Ronghai Yu

Flexible nanocomposites with enhanced microwave absorption properties based on Fe3O4/SiO2 nanorods and polyvinylidene fluoride

Fe3O4/SiO2 porous nanorods with one-dimensional core/shell structure and high aspect ratio have been synthesized as an absorber by a wet chemical method. Based on the nanorods, flexible Fe3O4/SiO2/PVDF nanocomposites have been first prepared by embedding the Fe3O4/SiO2 nanorods in a polyvinylidene fluoride (PVDF) matrix. The Fe3O4/SiO2/PVDF nanocomposite with a filler content of 40 wt% shows excellent microwave absorption performance over 2–18 GHz, and the minimum reflection loss (RL) can reach −28.6 dB when the thickness is 2.5 mm. The mechanisms for the enhanced microwave absorption performance were clarified based on the unique structural features as well as the synergetic effect between multiple components.

Enhanced high-frequency absorption of anisotropic Fe3O4/graphene nanocomposites.

Anisotropic Fe3O4 nanoparticle and a series of its graphene composites have been successfully prepared as high-frequency absorbers. The crystal structure, morphology and magnetic property of the samples were detailed characterized through X-ray diffractometer (XRD), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM). The high-frequency absorbing performance of the composites is evaluated within 2.0–18.0 GHz. Combining reduced graphene oxide (RGO) to Fe3O4 helps to adjust the permittivity and permeability of the composite, balance the dielectric loss and magnetic loss, consequently improve the absorbing performance in view of the impedance matching characteristic. The optimal reflection loss of the pure Fe3O4 sample reaches −38.1 dB with a thickness of 1.7 mm, and it increases to −65.1 dB for the sample grafted with 3 wt.% RGO. The addition of proper content of RGO both improves the reflection loss and expands the absorbing bandwidth. This work not only opens a new method and an idea for tuning the electromagnetic properties and enhancing the capacity of high-efficient absorbers, but also broadens the application of such kinds of lightweight absorbing materials frameworks.

Effects of local structure of Ce(3+) ions on luminescent properties of Y3Al5O12:Ce nanoparticles.

Ce3+-doped yttrium aluminum garnet (YAG:Ce) nanocrystals were successfully synthesized via a facile sol-gel method. Multiple characterization techniques were employed to study the structure, morphology, composition and photoluminescence properties of YAG:Ce nanophosphors. The YAG:Ce0.0055 sintered at 1030 °C exhibited a typical 5d1-4f1 emission band with the maximum peak located at 525 nm, and owned a short fluorescence lifetime τ1 (~28 ns) and a long fluorescence lifetime τ2 (~94 ns). Calcination temperature and Ce3+ doping concentration have significant effects on the photoluminescence properties of the YAG:Ce nanophosphors. The emission intensity was enhanced as the calcination temperature increased from 830 to 1030 °C, but decreased dramatically with the increase of Ce3+ doping concentration from 0.55 to 5.50 at.% due to the concentration quenching. By optimizing the synthesized condition, the strongest photoluminescence emission intensity was achieved at 1030 °C with Ce3+ concentration of 0.55 at.%.

Effect of Ti substitution on hydrogen storage properties of Zr1−xTixCo (x = 0, 0.1, 0.2, 0.3) alloys

Abstract Zr1−xTixCo (x = 0, 0.1, 0.2, 0.3) alloys were prepared by arc-melting method and the effect of Ti substitution on hydrogen storage properties was studied systematically. Hydrogen desorption pressure-composition-temperature (PCT) measurements were carried out using Sieverts type volumetric apparatus for ZrCo (at 473 K, 573 K and 673 K) and Zr1−xTixCo alloys (at 673 K), respectively. Products after dehydrogenation were characterized by X-ray diffraction (XRD). In addition, the kinetics of Zr1−xTixCo hydride was investigated at 473 K and 673 K, respectively, under hydrogen pressure of 5 MPa. Results showed that Ti substitution for Zr did not change the crystal structure of ZrCo phase. With the increase of temperature from 473 K to 673 K, the extent of disproportionation for ZrCo alloy increased. With Ti content increasing at 673 K, the desorption equilibrium pressure of Zr1−xTixCo-H2 systems elevated and the disproportionation reaction of Zr1−xTixCo alloys was inhibited effectively. Ti substitution decreased the kinetics rate and the effective hydrogen storage capacity of Zr1−xTixCo alloys slightly. Generally speaking, it was found that Zr0.8Ti0.2Co alloy had better anti-disproportionation property with less decrease of effective hydrogen storage capacity which was beneficial to tritium application in the International Thermonuclear Experimental Reactor (ITER).

Hierarchical NiCo2O4/Co3O4/NiO porous composite: a lightweight electromagnetic wave absorber with tunable absorbing performance

Hierarchical NiCo2O4/Co3O4/NiO composite was fabricated through a facile hydrothermal method, followed by a post-annealing treatment. The obtained ternary composite, with a flower-like morphology, consists of radially assembled porous nanosheets. Benefiting from the unique structure and the synergistic effect among multiple components, the NiCo2O4/Co3O4/NiO composite exhibited strong electromagnetic wave (EMW) absorption in both low-frequency and high-frequency regions by simply changing its filler loading in the matrix. The absorber with a filler loading of 30 wt% possessed a reflection loss of −28.6 dB at a small layer thickness of 1.64 mm in the Ku band, while the absorber with less filler loading at 20 wt% achieved a reflection loss of −57.0 dB in the C band. The light weight, small thickness, strong and tunable absorbing property of the NiCo2O4/Co3O4/NiO composite make it attractive for developing a high-performance electromagnetic wave absorber.

Controllable permittivity in 3D Fe3O4/CNTs network for remarkable microwave absorption performances

Carbon-based magnetic composites are promising alternatives to pure magnetic nanoparticles (MOx, M = Fe, Co, Ni) and spinel ferrites (MFe2O4, M = Fe, Co, Ni, Mn, Zn) as electromagnetic (EM) wave absorption materials, which however suffer from complex processes and have poor microwave absorption properties. Herein, we anchored Fe3O4 porous spheres onto carbon nanotubes (CNTs) via a simple solvothermal method. The formed Fe3O4/CNTs nanocomposites show a three-dimension (3D) network. The improved effective bandwidth (3.9 GHz) and, more importantly, remarkable EM microwave absorption performances (−51 dB at 5.52 GHz) are observed in 3D Fe3O4/5 wt% CNTs nanocomposites. The enhanced microwave absorption performances are attributed to the high surface areas and porous structure of magnetic Fe3O4 spheres, which presented a good synergetic role with CNTs. Furthermore, the controllable permittivity in nanocomposites was developed by adjusting the CNT content, which balanced the permeability to obtain a good impedance matching. This work demonstrates a simple approach to enhancing the microwave absorption performances of EM wave absorption materials.

Surfactant-free synthesis of octahedral ZnO/ZnFe2O4 heterostructure with ultrahigh and selective adsorption capacity of malachite green.

A new octahedral ZnO/ZnFe2O4 heterostructure has been fabricated through a facile surfactant-free solvothermal method followed by thermal treatment. It exhibits a record-high adsorption capacity (up to 4983.0 mg·g−1) of malachite green (MG), which is a potentially harmful dye in prevalence and should be removed from wastewater and other aqueous solutions before discharging into the environment. The octahedral ZnO/ZnFe2O4 heterostructure also demonstrates strong selective adsorption towards MG from two kinds of mixed solutions: MG/methyl orange (MO) and MG/rhodamine B (RhB) mixtures, indicating its promise in water treatment.

Magnetically Aligned Co–C/MWCNTs Composite Derived from MWCNT-Interconnected Zeolitic Imidazolate Frameworks for a Lightweight and Highly Efficient Electromagnetic Wave Absorber

Developing lightweight and highly efficient electromagnetic wave (EMW) absorbing materials is crucial but challenging for anti-electromagnetic irradiation and interference. Herein, we used multiwalled carbon nanotubes (MWCNTs) as templates for growth of Co-based zeolitic imidazolate frameworks (ZIFs) and obtained a Co-C/MWCNTs composite by postpyrolysis. The MWCNTs interconnected the ZIF-derived Co-C porous particles, constructing a conductive network for electron hopping and migration. Moreover, the Co-C/MWCNTs composite was aligned in paraffin matrix under an external magnetic field, which led to a stretch of the MWCNTs along the magnetic field direction. Due to the anisotropic permittivity of MWCNTs, the magnetic alignment considerably increased the dielectric loss of the Co-C/MWCNTs composite. Benefiting from the conductive network, the orientation-enhanced dielectric loss, and the synergistic effect between magnetic and dielectric components, the magnetically aligned Co-C/MWCNTs composite exhibited extremely strong EMW absorption, with a minimum reflection loss (RL) of -48.9 dB at a filler loading as low as 15 wt %. The specific RL value (RL/filler loading) of the composite was superior to that of the previous MOF-derived composite absorbers. It is expected that the proposed strategy can be extended to the fabrication of other lightweight and high-performance EMW-absorbing materials.

Flaky FeSiAl alloy-carbon nanotube composite with tunable electromagnetic properties for microwave absorption

Flaky FeSiAl alloy/multi-wall carbon nanotube (FeSiAl/MWCNT) composite was fabricated by facile and scalable ball milling method. The morphology and electromagnetic properties of the FeSiAl alloy can be well tuned by controlling the milling time. It is found that the magnetic loss of the FeSiAl alloy is improved by optimizing the milling time due to the increased anisotropy field. Meanwhile the addition of MWCNTs enhances the dielectric loss of the composite by increasing the interfacial polarizations, dipolar polarizations and conductive paths. Relative to conventional FeSiAl absorbers, the FeSiAl/MWCNT composite exhibits greatly improved microwave absorption performance with advantages of strong absorption and small thickness. The minimum reflection loss of the composite reaches −42.8 dB at 12.3 GHz at a very thin thickness of 1.9 mm.

Static and Dynamic Magnetization of Gradient FeNi Alloy Nanowire.

FeNi binary nanowires with gradient composition are fabricated by the electrodeposition method. The energy dispersive spec-trometer line-sweep results show that the composition changes gradually along the wire axis. The gradient FeNi nanowires exhibit polycrystalline and crystal twinning at different areas along the nanowire axis, with a textured face-centered cubic structure. The static and dynamic magnetization properties are characterized by a hysteresis loop and ferromagnetic reso-nance with pumping frequencies from 12– 40 GHz. The linear dispersion of the pumping frequency vs: the resonance field has been observed with the applied bias field higher than the saturation field, corresponding to the hysteresis loop. The field-sweep linewidths decrease with increasing pumping frequency, and the frequency-sweep linewidths stay nearly constant at the unsaturated region. The linewidth is a Gilbert type at the saturated state, with damping of 0.035 ± 0.003. Compared with the damping of the homogeneous composition FeNi nanowire (a = 0.044 ± 0.005), the gradient FeNi nanowire may have less eddy current damping, which could make it an alternative candidate for spintronics and microstrip antennas.