Abstract
This is the first systemized study on the design, melting, casting, activation and kinetics of hydrogen absorption of 4- to 7/8-multicomponent equal-mole hydrogen-storage alloys that are based on the composition of TiVFe with the addition of Zr, Co, Mn, Cr, and Ni in various combinations. As-cast alloys were activated under the available experiment condition in laboratory, i.e., 40 atm and 400℃ for 2 h. All that can significantly absorb hydrogen in kinetic experiment were carried out for PCT isotherms at room temperature (RT) and 80℃. The fourth component element, which is one of Zr, Cr and Mn, that was added in TiVFe to become TiVFeZr, TiVFeCr, and TiVFeMn alloys, only TiVFeZr can absorb hydrogen significantly. In comparison, quaternary TiVFeCr and TiVFeMn alloys cannot absorb hydrogen. On further adding the fifth to eighth element in sequence of Co, Mn, Zr, and Ni in TiVFeCr to become quinternary to octonary equal-mole alloys, only the as-cast 7-multicomponent Zr-containing alloy can absorb hydrogen. However, the as-cast octonary equal-mole alloy cannot. This obviously shows that the key function of Zr in this series of as-cast alloy on the hydrogen absorption. The aforementioned as-cast TiVFeZr and TiVFeMn-ZrCrCo alloys that can absorb hydrogen are designated as 4E and 7E alloys, respectively. In order to differentiate the absorbable from the unabsorbable, all those can absorb hydrogen in this study are designated with notation as nE-m, where n and m are number of components and different kind of alloys with same number of component, respectively. Thus adding Co and Mn to 4E becomes 5E-1and 5E-2, respectively. In turn, adding Cr and Mn to 5E-1 becomes 6E-1 and 6E-2, while adding Co, Cr, and Ni to 5E-2 becomes 6E-2, 6E-3, and 6E-4, respectively. At last, alloy formation by adding Mn in 6E-1 results in 7E alloy. Again, from the change in element adding sequence one can easily see the key function of Zr on hydrogen absorption. After the RT-PCT experiment, one can obtain the maximal RT H-capacities (H/M) for 4E, 5E-1, 5E-2, 6E-1, 6E-2, 6E-3, 6E-4, and 7E as-cast alloys are 1.12, 1.21, 1.07, 0.80, 0.79, 1.18, 1.01, and 0.84, respectively. By the calculation of averaging the formation enthalpies between hydrogen and each component element in each alloy, one can obtain enthalpies of 4E, 5E-1, 5E-2, 6E-1, 6E-2, 6E-3, 6E-4, and 7E are -87.6, -69.9, -70.7, -56.8, -60.7, -64.6, -65.0, and -62.3 kJ/mol, respectively. It shows roughly that the more negative the average formation enthalpies, the more the H/M ratio for alloys. On further calculation in averaging the absolute values of difference between two atomic radii in alloys, which manifests the degree of mismatch strain in lattice, one can explain why the low enthalpy of -64.6 kJ/mol for 6E-3 can have high H/M since it has the lowest mismatch strain among alloys studied. Finally we calculate the mixing configuration entropy energies at RT, and obtain -3.5, -4.0, -4.0, -4.5, -4.5, -4.5, -4.5, and -4.9 kJ/mol for 4E, 5E-1, 5E-2, 6E-1, 6E-2, 6E-3, 6E-4, and 7E, respectively. The influence of entropy on the H/M ratio is relatively small. The hydrogen absorption and desoption capacities for alloys in PCT isotherms at 80℃ are lower than those counterparts at RT. The plateaus in PCT isotherms for the alloys are also discussed in this study. SEM micrographs, EDS composition, and XRD patterns for alloys were also carried out in this experiment. It shows that the hydrogen absorption and desoption capacities for alloys studied in this experiment have something to do with the kinds and amount of the hydrogen absorbing and desorbing phases. All alloys in this study contain 3 to 5 phases such as disordered and ordered BCC and Laves phases that can absorb and desorb hydrogen. Except as-cast 7E alloy, all other 7 alloys generally occur hydrogen-induced phase transformation reactions, such as A → (H absorbing) AHx →(H desorbing) A’, during the hydrogen absorbing and desorbing processes. That is, there are irreversible phases after absorbing and desorbing hydrogen. Most alloys are diffusion-controlled in H absorbing kinetic study. Mechanically alloying the alloys of whatever H absorbing or not gives easily amorphous powders. XRD patterns for 800℃-heat treating the amorphized powders from both as-cast and as-H absorbing states of 6E-1alloy are the same, while those for 7E alloy at different states are not. This means that there must be complexity in this alloy series. The amorphized 6E-1 and 7E alloys followed by heat treatment also show little H absorption.