Ke-lan Yan

Negative permittivity behavior and magnetic performance of perovskite La1−xSrxMnO3 at high-frequency

For double negative materials (DNMs), these are usually observed in artificial metamaterials or some metal/ceramic composites. Here, an interesting negative permittivity behavior of single-phase perovskite La1−xSrxMnO3 (LSMO) is found. The LSMO bulks with an x value of 0.2 or 0.5 were prepared by a sol–gel/sintering process. From microstructure characterization, the bulks are nearly in single phase and the major phase belongs to the perovskite structure; meanwhile, the bulks are of high density. Furthermore, the experimental data of the plasma-like negative permittivity are fitted well by a lossy Drude model, suggesting a plasma frequency of 3.85 GHz (x = 0.5). The complex permeability of LSMO presents a negative susceptibility (μ′ < 1) at a frequency range of 0.2–1 GHz (x = 0.2) and 0.6–1 GHz (x = 0.5). These results have important implications for the realization of double negative properties in single-phase LSMO as a promising candidate for DNMs.

Experimental realization of simultaneous negative permittivity and permeability in Ag/Y3Fe5O12 random composites

Ag/Y3Fe5O12 random composites of silver particles randomly hosted in porous Y3Fe5O12 were prepared using a facile impregnation–calcination process. With the increase of silver content, the silver particles interconnected with each other near the percolation threshold, leading to the formation of electrical conductive silver networks. The plasma oscillation of delocalized electrons in interconnected silver particles leads to the negative permittivity. The negative permittivity behaviour was analyzed using the Drude model and equivalent circuit models. Meanwhile, the diamagnetic response of silver networks combined with the magnetic resonance of Y3Fe5O12 results in the negative permeability. That is to say, simultaneous negative permittivity and negative permeability were realized in the Ag/Y3Fe5O12 random composites. Hopefully, tunable double negative property could be realized by adjusting the microstructure and composition of the composites. External DC magnetic field could also be applied to control the electromagnetic properties of the composites.

The tunable negative permittivity and negative permeability of percolative Fe/Al2O3 composites in radio frequency range

The electromagnetic properties including ac conductivity, permittivity, and permeability of percolative Fe/Al2O3 composites, which were prepared by in-situ synthesis process, are investigated in the radio frequency range. There is an obvious percolation transition with the increase of iron contents. When iron content is beyond but near the percolation threshold, the negative permittivity and permeability are simultaneously obtained from 631 MHz to 1 GHz in sample FA30. Further increasing iron content, the Fano-like resonances are observed, and the resonance frequency where the permittivity changes from negative to positive shifts to lower frequency. In addition, the frequency region of negative permittivity and negative permeability are not overlapped any more. Hopefully, the percolative Fe/Al2O3 composites with tunable negative permittivity and negative permeability can be used as electromagnetic wave absorber.

Tunable negative permittivity behavior and conductor–insulator transition in dual composites prepared by selective reduction reaction

We use a selective reduction reaction to fabricate a new kind of dual composite, which has a “composite-within-a-composite” structure. Based on the different reduction behavior of Fe2O3, an Fe-rich structure (Fe–Fe3O4–Al2O3) is formed in an Al-rich structure (FeAl2O4–Al2O3). The Fe-rich structure can bring down the concentration of free electrons to reduce the energy loss, without losing its metallic behaviour at the same time. Near the percolation threshold, a conductor–insulator transition appears and the dual composites show totally different electromagnetic responses, which can be well explained by effective medium theory. By controlling the parameters of the selective reduction process, we can get a tunable negative permittivity, which implies the ability to continuously change their electromagnetic properties.

Experimental realization of tunable negative permittivity in percolative Fe78Si9B13/epoxy composites

Negative permittivity is one of the most important properties in the realization of double negative medium or negative index materials. In this paper, tunable negative permittivity in the radio frequency range has been obtained in composites with Fe78Si9B13 amorphous alloy dispersed in an epoxy matrix. The microstructure and dielectric properties of Fe78Si9B13/epoxy composites are investigated in detail. The results indicate that when the Fe78Si9B13 content is beyond the percolation threshold, the plasma oscillation of delocalized electrons in interconnected Fe78Si9B13 leads to negative permittivity. By controlling the effective concentration of free electrons, the negative permittivity of the Fe78Si9B13/epoxy composites could be easily adjusted. Additionally, the frequency dispersion behaviors of the conductivity conform to the Jonschers power law below percolation threshold, demonstrating that the conductive mechanism is hopping conduction. The realization of tunable negative permittivity in Fe78Si9B13/epoxy composites gives a new and high-efficient way toward double negative materials.

Microstructure and dielectric properties of ion-doped La0.7Sr0.3MnO3 lossy ceramics at radio frequencies

In this paper, the microstructure and dielectric properties of ion-doped La0.7Sr0.3MnO3 are investigated in detail. The polycrystalline ceramics of La0.7Sr0.3Mn1−yMyO3 (M = Fe, Ni, Cu; y = 0.3 or 0.5) are prepared by the sol–gel/sintering method. The symmetry of the crystal structure is improved and the dielectric properties obtained in the frequency range from 80 MHz to 1 GHz can be tuned by the doping of Fe, Ni, and Cu ions. With regard to the dielectric properties in lossy ceramics, the decrease of real permittivity e′r for the doping of Fe and Ni will be effective for impedance-matching; the enhanced dielectric losses in the Ni and Cu doping samples will promote stronger absorption. Thus, the improved dielectric properties make ion-doped La0.7Sr0.3MnO3 promising candidates for lossy ceramics in microwave electronics. In addition, the experimental results of permittivity were checked by K–K relations, and the frequency dispersion behaviors of the conductivity within a certain frequency range accord with the Jonschers power law in the form of σ′ac(f) ∝ (2πf)n, demonstrating that the conductive mechanism is hopping conduction.