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Conduction electron spin resonance studies of vapor-grown carbon fibers
Abstract
Vapor grown (VG) carbon fibers have been studied using Conduction Electron Spin Resonance (CESR). CESR is relatively easy to perform and the analysis of the spectral lineshape results in quantitative measurements concerning the fiber structure and electron mobility. CESR has been used to determine the basal plane alignment of the VG carbon fibers with respect to the long fiber axis. The standard deviation of the basal planes due to disorder can be determined by comparing real fibers to theoretical models derived from single crystal graphite. VG fibers heat treated at 2275 K have an angular standard deviation of 35$\sp\circ$, whereas fibers heat treated at 3375 K have a deviation of only 12$\sp\circ$. Fibers annealed between 2300 K and 3025 K exhibit a "two-dimensional" powder pattern. The full g-anisotropy of a single crystal of graphite is not observed in VG carbon fibers until the heat treatment temperature is greater than 2800 K. When the fiber axis is not collinear to the oscillating magnetic field there is a decrease in spectral intensity near g$\sb{\rm a}$, the resonance corresponding to the electrons located in the basal planes. This is due to the inability of the microwave field to completely penetrate the fibers because of eddy currents of the conduction electrons near the fiber surface. For fibers annealed above 3275 K or having diameters less than 20$\mu$, the "two-dimensional" powder pattern collapses to an isotropic line due to the motion of the electrons. Fibers heated above 3275 K result in motional narrowing due to the development of longer-range crystallite order. In the smaller diameter fibers, spectral narrowing occurs when the electron travels a distance comparable to the fiber radius. At room temperature, the migration of the electron is limited by electron-phonon scattering. Lineshape analysis estimates the product of the mean free path and Fermi velocity of conduction electrons in VG fibers. As the annealing temperature increases, the product approaches the value estimated for graphite.
Subject Area
Condensation|Chemistry|Electrical engineering
Recommended Citation
Marshik, Barbara Jean, "Conduction electron spin resonance studies of vapor-grown carbon fibers" (1989). ETD collection for University of Nebraska-Lincoln. AAI9013614.
https://digitalcommons.unl.edu/dissertations/AAI9013614