XPS and Sem Characterization of Pan-Based Carbon Fibers Treated in Oxygen and Nitric Oxide Plasmas
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studies have also introduced nitrogen functionality onto fiber surfaces by ammonia plasma treatments [6], but nitric oxide treatments have not been investigated. Because NO is a potential source of both oxygen and nitrogen, surface treatments yield many different combinations of nitrogen and oxygen species. All treated fibers are studied by XPS to quantitatively follow chemical changes and by SEM to qualitatively monitor the fiber diameter and surface morphology. EXPERIMENTAL The PAN-based carbon fibers used in this work are unsized, high strength, AU4 fibers obtained from the Hercules Corporation. These fibers are classified as type II materials and have a tensile strength and modulus of 3.98 GPa and 241 GPa, respectively [7]. Fibers were treated in a Harrick PDC-3XG radio frequency (rf) plasma chamber using either 5 Watts or 25 Watts and reaction pressures of ca. 0.5 Torr. The fibers were aligned side-by-side in multiple layers on an XPS sample holder and held down by a gold cover ring. The entire sample holder was placed inside the plasma chamber which was purged with either UHP 02 or NO several times before the rf power was turned on. Fibers were treated for 2 to 60 minutes. After treatment, samples were immediately transferred through the air and analyzed on a Perkin-Elmer 5300 XPS. Spectra were collected at a base pressure of 10-9 Torr using a Mg X-ray source (Kal,2 = 1253.6 eV) operated at 15 kV and 300 Watts. The total spectrometer broadening was of the same order as the X-ray line width, about 0.7 eV for Mg, at an electron pass energy of 8.95 eV. The binding energy scale was calibrated by setting the Au(4f7/2) and Cu(2p3/2) peak values to 84.0 eV and 932.7 eV, respectively. In order to determine the chemical state and quantity of specific species on fiber surfaces, XPS spectra were computer fit using a non-linear least squares program with a GaussianLorentzian (G/L) sum function. Intensities, peak positions, G/L mix (fraction Gaussian), and line widths were all optimized by the fitting routine. The only fitting parameters constrained by the computer program were the G/L mix for the oxidized carbon species which was set to 0.5, and the tail height and slope for the main graphitic carbon peak which were set to 0.5 and 0.4, respectively. The asymmetric character of the main graphitic peak arises because of core-hole coupling [4, 8]. SEM samples were prepared by placing several fibers on an aluminum stud using copper conducting tape. Sample studs were then placed in the plasma chamber along with the XPS sample holder and treated as mentioned above. SEM samples were then sputter coated with a thin Pd/Au layer of approximately 200 nm in order to avoid charging problems in the electron microscope. All SEM micrographs were taken on a JEOL JSM-840 microscope. RESULTS AND DISCUSSION Scanning Electron Microscopy Results Figures l a-d are SEM micrographs (xl0,000 magnification) for AU4 carbon fibers treated in NO plasmas at 25 Watts for 5, 15, 30 and 60 minutes. Micrographs for untreated and 2 minute treatments are n
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