A process-based molecular model of nano-porous silicon carbide membranes.

摘要

A broad class of important materials, such as silicon carbide (SiC), are fabricated by temperature-controlled pyrolysis of pre-ceramic polymers. In particular, fabrication of SiC membranes by pyrolysis of a polymer precursor that contains Si is quite attractive for separation of hydrogen from other gases. It has been quite difficult to extract atomistic-scale information about such SiC membranes, since they are amorphous. The research presented in this dissertation extends the ReaxFF reactive force field to describe the processes involved in the thermal decomposition of hydridopolycarbosilane (HPCS) to form SiC nanoporous membranes.;First, we carry out quantum mechanical calculations on models meant to capture the important reaction steps and structures. Then, we develop a model of the HPCS polymer and utilize ReaxFF to describe the thermal degradation and decomposition of the polymer as the system is heated by molecular dynamics (MD) simulations. In the next step, we use ReaxFF for reactive dynamics of HPCS over a wide range of temperature. We then simulate pyrolysis of the HPCS under conditions that closely mimic the conditions of the fabrication of nanoporous SiC membranes to produce amorphous SiC material. The pyrolysis results and the computed properties of the SiC ceramic are shown to be in good agreement with the experimental data.;To further test the validity of the model and to provide insight into improving the performance of the membrane, extensive MD simulations were carried out to compute the self-diffusivities of H2, CO2 and CH4 in the molecular model of SiC. The results indicate higher values of the diffusivity for H2. The morphology of the amorphous SiC layer is characterized by computing its accessible free volumes and the cavity distributions.;Finally, we use the molecular model of SiC and non-equilibrium MD simulations in order to study transport and separation in the membrane of two binary gaseous mixtures, namely, H2/CO2 and H2/CH4 at various temperatures and pressure drops, applied externally to the membrane. When compared with our own experimental data, the model is demonstrated to provide accurate predictions for various properties of interest and, in particular, for the separation factors of the mixtures. The model can be used to determine the optimal membrane's thickness.