Marshall H. Mendelsohn

LaNi5-xAlx is a versatile alloy system for metal hydride applications

METAL alloys which reversibly absorb and desorb hydrogen are being used or considered for a variety of energy applications1,2. Alloys of general composition AB5 are prime candidates because of their good hydrogen absorption/desorption kinetics and large hydrogen storage capacity. The LaNi5–H2 system, in particular, has been extensively investigated. In many applications, however, materials are required whose plateau pressures are different from those of LaNi5H6,7. The desorption pressure of LaNi5H6,7 may be modified by substitutions either of La or Ni with other elements. Previous work3 has shown that 20% substitutions of Ni by a variety of transition metals can lower plateau pressures by a factor of ∼4. We report here that Al can substitute for Ni in LaNi5 with dramatic results in lowering decomposition pressures without impairing the kinetics or the hydrogen carrying capacities. The important new result is that Al substitutions allow a wide range of decomposition pressure to be spanned in continuous fashion. In the range 0–20% Al, the plateau pressures of the LaNi5–LaNi4 Al hydride system are reduced by a factor of ∼ 300.

The effect of aluminum additions on the structural and hydrogen absorption properties of AB5 alloys with particular reference to the LaNi5−xAlx ternary alloy system

Abstract The dissociation pressures of the hydrides LaNi 4.6 Al 0.4 H 6 , LaNi 4.5 Al 0.5 H 6 , CFMNi 4.8 Al 0.2 H 6 (CFM = cerium-free mischmetal), CFMNi 4.6 Al 0.4 H 6 and YNi 4 AlH 4 were studied. In all cases, the aluminum additions were found greatly to reduce the plateau pressure of the ANi 5−x Al x hydride compared with that of the corresponding ANi 5 hydride (A = La, CFM, Y). LaNi 4.6 Al 0.4 and LaNi 4.5 Al 0.5 were specially prepared homogeneous alloys whose enthalpies and entropies of transition were calculated as − 8.7 kcal (mol H 2 ) −1 , − 9.2 kcal (mol H 2 ) −1 − 26.1 cal (mol H 2 ) −1 deg −1 and 26.6 cal (mol H 2 ) −1 deg −1 , respectively. A crystallographic model of LaNi 4 Al is shown and consideration of the nearest neighbor atom distribution leads to a rationalization of the observed linear relationship between the enthalpy change ΔH and the Al composition. A recently proposed model of hydrogen interstitial site occupation is examined in the light of the present data. Finally, correlations of hydride dissociation pressures with either alloy heats of formation or alloy cell volumes are discussed.

Preparation and properties of porous metal hydride compacts

Abstract Hydrides of LaNi5 were compacted to form solid porous metallic-matrix hydrides (PMHs). Pellets were obtained and were cycled for preliminary testing. The metal matrix was formed from aluminum, copper, nickel etc.; only PMHs using aluminum were tested. The method of preparation yielded pellets that did not disintegrate after repeated cycling. The pellets rapidly absorbed hydrogen from the first cycle, eliminating the need for activation. The kinetics were very fast and the hydrogen capacity was high. Preliminary metallographical analyses showed the microstructure actually obtained. The thermal conductivity was found to be 32.5 W m−1 K−1.

NMR studies of hydrogen diffusion in β-LaNi5−yAly hydrides

Proton relaxation times have been measured in β-phase LaNi5−yAlyHx, where 0<y<1.2, to determine the role of Al on hydrogen diffusion. Al substitution significantly increases the observed activation energy with a corresponding 100-fold decrease in the room temperature (300K) diffusion constant between y=0 and y=1.2. Comparisons are also made with previous studies of β-LaNi5Hx and possible diffusion mechanisms are suggested.

The effect on hydrogen decomposition pressures of group IIIA and IVA element substitutions for Ni In LaNi5 alloys

Abstract The alloys LaNi4.6In0.4 and LaNi4.6Sn0.4 were prepared and their hydrogen absorption properties studied. Hydrogen desorption pressures for both corresponding hydrides were found to be lower than those determined previously for LaNi4.6Al0.4 and LaNi4.6Ga0.4. Enthalpies and entropies of transition for both hydrides were calculated and their hydrogen absorption-desorption hysteresis appeared to be negligible.

Metal hydrides as chemical heat pumps

The refrigeration mode of operation for a two metal hydride chemical heat pump system is described. A derivation is presented for general heat pump equations which may be applied to any pair of compounds which show a linear relation of ln P vs 1/T for their heat pump performance. The coefficient of performance of a two metal hydride refrigerator is obtained. (SPH)

The pseudo-binary system Zr(V1−xCrx)2: Hydrogen absorption and stability considerations

Abstract Pressure-composition absorption isotherms were measured for the hydrides of Zr(V 1− x Cr x ) 2 ( x = 0.4, 0.5, 0.6). These cubic Laves phase alloys follow a linear relationship of free energy with chromium substitution. A free energy against cell volume plot of the cubic chromium-containing materials compared with earlier data on cubic as well as hexagonal iron-substituted alloys shows that electronic factors are probably more important than structural factors in determining the free energies of the ternary Zr(V 1− x B x ) 2 systems.

Role of aluminum substitution on hydrogen diffusion in β-LaNi5−yAlyHx☆

Proton relaxation times measured by nuclear magnetic resonance pulse techniques were used to investigate hydrogen diffusion behavior in β-LaNi5−y AlyHx for 0 < y < 1.5. Aluminum substitution increases the activation energy Ea with a subsequent reduction in the apparent diffusion constants by more than two orders of magnitude relative to those for β-LaNi5H6 when y ⩾ 1. The values of the proton relaxation times T1 and T1ρ from high purity samples suggest that two types of jump processes with differing activation energies are probably involved.

Temperature-programmed desorption (TPD) studies of ZrV1.6Fe0.4Hy and ZrV1.2Cr0.8Hy

Abstract Temperature-programmed desorption (TPD) of several hydride compositions in the systems ZrV 1.6 Fe 0.4 H y and ZrV 1.2 Cr 0.8 H y reveal two distinct peaks. The TPD results appear to correlate with changes in the pressure-composition isotherms when plotted on a log-log plot, which in turn may be related to changes in the thermodynamics of hydrogen desorption. The so called “traping model” (8,9) is presented as a possible explanation of the observed TPD behavior.

Intermetallic compounds as bulk getters

Abstract A brief history of “gettering” is given and a short discussion of the potential use of intermetallic compounds as bulk getters is presented. Specific studies of Zr(V1−xFex) 2 alloys were started and data on three different alloy compositions were obtained. The materials examined thus far are multiphase and exhibit non-Sieverts-type behavior. An approximately linear correlation of the dissociation pressure of the hydrogenated alloys with the iron content of the substituted alloys was observed. Preliminary indications point to these materials as useful practical getters; however, more specific data on their gettering properties remain to be determined.