Atomic hydrogen, diamond synthesis

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. 

In the case of hot-filament CVD, refractory metal filaments (e.g., W, Ta, Re, etc.) are electrically heated to very high temperatures (between 2000 and 2700°C) to produce the necessary amount of atomic hydrogen that is necessary for the reasons mentioned above for the synthesis of diamond. Except for combustion flame CVD, hot-filament CVD is considered the simplest of all of the methods and also the most inexpensive. Plasma-jet and laser-assisted CVD methods rely on a plasma torch or laser to attain the very high temperatures that are needed to... 

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 ... 

Pyrolysis of CH4 over a diamond surface was the first extensively studied process in which synthesis of diamond was documented in the laboratory (3,4). Molecules of CH4 at low pressure ( 0.1 torr) undergo decomposition on the hot (e.g., 1000°C) diamond surface. Carbon atoms are bonded to the surface and hydrogen atoms return to the gas phase after an abstraction process. 

---