Haiqian Zhang

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

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 presented outstanding electromagnetic absorption properties. For example, such special structure enabled an optimal reflection loss value of -50 dB with the frequency bandwidth ranged from 13.8 to 18 GHz. The excellent microwave absorption performance may attribute to the high impedance matching behavior and novel dielectric loss ability. Additionally, it can be supposed that this micrometer-size flower structure was more beneficial to scatter the incident electromagnetic wave. Meanwhile, the rough surface of the ultrathin flake is apt to increase the electromagnetic scattering among the leaves of the flower due to their large spacing and porous features.

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

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 −17.5 dB obtained with a thin coating thickness of 1.5 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.

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

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.

Coin-like α-Fe2O3@CoFe2O4 Core–Shell Composites with Excellent Electromagnetic Absorption Performance

In this paper, we designed a novel core-shell composite for microwave absorption application in which the α-Fe2O3 and the porous CoFe2O4 nanospheres served as the core and shell, respectively. Interestingly, during the solvothermal process, the solvent ratio (V) of PEG-200 to distilled water played a key role in the morphology of α-Fe2O3 for which irregular flake, coin-like, and thinner coin-like forms of α-Fe2O3 can be produced with the ratios of 1:7, 1:3, and 1:1, respectively. The porous 70 nm diameter CoFe2O4 nanospheres were generated as the shell of α-Fe2O3. It should be noted that the CoFe2O4 coating layer did not damage the original shape of α-Fe2O3. As compared with the uncoated α-Fe2O3, the Fe2O3@CoFe2O4 composites exhibited improved microwave absorption performance over the tested frequency range (2-18 GHz). In particular, the optimal reflection loss value of the flake-like composite can reach -60 dB at 16.5 GHz with a thin coating thickness of 2 mm. Furthermore, the frequency bandwidth corresponding to the RLmin value below -10 dB was up to 5 GHz (13-18 GHz). The enhanced microwave absorption properties of these composites may originate from the strong electron polarization effect (i.e., the electron polarization between Fe and Co) and the electromagnetic wave scattering on this special porous core-shell structure. In addition, the synergy effect between α-Fe2O3 and CoFe2O4 also favored balancing the electromagnetic parameters. Our results provided a promising approach for preparing an absorbent with good absorption intensity and a broad frequency that was lightweight.

Achieving excellent bandwidth absorption by a mirror growth process of magnetic porous polyhedron structures

A symmetrical Fe2O3/BaCO3 hexagonal cone structure having a height of 10 μm and an edge length of ~4 μm is reported, obtained using a common solvothermal process and a mirror growth process. Focused ion beam and high-resolution transmission electron microscopy techniques revealed that α-Fe2O3 was the single crystal feature present. Ba ions contributed to the formation of symmetrical structures exhibited in the final composites. Subsequently, porous magnetic symmetric hexagonal cone structures were used to study the observed intense electromagnetic wave interference. Electromagnetic absorption performance studies at 2–18 GHz indicated stronger attenuation electromagnetic wave ability as compared to other shapes such as spindles, spheres, cubes, and rods. The maximum absorption frequency bandwidth was at 7.2 GHz with a coating thickness d = 1.5 mm. Special structures and the absence of BaCO3 likely played a vital role in the excellent electromagnetic absorption properties described in this research.

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

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.

Enhancement photocatalytic activity of the graphite-like C3N4 coated hollow pencil-like ZnO

The pencil-like ZnO hollow tubes with 9-12 μm in length, 350-700 nm in width, 200 nm in wall thickness coating with g-C3N4 have been prepared via a chemical deposition process. As compared with uncoated ZnO or g-C3N4, these g-C3N4/ZnO composites showed the enhanced photocatalytic activity which can be attributed to the heterojunction structure. Furthermore, it is worth pointing out that the weight ratios of g-C3N4 to ZnO (g-C3N4/ZnO) played a significantly influence on the photodegradable properties. With increasing the mass ratio, the photocatalytic activity increased firstly and then decreased after reaching to an optimal photocatalytic performance. It can be inferred that the appreciation of g-C3N4 on the ZnO surface can improve the contact area which resulted in high separation of electrons and holes. However, excessive g-C3N4 may hinder the electrons transferring from the g-C3N4 to ZnO, and thus worse its photocatalytic performance. In our study, the g-C3N4/ZnO sample prepared with 10 wt% of g-C3N4 exhibited the optimal photodegradable efficiency which 94% of Rhodamine B (RhB) has been degraded just in 2 h.

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

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.

Facile synthesis of porous coin-like iron and its excellent electromagnetic absorption performance

In this paper a novel electromagnetic absorbent, porous coin-like iron with a diameter of ∼10 μm and a thickness of 2 μm, was fabricated using a hydrogen gas reduction process. This special porous coin-like structure was attributed to a decrease in density and exceeded the Snoek limitation. It was observed that these coin-like iron structures exhibit excellent microwave absorption properties. An optimal reflection loss value of −53.2 dB was obtained at 16 GHz, moreover, the effective frequency bandwidth could be up to 6.3 GHz (11.7–18 GHz) at a thickness of 1.4 mm. The microwave absorption mechanism may have originated from the following factors: firstly, these coin-like irons were favorable for obtaining a lower real part of permittivity value and thus gained the improvement of impedance matching behavior, as compared with other reported irons. Secondly, the coin-like morphology exhibited a strong magnetic loss ability. Further analysis revealed that the magnetic loss mechanism may rely mainly on the resonance. In addition, the porous feature of the coin-like iron offered a rough surface on the large size of the coin-like structure, which was beneficial for electromagnetic wave scattering and further enhanced their microwave absorption properties.

A proposed electron transmission mechanism between Fe3+/Co2+ and Fe3+/Fe3+ in the spinel structure and its practical evidence in quaternary Fe0.5Ni0.5Co2S4

This study is focused on the spinel structure of metal oxides and sulfides, from which the ternary (NiCo2O4/NiCo2S4) and quaternary (Fe0.5Ni0.5Co2O4/Fe0.5Ni0.5Co2S4) samples with hollow sphere structures were prepared. Among these samples, Fe0.5Ni0.5Co2S4 was highly effective in its ability to attenuate electromagnetic waves, wherein a broader absorption bandwidth of 6.2 GHz could be achieved with a thinner coating layer of 1.3 mm. The cation distribution rule for Fe, Co and Ni ions in the spinel structure is given according to hybrid orbital theory to support the excellent electromagnetic absorption properties. Relying on the distribution of Fe, Co and Ni cations, the probable electron transmission and coupling between Fe3+/Co2+ and Fe3+/Fe3+ adjacent cation ion pairs could occur at the octahedral site (B site), which reflects the enhanced dielectric loss.