Electric permittivity of carbon fiber

The electric permittivity is a fundamental material property that affects electrical, electromagnetic and electrochemical applications. This work provides the first determination of the permittivity of continuous carbon fibers. The measurement is conducted along the fiber axis by capacitance measurement at 2 kHz using an LCR meter, with a dielectric film between specimen and electrode (necessary because an LCR meter is not designed to measure the capacitance of an electrical conductor), and with decoupling of the contributions of the specimen volume and specimen-electrode interface to the measured capacitance. The relative permittivity is 4960 ± 662 and 3960 ± 450 for Thornel P-100 (more graphitic) and Thornel P-25 fibers (less graphitic), respectively. These values are high compared to those of discontinuous carbons, such as reduced graphite oxide (relative permittivity 1130), but are low compared to those of steels, which are more conductive than carbon fibers. The high permittivity of carbon fibers compared to discontinuous carbons is attributed to the continuity of the fibers and the consequent substantial distance that the electrons can move during polarization. The P-100/P-25 permittivity ratio is 1.3, whereas the P-100/P-25 conductivity ratio is 67. Thus, defects associated with a low degree of graphitization hinder conduction more than polarization.