K. Das Chowdhury

Raman scattering from nanoscale carbons generated in a cobalt-catalyzed carbon plasma

Abstract Carbonaceous material including nanoscale soot, carbon-coated nanoscale Co particles and nanotubes have been generated from a dc arc discharge between carbon electrodes. Sharp first- and second-order lines are observed in the Raman scattering spectra of the arc-derived carbons only when Co metal is present in the core of the anode. The sharp lines in the Raman spectrum of the Co-catalyzed, arc-derived carbons have not been observed previously in carbonaceous materials and are tentatively assigned to carbon nanotubes on the basis of a zone-folded model for the vibrational spectra of armchair tubules.

Fullerenic nanostructures in flames

High resolution transmission electron microscopy (HRTEM) was used to characterize nanostructures in soots produced in flames of benzene, acetylene, or ethylene premixed with oxygen and an inert diluent gas. The nanostructures ranged from {approximately}2 nm to {approximately}30 nm in size with a hollow core measuring about {approximately}1 nm to {approximately}10 nm in diameter and containing 5 to 20 shells. The shapes of the nanostructures included spherical, spheroidal, tubular, and trigonous. {copyright} {ital 1996 Materials Research Society.}

NANOSCALE CARBON BLACKS PRODUCED BY CO2 LASER PYROLYSIS

CO 2 laser pyrolysis has been used to synthesize carbon black (particle diameter ∼30 nm) via a catalytically driven pyrolysis of benzene vapor. The H : C ratio is found to be ∼1 : 10, which is unusually high for carbon blacks. Subsequent heat treatment of the “laser black” to temperatures up to ∼2800 °C produces well-graphitized faceted particles with central polygonal cavities. High resolution TEM lattice imaging, Raman scattering, and x-ray diffraction have been used to characterize the morphological structure of these carbon particles in their as-synthesized and heat-treated forms. Furthermore, KOH treatment at ∼800 °C has been employed to activate the as-synthesized particles, producing a tenfold increase in the surface area from 50 to 700 m 2 /g. Possible pore structures generated during this activation process have been identified by high resolution TEM imaging.

A comparative high-resolution study of interface chemistry in silicon-nitride-based ceramic matrix composites reinforced with silicon carbide whiskers

Abstract Silicon nitride/silicon carbide(w) ceramic matrix composites synthesized from common starting materials, except for the whiskers themselves, under the same processing conditions have been investigated. High-resolution electron microscopy and high-spatial-resolution electron energy loss nanospectroscopy were used to characterize the whisker/matrix interfaces in these composites both structurally and chemically. The presence of a discontinuous oxygen-rich amorphous layer at the two kinds of whisker/matrix interfaces examined in this paper appeared to be a general phenomenon. The wide difference (a factor of 10) between the structural image and chemical widths (oxygen distributions) of the whisker/matrix interfaces was attributed to oxygen solution into the silicon nitride matrix. The oxygen sources were the sintering aids and surface impurities.

Chemical Widths at Composite Interfaces: Relationships to Structural Widths and Methods for Measurement.

Energy selected imaging with a Zeiss 912 {Omega}-filter TEM was used to examine grain boundary solute distributions in an Si{sub 3}N{sub 4}/SiC(w) ceramic densified with Y{sub 2}O{sub 3} + Al{sub 2}O{sub 3} sintering aid. These results are compared to boundary region solute distributions in the same materials determined by field emission small probe electron energy loss spectroscopy and related methods. The intrinsic higher incident flux of the FEG small probe methods renders them the most useful for high spatial resolution local chemical width measurement. Energy selected imaging is fast and relatively simple for determining elemental distributions in boundaries at low magnifications. The methods are complementary.

A Microstructural Study of Reaction-Bonded Silicon Carbide

Interfaces in Reaction Bonded Silicon Carbide (RBSC) have been characterized by Analytical and High Resolution Electron Microscopy. Both Si/SiC and SiC/SiC interfaces were free of any oxygen impurity segregation, but contained metallic impurity precipitates. Oxygen was detected in the second phase particles in the SiC grains. A model is presented to explain the evolution of these second phase particles in the SiC grains.