Chuangui Jin

Enhanced microwave absorption of flower-like FeNi@C nanocomposites by dual dielectric relaxation and multiple magnetic resonance

Flower-like FeNi@C nanocomposites self-assembled by FeNi nanosheets and flocculent carbon were synthesized by the hydrothermal method. The electromagnetic parameters of FeNi@C–paraffin composites were measured at 0.03–18 GHz. Dual dielectric relaxations were observed in the composite system due to the cooperative consequence of the FeNi–C interfaces and dielectric carbon. The strong magnetic loss is from the coexistence of natural resonance and exchange resonance. For the 2.0 mm thickness layer, an optimal reflection loss (RL) of −46.7 dB is observed at 3.17 GHz, whereas the absorbent with a thickness of 1.3 mm has an RL of −32.78 dB at 13.78 GHz. The excellent microwave absorption abilities result from the synergy of dielectric and magnetic losses and quarter-wavelength cancellation.

Magnetic properties, exchange coupling and novel stripe domains in bulk SrFe12O19/(Ni,Zn)Fe2O4 composites

Bulk SrFe12O19/(Ni0.4Zn0.6)Fe2O4 composite ferrites with mass ratios Rm = 2 : 1, 1 : 1 and 1 : 2 were prepared using nanopowders obtained via a hydrothermal method, and their phase composition, magnetic properties, exchange coupling (EC) and magnetic microstructures were systematically investigated. It is found that all the bulk specimens sintered at either 700 or 900 °C are composed of two phases but exhibit typical single-phase magnetic behaviours, indicating the existence of EC between the magnetically hard and soft phases. However, too much (Rm = 2 : 1) or too little (Rm = 1 : 2) soft (Ni0.4Zn0.6)Fe2O4 phase weakens the EC in the composites. It is also proved that except for the EC, the strengthening of chemical polarization of the internal chemical bonds affects saturation magnetization, and the size of nanoscale grains significantly influences the EC and magnetic properties. In addition, novel stripe domains are found in all of the bulk composite specimens, which could be ascribed to the magnetization of the soft (Ni,Zn)Fe2O4 phase induced by the hard SrFe12O19 phase.

Facile hydrothermal synthesis of core/shell-like composite SrFe12O19/(Ni, Zn)Fe2O4 nanopowders and their magnetic properties

SrFe12O19/(Ni, Zn)Fe2O4 nanopowders with exchange coupling were synthesized via a facile hydrothermal method, and their structural and magnetic properties were studied. It is found that only when the mass ratio of SrFe12O19 to (Ni, Zn)Fe2O4 is 2 : 1, the specimen exhibits a two-phase composite structure. The synthesized composites are quite stable under high temperatures lower than 950 °C. It is also found that the nanocomposites exhibit a core/shell-like structure, and both the saturation magnetization and coercivity are greatly influenced by the content of soft phase and impurity.

A facile route to carbon-coated vanadium carbide nanocapsules as microwave absorbers

Vanadium carbide (VC) nanocapsules with carbon shells have been synthesized via an arc discharge process. The nanocapsules are spherical in shape with a diameter distribution of 10–40 nm. Reflection losses exceeding −20 dB can be realized in any interval within the 6.6–18 GHz range by choosing an appropriate thickness of the absorbent layer between 1.4 and 3.5 mm. An optimal absorption value of VC/C nanocapsules with a thickness of 2.2 mm is −45.94 dB at 10.8 GHz. The electromagnetic-wave absorption properties of the nanocomposite are illustrated in detail by means of an absorption-tube-map. The VC nanocapsules are attractive candidates for electromagnetic-wave absorption.

Synthesis, structure and magnetic properties of hexagonal BaFe12O19 ferrite obtained via a hydrothermal method

Hexagonal M-type BaFe12O19 ferrites were synthesized via a hydrothermal method with Ba/Fe ratio from 1:12 to 1:7 in the raw materials, and their structure and magnetic properties are studied. When the Ba/Fe ratio is smaller than 1:9, the impurity α-Fe2O3 and BaFe2O4 can be found in the as-synthesized specimens, and it seems hard to avoid the formation of impurity BaCO3 during a hydrothermal process. The grain size of as-synthesized specimens decreases obviously with the increase of Ba/Fe ratio. The phase composition and morphology affect the magnetic properties of specimens markedly. In addition, it is found that the acid washing can improve the magnetic properties of specimens.

Synthesis, characterization and microwave absorption of carbon-coated Cu nanocapsules

The microstructure and microwave absorption of carbon-coated Cu nanocapsules have been investigated. Carbon-coated Cu nanocapsules have been synthesized by an arc-discharge method. The paraffin-Cu/C nanocapsules composite shows excellent electromagnetic (EM) absorption properties. An optimal reflection loss (RL) value of –40.0 dB is reached at 10.52 GHz for a layer 1.9 mm thickness. RL exceeding –20 dB can be realized in any interval within the 1-18 GHz range by choosing an appropriate thickness of the absorbent layer between 1.1 and 10.0 mm. Theoretical simulation for the microwave absorption using the transmission line theory agrees reasonably well with the experimental results. The EM-wave absorption properties of nanocapsules materials are illustrated by means of an absorption-tube-map. The carbon-coated Cu nanocapsule is an attractive candidate for EM-wave absorption, which significantly enriches the family of EM-wave nanoabsorbents.

Hydrothermal hexagonal SrFe12O19 ferrite powders: Phase composition, microstructure and acid washing

A series of hexagonal m-type SrFe12O19 ferrite powders were hydrothermally synthesized, and their phase composition, microstructure and magnetic properties before/after acid washing were studied. In the synthesis of these specimens, the atomic ratio of Fe/Sr (RF/S) in starting materials was set to 4, 5 and 12, respectively. When RF/S = 12, the specimen has morphology of round flat cakes, not typical hexagonal plate-like structure. The results of SEM images and XRD patterns indicate that the specimen with RF/S = 12 was mostly composed of Fe2O3. When RF/S = 4 or 5, the hexagonal plate-like SrFe12O19 ferrite powders were successfully synthesized with only a small quantity of Fe2O3 and SrCO3 impurities. It is also found that acid washing can eliminate the impurities in as-synthesized specimens effectively, and also change their topography, which enhances the saturation magnetization. However, the coercivity changed irregularly after acid washing, which is ascribed to the combination of the changed morphology, introduced stress and lattice defects.

Giant Reversible Magnetocaloric Effect in Flower-like b-Co(OH)2 Hierarchical Superstructures Self-assembled by Nanosheets

A facile hydrothermal strategy is proposed to synthesize flower-like β-Co(OH)2 hierarchical microspherical superstructures with a diameter of 0.5-1.5 μm, which are self-assembled by β-Co(OH)2 nanosheets with the average thickness ranging between 20 and 40 nm. The magnetocaloric effect associated with magnetic phase transitions in Co(OH)2 superstructures has been investigated. A sign change in the magnetocaloric effect is induced by a magnetic field, which is related to a filed-induced transition from the antiferromagnetic to the ferromagnetic state below the Neel temperature. The large reversible magnetic-entropy change -ΔSm (13.4 J/kg K at 15 K for a field change of 5 T) indicates that flower-like Co(OH)2 superstructures is a potential candidate for application in magnetic refrigeration in the low-temperature range.

Synthesis, characterization and microwave dielectric properties of flower - like Co(OH)2/C nanocomposites

. Up to now, with the development of local electronic devices, microwave communication, and the rising pollution of electromagnetic (EM) interference, materials with EM-wave absorption in a wide frequency range, with strong absorption, low density, and low cost are more and more desirable. Among these nanocomposites, nanocapsules with magnetic nanoparticles as cores and dielectric shells are of great interest due to the synergetic effect between magnetic and dielectric losses. Recently, the EM-wave absorption properties of non-magnetic material-based nanocomposites have been driving considerable theoretical and experimental investigation. Wang et al synthesized the carbon coated Sn nanorods by arc discharge method and reported that reflection loss (RL) exceeding -20 dB could be realized in any interval within the 2-18 GHz range by choosing an appropriate thickness of the absorbent layer between 1.5 and 9 mm

Li-Cr substituted nickel-zinc-copper ferrite powders: structural and magnetic properties

Abstract Li–Cr substituted NiZnCu ferrite powder specimens with the nominal composition Ni0.25–2xLixCrxCu0.15Zn0.6Fe2O4 (x=0.00, 0.01, 0.02, 0.03, 0.04) were prepared primarily via chemical coprecipitation. X-ray diffractometry, scanning electron microscopy and vibrating sample magnetometry were used to study their structural or magnetic properties. It is found that all the specimens exhibit a single phase after annealing. The variation tendency of saturation magnetization with x exhibts a strange variation tendency and can be attributed to the different site occupation of Li+ions. It is also found that suitable Li–Cr substitutions help decrease the annealing temperature.