High-frequency transmission properties of carbon nanofibers with carbonization temperature variations

Carbon nanofibers (CNFs) are promising materials in many fields because they can form multi-purpose matrices of carbon materials and have attractive electric conductivity, dielectric permittivity, thermal stability, and ultrahigh specific surface area. These properties of CNFs significantly depend on the carbonization temperature (CT). Thus, the effect of CT can be primarily crucial for zero- and high-frequency devices and circuits with CNFs. In this study, we report high-frequency transmission properties of the coplanar waveguide line (CPW line) with a CNF film synthesized at CTs of 700 degrees C, 800 degrees C, and 900 degrees C (denoted CNF-700, CNF-800, and CNF-900, respectively) in the frequency ranging from 0.5 to 10 GHz. For all samples, a bare CPW line, a CPW line with a tape, and a CPW line with three kinds of CNF films, the S11 magnitude (reflection coefficient) is less than -10 dB in the frequency range, and the S21 magnitude (transmission coefficient) significantly decreases linearly to -1.3 dB (CNF-700), -3.0 dB (CNF-800), and -5.5 dB (CNF-900) at 10 GHz. Furthermore, the effective permittivity (epsilon eff) of CPW line with CNF films increases with CT, although epsilon eff decreases with increasing frequency. Conversely, due to dielectric and conductive loss of CNF film, the attenuation constant (alpha) of CPW line with CNF film increases linearly with CTs and frequency. These results indicate that the CPW line with a CNF-900 (CPWCNF-900) shows lossier characteristic than the other CNF films because of finer three-dimensional network structure, higher conductivity, higher attenuation constant, higher shielding effectiveness, and larger effective permittivity. Furthermore, this study shows the possibility that thin CNF films with different CT variations can regulate signal transmission for high-frequency and high-speed circuit and device applications.