Diamond Synthesis Routes

Diamond is everywhere; inside supernovae, with star dusts, in meteorites, around impact craters, and under ground, consequently, there are numerous ways that can form diamond. However, all these synthesis routes employ a common strategy, namely, by squeezing loosely held carbon atoms surrounded by contact neighbors of 0 (sp0, e.g. C), 1 (spa5, e.g. C2), 2 (sp1, e.g. car-byne, =C=C=), 3 (sp2, e.g. graphite), and 4 (sp3, e.g. methane) to make them more compact by forming a continuous network of tetrahedral (sp3) bonds. The difference between non-diamond carbon atoms and diamond carbon atoms are that the former contain either dangling valence electrons, or they are bonded to non-carbon atoms. In the case of diamond, there are no foreign atoms nor unbonded electrons or multiple bonded electrons, as all four valence electrons are involved in single covalent bonding.

There are two general ways to subject carbon atoms under pressure, by squeezing them simultaneously, or by squeezing them in turn. In order to squeeze carbon atoms together, the raw material should be solid. Such a solid may either form linear structures with sp1 bonding (e.g. carbyne or its hydrogenated form such as polyethane), or most commonly, it possesses a layered structures of sp2 bonding (graphite). The solid carbon atoms can be squeezed instantaneously, as in the case of explosive synthesis of diamond, or they may be squeezed continually as in the case of static high pressure synthesis of diamond. In either case, high temperature must be accompanied to facilitate the transformation of loose carbon atoms into compact diamond. However, the temperature of the dynamic process is transient, whereas it is steady in the static process.

Hydrocarbon molecules, such as methane, may also be dissociated at high temperature. The dissociated carbon atoms may be influenced by surrounding hydrogen atoms to maintain the tetrahedral bonding and they connect one-by-one to form a continuous diamond film. This process is known as CVD.

Table 4.2. Major diamond synthesis methods

State

Material

Pressure

Mechanism

Catalyst

S

Graphite

High

Puckering

None

L

Graphite

High

Puckering

Fe group

C solute

Low

Exsolution metal

G

Methane

Low

CVD

H

Plasma

C ions

Low

PVD none

Alternatively, ionized carbon atoms or hydrocarbon molecules may be smashed one-by-one onto a substrate, where they are accumulated to form a thin coating of diamond-like carbon with distorted tetrahedral bonding. This process is known as PVD.

Although high temperature is also required in CVD or PVD process, the temperature is uniform in the deposited film in the former case, but it is restricted locally in the latter case. In both cases, the ambient pressure is lower than atmospheric pressure. In the case of PVD process, the ambient temperature may also be low (e.g. <100°C).

Diamond may also be formed by other methods. For example, it could be produced by laser irradiation, accelerated quenching, ultrasonic booming, reduction pyrolysis, or hydrothermal reaction. Major diamond synthesis methods are listed in Table 4.2.

The commercial diamond production has been using primarily methods of static high pressure, dynamic explosion, CVD synthesis, and PVD deposition. Although the synthesis methods may differ, many share the common diamond formation route as shown in Fig. 4.10. Thus, it is the focus of this review to compare the mechanisms and kinetics of major diamond synthesis routes, with the examples based on commercial methods of production.

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