Kehe Su

Evaluation of the Thermodynamic Data of CH3SiCl3 Based on Quantum Chemistry Calculations

CH3SiCl3 (methyltrichlorosilane) (MTS) is one of the most important precursors for manufacturing both an oxidation resistant SiC coating and a functional SiC film by chemical vapor deposition (CVD). In order to analyze the decomposition products of MTS with a thermodynamic calculation, correct thermodynamic data must be obtained from the authoritative data sources. G3(MP2) has been applied to evaluate the thermodynamic data of MTS(gas). The calculated value of the Gibbs energy of formation, ΔfGm0(298.15K)=−490.13kJ∙mol−1, compares with a value, ΔfGm0(298.15K)=−468.02kJ∙mol−1 from the 4th edition of the NIST-JANAF Thermochemical Tables. Further analyses have been conducted: (1) by using G3, G3//B3LYP, and G3(MP2)//B3LYP theories; (2) by using variable scale factors for G3(MP2) theory; and (3) by investigating the accuracy of both experimental and calculated thermodynamic data. The calculated values can provide ΔfGm0 values for MTS above 1500K. The final fitted equation for MTS(gas) is: ΔfGm0=7.5763×10−6T2+...

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 BCl3–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 B4C, 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, BH2Cl, and B2Cl4 are found to be the most effective species in the CVD process, which is similar to those in the BCl3–CH4–H2 system. The results predicted in this work are consistent with the experiments.

Reaction Pathways of Propene Pyrolysis

The gas‐phase reaction pathways in preparing pyrolytic carbon with propene pyrolysis have been investigated in detail with a total number of 110 transition states and 50 intermediates. The structure of the species was determined with density functional theory at B3PW91/6‐311G(d,p) level. The transition states and their linked intermediates were confirmed with frequency and the intrinsic reaction coordinates analyses. The elementary reactions were explored in the pathways of both direct and the radical attacking decompositions. The energy barriers and the reaction energies were determined with accurate model chemistry method at G3(MP2) level after an examination of the nondynamic electronic correlations. The heat capacities and entropies were obtained with statistical thermodynamics. The Gibbs free energies at 298.15 K for all the reaction steps were reported. Those at any temperature can be developed with classical thermodynamics by using the fitted (as a function of temperature) heat capacities. It was found that the most favorable paths are mainly in the radical attacking chain reactions. The chain was proposed with 26 reaction steps including two steps of the initialization of the chain to produce H and CH3 radicals. For a typical temperature (1200 K) adopted in the experiments, the highest energy barriers were found in the production of C3 to be 203.4 and 193.7 kJ/mol. The highest energy barriers for the production of C2 and C were found 174.1 and 181.4 kJ/mol, respectively. These results are comparable with the most recent experimental observation of the apparent activation energy 201.9 ± 0.6 or 137 ± 25 kJ/mol.

Reaction rate of propene pyrolysis.

The reaction rate of propene pyrolysis was investigated based on the elementary reactions proposed in Qu et al., J Comput Chem 2009, 31, 1421. The overall reaction rate was developed with the steady‐state approximation and the rate constants of the elementary reactions were determined with the variational transition state theory. For the elementary reaction having transition state, the vibrational frequencies of the selected points along the minimum energy path were calculated with density functional theory at B3PW91/6‐311G(d,p) level and the energies were improved with the accurate model chemistry method G3(MP2). For the elementary reaction without transition state, the frequencies were calculated with CASSCF/6‐311G(d,p) and the energies were refined with the multireference configuration interaction method MRCISD/6‐311G(d,p). The rate constants were evaluated within 200–2000 K and the fitted three‐parameter expressions were obtained. The results are consistent with those in the literatures in most cases. For the overall rate, it was found that the logarithm of the rate and the reciprocal temperature have excellent linear relationship above 400 K, predicting that the rate follows a typical first‐order law at high temperatures of 800–2000 K, which is also consistent with the experiments. The apparent activation energy in 800–2000 K is 317.3 kJ/mol from the potential energy surface of zero Kelvin. This value is comparable with the energy barriers, 365.4 and 403.7 kJ/mol, of the rate control steps. However, the apparent activation energy, 215.7 kJ/mol, developed with the Gibbs free energy surface at 1200 K is consistent with the most recent experimental result 201.9 ± 0.6 kJ/mol.

Decomposition reaction rate of BCl3-C3H6(propene)-H2 in the gas phase.

The decomposition reaction rate in the BCl(3)-C(3)H(6)-H(2) gas phase reaction system in preparing boron carbides was investigated based on the most favorable reaction pathways proposed by Jiang et al. [Theor. Chem. Accs. 2010, 127, 519] and Yang et al. [J. Theor. Comput. Chem. 2012, 11, 53]. The rate constants of all the elementary reactions were evaluated with the variational transition state theory. The vibrational frequencies for the stationary points as well as the selected points along the minimum energy paths (MEPs) were calculated with density functional theory at the B3PW91/6-311G(d,p) level and the energies were refined with the accurate model chemistry method G3(MP2). For the elementary reaction associated with a transition state, the MEP was obtained with the intrinsic reaction coordinates, while for the elementary reaction without transition state, the relaxed potential energy surface scan was employed to obtain the MEP. The rate constants were calculated for temperatures within 200-2000 K and fitted into three-parameter Arrhenius expressions. The reaction rates were investigated by using the COMSOL software to solve numerically the coupled differential rate equations. The results show that the reactions are, consistent with the experiments, appropriate at 1100-1500 K with the reaction time of 30 s for 1100 K, 1.5 s for 1200 K, 0.12 s for 1300 K, 0.011 s for 1400 K, or 0.001 s for 1500 K, for propene being almost completely consumed. The completely dissociated species, boron carbides C(3)B, C(2)B, and CB, have very low concentrations, and C(3)B is the main product at higher temperatures, while C(2)B is the main product at lower temperatures. For the reaction time 1 s, all these concentrations approach into a nearly constant. The maximum value (in mol/m(3)) is for the highest temperature 1500 K with the orders of -13, -17, and -23 for C(3)B, C(2)B, and CB, respectively. It was also found that the logarithm of the overall reaction rate and reciprocal temperature have an excellent linear relationship within 700-2000 K with a correlation coefficient of 0.99996. This corresponds to an apparent activation energy 337.0 kJ/mol, which is comparable with the energy barrier 362.6 kJ/mol of the rate control reaction at 0 K but is higher than either of the experiments 208.7 kJ/mol or the Gibbs free energy barrier 226.2 kJ/mol at 1200 K.

Thermodynamic study on the chemical vapor deposition of silicon nitride from the SiCl4–NH3–H2 system

The gas-phase reaction thermodynamics in the chemical vapor deposition (CVD) process of preparing boron nitride with the precursors of BCl3–NH3–H2 was investigated with a relatively complete set of 144 species, in which the thermochemistry data were calculated with accurate model chemistry at G3(MP2) and G3//B3LYP levels combined with standard statistical thermodynamics. The data include the heat capacities, entropies, enthalpies of formation, and Gibbs free energies of formation. Three different modifications of condensed phase boron nitride (hexagonal h-BN, cubic c-BN, and wurtzite w-BN) were taken into consideration. Based on these data, the distribution of the equilibrium concentration of the 144 species was obtained with the principle of chemical equilibrium. It was concluded that c-BN is formed at temperatures up to 1,800 K, h-BN is the most stable above this temperature, and w-BN is unstable under considerable conditions.

Thermodynamics investigation of the gas-phase reactions in the chemical vapor deposition of silicon borides with BCl3–SiCl4–H2 precursors

The gas-phase reaction thermodynamics in the chemical vapor deposition (CVD) process of preparing silicon borides with the precursors of BCl3–SiCl4–H2 is investigated with a relatively complete set of 220 species, in which the thermochemistry data are calculated with accurate model chemistry at G3(MP2) and G3//B3LYP levels combined with standard statistical thermodynamics. The data include the heat capacities, entropies, enthalpies of formation, and Gibbs free energies of formation. Based on these data, the distribution of the equilibrium concentration of the 220 species is obtained with the principle of chemical equilibrium. BHCl2, SiHCl3, and BH2Cl are found to be the crucial intermediates. This work provides fundamental data for analyzing the thermochemistry of the CVD process of the BCl3–SiCl4–H2 system, which is instructive to optimize the input precursors and temperatures for controlling the composition of the condensed phase B, SiB6, and SiB14.