Svetlana Dimovski

Raman scattering of non-planar graphite : Arched edges, polyhedral crystals, whiskers and cones

Planar graphite has been extensively studied by Raman scattering for years. A comparative Raman study of several different and less common non–planar graphitic materials is given here. New kinds of graphite whiskers and tubular graphite cones (synthetic and natural) have been introduced. Raman spectroscopy has been applied to the characterization of natural graphite crystal edge planes, an individual graphite whisker, graphite polyhedral crystals and tubular graphite cones. Almost all of the observed Raman modes were assigned according to the selection rules and the double–resonance Raman mechanism. The polarization properties related to the structural features, the line shape of the first–order dispersive mode and its combination modes, the frequency variation of some modes in different carbon materials and other unique Raman spectral features are discussed here in detail.

Naturally occurring graphite cones

Abstract Carbon, boron nitride, and other materials that form nanotubes are also able to form conical shapes. Even though the potential applications of cone arrays as electron emitters and other devices are very promising, understanding of their structure and formation mechanisms is still very limited compared to nanotubes and other carbon structures. Moreover, the cones have only been synthesized in a mixture with other shapes, but never as continuous arrays. It appears, however, that we can learn from nature how to produce large carbon cone arrays. We here report the first-known natural occurrence of large arrays of conical graphite crystals. These occur on the surfaces of millimeter-sized polycrystalline spheroidal aggregates of graphite. Cone heights range from less then a micron to 40 μm, which is larger than any other carbon cones reported in the literature. They are also observed to dominate sample surfaces. The surface topography of the cones and petrologic relations of the samples suggest that the cones formed from a metamorphic fluid. Unlike most laboratory produced cones, the natural cones have a wide distribution of apex angles, which supports a disclination model for cone-helix structures.

Synthesis of graphite by chlorination of iron carbide at moderate temperatures

Synthesis of graphite by extraction of iron from iron carbide by chlorine is reported in this work. This process is attractive because it can produce well-ordered graphite at temperatures as low as 600 °C, providing an opportunity for low-temperature solid-state synthesis. Thermodynamic simulation was used to determine the composition of the reaction products under equilibrium conditions and select the initial process parameters such as temperature and chlorine/carbide molar ratio. The interlayer spacing and crystal size of the produced graphite were calculated from X-ray diffraction measurements. The degree of orientation of the graphitic layers was determined by Raman spectroscopy. Three temperature regimes have been identified. At temperatures below 500 °C, amorphous or disordered carbon is formed as shown by Raman spectroscopy and TEM studies. Well-ordered graphite microcrystals are formed by solid-state growth between 600 and 1100 °C. Above the eutectic temperature in the Fe/Fe3C system, 1130 °C, the growth of large graphite crystals occurs from the liquid phase, similar to the formation of kish graphite by precipitation of carbon at high temperatures from supersaturated molten iron. Iron chlorides, the main impurities in the material synthesized by the solid-state growth, can be removed by using excess chlorine gas or by a separate wet chemical purification step. Preparation of graphite doped with iron for catalytic purposes is also possible using this process.

Controlling dissociative adsorption for effective growth of carbon nanotubes

Dissociative adsorption has been widely simplified as part of the vapor–liquid–solid (VLS) growth model. We found that the addition of specific carrier gases can critically modify the growth rate and growth density of multiwall carbon nanotubes (MWNTs). These results were explained by dissociative adsorption of C2H2 molecules and a solid-core VLS growth model. Based on these integrated mechanisms, vertically aligned MWNTs were grown with an initial growth rate as high as ∼800μm∕h. This efficient growth process results at temperature and C2H2 partial pressures at which the decomposition and segregation rates of carbon are balanced. Appropriate use of carrier gas is one of the factors that could facilitate efficient and continuous growth of carbon nanotubes in the future.

Effect of Carrier Gas on the Growth Rate, Growth Density, and Structure of Carbon Nanotubes

We attempt to understand the fundamental factors that determine the growth rate of carbon nanotubes. In a series of experiments on growing multiwall carbon nanotubes (MWNTs) by thermal chemical vapor deposition, we found that the addition of carrier gas and the type of carrier gas can change the growth rate, growth density, and structures of MWNTs. We explain these results based on the dissociative adsorption of C2H2 on Fe nanoparticles and the vapor-liquid-solid (VLS) growth model. Finally, high-density, vertically aligned MWNTs were grown when decomposition and segregation rates of carbon were balanced.

Testing Multiwall Carbon Nanotubes on Ion Erosion for Advanced Space Propulsion

Are carbon nanotubes more resistant than diamonds against ion erosion? Here, we report an evaluation of multiwall carbon nanotubes (MWNTs) as the protective coating against plasma erosion in advanced space propulsion systems. We have compared polycrystalline diamond films with MWNTs, amorphous carbon (a-C) and boron nitride (BN) films. Two types of MWNTs were investigated including vertically aligned (VA) MWNTs, and those horizontally laid on the substrate surfaces. Only diamond films and VA-MWNTs survived erosion by 250 eV krypton ions of a flight-quality Hall-effect thruster. VA-MWNTs are found to bundle at their tips after ion erosion.