SiC Nanoporous Membranes: Experimental Results and Detailed Molecular Simulations of Their Preparation and Structure

SiC is a promising material for the preparation of high-temperature membranes due to its many unique properties, such as high thermochemical stability, high thermal conductivity, and resistance to abrasion. In our studies, SiC microporous membranes are fabricated via the pyrolysis of pre-ceramic polymeric films, coated on tubular SiC macroporous supports using a combination of slip-casting and dip-coating techniques, and with the aid of sacrificial interlayers. Membranes prepared by this technique exhibit single gas ideal separation factors of He over Ar >2,000, and He permeances ~ 10^-7 mol/m²sPa.

In this talk, we will first describe the techniques that we utilize for the preparation and characterization of such microporous membranes. These involve a variety of surface analytical techniques (SEM/TEM, EDAX, AFM, FTIR, XPS, etc.), TGA, and various chromatographic and mass spectrometric techniques useful for characterizing the membrane structure and its surface and bulk chemical composition. They are coupled to transport and sorption investigations, and molecular simulations of such processes. A critical first step of such simulations is the development of an accurate molecular-scale model of these membranes.

In our studies, we utilize a reactive force field (ReaxFF) in order to simulate the pyrolysis of the hydridopolycarbosilane (HPCS) polymeric precursor leading to the formation of such membranes. For that goal, we have expanded and further improved the ReaxFF of poly(dimethylsiloxane). The amorphous structure of SiC is generated by carrying out by molecular dynamics (MD) simulations using the developed ReaxFF in the NVT ensemble at the temperature of 2000 K. During the simulations the hydrogen radicals, the main product of the pyrolysis process, as well as any other gaseous molecules, such as CH₄, C₂H₆, etc., are removed from the simulation box. Then, MD simulations in the NPT ensemble are utilized to compress the system at 2000 K. Eventually the system is relaxed at ambient temperature to arrive at the amorphous structure of SiC, which is then utilized in the development of a molecular model of the SiC membrane, see above. To test the validity of the model, in addition to calculating the entropy and radial distribution function, we also compute X-ray diffraction spectra, and the melting point of the molecular structure. The results are compared with the experimental data.