The CVD Process

CVD is a versatile process, well adapted to the production of all the refi actory carbides and nitrides not only as coatings but also as powders, bulk/monolithic components, and fibers. It may be defined as the deposition of a solid on a heated surface firom a chemical reaction in the vapor phase. Its advantages are  

The by-products can also be toxic and corrosive and must be neutralized, which may be a costly operation 

Thermodynamic and Kinetic Considerations. As with all chemical reactions, the constraints of thermodynamics and kinetics apply to chemical v )or deposition, i.e., the reaction must have a negative heat of formation (-AG ). An analysis of these constraints is necessary before any CVD reaction is considered. 

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Refractory compound coatings of carbides, nitrides, and oxides on cemented carbide cutting tools, mainly by the CVD process, are estimated at 300 X 10 annually worldwide. 

The CVD process is accomplished using either a hot-wall or a cold-wall reactor (Fig. 13). In the former, the whole chamber is heated and thus a large volume of processing gases is heated as well as the substrate. In the latter, the substrate or substrate fixture is heated, often by inductive heating. This heats the gas locally. 

Reactions of boron ttihalides that are of commercial importance are those of BCl, and to a lesser extent BBr, with gases in chemical vapor deposition (CVD). CVD of boron by reduction, of boron nitride using NH, and of boron carbide using CH on transition metals and alloys are all technically important processes (34—38). The CVD process is normally supported by heating or by plasma formed by an arc or discharge (39,40). 

The book is divided into three major parts. The first covers a theoretical examination of the CVD process, a description of the major chemical reactions and a review of the CVD systems and equipment used in research and production, including the advanced subprocesses such as plasma, laser, and photon CVD. 

Until recently, most CVD operations were relatively simple and could be readily optimized experimentally by changing the reach on chemistry, the activation method, or the deposition variables until a satisfactory deposit was achieved. It is still possible to do just that and in some cases it is the most efficient way to proceed. However, many of the CVD processes are becoming increasingly complicated with much more exacting requirements, which would make the empirical approach too cumbersome. 

Such an analysis requires a clear understanding of the CVD process and a review of several fundamental considerations in the disciplines of thermodynamics, kinetics, and chemistry is in order. It is not the intent here to dwell in detail on these considerations but rather provide an overview which shouldbe generally adequate. More detailed investigations of the theoretical aspects of CVD are given in Refs. 1-3. 

CVD in Fiber, Powder, and Monolithic Applications 467 2.3 The CVD Process for Fiber Production... 

Independent of the CVD system, certain constants must be adhered to. The precursor is one of the most important components of the CVD system and is often referred to as the source. The first step in the CVD process is vaporization of the precursor, if it does not already exist as a gas at ambient conditions. The precursor should have sufficient vapor pressure, at least 100 mtorr at delivery temperature, to achieve reasonable deposition rates (16). Ambient temperature liquids are preferred to solids, since it is easier to maintain a constant flux of the precursor in the vapor phase. This is due to the fact that liquids rapidly reestablish equilibrium upon removal of vapors,... 

The concept to use CNT-LiMP04/LiM204 composite/hybrid to increase the rate performance (not the energy density) of LIBs gives some interesting and promising aspects. But the work in this research area is by far not as systematic as for the anodic side. As discussed above, the performance depends on many factors, and there is still no study on how the microstructure and aspect ratio of CNTs influence the performance of the obtained cathode. One other technical problem to use such composite/hybrid cathode materials is the impurity in CNTs remaining after the CVD process. 

SiC monofilaments produced by the CVD process is generally superior to Nicalon SiC fibers in mechanical properties because of its almost 100% 6-SiC purity while Nicalon is a mixture of SiC, Si02 and free carbon. Representative properties of SiC monofilaments and Nicalon fibers are given in Table 5.15. 

Stripping of chlorine from hydroxides such as Cl2Sn(OH)2 could eventually lead to gas-phase SnO or Sn02. However, at the relatively low temperatures typical of tin oxide CVD ( 873-973 K), we do not expect these oxides to form, based on the equilibrium calculations described above. Thus, the formation of tin hydroxides is not only thermodynamically favored (i.e., based on minimization of the Gibbs free energy), but there are also exothermic reaction pathways that we expect to be kinetically favorable. The primary tin carrier in the CVD process could therefore be a tin hydroxide. Complete conversion to Sn02 would most likely occur via reactions on the surface. 

Thus, the primary tin carrier in the CVD process is again expected to be a tin hydroxide, whose conversion to Sn02 most hkely on the deposition surface. 

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