method of the present invention, the reaction of the carbon source in the plasma is carried out in the presence of hydrogen gas, which is
It may be introduced into the reaction system as a gas for plasma generation, or may be introduced into the reaction system separately from the gas for plasma generation, for example in the form of a sheath gas in the examples described later. Of course, it may be introduced in both forms.

Furthermore, the carbon source may be introduced as a plasma generating gas component as described above, or may be separately introduced into the generated plasma.

A preferred embodiment of the method for introducing the plasma generating gas and the carbon source includes a method of subjecting hydrogen or a mixed gas of hydrogen and an inert gas to electric discharge to generate plasma, and introducing the carbon source into the plasma. .

In the method of the present invention, it is usually preferable that hydrogen gas and carbon source are continuously introduced into the reaction system, and the ratio of the introduced carbon source to hydrogen gas per unit time is determined by the number of carbon atoms. / number of hydrogen atoms is preferably in a range of 0.0001 to 10. If this ratio is too large, substances having a structure said to be made of non-diamond carbon, such as amorphous carbon and graphite, tend to precipitate. If it is too low, diamond formation will be difficult.

In the method of the present invention, it is necessary to carry out the reaction under reduced pressure, for example in the range of 10 to 400 torr.

Materials for the cooling body used in the method of the present invention include, for example, copper, molybdenum, tungsten, silicon, stainless steel, tantalum, graphite, etc.; quartz glass, alumina, silicon carbide, silicon nitride, boron carbide, aluminum nitride, Compounds such as boron nitride can be mentioned, and molybdenum, boron nitride, silicon, etc. are preferable. This cooling body is preferably cooled with a refrigerant such as water. Naoki Tai / 1 Zuijo 1J915ti 4t) O
The plasma cooling body is placed in the reactor so that the active species generated in the plasma come into efficient contact with it. For example, in the embodiment described later, a reaction mixture containing active species flows in the extending direction of the plasma flow and collides with a cooling body disposed in the extending direction, so that it efficiently contacts the cooling body and is cooled. As a result, after contact with the cooling body, the degree of supersaturation of the active species increases in the vicinity thereof, and diamond powder is efficiently precipitated.

Embodiments of the method of the present invention will be explained with reference to the drawings. FIG. 1 shows an example of an apparatus for carrying out the method of synthesizing diamond using direct current discharge and cooling with a water-cooled cooling body; The figure shows an example of an apparatus for synthesizing diamond powder using high-frequency discharge and cooling with a water-cooled cooling body.

In FIG. 1, l is a DC plasma torch, 2 is a DC power source, 3 is a water-cooled reaction vessel, 4 is a water-cooled cooling body that can move up and down and rotate, 5 is a gas inlet for plasma generation,
6 is a raw material gas inlet, 7 is a sheath gas (a gas that flows down in a spiral manner along the inner wall of the water-cooled reaction vessel) inlet (a nozzle (not shown) is connected from the diametrical direction to the tangential direction so that the sheath gas flows down in a spiral manner). 8 is a gas supply device;
Reference numeral 9 indicates a vacuum evacuation device capable of adjusting the pressure inside the reaction vessel by adjusting the opening/closing degree of an exhaust valve.

When synthesizing diamond powder, for example, the reaction system is evacuated to a vacuum level of 0.01 torr or more. Next, from step 5, argon is flowed as a plasma generation gas to cause discharge and generate plasma. Furthermore, from 7, a sheath gas, in this case hydrogen gas or a mixed gas of an inert gas and hydrogen gas, is flowed as an atmosphere adjustment and reaction gas. After the cooling body is installed at the predetermined position shown in the figure on the extension line of the plasma flow, the raw material is introduced into the plasma flame from 6. The reaction is carried out by controlling the internal pressure of the reaction vessel using a vacuum evacuation device (No. 9). The activated species generated in the plasma flame collide with the cooling body 4 and are cooled, thereby becoming solid, crystallized, and grain-grown to become diamond powder. The generated diamond powder is carried by the gas flow and deposited on the inner wall of the water-cooled reaction vessel around the cooling body.

Note that some of the active species may become solid on the cooling body, crystallize, and grow into grains to form a diamond film or diamond grains.

In FIG. 2, 10 is a coil for high frequency generation, 11 is a high frequency power source, 12 is a cooling gas inlet, and the other first
Elements with the same numbers as those in the figures are the same as in FIG. 1. When synthesizing diamond, the reaction system is kept at 0.
After evacuation to 0.01 torr, argon gas is supplied as a plasma generation gas through 5, and argon is supplied as a sheath gas through 7 to generate high-frequency plasma. Furthermore, hydrogen gas is added to the sheath gas as an atmosphere adjustment and reaction gas. Furthermore, raw materials are introduced into the plasma from step 6, and the internal pressure of the reaction vessel is adjusted to a predetermined pressure to carry out a reaction.

Similar to when a DC power source is used, diamond powder is deposited on the inner wall of the water-cooled reaction vessel around the cooling substrate.

〔Example〕

Example 1 Using the apparatus shown in FIG. 1, the inside of the reaction system was evacuated to 0.01 torr by the evacuation device 9, and then argon was supplied as plasma generation gas at 17 f/min, and hydrogen gas was supplied at 3 j! /s+in, to generate DC plasma, and hydrogen gas was flowed at 51/s+in from the sheath gas introduction lower. Next, 0.0% acetylene was introduced from the raw material gas inlet 6. 8j
! /sin, and the reaction was carried out for 1 hour. At this time,
DC power input is 8kW, reaction vessel internal pressure is 100 torr.
r, the position of the cooling body was set at a position 90 am below the DC torch. As a result, 6g of whitish powder was found on the inner wall of the reaction vessel.
was precipitated.

The results of X-ray diffraction and Raman scattering spectroscopy of the obtained powder are shown in FIGS. 3 and 4, respectively. In FIG. 4, a peak was observed only around 1333C11-', which is derived from diamond crystals. From these results, it was found that it was cubic diamond. Note that the diamond powder obtained through scanning electron microscopy revealed that the particles were not divided and maintained a perfect shape, indicating that they were nucleated and grown in the gas phase.

Example 2 Using the apparatus shown in FIG. 2, the inside of the apparatus was evacuated to approximately 0.01 torr by the evacuation device 9, and then argon gas was pumped in at 181/s+in from the plasma generation gas inlet 5.
, Argon gas is introduced from the sheath gas introduction lower at 301/l.
ll1n, and a high frequency plasma was generated.

After plasma generation, helium gas 31 /
n+in, , hydrogen gas 151/lll1 in sheath gas
n, was added. Ethanol vapor (0.75/win) was introduced into the plasma using 5 l/min of argon gas as a carrier gas through the raw material gas inlet, and a reaction was carried out for 30 minutes. At this time, the high-frequency power input was 60 to -, the internal pressure of the reaction vessel was 400 torr, and the cooling body was set at a position 100 m below the high-frequency coil. As a result, 4 g of white to light gray powder was obtained on the inner wall of the reaction vessel around the cooling body. The obtained powder was found to be a cubic diamond similar to that in Example 1, based on the results of X-ray diffraction and Raman scattering spectroscopy and scanning electron microscope observation.

Comparative Example 1 Diamond powder was synthesized for 1 hour under the same conditions as in Example 1 without using a cooling body.

As a result, 0.1 g of pale yellow powder was deposited on the inner wall of the reaction vessel. This powder was found to be cubic diamond according to the results of X-ray diffraction and Raman scattering spectroscopy.

Comparative Example 2 Using the apparatus of Example 2, hydrogen gas at 301/sin was introduced from the 12 cooling gas inlets without using a cooling body, and the diamond was rapidly cooled by the cooling gas, but the other conditions were the same as in Example 2. Powder synthesis was carried out for 30 minutes.

As a result, 0.2 g of gray to black powder was precipitated.

The obtained powder was found to be cubic diamond powder containing a small amount of amorphous carbon from the results of X-ray diffraction and Raman scattering spectra.

[Effects of the Invention] According to the method of the present invention, highly pure diamond powder containing no non-diamond prize carbon can be produced efficiently and with high productivity.

[Brief explanation of the drawing]

Figures 1 and 2 are schematic diagrams showing examples of equipment for carrying out the method of the present invention. Figure 1 shows the DC discharge used in Example 1, and Figure 2 shows the high frequency discharge used in Example 2. FIG. 3, which shows the discharge reactor, is an X-ray diffraction diagram of the diamond powder obtained in Example 1, and FIG. 4 shows its Raman scattering spectrum. ■, DC plasma torch 2, DC power supply 3, water-cooled reaction vessel 4, cooling body 5 that can move up and down and rotate, plasma generation gas inlet 6, raw material gas inlet a, sheath gas inlet 8, Gas supply device 9, vacuum exhaust device 10, coil 11 for high frequency generation, high frequency power supply 12, gas inlet for cooling

Claims (1)

[Claims] Diamond powder is precipitated by reacting a carbon source in plasma under reduced pressure in the presence of hydrogen gas, and rapidly cooling the gas phase reaction mixture containing the generated active species by contacting it with a solid cooling body. A method for synthesizing diamond powder.