Energy &Fuels 1994,8, 1296-1303
1296
Electrical Resistivity of Carbonized Coals Anna Marzec,* Sylwia Czajkowska, and Jan Moszynski Institute of Coal Chemistry, Polish Academy of Sciences, Sowinskiego 5, 44-102 Gliwice, Poland Received May 13, 1994@
The aim of the study was (i) to determine the relationship between the end temperature of carbonization (ETC) and electrical resistivity of the carbonized coals and (ii) t o find a minimal ETC which is satisfactory with regard to yielding low-resistivity ( ~ 0 . 2 ohdcm) 5 coke rods that could be used for fullerene preparation. Twenty-seven coals, 82-92% C daf, were studied. Coal samples were heated to various ETC in the 650-850 "C range. Electrical resistivities of the resultant coke rods were measured exclusively at ambient temperature. Ultimate and proximate data as well as content of optically anisotropic phase were determined for selected coke rods. A rapid falloff of the resistivity, from lo3 to 1-3.6 ohdcm, was found on increase of ETC from 650 to 750 "C. Further ETC increase to 850 "C led t o a moderate resistivity decrease to 0.1-0.3 o h d c m level. A formula was worked out that defined the relationship between concentration of electrical current carriers in the coke rods and the end temperature of carbonization. The Arrhenius thermal coefficients for various coals were 480-740 kJ/mol and 200-290 kJ/mol for ETC 1750 "C and ETC '750 "C, respectively. Resistivities of the cokes and individual aromatic compounds were compared. This led to a conclusion that a network of cokebasic aromatic units plays an important role in generating electrical current carriers.
Introduction So far, fullerenes have been prepared in macroscopic quantities from the electrical resistivity heating or arcing of graphite.lP2 Recently, the preparation of fullerenes from cokes was r e p ~ r t e d . In ~ , these ~ studies, coals or carbonized coal-pitch composites were converted to conductive rods by heating between 1010 and 1300 "C. The rods showed electrical resistivity in the 0.1-0.5 o h d c m range and were successfully used instead of graphite. Our preliminary experiments (W. Kraetschmer and A. Marzec, unpublished data, 1993) have shown that rods obtained by heating a coal at 850 "C could also be used for fullerene preparation. This result indicated there was a possibility of forming rods that have adequate electrical conductivity for fullerene production, by heating a coal at lower temperatures compared with the temperatures that are generally required for coking (950-1100 "C) or graphite production (above 2300 "C). Thus, the discovery of fullerenes and the methods of their preparation renewed interest in electrical resistivity of coal carbonization products. This issue is the subject of our paper. Published data on electrical properties of coals carbonized below 1000 0C5-s show a general trend: an extremely sharp decrease of electrical resistivity with Abstract published in Advance ACS Abstracts, October 1, 1994. (1)Kraetschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990,347, 354-358. (2)Hammond, G. S.,Kuck, V. J.,Eds. Fullerenes; American Chemical Society; Washington, DC, 1992. (3)Pang, L. S.K.; Vassallo,A. M.; Wilson, M. A. Nature 1991,352, 480. (4)Pang, L. S.K.; Vassallo, A. M.; Wilson, M. A. Energy Fuels 1992, 6, 176-179. (5) Sandor, J. Proc. Conf Ultrafine Struct. Coals Cokes, B.C.U.R.A. 1944, 342-350. (6) Pinnick, H. T. Proceedings of the First and Second Conferences on Carbon; University of Buffalo: Buffalo, NY,1956;pp 3-11. (7)Kroeger, C.; Dobmaier, N. Brennstof-Chem. 1959, 40, 1-10, ( 8 ) Duba, A. G. Fuel 1977, 56, 441-443. @
0887-0624l94l2508-1296$04.50/0
increasing temperature of carbonization. The resistivity of materials prepared by heating at 600 "C was about lo5 ohdcm and decreased to loF2ohdcm range when the carbonization was carried out to 1000 "C.6 Further temperature increase up to 3000 "C led only to a moderate decrease of resistivity to o h d c m range. Kroeger and Dobmaier' prepared a series of cokes by heating a coal (28%VM) at various temperatures in the 500-800 "C range. The resistivity decreased from lo9 t o about lo2 ohdcm. Dubas measured electrical conductivities of Wyodak coal and its carbonization products (the aim was to study a possibility of electrical monitoring of reaction zone during underground gasification). He found that the conductivity increased by a factor of lo9 on carbonization temperature increase from 500 to 1000 "C (corresponds to a decrease of resistivity by a factor of lop9 since resistivity is an inverse of conductivity). All these indicated the same trend. However, one can hardly derive from them information on ETC as low as possible that is adequate for obtaining coke rods with electrical resistivity
