Diamond clusters

A. Hirata, M. Igarashi, T. Kaito, Study on solid lubricant properties of carbon onions produced by heat treatment of diamond clusters and particles. Tribology Int., 37(11-12) (2004) 899-905. 

Other than single crystalline and polycrystalline diamonds, cluster diamond (CD) [96] is another variety that contains a core and an overgrown region containing a plurality of diamond crystallites extending outward from the core. Some CD, which consists of ultrafine diamond particles with a mean particle size of 5 nm, shows excellent lubricating ability [97]. 

The carbon atoms arriving at the substrate surface must exceed a certain concentration at the solid-gas interface to reach and exceed the critical nucleus size. Therefore the diamond nucleation density as well as the growth rate are dependent on the relative rates of bulk and surface diffusion of carbon atoms. ° These are different for different substrates. Thus, the nucleation process needs a temperature dependent incubation time which is related to the time required to form critical size diamond clusters on the substrate surface. The nucleation rate, which is initially negligible, reaches a maximum after a certain time period and tends to zero for longer deposition times. ... 

Vereschagin, A.L. Sakovich, G.V. Komarov, V.F. Petrov, E.A. Properties of ultrafine diamond clusters from detonation synthesis. Diamond Relat. Mater 1993, 3, 160-162. 

Little is known so far about the chemical properties, yet first results suggest a reactivity similar to that of multiwalled carbon nanotubes. Furthermore, a transformation of nano-onions into other forms of carbon can be achieved by heating (equihbration as faceted nanoparticles) or electron bombardment. In large carbon onions, a formation of small diamond clusters due to internal self-compression has been observed. These grow up to be nanoscale diamond particles under complete consumption of the onion structure. 

It was found that condueting the RF self-bias experiment with hydrocarbons in the presence of H2 results in production of small diamond clusters. This experimental result indicates that aC H could be considered as an intermediate compound formed during the transformation of hydrocarbons to diamond in the plasma processes [65]. 

Hence the volume of a mixture of heterogenous molecules interacting by the vdW mechanism exceeds the additive value, and according to the Le Chatelier principle such mixture under high pressure will be separated. It follows that the diamond cluster size depends on the separation extent of components of the detonation cloud. In this context, it seemed reasonable to perform a detonation synthesis of c-BN under similar conditions (in a large chamber). No attempt in this direction has been... 

An attempt to include the observed carbon cluster structure for carbon into the detonation equation of state has been made by Van Thiel and Ree . They replaced their diamond equation of state with an estimated diamond cluster treatment and found a larger compressibility, heat of formation and initial volume for the cluster compared to diamond. They concluded that a more accurate description of the surface layer will be required. 

Figure 2.72 shows the dependence of the heat of detonation for several explosives on the average size of the diamond clusters. It was calculated in reference 73 for the equilibrium chemical composition and the heat of formation of diamond-like clusters using... 

Figure 2.72 The relative heat of explosion as a function of the average diamond cluster size for TATB, PETN, HMX and Composition B.

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