Energy & Fuels 2004, 18, 365-370
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Structural Characterization of Graphite Materials Prepared from Anthracites of Different Characteristics: A Comparative Analysis David Gonza´lez, Miguel A. Montes-Mora´n, Isabel Sua´rez-Ruiz, and Ana B. Garcia1 Instituto Nacional del Carbo´ n, CSIC, Francisco Pintado Fe 26, 33011-Oviedo, Spain Received July 18, 2003
Graphite materials were prepared from two Spanish anthracites, AF and ATO, by heating at different temperatures within the range 2000-2800 °C. XRD and Raman spectroscopy were employed to characterize the degrees of crystallinity and crystal orientation of the materials. In addition to studying the evolution of typical crystal parameters such as interlayer spacing, d002, and crystallite sizes, La and Lc, with temperature, this work aimed to evaluate the influence of elemental composition, texture (as measured by optical microscopy), and mineral matter of the raw anthracites on their ability to graphitize. Two temperature segments were discerned during the development of crystallinity. The first segment exhibited major improvements in crystal parameters, which afterward reached a plateau value. Raman parameters indicated that further improvement in crystal orientation could be obtained after heating at the highest temperature (2800 °C). The limiting temperature at which the materials showed their highest degree of structural order, i.e., the temperature at which the plateau was reached, was lower for the most graphitizable anthracite (AF). This anthracite was found to have higher hydrogen and mineral matter (specifically Al, Fe, K, and Si) contents. However, the textural anisotropy of this most graphitizable anthracite was lower than that of the other anthracite under study (ATO). Optical microscopy characterization of the carbonized materials showed that this trend changed after heating the anthracites at 1000 °C, i.e., the anisotropy of the texture in the carbonized AF was higher than that of the corresponding carbonized material prepared from ATO. It was concluded that the structural and textural changes of the anthracites during carbonization, which are related with both their microtexture and hydrogen content, influence the graphitization process.
Introduction Synthetic graphite is a highly valuable material with many applications,1,2 its manufacturing process involving the selection of carbon materials (precursors) that graphitize readily.1 Currently, petroleum coke of various grades is used as the main filler material in the manufacturing of synthetic graphite. Coal, with known world reserves more than five times higher than that of oil,3 appears as a possible precursor of different carbon materials. Among the different classes of coals, anthracites were found to graphitize when heated at temperatures above 2000 °C.4,5 Anthracites carbon content is over 90%, which is arranged in a macromolecular structure of condensed aromatics rings forming large units (graphene layers) bridged or “cross-linked” by aliphatic and/or ether groups,6 conferring on them a * Corresponding author. Tel: +34 98 511 89 54. Fax: +34 98 529 76 62. E-mail:
[email protected]. (1) Pierson H. O. Handbook of carbon, graphite, diamonds and fullerenes; Noyes: Park Ridge, NJ, 1993; pp 87-121. (2) Kalyoncu, R. S. Graphite; U.S. Geological Survey Minerals Yearbook, U. S. Department of Interior: Reston, VA, 2001; p 314. In U.S. Geological Survey Minerals Yearbook, U.S. Department of Interior, Reston, VA, 2001: pp 341-343. (3) Schobert, H. H.; Song, C. Fuel 2002, 81, 15-32. (4) Franklin, R. E. Proc. R. Soc. 1951, 209, 196-218. (5) Oberlin, A.; Terriere, G. Carbon 1975, 13, 367-376.
certain structural order. The removal of the “cross-links” by heating the anthracite at high temperatures should facilitate the reorganization of the aromatic units into graphite-like structures. Conversion of anthracites into graphite materials offers a potential alternative to petroleum coke. Moreover, graphite materials with an acceptable degree of structural order were obtained from anthracites at temperatures g2700 °C.5,7,8 The most significant changes in the structure of the graphite materials prepared from anthracites were found to occur in the temperature range 2000-2700 °C. No significant improvements of the interlayer spacing were observed at higher graphitization temperatures, thus suggesting that a plateau may be reached at some temperature below 2700 °C.7 For potential use of anthracites as precursors of graphite materials, the knowledge of this limiting temperature at which the material prepared reaches its highest degree of crystallinity is more than interesting. To determine this temperature, a step-by-step study of the development of the structural order of the graphite materials in the (6) van Krevelen, D. W. Coal: Typology, Physics, Chemistry and Constitution; Elsevier: Amsterdam, 1993; pp 777-810. (7) Atria, J. V.; Rusinko, F., Jr.; Schobert, H. H. Energy Fuels 2002, 16, 1343-1347. (8) Bustin, R. M.; Rouzaud, J. N.; Ross, J. V. Carbon 1995, 33, 679691.
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above-mentioned temperature range should be done. In this research, Spanish anthracites of different characteristics were heated in the temperature interval 20002800 °C with the aim of studying the evolution with the temperature of the crystallinity and orientation of the materials obtained. While there are some data on the graphitization behavior of the anthracites at 2000 °C and the cited g2700 °C,5,7,8 to the authors’ knowledge, the information available at temperatures between those temperatures is very scarce and practically limited to experiments carried out at 2500 °C in which TEM was the main technique used to characterize the structural order of the materials.5,9,10 In addition to the treatment temperature, some characteristics of the anthracite influence the graphitization process. Among them, the coal mineral matter has been suggested to act as a graphitization catalyst.5,11 Thus, graphitized regions surrounding mineral particles were observed by HRTEM in heat-treated anthracites at temperatures as low as 1300 °C.11 Also a significant improvement of the structural order of the graphite materials by using anthracites with high mineral matter contents was found.12 These findings seem to indicate that the catalytic effect of the mineral matter is associated with both its amount and its distribution, a close proximity between the organic and the mineral matters of the anthracite improving that effect. The texture of the anthracites was also related to their ability to graphitize, specifically when there is preferential planar orientation of the polyaromatic basic structural units BSUs.5,13 Finally, more ordered graphite materials were prepared from anthracites with a higher H/C atomic ratio,7 inferring the influence of the elemental composition on the graphitization. Therefore, all these influencing factors should be taken into account to study the graphitization of the anthracites. On the basis of that consideration, the second objective of this research is to evaluate globally the influence of the elemental composition, texture as measured by optical microscopy, and mineral matter of the anthracite on its ability to graphitize. The interlayer spacing, d002, and crystallite sizes along the c axis, Lc, and the a axis, La, calculated from XRD, as well as the relative intensity of the D Raman band, ID/It, are used in this study to assess the degree of the structural order of the graphite materials prepared. Both X-ray diffraction and Raman spectroscopy techniques have been employed extensively for the characterization of carbon materials.14-22 Given that the (9) Duber, S.; Rouzaud, J. N.; Beny, C.; Dumas, D. Extended Abstracts, 21st Biennal Carbon Conf. 1993, 316. (10) Deubergue, A.; Oberlin, A.; Oh, J. H.; Rouzaud, J. N. Carbon 1987, 8, 375. (11) Evans, E. L.; Jenkins, J. L.; Thomas, J. M. Carbon 1972, 10, 637. (12) Zeng, S. M.; Rusinko, F.; Schobert, H. H. In Producing highquality carbon and/or graphite materials from anthracites by catalytic graphitization; Commonwealth of Pennsylvania, Pennsylvania Energy Development Authority, Harrisburg, PA, Final Technical Report (Grant 9303-4019), 1996. (13) Blanche, C.; Rouzaud, J. N.; Dumas, D. Extended Abstr. 22nd Biennal Carbon Conf. 1995, 694. (14) Kajiura, K.; Tanabe, Y.; Yasuda, E. Carbon 1997, 34, 169. (15) Oberlin, A. Carbon 1984, 22, 521. (16) Cuesta, A.; Dhamelincourt, P.; Laureyns, J.; Martı´nez-Alonso, A.; Tasco´n, J. M. D. J. Mater. Chem. 1998, 8, 2875. (17) Franklin, R. E. Acta Crystallogr. 1951, 4, 253. (18) Waldek Zerda, T.; Gruber, T. Rubber Chem. Technol. 2000, 73, 284.
Gonza´ lez et al. Table 1. Proximate and Elemental Analyses and Sulfur Forms of AF and ATO Anthracites proximate analyses (wt % db) ash volatile matter elemental analyses (wt % daf) carbon hydrogen nitrogen organic sulfur oxygen (diff.) sulfur forms (wt % db) total pyrite sulfate organic (diff.)
ATO
AF
10.12 4.12
19.74 8.72
93.13 2.03 0.87 1.01 2.96
91.00 3.01 1.40 0.92 3.67
1.07 0.15 0.01 0.91
1.08 0.33 0.02 0.74
Table 2. Concentrations of Al, Fe, K, and Si in AF and ATO Anthracites (wt % of coal organic matter) anthracite
Al
Fe
K
Si
AF ATO
2.70 1.48
2.32 0.57
0.50 0.26
5.03 2.85
presence of imperfections in the graphite materials leads to lower densities, helium densities were also measured to estimate the structural characteristics of these materials.23 Experimental Section Anthracites: Selection and Characterization. Two anthracites, denoted AF and ATO, from Villablino in northwest Spain were selected for this research. Their proximate and elemental analyses are reported in Table 1. Major inorganic elements Al, Fe, K, and Si present in the mineral matter of the anthracites were analyzed in the ashes by X-ray fluorescence spectrometry in a Siemens 3000 using fused glass disks. The ash sample (0.6 g) was fused with a mixture of lithium tetraborate and metaborate (6 g) to prepare the glass disks. The concentration of these inorganic elements, expressed as wt % of coal organic matter (100 - wt % ash) to better compare the results among the two anthracites, are given in Table 2. Graphitization. The anthracites at a particle size of e 212 µm were carbonized at 1000 °C in a tube furnace, under nitrogen flow, for 1 h with a heating rate of 2 °C min-1, and then graphitized. The graphitization experiments were carried out at 2000, 2200, 2300, 2400, 2500, 2600, 2700, and 2800 °C in a graphite furnace for 1 h under an argon flow. The heating rates were 20 °C min-1 from room temperature to 2000 °C, and 10 °C min-1 from 2000 °C to the prescribed temperature. Reflectance Measurements. The measurements were performed on particulate pellets with randomly oriented particles using a MPV-Combi (Leitz) microscope, reflected light, and oil immersion objectives (50×). The pellets were prepared by a modified procedure of the ISO 7404/02 standard. The statistical mean random reflectance (Ro) was obtained following the analytical method described in ISO 7404/5. The maximum and minimum apparent reflectances (R′max and R′min) were measured in polarized light and during a rotation of the microscope stage through 360°. For each sample, a minimum of 200 particles were measured. Based on these measured values, the principal axes of the reflectance indicat(19) Tunistra, F.; Koening, J. L. J. Chem. Phys. 1970, 53, 1126. (20) Lespade, P.; Marchand, A.; Couzi, M.; Cruege, F. Carbon 1984, 22, 375. (21) Cuesta, A.; Dhamelincourt, P.; Laureyns, J.; Martı´nez-Alonso, A.; Tasco´n, J. M. D. Carbon 1994, 32, 1523. (22) Montes-Mora´n, M. A.; Young, R. J. Carbon 2002, 40, 845. (23) Pierson, H. O. Handbook of carbon, graphite, diamonds and fullerenes; Noyes: Park Ridge, NJ, 1993; pp 43-69.
Characterization of Graphite Materials from Anthracites
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Table 3. Optical Parameters, Measured (%) and Calculated, of AF and ATO Anthracites and Their Carbonized (1000 °C) Samples AFC and ATOC measured values
RIS axes
RIS parameters
sample
Ro
R′max
R′min
RMAX
RINT
RMIN
Bwa
Rev
Rst
Ram
AF AFC ATO ATOC
2.37 7.17 3.63 7.01
2.65 7.31 4.22 7.65
2.10 5.47 3.19 6.10
2.82 7.67 4.70 7.90
2.55 7.05 3.93 7.43
1.62 3.70 2.40 4.94
1.21 3.97 2.30 2.97
2.26 5.84 3.53 6.61
-17.59 -22.19 -11.96 -21.40
0.900 0.115 0.105 0.078
a
Bw ) RMAX - RMIN.
ing surface RIS: RMAX, RMIN, and RINT, and the RIS parameters: Rev ) reflectance of equivalent volumen isotropic RIS, Rst ) RIS style, and Ram ) RIS anisotropy magnitude were calculated following the method described by Kilby 24 and Duber et al.25 In addition, BW (RMAX - RMIN) as a measure of the bireflectance was also calculated. The values of these optical parameters for the raw anthracites and their carbonized samples are shown in Table 3. X-ray Diffraction. The diffractograms of the samples were recorded in a Siemens D5000 powder diffractometer equipped with monochromatic Cu KR X-ray source and an internal standard of silicon powder. Diffraction data were collected by step scanning with a step size of 0.02° 2θ and a scan step time of 1 s. For each sample, five diffractograms were obtained, using a different representative batch of sample for each run. The interlayer spacing, d002, was evaluated from the position of the (002) peak applying Bragg’s equation. The crystallite sizes, Lc and La, were calculated from the (002) and (110) peaks, respectively, using the Scherrer formula, with values of K ) 0.9 for Lc and 1.84 for La.26 The broadening of diffraction peaks due to instrumental factors was corrected with the use of a silicon standard. Raman Spectroscopy. Raman spectra were obtained in a Renishaw 1000 System using the green line of an argon laser (λ ) 514.5 nm) as excitation source and equipped with a CCD camera. The 50× objective lens of an Olympus BH-2 optical microscope was used both to focus the laser beam (at a power of approx 25 mW) and to collect the scattered radiation. Extended scans from 3000 to 1000 cm-1 were performed to obtain the first- and second-order Raman bands of the samples, with typical exposure times of 30 s. To assess differences within samples, at least 21 measurements were performed on different particles of each individual sample. The intensity I, width W, and frequency ν of the bands were measured using a mixed Gaussian-Lorentzian curve-fitting procedure. Transmission Electron Microscopy (TEM). Transmission electron microscopy was performed using a Philips CM200 microscope working at 200 kV giving a resolution of 0.144 nm in lattice fringes. The 002 lattice fringes technique (002 LF) was used to image directly the profile of aromatic layers. Helium Density. Density measurements of the samples were made with a Helium Pycnometer Accupyc 1330 from Micromeritics.
Results and Discussion Effect of the Graphitization Temperature. The interlayer spacing, d002, and crystallite sizes, Lc and La, of AF and ATO anthracites after heat treatments are plotted versus temperature in Figures 1 and 2, respectively. Typical standard errors of crystallite sizes are less than 2% and 9% of the reported values for Lc and La, respectively. d002 values are much more precise, with standard errors lower than 0.1%. (24) Kilby, W. E. Int. J. Coal Geology 1988, 9, 267. (25) Duber, S.; Pusz, S.; Kwiecinska, B. K.; Rouzaud, J. N. Int. J. Coal Geol. 2000, 44, 227. (26) Biscoe, J.; Warren, B. J. Appl. Phys. 1942, 13, 364.
Figure 1. Values of the interlayer spacing, d002, of the graphite materials prepared from ATO and AF anthracites.
Two temperature segments may be distinguished in the evolution of the crystallinity of the graphite materials prepared from AF and ATO anthracites. Thus, as the graphitization temperature increases from 2000 °C to 2400 °C in AF and to 2500-2600 °C in ATO, the interlayer spacing, d002, decreases to reach plateau values of approximately 0.337 and 0.340 nm, respectively (Figure 1). In a parallel way, a growth of the crystallite sizes, particularly at temperatures above 2200 °C, is observed (Figure 2). A preferential lateral coalescence of the crystallites occurs in this temperature interval, as confirmed by the higher absolute growth of the La with regard to that of Lc. Basically, no significant improvements in the degree of crystallinity of the graphite materials prepared from AF and ATO anthracites were attained at temperatures above 2400 °C and 2500-2600 °C, respectively. As an example, crystallite sizes in the basal planes direction, La, of 25.3, 27.1, and 28.6 nm were calculated for the graphitized samples of ATO at 2500, 2600, and 2700 °C; the corresponding interlayer spacing, d002, being 0.3410, 0.3401, and 0.3404 nm. The variation with the temperature of the relative intensity of the D Raman band ID/It (It ) ID + IG + ID′) follows a similar tendency (Figure 3). Standard errors of this ratio are less than 6%. The drop of the D band relative intensity with the temperature indicates an improvement of the crystallite orientation of the graphite material.16,18-22 Similar to crystalline parameters, the ID/It Raman ratio remains almost unchanged during the second temperature segment. However, there are some slight differences between the information provided by XRD and Raman spectroscopy about the graphitization process of the anthracites. On the basis of Raman data, the second graphitization temperature zone of ATO anthracite clearly starts at 2500 °C with a
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Figure 4. Percentages of particles exhibiting the G′1, G′2 doublet in the second-order Raman spectrum, N*, of the graphite materials prepared from ATO and AF anthracites.
Figure 2. Crystallite sizes, Lc and La, of the graphite materials prepared from (a) AF and (b) ATO anthracites. Figure 5. Helium densities of the graphite materials prepared from ATO and AF anthracites.
Figure 3. Values of the relative intensity of the D Raman band, ID/It, calculated for the graphite materials prepared from ATO and AF anthracites.
plateau value of 18-19% for the ID/It ratio. Moreover, the graphitized sample from AF anthracite at the highest temperature (2800 °C) shows a further decrease of the ID/It ratio to 10.5% (Figure 3). Since the plateau value is 13-14% (Figure 3), the difference between both values is statistically significant, thus suggesting an improvement of the crystallite orientation of the material. Bands in the second order of the Raman spectrum are observable only in highly orientated carbons, the most
important feature being the G′ band occurring at approximately 2700 cm-1 that, in the case of graphitelike materials, tends to split into a doublet.27 Therefore, the percentage of measured particles exhibiting the doublet of the G′ band should constitute an indication of the degree of crystallite order for a given material. Those percentages are plotted in Figure 4 for the graphitized samples prepared from both anthracites. Practically, no particles with doublet of the G′ band were detected until AF and ATO anthracites were heated at 2300 and 2500 °C, respectively. Results of Figure 4 seem to confirm that, regardless of the crystallographic measurements, (i) the increase of graphitization temperature to 2800 °C improves slightly the structural order of the materials prepared from AF anthracite, and (ii) 2500 °C is the temperature at which ATO anthracite reaches its highest degree of structural order. Helium densities of the graphitized samples from AF and ATO anthracites tend to increase with the temperature of treatment (Figure 5), giving an indication that more graphite-like materials are being obtained. However, as in the evolution of the crystalline and Raman parameters (Figures 1-4), no significant improvements in the structural order of the materials produced from (27) Angoni, K. J. Mater. Sci. 1998, 33, 3693.
Characterization of Graphite Materials from Anthracites
AF and ATO anthracites, as indicated by the value of this physical property, were found at temperatures above 2400 °C and 2500 °C, respectively. According to TEM observations of graphitized anthracites5,10 and other hard carbon precursors such as polyimide films,28 polyaromatic basic structural units BSUs arrange around the walls of pores that would flatten progressively as the temperature increases. During this progressive flattening, the BSUs coalesce, leading to the appearance of imperfect flat lamellar areas which still retain pore walls normal to the basal plane. Therefore, the crystallinity degree of the material improves significantly as shown by the evolution of the XRD parameters during the graphitization of the mentioned polyimide films.28 The flattened pores collapse at a given temperature, allowing the stacking of the carbon layers in a graphite-like structure. From this temperature at which the interlayer spacing, d002, of the graphitized polyimide was close to the minimum one achieved, the value of this crystalline parameter was found to decrease gradually toward a plateau value.28,29 On the basis of the evolution of the crystallinity of the graphitized polyimide films with temperature, the flattened pores of AF and ATO anthracites would collapse at temperatures close to 2400 °C and 25002600 °C, respectively, at which the d002 plateau values of 0.337 and 0.340 nm were reached (Figure 1). The analysis of the XRD profiles showed that of the (10) band splits into (100) and (101) well-defined peaks, and an additional (112) peak appears in AF and ATO treated at temperatures g2400 °C, and g2500-2600 °C, respectively. These facts are associated with the development of a three-dimensional crystalline structure,17 thus confirming that the breakage of the pore walls occurs at these temperatures. Once the pores have collapsed and the carbon layers are stacked in a more or less graphitic structure, the progress of the graphitization of these anthracites is very limited, specifically in the case of AF (Figures 1-4). From this point on, the evolution of the crystallinity of the materials with the temperature depends on the activation energy needed to remove the crystal defects and to promote the coalescence of different groups of graphite-like crystallites along the c and a axis. According to the results of the present work, an increase in the temperature of treatment from 2400 to 2800 °C, such in the case of AF anthracite, is not enough to improve the crystallinity degree of the graphite material obtained (Figures 1-4). A plausible explanation for this behavior could be the presence of amorphous and/or low-organized carbon material between the crystallites. This type of carbon has been reported to prevent the coalescence of the crystallites by not favoring their complete orientation.30 In this sense, lamellar and amorphous carbon structures were observed by TEM to coexist even after heating AF anthracite at the highest temperature of 2800 °C (Figure 6). Nevertheless temperatures as high as 2800 °C seem to promote a better orientation of the (28) Inagaki, M.; Takeichi, T.; Hishiyama, Y.; Oberlin A. In Chemistry and Physics of Carbons; Thrower, P. A, Radovic, L. R., Eds.; Marcel Dekker: New York, 1999; Vol. 26, pp 245-333. (29) Hishiyama, Y.; Igarashi, K.; Kanaoka, I.; Fuji, H.; Kaneda, T.; Koidesawa, T.; Shimazawa, Y.; Yoshida, A. Carbon 1997, 35, 657. (30) Emmerich, F. G. Carbon 1995, 33, 1709.
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Figure 6. TEM lattice fringe imaging of AF anthracite graphitized at 2800 °C.
crystallites, as indicated by Raman measurements of ID/It ratio (Figure 3). Effect of Anthracite Characteristics. A comparative analysis of the graphitization results of the anthracites, shown in Figures 1 to 4, leads to the conclusion that materials with higher degree of crystallinity and orientation can be obtained from AF. Thus, d002 values of 0.3371 and 0.3404 nm were measured for samples of AF and ATO anthracites graphitized at 2700 °C, the corresponding Raman ID/It ratio being 13% and 19%, respectively. This significant difference in the degree of graphitizability can be explained attending to their different characteristics. Considering the elemental composition of the anthracites in Table 1, AF exhibits higher hydrogen content than ATO, the H/C atomic ratio being also higher. As the heat treatment of the anthracite proceeds, the heteroatomic (oxygen, nitrogen, and sulfur) cross-linkers are removed as their corresponding hydrogen compounds, “unlocking” the aromatic layers and, consequently, eliminating a significant obstacle to their structural reorganization. The residual hydrogen would be located in light molecules, probably of aromatic nature, that can act as a suspensive medium for the BSUs, facilitating their rearrangement, in a way similar to that occurring during the pyrolysis of other carbonaceous materials.31 This remaining hydrogen can be quantified approximately as the amount of hydrogen per hundred grams of carbon after the stoichiometric correction for the hydrogen that would be removed with the heteroatoms (net hydrogen).32 Anthracites having the highest net hydrogen content were found to be the most graphitizable ones and vice versa.7 It seems to be the case of AF and ATO anthracites with calculated hydrogen net contents of 2.38 wt % and 1.50 wt %, respectively. In addition to a higher amount of net hydrogen, AF also shows higher ash content than ATO (19.74 wt % against 10.12 wt % in Table 1). As mentioned in the introduction of this work, the coal mineral matter may behave as a graphitization catalyst.5,11 Improvements of the XRD crystalline parameters of the graphitized materials with increasing coal ash content have been previously found,12 thus agreeing well with our results. Among the major constituents of (31) Chermin, H. A. G.; Van Krevelen, D. W. Fuel 1957, 36, 85. (32) Donath, E. E. In Chemistry of Coal Utilization; Lowry, H. H., Ed.; Marcel Dekker: New York, 1963; pp 1041-1080.
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coal mineral matter, clay minerals are known to be one of the most effective graphitization catalysts of several carbon materials, particularly in the presence of iron.33 Different types of aluminum silicates such as illite and kaolinite, have been identified in the LTA residues of AF and ATO anthracites,34 the contents of Al, Si as well as K which is present in the illite, and Fe being higher in the most graphitizable AF (Table 2). The microtexture of the anthracites was related to their ability to graphitize, 9, 13 specifically when there is a preferential planar orientation of the polyaromatic basic structural units (BSUs). Apart from other methods such as TEM, 5,9,13,15 the microtexture can be studied on the basis of anthracite optical properties which are defined by the reflectance indicating surface (RIS). Principal axes (RMAX, RMIN and RINT) and RIS parameters (Rev, Rst, Ram) characterize the textural anisotropy and the structure of the BSUs (microtexture) of the anthracites as well as their changes during heat treatment.24,25,35 Assuming that the anisotropy of the texture (spatial arrangements of the BSUs), as estimated from bireflectance values (Bw), increases with the degree of planar orientation, anthracites with larger BW values should be expected to show a better degree of graphitizability. However, according to the BW values in Table 3, the most graphitizable AF anthracite exhibits a lower degree of textural anisotropy than ATO anthracite. On the other hand, due to the transformations caused by thermal treatment which include devolatilization, aromatization, and rearrangement of the BSUs,36 values of RMAX and RMIN of the AF and ATO anthracites increase during carbonization. The magnitude of this increase is higher for RMAX, leading in turn to carbonized samples with larger anisotropy of the texture as shown by the values of the BW parameter. Unlike raw anthracites, the carbonized AFC exhibits a higher degree of textural anisotropy than ATOC (Table 3). The RIS parameter Rev has been suggested to characterize the structure of the BSUs, 36 its improvement during heating being a consequence of their chemical structural ordering. As shown in Table 3, a higher value of this parameter is obtained for ATO, suggesting a more ordered structure of the BSUs in this anthracite which also shows a larger anisotropy of the texture when compared to that of AF, as measured by the BW parameter. At temperatures up to 1000 °C, changes in the structure of the BSUs are mainly of a chemical nature as a consequence of the removal of heteroatoms and other light compounds. The significance of these changes depends on the amount of hydrogen available and the microtexture of the initial anthracite. Therefore, (33) Marsh, H.; Warburton, A. F. J. Appl. Chem. 1970, 20, 133. (34) Gonza´lez, D. Doctoral Thesis, 2003. (35) Kilby, W. E. Int. J. Coal Geol. 1991, 19, 201. (36) Pusz, S.; Duber, S.; Kwiecinska, B. K. Fuel Process. Technol. 2002, 77-78, 173.
Gonza´ lez et al.
AF anthracite shows a greater growth of Rev during carbonization (Table 3). However, on the basis of the values of Rev, the structure of the BSUs is still more ordered in the carbonized sample of ATO anthracite (ATOC) than in AFC sample. Nevertheless, one should keep in mind that, although higher values of RIS parameter Rev were calculated for anthracites with a more anisotropic texture of the BSUs as determined by the bireflectance BW, no direct relation was found between these two optical parameters.36 In conclusion, the analysis of the optical properties of the samples in Table 3 suggests that the ability of the anthracites to graphitize depends, among other factors, on the anisotropy of the texture of their carbonized form, the presence of a higher amount of hydrogen in the raw anthracite facilitating both the structural ordering and the rearrangements of the BSUs in the carbonization process. Conclusions Two temperature segments, the second one being a plateau, were found to happen in the evolution of the crystallinity and degree of orientation of the graphite materials prepared from the anthracites. Once the carbon layers are stacked in a more or less graphitic structure after the pores collapse, the presence of amorphous and/or low-organized carbon between the crystallites prevents their coalescence, thus limiting the progress of the graphitization. More graphite-like materials were achieved from the anthracite with higher hydrogen and mineral matter contents. The degree of preferential planar orientation of the BSUs in the carbonized form of this anthracite, as measured by optical microscopy, was also higher. The structural and textural changes of the anthracites during carbonization, which are related with both their microtexture and hydrogen content, influence the graphitization process. The ability of the anthracites to graphitize was found to depend, among other factors, on the anisotropy of the texture of their carbonized form rather than on the anisotropy of the raw anthracite as reported previously. The limiting temperature at which the graphite materials prepared from the anthracites reach their highest degree of structural order is lower for the anthracite exhibiting higher anisotropy of the texture and/or degree of preferential planar orientation of the BSUs after the carbonization process. Acknowledgment. Financial support from DGICYT (project MAT2001-1843) and FICYT (project PB-EXP0101) is gratefully acknowledged. One of the authors (D. Gonzalez) thanks FICYT for a personal grant. Thanks are also due Dr. M. A. Ban˜ares of the Instituto de Cata´lisis, CSIC, Madrid (Spain) for providing access to the Raman spectrometer. EF030144+
