The CVD of pyrolytic graphite is theoretically simple and is based on the thermal decomposition of a hydrocarbon gas. The actual mechanism of decomposition however is complex and 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, rather than the deposition of a coating.
Although many studies of the CVD of graphite have been carried out, a better understanding of the pyrolysis reactions, a more accurate prediction of the results, and more complete experimental, thermodynamic, and kinetic investigations are still needed.
Thermodynamics and kinetics analyses: The CVD of graphite can be optimized by experimentation. The carbon source, the method of activating the decomposition reaction, and the deposition variables can be changed until a satisfactory deposit is achieved. However, this empirical approach may be too cumbersome and, for more accurate results, it should be combined with a theoretical analysis.
Such an analysis is a valuable step which, if properly carried out, predicts what will happen to the reaction, what the resulting composition of the deposit will be, what type of carbon structure to expect, and what the reaction mechanism is likely to be. The analysis generally includes two steps:
- The calculation of the change in the free energy of formation for a given temperature range; this is a preliminary, relatively simple step which provides information on the feasibility of the reaction.
- The minimization of the free energy of formation which is a more complete analysis carried out with a computer program.
Thermodynamics of CVD carbon. The CVD of carbon is governed by two factors: (a) thermodynamics, that is the driving force which indicates the direction the reaction is going to proceed, and (b) kinetics, which defines the transport process and determines the rate-control mechanism, i.e., how fast it is going.
Chemical thermodynamics is concerned with the interrelation of various forms of energy and the transfer of energy from one chemical system to another in accordance with the first and second laws of thermodynamics. In the case of CVD, this transfer occurs when the gaseous compounds, introduced in the deposition chamber, react to form the carbon deposit.