S. Duber

On the optically biaxial character and heterogeneity of anthracites

Abstract The results of the study of optical properties of 13 anthracites from different parts of the world are presented in this paper. Measurements of reflectance values were made on non-oriented vitrinite grains for a minimum of 300 points per sample. The reconstruction of Reflectance Indicating Surfaces (RIS) were made by Kilbys method [Kilby, W.E., 1988. Recognition of vitrinite with non-uniaxial negative reflectance characteristics. Int. J. Coal Geol. 9, 267–285; Kilby, W.E., 1991. Vitrinite reflectance measurement — some technique enhancements and relationships. Int. J. Coal Geol. 19, 201–218]. It was found that the use of Kilbys method for strongly anisotropic materials like anthracites did not give unambiguous results. Some improvement in Kilbys method, consisting of the division of the cumulative cross-plot into several elemental components, is suggested. Each elemental cross-plot corresponds to a textural class of anthracite, which is characterized by the values of RIS main axes RMAX(k), RINT(k) and RMIN(k) (k=1,2,…n; n — number of classes). The global texture of anthracite is characterized as a RIS with main axes calculated as the weighted means of R MAX , R INT and R MIN for each class of this anthracite. The division of cumulative Kilbys cross-plot on elemental components makes possible the calculation of new coefficients Ht and H10 characterizing the heterogeneity of the structure and texture of anthracites. The results of our study show that all anthracites have biaxial negative textures, but their heterogeneity varies in a wide range of Ht and H10 coefficients depending upon the individual coal basin.

Textural transformation of thermally treated anthracites

Abstract The organic matter of anthracites consists of turbostratic (two-dimensional crystalline order) stacks of three to five aromatic layers called the Basic Structural Units (BSU). The arrangement of BSU forms the texture of anthracites, which corresponds to their chemical and physical properties including optical properties (reflectance values). Thermal treatment leads to the changes in the arrangement of BSU. The character of structural and textural transformation of anthracites during heating can be determined based on the changes of reflectance values of anthracites. Several anthracites of various ranks were thermally treated over the temperature range of 400 (673 K)–1000 °C (1273 K) in an inert atmosphere (N2), at atmospheric pressure. The reflectance indicating surfaces (RIS) were reconstructed and reflectance indicatrix parameters were calculated according to the Kilby method specially modified for the study of anthracites. It was found that the relative increase of reflectance values (Rmax, Rmin) was greater for the lower metamorphosed samples than for the higher ones. However, the higher rank of initial samples, the greater their final reflectance values and the anisotropy of the texture attained at 1000 °C. It indicates that the texture of the higher metamorphosed anthracites (meta-anthracites) is more flexible for rearrangement during heating, than the texture of the lower metamorphosed anthracites. Moreover, the difference in the changes of reflectance values versus temperature for the higher and for the lower rank anthracites suggests that meta-anthracites have undergone stronger natural metamorphism equivalent to about 200 °C in comparison with the other anthracites. It could be said that the character of textural transformations of individual anthracites results from the degree of their metamorphism and, in a consequence, from the texture of parent sediments.

The study of textural and structural transformations of carbonized anthracites

Abstract Specific physical and chemical properties of anthracites may be explained by the differences in chemical structure of basic structural units (BSUs) and in various spatial arrangements of BSUs, that is, the texture. The structure and the texture of anthracites correspond with their optical properties. Thermal treatment causes the changes in the structure of BSUs and rearrangement of the texture of anthracites. The character of structural and textural transformation could be determined based on the reflectance values of anthracites. Various anthracites were thermally treated to the temperature of 1800 °C. Maximum ( R max ) and minimum ( R min ) reflectance values were measured and reflectance indicatrix parameters ( R ev , R st and R am ) were calculated for initial and heated samples. It was found that R max , R min and R ev values increase together with the temperature up to 1200–1400 °C. In higher temperatures, they start to decrease, while R am parameter depending on anisotropy of the texture still increases to the final temperature of 1800 °C. That indicates gradual and soft transformation of the structure and the texture of anthracites up to the temperature 1200–1400 °C (arrangement of turbostratic structure) and some disturbances in textural order before the stage of graphitization.

Microporosity and optical properties of some activated chars

Abstract The aim of this work was to understand the structural and microtextural changes responsible for microporosity formation in saccharose-based chars activated with CO 2 at 850 °C. High-resolution transmission electron microscopy (HRTEM) and optical microscopy (reflectance measurements) allow to follow the changes in the chars organisation from nanometric to micrometric scales. Quantitative structural and microtextural data can now be extracted from the HRTEM images with an in-house image analysis procedure. Our results allow to suggest that the development of the microporosity with an increasing burn-off could be due to the development of slit-shaped pores. Such changes in pore shape and dimensions affect optical properties; a relationship was established between BET surface area and mean reflectance. By coupling these experimental data and a theoretical approach, a model of activated carbons is proposed to explain the reflectance changes during activation.

Radial distribution function analysis of the graphitization process in carbon materials

X-ray scattering studies of the graphitization process of saccharose- and anthracene-based carbons are reported. The scattered intensities were recorded in the scattering vector range from 0.6 to 24 A−1, yielding a radial distribution function of good quality. Analysis of the first three peaks of the radial distribution function allowed us to establish a hexagonal structure within a single layer. In the early stage of preparation, both carbons exhibit a disordered structure in which graphite layers are stacked without spatial correlation, i.e. the turbostatic structure. The inter-layer spacings were estimated to be in the range from 3.6 to 3.37 A. At higher temperatures the graphite-like layers exhibit an increasing ordering tendency within a single layer and in the c-axis. However, the degree of ordering is much higher for the anthracene-based carbons than the carbons prepared from saccharose, which remain disordered even at 2300°C.

Raman scattering studies of the graphitization process in anthracene- and saccharose-based carbons

Abstract We report Raman scattering studies of the graphitization process in a series of carbons produced by the pyrolysis of saccharose and anthracene and then annealed at 1000, 1900 and 2300°C. Using 488 nm light, the first- and second-order Raman spectra of six samples have been examined in the range 50-4000cm−1. The low-frequency Raman range 50-lOOOcn−1 has been carefully examined using the laser excitation wavelengths 457.2, 476.5, 488.0 and 514.3 nm. A dependence of the Raman spectra in the first- and second-order ranges on annealing temperature and degree and size of ordered domains has been observed and correlated with neutron diffraction observations, indicating almost complete graphitization of the carbon prepared from anthracene and a disordered structure of the saccharose carbon at 2300°C. The occurrence of a sharp peak at about 464cm−1 and broadened peaks in the range 75-210cm−1, which shift with excitation energy as result of the one-dimensional auantum confinement effect of electrons in such structures, provide evidence for the presence of fullerene- and nanotube-like elements in the investigated materials. The presence of curved elements with odd-membered rings at an early stage of preparation and their sizeable increase at higher temperatures distinguish non-graphitizing from graphitizing carbons.

E.p.r. study of reduced and reductively ethylated coals

Abstract Two coals (78 and 85 wt % C) were reduced with potassium in tetrahydrofuran in the presence of naphthalene and also ethylated with ethyl iodide. The e.p.r. spectra of the coals and products consisted of two components: a narrow and a broad line. Both reduction and ethylation led to a decrease in concentration of paramagnetic centres; quenching was greater in centres related to the broad line component. A mechanism for the quenching is suggested.

Excited triplet states in bituminous coal

The intensities of the broad and narrow lines constituting the e.p.r. spectrum of coal depend on the temperature of measurement. The system of paramagnetic centres related to the broad line does not obey the Curie law and consists of free radicals (S = 12) and of centres with singlet ground state (S = 0) and lowest-excited triplet state (S = 1). The singlet-triplet excitation energy, J, is comparable with thermal energy, kT, of lattice vibrations. The system of paramagnetic centres related to the narrow line obeys the Curie law (S = 12).

Effects of organic solvents on the EPR spectrum of coal

Abstract EPR studies have been carried out on coal and the products of its extraction with ethylenediamine, pyridine and tetrahydrofuran. The spectrum of the coal and the residue after extraction consisted of two lines. In the case of the extraction residue the width of the narrow line remains practically unchanged compared to the initial coal whereas that of the broad component decreases. The extracts exhibit a single line with a width close to that of the broad component from the extraction residue. The concentration of paramagnetic centres in the residue and extracts is lower than in the coal. The decrease in concentration is higher when the extraction yield obtained with the solvent is higher. Quenching of paramagnetic centres is found to be related to the destruction of intermolecular bonds of the donor-acceptor type in the coal.

EPR Study of Excited Triplet States and Interactions between the Molecular Phases in Coal

EPR studies are performed for coal and its products of extraction with tetrahydrofurane /THF/, pyridine /Py/ and ethylenediamine /EtDA/. The spectra of the coal and the residues after extraction consisted of two lines, the one broad and the other narrow. Their intensities depend differently on the temperature of measurement. The system of paramagnetic centers related with the narrow line obeys the Curie law /S = 1/2 /, whereas that related with the broad line does not, and consists of free radicals /S = 1/2/and of centers with singlet ground state /S = 0/ and lowest excited triplet state /S = 1/. Quenching of paramagnetic centers during extraction is found to be related with the destruction of inter-molecular bonds of the electron donor-acceptor type between the two molecular phases in coal.