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A model has been developed to predict the microstructure of the carbon matrix deposited during forced flow-thermal gradient chemical vapor infiltration (FCVI). This method employs previous thermodynamic-microstructure and densification models to determine conditions throughout the preform as a function of time. The model was verified by comparison with samples prepared over a range of deposition...
We use atomic force microscopy (AFM) to characterize the structure of carbon black aggregates. From the images we determine the lateral sizes and the height of the aggregates and from this data we calculate the flatness index. The flatness indices reveal that aggregates generally exhibit anisotropy, in the form of a reduction of aggregate breadth, or flatness, in one direction. The flat sides tend...
Pyrolysis of acetylene over quartz plates coated with various metal catalysts resulted in the formation of all-carbon nanostructures which were observed directly by scanning electron microscopy (SEM). The nanotubes were studied in detail using transmission electron microscopy (TEM), and appear to grow as ultra-thin tubes with a central hollow core and considerable thickening due to secondary pyrolytic...
Mesoporous carbon xerogels were prepared from the sol-gel polymerization of resorcinol with formaldehyde (RF) followed by carbonization. The effect of the initial pH of the RF solution on the surface area, pore volume, pore size distribution and nanostructure was studied. A brief mechanism of polymerization is discussed. Typically, a lower initial pH yielded carbon xerogels with a higher surface...
Piezoresistance effect in different carbon and graphite fibers was examined. PAN-based carbon fibers laboratory prepared at various temperatures and pitch-based carbon and graphite fibers have been analyzed. Graphite fibers were intercalated with metal chlorides. The resistance changes during tensile loading were measured. The piezoresistance coefficient and coefficient of strain sensitivity were...
The microtextural characteristics of coke produced from raw as well as from their beneficiated/demineralised counterparts both under normal and elevated pressures (22 mPa) at two temperature levels (500 to 1000°C) were studied on two caking/coking and one non-caking coals. An account with emphasis on the development of well-ordered carbonforms under the influence of hydrothermal treatment as well...
Post spinning modifications play an important role in improving the mechanical properties, density and structure of PAN fibres. Post spinning modifications affect the thermal and kinetic parameters during stabilization and alter the overall stabilization mechanism. The present paper presents a review of the different types of post spinning modifications previously attempted on PAN fibres and the...
The mesoscopic textures of the mesophase pitch and its spun fiber were examined in the as-prepared and solvent extracted forms using high resolution scanning electron microscopy (HR-SEM). HR-SEM revealed spherical or elliptical grains of 10-50 nm size. Such units were defined as microdomains in the insoluble fraction of the mesophase pitch. The solvent of high dissolving activity reduced the size...
Co-oligomerized mesophase pitch from naphthalene and methylnaphthalene at mixing ratio of 7:3 by aid of HFBF 3 was studied in terms of its structure, flow behavior and stabilization reactivity as well as microstructure and mechanical properties of the resultant fiber. The molecular structure of co-oligomerized mesophase pitch was rather similar to that of naphthalene pitch because of major...
We propose a new form of carbon in which the atoms are arranged in a self-similar fashion. The hypothetical structure we consider is two-dimensional, having a fractal dimension df = 1 + log 2 log 3 ≈ 1.63 which we call the Davidene. Our results for the ground state geometries of the aggregate as a function of its size L indicate that the fractal cluster is stable, displaying a small “gap” (≈ 0.16...
Several TEM techniques are used to characterise the local structure of low dimensional forms of carbon. HREM is particularly useful to describe the defect structure of thin films of diamond or fullerenes and C 60 –C 70 nanoclusters. A columnar form of graphite is analysed, mainly by electron diffraction which allowed us to propose a growth mechanism. Diffraction contrast dark field...
We have performed high dose carbon ion implantation into polycrystalline silver substrates at high temperature (500–600 °C). The formation of a high density of spherical carbon onions on the nearby silver substrate was characterized by atomic force microscopy experiments and transmission electron microscopy. The influence of the implantation temperature as well as that of silver grain sizes and crystallographic...
Three-dimensional carbon/silicon carbide composites were fabricated by chemical vapor infiltration, and the microstructure and mechanical properties were investigated. For the composites (C/SiC) with no pyrolytic carbon interfacial layer, the mechanical properties (flexural strength, flexural elastic modulus, shear strength and fracture toughness) are increased with density of the composites. High...
High resolution electron microscopy (HREM) reveals that carbon nanotubes obtained by heating a polyethylene precursor in the presence of catalytic Ni particles have a structure consisting of stacked conical carbon layers. Two main types of nanotube structures were identified. When the conical angle along the tubes is in the range 16-35°, bamboo -shaped nanotubes are observed, characterized by...
The pyrolysis of four pitches of increasing quinoline insolubles (QI) content (0.5-11 wt%), prepared by blending, in different proportions, a commercial binder pitch with varying amounts of the same filtered pitch (almost QI free), was studied by hot stage microscopy and thermogravimetric analysis. Pyrolysis products of the pitches, obtained at 450°C for 30 minutes, were examined by optical microscopy...
A zirconium carbide (ZrC) single crytal was oxidized isothermally at temperatures of 600-1500°C in a mixed atmosphere of oxygen and argon with an oxygen partial pressure (P o2 ) of 0.02-2 kPa for 1-20 hours. Preferred orientation of tetragonal or monoclincal ZrO 2 (t- and m-ZrO 2 ) was occasionally observed on the surface of the oxidized crystal. The oxidized crystal...
Pyrolysis of hydrocarbon compounds on evaporated metal-based catalysts was used to produce carbon shells, hollow fibers and other hollow carbon forms. The carbon structures were investigated using transmission electron microscopy. A kinetic model for their formation and the mechanisms for obtaining different types of hollow carbons are proposed. In this model two competing processes, the linear...
The mechanical properties of flexible graphite sheets made by exfoliated graphite flakes have been investigated in term of the density changes and the flake size variation. The fracture toughness of flexible graphite sheets has been characterized using the concept of essential fracture work (w e ). The experimental results indicate that both tensile strength and the fracture toughness monotonically...
A detailed study of the interfacial microstructures of the intrabundles in as-receivedcarbon/carbon composites prepared by CVI was performed by high-resolution electron microscopy. Two kinds of carbon fibers, high strength carbon fiber T300 and high modulus carbon fiber T-50, were chosen for comparison. It was found that the interface between the carbon fiber and carbon matrix is not simply a mechanical...
The structure of a series of activated carbon materials prepared from a polymer of phenol formaldehyde resin with different degree of “burn-off” from 0 to 32% and variable porosity has been studied by wide-angle neutron scattering. The intensity measurements, extended to Q values of 50Å −1 yielded the radial distribution functions with the real space resolution of about 0.12Å. The obtained...
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