THE CVD OF GRAPHITE

The CVD of graphite is theoretically simple and is based on the thermal decomposition (pyrolysis) of a hydrocarbon gas. The actual mechanism of decomposition, however, is complex and still not completely understood. This may be due, in part, to the fact that most of the studies on the subject of hydrocarbon decomposition are focused on the improvement of fuel efficiency and the prevention of carbon formation (e.g., soot), rather than the deposition of a coating. 

Flexural strength (c direction), MPa 80-170 Tensile strength (ab direction), MPa 110 Young s modulus of elasticity, GPa 28-31 

The most common precursor is methane (CH4), which is generally pyrolyzed at 1100°C or above, over a wide range of pressure from about 100 Pa (0.001 atm) to 10 Pa (1 atm). The reaction in a simplified form is as follows  

Other common precursors are ethylene (C2Hg) and acetylene (C2H2). Acetylene can also be decomposed at lower temperatures (300-750°C) and at pressures up to 1 atm, in the presence of a nickel catalyst.Pl Another common precursor is propylene (CgH ), which decomposes in the 1000-1400°C temperature range at low pressure (- 1.3 X 104 Pa or 100 Torr).  

The activation energies for the decomposition of these precursor gases still are not accurately known and show a large scatter. Some reported values are as follows  

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It is only since the end of World War 11 that the CVD of graphite began to expand rapidly as researchers realized the potential of this technique for the formation of coatings and free-standing shapes. The importance and impact of pyrolytic graphite have been growing ever since. 

Although many studies of the CVD of graphite have been carried out, a better understanding of the pyrolysis. eactions, a more accurate p.redic-tion of the results, and more complete experimental, thermodynamic, and kinetic investigations are still needed. 

The deposition of graphite is also obtained by plasma CVD, with the following deposition parameters ]... 

In the early experiments of thermal CVD using methane -i- hydrogen gas mixtures, a temperature of about 1000°C and a pressure less than 0.1 atm meant that the formation of graphite was predominant.An activation energy is required to produce carbon and/or hydrogen radicals so that the gas phase and surface reactions leading to predominantly diamond deposition can proceed at a significant rate." ... 

Low-wavenumber Raman bands of MWCNT were assigned in terms of modes from coupling of radial breathing modes of individual tubes via van der Waals interactions.279 The Raman spectra of MWCNT prepared at 470°C showed the coexistence of graphite and amorphous carbon units.280 Micro-Raman spectra were used to characterise 13C-labelled MWCNT.281 The Raman spectra of MWCNT s subjected to plasma-etching were used to identify structural defects introduced thereby.282 Raman spectroscopy was used to compare the structures of MWCNT s prepared by high-temperature arc and low-temperature CVD methods. The former had a more graphite-like structure.283 Micro-Raman spectroscopy was used to characterise MWCNT obtained by electrophoretic deposition.284... 

The B-C-N compounds are attractive materials due to their unique properties associated with their layered, graphite-like structures [246, 247]. Such compositions can be deposited by the CVD of a mixture of BCI3, ammonia and a hydrocarbon (usually acetylene) at temperatures between 700-1700 °C [247-249]. A compound with the composition BC2N has been obtained by the CVD of a mixture of BCI3 and acetonitrile... 

An inevitable first highly endothermic step in the CVD reaction is the dissociation of molecular hydrogen by the application of energy, for example by means of a heated filament, a flame or an electrical gas discharge, reaction (2). The gasification of graphite, reaction (3) on the other hand, is strongly favored thermodynamically ... 

Brownson, D.A.C. and Banks, C.E. (2011) CVD graphene electrochemistry the role of graphitic islands. Phys. Chem, Chem. Phys., 13, 15825-15828. 

Highly crystalline hexagonal boron nitride layers can be formed on graphite layers which have been obtained from the CVD of benzene [128]. Low-pressure CVD originating from 2,4,6-trichloroborazine on graphite, metallic, and oxide ceramic substrates at 1050°C leads to dense, amorphous boron nitride deposits [129] see also [130]. Ceramics are frequently coated or infiltrated with a-BN by the different chemical vapor deposition (CVD) methods already... 

The CVD of pyrolytic graphite can be optimized by experimentation. The carbon source (hydrocarbon gas), the method of activating the decomposition reaction (thermal, plasma, laser, etc.), and the deposition variables (temperature, pressure, gas flow, etc.) can be changed until a satisfeictory deposit is achieved. However, this empirical eipproach may be too cumbersome and, for more accurate results, it should be combined with a theoretical analysis. 

The principles of thermodynamics and idnetics of CVD as they apply to the deposition of graphite were reviewed in Ch. 7, Sec. 2.0.( 1 These considerations are valid for the CVD of diamond as well. 

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