Structure And Properties Study On La(NiMMn)_5+x,Ti-Mo,Ti(Zr)/Mn₂ Base Laves Phase Hydrogen Storage Alloys

In the present work, over-stoichiometric alloys La(NiMMn)₅+x (M=Ni,Cu,Al,Fe,Sn), Ti-Mo solid solution alloys and Ti(Zr)/Mn based AB₂-type Laves phases were studied using both of neutron and x-ray powder diffraction techniques. And the crystal structure and the occupancy factor of deuterium have been determined, the influence of composition and phase transformation on the occupancy of deuterium has been analyzed, the relation of structure and hydrogen storage properties has been revealed. According to the experimental requirement, an absorption-desorption hydrogen (deuterium) device, a thermostatically controlled furnace and a null matrix sample cell for neutron diffraction, were designed and manufactured by ourselves. It is worth mentioning that it is first time to study the microstructures of deuteride domestically by the means of the method of null matrix neutron diffraction.Through the study of over-stoichiometric La(NiMMn)₅+x (M=Ni,Cu,Al,Fe,Sn) hydrogen storage alloys, a series of results were obtained as follows: The crystal structures of all hydrogen storage alloys are CaCu₅ type(Space Group: P6/mmm), but their occupancies are different from those of AB5 compounds with the CaCus structure. In AB₅-type alloys, atoms A occupy 1a site in P6/mmm space group, atoms B occupy 2c and 3g site; for the over-stoichiometric AB₅+X alloys, B atom dumbbells occupy site 2e in P6/mmm space group, orienting along the c-axis and replacing A atoms, moreover, the B atoms forming a hexagon in the z=0 plane around the dumbbell are no longer on site 2c, but shrink into position 6l. The occupation number of atoms B on site 2e and 6l goes up while that of atoms B on site 2c goes down with x increasing, 3g site is fully occupied by atomsB from beginning to end. The formation of the dumbbells induces the decrease of the a-axis length while the increase of the c-axis length, so that the unit cell volume also decreases ultimately. The crystal structure of over-stoichiometric alloys can be affected obviously by different preparing procedure, the main phenomenon is that the occupancy factor of atoms B on site 2c, 2e and 6/ in basal plane is quite different. By annealing, a-axis can be decreased, whereas c-axis can be increased, ultimately maximum hydrogen capacity increase and hysteresis reduce. The impact of Ni substitution by different elements in over-stoichiometric alloys is different in electrode activation properties, maximum discharge capability and charge-discharge cycling stability, but all of them can reduce hysteresis and decrease plateau pressure.Studying seven Ti-x at%Mo binary alloys (x = 5,10,15,20,25,33,40), we found that the Ti-Mo alloy is composed of a and p phases when Mo content is 5at%; Ti-Mo alloys form p single phase at room temperature when Mo content is not smaller than 10at%, and the lattice constant decreases linearly with increasing Mo contents; Deuterium atoms in Ti-Mo saturated deuteride distribute in order, which changes the symmetry oflattice, i.e. the space group changes from Fm3 m to P 4 3m, deuterium atoms occupy la, lb,3c and 3d sites; The structures of a null matrix Ti-33at%Mo deuteride are studied by neutron diffraction, the results reveal that the occupancy factor of la (0,0,0) site and lb (1/2,1/2,1/2) influence on the intensities of two satellite peaks (100) and (110); By milling the deuteride, the deuterium atoms can be partially desorbed, and the ordering of deuterium atoms can be eliminated; The insertion of deuterium atoms increases the lattice defect and microstrain.All of sttfdied hydrogen storage alloys of Ti(Zr)/Mn base AB2 Laves phase are C14 type. A obvious noncrystal phenomenon can be discovered in Zro.9Tio.iMnCr alloy after absorbing deuterium, the more deuterium content, the moreobvious, but the crystal phase is always C14-type Laves structure in process of absorption/desorption deuterium (hydrogen) cycles. The space group is P63/mmc, where four Ti(Zr) atoms, two Mn(Cr) atoms, and six Mn(Cr) atoms, are located on the 4f site, 2a site , and 6h site, respectively, and deuterium atoms occupy 6hi, 6h2, 12k and 24/ tetrahedral interstices. The occupancy factors of deuterium atoms are different at different absorption-desorption deuterium stages; the crystal constant a, c and unit cell volume V increase with deuterium content increasing, but the lattice constant of de-deuterium state is larger than that of absorbing deuterium state even the same deuterium concentration. The effects of the Ti/Zr ratio and Mn/Cr ratio on the hydrogen storage properties of Ti(Zr)/Mn2 base Laves phase alloys are obvious: With the decrease of the ratio of Ti/Zr, the hydrogen capacity increases, the plateau pressure decreases, and plateau of P'-C-T curve becomes steeper; With the decrease of the Mn/Cr ratio, the plateau pressure decreases, hysteresis becomes weaker and hydrogen storage capacity decreases.