High-temperature and high-pressure preparation method for hexagonal diamond

A high-temperature and high-pressure preparation method for hexagonal diamond, comprising:

• a) preparation of a precursor including:
• selecting high-purity graphite, processing the selected high-purity graphite into a cylindrical shape, cleaning and then vacuum drying the processed high-purity graphite, to obtain the precursor;
• b) assembly of a synthesis block including:
• placing a cylindrical diamond plug on an upper surface of the precursor, with a lower surface of the diamond plug coinciding with the upper surface of the precursor;
• tightly enclosing outer vertical sidewalls of the precursor and the diamond plug with an insulating tube which insulates the precursor and the diamond plug from a heating tube, to obtain a first assembly of the precursor, the insulating tube, and the diamond plug;
• placing the first assembly vertically in a chamber of a high-temperature and high-pressure apparatus, with the precursor positioned at a center of the chamber;
• placing a first cylindrical zirconia plug on an upper surface of the first assembly, with a lower surface of the zirconia plug coinciding with the upper surface of the first assembly;
• placing a cylindrical alumina plug on a lower surface of the first assembly, with an upper surface of the alumina plug coinciding with the lower surface of the first assembly;
• placing a second cylindrical zirconia plug on a lower surface of the alumina plug, with an upper surface of the second zirconia plug coinciding with the lower surface of the alumina plug;
• tightly enclosing outer vertical sidewalls of a second assembly of the precursor, the insulating tube, the diamond plug, the first and second zirconia plugs, and the alumina plug with the heating tube for subsequent indirect heating, wherein a temperature of the heating tube is measured by a thermocouple;
• and tightly enclosing an outer vertical sidewall of the heating tube with a zirconia tube, with an outer vertical sidewall of the zirconia tube in close contact with inner vertical sidewalls of a magnesium oxide octahedron of the high-temperature and high-pressure apparatus;
• and
• c) synthesis process including:
• increasing a pressure in the chamber to a first pressure at a rate of 1 GPa/h, then increasing a temperature in the chamber to a first temperature at a rate of 100° C./min;
• maintaining the first temperature for a certain period, then rapidly quenching to room temperature;
• maintaining the first pressure for a period, then reducing the pressure to zero at a rate of 1 GPa/h;
• after pressure release, removing a product from the chamber;
• and removing residual substances on a surface of the product to obtain a final product of hexagonal diamond.