UDC 539.538:621.921.34

A. Kurio (Yokohama, Matsuyama, Japan)
Y. Tanaka (Yokohama, Japan)
H. Sumiya (Itami, Japan)
T. Irifune, T. Shinmei, H. Ohfuji (Matsuyama, Japan)
H. Kagi (Tokyo, Japan)

Wear resistance of nano-polycrystalline diamond with various hexagonal diamond contents

Wear resistance of nano-polycrystalline diamond (NPD) rods containing various amounts of hexagonal diamond has been tested with a new method for practical evaluation of the wear–resistance rate of superhard ceramics, in addition to the measurements of their Knoop hardness. The wear resistance of NPD has been found to increase with increasing synthesis temperature and accordingly decreasing proportion of hexagonal diamond. A slight increase in Knoop hardness with the synthesis temperature also has been observed for these samples, consistent with the results of the wear–resistance measurements. These results suggest that the presence of hexagonal diamond would not yield any observable increase in both hardness and wear resistance of NPD, contradictory to a recent prediction suggesting that hexagonal diamond is harder than cubic diamond. It is also demonstrated that NPD is superior to single crystal diamond in terms of relatively homogeneous wearing without any significant chipping/cracking.

Keywords: nano-polycrystalline diamond (NPD), hexagonal diamond, hardness and wear resistance of NPD.

 
UDC 546.26-162

S. E. Boulfelfel, Q. Zhu (New York, US)
A. R. Oganov (New York, US, Moscow, Russia)

Novel sp³ forms of carbon predicted by evolutionary metadynamics and analysis of their synthesizability using transition path sampling

Experiments on cold compression of graphite have indicated the existence of a new superhard and transparent allotrope of carbon. Numerous metastable candidate structures featuring different topologies have been proposed for “superhard graphite”, showing a good agreement with experimental X-ray data. In order to determine the nature of this new allotrope, we use evolutionary metadynamics to systematically search for low-enthalpy sp³ carbon structures easily accessible from graphite and we employ molecular-dynamics transition path sampling to investigate the corresponding kinetic pathways starting from graphite at 15–20 GPa. Real transformation kinetics are computed and physically meaningful transition mechanisms are produced at the atomistic level of detail in order to demonstrate how nucleation mechanism and transformation kinetics lead to M-carbon as final product of cold compression of graphite. This establishes M-carbon as an experimentally synthesized carbon allotrope.

Keywords: high pressure, carbon, polymorphism, superhard material, molecular dynamics, metadynamics.

  

UDC 546.26-162:539.58

Y. Wang (Rochester, USA)
K. K. M. Lee (New Haven, USA)

From soft to superhard: fifty years of experiments on cold-compressed graphite

In recent years there have been numerous computational studies predicting the nature of cold-compressed graphite yielding a proverbial alphabet soup of carbon structures (e.g., bct-C₄, K₄-, M-, H-, R-, S-, T-, W- and Z-carbon). Although theoretical methods have improved, the inherent nature of graphite (i.e., low-Z) and the subsequent room-temperature, high-pressure phase transition (i.e., low symmetry, nanocrystalline and sluggish), make experimental measurements difficult to execute and interpret even with the current technology of 3rd generation synchrotron sources. The room-temperature, high-pressure phase transition of graphite has been detected by numerous kinds of experiments over the past fifty years, such as electrical resistance measurements, optical microscopy, X-ray diffraction, inelastic X-ray scattering, and Raman spectroscopy. However, the identification and characterization of high-pressure graphite is replete with controversy since its discovery more than fifty years ago. Recent experiments confirm that this phase has a monoclinic structure, consistent with the M-carbon phase predicted by theoretical computations. Meanwhile, experiments demonstrate that the phase transition is sluggish and kinetics is important in discerning the phase boundary. Additionally, the post-graphite phase appears to be superhard with hardness comparable to that of diamond.

Keywords: high-pressure graphite, post-graphite phase, phase transition, M-carbon, diamond–anvil cell experiments.

  

UDC 546.26:539.89

Z. S. Zhao (Qinhuangdao, Hebei, China)
X.-F. Zhou (Tianjin, China)
M. Hu, D. L. Yu, J. He (Qinhuangdao, Hebei, China)
H.-T. Wang (Tianjin, China)
Y. J. Tian, B. Xu (Qinhuangdao, Hebei , China)

High-pressure behaviors of carbon nanotubes

In this paper, we have reviewed the experimental and theoretical studies on pressure-induced polygonization, ovalization, racetrack–shape deformation, and polymerization of carbon nanotubes (CNTs). The corresponding electronic, optical, and mechanical changes accompanying these behaviors have been discussed. The transformations of armchair (n, n) CNT bundles (n = 2, 3, 4, 6, and 8) under hydrostatic or nonhydrostatic pressure into new carbons, including recently proposed superhard bct-C₄, Cco-C₈, and B-B₁A_L2R2 carbon phases have also been demonstrated. Given the diversity of CNTs from various chiralities, diameters, and arrangements, pressure-induced CNT polymerization provides a promising approach to produce numerous novel metastable carbons exhibiting unique electronic, optical, and mechanical characteristics.

Keywords: pressure-induced carbon nanotubes, polymerization, novel metastable carbons, electronic and mechanical characteristics.

  

UDC 546.26-162.001.1

Ch. He, L. Z. Sun, J. Zhong (Xiangtan, China)

Prediction of superhard carbon allotropes from the segment combination method

Many superhard allotropes of carbon have been proposed in recent years for the purpose of explaining the superhard carbon phases observed in the processes of cold compressing graphite and carbon nanotubes. In this paper, we have reviewed recent advances in searching for superhard phases of carbon from a segment combination view and found that they can be divided into two groups: (i) combinations of segments from cubic diamond and hexagonal diamond with 5-6-7 carbon rings and (ii) combinations of segments from hexagonal diamond and mutated hexagonal diamond with 4-6-8 carbon rings. Finally, an additional example of extending these allotropes of carbon to their corresponding boron nitride counterparts has been discussed.

Keywords: superhard material, carbon allotrope, cold compressing graphite, crystal structure prediction, first-principles method.

  

UDC 546.26-162:539.89

V. V. Brazhkin, A. G. Lyapin (Troitsk, Russia)

Hard and superhard carbon phases synthesised from fullerites under pressure

A review has been presented on the structural and mechanical properties of hard carbon phases synthesized from fullerite C₆₀ under pressure. The density and nanostructure have been recognized as the key parameters defining the mechanical properties of hard carbon phases. By suggesting a version of the transitional high-pressure diagram of C₆₀ (developed up to 20 GPa), the three areas of the formation of hard carbon phases have been highlighted. The corresponding phases of superhard carbon are (1) disordered sp²-type atomic structures at moderate pressures and high temperatures (> 1100 K), (2) three-dimensionally polymerized C₆₀ structures at moderate temperatures and high pressures (> 8 GPa), and (3) sp³-based amorphous and nanocomposite phases at high pressures and temperatures. First region can be in turn separated into 2 subparts with different peculiarities of sp² structures and properties: low pressure part (0.1–2 GPa) and high-pressure part (2–8 GPa). Temperature can be recognized as a factor responsible for the formation of nanostructures by the partial destruction of molecular phases, whereas pressure is a factor responsible for stimulating the formation of rigid polymerized structures consisting of covalently bonded C₆₀ molecules, whereas the combination of both factors leads to the formation of atomic-based phases with dominating sp³ bonding.

Keywords: carbon, diamond, fullerite C₆₀, nanostructure, polymerization, superhard phases.