The process of synthesizing diamond using iron-carbon compounds under high pressure and high temperature (HPHT) mainly consists of three core steps: preparation of iron-carbon compounds, HPHT assembly & synthesis, and growth and phase transformation mechanisms:

1. Preparation of Iron-Carbon Compounds

Melting and Mixing Raw Materials: Industrial pure iron (Fe >97) and graphite (ge >98%) are mixed in a specific proportion, melted in a medium-frequency induction furnace, and stirred thoroughly until homogeneous.

Rapid Quenching to Suppress Graphite Precipitation: To prevent graphite from precipitating during cooling, a high-pressure water jet atomizes the molten iron into fine droplets, which are rapidly cooled in a water bath.

Powder Preparation: After drying and rust removal, the mixture is pulverized into iron-carbon compound powders of 80–100 mesh. The carbon content of the specimen reaches 13.0 wt% (41.1 at%). Its microstructure consists mainly of various iron-carbon compounds and martensite supersaturated with carbon, containing no free graphite whatsoever.

2. HPHT Assembly and Synthesis Conditions

Diffusion-Barrier Assembly: The iron-carbon compound powder is placed in the center of the high-pressure synthesis cell, isolated from the surrounding graphite by titanium (Ti) foils at both ends. Since titanium has a high melting point and does not melt under high pressure, it effectively blocks the diffusion of external graphite, ensuring that the carbon source for diamond synthesis is supplied entirely by the iron-carbon compound itself.

Process Parameters (using a domestic cubic press, model KY-7200):

Synthesis Pressure: 5.5 GPa

Synthesis Temperature: 1620 K

Holding Time: 8 min

3. Synthesis Results and Decomposition/Growth Mechanism

Synthesized Product: Under HPHT conditions, the iron-carbon compound acts as both the carbon source and the catalyst, successfully synthesizing single-crystal diamond particles with grain sizes of 5–100 μm.

Precipitation and Growth Mechanism: At 5.5 GPa and 1620 K, the system operates within the thermodynamic stability region of diamond. The iron-carbon compound decomposes, releasing supersaturated carbon atoms that preferentially aggregate into stable diamond microcrystals, which grow as holding time increases.

Microscopic Morphology Features: A localized carbon-rich melt forms on the outer surface of the diamond particles. Upon cooling, a residual iron-rich annular zone is left between the diamond and the surrounding carbon-rich ring zone. This phenomenon closely resembles the encapsulating film found in conventional transition-metal catalyst diamond synthesis, preliminarily validating the feasibility of the TFDC (Cheng's Theory) concept of synthesizing large-grained diamonds without free graphite.