Crystallites diamond synthesis

Shock Synthesis. When graphite is strongly compressed and heated by the shock produced by an explosive charge, some (up to 10%) diamond may form (26,27). These crystallite diamonds are small (on the order of 1 -lm) and appear as a black powder. The peak pressures and temperatures, which are maintained for a few microseconds, are estimated to be about 30 GPa (300 kbar) and 1000 K. It is believed that the diamonds found in certain meteorites were produced by similar shock compression processes that occurred upon impact (5). 

Around the same time a similar technique was independendy developed whereby micrometer sized diamond crystallites were grown (161). What is required in essence for the low pressure diamond synthesis is a source of carbon (typically a hydrocarbon gas), hydrogen, and a temperature above 2000°C to convert molecular hydrogen to its atomic state. 

We have developed solvothermal synthesis as an important method in research of metastable structures. In the benzene-thermal synthesis of nanocrystalline GaN at 280°C through the metathesis reaction of GaClj and U3N, the ultrahigh pressure rocksalt type GaN metastable phase, which was previously prepared at 37 GPa, was obtained at ambient condition [5]. Diamond crystallites were prepared from catalytic reduction of CCI4 by metallic sodium in an autoclave at 700°C (Fig.l) [6]. In our recent studies, diamond was also prepared via the solvothermal process. In the solvothermal catalytic metathesis reaction of carbides of transition metals and CX4 (X = F, Cl, Br) at 600-700°C, Raman spectrum of the prepared sample shows a sharp peak at 1330 cm" (Fig. 1), indicating existence of diamond. In another process, multiwalled carbon nanotubes were synthesized at 350°C by the solvothermal catalytic reaction of CgCle with metallic potassium (Fig. 2) [7]. 

Depending on the method of their preparation, the individual nanodiamond particles do not exist as isolated crystallites, but they form tightly bound agglomerates. Apart from unordered sp - and sp -hybridized carbon, they may also include other impurities. The latter may originate either from synthesis or purification, for example, finely dispersed material from the reactor walls may contaminate the sample (Section 5.3). This is especially true for material produced by the detonation or shock wave method, whereas hydrogen-terminated diamond nanoparticles do not show this effect. 

This publication is a good example of the development of analytical techniques such as high resolution electron microscopy lattice imaging [108] and micrometre-area Raman spectroscopy (microRaman) [109] which permit the researcher to determine that diamond is present in crystallites of submicrometre, perhaps even nanometre size. Early scientists like Moissan [110] and Hannay [111] performed experiments in the pressure and temperature ranges now being cited as feasible for hydrothermal synthesis of diamond [38, 112, 113]. Had they been equipped to use the analytical techniques now available, they may have been proven correct in their claims, but in a particle size regime much finer than they were able to inspect. 

The idea underlying the dynamic synthesis of nanometer-sized diamonds essentially consists of providing the pressure and tanperature needed to drive the graphite-diamond-phase transition by means of a shock (explosive) wave. Since both the pressure and temperature in a shock wave are characteristic of the region of thermodynamic stability of diamond, the time of synthesis is necessarily very short (as a rule, a few fractions of a microsecond), and, hence, the diamond crystallites produced usually remain nanometer sized. 

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