Molecular Simulation Of Structure And Properties Of Typical Silicon-Based Ceramics

In this dissertation, the research focused on the relationship between atomic/electronic structure and properties of materials science with background of the silicon carbide fiber reinforced silicon carbide ceramic matrix composites (SiC/SiC). Molecular simulation was used in the present study in order to explore the methods and mechanisms of the improvement of the creep behavior and oxidation behavior of typical silicon-based structural ceramics. The properties of silicon-based structural ceramics with amorphous state or dopant were predicted by using molecular simulation. The intrinsic mechanism for the change of properties was discussed. The relationship between micro-atomic structure and properties were revealed. This work will give a new way to optimize the design for silicon-based structural ceramics.The research included: (1) the electronic structures of twin boundary substitution doping, cell substitution doping, and cell interstitial doping with B, N, A1 and Ti, and the doping effect on the bonding strength and creep esistance of system silicon carbide (SIC); (2) the atomic structures and the self-diffusion behavior of amorphous Si-B-C-N system at different temperatures, revealing the atomic reason for the excellent thermal stabiliby and creep resistance at elevated temperature; (3) the electronic structures and the mechanism of the adsorption of the surface of the doped-SiC (111) (2×3) absorbing with molecular oxygen, and (4) the electronic structures and the mechanism of the SiC (310) twin boundary of the doped-SiC interacting with atomic oxygen. The main results are shown as follows:1. In this dissertation, it is first time to calculate the electronic structures of SiC cells and their grain boundaries with B, N, Al, or Ti doping by using density functional theory (DFT) and pseudopotential plane wave (PPW). The interaction between atoms in matrix and additives was also analyzed. The results shows that the grain boundary structure and additives are the two important factors to affect the properties of SiC ceramics:(1) The interaction between additive atoms and Si, C atoms is determined by the doping type (grain boundary doping and cell doping) and the characters of bonds. The grain boundary was strengthened by B and N doping while the bond strength between cells was weakened by B, N, Al, and Ti doping inside. (2) In the system of substitution doped SiC (310) twin boundaries, the interaction of electrons on grain boundary was enhanced by B and N doping. This effect will promote the bondage between atoms on the grain boundary and will prevent atoms diffusing through. The creep resistance in the grain boundary area was enhanced. The effect of AI on the interaction of electrons on the grain boundary is limited. Hence, the bond strength was little increased. On the other contrary, Ti reduces the interaction of electrons obviously, so the bonds between atoms in grain boundary are weakened. It will result in lowering creep resistance in the area of grain boundary.(3) The bond strength of SiC was weakened after B, N, A1, and Ti substituted for silicon atoms or doped in octahedral interstitial positions. However, the extent of this influence is different and the order is B＜N＜Al＜Ti.(4) The combination of calculation methods of population, EDDs, and DOSs can serve as a criterion to determine the reinforcing effect or weakening effect of additives.2. The conclusion that the intrinsic effect on the atomic self-diffusion coefficients is bonds between atoms was obtained by comparison of the atomic diffusion behaviors in different ceramic systems, and the atomic structures (coordination number). Principally, the lager number of components in the system will result in the smaller atomic self-diffusion coefficients if the Moore ratio of them is constant. Thus, the phase transformation is difficult to happen at high temperatures. The thermal stability is good. It also indicated that the intrinsic reason for improving creep behavior of ceramics is to lower the atomic self-diffusion coefficients. These results provide theory instructions for the optimizing design of amorphous ceramic.3. In this dissertation, it is first time to study the electronic structural change after the oxygen molecular absorbing on the doped SiC (111) (2×3) surface by using DFT, and PPW. Oxygen molecular will dissociation chemical adsorb on the surface of SiC (111) (2×3) doped with any elements. The absorption positions of oxygen on the surface vary with the different doped elements. The absorption on the surface of Al doped SiC is hollow site adsorption with the lowest absorption energy, while the absorption on the surface of undoped SiC or B, Ti doped SiC is top site adsorption with the highest absorption energy. The analysis of DOS indicated the orbitals for interaction. The results revealed the intrinsic information for absorption.That is the interaction between oxygen valence orbitals and additive valence orbitals and the electron shifts between them.4. B, N, Al, and Ti doping on SiC (310) twin boundary have different effect on the oxidation resistance of SiC. If B was doped, the oxygen will prefer to combine with boron instead of silicon. The silicon avoids being oxidized to some extent. Although AI and Ti are easy to form bonds with oxygen, these bonds show more ionic character than Si-O bonds that have more interaction. N is more difficult to connect with oxygen compared with Si. Hence, N cannot protect silicon from oxidizing. Therefore, they have less protection for silicon than boron has. From the calculation results and analysis above, boron is the only one that promotes the oxidation resistance of SiC grain boundary among the four elements. It can be chosen as the primary element for improving oxidation resistance of SiC.