Carbon 40 (2002) 1475–1486

C hemical, microstructural and thermal analyses of anaphthalene-derived mesophase pitch

a a , a a a a*M. Dumont , G. Chollon , M.A. Dourges , R. Pailler , X. Bourrat , R. Naslain ,b bJ.L. Bruneel , M. Couzi

a ´Laboratoire des Composites Thermostructuraux (LCTS), UMR 5801 (CNRS-SNECMA-CEA-UB1) Universite Bordeaux-1,´ ´3 Allee de La Boetie, 33600 Pessac, France

b ´ ´ ´Laboratoire de Physico-Chimie Moleculaire (LPCM), UMR 5803, Universite Bordeaux-1, 351 Cours de la Liberation, 33405 Talence,France

Received 6 July 2001; accepted 30 November 2001

Abstract

A detailed characterisation of a synthetic naphthalene-derived mesophase pitch, in its as-received state and duringpyrolysis, has been performed. The study has been conducted by means of various techniques and with a particular attentionto Raman microspectroscopy. The Raman spectra show features in common with the naphthalene precursor, i.e., a broad and

21 21complex band at 1150–1500 cm and a multicomponent G band at 1600 cm . These features correspond to the vibrationmodes of the molecules of the pitch and more especially to the non-aromatic C–C bonds involved in alkyl groups, aryl–arylbonds or naphthenic rings. The pyrolysis of the pitch into coke takes place within a narrow temperature range (480–500 8C)through the elimination of hydrogen and light alkanes resulting from the breaking of homolytic C–H bonds and naphtheniccycles, respectively. This process initiates a swelling of the pitch. The analysis of the Raman features shows that thestructure of the pitch is only slightly affected within this temperature range. Conversely, significant structural changes of the

21material (as shown by the vanishing of the multicomponent bands at 1600 and 1150–1500 cm ) are evidenced beyond750 8C, simultaneously with a hydrogen release and an increase of the true density. This phenomenon corresponds to theextension of the graphene layers of the coke and the formation of a distorted carbon network. 2002 Elsevier ScienceLtd. All rights reserved.

Keywords: A. Mesophase pitch; B. Pyrolysis; C. Raman spectroscopy; D. Microstructure

1 . Introduction produced C–C composites. A wider application range forsuch materials is still difficult to achieve because of their

Carbon–carbon composites (C–C) exhibit a large vari- high manufacturing cost. Much research has been devotedety of valuable properties for structural applications, such to the development of faster, cheaper and effective pro-as high strength and stiffness at high temperature, a low cesses. Chemical vapour deposition (CVD) is generallydensity, a low thermal expansion, a wide range of thermal used to process high performance C–C composites [2]. Anconductivity, a high ablation–abrasion resistance and a interesting alternative is the impregnation and pyrolysis ofgood friction behaviour. Initially developed for a high a liquid precursor, such as pitch-based for instance. Pitchtechnology field, i.e., for military and space (rocket precursors are either derived from the distillation of coalnozzles, exhaust cones, etc.) [1], new generation C–C tar or petroleum. They are converted into carbon matrixcomposites are being developed for civil applications, according to appropriate heat treatments usually requiringimplying a significantly larger production. C–C brakes for severe conditions (e.g., very high pressures, typically 70–aircrafts represent the most important part of the currently 100 MPa) [3].

From 1987, on the basis of Mochida’s work, MitsubishiGas Chemical (MGC) and Mitsubishi Oil (MO) [4–7]*Corresponding author. Tel.: 133-5-56844-727; fax: 133-5-have simultaneously developed a new generation of syn-56841-225.

E-mail address: chollon@lcts.u-bordeaux.fr (G. Chollon). thetic pitches derived from polyaromatic systems. In

0008-6223/02/$ – see front matter 2002 Elsevier Science Ltd. All rights reserved.PI I : S0008-6223( 01 )00320-7

1476 M. Dumont et al. / Carbon 40 (2002) 1475 –1486

addition to the advantage of a better control of the [11,12]. They are assigned to defects within the carboncomposition, these synthetic pitches are fully anisotropic, microtexture (edges, distorted graphene layers, etc.) withthey are characterised by a relatively low viscosity and a regard to the ideal graphite structure [8–12].high coke yield (beyond 80 wt.%). One should therefore The MSR analyses were performed from the sameconsider such a precursor for the densification of three- polished samples analysed by optical microscopy. Thedimensional fibrous textures. This study mainly focuses on analyses were conducted with a Labram 10 spectrometer

1a 100% anisotropic commercial precursor synthesised from (Jobin Yvon) with the 514.5-nm emission line of an Arnaphthalene, the ara24r from MGC [4,5]. The physical and laser as the monochromatic excitation source. The lateralthe chemical properties of the as received ara24r precursor resolution of the laser probe was close to 1–2 mm (with ahave been first investigated and compared to other com- microscope objective 3100) and the thickness analysedmercial pitches. The mechanism of the low-pressure was estimated to be of the order of 100 nm [11]. Thepyrolysis of the pitch and its conversion into carbon has power was adjusted between 0.5 and 7.5 mW to minimisebeen subsequently studied to precisely define the optimal the acquisition time while preventing an excessive heatingconditions for the C–C composite processing. of the sample.

Thermal analyses, i.e., thermogravimetric and differen-tial thermal analyses (TGA-DTA) were carried-out with

2 . Experimental procedures STA409 Netzsch analyser. The thermal analysis devicewas coupled to a mass spectrometer equipped with a

The precursor was characterised by polarised light quadripolar analyser (Thermostar, Balzers), to simultan-optical microscopy. The observations were carried-out in eously identify and qualitatively assess the concentrationthe reflection mode on polished solid pitch samples of the volatile species released during the pyrolysis (m,

embedded in epoxy resin. The specimens were prepared 200 uma). The samples (10 mg) were set in an aluminafrom as-received pitch granules or from solidified samples crucible and heated up to 1000 8C with a rate of 5 8C

21after melting at 370 8C. The microscope (Leitz) was min under flowing pure argon (P5100 kPa). The TGAequipped with crossed polarisers and a tint l plate to and DTA curves were corrected by subtracting a blankclearly evidence the anisotropic character of the pitch, signal (i.e., a signal recorded during a similar thermalfrom the interference colours resulting from the birefring- cycle but without any sample) from the signal recordedence of the liquid crystal (quenched and observed at room during the analysis of the sample.temperature). The sample was set on a 0–1808 rotating Other ara24r pitch samples (about 5 g) were pyrolysed

21stage to analyse the optical texture of the various domains. according to the same thermal treatment (5 8C min underRaman microspectroscopy (RMS) is a non-destructive flowing argon) but interrupted at an intermediate tempera-

technique, very appropriate for studying at the micrometer ture ranging from 420 to 750 8C. Several characterisationscale the chemical bonding, the phase composition and the techniques were applied to the resulting cokes, such ascrystalline state of solids, from poorly organised to well elementary analyses, RMS analyses and density measure-crystallised structures. This technique is based on the ments.analysis of the spectral characteristics of vibrational modes The chemical composition of the pitch (C, H, S, N andassociated with the Raman peaks (wave number, band- O weight ratio) and the pitch-based cokes was assessed bywidth and intensity). The RMS technique has been widely elementary analysis (Service Central d’Analyse du CNRS,applied to the study of carbon materials and their graphiti- Vernaison).sation behaviour at high temperature [8–12]. The hexagon- The true density of the samples (i.e., while taking the

4al graphite structure (space group D ) is characterised by open porosity into account) was measured by helium6h

two Raman active vibrational modes with E symmetry pycnometry (Accupyc 1330 from Micromeretics).2g

(E and E ). Both modes correspond to in-plane During the pyrolysis of pitches, and more especially2g1 2g2

vibrations of carbon atoms. Whereas the former mode mesophase pitches, the release of gaseous species within acorresponds to shear displacement of whole graphene relatively narrow temperature range, associated to an

21layers (nE 542 cm , not investigated here), the latter increase of the viscosity, leads to a swelling phenomenon2g12involves the stretching of the sp C–C bond. Owing to the and the formation of foam. A method similar to that

strong C–C covalent bond (as compared to the weak developed by Kanno et al. was applied to assess theinterlayer bonding), the E mode (commonly referred to swelling of the pitch during pyrolysis [13]. A tablet of2g2

as the G band) is characterised by a much higher fre- compacted pitch powder ([513 mm, 1 g) was set at the21quency (nE 51582 cm ). In the case of disordered closed end of a vertical tubular furnace (amorphous SiO ,2g2 2

carbons (e.g., turbostratic), the Raman spectra commonly [520 mm) and the pyrolysis was carried out under21show an additional feature at about 1350 cm (D band) nitrogen (100 kPa) according to the thermal cycle de-

as well as, in some cases, another weak band around 1620 scribed above. The length of the foam column into the tube21cm (D9 band), close to the G band. Both D and D9 bands (swelling length) was measured after pyrolysis and the

are generally associated to a broadening of the G band residue was weighted to calculate the coke yield.

M. Dumont et al. / Carbon 40 (2002) 1475 –1486 1477

Table 1Characteristics of synthetic pitches derived from naphthalene (data provided by the manufacturer)

a b c dSP CY AC H/C Solubility (%) Price(8C) (%) (%) (atomic) (euro /kg)

TS TI-PS PI

Ara24r 290 83 100 0.61 30 13 57 –a SP, softening point (Mettler).b 21CY, coke yield (after pyrolysis: 5 8C min , 1000 8C, 1 h, N ).2c AC, anisotropic character.d TS, toluene soluble; TI-PS, toluene insoluble-pyridine soluble; PI, pyridine insoluble.

C–C composites were prepared from an ex-PAN 3D MGC from methylnaphthalene (5T14 from MGC) ortexture. The carbon preform was impregnated with molten alkylbenzene (MSP285) and two isotropic pitches, eitherara24r precursor. The composites were subsequently car- coal tar (V pitch, from HGD, Centre de Pyrolyse debonised at a very slow heating rate (3 8C/h) under nitrogen Marienau, France) or petroleum based (A240 from Ashlandat temperatures up to 750 8C. Some of the specimens were Petroleum, USA). The ara24r precursor essentially consistssubmitted to a further pyrolysis under flowing argon at of carbon and hydrogen. The synthesis route for this1400 8C (1 h) and to a final graphitisation treatment under particular pitch involves only pure naphthalene-basedargon at temperatures ranging from 2080 to 2700 8C (15 polyaromatic species. Conversely to other coal tar ormin). The matrix of the C–C composites treated at various petroleum pitches, this process gives rise to very lowtemperatures was locally analysed by RMS. The compos- amounts of heteroelements (0.23 wt.% sulphur and belowites investigated by RMS were processed from only one 0.1 wt.% for nitrogen and oxygen). The H/C atomic ratioimpregnation–pyrolysis cycle. The fibre and matrix frac- is a characteristic of the condensation state of the pitchtions were, respectively, about 25 and 33% of the total molecules. The H/C ratio for the ara24r precursor isapparent volume of the specimens. intermediate between the values for the V and the A240

pitches, the latter showing the most condensed state owingto its coal tar origin.

3 . Results and discussion3 .1.2. Structural properties

3 .1. As-received pitch properties3 .1.2.1. Optical microscopy analysis. The ara24r pre-

3 .1.1. General properties cursor shows a flow domain optical texture. Depending onThe synthetic mesophase pitch was provided by MGC the preparation procedure of the samples (e.g., after re-

under the reference ara24r (batch number 6T10). The solidification), the texture shows domains of a variableproperties of the pitch are shown in Table 1. The chemical size. The as-received precursor, in the form of granulescomposition, as assessed by elementary analysis, is pre- (d53 mm, L.7 mm), consists of very elongated fibroussented in Table 2. They are compared with the properties domains (d510 mm, L.1 mm). This texture is likelyof two other synthetic mesophase pitches synthesised by induced by an extrusion of the precursor during the

Table 2Elementary analysis of the various pitch precursors (present study)

aPitch type SP Elementary analysis (wt.%) H/C(8C) (atomic)

C H S N O

ara24r Synthetic 290 94.68 4.85 0.23 ,0.1 ,0.3 0.61(naphthalene)

5T14 Synthetic (methyl- 242 – – – – – –naphthalene)

MSP285 Synthetic 285 – – – – – –(alkylbenzene)

V Coal tar 90 92.71 4.25 0.52 0.95 1.5 0.55

A240 Petroleum 120 92.70 5.23 1.9 0.32 ,0.3 0.68a SP, softening point (Mettler).

1478 M. Dumont et al. / Carbon 40 (2002) 1475 –1486

processing. After a post-treatment at 370 8C and a re-solidification, the anisotropic domains show significantlylarger but still elongated shapes, resulting from the liquidflow. The rotation of the sample on the microscope stageresults in a change of the characteristic tints. The blue tintturns to magenta and subsequently to yellow after rotatingthe sample to an angle of 45 and 908, respectively. It isworthy of note that all the domains of the sample show achange in their characteristic tints while rotating the stage,confirming the 100% anisotropic character of the ara24rpitch.

3 .1.2.2. RMS analysis. 3 .1.2.2.1. As-received ara24rpitch The Raman analyses were carried-out on the ara24rprecursor as well as the two other mesophase pitchessynthesised from methylnaphthalene (5T14) or alkylben-zene (MSP285) and the two isotropic pitches A240 and V.

The control of the power of the incident laser (normal-ised to the spot size) is of great importance for the RMS Fig. 2. Raman spectra of the various pitches (inset: spectra afteranalyses, especially for this type of materials, very absorb- background subtraction).ing and sensitive to heat. As a matter of fact, an excessiveincident power may result in an early pyrolysis of the pitch

ground level of the ara24r and 5T14 synthetic pitches,into coke affecting the characteristic Raman features (Fig.derived, respectively, from naphthalene and methylnaph-1). For the following analyses, all spectra were recordedthalene species, is significantly lower than for the otherusing an incident laser power below 0.5 mW with apitches synthesised either from alkylbenzene (MSP285),magnification 3100 (corresponding to a laser spot of aboutcoal tar (V) or petroleum (A240) (Fig. 2). Such a1–2 mm) and the sample surface state under the laser spotdifference between the Raman spectra might be due to thewas systematically controlled after the measurements.variable type of molecules present in the precursors, i.e.,All the spectra recorded from the various precursorstheir aromaticity and the occurrence of alkyl chains.show an intense fluorescence background (Fig. 2). The

Except fluorescence, the spectrum for the ara24r pitchpresence of a large amount of hydrogen in the materials isexhibits features in common with those obtained fromthought to be responsible for such a fluorescence phenom-disordered carbon materials, i.e., a G band around 1600enon for a 514.3-nm exciting wavelength. Marchon et al.

21cm and a broad complex band, lying between 1150 andestablished empirical quantitative relations between the211500 cm and comprising the usual wavenumbers for theintensity of the background and the hydrogen content of

D band. However, a careful examination of the spectrumdiamond-like carbon films [14]. The fluorescence back-reveals noticeable differences in both the D and G bands.

21The relative intensity of the 1600 cm band is higher21(I /I 50.5) and its width about 50 cm narrower for theD G

ara24r pitch than for usual carbons (Fig. 2). The complex211150–1500-cm band also shows several well-defined

21components at approximately 1250, 1290 and 1375 cm .For a better analysis of the spectrum, a curve fitting wascarried-out using different functions (mixing Gaussian andLorentzian shapes). Besides the latter three functions, a

21broad Gaussian band around 1350 cm had to beconsidered to fit the spectrum. An example of fit isrepresented in Fig. 3, showing the three sharp components

21 21at 1248, 1288 and 1376 cm in addition to the 1350-cm21band (Table 3). The 1600-cm band actually consists of

two components G and G (n ,n ). The band fitting1 2 G1 G221leads to two distinct peaks at 1609 cm (the most intense)

21and 1580 cm , the latter value corresponding rather wellto the G band for graphite.Fig. 1. Influence of the power of the incident laser on the Raman

spectrum of the ara24r pitch. The Raman spectral features of the ara24r pitch in

M. Dumont et al. / Carbon 40 (2002) 1475 –1486 1479

Fig. 3. Band-fitting of the Raman spectrum of the ara24r pitch (see band-fitting parameters in Table 3).

common with those of usual disordered carbon materials (a Some of the characteristics of the ara24r pitch are common21sharp G peak at 1580 cm and large D band at 1350 to other similar (mesophase pitches) or related materials

21cm ) as well as the distinctive features (components at such as PAHs (polycyclic aromatic hydrocarbons) or211609, 1248, 1288 and 1376 cm ), can be explained by the polyparaphenylene-derived products. There is a certain

particular structure of the mesophase pitch. The ara24r similarity between the different vibration modes of thepitch consists of a combination of molecules resulting from C–C bonds within the plane of aromatic molecules, around

21the polycondensation of a unique pattern (i.e., naphthalene- 1570–1620 cm and the E mode for graphite at 15802g221based oligomers). The variety of molecules present in this cm . The difference between the vibration frequencies of

synthetic pitch is therefore limited, as compared to a the various systems is related to the variation of thenatural pitch such as the A240. Furthermore, the structure strength of the C–C bonds, resulting from distortions ofof these molecules significantly differs from that of a the polycyclic molecules [15] and/or a simple size effect

21graphene layer in a disordered carbon. They are indeed [16]. Hence, the two-component 1600-cm band for thecharacterised by smaller dimensions in the ara24r pitch and ara24r pitch (a feature already observed for other mesoph-by the presence of aryl–aryl C–C bonds, naphthenic rings ase pitches [15] and large PAHs [16]) would be due to theand possibly also alkyl groups at the periphery of the different types of covalent C–C bonds within the mole-molecules. Many C–C bonds, having a different nature cules, presenting different lengths, strengths and therefore,from that of the aromatic rings of the graphene layers different vibrational frequencies [15]. The relatively sharp

21(involving C–C double bonds) are present in the pitch. peak of the 1600-cm band for the ara24r pitch, ascompared to disordered carbons, is inherent to the molecu-lar character of the pitch (similarly to PAHs). Whereas the

Table 3 structure of disordered carbons corresponds to a particu-Band-fitting parameters as derived from the Raman spectrum of larly broad G band, owing to large distortions of carbonthe ara24r pitch (Fig. 3) network, the presence of hydrogen and alkyl groups at theFunction Position Full width at edges of the planes decreases the interactions between each

21(cm ) half maximum molecule (mainly Van der Waals type forces) and gives rise21(cm ) to much better defined vibrational modes.

21Similarly to the 1600-cm band, it is noteworthy thatG/L 1609 3721

G/L 1580 26 the different components of the complex 1150–1500-cmG/L 1376 36 band of the ara24r pitch presents some similarities withG 1350 204 spectra obtained from PAHs such as naphthalene orG/L 1288 20 perylene (containing the same units) showing components

21G/L 1248 33 located, respectively, at 1382 and 1374 cm [17,18].G, Gaussian; L, Lorentzian. Rogovoi and Amerik have also observed similar spectral

1480 M. Dumont et al. / Carbon 40 (2002) 1475 –1486

Table 4features for another mesophase pitch, but except a shoulder21 Band-fitting parameters as derived from the Raman spectra of thearound 1620 cm , the intensity of the various components

various fractions of the ara24r pitch (Fig. 4)were significantly weaker [15]. Other common featureswith polyparaphenylene and its derived products are I /I I /IG1 G2 D G2

worthwhile mentioning. Marucci et al. showed the pres-As received 0.47 0.38ence of characteristic vibrational modes of theTS 0.64 0.36paraphenylene units and more especially the vibration ofTI-PS 0.48 0.36the C–C bond linking the phenyl rings and also associatedPI 0.39 0.39to C–H bonds vibrations [19]. Just as for the poly-

paraphenylene-derived products, the various components21of the ara24r pitch at 1250, 1290 and 1375 cm are likely

21to be associated to the A type vibration of the specific features, in particular concerning the 1150–1500-cmg21benzenic skeleton of the molecules present in the pitch band. The relative intensity of the broad 1350-cm band

[16,19]. (relative to the G band) is approximately equal for allfractions. Conversely, the sharp components within the3 .1.2.2.2. ara24r pitch soluble /insoluble fractions In

211150–1500-cm range and G band more particularly,order to complete the RMS study of the ara24r pitch andshow different noticeable features. The position of theattempt to distinguish the different Raman characteristicsbands remain unchanged for the various fractions but theof the molecules of the pitch, three fractions obtained afterintensity of the G band (relative to the G band) increasessuccessive extractions with toluene and pyridine have also 1 2

from the PI fraction to the TS fraction (from I /I 50.39been characterised. The optical microscopy observations G1 G221revealed that only the pyridine insoluble (PI) fraction to 0.64) (Table 4). The 1250-, 1290- and 1375-cm bands

shows an anisotropic texture. The Raman spectra obtained are also apparently slightly sharper for the TS fraction, infrom the fractions soluble in toluene (TS), insoluble in agreement with the improved molecular character and thetoluene and soluble in pyridine (TI-PS) and insoluble in smaller average molecular size of this fraction. However,

21pyridine (PI) are shown in Fig. 4. The same fitting this analysis tends to show that the G band at 1580 cm1

procedure as described above was also applied to the is representative of the most soluble fractions, i.e., the21spectra, with a particular attention to the two components smallest molecules, whereas the G band at 1610 cm is2

of the G band, G and G . All the spectra display similar rather characteristic of the larger molecules. The Raman1 2

Fig. 4. Raman spectra of the various fractions of the ara24r pitch (see band-fitting parameters in Table 4).

M. Dumont et al. / Carbon 40 (2002) 1475 –1486 1481

features of the various fractions are apparently not relatedto the anisotropic character of the pitch and its derivedproducts. The molecular patterns responsible for theRaman vibrational modes observed for the ara24r pitch arealso apparently present in all the fractions. This feature isindeed consistent with the association of the 1250–1375-

21cm bands to the A vibration of the benzenic skeleton ofg

the molecules of the pitch, as suggested above.3 .1.2.2.3. Pitches from other origins Pitches from other

sources have also been studied. There is no markeddifference between the spectra of the ara24r and 5T14pitch synthesised from methylnaphthalene (Fig. 2). Al-though the existence of additional methyl groups in thelatter material, the respective structures of the pitches areclose enough to yield similar Raman features. In spite ofthe strong fluorescence observed for the alkylbenzene-based MSP285 pitch, making difficult the analysis, theRaman spectrum is not as complex as those previously

21mentioned. Hence, only a broad band around 1350 cm is21observed in the 1150–1500-cm region (Fig. 2). This

feature might be due to the cata-condensed structure of thispitch, consisting of a non-linear arrangement of about eightaromatic rings and the presence of peripheral methylgroups (four to five). Furthermore, the molecules of theMSP285 pitch are more condensed than those of the ara24rand do not contain naphthenic rings. The analysis of theRaman spectra recorded from various PAHs shows that thespectrum of phenanthrene, which is the closest molecule tothe MSP285 pitch pattern, displays features differing fromthose observed for the naphthalene or perylene within the

21 211100–1500-cm region, i.e., a sharp band at 1350 cm[17].

3 .2. Pyrolysis and graphitisation study of the pitch

Fig. 5. Thermal analyses during the pyrolysis of the ara24r pitchVarious investigations on the pyrolysis and the thermal (5 8C/min). (a) Swelling length, (b) H/C atomic ratio, (c) DTA

analysis of the pitch have been carried-out to estimate the and (d) TGA signals.optimal conditions for the use as a carbon matrix precursorfor 3D C–C composites. The gradual transformation of the

ular rearrangements and hydrogen transfer [20,21]. Thepitch into coke has been studied, the principal objectiveDTA curve therefore shows a mean behaviour (eitherbeing to prevent or limit the swelling, a major drawbackendothermic or exothermic) for a given temperature.inherent to mesophase pitches.Studies from Martinez-Alonzo et al. have shown that theshape of the DTA curves depends on the structure, the

3 .2.1. Thermal analyses (TGA-DTA–mass spectrometry) functionality and the molecular distribution, i.e., on theThe TGA curve recorded during the pyrolysis of the nature of the studied pitch [22].

ara24r pitch evidences a high coke yield (83 wt.%) at Pitch materials do not have a well-defined melting1000 8C. The release of gaseous species occurs within a temperature but a rather broad softening domain. A slightrelatively narrow 400–550 8C temperature range with a endothermic peak is however observed around 460 8Cmaximum at about 480 8C (Fig. 5). The analysis of the (arrow on Fig. 5c). This feature is related to the maximalDTA curve is more complex owing to the various phenom- weight loss observed on the TGA curve (Fig. 5), corre-ena occurring simultaneously and presenting either an sponding to the release of gaseous species. The thermalendothermic or exothermic character. Different physical behaviour is mainly exothermic between 460 and 560 8Cand chemical phenomena take place during the pyrolysis of and corresponds to polymerisation and polycondensationthe pitch between 200 and 800 8C: devolatilisation, poly- reactions of the molecules yielding an infusible material,merisation, polycondensation or cracking reactions, molec- the semi-coke. An endothermic behaviour is detected on

1482 M. Dumont et al. / Carbon 40 (2002) 1475 –1486

the DTA signal from 560 8C, with a very pronounced different from those of the naphthenic rings and giving risemaximum around 740 8C. This feature is related to the to a methane release. This mechanism might correspond toaromatisation of the semi-coke and to the starting trans- the breaking of methyl groups present in certain aromaticformation into coke. rings and requiring a higher thermal activation. The

Only masses below 57 uma were detected during the relatively high emission of gaseous species around 480 8Cmass spectrometry analysis of the gas phase during induces a significant swelling of the pitch occurring withinpyrolysis. The absence of higher masses discards the the same temperature range (Fig. 5). The simultaneousrelease of oligomers (such as toluene for instance) resulting increase of the viscosity then leads to the formation offrom the cleavage of the aryl–aryl bonds present in certain foam.molecules of the pitch. The analysis conditions allowed the Only hydrogen is detected beyond 750 8C, with ameasurement of masses up to only 200 uma and therefore maximum emission at 760 8C. An aromatisation of thethe presence of molecules of higher masses (.200 uma) is graphene layers takes place, corresponding to the largenot excluded. The gaseous species released during endothermic character of the DTA signal observed atpyrolysis mainly consist of hydrogen and light alkanes 740 8C (Fig. 5). This aromatisation process corresponds to(from methane to butane: C1–C4, Fig. 6). The release of an important structural transition of the semi-coke intogases is only effective beyond 400 8C and corresponds to coke, as confirmed by the significant increase of the truethe starting of the mass loss detected by TGA. density observed within this temperature range (Fig. 7).

The major emission of light alkanes takes place around480–500 8C (Fig. 6). Its occurs simultaneously with a first 3 .2.2. Structural analyses (RMS)hydrogen release resulting from the thermally inducedbreaking of C–H homolytic bonds. The light alkanes are 3 .2.2.1. Coke from ara24r pitch. The structural evolu-likely generated from the opening and the cleavage of the tion of the ara24r was investigated by Raman microspec-naphthenic rings according to the mechanism proposed by troscopy after pyrolysis under nitrogen up to 750 8C (5 8C

21Drhoblav and Stevenson [23]. The homolytic cleavage of min ). The recorded spectra show a gradual decrease ofthe naphthenic ring leads to the formation of radicals, the fluorescence background intensity with the pyrolysiswhich subsequently recombine to form a more condensed temperature (Fig. 8). This phenomenon is likely due to thestructure, after the elimination of hydrogen or alkanes such gradual elimination of hydrogen resulting from theas ethane, propane, etc. This mechanism is supported by aromatisation and the condensation of the moleculesthe evolution of the H/C atomic ratio assessed by elemen- during the early pyrolysis. An elemental analysis of thetary analysis (Fig. 5). A careful analysis of the spectra pitch was carried-out in order to better assess the chemicalshows that the methane release takes place mainly around changes occurring during pyrolysis. A similar approach as500 8C but is still effective at higher temperatures (be- that proposed by Marchon et al. was applied to quantifytween 600 and 750 8C). Conversely, the emission of other the fluorescence [14]. The spectra were first normalised toC2, C3 and C4 alkanes shows a maximum around 480 8C the maximal intensity of the G band. The intensity of theand is almost fully achieved beyond 550 8C. This different fluorescence background was calculated from the intensitybehaviour can be explained by the breaking of bonds

Fig. 6. Mass spectrometry analysis during the pyrolysis of the Fig. 7. Influence of the pyrolysis temperature on the true densityara24r pitch (5 8C/min). of the ara24r pitch.

M. Dumont et al. / Carbon 40 (2002) 1475 –1486 1483

centration and the fluorescence of the material, as assessedfrom the background intensity measurement by RMS.

A detailed analysis of the spectra during pyrolysis in the211150–1500-cm region more especially, shows only a

limited evolution up to 550 8C. The characteristic bands ofthe as-received pitch are still observed without noticeablechange of position. A slight decrease of the intensity of the

211250- and 1290-cm bands is observed at 550 8C (Fig. 8).The TGA curve however shows a major evolution between400 and 550 8C corresponding to an important gaseousrelease, the formation of an infusible system and itstransformation into coke.

A significant evolution of the Raman spectra appearsonly beyond 750 8C, with the vanishing of the 1250- and

211290-cm bands. Moreover, the intensity of the 1375-21cm band also decreases and the splitting of the 1600-21cm G band can no longer be detected (Fig. 8). The

thermal treatment of the pitch at a higher temperature leadsto a spectrum characteristic of disordered carbons, with a

21broad and unique D band at 1350 cm and a G band at211600 cm .

The first stage of the pyrolysis freezes the anisotropicstructure of the pitch pre-existing in the liquid state. Thisphenomenon, which occurs below 750 8C, has apparentlyno major influence on the Raman vibrational modes of theFig. 8. Influence of the pyrolysis temperature on the Ramancarbon structure. Conversely, beyond 750 8C, a significantspectra of the coke from the ara24r pitch.structural and chemical evolution of the material (largeincrease of density and hydrogen release) takes place,

21values measured at 800 and 1950 cm (respectively, y giving rise to an apparently less organised structure.1

and y ). The slope of the curve associated to fluorescence Although leading to an extension of the size of the carbon2

was then assessed from the parameter y /y (the classical domains, this phenomenon initiates the formation of a2 1

description of the slope being impossible since the mea- carbon network containing numerous defects as well assurement conditions may affect the intensity). Fig. 9 distortions of the graphene layers. The Raman characteris-clearly shows a linear relation between the H/C atomic tics of this type of structure significantly differ from thoseratio and the y /y parameter. Hence, as for DLC films, of molecular systems such as the ara24r pitch.2 1

there is a direct correlation between the hydrogen con-3 .2.2.2. C–C composite matrix from ara24r pitch. TheRMS study was also extended to the matrix of thecomposites impregnated with ara24r pitch and heat-treatedat temperatures corresponding to various stages of theirpreparation. The evolution of the Raman spectra of thematrix heat-treated at temperatures ranging from 430 to2700 8C is shown in Fig. 10. The matrix treated at 430 8Cconsists of an infusible system. This state of the material isobtained at a pyrolysis temperature slightly higher thanthat corresponding to the most intense gas release. Adetailed analysis of the Raman features as a function of theheat treatment temperature evidences two distinct domainsassociated to carbonisation, up to 1400 8C and graphitisa-tion at higher temperatures. These two stages are repre-sented in Fig. 11, showing the evolution of the frequency(n ), the full width at half maximum (Dn ) of the G bandG G

and the I /I ratio, as a function of the treatmentD G

temperature.Fig. 9. Correlation between the H/C atomic ratio and the relativefluorescence intensity (as derived from the Raman spectra of 3 .2.2.2.1. Carbonisation step (T ,1400 8C) For a lowpyrolysed ara24r pitch). heat-treatment temperature (T5430 8C), the Raman spec-

1484 M. Dumont et al. / Carbon 40 (2002) 1475 –1486

Fig. 10. Influence of the pyrolysis and the graphitising tempera-ture on the Raman spectra of the C–C composite matrix from theara24r pitch.

trum is similar to those recorded from the as-received21pitch. It shows the same complex 1150–1500-cm band

21and the two-component G band at 1600 cm . The G bandwas considered as a single component for the band fitting Fig. 11. Influence of the pyrolysis and the graphitising tempera-and the measurement of Dn , and the D band wasG ture on the carbon Raman features of the C–C composite matrixassociated to the broad Gaussian component within the from the ara24r pitch.

211150–1500-cm region. A gradual change of the Ramanfeatures, from those of the ara24r pitch to those of typicaldisordered carbon materials is observed up to 1400 8C. a decrease of the frequency of the G band from 1600 to

21More precisely, the evolution of the spectra up to 1400 8C 1580 cm (the value for graphite) and a decrease of DnG

is characterised by a decrease of the width of the D band and I /I . Besides the narrowing of the G and D bands,D G21from 290 to 65 cm and a large increase of the I /I the D9 band, also associated to disordered carbon, appearsD G

21ratio (the D band is more intense than the G band for at 1620 cm . It is worthy of note that both Dn and I /IG D G

T51400 8C, whereas it is the contrary for the as-received vary linearly with n during the graphitisation process.G

pitch). Only a slight decrease of the frequency of the G This evolution of the Raman features during graphitisationband is detected within the carbonisation step (430,T , has been often experimentally observed. It is related to the1400 8C), whereas a significant increase of Dn is observed increase of the size of the coherent domains (L ) inG a

up to 1400 8C. This phenomenon corresponds to the polycrystalline graphite [8]. However, I /I is stronglyD G

structural evolution of the material, i.e., the vanishing of dependent on the orientation of textured materials, such asthe mesophase character of the pitch and the formation of a the ex-pitch matrix, with the laser beam. For instance, themore disordered material. intensity of the D band is close to zero when the laser axis

3 .2.2.2.2. Graphitisation step (2080,T ,2700 8C) The is perpendicular to the graphene layers of the matrix.evolution of the Raman features during the heat-treatment Raman studies from highly orientated pyrolytic graphiteof the matrix at temperatures beyond 1400 8C, corresponds (HOPG) clearly evidenced the relation between I /I andD G

to the behaviour generally observed for graphitising dis- the orientation between the crystal and the incident laserordered carbons [11]. It is simultaneously characterised by [24]. Just as for HOPG, I /I directly depends on theD G

M. Dumont et al. / Carbon 40 (2002) 1475 –1486 1485

amount of graphene layer edges exposed to the laser. The The study of the pyrolysis of the pitch shows that thepresent analyses were performed from regions close to the transformation into semi-coke proceeds, within a narrowfibres, where large orientated domains were found by temperature range (480–500 8C), through the eliminationoptical microscopy. These matrix parts show a maximum of hydrogen and light alkanes (from C1 to C4) resultingof plane edges (the graphene planes lying flat upon the from the breaking of homolytic C–H bonds and naphthenicsurface of the fibres) and give rise to high I /I values. cycles, respectively. This process causes a significantD G

Conversely, n and Dn are almost independent on the swelling of the pitch during pyrolysis. The structure of theG G

orientation of the matrix. They are only influenced by the pitch is only slightly affected within this temperature rangetemperature treatment, i.e., by the structural organisation. (infusibilisation step), as shown by the only limited

Such a two-stage thermal behaviour evidenced by RMS evolution of the Raman features. Conversely, if only a(carbonisation-graphitisation) illustrates the transition from hydrogen release is observed beyond 750 8C, RMS revealsstage 2 to 1, in the three-stage model proposed by Ferrari a significant structural change of the material (vanishing of

21and Robertson, on the disorder characterisation in carbon the multicomponent bands at 1600 and 1150–1500 cm ).materials (from nanocrystalline graphite to amorphous This phenomenon, associated with an increase of the truecarbon) [25]. The graphitisation step corresponds to stage density, corresponds to the disappearing of the mesophase

211, i.e., a shift of the G peak from 1600 to 1581 cm and a character of the pitch, the extension of the graphene layersdecrease of the I /I ratio (varying inversely with La [8]), of the coke and the formation of a distorted carbonD G

from nanocrystalline to microcrystalline graphite. The network. The Raman spectra recorded from the pitch showcarbonisation step rather corresponds to stage 2, i.e., an a gradual decrease of fluorescence with the pyrolysisincrease of I /I from amorphous carbon to nanocrystal- temperature due to the elimination of hydrogen. ElementalD G

line graphite. The multicomponent G band of the pitch analyses clearly established a linear relation between thetreated at low temperature likely affects the fitting and the H/C atomic ratio and the slope the intensity of theaccurate measurement of the band position (a slight fluorescence background. RMS evidences the graphitisa-decrease of n is observed during carbonisation, whereas tion of the coke from T $2080 8C, yielding large domainsG

an increase should be expected during stage 2 [25]). oriented parallel to the fibres.

4 . Conclusion A cknowledgements

The various characterisations of the ara24r pitch, in the This work has been supported through a grant given byas-received state and during pyrolysis, have provided CNRS and Snecma-Moteur to M.D. The authors aredetailed information on the structure, composition and indebted to Professor I. Mochida for his assistance and forthermal behaviour of the pitch. These properties are helpful valuable discussions as well as to Mitsubishi Gas Chemicalto consider the application of this pitch as a carbon matrix for providing different batches of aromatic pitch.precursor. All the analyses and the pyrolysis studies of thepitch, either performed simultaneously (TGA, DTA, massspectrometry, etc.) or in parallel (elemental analyses,

R eferencesRaman spectroscopy, etc.) were obtained from the samebatch.

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