Radio Frequency Negative Permittivity in Random Carbon Nanotubes/Alumina Nanocomposites
While metal is the most common conductive constituent element in the preparation of metamaterials, one-dimensional conductive carbon nanotubes (CNTs) provide alternative building blocks. Here alumina (Al2O3) nanocomposites with multi-walled carbon nanotubes (MWCNTs) uniformly dispersed in the alumina matrix were prepared by the hot-pressing sintering. As the MWCNTs content increased, the formed conductive MWCNTs networks led to the appearance of percolation phenomenon and a change of conductive mechanism. Two different types of negative permittivity (i.e., resonance-induced and plasma-like) were observed in the composites. The resonance-induced negative permittivity behavior was obtained in the composite with low nanotube contents, and was ascribed to the induced electric dipole generated from the isolated MWCNTs. The frequency dispersions of such negative permittivity can be fitted well by Lorentz model. While a plasma-like negative permittivity behavior was observed in the composites with MWCNTs content exceeding percolation threshold, which could be well explained by the low frequency plasmonic state generated from conductive nanotube networks using Drude model. This work is favorable to proving up the generation mechanism of negative permittivity behavior and will greatly facilitate the practical application of metamaterials.