The Threshold Conditions Of Diamond Synthesis

Diamond may be converted directly from a non-diamond carbon source in microseconds by a displacive movement of atoms without diffusion, or it can be synthesized catalytically in minutes by reconstructive means with the transport of atoms. The diamond formation pressure and temperature are dependent on the time scale and the structure of the carbon source. The instant conversion may occur with zero-dimensional carbon (point-like sp0 atoms) by PVD, one-dimensional carbon (line-like sp1 chains) by dynamite detonation, or two-dimensional carbon (plane-like sp2 surface) by shock wave compaction. The timely synthesis may proceed with two-dimensional (sp2) graphite (e.g. by puckering recrystal-lized flakes in a molten catalyst), or with three-dimensional (sp3) diamond-like molecules such as solute atoms in a molten catalyst (e.g. by temperature gradient method), or caged atoms in methane molecules (e.g. by CVD).

The approximate activation energies (E) for the major diamond synthesis methods may be estimated from their reaction routes. The threshold pressure and temperature for each of these synthesis routines may be calculated based on the estimated activation energy. The estimated values are summarized in Table 4.4.

The pressure and temperature for diamond synthesis may be determined from the activation energy associated with the mechanism of the transition. For each activation energy, the higher the pressure is available, the lower the temperature is necessary. Hence, the inverse function of the P-T trace is an U-shaped curve. The position of such a curve is dependent on the time scale of

Table 4.4. Threshold conditions for diamond formation

C Source

State

Method

Route

Time

E(eV)

P (GPa)

T (°)

Plasma

spQ

PVD

Direct

Instant

Q.33

16

35QQ

Dynamite

sp

Explosion

Direct

Instant

Q.25

12

27QQ

Graphite

sp

Shock wave

Direct

Instant

Q.17

8.1

18QQ

Graphite

sp

Puckering

Catalytic

Timely

Q.13

6.2

125Q

Solute

sp3

Precipitation

Catalytic

Timely

Q.13

6.2

125Q

Methane

sp3

Adjoining

Catalytic

Timely

Q.Q9

4.3

8QQ

synthesis (Sung and Tai, 1996). For example, if the time for the synthesis is short, as in the case of explosion synthesis, the P/T trace for the transition would be displaced toward high values.

The activation energy for diamond synthesis depends on the original form of carbon and the route it takes to become diamond. In the case of direct graphite-diamond transition by the puckering mechanism, such as that may happen in the explosion synthesis, the activation energy is approximately 0.17 eV. If the puckering is assisted by the catalytic action of molten metal, this activation energy may be further decreased to about 0.13 eV, so the P/T requirements for the synthesis are greatly reduced. The longer durations for static high pressure processes may further depress this P/T regime required for the synthesis.

In the above example of PVD synthesis of diamond, the starting carbon is in the form of carbon atom. The activation energy for forming diamond is about 0.33 eV. This amount of high energy barrier, coupled with its extremely short duration of time in forming diamond bonds would shift the P/T curve significantly to the high side.

For the CVD process, the starting carbon is already in the diamond-like configuration. The activation energy is then equal to joining such diamond-like carbon atoms together. This activation energy (0.09 eV) is the lowest for all diamond synthesis processes. Table 4.4 summarizes the activation energies of major diamond synthesis routes and the representative pressure and temperature regimes for these processes to occur.

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