Propriedades opticas de carvões minerais e grafites

AUTOR(ES)
DATA DE PUBLICAÇÃO

1981

RESUMO

Coals from southern Brazil with high mineral matter content were studied with infrared absorption spectroscopy and reflectivity measurements. The results of Raman and photoluminescence experiments are also reported. Reference measurements were performed on coals with low mineral content and graphite?s (which are structurally similar to the coals with the highest degree of coalification). Coal samples were also heat treated to several temperatures up to 2000 ºC. Special attention was given to the influence of mineral matter in the interpretation of experimental results. Infrared spectroscopy was applied to determinations of molecular structure and composition of different coal samples. Between 600cm-1 and 1300cm-1 an intense band absorption structure was visible for the high mineral matter samples. These bands were used for ?in situ? identification and quantitative analysis of mineral components. Heat treatment induced drastic changes in the absorption bands which were related to pyrolysis and graphitzation processes of organic phases as well as with phase transitions of some of the minerals. It was developed a theoretical and experimental framework which allowed the determination of the index of refraction and the extinction coefficient, it is applicable to coals and also to highly absorbent materials. The method is based upon: i) measurements at the angle for wich the intensity of the inplane polarized reflected beam is minimum, and, ii) the function relating this angle with N = n + ik. This methodology was applied to natural and heat treated coals and also to several kinds of graphite?s. The results for natural coals were related to the samples carbon content and to their degree of aromaticity. Results for the heat treated samples were related to the growth of graphitic structures. The analysis of 1st and 2nd order Raman scattering spectra of graphite?s showed features detected for the first time. Several peaks of polycrystalline graphite were attributed to the relaxation of selection rules provocated by loss of symmetry induced by the finite size of graphitic micro-crystals. The polycrystalline graphite showed a wide luminescence band peaked at 2.3 eV, the effect being absent in the single crystals. Mineral coal and polycrystalline graphite showed anti-Stokes luminescence. Raman and luminescence spectra were almost identical for all coal samples in spite of different compositions and origins, which may indicate that these phenomena are originated in the aromatic matrix, which is the structural feature common to all coals. From the comparison of Raman and luminescence spectra of coals and polycrystalline graphite it may be concluded that the carbonaceous matrix originating these optical properties behaves in this respect similarly to polycrystalline graphite. The optical properties reported here for the heat treated samples were sensitive to the development of graphitic structures, which is inhibited before 1500 ºC in the Brazilian coals due to the high mineral matter content

ASSUNTO(S)

carvão - propriedades oticas grafita - propriedades oticas

ACESSO AO ARTIGO

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