Vapor-phase diamond

CVD (vapor-phase) diamond, potentially important but still basically at the laboratory stage withfew applications in production (reviewed in Ch. 13). 

The low-pressure vapor-phase process is based on chemical vapor deposition (CVD) and the material is often referred to as Vapor-phase diamond, diamond coating, or CVD diamond . CVD diamond will be used in this book. 

Another important function of metallic coatings is to provide wear resistance. Hard chromium, electroless nickel, composites of nickel and diamond, or diffusion or vapor-phase deposits of sUicon carbide [409-21-2], SiC , SiC tungsten carbide [56780-56-4], WC and boron carbide [12069-32-8], B4C, are examples. Chemical resistance at high temperatures is provided by aUoys of aluminum and platinum [7440-06-4] or other precious metals (10—14). 

Metastable vapor-phase deposition, of synthetic diamond, 8 538-539 Metastasis, 25 205... 

There are numerous materials, both metallic and ceramic, that are produced via CVD processes, including some exciting new applications such as CVD diamond, but they all involve deposition on some substrate, making them fundamentally composite materials. There are equally numerous modifications to the basic CVD processes, leading to such exotic-sounding processes as vapor-phase epitaxy (VPE), atomic-layer epitaxy (ALE), chemical-beam epitaxy (CBE), plasma-enhanced CVD (PECVD), laser-assisted CVD (LACVD), and metal-organic compound CVD (MOCVD). We will discuss the specifics of CVD processing equipment and more CVD materials in Chapter 7. 

The SiC layer grows in two steps. In the first step, a thin SiC layer is formed by the direct reaction between SiO(g) and diamond or MWCNTs. In the second step, nanometer-sized SiC granules are deposited on the SiC layer by the vapor phase reaction between SiO(g) and CO(g). 

Figure 1. Activated low-pressure diamond growth from the vapor phase.

Thermodynamics for low-pressure diamond growth from the vapor phase (in Chinese) (Science Press, Beijing, 2000) 202 pages. 

Wang J.-T., Carlsson J.-O., A thermochemical model for diamond growth from the vapor phase. Surf. Coat. Technol, 43/44 (1990) pp.1-9. 

Chemical Vapor Deposition (CVD) has been defined as a materials synthesis process whereby constituents of the vapor phase react chemically near or on a substrate surface to form a solid product. With these traditional processes a reaction chamber and secondary energy (heat) source are mandatory making them different from the Combustion CVD process. Numerous flame-based variations of CVD have been used to generate powders, perform spray pyrolysis, create glass forms, and form carbon films including diamond films. 

Nucleation from the vapor phase is not a determining factor in the relative kinetics of formation of either graphite or diamond on a substrate. Which of these two competing stmctures wins out depends on the kinetics of growth. 

It should be noted, however, that most conditions of deposition from the vapor phase have been shown to be such that classical nucleation theory is not well-suited to describe the nucleation kinetics of diamond, since the critical nucleus size is on the order of a few atoms.P The small size of the critical nucleus makes it quite inappropriate to use the classical thermodynamic variables to describe the nucleation processes. Under such conditions, the Gibbs free-energy of the formation of a critical nucleus carmot be expressed... 

In monotropy, one polymorph is always more stable than the other at all temperatures below their melting points (Fig. 7b). This definition is based on the assumption that the pressure remains eonstant. An alternative definition is that, if the pressure temperature phase diagram does not allow a polymorph to be in equilibrium with its vapor phase below the critical point, it is the unstable monotrope, otherwise it is an enantiotrope. This definition recognizes that some monotropes may be thermodynamically stable at elevated pressures and temperatures, e.g., diamond, which is the metastable polymorph of carbon under ambient conditions. 

Figure 5. Pressure and temperature conditions of the diamond synthesis (a) shock wave production of diamond (b) high temperature, high pressure regime for the synthesis of diamond (c) catalytic region for diamond formation (d) chemical vapor deposited diamond and (e) transformation of Cjo into diamond. The most recent review of the P, T phase diagram of carbon can be found elsewhere [151].

Identification of vapor grown diamond is accomplished by X-ray diffraction or electron diffraction using a transmission electron microscope (TEM) and by Raman spectroscopy. Diamond coatings show the characteristic first-order Raman peak for diamond at 1332cm [59]. In many cases, an additional peak located at 1550cm occurs in the Raman spectrum which is attributed to a highly disordered carbon phase (diamond-like carbon) [60]. This diamond-like carbon contains sp and sp hybridized C-atoms which do not coincide with graphite. 

Diamond /tungsten fiber endless solid one exper. vapor phase 2.4.6... 

Indentation twinning and possible structure change are the only indications that exist for any forms of silicon other than the cubic diamond modification which is in contrast to the polymorphism of carbon. Chemically deposited silicon films from the vapor phase are frequently encountered that are amorphous to X-radiation. 

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