Thermodynamic investigation of the gas-phase reactions in the chemical vapor deposition of boron carbide with BCl₃-CH₄-H₂ precursors

A relatively complete set of thermochemical data involving 154 species in the gas-phase chemical vapour deposition (CVD) reactions in preparing boron carbides via the precursors of BCl₃-CH₄-H₂ is obtained with high accurate model chemistry calculations. The data include the heat capacities, entropies, enthalpies of formation and Gibbs free energies of formation. The heat capacities and entropies at temperatures in 100-2000 K are evaluated with the standard statistical thermodynamics. The distribution of the equilibrium concentration of the 154 species is obtained with the principle of chemical equilibrium. By including the crystal B₄C, C(graphite) and B, the results for an example of the 5:1:5 precursors of BCl₃:CH₄:H₂ show that the crystal B₄C can be produced at temperatures higher than 760 K, while the graphite can also be produced at temperatures higher than 820 K. However, the production of graphite can be controlled by changing the ratio of the inject reactants. In the CVD process, BHCl₂, BCH₃Cl₂, BH₂Cl and B₂Cl₄ are found to be the most effective species. The results obtained in this work are consistent with the available experiments and the work is instructive for experiments with different conditions (ratio of the inject reactants, pressure and temperature).