Hydrogen absorption properties of Li–Mg–N–H system

Isothermal hydrogen absorption properties of the ball milled mixture of 3Mg(NH₂)₂ and 8LiH after dehydrogenation at 200 °C under high vacuum were investigated at two different temperatures of 150 and 200 °C. The pressure–composition isotherm (PCT) curve at 200 °C revealed a two-plateaus-like behavior, while the PCT curve at 150 °C showed a single-plateau-like behavior. The hydrogenated phases were composed of LiH and Mg(NH₂)₂ under 9 MPa at 200 °C, while those were observed as mixed phases of LiH and LiNH₂ at 150 °C without any trace of Mg(NH₂)₂ in XRD measurements. These results indicate that there are two-step hydrogenation processes corresponding to high and low pressures at 200 °C, but the kinetics at 150 °C is too slow to proceed with the second hydrogenating step at high pressure region.     

Characterization of NH3 formation in desorption of Li–Mg–N–H storage system

Hydrogen energy may provide the means to an environmentally friendly future. One of the problems related to its application for transportation is “on board” storage. Hydrogen storage in solids has long been recognized as one of the most practical approaches for this. Recently the hydrogen storage system, (Li₃N + 2H₂  LiNH₂ + 2LiH), was introduced by Chen et al. [P. Chen, Z. Xiong, J. Luo, J. Lin, K.L. Tan, Nature 420 (2002) 302–304. [1]]. This type of material has attracted a great attention of the researchers from the metal hydride research community due to its high reversible storage capacity, up to 11.5 wt%. Currently the Li–Mg–N–H system has been shown to be able to deliver 5.2 wt% reversibly at a H₂ pressure of 30 bar and temperature of 200 °C. The accessibility of the capacity beyond 5.2 wt% is being actively explored. One of the issues related to the application of the metal–N–H storage systems is NH₃ formation that takes place simultaneously with H₂ release. NH₃ formation will not only damage the catalyst in a fuel cell, but also accelerate the cyclic instability of the H-storage material since the metal–N–H system turns into a metal–H system after loosing nitrogen and, therefore, it would not function at the temperature and pressure range designed for the metal–N–H system. The accurate determination of the amounts of NH₃ in the H₂ is, therefore, very important and has not been previously reported. Here a novel method to quantify NH₃ in the desorbed H₂, the Draeger Tube, is reported as being suitable for this purpose. The results indicate that the concentration of NH₃ in desorbed H₂ increases with the desorption temperature. For the (2LiNH₂ + MgH₂) system the NH₃ concentration was found to be 180 ppm at 180 °C and 720 ppm at 240 °C.     

Formation of a tetragonal phase hydride in Ti–Mo–H system

The structural relationship between the hydride phases in Ti–Mo–H solid solution system (Mo content up to 15 at% in the alloy) during dehydrogenation process under annealing has been studied by conventional and in situ X-ray powder diffraction and transmission electron microscopy (TEM) analysis. During dehydrogenation, the saturated hydrides of the Ti–Mo alloys with fcc δ-phase structure transfer into bcc β-phase at higher temperatures. An associated hydrogen concentration reduction for the δ-phase hydride is observed in the process. However, as the hydrogen concentrations decrease to certain values (H/M  1.1–1.7), the unsaturated δ-phase formed at high temperature would become unstable at lower temperature, and transfer into a tetragonal phase (denoted the -phase here). Unlike that of the -phase in Ti–H system, the phase transition does not occur for the saturated δ-phase with hydrogen concentration close to the stoichiometric limit. The hydrogen concentration of this -phase hydride is in between that of the tetragonal γ and -phase in Ti–H system, but more close to the γ-phase. The occurrence region of this -phase expands along with the increase of the Mo content in the alloys. The phase has a lattice similar to that of the -phase in Ti–H system with corresponding fct unit-cell c/a < 1.     

Influence of the rare earth composition on the properties of Ni–MH electrodes

Lattice parameters, hydrogen absorption properties and electrochemical cycling properties up to 240 cycles have been measured as a function of the Ce content for alloys of composition La_0.82−xCe_xNd_0.15Pr_0.03Ni_3.55Mn_0.4Al_0.3Co_0.75 (0≤x≤0.82). The results show the strong increase of the plateau pressure correlated to the cell volume decrease as a function of x. On the other hand, the hydrogen capacity measured in solid–gas reaction as well as the electrochemical capacity decreases slightly. The results show that both La and Ce have to be present to achieve a good cycle life, the cycling degradation being almost independent of their relative quantities in a broad range of concentrations.     

Structural and thermodynamic properties of the pseudo-binary TiCr2−xVx compounds with 0.0≤x≤1.2

The TiCr_2−xV_x compounds with 0.0≤x≤1.2 series have been synthesised and characterised by X-ray powder diffraction. X-Ray qualitative and quantitative phase analysis has been carried out on the as-cast alloys using the Rietveld method. The refinements of the structure shows that the materials crystallise either in the hexagonal or in the cubic Laves phase type for low V contents. For x>0.6, the system is found of b.c.c.-type structure only. The pressure–composition–temperature (P–C–T) isotherms measured at 298 K show that the as-cast alloys absorb large amounts of hydrogen, from 4 to 5.2 H/f.u. The P–C–T diagrams reveal also the presence of a relatively flat plateau, and a large hysterisis effect, and correspondingly the hydride cannot be completely dehydrogenated.     

Influence of machining conditions on tensile stress and ductility of a mechanically alloyed Fe–40Al intermetallic

The influence of a variety of machining conditions on the tensile behaviour of a mechanically alloyed FeAl intermetallic has been examined. Yield behaviour is hardly affected by machining conditions, and ductility is only weakly sensitive to machining speed or cutting depth. This behaviour seems to be related more with the extent of subsurface damage or work hardening than with the roughness of the machined samples.     

Interfacial reactions between Sn–8Zn–3Bi–xAg lead-free solders and Cu substrate

The formation and the growth of the intermetallic compounds (IMCs) at the interface between the Sn–8Zn–3Bi–xAg (x = 0, 0.5, and 1 wt.%) lead-free solder alloys and Cu substrate soldered at 250 °C for different durations from 5 to 60 min were investigated. It was found that Cu₅Zn₈ and CuZn₅ formed at Sn–8Zn–3Bi/Cu interface, and Cu₅Zn₈ and AgZn₃ formed at the solder/Cu interface when the solder was added with Ag. The thickness of IMC layers in different solder/Cu systems increased with increasing the soldering time. And the growth of the IMCs was found to be mainly controlled by a diffusion mechanism. Additionally, the growth of the IMC layers decreased with increasing content of Ag in the soldering process.     

Pressure–composition–temperature hysteresis in C14 Laves phase alloys: Part 3. Empirical formula

In Part 1 and Part 2 of this series of papers, the pressure–concentration–temperature (PCT) isotherms hysteresis was found to be closely related to the axial ratio a/c for both simple ternary and more complicated multi-element C14 Laves phase based alloys. Furthermore, the particle pulverization rate, which is the major determining factor in the duration of metal hydride electrode cycling, was found to correlate well with PCT hysteresis. In the current Part 3, we discuss an empirical equation which was developed to predict the PCT hysteresis of battery alloys through the study of the lattice constant ratios of a series of ZrCr₂-based ternary alloys. The empirical formula can then be used to estimate the pulverization rate of metal hydride electrode. To fit the empirical formula, an equivalent number of outer shell electrons for some non-transition metals was calculated from the axial ratio of ZrCr_1.8M_0.2 ternary alloys, where M is an element from the group of Al, Si, Ga, Ge, and Sn. Other factors, such as the amount of substitution, the difference in A and B element electronegativities, atomic size, and the choice of A element, were also investigated.     

Pressure–composition–temperature hysteresis in C14 Laves phase alloys: Part 1. Simple ternary alloys

In Part 1 of a series of two papers, the a/c lattice constant ratio of ZrCr₂-based ternary alloys are shown to be strongly correlated not only to the number of outside electrons of substitutional elements but also to the PCT absorption/desorption hysteresis and the degree of pulverization during hydride/dehydride cycling of the alloy. In differentiation from AB5 alloys in which elongating the c-axis dimension of the unit cell extends the alloy's electrochemical cycle life, flattening the unit cell of an AB2 alloy extends its cycle life. This difference can be explained by the different hydrogen occlusion sites of the two structures. Adding small amounts (10%) of substituents such as Zn, Cr, Mo, Si, or Cu, was generally found to help the prevention of the alloy hydride/dehydride pulverization by maintaining a relatively high a/c lattice constant ratio. Application of these principles to more complicated electrochemical hydrogen storage alloys can be found in Part 2 of this series.     

Ti–V–Mn based alloys for hydrogen compression system

Ti–V–Mn based hydrides are one family of alloys with improved hydrogenation properties and they have a great potential to replace the AB₅ alloys as the sorption materials in hydrogen compression systems, although there still are many problems associated with their use, including unstable reversible hydrogen capacity and unfavorable thermodynamic properties. To gain a better understanding on the effect of the substitution elements and to optimize the alloy composition for high storage capacity, the influence of the alloy stoichiometry was investigated. Ti–Zr–V–Mn alloys were prepared by arc melting technique and were annealed in vacuum at temperature above 900 °C to obtain great sorption properties. Hydrogen absorption and desorption kinetics and PCT characteristics of these alloys at ambient temperature were measured and compared. These hydrogen storage features were also discussed in relation to the effect of alloy element compositions. Ti–Zr–V–Mn alloy cycling behavior was also examined.     

Activation of hydrogen storage materials in the Li–Mg–N–H system: Effect on storage properties

We investigate the thermodynamics, kinetics, and capacity of the hydrogen storage reaction: Li₂Mg(NH)₂ + 2H₂  Mg(NH₂)₂ + 2LiH. Starting with LiNH₂ and MgH₂, two distinct procedures have been previously proposed for activating samples to induce the reversible storage reaction. We clarify here the impact of these two activation procedures on the resulting capacity for the Li–Mg–N–H reaction. Additionally, we measure the temperature-dependent kinetic absorption data for this hydrogen storage system. Finally, our experiments confirm the previously reported formation enthalpy (ΔH), hydrogen capacity, and pressure–composition–isotherm (PCI) data, and suggest that this system represents a kinetically (but not thermodynamically) limited system for vehicular on-board storage applications.     

Phase equilibria on the ternary Mg–Mn–Ce system at the Mg-rich corner

Three isopleths at the Mg-rich corner of Mg–Mn–Ce ternary system were investigated via thermal analysis, SEM/EPMA and XRD. A ternary eutectic reaction was observed at 1 wt.% Mn and 23 wt.% Ce and 592 °C. A solid-solution type ternary intermetallic compound, (Mg,Mn)₁₂Ce, was observed with 0.5 at% solid solubility of Mn in the tetragonal Mg₁₂Ce. With the aid of thermodynamic modeling and experiments, a revised phase diagram for the binary Mg–Ce system and the isopleths of 0.6, 1.8 and 2.5 wt.% Mn were proposed up to 25 wt.% Ce.     

Investigation of hydrogen discharging and recharging processes of Ti-doped NaAlH4 by X-ray diffraction analysis (XRD) and solid-state NMR spectroscopy

The processes occurring in the course of two sequential hydrogen discharging and recharging cycles of Ti-doped sodium alanate were investigated in parallel using XRD analysis and solid-state NMR spectroscopy. Both methods demonstrate that in hydrogen storage cycles (Eq. (1)) the majority phases involved are NaAlH₄, Na₃AlH₆, Al and NaH. Only traces of other, as yet unidentified phases are observed, one of which has been tentatively assigned to an Al–Ti alloy on the basis of XRD analysis. The unsatisfactory hydrogen storage capacities heretofore observed in cycle tests are shown to be due entirely to the reaction of Na₃AlH₆ with Al and hydrogen to NaAlH₄ (Eq. (1), 2nd hydrogenation step) being incomplete. Using XRD and NMR methods it has been shown that a higher level of rehydrogenation can be achieved by adding an excess of Al powder.     

Thermodynamic modeling of the Al–Cr system

By means of calculation of phase diagram (CALPHAD) technique, the Al–Cr system was critically assessed. Three solution phases (liquid, body-centered cubic, face-centered cubic) were modeled with the Redlich–Kister equation. The intermetallic compounds Al₇Cr, Al₁₁Cr₂, Al₄Cr, Al₈Cr₅, AlCr₂, which have a homogeneity range, were treated as the formulae Al₇(Al,Cr), Al₁₁(Al,Cr)₂, Al₄(Al,Cr), (Al,Cr)₈(Al,Cr)₅, (Al,Cr)(Al,Cr)₂ using two-sublattice model, respectively. A set of self-consistent thermodynamic parameters describing the Gibbs energy of each individual phase as a function of composition and temperature for the Al–Cr system was obtained.     

Processing and mechanical properties of multiphase composites based on Mo–Si–Al–C system

Mo–Si–Al–C-based multiphase compounds and their composites reinforced by micro-SiC and TiC particulates were manufactured by means of reactive hot-pressed sintering method. Their microstructure and room temperature mechanical properties were studied. The results showed that Al addition and the ratio of Si/Al exerted a remarkable effect on the reaction products in the Mo–Si–Al–C systems. For the stoichiometric Mo₅(Si,Al)₃C mixed powders with a molar ratio of Mo:Si:Al:C as 5:1.5:1.5:1, the sintered body contained Mo₃Si, Mo₃Al₂C, and Mo₅Si₃C as the major reaction products whereas and the minor phases consisted of MoSi₂, Mo₂C, and Mo(Si,Al)₂ compounds. When the starting powder mixture was off-stoichiometric with a small amount of excess Si, only Mo₂C accounted for the minor product. Moreover, the relative contents of the former three major phases were affected by the changed Si/Al ratio, where the amounts of Mo₃Al₂C and Mo₅Si₃C compounds decreased and increased, respectively with increasing Si/Al ratio. The two multiphase alloys showed poor mechanical properties, due to the existence of residual porosity. In contrast, the composites exhibited superiority in both flexural strength and fracture toughness at room temperature to the Mo–Si–Al–C-based multiphase compounds. MSAC1/20 wt.%SiC and MSAC1/20 wt.%TiC composites had a respective flexural strength and fracture toughness of 454 and 438 MPa, 4.93 and 4.85 MPa.     

The characteristics of Li(CoxNi1 − x)O2 cathode powders formed from the fine-sized Co3O4/NiO precursor powders

Li(Co_xNi_1 − x)O₂ (0 ≤ x ≤ 1) cathode powders were prepared by solid state reaction method using Co₃O₄/NiO precursor powders obtained by spray pyrolysis. The effect of the ratios of cobalt and nickel components on the characteristics of Co₃O₄/NiO precursor and Li(Co_xNi_1 − x)O₂ cathode powders were investigated. The Co₃O₄/NiO precursor powders with the ratios of cobalt and nickel components as 1/0, 0.75/0.25 and 0.5/0.5 had submicron size and regular morphologies. On the other hand, the Co₃O₄/NiO powders with the high contents of nickel component had aggregated morphologies of submicron size primary powders. The fine-sized precursor powders formed the fine-sized LiCoO₂ and Li(Co_0.75Ni_0.25)O₂ cathode powders by solid state reaction with LiOH powders. However, the high contents of the nickel component of the Co₃O₄/NiO precursor powders formed the Li(Co_xNi_1 − x)O₂ (0 ≤ x ≤ 0.5) cathode powders with aggregated morphologies and large sizes. The discharge capacities of the powders increased with increasing the nickel content into the Li(Co_xNi_1 − x)O₂ cathode powders up to 188 mAh/g.     

Influence of high-energy impact actions on the elastic and anelastic properties of martensitic Cu–Al–Ni crystals

The influence of high-energy impact shock-wave loading on the microplasticity and macroscopic performance of the Cu–Al–Ni crystals in the β₁′ martensitic phase has been studied. Elastic and anelastic properties of quenched and aged polyvariant single crystals before and after impact shock-wave loading were measured in the temperature range 80–300 K, at a frequency of about 100 kHz in the strain amplitude-independent and amplitude-dependent ranges by means of the composite oscillator technique, and in the MHz frequency range using the pulse–echo technique. High-velocity impact loading of the specimens was realised by plane shock-waves with stress pulses with a duration of 2·10⁻⁶ s and stress amplitudes up to 5 GPa. A pronounced influence of impact shock-wave loading on the elastic and anelastic properties of the β₁′ martensite has been observed. A strongly marked softening of the material and an enhancement of damping properties are revealed up to the highest stress pulse amplitudes. This behaviour differs fundamentally from the one observed in ‘ordinary’ fcc metals. Changes of the defect structure induced by shock-wave loading, which may be responsible for the observed phenomena, have been discussed.     

Chemical and thermodynamic properties of several Al–Ni–R systems

The standard enthalpies of formation at 300 K of the RNiAl phases (R=rare earth) have been obtained by using a high temperature direct reaction drop calorimeter and an aneroid isoperibol calorimeter. State and composition of the samples were checked by X-ray diffraction analysis. Metallographic examination was performed and the phases were further identified by electron microscopy and electron probe microanalysis. The results obtained are discussed and compared with those available for the binary RNi₂ and RAl₂ compounds.     

Hydrogen absorption properties of the slurry system composed of liquid C6H6 and F-treated Mg2Ni

The hydrogenation characteristics of the slurry composed of the NH₄F solution treated Mg₂Ni and liquid C₆H₆ were studied. The F-treatment results in a net-shaped MgF₂ surface and higher nickel content in the sub-layer. It is found that the hydride of the NH₄F treated alloy has a much higher activity for the hydrogenation of benzene. The catalytic activity for hydrogenation of the alloy depended strongly on the surface properties of the catalyst. At 483 K and under a hydrogen pressure of 4.0 MPa, the alloy absorbed hydrogen first, transformed into hydride and then the benzene was hydrogenated to cyclohexane with the hydride as the catalyst. The hydrogen absorption capacity of slurry system composed of 20 wt.% treated alloy and benzene reached 6.4 wt.% and the hydrogenation completed in 20 min. Results of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) analysis on the crystal structure, surface composition and surface morphology of the untreated and treated alloy are presented and discussed.