Reaction paths of BCl3 + CH4 + H2 in the chemical vapor deposition process

The reaction paths in the chemical vapor deposition preparation of boron carbides with BCl3–CH4–H2 precursors were investigated theoretically in detail with a total number of 82 intermediates (IM) and 118 transition states (TS). The geometries of the species were optimized with B3PW91/6-311G(d,p) method and the TS as well as their linked IM were confirmed with the frequency and the intrinsic reaction coordinates analyses at the same theoretical level. The energy barriers and the reaction energies were determined with the accurate model chemistry method G3(MP2) after a diagnosis of the non-dynamic electronic correlations. The heat capacities and entropies were obtained with statistical thermodynamics. The Gibbs free energies at 298.15 K for all of the reaction steps were reported and the data at any temperature can be developed with the classical thermodynamics by using the fitted (as a function of temperature) heat capacities. All the possible elementary reactions, including both direct decomposition and the radical attacking dissociations for each reaction step were examined. It was found that there are nine reaction steps in the lowest reaction pathway to produce the final boron carbide and five steps to produce boron. The highest energy barrier in the lowest reaction pathway is 238.6 kJ mol−1 at 298.15 K and 346.0 kJ mol−1 at 1,200 K for producing BC, and is 294.7 kJ mol−1 at 298.15 K and 314.2 kJ mol−1 at 1,200 K for producing B.