Theoretical Study of the Pyrolysis of Methyltrichlorosilane in the Gas Phase. 1. Thermodynamics

Abstract

Structures and energies of the gas-phase species produced during and after the various unimolecular decomposition reactions of methyltrichlorosilane (MTS) with the presence of H2 carrier gas were determined using second-order perturbation theory (MP2). Single point energies were obtained using singles + doubles coupled cluster theory, augmented by perturbative triples, CCSD(T). Partition functions were obtained using the harmonic oscillator-rigid rotor approximation. A 114-reaction mechanism is proposed to account for the gas-phase chemistry of MTS decompositions. Reaction enthalpies, entropies, and Gibbs free energies for these reactions were obtained at 11 temperatures ranging from 0 to 2000 K including room temperature and typical chemical vapor deposition (CVD) temperatures. Calculated and experimental thermodynamic properties such as heat capacities and entropies of various species and reaction enthalpies are compared, and theory is found to provide good agreement with experiment. ReceiVed: August 23, 2006; In Final Form: NoVember 27, 2006 Structures and energies of the gas-phase species produced during and after the various unimolecular decomposition reactions of methyltrichlorosilane (MTS) with the presence of H 2 carrier gas were determined using second-order perturbation theory (MP2). Single point energies were obtained using singles + doubles coupled cluster theory, augmented by perturbative triples, CCSD(T). Partition functions were obtained using the harmonic oscillator-rigid rotor approximation. A 114-reaction mechanism is proposed to account for the gas-phase chemistry of MTS decompositions. Reaction enthalpies, entropies, and Gibbs free energies for these reactions were obtained at 11 temperatures ranging from 0 to 2000 K including room temperature and typical chemical vapor deposition (CVD) temperatures. Calculated and experimental thermodynamic properties such as heat capacities and entropies of various species and reaction enthalpies are compared, and theory is found to provide good agreement with experiment.

Theoretical Study of the Pyrolysis of Methyltrichlorosilane in the Gas Phase. 2. Reaction Paths and Transition States

“…1 In the present work, 41 reactions have been determined to have no well-defined transition state (TS) by the following means. First, bondbreaking reactions without significant reconstruction of electronic structure are assumed to have no well-defined transition state.…”

TRISO coated fuel particles with enhanced SiC properties

“…The effect that argon has on SiC deposition is not by participating in any reaction for the formation of SiC, but by reducing the concentration of hydrogen which is an important reactant in both the decomposition of MTS and the reactions leading to the formation of SiC [48,49]. As hydrogen concentration decreases, the heterogeneous reactions leading to the formation of carbon sites are favoured, therefore eliminating the excess Si and promoting the formation of stoichiometric SiC [44,46].…”

Control of stoichiometry, microstructure, and mechanical properties in SiC coatings produced by fluidized bed chemical vapor deposition

“…In addition to the codeposition of Si, porosity was also found to be formed for some cases at deposition temperatures below 1500°C. [20][21][22][23][24][25][26][27][28][29][30][31][32] Similarly, although not for this particular application, other reactants such as methane, ethane, and propane have been added to the reaction mixture in addition to MTS and hydrogen to produce SiC, providing aneffective method to modify the microstructure and stoichiometry of SiC. For these reasons typical SiC coatings are generally produced between 1500 and 1600°C with grain sizes between 1 and 8 m and with coating rates of around 0.2 m/min.…”