The method of zero-range potentials is used to investigate the band structure of the one-electron spectrum of zigzag single-walled carbon nanotubes. It is shown that the formation of a narrow forbidden band in ͑3N ,0͒ zigzag nanotubes is due to the influence of the curvature of their walls on the one-electron spectrum of the electrons. Expressions are obtained for the widths of the forbidden bands ͑band gaps͒ and effective masses of ͑3N ,0͒, ͑3N −1,0͒, and ͑3N +1,0͒ nanotubes as functions of their diameter.

Explicit expressions for the wavefunctions and dispersion equation for the band π-electrons in single-wall carbon nanotubes are obtained within the method of zero-range potentials. They are then used to investigate the absorption spectrum of polarized light caused by direct interband transitions in isolated nanotubes. It is shown that, at least, under the above approximations, circular dichroism is absent in chiral nanotubes for a light wave propagating along the tube axis. The results obtained are compared with those calculated in a similar way for a graphite plane.

The ground-state exciton binding energy for single-walled carbon nanotubes (SWCNTs) in vacuum calculated ignoring the screening of Coulomb interaction appears to be much greater than the corresponding band gap. The most essential contributions to the screening of electron-hole (e-h) interaction potential in semiconducting SWCNTs, which return the ground-state exciton binding energy into the energy gap, are considered. Our estimates on the screening effects and exciton binding energies are in satisfactory agreement with the corresponding experimental data for concrete nanotubes.PACS number(s): 78.67.Ch

Comparative characteristics are presented for the physicomechanical properties and oxidation resistance of refractory materials of Al 2 O 3 -Si 3 N 4 -C composition based on an ACPB for the original materials (fired at 1400°C) and after impregnation with a sol-gel composition and heat treatment at 800°C. A reduction in material porosity, increase in strength and reduction in carbon burn-off are due to development of a glassy phase in the pore space and on graphite flakes due to SiO 2 formation with thermal destruction of the organosilicon substance.

No abstract