Chunhua Tian

Metal organic framework-derived Fe/C nanocubes toward efficient microwave absorption

Composites of magnetic metal nanoparticles and carbon materials are highly desirable for high-performance microwave absorbers due to their compatible dielectric loss and magnetic loss abilities. In this article, novel nanocomposites, Fe/C nanocubes, have been successfully prepared through an in situ route from a metal–organic framework, Prussian blue, by controlled high-temperature pyrolysis. The resultant nanocubes are actually composed of a cubic framework of amorphous carbon and uniformly dispersed core–shell Fe@graphitic carbon nanoparticles. Within the studied pyrolysis temperature range (600–700 °C), the porous structure, iron content, magnetic properties, and graphitization degree of the Fe/C nanocubes can be well modulated. Particularly, the improved carbon graphitization degree, both in amorphous frameworks and graphitic shells, results in enhanced complex permittivity and dielectric loss properties. The homogeneous chemical composition and microstructure stimulate the formation of multiple dielectric resonances by regularizing various polarizations. The synergistic effect of dielectric loss, magnetic loss, matched impedance, and dielectric resonances accounts for the improved microwave absorption properties of the Fe/C nanocubes. The absorption bands of the optimum one obtained at 650 °C are superior to most composites ever reported. By considering the good chemical homogeneity and microwave absorption, we believe that the as-fabricated Fe/C nanocubes will be promising candidates as highly effective microwave absorbers.

Constructing Uniform Core–Shell PPy@PANI Composites with Tunable Shell Thickness toward Enhancement in Microwave Absorption

Highly uniform core-shell composites, polypyrrole@polyaniline (PPy@PANI), have been successfully constructed by directing the polymerization of aniline on the surface of PPy microspheres. The thickness of PANI shells, from 30 to 120 nm, can be well controlled by modulating the weight ratio of aniline and PPy microspheres. PPy microspheres with abundant carbonyl groups have very strong affinity to the conjugated chains of PANI, which is responsible for the spontaneous formation of uniform core-shell microstructures. However, the strong affinity between PPy microspheres and PANI shells does not promote the diffusion or reassembly of two kinds of conjugated chains. Coating PPy microspheres with PANI shells increases the complex permittivity and creates the mechanism of interfacial polarization, where the latter plays an important role in increasing the dielectric loss of PPy@PANI composites. With a proper thickness of PANI shells, the moderate dielectric loss will produce well matched characteristic impedance, so that the microwave absorption properties of these composites can be greatly enhanced. Although PPy@PANI composites herein consume the incident electromagnetic wave by absolute dielectric loss, their performances are still superior or comparable to most PANI-based composites ever reported, indicating that they can be taken as a new kind of promising lightweight microwave absorbers. More importantly, microwave absorption of PPy@PANI composites can be simply modulated not only by the thickness of the absorbers, but also the shell thickness to satisfy the applications in different frequency bands.

Synthesis and microwave absorption enhancement of yolk–shell Fe3O4@C microspheres

Rational design on the microstructure of microwave-absorbing materials is paving the way for upgrading their performances in electromagnetic pollution prevention. In this study, a Fe3O4/C composite with unique yolk–shell microstructure (YS-Fe3O4@C) is successfully fabricated by a silica-assisted route. It is found that carbon shells in this composite can make up the shortages of Fe3O4 microspheres in dielectric loss ability, while they may more or less attenuate the intrinsically magnetic loss of Fe3O4 microspheres. The microwave absorption properties of YS-Fe3O4@C are evaluated in the frequency range of 2.0–18.0xa0GHz in terms of the measured complex permittivity and complex permeability. The results demonstrate that YS-Fe3O4@C can exhibit much better performance than bare Fe3O4 microspheres and individual carbon materials, as well as core–shell Fe3O4/C composite (CS-Fe3O4@C), where strong reflection loss and wide response bandwidth can be achieved simultaneously. With an absorber thickness of 2.0xa0mm, the maximum reflection loss is −73.1xa0dB at 14.6xa0GHz and a bandwidth over −10.0xa0dB is in the range of 12.3–18.0xa0GHz. It can be proved that the unique yolk–shell microstructure is helpful to reinforce the dielectric loss ability and create an optimized matching of characteristic impedance in the composite.

FeCo alloy nanoparticles supported on ordered mesoporous carbon for enhanced microwave absorption

Composites of carbon and magnetic metal are always the most attractive candidates for high-performance microwave absorbing materials (MAMs) due to their synergetic loss mechanisms and tunable electromagnetic properties. Herein, FeCo alloy nanoparticles have been innovatively coupled with ordered mesoporous carbon through impregnation and in situ hydrogenthermal reduction. Incorporation of FeCo alloy nanoparticles into mesoporous carbon provides a significant enhancement in microwave absorption, and both strong reflection loss (−73.8xa0dB at 10.6xa0GHz) and wide response bandwidth (4.0–18.0xa0GHz over −10.0xa0dB) can be simultaneously achieved in the optimized composite. This improved microwave absorption property is much better than that of many mesoporous carbon-based composites ever reported. Electromagnetic analysis reveals that FeCo alloy nanoparticles display dual electromagnetic functions. On the one hand, they produce obvious magnetic loss ability in the composites, and on the other hand, they moderately weaken the dielectric loss ability as compared to pristine mesoporous carbon. A delta function calculation indicates that the narrowed gap between complex permittivity and complex permeability is beneficial to creating the matched characteristic impedance, which affords the prerequisite for the formation of desirable microwave absorption performance. This study not only identifies FeCo alloy/mesoporous carbon as promising MAMs and reveals the origin of the microwave absorption enhancement, but also paves a new way for designing magnetic carbon-based MAMs in the future.

Precursor-directed synthesis of porous cobalt assemblies with tunable close-packed hexagonal and face-centered cubic phases for the effective enhancement in microwave absorption

AbstractMetal cobalt is one of the most promising candidates for high-performance microwave absorbers due to its compatible dielectric loss and magnetic loss abilities. Rational design on the microstructure of metal cobalt became a popular way to upgrade its microwave absorption performance in the past decade, while much less attention has been paid to the electromagnetic functions derived from its different crystal structures. Herein, we report the microwave absorption of porous cobalt assemblies with varied composition of close-packed hexagonal (hcp) and face-centered cubic (fcc) phases. Electromagnetic analysis reveals that the change of phase composition can significantly impact the complex permittivity and complex permeability of metal cobalt, where hcp-cobalt favors high complex permittivity and fcc-cobalt produces high complex permeability. The optimum phase composition in these porous cobalt assemblies will promise well-matched characteristic impedance and good performance in strong reflection loss (−41.0xa0dB at 9.4xa0GHz) and wide response bandwidth (4.0–17.4xa0GHz over −10.0xa0dB). The enhanced microwave absorption is superior to many cobalt absorbers ever reported. It is believed that these results will provide a new pathway to the design and preparation of highly effective metal cobalt and cobalt-based composites as novel microwave absorbers in the future.n

Prussian blue analogues derived magnetic FeCo alloy/carbon composites with tunable chemical composition and enhanced microwave absorption

A series of magnetic FeCo alloy/carbon composites have been successfully prepared through in situ pyrolysis of Prussian blue analogues (PBAs) with different Fe/Co ratios. The Fe/Co ratio can affect the crystalline phase, particle size, and magnetic property of the FeCo alloy particles, as well as the relative graphitization degree of the carbon frameworks. As a result, the electromagnetic functions of these composites will be highly associated with the Fe/Co ratio, where high Co content is beneficial to the formation of strong dielectric loss and moderate Co content can facilitate the magnetic loss. When Fe/Co ratio reaches 1:1, the as-obtained composite (sample S4) displays excellent reflection loss characteristics with powerful absorption in a very broad frequency range (over -10u202fdB in 3.2-18.0u202fGHz), which is superior to those of single magnetic metal (Fe or Co)/carbon composite derived from PBAs, as well as many previously reported FeCo alloy/carbon composites. Electromagnetic analysis reveals that the excellent microwave absorption of sample S4 benefits from its preferable matching of characteristic impedance and good attenuation ability toward incident electromagnetic waves. These results provide new insight into the fabrication of carbon-based magnetic composites with enhanced microwave absorption by rationally manipulating the chemical composition of magnetic components.

Differential shrinkage induced formation of yolk-shell carbon microspheres toward enhanced microwave absorption

Rational design of the microstructure paves new ways for microwave absorbing materials because it can create more facilities for the attenuation of incident electromagnetic waves. In this study, a simple method is proposed to prepare yolk-shell carbon microspheres through differential shrinkage in the internal cores and external shells of polypyrrole microspheres with the assistance of outermost SiO2 coating. This method simplifies the preparation procedures and avoids strictly controlled conditions. The electromagnetic parameters, such as relative complex permittivity and permeability, of the as-prepared yolk-shell carbon microspheres, are investigated in the frequency range of 2.0–18.0u2009GHz. Compared with solid carbon microspheres, yolk-shell carbon microspheres exhibit significantly enhanced microwave absorption properties in terms of both the reflection loss intensity and absorption bandwidth. The minimum reflection loss value can reach up to −27.5u2009dB at 8.32u2009GHz with an absorber thickness of 2.96u2009mm. ...

Performance Vs Convenience of Magnetic Carbon-Metal Nanocomposites: A Low-Cost and Facile Citrate-Derived Strategy for Feco Alloy/Carbon Composites with High-Performance Microwave Absorption

Magnetic carbon-based composites reside at the frontier of high-performance microwave absorbing materials (MAMs) due to their dual loss mechanisms and tunable electromagnetic properties. In this study, we review the construction of magnetic carbon-metal composites in recent years, and introduce a facile citrate-derived strategy to produce FeCo alloy/carbon composites through a direct pyrolysis of the resultant gels, where citric acid is employed as both a carbon source and a complexing agent. It is found that the dosage of citric acid determines the crystalline phase and particle size of magnetic FeCo alloy particles, as well as the content of carbon components. Electromagnetic analysis indicates that the electromagnetic functions of these composites are highly dependent on their specific compositions, where carbon components favor high complex permittivity and FeCo alloy particles take charge of complex permeability. When the chemical composition is optimized, good attenuation ability and characteristic impedance matching are created in the composite, resulting in desirable microwave absorption properties. The absorption performance is comparable to most homologous composites reported. Considering the low-cost and easy preparative process, we believe that this citrate-derived strategy may be instructive and helpful for practical applications of various magnetic carbon-based composites in the field of microwave absorption. Graphical Abstract