Zhou Huaiying

Microwave Absorbing Properties of Ni/Ferrite Mixture

Abstract The Sr-W type planar hexagonal ferrite was synthesized by a sol-gel method, and its morphology and phase structure were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The microwave absorbing properties of the mixtures with different ratios of Ni/ferrite were studied by vector network analyzer (VAZ). The results demonstrate that with the increase of hexagonal ferrite content, the microwave absorbing peaks of the mixtures shift to higher frequencies, and when the mixture consists of 20% Ni and 80% Sr-W-type ferrite, its microwave absorbing performance is the most advantageous. When absorbing coating thickness is 2 mm, the minimum reflectivity and absorbing peak frequency is −20.69 dB at 12.08 GHz and the bandwidth less than −10 dB reaches 4 GHz. The absorbing peak shifts to low-frequency with the increase of absorbing coating thickness, and when the coating thickness reaches a certain value, the two absorbing peaks appear in the mixture in the frequency range of 2∼18 GHz, which is of significance to explore the absorbing material with bandwidth effect.

Effect of Temperature on Structure and Electrode Properties of LaNi3.8Co0.6Mn0.3M0.3 Hydrogen Storage Alloys

Abstract A series of experiments have been performed to investigate the temperature effects on the structure and the electrochemical properties of LaNi3.8Co0.6Mn0.3M0.3 (M=Ni, Al, Cu) hydrogen storage alloys at different temperatures of 238, 273, 303 and 323 K. Samples A, B and C were used to represent LaNi4.1Co0.6Mn0.3 (Ni substituted alloy), LaNi3.8Co0.6Mn0.3Al0.3 (Al substituted alloy) and LaNi3.8Co0.6Mn0.3Cu0.3 (Cu substituted alloy), respectively. The structures and the electrochemical properties of A, B and C hydrogen storage alloys were investigated by XRD and simulated battery test, respectively. The results reveal that all of the alloys are composed of the homogeneous LaNi5 phase with a CaCu5-type hexagonal structure. The low-temperature properties of the Cu substituted alloy are improved, and the high-temperature discharge capacity of the Al substituted alloy is enhanced. Electrochemical impedance spectroscopy (EIS) analysis shows that the improvement of high-temperature discharge capacity of alloy electrode B is attributed to the formation of dense oxide film to protect the active material from corrosion, which is not favorable to charge-transfer reaction at the interface of electrode-electrolyte. The deterioration in the high-rate dischargeability (HRD) of alloy electrodes B and C are attributed to the degradation of electrochemical kinetics, including both charge-transfer reactions on the electrode surface and hydrogen diffusion in the bulk. Furthermore, the good low-temperature performance of alloy electrode C is due to the improvement of charge-transfer reaction.

Effects of Pr on the structure and magnetic properties of FePt alloys

Abstract The effects of Pr on the structure and magnetic properties of PrxFe60.5-xPt39.5 alloys (x = 0, 0.5, 1.0. and 1.5) were investigated. X-ray diffraction data indicated that the phase transition temperature of FePt based alloys from disordered face-centered-cubic to ordered face-centered-tetragonal cubic decreases with the increase in Pr concentration. Pr plays the role of a grain refiner and it can enhance the exchange coupling between soft magnetic phase and hard magnetic phase. The results indicate that the replacement of Fe by Pr can significantly improve the remanence and coercivity of the Fe60.5Pt39.5 alloy. These results can be explained on the basis of phase transformation and microstructure. Both the remanence ratio and coercivity of the FePt based alloy as a function of the Pr content are increased by the optimum addition of 0.5 at.% Pr.

Mechanical Properties of Nb-Ti-Ni Alloy Membranes for Hydrogen Permeation

Abstract The microstructure and the mechanical property of Nb-Ti-Ni alloy membranes for hydrogen permeation were investigated. XRD and SEM were employed to characterize the structures of Nb-Ti-Ni alloys, and the bending property of the alloy membranes was measured by three-point bending tests. The results indicate that the as-prepared alloy is mainly composed of two phases, i.e. the primary bcc-Nb(Ni, Ti) solid solution phase, and [bcc-Nb(Ni, Ti)+β2-NiTi] eutectic phase. The former acts as functional cell for hydrogen permeation and the later is structural cell to keep the stability of alloy membrane. Mechanical properties of Nb-Ti-Ni alloy membranes have been improved to a certain degree by magnetic heat-treatment (MHT) along the three directions (0°, 45° and 90°). An obvious improvement has been made in the case of N7 sample by MHT at both of 0° and 45°, and in the case of N5 sample by MHT at 90° angle. MHT can lead to the change of grain sizes and grain orientations. In the case of MHT at 0° angle, a tiny bar-like region (consisting of eutectic phase) is formed and spreads along the direction of membrane surface, while some oral-like (or spot-like) region is regularly arranged on the surface, and the region extends vertically to the surface in the case of MHT at 90° angle. It is suggested that suitable MHT is beneficial to the improvement of the bending property of Nb-Ti-Ni alloy membranes.

Phase Transformation and Magnetic Properties of NdxFe60.5-xPt39.5 (x = 0, 0.5, 1.0, 1.5) Alloys

Abstract The effect of Nd addition on the structure, phase transformation and magnetic properties of FePt based alloys was investigated. The results indicated that the transition temperature from ordered FCT to disordered FCC phase decreased with increasing Nd concentration, but for alloys quenched rapidly from the γ phase region into ice-water, it increased with increasing Nd. The Nd element not only effectively reduced the grain size of the ordered phase but also decreased the degree of the ordered phase and refined the grains of the FCC matrix phase. The remanence ratio and coercivity of the FePt based alloy as a function of the Nd content had maximum values, respectively.

Microwave Absorbing Properties of MnAl Alloy Powder

Abstract Mn55Al45 alloy powder was prepared by the combined use of arc melting, high energy ball milling and tempering heat treatment process. The phase structure and the microstructure of the alloy powder were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), and then their microwave absorbing properties were tested by a vector network analyzer. The results show that an increase of phases for Al2Mn3 and Al11Mn14 are accompanied by the increased milling time; the resonant frequency and the absorption peaks of ɛ″ and μ″ shift towards lower frequency region simultaneously. A better result of electromagnetic properties is brought to the alloy powder when the ball milling time is set to 18 h in first attempt. While the ball milling time is increased to 24 h during later experiment, the minimum reflectivity value reaches to −28 dB at a frequency of 16 GHz. The Al2Mn3 phase increases with an artificial increase of tempering temperature. The Mn55Al45 alloy powder ball milled for 18 h and tempered at 400 °C exhibits preferably comprehensive microwave absorbing properties in the range of 6∼8 GHz. The minimum reflectivity value and absorption peak frequency at the coating thickness of 2.0 mm are −26.4 dB, 17 GHz, respectively.

Phase Transformation and Magnetic Properties of Dy0.5Fe60Pt39.5 Alloy

Abstract The effects of the annealing time on the microstructure and magnetic properties of Dy0.5Fe60Pt39. 5 were investigated. X-ray diffraction and M-T data indicated that the disordered face-centered-cubic phase (soft phase) is not truly disordered phase due to an increase in the degree of short-range order with increasing of annealing time. Both the coercivity and remanence ratio of the FePt based alloy are as a function of the annealing time. The optimum heat treatment condition for the alloy is at annealing temperature of 600 °C for 5 h. These results of magnetic properties can be explained on the basis of phase transformation and microstructure of the alloy.