JP2706492B2 - Method of synthesizing diamond powder - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for synthesizing diamond powder.

[Prior Art] Conventionally, the following two methods are known as methods for synthesizing diamond powder by a gas phase method.

(1) As shown in JP-A-62-158195, a method in which an organic compound containing a hydrocarbon or a carbon material is introduced into a high-temperature thermal plasma, and is 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 a non-limited space to precipitate diamond powder.

(2) As shown in JP-A-63-156009, a mixed gas of an organic compound containing carbon and hydrogen is introduced into a high-frequency plasma or a microwave plasma, and simultaneously, a nucleus for diamond generation is introduced into the plasma. Method of depositing diamond on it.

[Problems to be Solved by the Invention] However, these methods have the following disadvantages. That is, in the method (1), the cooling gas is mixed with the reaction mixture containing the active species in a non-limited, relatively free space to thereby cool and precipitate the diamond powder. The degree of supersaturation of the active species cannot be made too high, and the amount of diamond deposited is extremely small. Further, the method (2) has a disadvantage that the purity is poor because a nucleus for diamond generation is introduced from the beginning, and the generated powder is a composite powder or a mixed powder of diamond and a foreign substance used as a nucleus. Had.

Accordingly, an object of the present invention is to provide a synthesis method for solving the above-mentioned disadvantages and efficiently depositing high-purity diamond powder with high productivity.

[Means for Solving the Problems] As a result of intensive studies on a method for synthesizing diamond powder, the present inventors have found that a reaction mixture containing active species generated by reacting a carbon source in plasma under reduced pressure is used. It has been found that the above object can be achieved by contact with a solid for cooling, and the present invention has been completed.

That is, the present invention provides a diamond source by reacting a carbon source in a plasma in the presence of hydrogen gas under reduced pressure, bringing a gaseous reaction mixture containing the generated active species into contact with a solid cooling body, and rapidly cooling the diamond. An object of the present invention is to provide a method for synthesizing a diamond powder by depositing the powder.

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

The gas used for plasma generation in the present invention includes:
At least one selected from an inert gas such as hydrogen gas, argon, neon, helium, and xenon, and a substance serving as a carbon source can be used, and used alone or as a mixed gas of two or more types. Also, a gas such as nitrogen or ammonia may coexist. Typically, a hydrogen gas, an inert gas, or a mixed gas thereof is used.

The carbon source used in the present invention may be any of gas, liquid and solid as long as it dissociates in a plasma flame to generate active species such as ionic species and radical species containing carbon. As this carbon source, for example, methane, ethane, saturated hydrocarbons such as propane, ethylene, propylene, unsaturated aliphatics such as acetylene, aromatic hydrocarbons such as benzene,
Alicyclic hydrocarbons such as cyclohexane and cyclopropane,
Alcohols such as ethanol, propanol and 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, acetic acid Fatty acids such as oxalic acid, carboxylic acids such as malonic acid, esters such as methyl formate and formate, acid amides such as acetic acid amide, amines such as methylamine and trimethylamine, and high molecular compounds such as polyethylene and polypropylene;
Organic compounds containing sulfur (S) such as thiophin, organic compounds containing phosphorus (P) such as phosphine, carbon monoxide, carbon dioxide, graphite, and the like can be given. One or more of these carbon source substances can be used.
When the carbon source is a liquid or solid, an inert gas such as argon or helium or a hydrogen gas may be used as a carrier gas.

In the method of the present invention, the reaction of the carbon source in the plasma is performed in the presence of hydrogen gas.The hydrogen gas may be introduced into the reaction system as a gas for plasma generation, for example, as described below. May be introduced into the reaction system separately from the plasma generating gas in the form of a sheath gas in the embodiment. Of course, it may be introduced in both forms.

Further, the carbon source may be introduced as a gas component for plasma generation as described above, or may be separately introduced into the generated plasma.

As a preferred embodiment of the method of introducing the plasma generating gas and the carbon source, a method of subjecting hydrogen or a mixed gas of hydrogen and an inert gas to discharge to generate plasma, and introducing a carbon source into the plasma is given. .

In the method of the present invention, it is usually preferable that the hydrogen gas and the carbon source are continuously introduced into the reaction system, but the ratio of the introduced carbon source and the hydrogen gas per unit time is determined by the number of carbon atoms. It is preferable that the ratio of the number of hydrogen atoms be in the range of 0.0001 to 10. If the ratio is too large, a substance having a structure called non-diamond carbon such as amorphous carbon or graphite tends to be precipitated, and if it is too low, it becomes difficult to form diamond.

In the method of the present invention, the reaction needs to be performed under reduced pressure, for example, in the range of 10 to 400 torr.

As the material of the cooling body used in the method of the present invention, for example, a simple substance such as copper, molybdenum, tungsten, silicon, stainless steel, tantalum, graphite; quartz glass, alumina, silicon carbide, silicon nitride, boron carbide, aluminum nitride, Compounds such as boron nitride can be given, and preferred are molybdenum, boron nitride, silicon and the like.
This cooling body is desirably cooled by a coolant such as water. The temperature of the cooling body is, for example, 400 to 1400 ° C.

The cooling body is arranged in the reactor so that active species generated in the plasma come into efficient contact with each other. For example, in the embodiments described later, the reaction mixture containing the active species flows in the direction of extension of the plasma flow, and collides with the cooling member arranged in the direction of extension of the reaction mixture. 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 deposited.

An embodiment of the method of the present invention will be described with reference to the drawings.
FIG. 1 shows an example of an apparatus for implementing a method of synthesizing diamond by cooling with a water-cooled cooling body using a direct current discharge, and FIG. 2 shows a diamond cooled with a water-cooled cooling body using a high-frequency discharge. An apparatus example of a method for synthesizing a powder will be described.

In FIG. 1, 1 is a DC plasma torch, 2 is a DC power supply, 3 is a water-cooled reaction vessel, 4 is a water-cooled cooling body capable of moving up and down and rotating, 5 is a gas inlet for plasma generation, 6 is A raw material gas inlet 7 is a sheath gas (gas that spirally flows down along the inner wall of the water-cooled reaction vessel). An inlet (not shown) whose nozzle (not shown) is tangentially inclined from the diameter direction so that the sheath gas spirally flows down. , 8 denotes a gas supply device, and 9 denotes a vacuum exhaust device capable of adjusting the pressure in the reaction vessel by adjusting the degree of opening and closing of an exhaust valve.

In synthesizing the diamond powder, for example, the inside of the reaction system is evacuated to a vacuum of 0.01 torr or more. Next, from step 5, argon is flowed as a plasma generation gas to discharge and generate plasma. Further, a sheath gas, in this case, a hydrogen gas or a mixed gas of an inert gas and a hydrogen gas is flowed as an atmosphere control and a reaction gas in this case. After installing the cooling body at the predetermined position shown on the extension 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 by the evacuation device 9.
The active species generated in the plasma flame collides with the cooling body 4 and is cooled, whereby it becomes a solid, crystallizes and grows into a diamond powder. The generated diamond powder is carried by the gas flow and is deposited on the inner wall of the water-cooled reaction vessel around the cooling body. Some of the active species may become solid on the cooling body and crystallize and grow to form a diamond film or diamond grains.

In FIG. 2, reference numeral 10 denotes a high-frequency generating coil, 11 denotes a high-frequency power supply, 12 denotes a cooling gas inlet, and the same reference numerals as in FIG. 1 denote the same elements as those in FIG. is there. When synthesizing diamond, 0.01 to
After evacuating to rr, an argon gas is supplied as a plasma generation gas from 5 and an argon gas as a sheath gas from 7 to generate high frequency plasma. Further, hydrogen gas is added to the sheath gas as a gas for adjusting the atmosphere and as a reaction gas. Further, starting from 6, the raw material is introduced into the plasma, and the reaction is carried out by adjusting the internal pressure of the reaction vessel to a predetermined pressure. As in the case of using the DC power supply, diamond powder is deposited on the inner wall of the water-cooled reaction vessel around the cooling substrate.

〔Example〕

Example 1 Using the device shown in FIG.
After evacuation of the reaction system to 0.01 torr, 17 l / min of argon and 3 l / min of hydrogen gas were flowed as plasma generation gas to generate DC plasma, and 5 l / min of hydrogen gas was flowed from the sheath gas inlet 7. . Then, acetylene was flowed at 0.8 l / min. At this time, the DC power input was set to 8 kW, the internal pressure of the reaction vessel was set to 100 torr, and the position of the cooling body was set to a position 90 mm below the DC torch.
As a result, 6 g of whitish powder precipitated on the inner wall of the reaction vessel.

The results of X-ray diffraction and Raman scattering spectrum measurement of the obtained powder are shown in FIGS. 3 and 4, respectively. In FIG. 4, a peak was observed only at around 1333 cm ⁻¹ derived from the diamond crystal. From these results, it was found to be cubic diamond. In addition, it was found from the scanning electron microscope observation that the diamond powder obtained was not divided and kept a perfect shape, and had been nucleated and grown in the gas phase.

Example 2 Using the apparatus shown in FIG. 2, the inside of the apparatus was evacuated to about 0.01 torr by the vacuum evacuation apparatus 9, and then 18 l / min of argon gas was introduced from the plasma generation gas inlet 5. Gas was flowed at 30 l / min. To generate high frequency plasma. After plasma generation, helium gas 3 l / min. Was added to the plasma gas, and hydrogen gas 15 l / min. Was added to the sheath gas. Ethanol vapor 0.75 / min. Was introduced into the plasma using 5 l / min. Of argon gas as a carrier gas from the raw material gas inlet, and a reaction was carried out for 30 minutes. At this time, the high-frequency power input was 60 kW, the internal pressure of the reaction vessel was set at 400 torr, and the position of the cooling body was set at a position 100 mm below the high-frequency coil. As a result, 4 g of a white to light gray powder was obtained on the inner wall of the reaction vessel around the cooling body. The obtained powder was a cubic diamond similar to that of Example 1 based on the results of X-ray diffraction, Raman scattering spectrum measurement, and scanning electron microscope observation.

Comparative Example 1 A diamond powder was synthesized for one hour under the same conditions as in Example 1 except that no cooling body was used. as a result,
0.1 g of a pale yellow powder precipitated on the inner wall of the reaction vessel. The powder was a cubic diamond based on the results of X-ray diffraction and Raman scattering spectrum measurements.

Comparative Example 2 Using the apparatus of Example 2, 30 l / min of hydrogen gas was introduced from the 12 cooling gas inlets without using a cooling body, and quenched with the cooling gas. The synthesis of the powder was carried out for 30 minutes.

As a result, 0.2 g of a gray-black powder was precipitated. From the results of X-ray diffraction and Maran scattering spectra, the obtained powder was found to be a cubic diamond powder containing a small amount of amorphous carbon.

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

[Brief description of the drawings]

1 and 2 are schematic views showing an example of an apparatus for carrying out the method of the present invention. FIG. 1 is a direct current discharge used in Example 1, and FIG. 1 shows a discharge reactor. FIG. 3 is an X-ray diffraction diagram of the diamond powder obtained in Example 1, and FIG. 4 shows its Raman scattering spectrum. 1. DC plasma torch 2. DC power supply 3. Water-cooled reaction vessel 4. Cooling body that can move up and down and rotate 5. Gas inlet for plasma generation 6. Source gas inlet 7. Sheath gas inlet 8. Gas supply device 9. Vacuum exhaust device 10. Coil for high frequency generation 11. High frequency power supply 12. Gas inlet for cooling

Claims (1)

(57) [Claims] A diamond powder is prepared by reacting a carbon source in a plasma under reduced pressure in the presence of hydrogen gas in a plasma, bringing a produced gas-phase reaction mixture containing active species into contact with a solid cooling body and rapidly cooling the mixture. A method for synthesizing diamond powder, comprising precipitating diamond.