Silicon, diamond synthesis

The most conventional non-equilibrium plasma-chemical systems that produce diamond films use H2-CH4 mixture as a feed gas. Plasma activation of this mixture leads to the gas-phase formation of hydrogen atoms, methyl radicals (CH3), and acetylene (C2H2), which play a major role in further film growth. Transport of the gas-phase active species to the substrate is mostly provided by diffusion. The substrate is usually made from metal, silicon, or ceramics and is specially treated to create diamond nucleation centers. The temperature of the substrate is sustained at the level of 1000-1300 K to provide effective diamond synthesis. The synthesis of diamond films is provided by numerous elementary surface reactions. Four chemical reactions in particular describe the most general kinetic features of the process. First of all, surface recombination of atomic lydrogen from the gas phase into molecular hydrogen returns back to the gas phase ... 

The discussion of hydrothermal diamond synthesis is divided into two sections, dealing with synthesis from C-H-0 liquids and synthesis based on decomposition of silicon carbide, respectively. Both start with thermodynamic calculations in order to demonstrate the theoretical possibility of carbon formation before the experimental findings are summarized. Naturally, equilibrium calculations do not consider kinetic limitations. 

Successful hydrothermal diamond synthesis was carried out in autoclaves filled with a specially prepared carbon enriched water solution , the composition of which was not disclosed [15,45,46]. The carbon precursor should be fine-grained diamond, vitreous carbon or emulsion of crude oil and water [29]. The presence of free radical catalysts was mentioned and paragenetic crystallization of quartz needles and diamond indicate the presence of silicon [45]. The synthesis was described as a sol/gel colloidal process working in the range 200-600°C and 100-200 MPa. Healing and joining of diamond crystals was reported. After 21 days at 400°C and 170 MPa, thin colorless films of polycrystalline diamond were obtained on (111) surfaces of seed crystals (Fig. 3c). With a reported size of 15-40 pm, these are the largest diamond crystals from hydrothermal experiments. 

Dense parts made by hot-pressing are generally considered to have a very fine microstructure (see Fig. 12g) and the best mechanical properties. Completely pure silicon carbide does not exhibit any noticeable plastic behavior up to its decomposition temperature and can therefore be densified only under diamond synthesis conditions... 

A simple way of building nanotubes is the use of elements that are able to adopt a planar, layered structure. In this context, carbon is special due to its different possible hybridizations. Both two- and three-dimensional networks, such as graphene and diamond, can be built by the same element. Therefore, it has been a challenge for theoreticians and experimentalists to search for other nanotubular structures built up from just one element. Silicon has been a natural candidate since it belongs to the same main group as carbon. The stability of silicon nanotubes has been predicted theoreti-cally. However, an experimental observation or synthesis has not been achieved to date. 

Oxygen in a small concentration was found to be essential for the synthesis of diamond by hot-filament chemical vapor deposition on silicon in the presence of methane [18] and by magnetoactive microwave discharge [19]. 

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