A. Oberlin

FILAMENTOUS GROWTH OF CARBON THROUGH BENZENE DECOMPOSITION

Abstract Carbon fibres have been prepared by pyrolysing a mixture of benzene and hydrogen at about 1100°C. They have been studied by high resolution electron microscopy. These fibres have various external shapes and contain a hollow tube with a diameter ranging from 20 to more than 500 A along the fibre axis. They consist of turbostratic stacks of carbon layers, parallel to the fibre axis, and arranged in concentric sheets like the “annual ring structure of a tree”. These fibres have two textures resulting from different growth processes; core regions, made of long, straight and parallel carbon layers, are primarily formed by catalytic effect; the external regions correspond to a pyrolytic deposit occuring during the secondary thickening growth process. Very small cementite crystals, typically about 100 A in a diameter, have been identified by dark-field techniques at the tip of the central tube of each fibre. A model of fibre growth related to a surface diffusion of carbon species on the catalyst particle has been established.

Carbonization and graphitization

Abstract A review is made of recent electron microscope observations relating to the carbonization and graphitization of a variety of carbonaceous precursors. The different behaviors of graphitizing and non-graphitizing carbons are elucidated, and the effect of sulphur as a cross-linker is determined. The resulting processes are shown to apply to a wide variety of materials ranging from cokes to carbon fibers.

Structure, microtexture, and optical properties of anthracene and saccharose-based carbons

Abstract Graphitizing anthracene-based (AC) and nongraphitizing saccharose-based carbons (SC) were heat treated up to 2900°C. Their structure and microtexture were studied by TEM and compared with some of their properties. The data obtained show that graphitization of AC is prepared by an annealing of the aromatic layer distortions occurring in four stages. In the first stage, elemental basic structural units (BSU) ( 1 μm). However, BSU are separated by tilt and twist boundaries where defects and heteroatoms are gathered. Stage 2 begins at the end of heteroatom release. BSU coalesce face to face into distorted columns; the lateral coalescence is inhibited by misoriented single BSU. This stage ends when misoriented BSU disappear. Stage 3 corresponds to lateral coalescence of columns into distorted continuous layer stacks. Stage 4 begins when all in-plane defects are wiped out, entirely annealing the distortions. Graphitization occurs from the beginning of stage 4. The end of stage 1 corresponds to a reflectance equal to that of graphite and to an electrical resistivity almost at its minimum. The end of stage 2 corresponds to a fast increase of diamagnetic susceptibility. The beginning of stage 4 is the plateau of the diamagnetic susceptibility, the maximum of Hall effect, the minimum of the magnetoresistance, and the final decrease of resistivity. SC follows the same stages as AC, even though SC are nongraphitizing carbons. This is due to the very small extent of LMO (5–10 nm).

Morphology and crystalline order in catalytic carbons

Abstract The influence of temperature and of catalyst composition on the morphology and crystalline order has been studied for a series of catalytic carbons prepared by carbon monoxide disproportionation (2CO→CO2+C) over an iron-cobalt, an iron-nickel, and a nickel catalyst, and by methane decomposition (CH4→2H2+C) over a nickel catalyst. The morphology was studied by TEM in a bright field mode. The products are filamentous when prepared at low temperature and granular when prepared at high temperature. The ranges of temperature corresponding to each morphology depend both on the alloy composition and on the reacting gas. The structure of the catalytic carbons, as studied by TEM in SAD mode and by X-ray diffraction, is turbostratic (2D, i.e. biperiodic) when the carbons are prepared at low temperature and crystalline (3D, i.e. triperiodic) when prepared at high temperature. When the particle size is large, the temperature of transition is about 500°C, whatever the composition of the alloy and of the reacting gas are. However, small particles are always 2D. The carbon layers, as observed by TEM in lattice fringe mode, are short and curved for turbostratic, and large and stiff for crystalline phase. Biperiodic products are not graphitizable.

Crystallographic orientations of catalytic particles in filamentous carbon; Case of simple conical particles

Abstract Filamentous catalytic carbons in the form of simple tubes with conical metal particles at one end have been selectively prepared by CO disproportionation (2 CO → CO 2 + C) catalysed by iron-cobalt and iron-nickel alloys of various compositions and structures: 6 iron-cobalt alloys with the bcc structure, 1 fcc iron-cobalt and 4 fcc iron-nickel. The crystallographic relations between the metal particles and their carbon tubes have been determined by CTEM. In the case of all the metal composites prepared from alloys of bcc structure, the metal particle is a single crystal with a [100] axis parallel to the axis of the carbon tube, and the basal faces of the truncated cone, which appear free of carbon, are (100) faces. In the case of all the metal composites prepared from alloys of fcc structure (FeCo and FeNi), the metal particles are not always single crystals as they present sometimes (111) twin planes. However, there is always a [110] axis parallel to the axis of the carbon tube and the carbon-free basal planes of the cones are always (111) faces.

A possible mechanism for natural graphite formation

Abstract By extrapolating the Arrhenius plots for carbonization and experimental thermal progressive graphitization, it is shown that carbonization can go to completion in nature (ΔH ≈ 65 kcal/mole), whereas progressive graphitization is thermodynamically improbable (ΔH ≈ 260 kcal/mole). The mechanism of formation of natural graphite has thus to be determined. Since the geothermal gradient is not strong enough for producing graphite, the existence of shear stresses has to be taken into account. Metamorphism and tectonics create suitable conditions for this transformation. Series of samples of increasing rank from anthracites to metaanthracites, semigraphite and graphite (some of them from the same parent rocks) were compared with carbon, heat-treated experimentally under pressure (5 kbar). Anthracites are microporous materials. Their pores are flattened parallel to the bedding by a pressure effect which is responsible for a long-range statistical preferred orientation. They are anisotropic in texture but only biperiodically crystallized (turbostratic). Metaanthracites differ from anthracites only by an increasing coalescence between adjacent pores. They are thus either mesoporous or even macroporous. They are still turbostratic. Semi-graphites are suddenly obtained as a new phase by an increase in temperature, pressure and shear stresses. They are formed by single macropores, i.e. hollow distorted polyhedral shells. They are partially graphitized. Graphite is suddenly produced by a second phase change also due to an increase in temperature, pressure and shear stresses. The lamellar shape represents the limit of a flattened macropore.

Characterization of organic matter associated with uranium deposits in the Francevillian formation of Gabon (lower proterozoic)

Abstract Elemental analysis, organic petrography, and high-resolution transmission electron microscopy were used to study organic matter in Lower Proterozoic rocks of the Francevillian Series in Africa. Results show a convincing relationship between solid bitumens derived from thermal alteration of crude oil, and deposition of uraninite ores. Evidence is presented that suggests the presence of migration paths for crude oil in associated sandstones. Moreover, the solid bitumens appear to have been further altered by radiation damage as a consequence of oxidation and uranium mineralization.

Heavy petroleum products: Microtexture and ability to graphitize

Abstract Various asphalts extracted from crude oils by light n-alcanes were studied by elemental analysis, conventional transmission microscopy and X-ray diffraction. Like most carbonaceous materials, asphalts contain planar aromatic structures (4–12 rings in dia.) single or piled up by twos or threes (basic structural units). These units are distributed at random in the bulk and linked by non-aromatic groups. Heat-treatment was carried out under inert gas flow from room temperature to 3000°C. In the early stages of carbonization, the structural units acquire a local parallel orientation (just before the end of the tar release). The extent of the molecular orientation depends only on the opposing influence of the hydrogens and of heteroatoms such as oxygen, sulfur and nitrogen. The extent of orientation increases drastically from less than 50 A to a few μm as the percentage of cross-linking agents (oxygen and sulfur) and of nitrogen decreases. At the same time the ability to graphitize increases progressively. All kinds of intermediates between non-graphitizing and graphitizing carbons are found after heat-treatment.

Oxidation of carbonaceous matter—I: Elemental analysis (C, H, O) and IR spectrometry

Abstract Products of various elemental composition ( 1.56 ⪢ ( H C ) at ⪢ 0.45 and 0 O C ) at H C vs O C ), their elemental composition follows an oxidation path, the slope of which depends only on the original ( O H ) atomic ratio and on the oxidizing temperature. Oxidation reactions have an “apparent activation energy” of 20–40 kcal/mole. Cross-linking may be due either to ether bonding or to hydrogen bonding as indicated by IR spectrometry. The final product, named “oxychar”, has a constant composition ( H C ∼ - O C ∼- 0.5 ).

Graphitization of Korean anthracites as studied by transmission electron microscopy and X-ray diffraction

Abstract The different modes of high-resolution transmission electron microscopy (TEM) were applied to Korean anthracites, semi-graphites and graphites. Whereas X-ray diffraction data yield averaged values, TEM is the only technique able to bring out the heterogeneity of phases which are different in morphology and in microtexture. The thermal behavior of samples was studied using laboratory heat-treatments up to 2800°C. In heat-treated anthracites for example, an increasing degree of graphitization results in phase changes which can be quantified by TEM. The study of a larger sampling appears however necessary to relate crystallographic variations to geological data.