Frequency-Independent and Colossal Dielectric Permittivity of Platy Alumina/Few-Layer Graphene Multilayered Composites

In this work, we investigated the dielectric property of layer-structured Al₂O₃/few-layer graphene (FLG) composite by high temperature impedance spectroscopic analysis. The sintered composites have highly enhanced permittivity (ε = 17.3) compared to pure platy alumina (ε = 7.3) with low dielectric loss (tanδ ~ 10⁻³). Percolative permittivity of the composites exhibits almost frequency-independent behavior at 1 Hz to 1 MHz even though the real permittivity was increased, compared to that of pure platy alumina. Such behavior can be explained by intervening dielectric interface between alumina and FLG based on multilayer filler model. After percolation threshold (FLG > 0.75 vol %), the permittivity is further enhanced up to ~260 (@1 kHz), which amounts to >35 fold increase, compared to pure platy alumina because of interfacial capacitance between FLG platelets and alumina. According to temperature-dependent impedance analysis, a dielectric relaxation due to Al₂O₃/FLG interface is observed in the frequency range of 1 Hz to 1 kHz at high temperature (RT = 400°C) besides that due to Al₂O₃ grains (>10 MHz). Real permittivity increases up to >20 000 at 400°C at low frequency, whereas dielectric loss remains as tanδ ~ 3 × 10⁻⁴. High temperature impedance analysis shows that the effect of interface between Al₂O₃ and FLG is responsible for such dielectric relaxation at high temperature.