Energy & Fuels 2002, 16, 1087-1094
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Production of High-Strength Carbon Artifacts from Petroleum Residues: Influence of the Solvent Used to Prepare Mesophase Powder M. A. Rodrı´guez-Valero, M. Martı´nez-Escandell, and F. Rodrı´guez-Reinoso* Departamento de Quı´mica Inorga´ nica, Universidad de Alicante, E-03080-Alicante, Spain Received December 13, 2001
The preparation process and sintering behavior of different binderless polyaromatic mesophase (BPAM) powders, obtained by extraction of a semicoke (product of pyrolysis of a petroleum residue) with different solvents, are described. Most of the mesophase is in the form of domains rather than in spherical shape. None of the solvents used completely separates mesophase from the isotropic matrix, 1-methyl-2-pyrrolidone being the most and diethylamine the less effective. The amount of extracted material and of solvent remains are important for the behavior of the sintered carbons obtained with BPAM, as they control the fusibility of the powders. If the strength of the solvent is low, compacts may swell, but if it is too high, compacts may have sintering problems. Thus, it has been possible to obtain graphitized compacts with high density (∼1.90 gcm-3) and bending strength (100 MPa), even using a relatively large particle size (175 202
carbon particles become strongly bonded and the compacts achieve high density. Sintering of mesophase powders can progress in two steps: (i) A fluid phase sintering step, in which adhesion of individual particles and filling of existing pores take place through the partial fusion of the powders, viscosity controlling the extent of the process.3,6-9,12,1312-13 (ii) A solid-state sintering step in which a large shrinkage of the grains takes place and the compact densifies as a result of the densification of the particles. Porosity is also reduced by movement of atoms in a process similar to that of ceramics.3,6,13 The composition of the mesophase powder can control the fusibility of the material upon carbonization and so the extent of the fluid phase sintering step. Additionally, forming pressure controls the density and porosity of the green compact, which is important for sintering of grains and for the evolution of volatile matter. The control of heating rate is also important, as it governs the rate and the temperature of evolution of volatile matter. If heating rate is fast, light material can remain in the compacts at higher temperature and thus modify the rheological behavior. Forming pressure and heating rate of the sintered carbons must be adequate to the composition of the powder. The purpose of this paper is to (i) prepare a selfsintering material (it will be called binderless polyaromatic mesophase powder, BPAM, to differentiate it from MCMB) using the solid product of pyrolysis of a petroleum residue; (ii) modify the composition of the BPAM powder by changing the extraction conditions; (iii) study the influence of the composition of BPAM powder, the forming pressure, and heat treatment conditions, on the sintering behavior of compacts made out of these powders. Experimental Section Raw Semicoke. The semicoke was supplied by REPSOLYPF. This material was obtained by pyrolyzing a petroleum residue in a pilot plant working under mild conditions. Information about the characteristics of the petroleum residue used in this process (R1) has already been published.14-16 Details of the experimental conditions of the carbonization (12) Hoffmann, W. R.; Hu¨ttinger, K. J. Demonstration of Spontaneous Liquid-Phase Sintering of Mesophase Powders. Carbon 1993, 31, 259-262. (13) Dollin, P.; Ngi, D.; Rand, B. Sintered Carbons-A Review. In Extended Abstracts European Carbon Conference, Carbon 96, Newcastle, UK; The British Carbon Group: London, 1996; pp 421-422. (14) Martı´nez Escandell, M.; Torregrosa, P.; Rodrı´guez Reinoso, F.; Romero, E.; Go´mez de Salazar, C.; Marsh H. Pyrolysis of Petroleum Residues: I. Yields andcalyses. Carbon 1999, 37, 1567-82. (15) Torregrosa, P.; Martı´nez Escandell, M.; Rodrı´guez Reinoso, F.; Romero, E.; Go´mez de Salazar, C.; Marsh, H. Pyrolysis of Petroleum Residues: II. Chemistry of Pyrolysis. Carbon 2000, 38, 535-546. (16) Rodrı´guez Reinoso, F.; Martı´nez Escandell, M.; Torregrosa, P.; Romero, E.; Gomez de Salazar, C.; Marsh, H. Pyrolysis of Petroleum Residues: III. Kinetics of Pyrolysis. Carbon 2001, 39, 61-71.
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process to obtain the semicoke were not provided by the supplier. Preparation of BPAM Powders: Solvent Extraction. The semicoke was ground to DEA, the latter being the weakest and NMP the strongest solvent. In principle, powders obtained with DEA, are supposed to have the largest thermoplasticity, as a result of a weaker extraction, whereas powders obtained with NMP are supposed to have the lowest thermoplasticity. Solubility in toluene can give an idea or the amount of light material remaining after extraction. Toluene insoluble material (TI) of BPAM is low for the powder obtained using DEA due to its low extraction yield, but it is almost constant for the rest of the powders (Table 3). The control of NMP insoluble (NMPI) material is also important, as it is an approximate measure of the amount of mesophase in the sample. The value of NMPI is more sensitive to the extraction solvent, as BPAM (18) Martı´nez Escandell, M.; Carreira, P.; Rodrı´guez-Valero, M. A.; Rodrı´guez Reinoso, F. Self-Sintering in Carbon Mesophase Powders: Effect of Extraction/Washing with Solvents. Carbon 1999, 37, 16621666.
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Rodrı´guez-Valero et al. Table 3. Analysis of BPAM
solvent
extraction yield (wt %)
TI (wt %)
NMPI (wt %)
β-resins (wt %)
He density (g cm-3)
volatile matter (%)
mesophase content (%)
C/H ratio
DEA T D THF I NMP
79 63 61 60 55 46
78 92 94 94 94 95
56 75 75 77 84 91
22 17 19 17 10 4
1.29 1.30 1.30 1.30 1.30 1.31
16 12 17 13 14 14
33 39 42 38 42 47
1.66 1.80 1.51 1.82 1.80 1.78
have been prepared using solvents that have lower extraction yield than NMP. Thus, NMPI values increase in almost the same order as extraction yield (Table 3). The binding properties of the powders are usually assigned to the β-resins fraction2,10,19 (normally defined as the benzene insoluble-quinoline soluble material, but it can also be defined as toluene insoluble-NMP soluble material since the values are almost identical). This parameter should be larger for DEA (the weakest solvent), and smaller for NMP (the strongest solvent). For some of the powders the values of β-resin do not differ much, i.e., DEA and D, but the composition of these β-resins must be different (and consequently the rheological behavior of this component), as these powders show very different TI and NMPI values (Table 3). Thus, the evaluation of β-resins content is not enough to explain the different dimensional changes observed for the different powders when analyzing the TMA curves, which will be presented somewhere below. The control of volatile matter of BPAM (amount, temperature and rate at which the evolution is produced) is very important for the properties of the sintered carbons. Volatile matter of the powders (Table 3) show unexpected differences, as powders obtained with DEA (highest extraction yield) should have the largest amount of volatile matter and those extracted with NMP the lowest. However, powders extracted with D and NMP showed higher volatile matter evolution than expected. A more careful analysis of the weight loss with temperature carried out by TGA indicates that the highest amount of volatile matter lost at 1000 °C is for powders extracted with DEA, and the lowest for toluene (Figure 1), the order being T < THF < NMP < I < D < DEA. However, differences in volatile matter content of the powders are only observed below 600 °C, the main differences occurring below 300 °C. Thus, powders extracted with the more volatile solvents (T and THF), evolve the lowest amount of volatile matter below 300°, while powders obtained with the less volatile solvents (I, D, and NMP), show a significant weight loss (Figure 1). This initial weight loss is originated from solvent trapped in the carbon powder, which is more difficult to eliminate due to the high boiling point of the solvents. When analyzing the amount of volatile lost above 300 °C the order found is as follows: NMP < D < I < T < THF < DEA, which adjusts better to the results deduced from the extraction yield. A more detailed analysis of the initial weight loss indicates that D and NMP are almost eliminated below 200 °C, but the industrial solvent (I) seems to be difficult to eliminate, as there is an almost continuous weight loss up (19) Jones, S. S. Anode-Carbon Usage in the Aluminium Industry. Petroleum Derived Carbons; Bacha, J. D., Newman, J. W., White, J. L., Eds.; ACS Symposium Series 303; American Chemical Society: Washington, DC, 1986; pp 234-249.
Figure 1. Weight loss vs temperature for different BPAM obtained by extraction with DEA, D, THF, toluene, and NMP.
to 400 °C. DEA (low boiling point) however presents a more continuous evolution of volatile matter, as a result of the low capacity of this solvent to extract isotropic material from the semicoke. Consequently, toluene and THF seem to be adequate solvents, as they are more easily removed. The C/H ratio values and the aromaticity parameter, obtained by FTIR, for BPAM powders (Table 3) increase as expected, except for D and NMP insoluble material, with lower values than expected, this being caused by the presence of solvent remains. The powders were analyzed by optical microscopy in order to determine the degree of separation of mesophase (Table 3). None of the solvents completely separates the mesophase from the isotropic phase, the powders still having an important content of isotropic phase (Figure 2). Certain solubility of mesophase may also have occurred. The extracted powders were analyzed by SEM. To abbreviate, only photographs of DEA, toluene, and NMP insoluble materials are included (Figure 3). Large differences between the extracted powders are observed. Powders insoluble in DEA are of irregular shape and present sharp edges. Most particles do not show apparent signs of solvent attack, and separation of mesophase is not observed. Toluene powders are also of irregular shape. The surface of the particles seems to be smooth, although a large number of particles still exhibit sharp edges. In some particles, erosion of the surface by the solvent has taken place, leaving a surface constituted by small grains of insoluble material (micro and submicron size) of irregular shape, which seem to be agglomerated. These small particles may be nuclei of mesophase, but since they cannot be detected by optical microscopy ( THF > D > T > I > NMP. In the case of the powder extracted with D, it presents a large shrinkage but also a relative large weight loss, so it is difficult to explain if this larger shrinkage is being originated by a higher thermoplasticity or by the larger weight loss. Thus, it is expected that samples that present the highest fusibility and shrink-
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Table 4. Properties of the Carbonized (1000 °C) and Graphitized (2500 °C) Compacts
1000 °C
2500 °C
sample
weight loss (%)
shrinkage (% vol)
density (g cm-3)
open porosity (%)
bending strength (MPa)
elastic module (GPa)
strain-to -failure (%)
THF D D200 T I NMP THF D T I NMP
12 20 15 12 15 15 14 22 14 17 17
29 31 29 28 28 27 37 40 38 37 35
1.65 1.65 1.64 1.63 1.65 1.60 1.87 1.90 1.88 1.86 1.79
8 8 9 9 9 11 8 8 9 7 11
85 65 105 85 105 40 90
16 14 17 16 18 14 10
0.53 0.46 0.63 0.53 0.58 0.29 0.90
90 100 50
12 14 9
0.75 0.71 0.56
Lc (nm)
electrical resistivity (µΩ m) 49 53
12.2 12.0 12.0 12.9 14.8
53 55 66 17 19 17 20 22
age without swelling will provide compacts with the best mechanical properties. Behavior of the Carbonized (1000 °C) and Graphitized (2500 °C) Compacts. The BPAMs extracted with the six solvents (DEA, THF, D, T, I and NMP) were conformed at 150 MPa to give compacts of 5 × 10 × 50 mm which were heat treated up to 1000 °C (1 °C/min), and afterward graphitized (2500 °C, 1 °C/ min) under argon atmosphere. Heating rate and pressure applied were those that offered the best results with small samples. It was not possible to obtain compacts with DEA powder, as they deform during carbonization, even when the powder is washed twice. THF powder also swelled but it was possible to obtain compacts when the powder was washed twice. Table 4 shows the properties of the carbonized and graphitized compacts. As expected, large differences were obtained in volatile matter content and shrinkage of the compacts when using different BPAM, the tendency being identical to that obtained by TG and TMA. Bulk density of carbonized and graphitized samples is high, between 1.6 and 1.65 g/cm3 for the carbonized compacts (1000 °C), and between 1.79 and 1.90 g/cm3 for graphitized compacts (2500 °C), the lowest being for NMPI and the highest for D powders. Densification occurs as expected from TMA results. Thus, the highest densities are obtained for the samples that exhibit the highest shrinkage in TMA analysis (THF powders did not swell because they were washed twice). Analysis of the results of mechanical properties showed one apparently unexpected result, as it was not possible to obtain compacts with acceptable mechanical properties from D powders, despite the fact that these compacts present the highest density. The poor mechanical properties of D compacts may be originated by the existence of microcracks that may form during thermal treatment, due to the high shrinkage and high volatile matter evolution exhibited. Because this sample evolved a large amount of volatile matter below 200 °C (about 9% in TG curves), it is necessary to process D powders with a lower heating rate or carry out a pretreatment of the powder21 to eliminate solvent remains in order to obtain compacts with acceptable mechanical properties. The latter treatment has been carried out to eliminate solvent remains. The powder was heat-treated at 200 °C under nitrogen atmosphere,
loosing 4% of its mass (sample D-200). The properties of this sample heat-treated at 1000 °C are presented in Table 4. The heat-treated compact using D-200 has a similar density as the sample without pretreatment, but its mechanical properties have improved. In this case, it seems that the elimination of rest of solvents has improved the behavior of the compacts. Mechanical properties of all other compacts were excellent, bending strength ranging from 50 to 100 MPa in the graphitized samples. The highest values were obtained for the powders extracted with industrial solvent (100 MPa), rather than for toluene or THF powders (90 MPa), which present a slightly higher shrinkage and density and a lower amount of solvent remains. In the case of BPAM obtained with industrial solvent, the presence of solvent remains does not seem to be as critical as with D powder, and the properties of the compacts are even better that those samples obtained with toluene and THF. In fact, the presence of solvent remains such as light oil fractions (tar middle oil) was considered to improve the sintering abilities of MCMB.22 As expected, NMPI powders, which present the lowest shrinkage and density, present the lowest mechanical properties, indicating that a good sintering among the particles has not occurred. Higher densities and mechanical properties are expected for all the samples if the particle size was reduced to lower than 10 µm, and conforming were carried using an isostatic mode. The elastic modulus of the powders (Table 4) has been obtained from the strength-strain curves. Values of the carbonized compacts are higher than those for graphitized compacts. For both carbonized and graphitized compacts the highest values are for BPAM obtained with industrial solvent and the lowest with NMP, as for mechanical properties, also indicating that a higher degree of sintering has been reached with the former sample. The fracture surface of the graphitized compacts has been analyzed by SEM. It is observed that compacts obtained with toluene present a good sintering between the grains and the existence of submicron size particles observed in the powder is not detected. In this way, the degree of sintering seems to be decreasing as the strength of the solvent increases, the sample obtained with NMP presenting sintering problems between the
(21) Aggarwal, R. K.; Bhatia, G.; Bahl, O. P.; Punjabi, N. Effect of Calcination Conditions of Self-Sintering Mesocarbon Microbeads on the Characteristics of Resulting Graphite. J. Mater. Sci. 2000, 35, 5437-5442.
(22) Nagayama, K.; Torii, T.; Hatano, H.; Fukuda, N. A Study of the Sintering Mechanism of KMFC Compact Made of Mesocarbonmicrobeads. Extended Abstracts 20th Biennial Conference on Carbon; American Carbon Society: Santa Barbara, CA, 1991; pp 206-207.
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Figure 5. (a-c). Evolution of weight loss, density, and shrinkage with HTT of compacts obtained from BPAM powders conformed at 38, 94, and 151 MPa.
grains, specially when large particles of mesophase are present. The conductivity of the graphitized compacts has been determined. No significant differences were obtained when varying the solvent. Lc values for the graphitized compacts have also been determined, values being similar, although slightly increasing with the strength of the solvent. Behavior on Heat Treatment of Compacts Obtained with Toluene-Extracted BPAM. Toluene BPAM powder was selected to carry out studies of the sintering behavior during heat treatment (HTT), as this powder presents the lowest volatile matter and a relatively high shrinkage. Small disks (13 × 5 mm) were prepared using different pressures (38, 94 and 151 MPa). Weight loss, density, total and water porosity and volumetric shrinkage have been determined every 200 °C, from 25 to 1400 °C (heating rate 1 °C/min). Figure 5 shows the evolution of weight loss, density, and volumetric shrinkage with HTT. Main weight loss takes place in the 400-600 °C temperature range, differences with forming pressure being small (Figure 5a) and only at temperatures above 1000 °C. Density curves (Figure 5b) have the same shape for all the pressures studied and they are parallel, densification initiating at approximately 400 °C; maximum densification takes place between 600 and 800 °C. The differences in density with pressure originate in the green sample, and they are maintained along the heat-treatment. Thus, the curves showing the evolution of volumetric shrinkage with HTT are similar to those of density and coincident for the three pressures (Figure 5c). This indicates that the sintering of the grains is taking place similarly and that the differences in density with pressure are those that existed in the green compact. The total and open porosity of the compacts have also been studied, and Figure 8 indicates that total porosity remains almost constant during heat treatment, being much larger for the 38 MPa series, and decreasing with pressure. This behavior explains the similarity observed in the shrinkage curves. However, the open porosity (measured in water) varies with HTT and also with pressure. At 38 MPa, the amount of open porosity is small at low temperature, and consequently, all the porosity is in the form of closed porosity. At temperatures above 400 °C there is an increase in open porosity, to reach a maximum at temperatures between 1200 and 1400 °C, thus indicating that close pores are intercon-
Figure 6. Evolution of total porosity (symbols) and open porosity (bars) with HTT of compacts obtained with BPAM powders conformed at 38, 94, and 151 MPa.
necting when increasing temperature, so that most of the final porosity at 1400 °C is open. At 94 and 151 MPa, there is a similar behavior, but the maximum of open porosity occurs at 800-1000 °C, decreasing for higher temperatures. Influence of Forming Pressure and Heating Rate for BPAM Extracted with Toluene. BPAM extracted with toluene was also used to carry out a study of the influence of both forming pressure and heating rate. Small disks (13 × 5 mm) were prepared varying forming pressure, 37-221 MPa, and were heattreated under inert atmosphere to 1000 °C using different heating rates (0.5-10 °C/min). Figure 7a shows that for heating rates equal or lower than 5 °C/min, the density of the compacts increases with pressure to reach a maximum, remaining almost constant up to 225 MPa. The minimum pressure to obtain the maximum density for this material is about 175 MPa. For this powder, when HTT is carried out at 10 °C/min, a decrease in the density is observed when operating at pressures over 100 MPa, it being more pronounced as pressure increases. This behavior is thought to be caused by an increase in the thermofusibility of the material as a result of (i) the retention of low molecular weight cracking products by a faster heating rate and (ii) a larger difficulty for gases to get out of the body due to the higher forming pressure. Thus, the material may go through a viscous liquid state in which sintering of the grain takes place, reducing the intergranular space, but bubbles of volatile matter may be trapped thus increasing closed porosity. There is no
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Figure 7. (a,b). Evolution of density with pressure and heating rate of compacts obtained using BPAM powders extracted with toluene.
Figure 8. Optical micrographs of a transversal cross-section of compacts obtained with BPAM extracted with toluene: (a) conformed at 148 MPa, heating rate 1 °C/min; (b) conformed at 148 MPa, heating rate 10 °C/min; (c) conformed at 185 MPa, heating rate 10 °C/min.
difference in open porosity between samples conformed at pressures over 150 MPa and heating rate between 0.5 and 10 C/min (Figure 7b), so the decrease in density must be caused by an increase in internal porosity. This is observed in Figure 8a-c, in which there is an enlargement of the internal porosity as pressure and heating rate increase. Additionally, grains seem to be better bonded as a result of a fluid phase sintering of the grains and intragranular fusion also seems to occur, which lead to transformation of the remaining isotropic phase into a very fine mosaic structure. The original mesophase of the powder does not seem to change during HTT. Thus, the combination of high pressure and fast heat treatment may improve the sintering of grains, but thermal treatment must be optimized to avoid entrapment of volatile matter or even swelling of compacts. Conclusions BPAM consisted of particles of irregular shape rather than spheres. Observation of the green compacts by polarized optical microscopy reveals that neither of the solvents used completely separates mesophase of the isotropic matrix. There is an important amount of isotropic phase after extraction, 1-methyl-2-pyrrolidone (NMP) being the most effective solvent in the separation and diethylamine (DEA), the least effective. Composition of BPAM varies with the strength of the solvent used to separate mesophase, as the effectiveness of this stage controls the amount and composition of
light material in the powders, i.e., volatile matter and β-resin. The sintering ability of BPAM is controlled by the thermofusibility of the powders, and this is dependent on the solvent used to separate mesophase. Thus, if the strength of the solvent is low, i.e., diethylamine, compacts may swell, but if it is too large, i.e., NMP, compacts may have sintering problems. Graphitized compacts of high density (1.85-1.90 g/cm3) and good mechanical properties have been obtained (BS, 90-100 MPa) when using THF, toluene, and industrial solvent to separate mesophase. Physical and mechanical properties must improve if lower particle size and isostatic pressing were used. Heating rate and forming pressure is also important for the sintering behavior of the compacts. The combination of high pressure and fast heating rate can retain light material inside the compacts, affecting to the fusibility of the material and the sintering behavior. For materials that have low fusibility the application of high pressure and fast heating rate improves sintering between particles. Acknowledgment. This work was supported by the Brite-Euram program (Project BRPRCT-97-04829) and the Spanish CICYT (Project PETRI 95-0267-OP). Also acknowledge Repsol-YPF for supplying the semicoke and Schunk Kohlenstofftechnik for graphitizing the samples. EF010293X
