The Formation of Carbon Materials under Extreme Conditions

Carbon is a versatile element due to its ability to form numerous types of bonds with itself, and other elements, enabling it to form a wide variety of materials. The two most well-known pure carbon phases are graphite and diamond; however, other carbon phases have been reported including lonsdaleite (a hexagonal form of diamond), amorphous diamond and a monoclinic form of carbon known as M-carbon. Controversy surrounds the existence of these phases and the mechanism for their formation is not known. For example, it has been suggested that lonsdaleite is not a discrete phase of carbon while evidence for M-carbon has only been found in situ during high-pressure experiments. Extreme conditions, including high pressures, temperatures, and shear, are likely to be required to form many of these exotic forms of carbon. However, there are still many questions as to the extreme conditions that are necessary to selectively produce different carbon structures. This thesis aims to provide new knowledge in how carbon materials behave under extreme conditions by investigating carbon phases in meteorites and under extreme conditions in the laboratory. In the initial part of this thesis, a variety of characterization techniques were employed to study carbon phases in ureilite meteorites, aiming to discern the microstructure of these phases and infer their synthesis conditions. The size and composition of the ureilite parent body, alongside the processes culminating in the observed carbon structures, remain poorly defined and contentious. Euhedral graphite, marking the peak metamorphic carbon state in the ureilite parent body, and microcrystalline graphite, remaining post-impact, were identified around diamonds and lonsdaleite. Diamonds, up to 20 μm in size, were found and determined to form via a chemical vapour deposition-like process with a relationship to troilite and pentlandite, implying a metal-catalytic dependence for diamond growth in ureilites. Furthermore, the identification of large single-crystal lonsdaleite, up to 1 μm, maintaining a crystallographic relationship with adjacent graphite, supports a formation mechanism involving the direct replacement of graphite. The finding of lonsdaleite also challenges the notion that lonsdaleite does not exist as a distinct carbon phase. These insights, particularly regarding the ureilite parent body’s substantial size comparable to Mars or the Moon, emphasize its intricate geological evolution and capacity for sustaining conditions conducive to such carbon phase transformations. The second part of this thesis investigated the formation of amorphous carbon thin films synthesised using an energetic plasma. It was found that the presence of a high compressive stress was required to form the highest density amorphous carbon films that contained the highest fraction of diamond-like (sp3 hybridised) bonding. A particularly notable outcome was the creation of an almost entirely sp3 bonded hydrogenated amorphous diamond structure at room temperature. This thesis also investigated the effects of high-pressure treatment on disordered carbon precursors, since non-crystalline precursors may lower the barrier to the formation of new phases. Hydrogenated amorphous carbon and polyethylene were subject to high-pressure and examined using a variety of in situ and ex situ analysis methods. No new crystalline phases were identified during or post-compression, with the main finding being that the C-H bonds were far more compressible than the C-C bonds. High-pressure studies of polyethylene showed that the semi-crystalline structure transformed to an unstable reversible phase at 6.8 GPa, after which the polyethylene structure was largely recovered. In the final part of this Thesis, the effects of extreme pressures of 145 GPa on a glassy carbon precursor at room temperature was investigated. Previous work had demonstrated that compressing non-crystalline graphitic precursor including glassy carbon, C60 and carbon nanotubes can result in interesting new forms of carbon, however the mechanism for their formation was not understood. By using advanced microscopy and microanalysis techniques on the recovered sample, the work presented in this thesis revealed the formation of shear bands containing diamonds and lonsdaleite. Interestingly, the presence of an amorphous form of diamond was also found, which had not been previously identified following the room temperature compression of graphitic precursors. An energy landscape model was employed to provide insights into the different transition pathways between phases observed. This approach helps understand the synthesis conditions required to produce specific desired carbon structures.