The activation, hydrogen storage and thermodynamic properties of Ti-Zr-Mn-Cr alloys were systematically studied. The structures of the alloys and hydrides/deuterides were also investigated by X-ray diffraction, neutron diffraction technologies and Rietveld refinements. The structural parameters and the occupancies of metal atoms and H/D atoms were analyzed. The H/D distribution mechanism in the alloys was discussed. A method of spontaneous growth of Mn nanowhiskers from hydrogen activated Laves phase alloys was established. The images, structure and growth mechanism of Mn nanowiskers were investigated by TEM experiments.
The crystal structures of all the Ti1-xZrx(Mn1-yCry)2 alloys were C14-type Laves phase (space group P63/mmc). The lattice parameters of the unit-cell increased isotropically and the associated volume cells expanded proportionally with the increase of Zr/Cr contents in the alloys. The introduction of Zr induced microstrains in the alloys.
A hydrogenator was designed and constructed based on the consideration of its performance, operation and maintenance. The hydrogenator could not only be used in the measurements of the conventional p-c-T curves, but also in the neutron diffraction for in-situ hydrogenation measurements. The null-matrix Ti-Zr alloy sample cell was used to enhance the quality of diffraction results of metal hydrides/deuterides.
The substitution of Ti by Zr resulted in the properties variations for the Ti1-xZrx(Mn0.5Cr0.5)2 (x = 0 ~ 1.0) alloys as follows: (1) the activation became easier and most of the alloys reacted with hydrogen under conditions of room temperature and 3 MPa; (2) absorption/desorption plateau decreased monotonously and the slope of the plateau increased firstly, then decreased, forming a maximum slope at x = 0.5; (3) the hysteresis of the absorption/desorption curves reduced to a minimum at x = 0.32, and then raised again at higher x. (4) the hydrogen absorption capacity increased, but the corresponding desorption decreased at different scales, the alloy at x = 0.32 had the maximum reversible capacity. (5) the average value of H increased linearly, and the average value of S decreased slightly, all the H values in the Ti1-xZrx(Mn0.5Cr0.5)2-H2 system were considerably lower than those in most Laves phase hydride systems.
The activation and hydrogen storage properties of Ti0.68Zr0.32(Mn1-yCry)2 (y = 0.1 ~ 0.9) alloys were studied. With the increase of Cr contents, the properties of alloys varied as follows: (1) the hydrogen pressures of activation decreased, the maximum pressure was 8 MPa; (2) absorption/desorption plateau decreased and the slope of the plateau decreased firstly, then increased, forming a minimum slope at y = 0.5; (3) the hysteresis of the absorption/desorption curves reduced drastically; (4) the hydrogen absorption capacity increased，but the corresponding desorption decreased, inducing a maximum reversible capacity at y = 0.5.
The Ti0.68Zr0.32(Mn0.5Cr0.5)2 alloy exhibited the best hydrogen storage properties with a reversible hydrogen capacity of 216ml/g, good plateau characteristics, low hysteresis and low enthalpy.
The distribution of H/D atoms in the Ti0.1Zr0.9(Mn0.5Cr0.5)2 alloy was investigated. The H/D occupied the A2B2 tetrahedral interstices. The occupancies of H/D atoms in the 6h1 and 12k sites were higher than those in the 6h2 and 24l sites. The H/D atoms diffused in the interstices surrounding A atoms, formed a 3-dimensional infinite network which extended along the c-axis of the Laves phase structure. The D atoms showed two phases, one was crystallized, and the other was non-crystallized or amorphous induced by the “short-range order distribution”.
Mn nanowhiskers were segregated from the hydrogen activated Ti1-xZrx(Mn0.5Cr0.5)2 (x = 0.5～1.0) alloys after a period of exposure in air. The shapes of the whiskers were nanorod and nanowire. The whiskers had a new crystal structure, with a primitive hexagonal unit cell of a = 1.05 nm, c = 0.31 nm. It was suggested that the oxygen and trapped hydrogen in the alloys induced a frictional compressive force to extrude Mn nanawhiskers out of the alloys.
修改评论