Modeling and simulation of the structural evolution and thermal properties of ultralight aerogel and 2D materials

Abstract

In this thesis, the thermal properties of novel ultralight materials, namely silica aerogels and 2D graphene and silicene, are investigated and established through classical molecular dynamics (MD). In addition, the construct and evolution of the atomic/molecular structures of these ultralight aerogels and 2D materials are explored and realized through various MD simulation schemes. The aerogel study involves two simulation steps, where the percolated structures of silica aerogel at various densities are first modeled using negative pressure rupturing, while reverse non-equilibrium molecular dynamics (RNEMD) is performed to determine the thermal conductivities of these aerogel samples. Two potential functions were employed in this study, namely the widely used Beest-Kramer-Santen (BKS), and the re-parameterized Tersoff. Results obtained based on the BKS potential followed the power-law variation reminiscent of sintered aerogel characteristics. To achieve a higher accuracy for bulk amorphous silica, the re-parameterized Tersoff potential was found to be superior to the BKS potential, when comparing the vibrational density of states with previous experimental studies. By fitting the results using a power-law variation, the trend of the thermal conductivities successfully mirrored power-law trends of experimental bulk aerogel. Due to finite sizes of pores that can be represented, where increasing simulation length scales leads to an increase in the largest pore size that can be modeled, current quantitative results are consistently higher but within the same order of magnitude as experimental bulk aerogel. For the analysis of 2D materials, graphene and silicene are considered. There are two major components in this 2D materials simulation study. Firstly, the exfoliation phenomenon of graphite is investigated. This is modeled through MD simulations, where the compression of AB-stacked graphite flakes between hydrogen-terminated silicon substrates leads to the exfoliation of graphene layers. It is shown that this phenomenon occurs through the activation of an inter-layer superlubricity regime, caused by torque-induced spontaneous rotations of the layers due to in-plane shear of graphite during compression. Secondly, the thermal properties of graphene and silicene are established through RNEMD simulations using the adaptive intermolecular reactive empirical bond order (AIREBO) potential. Of special interest in this study is the effects of defects, such as double vacancies and Stone-Thrower-Wales (STW) defects, on the thermal conductivities of graphene nanoribbons when compared with their pristine structures. It was found that thermal conductivities can fall by more than 80% with just 10% coverage of defect density, and these reductions occur over a wide temperature range of 100K to 600K. Finally, the thermal conductivity of nano- to micron-sized silicene and graphene monolayers are analyzed and compared. Across all length scales, silicene has significantly lower thermal conductivity than graphene, and this was found to be attributed to the lower phonon group velocities of the dominant acoustic modes in silicene.