Carbonization in Polyacrylonitrile (PAN) Based Carbon Fibers Studied by ReaxFF Molecular Dynamics Simulations

Abstract

The carbonization mechanism in polyacrylonitrile (PAN) based carbon nanofibers is studied using ReaxFF molecular dynamics simulations. Simulations are performed at two carbonization temperatures, 2500 and 2800 K, and also at two densities, 1.6 and 2.1 g/cm(3), that are relevant to the experimental carbonization conditions. The results are analyzed by examining the evolution of species with time, including carbon-only ring structures and gaseous species. Formation mechanisms are proposed for species like N(2), H(2), NH(3), and HCN and five-, six-, and seven-membered carbon-only rings, along with polycyclic structures. Interestingly, the formation of five-membered rings follows N(2) formation and usually occurs as a precursor to six-membered rings. Elimination mechanisms for the gaseous molecules are found that are in agreement with previously proposed mechanisms; however, alternative mechanisms are also proposed.

Multi-step mechanism of carbonization in templated polyacrylonitrile derived fibers: ReaxFF model uncovers origins of graphite alignment

“…Other valence terms (angle and torsion) are defined in terms of the same bond orders so that all interactions go to zero smoothly as the bonds break. ReaxFF parameters have previously been developed for systems consisting of C, N, O and H atoms [35][36][37][38], and ReaxFF simulations based on these parameters show good qualitative correspondence with experiment in the initial stages of PAN [2,25] carbonization (without nanofillers) and phenolic pyrolysis [38]. The accuracy of the force field has also been tested [2,[39][40][41] through comparisons with density functional theory as well as density functional tight binding theory results, and was considered adequate for simulations at the temperatures of interest in this work.…”

“…Motivation for this choice, which is derived from models that were developed in earlier studies of PAN stabilization and carbonization, [22] has been given previously. [2] In order to study templating effects in carbonization, we considered systems consisting of SPP/DWCNT and SPP/AB-stacked double layer graphene (DLG). The DWCNT and graphene sheets were assumed to be finite structures, and therefore were hydrogen terminated.…”

“…The total number of atoms in our models was 13690 and 13540 for the SPP/DWCNT and SPP/ DLG systems, respectively, which is over an order of magnitude larger than was considered in any earlier study of carbonization. [2,8] Therefore, all MD simulations were performed in parallel using 64 processors. Periodic boundary conditions were applied in all directions of the simulation cell.…”

“…[1] Extensive research and development in the 1960-1980s resulted in scalable industrial manufacturing processes producing micron-sized fibers with uniform structure and properties. However, the detailed mechanisms of carbon structure formation during the complex multi-step carbonization process are not yet fully understood [2]. Perhaps due to this lack of fundamental understanding, the optimization of carbon fiber mechanical properties has plateaued with no major breakthroughs reported since the mid 1980s.…”

First principles-based multiparadigm, multiscale strategy for simulating complex materials processes with applications to amorphous SiC films

“…14 studied the pyrolysis of phenolic polymer in the presence of carbon nanotubes, while Saha and Schatz studied 15 thermal decomposition of polyacrylonitrile in the carbonization process.…”