Negative permittivity at kHz regulated by length-diameter ratio of short carbon fibers in phenolic resin composites

Abstract

Negative dielectric materials show bright application prospects in the electromagnetic protection, energy storage and radio transmission fields. However, high negative permittivity and expensive manufacturing costs limit their applications in above fields. Here, phenolic resin composites containing short carbon fibers (SCF) with different length-diameter ratios were prepared by mechanical mixing and dry pressing. The effects of length-diameter ratio and mass fraction of SCF on conductivity, permittivity and impedance were investigated. The AC conduction mechanism changed from jump conduction to metal-like conduction due to the increased carrier concentration resulting from improved connectivity of the 3D conductive network. In addition, a weak negative permittivity was obtained in SCF429-10/PF composite. At SCF of 20 wt%, when the length-diameter ratio increased from 114 to 429, the negative permittivity mechanism changed from Lorentz & Drude to Drude model, indicating that the plasma oscillation of delocalized electrons was the main cause of negative permittivity. The relationship between reactance and permittivity were discussed by impedance and equivalent circuit analysis. This work provided a new strategy for obtaining weak negative permittivity, which exhibited potential applications in the fields of electromagnetic protection, high dielectric capacitors, and perfect absorption.