Nucleation diamond

The main siUcate iaclusions ia natural diamond are pyroxenes and garnet [12178-41 -5] and the understanding of the conditions of their formation from laboratory studies is the basis for the determination of the P—T conditions when diamond was formed (2—6). CO, CO2, H2, H2O are also found ia diamond (20), and it is possible that diamond nucleated and grew ia a Hquid ia a C—H—O system, perhaps immiscible, but ia equiUbrium with the siUcate matrix (4). Graphite [7440-44-0] is also a common iaclusion ia natural diamond. 

A new, low-pressure, plasma-assisted proeess for synthesising diamonds has been found by Roy et al [83,84]. An intimate mixture of various forms of carbon with one of many metals (e.g., Au, Ag, Fe, Cu, Ni) is exposed to a microwave plasma derived from pure hydrogen at temperatures ranging from 600-1000 °C. Roy et al postulate a mechanism in which a solid solution of atomic hydrogen and the metal. Me, facilitates dissolution of carbon to form molten droplets of Me -Cj,-H. Diamonds nucleate at the surface of the droplets as the temperature is reduced. 

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... 

Li, D. M., Hemberg, R. and Mantyla, T. (1998), Diamond nucleation under high CH4 concentration and high filament temperature. Diam. Relat. Mater., 7(2-5) 188-192. 

In 1976, Derjaguin, Spitzyn and Bouilov showed that diamond nucleation is possible on nondiamond substrates such as copper. This proved to be a major breakthrough attracting the attention of the entire materials science community towards this field. In the early 1980 s a team of scientists led by Matsumoto in Japan demonstrated the nucleation and continuous low pressure growth of good quality diamond on various nondiamond substrates using different gas activation techniques. This set off an intensive research activity in Japan, Western Europe and the U.S.A. Since then a number of experimental procedures for producing well-crystallized diamond have been demonstrated. Since the last decade or so, scientists all over the world have actively taken up research in this field. 

The carbon atoms arriving at the substrate surface must exceed a certain concentration at the solid-gas interface to reach and exceed the critical nucleus size. Therefore the diamond nucleation density as well as the growth rate are dependent on the relative rates of bulk and surface diffusion of carbon atoms. ° These are different for different substrates. Thus, the nucleation process needs a temperature dependent incubation time which is related to the time required to form critical size diamond clusters on the substrate surface. The nucleation rate, which is initially negligible, reaches a maximum after a certain time period and tends to zero for longer deposition times. ... 

A bias nucleation process has been developed to overcome the problem of the low diamond nucleation density on untreated nondiamond substrates wherein a negative bias is apphed to the substrate during deposition. ... 

In contrast to the B-terminated (111) face, only randomly oriented diamond particles were grown on the N-terminated cBN(lll) face with a number density of only 10 /cm. Thus, diamond nucleation was difficult on the N-terminated cBN(lll) face. The difference in nucleation density between the B- and N-terminated cBN(lll) faces was attributed to the fact that the formation of energy of the B-C bond. Eg c (= 348 kJ/mol), is greater than that of the B-H bond, Eb-h (=320kJ/mol), while the formation of energy of the N-C bond,... 

Step 1 (pretreatment)-. The method of seeding for diamond nucleation centers on Ni surface has been optimized as the research was developed. In Ref. [170], the Ni surface was scratched with diamond powder of 0.25 pm in size. In their successive experiments, instead of scratching, diamond powder was sprinkled on the... 

The outline of known diamond nucleation methods and process has been described in Section. 2.2, and it is briefly summarized below ... 

In Figure 10.8, reference spectra of AES and XPS-EELS from various materials observed by Stoner et al. [2] are shown. Based on these data, it is clearly seen that the specimen spectra of Figure 10.7 exhibit a transition process from P-SiC formed by CVD to diamond. This transition process was also confirmed by Raman spectroscopy. According to an XTEM observation for the specimen after a 1-h biasing followed by a 5-h diamond CVD, an a-SiC layer of 6-nm (maximum 10-nm) thickness was present between the Si substrate surface and the diamond layer. An HRTEM indicated that diamonds nucleated within the interfacial layer but above the Si substrate. From the observed data, a model of diamond nucleation by BEN was proposed, as shown in Figure 10.9. 

The deposition of carbon particles under a positive bias voltage to the substrate was attributed to the electron flux to the substrate, giving the same effects as the electron-assisted HFCVD [219], in which a negative bias was applied to the filament and a high density of diamond nucleation occurred even without any pretreatment of Si substrates. Figure 10.11 shows the I-V characteristics of the plasma without substrate, indicating that the electron flux was approximately 10 times higher than the ion flux. 

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