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carbon composite
Carbon 43 (2005) 229–239 www.elsevier.com/locate/carbon
Effect of carbonization rate on the properties of a PAN/phenolic-based carbon/carbon composite H.H. Kuo, J.H. Chern Lin, C.P. Ju
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Department of Materials Science and Engineering, National Cheng-Kung University, No. 1 University Road, Tainan City 701, Taiwan, ROC Received 17 February 2004; accepted 21 August 2004
Abstract The effect of carbonization rate in a wide range (1, 100 and 1000 C/min) on the properties of a PAN/phenolic-based carbon/carbon (C/C) composite was studied. The results indicated that the composite processed at a higher carbonization rate had a higher porosity level, more large pores and a more graphitic structure than that processed at a lower carbonization rate. After second graphitization the bending properties of composites carbonized at 1 C/min and 1000 C/min were comparable. The composite carbonized at 1000 C/min had the highest fracture energy. The composite carbonized at 100 C/min showed the worst mechanical performance among three. The large increase in carbonization rate can be beneficial to the industry from an economic point of view. 2004 Elsevier Ltd. All rights reserved. Keywords: A. Carbon/carbon composites; B. Carbonization; C. Scanning electron microscopy; D. Mechanical properties
1. Introduction Carbonization plays an important role in preparing a carbon/carbon (C/C) composite. Not only the properties, the economical efficacy of a C/C composite is also critically dependent on a careful control of its carbonization process due to the fact that carbonization is one of the most time and energy-consuming steps in the entire fabrication process of C/C composites, especially for those densified by liquid phases. One way to reduce the high cost, which largely limits the application of a C/C composite, is the use of precursors with a high carbon yield, such as polyphenylene and phthalonitrile resins, during carbonization [1–3]. The higher carbon yield leads to the fewer densification/carbonization cycles needed, thereby reducing the manufacturing cost the opposite.
*
Corresponding author. Tel./fax: +886 6 2748086. E-mail address: [email protected] (C.P. Ju).
0008-6223/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.08.024
Logically the simplest way to reduce the manufacturing cost of C/C composites is to increase the carbonization rate, which is usually very low. However, carbonization is primarily a process of pyrolysis of hydrocarbons of a carbon precursor. The pyrolysis of hydrocarbons generally involves such processes as the cleavage of C–H and C–C bonds to form reactive free radicals, molecular rearrangement, thermal polymerization, aromatic condensation and elimination of side chains (e.g., H2) [4,5]. To minimize such adverse effects as shrinkage, cracking and thermal stresses that may build up during carbonization, low carbonization/heating rates (typically <20 C/min) are usually required [1,3,6–13]. Ko and Chen [6] studied the pyrolysis of a planewoven PAN (polyacrylonitrile)-based carbon fabric/ phenolic resin composite, and observed that in their heating rate range (0.1–5 C/min), the heating rates below 3 C/min had no effect on the carbon yield of the composite carbonized to 1000 C. Roy et al. [7] found that in the heating rate range of 0.03–0.8 C/min, the
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interlaminar tensile strength of their 8H satin woven PAN-based UHM carbon fabric/phenolic resin composite was not affected by the heating rate. Chang et al. [8] revealed that in the range of 0.5–3 C/min, the density and flexural strength of their plane-woven PAN-based carbon fabric/phenolic resin composite declined by 0.7% and 6.6% respectively, when a higher heating rate was used. The weight loss and flexural modulus were not affected. In their research on an 8H satin woven PAN-based carbon fabric/phenolic resin composite carbonized to 1000 C at a heating rate of 2–7 C/min, Nam and Seferis [14] discovered that a higher heating rate and a larger thickness of the composite resulted in a larger temperature gradient within the composite. This temperature gradient could result in a non-uniform carbonization, creating internal stresses through the laminates, which in turn, might lead to localized delamination and/or other damages to the char structure of the matrix. As described above, although a number of different results have been reported, the effects of carbonization rate on the properties of C/C composites have been investigated in low carbonization rate range. It has generally been agreed that high carbonization rates can deteriorate the mechanical performance of C/C composites, therefore the practicability of reducing the manufacturing cost by the increase of carbonization rate seem to be limited. It is the purpose of the present work to study the effect of carbonization rate in a much larger range (up to 1000 C/min) on properties of a PAN/phenolic based C/ C composite and to explore the possibility of manufacturing a C/C composite with a carbonization rate far higher than the conventional ones.
fiber (Torayca T700S, 12K, Toray Co., Japan) and a resole-type phenolic resin (PF-650, Chang Chun Petrochemical Industry, Taiwan), respectively. The chopped carbon fibers were first impregnated with the phenolic resin in a plastic mold in vacuum to form a 110 mm · 110 mm prepreg. The polymer matrix prepreg was placed in an oven at 70 C for 6 h to remove excess solvent, followed by a vacuum bag hot press curing process in a stainless steel mold at 160 C for 30 min under a pressure of 2.76 MPa (400 psi). The cured composite was sectioned into 50 mm · 10 mm coupons using a water-cooled diamond saw, followed by a post-curing process at 230 C for 8 h in an air-circulated oven [15].
2.1. Sample preparation
2.1.2. Carbonization Carbonization was conducted by heating the postcured composite under a nitrogen atmosphere to 1200 C. To study the effect of carbonization (heating) rate on properties of the composite, three different heating rates, i.e., 1 C/min, 100 C/min, and 1000 C/min, were used. The carbonization at low heating rate (1 C/ min) was conducted in an ordinary furnace heated by SiC heating element. The nitrogen gas was introduced continuously into the furnace at a constant flow rate of 0.3 L/min. Carbonization at higher heating rates (100 and 1000 C/min) was conducted using an apparatus specially designed for the study. As sketched in Fig. 1, a 25 mm diameter quartz tube that could be slid in and out a MoSi2-heated tube furnace at a controlled speed to adjust the heating rate of the C/C sample placed within the quartz tube. The temperature of the C/C sample was determined using a K type thermocouple inserted into the quartz tube. When the desired furnace temperature (for this study, 1200 C) was reached, the quartz tube was pushed into the center of the furnace at a controlled, constant speed. Nitrogen gas was introduced continuously into the quartz tube at a flow rate of 0.7 L/min during the entire carbonization process.
2.1.1. Composite forming The fiber and resin used for the preparation of the present PAN/phenolic based C/C composite are a randomly oriented chopped (4.5 mm) PAN-based carbon
2.1.3. Graphitization The carbonized composite was subsequently graphitized to 2200 C in a helium-purged graphitization furnace. (Note: The term ‘‘graphitized’’ or ‘‘graphitiza-
2. Experimental procedure
Fig. 1. Schematic drawing of apparatus for high speed carbonization.
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tion’’ was conveniently used throughout text to stand for the heat treatment at 2200 C, not an indication of a major three-dimensional ordering in crystal structure) All graphitization processes were conducted by a heating rate of 10 C/min, a holding time of 30 min at 2200 C and a cooling rate of 20 C/min. 2.1.4. Densification/carbonization cycles The porous graphitized composite was then densified by re-impregnation with phenolic resin in vacuum, followed by pre-curing at 70 C for 6 h to remove excess solvent; curing at 160 C for 30 min; and post-curing at 230 C for 2 h in an air-circulated oven. The post-cured composite was re-carbonized at the same carbonization rate as that used in the first carbonization process. To improve the density and properties of the composite, four such densification/carbonization cycles were applied, and a further graphitization process were applied to some selected samples. Table 1 lists sample designations for the composite processed at different stages at different carbonization rates. 2.2. Measurements and analytical methods 2.2.1. Morphology An optical microscope and a scanning electron microscope (SEM) were used to examine the surfaces morphology of composites. 2.2.2. Density and porosity According to ASTM C830, bulk density and open porosity of the composite processed at different stages were measured by a water saturation method. (Note: ‘‘density’’ and ‘‘porosity’’ will stand for ‘‘bulk density’’ and ‘‘open porosity’’ in the following discussion) A one-way ANOVA method was used to evaluate the statistical significance of density and porosity data. In all cases, the results were considered statistically different with p < 0.05. The distribution of open porosity at a smaller scale (termed ‘‘microporosity’’ in the following discussion) was also determined using a mercury porosimeter.
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2.2.3. XRD and ESCA An X-ray diffraction (XRD) study was carried out using an X-ray diffractometer. The data were collected at a scan rate of 2 /min under the 2h step scanning mode with a step size of 0.01 using Cu Ka radiation at 30 kV and 20 mA. The degree of randomness were determined by Franklin model [16]: d 002 ¼ 3:44 0:086 ð1 p2 Þ where p represents the degree of randomness. The value ˚ ) and of p for ideal graphite is zero (p = 0, d002 = 3.354 A for a non-graphitic carbon, i.e. the turbostratic struc˚ ). When the ture, the value is one (p = 1, d002 = 3.440 A interlayer spacing of carbon structure is greater than ˚ , the value of p became larger than one, even 3.440 A though the value p > 1 was not defined in the model, this paper only use it to represent a structure that is far from ideal graphite structure. The spectra of electron spectroscopy for chemical analysis (ESCA) were collected on a electron spectrometer with a base pressure of about 108 Torr. The X-ray source was Al Ka radiation (1486.6 eV) and the wide scan spectra in the range of binding energy 1400– 0 eV were recorded for samples. The high-resolution spectra of the C 1s and O 1s signals were recorded in the step of 0.2 eV within the range of binding energy 290–282 eV and 542–522 eV. All samples for ESCA analysis were in a size of 10 mm · 10 mm. 2.2.4. Bending properties The flexural strength and modulus of the composite were measured by three-point bending test based on ASTM D790 with the span-to-depth ratio of 16. Samples for this test were in a size of 50 mm · 10 mm · 2.2 mm. A Shimadzu AGS-500 D universal tester was operated at a crosshead speed of 1 mm/min with a support span of 40 mm. All the bending test data shown in this paper are averages of five samples. Again, one-way ANOVA was used to analyze the statistical significance/insignificance of the data.
3. Experimental results Table 1 Sample designations for composite under different process stages at different carbonization rates Process stage
Carbonization rate (C/min) 1
100
1000
First carbonization First graphitization Second densification/carbonization Fourth densification/carbonization Second graphitization
C1-1 G1-1 D2-1 D4-1 G2-1
C1-100 G1-100 D2-100 D4-100 G2-100
C1-1000 G1-1000 D2-1000 D4-1000 G2-1000
3.1. Morphology The cross-sectional morphology of the C/C composite at various process stages is shown in Fig. 2. These low magnification images exhibited such information as fiber orientation, bubble pores, dry zones, shrinkage and thermal stress-induced cracks, and interfaces of fiber bundles. According to Jortner [17], in the cured stage, the major porosities were bubble pores and dry zones formed during curing process. The interfaces
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Fig. 2. Cross-sectional optical micrographs of composite under different process stages at different carbonization rates. (a) Cured, (b) C1-1, (c) C1100, (d) C1-1000, (e) G1-1, (f) G1-100, (g) G1-1000, (h) D4-1, (i) D4-100, (j) D4-1000, (k) G2-1, (l) G2-100, (m) G2-1000.
between fiber bundles in different orientations could be recognized (marked by arrow), where those of bundles in similar orientation were not able to recognize. After the cured composites were first carbonized (Fig. 2(b)–(d)), the interfaces between fiber bundles in similar orientation became more visible under this magnification. When the samples were further graphitized (Fig. 2(e)–(g)), more thermal-stress cracks formed at or near the interfaces between bundles, therefore the bundlebundle interfaces became easier to recognize. In general, the C/C samples processed at a higher carbonization rate yield more bubble pores than that carbonized at a lower rate. The composite carbonized at a higher heating rate also seems to generate a larger amount of porosity than that carbonized at a lower rate, especially after first graphitization treatment. It is to be noted that the
micrographs in this figure only present a general morphology of the composite processed at different stages. The effects of process stage and carbonization rate on the density/porosity in a quantitative way will be presented in the following section. 3.2. Density and porosity As indicated in Figs. 3 and 4, after first carbonization, the density of the composite largely decreased, while the porosity level largely increased. Sample C11000 had a lower density and lower porosity level than C1-1 and C1-100. After first graphitization, the difference in density change is small, but the porosity values of the G1-1 and G1-100 largely decreased. As expected, the density of the composite continued to increase and
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Fig. 3. Bulk densities of composite under different process stages at different carbonization rates. Same symbol indicate no statistical difference.
Fig. 5. Pore size distributions of composite under process stage C1 at different carbonization rates. (a) Incremental volume, (b) Cumulative volume.
Fig. 4. Open porosity values of composite under different process stages at different carbonization rates. Same symbol indicate no statistical difference.
the porosity content continued to decrease when the densification process continued. After four densification cycles the porosity content of D4-1000 was significantly larger than other two composites. After second graphitization this difference became even more dramatic. Figs. 5–8 demonstrate a series of micron/sub-micron pore size distributions of the composite under different process stages. (In the following discussion ‘‘micropores’’ or ‘‘microporosity’’ is used to stand for the pores in this region). As shown in Fig. 5, the C1-1 had the smallest micropore volume, while the C1-1000 had the largest volume. After first graphitization, the highest incremental-volume peaks of all three composites shifted toward the larger-diameter side (Fig. 6). This result indicating that the existing micropores expanded in
size and/or additional larger micropores were created during the graphitization process. The peaks of G1-100 and G1-1000 shifted more toward the largerdiameter side than G1-1 did. As expected, after four densification/carbonization cycles, the larger micropores in all composites largely diminished (Fig. 7). After second graphitization, G2-100 and G2-1000 showed larger micropore volumes again (Fig. 8). 3.3. XRD and ESCA The XRD was performed on the fiber, bulk resin and C/C composites processed at 1 C/min and 1000 C/min. As presented in Table 2, the d002 values of the carbons processed at 1000 C/min were consistently lower than those processed at 1 C/min, no matter such carbons are fiber, matrix resin, or C/C composite. This indicates that the material processed at higher carbonization rate has a more graphitic structure than the material processed at lower carbonization rate.
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Fig. 6. Pore size distributions of composite under process stage G1 at different carbonization rates. (a) Incremental volume, (b) Cumulative volume.
The results of ESCA analysis on C 1s spectra are shown in Table 3. The position, FWHM (full width at half maximum) and symmetry of C 1s peak are often used for the evaluation of surface chemistry of carbon materials. The C 1s binding energy of the graphitic peak is 284.6 eV [18]. The broadening of the graphitic peak is an indication of a less ordered structure [18]. According to La´szlo´ et al. [19], the existence of functional groups based on carbon such as C–OH, C–O–C and C@O also leads to a broadening and asymmetric C 1s peak. The oxygen-containing functional groups could be recognized by C 1s and O 1s binding energies. Oxide peak of hydroxyl (C–OH) or ether (C–O–C) is shown in the range of 285.3–286.1 eV for C 1s and 533 eV for O 1s, while the oxide peak of carbonyl (C@O) is shown in 287.6 eV for C 1s and 531.5 eV for O 1s [18]. At stage C1, the C 1s binding energy decreased as the carbonization rate increased from 1 C/min to 1000 C/ min. After first graphitization, almost all C 1 s binding energies reached 284.6 eV. At this stage the O 1s binding
Fig. 7. Pore size distributions of composite under process stage D4 at different carbonization rates. (a) Incremental volume, (b) Cumulative volume.
energy also decreased when the carbonization rate increased. After first graphitization, the O 1s binding energies of G1-1 and G1-100 were larger than that of G1-1000. The C 1s FWHM slightly increased at stage C1 when the carbonization rate increased from 1 C/ min to 100 C/min. When the carbonization rate further increased to 1000 C/min, the FWHM dropped. The O 1s/C 1s peak height ratios at stages C1 and D4 both gradually increased with increasing carbonization rate, whereas at stages G1 and G2 the ratios gradually decreased. The results indicate that carbonized samples prepared from higher carbonization rates had more oxygen-containing functional groups. With subsequent graphitization, the oxygen-containing functional groups of samples prepared from higher carbonization rates turned less. 3.4. Bending properties The flexural strengths and moduli of the composites at different process stages are shown in Figs. 9 and 10,
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the strength of D4-100 was lower than other two composites. This effect became even more dramatic after second graphitization. As clearly shown in Figs. 9 and 10, both flexural strength and modulus of G2-1000 were not much different from those of G2-1. Again, the strength and modulus of G2-100 were respectively much lower than other two composites. Typical bending profiles of the three composites are shown in Fig. 11.
4. Discussion
Fig. 8. Pore size distributions of composite under process stage G2 at different carbonization rates. (a) Incremental volume, (b) Cumulative volume.
respectively. Compared with the cured composite, the post-cured composite had a similar flexural strength but a much higher modulus. After first carbonization, both strengths and moduli of all composites largely decreased. After four densification/carbonization cycles, the strengths and moduli of all composites largely increased, as expected. However, quite unexpectedly, the strength of the composite did not decrease with increasing carbonization rate. The average flexural strength of D4-1000 appeared to be similar to that of D4-1, while
To help learn the effect of carbonization rate on bubble pore formation in matrix carbon during carbonization process, a neat phenolic resin (in the absence of fiber) was carbonized using the same carbonization process as for the preparation of C/C samples. The SEM images in Fig. 12 clearly show that the size and distribution of bubble pores in the carbonized resin were sensitive to the carbonization rate. When the resin was carbonized at 1 C/min, there were only a few large (tens of microns in width and hundreds of microns in length) cracks and small (2–5 lm) pores were observed in the cross-sectional micrograph. As the carbonization rate increased to 100 C/min, pores of 10–100 lm could be found. For the resin carbonized at 1000 C/min, more smaller-pores were observed throughout the entire cross-section. This helps to explain the existence of the generally observed lower density/higher porosity level in the samples prepared by higher carbonization rates. According to the study of Gupta and Harrison [9], macroporosity of tens to several hundreds of microns developed primarily in the curing stage of the resin, at temperatures lower than 200 C. As HTT was raised to 450 C (the resin was already in a solid state), there was an increase in pore size and, simultaneously, small pores coalesced to form larger pores. They also found a higher heating rate (15 C/min) yielded a more random distribution of smaller pores in the bulk. Besides the above research, Lausˇevic´ and Marinkovic´ [5] found earlier that during the carbonization process of phenolic resin, water evolved and reacts with the surrounding polymer structure producing H2 and CO2. They compared the water evolution of phenolic resin on powdered and bulk samples and found that water was easier to
Table 2 Effect of carbonization rate on 2h and d002 values of composite under different process stages at different carbonization rates 2h d002 Pa a
Carbonization rate (C/min)
C/C C1
Fiber G1
Resin G1
C/C G1
C/C G2
1 1000 1 1000 1 1000
25.366 25.454 3.512 3.500 1.356 1.303
25.857 25.892 3.446 3.441 1.034 1.006
25.630 25.980 3.476 3.430 1.191 0.941
25.903 25.943 3.440 3.435 1.000 0.970
26.021 26.044 3.425 3.422 0.907 0.887
Degree of randomness, p, is calculated according to Franklin model [16].
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Table 3 ESCA results of composite under different process stages at different carbonization rates
C 1s BE
O 1s BE
C 1s FWHM
O 1s/C 1s Peak height ratio
Carbonization rate (C/min)
C1
G1
D4
G2
1 100 1000 1 100 1000 1 100 1000 1 100 1000
285.4 285.2 284.6 533.2 532.6 532.4 1.61 1.65 1.47 0.19 0.22 0.32
284.8 284.6 284.6 533.6 533.8 532.6 1.91 1.38 1.38 0.55 0.32 0.07
284.6 284.6 284.6 532.2 532.2 532.2 1.36 1.55 1.56 0.24 0.34 0.35
284.6 284.6 284.6 532.6 532.8 533.0 1.63 1.39 1.35 0.46 0.29 0.14
Fig. 9. Flexural strengths of composite under different process stages at different carbonization rates. The symbol ‘‘’’ indicate that the data are significantly different from the neighboring data.
Fig. 10. Flexural moduli of composite under different process stages at different carbonization rates. The symbol ‘‘’’ indicate that the data are significantly different from the neighboring data.
Fig. 11. Typical bending profiles of composite under process stage G2 at different carbonization rates.
stay in the bulk sample, therefore this reaction take place mainly in the bulk samples. The presently observed heating rate effect on pore formation might be explained by the amount of water vapor/gases stay in the samples. During a carbonization process with a higher heating rate, there was insufficient time for the vapor/gases to leave the samples. As a result, the vapor/gases were reacts with the surrounding polymer structure and formed the observed pores in the resin. Jortner [17] proposed that if relaxation of stresses by creep occurs at high temperatures (perhaps above 2000 C), the void volume of cracks at room temperature would be larger than that without creep. Chłopek and Bł_zewicz [20] also found that graphitization process results in the opening of closed pores. These findings might explain the present observation that, prior to the first graphitization process, C1-1000 had both a lower open porosity and lower density than C1-100 and C11. A portion of closed pores contained in C1-1000 broke open during the first graphitization process (G1-1000), resulting in a higher porosity level.
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Fig. 12. Scanning electron micrographs of phenolic resin bulk carbonized at different carbonization rates. (a) 1 C/min, (b) 100 C/ min, (c) 1000 C/min.
The mercury porosimetry results clearly indicated that the volume of pores increased after graphitization, especially after first graphitization. The effect of second graphitization was not as dramatic, since after four cycles of densification, majority of large pores were already filled. The mercury porosimetry data also indicate that composite carbonized at higher heating rates had more large pores than those carbonized at lower heating rates, supporting the argument that the composite carbonized at higher heating rates generate a higher thermal stress level that, in turn, results in larger cracks/pores when the stress was released during heat treatment [17]. In this study, both XRD and ESCA results indicate that the composite processed at a higher carbonization
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rate consistently has a more graphitic structure than that processed at a lower carbonization rate. Again, this is primarily due to the larger internal stress build-up in the composite carbonized at higher heating rate that assists in a ‘‘stress-graphitization’’ process [21,22]. The trends of C 1s FWHM data were not as straightforward as C 1s binding energy; possibly due to the effect of oxygen presence (causing the lineshape to be more asymmetric) that somewhat offsets the heating rate effect. This phenomenon is particularly clear for C1 and D4 groups, where O 1s/C 1s peak height ratios increased with increasing carbonization rate. Given that there was insufficient time for the water vapor/gases to leave the samples during a carbonization process with a higher heating rate, it seems reasonable to suppose that the further reaction of water with the surrounding matrix will not only create more pores in the resin, but also increase the amount of oxygen-containing functional groups [5,23,24]. Park et al. [25] and Tanabe et al. [26] also observed that more oxygen-containing in a thermosetting resin derived carbon generate a more graphitic structure after graphitization process. It is possibly the reason for the opposite carbonization rate effects on O 1s/C 1s peak height ratio between G1/G2 group (decreasing with increasing carbonization rate) and C1/D4 group (increasing with increasing carbonization rate). As mentioned earlier, in the heating rate range of previous studies, the mechanical properties tends to decline as the carbonization rate rise. This seems to be true for carbonization rates up to 100 C/min in this study. However, while the heating rate further increased to 1000 C/ min, the mechanical performance of the composite showed an unexpected ‘‘down and up’’ phenomenon. This phenomenon was most obvious under G2 condition. For ensure the phenomenon is true, we repeat some experiments again, the same trend presents. The ‘‘down and up’’ phenomenon at high carbonization rates may be interpreted as a result of an internal stress build-up effect that caused a stress-induced graphitization process at high carbonization rates, as mentioned earlier [21,22]. At 1000 C/min, this stress graphitization effect became significant enough to cause the strength and modulus of the composite to increase. Another interesting observation, that might help explain the improved mechanical performance of the composite processed at 1000 C/min, is presented below. As shown in the SEM fractography of samples at stages D4 and G2 (Fig. 13), numerous micropores (majority smaller than 5 lm) were observed in the matrix of D41000 and G2-1000, while in D4-1 and G2-1 much fewer such micropores were observed. This result is consistent with the microporosity data as indicated in Figs. 5–8. The presence of such micropores (particularly in G21000) might effectively increase the fracture energy of the composite [27].
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Fig. 13. Scanning electron micrographs of composite under different process stages at different carbonization rates. (a) D4-1, (b) D4-1000, (c) G2-1, (d) G2-1000.
As a final remark, while a stress-induced graphitization in matrix might improve the mechanical properties of a C/C composite, the literature has repeatedly shown that a rapid-heating pyrolysis process could cause cracking, delamination, etc. to degrade the mechanical performance of the composite [14]. In the present work it was found that largely increasing the carbonization rate to 1000 C/min caused the degradation in mechanical properties to recover to the level of 1 C/min. Furthermore, the high carbonization rate (1000 C/min) resulted in higher fracture energy. Obviously under such condition the advantage of the high carbonization rate has overcome its disadvantage. As mentioned earlier, carbonization is perhaps the most time and energy-consuming step in the entire conventional fabrication process of C/C composites. The large increase in carbonization rate can be greatly beneficial to the industry from an economic point of view.
2. The composite processed at a higher carbonization rate had a more graphitic structure than that processed at a lower carbonization rate. 3. After second graphitization the bending properties of composites carbonized at 1 C/min and 1000 C/min were comparable. The composite carbonized at 1000 C/min had the highest fracture energy. The composite carbonized at 100 C/min showed the worst mechanical performance among three.
Acknowledgment The authors are grateful to the National Science Council of the Republic of China for support of this research under contract no. NSC 90-2216-E-006-037.
References 5. Conclusions 1. The composite processed at a higher carbonization rate had a higher porosity level and more large pores than that processed at a lower carbonization rate.
[1] Olsen RE, Reese HF, Backlund SJ. Process for forming carbon– carbon composite. US Patent 5686027, 1997. [2] Sastri SB, Armistead JP, Keller TM. Carbonization of hightemperature resin. Carbon 1993;31(4):617–22. [3] Sastri SB, Armistead JP, Keller TM. Carbon-based composites derived from phthalonitrile resins. US Patent 5965268, 1999.
H.H. Kuo et al. / Carbon 43 (2005) 229–239 [4] Lewis IC. Chemistry of carbonization. Carbon 1982;20(6):519–29. [5] Lausˇevic´ Z, Marinkovic´ S. Mechanical properties and chemistry of carbonization of phenol formaldehyde resin. Carbon 1986;24(5):575–80. [6] Ko TH, Chen PC. Study of the pyrolysis of phenolic resin reinforced with two-dimensional plain-woven carbon fabric. J Mater Sci Lett 1991;10:301–3. [7] Roy AK. Effect of carbonization rates on the interlaminar tensile stiffness and strength of two-dimensional carbon–carbon composites, AD-vol. 34/AMD-vol. 173. Thermomechanical Behavior of Advanced Structural Materials. ASME; 1993. [8] Chang WC, Ma CCM, Tai NH, Chen CB. Effect of processing methods and parameters on the mechanical properties and microstructure of carbon/carbon composites. J Mater Sci 1994;29:5859–67. [9] Gupta A, Harrison IR. Small-angle X-ray scattering (SAXS) in carbonized phenolic resins. Carbon 1994;32(5):953–60. [10] Prevorsek DC, Li HL. Carbon fiber-reinforced carbon composite material. US patent 5556704, 1996. [11] Lewis IC, Howard RA. Carbon–carbon composites containing poorly graphitizing pitch as a binder and/or impregnant having a reduced coefficient of thermal expansion and improved flexural strength. US Patent 5607770, 1997. [12] Hager JW, Newman JW, Johannes N, Turrill FH. Carbon artifacts and compositions and processes for their manufacture. US Patent 6013371, 2000. [13] Murdie N, Parker CA, Pigford JF, Narasimhan D, Dillon F. Process of making carbon–carbon composite material made from densified carbon foam. US Patent 6077464, 2000. [14] Nam JD, Seferis JC. Initial polymer degradation as a process in the manufacture of carbon–carbon composites. Carbon 1992;30(5):751–61. [15] Ko TH, Ma TS. Effect of post-curing on the mechanical properties of carbonized phenolic resins. Polym Compos 1998;19(4):456–62.
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[16] Franklin RE. The structure of graphitic carbons. Acta Crystallogr 1951;4:253–61. [17] Jortner J. Macroporosity and interface cracking in multi-directional carbon–carbons. Carbon 1986;24(5):603–13. [18] Xie Y, Sherwood PMA. X-ray photoelectron spectroscopic studies of carbon fibers. Part XIV: Electrochemical treatment of pitch-based fibers and the surface and bulk structure changes monitored by XPS, XRD, and SEM. Appl Spectros 1990;44(10):1621–8. [19] La´szlo´ K, Josepovits K, Tomba´cz E. Analysis of active sites on synthetic carbon surfaces by various methods. Anal Sci 2001;17(supplement):i1741–4. [20] ChŁopek L, BŁ_zewicz S. Effect of processing variables on the properties of carbon–carbon composites. Carbon 1991;29(2):127–31. [21] Zaldivar RJ, Rellick GS. Some observations on stress graphitization in carbon–carbon composites. Carbon 1991;29(8):1155–63. [22] Ju CP, Watahiki S, Takano S. Stress-induced preferential alignment of basal planes in phenolic-based carbon–carbon composite. Mater Chem Phys 1993;36:150–5. [23] Kurosaki F, Ishimaru K, Hata T, Bronsveld P, Kobayashi E, Imamura Y. Microstructure of wood charcoal prepared by flash heating. Carbon 2003;41(14):3057–62. [24] Graham RG, Bergougnou MA, Overend RP. Fast pyrolysis of wood. Review. J Anal Appl Pyrol 1984;9:95–135. [25] Park SM, Yasuda E, Akatsu T, Tanabe Y, Kakihana M, Ozawa K. Evolution of microtexture in furan resin-derived carbon with heat-treatment. Carbon 1995;33(10):1377–81. [26] Tanabe Y, Yamanaka J, Hoshi K, Migita H, Yasuda E. Surface graphitization of Furan-resin-derived carbon. Carbon 2001;39(15):2347–53. [27] Dhakate SR, Mathur RB, Bahl OP. Catalytic effect of iron oxide on carbon/carbon composites during graphitization. Carbon 1997;35(12):1753–6.
Effect of carbonization rate on the properties of a PAN/phenolic-based carbon/carbon composite H.H. Kuo, J.H. Chern Lin, C.P. Ju
*
Department of Materials Science and Engineering, National Cheng-Kung University, No. 1 University Road, Tainan City 701, Taiwan, ROC Received 17 February 2004; accepted 21 August 2004
Abstract The effect of carbonization rate in a wide range (1, 100 and 1000 C/min) on the properties of a PAN/phenolic-based carbon/carbon (C/C) composite was studied. The results indicated that the composite processed at a higher carbonization rate had a higher porosity level, more large pores and a more graphitic structure than that processed at a lower carbonization rate. After second graphitization the bending properties of composites carbonized at 1 C/min and 1000 C/min were comparable. The composite carbonized at 1000 C/min had the highest fracture energy. The composite carbonized at 100 C/min showed the worst mechanical performance among three. The large increase in carbonization rate can be beneficial to the industry from an economic point of view. 2004 Elsevier Ltd. All rights reserved. Keywords: A. Carbon/carbon composites; B. Carbonization; C. Scanning electron microscopy; D. Mechanical properties
1. Introduction Carbonization plays an important role in preparing a carbon/carbon (C/C) composite. Not only the properties, the economical efficacy of a C/C composite is also critically dependent on a careful control of its carbonization process due to the fact that carbonization is one of the most time and energy-consuming steps in the entire fabrication process of C/C composites, especially for those densified by liquid phases. One way to reduce the high cost, which largely limits the application of a C/C composite, is the use of precursors with a high carbon yield, such as polyphenylene and phthalonitrile resins, during carbonization [1–3]. The higher carbon yield leads to the fewer densification/carbonization cycles needed, thereby reducing the manufacturing cost the opposite.
*
Corresponding author. Tel./fax: +886 6 2748086. E-mail address: [email protected] (C.P. Ju).
0008-6223/$ - see front matter 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2004.08.024
Logically the simplest way to reduce the manufacturing cost of C/C composites is to increase the carbonization rate, which is usually very low. However, carbonization is primarily a process of pyrolysis of hydrocarbons of a carbon precursor. The pyrolysis of hydrocarbons generally involves such processes as the cleavage of C–H and C–C bonds to form reactive free radicals, molecular rearrangement, thermal polymerization, aromatic condensation and elimination of side chains (e.g., H2) [4,5]. To minimize such adverse effects as shrinkage, cracking and thermal stresses that may build up during carbonization, low carbonization/heating rates (typically <20 C/min) are usually required [1,3,6–13]. Ko and Chen [6] studied the pyrolysis of a planewoven PAN (polyacrylonitrile)-based carbon fabric/ phenolic resin composite, and observed that in their heating rate range (0.1–5 C/min), the heating rates below 3 C/min had no effect on the carbon yield of the composite carbonized to 1000 C. Roy et al. [7] found that in the heating rate range of 0.03–0.8 C/min, the
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interlaminar tensile strength of their 8H satin woven PAN-based UHM carbon fabric/phenolic resin composite was not affected by the heating rate. Chang et al. [8] revealed that in the range of 0.5–3 C/min, the density and flexural strength of their plane-woven PAN-based carbon fabric/phenolic resin composite declined by 0.7% and 6.6% respectively, when a higher heating rate was used. The weight loss and flexural modulus were not affected. In their research on an 8H satin woven PAN-based carbon fabric/phenolic resin composite carbonized to 1000 C at a heating rate of 2–7 C/min, Nam and Seferis [14] discovered that a higher heating rate and a larger thickness of the composite resulted in a larger temperature gradient within the composite. This temperature gradient could result in a non-uniform carbonization, creating internal stresses through the laminates, which in turn, might lead to localized delamination and/or other damages to the char structure of the matrix. As described above, although a number of different results have been reported, the effects of carbonization rate on the properties of C/C composites have been investigated in low carbonization rate range. It has generally been agreed that high carbonization rates can deteriorate the mechanical performance of C/C composites, therefore the practicability of reducing the manufacturing cost by the increase of carbonization rate seem to be limited. It is the purpose of the present work to study the effect of carbonization rate in a much larger range (up to 1000 C/min) on properties of a PAN/phenolic based C/ C composite and to explore the possibility of manufacturing a C/C composite with a carbonization rate far higher than the conventional ones.
fiber (Torayca T700S, 12K, Toray Co., Japan) and a resole-type phenolic resin (PF-650, Chang Chun Petrochemical Industry, Taiwan), respectively. The chopped carbon fibers were first impregnated with the phenolic resin in a plastic mold in vacuum to form a 110 mm · 110 mm prepreg. The polymer matrix prepreg was placed in an oven at 70 C for 6 h to remove excess solvent, followed by a vacuum bag hot press curing process in a stainless steel mold at 160 C for 30 min under a pressure of 2.76 MPa (400 psi). The cured composite was sectioned into 50 mm · 10 mm coupons using a water-cooled diamond saw, followed by a post-curing process at 230 C for 8 h in an air-circulated oven [15].
2.1. Sample preparation
2.1.2. Carbonization Carbonization was conducted by heating the postcured composite under a nitrogen atmosphere to 1200 C. To study the effect of carbonization (heating) rate on properties of the composite, three different heating rates, i.e., 1 C/min, 100 C/min, and 1000 C/min, were used. The carbonization at low heating rate (1 C/ min) was conducted in an ordinary furnace heated by SiC heating element. The nitrogen gas was introduced continuously into the furnace at a constant flow rate of 0.3 L/min. Carbonization at higher heating rates (100 and 1000 C/min) was conducted using an apparatus specially designed for the study. As sketched in Fig. 1, a 25 mm diameter quartz tube that could be slid in and out a MoSi2-heated tube furnace at a controlled speed to adjust the heating rate of the C/C sample placed within the quartz tube. The temperature of the C/C sample was determined using a K type thermocouple inserted into the quartz tube. When the desired furnace temperature (for this study, 1200 C) was reached, the quartz tube was pushed into the center of the furnace at a controlled, constant speed. Nitrogen gas was introduced continuously into the quartz tube at a flow rate of 0.7 L/min during the entire carbonization process.
2.1.1. Composite forming The fiber and resin used for the preparation of the present PAN/phenolic based C/C composite are a randomly oriented chopped (4.5 mm) PAN-based carbon
2.1.3. Graphitization The carbonized composite was subsequently graphitized to 2200 C in a helium-purged graphitization furnace. (Note: The term ‘‘graphitized’’ or ‘‘graphitiza-
2. Experimental procedure
Fig. 1. Schematic drawing of apparatus for high speed carbonization.
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tion’’ was conveniently used throughout text to stand for the heat treatment at 2200 C, not an indication of a major three-dimensional ordering in crystal structure) All graphitization processes were conducted by a heating rate of 10 C/min, a holding time of 30 min at 2200 C and a cooling rate of 20 C/min. 2.1.4. Densification/carbonization cycles The porous graphitized composite was then densified by re-impregnation with phenolic resin in vacuum, followed by pre-curing at 70 C for 6 h to remove excess solvent; curing at 160 C for 30 min; and post-curing at 230 C for 2 h in an air-circulated oven. The post-cured composite was re-carbonized at the same carbonization rate as that used in the first carbonization process. To improve the density and properties of the composite, four such densification/carbonization cycles were applied, and a further graphitization process were applied to some selected samples. Table 1 lists sample designations for the composite processed at different stages at different carbonization rates. 2.2. Measurements and analytical methods 2.2.1. Morphology An optical microscope and a scanning electron microscope (SEM) were used to examine the surfaces morphology of composites. 2.2.2. Density and porosity According to ASTM C830, bulk density and open porosity of the composite processed at different stages were measured by a water saturation method. (Note: ‘‘density’’ and ‘‘porosity’’ will stand for ‘‘bulk density’’ and ‘‘open porosity’’ in the following discussion) A one-way ANOVA method was used to evaluate the statistical significance of density and porosity data. In all cases, the results were considered statistically different with p < 0.05. The distribution of open porosity at a smaller scale (termed ‘‘microporosity’’ in the following discussion) was also determined using a mercury porosimeter.
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2.2.3. XRD and ESCA An X-ray diffraction (XRD) study was carried out using an X-ray diffractometer. The data were collected at a scan rate of 2 /min under the 2h step scanning mode with a step size of 0.01 using Cu Ka radiation at 30 kV and 20 mA. The degree of randomness were determined by Franklin model [16]: d 002 ¼ 3:44 0:086 ð1 p2 Þ where p represents the degree of randomness. The value ˚ ) and of p for ideal graphite is zero (p = 0, d002 = 3.354 A for a non-graphitic carbon, i.e. the turbostratic struc˚ ). When the ture, the value is one (p = 1, d002 = 3.440 A interlayer spacing of carbon structure is greater than ˚ , the value of p became larger than one, even 3.440 A though the value p > 1 was not defined in the model, this paper only use it to represent a structure that is far from ideal graphite structure. The spectra of electron spectroscopy for chemical analysis (ESCA) were collected on a electron spectrometer with a base pressure of about 108 Torr. The X-ray source was Al Ka radiation (1486.6 eV) and the wide scan spectra in the range of binding energy 1400– 0 eV were recorded for samples. The high-resolution spectra of the C 1s and O 1s signals were recorded in the step of 0.2 eV within the range of binding energy 290–282 eV and 542–522 eV. All samples for ESCA analysis were in a size of 10 mm · 10 mm. 2.2.4. Bending properties The flexural strength and modulus of the composite were measured by three-point bending test based on ASTM D790 with the span-to-depth ratio of 16. Samples for this test were in a size of 50 mm · 10 mm · 2.2 mm. A Shimadzu AGS-500 D universal tester was operated at a crosshead speed of 1 mm/min with a support span of 40 mm. All the bending test data shown in this paper are averages of five samples. Again, one-way ANOVA was used to analyze the statistical significance/insignificance of the data.
3. Experimental results Table 1 Sample designations for composite under different process stages at different carbonization rates Process stage
Carbonization rate (C/min) 1
100
1000
First carbonization First graphitization Second densification/carbonization Fourth densification/carbonization Second graphitization
C1-1 G1-1 D2-1 D4-1 G2-1
C1-100 G1-100 D2-100 D4-100 G2-100
C1-1000 G1-1000 D2-1000 D4-1000 G2-1000
3.1. Morphology The cross-sectional morphology of the C/C composite at various process stages is shown in Fig. 2. These low magnification images exhibited such information as fiber orientation, bubble pores, dry zones, shrinkage and thermal stress-induced cracks, and interfaces of fiber bundles. According to Jortner [17], in the cured stage, the major porosities were bubble pores and dry zones formed during curing process. The interfaces
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Fig. 2. Cross-sectional optical micrographs of composite under different process stages at different carbonization rates. (a) Cured, (b) C1-1, (c) C1100, (d) C1-1000, (e) G1-1, (f) G1-100, (g) G1-1000, (h) D4-1, (i) D4-100, (j) D4-1000, (k) G2-1, (l) G2-100, (m) G2-1000.
between fiber bundles in different orientations could be recognized (marked by arrow), where those of bundles in similar orientation were not able to recognize. After the cured composites were first carbonized (Fig. 2(b)–(d)), the interfaces between fiber bundles in similar orientation became more visible under this magnification. When the samples were further graphitized (Fig. 2(e)–(g)), more thermal-stress cracks formed at or near the interfaces between bundles, therefore the bundlebundle interfaces became easier to recognize. In general, the C/C samples processed at a higher carbonization rate yield more bubble pores than that carbonized at a lower rate. The composite carbonized at a higher heating rate also seems to generate a larger amount of porosity than that carbonized at a lower rate, especially after first graphitization treatment. It is to be noted that the
micrographs in this figure only present a general morphology of the composite processed at different stages. The effects of process stage and carbonization rate on the density/porosity in a quantitative way will be presented in the following section. 3.2. Density and porosity As indicated in Figs. 3 and 4, after first carbonization, the density of the composite largely decreased, while the porosity level largely increased. Sample C11000 had a lower density and lower porosity level than C1-1 and C1-100. After first graphitization, the difference in density change is small, but the porosity values of the G1-1 and G1-100 largely decreased. As expected, the density of the composite continued to increase and
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Fig. 3. Bulk densities of composite under different process stages at different carbonization rates. Same symbol indicate no statistical difference.
Fig. 5. Pore size distributions of composite under process stage C1 at different carbonization rates. (a) Incremental volume, (b) Cumulative volume.
Fig. 4. Open porosity values of composite under different process stages at different carbonization rates. Same symbol indicate no statistical difference.
the porosity content continued to decrease when the densification process continued. After four densification cycles the porosity content of D4-1000 was significantly larger than other two composites. After second graphitization this difference became even more dramatic. Figs. 5–8 demonstrate a series of micron/sub-micron pore size distributions of the composite under different process stages. (In the following discussion ‘‘micropores’’ or ‘‘microporosity’’ is used to stand for the pores in this region). As shown in Fig. 5, the C1-1 had the smallest micropore volume, while the C1-1000 had the largest volume. After first graphitization, the highest incremental-volume peaks of all three composites shifted toward the larger-diameter side (Fig. 6). This result indicating that the existing micropores expanded in
size and/or additional larger micropores were created during the graphitization process. The peaks of G1-100 and G1-1000 shifted more toward the largerdiameter side than G1-1 did. As expected, after four densification/carbonization cycles, the larger micropores in all composites largely diminished (Fig. 7). After second graphitization, G2-100 and G2-1000 showed larger micropore volumes again (Fig. 8). 3.3. XRD and ESCA The XRD was performed on the fiber, bulk resin and C/C composites processed at 1 C/min and 1000 C/min. As presented in Table 2, the d002 values of the carbons processed at 1000 C/min were consistently lower than those processed at 1 C/min, no matter such carbons are fiber, matrix resin, or C/C composite. This indicates that the material processed at higher carbonization rate has a more graphitic structure than the material processed at lower carbonization rate.
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Fig. 6. Pore size distributions of composite under process stage G1 at different carbonization rates. (a) Incremental volume, (b) Cumulative volume.
The results of ESCA analysis on C 1s spectra are shown in Table 3. The position, FWHM (full width at half maximum) and symmetry of C 1s peak are often used for the evaluation of surface chemistry of carbon materials. The C 1s binding energy of the graphitic peak is 284.6 eV [18]. The broadening of the graphitic peak is an indication of a less ordered structure [18]. According to La´szlo´ et al. [19], the existence of functional groups based on carbon such as C–OH, C–O–C and C@O also leads to a broadening and asymmetric C 1s peak. The oxygen-containing functional groups could be recognized by C 1s and O 1s binding energies. Oxide peak of hydroxyl (C–OH) or ether (C–O–C) is shown in the range of 285.3–286.1 eV for C 1s and 533 eV for O 1s, while the oxide peak of carbonyl (C@O) is shown in 287.6 eV for C 1s and 531.5 eV for O 1s [18]. At stage C1, the C 1s binding energy decreased as the carbonization rate increased from 1 C/min to 1000 C/ min. After first graphitization, almost all C 1 s binding energies reached 284.6 eV. At this stage the O 1s binding
Fig. 7. Pore size distributions of composite under process stage D4 at different carbonization rates. (a) Incremental volume, (b) Cumulative volume.
energy also decreased when the carbonization rate increased. After first graphitization, the O 1s binding energies of G1-1 and G1-100 were larger than that of G1-1000. The C 1s FWHM slightly increased at stage C1 when the carbonization rate increased from 1 C/ min to 100 C/min. When the carbonization rate further increased to 1000 C/min, the FWHM dropped. The O 1s/C 1s peak height ratios at stages C1 and D4 both gradually increased with increasing carbonization rate, whereas at stages G1 and G2 the ratios gradually decreased. The results indicate that carbonized samples prepared from higher carbonization rates had more oxygen-containing functional groups. With subsequent graphitization, the oxygen-containing functional groups of samples prepared from higher carbonization rates turned less. 3.4. Bending properties The flexural strengths and moduli of the composites at different process stages are shown in Figs. 9 and 10,
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the strength of D4-100 was lower than other two composites. This effect became even more dramatic after second graphitization. As clearly shown in Figs. 9 and 10, both flexural strength and modulus of G2-1000 were not much different from those of G2-1. Again, the strength and modulus of G2-100 were respectively much lower than other two composites. Typical bending profiles of the three composites are shown in Fig. 11.
4. Discussion
Fig. 8. Pore size distributions of composite under process stage G2 at different carbonization rates. (a) Incremental volume, (b) Cumulative volume.
respectively. Compared with the cured composite, the post-cured composite had a similar flexural strength but a much higher modulus. After first carbonization, both strengths and moduli of all composites largely decreased. After four densification/carbonization cycles, the strengths and moduli of all composites largely increased, as expected. However, quite unexpectedly, the strength of the composite did not decrease with increasing carbonization rate. The average flexural strength of D4-1000 appeared to be similar to that of D4-1, while
To help learn the effect of carbonization rate on bubble pore formation in matrix carbon during carbonization process, a neat phenolic resin (in the absence of fiber) was carbonized using the same carbonization process as for the preparation of C/C samples. The SEM images in Fig. 12 clearly show that the size and distribution of bubble pores in the carbonized resin were sensitive to the carbonization rate. When the resin was carbonized at 1 C/min, there were only a few large (tens of microns in width and hundreds of microns in length) cracks and small (2–5 lm) pores were observed in the cross-sectional micrograph. As the carbonization rate increased to 100 C/min, pores of 10–100 lm could be found. For the resin carbonized at 1000 C/min, more smaller-pores were observed throughout the entire cross-section. This helps to explain the existence of the generally observed lower density/higher porosity level in the samples prepared by higher carbonization rates. According to the study of Gupta and Harrison [9], macroporosity of tens to several hundreds of microns developed primarily in the curing stage of the resin, at temperatures lower than 200 C. As HTT was raised to 450 C (the resin was already in a solid state), there was an increase in pore size and, simultaneously, small pores coalesced to form larger pores. They also found a higher heating rate (15 C/min) yielded a more random distribution of smaller pores in the bulk. Besides the above research, Lausˇevic´ and Marinkovic´ [5] found earlier that during the carbonization process of phenolic resin, water evolved and reacts with the surrounding polymer structure producing H2 and CO2. They compared the water evolution of phenolic resin on powdered and bulk samples and found that water was easier to
Table 2 Effect of carbonization rate on 2h and d002 values of composite under different process stages at different carbonization rates 2h d002 Pa a
Carbonization rate (C/min)
C/C C1
Fiber G1
Resin G1
C/C G1
C/C G2
1 1000 1 1000 1 1000
25.366 25.454 3.512 3.500 1.356 1.303
25.857 25.892 3.446 3.441 1.034 1.006
25.630 25.980 3.476 3.430 1.191 0.941
25.903 25.943 3.440 3.435 1.000 0.970
26.021 26.044 3.425 3.422 0.907 0.887
Degree of randomness, p, is calculated according to Franklin model [16].
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Table 3 ESCA results of composite under different process stages at different carbonization rates
C 1s BE
O 1s BE
C 1s FWHM
O 1s/C 1s Peak height ratio
Carbonization rate (C/min)
C1
G1
D4
G2
1 100 1000 1 100 1000 1 100 1000 1 100 1000
285.4 285.2 284.6 533.2 532.6 532.4 1.61 1.65 1.47 0.19 0.22 0.32
284.8 284.6 284.6 533.6 533.8 532.6 1.91 1.38 1.38 0.55 0.32 0.07
284.6 284.6 284.6 532.2 532.2 532.2 1.36 1.55 1.56 0.24 0.34 0.35
284.6 284.6 284.6 532.6 532.8 533.0 1.63 1.39 1.35 0.46 0.29 0.14
Fig. 9. Flexural strengths of composite under different process stages at different carbonization rates. The symbol ‘‘’’ indicate that the data are significantly different from the neighboring data.
Fig. 10. Flexural moduli of composite under different process stages at different carbonization rates. The symbol ‘‘’’ indicate that the data are significantly different from the neighboring data.
Fig. 11. Typical bending profiles of composite under process stage G2 at different carbonization rates.
stay in the bulk sample, therefore this reaction take place mainly in the bulk samples. The presently observed heating rate effect on pore formation might be explained by the amount of water vapor/gases stay in the samples. During a carbonization process with a higher heating rate, there was insufficient time for the vapor/gases to leave the samples. As a result, the vapor/gases were reacts with the surrounding polymer structure and formed the observed pores in the resin. Jortner [17] proposed that if relaxation of stresses by creep occurs at high temperatures (perhaps above 2000 C), the void volume of cracks at room temperature would be larger than that without creep. Chłopek and Bł_zewicz [20] also found that graphitization process results in the opening of closed pores. These findings might explain the present observation that, prior to the first graphitization process, C1-1000 had both a lower open porosity and lower density than C1-100 and C11. A portion of closed pores contained in C1-1000 broke open during the first graphitization process (G1-1000), resulting in a higher porosity level.
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Fig. 12. Scanning electron micrographs of phenolic resin bulk carbonized at different carbonization rates. (a) 1 C/min, (b) 100 C/ min, (c) 1000 C/min.
The mercury porosimetry results clearly indicated that the volume of pores increased after graphitization, especially after first graphitization. The effect of second graphitization was not as dramatic, since after four cycles of densification, majority of large pores were already filled. The mercury porosimetry data also indicate that composite carbonized at higher heating rates had more large pores than those carbonized at lower heating rates, supporting the argument that the composite carbonized at higher heating rates generate a higher thermal stress level that, in turn, results in larger cracks/pores when the stress was released during heat treatment [17]. In this study, both XRD and ESCA results indicate that the composite processed at a higher carbonization
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rate consistently has a more graphitic structure than that processed at a lower carbonization rate. Again, this is primarily due to the larger internal stress build-up in the composite carbonized at higher heating rate that assists in a ‘‘stress-graphitization’’ process [21,22]. The trends of C 1s FWHM data were not as straightforward as C 1s binding energy; possibly due to the effect of oxygen presence (causing the lineshape to be more asymmetric) that somewhat offsets the heating rate effect. This phenomenon is particularly clear for C1 and D4 groups, where O 1s/C 1s peak height ratios increased with increasing carbonization rate. Given that there was insufficient time for the water vapor/gases to leave the samples during a carbonization process with a higher heating rate, it seems reasonable to suppose that the further reaction of water with the surrounding matrix will not only create more pores in the resin, but also increase the amount of oxygen-containing functional groups [5,23,24]. Park et al. [25] and Tanabe et al. [26] also observed that more oxygen-containing in a thermosetting resin derived carbon generate a more graphitic structure after graphitization process. It is possibly the reason for the opposite carbonization rate effects on O 1s/C 1s peak height ratio between G1/G2 group (decreasing with increasing carbonization rate) and C1/D4 group (increasing with increasing carbonization rate). As mentioned earlier, in the heating rate range of previous studies, the mechanical properties tends to decline as the carbonization rate rise. This seems to be true for carbonization rates up to 100 C/min in this study. However, while the heating rate further increased to 1000 C/ min, the mechanical performance of the composite showed an unexpected ‘‘down and up’’ phenomenon. This phenomenon was most obvious under G2 condition. For ensure the phenomenon is true, we repeat some experiments again, the same trend presents. The ‘‘down and up’’ phenomenon at high carbonization rates may be interpreted as a result of an internal stress build-up effect that caused a stress-induced graphitization process at high carbonization rates, as mentioned earlier [21,22]. At 1000 C/min, this stress graphitization effect became significant enough to cause the strength and modulus of the composite to increase. Another interesting observation, that might help explain the improved mechanical performance of the composite processed at 1000 C/min, is presented below. As shown in the SEM fractography of samples at stages D4 and G2 (Fig. 13), numerous micropores (majority smaller than 5 lm) were observed in the matrix of D41000 and G2-1000, while in D4-1 and G2-1 much fewer such micropores were observed. This result is consistent with the microporosity data as indicated in Figs. 5–8. The presence of such micropores (particularly in G21000) might effectively increase the fracture energy of the composite [27].
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Fig. 13. Scanning electron micrographs of composite under different process stages at different carbonization rates. (a) D4-1, (b) D4-1000, (c) G2-1, (d) G2-1000.
As a final remark, while a stress-induced graphitization in matrix might improve the mechanical properties of a C/C composite, the literature has repeatedly shown that a rapid-heating pyrolysis process could cause cracking, delamination, etc. to degrade the mechanical performance of the composite [14]. In the present work it was found that largely increasing the carbonization rate to 1000 C/min caused the degradation in mechanical properties to recover to the level of 1 C/min. Furthermore, the high carbonization rate (1000 C/min) resulted in higher fracture energy. Obviously under such condition the advantage of the high carbonization rate has overcome its disadvantage. As mentioned earlier, carbonization is perhaps the most time and energy-consuming step in the entire conventional fabrication process of C/C composites. The large increase in carbonization rate can be greatly beneficial to the industry from an economic point of view.
2. The composite processed at a higher carbonization rate had a more graphitic structure than that processed at a lower carbonization rate. 3. After second graphitization the bending properties of composites carbonized at 1 C/min and 1000 C/min were comparable. The composite carbonized at 1000 C/min had the highest fracture energy. The composite carbonized at 100 C/min showed the worst mechanical performance among three.
Acknowledgment The authors are grateful to the National Science Council of the Republic of China for support of this research under contract no. NSC 90-2216-E-006-037.
References 5. Conclusions 1. The composite processed at a higher carbonization rate had a higher porosity level and more large pores than that processed at a lower carbonization rate.
[1] Olsen RE, Reese HF, Backlund SJ. Process for forming carbon– carbon composite. US Patent 5686027, 1997. [2] Sastri SB, Armistead JP, Keller TM. Carbonization of hightemperature resin. Carbon 1993;31(4):617–22. [3] Sastri SB, Armistead JP, Keller TM. Carbon-based composites derived from phthalonitrile resins. US Patent 5965268, 1999.
H.H. Kuo et al. / Carbon 43 (2005) 229–239 [4] Lewis IC. Chemistry of carbonization. Carbon 1982;20(6):519–29. [5] Lausˇevic´ Z, Marinkovic´ S. Mechanical properties and chemistry of carbonization of phenol formaldehyde resin. Carbon 1986;24(5):575–80. [6] Ko TH, Chen PC. Study of the pyrolysis of phenolic resin reinforced with two-dimensional plain-woven carbon fabric. J Mater Sci Lett 1991;10:301–3. [7] Roy AK. Effect of carbonization rates on the interlaminar tensile stiffness and strength of two-dimensional carbon–carbon composites, AD-vol. 34/AMD-vol. 173. Thermomechanical Behavior of Advanced Structural Materials. ASME; 1993. [8] Chang WC, Ma CCM, Tai NH, Chen CB. Effect of processing methods and parameters on the mechanical properties and microstructure of carbon/carbon composites. J Mater Sci 1994;29:5859–67. [9] Gupta A, Harrison IR. Small-angle X-ray scattering (SAXS) in carbonized phenolic resins. Carbon 1994;32(5):953–60. [10] Prevorsek DC, Li HL. Carbon fiber-reinforced carbon composite material. US patent 5556704, 1996. [11] Lewis IC, Howard RA. Carbon–carbon composites containing poorly graphitizing pitch as a binder and/or impregnant having a reduced coefficient of thermal expansion and improved flexural strength. US Patent 5607770, 1997. [12] Hager JW, Newman JW, Johannes N, Turrill FH. Carbon artifacts and compositions and processes for their manufacture. US Patent 6013371, 2000. [13] Murdie N, Parker CA, Pigford JF, Narasimhan D, Dillon F. Process of making carbon–carbon composite material made from densified carbon foam. US Patent 6077464, 2000. [14] Nam JD, Seferis JC. Initial polymer degradation as a process in the manufacture of carbon–carbon composites. Carbon 1992;30(5):751–61. [15] Ko TH, Ma TS. Effect of post-curing on the mechanical properties of carbonized phenolic resins. Polym Compos 1998;19(4):456–62.
239
[16] Franklin RE. The structure of graphitic carbons. Acta Crystallogr 1951;4:253–61. [17] Jortner J. Macroporosity and interface cracking in multi-directional carbon–carbons. Carbon 1986;24(5):603–13. [18] Xie Y, Sherwood PMA. X-ray photoelectron spectroscopic studies of carbon fibers. Part XIV: Electrochemical treatment of pitch-based fibers and the surface and bulk structure changes monitored by XPS, XRD, and SEM. Appl Spectros 1990;44(10):1621–8. [19] La´szlo´ K, Josepovits K, Tomba´cz E. Analysis of active sites on synthetic carbon surfaces by various methods. Anal Sci 2001;17(supplement):i1741–4. [20] ChŁopek L, BŁ_zewicz S. Effect of processing variables on the properties of carbon–carbon composites. Carbon 1991;29(2):127–31. [21] Zaldivar RJ, Rellick GS. Some observations on stress graphitization in carbon–carbon composites. Carbon 1991;29(8):1155–63. [22] Ju CP, Watahiki S, Takano S. Stress-induced preferential alignment of basal planes in phenolic-based carbon–carbon composite. Mater Chem Phys 1993;36:150–5. [23] Kurosaki F, Ishimaru K, Hata T, Bronsveld P, Kobayashi E, Imamura Y. Microstructure of wood charcoal prepared by flash heating. Carbon 2003;41(14):3057–62. [24] Graham RG, Bergougnou MA, Overend RP. Fast pyrolysis of wood. Review. J Anal Appl Pyrol 1984;9:95–135. [25] Park SM, Yasuda E, Akatsu T, Tanabe Y, Kakihana M, Ozawa K. Evolution of microtexture in furan resin-derived carbon with heat-treatment. Carbon 1995;33(10):1377–81. [26] Tanabe Y, Yamanaka J, Hoshi K, Migita H, Yasuda E. Surface graphitization of Furan-resin-derived carbon. Carbon 2001;39(15):2347–53. [27] Dhakate SR, Mathur RB, Bahl OP. Catalytic effect of iron oxide on carbon/carbon composites during graphitization. Carbon 1997;35(12):1753–6.