Magnetoresistance and Microstructure of Stress-graphitized Carbon Fibers

Abstract

The microstructure of stress-graphitized polyacrylonitrile-based carbon fibers has been studied through the measurements of magnetoresistance at 77K and by scanning electron microscopy. The carbon fibers were stretched throughout oxcidation, carbonization and graphitization (at 2700°C). The fibers were then annealed at 3000°C. The transverse magnetoresistance of the fibers was positive. The magnetoresistance changes linearly with increasing magnetic field strength above 3kG. The low field behavior of the magnetoresistance suggested the existence of the light minority carriers in the samples. The maximum transverse magnetoresistance at 10kG is 10.45%, which indicates the presence of the extensively grown crystallites with three dimensional crystallinity. Preferred orientation of the samples was determined by the anisotropy of the magnetoresistance as follows: (1) The c-axes of the crystallites are perpendicular to the fiber axis.(2) The c-axes of the crystallites are parallel to the radial direction in the cross-section of the fibers. The preferred orientation of the crystallites in the cross-section was confirmed by scanning electron microscopy. The average crystallite size of the samples was estimated by the scanning electron micrographs of the cross-section to be about 0.6-1.2μm in width and 500-700A in thickness.