Ti-Zr-Ni (-Pd) Icosahedral Quasicrystal Formation And Storage Deuterium

Bergman-type Ti-Zr-Ni quasicrystals can absorb hydrogen rapidly in a large quantity, and have been regarded as a highly promising hydrogen storage material, which are unveiling their potential applications in fields of hydrogen energy or deuterium-tritium nuclear fusion energy. However, their potential applications have been restricted due to the reported large differences in hydrogen storage quantity in literature for various Ti-Zr-Ni quasicrystals and inferior hydrogen desorption kinetics. Palladium, as an important alloying element for quasicrystal formation, possesses superior hydrogen absorption/desorption kinetics. Therefore, this thesis aims to study the effect of Pd addition on the structures and deuterium storage/release properties of Ti-Zr-Ni quasicrystals.Firstly, the formation characteristics of Ti-Zr-Ni-Pd quasicrystals prepared by suction-casting method were investigated by using X-ray diffraction (XRD), Optical Microscopy (OM), Transition Electron Microscopy (TEM), Differential Scanning Calorimeter (DSC) apparatus. The effects of Pd addition on quasicrystal formation, atomic structure, electronic structure and thermodynamic stability were also evaluated. Secondly, the deuteration behaviors before/after Pd addition of Ti-Zr-Ni quasicrystalline alloys were studied by using home-made solid-gas reaction systems, Temperature Programmed Desorption (TPD) and X-ray Photoelectron Spectroscopy (XPS) techniques. Besides, the common features of hydrogen storage for Ti-Zr-Ni based quasicrystals were discussed.The results are concluded as follows:1) The quasicrystal forming abilities of Ti4oZr4oNi2o, Ti45Zr38Ni17 and Ti41.5Zr41.5Ni17 are close related to the cooling rate during preparation. Directly cooling the alloy melt led to the formation of a MgZn2-typed C14 Laves phase (abbr. C14), while suction casting it resulted in the formation of a main phase of icosahedral quasicrystal(IQC) together with the presence of a few hcp a-Ti, bcc (3-(Ti, Zr) and C14 phases. Moderate Pd substitution for Ti/Zr conduced to the formation of a single IQC phase in the Ti4oZr4oNi2o and Ti45Zr38Ni17 alloys. In the former alloy, the substitution quantity is 2 at.%-4 at.% for Ti and 2 at.% for Zr, while it is 4 at.%-6 at.% for Ti and 4at.% for Zr in the latter. Ti is more prone to be substituted than Zr, which should be owing to that Ti possesses the comparable atom size with that of Pd.2) Pd addition affects the close packness of atoms, electronic structure and thermal stability of Ti-Zr-Ni quasicrystals. Possessing a moderate atom size, Pd can increase the topological close packness of atoms, which makes the alloys tend to form a topologically densely-packed phase having higher coordination number and the deviation of the electronic structure from the Hume-Rothery rule for quasicrystal formation. The substitution of Pd for Ti caused Ti-Zr-Ni quasicrystals less stable and resulted in a decrease of 300℃for the initial phase transformation temperature, which could be attributed to easier diffusion of Pd with a smaller atom size.3) At room temperature, double-phase alloys Ti40Zr40Ni20, Ti41.5Zr41.5Ni17 and Ti45Zr38Ni17 can load deuterium up to 11.6 mmol/g,12.9 mmol/g and 12.7 mmol/g, respectively. The first value is 2 times larger than that in literature while the second is about 65% of the result reported in literature. For Ti36Zr4oNi2oPd4 and Ti39Zr38Ni17Pd6 containing a pure IQC phase, the saturated deuterium concentration is 11.0 mmol/g (-4.4wt%), which corresponds to the deuterium/metal ratio D/M of 1.56, close to the number ratio between the interstices and the metal atoms in Bergman-type cluster. Thus, it can be deduced that ideal Bergman-type Ti-Zr-Ni-based quasicrystals must possess almost the same deuterium (hydrogen)-absorbing ability～11.0mmol/g (corresponding to 2.2 wt.%). The difference in the amount of deuterium (or hydrogen) storage must lie in the variation in structure defects in the quasicrystal alloys.4) TiD2 and ZrD2 phases were not observed in the Ti-Zr-Ni(-Pd) quasicrystal after deuterium absorption. Most of the deuterium atoms are located in the tetragonal interstice. A dissolution of about 4.4 wt.% of deuterium caused the expansion of quasicrystal lattice of less than 7% without phase transformation, which reflects the structural stability of quasicrystal during hydrogenation. The chemical shift due to deuterium dissolution reveals that deuterium atoms are preferentially located near Zr and Ti, which is related to their strong chemical affinities.5) The characteristics of rapid absorption and difficult desorption for Ti-Zr-Ni based quasicrystals were confirmed in the study. The hydrogen absorption obeys the law of 1-stage reaction. Rapid absorption should be due to the low activation energy for deuterium diffusion in the quasicrystals. Pd addition improved the kinetic property of deuterium absorption for Ti-Zr-Ni quasicrystals, resulting in that the deuterium absorption velocity constant for Ti36Zr40Ni20Pd4 (0.03s-1) is almost two times of that for Ti40Zr40Ni20. The difficulty in deuterium desorption should be related to the shrink of the quasicrystal lattice.6) Preliminary results show similar deuterium-release equilibrium pressure between Ti-Zr-Ni-Pd quasicrystal and U/Ti, revealing the high thermo-stability of deuterium in the quasicrystals. In addition, due to the negative hydrogen isotope effect of Ti-Zr-Ni-Pd quasicrystals with H-D separate factor of about 0.8, higher thermodynamic stability of tritium in quasicrystals than that of hydrogen/deuterium can be expected.