Colossal electric permittivity discovered in polyacrylonitrile (PAN) based carbon fiber, with comparison of PAN-based and pitch-based carbon fibers

The electric permittivity is a fundamental material property concerning the dielectric behavior. Colossal permittivity (relative permittivity exceeding 10,000) is attractive for capacitors, sensors, actuators and mechanical energy harvesters. It was discovered in this work in continuous polyacrylonitrile (PAN) based carbon fiber (Teijin's Tenax HTS45) along the fiber axis. This is the first report of colossal permittivity in carbon materials. The relative permittivity (12230 ± 990, 2 kHz) is much higher than that of mesophase-pitch-based carbon fiber (Cytec's Thornel P-25, 4384 ± 257, 2 kHz). The two fibers exhibit similar resistivity (1.52 × 10 ⁻³ and 1.24 × 10 ⁻³ Ω.cm for the PAN-based and pitch-based fibers, respectively). For both fibers, the permittivity is high, due to the physical continuity and mobile charge carriers. The higher permittivity of the PAN-based fiber compared to the pitch-based fiber is attributed to the higher degree of graphitization (as indicated by the lower interplanar spacing) of the PAN-based fiber. However, the smaller crystallite sizes and larger azimuthal spread of the carbon layers around the fiber axis cause the PAN-based fiber to be not higher in conductivity than the pitch-based fiber, in spite of the higher degree of graphitization. The smaller crystallite sizes and larger azimuthal spread are detrimental more to the conductivity than the polarization.