Reaction thermodynamics in chemical vapor deposition of boron carbides with BCl₃-C₃H₆ (propene)-H₂ precursors

The gas phase reaction thermodynamics in the chemical vapor deposition (CVD) process of preparing boron carbides via the precursors of BCl ₃-C₃H₆(propene)-H₂ is investigated with a set of 325 gaseous species, in which the data for 135 species are evaluated in this work. The thermochemistry data are calculated with accurate model chemistry at G3(MP2) and G3//B3LYP levels. The concentration distribution of all of the 325 species is obtained with the principle of chemical equilibrium. The thermochemistry data include the heat capacities, entropies, enthalpies of formation, and Gibbs free energies of formation. The heat capacities and entropies at temperatures in 298.15-2000 K are evaluated with the standard statistical thermodynamics. The Gibbs free energies of formation in 298.15-2000 K are calculated with the classical thermodynamics based on the developed heat capacities and entropies. By including the crystal B ₄C, C(graphite), and B, the results for an example of the 3:1:2 precursors of BCl₃:C₃H₆:H₂ show that the crystal B₄C can be produced at temperatures higher than 700 K while the graphite has a higher molar value and can be produced at lower temperatures. It is also examined that the production of graphite can be controlled by changing the ratio of the injected reactants or pressure. It is interesting that BHCl₂, BCH₃Cl₂, BH ₂Cl, and B₂Cl₄ are found to be the most effective species in the CVD process, which is similar to those in the BCl ₃-CH₄-H₂ system. The results predicted in this work are consistent with the experiments.