The Kinetics Of Diamond Formation

Based on the above activation energies, the kinetic diagram for diamond synthesis can then be constructed as shown in Fig. 4.13.

Figure 4.13 indicates that all diamond syntheses are performed under high pressure, and the P/T regimes for their syntheses are primarily determined by the energy barrier and the conversion time.

In summary, there are numerous ways to synthesize diamond. They differ in the selection of starting material and the duration of the synthesis. The starting material will affect the activation energy for the synthesis. This energy barrier and the duration of synthesis may determine the pressure and temperature for the synthesis.

Figure 4.13. The kinetic diagram of diamond synthesis that shows the pressure and temperature conditions of in situ at real time for the formation of diamond by taking different transition routes. The actual pressure and temperature required for the actual synthesis will increase with increasing activation energy and they will decrease with the increasing of synthesis time.

Temperalure (°C)

Figure 4.13. The kinetic diagram of diamond synthesis that shows the pressure and temperature conditions of in situ at real time for the formation of diamond by taking different transition routes. The actual pressure and temperature required for the actual synthesis will increase with increasing activation energy and they will decrease with the increasing of synthesis time.

If the starting material contains carbon atoms/ions that are generated by electric arc, lasers, or dynamite explosion, the activation energy is high (0.33 eV), and the synthesis duration can be very short (microseconds). As a result, the pressure and temperature for the synthesis are high.

If the starting material is graphite that is puckered instantly by either shock wave or flash heating, the activation energy is moderate (0.17 eV), so are pressure and temperature for the synthesis.

If the starting material is graphite that is disintegrated into flakes or dissolved atoms, the synthesis time is long. The activation energy is relatively low (0.13 eV), so are pressure and temperature for the synthesis.

If the starting material is diamond-like carbon as in the case of ionized methane gas, the synthesis time is also relatively long.

Diamond

Figure 4.14. Different precursors of carbon may transform into diamond with various activation energies. The term "flake" refers to the recrystal-ized graphite micro particles that are suspended in a molten metal catalyst. These flakes may be rhombohedral graphite that possesses the layer configuration easily puckerable into diamond structure.

In this case, the activation energy is the lowest (0.09 eV), so are pressure and temperature for the synthesis.

The above four major diamond formation routes and their associated activation energies are shown schematically in Fig. 4.14.

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