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. 2004 Sep 21;101(38):13699-702.
doi: 10.1073/pnas.0405877101. Epub 2004 Sep 10.

A Quenchable Superhard Carbon Phase Synthesized by Cold Compression of Carbon Nanotubes

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Free PMC article

A Quenchable Superhard Carbon Phase Synthesized by Cold Compression of Carbon Nanotubes

Zhongwu Wang et al. Proc Natl Acad Sci U S A. .
Free PMC article

Abstract

A quenchable superhard high-pressure carbon phase was synthesized by cold compression of carbon nanotubes. Carbon nanotubes were placed in a diamond anvil cell, and x-ray diffraction measurements were conducted to pressures of approximately 100 GPa. A hexagonal carbon phase was formed at approximately 75 GPa and preserved at room conditions. X-ray and transmission electron microscopy electron diffraction, as well as Raman spectroscopy at ambient conditions, explicitly indicate that this phase is a sp(3)-rich hexagonal carbon polymorph, rather than hexagonal diamond. The cell parameters were refined to a(0) = 2.496(4) A, c(0) = 4.123(8) A, and V(0) = 22.24(7) A (3). There is a significant ratio of defects in this nonhomogeneous sample that contains regions with different stacking faults. In addition to the possibly existing amorphous carbon, an average density was estimated to be 3.6 +/- 0.2 g/cm(3), which is at least compatible to that of diamond (3.52 g/cm(3)). The bulk modulus was determined to be 447 GPa at fixed K' identical with 4, slightly greater than the reported value for diamond of approximately 440-442 GPa. An indented mark, along with radial cracks on the diamond anvils, demonstrates that this hexagonal carbon is a superhard material, at least comparable in hardness to cubic diamond.

Figures

Fig. 1.
Fig. 1.
TEM images of starting and recovered carbon nanotubes assembled into bundles. (A and A′) Low- and high-magnification images of the starting carbon nanotubes, showing the wall d-spacing of 0.34 nm in A′.(B and B′) The compact morphology of the recovered sample shows that carbon nanotube characteristics still remained at the edge of the sample. (C) Electron diffraction pattern that indicates a hexagonal structure (a0 = 0.24 nm, c0 = 0.43 nm), differing from graphite in which the strongest peak, (002) with d-spacing 0.34 nm, should be observed. (D) Filtered high-resolution TEM image of the hexagonal carbon polymorph. Highly imperfect fringes with lattice distances of 0.21 nm and hexagonal structural morphologies are clearly seen.
Fig. 2.
Fig. 2.
X-ray diffraction patterns of the starting sample and recovered high-pressure polymorph of nanotube released from ≈100 GPa. The sharp noise is overlapping with the carbon nanotube peak of (002).
Fig. 3.
Fig. 3.
Raman spectra of several structural polymorphs of carbon collected at ambient conditions. (Inset) A representation of the Raman spectrum of hexagonal diamond made by shock-wave impact on diamond (7).
Fig. 4.
Fig. 4.
Comparison of compressibilities between cubic diamond and the observed high-pressure hexagonal carbon. Hexagonal carbon is slightly more incompressible than cubic diamond.
Fig. 5.
Fig. 5.
Photomicrograph showing indentation (crack in center and related radial crack lines) of the diamond anvil by the high-pressure hexagonal polymorph of cold-compressed carbon nanotubes. The large circle represents the gasket hole; the small circle highlights the indentation mark with the depth of ≈3 μm (e.g., estimated from microscopy focusing) at the center of the diamond anvil that has the highest pressure).

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