“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.
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.
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