The Activation Energies Of Diamond Formation

Carbon can form free atoms with no bond (sp0), linear chains (e.g. carbyne) with two bonds (sp1), planar structure (e.g. graphite) with three bonds (sp2), or volumetric framework (e.g. diamond) with four bonds (sp3). Thus, the three-dimensional diamond can be

Diamond Synthesis Routes Dynamic

Figure 4.10. Diamond synthesis routes from various bonding statuses of carbon atoms and the possible use of catalysts. Abbreviations: S = solid, L = liquid, G = gas, HP = high pressure, M = catalyst metal, H = hydrogen atoms. The energy source can be chemical, electrical, heat, electromagnetic, plasma, sound, etc.

Static

Figure 4.10. Diamond synthesis routes from various bonding statuses of carbon atoms and the possible use of catalysts. Abbreviations: S = solid, L = liquid, G = gas, HP = high pressure, M = catalyst metal, H = hydrogen atoms. The energy source can be chemical, electrical, heat, electromagnetic, plasma, sound, etc.

derived from zero-, one-, two- or three-dimensional carbon structures as precursors.

If the source material is free carbon atoms, they can form diamond by hitting one another. The diamond bond has an energy of

3.7 eV. However, the activation energy above the diamond energy for joining two carbon atoms to form a diamond bond is only with the magnitude of about 0.33 eV. This energy barrier is reached when two carbon atoms are approaching each other at a distance of about

Carbon atoms may also be formed by detonating a TNT with a high carbon content. In this case, the carbon containing molecules in the TNT themselves are smashed together to form nano-crystalline diamond (e.g. 4 nm). This direct explosion process is in contrast to the indirect process that explosion is used to create shock waves. It is the latter that puckers graphite into diamond. In the case of shock synthesis, the diamond crystals formed are

0.33

1.54 (Diamond)

1.80

0.33

1.54 (Diamond)

1.80

3.35 (Graphite)

Figure 4.11. The activation energy of two approaching carbon atoms. The highest energy (0.33 eV) exists at an inter-atomic distance of 1.8 A that is somewhere between a graphitic van der Waals bond of 3.35 A and a diamond's covalent bond of 1.54 A.

microns in size. The activation energy (2.5 eV) for smashing linear carbon structures together to form imperfect diamond is intermediate between that for bombarding with carbon atoms (0.33 eV) and for puckering graphite (0.17 eV).

Diamond may also be generated by dissociating carbonaceous gases in an atmosphere that does not contain hydrogen atoms. For example, if methane is mixed with argon or nitrogen and the gas is energized by microwave agitation, the carbonaceous gases may form dimers of carbon atoms (C2) analogous of nitrogen (N2), oxygen (O2) or fluorine (F2). Unlike these dimer molecules of gases that hold two atoms by single (F2), double (O2), or triple (N2) covalent bonds, the C2 dimer is held together by quadruple covalent bonds that is metastable. At high temperature, C2 dimers will decompose to form graphitic carbon. But at low temperature (e.g. 700°C), their covalent bonds may reconnect to form diamond if they have sufficient concentration. However, because the temperature is not high enough to allow long range diffusion of diamond atoms, so the diamond grains formed are nano-crystalline. The diamond film so formed is typically very thin (e.g. 1 fim). Because the grains are so tiny, a significant proportion of atoms is actually located on grain boundary rather than inside the crystal.

Another unusual CVD process capable of depositing diamond film continually on a steel substrate at ambient condition involve using multiple lasers. This method known as QQC (Mistry et al., 1996) has been used to grow diamond or diamond-like materials by bombarding CO2 and N2 gases with three lasers (CO2, YAG, and Excimer) simultaneously. Although the mechanism of diamond formation is unclear, but it is possible that C2 dimers are also involved as the nutrient for diamond growth. Apparently, the reaction induced by laser ablation is so violent that the dimers can collide among themselves at a rate fast enough to sustain the diamond growth to a significant size (e.g. 100 ¡m). Subsequently, the quenching rate from the high temperature of laser ablation is sufficiently high so the diamond formed is preserved without much oxidation or back conversion.

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