The flexibility of the carbon atom in forming covalent bonds with different hybridisation states leads to carbon solids that exhibit a range of useful properties, from the extreme hardness of diamond to the extreme ‘slipperiness’ of graphite. In recent years a series of studies have been conducted to better understand the high-pressure transformation process between sp2 bonded graphitic precursor materials to the sp3 bonded diamond phase of carbon. The aim of this work is to use disordered and amorphous carbon materials as precursors in high pressure experiments in order to explore the fundamental phase diagram of carbon and to form new materials with interesting properties – such as harder-than-diamond materials
This presentation includes in depth analysis of the precursor, glassy carbon, followed by a detailed description of the technique of using diamond anvil cells to generate the extremely high pressures. Raman spectroscopy and electron microscopy measurements of the materials recovered following compression show that the glassy carbon precursor loses its fundamental properties when compressed beyond 35-45 GPa. In situ X-ray diffraction and molecular dynamics simulations show that the transformation observed across this pressure threshold signifies a sharp increase in diamond-like bonding and the initial nucleation of isolated metastable nanodiamonds. Further high pressure experiments show that this metastable nanodiamond phase is recoverable at ambient when annealed at 400°C whilst at high pressure. This recovered nanodiamond material is shown to be comprised of hexagonally stacked diamond, a material thought to be harder than regular cubic diamond.