Formation Of Hydride Phase Research Articles

Metastability of the β-phase in Zr-rich Zr-Nb alloys

The decomposition of the β-phase in Zr-rich Zr-Nb alloys by three processes viz., ω formation, α-formation and hydride precipitation has been examined. In the Zr-20Nb alloy, ω-formation has been examined after thermal treatment as well as after electron irradiation and a comparison has been made between the kinetics of ω-phase formation under these two conditions. The morphology of the α-precipitates and their internal structures has been found to depend upon the type of thermal treatment with step quenching from the β-phase field leading to an allotriomorphic morphology and quenching and aging leading to internally twinned Widmanstätten α. The different morphologies obtained due to change in thermal treatment and composition of the Zr Nb alloys has been rationalized. Hydride formation has been examined in α-Zr, β-Zr and in α + β microstructures. A comparison has been made between the mechanism of formation of hydride phase in these three types of microstructures and their morphology and internal structures have been explained.

Prevention of hydrogen degradation in titanium by deposition of TiN thin film

This paper describes the effects of using a TiN film as a possible protection layer for titanium in an aggressive chemical environment. The TiN film as a barrier of hydrogen permeation into the material was evaluated by electrochemical cathodic charging of the specimens in solutions with different pH values and temperatures. Results revealed that the thickness of hydride on the surface of the uncoated titanium increased with the increase of charging time. Higher charging temperature and lower pH value promoted the formation of hydride phases. For titanium coated with a TiN film, it effectively retarded the permeation of hydrogen into the titanium substrate. However, this was at the sacrifice of the TiN film itself.

Hydrogen absorption and thermodynamic properties of hydrogen in low rh content Pd‐Rh alloys

AbstractThe hydrogen absorption characteristics of fully annealed Pd‐Rh alloys containing up to 10.0 at.% Rh have been investigated at temperatures between 273 K and 433 K and hydrogen pressures up to 1000 Torr by means of pressure‐composition isotherm measurements. For the p‐c isotherms of Pd‐7.5 at.% Rh alloy, in addition to the normal (α+β) two‐phase plateaus, “anomalous plateaus” are observed near the normal βmin phase boundaries. The formation of this anomalous plateau implies that in the fully annealed alloys some precursory phase separation exists, together with the metastable, homogeneous solid solution phase. The low pressure hydrogen solubilities decrease with an increase of Rh content and the main plateau pressures increase with the solute metal content. The hydrogen solubilities exhibit a maximum at about 7.5 at.% Rh at 1000 Torr. The derived relative partial molar thermodynamic quantities at infinite dilution, and also the plateau thermodynamic parameters are in agreement with the previously reported results. These thermodynamic quantities are not affected by the formation of the anomalous hydride phase, because the hydride does not form in the dilute phase region, and because the amount of hydrogen involved in the anomalous hydride phase is small compared to the main hydride phase.

Stress corrosion cracking of U-0.1% Cr in humid helium atmosphere

Rivets were matched into adapted drilled holes in plates, both made of U-0.1% Cr alloy and were placed in different environments containing dry air and helium and humid air and helium for a variety of exposure times. After opening, the most significant amounts of corrosion products were detected in the specimens that stayed for three years in humid helium (5% RH) environment. Radial cracks, developed in the bore edge, were detected in the specimens. X-ray diffraction patterns of the corrosion products gave the composition of UH 3 and UO 2. The microstructure was examined using light and electron microscopy techniques. The hydride phase that was observed, formed mainly beneath the oxide layer and penetrated into the metal matrix as needle-like forms. The formation of a lower density hydride phase, yielded in a large volume change causing the development of high stresses at the rivet-bore interface. The combination of the high stress and the weakening of the bore edge due to the presence of the brittle hydride phase led to radial crack formation around the bore edge.

Thermal hydriding of some Ni-based intermetallic compounds

The behavior of various types of Ni-based intermetallic alloys, namely NiZr, NiZr 2 and LaNi 5 , in thermal hydriding-dehydriding cycles has been investigated from room temperature to 1173K at various initial pressures ranging from 0.2 to 3.3 MPa absolute. It has been found that, upon hydriding, the rate of hydrogen absorption becomes enormous at specific temperatures, regardless of the initial hydrogen pressure over the samples. The same phenomenon has been observed upon dehydriding (i.e. the rate of hydrogen release becomes high at certain temperatures). The incident has been found to be reproducible. The specific temperature changes from one alloy to another during hydriding cycles. A method by which to record and present the phenomenon has been suggested, plotting the rate of change of hydrogen in the hydride with the temperature, where these sudden changes appear as sharp and distinctive peaks. This phenomenon has been attributed to the formation of a stable hydride phase in the case of LaNi 5 , and a phase transition in both NiZr and NiZr 2 .

Hydrogen-induced amorphisation in the Ce(Fe 1− xAl x) 2 system

Hydriding behaviour of cubic Laves phase pseudo-binary Ce(Fe 1− x Al x ) 2 system has been studied for the iron-rich (0 ⩽ x ⩽ 0.1) as well as for the aluminium-rich compositions (0.7 ⩽ x ⩽ 0.85) at temperatures in the range of 223–298 K, and hydrogen pressure (≲ 0.5 atm), ensuring fast withdrawal of exothermic heat. The maximum hydrogen absorption capacity is found to decrease with increasing aluminium concentration. The results of X-ray diffraction and Mossbauer studies are reported. This system shows the unique feature of stabilising the amorphous ternary, for the Fe-rich region and the crystalline ternary hydride phases for the Al-rich compositions. The role of the maximum hydrogen absorption capacity, the rate and sequence of H atom occupancy and the volume associated with each hydrogen atom is discussed in relation to the amorphous ternary vis-a-vis the crystalline ternary hydride phase formation. It is shown that by restricting the maximum hydrogen absorption capacity such that the H-atom occupancy is confined to the g-type interstitial sites only, the crystalline ternary hydride phase can be stabilised in this system.

Hydrogen-induced amorphization of the Laves compound CeNi 2 and the structural and thermal characteristics of the amorphous phase

The hydrogen-induced amorphization behaviour of the C15 Laves compound CeNi 2 were investigated by X-ray, differential scanning calorimetry, transmission electron microscopy and thermal desorption techniques. From the X-ray measurements, it was observed that the CeNi 2 compound is amorphized by hydrogenation, and the transformation to an amorphous state takes place without the formation of a crystalline hydride phase. At high reaction temperatures (400 °C), the sample decomposes into the CeNi 5 and CeH 2 phases, which are believed to be equilibrium phases in a hydrogen atmosphere. Amorphization is possible even when the reaction temperature is as low as −76 °C. From this low temperature amorphization behavior, it is suggested that the elastic strain due to hydrogen absorption may play a key role in the amorphization. Electron diffraction of the hydrogenated sample shows two diffuse halos, which are proposed to be caused by phase separation of the amorphous phase. One of these is considered to be a cerium-rich phase (close to CeH 2) and the other is a nickelrich phase (close to CeNi 5). Therefore, it is suggested that the amorphization of CeNi 2 by hydrogénation may occur by lattice distortion in the course of the phase decomposition. The thermal decomposition and hydrogen desorption behaviors of the amorphous CeNi 2 hydride was examined by differential scanning calorimetry and a thermal desorption technique using gas chromatography. The first cystallization product is the CeNi 5 phase followed by the formation of CeH 2 at higher temperatures. Through the formation of intermediate compounds of varying [Ce] [Ni] ratios, the CeNi 5 and CeH 2 phases combine into the original CeNi 2 compound.

Studies of sorption-desorption processes in the Ce xLa 1− xNi 5 −H 2 system by DSC

The dependence of the integral enthalpy of hydrogenation (or dehydrogenation) of the intermetallic compounds (IMCs) Ce x La 1− x Ni 5 ( x = 0, 0.05, 0.1 or 0.3) on pressure in the range 0.5–6 MPa was studied by the method of differential scanning calorimetry. It has been shown that at x = 0 and x = 0.05 the absorption (or desorption) proceeds via the formation of a stable intermediate hydride phase.

A change in magnetic properties of R 2Co 7 intermetallic compounds (R = Pr, Sm, Tb and Ho) upon hydrogen absorption

The magnetic properties of the compounds R 2Co 7 (R = Pr, Sm, Ho and Tb) and their hydrides with hydrogen concentration corresponding to β and γ phases are studied. Ferro to spin-glass magnetic transitions are found upon the β hydride phase formation in Tb 2Co 7 and Ho 2Co 7-ferri-magnetics. Strong reductions in exchange interactions and T c for R 2Co 7 hydrides phases are observed.

Interaction of hydrogen and deuterium with CoTi surfaces.

The interaction of ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ with polycrystalline CoTi in the annealed and unannealed states, and at 300 and 80 K, has been studied using ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy, electron-energy-loss spectroscopy (EELS), thermal desorption spectroscopy, and measurement of work function. The two surface conditions, annealed and unannealed, were found to be slightly enriched in Ti and Co, respectively. Both ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ were found to react readily with CoTi at both 300 and 80 K, causing changes in the UPS spectra that could be explained from density-of-states calculations on CoTi and (by extension) on ${\mathrm{FeTiH}}_{\mathrm{x}}$. The surface stoichiometry was unaltered by the interaction, indicating no adsorbate-induced segregation, and there was no evidence from EELS of the formation of a separate hydride phase. However, EELS was able to demonstrate the presence of ${\mathrm{H}}_{2}$ or ${\mathrm{D}}_{2}$ at the surface by virtue of the rapid quenching of a surface loss feature on initial exposure. On heating above \ensuremath{\approxeq}500 K ${\mathrm{H}}_{2}$ or ${\mathrm{D}}_{2}$ were desorbed, the desorption threshold temperature increasing progressively with successive cycles of heating. For both ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ interaction the work function increased on exposure, but reached different saturation levels at 300 and 80 K, respectively. All these results are consistent with a mechanism of ${\mathrm{H}}_{2}$ or ${\mathrm{D}}_{2}$ chemisorption followed by inward diffusion and the formation of an \ensuremath{\alpha}-phase solid solution, but not with formation of a hydride phase.

An in situ X-ray diffraction study of the activation and performance of methanol synthesis catalysts derived from rare earth-copper alloys

The activation of NdCu, NdCu 2, NdCu 5, and CeCu 2 alloy precursors for methanol synthesis catalysts has been investigated by in situ XRD observations and concurrent measurements of methanol activity. Pressures of 2–30 bar and temperatures in the range 300–573 K were employed. It is shown that the formation of certain intermediate hydride phases is crucial to the eventual production of highly active catalysts and that methanol activity does not correlate with Cu crystallite size. CO 2 is a very effective poison which does not visibly affect the morphology of the metal phase. These findings strongly suggest that the reaction mechanism which obtains here is quite different from that which operates on conventional Cu ZnO Al 2 O 3 methanol synthesis Catalysts.

Effect of surface temperature on the sorption of hydrogen by Pd(111)

Temperature programmed desorption (TPD) subsequent to various hydrogen exposure conditions indicates the formation of chemisorption, solid solution, and hydride phases of hydrogen in the near surface region of Pd(111). Variation of the sample exposure temperature ( T e) between 80 and 300 K has a strong effect on the subsequent TPD spectra. At T e = 80 K a single desorption peak, β, appears at 310 K. Coverage variation of the β peak is consistent with second-order recombinative desorption of chemisorbed hydrogen. For T e between 90 and 140 K a slight enhancement of the β peak occurs and a new peak, α, appears initially near 170 K. It does not saturate, exhibits near-zeroth-order desorption kinetics, and is assigned to the decomposition of a near surface palladium hydride phase. Population of the α peak is thermally activated with a maximum at T e ≈ 115 K. For T e, greater than 140 K, α disappears while the total amount of absorbed hydrogen increases significantly. At these temperatures, the concentration of absorbed hydrogen decreases significantly if the sample is held in vacuo at T e after completion of the hydrogen exposure. At all exposure temperatures there is also a broad desorption feature near 800 K which is enhanced by higher T e and is associated with hydrogen in solid solution.

Effect of surface temperature on the sorption of hydrogen by Pd(111)

Temperature programmed desorption (TPD) subsequent to various hydrogen exposure conditions indicates the formation of chemisorption, solid solution, and hydride phases of hydrogen in the near surface region of Pd(111). Variation of the sample exposure temperature ( T e ) between 80 and 300 K has a strong effect on the subsequent TPD spectra. At T e = 80 K a single desorption peak, β, appears at 310 K. Coverage variation of the β peak is consistent with second-order recombinative desorption of chemisorbed hydrogen. For T e between 90 and 140 K a slight saturate, exhibits near-zeroth-order desorption kinetics, and is assigned to the decomposition of a near surface palladium hydride phase. Population of the α peak is thermally activated with a maximum at T e ≈115 K. For T e greater than 140 K, α disappears while the total amount of absorbed hydrogen increases significantly. At these temperatures, the concentration of absorbed hydrogen decreases significantly if the sample is held in vacuo at T e after completion of the hydrogen exposure. At all exposure temperatures there is also a broad desorption feature near 800 K which is enhanced by higher T e and is associated with hydrogen in solid solution.

TiC amorphization under H implantation at low temperature

In situ transmission electron microscope on line with an implantor was used to study the formation of an amorphous hydride phase during H implantation of TiC. We first observed the amorphization of polycrystalline TiC during low temperature (15 K) H implantation (7 keV): amorphization started at a fluence of - 3 x 1017 H cm−2 (concentration > 30 at.%). No amorphization occurs without the presence of hydrogen in the material. This has been demonstrated both by in situ He implantation and 2 MeV in situ electron microscopy irradiation. We found that the amorphous phase is stable after annealing at room temperature. Moreover EELS analysis of this amorphous phase did not reveal any change in the C/Ti ratio.

Initial stages of hydride formation: Hydrogen on Ce(001).

The interaction of hydrogen with a Ce(001) single-crystal surface has been investigated with use of angle-resolved uv photoemission spectroscopy (ARUPS), electron-energy-loss spectroscopy (ELS), and low-energy electron diffraction (LEED). Hydrogen uptake from the gas phase results in a cerium-hydrogen solid-solution phase, which is evidenced by a 4.2-eV structure in UPS and a 3-eV H-derived interband transition in ELS. After larger H/sub 2/ exposures (>500 L) a low-energy plasmon feature at 1.6-eV loss energy signals the formation of a surface hydride phase of dihydridelike stoichiometry (1 langmuir (L)equivalent10/sup -6/ TorrXc). The surface disorders as a result of hydride formation. It is suggested that hydrogen occupies tetrahedral sites in the solid solution and dihydridelike phases. Annealing of H/sub 2/-exposed Ce surfaces to 300--400XdeC induces the population of octahedral sites by hydrogen in an activated process, and a corresponding UPS band is detected at 3.4 eV. A H-induced hexagonal reconstruction of the Ce(001) surface with a complex LEED pattern is observed after annealing to >400/sup 0/C, and the characteristic features of the UPS of this reconstructed surface suggest hydrogen in both tetrahedral and octahedral sites near the surface.

Hydriding properties of Zr(Fe xCr 1− x) 2 intermetallic compounds

One group of potential hydrogen storage compounds, i.e. Zr(Fe x Cr 1− x ) 2 intermetallics, are investigated. These intermetallics are predominantly single-phased, identified as the hexagonal Laves phase. Hydrogen is absorbed quite readily, with no special activation treatment. Hydrogen absorption results in the formation of a distinct hydride phase, with the same crystal structure as the parent compound. Of these intermetallics, Zr(Fe 0.75Cr 0.25) 2 demonstrates the best overall energy storage characteristics, i.e. a relatively high sorption capacity (H/M = 1) and low hydride stability ( ΔH = −25 kJ mol −1 H 2). This composition compares quite favorably to LaNi 5 in terms of hydrogen capacity, hydride stability and reaction kinetics.

Photoemission and x-ray studies of metal hydrides and hydride formation at metal/hydride interfaces

Synchrotron radiation photoemission was used to investigate the concentration dependent intensity of hydrogen-induced states for a series of niobium hydrides. These intensities were found to be linearly related to the fraction of β phase hydride present in the sample, as deduced from the bulk phase diagram, and we conclude that fracturing did not produce a self-sustaining surface hydride layer. Photoemission studies of V overlayers on NbH 0.799 showed strong hydrogen outdiffusion and the formation of the interface hydride phase which is more stable than VH prepared in bulk form. X-ray diffraction studies of thick V-H overlayers subsequently showed the absence of long-range order. These results demonstrate that interfacial diffusion of hydrogen from NbH 0.799 into the V overlayer produced a stable hydride with electronic properties analogous to those of crystalline VH but with dominant local interactions.

Hysteresis behaviour and thermodynamic properties of the Pa-H system

The enthalpies and entropies of formation of the non-stoichiometric protactinium hydride phases were obtained from temperature-pressure-composition data. Severe hysteresis observed in the pressure-composition isotherms measured in the hydriding and dehydriding directions correlates with the occurrence of sluggish structural transitions during hydriding. The dehydriding pressures are path independent and were adopted for the evaluation of the thermodynamic properties of the MgCu 2(C15)-related phase II ( 1.3 ⩽ [H] [Pa] ⩽ 1.7 ) and the β-UH 3-related phase III ( 2.0 ⩽ [H] [Pa] ⩽ 3.0 ) which form at temperatures above 500 K. ΔH° f (II, PaH 1.3, 675 K) is −62.2 ± 2.0 kJ mol −1 and ΔH° f (III, PaH 3.0, 657 K) is −128.6 ± 6.9 kJ mol −1, while the corresponding ΔSδ f values are −80.3 ± 2.9 J K −1 mol −1 and −180.8 ± 9.9 J K −1 mol −1 respectively. Only hydriding pressures and the associated thermodynamic values were obtained for the α-UH 3-related phase IV ( 1.3 ⩽ [H] [Pa] ⩽ 3.0 ) which forms at temperatures below 500 K. In the absence of equilibrium thermodynamic results for phase IV, the relative stabilities of phases II and IV and of phases III and IV at low temperatures cannot be established.

Thermodynamics and Kinetics of Crystal-Amorphous Transformation in A1–x Bx Alloys During Reaction with Hydrogen

ABSTRACTThe kinetics of amorphous phase formation in polycrystalline A1–xBx (A=Zr, Hf, B=Pd, Rh,.15<x<.25) alloys during reaction with hydrogen has been studied by x-ray diffraction, +19F nuclear reaction depth profiling, TEM and electron diffraction. The formation of the amorphous hydride phase is observed by TEM to begin at grain bounderies of the polycrystalline Zr1–xRhx much in the same manner that “melting” nucleates at grain boundaries. TEM micrographs further show that the phase boundary between the crystalline and amorphous phases remains sharp during the growth of the amorphous phase. Both x-ray diffraction and nuclear depth profiling studies suggest that the overall rate of transformation to the amorphous hydride phase is limited by the rate of hydrogen permeation through the sample surface.Based on the present experiments and an analysis of the relevant free energy curves, we discuss the thermodynamic and kinetic aspects of this effect to explain why an amorphous phase is formed.

Hydrogen and deuterium in transition metal-p element compounds: Crystal chemical aspects of interstitial solid solubility and hydride phase formation

Compounds of transition metals with p elements of groups IIIb–VIIb which have been reported to exhibit hydrogen-absorbing properties are surveyed. Hydrogen is taken up only by those materials that have metallic properties or contain metal clusters with delocalized bonding. Almost all p elements can be constituents of these materials, and at least one transition metal component which is itself capable of dissolving hydrogen or forming hydride phases must be present. The crystallography of the hydride phases is presented and discussed with particular emphasis on results obtained by neutron diffraction methods. The hydrogen atoms are accommodated either in interstitial solid solutions with only minor changes in the host lattices or in new phases formed by major rearrangements of both transition metal and p element atoms. The distribution of the hydrogen atoms on the interstitial sites in the structures appears to be governed by two empirical rules: the hydrogen atoms are always more than 2 Å apart and they preferentially occupy those sites that are most distant from the p element atoms.