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Here we constructed a charged gravastar model formed by an interior de Sitter spacetime, a charged dynamical infinitely thin shell with an equation of state and an exterior de Sitter-Reissner-Nordstrom spacetime. We find that the presence of the charge is crucial to the stability of these structures. It can as much favor the stability of a bounded excursion gravastar, and still converting it in a stable gravastar, as make disappear a stable gravastar, depending on the range of the charge considered. There is also formation of black holes and, above certain values, the presence of the charge allows the formation of naked singularity. This is an important example in which a naked singularity emerges as a consequence of unstabilities of a gravastar model, which reinforces that gravastar is not an alternative model to black hole.
In this work, we present a study of growth and
characterization of nanocomposites, based on multiwalled carbon nanotubes and
metal nanoparticles (Al, Ag, Au, Co, Cu, Fe, Ni and Ti). We observe a very
different behavior between noble and transitions metals. All the nanocomposites
are characterized by a network of carbon nanotubes with randomly insertion of
spherical metal particles with dimensions of about 100 nm (clearly visible in
SEM images). In particular, in transition metal nanocomposites, each
tube on sheet surface is covered by particles of about 40 - 50 nm
and for all metals the XPS measurements indicate the absence of chemical bonds
and the simply physisorption of nanoparticle on carbon nanotube buckypaper.
Furthermore, the nanocomposites show very different properties respect to pure
carbon nanotubes: they are hydrophobic, their roughness is about 50% smaller
than carbon nanotube and they exhibit a strong visible photoluminescence, which
is absent in pure nanotube.