High Pressure Study On The Phase Transitions Of Onion-like Carbon Nanospheres

Elemental carbon can exist in various allotropes(from zero dimensional to three-dimensional), due to its unique bond-hybridized modes of sp, sp2 and sp3, with its property from the hard to extremely soft, from insulator to semiconductor, even to super conductor, from adiabatic to good heat conductor, and so on. As a new member of fullerenes family, carbon nano-onions come after C60 s and carbon nanotubes. An ideal carbon nano-onion is nested by concentric graphite shells, and the innermost layer is composed of 60 carbon atoms, and carbon atoms of the other layers increase in turn by 60n2(n indicates the number of layer). The distance between the adjacent layers is about 0.34 nm. Such onion-like carbon materials have various potential applications such as solid lubrication, electromagnetic shielding, gas and energy storage, superhard materials and electro-optical devices owing to their outstanding structural and physical properties.High pressure is an effective means to change the structure and properties of materials. It can change the distances between molecules or atoms, and then affect their interactions and bondings to obtain new high pressure phases. For instance, fullerene C60 becomes very reactive and forms various polymeric structures when subjected to high pressure. The reactions are associated with a partial transformation of sp2 C into sp3 C. It has also been reported that C60 was converted into amorphous or polycrystalline diamond under high pressure. From the view of structure and bonding state, multilayer fullerenes nested structure of carbon nano-onions and their sp2 carbon bonds of highly curved state, the structural transformation under high pressure may be different from that of other sp2 carbon materials, which is expected to find new structure or new bonding changes. However, up to now, few studies have been carried on the onion-like carbon nanospheres under pressure, and previous researches are mainly concentrated in the low pressure(<20 GPa), no structural or bonding change has been observed in the onion-like carbons, which shows better high-pressure structural stability. In addition, it is very important to study the structural stability of onion-like carbon nanospheres under extreme conditions, and the possible structural phase transition, which is of great significance to understand the structure and properties of the materials. Therefore, in this paper, the high pressure transition behavior of onion-like carbon nanospheres with different sizes is studied by using high pressure experimental technique.1. The structural and bonding changes of onion-like carbon nanospheres(OCNSs) have been studied under high pressure up to 48 GPa. A transformation related to a change from sp2 to sp3 bonding of carbons in OCNSs was observed at pressures above 20 GPa. Due to the unique onion-like configuration, the interlayer bonds are formed on the nanoscale between buckled layers in the OCNSs, forming Moire-like patterns. The onion-like carbon structure is extremely stable and can be recovered even after a compression cycle to 48 GPa. Such a high stability, far more than other sp2 carbon materials, is related to the unique onion-like configuration and to the interlayer concentric sp3 bonding formation, which also makes a significant increase in the hardness of the samples.2. The size effect of carbon nano-onions(CNOs) on the high pressure behavior was studied in situ high pressure technique. At ambient conditions, the interlayer d-spacings of small CNOs(with an average diameter of 43 nm) are 0.36 nm, which are expanded by about 3% compared to the larger ones(with an average diameter of 150 nm). High pressure study on the G-band of the small CNOs indicates that a bonding change was initiated at 23.4 GPa, which is about 3 GPa higher than that of the larger CNOs. The small CNOs were destroyed into amorphous fragments at above 48 GPa with a large applied deviatoric stress, showing a lower high pressure stability compared with the larger CNOs. The results for CNOs can be rationalized by the interlayer expansion and the highly turbostratic layer structure of the studied material. These features are qualitatively similar to the size effects observed in the compression behavior of nanocrystalline materials, showing that a reduced cluster size gives similar physical effects in the two classes of materials. Our results present important data on the transformation of CNOs under pressure and suggest that it is possible to optimize the synthesis of nanodiamond under HPHT when suitably sized CNOs are used as carbon source.3. The structural transformation of onion-like carbon nanospheres under high pressure and high temperature was studied by using large cavity press. Preliminary studies have found that OCNSs can transform into diamond-like structure under a certain pressure and temperature. The results show that the onion-like carbon nanospheres contains a small amount of diamond structure when released from 20 GPa, 1800 oC, 3 min conditions.