V.K. Sinha

Hydrogenation characteristics of Zr1−xTixMnFe alloys

Abstract The Zr 1− x Ti x MnFe-H 2 systems with x = 0.2 and 0.3 were studied as a function of composition, temperature (23–200 °C) and hydrogen dissociation pressure (0.01–50 atm). The hydrogen vapor pressure of the ZrMn 2 -H 2 system is raised approximately 400-fold by the replacement of zirconium by titanium and of manganese by iron without significantly impairing its hydrogen capacity. The hydrides of Zr 1− x Ti x MnFe alloys have dissociation pressures in the range 1–2 atm at room temperature (23 °C). The hydrogen densities of the alloys compared with that of liquid hydrogen range from about 1.0 to 1.4. The enthalpies and entropies of dehydrogenation are exceptionally low (10–11 kJ (mol H 2 ) −1 and 46–50 J (mol H 2 ) −1 K −1 respectively). The kinetics of hydrogen absorption and desorption are extremely fast. Desorption of hydrogen in the two-phase field α + β follows first-order kinetics with activation energies of 34–38 kJ mol −1 . The energetics of hydrogen charging and discharging for the Zr 0.8 Ti 0.2 MnFe alloy are smaller than those for LaNi 5 or other accepted hydrogen storage materials. Similarly, a hydride compressor using Zr 0.8 Ti 0.2 MnFe alloy appears to be significantly more efficient than the LaNi 5 hydride compressor.

Effect of hydrogen absorption on the magnetic properties of ZrMn2−xFex ternaries with a C14 structure

Abstract Magnetization measurements were carried out on ZrMn 2− x Fe x ( x = 0.4, 0.7 and 1.0) and their hydrides. All systems possess a hexagonal Laves phase structure (C14 structure). These compounds are found to absorb about 3.6 hydrogen atoms per formula unit. The hydrogen absorption is accompanied by a large expansion in volume ranging from 26% to 28% with no change in crystal structure. In this system, both the magnetic moments and the transition temperatures were found to be significantly larger in the hydride phases than in the corresponding host compounds. An increase in the magnetic moments on hydrogenation can be interpreted as a result of charge transfer from the transition metal atoms to hydrogen due to the difference of electronegativity. An increase in the magnetic ordering temperatures is attributed to a decrease in the electron concentration of 3d bands as well as to an increase in the lattice constants on hydrogenation. Spin glass behavior is exhibited by all the systems studied. The spin glass behavior is thought to originate from iron-rich clusters which occur as a consequence of statistical fluctuations in the manganese sublattice.

Hydrogen sorption properties of non-stoichiometric ZrMn2-based systems

Abstract Hydrogen sorption by several non-stoichiometric ZrMn 2 -based alloys was studied at pressures up to 50 atm and over a temperature range from 23 to about 200°C. The dissociation pressure of the hydrides is raised by a factor of 500–1000 for ZrMn 2 T 0.8 or ZrMn 2 T 1.2 (T ≡; transition element or Cu) as the host material compared with that for ZrMn 2 as the host material. Among the hydrides studied, ZrMn 2 Co 0.8 -H exhibited the highest value for the plateau pressure. Measurements of the experimental densities of the non-stoichiometric host materials show good agreement with the substitutional model in which manganese and/or T partially replace zirconium at the zirconium sites. The hydrides have remarkably low heats of formation and entropies of 12–19 kJ (mol H 2 ) −1 and 50–80 J (mol H 2 ) −1 K −1 respectively. The hydrogen absorption or desorption is extremely rapid, e.g. 90% of the hydrogen was released or absorbed in about 1 min. The hydrides studied exhibit features which strongly suggest that they have technological potential.

The hyperstoichiometric ZrMn1+xFe1+y−H2 system II: Hysteresis effect

Abstract The absorption pressure-composition isotherms in the temperature range 23–200 °C were determined for the ZrMn 1.22 Fe 1.11 −H 2 and ZrMn 1.11 Fe 1.22 −H 2 systems using a gasometric technique. Marked hysteresis was observed. The pressure P a needed for hydride formation exceeded the decomposition pressure P d by a factor of 5–6 at room temperature. To explain the hysteresis, the strain energies associated with various sites were computed. These results provided a basis for understanding the hydrogen atom occupancy in the ZrMn 2 hydride. Strain energies at room temperature were computed to be 8.7 kJ (mol H 2 ) −1 and 9.1 kJ (mol H 2 ) −1 for the ZrMn 1.22 Fe 1.11 −H 2 and ZrMn 1.11 Fe 1.23 −H 2 systems respectively. These values are in reasonable agreement with the observed freeenergy difference or the enthalpy difference derived from the absorption and desorption pressure-composition isotherms. This analysis makes it appear almost certain that hysteresis is primarily a consequence of strain energy.

Kinetics and thermodynamics of ZrMn2-based hydrides☆

Abstract The thermodynamics and kinetics of hydride formation and decomposition are presented for ZrMn 2+ x (or Zr 1− y Mn y Mn 2 ) and Zr 1− x Ti x Mn 2 . Making ZrMn 2 non-stoichiometric or replacing the zirconium by titanium raises the hydrogen escape tendency dramatically. ΔH is small (about 16 kJ (mol H 2 ) −1 ) and the configurational entropy is large for the Zr 1− y Mn y Mn 2 hydrides. The oxide coatings as determined by Auger spectroscopy are thin. The kinetics are very rapid probably because the oxide coating is thin.

Magnetic properties of Zr(Cr1−xCox)2 alloys and their hydrides

Abstract Magnetic properties of Zr(Cr 1− x Co x ) 2 alloys, with 0 ⩽ x ⩽ 0.75, and their hydrides were determined over the temperature range 4 to 275 K. All systems were paramagnetic. The susceptibility (ϰ) is significantly enhanced both by increase of the Co content and by hydrogenation in all cases except cubic ZrCr 2 , for which ϰ is virtually unchanged by hydrogenation. The rise in ϰ is ascribed to electron transfer from Co to Cr and to H. This transfer increases the local density of states for Co at the Fermi energy. The temperature dependence of the susceptibility can be represented by an equation of the form ϰ = ϰ 0 + C /( T − θ), indicating Pauli and Curie-Weiss contributions to ϰ. The contributions are attributed to the ZrCr 2 matrix and the solute Co atoms, respectively. The Co atoms on Zr sites are weakly coupled antiferromagnetically. Pressure-composition isotherms for the ZrCrCo-H 2 system are given at 80, 120 and 165°C. At 80°C this alloy hydrogenates to the composition ZrCoCrH 3.25 at a pressure of 50 atm.

Zr0.7Ti0.3Mn2Fe0.8 as a material for hydrogen storage

Zr0.7Ti0.3Mn2Fe0.8 hydride was prepared and studied as a function of temperature (25–200 °C) and hydrogen equilibrium pressure (0.01–50 atm). The vapor pressure of the quaternary hydride system is approximately 1000 times larger than that of the ZrMn2-H2 system. The hydrogen density of the alloy relative to liquid hydrogen is 0.7. The enthalpy and entropy of hydrogenation are very low, being −14.1 kJ mol-1 and−59.5 J mol-1 K-1 respectively. The low value of ΔH α → β and the rapid kinetics of hydrogen absorption-desorption processes are particularly significant in regard to practical applications of Zr1−xTixMn2Fey alloys for hydrogen storage.

Hydrogen storage in some ternary and quaternary zirconium-based alloys with the C14 structure

The Zr0.8Ti0.2MnCr1.25, ZrMnFeTx (T ≡ Cr, Ni, Co; 0 < x < 0.4) and ZrCrFe1 + x (0 < x < 0.5) alloys were studied for their hydrogen storage characteristics in the temperature range 23–150°C and hydrogen equilibrium pressure (0.01–50 atm). The kinetics of hydrogen sorption by these alloys are observed to be extremely rapid, and the processes are almost complete in 60–200 s. Hydrogen capacities of these alloys are in the range 92–222 cm3 H2 (g alloy)−1. The hyperstoichiometric elements appear to substitute at the zirconium site in these AB2-type alloys with a net effect of raising the decomposition pressure of their hydrides several-fold over that of the ZrMn2 and ZrCr2 hydrides. The ZrMnFeCo0.4 alloy does not absorb measurable quantities of hydrogen, presumably because of the high decomposition pressure of its hypothetical hydride. The effectiveness of alloying elements in destabilizing the ZrMn2 hydride increases in the following order: Cr < Mn < Fe < Ni < Co. It is found that, unlike ZrMn2, ZrCr2 cannot be made hyperstoichiometric with chromium or manganese. The hydrides of ZrCrFe1 + x alloys have very favorable dissociation pressures, e.g. 0.9–2 atm at room temperature. The enthalpies and entropies of dehydrogenation of their hydrides are in the range 20–23 kJ (mol H2)−1 and 66–80 J mol−1 K−1 respectively, which are advantageous for application purposes.

Effect of neodymium, copper and aluminum on the hydriding characteristics of CeNi5

Abstract The hydrides of Ce0.7Nd0.3Ni2.5Cu2.5 and the CeNi5−x−yCuxAly quaternaries with 2

Hydrogenation Entropies of the ZrMn2+y System

Hydrogenation of Mn-containing intermetallics leads to striking changes in magnetic properties. For example, Th6Mn23 is a Pauli paramagnetic but becomes ferrimagnetic upon hydrogenation. Vapor pressure studies of ZrMn2+y hydrides. have suggested unusual ΔH and ΔS values for hydrogen release. The values are about 40 and 50%, respectively, lower than the corresponding values for the paradigm hydride material LaNi5H6. The unusual thermodynamics could originate with either a large magnetic contribution or irreversibility for the system. Recent calorimetric measurements indicate that irreversibility is the major factor. Entropies of absorbed hydrogen derived from the calorimetric results are about 17 J/K g.atom of H, which is in good agreement with the computed values, 18.3 J/K g. atom H.