Elsevier

Carbon

Volume 68, March 2014, Pages 426-439
Carbon

The structure and properties of ribbon-shaped carbon fibers with high orientation

https://doi.org/10.1016/j.carbon.2013.11.019Get rights and content

Abstract

Using a naphthalene-derived mesophase pitch as a starting material, highly oriented ribbon-shaped carbon fibers with a smooth and flat surface were prepared by melt-spinning, oxidative stabilization, carbonization, and graphitization. The preferred orientation, morphology, and microstructure, as well as physical properties, of the ribbon-shaped carbon fibers were characterized. The results show that, the ribbon-shaped fibers possessed uniform shrinkage upon heat treatment, thereby avoiding shrinkage cracking commonly observed in round-shaped fibers. As heat treatment progressed, the ribbon-shaped graphite fibers displayed larger crystallite sizes and higher orientation of graphene layers along the main surface of the ribbon-shaped fiber in comparison with corresponding round-shaped fibers. The stability of the ribbon-shaped graphite fibers towards thermal oxidation was significantly higher than that of K-1100 graphite fibers. The longitudinal thermal conductivity of the ribbon fibers increased, and electrical resistivity decreased, with increasing the heat treatment temperatures. The longitudinal electrical resistivity and the calculated thermal conductivity of the ribbon-shaped fibers graphitized at 3000 °C are about 1.1 μΩ m and above 1100 W/m K at room temperature, respectively. The tensile strength and Young’s modulus of these fibers approach 2.53 and 842 GPa, respectively.

Introduction

Mesophase pitch-based carbon fiber has been recognized as a strategic material because of its higher Young’s modulus and greater thermal and electrical conductivity compared with carbon fibers derived from polyacrylonitrile [1], [2], [3]. Its well-developed graphitic structure is known to be the origin of its superior stiffness and conductivity which is attractive for many applications; one example is in the thermal management field due to its excellent thermal transport properties [4], [5], [6]. The most thermally conductive, commercially available mesophase pitch-based graphite fiber, K-1100 (produced by BP-Amoco), has a nominal thermal conductivity value over 1000 W/m K [7]. This high value of thermal conductivity is a direct result of its highly crystalline graphitic structure and a high degree of orientation parallel to the fiber axis. However, preparing such small fibers with a uniform diameter of ∼7 μm requires a complex combination of processing parameters [8], in particular precise control of the transverse texture of the as-spun pitch fibers. It has been well documented that fibers with a radial transverse texture offer a high degree of graphitization and thermal conductivity [5]. However, open wedge cracks often are formed in fibers with radial transverse structure during their subsequent high temperature heat-treatment which may affect their final properties [9], [10]. On the other hand, as-spun mesophase pitch fibers with larger diameters may be beneficial for development of a higher degree of preferred orientation of graphite crystals, but stabilization of these fibers requires a longer time compared with those of smaller diameters and some randomization of the crystal preferred orientation in the fibers often occurs [9], [11], [12]. Thus the diameters of carbon fibers that can be achieved in commercial processes are restricted by the time-consuming stabilization process. Therefore, carbon fibers with average diameters larger than 15 μm are rarely produced.

Ribbon-shaped carbon fibers can efficiently overcome the limitations imposed by crack formation and maximum diameter because, through the control of processing parameters, they can be made to possess a highly ordered structure with the basal plane layers oriented predominantly perpendicular [5], [7] or parallel [7] to the main surface of the ribbon-shaped fiber. Previous research [7], [13] indicates, qualitatively, that low shear rates in a slit-shaped die tend to cause alignment of the microstructure parallel to the tape axis and to the main surface of the tape, whereas higher shear rates tend to cause alignment parallel to the tape axis but approximately perpendicular to the main surface of the tape. In the case of low shear rates, the latter texture is also observed at the tape edges. Also, at least for low shear rates, the degree of misorientation in the microstructure is larger for wider tapes (i.e. from wider slit-shaped dies), although this disparity between wide and narrow tapes decreases with increasing heat treatment. The graphite crystal orientation in a mesophase pitch-based ribbon fiber is superior parallel to the longitudinal direction of fiber, compared with that of traditional commercial round-shaped fibers [4], [5]. This allows these ribbon-shaped fibers to be graphitized more easily than round-shaped fibers at lower graphitization temperatures, and hence with lower production costs. Ribbon-shaped fibers graphitized at only 2400 °C exhibit transport properties comparable to those of commercial round-shaped fibers graphitized at temperatures above 3000 °C [14]. Furthermore, ribbon-shaped carbon fibers can be made with large cross-sectional areas of ∼200 times of those of conventional round-shaped carbon fibers [7] but their low thickness, perpendicular to the main fiber surface, offers a short diffusion pathway for oxygen, enabling the ribbon-shaped fibers to be easily stabilized in air or oxygen. It is well known that oxidative stabilization is diffusion-controlled over larger pathways [15]. In addition, for thicker round-shaped fibers in particular, the rate of reaction in the pitch not only becomes slower and slower with increasing distance from the interface between the pitch and the oxidizing atmosphere but, at high stabilization temperatures, it may be further hindered as the outer surface becomes stabilized rapidly and so becomes a diffusion barrier to deeper stabilization [12]. Thus the thickness of as-spun ribbon-shaped pitch fibers is commonly controlled so as to be less than 30 μm in order to achieve complete stabilization. Up to now, there are few reports detailing the structure and properties of ribbon-shaped carbon fibers with a large transverse area.

In this paper, highly oriented ribbon-shaped carbon fibers, with a smooth and flat surface, were prepared by melt spinning a naphthalene-derived mesophase pitch. The preferred orientation, morphology, microstructure, and texture of the ribbon-shaped carbon fibers heat treated at various temperatures were then characterized. In particular, the electrical and thermal transport properties, as well as the mechanical properties, of the ribbon-shaped fibers produced under various conditions are also reported.

Section snippets

Preparation of ribbon-shaped pitch fibers

A commercial naphthalene-derived synthetic mesophase pitch produced by Mitsubishi Gas Chemical Corporation was used directly as a raw material for spinning of pitch fibers. This type of mesophase pitch is 100% anisotropic and has a softening point of 265 °C. Uniformly molten mesophase pitch was extruded under nitrogen pressure of ∼0.2 MPa through a slit-shaped die with an aspect ratio of about 80 at a spinning temperature of 320–330 °C to form ribbon-shaped pitch fibers which were drawn and

XRD and Raman analyses of the ribbon-shaped carbon fibers

Fig. 1(a) shows a diagram of three different methods (random scan, equatorial scan, and meridional scan [19]) used for XRD characterization to determine the degree of structural orientation of ribbon-shaped fibers. The XRD profiles from the three different methods of analysis of the ribbon-shaped fibers, carbonized, and graphitized at 1600 and 3000 °C, respectively, are shown in Fig. 1(b–d). The XRD random scan profiles show that the samples are pure carbon. There is one sharp diffraction at

Conclusions

Highly oriented ribbon-shaped carbon fibers with a smooth and flat surface were prepared by melt-spinning, oxidative stabilization, carbonization, and graphitization. XRD shows that graphite crystallites within the ribbon-shaped fibers develop upon progressive heat treatment, as confirmed by Raman spectroscopy, and are well aligned with their graphitic layers parallel to the main surfaces of the ribbon fiber (despite the differentia of crystal orientation at the center and edges of the ribbon

Acknowledgments

This work was sponsored by the Key Program of the Major Research Plan of the National Natural Science Foundation (Grant No. 91016003) and the National Natural Science Foundation (Grant No. 51372177) of China.

References (35)

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