Yanli Nan

Structural characterization of individual graphene sheets formed by arc discharge and their growth mechanisms

Graphene sheets formed by arc discharge are hard to characterize in detail due to their complex pristine states in raw soot, which always exhibit an overall morphology of overlapping aggregation, together with other carbonaceous by-products. Here we used an improved arc method and simple separation procedure to obtain a large number of individual graphene sheets with single- to few-layers, and further probed their structural details using optical microscopy, transmission electron microscopy (TEM), atomic force microscopy and Raman spectroscopy. By TEM characterization, two major types of graphene sheets are shown; one is observed with folded fringes and polycrystalline structure, whereas the other is with an even graphene plane and single crystalline structure. In contrast to that of supported graphene, the Raman spectra of these graphene sheets show some different characteristics such as opposite shift of G band frequency as the layers increase. With increasing layers, the frequencies of G and G′ bands and the full width at half maximum (FWHM) of the G′ band totally exhibit layer-dependence. According to the FWHM of G′ bands, the folding within graphene sheets is also discussed. In addition, the defect types for arc graphene are analysed based on the D and D′ bands. Our results suggest that the D bands of such graphene sheets result from edges, rather than topological defects or disorder. Based on the findings, a new growth mechanism of arc graphene is proposed rationally responsible for the difference of two types of graphene sheets.

Carbon nanohorns under cold compression to 40 GPa: Raman scattering and X-ray diffraction experiments

We report a high-pressure behavior of carbon nanohorns (CNHs) to 40 GPa at ambient temperature by in situ Raman spectroscopy and synchrotron radiation x-ray diffraction (XRD) in a diamond anvil cell. In Raman measurement, multiple structural transitions are observed. In particular, an additional band at ∼1540 cm−1 indicative of sp3 bonding is shown above 35 GPa, but it reverses upon releasing pressure, implying the formation of a metastable carbon phase having both sp2 and sp3 bonds. Raman frequencies of all bands (G, 2D, D + G, and 2D′) are dependent upon pressure with respective pressure coefficients, among which the value for the G band is as small as ∼2.65 cm−1 GPa−1 above 10 GPa, showing a superior high-pressure structural stability. Analysis based on mode Gruneisen parameter demonstrates the similarity of high-pressure behavior between CNHs and single-walled carbon nanotubes. Furthermore, the bulk modulus and Gruneisen parameter for the G band of CNHs are calculated to be ∼33.3 GPa and 0.1, respectively. In addition, XRD data demonstrate that the structure of post-graphite phase derives from surface nanohorns. Based on topological defects within conical graphene lattice, a reasonable transformation route from nanohorns to the post-graphite phase is proposed.We report a high-pressure behavior of carbon nanohorns (CNHs) to 40 GPa at ambient temperature by in situ Raman spectroscopy and synchrotron radiation x-ray diffraction (XRD) in a diamond anvil cell. In Raman measurement, multiple structural transitions are observed. In particular, an additional band at ∼1540 cm−1 indicative of sp3 bonding is shown above 35 GPa, but it reverses upon releasing pressure, implying the formation of a metastable carbon phase having both sp2 and sp3 bonds. Raman frequencies of all bands (G, 2D, D + G, and 2D′) are dependent upon pressure with respective pressure coefficients, among which the value for the G band is as small as ∼2.65 cm−1 GPa−1 above 10 GPa, showing a superior high-pressure structural stability. Analysis based on mode Gruneisen parameter demonstrates the similarity of high-pressure behavior between CNHs and single-walled carbon nanotubes. Furthermore, the bulk modulus and Gruneisen parameter for the G band of CNHs are calculated to be ∼33.3 GPa and 0.1, respecti...

Failure Analysis and Fatigue Investigation on Titanium Tubes in a Condenser

This paper presents an analysis of condenser titanium tube leakage in nuclear power plants. Chemical compositions, mechanical properties, metallographic structures, and microscopic morphologies were analyzed. The results show that the titanium tube leakage was mainly caused by fatigue failure on the basis of the fatigue fracture features. Fatigue tests had been carried out in both air and steam environments, and the fatigue resistance of titanium tubes decreased distinctly in a steam environment. Based on the investigations, proper recommendations have been proposed to enhance the prevention of fatigue fracture of titanium tubes in condensers.