L. Zhen

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.

DSC analyses of the precipitation behavior of two Al-Mg-Si alloys naturally aged for different times

Abstract The influence of natural ageing on the overall subsequent precipitation processes of two Al–Mg–Si alloys has been studied by a series of differential scanning calorimeter (DSC) analyses. The shape of DSC thermograms changes distinctly with increasing natural ageing times: there are seven exothermic peaks in as-quenched condition, while, there are only three for specimens naturally aged for more than four weeks. The reason for the strong influence of natural ageing on precipitation behavior of the two Al–Mg–Si alloys is assumed to be the formation of clusters during natural ageing.

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 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.

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.

TEM observation of the alpha(2)/O interface in a Ti3Al-Nb alloy

A study of the microstructures of a Ti-24Al-14Nb-3V-0.5Mo (at%) alloy was carried out using X-ray diffraction (XRD), transmission electron microscopy (TEM), anc high-resolution electron microscopy (HREM). The results indicate that the O-phase is directly derived from the alpha(2)-phase and is distributed all over the primary alpha(2)-grains with fine plate-like form. Transformation of the alpha(2) to the O-phase is explained by a diffusional growth mechanism, and the interface between the alpha(2) and O-phases is completely coherent

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.

Microstructure evolution in hot deformation of 7050 aluminium alloy with coarse elongated grains

Abstract Hot deformation of 7050 aluminium alloy with coarse elongated grains has been investigated by tensile tests conducted at 340 and 460°C and the strain rate of 1˙0 × 10–4–1˙0 × 10–1 s–1. When the 7050 aluminium alloy was conducted at 460°C and 1˙0 × 10–2 s–1, the maximum elongation of 273% is achieved and large plastic deformation was carried out at an almost constant stress. The microstructure evolution under the deformation condition is characterised using OM, SEM, EBSD and TEM in detail. The results show that the microstructure evolution is realised by continuous dynamic recrystallisation in coarse grains. The increase in misorientation is proportional to the increase in the true strain, and the k value, the increasing rate of average misorientation angle, is 15˙7°. The primary hot deformation mechanisms of the 7050 aluminium alloy are localised grain boundary sliding and dislocation gliding, which can increase the grain boundary misorientation continuously.

Deformation and fracture behavior of a RSP Al–Li alloy

Abstract The deformation and fracture behaviors in a rapid solidification processed (RSP) Al–Li alloy have been studied by tensile tests, transmission electron microscope (TEM) and scanning electron microscope (SEM) observations. The results show that the interaction mechanism between dislocations and δ′ precipitates is shearing in underaged, peak aged and some overaged conditions. In the early stage of deformation (δ 5%) of the alloy, δ′ precipitates are cut into pieces by dislocations from different directions, suggesting that cross slip is predominant. In severely overaged conditions, the deformation is mainly concentrated on the precipitate-free zones (PFZs), and only smaller δ′ precipitates near PFZs are sheared. The fine grain size (2–3 μm) produced by rapid solidification cannot change the inherent intergranular fracture mode of Al–Li alloys. TEM observations show that the nominal intergranular fracture in the alloy is actually caused by the cracking of PFZs but not by the fracture of grain boundary. Therefore, it is suggested that in alloys with PFZs in their microstructure, PFZ is the main reason that results in the intergranular fracture, and the width of PFZ is not an important factor in controlling the fracture mode. Tensile tests show that the peak strength of the alloy does not correspond to the transition stage of dislocations’ shearing to bypassing of δ′ precipitates. The high volume fraction of PFZ and the small spacing of δ′ precipitates are thought to be reasons for such a phenomenon.