Atomistic scale computational insights into structure-property relationships in silica, amorphous carbon, and collapsed carbon nanotubes

Abstract

This thesis investigates the structure-property relationships in complex structural materials—silica, amorphous carbon, and collapsed carbon nanotubes—using three established atomistic methods: density functional tight binding (DFTB), ReaxFF molecular dynamics, and density functional theory (DFT).First part of the thesis examines with DFTB how adding aluminum (Al) into amorphous silica (a-SiO₂) influences its microscopic structure and properties through density functional tight-binding simulations. The inclusion of Al promotes densification, introducing Si-centered coordination defects, including unquenchable pentahedra and hexahedra. Al atoms act both as network formers and centers for these coordination defects, leading to structural characteristics similar to pressure-densified silica. The findings highlight that Al-modified silica can achieve enhanced mechanical and dielectric properties, useful for advanced electronic applications. In the second part, ReaxFF molecular dynamics simulations were used to explore the structural and mechanical properties of amorphous carbon across a wide density range. Low-density amorphous carbon, dominated by sp² bonding, shows elastic isotropy and fails via crack formation perpendicular to strain direction, while higher-density sp³-rich carbon behaves differently. Nanostructures, including slabs and nanotubes, show varied mechanical responses depending on sp²/sp³ ratios and density. The findings provide detailed insights into crack propagation mechanisms, offering a foundation for designing amorphous carbon materials with tailored mechanical properties. Third part of this work investigates a graphite-like phase formed by collapsed large-radius carbon nanotubes (CNTs) through DFT simulations. These CNTs self-assemble into unique structures with anomalous grain boundaries, which exhibit strong van der Waals coupling, mechanical rigidity, and flexoelectric properties. The grain boundary characteristics, including flexoelectricity and reactivity, make these materials promising candidates for ultra-strong composites. The study underscores the potential of collapsed CNTs as a novel structural material with applications in advanced composite materials.