Aligning vapor-grown carbon fibers in polydimethylsiloxane using dc electric or magnetic field
Introduction
Research on alignment of various carbonaceous materials, e.g., carbon black (CB), carbon fiber (CF), carbon nanotubes (CNTs), has received considerable attention. In particular, carbon materials that exhibit a graphite-layered structure have a strong structural anisotropy, and the alignment of CNTs along the direction of an applied magnetic field has been attempted by utilizing this strong anisotropic diamagnetic property [1]. Fujiwara et al. have reported magnetic orientation of CNTs in a carbon tetrachloride suspension [2]. Kimura et al. and Choi et al. have produced polymer composites including magnetically aligned multiwalled (MW) CNTs (under a field of 10–25 T), and evaluated their electric conductivity [3], [4]. Although some improvement in the electroconductivity of the composite was achieved by magnetic processing when the CNT concentration was less than the percolation threshold, the enhancement was only one or two orders of magnitude.
Another possible way to align carbonaceous materials is by using either a dc or ac electric field [5], [6], [7], [8], [9], [10], [11], [12], [13]. Experiments aimed at aligning CNTs have been performed in organic solvent suspensions under a dc or ac electric field. Organic solvents used to date include isopropyl alcohol (IPA) [5], [6], ethanol [7], dimethylformamide (DMF) [8], and tetrahydrofran (THF) [9]. Yamamoto et al. have reported electrophoresis of CNTs in IPA [5], [6]. The CNTs in IPA moved towards the cathode under a dc electric field [5], and the electrophoresis strongly depended on frequency under an ac electric field [6]. Chen et al. and Kumar et al. have achieved highly aligned structures of CNTs between electrodes (having 25 μm and 3 μm gaps, respectively) under an ac electric field by completely removing the solvents [7], [8]. Kamat et al. have reported the reversible assembly of single-walled CNTs in a bundle in THF by applying an ON–OFF cycle of a dc electric field.
Electric fields have also been applied to achieve oriented carbonaceous structures in a polymer matrix as well. A study on CB–epoxy composites showed that CB moves toward the anode under a dc electric field, and the growth of dendrites from the anode into the polymer matrix was observed [10]. In another study carried out under a dc electric field on CB–epoxy composites, it was revealed that the charge on the surface structure of CB influences whether or not a network is formed, and that the network structure formed, either a ramified or straight chain network structure, depends on this charge on the surface structure [11]. In contrast, it was observed that a network structure, either a ramified or chain network structure, is always formed under an ac electric field [11]. Prasse et al. found that in an MWCNT–epoxy system, the MWCNTs moved toward the anode under a dc electric field and formed a straight chain network structure under an ac electric field [12]. In a study on another MWCNT–epoxy composite by Martin et al., MWCNTs were observed to move toward the anode under a dc electric field, resulting in the growth of dendrites from the anode [13]. On the other hand, a relatively uniform ramified alignment of MWCNTs was observed under an ac electric field [13]. The structural development of carbonaceous materials in various matrixes under an electric field is complex, particularly under a dc electric field, and therefore further investigation is required to realize a thorough understanding. To date, there have been almost no reports detailing a comparison in structural development among carbonaceous materials under electric and magnetic fields.
Vapor-grown carbon fibers (VGCFs) are an important carbonaceous material, which are grown by the decomposition of hydrocarbons using transition metal particles, e.g., iron, as a catalyst at a growth temperature of 1000–1300 °C [14]. After a VGCF is graphitized at 2800 °C under an inert atmosphere, it has highly graphitic basal planes parallel to the fiber axis [14]. Regarding matrix media, polydimethylsiloxane is a nonpolar transparent viscous polymer at room temperature with zero reactivity. This article details in situ optical microscope observations carried out along two directions, parallel and perpendicular to the direction in which the field was applied, which revealed the structural development of graphitized VGCFs in polydimethylsiloxane under a dc electric or magnetic field. The initial degree of dispersion and the stability of dispersed VGCFs were carefully checked. The volume electric resistivity, which reached saturation with passage of time, was measured as a function of VGCF concentration. In addition, the effect of the matrix viscosity on the structural development was examined.
Section snippets
Materials
Commercially available VGCFs, VGCFs® (trade name of Showa Denko K.K. Japan, average diameter = 150 nm) were used as received. Pyrolysis of hydrocarbons was carried out prior to the heat treatment, i.e., graphitization at a temperature up to 2800 °C, resulting in highly ordered graphite layers along the axial direction [14]. The diffraction pattern (0 0 2), as obtained by wide-angle X-ray diffraction (WAXD) analysis, had a very sharp peak at 26.2°. According to the metal analysis performed by the
Dispersion of VGCFs in silicone oil
To study the effect of the application of a dc electric or magnetic field on the alignment of VGCFs, a dispersion of uniformly dispersed VGCFs® was prepared. It is known that the stability of the dispersion was affected by the post-preparation condition of the dispersion. Therefore, the influence of post-preparation shear deformation on the induced re-aggregation of uniformly dispersed VGCFs® in silicone oil was examined. The starting dispersion was quite uniform, as shown in Fig. 1(c). Fig. 3
Conclusions
A trace amount of vapor-grown carbon fibers (VGCFs) was used as carbon filler in a silicone oil system, giving a uniform dispersion of fibers without any initial lumps of VGCF powder. It was successfully demonstrated that the application of a dc electric field to this system allowed the formation of a relatively uniform aligned network structure without the occurrence of electrophoresis. The resulting network had a markedly reduced percolation threshold (along the thickness direction) of below 1
Acknowledgements
This research was supported by the CLUSTER of the Ministry of Education, Culture, Sports, Science and Technology, Japan. It was also partially supported by Grants-in-Aid (No. 17560603, 16550171) from the Japan Society for the Promotion of Science, and the Fukushima Technology Center, Japan.
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