carbon foams from different coals

Carbon Foams from Different Coals

Montserrat Calvo, Roberto Garcıa,* and Sabino R. Moinelo

Instituto Nacional del Carbon (INCAR), Consejo Superior de InVestigaciones Cientıficas (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 mattercontent by a two-stage thermal process. The first stage, a controlled carbonization treatment under pressure at450-500 °C, is responsible for the final textural properties of the foam. In the second stage, the carbonizationproduct was baked at 1100 °C. The foams produced display a macroporous texture with fluidity, volatilematter content, and maceral composition of the precursor coals, having an influence on the apparent densityand the pore size of the resultant porous products. Coals with low fluidity, volatile matter content, and liptinitecontent 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 carbonfoams obtained in this study display good electrical properties (electrical resistivity comparable to that ofcommercial foams).

Introduction

Carbon foams are lightweight (0.2-0.8 g cm-3) and excep-tionally strong cellular materials, with high-temperature resis-tance (up to 3000 °C under reducing or inert atmosphere) andthermal and electrical conductivity adjustable by the foamingthermal treatment. Their low production costs and flexiblephysical properties make them ideal for a wide range ofapplications in such diverse areas as thermal management,electromagnetic interference, acoustic shielding, energy absorp-tion and storage, catalysts supports, filtration, etc.1–4

Carbon foams were first developed as reticulated carbons5,6

from thermosetting organic polymer foams through a thermaltreatment that rendered macroporous refractory materials witha uniform cell size and moderate mechanical strength, especiallyintended for thermal insulating applications or as lightweightconstruction materials for the aerospace industry.

Graphitized carbon foams obtained from mesophase pitchesare most appropriate to produce high strength and thermal andelectrical conductivity because of the interconnected graphiticligament network.1–3,7 Traditionally, a previous thermosettingstep is mandatory to improve the plastic properties of theprecursor by modifying the size of the anisotropic domains

generated in the foaming process.8,9 Then, an oxidative cross-linking stabilization of the foam structure was usually appliedto prevent the porous structure of the foam from melting inany subsequent treatment or use.10 Recently, however, proce-dures have been developed that obviate the time-consuming andexpensive stabilization step.11 These mesophase pitch-basedcarbon foams display thermal conductivities of up to 180 Wm-1 K-1, very appropriate for heat dissipation applications.1,11,12

Carbon foams can be also made straight from coal.8,13,14 Thestructural properties of coal-based carbon foams make themperfectly useful in numerous applications, when very highconductivity is not required and the manufacturing costs canbe considerably reduced because coal can be used as a precursorwithout any previous preparation process. Possible applicationsare thermal isolation, energy adsorption, fire-resistant construc-tions, structural materials, etc. They can even be considered asgood materials for the manufacture of electrodes for battery andfuel cell applications after an appropriate graphitization step.15

* To whom correspondence should be addressed. E-mail: [email protected].

(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 ofthe electrochemical stability of graphite foams as current collectors for leadacid batteries. J. Power Sources 2006, 161 (2), 1392–1399.

(3) Min, G.; Zengmin, S.; Weidong, C.; Hui, L. Anisotropy of mesophasepitch-derived carbon foams. Carbon 2007, 45 (1), 141–145.

(4) Fang, Z.; Li, C.; Sun, J.; Zhang, H.; Zhang, J. The electromagneticcharacteristics of carbon foams. Carbon 2007, 45 (15), 2873–2879.

(5) Ford, W. D. Method of making cellular refractory thermal insulatingmaterial. U.S. Patent 3,121,050, 1964.

(6) Googin, J. M.; Napier, J. M.; Scrivner, M. E. Method for manufac-turing foam carbon products. U.S. Patent 3,345,440, 1967.

(7) Klett, J.; Hardy, R.; Romine, E.; Walls, C.; Burchell, T. High-thermal-conductivity, mesophase-pitch-derived carbon foams: Effect ofprecursor on structure and properties. Carbon 2000, 38 (7), 953–973.

(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 thestabilization conditions on the structural properties of mesophase-pitch-based carbon foams. Carbon 2006, 44 (15), 3371–3372.

(10) Kearns, K. M. Process for preparing pitch foams. U.S. Patent5,868,974, 1999.

(11) Klett, J. W.; Burchell, T. D. Pitch-based carbon foam heat sinkwith 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 propertiesof mesophase pitch-derived graphitic foams. Carbon 2004, 42 (8-9), 1849–1852.

(13) Rogers, D. K.; Plucinski, J. W.; Handley, R. A. Preparation andgraphitization of high-performance carbon foams from coal. In 2001 CarbonConference, Lexington, KY, 2001.

(14) Calvo, M.; Garcıa, R.; Arenillas, A.; Suarez, 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 electrodescomprising coal-based carbon foams. U.S. Patent 6,899,970 B1, 2005.

Energy & Fuels 2008, 22, 3376–33833376

10.1021/ef8000778 CCC: $40.75 2008 American Chemical SocietyPublished on Web 07/29/2008

With previous steps of hydrogenation and separation in theprecursor coal, the production of anisotropic carbon foams wasclaimed.16,17

In this work, carbon foams were obtained from 10 high-,medium-, and low-volatile bituminous coals through a simplethermal procedure. Neither previous modification of the coalsnor further stabilization step of the green foams was carriedout. The aim of this study is to find correlations between theraw coal properties (including the maceral composition) andthe texture and properties of the carbon foams.

Experimental Section

Precursor Characterization. The coals used as carbon foamprecursors were Polio (PL) (Spain), Batan (BA) (Spain), NorwichPark (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 plasticitytest (ASTM D2639-98) and the crucible swelling test (ISO 501:2003) were used to measure the fluidity and dilatation characteristicsof each precursor. Thermogravimetric analysis was carried out witha TA Instruments equipment, model SDT 2960, at a heating rateof 2 °C min-1 from ambient temperature to 1000 °C using 15 mgof sample. The mean random vitrinite reflectance measurement (%Ro) and maceral analysis (Table 2) were carried out on a MPV-Combi Leitz microscope in accordance with the ISO 7404/05 andISO 7404/03 standard procedures, respectively.

Foaming Process. In these experiments, the precursor coal (80g), pulverized at < 212 µm, was pressed at 100 kg cm-2 into acylinder and fed into a 50 × 100 mm cylindrical stainless-steelreactor. Then, it was purged with argon to provide an inert

atmosphere and heated at 2 °C min-1 up to around the temperatureof maximum fluidity (470 °C for NP, L, and BU and 450 °C forthe rest) that was held for 2 h. The outlet valve is kept open untilthe highest volatile release starting temperature was reached (Table1), to let coal moisture and other minor volatiles leave the reactorbefore the foaming process. At that moment, the valve was closed

(16) Stiller, A. H.; Stansberry, P. G.; Zondlo, J. W. Method of makinga 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 carbonfoam material and resultant product. U.S. Patent 6,506,354 B1, 2003.

Table 1. Analytical Data of the Bituminous Coal Used as Precursors for Carbon Foams

coal PL BA NP Z NC AB L BU W PF

Gieseler plasticity testsoftening temperature (°C) 414 388 441 387 380 393 432 446 389 393solidification temperature (°C) 470 476 500 467 482 470 498 502 476 486plastic range (°C) 56 88 59 80 102 77 66 56 87 93maximum fluidity temperature (°C) 444 436 474 427 442 438 470 475 436 439fluidity (ddpm)a 43 1696 30 3019 26 695 7307 80 15 11 883 13 037

swelling index 7.75 5.75 8 3.5 6.5 6 7.5 6.75 5 6.5volatile matter (% db)a 34.1 26.6 17.8 34.5 32.5 34.8 19.0 17.6 34.1 30.8ash (% db)a 5.74 5.28 9.7 7.5 8.4 6.6 7.0 5.6 6.1 6.1thermogravimetric analysis

initial weight loss temperature (°C) 370 370 395 345 368 365 384 400 354 374maximum weight loss temperature (°C) 444 436 477 429 438 432 473 477 436 446final weight loss temperature (°C) 537 540 571 558 522 525 626 585 552 525

a ddpm, dial divisions per minute as obtained by the Gieseler plasticity test (ASTM D2639-98); db, dry basis.

Table 2. Petrographic Analysis Data

maceral composition (%)

coal

vitrinitereflectance

(% Ro)coalranka vitrinite liptinite inertinite mineral matter

PL 0.90 HVB 83.3 2.3 3.4 11.0BA 0.97 HVB 73.7 9.0 13.7 3.6NP 1.49 LVB 71.4 0.0 23.5 5.1Z 0.73 HVB 36.8 10.5 45.7 7.0NC 0.89 HVB 64.7 6.6 24.1 4.6AB 0.86 HVB 64.1 8.8 22.0 5.1L 1.43 MVB 71.1 0.1 24.8 4.0BU 1.57 LVB 75.2 0.0 21.0 3.8W 0.88 HVB 62.8 7.7 23.6 5.9PF 0.99 HVB 66.3 3.4 24.4 5.9

a HVB, high-volatile bituminous coal; LVB, low-volatile bituminouscoal; MVB, medium-volatile bituminous coal.

Table 3. Properties of Carbon Foams

true density (FHe, g cm-3)

foamgrindedfoams

foamin

pieces

apparentdensity(FHg, gcm-3)

openporosity(ε, %)

totalpore

volume(VT, cm3

g-1)

majorityporesize(µm)

PLF 1.85 1.84 0.51 72.3 1.42 19BAF 1.91 1.90 0.84 53.7 0.64 97NPF 1.97 1.82 0.56 67.4 1.21 21ZF 1.88 1.90 0.87 48.5 0.56 132NCF 1.91 1.90 0.61 66.6 1.10 107ABF 1.89 1.89 0.71 61.4 0.86 107LF 1.98 1.89 0.50 71.2 1.42 4BUF 1.94 1.75 0.60 63.6 1.06 11WF 1.88 1.88 0.63 62.9 0.99 108PFF 1.90 1.90 0.67 62.5 0.93 60

Figure 1. Variation of the apparent density of the carbon foams withthe fluidity of the precursor coals. (a) Low-rank (high- and medium-volatile bituminous) coals (fluidity > 3000 ddpm) and (b) high-rank(low-volatile bituminous) coals (fluidity < 100 ddpm). Tentative trendlines are given.

Carbon Foams from Different Coals Energy & Fuels, Vol. 22, No. 5, 2008 3377

and the pressure increased because of the release of volatile matter,which acts as the foaming agent during the coal plastic range. Thegreen foams thus obtained were carbonized under argon flow at1100 °C, with a heating rate of 1 °C min-1 and a soaking time of2 h. The final carbon foams are designated with the letterscorresponding to the precursor coal followed by a F.

Carbon Foams Characterization. The foams were analyzedby scanning electron microscopy (SEM) using a Zeiss microscope,

model DSM-942, provided with an EDS detector OXFORD, modelLink-Isis II. The true density, determined by He displacement, wasmeasured using a pycnometer, Accupyc 1330 from Micromeritics;either foams in pieces or grinded samples were used for themeasurements. Apparent density and pore volume distributions wereevaluated with a mercury porosimeter (AutoPore IV, from Mi-cromeritics), which provides a maximum operating pressure of 227MPa. The percentage of open cells was calculated by eq 1, where

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.

3378 Energy & Fuels, Vol. 22, No. 5, 2008 CalVo et al.

Figure 4. SEM microphotographs of the carbon foams obtained from the different coals.


ε is the open porosity (%) and FHg and FHe are the apparent andtrue densities (g cm-3) determined in Hg and He, respectively.Similarly, the total open pore volume, VT, was obtained by eq 2.

ε) (1-FHg

FHe)100 (1)

VT ) ( 1FHg

- 1FHe

) (2)

Flexural Strength Test. Foams were cut into at least three beamsof 8 × 8 × 50 mm3 for flexure testing. Four-point bending testswere carried out at a constant speed in an Instron Model 450, witha stainless-steal four-point bending fixture. The inner load span was10 mm, and the outer load span was 21 mm; the loading rate was0.002 mm s-1. Failure strength (σf) was calculated by eq 3

σf )3R(L- l)

2bh2(3)

where, R is the rupture load, b and h are the width and the heightof the beam, respectively, and L and l are the outer and inner spans,respectively.

Electrical Resistivity. The procedure followed to determine theelectrical resistivity of the carbon foams is based on the ASTMD6120-97 standard method. Once the foam cylinder has been cutwith perfectly parallel bases, its diameter was measured twice with90° intervals, at four different heights. The average of the eightvalues is used to calculate the value of the area of the cylinder

section (s). Then, the foam was placed in a press between twoconductive plates attached to the pistons, and after being pressedat 50 bar, an electric current with an intensity I was applied throughit. A multimeter, with the connectors placed at a constant distance(d), is used to determine the voltage (V) through the foam bycalculating the average of three measurements in the foam cylinderat intervals of 120°. The electrical resistivity (F) is calculated witheq 4.

F) 10VSld

(4)

Results and Discussion

Visual examination reveals that all of the precursors producedgood quality foams, except NP and BU. Foams NPF and BUFexhibit several cracks, maybe because of an incomplete ag-glomeration. In fact, these are the coals with the lowest fluidity(30 and 15 ddpm, respectively, Table 1) and without liptinitein their maceral composition (Table 2).

The properties of the carbon foams carbonized at 1100 °Care listed in Table 3. The apparent density ranges from 0.50 to0.87 g cm-3. Previous results14 indicate that the apparent densityof the foam decreases with the increasing fluidity of theprecursor coal. The results of this study follow the same trendbut only if low fluidity and high fluidity coals are consideredseparately. Figure 1 displays two apparent density versus fluidityplots: one for the coals with fluidity > 3000 ddpm (high-volatile

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. Tentativetrend lines are given.


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 bitumi-nous coal PL, which displays low fluidity because of aerialoxidation exposition,18 is displayed in Figure 1b. In both plots,the apparent density of the resultant foam decreases significantlywith the increasing fluidity of the precursor coal, reaching aplateau at high fluidity values. The drop is more pronounced inthe lower rank coals (Figure 1a), with the apparent densityof 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 radicalsproducing cracking reactions take place but, simultaneously,some of the latter are involved in condensation reactions witharomatic molecules. On the other hand, the hydrogen-richspecies present in the coal are able to stabilize the fragmentsand convert them into “solvating” species, which make largersize molecules dissolve easier with a concomitant increase offluidity. If there is not enough hydrogen available, the radicalfragments join each other, generating larger molecules. This

would reduce the fluidity, hindering the foaming process andrendering high-density foams. However, converse to the ex-pected, the low-volatile bituminous coals produce the lowestdensity foams, despite having very low fluidity. This fact canbe explained taking into account how the foam cellular structuredevelops in these experiments.

The medium- and high-volatile bituminous coals displayhigher fluidity but also generate higher pressure during thefoaming carbonization experiments. As a consequence, porositydevelopment is hindered and the resulting foams present lowervalues of open porosity and total pore volume (Table 3). Onthe other hand, in the carbonization of the low-volatile precursorstested in this study (NP, L, and BU), the reduced pressure allowsfor 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 ofthe coals tested (62.8-83.3%, Table 2), with the only exceptionof Z (36.8%), in which inertinite dominates (45.7%). Vitrinitedisplays values of the H/C ratio slightly higher (0.70-0.80 dafbasis)19 than inertinite but much lower than liptinite (typically0.95-1.20),19 which is the maceral group with the highest

(18) Ignasiak, B. S.; Szladow, A. J.; Montgomery, D. S. Oxidationstudies on coking coal related to weathering. 3. The influence of acidichydroxyl groups, created during oxidation, on the plasticity and dilatationof the weathered coking coal. Fuel 1974, 53 (1), 12–15.

(19) White, A.; Davies, M. R.; Jones, S. D. Reactivity and characteriza-tion of coal maceral concentrates. Fuel 1989, 68 (4), 511–519.

Figure 6. Variation of the flexural strength with the relative density in the carbonized foams. The error bars correspond to confidence intervals of95%. 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 toconfidence 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.


content of hydrogen.19–22 Qualitatively, hydrogen in liptinitemacerals is mainly aliphatic, whereas aromatic hydrogenpredominates in vitrinites.19–22 The abundant aliphatic groupsof the liptinites render tar and oils during carbonization, helpingto increase fluidity23 and producing lower density foams.However, as mentioned above, PL, NP, L, and BU, despitehaving very low contents of liptinite macerals in their composi-tion and displaying low fluidity, produce foams with the lowestapparent density. This observation may be considered anoma-lous, because lower apparent densities would be expected forthe foams derived from coals with higher liptinite contents. Ithas to be taken into account that, in the batch reaction systemused in this study, the volatiles released by cracking reactionsduring carbonization remain in the reactor contributing to theincrease in the internal pressure. The enhanced concentrationof aliphatic chains in liptinite macerals will contribute more tothe volatilized fraction than to the final structure of the foamsand, therefore, to the increase of pressure in the reactor duringfoaming. Also, the aliphatic hydrogen is more reactive than thearomatic at these temperatures,19 being available for thestabilization of the free radicals formed when coal cross-linksare broken by thermal effect. In fact, it has been previouslyreported that, under carbonization conditions, the conversionsand volatiles release of the different maceral groups decreasein the order: liptinite > vitrinite > inertinite.24,25 Despite thelow liptinite concentrations (Table 2) of the coals studied, ithas an effect on the pressure reached in the foam formationprocess, as shown by the temperature/liptinite content correlationdisplayed in Figure 2.

The pressure increase resulting from the release of volatilematter hinders the porosity development, and as a consequence,higher density foams will be generated when higher amountsof liptinite are present (Figure 3a). The opposite trend isobserved when the content of vitrinite group is considered(Figure 3b), with lower density foams being obtained from coals

with higher proportions of vitrinite. Then, the good correlationfound between apparent density and liptinite content cor-roborates that the fluidity previously determined on the precursorcoal is not enough to predict the texture of the resultant foamand that the volatile matter content also plays a significant roleunder 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 determinedby mercury intrusion up to 227 MPa. On the whole, carbonfoams have quite a homogeneous cell size, spherical structure,and open interconnected pores in most of the cells, as shownin the SEM microphotographs displayed in Figure 4. Most ofthe samples display macropores of around 100 µm, except PLF,NPF, LF, and BUF, whose pore size is closer to 20 µm. Thesesamples also present high values of open porosity and total porevolume.

Again, a trend was found between the pore size and theliptinite content (Figure 5a). High liptinite content coals pro-duce higher macropore diameter foams. As it was stated above,the presence of liptinite macerals has two opposite effects. Onone hand, it hinders the development of porosity by the pressure-rising 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 porecoalescence under carbonization conditions and, consequently,in higher pore sizes (Figure 5a). Again, the correlation withthe content of the vitrinite maceral group turns out to be theopposite (Figure 5b).

In summary, coals with low fluidity (PL and the high-rankNP, L, and BU) render carbon foams of lower pore size.However, they display lower apparent density values and highertotal pore volumes. This can only be explained by the presenceof a larger number of pores than in the other foams.

As it has been previously reported for precursors, such ascoals8 and ptiches,26 the textural characteristics of the foamsdictate their physical properties, which are a consequence ofthe coal behavior during the carbonization process. Moreprecisely, the mechanical strength depends upon its relativedensity, which, in turn, is related to the cell edge length andthe cell-wall thickness.27 Comparatively, the most resistantfoams are those with higher relative density,8,26 and goodcorrelations are typically found for foams with similar origin.Figure 6 shows the relationship between flexural strength andrelative density in the foams of this study. Several points escapefrom the typical trend (foams PLF, NPF, and LF), but all ofthem correspond to precursor coals with low fluidity. The restof the foams adjust to an increase of the mechanical strengthwith the increasing relative density. The difference in fluiditybetween the two groups of coals is so significant that they canbe considered as two different kinds of precursors. Those derivedfrom low-fluidity coals, NPF, LF, and BUF, are the only foamsthat contain a significant proportion of closed porosity. Thishas been deduced from the measurement of true density. Table3 lists the true density of the foams for both grinded samplesand pieces. NPF, LF, and BUF render higher values whengrinded, indicating the existence of closed pores that He cannotreach in the measurement. This kind of porosity might have anenhancing effect on the flexural strength, but, in any case, thesefoams should be considered apart from those derived from high-fluidity coals.

(20) Dyrkacz, G. R.; Bloomquist, C. A. A.; Solomon, P. R. Fouriertransform 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. Infraredspectroscopy of coal maceral concentrates at elevated temperatures. Fuel1991, 70 (5), 635–639.

(22) Strugnell, B.; Patrick, J. W. Rapid hydropyrolysis studies on coaland maceral concentrates. Fuel 1996, 75 (3), 300–306.

(23) Walker, R.; Mastalerz, M. Functional group and individual maceralchemistry of high volatile bituminous coals from southern Indiana: Controlson coking. Int. J. Coal Geol. 2004, 58 (3), 181–191.

(24) Megaritis, A.; Messenbock, R. C.; Chatzakis, I. N.; Dugwell, D. R.;Kandiyoti, R. High-pressure pyrolysis and CO2-gasification of coal maceralconcentrates: Conversions and char combustion reactivities. Fuel 1999, 78(8), 871–882.

(25) Cai, H.-Y.; Megaritis, A.; Messenbock, R.; Dix, M.; Dugwell, D. R.;Kandiyoti, R. Pyrolysis of coal maceral concentrates under pf-combustionconditions (I): Changes in volatile release and char combustibility as afunction of rank. Fuel 1998, 77 (12), 1273–1282.

(26) Eksilioglu, A.; Gencay, N.; Yardim, M. F.; Ekinci, E. MesophaseAR pitch derived carbon foam: Effect of temperature, pressure and pressurerelease time. J. Mater. Sci. 2006, 41 (10), 2743–2748.

(27) Gibson, L. J.; Ashby, M. F. Cellular Solids. Structure andProperties, 2nd ed.; Cambridge University Press: Cambridge, U.K., 1997.

Table 4. Electrical Resistivity of Some of the Foams of ThisStudy, Heat-Treated at 2200 °C, and Two Commercial

Graphitic Foams28,29

foam electrical resistivity (Ω mm)

PLF 47.53 × 10-3

BAF 108.06 × 10-3

ZF 94.21 × 10-3

NCF 119.15 × 10-3

ABF 101.65 × 10-3

WF 80.36 × 10-3

PFF 142.59 × 10-3

Cfoama 0.1-1 × 108

PocoFoama 7.62 × 10-3-25.40 × 10-3

a Commercial graphitic foams.


The case of PLF is different because the precursor coalpossesses high mineral matter content (Table 2) and, further-more, it has been subjected to aerial oxidation. One of the effectsof oxidation on coal is a reduction of the fluidity.18 Conse-quently, the behavior of PL under carbonization and thecharacteristics of its foam may be, for those reasons, differentfrom that of the rest of the foams.

The influence of the maceral composition of the precursorcoal on the textural properties of the carbonized foams (observedin Figures 3 and 5) should also be noticed in the physicalproperties. Because there is a relationship between apparentdensity of the foams and the liptinite content of the precursorcoals (Figure 3a), the latter might be beneficial if foams withhigh flexural strength are wanted (Figure 7). The foams fromlow-fluidity coals remain away from the correlation.

The foams obtained in this study were subjected to agraphitization process at 2200 °C under inert atmosphere, andthe electrical resistivity was measured in the final samples. Table4 lists the values obtained and compares them to the resistivityshown in the specifications of two commercial graphiticfoams.28,29 The lowest electrical resistivity is observed in foamPLF, despite displaying the highest porosity and the lowestapparent density (Table 3) among the foams obtained after the1100 °C carbonization step in this study. When compared to

commercial graphitic foams, the values of electrical resistivityfound in this study are inside the companies specifications.28,29

Conclusions

Carbon foams have been obtained from several coals rangingfrom high- to low-volatile bituminous coals. The texturalcharacteristics of the resultant foams are influenced by theproperties of the precursor coals, with fluidity and maceralcomposition playing a significant role in determining the density,the pore size, and the pore volume of the products. Theincreasing fluidity gives rise to foams of lower apparent densitywhen considering coals of similar volatile matter content.However, the higher rank coals, with higher volatile mattercontent and lower fluidity render foams with lower apparentdensity. The high hydrogen content of the liptinite maceral grouppromotes an increase of the pore size but, also, an increase ofthe apparent density as a consequence of condensation reactions,being favored by the increased pressure. Flexural strength ishigher in the foams with higher relative density, and all of thefoams display low electrical resistivity as compared to com-mercial ones.

Acknowledgment. The authors thank the Spanish Ministry ofEducation and Science (MEC) (project NAT2005-04658) forfinancial support. M.C. acknowledges CSIC-ESF for the awardof an I3P contract.

EF8000778

(28) Touchstone Research Laboratory, Ltd. Product Data Sheet: CFOAMCarbon Foams. http://www.cfoam.com/pdf/CFOAMProductDataSheet.pdf.

(29) Poco Graphite. Poco GraphitesThermal Management Materials.http://www.poco.com/tabid/130/Default.aspx.


carbon foams from different coals

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