Tunable radio-frequency negative permittivity in nickel-alumina “natural” meta-composites

“Natural” metamaterials with intrinsic negative permittivity and permeability have attracted significant attention because of their wide promising applications and facile preparation processes. In order to match the negative permeability band which is usually located in the radio-frequency range, radio-frequency negative permittivity is desired. Moreover, the negative permittivity should be tunable to satisfy different application circumstances. Herein, nickel/alumina composites consisting of nickel particles homogeneously dispersed in alumina were prepared using a facile wet chemical process. A percolation phenomenon appears, and expected radio-frequency negative permittivity is obtained. Furthermore, the dependences of negative permittivity on the composites' compositions and microstructures are investigated in detail. It is shown that higher nickel content results in higher negative permittivity magnitude and lower negative permittivity frequency. Besides, the addition of alumina particles leads to decreasing negative permittivity magnitude and higher negative permittivity frequency band owing to the deteriorated interconnectivity between nickel particles. The present work will greatly facilitate the fabrication and application of “natural” metamaterials with tailored properties.