F. Pourarian

A neutron diffraction and Mössbauer effect study of the Nd2Fe17−xSix solid solutions

Abstract The substitution of silicon for iron in Nd 2 Fe 17 strongly raises the Curie temperature but leads to a reduction in the unit cell volume. Refinement of the neutron-diffraction pattern for Nd 2 Fe 12.91 Si 4.09 indicates that silicon preferentially occupies the 18h site in the Nd 2 Fe 17 structure, the site with the most neodymium near neighbors. This occupation is surprising because conventional arguments would suggest that replacement of iron on the 6c site, which has a very short iron to near-neighbor iron bond length, would yield an increase in the Curie temperature.

Influence of hydrogen on the magnetic characteristics of R2Fe14B (R Ce, Pr, Nd, Sm or Y) systems

Abstract The effect of absorbed hydrogen on the magnetic behavior of R2Fe14B (R ue5fc Ce, Pr, Nd, Sm and Y) has been investigated. These compounds are found to absorb 4.4 to about 6 H per formula unit. Hydrogen absorption does not change the structure of these compounds but is accompanied by an expansion in unit cell volume ranging from 4% to 6%. The measurements show that the hydrogen absorption leads to an increase in the iron moment in all the hydrides investigated and also a pronounced change in the room temperature easy direction of magnetization of the yttrium- and praseodymium-containing compounds. Anisotropy fields for Nd2Fe14B and Ce2Fe14B at 300 K are significantly reduced on hydrogenation. The results are taken to imply a weakening of 4f-3d exchange interaction with the existence of reasonably large 4f crystal field interactions at low temperatures.

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.

Hydrogen storage in CeNi5−xCux

Abstract Hydrides of CeNi5−xCux (x = 2, 2.5, 3) were studied in the temperature range of 0–50 °C. Pressure-composition isotherms with well-defined flat plateaux were obtained in this temperature range for x = 2 and 2.5. Substitution of copper for nickel causes a substantial decrease in the hydrogen dissociation pressure. The substitution also lowers the alloy cost. The kinetics of absorption and desorption are extremely fast (90% of the absorption takes place in about 50 s). The heats ΔH of formation of the hydride system are low relative to the value for LaNi5-H2. The low value of ΔH together with the high absorption and desorption rates makes these compounds promising materials for technical applications, especially hydrogen storage.

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.

Influence of hydrogen on structure and magnetic properties of Ho6Fe23 and Er6Fe23

Abstract The pressure composition isotherm of the Er6Fe23-H2 system studied at 0°C reveals the presence of three different hydride phases. Maximum absorption is typically 14 hydrogen atoms/formula unit in Er6Fe23 and Ho6Fe23 compared to about 22 in the isostructural Mn compounds. During hydrogenation Ho6Fe23 remains cubic with lattice parameter which increases from 12.034 A to 12.399 A. Upon hydrogenation Er6Fe23 exhibits the very surprising behavior of transforming from a cubic ( a = 12.004 A ) to a tetragonal structure ( a = 12.131 A , c = 12.601 A ). In spite of their contrasting crystallographic features both hydrides exhibit similar magnetic behavior, relative to the parent compounds, namely, lowering of both the saturation moment at 4.2 and the compensation temperature and an increase in the saturation moment at room temperature. The results of magnetic measurements may be understood by assuming an increase in the iron moment in these compounds on hydrogen absorption.

Magnetic characteristics of CeNi5−xCux (x=0, 1, 2, 2.5, 3, and 4) alloys and their hydrides

Susceptibilities were measured for the series of CeNi5−xCux (x=0, 1, 2, 2.5, 3, and 4) alloys and their hydrides over the temperature range 4.2–300 K in an applied field up to 20 kOe. All materials studied were found to crystallize in CaCu5‐type crystal structure. A pronounced change in lattice parameters was observed for x=2.5. Materials with x=0, 1, and 2 exhibit Pauli paramagnetism, while for x>2 the total susceptibility can be regarded as a sum of Pauli paramagnetism and Curie–Weiss paramagnetism associated with the Ce3+ ions. The effective fraction of Ce3+ in all compounds was calculated. Hydrogenation increased the unit cell volume ∼15% without a change of crystal structure. It also caused an increase in effective moment for x=2.5 and x=3 alloys, but a small decrease was observed for x=4.

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.

Magnetic characteristics of RCo2−xFex hydrides (R ≡ Tb,Dy)

Abstract The Laves phase compounds RCo2−xFex (R ≡ Tb, Dy and x = 0, 0.6 and 2) are found to absorb about 3–3.5 hydrogen atoms per formula unit. The hydrogen absorption results in a large increase in cell volume (about 20%–25%) without a change in the structure. Detailed magnetization studies on the parent compounds and the hydrides have been carried out in the temperature interval 4.2–300 K. From these measurements it is inferred that the hydrogen absorption leads to a reduction in the cobalt magnetic moment in TbCo2 and DyCo2 but to an increase in the iron moment in TbFe2 and DyFe2 as well as in the cobalt-rich ternaries TbCo1.4Fe0.6 and DyCo1.4Fe0.6. Magnetization of all the hydrides at 4.2 K shows no tendency to saturate even in fields up to 20 kOe. This suggests a possible fanning of the rare earth moments in the hydrides due to the weakening of the Rue5f8Co and Rue5f8Fe exchange. A change from the first-order magnetic transition in DyCo2 at the Curie temperature to the second order, on hydrogenation, is observed.

Magnetization behavior of RFe3-hydrides (R = Tb, Er and Tm)

The RFe/sub 3/ compounds (R = heavy rare earth), crystallizing in rhombohedral PuNi/sub 3/ type structure, absorb typically three hydrogens per formula unit at room temperature. The hydrogen absorption does not change the structure of these compounds but merely causes an expansion of the lattice. Magnetization studies have been carried out on the hydrides in the temperature range 4.2 to 300 K and in applied fields up to 21 kOe. The parent compounds are known to be ferrimagnetically ordered with reasonably high Curie temperatures. The magnetization of the hydrides at 4.2 K is observed to be smaller than that of the corresponding parent compounds. The compensation temperature also decrease on hydrogen absorption. These results may be taken to imply either an increase in moment on iron and/or a decrease in rare earth moment. In ErFe/sub 3/-hydride, a sharp increase in magnetization is observed close to room temperature. This is attributed to the change in easy direction of magnetization.