Tunable Electromagnetic Properties in Co/Al₂O₃ Cermets Prepared by Wet Chemical Method

Abstract

Cobalt/alumina composites with three‐dimensional cobalt networks hosted in porous alumina matrix were prepared by a facile impregnation process under low temperature. The effects of composition and microstructure on their electromagnetic properties were investigated. A percolation phenomenon appeared with cobalt content increasing, and an interesting feature of negative permittivity was obtained in the percolative composites due to the appearance of equivalent inductances, while the composites were equivalent to circuit models composed of capacitance and resistance with Co content below the percolation threshold. Meanwhile, negative permeability was obtained in the frequency range of 500 MHz ~ 1 GHz. Further investigations showed that the electromagnetic properties of the composites could be changed significantly when the cobalt particles networks turned into sheet structure. That was to say, the electromagnetic properties of metal–ceramic composites could be tailored by their compositions and microstructures, which make it available for microwave‐absorbing materials, attenuation ceramics and microwave dielectrics, etc.

Negative permittivity adjusted by SiO2-coated metallic particles in percolative composites

“…3a) and agrees well with the experimental data, indicating a hopping conductive behavior. Above the percolation threshold, with the formation of percolative conductive network, the decreasing trend of σ ac was attributed to the skin effect at high frequency, which could be described by skin depth [38]: leading to the decreasing of conductivity. Fig.…”

Negative permittivity behavior in the carbon/silicon nitride composites prepared by impregnation-carbonization approach

“…5a), meaning a lag of the voltage phase behind the current phase, hence the composites manifests capacitive behavior. The equivalent circuit models consisting of capacitors(C), resistors (R), and inductors (L), are frequently employed to analyze the reactance of materials [45]. The composites below percolation threshold can be simulated by a corresponding equivalent circuit model composed of a parallel combination of resistor R and capacitor C. When the reactance of composites are positive (Z″ 0) in the whole tested frequency range (Fig.…”

Negative permittivity in titanium nitride‐alumina composite for functionalized structural ceramics

“…42 Similarly, negative dielectric behavior was also observed in some cermets and polymer composites. 4,[16][17][18]22,43 Moreover, negative permittivity with plasma character can be described by the following Drude model 44 and the solid lines in Figure 4B were fitted with Equation (2). where ω p (=2πf p ) is angular plasma frequency, γ is a damping term representing dissipation of plasmon's energy to the system, i is the pure imaginary unit, ε 0 (=8.85 × 10 −12 F/m) is vacuum dielectric constant, e (=1.6 × 10 −19 C) is electron charge, n and m eff is the electrons density and effective weight.…”