The Catalytic Mechanism

Saw grits are synthesized in the stability of diamond. According to common understanding, graphite will dissolve into molten metal and diamond will precipitate out as it has a lower solubility. However, this simple solution model cannot explain why the graphite microstructure can drastically affect the type of diamond formed. In fact, it has been observed that non-graphitized carbon can only form diamond, if any, under a much higher pressure. Moreover, amorphous glassy carbon must first crystallize to form graphite before transforming into diamond (Hirano et al., 1982). Furthermore, the higher the degree of graphitization in the carbon source, the faster the diamond can nucleate and grow (Hirano et al., 1984).

The solution mechanism also cannot explain why certain metals such as Cu or Zn cannot form diamond at all, although they may dissolve a minute amount of carbon. The apparent dependency of diamond formation from graphitic structures and metal chemistry can be attributed to the catalytic action. This catalytic mechanism was revealed by Gou in 1972 based on the electronic interaction between the empty 3d orbitals of transition metals and the unbonded (n bond) 2p electron of carbon.

According to Gou's model, when the catalyst metal melts, its atoms can assume a configuration of pseudo-closest packing in short ranges, resembling the (111) face of a CCP (FCC) lattice. If the metal atoms can cover roughly every other carbon atom on the graphite basal plane, then their empty 3d orbitals may attract the dangling 2p electrons of carbon. This attraction could pull every other carbon atoms toward the metal atoms. As a result, the graphite hexagon may pucker in such a way that the unmatched carbon atoms would move toward the opposite direction, i.e. toward the next layer of the graphite basal plane. This puckering can deform a plane hexagon of graphite to a "chair" of diamond.

If graphite has the right stacking sequence of hexagon layers, the away moving carbon atoms may interact with matching carbon atoms in the next layer to form diamond bonds. This puckering action may sweep through the graphite lattice like a wave (Fig. 4.6). As a consequence of this domino effect, an entire grain of graphite may transform into diamond.

The above-mentioned model explains the catalytic mechanism of the graphite-diamond transition. In order for this mechanism to operate, carbon must first form the graphite structure, and a catalyst must possess empty d orbitals. Both requirements are consistent with the empirical observations. For example, glassy carbon cannot form diamond, nor can d-orbital filled transition metals (e.g. Cu and Zn). In order to facilitate the formation of diamond, the graphite structure must be well developed and the catalyst must contain empty d-orbitals.

However, the above model falls in two deficiencies. First, the common graphite is predominantly 2H type, i.e. the stacking sequence is ABAB, etc. As such, only half of the atoms can find matching ones in its adjacent layers, and therefore the puckering cannot transmit onto the next layer. In order to allow the puckering to sweep through the structure, the graphite must be either the rhombohedral 3C type with an ABCABC, etc. sequence, or a hexagonal IH type with the AAA, etc. sequence. 3C type graphite can pucker into cubic diamond; and IH type graphite into hexagonal diamond (lonsdaleite) (Fig. 4.6).

As a diamond synthesized under high pressure is exclusively cubic in structure, it follows that ordinary graphite must first slide into the rhombohedral form before puckering into diamond. Graphite basal plans are held together by weak van der Waals bonding. The activation energy for shuffling these planes is 0.076 eV, much less than 2% of the energy required to break a graphite bond (4.8 eV). This activation energy is easily overcome at a temperature of 883°C. Moreover, the sliding of basal planes can be conveniently accomplished when graphite recrystallizes under the influence of a molten catalyst, a common phenomenon taking place during the formation of diamond.

The more empty d-orbitals a catalyst contains, the stronger its attraction toward carbon. As a result, when the deficiency of d-orbitals is too high, there is a tendency for the transition metal to form carbide. It has been observed that strong carbide former such as Ti or V cannot catalyze diamond formation. This observation must be factored in with the above catalytic model.

In order for a catalyst to pucker the graphite net, it must do so without sticking to it permanently. Thus, the golden rule for a diamond catalyst is "touch and go." The best manifestation of this phenomenon is the dissolution of carbon in a catalyst. When carbon atoms are dissolved as solute, they "touch and go" without forming carbide. The ability of the catalyst to moderate its interaction with graphite is also revealed in its solubility of carbon. Hence, the higher the solubility of a catalyst, the stronger its interaction with carbon without the formation of a compound. In other words, the solubility of carbon in a transition metal is the indicator of its catalytic power for converting graphite into diamond. This power may be manifested in the thickness of a graphite flake that it can pucker into a diamond nucleus.

Figure 4.6. Schematic showing the puckering of graphite into diamond. An effective metal catalyst has the right-sized atoms (large circles) that match every other carbon atoms (small circles) on graphite's basal plane (001) (top diagram). The metal atoms will pull half of the carbon atoms (lightly dotted) toward them. The result is an accordion effect that puckers the graphite structure into that of diamond (lower diagram). The middle diagram illustrates the catalytic conversion (all black atoms moving upward; and white ones, downward) of a rhombohedral graphite (ABC type) into a cubic diamond. The lower diagram shows a similar catalytic conversion (all white atoms moving upward; and black ones, downward) of a hexagonal graphite (AAA type) into lonsdaleite (hexagonal diamond) (Sung et al, 1996).

Figure 4.6. Schematic showing the puckering of graphite into diamond. An effective metal catalyst has the right-sized atoms (large circles) that match every other carbon atoms (small circles) on graphite's basal plane (001) (top diagram). The metal atoms will pull half of the carbon atoms (lightly dotted) toward them. The result is an accordion effect that puckers the graphite structure into that of diamond (lower diagram). The middle diagram illustrates the catalytic conversion (all black atoms moving upward; and white ones, downward) of a rhombohedral graphite (ABC type) into a cubic diamond. The lower diagram shows a similar catalytic conversion (all white atoms moving upward; and black ones, downward) of a hexagonal graphite (AAA type) into lonsdaleite (hexagonal diamond) (Sung et al, 1996).

0 0

Post a comment