Han Bi

CoNi@SiO2@TiO2 and CoNi@Air@TiO2 Microspheres with Strong Wideband Microwave Absorption

The synthesis of CoNi@SiO2 @TiO2 core-shell and CoNi@Air@TiO2 yolk-shell microspheres is reported for the first time. Owing to the magnetic-dielectric synergistic effect, the obtained CoNi@SiO2 @TiO2 microspheres exhibit outstanding microwave absorption performance with a maximum reflection loss of -58.2 dB and wide bandwidth of 8.1 GHz (8.0-16.1 GHz, < -10 dB).

Tunable Microwave Absorption Frequency by Aspect Ratio of Hollow Polydopamine@α-MnO2 Microspindles Studied by Electron Holography

A tunable response frequency is highly desirable for practical applications of microwave absorption materials but remains a great challenge. Here, hollow lightweight polydopamine@α-MnO2 microspindles were facilely synthesized with the tunable absorption frequency governed by the aspect ratio. The size of the hard template is a key factor to achieve the unique shape; the polymer layer with uniform thickness plays an important role in obtaining spindles with homogeneous size. With the aspect ratio increasing, the maximum reflection loss, as well as the absorption bandwidth (<-10 dB), increases and then decreases; meanwhile, the microwave absorption band shifts to the low frequency. The optimized aspect ratio of the cavity about the hollow polydopamine@α-MnO2 microspindles is ∼2.8. With 3 mm thickness at 9.7 GHz, the strongest reflection reaches -21.8 dB, and the width of the absorbing band (<-10 dB) is as wide as 3.3 GHz. Via electron holography, it is confirmed that strong charge accumulates around the interface between the polydopamine and α-MnO2 layers, which mainly contributes to the dielectric polarization absorption. This study proposes a reliable strategy to tune the absorption frequency via different aspect ratio polymer@α-MnO2 microspindles.

Dipolar-Distribution Cavity γ-Fe2O3@C@α-MnO2 Nanospindle with Broadened Microwave Absorption Bandwidth by Chemically Etching

Developing microwave absorption materials with ultrawide bandwidth and low density still remains a challenge, which restricts their actual application in electromagnetic signal anticontamination and defense stealth technology. Here a series of olive-like γ-Fe2 O3 @C core-shell spindles with different shell thickness and γ-Fe2 O3 @C@α-MnO2 spindles with different volumes of dipolar-distribution cavities were successfully prepared. Both series of absorbers exhibit excellent absorption properties. The γ-Fe2 O3 @C@α-MnO2 spindle with controllable cavity volume exhibits an effective absorption (<-10 dB) bandwidth as wide as 9.2 GHz due to the chemically dipolar etching of the core. Reflection loss of the γ-Fe2 O3 @C spindle reaches as high as -45 dB because of the optimized electromagnetic impedance balance between polymer shell and γ-Fe2 O3 core. Intrinsic ferromagnetism of the anisotropy spindle is confirmed by electron holography. Strong coupling of magnetic flux stray lines between spindles is directly imaged. This unique morphology and facile etching technique might facilitate the study of core-shell type microwave absorbers.

Fabrication of hierarchical TiO2 coated Co20Ni80 particles with tunable core sizes as high-performance wide-band microwave absorbers.

Multifunctional composite microspheres with a Co20Ni80 core and anatase TiO2 shells (Co20Ni80@TiO2) are synthesized by combining a solvothermal reaction and a calcination process, and include a series of microspheres with different core sizes (100 nm, 500 nm and 1 μm). The mechanism of self-assembly of the primary particles has been effective in both the fabrication of the core and the process of coating. The obtained core-shell particles possess superior monodispersity, size uniformity, and tailored core sizes, and are characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Furthermore, the electromagnetic shielding performance of the microspheres is investigated in terms of the theory of transmission lines. The Co20Ni80@TiO2 core-shell particle (CoNi@TiO2) with a well-defined core size of 500 nm demonstrates a remarkable wide-band electromagnetic shielding performance of up to 6.2 GHz (10.0-16.2 GHz, <-10 dB) within 2-18 GHz, which is due to the tunable multi-component hierarchical structure of the particles and contributes to the complex permittivity and permeability and the multiple scattering loss of the microwave. The Co20Ni80@TiO2 particle with a specific core size (500 nm) is a promising candidate for the wide-band electromagnetic shielding materials, gathering increasing interest from researchers.

Dual-ligand mediated one-pot self-assembly of Cu/ZnO core/shell structures for enhanced microwave absorption

Encapsulation of a Cu core into a ZnO shell has been rarely reported by a one-pot method, which is expected to be a novel combination of a reflection core and a dielectric microwave absorber. Here, a facile one-pot strategy has been developed for assembling Cu/ZnO core/shell nanocomposites with different sizes and aspect ratios. The temperature acts as the switch for starting ZnO encapsulation in this strategy, and the sizes and aspect ratios of the resultant nanocomposites depend sensitively on the heating rate in 220–250 °C. The different morphologies and structures of the nanocomposites have been characterized comprehensively, and the relevant growth mechanism is also discussed in this paper. The microwave absorption performances in both as-synthesized Cu@ZnO products (spherical-like and rod-like shapes) are significantly enhanced by comparing with that of pure ZnO due to the enhanced interfacial scattering and dielectric interface polarization in the ZnO shell. This one-pot method can complement for rational methodology in constructing metal/semiconductor core/shell nanocomposites.

Insight into the atomic structure of Li2MnO3 in Li-rich Mn-based cathode materials and the impact of its atomic arrangement on electrochemical performance

Li-rich Mn-based cathode materials have been considered as promising candidates for next generation Li-ion batteries due to their high-energy density, low cost and non-toxicity. However, the atomic arrangement of such materials and the relationship between the microstructure and electrochemical performance are still not fully understood. In this paper, local heterogeneity in the crystal lattice is directly observed in synthesized Li2MnO3/LiMO2 (M = Ni and Mn) cathode materials. With SAED application, for the first time, we accordingly uncover that the lattice heterogeneity is induced by different Li2MnO3 atomic arrangements coexisting in the same crystal domain. The co-growth of Li2MnO3 with different orientations is proved to be a defective feature, which would induce atomic vacancy concentration in the lattice and increase the risk of layered structure collapse in the cycling process. The electrochemical test results also suggest that the composition with a relatively uniform Li2MnO3 arrangement exhibits better cycling performance (the capacity retention is as high as 95.1% after 50 cycles at 0.1C), oppositely, the coexistence of multiple complex Li2MnO3 arrangements results in poor cycling performance (the capacity retention is below 70% after 50 cycles at 0.1C). The crystal lattice structure comparison between primary and cycled is shown to manifest the effect of Li2MnO3 arrangement on the electrochemical performance and structural stability, providing one possible explanation for the capacity degradation of the Li-rich materials.

Crystal defect-mediated band-gap engineering: a new strategy for tuning the optical properties of Ag2Se quantum dots toward enhanced hydrogen evolution performance

Crystal defects have been introduced into inherently narrow-band-gap and non-toxic Ag2Se quantum dots (QDs) via a facile and efficient thermal vibration approach during synthesis and studied by using electron holography and geometric phase analysis techniques. These crystal defects consequently demonstrated a solid possibility for tuning the optical band-gaps of Ag2Se QDs, and thereby enhancing the visible-light-driven water splitting and hydrogen evolution performance of Ag2Se QD-sensitized TiO2 photocatalysts.

High-temperature annealing of an iron microplate with excellent microwave absorption performance and its direct micromagnetic analysis by electron holography and Lorentz microscopy

To meet the demand of electromagnetic interference shielding, cheap and easily available microwave absorbers are urgently required. Recently, most of the related research has been focussed on a number of complicated absorbers comprising multi-components because of their better electromagnetic match. However, it is still a great challenge to develop an absorber that simultaneously possesses the advantages of easy fabrication, low-cost, ultra-wide bandwidth, and strong absorption. Hence, development of a simple and convenient absorber with efficient performance is attracting significant attention because of the urgent requirement of this type of absorbers. Herein, a series of single-component iron-based absorbers with different morphologies and grain sizes was successfully prepared. Strong absorption intensity (∼−43.4 dB) was found in plate-like samples, which could even match those of some multi-component absorbers. Electron holography and Lorentz microscopy analysis were used for the further comprehension of the relationships among the microstructure, electromagnetic property, and microwave absorption performance. The primary grain size of the present iron microplate was found fundamentally important for microwave absorption performance. This cheap and available absorber is believed to be an optimal choice for single-component absorbers and useful in the research of absorption mechanism.

Facile preparation of 3D hierarchical coaxial-cable-like Ni-CNTs@beta-(Ni, Co) binary hydroxides for supercapacitors with ultrahigh specific capacitance

A facile chemical method for Co doping Ni-CNTs@α-Ni(OH)2 combining with an in situ phase transformation process is successfully proposed and employed to synthesize three-dimensional (3D) hierarchical Ni-CNTs@β-(Ni, Co) binary hydroxides. This strategy can effectively maintain the coaxial-cable-like structure of Ni-CNTs@α-Ni(OH)2 and meanwhile increase the content of Co as much as possible. Eventually, the specific capacitances and electrical conductivity of the composites are remarkably enhanced. The optimized composite exhibits high specific capacitances of 2861.8F g-1 at 1A g-1 (39.48F cm-2 at 15mAcm-2), good rate capabilities of 1221.8F g-1 at 20A g-1 and cycling stabilities (87.6% of capacitance retention after 5000cycles at 5A g-1). The asymmetric supercapacitor (ASC) constructed with the as-synthesized composite and activated carbon as positive and negative electrode delivers a high specific capacitance of 287.7F g-1 at 1A g-1. The device demonstrates remarkable energy density (96Whkg-1) and high power density (15829.4Wkg-1). The retention of capacitance remains 83.5% at the current density of 5A g-1 after 5000cycles. The charged and discharged samples are further studied by ex situ electron energy loss spectroscopy (EELS) analysis, XRD and SEM to figure out the reasons of capacitance fading. Overall, it is believable that this facile synthetic strategy can be applied to prepare various nanostructured metal hydroxide/CNT composites for high performance supercapacitor electrode materials.

Microstructure research for ferroelectric origin in the strained Hf0.5Zr0.5O2 thin film via geometric phase analysis

Recently, non-volatile semiconductor memory devices using a ferroelectric Hf0.5Zr0.5O2 film have been attracting extensive attention. However, at the nano-scale, the phase structure remains unclear in a thin Hf0.5Zr0.5O2 film, which stands in the way of the sustained development of ferroelectric memory nano-devices. Here, a series of electron microscopy evidences have illustrated that the interfacial strain played a key role in inducing the orthorhombic phase and the distorted tetragonal phase, which was the origin of the ferroelectricity in the Hf0.5Zr0.5O2 film. Our results provide insight into understanding the association between ferroelectric performances and microstructures of Hf0.5Zr0.5O2-based systems.