ABSTRACT of the Doctoral Thesis

Continuum Shell Models for Multi-Layer Carbon Nanostructures

by Melanie Todt (2012)

Carbon nanostructures are nanometer-sized molecular structures made-up of pure carbon such as graphene, single- and multi-walled carbon nanotubes, fullerenes and carbon onions. They posses exceptional mechanical and electronic properties as well as high thermal conductivity which make them promising materials for semiconducting devices and fillers in conductive nanocomposites. To exploit the remarkable properties of these structures and to ensure their reliability and performance, their mechanical properties have to be well understood.

In the present thesis continuum mechanical shell models are used for studying the mechanical behavior of multi-layer carbon nanostructures. These models cause relatively low computational costs compared to atomistic simulation techniques and are - within certain limits - appropriate for studying the mechanical behavior of nanostructures. In continuum shell models of carbon nanostructures the atomic layers are represented by thin elastic shells and the van der Waals interactions between the layers are modeled by appropriate pressure-distance relations.

Different sets of shell parameters found in literature for carbon nanostructures are investigated regarding their suitability for describing the mechanical behavior of a single carbon layer subjected to different mechanical loads. Furthermore, the implication of layer curvature on the formulation of the van der Waals models are discussed and a new van der Waals model for spherical carbon nanostructures is derived. The findings made for the shell parameters and van der Waals interactions are then used for studying the compressive behavior of carbon crystallites and a possible growth limit of carbon onions.

The results obtained for the carbon crystallites are in good agreement with experimental observations made on bent carbon fibers that consist of such crystallites. Furthermore, these results lead to a better insight to the mechanisms determining the compressive behavior of carbon fibers. For carbon onions the occurrence of a structural instability due to mutual accommodation of onion layers is identified as a possible reason for their limited size. The obtained critical sizes are comparable to those observed in experiments.

For both carbon nanostructures experimental observations can be well predicted, confirming that continuum shell models can be used for investigating the mechanical behavior of carbon nanostructures.

revised 120906 (hjb)