Random composites with nickel networks hosted randomly in porous alumina are proposed to realize double negative materials. The random composite for DNMs (RC-DNMs) can be prepared by typical processing of material, which makes it possible to explore new DNMs and potential applications, and to feasibly tune their electromagnetic parameters by controlling their composition and microstructure. Hopefully, various new RC-DNMs with improved performance will be proposed in the future.

Random composites of iron particles hosted in porous alumina were prepared from a facile impregnation‐reduction process. Interestingly, when the iron content exceeds the percolation threshold, the interconnection of iron particles results in the formation of iron networks. The composites then change from capacitive to inductive and the conductive mechanism changes from hopping conduction to metal‐like conduction. The negative permittivity was attributed to the plasma oscillation of delocalized electrons in iron networks, while the negative permeability could be ascribed to the strong diamagnetic response of current loops in iron networks. The negative permittivity behavior of the iron/alumina composite was analyzed using Drude model. Additionally, the fitting results indicated that the effective plasma frequency of the iron/alumina composite is much lower than bulk iron. Further investigations show that, the iron content and reduction temperature can easily tune the amplitude and frequency ranges of the negative permittivity and permeability. Moreover, the negative permittivity region and the negative permeability region can be pushed to the same frequency region by adjusting the iron content and reduction temperature. The impregnation‐reduction process opens a new way for the realization of tunable negative permittivity and permeability in random composites, and has great potential for the preparation of new types of double negative materials.

The schematic of metamaterials applied in wireless power transfer.

Controllable designing of heteroatom-doped carbon catalysts provides an insightful strategy for boosting the performance and kinetics of the oxygen reduction/evolution reaction (ORR/OER). However, the role of oxygen species is usually omitted. Herein, a facile oxygen engineering strategy is proposed to tune the oxygen species in N-doped porous carbon nanofibers (NPCNFs-O) via a facile electrospinning method, in which βcyclodextrin acts as the pore inducer and oxygen regulator. Benefitting from the large specific surface area and synergistic effect of N,O codoping, the NPCNF-O catalyst exhibits superior ORR (E 1/2 = 0.85 V vs reversible hydrogen electrode (RHE)) and OER (E j = 10 = 1.556 V vs RHE) activities with excellent stability. Both experimental and theoretical calculations verify the crucial role of carboxyl groups, which regulate the local charge density and reduce the reaction energy barrier for enhancing the oxygen electrocatalytic activity. Moreover, a rechargeable zinc−air battery using NPCNF-O as the air cathode demonstrates a maximum power density of 125.1 mW cm −2 and long-term durability. Importantly, NPCNF-O can be served as an integrated freestanding electrode for portable zinc−air batteries. The work brings brilliant fundamental insights for constructing efficient metal-free carbon material catalysts for future energy conversion and storage systems.

No abstract

A significantly enhanced dielectric constant and suppressed loss were simultaneously achieved in sandwich-structured composites consisting of alternating positive-k and negative-k layers.

No abstract

No abstract