Toward a Process-Based Molecular Model of SiC Membranes. 1. Development of a Reactive Force Field

A broad class of important materials, such as carbon molecular sieves, silicon carbide (SiC), and silicon nitride, are fabricated by temperature-controlled pyrolysis of preceramic polymers. In particular, the 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. In principle, ab initio quantum mechanics (QM) can provide information about the structure of the amorphous systems. However, to determine the structure of the SiC membrane layer one should capture in the simulations the various reactive processes involved in forming the layer. This requires QM simulations on systems with about 3000 atoms per cell at temperature of 1200 K for microseconds, which are far beyond the current QM capabilities. Instead, this paper extends the ReaxFF reactive force field, validated for high temperature reactions of other materials, to describe the processes involved in the thermal decomposition of hydridopolycarbosilane (HPCS) to form SiC nanoporous membranes. First, we carry out QM calculations on models meant to capture 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 in the molecular dynamics (MD) simulations. Analysis of the pyrolysis studies and their results leads to various quantities that can be compared with experimental data. Good agreement is found between the data and the results of the MD simulations.