CeNi3-based Intermetallic hydrides

A hydride phase containing 5.0 H atoms per formula unit has been synthesized in the CeNi₃-H₂ system at a hydrogen pressure of 5 MPa and a temperature of 273 K. The hydride is very stable during storage in air, with no hydrogen release. Its lattice parameters have been determined by X-ray diffraction for different synthesis conditions.     

Hydride formation under cathodic charging of titanium and TiAl-based alloys in alkaline solutions

The electrochemical methods and the data of X-ray diffraction analysis are used to determine the parameters of cathodic polarization for the hydrogenation of α-Ti and alloys based on the TiAl intermetallic phase without formation of the hydride phase or with formation of hydrides. In α-Ti, the increasing cathodic polarization in a 0.1 M NaOH solution leads to the dissolution of hydrogen in the metal lattice and its modification and to the increase in the amount of hydrides. The hydride phase is not recorded TiAl-based alloys even for much higher levels of absorption of hydrogen as compared with pure titanium. However, hydrogen affects the phase composition of alloys and the lattice parameters of the phases. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 44, No. 3, pp. 103–106, May–June, 2008.     

Structure of thermally desorbed CeNi<Subscript>3</Subscript>-based hydrides

CeNi₃H _x (x = 0.7, 0.8, 1.0, 1.8, 3.4, 3.8) hydrides have been prepared through hydrogen desorption from CeNi₃ hydrogenated at low (p _{H 2} = 0.01 GPa) and high (p _{H 2} = 0.2 GPa) hydrogen pressures. Using X-ray and neutron diffraction, the hydrides are shown to be isostructural with CeNi₃ (sp. gr. P6₃/mmc, no. 194). The lattice parameters of the hydrides vary appreciably with hydrogen content. The sequence of hydrogen release from different interstices in the desorption process is shown to be opposite to that of hydrogen uptake in the hydrogenation process. The solid-solution range in the desorbed hydrides is much broader than that upon hydrogenation. The extent of the solid solution is influenced by the phase composition of the parent intermetallic compound.     

Magnetic properties of the intermetallic compounds RNi (R=Gd,Tb, Dy,Sm) and their hydrides

Hydrogen interaction with RNi intermetallic compounds and the influence of hydrogen on magnetic properties of these compounds were investigated. Ternary hydrides GdNiH_3.2, TbNiH_3.4, DyNiH_3.4 and SmNiH_3.7 were prepared by hydrogenation of the initial alloys at room temperature and hydrogen pressure up to 0.1 MPa. Hydrides possess orthorhombic CrB-type structure (S.G. Cmcm). The formation of hydrides results in substantial expansion of the metallic sublattices, weakening of the ferromagnetic interactions and decreasing of the paramagnetic Curie temperatures. The article was translated by the author.     

Structure and bonding configuration of hydrided ErNi3 and CeCo3

The intermetallic compounds ErNi₃ and CeCo₃ has been hydrided at low (p _{H 2} ≤ 0.01 GPa) and high (p _{H 2} up to 0.2 GPa) hydrogen pressures. X-ray and neutron diffraction characterization has shown that the resultant hydrides have structures of the same type (PuNi₃) as the parent intermetallics and have a larger unitcell volume. We have identified the positions occupied by the metal and hydrogen atoms and have determined their positional parameters. The lattice anisotropy has been shown to vary little at high hydrogen concentrations. Our results indicate that the metal-hydrogen bonds in the hydrides studied are predominantly ionic for the rare-earth metals (Er and Ce) and predominantly metallic for the transition metals (Ni and Co).     

Hydrogenation of Zr3Al2 in hydrogen and ammonia

We have identified conditions for the formation of Zr₃Al₂-based intermetallic hydrides through reactions with hydrogen and ammonia at temperatures from 150 to 300°C. The use of ammonia is shown to reduce the onset temperature for the formation of a hydride phase by 100°C compared to hydrogenation with hydrogen. Increasing the ammonia-Zr₃Al₂ reaction temperature to 500°C in the presence of NH₄Cl as an activator leads to the decomposition of the intermetallic compound and the formation of finely dispersed zirconium hydride and zirconium nitride powders.     

Synthesis and Determination of the Crystal Structure of Hydrides of Er(M,V)<Subscript>2</Subscript>, where M = Fe or Co,Compounds

We study the process of formation of the Laves phases AB₂ in Er(M, V)₂ (M = Fe or Co) systems and determine their hydrogen-sorption properties. The crystal structure of parent compounds and their saturated hydrides is analyzed by the X-ray diffraction method. It is shown that these compounds absorb hydrogen at pressures of 0.1– 0.12 MPa without amorphization. The formation of the ErFe₂ hydride is accompanied by the transformation of a cubic MgCu₂-type structure into a trigonal TbFe₂-type structure. Even an insignificant substitution of Fe with V or Co leads to an increase in the hydrogen-sorption capacity and prevents the changes in the crystal structure.__________Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 6, pp. 62–66, November–December, 2004.     

Hydrides and deuterides of CaNi5

CaNi₅ is unique among the known AB₅ hydrogen storage compounds in that three distinct hydride phases can be formed at room temperature and modest H₂ pressures. These hydrides have average compositions of approximately CaNi₅H, CaNi₅H₅ and CaNi₅H₆, designated herein as β, γ and δ hydride respectively. Deuteride phases of identical composition can be formed but the isotopic pressure effects are different for each of the three phases. Thermodynamics of the β and γ hydrides are presented along with comments on the tendency for CaNi₅ to disproportionate when hydrided.     

Hydrogen-induced phase transformations in alloys based on Sm Co<Subscript>5</Subscript> under pressures of up to 650 kPA

The interaction of alloys based on SmCo₅ with hydrogen is studied by the methods of differential thermal and X-ray phase diffraction analyses under initial pressures of hydrogen of 200, 300, 400, 500, and 650 kPa at temperatures of up to 1223°K. The hydride of the alloy is formed up to a temperature of 343°K. Within the temperature ranges 388–408°K and 488–523°K, hydrogen is released from the hydrides of phases of the alloy. Within the temperature range 823–863°K, the alloy partially disproportionates into Sm H_x and Co. At 1008–1053°K, SmH_x undergoes partial decomposition and the SmCo₅ and Sm₂Co₁₇ phases are detected. The Co, SmCo₅, and Sm₂Co₁₇ phases exist at temperatures above 1168–1188°K. The compositions of the phases depend on the duration of interaction of the alloy with hydrogen. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 1, pp. 94–98, January–February, 2007.     

Mechanochemical Synthesis of Intermetallic Hydrides at Elevated Hydrogen Pressures

An apparatus is described for producing gas pressures of up to 10 MPa in the grinding vessels of a planetary mill. Mechanochemical reactions are carried out for the first time at gas pressures above 1 MPa. A novel approach to the synthesis of intermetallic hydrides is proposed which involves mechanical activation of a metal mixture at a high hydrogen pressure. Using this approach, two new hydrides are synthesized: Mg₂NiH₆ and a magnesium copper hydride with the approximate composition MgCuH₂.     

Hydriding of TiMo alloys at high hydrogen pressures

We studied the interaction of Ti_0.40Mo_0.60 and Ti_0.34Mo_0.66 alloys with hydrogen and obtained hydrogen desorption isotherms at pressures of up to 250 MPa. At high hydrogen pressures, we observed the formation of Ti_0.40Mo_0.60Н_1.1 and Ti_0.34Mo_0.66Н_0.8 hydride phases. According to X-ray diffraction data, the hydrides consisted of phases with a body-centered cubic and face-centered cubic (CaF₂ structure) lattices. The structure of the deuteride based on the Ti_0.40Mo_0.60 alloy was studied by neutron diffraction. We identified the sites occupied by deuterium atoms and determined their occupancies.     

Precipitation behaviour of a sensitized AISI type 316 austenitic stainless steel in hydrogen

The purpose of this study was to characterize the precipitation behaviour of AISI type 316 steel in hydrogen. The different precipitates (M₂₃C₆, M₆C), the intermetallicχ-phase and the martensitic phase (α′,ε) were determined by using transmission electron microscopy (TEM) and X-ray diffraction techniques. All the specimens were sensitized at 650? C for 24 h. Some samples were carburized up to 2 wt% C. Additions of carbon content decrease the time required for sensitization. Short-term (24 h) exposure of this steel to sensitization temperature results in a complex precipitation reaction of various carbides and intermetallic phases. Hydrogen was introduced by severe cathodic charging at room temperature. This study indicates that by conventional X-ray techniques it is possible to detect those precipitates and their behaviour in a hydrogen environment. The zero shift as observed by X-ray diffraction from the carbides (M₂₃C₆, M₆C) and the intermetallicχ-phase, indicates that those phases absorb far less hydrogen than the austenitic matrix. TEM studies reveal that hydrogen inducesα′ martensite at chromium-depleted grain-boundary zones, near the formation of the carbides.     

Hydrogenation of Zr6MeX2 intermetallic compounds: Absorption and desorption characteristics, crystal structure, and magnetic properties (Me-Fe, Co, Ni; X-Al, Ga, Sn)

We study the process of formation of the hydrides of Zr₆MeX₂ ternary intermetallic compounds (Me-Fe, Co, Ni; X-Al, Ga, Sn). It is shown that they are characterized by high hydrogen-storage capacity (9.4–10.8 atoms of hydrogen per formula unit). In the process hydrogenation, by the method of X-ray powder diffraction analysis, we revealed the transition of the crystal structure of Zr₆MeX₂ from the {ie528-1} space group of symmetries into {ie528-2} with doubling of the unit cell in the [001]-direction. The analysis of the crystal structure of Zr₆NiAl(Sn)₂H _x hydrides by using the Rietveld improvement of the data of X-ray powder diffraction analysis shows that it is identical to the structure of Zr₆FeAl₂D₁₀ deuteride studied earlier by the method of neutron diffraction analysis. The thermal desorption of hydrogen from the synthesized Zr₆MeX₂ hydrides was observed in the temperature range 400–900°K. The dependence of the magnetic susceptibility of the Zr₆FeAl₂ compound on temperature is characterized by the presence of a peak at 50°K. The character of these curves undergoes significant changes in the processes of hydrogenation and dehydrogenation of the specimen. These results were presented by the author at the 5th International Conference “Hydrogen Materials Science and Chemistry of Metal Hydrides” held on September 2–8, 1997 in Katsiveli (Crimea, Ukraine). Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Published in Fizyko-Khimichna Mekhanika Materialiv, Vol. 34, No. 4, pp. 71–78, July–August 1998.     

Decomposition of metastable ternary hydrides

Most ternary hydrides RM_nH_x are metastable and tend to decompose into the stable phases consisting of the binary hydride of the strongly hydrogen-attracting component (RH₂ or RH₃) and either pure M metal or an intermetallic compound of higher M content than RM_n. This decomposition or disproportionation is described in terms of an activation energy for diffusion of metal atoms in the ternary hydrides.     

Hydrogen-absorbing alloys

Improvement of hydrogen capacity in hydrogen-absorbing alloys has been achieved in recent years. Mg-based alloys which were synthesized by ball milling showed lower dehydrogenation temperatures than intermetallic Mg-based alloys. This technique is also effective for preparing a novel Mg-based amorphous alloy, MgNi, and its hydride. Besides conventional intermetallic compounds such as LaNi₅, solid solution alloy, ‘Laves phase related BCC solid solution’ with body-centered-cubic structure showed a hydrogen capacity of 2.2 mass% at room temperature. Alanate, which is not an interstitial hydride, was found to react with gaseous hydrogen reversibly with a catalyst, and its hydrogen capacity was more than 3 mass%.     

Interaction of NbVNi with Hydrogen

The interaction of NbVNi with hydrogen is studied over wide pressure and temperature ranges. Hydrogen absorption–desorption isotherms are measured, and the compositions of the resulting hydride phases are determined. X-ray diffraction analysis demonstrates that the incorporation of hydrogen into NbVNi leads to an isotropic expansion of its crystal lattice.     

Interactions of hydrogen with Tb3Ni6M2 (M= Al,Ga, or Si) alloys with structures of Ce3Ni6Si2 type

Summary A crystallochemical analysis has been performed on the structures of Tb₃Ni₆M₂ (M = Al, Ga, Si), which crystallize in the Ce₃Ni₆Si₂ structure type, as they are potential hydrogen absorbers. There are seven types of cavity suitable for hydrogen atoms. When the Tb₃Ni₆M₂ intermetallides interact with hydrogen, hydride phases are formed that contain 0.32-0.95 mass % H and which decompose completely in the range 20–300°C. The hydrogen contents in the hydrides and the thermal stabilities decrease in the aluminide-gallide-silicide sequence. Hydrogenation produces isotropic expansion in the initial body-centered cubic structures. Neutron diffraction indicates that the deuterium atoms in Tb₃Ni₆Al₂D_6.5 occupy the Tb₃Al, TbNi₃, and Ni₄ cavities.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 27, No. 4, pp. 22–25, July–August, 1991.     

NdRh<Subscript>3</Subscript>-based intermetallic hydrides

We have studied chemical interaction of the intermetallic compound NdRh₃ with hydrogen at pressures of up to 0.2 GPa. The structural changes in the intermetallic matrix in different stages of the hydrogenation process have been followed using synchrotron X-ray diffraction. The results demonstrate that hydrogen absorption leads to an irreversible change in lattice symmetry from hexagonal to cubic.     

Hydrogenation of the intermetallic compound Zr2Ni

We have determined conditions for the preparation of hydride phases with the composition Zr₂NiH_～5 by reacting the intermetallic compound Zr₂Ni with hydrogen or ammonia and identified the products of the reaction between the intermetallic compound and ammonia in the temperature range 150–500°C in the presence of NH₄Cl as an activator. The results demonstrate that the use of ammonia at 500°C leads to decomposition of the intermetallic compound and formation of zirconium hydride, zirconium nitride, and metallic nickel.     

Sorption and desorption of hydrogen in the alloys based on the ErNi<Subscript>2</Subscript> compound

We study the process of formation of pseudobinary Laves phases on the basis of the ErNi₂ compound realized as a result of the substitution of Y for Er and V for Ni and determine the hydrogen-sorption properties of these phases. The crystal structures of the original compounds and their saturated hydrides are investigated by the X-ray diffraction method. It is shown that these compounds absorb hydrogen under a pressure of 0.1–0.12 MPa without amorphization. The maximum hydrogen-sorption capacity (3.4 at. H/f.u.) is obtained for the Er_0.85Y_0.15Ni₂ compound. The thermal stability of hydrides is determined by the method of thermodesorption spectroscopy in gaseous hydrogen at a heating rate of 5°C / min. An insignificant substitution o Y for Er and V for Ni improves the parameters of hydrogen sorption and desorption for these compounds. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 5, pp. 76–80, September–October, 2007.