A method for chemical vapor deposition for preparing spherical or spheroidal polycrystalline diamond and products thereof

Abstract

The application discloses a method for preparing spherical or spheroidal polycrystalline diamond by chemical vapor deposition and a product thereof, and belongs to the technical field of artificial diamond. The method comprises the following steps: (1) providing a sheet with nano-particles as nucleation points on the surface of the sheet; (2) taking the sheet with the nano-particles as nucleation points on the surface as a substrate, and using a chemical vapor deposition method to first raise the temperature of the substrate to 700-1200 DEG C, then introduce hydrogen and methane into a reaction cavity, the volume flow ratio of the methane and the hydrogen is 1-7:100, and spherical or spheroidal polycrystalline diamond is deposited; and (3) separating the spherical or spheroidal polycrystalline diamond from the surface of the substrate processed in the step (2), and obtaining a product, i.e., spherical or spheroidal polycrystalline diamond powder, after further purification. The method has the advantages of precise control on the diamond particle size and good maintenance of the spherical degree, and the obtained spherical or spheroidal polycrystalline diamond is micron-level, the size is adjustable, the spherical degree is high, and the uniformity is good.

Description

A method for preparing spherical or near-spherical polycrystalline diamond by chemical vapor deposition and the product thereof. Technical Field

[0001] This invention belongs to the field of synthetic diamond technology, specifically relating to a method for preparing spherical or near-spherical polycrystalline diamond by chemical vapor deposition and the product thereof. Background Technology

[0002] In the preparation of composite materials, diamond micropowder is often used as a filler to improve the strength, hardness, thermal conductivity, and other properties of the composite material. To better achieve the density of diamond micropowder packing and controllability of the filling process, it is generally desirable for diamond particles to have a near-spherical structure. Under high temperature and high pressure synthesis conditions, diamond single crystals generally form regular crystal morphologies, whose specific morphology evolves with temperature gradients, typically exhibiting a millimeter-scale hexagonal octahedral shape. Micron-scale diamond powder, on the other hand, is obtained by crushing and classifying millimeter-scale diamond single crystals. Therefore, micron-scale diamond powder usually has an irregular shape after crushing. When filling composite materials, the arrangement and flowability of this irregularly shaped diamond micropowder are difficult to control, resulting in deficiencies in the microstructural uniformity and forming flowability of polycrystalline diamond (PCD) tools and polycrystalline diamond composite (PDC) drill bits prepared from it. Therefore, how to effectively prepare spherical or near-spherical diamond micropowder has always been a problem that the industry urgently needs to solve.

[0003] Currently, there are two main methods for obtaining spherical or near-spherical diamond micropowder: one method is mechanical ball milling, which uses a high-speed rotating ball mill to grind irregular diamond micropowder raw materials, removing the sharp corners of irregular shapes and gradually making the particles approach a near-spherical shape. However, impurities are easily introduced during the ball milling process, and it is difficult to accurately control the particle size and shape. The resulting spherical diamond particles have a wide particle size distribution and poor surface quality. The other method is granulation technology, which uses a binder to bind diamond micropowder into spherical clusters in a liquid or gaseous medium. However, the diamond micropowder obtained by this method relies on the binder to hold them together, resulting in a large size (usually tens to hundreds of micrometers), low interparticle strength, and the introduction of other impurities.

[0004] Chinese patent document CN115194160A discloses a method for preparing spherical polycrystalline diamond sintered bodies. The method involves adding metallic cobalt as a binder in a granulator and forming tiny diamond spherical powder particles with a size of 0.2 mm to 1 mm by continuous vibration and rolling of diamond micro powder and binder. The literature (Liu Ju, Wan Long, Hong Qiu, et al. Preparation of spherical diamond deposited abrasive by reverse microemulsion method and performance study [J]. Functional Materials, 2018, 49(10):140-144.) disperses irregular diamond micropowder in silica sol to form a mixed slurry, and then slowly adds the mixed slurry to a microemulsion system that is stirred at high speed. In the microemulsion system, the mixed slurry will spontaneously disperse into countless spherical droplets. After thorough stirring, it reacts with anhydrous ethanol and isooctyl alcohol in the microemulsion system to form spherical gel particles. After drying and high-temperature calcination, the organic matter such as isooctyl alcohol is fully decomposed, and finally diamond deposited abrasive coated with inorganic ceramic binder is obtained. In addition, there is a report (Chang Chuang. Preparation and performance study of ceramic-bonded abrasives by pressure spray drying [D]. Hunan University.) that mixed ceramic binders of different proportions with diamond micropowder to form a suspension, and obtained spherical diamond micropowder agglomerates bound by ceramic binders by spray drying equipment. However, both ball milling and granulation techniques have inherent defects such as introducing impurities, irregular morphology and non-uniform size, and the particle size range of the products is large (0.3 μm-12 mm). Summary of the Invention

[0005] To address the shortcomings of the existing technology, this invention provides a method for synthesizing spherical or near-spherical polycrystalline diamond using chemical vapor deposition (CVD). This method combines precise control of diamond particle size with good sphericity maintenance.

[0006] The specific technical solution adopted is as follows:

[0007] A method for preparing spherical or near-spherical polycrystalline diamond by chemical vapor deposition includes the following steps:

[0008] (1) A sheet material is provided, wherein the surface of the sheet material has nanoparticles as nucleation sites, and the constituent elements of the nanoparticles have a thermal expansion coefficient greater than 2.5 × 10⁻⁶. -6 Elements at / ℃;

[0009] (2) Using a sheet with nanoparticle nucleation points on the surface as a substrate, the substrate temperature is first raised to 700-1200℃ by chemical vapor deposition, and then hydrogen and methane are introduced into the reaction chamber. The volume flow rate ratio of methane to hydrogen is 1-7:100, and spherical or near-spherical polycrystalline diamond is deposited.

[0010] (3) Stop heating and gas introduction, cool down and take out the treated substrate, separate spherical or near-spherical polycrystalline diamond from the surface of the substrate treated in step (2), and further purify to obtain the product spherical or near-spherical polycrystalline diamond powder.

[0011] This invention discovers that, in a CH4 and H2 atmosphere that meets the requirements for diamond deposition, a heating source causes the CH bonds to ionize into carbon-based plasma, which can further deposit spherical or near-spherical polycrystalline diamonds using nanoparticles as nucleation sites.

[0012] Furthermore, the nanoparticles are preferably cobalt nanoparticles or silicon nanoparticles (cobalt and silicon have thermal expansion coefficients of ~13×10⁻⁶). -6 / ℃ and ~2.6×10 -6 / ℃), the particle size of the nanoparticles on the surface of the sheet is 10-100 nm.

[0013] Optionally, elements with higher coefficients of thermal expansion in the composite sheet can be heated and expanded to precipitate out, forming nanoparticles on the surface of the sheet.

[0014] Preferably, the preparation method of the sheet with nanoparticle nucleation points on the surface is as follows: silicon or cobalt in silicon-diamond composite sheet (silicon content range 5-30wt%) or cobalt-based polycrystalline diamond composite sheet (cobalt content range 5-40wt%) is melted and precipitated by hot filament chemical vapor deposition to form cobalt nanoparticles or silicon nanoparticles on the surface of the sheet.

[0015] In a further preferred embodiment, during the hot-wire chemical vapor deposition process described above, the distance between the metal wire (hot wire) and the silicon-diamond composite sheet or cobalt-based polycrystalline diamond composite sheet is maintained at 4 mm-10 mm. Hydrogen gas is introduced into the reaction chamber at a volumetric flow rate of 400 sccm-500 sccm, and the temperature is maintained for 1-2 hours under a power of 4.0-5.0 kW. After heating is stopped or the power is reduced, silicon or cobalt melts and precipitates, forming cobalt nanoparticles or silicon nanoparticles on the surface of the sheet.

[0016] Optionally, the preparation method of the sheet with nanoparticle nucleation points on the surface is as follows: cobalt nanoparticles or silicon nanoparticles are obtained on the surface of the sheet by magnetron sputtering deposition or vapor deposition. The sheet material includes, but is not limited to, stainless steel, molybdenum sheet, titanium sheet, cemented carbide, etc.

[0017] Optionally, the preparation method of the sheet with nanoparticle nucleation points on the surface is as follows: using hydrogen plasma to bombard the silicon-diamond composite sheet or the cobalt-based polycrystalline diamond composite sheet to form cobalt nanoparticles or silicon nanoparticles on the surface of the sheet.

[0018] Preferably, during the bombardment process, the microwave power is 3-6kW, the cavity pressure is 5-10kPa, and the bombardment time is 30-60 minutes.

[0019] Furthermore, in step (2), the chemical vapor deposition method selected is either hot filament chemical vapor deposition or microwave plasma chemical vapor deposition.

[0020] In step (2), during the chemical vapor deposition process, nanoparticles on the sheet surface serve as nucleation sites to deposit spherical or near-spherical polycrystalline diamonds. As the holding time increases, the deposited spherical or near-spherical polycrystalline diamonds gradually grow larger.

[0021] Preferably, in step (2), the power is adjusted to 3.6-6.5 kW to raise the substrate temperature to 700-1200℃, and hydrogen and methane are introduced into the reaction chamber. The volumetric flow rate of hydrogen is 400-500 sccm, and the volumetric flow rate ratio of methane to hydrogen is 1-7:100. Under a pressure of 2-12 kPa, spherical or near-spherical polycrystalline diamond is deposited for 1-50 h.

[0022] Furthermore, in step (3), the method for separating spherical or near-spherical polycrystalline diamond from the substrate surface is physical exfoliation (preferably ultrasonic oscillation), and the purification method includes acid washing, centrifugation, magnetic separation or hydrocyclone separation.

[0023] Preferably, the magnetic rods used for magnetic separation are not limited to neodymium iron boron magnetic rods, cobalt magnetic rods, ferrite magnetic rods, etc.

[0024] Preferably, the feed pressure for hydrocyclone separation is 0.05-0.3 MPa.

[0025] The present invention also provides a spherical or near-spherical polycrystalline diamond, which is prepared by the method for preparing spherical or near-spherical polycrystalline diamond by chemical vapor deposition.

[0026] The spherical or near-spherical polycrystalline diamond has a particle size of 0.1-10 μm and a purity of ≥99%. Compared with spherical or near-spherical polycrystalline diamond prepared by traditional methods (such as ball milling, granulation technology, or reverse emulsion method), the spherical or near-spherical polycrystalline diamond prepared by the method of this invention has a smoother and more uniform particle size distribution, regular shape, no internal cracks and no obvious defects; the purity can reach more than 99%, and no metal impurities are introduced.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] (1) The method of the present invention uses nanoparticles on the surface of the sheet as nucleation points and uses chemical vapor deposition to grow carbon atoms epitaxially into spherical or near-spherical polycrystalline diamonds under specific atmospheric conditions (diamond bonds are formed between diamond grains). The resulting spherical or near-spherical polycrystalline diamonds are in the micrometer range, with adjustable size, high sphericity, and good uniformity.

[0029] (2) This invention has developed a variety of methods for preparing sheets with nanoparticle nucleation sites on the surface. In particular, hot filament chemical vapor deposition can be used to melt and precipitate the second phase in the composite material to form nanoparticle nucleation sites. The method is simple and efficient.

[0030] (3) The method of the present invention is simple, efficient and easy to implement. Spherical or near-spherical polycrystalline diamond can be obtained by two-step or one-step method. First, a sheet with nanoparticle nucleation points on the surface can be prepared, and then spherical or near-spherical polycrystalline diamond can be prepared by chemical vapor deposition. Alternatively, when the thermal expansion coefficient of the second phase is similar to that of diamond, methane gas can be directly introduced after the formation of nanoparticles to prepare spherical or near-spherical polycrystalline diamond by chemical vapor deposition. Attached Figure Description

[0031] Figure 1 is a schematic diagram of a representative chemical vapor deposition process for preparing spherical or near-spherical polycrystalline diamond (NCD represents nanocrystalline diamond).

[0032] Figure 2 is an SEM image of cobalt nanoparticles formed on the PDC surface in Example 1.

[0033] Figure 3 is a SEM image of the micron-sized spherical or near-spherical polycrystalline diamond deposited in Example 1.

[0034] Figure 4 is a SEM image of the nanoscale spherical or near-spherical polycrystalline diamond deposited in Example 1.

[0035] Figure 5 shows EDS scan images of the micron-sized spherical or near-spherical polycrystalline diamonds deposited in Example 1, with a scale bar of 1 μm.

[0036] Figure 6 is an optical image of the spherical or near-spherical polycrystalline diamond particles obtained in Example 3.

[0037] Figure 7 is an optical image of the spherical or near-spherical polycrystalline diamond particles obtained in Example 4.

[0038] Figure 8 is an optical image of the spherical or near-spherical polycrystalline diamond particles obtained in Example 5.

[0039] Figure 9 shows the morphology of micron-sized diamond particles obtained after crushing and grading diamond single crystals. Detailed Implementation

[0040] To make the objectives, features, and advantages of this invention more apparent and understandable, a detailed description is provided below through specific embodiments. Many specific details are set forth in the following description to provide a thorough understanding of the invention. However, the invention can be practiced in many other ways different from those described herein, and those skilled in the art can make similar modifications without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below. Technical features in various embodiments of the invention can be combined appropriately without mutual conflict.

[0041] Unless otherwise specified, the operating methods in the following examples are generally performed under conventional conditions or as recommended by the manufacturer. Contents not described in detail in this specification are prior art known to those skilled in the art. Unless otherwise specified, the experimental materials used in the examples below can be purchased from conventional biochemical reagent companies.

[0042] Example 1

[0043] Spherical or near-spherical polycrystalline diamonds were prepared by HFCVD deposition using cobalt-based polycrystalline diamond composite sheets as a substrate (flowchart shown in Figure 1). The cobalt-based polycrystalline diamond composite sheets were synthesized under high temperature and high pressure, and the mass percentage of the cobalt phase was 10 wt%.

[0044] Step 1: Take a polycrystalline diamond composite (PDC) sheet containing 10 wt% Co, clean the surface, place it in a hot-wire chemical vapor deposition furnace, adjust the distance between the tantalum wire and the PDC to 4 mm, and start the vacuum system to evacuate the base vacuum to 10. -3 To minimize the impact of impurity gases, 400 sccm of hydrogen gas was then introduced to stabilize the chamber pressure at 2 kPa. The power was adjusted to 4000 W, and the chamber was held at this temperature for 1 hour to allow the cobalt metal phase to melt and precipitate. Heating was then stopped to lower the temperature inside the hot filament chemical vapor deposition furnace. The molten cobalt phase cooled and formed cobalt nanoparticles on the PDC surface (relieving the thermal stress of the Co phase and preventing diamond from failing to adhere to the substrate surface due to the difference in thermal expansion coefficients).

[0045] Step 2: Hydrogen and methane gas are introduced into the hot-wire chemical vapor deposition furnace. The volumetric flow rate ratio of hydrogen to methane is adjusted to 100:3, and the volumetric flow rate of hydrogen remains at 400 sccm. The power is adjusted to 3.6-4.4 kW to raise the substrate temperature to ~1000℃. Under a pressure of 2.3 kPa, cobalt nanoparticles precipitated on the PDC surface serve as nucleation sites. Spherical or near-spherical polycrystalline diamonds are formed after 3 hours of deposition. The size of the corresponding spherical or near-spherical polycrystalline diamonds is 0.3-3.2 μm.

[0046] Step 3: Stop heating and gas introduction. When the furnace temperature drops to near room temperature, take out the sample and place it in a glass filled with alcohol. Use an ultrasonic cleaner to shake for 1 hour to separate the spherical or near-spherical polycrystalline diamond from the PDC substrate. Take out the suspension and use a high-speed centrifuge (8000 rpm) for coarse separation for 10 minutes. Filter to obtain powder.

[0047] Step 4: Use a neodymium iron boron magnetic rod to extract the residual cobalt metal from the powder, then put the remaining powder into a hydrocyclone and treat it at a pressure of 0.2 MPa for 10 minutes. The fine powder flows out from the bottom outlet, yielding 0.7 g of spherical or near-spherical polycrystalline diamond particles.

[0048] Figure 2 shows the SEM image of the cobalt nanoparticles formed on the PDC surface in step 1. Their particle size is small, ranging from 10 to 100 nm. Figure 3 shows the SEM image of the deposited micron-sized spherical or near-spherical polycrystalline diamond. Figure 4 shows the SEM image of the deposited nano-sized spherical or near-spherical polycrystalline diamond (micron-sized spherical or near-spherical polycrystalline diamond was obtained with 4.4 kW power, and nano-sized spherical or near-spherical polycrystalline diamond was obtained with 3.6 kW power). Figure 5 shows the EDS elemental analysis diagram, which clearly shows the morphology of the spherical polycrystalline diamond. Figure 9 shows the morphology of the micron-sized diamond particles obtained after diamond single-crystal crushing and grading.

[0049] Example 2

[0050] Spherical or near-spherical polycrystalline diamonds were prepared by HFCVD deposition using silicon-diamond composite sheets as a substrate. The silicon-diamond composite sheets were synthesized under high temperature and high pressure, with silicon comprising 15 wt% by mass.

[0051] Step 1: Take the silicon-diamond composite wafer, clean its surface, and place it in a hot-wire chemical vapor deposition furnace. Adjust the distance between the tantalum wire and the silicon-diamond composite wafer to 4 mm, and start the vacuum system to evacuate the base vacuum to 10. -3 To minimize the impact of impurity gases, hydrogen gas at 400 sccm was introduced to stabilize the chamber pressure at 2 kPa. The power was adjusted to 4500 W, and the chamber was kept at this temperature for 1 hour to allow silicon to thermally expand and precipitate, forming silicon nanoparticles on the surface of the silicon-diamond composite. Heating was then stopped, allowing the temperature inside the hot-wire chemical vapor deposition furnace to decrease, and the precipitated silicon phase to cool, forming silicon nanoparticles on the substrate surface.

[0052] Step 2: Adjust the wire pitch to 6 mm using the lifting platform. While ensuring a hydrogen volume flow rate of 400 sccm, introduce 16 sccm of methane into the furnace. Adjust the power to 4000 W to raise the substrate temperature to 1000℃. Under a pressure of 2.3 kPa, use silicon nanoparticles on the surface of the silicon-diamond composite sheet as nucleation sites to deposit spherical or near-spherical polycrystalline diamond for 3 hours.

[0053] Step 3: Stop heating and gas introduction. When the furnace temperature drops to close to room temperature (26°C), take out the sample and place it in a glass filled with alcohol. Use an ultrasonic cleaner to agitate for 2 hours to separate the spherical or near-spherical polycrystalline diamond from the substrate. Take out the suspension and centrifuge it for 10 minutes using a high-speed centrifuge (10,000 rpm) to achieve coarse separation. The diamond will quickly settle to the bottom and be filtered to obtain powder.

[0054] Step 4: Place the powder into a hydrocyclone, set the feed pressure to 0.3 MPa and process for 10 minutes. The fine powder flows out from the bottom outlet, yielding 0.5 g of spherical or near-spherical polycrystalline diamond particles.

[0055] Example 3

[0056] Using a steel substrate as the base, silicon nanoparticles are first vapor-deposited on the steel substrate, and then spherical or near-spherical polycrystalline diamonds are prepared by HFCVD deposition.

[0057] Step 1: Clean the steel substrate sequentially with acetone and alcohol using ultrasonic cleaning for 15 minutes to remove surface oil and oxides. After drying, place it in a hot-wire chemical vapor deposition furnace. Adjust the distance between the tantalum wire and the substrate to 5 mm, introduce 500 sccm of hydrogen gas, gradually increase the power to 3000 W, stabilize the chamber pressure at 2.5 kPa, and heat to 600 ℃ for 30 minutes to further clean and activate the surface.

[0058] Step 2: Maintain a hydrogen flow rate of 500 sccm, increase the power to 3800 W, stabilize the substrate temperature at 850℃, stabilize the chamber pressure at 3 kPa, and introduce 10 sccm of silane (SiH4) as a silicon source. Deposit for 40 minutes. The silane decomposes under the action of the hot filament, and silicon atoms nucleate and grow into silicon nanoparticles of 50-100 nm on the steel surface. After deposition, stop the silane gas supply, maintain hydrogen and power to continue annealing for 10 minutes to stabilize the silicon particles, stop heating, maintain a hydrogen flow rate of 500 sccm, and wait for the substrate temperature to drop naturally to 600℃ (approximately 15 minutes). Then, turn on the water cooling system to accelerate cooling to below 300℃ to prevent the steel substrate from overheating and deforming and to fix the distribution of silicon nanoparticles.

[0059] Step 3: Readjust the distance between the tantalum wire and the substrate to 7 mm, introduce 400 sccm of hydrogen and 18 sccm of methane, adjust the power to 3600 W to raise the substrate temperature to 820℃, and deposit for 13 hours using silicon nanoparticles on the substrate surface as nucleation sites under a pressure of 4 kPa. The methane is decomposed under the action of the hot wire, and carbon atoms are epitaxially grown into spherical or near-spherical polycrystalline diamonds using silicon nanoparticles as nucleation sites.

[0060] Step 4: Stop heating and gas introduction. When the furnace temperature drops to near room temperature, take out the sample and place it in a glass filled with alcohol. Use an ultrasonic cleaner to agitate for 2 hours to allow the spherical or near-spherical polycrystalline diamond to peel off from the steel surface. Take out the suspension and use a high-speed centrifuge (10,000 rpm) for coarse separation for 10 minutes (diamond particles with high density settle to the bottom, while metal impurities and silicon particles concentrate in the supernatant). Filter to obtain powder.

[0061] Step 5: Place the powder into a hydrocyclone and treat it for 10 minutes at a feed pressure of 0.3 MPa. The fine powder flows out from the bottom outlet, yielding 0.6 g of spherical or near-spherical polycrystalline diamond particles. The optical image of the particles is shown in Figure 6.

[0062] Example 4

[0063] Spherical or near-spherical polycrystalline diamonds were prepared by MPCVD using cobalt-based polycrystalline diamond composite sheets as a substrate. The cobalt-based polycrystalline diamond composite sheets were the same as those in Example 1.

[0064] Step 1: Clean the cobalt-based polycrystalline diamond composite substrate sequentially with acetone and alcohol for 15 minutes using ultrasonic cleaning to thoroughly remove surface oil and adsorbents. After drying with high-purity nitrogen, place it on the substrate stage of the MPCVD reaction chamber and adjust the height of the substrate stage so that the substrate is located in the concentrated area of ​​the microwave plasma ball.

[0065] Step 2: Close the reaction chamber and start the vacuum system to evacuate the base vacuum to 10. -3 To eliminate interference from air and moisture, high-purity hydrogen gas was introduced at a flow rate of 500 sccm to stabilize the cavity pressure at 5 kPa. The microwave power was then slowly increased to 3 kW to ignite and form a stable hydrogen plasma. The substrate was then bombarded and cleaned for 30 minutes to further activate the surface and form cobalt nanoparticles on the substrate surface.

[0066] Step 3: Increase the microwave power to 5.5 kW and maintain the reaction pressure at 8 kPa. Introduce hydrogen and methane gas into the MPCVD reaction chamber. The CH4 flow rate is 15 sccm and the H2 flow rate is 500 sccm (CH4 / H2 volume ratio is approximately 3%). Raise the substrate temperature to 850℃. Under a pressure of 8 kPa, deposit cobalt nanoparticles as nucleation sites for 10 hours to form spherical or near-spherical polycrystalline diamond. After deposition, first turn off the methane gas flow and maintain the sample in a pure hydrogen plasma environment for 5 minutes to perform hydrogen termination treatment on the surface of the newly grown diamond to stabilize its surface properties. Then, turn off the microwave power, stop heating, and allow the sample to cool naturally to below 150℃ under a continuous hydrogen gas flow.

[0067] Step 4: Stop the gas flow, cool down, and remove the treated sample. A black or grayish-black velvety layer is visible on the substrate surface, which is spherical or near-spherical polycrystalline diamond. Gently scrape it off with a soft brush or scraper and collect it. Place the collected powder in a beaker, add a mixture of concentrated nitric acid (HNO3) and concentrated sulfuric acid (H2SO4) (volume ratio 1:3), and stir for 4 hours in an 80°C water bath to thoroughly remove any possible cobalt metal catalyst and amorphous carbon. After cooling, rinse repeatedly with deionized water until neutral. Centrifuge the resulting suspension for 10 minutes using a high-speed centrifuge (10,000 rpm) to achieve coarse separation. After filtration, place the powder in a hydrocyclone and treat for 10 minutes at a feed pressure of 0.3 MPa. Fine powder flows out from the bottom outlet, yielding 0.4 g of spherical or near-spherical polycrystalline diamond particles, as shown in Figure 7.

[0068] Example 5

[0069] Spherical or near-spherical polycrystalline diamonds were prepared by MPCVD using silicon-diamond composite sheets as a substrate. The silicon-diamond composite sheets were synthesized under high temperature and high pressure, with silicon comprising 20 wt% by mass.

[0070] Step 1: Clean the silicon-diamond composite wafer (with a layer of micron / nano diamond coating on its surface) with acetone and alcohol for 15 minutes in sequence to thoroughly remove surface oil and adsorbents. After drying with high-purity nitrogen, place it on the substrate stage of the MPCVD reaction chamber and adjust the height of the substrate stage so that the substrate is located in the concentrated area of ​​the microwave plasma ball.

[0071] Step 2: Close the reaction chamber and start the vacuum system to evacuate the base vacuum to 10. -3To eliminate interference from air and moisture, high-purity hydrogen gas was introduced at a flow rate of 400 sccm to stabilize the chamber pressure at 6 kPa. The microwave power was then slowly increased to 4 kW to ignite and form a stable hydrogen plasma, which was used to bombard and etch the substrate for 30 minutes to further activate the surface and form silicon nanoparticles on the substrate surface.

[0072] Step 3: Increase the microwave power to 6.5 kW, maintain the reaction pressure at 12 kPa, and introduce hydrogen and methane gas into the MPCVD reaction chamber. The CH4 flow rate is 25 sccm, the H2 flow rate is 400 sccm, and the substrate temperature is raised to 900℃. Under a pressure of 12 kPa, silicon nanoparticles are used as nucleation sites to deposit spherical or near-spherical polycrystalline diamond for 3 hours. After deposition, the methane gas flow is turned off first, and the sample is maintained in a pure hydrogen plasma environment for 10 minutes to perform hydrogen terminal treatment on the surface of the newly grown diamond to stabilize its surface properties and remove surface amorphous carbon. Then, the microwave power is turned off, heating is stopped, and the sample is allowed to cool naturally to below 200℃ under a continuous hydrogen gas flow.

[0073] Step 4: Stop the gas supply, cool down, and remove the treated sample. A black or black velvety layer is visible on the substrate surface, which is spherical or near-spherical polycrystalline diamond. Gently scrape it off with a soft brush or scraper and collect it. Place the collected powder in a polytetrafluoroethylene beaker, add a mixture of concentrated nitric acid (HNO3) and hydrofluoric acid (HF) (volume ratio HNO3:HF = 1:1), and gently stir for 2 hours in a 60℃ water bath to thoroughly remove any trace silicon particles and amorphous carbon that may be entrained. After cooling, rinse repeatedly with deionized water until neutral. Centrifuge the resulting suspension for 15 minutes using a high-speed centrifuge (12000 rpm) to achieve coarse separation. After filtration, place the powder in a hydrocyclone and treat it for 10 minutes at a feed pressure of 0.4 MPa. Fine powder flows out from the bottom outlet, yielding 0.45 g of spherical or near-spherical polycrystalline diamond particles, the optical image of which is shown in Figure 8.

[0074] The embodiments described above provide a detailed explanation of the technical solutions of the present invention. It should be understood that the above descriptions are merely specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, additions, or similar substitutions made within the scope of the principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing spherical or near-spherical polycrystalline diamond by chemical vapor deposition, characterized in that, The process includes the following steps: (1) providing a sheet material, wherein the surface of the sheet material has nanoparticles as nucleation sites; the nanoparticles are cobalt nanoparticles or silicon nanoparticles, and the particle size of the nanoparticles on the surface of the sheet material is 10-100 nm; (2) using the sheet material with nanoparticle nucleation sites as a substrate, using a chemical vapor deposition method, first raising the substrate temperature to 700-1200℃, and then introducing hydrogen and methane into the reaction chamber, wherein the volumetric flow rate of hydrogen is 400-500 sccm, and the volumetric flow rate ratio of methane to hydrogen is 1-7:100, and depositing for 1-50 h to form spherical or near-spherical polycrystalline diamond; (3) separating the spherical or near-spherical polycrystalline diamond from the surface of the substrate material after step (2), and further purifying it to obtain the product spherical or near-spherical polycrystalline diamond powder; wherein the particle size of the spherical or near-spherical polycrystalline diamond is 0.1-10 μm, and the purity is ≥99%.

2. The method for preparing spherical or near-spherical polycrystalline diamond by chemical vapor deposition according to claim 1, characterized in that, The preparation method of the sheet with nanoparticle nucleation points on the surface is as follows: silicon or cobalt in silicon-diamond composite sheet or cobalt-based polycrystalline diamond composite sheet is melted and precipitated by hot filament chemical vapor deposition to form cobalt nanoparticles or silicon nanoparticles on the surface of the sheet.

3. The method for preparing spherical or near-spherical polycrystalline diamond by chemical vapor deposition according to claim 1, characterized in that, The method for preparing a sheet with nanoparticle nucleation sites on its surface is as follows: depositing cobalt nanoparticles or silicon nanoparticles on the surface of the sheet.

4. The method for preparing spherical or near-spherical polycrystalline diamond by chemical vapor deposition according to claim 1, characterized in that, The preparation method of the sheet with nanoparticle nucleation points on the surface is as follows: using hydrogen plasma to bombard silicon-diamond composite sheet or cobalt-based polycrystalline diamond composite sheet to form cobalt nanoparticles or silicon nanoparticles on the surface of the sheet.

5. The method for preparing spherical or near-spherical polycrystalline diamond by chemical vapor deposition according to claim 1, characterized in that, In step (2), the chemical vapor deposition method selected is either hot filament chemical vapor deposition or microwave plasma chemical vapor deposition.

6. The method for preparing spherical or near-spherical polycrystalline diamond by chemical vapor deposition according to claim 1, characterized in that, In step (3), the method for separating spherical or near-spherical polycrystalline diamond from the substrate surface is physical exfoliation, and the purification methods include centrifugation, magnetic separation or hydrocyclone separation.

7. A spherical or near-spherical polycrystalline diamond, characterized in that, It is prepared by the method for preparing spherical or near-spherical polycrystalline diamond by chemical vapor deposition according to any one of claims 1-6.

8. The spherical or near-spherical polycrystalline diamond according to claim 7, characterized in that, The spherical or near-spherical polycrystalline diamond has a particle size of 0.1-10 μm and a purity of ≥99%.

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