Enhancing the thermal conductivity of polyacrylonitrile- and pitch-based carbon fibers by grafting carbon nanotubes on them
Introduction
Carbon fibers are widely used as a reinforcement in composite materials because of their high specific strength and modulus. Such composites have become a dominant material in the aerospace, automotive and sporting goods industries [1], [2], [3]. Current trends toward the development of carbon fibers have been driven in two directions; ultrahigh tensile strength fiber with a fairly high strain to failure (∼2%), and ultrahigh modulus fiber with high thermal conductivity. Today, a number of ultrahigh strength polyacrylonitrile (PAN)-based (more than 6 GPa), and ultrahigh modulus pitch-based (more than 900 GPa) carbon fibers have been commercially available. Carbon fibers with exceptionally high thermal conductivity are critical for many thermal control applications in the aerospace and electronics industries [4], [5], [6]. The thermal conductivity of carbon fibers was found to increase asymptotically as the degree of preferred orientation of the crystalline parts in the fiber increases [7]. However, further improvement of thermal conductivity over the existing highly-oriented pitch-based carbon fiber while retaining the desired mechanical property has proven to be very challenging.
One of the most effective approaches to further increase the thermal conductivity is to graft carbon nanotubes (CNTs) on the carbon fibers. CNTs have an extremely high thermal conductivity in the axial direction, and the thermal conductivities of multi-walled CNTs had been reported to be as high as 3000 W/m K [8]. The grafting of CNTs on carbon fibers using chemical vapor deposition and electrodeposition has been reported in the literature [9], [10], [11], [12], [13]. Naito et al. reported that the grafting of CNTs improves the mechanical properties and Weibull modulus of ultrahigh strength PAN-based and ultrahigh modulus pitch-based carbon fibers [14]. In the present work, the effect of grafting CNTs on the thermal conductivity of T1000GB PAN-based and K13D pitch-based carbon fibers were investigated.
Section snippets
Materials
Carbon fibers used in this study are (i) a low thermal conductivity and ultrahigh tensile strength PAN-based (T1000GB) carbon fiber and (ii) a high thermal conductivity and ultrahigh modulus pitch-based (K13D) carbon fiber. The T1000GB PAN-based carbon fiber was supplied from Toray Industries, Inc. and the K13D pitch-based carbon fiber was supplied from Mitsubishi Chemical Functional Products, Inc. Note that as-received, both fibers had been subjected to commercial surface treatments and sizing
Results and discussions
Fig. 2 shows the X-ray diffraction (XRD) results for the as-received T1000GB PAN-based and K13D pitch-based carbon fiber bundles.
For K13D pitch-based carbon fiber, the intensities of (0 0 2), (0 0 4), (1 0 0) and (1 1 0) reflections at φ of 0° and 90° were very strong and the peaks for (1 0 1) and (1 1 2) reflections at φ of 70° were also observed. This indicated that the K13D pitch-based carbon fiber has highly-oriented structure. Fibers which demonstrate a peak for the (1 0 1) and/or (1 1 2) reflection in
Concluding remarks
The thermal conductivities for the carbon nanotubes (CNTs)-grafted T1000GB and K13D fibers were measured to be 18.6 ± 1.7 and 967.1 ± 29.7 W/m K, which corresponds to 47% and 30% improvement. The results clearly show that the grafting of high thermal conductivity CNTs is very effective in improving the thermal conductivity of PAN- and pitch-based carbon fibers.
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