Growth diamond synthesis

Figure 15 shows the variation of diamond deposition rates by various activated CVD techniques as well as the HP—HT technique (165). It can be seen that the highest growth rate of activated CVD diamond synthesis is stiU an order of magnitude lower than the HP—HT technique. However, CVD has the potential to become an alternative for diamond growth ia view of the significantly lower cost of activated CVD equipmeat and lower miming and maintenance costs. 

In the last 40 yr, the development of synthetic diamond in various forms has fueled a revolution in the use of diamond as an engineering material. The process of HPHT diamond synthesis was responsible for stunning growth in the abrasives market. During that time, the world s consumption of diamond abrasive materials increased from 5 to over lOOtons/yr. 

Figure 1. Typical growth rales versus gas-phase temperatures in various diamond CVD processes, illustrating the importance of gas-phase temperature for high rate diamond synthesis. (Repr uced with permission.)...

M. Kamo, Y. Sato, S. Matsumoto, and N. Setaka, Diamond synthesis from gas phase in microwave plasma, J. Cryst. Growth, 62(3) 642 (1983)... 

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 ... 

While HPHT, shock wave, and CVD synthesis of diamond have been commercialized, these methods have serious limitations. None of them allows economical growth of large single crystals and the diamond quality does not satisfy the requirements for electronic or gem applications. Since the quality and size of the best natural diamonds have not yet teen reproduced in the lab, the search for a tetter method of diamond synthesis continues. Recent work demonstrates that diamond can be formed metastably under a variety of conditions. 

Crystal perfection is determined by a reasonably low growth rate which is achieved by optimizing, in combination, the space between seeds, the axial temperature gradient and the solvent/catalyst under diamond synthesis conditions. 

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... 

Goodwin DG. Simulations of high-rate diamond synthesis methyl as growth species. Appl Phys Lett. 1991 59 277-9. 

There are no obstacles in searching for novel approaches to diamond synthesis by studying new growth processes electrolysis, hydrothermal (15) and laser assisted. Graphite has been transformed to diamond by laser process (16,17) and some laser assisted CVD processes were... 

Diamond synthesis was developed with limited scientific understanding of the growth processes. Despite this, scientists had the courage to put forth risky hypotheses and work with them. In this way, enormous progress has been made in controlling diamond growth processes as weU as developing a comprehensive picture of associated phenomena. 

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