טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentSorkin Anastasia
SubjectComputer Simulation of the Nucleation of Diamond from Liquid
Carbon under Extreme Pressures
DepartmentDepartment of Physics
Supervisors Dr. Joan Adler
Professor Emeritus Rafael Kalish


Abstract

In the present study we simulate the precipitation and growth of diamond clusters inside an amorphous carbon or hydrogenated amorphous carbon network by rapid quenching of the compressed liquid phase, followed by volume expansion. This procedure is similar to that occurring during the bias-enhanced nucleation process. Our computational method is tight-binding molecular dynamics.

This method incorporates electronic structure calculations in the molecular dynamics through an empirical tight-binding Hamiltonian.

The simulations of diamond nucleation are carried out under both hydrostatic (in all three directions) and uniaxial pressure. At fast cooling rates (500-1000 K/ps) and high ensities (3.7-3.9 g/cc), large diamond crystallites (containing up to 120 atoms) are formed. We find that the probability of precipitation of diamond crystallites increases with density and with cooling rate. Uniaxial compression of the samples does not lead to nucleation of the hexagonal form of diamond;

all uniaxially compressed ordered four-folded clusters were identified to be cubic diamond.

The samples of hydrogenated amorphous carbon were prepared  in the same way. The diamond clusters generated inside hydrogenated amorphous carbon network are smaller and of lower quality than those formed without hydrogen atoms. Hydrogen atoms are bonded with three- and four-folded atoms, and are expelled from the four-folded amorphous or diamond clusters.

At slower cooling rates (200-500 K/ps), some samples (both with and without hydrogen) transformed to graphite with an interplanar distance smaller than that of perfect graphite. The graphite formed under hydrostatic pressure had planes with random orientation whereas the planes of graphite formed under uniaxial pressure were oriented parallel to the direction of compression. We suggest that this graphitic configuration is formed as a result of a structural phase transition occurring in liquid carbon under very high pressure.

In order to study the growth of diamond, samples of compressed  amorphous carbon with embedded frozen diamond clusters were generated. We observed new carbon  atoms joining the diamond core as the diamond grew. This epitaxial growth of diamond is more favorable at higher pressures. Quantum confinement effects were not found in our  iamond clusters or in diamond layers surrounded by an amorphous carbon phase.