| Owing to their excellent mechanical, corrosion, and oxidation resistant properties, continuous fiber reinforced ceramic matrix composites(CMC) have been used as advanced thermal protection materials, brake materials in high-temperature structural applications. Silicon, boron, carbon and compounds of the Si–B–C–N system(e.g. Si3N4, Si C, B4 C, BN, Si Bx) are the most common matrix materials for CMC, and the basis for the development of technically important refractory ceramics and hard materials. They are generally prepared by chemical vapor deposition(CVD) which has the potential of further development. Since the mechanisms in the CVD process are far from clear, it is desirable to understand the relationship between the deposition condition(e.g. temperature, pressure, or input gas concentrations) and the property of the materials. This thesis gives an overview of the thermodynamic calculations in the Si-B-C-N-H-Cl system and its binary(B-N, Si-N and Si-B) and ternary(Si-C-N) subsystems. The thermochemistry data include the heat capacities, entropies, enthalpies of formation and Gibbs free energies of formation, which are calculated with the reliable theoretical method of quantum mechanics combined with standard statistical thermodynamics. Based on this data, with the principle of chemical equilibrium, the distribution of the equilibrium concentration of the species and the production of the specific solid phases at different reaction parameters of the CVD process in each system are obtained respectively. The main subjects and results are summarized as follows:(1) The equilibrium distribution of a relatively complete set of the 144 species(4 atoms and 136 gaseous species, 4 condensed phase species) that might be involved in the CVD preparation of the boron nitride in the BCl3–NH3–H2 system was studied thermodynamically. The structures and the thermochemical data(include the heat capacities, entropies, enthalpies of formation and Gibbs free energies of formation) for 87(among the 144) species were determined theoretically. The structures were optimized with DFT B3PW91/6-31G(d) method. The heat capacities and entropies were evaluated with the standard statistical thermodynamics by using the structures and vibrational frequencies obtained at B3PW91/6-31G(d) level. The electronic excitation energies from TD-DFT at B3PW91/6-31G(d) level were involved in the statistical thermodynamics treatments. Accurate model chemistry G3(MP2) and G3//B3 LYP theories were employed to calculate the accurate molecular energies. The heat capacities and entropies at temperatures in 298.15-2000 K were evaluated with the standard statistical thermodynamics. The Gibbs free energies of formation in 298.15-2000 K were calculated with the classical thermodynamics based on the developed heat capacities and entropies.The equilibrium concentration distribution of the 144 species which might be involved in the BCl3–NH3–H2 system as a function of temperature by the conditions(total pressure is 1000 Pa and the molar ratio of the input gases BCl3:NH3:H2 is 1:3:6) have be obtained according to the principle of chemical equilibrium by minimizing the total Gibbs free energy of the system. And the production of the condensed phases B, c-BN and h-BN as a function of temperatures within 300-2000 K and the ratios of r= BCl3/(BCl3 + NH3) within 0-1.0 were obtained too. The results showed that NH3 and BCl3 could be reacted at the start temperature, the condensed boron appeared at temperature 2160 K, cubic boron nitride(c-BN) could be formed at temperatures below 1,800 K, hexagonal boron nitride(h-BN) existed above this temperature, and wurtzite boron nitride(w-BN) was unstable under considerable conditions. The production of the condensed phases strongly depended on the molar ratio of r= BCl3/(BCl3 + NH3) and was quite sensitive to temperature. The ideal deposition ratio r for BN was found to be 0.5.(2) Similar to Si Cl4–NH3–H2 system, the thermochemistry data of the 118(among the 161) species which might be involved in the Si Cl4–NH3–H2 system were calculated. The equilibrium concentration distribution of the 161 species which might be involved in the Si Cl4–NH3–H2 system as a function of temperature by the conditions(total pressure is 1000 Pa and the molar ratio of the input gases Si Cl4:NH3:H2 is 1:3:5) have be obtained, as well as the production of the condensed phases Si and Si3N4 as a function of temperatures within 300-2000 K and the ratios of r= Si Cl4/(Si Cl4 + NH3) within 0-1.0. The results showed that condensed Si3N4 could be produced at the initial temperature of 300 K up to the temperature 1560 K. Pure silicon could be produced only if the r ratio reached to about 0.99 at the temperature about 1000 K. The ideal deposition ratio r for Si3N4 was found to be about 0.5.(3) In the BCl3–Si Cl4–H2–Ar system, it might be involved 220 species, among which, the thermochemical data for 128 new species were determined theoretically. The equilibrium concentration distribution of the 220 species which might be involved in the BCl3–Si Cl4–H2 system as a function of temperature by the conditions(total pressure is 1 atm and the molar ratio of the input gases BCl3:Si Cl4:H2:Ar is 4:1:5:5) have be obtained, and the production of the condensed phases B, Si B6 and Si B14 as a function of temperatures within 300-2000 K and the ratios of r= BCl4/(Si Cl4 + BCl3) within 0-1.0 were obtained too. The results showed that Si Cl4 and BCl3 could be initially reacted at temperatures higher than 500 K and the condensed boron appeared at temperatures in 800-900 K by the typical conditions.The ideal deposition ratio r for condensed B should be above 0.8. The amount of Si B6 increases sharply with r in 0.1-0.8 and temperatures in 700-1550 K. Formation of a larger amount of the condensed phase Si B14 should have a higher r(>0.7) and a higher temperature(>1400 K).(4) And the Si Cl3CH3–NH3–H2 system, which might be involved a relatively complete set of 443 species, the structures and thermochemical data of 99 new species were determined. The equilibrium concentration distribution of the system showed that by the conditions(total pressure was 1000 Pa, the input gases Si Cl3CH3, NH3 and H2 were 1, 3 and 5 moles, respectively). The results showed Si3N4 could be produced at the initial temperature of 300 K up to the temperature 1200 K, and β-Si C could be produced above the temperature 1200 K. The production of all the condensed phases(graphite C, Si3N4 and β-Si C) strongly depended on the molar ratio of r= Si Cl3CH3/(Si Cl3CH3 + NH3) and was quite sensitive to temperature.(5) The Si Cl3CH3–BCl3–NH3–H2 system has been investigated by the conditions(total pressure was 1000 Pa, the input gases Si Cl3CH3, BCl3, NH3 and H2 were 5, 1, 4 and 50 moles, respectively).(6) Finally, according to the results of the thermodynamic calculation of each system, the experimental verification are undertaken with CVD methods, the experimental results are in good agreement with the thermodynamic calculation.This work provides more fundamental data for analyzing the thermochemistry of the CVD process of the Si–B–C–N–H–Cl system and its subsystems at any ratio of the input precursors to control the formation of the condensed phases. It is instructive to experiments at any new conditions. |
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