A high temperature and high pressure preparation method for hexagonal diamond

Abstract

The present invention discloses a high-temperature and high-pressure preparation method of hexagonal diamond, and belongs to the technical field of superhard material synthesis. The present invention first prepares a precursor from high-purity graphite, and then assembles the synthesis block. After the assembly is completed, the hexagonal diamond is obtained by heating, pressurizing, heat preservation, and pressure maintenance. The hexagonal diamond preparation method of the present invention enables the high-purity graphite to obtain a higher pressure along the c-axis direction, further promotes the phase change of the graphite, makes the temperature field where the precursor is located have a certain temperature gradient, promotes the transformation of high-purity graphite into hexagonal diamond, and has a better hexagonal diamond conversion rate.

Description

Technical Field

[0001] The invention belongs to the technical field of superhard material synthesis, and particularly relates to a method for preparing hexagonal diamond. Background Art

[0002] Diamond is the hardest material discovered in nature. In the process of continuous development and innovation of modern industry, superhard materials led by diamond have become the "sharpest industrial teeth" and are also the indispensable "hard support" to support the transformation and upgrading of the manufacturing industry.

[0003] Before 1967, all the diamonds discovered and manufactured were cubic in structure. In 1967, a type of diamond with a hexagonal structure, also called Lonsdaleite, was discovered in the Barringer Crater in Arizona, USA. Its bonding characteristics are similar to those of cubic diamonds, but its hardness is much higher. Therefore, it has aroused great research interest from both production applications and basic scientific research. Theoretical calculations show that the hardness of hexagonal diamonds is 58% higher than that of cubic diamonds, which has inspired a lot of synthetic work.

[0004] A research team from Osaka University in Japan conducted high temperature and high pressure experiments on single crystal graphite with larger grains, and conducted synthesis experiments under temperature and pressure conditions of 20GPa and 800℃ to 1800℃. Hexagonal diamond was found in all samples, but its content was generally very low. The sample with the highest hexagonal structure content was synthesized under 20GPa and 1200℃. Through X-ray diffraction analysis, the content of hexagonal diamond in this sample was only 50%, and the rest were graphite and cubic diamond structures. DOI: 10.1143 / JJAP.42.1694

[0005] The US research team used transmission electron microscopy to characterize the microstructure of the meteorite where hexagonal diamond was found, and believed that hexagonal diamond is only a stacking fault or twin boundary structure of cubic diamond, and cannot exist alone. At the same time, by verifying previous experiments, the graphite was treated at 19GPa and 2273K, and the structural analysis of the obtained samples was performed, and no hexagonal diamond structure was found to exist alone in the decompression samples. https: / / doi.org / 10.1038 / ncomms6447

[0006] The French and Japanese research teams conducted high temperature and high pressure experiments using precursors such as polycrystalline graphite, highly oriented pyrolytic graphite, carbon black, quasi-amorphous carbon, and heat-treated carbon black. The experiments were carried out under experimental conditions of 15GPa and 1500℃~1900℃. They found the existence of hexagonal diamond structure in the synthetic samples using polycrystalline graphite and highly oriented pyrolytic graphite as precursors, while hexagonal diamond was not found in other precursors. https: / / doi.org / 10.1016 / j.carbon.2006.10.005

[0007] Despite great efforts in the artificial synthesis of hexagonal diamonds, the purity of the obtained hexagonal diamonds is still very low. Usually, a very large proportion of cubic diamonds are mixed into the main products. Some people even question the existence of hexagonal diamonds, believing that hexagonal diamonds exist in cubic diamonds in the form of stacked faults or twins, and cannot be independent structures. One of the reasons is that cubic diamonds are more stable and easier to form than hexagonal diamonds under high pressure and high temperature, so cubic diamonds always dominate or the two structures coexist in synthetic samples. Therefore, a new artificial preparation method is urgently needed to solve this long-standing challenge to synthesize pure hexagonal diamonds. Summary of the invention

[0008] The technical problem to be solved by the present invention is to provide a preparation method with high conversion rate and capable of obtaining high-purity hexagonal diamond.

[0009] In order to solve the above technical problems, the present invention provides a method for preparing hexagonal diamond at high temperature and high pressure, and the technical scheme is as follows:

[0010] (I) Preparation of Precursors

[0011] High-purity graphite is selected and processed into a cylindrical shape, and then the processed high-purity graphite is cleaned and vacuum-dried to obtain a precursor.

[0012] The high-purity graphite has a purity greater than or equal to 99.99% and has AB stacking.

[0013] The graphite sheets of the precursor are kept horizontal, and the graphite c-axis of the precursor is always vertically upward;

[0014] (II) Assembly of synthetic blocks

[0015] A cylindrical diamond plug is placed on the upper surface of the precursor, and the lower surface of the diamond plug coincides with the upper surface of the precursor. The outer side of the vertical side wall of the precursor and the diamond plug is tightly wrapped with an insulating tube. The insulating tube insulates the precursor and the diamond plug from the heating tube described below. The precursor, the insulating tube and the diamond plug are placed vertically in the cavity of the high-temperature and high-pressure device as a whole. The precursor is located in the center of the cavity. A cylindrical zirconia plug is placed on the upper surface of the precursor, the insulating tube and the diamond plug as a whole. The lower surface of the zirconia plug coincides with the upper surface of the above-mentioned whole. The lower surface of the above-mentioned whole A cylindrical alumina plug is placed, and the upper surface of the alumina plug coincides with the lower surface of the above-mentioned whole. Another cylindrical zirconia plug is placed on the lower surface of the alumina plug, and the upper surface of the zirconia plug coincides with the lower surface of the alumina plug. A heating tube is tightly wrapped around the outer side of the vertical side wall of the structure composed of the precursor, the insulating tube, the diamond plug, the two zirconia plugs and the alumina plug for indirect heating. The temperature of the heating tube is measured with a thermocouple. The zirconia tube is tightly wrapped around the outer side of the vertical side wall of the heating tube, and the outer side of the vertical side wall of the zirconia tube is in close contact with the vertical inner side surface of the magnesium oxide octahedron.

[0016] (III) Synthesis process

[0017] First, the pressure is increased to 30 GPa at a pressure increase rate of 1 GPa / min, and then the temperature is increased to high temperature A at a heating rate of 100°C / min, and kept at this temperature for a certain period of time. After the end of the heat preservation, the temperature is directly quenched to rapidly reduce to room temperature; after maintaining the pressure for a period of time, the pressure is reduced to zero pressure at a pressure reduction rate of 1 GPa / min, and the sample is taken out after the pressure is released, and the residue on the surface of the sample is removed to obtain the final sample.

[0018] The room temperature is 18°C ​​to 32°C.

[0019] The insulation time is 15 min to 20 min.

[0020] The pressure holding time is 5 minutes to 10 minutes.

[0021] The high temperature A is 1400°C to 1500°C.

[0022] Preferably, the high temperature A is 1400°C.

[0023] Through the above design scheme, the present invention can bring the following beneficial effects:

[0024] 1. Diamond plugs and alumina plugs with higher hardness than zirconia are installed on the upper and lower sides of the precursor. The diamond plugs and alumina plugs are in direct contact with the sample, which greatly improves the pressure environment in the precursor area. Since the graphite flakes of the precursor remain horizontal, the c-axis of the graphite of the precursor is always vertically upward, which can make the graphite obtain higher pressure along the c-axis direction, further promoting the phase change of graphite.

[0025] 2. The two sides of the precursor are directly in contact with the diamond plug and the alumina plug. When additional pressure is applied, due to their different thermal conductivities, the temperature field of the entire precursor has a certain temperature gradient field during the heating process. This temperature gradient field is a favorable factor in promoting the transformation of graphite to hexagonal diamond. Compared with the experimental results using all-diamond plugs, it has a better hexagonal diamond conversion rate.

[0026] The hexagonal diamond synthesized according to the hexagonal diamond preparation method has extremely high hardness, and its Vickers hardness converges to 155GPa under a load of 1kg, which is far higher than that of cubic diamond. The high-temperature and high-pressure preparation method of hexagonal diamond provided by the present invention provides powerful guidance for the production and application of hexagonal diamond. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The present invention is further described below with reference to the accompanying drawings and specific embodiments:

[0028] Figure 1 It is a schematic diagram of the octahedron assembly of the high temperature and high pressure device in Example 1, Example 2, Comparative Example 1, Comparative Example 2, and Comparative Example 3.

[0029] Figure 2 325nm Raman spectra of samples obtained in Example 1 and Example 2.

[0030] Figure 3 These are the XRD diagrams of the samples obtained in Examples 1 and 2 in two orientations.

[0031] Figure 4 These are high-resolution transmission electron microscopy images of the samples obtained in Example 1 and Example 2.

[0032] Figure 5 This is the Vickers hardness diagram of the samples obtained in Example 1 and Example 2.

[0033] Figure 6 The XRD spectra of the samples obtained in Example 1 and Comparative Example 4 are shown.

[0034] Figure 7 The XRD spectra of the samples obtained in Example 1, Comparative Example 1, Comparative Example 2 and Comparative Example 3 are shown.

[0035] In the figure, 1-magnesium oxide octahedron, 2-precursor, 3-diamond plug, 4-aluminum oxide plug, 5-zirconia plug, 6-magnesium oxide tube, 7-rhenium heating tube, 8-copper tube protective sleeve, 9-tungsten rhenium thermocouple, 10-zirconia tube. DETAILED DESCRIPTION

[0036] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the following is a further detailed description of a high temperature and high pressure preparation method of hexagonal diamond of the present invention in conjunction with the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present invention and do not constitute a limitation of the present invention.

[0037] Example 1

[0038] (I) Preparation of Precursor 2

[0039] Grade A highly oriented pyrolytic graphite with a purity greater than or equal to 99.99% and AB stacking is processed into a cylinder with a diameter of 2 mm and a height of 2 mm. The processed grade A highly oriented pyrolytic graphite flakes remain horizontal, and the c-axis is always vertically upward. The processed grade A highly oriented pyrolytic graphite is placed in anhydrous ethanol and ultrasonically removed from the edge debris. Then the sample is placed in a drying oven and vacuum dried at 120°C for 2 hours to obtain the precursor 2 used for assembly.

[0040] (II) Assembly of synthetic blocks

[0041] A cylindrical diamond plug 3 is placed on the upper surface of the precursor 2, and the lower surface of the diamond plug 3 coincides with the upper surface of the precursor 2. The outer sides of the vertical side walls of the precursor 2 and the diamond plug 3 are tightly wrapped with a magnesium oxide tube 6. The magnesium oxide tube 6 insulates the precursor 2, the diamond plug 3 and the rhenium heating tube 7 described below. The precursor 2, the magnesium oxide tube 6 and the diamond plug 3 are placed vertically in the cavity of the high-temperature and high-pressure device as a whole. The precursor 2 is located in the center of the cavity. The precursor 2, the magnesium oxide tube 6 and the diamond plug 3 are placed as a whole. A cylindrical zirconia plug 5 is placed on the upper surface of the above-mentioned whole. The lower surface of the zirconia plug 5 coincides with the upper surface of the above-mentioned whole. A cylindrical An alumina plug 4, the upper surface of which coincides with the lower surface of the above-mentioned whole, and another cylindrical zirconia plug 5 is placed on the lower surface of the alumina plug 4, the upper surface of which coincides with the lower surface of the alumina plug 4, and a thin rhenium heating tube 7 is tightly wrapped around the outer side of the vertical side wall of the structure composed of the aforementioned precursor 2, magnesium oxide tube 6, diamond plug 3, two zirconia plugs 5, and alumina plug 4 for indirect heating, and a tungsten-rhenium thermocouple 9 is used to measure the temperature of the rhenium heating tube 7, the outer side of the tungsten-rhenium thermocouple 9 is tightly wrapped with a copper tube protective sleeve 8, and a zirconia tube 10 is tightly wrapped around the outer side of the vertical side wall of the rhenium heating tube 7, and the outer side of the vertical side wall of the zirconia tube 10 is in close contact with the magnesium oxide octahedron 1.

[0042] (III) Synthesis process

[0043] First, the pressure was increased to 30 GPa at a pressure increase rate of 1 GPa / min, and then the temperature was increased to 1400°C at a heating rate of 100°C / min and kept at this temperature for 15 minutes. After the end of the heat preservation, the temperature was directly quenched to quickly reduce to 25°C. After maintaining the pressure for 10 minutes, the pressure was reduced to zero pressure at a pressure reduction rate of 1 GPa / min. After unloading the pressure, the sample was taken out and the residue on the surface of the sample was removed to obtain the final sample.

[0044] Example 2

[0045] (I) Preparation of Precursors

[0046] Grade A highly oriented pyrolytic graphite with a purity greater than or equal to 99.99% and AB stacking is processed into a cylinder with a diameter of 2 mm and a height of 2 mm. The processed grade A highly oriented pyrolytic graphite flakes remain horizontal, and the c-axis is always vertically upward. The processed grade A highly oriented pyrolytic graphite is placed in anhydrous ethanol and ultrasonically removed from the edge debris. Then the sample is placed in a drying oven and vacuum dried at 120°C for 2 hours to finally obtain the precursor used for assembly.

[0047] (II) Assembly of synthetic blocks

[0048] A cylindrical diamond plug 3 is placed on the upper surface of the precursor 2, and the lower surface of the diamond plug 3 coincides with the upper surface of the precursor 2. The outer sides of the vertical side walls of the precursor 2 and the diamond plug 3 are tightly wrapped with a magnesium oxide tube 6. The magnesium oxide tube 6 insulates the precursor 2, the diamond plug 3 and the rhenium heating tube 7 described below. The precursor 2, the magnesium oxide tube 6 and the diamond plug 3 are placed vertically in the cavity of the high-temperature and high-pressure device as a whole. The precursor 2 is located in the center of the cavity. The precursor 2, the magnesium oxide tube 6 and the diamond plug 3 are placed as a whole. A cylindrical zirconia plug 5 is placed on the upper surface of the above-mentioned whole. The lower surface of the zirconia plug 5 coincides with the upper surface of the above-mentioned whole. A cylindrical An alumina plug 4, the upper surface of which coincides with the lower surface of the above-mentioned whole, and another cylindrical zirconia plug 5 is placed on the lower surface of the alumina plug 4, the upper surface of which coincides with the lower surface of the alumina plug 4, and a thin rhenium heating tube 7 is tightly wrapped around the outer side of the vertical side wall of the structure composed of the aforementioned precursor 2, magnesium oxide tube 6, diamond plug 3, two zirconia plugs 5, and alumina plug 4 for indirect heating, and a tungsten-rhenium thermocouple 9 is used to measure the temperature of the rhenium heating tube 7, the outer side of the tungsten-rhenium thermocouple 9 is tightly wrapped with a copper tube protective sleeve 8, and a zirconia tube 10 is tightly wrapped around the outer side of the vertical side wall of the rhenium heating tube 7, and the outer side of the vertical side wall of the zirconia tube 10 is in close contact with the magnesium oxide octahedron 1.

[0049] (III) Synthesis process

[0050] First, the pressure was increased to 30 GPa at a pressure increase rate of 1 GPa / min, and then the temperature was increased to 1400°C at a heating rate of 100°C / min and kept warm for 20 minutes. After the end of the heat preservation, the temperature was directly quenched to quickly reduce to 25°C. After holding the pressure for 5 minutes, the pressure was reduced to zero pressure at a pressure reduction rate of 1 GPa / min. After unloading the pressure, the sample was taken out and the residue on the surface of the sample was removed to obtain the final sample.

[0051] Comparative Example 1

[0052] (I) Preparation of Precursors

[0053] Grade A highly oriented pyrolytic graphite with a purity greater than or equal to 99.99% and AB stacking is processed into a cylinder with a diameter of 2 mm and a height of 2 mm. The processed grade A highly oriented pyrolytic graphite flakes remain horizontal, and the c-axis is always vertically upward. The processed grade A highly oriented pyrolytic graphite is placed in anhydrous ethanol and ultrasonically removed from the edge debris. Then the sample is placed in a drying oven and vacuum dried at 120°C for 2 hours to obtain the precursor 2 used for assembly.

[0054] (II) Assembly of synthetic blocks

[0055] A cylindrical diamond plug 3 is placed on the upper surface of the precursor 2, and the lower surface of the diamond plug 3 coincides with the upper surface of the precursor 2. The outer sides of the vertical side walls of the precursor 2 and the diamond plug 3 are tightly wrapped with a magnesium oxide tube 6. The magnesium oxide tube 6 insulates the precursor 2, the diamond plug 3 and the rhenium heating tube 7 described below. The precursor 2, the magnesium oxide tube 6 and the diamond plug 3 are placed vertically in the cavity of the high-temperature and high-pressure device as a whole. The precursor 2 is located in the center of the cavity. The precursor 2, the magnesium oxide tube 6 and the diamond plug 3 are placed as a whole. A cylindrical zirconia plug 5 is placed on the upper surface of the above-mentioned whole. The lower surface of the zirconia plug 5 coincides with the upper surface of the above-mentioned whole. A cylindrical An alumina plug 4, the upper surface of which coincides with the lower surface of the above-mentioned whole, and another cylindrical zirconia plug 5 is placed on the lower surface of the alumina plug 4, the upper surface of which coincides with the lower surface of the alumina plug 4, and a thin rhenium heating tube 7 is tightly wrapped around the outer side of the vertical side wall of the structure composed of the aforementioned precursor 2, magnesium oxide tube 6, diamond plug 3, two zirconia plugs 5, and alumina plug 4 for indirect heating, and a tungsten-rhenium thermocouple 9 is used to measure the temperature of the rhenium heating tube 7, the outer side of the tungsten-rhenium thermocouple 9 is tightly wrapped with a copper tube protective sleeve 8, and a zirconia tube 10 is tightly wrapped around the outer side of the vertical side wall of the rhenium heating tube 7, and the outer side of the vertical side wall of the zirconia tube 10 is in close contact with the magnesium oxide octahedron 1.

[0056] (III) Synthesis process

[0057] First, the pressure was increased to 30 GPa at a pressure increase rate of 1 GPa / min, and then the temperature was increased to 1200°C at a heating rate of 100°C / min and kept warm for 15 minutes. After the end of the heat preservation, the temperature was directly quenched to quickly reduce to 25°C. After holding the pressure for 10 minutes, the pressure was reduced to zero pressure at a pressure reduction rate of 1 GPa / min. After unloading the pressure, the sample was taken out and the residue on the surface of the sample was removed to obtain the final sample.

[0058] Comparative Example 2

[0059] (I) Preparation of Precursors

[0060] Grade A highly oriented pyrolytic graphite with a purity greater than or equal to 99.99% and AB stacking is processed into a cylinder with a diameter of 2 mm and a height of 2 mm. The processed grade A highly oriented pyrolytic graphite flakes remain horizontal, and the c-axis is always vertically upward. The processed grade A highly oriented pyrolytic graphite is placed in anhydrous ethanol and ultrasonically removed from the edge debris. Then the sample is placed in a drying oven and vacuum dried at 120°C for 2 hours to obtain the precursor 2 used for assembly.

[0061] (II) Assembly of synthetic blocks

[0062] A cylindrical diamond plug 3 is placed on the upper surface of the precursor 2, and the lower surface of the diamond plug 3 coincides with the upper surface of the precursor 2. The outer sides of the vertical side walls of the precursor 2 and the diamond plug 3 are tightly wrapped with a magnesium oxide tube 6. The magnesium oxide tube 6 insulates the precursor 2, the diamond plug 3 and the rhenium heating tube 7 described below. The precursor 2, the magnesium oxide tube 6 and the diamond plug 3 are placed vertically in the cavity of the high-temperature and high-pressure device as a whole. The precursor 2 is located in the center of the cavity. The precursor 2, the magnesium oxide tube 6 and the diamond plug 3 are placed as a whole. A cylindrical zirconia plug 5 is placed on the upper surface of the above-mentioned whole. The lower surface of the zirconia plug 5 coincides with the upper surface of the above-mentioned whole. A cylindrical An alumina plug 4, the upper surface of which coincides with the lower surface of the above-mentioned whole, and another cylindrical zirconia plug 5 is placed on the lower surface of the alumina plug 4, the upper surface of which coincides with the lower surface of the alumina plug 4, and a thin rhenium heating tube 7 is tightly wrapped around the outer side of the vertical side wall of the structure composed of the aforementioned precursor 2, magnesium oxide tube 6, diamond plug 3, two zirconia plugs 5, and alumina plug 4 for indirect heating, and a tungsten-rhenium thermocouple 9 is used to measure the temperature of the rhenium heating tube 7, the outer side of the tungsten-rhenium thermocouple 9 is tightly wrapped with a copper tube protective sleeve 8, and a zirconia tube 10 is tightly wrapped around the outer side of the vertical side wall of the rhenium heating tube 7, and the outer side of the vertical side wall of the zirconia tube 10 is in close contact with the magnesium oxide octahedron 1.

[0063] (III) Synthesis process

[0064] First, the pressure was increased to 30 GPa at a pressure increase rate of 1 GPa / min, and then the temperature was increased to 1500°C at a heating rate of 100°C / min and kept warm for 15 minutes. After the end of the heat preservation, the temperature was directly quenched to quickly reduce to 25°C. After holding the pressure for 10 minutes, the pressure was reduced to zero pressure at a pressure reduction rate of 1 GPa / min. After unloading the pressure, the sample was taken out and the residue on the surface of the sample was removed to obtain the final sample.

[0065] Comparative Example 3

[0066] (I) Preparation of Precursors

[0067] Grade A highly oriented pyrolytic graphite with a purity greater than or equal to 99.99% and AB stacking is processed into a cylinder with a diameter of 2 mm and a height of 2 mm. The processed grade A highly oriented pyrolytic graphite flakes remain horizontal, and the c-axis is always vertically upward. The processed grade A highly oriented pyrolytic graphite is placed in anhydrous ethanol and ultrasonically removed from the edge debris. Then the sample is placed in a drying oven and vacuum dried at 120°C for 2 hours to obtain the precursor 2 used for assembly.

[0068] (II) Assembly of synthetic blocks

[0069] A cylindrical diamond plug 3 is placed on the upper surface of the precursor 2, and the lower surface of the diamond plug 3 coincides with the upper surface of the precursor 2. The outer sides of the vertical side walls of the precursor 2 and the diamond plug 3 are tightly wrapped with a magnesium oxide tube 6. The magnesium oxide tube 6 insulates the precursor 2, the diamond plug 3 and the rhenium heating tube 7 described below. The precursor 2, the magnesium oxide tube 6 and the diamond plug 3 are placed vertically in the cavity of the high-temperature and high-pressure device as a whole. The precursor 2 is located in the center of the cavity. The precursor 2, the magnesium oxide tube 6 and the diamond plug 3 are placed as a whole. A cylindrical zirconia plug 5 is placed on the upper surface of the above-mentioned whole. The lower surface of the zirconia plug 5 coincides with the upper surface of the above-mentioned whole. A cylindrical An alumina plug 4, the upper surface of which coincides with the lower surface of the above-mentioned whole, and another cylindrical zirconia plug 5 is placed on the lower surface of the alumina plug 4, the upper surface of which coincides with the lower surface of the alumina plug 4, and a thin rhenium heating tube 7 is tightly wrapped around the outer side of the vertical side wall of the structure composed of the aforementioned precursor 2, magnesium oxide tube 6, diamond plug 3, two zirconia plugs 5, and alumina plug 4 for indirect heating, and a tungsten-rhenium thermocouple 9 is used to measure the temperature of the rhenium heating tube 7, the outer side of the tungsten-rhenium thermocouple 9 is tightly wrapped with a copper tube protective sleeve 8, and a zirconia tube 10 is tightly wrapped around the outer side of the vertical side wall of the rhenium heating tube 7, and the outer side of the vertical side wall of the zirconia tube 10 is in close contact with the magnesium oxide octahedron 1.

[0070] (III) Synthesis process

[0071] First, the pressure was increased to 30 GPa at a pressure increase rate of 1 GPa / min, and then the temperature was increased to 1700°C at a heating rate of 100°C / min and kept at this temperature for 15 minutes. After the end of the heat preservation, the temperature was directly quenched to quickly reduce to 25°C. After maintaining the pressure for 10 minutes, the pressure was reduced to zero pressure at a pressure reduction rate of 1 GPa / min. After unloading the pressure, the sample was taken out and the residue on the surface of the sample was removed to obtain the final sample.

[0072] Comparative Example 4

[0073] (I) Preparation of Precursors

[0074] Grade A highly oriented pyrolytic graphite with a purity greater than or equal to 99.99% and AB stacking is processed into a cylinder with a diameter of 2 mm and a height of 2 mm. The processed grade A highly oriented pyrolytic graphite flakes remain horizontal, and the c-axis is always vertically upward. The processed grade A highly oriented pyrolytic graphite is placed in anhydrous ethanol and ultrasonically removed from the edge debris. Then the sample is placed in a drying oven and vacuum dried at 120°C for 2 hours to finally obtain the precursor used for assembly.

[0075] (II) Assembly of synthetic blocks

[0076] A cylindrical alumina plug is placed on the upper surface of the precursor, and the lower surface of the alumina plug coincides with the upper surface of the precursor. The outer side of the vertical side walls of the precursor and the alumina plug are tightly wrapped with a magnesium oxide tube. The magnesium oxide tube insulates the precursor, the alumina plug and the rhenium heating tube described below. The precursor, the magnesium oxide tube and the alumina plug are placed vertically in the cavity of the high-temperature and high-pressure device as a whole. The precursor is located in the center of the cavity. The precursor, the magnesium oxide tube and the alumina plug are placed as a whole. A cylindrical zirconia plug is placed on the upper surface of the above whole. The lower surface of the zirconia plug coincides with the upper surface of the above whole. Another cylindrical zirconia plug is placed on the lower surface of the above whole. A cylindrical alumina plug, the upper surface of which coincides with the lower surface of the above-mentioned whole, another cylindrical zirconia plug is placed on the lower surface of the alumina plug, and the lower surface of the alumina plug coincides with the upper surface of the zirconia plug, and a thin rhenium heating tube is tightly wrapped around the outer side of the vertical side wall of the structure consisting of the aforementioned precursor, the magnesium oxide tube, the two alumina plugs and the two zirconia plugs for indirect heating, and a tungsten-rhenium thermocouple is used to measure the temperature of the rhenium heating tube, and the outer side of the tungsten-rhenium thermocouple is tightly wrapped with a copper tube protective sleeve, and the zirconia tube is tightly wrapped around the outer side of the vertical side wall of the rhenium heating tube, and the outer side of the vertical side wall of the zirconia tube is in close contact with the magnesium oxide octahedron.

[0077] (III) Synthesis process

[0078] First, the pressure was increased to 30 GPa at a pressure increase rate of 1 GPa / min, and then the temperature was increased to 1400°C at a heating rate of 100°C / min and kept at this temperature for 15 minutes. After the end of the heat preservation, the temperature was directly quenched to quickly reduce to 25°C. After maintaining the pressure for 10 minutes, the pressure was reduced to zero pressure at a pressure reduction rate of 1 GPa / min. After unloading the pressure, the sample was taken out and the residue on the surface of the sample was removed to obtain the final sample.

[0079] The upper and lower surfaces and side walls of the samples obtained in the embodiment and the comparative example were polished to facilitate Vickers hardness and XRD spectrum testing. The XRD spectrum is an X-ray diffraction spectrum. The Vickers hardness value of the samples obtained in Example 1 and Example 2 under a load of 1 kg is 155 GPa. Figure 5 As shown, its XRD spectrum is consistent with the theoretical XRD of hexagonal diamond; in addition, the samples obtained in Example 1 and Example 2 are processed by focused ion beam to obtain transmission samples, such as Figure 2 , Figure 3 , Figure 4 As shown, the structure of the transmission sample was fully characterized by transmission electron microscopy, and the results further confirmed that the samples obtained in Example 1 and Example 2 were hexagonal diamond blocks, wherein Figure 3 The refined content of the XRD spectrum shows that the hexagonal diamond content is more than 95%.

[0080] By comparing Example 1 with Comparative Example 4, and combining Figure 6 , and by comparing Example 1 with Comparative Example 1, Comparative Example 2, Comparative Example 3, and combining Figure 7 It can be seen that the purity of hexagonal diamond in the sample obtained by the high temperature and high pressure preparation method of hexagonal diamond of the present invention is the highest.

Claims

1. A high temperature and high pressure preparation method of hexagonal diamond, characterized in that: (I) Preparation of Precursors Select high-purity graphite, process it into a cylindrical shape, then clean the processed high-purity graphite, and vacuum dry it to obtain a precursor; (II) Assembly of synthetic blocks A cylindrical diamond plug is placed on the upper surface of the precursor, and the lower surface of the diamond plug coincides with the upper surface of the precursor. The outer side of the vertical side wall of the precursor and the diamond plug is tightly wrapped with an insulating tube. The insulating tube insulates the precursor and the diamond plug from the heating tube described below. The precursor, the insulating tube and the diamond plug are placed vertically in the cavity of the high-temperature and high-pressure device as a whole. The precursor is located in the center of the cavity. A cylindrical zirconia plug is placed on the upper surface of the precursor, the insulating tube and the diamond plug as a whole. The lower surface of the zirconia plug coincides with the upper surface of the above-mentioned whole. The lower surface of the above-mentioned whole A cylindrical alumina plug is placed, the upper surface of which coincides with the lower surface of the whole, another cylindrical zirconia plug is placed on the lower surface of the alumina plug, the upper surface of which coincides with the lower surface of the alumina plug, a heating tube is tightly wrapped around the outer side of the vertical side wall of the structure consisting of the precursor, the insulating tube, the diamond plug, the two zirconia plugs and the alumina plug for indirect heating, a thermocouple is used to measure the temperature of the heating tube, a zirconia tube is tightly wrapped around the outer side of the vertical side wall of the heating tube, and the outer side of the vertical side wall of the zirconia tube is in close contact with the vertical inner side surface of the magnesium oxide octahedron; (III) Synthesis First, the pressure is increased to a high pressure of 30 GPa at a pressure increase rate of 1 GPa / h, and then the temperature is increased to a high temperature of 1400°C to 1500°C at a heating rate of 100°C / min, and the temperature is kept for a certain period of time. After the insulation is completed, the temperature is directly quenched to quickly reduce to room temperature; after maintaining the pressure for a period of time, the pressure is reduced to zero pressure at a pressure reduction rate of 1 GPa / h, and the sample is taken out after the pressure is released, and the residue on the surface of the sample is removed to obtain the final sample.

2. A high temperature and high pressure method for preparing hexagonal diamond according to claim 1, characterized in that: The high-purity graphite has a purity greater than or equal to 99.99% and has AB stacking.

3. A high temperature and high pressure method for preparing hexagonal diamond according to claim 1, characterized in that: The graphite sheets of the precursor remain horizontal, and the graphite c-axis of the precursor is always vertically upward.

4. A high temperature and high pressure method for preparing hexagonal diamond according to claim 1, characterized in that: The insulation time is 15 min to 20 min.

5. A high temperature and high pressure method for preparing hexagonal diamond according to claim 1, characterized in that: The pressure holding time is 5 minutes to 10 minutes.

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