SiC formation

The carbothermic reduction can also be carried out at lower (about 1500°C—1600°C) temperature resulting in P SiC formation (72). 

Based on these analyses on the SiC coating, the growth mechanism of the SiC layer on diamond is considered as follows. In the early stage of the SiC formation on diamond, a very thin SiC layer is formed on the diamond surface according to reaction (10.2) between diamond and SiO(g). Once the SiC layer is formed, this reaction does not proceed due to the protective layer of SiC. The carbon sheet and felt in an alumina crucible act as the carbon source. The reaction of C02(g) with these carbon sources will produce further CO(g) and deposit SiC(s) by reaction (10.7). Thin j3-SiC whiskers are observed on the surface of the SiC-coated diamond, suggesting the vapor growth of SiC. 

The apparent activation energy of the SiC formation reaction is obtained by an Arrhenius plot of the rate constants that can be calculated using the mass gain data as a function of the coating temperature using the least-... 

From the above results, the growth model of the SiC formation on MWCNTs can be proposed as illustrated in Fig. 10.8. The growth of SiC is influenced by the existence of the carbon source in the crucible. In the early stage of the reaction, SiO(g) reaches the surface of MWCNTs and forms a thin SiC layer... 

Carburization P-SiC formation (step 1) Diamond nucleation (step 2)... 

In all three experiments, about seven holes (7.3 in EXP. 1 6.9 in EXP. 2 7.1 in EXP. 3) on average are needed for a single SiC pair to be removed. These results are also consistent with the hole number calculated for triangular porous SiC formation and the PECE reported by Shor and Kurtz [8]. This implies that all the photoelectrochemical reactions are similar, at least in the number of holes needed to etch each SiC pair. The different porous morphologies obtained here are unlikely to be related to different chemical reactions. 

Kim et al.f studied the effect of gas pressure on the nucleation behavior of diamond on a Si(lOO) substrate in HFCVD. The pressure was varied from 2 to 50 torr, while a filament temperature of2200°C, a substrate temperature of 850°C, a total flow rate of 20 seem and a CH4 concentration of 0.8 vol.% were used. The characterization of diamond deposits using micro-Raman spectroscopy, SEM and OM revealed that the maximum nucleation density of diamond parades on the unscratched Si substrate occurred at a pressure of 5 torr. The pressure dependence of the nucleation density was explained by the competition effect between P-SiC formation, which increases the diamond nucleation density, and atomic-hydrogen etching, which decreases the nmnber of nucleation sites. On the basis of this finding, a new fabrication approach for high-quality diamond films without... 

The theoretical model ing of the kinetic aspects of diamond nucleation processes is indeed scarce in published literature. Although attempts have been made to model the nucleation kinetics,as reviewed above, the approaches require an accurate estimation of the kinetic rate constants, necessitating fitting the kinetic model to experimental data, thereby making the model system- (or experiment-) dependent. In addition, the kinetics of surface diffusion of adatoms and the formation of intermediate carbonaceous phases were not considered in these studies. As indicated in Ref. 217, a kinetic model is expected to contribute to a better understanding of the role of SiC formation in the nucleation of diamond on a Si substrate. However, the kinetic scheme employed in these studies was, in fact, unable to distinguish between a Si and a SiC surface. To capture the possibil ity that an intermediate carbonaceous phase (such as DLC, carbide or graphite) may form prior to diamond nucleation, the kinetic model should be modified to includea time dependence of the density of nucleation sites determined by the kinetics of the formation of the intermediate phase. Further studies are therefore needed to construct a clear picture of the kinetics of diamond nucleation processes in CVD. 

FIGURE 8.2 Sic formation vs time using (a) microwave heating and (b) conventional heating. (Reproduced with permission from Ebadzadeh, T. Marzban-Rad, E., Mat. Char., 2009, 60, 69-72. Copyright Elsevier.)... 

Additionally, the LSI process allows the implementation of reaction bonded SiSiC-coatings on the component s surface. Adding porous carbonaceous layers and additional silicon granulate on the C/C surfaces to be protected, extremely wear resistant coatings can be formed simultaneously to the SiC formation inside the C/C composite in an easy and economic way [39]. 

Many different VS-processes are known, depending on the starting raw materials and the catalysts species involved. In all cases, the use of a catalyst for whisker formation is favored over particulate SiC formation [93-96]. 

The optimal temperature for SiC formation has been determined by performing experiments at 1075, 1250, and 1378 K and utilizing 2w% Ni on carbon. The results of XRD analysis of the products are displayed in Figure 2. At low temperatures (1075 K) mainly silicon is formed by reaction 3. 

Figure 3. CH4 selectivity to SiC formation (+) and total carbon conversion (a) as a function of nickel content (1 h reacticm, extrudates)...

Another useful diagram is the one reported by Fan et al. (2000) where MoSi and C are considered. Above 1700°C, SiC formation and the presence of a liquid phase is predicted at any C/MoSi ratio. 

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