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  • Volume 18
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  • Pages.18-23
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  • 2016
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  • 1976-4251(pISSN)
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  • 2233-4998(eISSN)
Korean Carbon Society (한국탄소학회)
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Microstructural changes of polyacrylonitrile-based carbon fibers (T300 and T700) due to isothermal oxidation (1): focusing on morphological changes using scanning electron microscopy

  • Oh, Seong-Moon (School of Materials Science and Engineering, Kumoh National Institute of Technology) ;
  • Lee, Sang-Min (School of Materials Science and Engineering, Kumoh National Institute of Technology) ;
  • Kang, Dong-Su (School of Materials Science and Engineering, Kumoh National Institute of Technology) ;
  • Roh, Jae-Seung (School of Materials Science and Engineering, Kumoh National Institute of Technology)
  • Received : 2015.04.29
  • Accepted : 2015.10.25
  • Published : 2016.04.30
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Abstract

Polyacrylonitrile (PAN)-based carbon fibers have high specific strength, elastic modulus, thermal resistance, and thermal conductivity. Due to these properties, they have been increasingly widely used in various spheres including leisure, aviation, aerospace, military, and energy applications. However, if exposed to air at high temperatures, they are oxidized, thus weakening the properties of carbon fibers and carbon composite materials. As such, it is important to understand the oxidation reactions of carbon fibers, which are often used as a reinforcement for composite materials. PAN-based carbon fibers T300 and T700 were isothermally oxidized in air, and microstructural changes caused by oxidation reactions were examined. The results showed a decrease in the rate of oxidation with increasing burn-off for both T300 and T700 fibers. The rate of oxidation of T300 fibers was two times faster than that of T700 fibers. The diameter of T700 fibers decreased linearly with increasing burn-off. The diameter of T300 also decreased with increasing burn-off but at slower rates over time. Cross-sectional observations after oxidation reactions revealed hollow cores in the longitudinal direction for both T300 and T700 fibers. The formation of hollow cores after oxidation can be traced to differences in the fabrication process such as the starting material and final heat treatment temperature.

Keywords

1. Introduction

Polyacrylonitrile (PAN)-based carbon fibers, which use PAN as a precursor, are fabricated into lightweight, high-strength carbon fibers through spinning, stabilization, and carbonization [1,2]. They are known for their high specific strength, melting point, elastic modulus, thermal resistance, and thermal conductivity [3-5]. Recently, PAN-based carbon fibers have seen wide use in leisure, aviation, aerospace, military, and energy sectors because they satisfy industrial demands for lightweight and high-strength materials [6,7].

However, carbon fibers and other carbon materials break down into CO or CO2 when oxidized in air at temperatures higher than 500℃ [8]. Since oxidation reactions at high temperatures weaken the properties of carbon fiber composite materials, various studies have been conducted to resolve this issue [9,10]. This highlights the importance of understanding the oxidation reactions of carbon fibers, which are used as a reinforcement for composite materials.

Most research on the oxidation reactions of carbon fibers has focused on the production of activated carbon fibers or the oxidation mechanism of pitch-based carbon fibers. On the other hand, there have been few studies on the oxidation reaction of PNA-based carbon fibers, which are the most widely used carbon fibers commercially [11-17].

In this study, PAN-based carbon fibers, most commonly used as a reinforcement for C-C composite materials or carbon fiber reinforced plastics (CFRP), were isothermally oxidized in air. The microstructural changes resulting from oxidation reactions were examined. Among the various microstructural changes, this study focused on morphological changes using scanning electron microscope (SEM).

 

2. Experimental

2.1. Raw materials and sample preparations

The raw carbon fibers used in this study were PAN-based carbon fibers T300 (3k) and T700 (12k) fabricated by Toray Advanced Materials (Seoul, Korea). The surface of the raw materials and desized carbon fibers was observed using a SEM (JSM-6500F, JEOL, Japan), and the results are shown in Fig. 1. As shown in Fig. 1a, the texture on the surface of T300 fibers was well-developed. The T700 fibers shown in Fig. 1b had a smoother surface with less texture. Fig. 1c and d present observations of the desized fibers after exposing them to air at 400℃ for 1 h. The desized T300 fibers had more distinct texture lines than the raw fibers. The desized T700 fibers showed some texture, but this was not as developed as the texture of the T300 fibers. Isothermal oxidation reactions in this study were carried out using the desized carbon fibers.

Fig. 1.Scanning electron microscope images (×10k); (a) raw T300 fiber, (b) raw T700 fiber, (c) desized T300 fiber, (d) desized T700 fiber.

2.2. Isothermal oxidation

The desized carbon fibers were isothermally oxidized using a thermogravimetric analyzer (TGA) (Auto TGA Q502, TA Instruments, USA). The fibers used in the TGA analysis were cut to 5.0 mm with an initial weight of 4.5 ± 0.1 mg. The gas for oxidation was air, with a flow rate of 50 mL/min. The isothermal oxidation reactions were carried out at 700℃, and a graph of the results is shown in Fig. 2. As can be seen from the isothermal oxidation graph, oxidation reactions were faster for T300 than T700 at 700℃. T300 fibers completed oxidation in 46.24 min, and T700 fibers did so in 82.25 min.

Fig. 2.Thermogravimetric analyzer plot for full oxidation of T300 and T700.

2.3. Microstructure

Microstructural changes of the carbon fibers were observed with SEM after isothermal oxidation. Oxidation samples were obtained by adjusting the oxidation time based on the graph of Fig. 2. The amount of oxidation of the samples in relation to the oxidation time is represented as burn-off (%), and the conditions are presented in Table 1. SEM was employed to observe the surface and cross-section to evaluate morphological changes.

Table 1.Burn-off (%) as a function of oxidation time

 

3. Results and Discussion

3.1. Oxidation rate

For a quantitative comparison of oxidation rates between the two fibers, the oxidation rate was calculated from the isothermal oxidation graph. As shown in (1), the oxidation rate was expressed as the change in weight with burn-off.

where mgm is measured weight, mgi is initial weight, and min is oxidation time.Table 2 shows the oxidation rates for varying burn-off values of 20%, 50%, and 80% in the isothermal oxidation graph of Fig. 2. This relationship is presented as a graph in Fig. 3a. The normalized rates in Table 2 are the standardized values of other samples when the rate of T700 fibers at 80% burn-off is set as 1.0. The relationship between oxidation rate and burn-off is given in Fig. 3b.

Table 2.mgm, measured weight; mgi, initial weight; min, oxidation time.

Fig. 3.Plots for oxidation rate and normalized rate as a function of burn-off.

As shown in Fig. 3 and Table 2, both the T300 and T700 fibers showed a decrease in the rate of oxidation with increasing burn-off. The normalized rate of T300 fibers at 20% burn-off was 29.00, and this decreased by 14.87 times to 1.95 at 80% burn-off. The normalizing rate of T700 fibers at 20% burn-off was 14.59 times faster than the rate at 80% burn-off.

The oxidation rate tends to decrease with increasing burn-off because the reduced weight results in a fewer number of carbon atoms of the fiber contributing to reactions with O2 in air. Another possibility is that there is sufficient O2 adsorbed on the surface of carbon fibers, but the emission of CO from the surface over time interferes with the adsorption of the reacting gas.

At all burn-off amounts, the rate of oxidation of T300 fibers was two times faster than that of T700 fibers. This indicates that T700 fibers have a greater resistance to oxidation than T300 fibers.

3.2. Surface morphology

Fig. 4 presents ×1000 magnification of the surface of T300 and T700 fibers using SEM. As shown in the image, the diameter decreases with increasing burn-off. The change in diameter was based on measurements of 50 carbon fibers at least 30 μm away from the cut area shown in Fig. 4. The measurement results are presented in Fig. 5, and organized in Table 3.

Fig. 4.Scanning electron microscope images (×1k); (a) desized T300 fiber, (b) 27.10% oxidized T300 fiber, (c) 45.16% oxidized T300 fiber, (d) 65.96% oxidized T300 fiber, (e) desized T700 fiber, (f ) 28.06% oxidized T700 fiber, (g) 51.56% oxidized T700 fiber, (h) 71.66% oxidized T700 fiber.

Fig. 5.Change in diameter as a function of burn-off.

Table 3.Average diameters and standard deviations as a function of burn-off

The diameter of desized T700 fibers was 7.28 μm. At 71.66% burn-off, the diameter decreased linearly to 4.64 μm. The diameter of T300 fibers was 7.43 μm, and decreased to 5.78 μm at 65.96% burn-off. Meanwhile, the diameter of T300 fibers decreased with increasing burn-off but at slower rates over time.

Fig. 6 shows a SEM magnification of the carbon fiber surfaces before and after isothermal oxidation. The texture of T300 fibers was more distinct and developed than that of raw fibers. As oxidation reactions progressed, the textural lines on the surface grew further apart. The smooth surface of T700 fibers grew rougher with oxidation.

Fig. 6.Surface morphologies (×10k); (a) desized T300 fiber, (b) 27.10% oxidized T300 fiber, (c) 45.16% oxidized T300 fiber, (d) 65.96% oxidized T300 fiber, (e) desized T700 fiber, (f) 28.06% oxidized T700 fiber, (g) 51.56% oxidized T700 fiber, (h) 71.66% oxidized T700 fiber.

From the textural development on the surface of the fibers, we can presume that pores are present in the fibers [18]. The slower rate of decrease in diameter of T300 fibers compared to T700 fibers indicates that the former is likely to have a greater development of pores.

3.3. Cross-sectional morphology

Fig. 7 shows the cross-section of carbon fibers before and after isothermal oxidation. The cross-sectional observations revealed hollow cores in the longitudinal direction after isothermal oxidation, similar to the shape of hollow fibers [19,20]. According to the model proposed by Barnet and Norr [21], the crystallinity weakens towards the center of PAN-based carbon fibers. Thus, the formation of hollow cores, as shown in Fig. 7, can be explained by the difference in oxidation rates, arising from the difference in crystallinity between the surface and the center. This results from differences in the fabrication process such as the starting material and the final heat treatment temperature.

Fig. 7.Cross-sectional morphologies (×5k); (a) desized T300 fiber, (b) 27.10% oxidized T300 fiber, (c) 45.16% oxidized T300 fiber, (d) 65.96% oxidized T300 fiber, (e) desized T700 fiber, (f) 28.06% oxidized T700 fiber, (g) 51.56% oxidized T700 fiber, (h) 71.66% oxidized T700 fiber.

This study employed SEM to observe morphological changes of T300 and T700 carbon fibers after isothermal oxidation. The difference in oxidation behavior between the two fibers is due to the difference in crystallinity between the surface and the center. As future work, Ramam spectroscopy and X-ray diffraction spectroscopy will be used to examine the influence of crystallinity on oxidation, along with pore development based on a Brunauer-Emmett-Teller analysis.

 

4. Conclusions

After isothermal oxidation of PAN-based carbon fibers T300 and T700 in air, morphological changes were observed in relation to oxidation reactions. The following conclusions were derived.

The results showed a decrease in the rate of oxidation with increasing burn-off for both T300 and T700 fibers. The rate of oxidation of T300 fibers was two times faster than that of T700 fibers. The diameter of T700 fibers decreased linearly with increasing burn-off. The diameter of T300 also decreased with increasing burn-off but at slower rates over time, indicating that more pores were present in the fibers. Cross-sectional observations after oxidation reactions revealed hollow cores in the longitudinal direction for both T300 and T700 fibers. The formation of hollow cores after oxidation is due to the difference in crystallinity between the surface and the center, which can be traced to differences in the fabrication process such as the starting material and the final heat treatment temperature.

References

  1. Hameed N, Sharp J, Nunna S, Creighton C, Magniez K, Jyotishkumar P, Salim NV, Fox B. Structural transformation of polyacrylonitrile fibers during stabilization and low temperature carbonization. Polym Degrad Stab, 128, 39 (2016). http://dx.doi.org/10.1016/j.polymdegradstab.2016.02.029.
  2. Rahaman MSA, Ismail AF, Mustafa A. A review of heat treatment on polyacrylonitrile fiber. Polym Degrad Stab, 92, 1421 (2007). http://dx.doi.org/10.1016/j.polymdegradstab.2007.03.023.
  3. Moon SC, Farris RJ. Strong electrospun nanometer-diameter polyacrylonitrile carbon fiber yarns. Carbon, 47, 2829 (2009). http://dx.doi.org/10.1016/j.carbon.2009.06.027.
  4. Yusof M, Ismail AF. Post spinning and pyrolysis processes of polyacrylonitrile (PAN)-based carbon fiber and activated carbon fiber: a review. J Anal Appl Pyrolysis, 93, 1 (2012). http://dx.doi.org/10.1016/j.jaap.2011.10.001.
  5. Wangxi Z, Jie L, Gang W. Evolution of structure and properties of PAN precursors during their conversion to carbon fibers. Carbon, 41, 2805 (2003). http://dx.doi.org/10.1016/s0008-6223(03)00391-9.
  6. Wang H, Wang Y, Li T, Wu S, Xu L. Gradient distribution of radial structure of PAN-based carbon fiber treated by high temperature. Prog Nat Sci Mater Int, 24, 31 (2014). http://dx.doi.org/10.1016/j.pnsc.2014.01.009.
  7. Lee MG, Jang SY, Kim SY, Cho SK. Design allowable value development of high modulus carbon/epoxy composite lamina material. Korea Soc Aeronaut Space Sci, 40, 1651 (2012).
  8. Yu Y, Luo R, Shang H, Wang T, Wang J, Wang L. Oxidation behavior of carbon/carbon composites coated with a Si-SiOx/BN-B2O3-SiO2-Al2O3 oxidation protection system at intermediate temperature. Vacuum, 128, 9 (2016). http://dx.doi.org/10.1016/j.vacuum.2016.02.016.
  9. Park SJ, Seo MK, Lee JR. Influence of oxidation inhibitor on carbon-carbon composites: 6. studies on friction and wear properties of carbon-carbon composites. Polym Korea, 25, 133 (2001).
  10. Kim GS. Study on the development and application of advanced carbon composites. Ceramist, 12, 91 (1997).
  11. Dawson EA, Parkes GMB, Barnes PA, Chinn MJ, Pears LA, Hindmarsh CJ. A study evolved gas control and its effect on carbon yield during the activation of carbon fibers by controlled rate methods. Carbon, 40, 2897 (2002). http://dx.doi.org/10.1016/s0008-6223(02)00220-8.
  12. Oya A, Yoshida S, Alcaniz-Monge J, Linares-Solano A. Formation of mesopores in phenolic resin-derived carbon fiber by catalytic activation using cobalt. Carbon, 33, 1085 (1995). http://dx.doi.org/10.1016/0008-6223(95)00054-h.
  13. Maciá-Agulló JA, Moore BC, Cazorla-Amorós D, Linares-Solano A. Activation of coal tar pitch carbon fibers: physical activation vs. chemical activation. Carbon, 42, 1367 (2004). http://dx.doi.org/10.1016/j.carbon.2004.01.013.
  14. Roh JS. Microstructural changes during activation process of isotopic carbon fibers using CO2 gas (II)-TEM Study. Korean J Mater Res, 13, 749 (2003). http://dx.doi.org/10.3740/mrsk.2003.13.11.749.
  15. Tekinalp HL, Cervo EG, Fathollahi B, Thies MC. The effect of molecular composition and structure on the development of porosity in pitch-based activated carbon fibers. Carbon, 52, 267 (2013). http://dx.doi.org/10.1016/j.carbon.2012.09.028.
  16. Greene ML, Schwartz RW, Treleaven JW. Short residence time graphitization of mesophase pitch-based carbon fibers. Carbon, 40, 1217 (2002). http://dx.doi.org/10.1016/s0008-6223(01)00301-3.
  17. Mochida I, Kawabuchi Y, Kawano S, Matsumura Y, Yoshikawa M. High catalytic activity of pitch-based activated carbon fibres of moderate surface area for oxidation of NO to NO2 at room temperature. Fuel, 76, 543 (1997). http://dx.doi.org/10.1016/s0016-2361(96)00223-2.
  18. Suzuki T, Kaneko K. The dynamic structural change of high surface area microcrystals of graphite with N2 adsorption. J Colloid Interface Sci, 138, 590 (1990). http://dx.doi.org/10.1016/0021-9797(90)90243-h.
  19. Qiu J, Wu X, Qiu T. High electromagnetic wave absorbing performance of activated hollow carbon fibers decorated with CNTs and Ni nanoparticles. Ceram Int, 42, 5278 (2016). http://dx.doi.org/10.1016/j.ceramint.2015.12.056.
  20. Favvas EP, Heliopoulos NS, Papageorgiou SK, Mitropoulos AC, Kapantaidakis GC, Kanellopoulos NK. Helium and hydrogen selective carbon hollow fiber membranes: the effect of pyrolysis isothermal time. Sep Purif Technol, 142, 176 (2015). http://dx.doi.org/10.1016/j.seppur.2014.12.048.
  21. Morgan P. Carbon Fibers and Their Composites, Taylor & Francis Group, Boca Raton, FL, 185, (2005).