Nucleation 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 ... 

There are continuous theoretical attempts to describe the mechanism of CVD-diamond synthesis including mechanisms of surface reactions, diamond nucleation, and film growth. To achieve this aim various phenomenological or first-principles models, molecular dynamics and Monte Carlo simulations have been used [57,58]. 

Diamond research in these areas continued to advance into the 1990s, less so in the United States and more so in Europe and Japan. During this period, the cost of producing diamond was significantly reduced due to improvements in the nucleation and growth rates. More was learned about the mechanisms and kinetics of diamond synthesis. The use of diamond as an electron emitter became an active area of research due to the negative... 

Crystal Morphology. Size, shape, color, and impurities are dependent on the conditions of synthesis (14—17). Lower temperatures favor dark colored, less pure crystals higher temperatures promote paler, purer crystals. Low pressures (5 GPa) and temperatures favor the development of cube faces, whereas higher pressures and temperatures produce octahedral faces. Nucleation and growth rates increase rapidly as the process pressure is raised above the diamond—graphite equiUbrium pressure. 

The higher the pressure over equilibrium, the higher the diamond nucleation and growth rate and the smaller and less perfect the crystal. Lower synthesis temperatures favor cubes and higher ones, octahedra. Suitable control of these variables permits the growth of selected types of... 

Jiang H.M., Hwang N.M., Theory of the charged cluster formation in the low pressure synthesis of diamond Part I. Charged-induced nucleation. J. Materials Res., 13(12) (1998) 3527-3535. 

Diamond nucleation rates on non-diamond substrates vary from 10 to 10 cm h, depending on synthesis conditions, substrate materials and surface pretreatment methods (polishing, etching, seeding, or annealing). 

Surface nucleation rates and densities of diamond on non-diamond substrates vary fi-om 10 to 10 cm h and from 10 to 10 cm, respectively, depending on substrate materials, surface pretreatment methods, and synthesis conditions. The possible maximum nucleation density of diamond would be 10 cm . ... 

The possible success of a synthesis route using nongraphitic carbon would seem to depend upon the suppression of graphite nucleation and purity factors, since growth of high quality diamond requires low levels of impurities. This is more difficult to achieve in a carbon which has not been graphitized. 

There is experimental support for this in the finding that nanometre diamonds are abundant in primitive meteorites and possibly in the interstellar dust clouds [104]. Similarly, growth of graphite in the diamond-stable region during high pressure, high temperature synthesis, and the corollary, an overpressure for nucleation of... 

For substrates other than Si, and technologically more feasible, is the seeding of substrates with nanometer-sized diamond powder applied in an ultrasonic bath, for example. This is the nucleation method generally employed in the CVD synthesis of polycrystaUine films and coatings. 

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