CONDUCTION ESR OF VAPOR GROWN CARBON-FIBERS

Conduction electron spin resonance (CESR) spectra have been obtained for vapor-grown carbon fibers. Large diameter fibers annealed at temperatures between 2300 and 3025 K exhibit broad "two-dimensional" powder patterns when the fiber axis is perpendicular to the magnetic field. The CESR of fibers heat treated at 3175 K and having diameters less than 40 μm show a collapse of the two-dimensional powder pattern into a relatively sharp line at the isotropic value. This narrowing results from the diffusion of the electron in the basal planes. The r.m.s. migration of the electron on the ESR timescale must be on the order of the fiber diameter for motional narrowing to occur. Electron migration is limited by electron-phonon scattering at room temperature. Products of the electron mean free paths and Fermi velocities are determined from the narrowing of the ESR signals. They are found to increase with increasing annealing temperatures. For all fibers this product is less than that estimated for pure graphite. © 1990.