Design Of New Superhard Materials Based On Typical Carbon Structures

Carbon nanomaterials are now available with rich variety of structures, such as diamond, fullerenes, nanotubes, graphite and graphene, which has a wide range of applications in many fields. The difference in structures can lead to different properties in them, such as electronic and mechanical properties.High pressure is the most powerful tool for exploring novel superhard structures, and a typical example in carbon materials is the transformation of graphite to diamond. Therefore, in this thesis, studies were carried out on the design of new superhard materials based on fullerenes, nanotubes, diamond and graphite by first principle methods, the CALYPSO structural prediction method and Vickers hardness calculations.1. We found that the C60@SWCNT(11,11) peapod and C70@SWCNT(10,10) peapod could transform to two novel structures under appropriate pressure condition, U carbon and V carbon, respectively. The transformations also suggest that the intentional inclusion of five-membered carbon rings in the starting material facilitate the creation of new structures. U carbon and V carbon adopt monoclinic structures with pure sp3-hybridized bonds, and they are more favorable than M carbon in a certain pressure range. U carbon(87.6GPa, 370GPa) and V carbon(89.4GPa, 411GPa) possess high hardnesses and bulk moduli, comparable to those of diamond. By simulating the x-ray diffraction patterns, we propose that U carbon would be the unidentified new quenchable superhard carbon phases synthesized by compressing C60 peapods. Our findings give us a better understanding on the high pressure behaviors of peapods and the formation mechanism of the superhard phases. This study is extremely helpful for future experiments and is helpful for achieving more insight on the superhard materials.2. We report a new 3D orthorhombic phase(denoted C14 carbon), which is both metallic and superhard(Vickers hardness 55.8 GPa). This allotrope can be derived by introducing proper configurations constructed by sp2-carbon into cubic diamond structure, for which sp3 bonded C atoms guarantee its high hardness and the sp2 bonded carbons ensure its conductivity. When compared to other metallic carbon allotropes, C14 carbon has a larger density(3.37 g/cm3)close to diamond(3.48 g/cm3), which implies the better mechanical properties of C14 carbon structure. It is dynamically stable and energetically more preferable than most theoretically predicted carbon. By simulating the x-ray diffraction patterns, we propose that C14-carbon would be the unidentified carbon phases observed in shock compression of tetracyanoethylene(TCE) powder experiment and we also have propose a possible transition of TCE to C14-carbon. This study is extremely helpful for future experiments and is helpful for achieving more insight on the superhard metallic materials.