Youwei Du

Influences of La3+ substitution on the structure and magnetic properties of M-type strontium ferrites

Abstract M-type strontium ferrites with substitution of Sr 2+ by rare-earth La 3+ , according to the formula Sr 1− x La x Fe 12 O 19 , are prepared by the ceramic process. Influences of the substituted amount of La 3+ on structure and magnetic properties of Sr 1− x La x Fe 12 O 19 compounds have systematically been investigated by XRD, VSM and Mossbauer spectrum. When the substituted amount x is below 0.30, X-ray diffraction shows that the samples are single M-type hexagonal ferrites. It is found that the suitable amount of La 3+ substitution may remarkably increase saturation magnetization σ s and intrinsic coercivity H cJ . With the La 3+ addition for the same sintering temperature, σ s and H cJ increase at first, then decrease gradually. However, the substituted amount x at the maximum value of H cJ is bigger than that of σ s . Mossbauer spectroscopy of 57 Fe in Sr 1− x La x Fe 12 O 19 has shown that the substitution of Sr 2+ by La 3+ is associated with a valence change of Fe 3+ to Fe 2+ at 2a or 4 f 2 site. The magnetic properties are reflected in the Mossbauer spectra which indicate that the magnetic hyperfine field ( H hf ) is detected with the highest value at x =0.20. The different exchange paths between the iron sublattices are discussed according to the increased hyperfine fields of the 12k- and 2b-site. The Curie temperature of Sr 1− x La x Fe 12 O 19 decreases linearly with increasing La 3+ substitution.

Key step in synthesis of ultrafine BaFe12O19 by sol-gel technique

Abstract Barium hexaferrite powders have been prepared by sol-gel technique. Different ways in heat treatment of the gel are adopted to study the factors that influence the formation mechanism, morphology and magnetic properties of BaFe12O19. Results show that preheating the gel between 400 and 500°C for several hours is a key step, it can prevent the formation of α-Fe2O3 intermediate and obtain ultrafine BaFe12O19 single phase with improved magnetic properties and narrow size distribution at a low temperature. The barium ferrite powder had a coercive force of 5950 Oe and a magnetization of 70 emu/g, both are in good agreement with the expected values theoretically.

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.

Synthesis and upconversion luminescence of N-doped graphene quantum dots

A hydrothermal approach was developed for the synthesis of N-doped graphene quantum dots (N-GQDs) by cutting N-doped graphene. The N-GQDs obtained have a N/C atomic ratio of ca. 5.6% and diameter of 1–7 nm. The photoluminescence (PL) properties of the N-GQDs were investigated. It was found that the N-GQDs possess bright blue PL and excellent upconversion PL properties.

Excellent microwave absorption property of Graphene-coated Fe nanocomposites

Graphene has evoked extensive interests for its abundant physical properties and potential applications. It is reported that the interfacial electronic interaction between metal and graphene would give rise to charge transfer and change the electronic properties of graphene, leading to some novel electrical and magnetic properties in metal-graphene heterostructure. In addition, large specific surface area, low density and high chemical stability make graphene act as an ideal coating material. Taking full advantage of the aforementioned features of graphene, we synthesized graphene-coated Fe nanocomposites for the first time and investigated their microwave absorption properties. Due to the charge transfer at Fe-graphene interface in Fe/G, the nanocomposites show distinct dielectric properties, which result in excellent microwave absorption performance in a wide frequency range. This work provides a novel approach for exploring high-performance microwave absorption material as well as expands the application field of graphene-based materials.

Preparation and magnetic property of Fe nanowire array

Fe was electrodeposited into the holes of porous anodic aluminium oxide (AAO) which was prepared electrochemically. X-ray diffraction (XRD) spectra illustrated that the deposited material was α-Fe. TEM observation showed that the aspect ratio of the nanowires was larger than 1000. The diameter of the wire was about 35 nm corresponding to that of the holes in the AAO. Magnetic measurements of the Fe nanowire array showed that its easy magnetization direction is perpendicular to the sample plane. This kind of nanowire array has potential applications in perpendicular magnetic recording.

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.

A novel Co/TiO2 nanocomposite derived from a metal–organic framework: synthesis and efficient microwave absorption

To overcome the shortcomings (poor impedance mismatching and weak electromagnetic wave attenuation) of the Co nanoparticles embedded into nanoporous carbon (Co@NPC) derived from the thermal decomposition of zeolitic imidazolate framework-67 (ZIF-67), two coated titanium oxide (TiO2) routes are designed to prepare core–shell Co@NPC@TiO2 and multi-interfaced yolk–shell C–ZIF-67@TiO2 (obtained from the thermal decomposition of ZIF-67@TiO2) structures. The permittivity and permeability of C–ZIF-67@TiO2 significantly depend on the thickness of the TiO2 shell in ZIF-67@TiO2, and the thickness of the TiO2 shell in the as-obtained samples can be easily controlled via changing the addition content of tetrabutyl titanate in the hydrolyzation process. The as-prepared samples have remarkable absorbing characteristics in wide frequency bands from 2–18 GHz with thicknesses of 1.0–5.0 mm. 50 wt% of the C–ZIF-67@TiO2-2 (the addition amount of tetrabutyl titanate is 2 mL) nanocomposite filled within paraffin shows a maximum reflection loss (RL) of −51.7 dB at an absorbing thickness of 1.65 mm, meanwhile, for the Co@NPC@TiO2-1.2 (the addition amount of tetrabutyl titanate is 1.2 mL) nanocomposite, a maximum RL can be achieved of −31.7 dB at 1.5 mm. This study provides a good reference for the future preparation of other carbon-based lightweight microwave absorbing materials derived from metal organic frameworks.