Fabrication of Graphene Network in Alumina Ceramics with Adjustable Negative Permittivity by Spark Plasma Sintering

The graphene/alumina (GR/Al2O3) composites with adjustable negative permittivity were prepared by spark plasma sintering method. The microstructures of the composites with different GR contents were investigated by field emission scanning electron microscopy. With increasing the GR content, the grain size of Al2O3 tended to decrease and the grain shape transformed from granulous to ellipsoidal. The radio frequency dielectric properties of the GR/Al2O3 composites including permittivity (e′ and e″), dielectric loss tangent (tan δ) and reactance (Z″) were investigated. The negative permittivity appeared when the GR content exceeded 15.38 wt %. The plasma oscillation of conduction electrons in the GR networks was considered to cause the negative permittivity. The maximum dielectric loss tangent for the GR/Al2O3 composites with the GR mass fraction of 15.38 and 18.46 wt % appeared near 40 and 70 MHz, respectively, corresponding to the transition of e′ from negative to positive, which was produced by the LC res...