SnO2: A comprehensive review on structures and gas sensors

Rational design of core-shell Co@C microspheres for high-performance microwave absorption

Synthesis of lightweight N-doped graphene foams with open reticular structure for high-efficiency electromagnetic wave absorption

With the rapid arising of information technology, microwave absorbing materials (MAMs) are playing an increasingly significant role in electronic reliability, healthcare, and national defense security. Hence, development of high performance MAMs with thin thickness, low density, wide bandwidth, and strong absorption has attracted great interests. Recently, taking graphene as MAMs for high-performance electromagnetic (EM) wave attenuation has grabbed considerable attention, owing to their low density, high specific surface area, strong dielectric loss, and high electronic conductivity. Furthermore, in order to address the interfacial impedance mismatching of the sole graphene materials, incorporation of other lossy materials has been widely studied as the imperative solution to improve its MA performance. In this review, we introduce the theory of microwave absorption and summarize recent advances in the fabrication of graphene-based MAMs, including rational design of the microstructure of pure graphene and tunable chemical integrations with polymers, magnetic metals, ferrites, ceramics, and multicomponents composites. The key point of enhancing MA in graphene-based MAMs is to regulate their EM properties, improve of impedance matching, and create diversified loss mechanisms. Furthermore, the shortcomings, challenges, and prospects of graphene-based MAMs are also put forward, which will be helpful to people working in the related fields.

Graphene-based microwave absorbing composites: A review and prospective

The porous three-dimensional (3-D) flower structures assembled by numerous ultrathin flakes were favor for strengthen electromagnetic absorption capability. However, it still remains a big challenge to fabricate such kind of materials. In this study, an easy and flexible two-step method consisting of hydrothermal and subsequent annealing process have been developed to synthesize the porous 3-D flower-like Co/CoO. Interestingly, we found that the suitable heat treatment temperature played a vital role on the flower-like structure, composition, and electromagnetic absorption properties. In detail, only in the composite treated with 400 °C can we gain the porous 3-D flower structure. If the annealing temperature were heated to 300 °C, the Co element was unable to generate. Moreover, when the annealing temperature increased from 400 to 500 °C, these flower-like structures were unable to be kept because the enlarged porous diameter would wreck the flower frame. Moreover, these 3-D porous flower-like structures...

Porous Three-Dimensional Flower-like Co/CoO and Its Excellent Electromagnetic Absorption Properties

Hierarchical hollow carbon@Fe@Fe3O4 nanospheres were synthesized by a simple template method and another pyrolysis process. Interestingly, the thickness of hollow carbon spheres is tunable by a simple hydrothermal approach. The as-prepared carbon@Fe@Fe3O4 shows excellent microwave absorption properties. In detail, the maximum effective frequency is up to 5.2 GHz with an optimal reflection loss value of −40 dB while the coating thickness is just 1.5 mm. Meanwhile, such absorption properties can be maintained via controlling the thickness of the hollow carbon. For instance, in another coating layer of 2 mm, the effective frequency is still more than 5 GHz as the carbon thickness declines to 12 nm. As novel electromagnetic absorbers, the composites also present the lower density feature due to the hollow carbon sphere frame. The excellent electromagnetic absorption mechanism may be attributed to the obvious interface polarization, and strong magnetic loss ability resulting from the Fe and Fe3O4 shell. Besides, owing to the dielectric feature of carbon, the hollow carbon core is beneficial for the attenuation ability.

Achieving hierarchical hollow carbon@Fe@Fe3O4 nanospheres with superior microwave absorption properties and lightweight features

Impedance matching and the attenuation constant, α, are two key parameters in determining electromagnetic absorption properties Although materials with single magnetic or dielectric loss properties have a high α value, they nonetheless suffer from poor impedance matching The design of magnetic and dielectric composites might possibly be an effective method of solving this problem, but unfortunately the introduction of magnetic material may give a poor value of α In order to obtain absorptive materials with high impedance matching and a high value of α, we have designed a novel ternary composite of MnO2@Fe–graphene A 30 nm wide rod-like strip of MnO2 was first obtained by a simple liquid process Liquid decomposition of Fe(CO)5 was then carried out to deposit iron on the surface of the rod-like structure, and the MnO2@Fe was finally loaded on graphene by a liquid deposition technique The resulting ternary composite exhibited attractive electromagnetic absorption properties, in which the optimal reflection loss of up to −175 dB obtained with a thin coating thickness of 15 mm was able to satisfy the requirements of lightness of weight and a high degree of absorption The effective bandwidth frequency of MnO2@Fe–GNS is broader than that of pure MnO2 or MnO2@Fe, possibly due to its moderate impedance matching and attenuation ability The possible attenuation mechanism will also be discussed

A novel rod-like MnO2@Fe loading on graphene giving excellent electromagnetic absorption properties

Hydrothermal synthesis Ni-doped ZnO nanorods with room-temperature ferromagnetism

The shell on the nano-magnetic absorber can prevent oxidation, which is very important for its practical utilization. Generally, the nonmagnetic shell will decrease the integral magnetic loss and thus weaken the electromagnetic absorption. However, maintaining the original absorption properties of the magnetic core is a major challenge. Here, we designed novel and facile CoxFey@C composites by reducing CoxFe3−xO4@phenolic resin (x = 1, 0.5 and 0.25). High saturation magnetization value (Ms) of CoxFey particle, as a core, shows the interesting magnetic loss ability. Meanwhile, the carbon shell may increase the integral dielectric loss. The resulting composite shows excellent electromagnetic absorption properties. For example, at a coating thickness of 2 mm, the RLmin value can reach to −23 dB with an effective frequency range of 7 GHz (11–18 GHz). The mechanisms of the improved microwave absorption properties are discussed.

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CoxFey@C Composites with Tunable Atomic Ratios for Excellent Electromagnetic Absorption Properties

FeCo/ZnO composites were successfully synthesized by a simple one-step hydrothermal process. It is clearly seen that the pencil-like ZnO particles with 3–4 μm in length and 200–300 nm in width are found to grow along the surface of the hexagonal-cone FeCo particles, which form a discontinuous conductive network. Enhanced microwave absorption properties can be obtained for the FeCo/ZnO composites as compared to those of pure FeCo alloy, which is mainly attributed to the better resonance according to an isotropic antenna mechanism. Further investigation confirms that the hydrothermal temperature (T) and time (t) play a key role on the real part of the permittivity values of the FeCo/ZnO composites and indirectly affect the microwave absorbing properties. It is surprising to find out that the composite prepared at 150 °C for 12 hours, exhibited the optimal reflection loss of −31 dB with a 5.5 GHz effective frequency bandwidth.

Haiqian Zhang

Papers

Achieving hierarchical hollow carbon@Fe@Fe3O4 nanospheres with superior microwave absorption properties and lightweight features

A novel rod-like MnO2@Fe loading on graphene giving excellent electromagnetic absorption properties

Hydrothermal synthesis Ni-doped ZnO nanorods with room-temperature ferromagnetism

CoxFey@C Composites with Tunable Atomic Ratios for Excellent Electromagnetic Absorption Properties

FeCo/ZnO composites with enhancing microwave absorbing properties: effect of hydrothermal temperature and time