Meikang Han

Graphene-wrapped ZnO hollow spheres with enhanced electromagnetic wave absorption properties

Graphene-wrapped ZnO hollow spheres were synthesized by a two-step process, which combined a hydrothermal reaction with surface modification. The experimental results show that reduced graphene oxide sheets adhere entirely to the surface of the ZnO hollow spheres consisting of nanoparticles. The unique structure effectively decreases the density of the composite without sacrificing the contact between graphene and the nanoparticles. Different mass ratios of graphene to ZnO hollow spheres mixed in a paraffin wax matrix (50 wt%) were prepared to investigate the electromagnetic wave absorption properties in the X-band region. When the mass ratio of graphene oxide to ZnO is 12 : 88, the composite exhibits a maximum absorption of −45.05 dB at 9.7 GHz with a sample thickness of only 2.2 mm. The fundamental mechanism based on electrical conductivity and the polarization between the graphene sheets and ZnO nanoparticles is discussed. The hierarchical structure of graphene-wrapped ZnO hollow spheres exhibits a promising designable approach to lightweight electromagnetic wave absorbing materials.

Ti3C2 MXenes with Modified Surface for High-Performance Electromagnetic Absorption and Shielding in the X-Band

Electromagnetic (EM) absorbing and shielding composites with tunable absorbing behaviors based on Ti3C2 MXenes are fabricated via HF etching and annealing treatment. Localized sandwich structure without sacrificing the original layered morphology is realized, which is responsible for the enhancement of EM absorbing capability in the X-band. The composite with 50 wt % annealed MXenes exhibits a minimum reflection loss of -48.4 dB at 11.6 GHz, because of the formation of TiO2 nanocrystals and amorphous carbon. Moreover, superior shielding effectiveness with high absorption effectiveness is achieved. The total and absorbing shielding effectiveness of Ti3C2 MXenes in a wax matrix with a thickness of only 1 mm reach values of 76.1 and 67.3 dB, while those of annealed Ti3C2 MXenes/wax composites are 32 and 24.2 dB, respectively. Considering the promising performance of Ti3C2 MXenes with the modified surface, this work is expected to open the door for the expanded applications of MXenes family in EM absorbing and shielding fields.

Carbon Hollow Microspheres with a Designable Mesoporous Shell for High-Performance Electromagnetic Wave Absorption

In this work, mesoporous carbon hollow microspheres (PCHMs) with designable mesoporous shell and interior void are constructed by a facile in situ stöber templating approach and a pyrolysis-etching process. The PCHMs are characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectra, Raman spectroscopy, and nitrogen adsorption and desorption system. A uniform mesoporous shell (pore size 4.7 nm) with a thickness of 55 nm and a cavity size of 345 nm is realized. The composite of paraffin mixed with 20 wt % PCHMs exhibits a minimum reflection coefficient (RCmin) of -84 dB at 8.2 GHz with a sample thickness of 3.9 mm and an effective absorption bandwidth (EAB) of 4.8 GHz below -10 dB (>90% electromagnetic wave is attenuated). Moreover, the composite of phenolic resin mixed with 20 wt % PCHMs exhibits an ultrawide EAB of 8 GHz below -10 dB with a thinner thickness of 2.15 mm. Such excellent electromagnetic wave absorption properties are ascribed to the large carbon-air interface in the mesoporous shell and interior void, which is favorable for the matching of characteristic impedance as compared with carbon hollow microspheres and carbon solid microspheres. Considering the excellent performance of PCHMs, we believe the as-fabricated PCHMs can be promising candidates as highly effective microwave absorbers, and the design philosophy can be extended to other spherical absorbers.

Core/shell structured C/ZnO nanoparticles composites for effective electromagnetic wave absorption

Core/shell structured C/ZnO nanoparticles were synthesized by a two-step process based on hydrothermal method. The experimental results show that ZnO nanoparticles attach on the surface of carbon spheres through the surfacial functional groups. The core/shell structure enhances the electromagnetic wave attenuation capability owing to defects, multiple interfaces and optimal impedance match. Different mass percentages of C/ZnO nanoparticles were mixed in paraffin wax to investigate the electromagnetic wave absorbing and shielding performance. When the filler loading is 40 wt%, the composite shows a minimum reflection coefficient of −52 dB at 11 GHz with a sample thickness of 1.75 mm. When the mass ratio is 50 wt%, the sample has an electromagnetic shielding performance of 14.85 dB dominated by absorption. Compared with pure carbon spheres and ZnO hollow spheres, the core/shell structure of C/ZnO composites exhibits a promising route to design electromagnetic wave absorbing materials with high dielectric loss and moderate impedance match.

Laminated and Two-Dimensional Carbon-Supported Microwave Absorbers Derived from MXenes

Microwave absorbers with layered structures that can provide abundant interfaces are highly desirable for enhancing electromagnetic absorbing capability and decreasing the thickness. The atomically thin layers of two-dimensional (2D) transition-metal carbides (MXenes) make them a convenient precursor for synthesis of other 2D and layered structures. Here, laminated carbon/TiO2 hybrid materials composed of well-aligned 2D carbon sheets with embedded TiO2 nanoparticles were synthesized and showed excellent microwave absorption. Disordered 2D carbon layers with an unusual structure were obtained by annealing multilayer Ti3C2 MXene in a CO2 atmosphere. The minimum reflection coefficient of laminated carbon/TiO2 composites reaches -36 dB, and the effective absorption bandwidth ranges from 3.6 to 18 GHz with the tunable thickness from 1.7 to 5 mm. The effective absorption bandwidth covers the whole Ku band (12.4-18 GHz) when the thickness of carbon/TiO2/paraffin composite is 1.7 mm. This study is expected to pave the way to the synthesis of carbon-supported absorbing materials using a large family of 2D carbides.

Flexible and Thermostable Graphene/SiC Nanowire Foam Composites with Tunable Electromagnetic Wave Absorption Properties

Three-dimensional (3D) flexible foams consisting of reduced graphene oxides (rGO) and in situ grown SiC nanowires (NWs) were prepared using freeze-drying and carbothermal reduction processes. By means of incorporating SiC nanowires into rGO foams, both the thermostability and electromagnetic absorption of the composites were improved. It was demonstrated that rGO/SiC NW foams were thermostable beyond ∼630 °C (90% weight retention in air atmosphere). As expected, rGO/SiC NW foams in the poly(dimethylsiloxane) matrix achieved effective absorption in the entire X-band (8.2-12.4 GHz) with a thinner thickness (3 mm) in comparison with those of the pure rGO foams. It is revealed that SiC nanowires with abundant stacking faults, twinning interfaces, and bridged junctions play an important role in the enhanced electromagnetic absorption performance, in addition to the contribution of interconnected graphene networks. Hierarchical rGO/SiC NW foams not only are efficient absorbers in the critical environments but also can be applied in photocatalytic and thermal-management fields.

Synthesis and EMW absorbing properties of nano SiC modified PDC–SiOC

Nano SiC modified silicon oxycarbide (n-SiC/SiOC) ceramics were prepared through the pyrolysis of a mixture of liquid polysiloxane and n-SiC with an average grain diameter of 30 nm. After adding n-SiC, palingenetic SiC nanograins with an average grain diameter smaller than 10 nm, and nanosized free carbon were gradually separated from the amorphous SiOC phase when the annealing temperature increased from 1100 °C to 1450 °C. The various interfaces among n-SiC, in situ formed SiC nanograins, nanosized carbon and amorphous SiOC phases can obtain interfacial scattering. Eventually, the electric dipole polarization and interfacial scattering enhanced the absorption properties. The minimal reflection coefficient (RCmin) of the n-SiC/SiOC ceramics annealed at 1400 °C (n-SiC/SiOC-1400) reached −61 dB at 8.6 GHz. The widest effective absorption bandwidth (EAB) reached 3.5 GHz in the X-band, which indicates that the n-SiC/SiOC ceramics can be considered as high-performance microwave absorbing materials because of the strong absorption capability and wide absorption bandwidth.

Ti3C2 MXenes modified with in situ grown carbon nanotubes for enhanced electromagnetic wave absorption properties

Ti3C2Tx MXenes modified with in situ grown carbon nanotubes (CNTs) are fabricated via a simple catalytic chemical vapor deposition (CVD) process. The as-prepared Ti3C2Tx/CNT nanocomposites show that one-dimensional (1D) carbon nanotubes are uniformly distributed in the interlayers of two-dimensional (2D) Ti3C2Tx MXene flakes. Compared with the pristine Ti3C2Tx MXenes, the hierarchical sandwich microstructure makes a contribution to the excellent electromagnetic wave absorption performance in the frequency range of 2–18 GHz, including higher absorption intensity (the minimum reflection coefficient reaches −52.9 dB, ∼99.999% absorption), broader effective absorption bandwidth (4.46 GHz), lower filler loading (35 wt%) and thinner thickness (only 1.55 mm). In addition, with the adjustment of thickness from 1.55 to 5 mm, the effective absorption bandwidth can reach up to 14.54 GHz (3.46–18 GHz). Different absorption mechanisms mainly based on polarization behaviors and conductivity loss are discussed. This work not only proposes the design of a novel electromagnetic wave absorber, but also provides an effective route for extending further the applications of 2D MXene materials in the field of electromagnetic wave absorption.

A controllable heterogeneous structure and electromagnetic wave absorption properties of Ti2CTx MXene

Herein, Ti2CTx MXene and its derivatives with various heterogeneous structures were constructed via etching and a facile oxidation treatment. The effect of different oxidation conditions on their structural evolution and phase composition was studied in detail. Compared with that of pristine Ti2CTx MXene, the improvement in the electromagnetic wave absorption capability of the as-prepared Ti2CTx/TiO2 and C/TiO2 nanocomposites was attributed to their enhanced polarization loss and stronger conductivity loss. The enhanced polarization loss is caused by the generated heterogeneous interfaces and higher specific surface area, and the stronger conductivity loss is due to the completely exfoliated carbon layers. Additionally, the remaining multilayered structure after exfoliation of the carbon layers favors energy dissipation. The C/TiO2 nanocomposites attain a minimum reflection coefficient of −50.3 dB at 7.1 and 14.2 GHz, and an effective absorption bandwidth of 4.7 GHz (covering the whole X-band) with a matching thickness of 2.1 mm; this indicates their excellent electromagnetic wave absorption properties. We believe that these nanocomposites with a heterogeneous structure also hold great promise for application in the fields of photocatalysis, lithium batteries, water purification, etc.

Structure of A-C Type Intervariant Interface in Nonmodulated Martensite in a Ni-Mn-Ga Alloy.

The structure of A-C type intervariant interface in nonmodulated martensite in the Ni54Mn25Ga21 alloy was studied using high resolution transmission electron microscopy. The A-C interface is between the martensitic variants A and C, each of which has a nanoscale substructure of twin-related lamellae. According to their different thicknesses, the nanoscale lamellae in each variant can be classified into major and minor lamellae. It is the boundaries between these lamellae in different variants that constitute the A-C interface, which is thus composed of major-major, minor-minor, and major-minor lamellar boundaries. The volume fraction of the minor lamellae, λ, plays an important role in the structure of A-C interfaces. For major-major and minor-minor lamellar boundaries, they are symmetrical or asymmetrical tilt boundaries; for major-minor boundary, as λ increases, it changes from a symmetrical tilt boundary to two asymmetrical microfacets. Moreover, both lattice and misfit dislocations were observed in the A-C interfaces. On the basis of experimental observations and dislocation theory, we explain how different morphologies of the A-C interface are formed and describe the formation process of the A-C interfaces from λ ≈ 0 to λ ≈ 0.5 in terms of dislocation-boundary interaction, and we infer that low density of interfacial dislocations would lead to high mobility of the A-C interface.