Ke Bi

Magnetically tunable Mie resonance-based dielectric metamaterials

Electromagnetic materials with tunable permeability and permittivity are highly desirable for wireless communication and radar technology. However, the tunability of electromagnetic parameters is an immense challenge for conventional materials and metamaterials. Here, we demonstrate a magnetically tunable Mie resonance-based dielectric metamaterials. The magnetically tunable property is derived from the coupling of the Mie resonance of dielectric cube and ferromagnetic precession of ferrite cuboid. Both the simulated and experimental results indicate that the effective permeability and permittivity of the metamaterial can be tuned by modifying the applied magnetic field. This mechanism offers a promising means of constructing microwave devices with large tunable ranges and considerable potential for tailoring via a metamaterial route.

Novel VN/C nanocomposites as methanol-tolerant oxygen reduction electrocatalyst in alkaline electrolyte

A novel VN/C nanostructure consisting of VN nanoparticles and graphite-dominant carbon layers is synthesized by nitridation of V2O5 using melamine as reductant under inert atmosphere. High crystalline VN nanoparticles are observed to be uniformly distributed in carbon layers with an average size of ca13.45u2009nm. Moreover, the electrocatalytic performance of VN/C towards oxygen reduction reaction (ORR) in alkaline electrolyte is fascinating. The results show that VN/C has a considerable ORR activity, including a 75 percent value of the diffusion-limited current density and a 0.11u2009V smaller value about the onset potential with respect to Pt/C catalyst. Moreover, the excellent methanol-tolerance performance of VN/C has also been verified with 3u2009M methanol. Combined with the competitive prices, this VN/C nanocomposite can serve as an appropriate non-precious methanol-tolerant ORR catalyst for alkaline fuel cells.

Magnetically tunable wideband microwave filter using ferrite-based metamaterials

Magnetically tunable wideband microwave filters have been designed and prepared by using ferrite-based metamaterial structures. The microwave properties of the filters have been investigated by experiments and simulations. The negative permeability appears around the ferromagnetic resonance frequency, which leads to a remarkable stopband for the bandstop filter. The bandpass filter is composed of two kinds of ferrite rods with different saturation magnetization. The bandwidth of the passband can be tuned by adjusting the saturation magnetization of the ferrite rods. Both the experimental and the simulated results show that those filters possess magnetically tunable property. This approach opens a way for designing tunable wideband microwave filters.

Negative and near zero refraction metamaterials based on permanent magnetic ferrites

Ferrite metamaterials based on the negative permeability of ferromagnetic resonance in ferrites are of great interest. However, such metamaterials face a limitation that the ferromagnetic resonance can only take place while an external magnetic field applied. Here, we demonstrate a metamaterial based on permanent magnetic ferrite which exhibits not only negative refraction but also near zero refraction without applied magnetic field. The wedge-shaped and slab-shaped structures of permanent magnetic ferrite-based metamaterials were prepared and the refraction properties were measured in a near-field scanning system. The negative and near zero refractive behaviors are confirmed by the measured spatial electric field maps. This work offers new opportunities for the development of ferrite-based metamaterials.

Ferrite based metamaterials with thermo-tunable negative refractive index

Thermo-tunable negative refractive index (NRI) metamaterials with single- and dual-band based on ferrite have been designed and prepared; the scattering parameters of these metamaterials have been measured at various temperatures to demonstrate the thermo-tunable property. As the temperature increases, the peak value of passband decreases and the passband frequency region shifts to the lower frequency. This temperature dependence is attributed to ferrimagnetic/paramagnetic transition of the ferrites. The thermo-tunable NRI characteristic makes these metamaterials suitable for practical applications in thermo-tunable NRI devices.

Dual band metamaterial perfect absorber based on Mie resonances

We numerically and experimentally demonstrated a polarization insensitive dual-band metamaterial perfect absorber working in wide incident angles based on the two magnetic Mie resonances of a single dielectric “atom” with simple structure. Two absorption bands with simulated absorptivity of 99% and 96%, experimental absorptivity of 97% and 94% at 8.45 and 11.97u2009GHz were achieved due to the simultaneous magnetic and electric resonances in dielectric “atom” and copper plate. Mie resonances of dielectric “atom” provide a simple way to design metamaterial perfect absorbers with high symmetry.

Magnetically tunable metamaterial perfect absorber

A magnetically tunable metamaterial perfect absorber (MPA) based on ferromagnetic resonance is experimentally and numerically demonstrated. The ferrite-based MPA is composed of an array of ferrite rods and a metallic ground plane. Frequency dependent absorption of the ferrite-based MPA under a series of applied magnetic fields is discussed. An absorption peak induced by ferromagnetic resonance appears in the range of 8–12u2009GHz under a certain magnetic field. Both the simulated and experimental results demonstrate that the absorption frequency of the ferrite-based MPA can be tuned by the applied magnetic field. This work provides an effective way to fabricate the magnetically tunable metamaterial perfect absorber.

Interface structure, precursor rheology and dielectric properties of BaTiO3/PVDF–hfp nanocomposite films prepared from colloidal perovskite nanoparticles

Nanocomposite materials with uniform microstructure and high permittivity have attracted extensive interest in modern electronics. The compatibility between the filler phase and the polymer matrix is crucial in preparing high-performance composites. As an alternative to conventional surface modification treatment methodologies, hydroxylated colloidal BaTiO3 (BT) nanoparticles synthesized via a green and scalable process were directly used to fabricate high-permittivity nanocomposites. Interfacial interaction analyses between the BT nanoparticles and polymer matrix reveals that due to strong hydrogen bonding at the interface, transparent composite sols with excellent flow behavior can be observed. The sols are ideal formulations for the preparation of BT/PVDF–hfp 0–3 nanocomposite films. Owing to the unique interface structure, the composite films show a dense and uniform microstructure and superior dielectric properties. Parallel plate capacitor devices and the co-development of a sandwich architecture leads to the ability to prepare dielectric films with favorable performance characteristics. This method provides a novel and greatly simplified strategy for the fabrication of high-permittivity dielectric nanocomposites.

Porous VO(x)N(y) nanoribbons supported on CNTs as efficient and stable non-noble electrocatalysts for the oxygen reduction reaction.

Novel nanocomposites of carbon nanotubes supported porous VOxNy nonoribbons (VOxNy-CNTs) have been synthesized by the annealing of the sol-gel mixture of CNTs and V2O5 under NH3 atmosphere as well as the ageing process in air. Besides the morphological and structural characterizations revealed by TEM, SEAD, EDS, XRD and XPS measurements, typical electrochemical tests including cyclic voltammetry (CV), rotating disk electrode (RDE) and chronoamperometry have been employed to determine the oxygen reduction reaction (ORR) performance of VOxNy-CNTs. Inspiringly, the results indicate that VOxNy-CNTs catalyst exhibits a 0.4u2009mA/cm2 larger diffusion-limited current density, a 0.10 u2009V smaller onset potential value, a 10.73% less of ORR current decay and an excellent methanol-tolerance compared with commercial Pt/C catalyst. Therefore, we have reasonable grounds to believe that this new VOxNy-CNTs nanocomposites can be regarded as a promising non-precious methanol-tolerant ORR catalyst candidate for alkaline fuel cells.

Temperature-dependent terahertz magnetic dipole radiation from antiferromagnetic GdFeO3 ceramics

Temperature-dependent terahertz magnetic dipole radiation in antiferromagnetic GdFeO3 ceramic is investigated both theoretically and experimentally in this work. A two-level quantum transition mechanism is introduced to describe the excitation-radiation process, and radiative lifetime is derived analytically from the change of spin state density during this process. Terahertz spectral measurements demonstrate that the radiative frequency exhibits a red-shift and lifetime shortens as temperature increases, which is in good agreement with theoretical predictions. The temperature-sensitive radiative frequency and excellent terahertz emission mean that the antiferromagnetic ceramics show potential for application in terahertz sensors and frequency-tunable terahertz lasers.