The specific method and key steps for preparing nano-polycrystalline diamond using an ultra-high pressure HPHT Hydraulic Cubic Press (such as the Y75–4800 hinged cubic press) are as follows:
1. Preparation and Treatment of Raw Materials
The main raw materials and auxiliary materials used in the synthesis experiments include:
Raw material: Nano-diamond powder with a particle size of 5–10 nm, which needs to be acid-base treated.
Sintering Agent/Catalyst: Iron-based (Fe-based) or Nickel-based (Ni-based) metal powder or alloy powder. Research indicates that the size of the synthesized polycrystalline diamond is related not only to the kind of catalyst but also to the particle size (fineness) of the catalyst.
Auxiliary materials: Pyrophyllite blocks, carbon tubes, titanium sheets, and carbon sheets.
2. Experimental Assembly and Mixing
The experimental procedure includes steps such as ingredient proportioning, mixing, purification treatment of raw and auxiliary materials, molding, pre-pressing, and assembly of the synthesis block:
A. Proportioning and Mixing: The treated nano-diamond powder and catalyst alloy are uniformly mixed according to a certain ratio.
B. Pre-pressing: The mixed powder is pre-pressed into rods.
C. Loading into Carbon Tubes: The formed rods are layered into carbon tubes. The carbon tube serves two functions: acting as a heating source and helping to set a reasonable temperature gradient.
D. Sealing and Assembly: The carbon tube ends are plugged with carbon sheets and titanium sheets, and then the assembly is placed into the composite pyrophyllite block according to the assembly diagram (Figure 1 in the source).
3. High-Temperature and High-Pressure Synthesis
The synthesis process uses the simultaneous pressure-to-temperature procedure in a high-pressure device:
Equipment: An ultra-high pressure cubic press is used.
Condition Selection: Process parameters such as the proportion of the sintering agent, sintering temperature, pressure, and sintering time significantly impact the synthesis results.
Optimal Synthesis Conditions: Studies show that the nano-polycrystalline diamond material prepared under the following conditions is relatively uniform and compact:
    Pressure: 6 GPa
    Temperature: 1 100℃
    Synthesis Time: 60 S
Results Analysis: Under these suitable conditions, the synthesized polycrystals are composed of nano-diamond, and the diamond particles are mainly connected by D–D bonding (diamond-to-diamond bonding), resulting in a uniform and compact structure.
Note: If the sintering temperature is too high (e.g., 6 GPa, 1 500℃, 3 min) or the synthesis time is too long, the nano-diamond can very easily graphitize. A sample synthesized at 6 GPa, 1 500°C, and 3 min showed a sheet-like microstructure similar to graphite and exhibited strong graphite diffraction peaks, confirming almost complete graphitization. Conversely, if the temperature is too low or the time is too short, insufficient sintering (under-sintering) may occur.
4. Post-Processing and Purification
After the synthesized block is removed from the mold and crushed, purification is required to remove impurities and residual graphite:
A. Crushing: The synthesized block is crushed.
B. Acid Washing (Purification): The sample is treated using a mixed acid prepared from concentrated nitric acid and concentrated sulfuric acid. This mixed acid treatment is effective for removing impurities and most of the graphite from the sample.
C. Neutralization and Drying: The treated sample is subsequently neutralized and dried to obtain the final nano-polycrystalline diamond product.