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A method for designing and fabricating a dual-section step-impedance frequency selective surface (DSSI-FSS) absorber for oblique incidence applications is proposed. The features of dual-section and step-impedance are realized by using two dielectric layers, which show better impedance matching characteristics compared with a single-section matching layer. We also propose a transmission line model that can be used for a DSSI-FSS absorber at different incidence angles. The model can be especially helpful for the rapid design of broadband and wide-angle absorbers. The proposed absorber structure has a −10 dB bandwidth in the range of 2.3–13.3 GHz at normal incidence (144.0% in fractional bandwidth), 2.5–14 GHz (139.4%) at transverse electric (TE) 45°, and 3.1–14 GHz (127.5%) at transverse magnetic (TM) 45°. In view of the difficulty in obtaining low-permittivity materials, the dielectric layers were fabricated with a honeycomb structure, which ensures high structural strength and low weight. Reflectivity simulation results were in good agreement with measurement results, which validated the design principle and fabrication process. The proposed absorber has a wide absorption band and a wide scan angle, which render it competitive in practical applications.

In this letter, we provide a physical insight into the operating mechanism of a reconfigurable bistate active frequency selective surface (AFSS) based on characteristic mode analysis. At first, we study the modal behavior of the unit structure under different load resistances and found that the modal significance bandwidths of the fundamental mode and the high-order modes increase with the load resistance. When the load resistance is very high, the fundamental resonant frequency will move to high frequencies and the high-order modes may not change. Such features can be used in the design of the reconfigurable transmission/reflection frequency selective surface. It is found that the isolation between transparent and reflective states is mainly determined by the radiated field from fundamental mode. At the same time, we provide two improvement approaches to design this kind of frequency selective surfaces. And a reconfigurable AFSS operating at 2.1 GHz is designed, fabricated, and measured. The letter makes sense for the design of reconfigurable bistate AFSSs.

In this article, a patterned honeycomb absorber (PHA) with an embedded frequency-selective surface (FSS) was proposed for broadband applications. The impedance of the FSS embedded in a homogeneous honeycomb absorber (HA) was first analyzed, which suggested that directly combining an FSS with a homogeneous HA was unreliable. Subsequently, a PHA was developed to adjust the input impedance. Using the PHA, an FSS embedded in patterned honeycomb absorber structure with a wide absorption band was developed. The consistency between the simulated and measured results verifies the analysis and design principle. The proposed absorber may be very suitable for various microwave applications owing to its low cost, lightweight, high strength, and environmental friendliness.

In this paper, the physical model of electrically thin weakly conductive film with intrinsic surface impedance is established, indicating that the imaginary part of high surface impedance is non-negligible at microwave frequencies. In the design of lossy frequency selective surface absorbers, we introduce the imaginary part of intrinsic surface impedance for the first time. With the experimentally established relationship between the complex surface impedance and the DC square resistance, this complex surface impedance allows us to accurately predict the electromagnetic response of high surface impedance film at microwave frequencies and provides an advantage in reducing the thickness of absorber. The proposed ultra-thin absorber can provide −10 dB reduction over the frequency range of 4.5–13.3 GHz. Total thickness of microwave absorber is only 0.06λ at lowest operating frequency, which is close to the theoretical limitation. The measurement is provided to verify the validity of the equivalent relationship and the reliability of the full-wave model. This study provides a new way to reduce the thickness of absorber, exhibiting promising potential for stealth technique.

In this paper, we demonstrate a multimode and broadband absorber that is fabricated directly on PET substrate using a commercial direct-to-garment (DTG) inkjet printer. A design procedure of this kind of absorber is presented. Based on the theory of characteristic mode, the underlying modal behaviors of the absorber structure are firstly analyzed to guide the design of multimode absorber. Two modes on the absorber structure are designed to resonate around 1.83 GHz and 4.28 GHz to cover the working frequency range. Simulation and measurement results show that the multimode absorber with a total thickness of 0.0883λL at the lowest operating frequency can achieve broadband microwave absorption with efficiency over 90% in the frequency band of 1.0 ∼ 4.5 GHz (127.3% in fractional bandwidth) through deliberate design. Both the simulated and experimental results demonstrate the validity of the proposed method and indicate that the method can be applied to other microwave and millimeter-wave regions.

The strong desire for a clean and secure energy future has stimulated great interest in searching for low-cost, highly efficient electrocatalysts for regenerative fuel cells and rechargeable metal-air batteries. Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) play a paramount role in the operation of these devices, but are notoriously sluggish even with precious metals-based catalysts. Here we report our findings in the development of a bifunctional catalyst based on phosphorus-doped LaFeO3-δ for ORR and OER in alkaline solutions. The remarkable electrocatalytic performance is attributed to larger amount of O22−/O− species, trace amount of Fe4+ species, and optimized eg electron filling (≈ 1), benefiting from the doping effect of phosphorus. Both the mass activity and the specific activity are nearly doubled after P-doping. Density functional theory calculations also confirm that the increase in valence state of Fe ions is due mainly to phosphorus doping. These results demonstrate that non-metal element doping is an effective approach to optimizing the electronic configuration, changing valence states of ions, and thus enhancing activities of perovskite electrocatalysts.

Oxygen is the most abundant element in the Earth’s crust. The oxygen reduction reaction (ORR) is also the most important reaction in life processes and energy converting/storage systems. Developing techniques toward high-efficiency ORR remains highly desired and a challenge. Here, we report a N-doped carbon (NC) encapsulated CeO2/Co interfacial hollow structure (CeO2–Co–NC) via a generalized strategy for largely increased oxygen species adsorption and improved ORR activities. First, the metallic Co nanoparticles not only provide high conductivity but also serve as electron donors to largely create oxygen vacancies in CeO2. Second, the outer carbon layer can effectively protect cobalt from oxidation and dissociation in alkaline media and as well imparts its higher ORR activity. In the meanwhile, the electronic interactions between CeO2 and Co in the CeO2/Co interface are unveiled theoretically by density functional theory calculations to justify the increased oxygen absorption for ORR activity improvement....

Designing low-cost and highly efficient bifunctional electrocatalysts for compatible integration with the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) for overall water splitting is critical but challenging. Herein, mutually beneficial Co3O4@MoS2 heterostructures were adopted to efficiently balance both HER and OER performance by improving the sluggish kinetics. These heterostructures synergistically favoured the reduction of the energy barrier of the initial water dissociation step and optimization of the subsequent H adsorption/desorption for MoS2 in alkaline HER. Moreover, the adsorption of oxygen intermediates was enhanced for Co3O4 in the OER process. As a result, the Co3O4@MoS2 heterostructures showed excellent overall water splitting performance with a low overpotential and Tafel slope.

Transition metal sulfides such as NiS and Co 9 S 8 are gaining popularity in electrochemical capacitors owning to low-cost, earth-abundant, and environmentally friendly nature, but their specific capacity/capacitance should be further improved. Here, by doping Al atoms into β-NiS mesoporous nanoflowers, we achieve reversible specific capacity of 697.3C g −1 at a current density of 1 A g −1 , while the pure β-NiS electrode only delivers a specific capacity of 311.5C g −1 . The improved performance of Al-doped β-NiS could be assigned to its large surface area and mesoporous characteristics, providing abundant electroactive sites and shortening ions transport pathway. Besides, new observed multiple electrochemical redox reactions are also deem as another possible reason to enrich the electrochemical redox species for the improved electrochemical performance caused by Al doping. Theoretical calculation manifests that charges are transferred from Al atoms to both Ni and S atoms that are close to Al atoms, resulting in the appearance of Ni ions-based multiple redox reactions. Moreover, a hybrid supercapacitor composed of Al-doped NiS positive electrode and carbon-based negative electrode is configured, delivering a high energy density of 39.6 W h kg −1 at a power density of 918.8 W kg −1 . Even at a high power density of 12296 W kg −1 , an energy density of 11.4 W h kg −1 can be still achieved, indicating its great potential for practical applications. Our developed doping approach may reverse the lagging status of nickel sulfides towards high theoretical-capacity capacitors.

In this paper, a broadband antenna array is firstly proposed and analyzed using a combination of characteristic mode analysis (CMA) and the theory of frequency selective surface. The element of the broadband antenna consists of crossed dipoles with end loading. The CMA is used to analyze the underlying modal behaviors and the mode excitation on the proposed antenna structure in order to guide the antenna design. When it obtains the best transmission or radiation performances, it’s configured as an absorber by reciprocity. In order to validate the theoretical solutions, a 10×10 cell prototype is fabricated and measured. The agreement between the simulated and measured results demonstrates the validity of the proposed method.

This paper presents a tunable graphene-based absorber with near-unity absorption in terahertz range. The absorber is composed of the patterned graphene and a metallic reflecting plate spaced by a dielectric layer. The results show that the absorber’s normalized bandwidth of 90% terahertz absorbance is over 86% under normal incidence for both TE and TM polarizations when the graphene chemical potential is set as 0.5 eV. The absorber also has the advantage of fexible tunability via electrostatic doping of the patterned graphene. By controlling the chemical potential from 0 eV to 0.6 eV, peak absorbance can be continuously tuned from 0% to 100%. The design scheme is scalable to develop various graphene-based tunable absorber at other terahertz, infrared, and visible frequencies, which may have promising applications in sensing, detecting, and optoelectronic devices.

In this paper, the modal behaviors and the frequency properties of resistor-loaded frequency selective surfaces have been systematically studied based on the periodic moment method and characteristic mode analysis. Through the observation of the phase curves of the reflection coefficients, we found that as the resistance value increases, the resonant frequency gradually moves toward higher frequencies. By calculating and analyzing the modal currents and the modal reflection coefficients of each mode, we know that the phenomenon is caused by higher-order modes. In order to reduce the influence of higher-order modes on the resonant frequency, a method of using more load resistors is proposed. To verify the proposed method, an absorber with high absorptivity is designed and measured. The measurement shows that the absorptivity of the designed absorber is more than −18dB in the range of 1.92– 3.99GHz. The experiment proves that the proposed loading method can be used to flexibly regularize the equivalent impedance of the lossy frequency selective surface without worrying about the negative effects of load resistors in the design of a circuit analog absorber.

A multisection step-impedance honeycomb absorbing structure is investigated both theoretically and experimentally. Excellent intrinsic impedance matching and low reflection are realized by providing a step-impedance transition between air and metal in the microwave band. The wideband absorption mechanism of the honeycomb absorbing structure is analyzed using a step-impedance model, and the particle swarm optimization algorithm is used to obtain the optimal step impedance. Two absorbing structures were simulated: The double-layer structure had an absorption frequency band of 3.9–12.4 GHz and a thickness of 9 mm, and the triple-layer structure had an absorption frequency band of 2.2–12.4 GHz and a thickness of 15 mm. The measured reflectivity values were consistent with the calculated results, thereby confirming the validity of our designs for the absorbing structure.

Most investigations of polarizers consider a normal angle of incidence only. This paper expands our investigation to also include an oblique angle of incidence. Essential problems are encountered for an oblique angle of incidence: there is an upward shift of the center frequency, and the impedance match gets worse as the angle of incidence increases. So it is apparent that polarizers gradually deteriorate for higher angles of incidence. The new contribution given in this paper is that it considers ways to greatly improve oblique incidence performance. These ways are well confirmed by the design process of the proposed graphene-based polarizer for large-angle oblique incident stability and high efficiency.

The effect of chemical doping on the ZSiNRs with Mn as passivating element replacing H atoms at one edge are investigated by first principles calculations.The structures optimized in the typical ferromagnetic and antiferromagnetic coupling show that the system leads to an AFM state and achieve half metallic properties.Also,our first principle approach based on the Keldysh nonequilibrium Greens function method gives the spin dependent transport properties of the device. When the system changes from parallel to antiparallel configuration. the spin up current increases rapidly while the spin down current is still depressed. Further, it is found that the system is a quite good spin filtering device with nearly 80 percent spin filtering efficiency at a wide bias voltage region and therefore suitable for applications. The mechanisms for these phenomena are proposed in detail., 9 pages, 8 figures,published by Chemical Physics Letters

Using density functional theory and non-equilibrium Greens function technique, we performed theoretical investigations on the structural and transport properties of zigzag silicene nanoribbons with Stone-Wales defect. The calculated formation energy is significantly lower than that of graphene and silicene, which implies the high stability of such defect in SiNRs. Negative differential resistance can be observed within certain bias voltage range in both perfect and SW defected SiNRs. In order to elucidate the mechanism the NDR behavior,the transmission spectra and molecular projected self-consistent Hamiltonian states are discussed in details., Comment: 16 pages, 4 figures,1 table

Using density functional theory and non-equilibrium Greens function technique, we performed theoretical investigations on the transport properties of several ZAZ SiNRs junctions,a similar kind of silicene molecules junction combined by zigzag and armchair silicene nanoribbons. It is found that the differential conductances of the three systems decrease with an order of 5-ZAZ>4-ZAZ>3-ZAZ.Particularly,the Negative differential resistance can be observed within certain bias voltage range only in 3-ZAZ SiNRs. In order to elucidate the mechanism the NDR behavior, the transmission spectra and molecular projected selfconsistent Hamiltonian states are discussed in details., 15 pages, 6 figures