MESOPHASE-PITCH FOR LOW PRESSURE CARBON/CARBON COMPOSITE PROCESSING Mickael Dumont, René Pailler and Xavier Bourrat LCTS, University Bordeaux 1, 3 Allée de la Boétie, 33600 Pessac, France Corresponding author e-mail address :
[email protected] 1. Introduction Densification of composites by the liquid way requires numerous cycles of impregnation, stabilization and carbonization. Mineralization of pitch under the form of carbon with a graphitic structure has a low volume yield, a major drawback with this processing. This is related to the fact that precursors possess a low density (of the order of 1.2 to 1.3) whilst the final density of carbon to reach is about 2.1 to 2.2. Some carbon/carbon applications require high composite density, higher than 1.9 or even 2.0. In this later case, a process involving autoclave is necessary for the carbonization step at very high pressure. Pressure increases the carbon yield and suppresses a second drawback, the swelling and rejection of the pitch out of the porosity, especially in the last cycles. A high pressure does allow the carbonization of the precursor to proceed within the porosity without rejection out of the preform : typically 100MPa. A process involving autoclave at high pressure is known as the HIPIC process for Hot Isostatic Pressure Impregnation Carbonization process. In this work an aromatic resin replaces the regular coal tar pitch used with the HIPIC process. The processing is conducted at low pressure and developed for needled fiber preforms. The aim is to test a possible low-cost alternative process (no autoclave) by using a much-sophisticated precursor. 2. Experimental conditions The dry preforms used are Novoltex™ needled preforms. The fiber is a high strength PAN-based carbon fiber with a density of 1.75. It possesses a porosity of 75%. The pitch used is an aromatic resin [1, 2] produced by Mitsubishi Gas Chemical in Japan : Ara 24r. It has already been the object of studies in our group [3]. After J. L. White and coworkers [4] we proposed recently a processing window which can be used to compare any type of pitch, whatever their state, i.e. isotopic or mesophasic, and their softening point [5]. Ara 24r has a good infiltrability potential together with a high reactivity regarding stabilization step. Density of this pitch is high : 1.31, this is not negligible for the volume report during the total processing [6]. Impregnation is managed by vacuum transfer in a home made device. This apparatus is a double compartment device (Fig.1). The preform is put in the vessel in the lower chamber. It is degassed under vacuum at 370°C. The upper part is heated up in the same time to 370°C under nitrogen flow. Powder of pitch is introduced in the upper chamber. After its melting, the pitch is degassed when over 350°C and 370°C during 15mn. Then, an over pressure of 0.2 to 0.3MPa is kept in the upper chamber (the lower one is still under vacuum). The transfer of the pitch occurs by gravity (opening of the valve in-between the two chambers). A pumping under vacuum during 15mn to enhance the impregnation follows it. Then the pressure in the lower chamber is increased up to 0.3MPa during 10mn with the temperature still at 370°C. Finally, all is cooled down under nitrogen flow. The crust over the impregnated preform is removed. The preform is weighed. The thermal treatments are of 3 types. The pyrolysis is performed after impregnation following two different ways. The first one is conducted as it, under an atmospheric pressure of nitrogen up to 750°C with a stay of 1hour and a low increasing ramp. The second one, under a 2MPa-nitrogen pressure up to
550°C and a ramp of 3°C/hour. Carbonization is conducted in an inductive oven under argon with a stay of 1hour.
thermocouple
mobile rod N2
feeding inlet
higher compartment pitch N2 thermocouple
vacu um Lower
Aluminium protection (Ø=75 mm, h=200 mm)
compartment
heater
preform Figure 1. Two-stage Impregnation device
Initial porosity a
b
c
Figure 2. Volumetric evaluation of Ara 24r thermal treatment : a) impregnation, b) pyrolysis, c) carbonization (in white : porosity, density in bracket) This sequence (3 steps of (i) impregnation, (ii) pyrolysis and (iii) carbonization) constitutes what is called the IPC cycle. As the amount of cycles can reach 3 or 4, they are numbered as : IPCi. A systematic weighing and sampling allow computing the increasing mass gain and density (by means of helium pycnometry) and metallographic characterization of the process. Figure 2 gives the different yields (mass and volume) obtained with Ara24r along the IPC cycle. Figure 3 is drawn to simulate the porosity opening during the first cycle. The origin of the curves is 370°C, which corresponds to the temperature of
2
impregnation. After the impregnation, if the yield were optimum then P1 = 0 and the curve should be superposed onto the abscissa (P1 is the porosity left by a poor impregnation). In Fig. 3 the impregnation yield was given the best experimental value, i.e. : YI = 0,93, then P1 = 7%. The 'envelope' curve P1+P2 can be drawn on this basis (P2 is the porosity produced by the mass-loss which occurs during the heat treatments). The main losses occur under the gaseous form during the pyrolysis : organic to mineral transition. Finally, the third envelope curve P = P1+P2 +P3 gives the total final porosity. The difference between the 2 last curves gives the porosity related to the structural ordering of carbon. Porosity increases strongly between 500° and 900°C. It corresponds linearly to the strong increase of density of carbon in the same range of temperatures. This is a typical behavior for graphitizable carbons. The dashed line corresponds to the same P1+P2+P3 when the impregnation yield is optimal (i.e. P1 = 0). The straightforward conclusion is that any lack of impregnation at the beginning of the process is highly damageable in term of residual porosity at the end. 50
Porosité (%)
Po ro sit y (% )
45
YI=93%
40
P1 P1+P2 P
35
shrinkage
30 25
YI =100%
20
P
mass-loss
15 10 5 0 300
lack of mpregnation 500
700
900
1100
1300
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1700
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2100
Température (°C) Temperature (°C)
Figure 3. Porosity evaluation during an experimental processing (small preform with an impregnation yield of 93% for the first cycle with no rejection) 3. Experimental study of the densification 3.1 Optimization of the impregnation step Mainly 2 different protocols were tested in this work. a) Capillary process : the impregnation occurs at a temperature higher than the stability limit, i.e. 390°C, heating rate is 30°C/mn and the stay is 30mn. b) Transfer process : temperature is kept under the stability limit, i.e. 370°C but the preform is degassed under vacuum, a pressure of 0.3MPa is applied during the 10mn holding time. In both cases, the impregnation is followed by a slow pyrolysis at atmospheric pressure up to 750°C, followed by a carbonization at 1400°C. The different yields are plotted in Fig. 4. (i) Impregnation yield. The analysis shows that capillary impregnation is less efficient. The efficiency decreases with the number of cycles. The pitch does not succeed penetrating to the core of the sample after the second cycle. Comparison between the two processes, point out the role of vacuum in eliminating the bubbles of gas trapped inside the preform. Penetration of the pitch is much better with transfer. Any way, the transfer protocol is not even sufficient to get an optimal impregnation. This is the bad side of the excellent reactivity of this pitch. Figure 5 shows a micrograph of the sample just after the 2d impregnation (transfer). Bubbles are forming and gathering in the large macropores. These bubbles are
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supposed to form by the reaction of the pitch on the coke or due to the de-polymerization of the pitch under vacuum conditions. The wetability of the coke by the pitch is excellent. All the fine and medium porosity opened during the carbonization in the previous coke is well impregnated, even the thinner. 91
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81
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tranfer
84 78
62
capillarity
44 44 38 32
73
33 26
36
19 59
23
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2
cycle
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4
1
b
YI (vol %)
2
cycle
3
YC (mass %)
11
1
4
c
2
cycle
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YIPC (vol %)
Figure 4. Experimental densification : a) impregnation yield (YI, volume %); b) coke yield (YC, mass %) and c) full cycle yield (YIPC, volume %). pictch 2
Porosity
matrix 1
Figure 5. Composite after the second impregnation, porosity (as macropores) is that left by the lack of impregnation (optical micrograph under cross-polars with a lambda plate) (ii) Carbon yield during the different cycles (YIPCi). Beyond the first cycle, yield is systematically better with the transfer. For example, during the 4th cycle, it can be seen that 26% of the residual porosity is fill in with transfer regarding only 11% by the capillary protocol. (iii) Coke yield within the preform (YC). The capillary impregnation shows a nearly constant efficiency whatever the cycle. The coke yield is comparable to the best yield obtained for the pitch alone. There is no rejection due to the poor impregnation. Finally, the total cycle yield is not good. In the case of transfer, the coke yield decreases progressively and keeps any time under the coke yield of the pitch alone. This indicates that the pitch is rejected with the gas, related to a better impregnation especially in the macropores. 3.2 Influence of the ramp on the pyrolysis. Pyrolysis is studied on the first cycle. A series of small cylindrical preform (10x18mm2) is used, impregnated by capillary with the same yield (YI = 0.92). Different pyrolysis are compared. If the heating rate is too rapid, a phenomenon of swelling and rejection occurs very strongly. For example, the coke yield
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increases from 0.57 up to 0.81 by simply decreasing the ramp from 3°C/mn to 3°C/hour with the same holding time of 1 hour at 750°C. In the case of slow heating rate, there is no more carbon foam on the surface of the sample. The coke yield is close to that of the pitch alone, in same condition (0.83). Polished sections show a dense coke without foaming features inside. A slow pyrolysis is supposed to stagger the evolving of volatile in the time : decomposition starts at lower temperature. Also, the thermosetting of the system occurs at lower temperature : the semi-coke stage is reached at 430°C. All these phenomena limit the swelling and the rejection. The first conclusion of this study concerns the critical size of the porosity beyond which rejection occurs. It is possible to pyrolyse the pitch in larger pores when the heating rate decreases. The second conclusion is that stabilization of the pitch is not required any way during the first cycle. The best coke yields are obtained during the first cycle. There is even a great interest to let the pitch free, owing to the phenomena of binder migration. This is discussed later on, in § 3.4. 3.3 Pyrolysis : role of pitch stabilization Different attempts were conducted on the pitch powder to optimize the process following the published conditions. A treatment at 200°C under air provides a good stabilization for a mass gain of 6% [7]. 10
dm/mbrai (%)
8 6 4 2 0 0
2
4
6
8
10
12
14
sqrt (temps (h)) √(exposure time) 1/2
Figure 6. Mass gain versus the square root of the exposure time at 200°C under air, for a preform (10x35x24mm3) just after the second impregnation (mass gain is given here related to the effective mass of pitch impregnated during cycle 2 noted : mbrai) Stabilization is then tested on cycle 2. Apparent density is 1.1 and residual porosity is 42% before impregnation. For these experiments the size of the preforms is : 10x35x24mm3. Impregnation of cycle 2 is conducted by transfer at 370°C with a yield of 70%. Figure 6 plots the evolution with the time of the mass gain divided by the mass of pitch impregnated. At the beginning this ratio is linear. A plateau is reached for 8 to 9%. Four samples were stabilized with increasing time, then were pyrolyzed at 5°C/mn up to 750°C as a test to evaluate the rejection out of the fiber preform. Results are gathered in Tab.1. In the case of the reference sample, the rejection is important : the coke yield is only 44%, rejection is 37%, the rest is volatilized. The stabilized samples are spectacularly improved. The coke yield is maximum for the 6%-mass gain sample reached in 30hours for very small samples (12H for the powder). A coke yield of 85% is reached. The interest of stabilization is not total, mainly because it increases the processing time with a discontinuous step. Also one can fear to get a skin/core effect with the difficulty to impregnate the core at next cycle (no experiment). On the contrary it is interesting during the last cycle(s) to fill in the residual macropores (see below).
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Table 1. Four stabilization tests conducted after impregnation 2. Stabilization sample time dm/mbrai Yc (%) (%) (h) 750°C 0 0 44* 1(ref) 3.1 67 2 10 5,8 89 3 30 65 8.8 88 4
Pyrolysis/Carbonization Yc (%) Swelling Yc(with the foam) (%) 1400°C (observation) 750°C abundant 81 63 presence 85 85 minute 90 85 non -
dm/mbrai = mass gain (%) during stabilization related to the mass of impregnated pitch Yc = coke yield related to the mass of impregnated pitch, after the pyrolysis at 750°C/°C/min,1H Yc(with the foam) = coke yield at 750°C related to the mass of impregnated pitch taking into account the mass of foam rejected out of the sample. 3 Sample size : 10x35x24 mm * very low yield due to rejection, emphasized by the pyrolysis conditions : 5°C/min
1,66
1,76
1,48 matrix 4 4,6%
1,14
residual porosity 13,3%
fibers 25,0%
matrix 3 8,7% 0,44 matrix 2 15,3%
(a)
prefor (dry)
cycle 1
cycle 2
cycle 3
matrix 1 33,1%
cycle 4
(b)
Figure 7. Experimental results of the processing conducted with Ara24r : a) rise of the apparent density of the composite with the number of cycles and b) volumetric fraction of the different matrices 3.4 Experimental evolution of porosity and density Porosity and density were studied by sampling small pieces (10x10x20mm3) inside the preform disk (50mm in diameter and 23mm in thickness). This series of measurements was conducted with the following protocol : (i) impregnation by transfer at 370°C (ii) slow pyrolysis (not dipped) under 0.1MP of nitrogen and 1H at 750°C (iii) carbonization at 20°C/mn, 1400°C under argon and 1H holding time Results are plotted in Fig. 7. It can be seen that the use of mesophase pitch at low pressure is a credible alternative to the HIPIC in autoclave : 4 cycles were sufficient to reach a density higher than 1.75 with a PAN-based fiber of approximately the same density. The lack of impregnation and the rejection make the process nearly two cycles longer than the theoretical assessment. With the addition of a stabilization step in the last cycle the difference is only 1 cycle. This result was obtained on 3D cylindrical preform of 50mm in diameter (23mm in thickness). In the literature, Matzimos et al. [8, 9] reached the same density on a 2D preform by using hot pressing and 5 cycles of impregnation/carbonization.
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coke 1
residual porosity Figure 8. Optical micrograph of the composite in the first cycle after impregnation and pyrolysis at 500°C (note the meniscus on isolated fibers around the tows) 3.5 Characterization of porosity opening During the first cycle, at 430°C (Fig. 8), the matrix was already hardened (semi-coke). All the intra- or inter-bundle porosity is perfectly impregnated. All the residual porosity is concentrated as macro-pores with a mean opening of approximately 50µm. At this temperature most of the volatile have been already eliminated. In small pores (e.g. intra-bundles ones) this departure (~20% in volume) must be balanced by the contribution of mater from the outside, because the small pores are seen well impregnated. During the pyrolysis and till the solidification (semi-coke), macropores are supposed to behave like pitch-tank, refeeding the mass loss in the bundles. A comparable phenomenon is known in ceramic processing : binder sucking-up (déliantage in French) which was also observed in the black ceramic processing [9]. This phenomenon is related to the capillary suction of the binder. This hypothesis is also coherent with the porosimetry profiles (Fig. 9) : the macro-porosity in-between 5 and 50µm opens progressively between 430 and 1400°C. An other argument is provided by the direct observation of the cross section in the material. It can be seen in Fig. 8 that the bubble walls are concave inside the macropores and their curvature, a function of the fiber density. This proves that the wettability of fibers is excellent and the capillary forces very strong to keep the liquid phase in place.
Figure 9. Porosimetry profiles at the main steps of the first cycle. Note the role of high temperature treatment in re-opening porosity due to structural shrinkage.
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These phenomena of binder migration are benefic for the densification of the bundles (and the finer porosity) as shown in Tab. 2. Between 430°C and 1400°C the density of the matrix increases from 1.5 to 1.97. These shrinking control 15% of the porosity that opens after impregnation. This family of pores was characterized by means of mercury porosimetry in-between 0.5 and 6µm and a mean value of 2µm corresponding to a multi-cracking phenomenon within the matrix. Table 2. Density and porosity data obtained during the first cycle
At the beginning of the 2d cycle, the material possesses 3 different classes of porosity : (i) intra-bundle porosity : cooling cracks (2µm) (ii) inter-bundle porosity : mean diameter (t50µm) At the end of the 2d cycle, the cooling cracks inside the bundles are fill in : the bundles are fully densified and the macropores keep hollow.
500 µm
Figure 10. Optical micrograph of the composite after 4cycles of densification (macropores are still present) During the 3d and 4th cycles, the last microcracks still present in the bundles are fill in, as well as the small inter-bundle porosity. Then the macropores are broken in smaller pores. The largest pores (higher than 50µm) are still present (Fig. 10). After the 4th cycle, the bundles are perfectly densified. There is not any shrinking crack inside the matrix or debonding crack at the fiber/matrix interface. The only pores that
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survive are the very large macropores (t=100µm, L>500µm) especially in crimp zones. All the measurement obtained during the evolution of the material, i.e. density and mercury porosimetry, are coherent (Tab. 3). With a regular pitch, it is necessary to thermoset the pitch after impregnation [10, 11]. Here our results with the mesophase pitch suggest proceeding only at the 3d and 4th cycle. In Figure 11, 2 composites are compared after the 3d cycle. One was stabilized after impregnation and not the other. The pore distribution is improved by the stabilization : the mean diameter decreases from 50 to 10µm : there does be a breaking down of macro pores by acting at the 3d cycle. Table 3. Density and porosity after the different cycles
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Discussion and conclusion
The aromatic resins as Ara 24r are thermoplastic and present a 'mesophase glass' state [13] at room temperature. When heated beyond its softening point (290°C), that resin presents the advantage to be liquid, to wet the carbon fiber and have a liquid crystal structure. This means that its density is among the highest of all the resins, 1.31. Also, it means that the structure of the carbon left in the porosity is that of a graphitizable carbon. To bring this pitch into play, it was shown in a previous work that the temperature has to be high enough so that the viscosity is lower than 1Pa.s [5]. The processing temperature of these precursors is much higher than that of the isotropic pitches : 350 to 390°C against 80 to 180°C for the coal tar pitch or equivalent petroleum isotropic pitches. The advantage is the coke yield : 80 to 85% for Ara 24r against 45 to 50% for A240 at atmospheric pressure and at 1000°C (in mass). More importantly is the volume yield provided by the density variation between the final carbon state and the precursor (in this process the goal is a graphitizable carbon). The use of a mesophase precursor of high quality (Ara 24r) allows the processing of carbon/carbon composites with a density better than 1.75 and a residual porosity of less than 15% and a fiber of ~1.75 in only 4 cycles of impregnation/pyrolysis/carbonization and a constant volume of the 3D preform. The use of an aromatic resin following the protocol defined in this work is a credible alternative at low pressure to the HIPIC process. The stability of the pitch at processing temperature is an important factor to select the right pitch or to optimize the quality for the impregnation-type pitch : the pitch has to reach a viscosity as low as 1Pa.s and being stable for long processing time. The formation of bubbles during the impregnation is damageable for the total yield of the process. Also the reaction of the pitch on the fiber surface is an important issue. In this work the most efficient way to impregnate the pitch was conducted by vacuum transfer. This process enhances the degassing of the preform. The drawback of this process is the stability of the pitch during the holding time of impregnation. This could be improved by applying a small pressure during that time. A good suggestion is the use of the process by injection at much lower temperature [4]. The pyrolysis optimized here is conducted at atmospheric pressure and very slow heating rate : 3°C/hour. Thermosetting temperature of the pitch is lowered and coke yield increased. Different processes were tested for the pyrolysis. At moderate pressure (2MPa) dipped into the pitch increases a little bit the coke yield. Meanwhile, the pyrolysis conducted under pressure did not overcome the rejection of the pitch out of the preform especially during the last cycles.
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100 µm
100 µm
Figure 11. Composite stabilized during the 3d cycle (a) and the reference without stabilization (b) : densification of large pores (arrow). (Optical micrograph after carbonization of 3d cycle) An extra step of stabilization during the 3d and the 4th cycle was the best response to the issue of rejection. Stabilization is clearly not necessary during the first cycle. It is conducted at 200°C during a time depending on the size of the preform, which corresponds to a mass gain of 6%. When conducted in the last cycles, stabilization is efficient to break up the macroporosity. Acknowledgements Authors wish to thank Pr I. Mochida and M K. Izaki from MGC for providing samples and scientific advises and the Conseil Régional d’Aquitaine for its support through a contract DAEESR N°980201109 References [1] Mochida I, Korai Y, Ku C-H, Watanabe F, Sakai Y. Chemistry of synthesis, structure, preparation and application of aromatic-derived mesophase pitch. Carbon, 2000;38:305-328 [2] Korai Y, Nakamura M, Mochida I. Mesophase pitches prepared from methylnaphtalene by the aid of HF/BF3. Carbon, 1991:29:561-567 [3] Dumont M, Chollon G, Dourge MA, Pailler R, Bourrat X, Naslain R, Bruneel JL, Couzi M. Chemical, microstructural and thermal analyses of a naphtalene-derived mesophase pitch, Carbon, 2002;40:1475-1486 [4] White JL., Gopalakrishnan M. K., Fathollahi B., A processing window for injection of mesophase pitch into a fiber preform. Carbon 1994;32:301-310 [5] Dumont M, Dourges MA, Pailler R, Bourrat X. Mesophase pitch for 3D-carbon texture densification : rheology and processability, Fuel 2002;82:1523-1529 [6] Rellick.G, Densification efficiency of C/C composites, Carbon 1990;28:589-594 [7] Fathollahi B. personal communication [8] Matzinos PD, Patrick JW, Walker A. The void structure of 2-D C/C performs and composites: effect of the nature of the matrix precursor coal-tar pitch, Carbon 1997;35:507-513 [9] Matzinos PD, Patrick JW, Walker A. Coal-tar pitch as a matrix precursor for 2-D C/C composites, Carbon 1996;34:639-644 [10] Ehrburger P, Sanseigne E, Tahon B, Formation of porosity and change in binder properties during thermal treatment of green carbon materials, Carbon 1996;34:1493-1499 [11] White JL, Scheaffer PM. Pitch based processing of carbon-carbon composites. Carbon 1989;27:697-707 [12] Matzinos P. D., Patrick J. W., Walker A., The efficiency and mechanism of densification of 2-D C/C composites by coal-tar pitch impregnation, Carbon 2000;38:1123-1128 [13] White JL, Fathollahi B, Bourrat X. Formation of microstructure in mesophase carbon fibers. In ‘Fibers and composites’, P. Delhaes ed, 2003:3-23, Word of Carbon, Taylor&Francis New York, NY ISBN 0-415-30826-7
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