Electric Discharge Plasmochemical Synthesis of Carbon Nanomaterials

High-energy electric discharge technologies (electric breakdown and HF volume discharge in organic media) for a large scale synthesis of amorphous carbon (AC) are developed. A destruction of hydrocarbon molecules into separate fragments occurs during such processing of organic media, what results in AC formation in the process of ultra-fast cooling of the clusters. To investigate the influence of chemical nature of working media, organic liquids and gases from the class of arenes with sp2-hybridisation of carbon atoms in molecule and alkanes with sp3-hybridisation were used. Performed XRD, HRTEM and Raman studies showed that produced powders are typical amorphous materials with significant degree of disorder. But only in the case of electric breakdown of alkanes, carbon nanomaterials with complex core-shell structure were discovered. Individual particles of onion-like carbon (OLC) consist of ~ 5 nm core surrounded by graphitic shell of 5-6 layers. Synthesized OLC is used as antifriction additives to industrial oils and as material for electromagnetic waves shielding. The statistical analysis of the atomic structure of the synthesized materials using reverse Monte Carlo and Voronoi-Delaunay methods was performed. When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3509

The influence of reacting gases on the motion of collapsing cavities

An analysis was performed on a collapsing cavity containing reacting gases. It was determined that with reacting gases in the cavity there was significant departure from the fluid mechanics of collapse of a cavity containing inert gases. When reacting gases are present, the nonlinear differential equations of motion, energy, and kinetics must be solved simultaneously. It was found that the time required to reach a given radius during collapse was greater for an exothermic reaction occurring in the gas phase than for the cavity containing a nonreacting gas. For the exothermic reaction the collapse time increases with increasing magnitude of the heat of reaction, while for an endothermic reaction the collapse time does not increase indefinitely with increasing heat of reaction but instead decreases and approaches a finite limit. For small heats of reaction the influence of a reacting gas on cavity motion is only readily detectable during the rebound of the cavity. When the collapse time approaches the half period of the wave, the conversion increases with acoustic pressure. However, when the collapse time approaches the period of the wave, the conversion first decreases, then increases with increasing acoustic pressure. Additional description are presented in the text delineating the behavior of the cavity during the collapse when a gas is reacting within the cavity.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/32946/1/0000329.pd

Experimental implementation of a four-level N-type scheme for the observation of Electromagnetically Induced Transparency

A nondegenerate four-level N-type scheme was experimentally implemented to observe electromagnetically induced transparency (EIT) at the $^{87}$Rb D$_{2}$ line. Radiations of two independent external-cavity semiconductor lasers were used in the experiment, the current of one of them being modulated at a frequency equal to the hyperfine-splitting frequency of the excited 5P$_{3/2}$ level. In this case, apart from the main EIT dip corresponding to the two-photon Raman resonance in a three-level $\Lambda$-scheme, additional dips detuned from the main dip by a frequency equal to the frequency of the HF generator were observed in the absorption spectrum. These dips were due to an increase in the medium transparency at frequencies corresponding to the three-photon Raman resonances in four-level N-type schemes. The resonance shapes are analyzed as functions of generator frequency and magnetic field.Comment: 3 pages, 2 figure

A ``Tetris''-like model for the Compaction of Dry Granular Media

We propose a two-dimensional geometrical model, based on the concept of geometrical frustration, conceived for the study of compaction in granular media. The dynamics exhibits an interesting inverse logarithmic law that is well known from real experiments. Moreover we present a simple dynamical model of $N$ planes exchanging particles with excluded volume problems, which allows to clarify the origin of the logarithmic relaxations and the stationary density distribution. A simple mapping allows us to cast this Tetris-like model in the form of an Ising-like spin systems with vacancies.Comment: 4 pages, Latex including 2 PS figures (reference corrected). Subm. to Phys. Rev. Lett. (1997

Effect of mesoscopic inhomogeneities on the critical current of bulk melt-textured YBCO

The downsizing 211-inclusions and an increase of their density leads to rise in mean critical current value in Y-based melt textured material. Very often 211-inclusion are spread in the material volume non-homogeneous, with typical scale 50 - 100 micrometer. Therefore it is difficult to find the real correlation between local critical current and the inclusions distribution. We performed a study of a local critical current using modified magneto-optic technique on a melt-textured YBaCuO ceramic, found the areas with constant current and studied the real structure of the material in the areas, inclusions distribution and their sizes, by scanning electron microscopy and X-ray microanalysis. The estimation of a pinning in these places, by taking into account the amount of inclusions and the length of their boundaries, and comparison with the value of local critical current reveals a strait correlation between the density of inclusions and the current but shows remarkable quantitative disagreement.Comment: PDF (8 pages, 4 figures