The broadband and tunable high-performance microwave absorption properties of an ultralight and highly compressible graphene foam (GF) are investigated. Simply via physical compression, the microwave absorption performance can be tuned. The qualified bandwidth coverage of 93.8% (60.5 GHz/64.5 GHz) is achieved for the GF under 90% compressive strain (1.0 mm thickness). This mainly because of the 3D conductive network.

Broadband and Tunable High‐Performance Microwave Absorption of an Ultralight and Highly Compressible Graphene Foam

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).

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

Temperature dependent microwave attenuation behavior for carbon-nanotube/silica composites

Electromagnetic energy radiation is becoming a "health-killer" of living bodies, especially around industrial transformer substation and electricity pylon. Harvesting, converting, and storing waste energy for recycling are considered the ideal ways to control electromagnetic radiation. However, heat-generation and temperature-rising with performance degradation remain big problems. Herein, graphene-silica xerogel is dissected hierarchically from functions to "genes," thermally driven relaxation and charge transport, experimentally and theoretically, demonstrating a competitive synergy on energy conversion. A generic approach of "material genes sequencing" is proposed, tactfully transforming the negative effects of heat energy to superiority for switching self-powered and self-circulated electromagnetic devices, beneficial for waste energy harvesting, conversion, and storage. Graphene networks with "well-sequencing genes" (w = Pc /Pp > 0.2) can serve as nanogenerators, thermally promoting electromagnetic wave absorption by 250%, with broadened bandwidth covering the whole investigated frequency. This finding of nonionic energy conversion opens up an unexpected horizon for converting, storing, and reusing waste electromagnetic energy, providing the most promising way for governing electromagnetic pollution with self-powered and self-circulated electromagnetic devices.

Thermally Driven Transport and Relaxation Switching Self-Powered Electromagnetic Energy Conversion.

In this work, atomic layer deposition is applied to coat carbon nanocoils with magnetic Fe(3)O(4) or Ni. The coatings have a uniform and highly controlled thickness. The coated nanocoils with coaxial multilayer nanostructures exhibit remarkably improved microwave absorption properties compared to the pristine carbon nanocoils. The enhanced absorption ability arises from the efficient complementarity between complex permittivity and permeability, chiral morphology, and multilayer structure of the products. This method can be extended to exploit other composite materials benefiting from its convenient control of the impedance matching and combination of dielectric-magnetic multiple loss mechanisms for microwave absorption applications.

Microwave Absorption Properties of Carbon Nanocoils Coated with Highly Controlled Magnetic Materials by Atomic Layer Deposition

Multifunctional composite microspheres with spinel Fe(3)O(4) cores and anatase TiO(2) shells (Fe(3)O(4)@TiO(2)) are synthesized by combining a solvothermal reaction and calcination process. The size, morphology, microstructure, phase purity, and magnetic properties are characterized by scanning electron microscopy, transmission electron microscopy (TEM), high-resolution TEM, selected-area electron diffraction, electron energy loss spectroscopy, powder X-ray diffraction, and superconducting quantum interference device magnetometry. The results show that the as-synthesized microspheres have a unique morphology, uniform size, good crystallinity, favorable superparamagnetism, and high magnetization. By varying the experimental conditions such as Fe(3)O(4) size and concentration, microspheres with different core sizes and shell thickneses can be readily synthesized. Furthermore, the microwave absorption properties of these microspheres are investigated in terms of complex permittivity and permeability. By integration of the chemical composition and unique structure, the Fe(3)O(4)@TiO(2) microspheres possess lower reflection loss and a wider absorption frequency range than pure Fe(3)O(4). Moreover, the electromagnetic data demonstrate that Fe(3)O(4@TiO(2) microspheres with thicker TiO(2) shells exhibit significantly enhanced microwave absorption properties compared to those with thinner TiO(2) shells, which may result from effective complementarities between dielectric loss and magnetic loss. All the results indicate that these Fe(3)O(4)@TiO(2) microspheres may be attractive candidate materials for microwave absorption applications.

Microwave Absorption Enhancement of Multifunctional Composite Microspheres with Spinel Fe3O4 Cores and Anatase TiO2 Shells

Uniform BaTiO3 nano-torus with either concave or epicenter holes were synthesized by a hydrothermal method. Experimental observations indicated that the BaTiO3 nano-torus with an average diameter ranging from 50 to 100 nm was of tetragonal phases at room temperature. The morphology of the BaTiO3 nano-torus depends on the shape of the original titanium dioxide precursor and reaction time. The microwave absorption properties of both the BaTiO3 nano-torus and the BaTiO3 solid nanoparticles were examined between 2–18 GHz microwave frequency bands. The maximum reflection loss of the BaTiO3 nano-torus reached −28.38 dB at 11.36 GHz, compared to that of −12.87 dB at 16.32 GHz of the BaTiO3 solid nanoparticles. The nearly 120% enhancement of the reflection loss in the range of 8–12 GHz was probably attributed to the hollow volume inside the BaTiO3 nano-torus which might contribute more dissipation and scattering effects of the microwave. Growth mechanisms of the BaTiO3 nano-torus were also investigated by changing both the reaction time from 0.5 h to 48 h and the reactants concentration ratio between Ba(OH)2·8H2O and titanium dioxide. Both an “in situ transformation” mechanism and a “dissolution-precipitation” growth mode were proposed.

Microwave absorption enhancement and electron microscopy characterization of BaTiO₃ nano-torus.

A facile “hydrothermal-assisted crystallization” route for the synthesis of hierarchical magnetic yolk–shell microspheres with mixed barium silicate and barium titanium oxide shells has been developed. Various yolk–shell microspheres with different shell thicknesses and core sizes have been successfully synthesized by controlling the synthetic parameters. The as-synthesized yolk–shell microspheres possess high magnetization, tailored shells, large specific surface area and high porosity, and are demonstrated to be attractive candidate materials for microwave absorption enhancement. Moreover, a possible formation mechanism based on the sacrificial-templating crystallization, dissolution–precipitation and in situ transformation processes is proposed.

Feng Xia

Papers

Microwave Absorption Enhancement of Multifunctional Composite Microspheres with Spinel Fe3O4 Cores and Anatase TiO2 Shells

Microwave absorption enhancement and electron microscopy characterization of BaTiO₃ nano-torus.

Hierarchical magnetic yolk–shell microspheres with mixed barium silicate and barium titanium oxide shells for microwave absorption enhancement