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Energy & Fuels 2009, 23, 942–950
High-Resolution Transmission Electron Microscopy Studies of Graphite Materials Prepared by High-Temperature Treatment of Unburned Carbon Concentrates from Combustion Fly Ashes Miguel Cabielles,† Jean-Noe¨l Rouzaud,‡ and Ana B. Garcia*,† Instituto Nacional del Carbo´n (INCAR), CSIC, Apdo. 73, 33080 OViedo, Spain, and Laboratoire de Ge´ologie, ENS, CNRS, 24 rue Lhomond, F-75231 Paris cedex 5, France ReceiVed September 12, 2008. ReVised Manuscript ReceiVed October 29, 2008
High-resolution transmission electron microscopy (HRTEM) has been used in this work to study the microstructural (structure and microtexture) changes occurring during the high-temperature treatment of the unburned carbon concentrates from coal combustion fly ashes. Emphasis was placed on two aspects: (i) the development of graphitic carbon structures and (ii) the disordered carbon forms remaining in the graphitized samples. In addition, by coupling HRTEM with energy-dispersive spectroscopy, the transformations with the temperature of the inorganic matter (mainly iron- and silicon-based phases) of the unburned carbon concentrates were evidenced. The HRTEM results were compared to the averaged structural order of the materials as evaluated by X-ray diffraction (XRD) and Raman spectroscopy. As indicated by XRD and Raman parameters, moreordered materials were obtained from the unburned carbon concentrates with higher mineral/inorganic matter, thus inferring the catalytic effect of some of their components. However, the average character of the information provided by these instrumental techniques seems to be inconclusive in discriminating between carbon structures with different degrees of order (stricto sensu graphite, graphitic, turbostratic, etc.) in a given graphitized unburned carbon. Unlike XRD and Raman, HRTEM is a useful tool for imaging directly the profile of the polyaromatic layers (graphene planes), thus allowing the sample heterogeneity to be looked at, specifically the presence of disordered carbon phases.
Introduction Fly ashes are the main solid wastes generated in coal combustion plants, their total production in the United States1 and Europe (EU15)2 in 2005 amounting to ∼135 × 106 t. These solid wastes need to be disposed of and/or reused. Coal fly ash disposal in dumps may lead to environmental and economic problems (such as pollutant compound lixiviation, landscape changes and filling, etc.). Reuse in the production of concrete and cement (the main route of coal fly ash utilization) is limited by the amount of unburned carbon present, which has increased as a consequence of the greater implementation of the NOx control systems in the utility industry to meet environmental regulations.1-6 Therefore, an increase of the proportion of coal fly ash utilization is linked to the removal of unburned carbon. Although several coal beneficiation methodologies have been successfully employed for removing the unburned carbon from * To whom correspondence should be addressed. E-mail: anabgs@ incar.csic.es. † CSIC. ‡ CNRS. (1) American Coal Ash Association (ACAA). 2005 Coal Combustion Product (CCP): Production and Use SurVey; Aurora, CO, September 2006. (2) European Coal Combustion Products Association Production (ECOBA). 2005 Production and Utilization; Essen, Germany, 2006. (3) Manz, O. E. Fuel 1999, 78, 133–136. (4) Smith, I. M. Cement and ConcretesBenefits and Barriers in Coal Ash Utilization; IEA Clean Coal Centre Reports, CCC/94; International Energy Agency: London, January 2005. (5) Hall, M. L.; Livingston, W. R. J. Chem. Technol. Biotechnol. 2002, 77, 234–239. (6) Hower, J. C.; Robl, T. L.; Thomas, G. A. Fuel 1999, 78, 701–712.
fly ashes,7-9 the up-to-date low value of these coal combustion byproducts has hindered their implementation in the power plants. However, the utilization of the unburned carbon as a precursor for value-added products would offset the cost of the beneficiation process. As indicated by X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) fringe imaging,10 the unburned carbon from high-rank coal combustion fly ashes shows carbon contents of >90%, a high degree of turbostratic structural order, and a lamellar microtexture. Moreover, such a lamellar microtexture was reported to be the most graphitizable carbon component of the anthracites.11-14 On this basis, the unburned carbon present in fly ashes appears as a potential precursor for the preparation of synthetic graphite. Thus, in a previous work,15 unburned carbon concentrates (UCCs) were obtained from anthracite combustion (7) Gray, M. L.; Champagne, K. J.; Soong, Y.; Killmeyer, R. P.; MarotoValer, M. M.; Andresen, J. M.; Ciocco, M. V.; Zandhuis, P. H. Fuel Process. Technol. 2002, 76, 11–21. (8) Soong, Y.; Schoffstall, M. R.; Gray, M. L.; Knoer, J. P.; Champagne, K. J.; Jones, R. J.; Fauth, D. J. Sep. Purif. Technol. 2002, 26, 177–184. (9) Gray, M. L.; Champagne, K. L.; Soong, Y.; Finseth, D. Fuel 2001, 80, 867–871. (10) Hurt, R. H.; Davis, K. A.; Yang, N. Y. C.; Headley, T. J.; Mitchell, G. D. Fuel 1995, 74, 1297–1306. (11) Blanche, C.; Dumas, C.; Rouzaud, J. N. Coal Science and Technology; Elsevier: New York, 1995; pp 43-46. (12) Rouzaud, J. N.; Duval, B.; Leroy, J. In Fundamental Issues in Control of Carbon Gasification ReactiVity; Lahaye, J., Ehrburger, P., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1991; pp 257268. (13) Rouzaud, J. N.; Oberlin, A. Carbon 1989, 27, 517–529. (14) Oberlin, A. In Chemistry and Physics of Carbon; Thrower, P. A., Ed.; Marcel Dekker: New York, 1989; Vol. 22, p 1. (15) Cabielles, M.; Montes-Mora´n, M. A.; Garcia, A. B. Energy Fuels 2008, 22, 1239–1243.
10.1021/ef800763s CCC: $40.75 2009 American Chemical Society Published on Web 12/12/2008
HRTEM Studies of Graphite Materials
fly ashes and then heated in the temperature interval 1800-2700 °C to explore their ability to graphitize. The mean interlayer spacing, d002, crystallite sizes along the c-axis, Lc, and the a-axis, La, calculated from XRD, and relative intensity of the Raman D-band, ID/It, were used to assess the degree of structural order of the materials prepared. Both XRD and Raman spectroscopy have been used extensively in the characterization of carbon materials.16-19 According to the crystalline parameters, the heat treatment of the UCC at temperatures g2400 °C can result in graphite-like materials that have structural characteristics comparable to those of petroleum coke derived synthetic graphites, which are commercially available for different applications.15 However, the average character of the information provided by these instrumental techniques seems to be inconclusive in discriminating between carbon structures with different degrees of order (stricto sensu graphite, graphitic, turbostratic, etc.) in a given graphitized unburned carbon. Unlike XRD and Raman spectroscopy, HRTEM is a useful tool for imaging directly the profile of the polyaromatic layers (graphene planes), thus allowing the sample heterogeneity to be looked at, specifically the presence of disordered carbon phases.13-24 Therefore, HRTEM has been used in this work to study the microstructural (structure and microtexture) changes occurring during the high-temperature treatment (HTT) of the unburned carbon concentrates from coal combustion fly ashes. Emphasis was placed on two aspects: (i) the development of graphitic carbon structures and (ii) the disordered carbon forms remaining in the graphitized samples. In addition, by coupling HRTEM with local (up to 100 nm2 areas, i.e., 103 nm3 volumes) elemental analyses by energy-dispersive spectroscopy (EDS), the transformations with the temperature of the inorganic matter (mainly iron- and silicon-based phases) of the unburned carbon concentrates were evidenced. As indicated by XRD and Raman parameters,15 more-ordered materials were obtained from the unburned carbon concentrates with higher mineral/inorganic matter, thus inferring the catalytic effect of some of their components. Regarding the composition of the coal mineral matter, most of its inorganic elements (such as aluminum, calcium, iron, magnesium, silicon, and titanium) are known to be graphitization catalysts of various carbon materials.20,21 The HRTEM results were compared to the averaged structural order of the materials as evaluated by XRD and Raman spectroscopy. Experimental Section Unburned Carbon Concentrates: Preparation and Characterization. Coal fly ashes from two pulverized coal combustion plants (labeled A and C) which are mainly fed with anthracites were selected for this research. Three UCCs denoted A/CVP, C/CIQ5, and C/IQS were prepared from A or C coal fly ashes as follows: A/CVP by screening out the e80 µm fraction, C/CIQ5 following an oil agglomeration methodology described previously25 by using a waste vegetable oil at a concentration of 5 wt %, and (16) Cuesta, A.; Dhamelincourt, P.; Laureyns, J.; Martı´nez-Alonso, A.; Tasco´n, J. M. D. J. Mater. Chem. 1998, 8, 2875–2879. (17) Zerda, W.; Gruber, T. Rubber Chem. Technol. 2000, 73, 289–292. (18) Cuesta, A.; Dhamelincourt, P.; Laureyns, J.; Martı´nez-Alonso, A.; Tasco´n, J. M. D. Carbon 1994, 32, 1523–1532. (19) Montes-Mora´n, M. A.; Young, R. J. Carbon 2002, 40, 845–855. (20) Marsh, H.; Warburton, A. F. J. Appl. Chem. 1970, 20, 133–142. (21) Oya, A.; Marsh, H. J. Mater. Sci. 1982, 17, 309–322. (22) Salver-Disma, F.; Tarascon, J. M.; Clinard, C.; Rouzaud, J. N. Carbon 1999, 37, 1941–1959. (23) Le Guillou, C.; Brunet, F.; Irifune, T.; Ohfuji, H.; Rouzaud, J. N. Carbon 2007, 45, 636–648. (24) Lillo-Rodenas, M. A.; Cazorla-Amoro´s, D.; Linares-Solano, A.; Be´guin, F.; Clinard, C.; Rouzaud, J. N. Carbon 2004, 42, 1305–1310. (25) Valde´s, A. F.; Garcia, A. B. Fuel 2006, 85, 607–614.
Energy & Fuels, Vol. 23, 2009 943 Table 1. Unburned Carbon (LOI Value) and Mineral Matter Contents of the A/CVP, C/CIQ5, and C/IQS Concentrates from Coal Combustion Fly Ashes concentrate
LOI (wt %, db)
mineral matter content (wt %, db)
A/CVP C/CIQ5 C/IQS
54.64 62.06 97.43
49.83 38.10 2.50
Table 2. Concentrations (wt %) of Al, Ca, Fe, Mg, Si, and Ti in the Ashes of A/CVP, C/CIQ5, and C/IQS Unburned Carbon Concentrates from Coal Combustion Fly Ashes element
A/CVP
C/CIQ5
C/IQS
Al Ca Fe K Mg Si Ti
10.99 1.09 3.80 2.69 0.90 30.22 0.67
12.49 1.81 6.47 2.77 1.19 25.85 0.63
13.80 5.10 5.07 1.75 2.26 21.77 1.89
C/IQS by demineralization after an acidic treatment with HCl/HF according to the standard ISO 602-1983 procedure for the determination of the mineral matter in coals. The unburned carbon content of the concentrates was calculated from the weight loss associated with firing the predried sample in an ashing furnace with an air atmosphere for 2 h at 815 °C, which is known as the loss of ignition (LOI). The unburned carbon (LOI values) and the mineral matter contents of A/CVP, C/CIQ5, and C/IQS concentrates appear in Table 1. The mineral matter of the samples was determined by the above-mentioned standard ISO 602-1983. The major inorganic elements (aluminum, calcium, iron, potassium, magnesium, silicon, and titanium) present in the mineral matter of the UCCs were analyzed in the ashes using atomic absorption spectrometry (AAS). The concentrations of these inorganic elements in the ashes of the UCCs are given in Table 2. High-Temperature Treatments of the Unburned Carbon Concentrates. The HTT experiments of A/CVP, C/CIQ5, and C/IQS were carried out at 1800, 2000, 2200, 2300, 2400, 2500, 2600, and 2700 °C in a graphite furnace, as described elsewhere.15 The carbon materials thus prepared were identified by the unburned carbon concentrate (precursor) designation and the treatment temperature, such as A/CVP/2400 or C/CIQ5/2600. Transmission Electron Microscopy and Energy Dispersive Spectroscopy. TEM has been carried out on a Jeol 2011 microscope equipped with a LaB6 gun and operating at 200 kV. Magnification up to 400000× was necessary to easily observe the 002 lattice fringe (named here the “high-resolution” mode) of both graphitic and disordered carbons. The resolution in the lattice fringe mode is 0.14 nm. Such a 002 lattice fringe mode was used to image the profile of the aromatic layers (extent and shape) and thus the structure and the microtexture. Local elemental analysis by the EDS mode has been performed thanks to a PGT energy-selective X-ray analyzer allowing in situ analysis on volumes smaller than 103 nm3, with a detection limit of 1%. The samples were first finely ground and dispersed in ethanol, and a drop of solution was then deposited on a classical TEM copper grid, previously covered by a holey amorphous carbon film. Examination of the sample was focused on parts of the samples lying across the holes to obtain information free of the contribution of the supporting carbon film. Different modes of TEM were used to study the structure (organization at the atomic scale) and the microtexture (organization from the nanometric to the micrometric scales, resulting from the mutual orientation in space of crystallites or basic structural units, BSUs) of the materials.13,14 Thus, bright-field (BF) techniques were employed to image the particle morphology (porous, lamellar, spherical). The dark-field (DF) mode is a medium-resolution (about 1 nm) amplitude contrast technique allowing easy access to deaveraged information on these heterogeneous carbon materials. For instance, nanostructural information such as the degree of preferential planar orientation of the polyaromatic layers in anthracites can be evaluated by comparing two 002 DF images
944 Energy & Fuels, Vol. 23, 2009 corresponding to two orthogonal directions.26 As far as heat-treated carbons are concerned, structural information such as the crystallite diameter can be obtained, enabling moire´ fringe imaging formed by superimposed crystallites lying flat on the grid.13 In some cases, the images were coupled to selected area electron diffraction (SAED) for structural analyses of a volume less than 1 µm in diameter and 0.1 µm in thickness; this allows the improvement of the stricto sensu graphitization to be followed13 and some mineral phases to be identified. The ImageJ software was used to help in the measurements of the crystalline parameters of the aromatic layers as imaged by the 002 lattice fringe mode as well as the mean dhkl values from the FFT (fast Fourier transform) of small areas cut in high-resolution images of crystalline phases [Rdhkl ) K (R ) measured distance), with the constant K ) 0.0453 for the squared images of 16 nm × 16 nm recorded at 400000×]. Such so-obtained local data, punctually measured on nanometer-sized areas, were compared to average XRD data representative of the bulk. X-ray Diffraction and Raman Spectroscopy. The diffractograms and Raman spectra of the carbon materials prepared were recorded as reported previously.27 The mean interlayer spacing, d002, was evaluated from the position of the (002) peak applying Bragg’s equation. The mean 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.28 Typical standard errors of the XRD parameters are 60 nm. Moreover, large crystallites with a height up to 600 nm (as measured perpendicular to the carbon layers) were observed by the darkfield TEM mode in the A/CVP/2000 material (Figure 5b). Disordered porous microtextures formed by four to ten stacked layers with a high misorientation were also found to remain after the UCCs were heated at 2000 °C, particularly in C/IQS/ 2000 (Figure 5d), thus explaining the much higher ID/It Raman ratio of this material (Table 3). Crystallized silicon (Figure 5e,f) and iron/silicon compounds (Figure 5g,h) were the main remaining inorganic impurities detected by HRTEM/EDS in the materials prepared from the unburned carbon concentrates at 2000 °C. Considering the results of the analysis of the XRD profiles of these materials (Figure 2) and of the local elemental analysis by EDS, the crystallized phase in Figure 5h may correspond to an iron silicide which has been suggested to be an intermediate stage for the formation of silicon carbide in
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Figure 6. HRTEM images and SAED pattern of A/CVP/2200 (a, graphitic lamellar microtexture; g, crystallized silicon/carbon phase; h, SAED pattern of the silicon/carbide particle in (g)), C/IQS/2200 (b, graphite-like lamella seen edge on; d, disordered carbon), and C/CIQ5/2200 (c, porous carbon areas around a graphite lamella at the top right corner; e, crystallized silicon; f, silicon/carbon structure) materials.
Figure 7. TEM images and SAED pattern of A/CVP/2400 (a, bright-field image showing a graphitic lamellar microtexture; b, dark-field image of a big graphite crystallite; e, silicon/carbon particle; g, titanium/carbon particle; h, SAED pattern of the particle in (g)), C/IQS/2400 (c, graphitic structure; d, turbostratic carbon lamellae), and C/CIQ5/2400 (f, silicon/iron particle) materials.
the presence of iron.43,44 As in the case of the iron/iron carbide compounds in Figure 4c, this iron/silicon phase appears surrounded by parallel carbon sheets. A graphite particle (flat/crumpled lamellae) can be observed in the bright-field image of A/CVP/2200 in Figure 6a. Graphitelike domains with measured crystallite sizes Lc in the interval 20-24 nm and La > 125 nm were identified by HRTEM in the materials prepared at 2200 °C, even in that from the C/IQS precursor (Figure 6b) with the lower degree of structural order as shown by the calculated XRD parameters in Table 3. Some disordered/microporous turbostratic carbon areas in the edge of the graphite lamellae were also found to be present in the more ordered A/CVP/2200 and C/CIQ5/2200 materials (Figure 6c), while this type of microporous microtexture appears isolated in C/IQS/2200 material (Figure 6d). Several crystallized silicon (Figure 6e) and silicon/carbon (Figure 6g,h) phases were identified by HRTEM/EDS in A/CVP/2200 and C/CIQ5/2200. Unlike in the materials prepared at 1800-2000 °C, iron compounds such as carbides or silicides surrounded by parallel carbon sheets were not detected, thus suggesting their massive decomposition during the HTT at 2200 °C to form graphitic carbons and silicon carbide, respectively. As expected, the proportion of mineral impurities observed by HRTEM in the C/IQS/2200 material was scarce. However, crystallized silicon/ carbon and titanium phases were identified by HRTEM coupled with EDS. Mainly strongly graphitized structures were observed by HRTEM in the materials prepared at 2400 °C from the A/CVP
and C/CIQ5 unburned carbon concentrates (Figure 7a). Large crystallites were imaged in both A/CVP/2400 and C/CIQ5/2400 materials by using their moire´ fringe images in the dark-field mode (Figure 7b). Unlike these materials, the structure of C/IQS/ 2400 appears very heterogeneous, showing both strongly graphitized domains with average thickness (Lc) values up to 25-30 nm as directly measured from several 002 lattice fringe images (Figure 7c) and still very disordered carbon areas (Figure 7d). Silicon/carbon phases, generally well crystallized, thus inferring the presence of silicon carbide, were identified by EDS/ HRTEM as the main residual inorganic impurity in the materials prepared at 2400 °C (Figure 7e). Even so, some silicon/iron (probably iron silicide) particles were still found to remain in the material prepared from C/CIQ5 (Figure 7f), whereas wellcrystallized titanium/carbon structures appeared in the A/CVP/ 2400 material (Figure 7g,h). Several stricto sensu graphite structures with a very high degree of layer parallelism and almost no defects were found in the materials prepared at 2600-2700 °C, particularly in those from A/CVP and C/CIQ5 precursors. Values of Lc of ∼30, ∼44, and >60 nm were directly measured for these graphite structures, thus suggesting the progress of the graphitization beyond 2400 °C (for comparison, see Table 3 with XRD and Raman data). As an example, an HRTEM image of a perfect graphite structure of A/CVP/2700 material appears in Figure 8a, its corresponding SAED in Figure 8b showing the presence of 112 reflections (indicating the graphite triperiodic order) as well as 100 and 110 rings which are characteristic of the
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Figure 8. TEM images and SAED patterns of A/CVP/2700 (a, 002 lattice fringe image of a graphite structure; b, SAED pattern of the structure appearing in (a); c, graphite structures imaged by the 002 dark-field mode; e, graphite crystals imaged by the 11 dark-field mode; f, SAED pattern of the crystal in (e); g, microporous/disordered carbon and polygon-shaped graphite), C/CIQ5/2600 (d, crystal structure imaged by the dark-field mode), and C/IQS/2600 (h, disordered microporous carbon area; i, polyhedral graphitic microtexture) materials.
Figure 9. TEM images and SAED pattern of the reference graphite GS4 (a, graphite structure; b, SAED pattern of the structure in (a); c, d, bright-field images of superimposed graphite lamellae showing large moire´ fringes), A/CVP/2700 (e, f, bright-field images of graphitic microtextures at different magnifications; g, bright-field image of superimposed graphite lamellae showing moire´ fringes), and C/CIQ5/2600 (h, bright-field image of superimposed graphite lamellae showing moire´ fringes) materials.
polycrystalline graphite. A d002 value of 0.3356 nm was calculated for this graphite structure from the FFT of the highresolution TEM image. Moreover, as measured in the dark-field 002 image in Figure 8c, these graphite structures may reach La values up to 350 nm. Several very large crystals were also observed with the dark-field 11 mode in A/CVP/2700 and C/CIQ5/2600 materials (Figure 8d,e), the SAED pattern corresponding to a single crystal lying on the 001 plane (Figure 8f). Even so, some microporous/disordered carbon areas coexisting with graphitic structures, mainly with a polygonal shape, were still found in the A/CVP/2700 and C/CIQ5/2700 materials (Figure 8g). Unlike these, the less ordered C/IQS/2600 material
has several microporous carbon areas (Figure 8h) as well as graphitic, mainly polygonal/polyhedral particles (Figure 8i), thus indicating a high degree of structural heterogeneity. Only a few particles of not-well-crystallized silicon/carbon and titanium/ carbon compounds were identified by HRTEM/EDS to be present in the A/CVP/2700 and C/CIQ5/2600 materials. To complete the present study, it was found interesting to compare the HRTEM/EDS characterization results of the A/CVP/2700 and C/CIQ5/2600 materials, already discussed, to those determined for the reference synthetic graphite GS4, which shows a similar degree of structural order as determined from XRD and Raman spectroscopy (Table 3). Graphite structures
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with a high degree of layer parallelism were also imaged in this reference material by using the HRTEM 002 lattice fringe mode (Figure 9a), their SAED patterns corresponding mainly to single graphite crystals lying on the 001 plane (Figure 9b). Moreover, as shown even in the bright-field mode images (Figure 9c,d), this lamellar microtexture exhibits large crystal moire´ fringes, thus indicating a high degree of crystallinity. Values of Lc of ∼19 and >60 nm were directly measured in some of the images of the graphite structures, thus showing the presence of a high range of crystallite sizes which were not detected by XRD (Table 3). Similar to GS4 reference graphite, a lamellar microtexture with large crystal moire´ fringes is also observed in the bright-field images of the A/CVP/2700 (Figure 9e-g) and C/CIQ5/2600 (Figure 9h) materials. However, on the basis of the information provided by the observation of the bright-field mode images, there is a difference between the microtexture of the reference graphite, GS4, and the materials prepared. As can be seen, only flat graphite lamellae are present in the GS4 reference material (Figure 9c), while some polyhedral carbons showing slightly wrinkled areas are still noticeable in the materials prepared from the unburned carbon concentrates (Figure 9f). Even so, the presence in the UCC-based materials of graphite lamellae near the ideal or theoretic crystal graphite structure can be inferred from the clearer moire´ fringes observed in the bright-field images of these materials (Figure 9). Conclusions A significant degree of microtexture heterogeneity was observed by coupling the different modes of TEM in the materials prepared from the unburned carbon concentrates by HTT, particularly at temperatures below 2400 °C. Thus, graphitic domains with sizes much larger than the averaged values calculated from XRD as well as disordered structures formed by a few imperfectly stacked layers were imaged in these materials. Moreover, some minor porous/disordered carbon microtextures coexisting with graphitic structures were still distinguished in those materials showing the highest structural order and crystal orientation degrees as estimated from the XRD and Raman parameters. Although no noticeable improvement in the XRD average degree of structural order was attained in the materials prepared
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at temperatures above 2400 °C, the appearance of stricto sensu graphite structures (with the characteristic lamellar microtexture) in these materials as imaged by HRTEM strongly suggests the progress of the unburned carbon concentrate graphitization beyond this temperature. Because of the catalytic effect of the mineral impurities, specifically iron and silicon, more-ordered carbon materials were obtained from those unburned carbon concentrates with higher mineral matter content. However, the ability of the UCCs to graphitize seems to also be related to the preferential planar orientation of the BSUs. This type of microtexture may provide an explanation for the structural evolution during HTT of the unburned carbon concentrate with a very small content of mineral matter in which large graphite domains were observed by HRTEM at temperatures as low as 1800 °C. Different crystallized mineral phases such as iron carbide, iron silicide, and silicon carbide were identified in the materials by both XRD and HRTEM coupled with EDS. Moreover, silicon and iron were directly and locally observed by the latter technique. Iron carbide was only detected in the materials prepared at 1800 °C, while iron silicide was imaged after the treatment of the unburned carbon concentrates at 2000 °C. Considering that both iron carbide and iron silicide were surrounded by parallel graphitic carbon layers, the formation of the iron silicide at the expense of the iron carbide decomposition seems to be plausible. Iron silicide remains in the materials up to 2200-2400 °C, at which temperatures crystallized silicon carbide first appears, thus suggesting the intermediate stage role of the iron silicide in the formation of silicon carbide (which further decomposes to give graphite) during the catalytic graphitization of the unburned carbon concentrates. Acknowledgment. Financial support from the Spanish Ministry of Education and Science MEC (under Projects MAT2001-1843 and MAT2004-01094) and FICYT (under Project PB-EXP01-01) is gratefully acknowledged. A.B.G. thanks MEC and CSIC for a personal grant (Spanish Scientists Mobility Program) to develop the HRTEM work at ENS/CNRS. EF800763S
