Superhard phase composed of single-wall carbon nanotubes

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Abstract

Single-wall carbon nanotubes (SWNT's) have been studied under pressure up to 55 GPa. We report experimental data on irreversible changes of mechanical and structure properties of SWNT under pressure. The new superhard phase (SP-SWNT) composed of single-wall carbon nanotubes has been studied which exhibits a bulk modulus exceeding or comparable with diamond and hardness belongs to the range between cubic BN and diamond. The SP-SWNT were synthesized by applying a shear deformation under load in a diamond-anvil cell; the procedure of stress tensor variation. After intermediate phase transitions, single-wall carbon nanotubes are transformed to SP-SWNT at pressure of 24 GPa. The transformation is accompanied by irreversible changes in the Raman spectra. The nanotubes do not collapse at least up to pressure 55 GPa (maximum pressure of the study). Bulk modulus of 462 to 546 GPa was found out for SP-SWNT from the comparative study of pressure dependence of the Raman modes of SP-SWNT (high-energy mode \ensuremath{\sim}1590 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) and diamond (1333 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$). This value exceeds the bulk modulus of diamond (420 GPa for single diamond crystal). Hardness measurements were performed using nanoindentation technique. Hardness of SP-SWNT (62 to 150 GPa) was found out from comparative study of SP-SWNT, diamond and cubic BN.

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