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. 

C. Chen, and T. Hong, The role of hydrogen in diamond synthesis from carbon dioxide-hydrocarbon gases by microwave plasma chemical vapor deposition, Sutf. Coat Technol, 54-55(l-3) 368-373 (1992)... 

The influence of the initial C/H/0 composition of the carbon compounds used for non-equilibrium diamond synthesis on the composition of the deposited films is usually illustratedby the so-called Bachmann triangle, showninFig. 9-45. The Bachmanntriangle summarizes experimental data and indicates the C/H/0 compositions corresponding to deposition of non-diamond films, diamond films, or no deposition at all (which corresponds to domination of etching over deposition). Deposition of diamond films from the gas phase is typically performed at pressures of 10-100 Torr in a mixture of hydrogen with about 1-5% methane. The plasma-chemical conversion of carbon compounds can lead not only to the formation of diamond films but also to so-called diamond-like carbon (DLC). DLC films are... 

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 ... 

M. Murayama and K. Uchida. Synthesis of Uniform Diamond Films by Flat Flame Combustion of Acetylene/Hydrogen/Oxygen Mixtures. Combust. Flame, 91 239-245,1992. 

Abstract Boron-doped diamond (BDD) electrodes provide an unusually wide electrochemical window in protic media, since there exist large offset potentials for the evolution of molecular hydrogen and oxygen, respectively. At the anode, alcohols are specifically converted to alkoxyl radicals. These can be used for chemical synthesis. When the enormous reactivity of such intermediate spin centers is not controlled, mineralization or electrochemical incineration dominates. Efficient strategies include either high substrate concentrations or fluorinated alcohols which seem to stabilize the spin centers in the course of reaction. 

Diamond, the hardest material known, is the most representative inorganic product of natural and artificial high pressure synthesis. As illustrated later, it can be obtained from graphite at 5.4 GPa and 1667 K, typically, while it is not diamond but graphite that is thermodynamically stable at ambient conditions. Though solid hydrogen cannot be quenched to ambient conditions, diamond can be and remains stable, in effect, to about 600 K. From practical viewpoints, high pressme synthesis is of value if the product has a particular... 

Low-pressure synthesis of diamond coatings involves the use of hydrogen and methane gas mixtures. Typical deposition pressures in these syntheses are 10 -10 Pa, and... 

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... 

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