W.Z. Shao

Microwave absorption properties of FeNi3 submicrometre spheres and SiO2@FeNi3 core–shell structures

Nearly monodispersed FeNi3 submicrometre spheres with an average diameter of 220 nm were synthesized by a simple low temperature reduction method. SiO2@FeNi3 core–shell structured submicrometre spheres with 25 nm thick SiO2 shell were then fabricated by a sol–gel process. A significant enhancement of electromagnetic absorption (EMA) performance was achieved by the silica coating over the 2–18 GHz. The reflection loss (RL) exceeding −20 dB of the composite was obtained over 6.7–15.1 GHz by choosing an appropriate sample thickness between 2.1 and 3.3 mm, and an optimal RL of −61.3 dB was obtained at 8.7 GHz with a thin absorber thickness of 2.9 mm. The coating of the dielectric silica shell significantly enhanced the EMA performance due to the enhancement of interface polarization at the alloys and dielectric interfaces.

Resonance-antiresonance electromagnetic behavior in a disordered dielectric composite

In the present study, a kind of SiCw/wax composite was prepared and its electromagnetic properties were studied experimentally. It is found that Mie resonance can occur in this composite in spite of its disordered structure. The Mie resonance is believed to lead to the resonant-antiresonant electromagnetic behavior accompanied by a negative e″ in the SiCw/wax composite in the 2–18GHz band range. The resulted magnetic behavior in the current composite is believed to be weakened by the random orientation of the whiskers.

A study on graphitization of diamond in copper-diamond composite materials

Abstract The graphitization of diamond particles in the copper–diamond composite materials used for low voltage electro-contacts was investigated by SEM, TEM and X-ray diffraction (XRD) analyses. The results show that although diamond is a metastable allotropic modification of carbon, the diamond particles are not graphitized to an apparent extent after sintering at 1150–1220 K in the copper–diamond composite. The reason for this phenomenon was discussed. The diamond powders prepared by explosion synthetic method are supposed to possess higher resistance to graphitization and thus benefit the performance of the copper–diamond composite used for electro-contacts.

Electromagnetic properties of FeNi alloy nanoparticles prepared by hydrogen-thermal reduction method

FeNi alloy nanoparticles were prepared by hydrogen-thermal reduction in nickel ferrite nanoparticles at 400 °C for 1 h. FeNi alloy nanoparticles with average particle size of 150 nm in diameter are composed of Fe, Ni, and FeNi solid solution. The effective complex permittivity and complex permeability of FeNi alloy nanoparticles in the frequency range of 2–18 GHz were measured, and the microwave reflection loss of FeNi/wax with different assumed coating thicknesses was calculated according to the transmission line theory. The hydrogen-thermal reduced FeNi alloy nanoparticles were found to possess high permeability and superior microwave absorption performance in the microwave band.

Synthesis and microwave electromagnetic properties of CoFe alloy nanoflakes prepared with hydrogen-thermal reduction method

CoFe alloy nanoflakes (NFs) with diameter and thickness on nanoscale were prepared by hydrogen-thermal reduction in CoFe2O4 flakes at 400 °C for 60 min. The effective complex permittivity and permeability of CoFe alloy NFs/paraffin composites were measured and compared with that of CoFe alloy nanoparticles (NPs)/paraffin composites. Due to the two-dimensional shape character, the real part of permittivity and permeability of CoFe alloy NFs was rather higher than that of CoFe alloy NPs. Electromagnetic wave absorbing (EMA) performance of both CoFe alloy NFs and NPs was evaluated by using transmission line theory. The effective EMA band position of the coating with CoFe alloy NFs as fillers was found to locate in the range of 2–4 GHz, while the effective EMA band position of the coating containing CoFe alloy NPs as fillers was located in the range 8–18 GHz. A maximum reflection loss (RLmax) of −57.8 dB was achieved in a coating containing CoFe alloy NFs as fillers, which is much higher than the −16.6 dB of ...

Giant electrocaloric effect in BaZr0.2Ti0.8O3 thick film

We report the giant electrocaloric effect (ECE) of BaZr0.2Ti0.8O3 (BZT) thick film near room temperature. The BZT thick film was fabricated by the tape casting method with the thickness of 12.0 μm. Due to the near invariant critical point composition, relaxor behavior, and the stress generated between the film and the substrate, the thick film exhibits a large adiabatic temperature drop ΔT = −7 °C under 19.5 MV/m electric field, large EC coefficient ΔT/ΔE = 0.50 × 10−6 K · m · V−1, ΔS/ΔE = 0.88 × 10−6 J · m · kg−1 · K−1 · V−1 over a wide temperature range near room temperature, where ΔS is the isothermal entropy change and ΔE is the applied field. These high EC properties and possibility of fabrication of the EC ceramics into multilayer ceramic capacitor configuration provide solution for the application of the EC material for practical cooling device applications.

Synthesis of CoFe/Al2O3 composite nanoparticles as the impedance matching layer of wideband multilayer absorber

CoFe/Al2O3 composite nanoparticles were successfully prepared by hydrogen-thermally reducing cobalt aluminum ferrite. Compared with CoFe alloy nanoparticles, the permeability of CoFe/Al2O3 composite nanoparticles was remarkably enhanced and an improved impedance characteristic was achieved due to the introduction of insulated Al2O3. A multilayer absorber with CoFe/Al2O3 composite nanoparticles as the impedance matching layer and CoFe nanoflake as the dissipation layer was designed by using genetic algorithm, in which an ultrawide operation frequency bandwidth over 2.5–18 GHz was obtained. The microwave absorption performance in both normal and oblique incident case was evaluated by using electromagnetic simulator. The backward radar cross-section (RCS) was decreased at least 10 dB over a wide frequency range by covering the multilayer absorber on the surface of perfect electrical conductive plate.

Artificial Ground Planes for Performance Enhancement of Microstrip Antennas

In this work we investigate three different artificial ground planes for microstrip antenna design, which are useful for different applications. The three different ground plane configurations studied include: (i) conventional dielectric substrate backed by a perfect conductor; (ii) a loop type of artificial magnetic conductor (AMC); and a spiral-type of AMC. Both the loop and spiral types of AMCs tend to lower the resonant frequency of the microstrip patch antenna (MPA), as compared to that of the dielectric substrate, for the linearly polarized case. However, the spiral AMC can be used to realize circular polarization as well, at a different frequency band, hence, is useful for designing dual-band dual-polarization microstrip antennas.

The influence of Fe content on the magnetic and electromagnetic characteristics for Fex(CoNi)1−x ternary alloy nanoparticles

Fex(CoNi)1−x (x = 0.14, 0.20, 0.25) ternary alloy nanoparticles were fabricated by a self-catalyzed reduction method at a low temperature. The investigation of static magnetic properties revealed both saturation magnetization and coercive force enhanced with increasing Fe content. By dispersing alloy nanoparticles into paraffin matrix homogeneously, the electromagnetic properties of them were investigated experimentally and the electromagnetic absorption performances were calculated according to transmission line theory. Significant dielectric relaxation was found in the low Fe content sample, which is dominant in dielectric loss. The magnetic loss was attributed to natural resonance and the resonance peaks’ shift to high frequency region with increasing Fecontent. The enhanced electromagnetic absorption performances were obtained by adjusting Fe content to balance electromagnetic parameters, because the electromagnetic parameters can be varied by structural and magnetic properties.

Study of γ-ray irradiation effect on permanent magnets

Irradiation damage of FeCrCo, AlNiCo, SmCo, and NdFeB permanent magnets was investigated by using γ-ray irradiation. Results of magnetic property measurement show that FeCrCo and NdFeB have more demagnetization than AlNiCo and SmCo. X-ray diffraction analysis shows that γ-ray irradiation leads to the increase of FeCrCo crystal mismatch and decrease of AlNiCo ordering degree. The investigations by positron annihilation spectroscopy technique show that the defects in AlNiCo and SmCo increase after γ-ray irradiation. The defects induced by γ-ray irradiation in NdFeB magnets are not the main source that leads to partial demagnetization. The irradiation resistances of these magnets are discussed in detail considering the thermal stability, coercivity mechanisms, and defect damage.