Surface treatments of vapor-grown carbon fibers produced on a substrate: Part II: Atomic force microscopy
Introduction
Surface oxidation of carbon fibers is commonly used to improve the adhesion between fiber and matrix in composites [1], [2], and the activation of carbon fibers to produce an adsorbent material relies on carbon dioxide or steam gasification, in order to develop an adequate porous texture [3], [4].
In a comparative study of the oxidation of different types of fibers (in air, at 973 K), the peculiar behavior of the VGCF was evidenced [5]. In particular, it was found that these fibers were attacked by a pitting mechanism, which was not observed either with PAN or with pitch fibers. Later, this phenomenon was investigated in more detail, and it was tentatively proposed that such a mechanism was due to the catalytic action of iron traces on the surface of the VGCF [6]. The presence of iron was not unexpected, as it was the catalyst used for fiber growth, but it could not be detected either by XPS or by EDS.
In Part I of this investigation [7] we were able to confirm the presence of iron by time-of-flight secondary ion mass spectrometry (ToF–SIMS). It was also shown that the metal could be eliminated by HCl washing. This procedure originated a clear change in the gasification mechanism, as the large pits were no longer formed. Furthermore, the fibers were submitted to different post-production treatments in order to obtain modifications of their texture and/or surface chemistry, and characterized by SEM, XPS and nitrogen adsorption. These previous studies are complemented here with an investigation of the surface topography of the same VGCF samples by atomic force microscopy. Although the usefulness of scanning probe microscopy and derived techniques for the study of carbon fiber surfaces has been recognized [8], the authors are unaware of any investigation of VGCF by AFM.
Section snippets
Experimental
Vapor-grown carbon fibers were grown from a methane–hydrogen mixture on a Grafoil® support seeded with an iron catalyst, Fe3(CO)12 being used as the catalyst precursor. The experimental method used for fiber growth was previously described [9]. The fibers as produced were submitted to different oxidative treatments: Oxidation with nitric acid (70 wt. %) was performed in a Soxhlet at 80°C during 5 h on 2 g of material (ACF1). Oxygen plasma treatments (ACF2) were realized at INCAR, Oviedo, Spain,
Results and discussion
Typical surface compositions of the VGCF obtained by XPS before and after the oxidative treatments are given in Table 1, together with their BET surface areas. The effect of oxidation is obvious, as indicated by the increase in the surface oxygen concentration.
The VGCF are produced with a variety of morphologies, and frequently show numerous crenulations [10]. As the fibers were not submitted to any graphitisation treatment, and the size of the ordered domains was small (Lc of the order of 4
Conclusions
AFM examination of the surface of VGCF revealed a granular texture which is not observed on the other fiber types (pitch, PAN). Nitric acid oxidation does not change this surface topography, but the plasma treatment increases the size of the grains.
Examination of the fibers after partial gasification by carbon dioxide revealed the presence of holes or pits in the sub-micrometer scale; at the bottom of each pit a particle of about 10 nm height was clearly visible. Both the large pits and the
Acknowledgments
Financial support by the EC through the Human Capital and Mobility programme (contract number CHRX-CT94-0457) is gratefully acknowledged. The authors are indebted to Drs. J.M.D. Tascón and A. Martı́nez-Alonso (INCAR, Oviedo, Spain) for the plasma treatments, and to UCAR (USA) for providing the Grafoil®.
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