Pyrolysis behaviour of stabilized self-sintering mesophase

Carbon 41 (2003) 413–422

Pyrolysis behaviour of stabilized self-sintering mesophase ´ R. Menendez ´ F. Fanjul, M. Granda*, R. Santamarıa, ´ , CSIC, C /Francisco Pintado Fe 26, 33011 Oviedo, Spain Instituto Nacional del Carbon Received 23 March 2002; accepted 1 October 2002

Abstract A coal-tar pitch-based mesophase and a naphthalene-based mesophase were stabilized with air, using a multi-step temperature / time program from 200 to 300 8C. The extent of the stabilization was monitored by elemental analysis. The pyrolysis behaviour of the stabilized samples was studied by thermogravimetric analysis and differential scanning calorimetry. The results showed that the different chemical composition of parent mesophases affected the degree and effectiveness of stabilization, and consequently, the plasticity of the stabilized samples and the intensity of the exothermic effects during their pyrolysis. Naphthalene-based mesophase is more aliphatic and oxygen is taken more rapidly and in a larger amount than in coal-tar pitch-based mesophase. Consequently, the naphthalene-based mesophase stabilized more easily but stabilization was more difficult to control. Moreover, the variations in oxygen uptake and weight gain during stabilization reveal that in this sample, above 250 8C, degradation competes with stabilization. The pyrolysis behaviour of the mesophases is extremely sensitive to the changes produced by stabilization. In the coal-tar pitch-based mesophase the weight loss decreased, and the temperature of maximum rate of weight loss and temperature of initial weight loss increased with the severity of stabilization, while in the naphthalene-based mesophase this tendency was not observed. The competition between stabilization and degradation seems to be the responsible factor for this different behaviour. It was also found that the sinterability of the stabilized samples was mainly governed by their plasticity.  2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Mesophase; Isotropic graphite; B. Stabilization; Carbonization; C. Thermal analysis

1. Introduction Polygranular carbons and graphites are attractive materials that find applications in high performance situations where excellent mechanical, electrical and thermal properties are required (e.g., pistons, electrical brushes, electrodes for electrical discharge machines, components of nuclear reactors [1]). Carbonaceous mesophase is a nematic and discotic liquid crystal phase, which is formed as an intermediate during the carbonization of some aromatic compounds or industrial products, such as pitches [2]. Carbonaceous mesophase is a self-sintering precursor, which is able to produce polygranular carbons and graphites [1,3–10]. The sinterability of mesophase derives from its intrinsic thermoplastic properties that allow mesophase to be moulded and compacted, giving rise to high-density and high-strength artefacts. However, the *Corresponding author. Tel.: 134-985-118-978; fax: 134985-297-662. E-mail address: [email protected] (M. Granda).

thermoplasticity of the mesophase is usually too high, and consequently, after being moulded into rigid pieces, the mesophase deforms and distorts in subsequent carbonizations. It is necessary, therefore, to modify the thermoplastic properties of the mesophase before sintering. A process widely used to reduce plasticity in carbon precursors, such as fibres, is oxidative stabilization at moderate temperatures (,350 8C) [11–17]. This stabilization is usually performed in an oxygen-rich atmosphere like air. The oxygen of the air reacts with the compounds, preferentially via aliphatic hydrogens [18], forming oxygenated groups (i.e., aldehydes, ketones, carboxylic acids, esters [19,20]), which then give rise to polymerization / condensation reactions with the removal of gases, mainly CO, CO 2 and H 2 O, in subsequent carbonization [21,22]. In the case of fibres, stabilization produces an infusible material. However, when dealing with polygranular carbons, the material must retain a certain plasticity [23]. Processes and mechanisms to stabilize mesophase and other carbon precursors have been widely described in the literature. However, to our knowledge there is little

0008-6223 / 02 / $ – see front matter  2002 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 02 )00343-3

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information available about the behaviour of stabilized samples during carbonization. The aim of this work is to study the pyrolysis behaviour of a coal-tar pitch-based mesophase and a naphthalene-based mesophase stabilized at different temperatures (200–300 8C). Pyrolysis parameters obtained by thermogravimetric analysis and differential scanning calorimetry are related with the stabilization temperature and the plasticity and sinterability of the stabilized samples.

2. Experimental

2.1. Materials used The raw materials used in this study were two mesophase samples (M-A and M-B). M-A was obtained from a commercial impregnating-grade coal-tar pitch by thermal treatment at 430 8C for 4 h and subsequent filtration at 300–350 8C under 0.5 MPa [18]. M-A contains 86.6 vol% of mesophase, mainly made up of microspheres of 10–50 mm in diameter. M-B was a commercial naphthalene-based mesophase pitch produced by Mitsubishi Gas Chemical Corporation.

2.2. Mesophase stabilization Ten grams of powdered mesophase, ground and sieved to ,200 mm, was placed in a 2203110340-mm crucible tray, made of 25-mm mesh wire-cloth, and covered with a wire-cloth lid (200-mm mesh) to prevent the sample from blowing away. The mesophase was stabilized in an oven under an air flow of 20 l h 21 to ensure an oxygen-rich atmosphere, using the following multi-step program: heating at 5 8C min 21 to 200 8C, maintaining this temperature for 1 h, and then heating at 1 8C min 21 to 225, 250, 275 and 300 8C, and maintaining 1 h of soaking time for each of these temperatures. Two grams of sample were taken for analyses at each step of the stabilization. The samples were labelled according to mesophase origin and stabilization temperature as follows: M-A200, M-A225, M-A250, MA275 and M-A300, for samples obtained from M-A; and M-B200, M-B225, M-B250, M-B275 and M-B300, for samples obtained from M-B. In order to evaluate the effect of sampling on stabilization, both mesophase samples were stabilized to 250 8C for 1 h without sampling. Since the characteristics of the samples with and without sampling were virtually the same, it may be assumed that sampling did not affect the stabilization process.

The softening point was measured using a Mettler Toledo equipment adopting the ASTM D3104 standard. The toluene insolubles were calculated according to the Pechiney B-16 (series PT-7 / 79 of STPTC) standard. The 1-methyl-2-pyrrolidinone (NMP) insolubles were determined according to the same standard as for quinoline insolubles (ASTM D2318), but using NMP instead of quinoline. The carbon yield was calculated adopting the Alcan method (ASTM D4715). Diffuse reflectance infrared spectra were recorded with a Nicolet Magna IR-560 spectrometer equipped with a mercury–cadmium telluride detector operating at a resolution of 4 cm 21 by averaging 300 scans. The thermoplasticity of the stabilized samples was determined by means of a specifically designed device [18]. Penetrability was calculated according to the following equation: (l /l o ) ? 100% where l o is the initial height of the pellet and l the insertion of the push rod into the pellet at any stage of the experiment. Thermogravimetric analysis (TG / DTG) was carried out in a TA Instruments thermal analyzer by heating to 1000 8C at 10 8C min 21 in a nitrogen flow of 150 ml min 21 . Thermogravimetric analysis was also used to monitor the changes produced in the mesophase samples by stabilization at different temperatures. The experiments were carried out in a Perkin-Elmer TGA7 thermal analyser, following the same temperature / time program as for the stabilization process (Section 2.2), using an air flow of 110 ml min 21 . Differential scanning calorimetry was performed with a TA Instruments thermal analyser by heating to 600 8C at 10 8C min 21 under an air flow of 52 ml min 21 .

2.4. Preparation of polygranular carbons obtained from stabilized samples Polygranular carbons were prepared by the sintering of stabilized mesophase samples moulded into cylindrical pieces. About 0.3 g of sample was moulded into cylindrical pieces (0.96 mm diameter and 0.33–0.34 mm height) by applying a uniaxial pressure of 140 MPa. The conformed pieces were carbonized under a nitrogen atmosphere at 0.3 8C min 21 to 1000 8C. After 30 min at this temperature, the materials were cooled at 0.3 8C min 21 to 500 8C and then at 1 8C min 21 to room temperature.

2.3. Characterization of the samples 2.5. Characterization of polygranular carbons The carbon, hydrogen, nitrogen and sulphur contents of the samples were determined using a LECO-CHNS-932 elemental analyzer. The oxygen content was determined directly using a LECO-VTF-900 graphite furnace.

The materials were characterized by density determinations (bulk density and helium pycnometry) and optical microscopy. Helium pycnometry was performed on car-

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bonized specimens (0.85 mm diameter and 0.30 mm in height). Porosity accessible to helium was calculated from bulk and helium densities.

3. Results and discussion

3.1. Composition of the parent mesophases Table 1 summarizes the main characteristics of the parent mesophases. Naphthalene-based mesophase (M-B) is partially soluble in toluene and NMP, and softens at 292 8C. Coal-tar pitch-based mesophase (M-A) does not have a defined softening point, at least below 350 8C, and its solubility in toluene and NMP is lower than that of M-B. Despite this, the carbon yield in M-B is greater than in M-A (88.0 and 83.3 wt.%, respectively). Both samples are mainly composed of carbon and hydrogen, especially in the case of M-B. One of the most significant parameters, of particular relevance in the stabilization process, is the content and type of hydrogen. Hydrogen content is higher in M-B than in M-A. Moreover, the hydrogen is more aliphatic in the former than in the latter. Consequently, the aromaticity index, determined as the aromatic hydrogen / total hydrogen ratio, is lower than 0.5 in M-B (Table 1). By contrast, the hydrogen in M-A is mainly aromatic, and the aromaticity index reaches 0.719. Studies carried out by other authors [24,25] revealed that the aliphatic hydrogen in the naphthalene-based mesophase belongs mainly to naphthenic structures, alkyl substituents being less abundant. It is these important differences in content and type of hydrogen that determine the stabilization capacity of the samples.

3.2. Monitoring the stabilization process by thermogravimetric analysis Fig. 1 shows the variation in weight gain by the mesophases with oxidative stabilization temperature, as determined by thermogravimetric analysis. In general terms, stabilization with air up to 300 8C causes a positive weight gain balance. Coal-tar pitch-based mesophase gains

Fig. 1. Step-by-step (bars) and accumulative (lines) weight gain by coal-tar pitch-based mesophase and naphthalene-based mesophase during stabilization.

0.82 wt.% and naphthalene-based mesophase 4.64 wt.%. This weight gain is accompanied by an increase in the oxygen content and a decrease in hydrogen content (Table 2). Oxygen is incorporated as different functional groups and the hydrogen removed is predominantly aliphatic [18]. When the stabilization is considered step-by-step (Fig. 1), it can be observed that the coal-tar pitch-based mesophase gains weight progressively at each step up to 250 8C, the temperature at which M-A reaches a maximum of weight gain. Above this temperature, M-A still continues to increase in weight but this increase is smaller than in the previous steps. In the case of the naphthalene-based mesophase, the maximum weight gain occurs during the heating to 200 8C. After this temperature the weight gain gradually decreases and at 300 8C the weight balance is clearly negative. It is evident, therefore, that oxidative stabilization begins at a lower temperature and occurs to a larger extent in M-B than in M-A. However, there is a critical temperature at which the weight-gain levels off. This could be due to one of two reasons: (i) the decomposition of oxygenated groups and / or (ii) the absence of available hydrogen. From the hydrogen content of the

Table 1 Main characteristics of the parent mesophases Sample

M-A M-B a

Elemental analysis (wt.%) C

H

N

S

O

93.97 94.46

3.50 4.86

1.13 0.00

0.45 0.18

0.95 0.50

Carbon / hydrogen atomic ratio. Softening point (8C). c Toluene insolubles (wt.%). d 1-Methyl-2-pyrrolidinone insolubles (wt.%). e Carbon yield (wt.%). f Aromaticity index determined by FTIR spectroscopy. b

C / Ha

SP b

TI c

NMPI d

CY e

IAr f

2.26 1.63

– 292

76.1 71.7

64.6 48.7

83.3 88.0

0.719 0.326

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Table 2 Elemental analysis of parent and stabilized mesophases Sample

M-A M-A200 M-A225 M-A250 M-A275 M-A300 M-B M-B200 M-B225 M-B250 M-B275 M-B300 a b

Elemental analysis (wt.%) C

H

O

93.81 93.18 92.19 91.21 90.00 88.58 94.46 92.76 90.76 87.01 84.68 83.14

3.50 3.33 3.26 3.14 2.98 2.81 4.86 4.53 4.18 3.76 3.31 3.06

0.95 1.98 2.96 4.05 5.45 7.00 0.50 2.54 4.90 9.05 11.85 13.64

C / Ha

C / Ob

2.26 2.35 2.37 2.44 2.53 2.65 1.63 1.72 1.82 1.94 2.15 2.28

131.51 62.84 41.92 29.99 22.01 16.86 252.56 48.67 24.66 12.80 9.52 8.12

Carbon / hydrogen atomic ratio. Carbon / oxygen atomic ratio.

samples (determined by elemental analysis) and the type of hydrogen (determined by FTIR spectroscopy), the aliphatic hydrogen in M-A and M-B were estimated to be 0.98 and 3.28 wt.%, respectively. These values are higher than the hydrogen consumed during stabilization at 300 8C (Table 2). This suggests that stabilization is competing with degradation, the latter playing a more important role with increasing temperature. Although M-B is easier to stabilize with air than M-A, some of the oxygenated groups formed in the naphthalenebased mesophase are more thermally unstable than those formed in the coal-tar pitch-based mesophase. This is corroborated by the variation in oxygen uptake with temperature (Fig. 2). The oxygen uptake is positive over the entire range of temperatures studied in both samples.

Fig. 2. Step-by-step (bars) and accumulative (lines) oxygen uptake by coal-tar pitch-based mesophase and naphthalene-based mesophase during stabilization.

However, while in M-A the oxygen uptake increases continuously, in M-B it reaches a maximum at 250 8C, after which the oxygen uptake decreases slightly. This suggests again that, although oxygen-containing functional groups are being formed even at 300 8C, other oxygencontaining functional groups are decomposing at the same time. This competition between the formation and decomposition of functional groups is more acute in the naphthalene-based mesophase.

3.3. Reactivity of stabilized mesophases determined by differential scanning calorimetry The reactivity in air of the stabilized samples was studied by differential scanning calorimetry (DSC). It is well-established that oxidation processes are exothermic. It is not surprising, therefore, that the DSC curves of stabilized samples in air are exothermic over the entire range of temperatures studied. However, in these curves two different regions can be distinguished, |400 8C being the temperature that defines these regions. The most intense band is, of course, the one above 400 8C, which is due to the combustion of the material. However, for the purpose of this study, the effects detected below 400 8C are more relevant (Fig. 3). The fact that the intensity of this effect decreases with the severity of the stabilization indicates that it is associated with the stabilization process itself [17]. The derivative of the DSC curves with respect to temperature is extremely effective for studying the air reactivity of stabilized samples, because by means of the DSC-derivative the temperature at which changes in the slope of DSC occur can be determined. The DSC-derivative curve of the coal-tar pitch-based mesophase (Fig. 4a) is characterized by three peaks centred at 217, 312 and 372 8C (Table 3). With increasing stabilization temperature, the exothermic effects between 150 and 400 8C are concentrated over a shorter range of temperatures. Moreover, these effects manifest themselves in a smaller number of peaks. Thus, M-A275 is characterized by a single peak at 341 8C, while in M-A300 no peaks are apparent. A similar tendency is observed in the naphthalene-based mesophase series (Fig. 4b). However, in this series, the peaks fit into a narrower range of temperatures than in the M-A series (Table 3) and the exothermic effects are of a higher magnitude (Figs. 3 and 4). This corroborates what was previously mentioned concerning the flexibility and capacity for stabilization in both samples. It is not easy to explain why the temperature at which the first peak appears increases and the number of peaks decreases with the severity of stabilization. In a previous work [18] it was stated that the oxygen-containing functional groups are formed progressively as the stabilization temperature is increased. Moreover, the decomposition of these groups during subsequent carbonization also occurs gradually. It is reasonable, therefore, to suppose that changes in DSC-derivative curves are associated with the

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Fig. 3. DSC curves in air of (a) coal-tar pitch-based mesophase and (b) naphthalene-based mesophase stabilized at different temperatures.

formation and decomposition of oxygenated groups, such as aldehydes, ketones, esters, etc. Similar conclusions were reached by other authors concerning the stabilization of mesophase pitch fibres [13] and PAN-based fibres [15]. The weight gain of the mesophases during stabilization reflects the extent to which oxidative reactions take place. Thus, the exothermic effect between 150 and 400 8C becomes less intense as the severity of stabilization increases (Figs. 3 and 4). This seems to indicate that during the DSC experiment, oxygen reacts with the samples thereby completing the stabilization initiated previously. This phenomenon is more pronounced, the less stabilized the sample is. Fig. 5 displays the variation in the area subjected to the exothermic effect between 150 and 400 8C with oxygen uptake (Fig. 5a) and hydrogen loss (Fig. 5b). These graphs show a linear relationship between the DSC parameters and oxygen uptake and hydrogen loss, in agreement with what has just been discussed.

3.4. Pyrolysis behaviour of stabilized mesophases by thermogravimetric analysis The oxidative stabilization with air produces significant changes in the composition of mesophases. These changes are clearly reflected in their pyrolysis behaviour. Figs. 6 and 7 show the thermogravimetric curves (TG / DTG) of the coal-tar pitch and naphthalene-based mesophase stabilized at different temperatures. The different pyrolysis behaviour is already patent in the parent samples. M-A gives a higher carbonaceous residue at 1000 8C and loses weight over a wider range of temperatures than M-B. The TG curves show that both samples lose weight continuously, but the coal-tar pitch-based mesophase has two peaks of maximum rate of weight loss (Fig. 7a) while naphthalenebased mesophase shows a single peak (Fig. 7b). It is well-known that weight loss at the initial stages of pitch pyrolysis is related with the removal of light components

Fig. 4. DSC-derivative curves of (a) coal-tar pitch-based mesophase and (b) naphthalene-based mesophase stabilized at different temperatures.

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Table 3 Differential scanning calorimetric (DSC) and thermogravimetric (TG / DTG) data for parent and stabilized mesophases Sample

M-A M-A200 M-A225 M-A250 M-A275 M-A300 M-B M-B200 M-B225 M-B250 M-B275 M-B300

DSC peaks

217, 312, 372 243, 312, 360 272, 351 305, 338 341 – 204, 261, 307 244, 316 272 306 329 376

TG / DTG Ti a

T max b

CRc

2DW350 d

2DW350 – 550 e

2DW550 – 1000 f

251 273 288 312 354 376 352 304 308 321 342 357

325, 500 354 357 369 415 – 504 481 476 454 440 445

75.20 77.47 79.46 81.70 84.52 85.09 71.93 76.12 82.72 82.88 80.67 77.99

7.73 6.70 5.01 2.88 0.89 0.73 0.95 1.95 2.04 1.69 1.19 1.38

13.51 11.77 10.23 9.49 7.86 5.70 24.08 19.28 11.46 9.57 10.49 10.86

3.56 4.09 5.30 5.93 6.73 8.48 3.04 2.65 3.78 5.86 7.65 9.77

a

Temperature of initial weight loss (8C). Temperature of maximum rate of weight loss (8C). c Carbonaceous residue at 1000 8C (wt.%). d Weight loss below 350 8C (wt.%). e Weight loss between 350 and 550 8C (wt.%). f Weight loss above 550 8C (wt.%). b

by distillation, whereas the weight loss at more advanced stages of pyrolysis corresponds to the removal of molecules generated during the polymerization / condensation reactions which take place on carbonization [26]. These two steps are clearly separated in the case of M-A (Figs. 6a and 7a), whereas they completely overlap in M-B (Figs. 6b and 7b). The changes caused by stabilization are evidenced by a significant increase in carbonaceous residue at 1000 8C and a shift in the temperature of initial weight loss (T i ) and temperature of minimum rate of weight loss (T max ) (Table 3 and Figs. 6 and 7). In the M-A series, the carbonaceous residue at 1000 8C and the Ti increase from M-A to M-A300 (Table 3). The DTG curves show a single peak of maximum rate of

weight loss (Fig. 7a). This peak is observed at a temperature intermediate between the two peaks in the parent mesophase. Studies carried out elsewhere [17] have demonstrated that the first peak in the non-stabilized mesophase attenuates and shifts to higher temperatures due to the effect of the temperature of stabilization, while the second peak is gradually reduced and even eliminated by the effect of oxygen during stabilization. In the most stabilized samples it can be seen that the peak of maximum rate of weight loss has widened with respect to samples stabilized under milder conditions (Fig. 7a). This is because the weight loss of the stabilized samples is due to the volatiles that are still present and the gases generated by the decomposition of the functional groups introduced during stabilization (mainly CO, CO 2 and H 2 O [20,27]). This

Fig. 5. Variation of the stabilization heat with (a) oxygen uptake and (b) hydrogen loss for coal-tar pitch-based mesophase and naphthalene-based mesophase stabilized at different temperatures.

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peak, therefore, widens with the increase in stabilization temperature. On the other hand, in the M-B series, the carbonaceous residue at 1000 8C increases from parent mesophase to M-B225 / M-B250 and from then on progressively decreases (Table 3). This could be related with the ‘competition’ between degradation and stabilization previously mentioned.

3.5. Effect of stabilization on the plasticity and sinterability of stabilized mesophases

Fig. 6. TG curves of (a) coal-tar pitch-based mesophase and (b) naphthalene-based mesophase stabilized at different temperatures.

Fig. 8 shows the variation in penetrability with temperature for the M-A and M-B series. In general terms, penetrability is drastically reduced as stabilization temperature increases. This effect is patent in the samples stabilized above 200 8C. Stabilization at 200 8C only causes a shift to higher values of the temperature of total penetration. However, the different behaviour exhibited by the two series is significant. Total penetration in M-B200 occurs at a lower temperature than in M-A200. Moreover, in the former a sharp displacement is detected near 250 8C. This displacement is not observed in M-B, probably because at 250 8C penetrability is already 100%. This peculiar behaviour is due to the swelling of the sample during the experiment. The displacement is also detected, but at a lower magnitude, in M-B225, while in M-B250 no displacement is observed. In the case of M-B two possible limitations must be considered: (i) plasticity is more rapidly reduced in M-B than in M-A and (ii) M-B is susceptible to swelling. These limitations explain why stabilization is more difficult to control in the naphthalenebased mesophase.

Fig. 7. DTG curves of (a) coal-tar pitch-based mesophase and (b) naphthalene-based mesophase stabilized at different temperatures.

420

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Fig. 8. Variation of penetrability with temperature for (a) coal-tar pitch-based mesophase and (b) naphthalene-based mesophase stabilized at different temperatures.

The plasticity of a sample is a property that is mainly determined by its content in light compounds [28,29]. For this reason, the weight loss of the stabilized mesophases must have something to do with their plasticity. Table 3 shows the weight loss of the samples below 350 8C, between 350 and 550 8C and above 550 8C, as determined by thermogravimetric analysis. In the case of the M-A series there is a relationship between weight loss at ,350 8C and penetrability. M-A and M-A200 lose 7.73 and 6.70 wt.% below 350 8C. In M-A225, M-A250 and M-A275 the loss decreases to 5.01, 2.88 and 0.89 wt.%, respectively, and penetrability falls to 51, 7 and 4 wt.%, respectively. In the M-B series, weight loss below 350 8C follows a random tendency. This is because competition between oxygen uptake and the decomposition of functional groups in this series takes place during the stabilization process itself, as previously mentioned. Consequently, an unknown portion of the weight loss in this temperature range is due to the decomposition of oxygenated groups. When stabilized samples are used for the preparation of polygranular carbons, the sinterability of the materials depends on the plasticity of the precursors. Fig. 9 shows the polygranular carbons obtained from the samples stabilized at different temperatures. Given the plasticity results (Fig. 8), it is not surprising to find that samples stabilized at 200 8C distort on carbonization as they soften. Samples stabilized above 200 8C give rise to undistorted materials in the case of coal-tar pitch-based mesophase. Materials from naphthalene-based mesophase stabilized above 200 8C are partially distorted, except for those obtained from M-B250. This again confirms the greater stabilization flexibility of M-A. Materials obtained from samples stabilized at 250 8C show the highest bulk densities (1.39 and 1.57 g cm 23 , for M-A250 and M-B250, respectively) and the lowest porosities (28.5 and 17.6 vol%). Samples stabilized at other temperatures different to 250 8C give

rise to materials with lower densities and higher porosities. Micrographs in Fig. 9 show that, although materials derived from M-A exhibit pores and cracks of a lower size than materials derived from M-B, the pores and cracks in the former are more numerous than in the latter. Interestingly, the materials obtained from stabilized naphthalenebased mesophase have higher values of bulk density and lower values of porosity than materials obtained from stabilized coal-tar pitch-based mesophase. The development of porosity in these materials can be attributed to both an excess of plasticity in the case of M-A225 and M-B225, and to a lack of plasticity in the case of samples stabilized at 275 and 300 8C. The excess of plasticity might lead to swelling, while the lack of plasticity makes sintering less effective because of the poor joint among mesophase particles. Additionally, samples stabilized at the latter temperatures undergo an important weight loss above 550 8C (Table 3), which can be attributed to the release of gases, such as CO and CO 2 [20,27]. These gases presumably contribute to a large extent to a deterioration of the structure of the final material.

4. Conclusions The oxygen uptake and hydrogen loss during the oxidative stabilization between 200 and 300 8C are double in the naphthalene-based mesophase than in the coal-tar pitch-based mesophase, because of the higher aliphatic character of the naphthalene-based mesophase. Moreover, in the naphthalene-based mesophase the maximum oxygen uptake occurs at lower temperature and the degradation of the oxygenated groups competes with stabilization. For these reasons, coal-tar pitch-based mesophase is more difficult to stabilize, but at the same time, easier to control the stabilization process.

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Fig. 9. Optical micrographs of cross-sections of polygranular carbons obtained from coal-tar pitch and naphthalene-based mesophases stabilized at different temperatures.

The relationship observed between the intensity of the DSC exothermic effect below 400 8C, the oxygen uptake and the hydrogen loss during stabilization of both mesophases allows a quick selection of the optimum temperatures of stabilization. Thermogravimetric analysis clearly shows that in the stabilized coal-tar pitch-based mesophases, the weight loss initiates a lower temperature and extends in a wider range of temperature than in the naphthalene-based mesophase,

which can give a higher flexibility in the further preparation of polygranular carbons. The different composition and pyrolysis behaviour of both stabilized mesophases affects their plasticity. Thus, in the naphthalene-based mesophase the plasticity is drastically reduced at a specific temperature of stabilization (225 8C), while in the coal-tar pitch-based mesophase plasticity is smoothly reduced in a wider range of stabilization temperatures (between 225 and 275 8C).

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Acknowledgements The authors would like to thank CICYT-FEDER (Project Ref. 1FD1997-1657 MAT) for financial support.

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