Xiaoqin Guo

Yolk–Shell Ni@SnO2 Composites with a Designable Interspace To Improve the Electromagnetic Wave Absorption Properties

In this study, yolk-shell Ni@SnO2 composites with a designable interspace were successfully prepared by the simple acid etching hydrothermal method. The Ni@void@SnO2 composites were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The results indicate that interspaces exist between the Ni cores and SnO2 shells. Moreover, the void can be adjusted by controlling the hydrothermal reaction time. The unique yolk-shell Ni@void@SnO2 composites show outstanding electromagnetic wave absorption properties. A minimum reflection loss (RLmin) of -50.2 dB was obtained at 17.4 GHz with absorber thickness of 1.5 mm. In addition, considering the absorber thickness, minimal reflection loss, and effective bandwidth, a novel method to judge the effective microwave absorption properties is proposed. On the basis of this method, the best microwave absorption properties were obtained with a 1.7 mm thick absorber layer (RLmin= -29.7 dB, bandwidth of 4.8 GHz). The outstanding electromagnetic wave absorption properties stem from the unique yolk-shell structure. These yolk-shell structures can tune the dielectric properties of the Ni@air@SnO2 composite to achieve good impedance matching. Moreover, the designable interspace can induce interfacial polarization, multiple reflections, and microwave plasma.

Facile synthesis of yolk–shell Ni@void@SnO2(Ni3Sn2) ternary composites via galvanic replacement/Kirkendall effect and their enhanced microwave absorption properties

Yolk–shell ternary composites composed of a Ni sphere core and a SnO2(Ni3Sn2) shell were successfully prepared by a facile two-step method. The size, morphology, microstructure, and phase purity of the resulting composites were characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, transmission electron microscopy (TEM), high-resolution TEM, selected-area electron diffraction, and powder X-ray diffraction. The core sizes, interstitial void volumes, and constituents of the yolk–shell structures varied by varying the reaction time. A mechanism based on the time-dependent experiments was proposed for the formation of the yolk–shell structures. The yolk–shell structures were formed by a synergistic combination of an etching reaction, a galvanic replacement reaction, and the Kirkendall effect. The yolk–shell ternary SnO2 (Ni3Sn2)@Ni composites synthesized at a reaction time of 15 h showed excellent microwave absorption properties. The reflection loss was found to be as low as–43 dB at 6.1 GHz. The enhanced microwave absorption properties may be attributed to the good impedance match, multiple reflections, the scattering owing to the voids between the core and the shell, and the effective complementarities between the dielectric loss and the magnetic loss. Thus, the yolk–shell ternary composites are expected to be promising candidates for microwave absorption applications, lithium ion batteries, and photocatalysis.

Constructing hierarchical hollow CuS microspheres via a galvanic replacement reaction and their use as wide-band microwave absorbers

In this study, hollow three-dimensional CuS hierarchical microspheres were prepared via a facile galvanic replacement reaction. The hollow flower-like CuS microspheres were characterized by XRD, Raman, XPS, SEM and TEM techniques, which revealed that numerous nanoflakes were self-assembled to construct hollow flowers. Based on time-dependent experiments, a plausible formation mechanism (galvanic replacement reaction) was proposed. Paraffin-based composites, containing 50 wt% hollow CuS, exhibited outstanding microwave absorption capabilities, which were attributed to the suitable impedance match and dielectric loss. The minimal reflection loss was −17.5 dB and the effective bandwidth was 3.0 GHz with thin absorber thickness of 1.1 mm. In addition, we put forward a novel solution to evaluate the microwave absorption efficiency. The high efficiency microwave absorption properties originate from the electric/dielectric polarization and unique hollow flower-like structure. The hollow structures can adjust the dielectric properties to obtain good impedance matching. Moreover, the two-dimensional flakes and hollow flowers can induce more multiple reflection and scattering, which consumes more microwave energy. This study has led to a novel useful method for the design of hollow structures used as high efficiency microwave absorbers.

An impedance match method used to tune the electromagnetic wave absorption properties of hierarchical ZnO assembled by porous nanosheets

In this study, three hierarchical porous ZnO flowers were prepared via calcination of the precursor at different temperatures (400 °C, 500 °C and 600 °C). It can be found that the calcination temperature influences the porous structure and size of the ZnO nanoparticles. The porous structure can effectively tune the permittivity to improve the impedance match. The porous ZnO flowers obtained at 400 °C show a large surface area and small size of ZnO nanoparticles and thus, hold good impedance match. Moreover, the porous ZnO flowers self-assembled by nanosheets can induce multiple reflection and scattering, which prolong the microwave travel path and consume more electromagnetic energy. Therefore, the porous ZnO flowers obtained at 400 °C exhibit outstanding microwave absorption properties. The minimal reflection loss is −46.1 dB at 13.6 GHz with only a thickness of 1.9 mm. The absorption bandwidth of RL less than −10 dB can reach up to 11.5 GHz (6.5–18.0 GHz) by adjusting the absorber thickness (1.5–4.0 mm). The lightweight, small thickness, and strong and tunable absorbing properties of the porous ZnO flowers make it attractive for developing a high-performance electromagnetic wave absorber.

1D Cu@Ni nanorods anchored on 2D reduced graphene oxide with interfacial engineering to enhance microwave absorption properties

In this work, one-dimensional core–shell Cu@Ni nanorods which were anchored on two dimensional reduced graphene oxide (rGO) heterostructures were successfully prepared by a simple co-reduction method. The characteristics of Cu@Ni/rGO composites were measured and collected by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, Fourier transform infrared spectroscopy and Raman spectroscopy. The formation mechanisms of these unique Cu@Ni nanorods–rGO heterostructures were discussed in detail. Compared with pure Cu@Ni and rGO, the Cu@Ni/rGO composite showed strong microwave absorption performance with a minimal reflection loss value of −41.2 dB at 8.8 GHz. The excellent microwave absorption properties could be attributed to the suitable impedance match, eddy current loss, multiple reflection and intense interfacial polarization of multiple interfaces existing in Ni/Cu, Cu@Ni/rGO, Ni/rGO, and Cu/rGO systems. The effective bandwidth with a reflection loss lower than −10 dB can be tuned in a frequency range of 3.8–18 GHz with an absorber thickness of 1.0–4.0 mm. This proves that the Cu@Ni/rGO heterostructure is a promising candidate as a highly efficient and lightweight absorber in C (4–8 GHz), X (8–12 GHz) and Ku (12–18 GHz) bands.

Lightweight porous Co3O4 and Co/CoO nanofibers with tunable impedance match and configuration-dependent microwave absorption properties

In this study, we fabricated one-dimensional porous Co3O4 and Co/CoO nanofibers by calcination of cobalt(II) oxalate dehydrate precursors in an environment filled with air and N2, respectively. The porous configurations of Co3O4 and Co/CoO nanofibers are determined by the calcination temperatures, which can effectively tune complex permittivity and impedance match. The minimal reflection loss of porous Co3O4 nanofibers is −23.8 dB at 11.4 GHz with a thickness of 2.0 mm, which results from good impedance match, dielectric loss (dipole polarization) and one-dimensional shape effect (point discharge as an antenna receiver, and long microwave travel distance due to multiple reflections and scattering). For the porous Co/CoO nanofibers, in comparison with Co3O4, due to the existence of magnetic Co, they exhibit better microwave absorption properties. The minimal RL value is −48.4 dB at 16.1 GHz, and the effective absorption (RL below −10 dB) can reach 4.2 GHz (13.8–18 GHz) with a thickness of only 1.5 mm. Besides, in terms of the above mentioned absorption mechanisms of porous Co3O4, the magnetic loss (natural and exchange resonance) and interfacial polarization between Co and CoO also contribute to the microwave absorption. These one-dimensional porous Co3O4 and Co/CoO nanofibers are proved to be efficient and lightweight absorbers with promising applications.