Cvd Diamond Formation

Although graphite is stable at low pressures but this refers to large structures of more than hundreds atoms. If the clusters of atoms are less than several hundreds, diamond, bucky balls, or carbon nano tubes may still be more stable. This is because the edges of two-dimensional graphene planes are more energetic than three-dimensional diamond surfaces, bucky balls with no ends, and rolled graphene planes with less ends. Hence, diamond, bucky balls and carbon nanotubes may form in smaller clusters of carbon atoms. Thus, in terms of thermodynamics, small-sized diamond is still more stable than small graphite even at low pressures (Fig. 11.15).

In addition, the atomic distance (1.54 A) in diamond lattice is much smaller than inter planar distance (3.35 A) between graphene planes, hence, if the decomposed carbon atoms have a long mean free path, it can attach to a diamond nucleus much quicker than starting a new graphene plane. Consequently, diamond may grow prior the formation of graphite. Thus, in terms of kinetics, diamond can grow faster than graphite.

Based on the above consideration, diamond may be preferentially grown from energetic carbon atoms formed from gas if the deposition rate is kept so slow that graphite is difficult to form. In 1952, William Eversole of Union Carbide was attempting to grow diamond this way by pyrolysis of methane at high temperature. In order to promote the diamond formation, he used seeded diamond as the precursor. Eversole might have found the window of diamond growth by decomposing methane gas at very slow rate. If so, he would have made diamond about a year before ASEA's first artificial diamond made under high pressure; and it was 2 years before the widely publicized GE's synthesis of diamond by converting graphite with metal catalyst.

However, Eversole's pyrolysis process could grow diamond only a few nanometers per hour, as any further rush of the process would yield deposition of only amorphous carbon. The characterization of nano-sized diamond was not possible in 1950s, particularly, it was a thin coating that covered a diamond seed,

Was this article helpful?

0 0

Post a comment