On the Stability of Old and Novel Carbon Phases; A computational study

Promotion on 7 June 2011

Summary of thesis

Carbon is one of the most abundant elements on Earth, where it is present in a great variety of molecules and materials. A remarkable feature of carbon is that it can adopt different forms. For centuries, graphite and diamond have been the only known crystalline forms of carbon. Since the discovery of fullerenes in the 80s of the last century, however, a completely new "world of carbon" has been unfolding before the eyes of scientists, and a multitude of previously unknown carbon structures have been discovered. Most of these new structures are "nano-materials", meaning that their size is on the scale of the nano-meter. Because of their unusual mechanical and electronic properties, carbon-based nano-materials are expected to have a major impact on technology. It is hard to predict which directions the science of carbon materials will take in the future. At least in part, this is due to our incomplete knowledge of the physical processes that take place at the molecular scale. As a matter of fact, there are many questions to which scientists are currently unable to give an answer. As an example, it is known that a bundle of nanotubes can be transformed into graphite and diamond by heating and compressing, but we do not know how this happens. We know that graphite melts at high temperature, but it is hard to determine this temperature precisely. Some scientists believe that nanotubes can grow from droplets of melted graphite, but many disagree.

The aim research reported in this thesis is to provide some of the missing pieces of information, addressing an important set of specific questions about the high-temperature high-pressure stability of carbon materials. To this end we performed large-scale atomistic simulations, mostly based on the LCBOPII potential, an accurate model of the interactions among carbon atoms. Our approach allows for the simulation of structural, mechanical, and thermodynamic properties of complex material, including the transformations of one carbon material to another.

The first part of the thesis is dedicated to the thermodynamic of graphite at temperatures on the scale of the GPa and temperatures up to ~4000K. Results include a parameterization of thermoelastic properties, an accurate estimate of the melting line, and a study of the structural and dynamic properties of melted graphite. In the second part of the thesis we study the stability of bundles of single-walled and double-walled nanotubes. We find that depending on the kind of tubes and on the thermodynamic conditions, the bundles might transform to graphite or to carbon nano-foams. We provide a reaction diagram, an analysis of the transformation mechanisms, and a prediction of the mechanical properties of the carbon nano-foams. Lastly, we studied the thermal stability of carbon clathrates, a hypothetical material with a great potential for technological applications. We calculate the equation of state up to high pressure and temperature, showing that spontaneous melting occurs only above 4000 K. This is strong evidence of the stability of the carbon clathrate at high temperature, indicating that its synthesis should be possible and suggesting that it might nucleate from high-pressure liquid carbon.

PhD promotion of
Francesco Colonna
Prof. Dr. A. Fasolino
Dr. E. J. Meijer
10:00, Tuesday, 7 June 2011
Agnietenkapel, UvA, Amsterdam