The high pressure behaviour of glassy carbon

Abstract

Carbon is the fourth most abundant element in the universe, commonly found in stars, comets, and the atmosphere of most planets. On Earth it is found in the soil, oceans, atmosphere, planetary interior, and it is the major constituent of all organic matter. There are two well-known naturally occurring pure carbon structures, graphite and diamond. By subjecting graphite to high pressure and high temperature it is possible to transform it into a variety of other structures. These other pure carbon structures, which are dissimilar to either graphite or cubic-diamond, may have useful and applicable physical properties. Interestingly, it is known that in some elemental systems using disordered precursors can lower the kinetic energy barriers which act to prevent high pressure phase transformations. So by using a precursor that is pure carbon and highly disordered, it may be possible to induce phase transformation to new structures using less extreme pressures and temperatures, and also to access transformation pathways that are not possible from rigid crystal structures.

The work presented in this thesis is focused on exploring and creating new high pressure carbon phases. This is done by compressing a disordered graphitic precursor, called glassy carbon, at room temperature using a diamond anvil cell. Analysis of the composition and structure of different glassy carbons at ambient revealed that a glassy carbon produced at 2500C was the most suitable for high pressure experiments. This was used as the precursor from this point onward. Ex situ analysis techniques such as Raman spectroscopy and electron microscopy are used to analyse the recovered samples following compression and decompression, revealing a pressure threshold that represents a permanent structural transformation between 35-45 GPa. When recovered from compression below 35 GPa the glassy carbon structure is retained. However, when compressed to 45 GPa and above nanocrystalline graphite is recovered. In situ X-ray diffraction measurements of glassy carbon under compression reveal that the material evolves continuously toward this threshold, with regard to both structure and bonding. On compression the graphitic layers gradually orient perpendicular to the compression axis of the diamond anvil cell. Beyond 35 GPa small isolated regions of sp3 bonded atoms are formed, which revert to sp2 bonds on decompression. It is further shown by annealing GC at a moderate temperature of 400C at 100 GPa that a different crystalline structure, hexagonal diamond, can be recovered at ambient. The recovered sample is nanocrystalline, and is shown to have the highest purity of hexagonal stacking of any hexagonal diamond formed under static compression.

Further detailed characterisation of recovered hexagonal-diamond samples using electron microscopy shows that shear strain (which is present as a result of the non-hydrostatic compression environment) is an important driver that promotes the pressure induced kinetic phase transformation from the disordered graphitic precursor. These results suggest that shear strain, which is usually minimised in high pressure experiments by the use of hydrostatic pressure media, is a useful and potentially underutilised tool. This knowledge can be applied to other materials to lower energy barriers which impede the formation of new structures that form via kinetic transformation pathways.