Effect of Oxygen‐Gas Concentration in the Heat‐Treatment Atmosphere and High‐Voltage Pulse Application on the Electrical Properties of Carbon Fibers

Semiconductive carbon fibers suitable for infrared-ray sensor materials were prepared by final heat treatments at 600--700 C in an atmosphere with a controlled oxygen-gas (O{sub 2}) concentration, and the influence of O{sub 2} concentration on the electrical properties and surface composition/microstructure was investigated. Resistivities at 20 C were almost equal and independent of the O{sub 2} concentration of heat treatment in all cases, but the thermistor constant decreased as the O{sub 2} concentration increased. Carbon fibers heat-treated in O{sub 2}-mixed nitrogen gas (N{sub 2}) exhibited higher surface oxygen concentrations than did those treated in pure N{sub 2}, and surface microstructures differed between the two fibers. A high-voltage pulse application more effectively decreased resistivity in fibers heat-treated in an atmosphere with a higher O{sub 2} concentration. Apparently, control of the O{sub 2} concentration in the heat-treatment atmosphere and high-voltage pulse application can produce a desirable resistivity and thermistor constant independently in carbon fibers.