Anomalous electrical behavior of partially carbonized polyacrylonitrile fibers
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Partially carbonized polyacrylonitrile fibers were observed to undergo a resistivity change of 2 to 4 orders of magnitude at a transition temperature typically in the range of 98 "C to 200 "C. The current-voltage curves exhibited an initial supercurrent-like increase, followed by a rapid drop to a high resistance state, and then a rise in current again at a later stage. These phenomena cannot be interpreted by existing theories on switching in inorganic amorphous semiconductors. They are explainable if the microstructure of the pyrolyzed fiber is viewed as comprising nanometer-scale superconducting phases interspersed with semiconducting phases, much like a large number of Josephson junctions connected in series.
I. INTRODUCTION
A commonly used graphite fiber manufacturing process involves the oxidation of polyacrylonitrile (PAN) fiber at a temperature between 200" and 300 "C, carbonization at 1200"- 1500 "C, and graphitization at a temperature greater than 2500 "C. The oxidation of PAN fibers is known to induce the reorganization of polymer chains to form a ladder or cyclic structure. During the subsequent carbonization treatments, the cyclic structures presumably join to form aromatic rings, representing the inception of a graphite-like structure.',* A complete carbonization and graphitization process converts the electrically insulating PAN to highly conducting graphite fiber, covering a conductivity range of up to 14 orders of magnitude. Many novel electrical behaviors are therefore expected to be observable with partially carbonized PAN fibers. Reported herein are some unique phenomena that are of great fundamental and technological significance. II. EXPERIMENTAL
PAN filament yarns were heat stabilized at approximately 200"-230 "C for 1-3 h in laboratory air and then partially carbonized at a temperature between 400" and 950 "C in an inert atmosphere for 1-6 h. Silver wire electrodes were connected to the sample with silver paint. Selected samples were subjected to a voltage excitation treatment, the forming process, that entailed increasing the voltage up to a high-current or low-resistance state at a temperature typically between 100" and 120 "C and then quickly decreasing the applied voltage and temperature back to zero volt and room temperature. Electrical resistivity was measured a)Author to whom all correspondence should be addressed. J. Mater. Res., Vol. 10, No. 10, Oct 1995 http://journals.cambridge.org
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by the four-point method with a Keithley Model-617 electrometer and a Hewlett-Packard (HP) Model-3497A constant dc current source (0.1 mA or 1 mA). The applied current density was estimated to be 1.6 A/cm2 (100 fibers), 0.16 A/cm2 (1000 fibers), and 0.13 A/cm2 (12,000 fibers), respectively. A HP Model-4245A semiconductor parameter analyzer was used to measure the characteristic I-V curve of a material. The samples used were typically 1 cm long, each sample containing 100, 1000, or 12,000 fibers (fiber diameter about 8.84 pm). 111. RESULTS AND DISCUSSION
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