Diamond growth

J.m/h. Because the diamond growth takes place under atmospheric conditions, expensive vacuum chambers and associated equipment are not needed. The flame provides its own environment for diamond growth and the quaUty of the film is dependent on such process variables as the gas flow rates, gas flow ratios, substrate temperature and its distribution, purity of the gases, distance from the flame to the substrate, etc. 

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. 

Fig. 1. Carbon-phase diagram where A, solvent-cataly2ed diamond growth B—G, diamond formation direcdy from graphite C, graphite formation from diamond, D, approximate region where formation of Lonsdaleite occurs from weU-ordered graphite crystals (7,8). To convert GPa to atm, multiply by...

Halogen-Based Deposition. Diamond growth also occurs in several halogen-based reactions such as ... 

Yarborough, W. A., Diamond Growth with Locally Supplied Methane and Acetylene, /. Mater. Res., 7(2) 379-383 (1992)... 

Yarborough, W.A., Thermochemical and Kinetic Considerations in Diamond Growth, Diamond Films and Technology, 1(3) 165-180 (1992)... 

Microwave Plasma CVD reactors use very similar conditions to hot filament reactors, and despite being significantly more expensive, are now among the most widely used techniques for diamond growth. In these... 

Figure 5.5. Electron micrographs of different types of diamond film grown on silicon. The white bar shows the scale in micrometres (p.m) (thousandths of a millimetre), (a) The initial stages of diamond growth on a nickel substrate, showing individual diamond crystallites nucleating in scratches and crevices created on the surface by mechanical abrasion, (b) a randomly oriented him,...

From these observations, we may conclude that, in the growth of natural diamond crystals, three faces, 111, 110, and 100, behave and show characteristics completely in agreement with the characteristics expected from PBC analysis. Therefore, we may conclude that, under the environmental conditions of natural diamond growth (principally in the silicate solution phase), 111 always behaves as a smooth interface under A/r/kT conditions, whereas the Ap,/lcT of 110, and particularly of 100, stays close to the origin under any conditions, and these faces behave exclusively as rough interfaces. 

There are three types of rocks that are known to fulfil the conditions for diamond growth, and, in fact, diamonds are found in all of them. 

It should be noted that not all flames have the behaviors discussed above. For example, the equilibrium species distribution in some H2-N20-Ar flames has essentially the same mole number as the reactants. As a result the adiabatic flame temperature is achieved directly in the flame front with no long recombination tail. Ammonia-oxygen flames exhibit a slow approach to chemical equilibrium, albeit with a long dissociation, not recombination, tail [279], Here the temperature in the flame front overshoots the adiabatic flame temperature, with the equilibrium temperature being approached from above as the dissociation reactions proceed. In certain highly strained, rich, hydrocarbon flames (e.g., C2H2-H2-O2), such as those used for flame-based diamond growth, the temperature can also overshoot the adiabatic flame temperature in the flame front. Here the overshoot is caused by the relatively slow dissociation of the excess acetylene [270]. 

D.S. Dandy and M.E. Coltrin. A Simplified Analytical Model of Diamond Growth in Direct Current Arcjet Reactors. J. Mater. Res., 10(8) 1993-2010,1995. 

Prior to deposition, polish the Si substrate for 15 min with diamond powder on a felt polishing pad. The scratches serve as nucleation sites for the diamond growth. Clean the polished substrate with 2-propanol twice, and dry it by air blowing. Then, place on molybdenum holder in the CYD reactor, and reduce the system pressure to less than ICC3 Torr. 

Another candidate for a useful material from very high pressure synthesis is the gem material, jadeite (NaAlSi206). The natural material of Imperial quality can cost as much as 2000 per carat. Jadeite can be synthesized at about 30 kb and above in equipment similar to that used for diamond growth, and it has been made into pieces of jewelry. Since jadeite is used as a poly-crystalline aggregate, synthesis is essentially hot pressing and sintering, much simpler than if single crystals were needed. However, it does not appear to be a commercial product in competition with the natural supply. 

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