Study On The Modification Of Overall Properties And Related Mechanism Of Nd-Mg-Ni-Co Hydrogen Storage Alloys By Magnetic Annealing

In this thesis, based on a comprehensive review of the research and development of the La-Mg-Ni hydrogen storage alloys, the Nd-Mg-Ni-Co system hydrogen storage alloys have been selected and studied in order to optimize the overall electrochemical properties and to explore new method and mechanism to modify the rate properties. The effect of non-stoichiometry, magnetic annealing, annealing treatment and rapid solidification on the structural, electrochemical and magnetic properties of Nd-Mg-Ni-Co system alloys have been investigated by means of XRD, TEM, SEM, PCT, EIS and VSM analyses.The Nd0.75Mg0.25(Ni0.8Co0.2)3.8 hydrogen storage alloy has better overall electrochemical properties by non-stoichiometric optimization. The Ce2Ni7 type superstructure of alloys was characterized by TEM. The thermal dynamic parameters of hydrogen absorption and desorption have been obtained by modeling according to Van't Hoff equations for the Nd0.75Mg0.25(Ni0.8Co0.2)3.8 hydrogen storage alloys.The effect of temperature and magnetic field strength on the structural, electrochemical and magnetic properties have been studied systemically for Nd0.75Mg0.25(Ni0.8Co0.2)3.8 hydrogen storage alloys. The results show that magnetic annealing can improve the rate properties and hydrogen absorption rate largely. It is better to do magetic annealing for 3 hours at 10 T and 650 oC. The as-cast, magnetic annealing and annealing Nd0.75Mg0.25(Ni0.8Co0.2)3.8 hydrogen storage alloys have been studied for comparison. Rietveld analyses results show that magnetic annealing has little effect on the phase composition for the Nd0.75Mg0.25 (Ni0.8Co0.2)3.8 hydrogen storage alloys, the lattice parameter c of Ce2Ni7 type (Nd,Mg)2(Ni,Co)7 phase increases, but the lattice parameter a has little change, the size of tetrahedral interstitial increase accordingly, the results are identical with TEM characterization. The decrease of reaction resistance and the increases of polar current for the alloys have been obtained by the test of EIS and polar curves respectively. So the resistance of hydrogen diffusion in the alloys decreases. The dynamic property increases largely. The c-axis was elongated by uniaxial anisotropy along the c-axis in magnetic annealing. In addition, the results which magnetic moments is increased for (Nd,Mg)2(Ni,Co)7 phase by magnetic annealing are confirmed by the first principle calculation.The cycle stability has been improved by annealing for Nd0.75Mg0.25(Ni0.8 Co0.2)3.5 hydrogen storage alloys. The three elements which include stability of crystal structure, grain pulverization, and surface corrosion have been analyzed systemically during the electrochemical cycle. After annealing treatment, the rate of volume expansion during of hydrogenation, the grain pulverization and the inter stress of grain decreases, the element composition is more uniformly, the structural stability is improved, the corrosion of alloy element is restrained, so the cycle stability properties is improved.The cycle stability of Nd0.75Mg0.25(Ni0.8 Co0.2)3.8 hydrogen storage alloy electrode has been modified by rapid solidification. The speed of melt-spun is higher, the grain size is smaller or some amorphous grain is produced, the element composition is more uniformly, the inter grain stress increases. It exhibits good overall electrochemical properties when the speed of melt-spun is 20 m/s.