Lianxi Hu

Hydrogen desorption behaviour and microstructure evolution of a γ-AlH3/MgCl2 nano-composite during dehydriding

A γ-AlH3/MgCl2 nano-composite, without or with Zn and Zr doping, was synthesized by solid state reaction milling using MgH2 and AlCl3 as reagents. The hydrogen desorption behaviour of the nano-composite was investigated by temperature programmed de-hydriding (TPD) and differential scanning calorimetry (DSC) tests, and the microstructure evolution due to de-hydriding was characterized by XRD, SEM, and TEM respectively. For de-hydriding by heating from 40 to 320 °C, a three-stage featured TPD curve was observed, with the maximum hydrogen desorption capacity excluding the MgCl2 in the composite achieving about 9.71 wt% when the temperature was raised to 240 °C and then remaining unchanged for the subsequent heating process to 320 °C. DSC and XRD tests revealed that three individual events, i.e., the transformation of γ-AlH3 to α-AlH3 phase, the direct decomposition of γ-AlH3 phase, and the subsequent decomposition of the α-AlH3 phase, take place during the de-hydriding process of the γ-AlH3/MgCl2 nano-composite. By doping the nano-composite with elements Zn and Zr, an improvement in the de-hydriding kinetics was observed. According to the calculation based on the fitting of de-hydriding kinetics data, it was found that the addition of Zn and Zr can reduce the activation energy for the de-hydriding reaction. Also, both the de-hydriding mechanism and the role that Zn and Zr play in the de-hydriding process were interpreted based on TEM observations.

GRAIN TEXTURE, Nd CONTENT AND PROCESSING CONDITION EFFECTS ON MAGNETIC PROPERTIES OF NdFeB COMPOSITE MAGNETS

The effects of grain texture, Nd content, processing conditions on magnetic properties and behavior of NdFeB nanocomposite magnet were investigated. The results demonstrate that mechanically activated disproportionation and desorption–recombination combined with plastic deformation process (M-HDDR & D) is an effective way to produce anisotropic NdFeB-type magnet. Magnetic anisotropy significantly enhances the magnetic properties of Nd16Fe76B8 nanocomposite magnet, with the intrinsic coercivity (Hci), magnetic induction (Br), and the energy product (BH)max achieving 0.88 T, 676.8 kA/m, and 135.2 kJ/m3, respectively. Decrease in Nd content improves the remanence but deteriorates the coercivity of NdFeB ternary composites. A low annealing temperature or short annealing time leads to an incomplete recombination, while a high temperature or long annealing time may result in grain overgrowth, and the optimum processing condition is found to be 780°C × 30 min. Pinning mechanism dominates in the magnetic hardenin...

EFFECTS OF INITIAL INHOMOGENEOUS DENSITY DISTRIBUTION AND VOID SHAPE ON POWDER FORGING OF POROUS METAL

The deformation behavior during axisymmetric upsetting of sintered metals has been studied based on the finite-element method. The investigation on the effects of the initial density distribution, void shape and die friction on the density distribution and punch force during deformation have been conducted. It was found that under low-friction conditions, the initial density distribution affects the deformation geometry and the density distribution. However, the effect of the initial density distribution was found to be negligible under high-friction conditions. The initial density distribution did not affect the punch force or the average density, regardless of the friction conditions. When the force is perpendicular to semi-major axis of elliptical void, it is not only good for densification but also decrease the punch force in forging of porous metal.