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Carbon Foams from Different Coals Montserrat Calvo, Roberto Garcı´a,* and Sabino R. Moinelo Instituto Nacional del Carbo´n (INCAR), Consejo Superior de InVestigaciones Cientı´ficas (CSIC), Apartado 73, 33080 OViedo, Spain ReceiVed February 4, 2008. ReVised Manuscript ReceiVed May 23, 2008
Carbon foams were obtained from several bituminous coals with different plasticity and volatile matter content by a two-stage thermal process. The first stage, a controlled carbonization treatment under pressure at 450-500 °C, is responsible for the final textural properties of the foam. In the second stage, the carbonization product was baked at 1100 °C. The foams produced display a macroporous texture with fluidity, volatile matter content, and maceral composition of the precursor coals, having an influence on the apparent density and the pore size of the resultant porous products. Coals with low fluidity, volatile matter content, and liptinite content give rise to foams with lower pore size and lower apparent density. In the case of high fluidity coals, their foams display an increase of flexural strength with the increasing relative density. In general, the carbon foams obtained in this study display good electrical properties (electrical resistivity comparable to that of commercial foams).
Introduction Carbon foams are lightweight (0.2-0.8 g cm-3) and exceptionally strong cellular materials, with high-temperature resistance (up to 3000 °C under reducing or inert atmosphere) and thermal and electrical conductivity adjustable by the foaming thermal treatment. Their low production costs and flexible physical properties make them ideal for a wide range of applications in such diverse areas as thermal management, electromagnetic interference, acoustic shielding, energy absorption and storage, catalysts supports, filtration, etc.1–4 Carbon foams were first developed as reticulated carbons5,6 from thermosetting organic polymer foams through a thermal treatment that rendered macroporous refractory materials with a uniform cell size and moderate mechanical strength, especially intended for thermal insulating applications or as lightweight construction materials for the aerospace industry. Graphitized carbon foams obtained from mesophase pitches are most appropriate to produce high strength and thermal and electrical conductivity because of the interconnected graphitic ligament network.1–3,7 Traditionally, a previous thermosetting step is mandatory to improve the plastic properties of the precursor by modifying the size of the anisotropic domains * To whom correspondence should be addressed. E-mail: robo@ incar.csic.es. (1) Gallego, N. C.; Klett, J. W. Carbon foams for thermal management. Carbon 2003, 41 (7), 1461–1466. (2) Jang, Y.-I.; Dudney, N. J.; Tiegs, T. N.; Klett, J. W. Evaluation of the electrochemical stability of graphite foams as current collectors for lead acid batteries. J. Power Sources 2006, 161 (2), 1392–1399. (3) Min, G.; Zengmin, S.; Weidong, C.; Hui, L. Anisotropy of mesophase pitch-derived carbon foams. Carbon 2007, 45 (1), 141–145. (4) Fang, Z.; Li, C.; Sun, J.; Zhang, H.; Zhang, J. The electromagnetic characteristics of carbon foams. Carbon 2007, 45 (15), 2873–2879. (5) Ford, W. D. Method of making cellular refractory thermal insulating material. U.S. Patent 3,121,050, 1964. (6) Googin, J. M.; Napier, J. M.; Scrivner, M. E. Method for manufacturing foam carbon products. U.S. Patent 3,345,440, 1967. (7) Klett, J.; Hardy, R.; Romine, E.; Walls, C.; Burchell, T. Highthermal-conductivity, mesophase-pitch-derived carbon foams: Effect of precursor on structure and properties. Carbon 2000, 38 (7), 953–973.
generated in the foaming process.8,9 Then, an oxidative crosslinking stabilization of the foam structure was usually applied to prevent the porous structure of the foam from melting in any subsequent treatment or use.10 Recently, however, procedures have been developed that obviate the time-consuming and expensive stabilization step.11 These mesophase pitch-based carbon foams display thermal conductivities of up to 180 W m-1 K-1, very appropriate for heat dissipation applications.1,11,12 Carbon foams can be also made straight from coal.8,13,14 The structural properties of coal-based carbon foams make them perfectly useful in numerous applications, when very high conductivity is not required and the manufacturing costs can be considerably reduced because coal can be used as a precursor without any previous preparation process. Possible applications are thermal isolation, energy adsorption, fire-resistant constructions, structural materials, etc. They can even be considered as good materials for the manufacture of electrodes for battery and fuel cell applications after an appropriate graphitization step.15
(8) Chen, C.; Kennel, E. B.; Stiller, A. H.; Stansberry, P. G.; Zondlo, J. W. Carbon foam derived from various precursors. Carbon 2006, 44 (8), 1535–1543. (9) Wang, M.-X.; Wang, C.-Y.; Zhang, X.-L.; Zhang, W. Effects of the stabilization conditions on the structural properties of mesophase-pitchbased carbon foams. Carbon 2006, 44 (15), 3371–3372. (10) Kearns, K. M. Process for preparing pitch foams. U.S. Patent 5,868,974, 1999. (11) Klett, J. W.; Burchell, T. D. Pitch-based carbon foam heat sink with phase change material. U.S. Patent 7,166,237 B2, 2007. (12) Klett, J. W.; McMillan, A. D.; Gallego, N. C.; Burchell, T. D.; Walls, C. A. Effects of heat treatment conditions on the thermal properties of mesophase pitch-derived graphitic foams. Carbon 2004, 42 (8-9), 1849– 1852. (13) Rogers, D. K.; Plucinski, J. W.; Handley, R. A. Preparation and graphitization of high-performance carbon foams from coal. In 2001 Carbon Conference, Lexington, KY, 2001. (14) Calvo, M.; Garcı´a, R.; Arenillas, A.; Sua´rez, I.; Moinelo, S. R. Carbon foams from coals. A preliminary study. Fuel 2005, 84 (17), 2184– 2189. (15) Rogers, D. K.; Plucinski, J. W. Electrochemical cell electrodes comprising coal-based carbon foams. U.S. Patent 6,899,970 B1, 2005.
10.1021/ef8000778 CCC: $40.75 2008 American Chemical Society Published on Web 07/29/2008
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Table 1. Analytical Data of the Bituminous Coal Used as Precursors for Carbon Foams coal Gieseler plasticity test softening temperature (°C) solidification temperature (°C) plastic range (°C) maximum fluidity temperature (°C) fluidity (ddpm)a swelling index volatile matter (% db)a ash (% db)a thermogravimetric analysis initial weight loss temperature (°C) maximum weight loss temperature (°C) final weight loss temperature (°C) a
PL
BA
NP
Z
NC
AB
L
BU
W
PF
414 470 56 444 43 7.75 34.1 5.74
388 476 88 436 1696 5.75 26.6 5.28
441 500 59 474 30 8 17.8 9.7
387 467 80 427 3019 3.5 34.5 7.5
380 482 102 442 26 695 6.5 32.5 8.4
393 470 77 438 7307 6 34.8 6.6
432 498 66 470 80 7.5 19.0 7.0
446 502 56 475 15 6.75 17.6 5.6
389 476 87 436 11 883 5 34.1 6.1
393 486 93 439 13 037 6.5 30.8 6.1
370 444 537
370 436 540
395 477 571
345 429 558
368 438 522
365 432 525
384 473 626
400 477 585
354 436 552
374 446 525
ddpm, dial divisions per minute as obtained by the Gieseler plasticity test (ASTM D2639-98); db, dry basis.
Table 2. Petrographic Analysis Data vitrinite reflectance coal (% Ro)
coal ranka
PL BA NP Z NC AB L BU W PF
HVB HVB LVB HVB HVB HVB MVB LVB HVB HVB
0.90 0.97 1.49 0.73 0.89 0.86 1.43 1.57 0.88 0.99
Table 3. Properties of Carbon Foams true density (FHe, g cm-3)
maceral composition (%)
foam
grinded foams
foam in pieces
PLF BAF NPF ZF NCF ABF LF BUF WF PFF
1.85 1.91 1.97 1.88 1.91 1.89 1.98 1.94 1.88 1.90
1.84 1.90 1.82 1.90 1.90 1.89 1.89 1.75 1.88 1.90
vitrinite liptinite inertinite mineral matter 83.3 73.7 71.4 36.8 64.7 64.1 71.1 75.2 62.8 66.3
2.3 9.0 0.0 10.5 6.6 8.8 0.1 0.0 7.7 3.4
3.4 13.7 23.5 45.7 24.1 22.0 24.8 21.0 23.6 24.4
11.0 3.6 5.1 7.0 4.6 5.1 4.0 3.8 5.9 5.9
a HVB, high-volatile bituminous coal; LVB, low-volatile bituminous coal; MVB, medium-volatile bituminous coal.
With previous steps of hydrogenation and separation in the precursor coal, the production of anisotropic carbon foams was claimed.16,17 In this work, carbon foams were obtained from 10 high-, medium-, and low-volatile bituminous coals through a simple thermal procedure. Neither previous modification of the coals nor further stabilization step of the green foams was carried out. The aim of this study is to find correlations between the raw coal properties (including the maceral composition) and the texture and properties of the carbon foams.
total apparent pore majority density open volume pore (FHg, g porosity (VT, cm3 size -3 -1 cm ) (ε, %) (µm) g ) 0.51 0.84 0.56 0.87 0.61 0.71 0.50 0.60 0.63 0.67
72.3 53.7 67.4 48.5 66.6 61.4 71.2 63.6 62.9 62.5
1.42 0.64 1.21 0.56 1.10 0.86 1.42 1.06 0.99 0.93
19 97 21 132 107 107 4 11 108 60
atmosphere and heated at 2 °C min-1 up to around the temperature of maximum fluidity (470 °C for NP, L, and BU and 450 °C for the rest) that was held for 2 h. The outlet valve is kept open until the highest volatile release starting temperature was reached (Table 1), to let coal moisture and other minor volatiles leave the reactor before the foaming process. At that moment, the valve was closed
Experimental Section Precursor Characterization. The coals used as carbon foam precursors were Polio (PL) (Spain), Batan (BA) (Spain), Norwich Park (NP) (Australia), Zaochai (Z) (China), Neec Creek (NC) (U.S.A.), Arch Blend (AB) (U.S.A.), Litwak (L) (U.S.A.), Buchanan (BU) (U.S.A.), Wells (W) (U.S.A.), and Pond Fork (PF) (U.S.A.). Some of their properties are listed in Table 1. The Gieseler plasticity test (ASTM D2639-98) and the crucible swelling test (ISO 501: 2003) were used to measure the fluidity and dilatation characteristics of each precursor. Thermogravimetric analysis was carried out with a TA Instruments equipment, model SDT 2960, at a heating rate of 2 °C min-1 from ambient temperature to 1000 °C using 15 mg of sample. The mean random vitrinite reflectance measurement (% Ro) and maceral analysis (Table 2) were carried out on a MPVCombi Leitz microscope in accordance with the ISO 7404/05 and ISO 7404/03 standard procedures, respectively. Foaming Process. In these experiments, the precursor coal (80 g), pulverized at < 212 µm, was pressed at 100 kg cm-2 into a cylinder and fed into a 50 × 100 mm cylindrical stainless-steel reactor. Then, it was purged with argon to provide an inert (16) Stiller, A. H.; Stansberry, P. G.; Zondlo, J. W. Method of making a carbon foam material and resultant product. U.S. Patent 5,888,469, 1999. (17) Stiller, A. H.; Plucinski, J.; Yocum, A. Method of making a carbon foam material and resultant product. U.S. Patent 6,506,354 B1, 2003.
Figure 1. Variation of the apparent density of the carbon foams with the fluidity of the precursor coals. (a) Low-rank (high- and mediumvolatile bituminous) coals (fluidity > 3000 ddpm) and (b) high-rank (low-volatile bituminous) coals (fluidity < 100 ddpm). Tentative trend lines are given.
3378 Energy & Fuels, Vol. 22, No. 5, 2008
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Figure 2. Variation of the pressure reached in the reactor at the end of the foam formation process with the liptinite content of the precursor coal. A tentative trend line is given.
Figure 3. Variation of the apparent density of the carbon foams with (a) the liptinite content and (b) the vitrinite content of the precursor coal. Tentative trend lines are given.
and the pressure increased because of the release of volatile matter, which acts as the foaming agent during the coal plastic range. The green foams thus obtained were carbonized under argon flow at 1100 °C, with a heating rate of 1 °C min-1 and a soaking time of 2 h. The final carbon foams are designated with the letters corresponding to the precursor coal followed by a F. Carbon Foams Characterization. The foams were analyzed by scanning electron microscopy (SEM) using a Zeiss microscope,
model DSM-942, provided with an EDS detector OXFORD, model Link-Isis II. The true density, determined by He displacement, was measured using a pycnometer, Accupyc 1330 from Micromeritics; either foams in pieces or grinded samples were used for the measurements. Apparent density and pore volume distributions were evaluated with a mercury porosimeter (AutoPore IV, from Micromeritics), which provides a maximum operating pressure of 227 MPa. The percentage of open cells was calculated by eq 1, where
Carbon Foams from Different Coals
Figure 4. SEM microphotographs of the carbon foams obtained from the different coals.
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Figure 5. Variation of the pore size of the carbon foams with (a) the liptinite content and (b) the vitrinite content of the precursor coal. Tentative trend lines are given.
ε is the open porosity (%) and FHg and FHe are the apparent and true densities (g cm-3) determined in Hg and He, respectively. Similarly, the total open pore volume, VT, was obtained by eq 2.
(
ε) 1VT )
(
)
FHg 100 FHe
(1)
)
(2)
1 1 FHg FHe
F)
Flexural Strength Test. Foams were cut into at least three beams of 8 × 8 × 50 mm3 for flexure testing. Four-point bending tests were carried out at a constant speed in an Instron Model 450, with a stainless-steal four-point bending fixture. The inner load span was 10 mm, and the outer load span was 21 mm; the loading rate was 0.002 mm s-1. Failure strength (σf) was calculated by eq 3
σf )
3R(L - l) 2bh2
section (s). Then, the foam was placed in a press between two conductive plates attached to the pistons, and after being pressed at 50 bar, an electric current with an intensity I was applied through it. A multimeter, with the connectors placed at a constant distance (d), is used to determine the voltage (V) through the foam by calculating the average of three measurements in the foam cylinder at intervals of 120°. The electrical resistivity (F) is calculated with eq 4.
(3)
where, R is the rupture load, b and h are the width and the height of the beam, respectively, and L and l are the outer and inner spans, respectively. Electrical Resistivity. The procedure followed to determine the electrical resistivity of the carbon foams is based on the ASTM D6120-97 standard method. Once the foam cylinder has been cut with perfectly parallel bases, its diameter was measured twice with 90° intervals, at four different heights. The average of the eight values is used to calculate the value of the area of the cylinder
10VS ld
(4)
Results and Discussion Visual examination reveals that all of the precursors produced good quality foams, except NP and BU. Foams NPF and BUF exhibit several cracks, maybe because of an incomplete agglomeration. In fact, these are the coals with the lowest fluidity (30 and 15 ddpm, respectively, Table 1) and without liptinite in their maceral composition (Table 2). The properties of the carbon foams carbonized at 1100 °C are listed in Table 3. The apparent density ranges from 0.50 to 0.87 g cm-3. Previous results14 indicate that the apparent density of the foam decreases with the increasing fluidity of the precursor coal. The results of this study follow the same trend but only if low fluidity and high fluidity coals are considered separately. Figure 1 displays two apparent density versus fluidity plots: one for the coals with fluidity > 3000 ddpm (high-volatile
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Figure 6. Variation of the flexural strength with the relative density in the carbonized foams. The error bars correspond to confidence intervals of 95%. A tentative trend line is given for the foams derived from high-fluidity coals (filled dots). Blank dots represent low-fluidity coals.
Figure 7. Influence of the liptinite content of the precursor coal on the flexural strength of the resultant foams. The error bars correspond to confidence intervals of 95%. A tentative trend line is given for the foams derived from high-fluidity coals (filled dots). Blank dots represent lowfluidity coals.
bituminous coals) (Figure 1a) and the other for those with values < 100 ddpm (low- and medium-volatile bituminous coals) (Figure 1b). The foam obtained from the high-volatile bituminous coal PL, which displays low fluidity because of aerial oxidation exposition,18 is displayed in Figure 1b. In both plots, the apparent density of the resultant foam decreases significantly with the increasing fluidity of the precursor coal, reaching a plateau at high fluidity values. The drop is more pronounced in the lower rank coals (Figure 1a), with the apparent density of the foam decreasing from 0.87 (ZF) to 0.61 (NCF) g cm-3 as the fluidity of the coal rises from 3000 (Z) to 27 000 (NC) ddpm. As coal is heated under inert atmosphere, free radicals producing cracking reactions take place but, simultaneously, some of the latter are involved in condensation reactions with aromatic molecules. On the other hand, the hydrogen-rich species present in the coal are able to stabilize the fragments and convert them into “solvating” species, which make larger size molecules dissolve easier with a concomitant increase of fluidity. If there is not enough hydrogen available, the radical fragments join each other, generating larger molecules. This (18) Ignasiak, B. S.; Szladow, A. J.; Montgomery, D. S. Oxidation studies on coking coal related to weathering. 3. The influence of acidic hydroxyl groups, created during oxidation, on the plasticity and dilatation of the weathered coking coal. Fuel 1974, 53 (1), 12–15.
would reduce the fluidity, hindering the foaming process and rendering high-density foams. However, converse to the expected, the low-volatile bituminous coals produce the lowest density foams, despite having very low fluidity. This fact can be explained taking into account how the foam cellular structure develops in these experiments. The medium- and high-volatile bituminous coals display higher fluidity but also generate higher pressure during the foaming carbonization experiments. As a consequence, porosity development is hindered and the resulting foams present lower values of open porosity and total pore volume (Table 3). On the other hand, in the carbonization of the low-volatile precursors tested in this study (NP, L, and BU), the reduced pressure allows for a better development of porosity despite the lower fluidity. This is confirmed when the maceral contents are considered. Vitrinite is the most abundant maceral group in almost all of the coals tested (62.8-83.3%, Table 2), with the only exception of Z (36.8%), in which inertinite dominates (45.7%). Vitrinite displays values of the H/C ratio slightly higher (0.70-0.80 daf basis)19 than inertinite but much lower than liptinite (typically 0.95-1.20),19 which is the maceral group with the highest (19) White, A.; Davies, M. R.; Jones, S. D. Reactivity and characterization of coal maceral concentrates. Fuel 1989, 68 (4), 511–519.
3382 Energy & Fuels, Vol. 22, No. 5, 2008 Table 4. Electrical Resistivity of Some of the Foams of This Study, Heat-Treated at 2200 °C, and Two Commercial Graphitic Foams28,29
a
foam
electrical resistivity (Ω mm)
PLF BAF ZF NCF ABF WF PFF Cfoama PocoFoama
47.53 × 10-3 108.06 × 10-3 94.21 × 10-3 119.15 × 10-3 101.65 × 10-3 80.36 × 10-3 142.59 × 10-3 0.1-1 × 108 7.62 × 10-3-25.40 × 10-3
Commercial graphitic foams.
content of hydrogen.19–22 Qualitatively, hydrogen in liptinite macerals is mainly aliphatic, whereas aromatic hydrogen predominates in vitrinites.19–22 The abundant aliphatic groups of the liptinites render tar and oils during carbonization, helping to increase fluidity23 and producing lower density foams. However, as mentioned above, PL, NP, L, and BU, despite having very low contents of liptinite macerals in their composition and displaying low fluidity, produce foams with the lowest apparent density. This observation may be considered anomalous, because lower apparent densities would be expected for the foams derived from coals with higher liptinite contents. It has to be taken into account that, in the batch reaction system used in this study, the volatiles released by cracking reactions during carbonization remain in the reactor contributing to the increase in the internal pressure. The enhanced concentration of aliphatic chains in liptinite macerals will contribute more to the volatilized fraction than to the final structure of the foams and, therefore, to the increase of pressure in the reactor during foaming. Also, the aliphatic hydrogen is more reactive than the aromatic at these temperatures,19 being available for the stabilization of the free radicals formed when coal cross-links are broken by thermal effect. In fact, it has been previously reported that, under carbonization conditions, the conversions and volatiles release of the different maceral groups decrease in the order: liptinite > vitrinite > inertinite.24,25 Despite the low liptinite concentrations (Table 2) of the coals studied, it has an effect on the pressure reached in the foam formation process, as shown by the temperature/liptinite content correlation displayed in Figure 2. The pressure increase resulting from the release of volatile matter hinders the porosity development, and as a consequence, higher density foams will be generated when higher amounts of liptinite are present (Figure 3a). The opposite trend is observed when the content of vitrinite group is considered (Figure 3b), with lower density foams being obtained from coals (20) Dyrkacz, G. R.; Bloomquist, C. A. A.; Solomon, P. R. Fourier transform infrared study of high-purity maceral types. Fuel 1984, 63 (4), 536–542. (21) Vassallo, A. M.; Liu, Y. L.; Pang, L. S. K.; Wilson, M. A. Infrared spectroscopy of coal maceral concentrates at elevated temperatures. Fuel 1991, 70 (5), 635–639. (22) Strugnell, B.; Patrick, J. W. Rapid hydropyrolysis studies on coal and maceral concentrates. Fuel 1996, 75 (3), 300–306. (23) Walker, R.; Mastalerz, M. Functional group and individual maceral chemistry of high volatile bituminous coals from southern Indiana: Controls on coking. Int. J. Coal Geol. 2004, 58 (3), 181–191. (24) Megaritis, A.; Messenbo¨ck, R. C.; Chatzakis, I. N.; Dugwell, D. R.; Kandiyoti, R. High-pressure pyrolysis and CO2-gasification of coal maceral concentrates: Conversions and char combustion reactivities. Fuel 1999, 78 (8), 871–882. (25) Cai, H.-Y.; Megaritis, A.; Messenbo¨ck, R.; Dix, M.; Dugwell, D. R.; Kandiyoti, R. Pyrolysis of coal maceral concentrates under pf-combustion conditions (I): Changes in volatile release and char combustibility as a function of rank. Fuel 1998, 77 (12), 1273–1282.
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with higher proportions of vitrinite. Then, the good correlation found between apparent density and liptinite content corroborates that the fluidity previously determined on the precursor coal is not enough to predict the texture of the resultant foam and that the volatile matter content also plays a significant role under the conditions of the batch reactor used in this study. Table 3 shows the most abundant pore diameter of the foams, obtained from the pore size distribution graphics determined by mercury intrusion up to 227 MPa. On the whole, carbon foams have quite a homogeneous cell size, spherical structure, and open interconnected pores in most of the cells, as shown in the SEM microphotographs displayed in Figure 4. Most of the samples display macropores of around 100 µm, except PLF, NPF, LF, and BUF, whose pore size is closer to 20 µm. These samples also present high values of open porosity and total pore volume. Again, a trend was found between the pore size and the liptinite content (Figure 5a). High liptinite content coals produce higher macropore diameter foams. As it was stated above, the presence of liptinite macerals has two opposite effects. On one hand, it hinders the development of porosity by the pressurerising release of volatile matter originating more dense foams (Figure 3a), with lower total pore volume (Table 3). However, it also favors the fluidity of the coal, resulting in a better pore coalescence under carbonization conditions and, consequently, in higher pore sizes (Figure 5a). Again, the correlation with the content of the vitrinite maceral group turns out to be the opposite (Figure 5b). In summary, coals with low fluidity (PL and the high-rank NP, L, and BU) render carbon foams of lower pore size. However, they display lower apparent density values and higher total pore volumes. This can only be explained by the presence of a larger number of pores than in the other foams. As it has been previously reported for precursors, such as coals8 and ptiches,26 the textural characteristics of the foams dictate their physical properties, which are a consequence of the coal behavior during the carbonization process. More precisely, the mechanical strength depends upon its relative density, which, in turn, is related to the cell edge length and the cell-wall thickness.27 Comparatively, the most resistant foams are those with higher relative density,8,26 and good correlations are typically found for foams with similar origin. Figure 6 shows the relationship between flexural strength and relative density in the foams of this study. Several points escape from the typical trend (foams PLF, NPF, and LF), but all of them correspond to precursor coals with low fluidity. The rest of the foams adjust to an increase of the mechanical strength with the increasing relative density. The difference in fluidity between the two groups of coals is so significant that they can be considered as two different kinds of precursors. Those derived from low-fluidity coals, NPF, LF, and BUF, are the only foams that contain a significant proportion of closed porosity. This has been deduced from the measurement of true density. Table 3 lists the true density of the foams for both grinded samples and pieces. NPF, LF, and BUF render higher values when grinded, indicating the existence of closed pores that He cannot reach in the measurement. This kind of porosity might have an enhancing effect on the flexural strength, but, in any case, these foams should be considered apart from those derived from highfluidity coals. (26) Eksilioglu, A.; Gencay, N.; Yardim, M. F.; Ekinci, E. Mesophase AR pitch derived carbon foam: Effect of temperature, pressure and pressure release time. J. Mater. Sci. 2006, 41 (10), 2743–2748. (27) Gibson, L. J.; Ashby, M. F. Cellular Solids. Structure and Properties, 2nd ed.; Cambridge University Press: Cambridge, U.K., 1997.
Carbon Foams from Different Coals
The case of PLF is different because the precursor coal possesses high mineral matter content (Table 2) and, furthermore, it has been subjected to aerial oxidation. One of the effects of oxidation on coal is a reduction of the fluidity.18 Consequently, the behavior of PL under carbonization and the characteristics of its foam may be, for those reasons, different from that of the rest of the foams. The influence of the maceral composition of the precursor coal on the textural properties of the carbonized foams (observed in Figures 3 and 5) should also be noticed in the physical properties. Because there is a relationship between apparent density of the foams and the liptinite content of the precursor coals (Figure 3a), the latter might be beneficial if foams with high flexural strength are wanted (Figure 7). The foams from low-fluidity coals remain away from the correlation. The foams obtained in this study were subjected to a graphitization process at 2200 °C under inert atmosphere, and the electrical resistivity was measured in the final samples. Table 4 lists the values obtained and compares them to the resistivity shown in the specifications of two commercial graphitic foams.28,29 The lowest electrical resistivity is observed in foam PLF, despite displaying the highest porosity and the lowest apparent density (Table 3) among the foams obtained after the 1100 °C carbonization step in this study. When compared to (28) Touchstone Research Laboratory, Ltd. Product Data Sheet: CFOAM Carbon Foams. http://www.cfoam.com/pdf/CFOAMProductDataSheet.pdf. (29) Poco Graphite. Poco GraphitesThermal Management Materials. http://www.poco.com/tabid/130/Default.aspx.
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commercial graphitic foams, the values of electrical resistivity found in this study are inside the companies specifications.28,29 Conclusions Carbon foams have been obtained from several coals ranging from high- to low-volatile bituminous coals. The textural characteristics of the resultant foams are influenced by the properties of the precursor coals, with fluidity and maceral composition playing a significant role in determining the density, the pore size, and the pore volume of the products. The increasing fluidity gives rise to foams of lower apparent density when considering coals of similar volatile matter content. However, the higher rank coals, with higher volatile matter content and lower fluidity render foams with lower apparent density. The high hydrogen content of the liptinite maceral group promotes an increase of the pore size but, also, an increase of the apparent density as a consequence of condensation reactions, being favored by the increased pressure. Flexural strength is higher in the foams with higher relative density, and all of the foams display low electrical resistivity as compared to commercial ones. Acknowledgment. The authors thank the Spanish Ministry of Education and Science (MEC) (project NAT2005-04658) for financial support. M.C. acknowledges CSIC-ESF for the award of an I3P contract. EF8000778
