The temperature dependence of electrical conductivity and the shielding effectiveness of shungites have been studied at temperatures ranging from 77 to 300 K. The idiosyncrasies of temperature dependences for measuring electrical parameters were determined. Correlations of the π + σ plasmon energies with changes in the frequency dependence of shielding effectiveness on temperature were determined.
Shungite rocks of two different types were treated at ∼ 1400 • C and a set of nanomaterials have been obtained. Among the different materials obtained were: carbon hollow fibers; spherical or ellipsoid particles; silicon carbide amorphous; crystalline nanofibers and nanoparticles having different morphologies; iron and iron silicide nanoparticles encapsulated into carbon shells. Measurements were performed for shielding effectiveness (SE) and the electrical conductivity (σ) of untreated and heat-treated shungite rocks. The shungite rock with dominated hyperfullerene carbon is remarkable for a twofold increase in the σ and a 10 dB increase in SE with a slight decrease of the carbon content by 1.5 % in relation to the untreated sample. In contrast, the treated shungite rock with high SiC nanofiber content is characterized by a halving of the σ and a 10 dB decrease in SE with a decrease of the carbon content by 6 % relative to the original sample.
Shungites have been studied by high-resolution transmission electron microscopy (HRTEM). Two types of carbon layers have been revealed in shungites by HRTEM. The first type, graphite-like carbon layers are characterized by strongly marked hexagonal symmetry. The second type are remarkable for imperfections connected with insignificant disorder in the direction of fringes and the distance between them, and can result from point defects or the pentagonal and heptagonal carbon rings that are signs of fullerene-like structures.
Abstract. Four types of shungite rocks containing 98.2 wt %, 22.2 wt %, 21.6 wt % and 22.4 wt % C and 416, 311, 78 and 182 ppm V were studied, while the ash from
these samples contained 23120, 400, 100 and 234 ppm V, respectively. The
presence of two vanadium carbides, V2C and V6C5, in samples of
shungite with a content of 98.2 wt % C has been revealed by energy
dispersive spectroscopy (EDS) and selected-area electron diffraction (SAED).
The vanadium carbides revealed are monocrystalline and encapsulated in a
carbon shell and thus are the first well-crystallized natural examples of
V carbides. The shungite-bearing rocks are Lower Proterozoic in age (2.0–2.1 Ga), and the encapsulation of vanadium carbides in carbon shells explains
their good preservation during this time. Parageneses of vanadium carbide
and roscoelite have also been found, indicating that roscoelite in shungite
rocks may be a secondary mineral formed during the decomposition of vanadium
carbide. It is possible that the decomposition of vanadium carbides due to
the destruction of carbon shells with the formation of roscoelite occurred
during the 1.8 Ga Svecofennian orogeny when the sediments were affected by
greenschist-facies metamorphism. Particles encapsulated in carbon shells
were also revealed in the shungite rock containing 22.2 wt % C and 311 ppm V, for which high shielding effectiveness is observed in a wide
frequency range.
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