AB2 Phases Research Articles

Hydrogenation behavior of a C14 Laves phase under ultra-high hydrogen pressure

For high pressure metal hydride (MH) compressor applications, it is important to have a thorough understanding of the hydrogenation properties of MH forming compounds under high pressure and temperature conditions, which are still little studied. Here, a Ti0.90V0.30Mn1.00Ni0.80 compound with MgZn2-type AB2 structure has been investigated by Sieverts’ method over a wide pressure and temperature range (up to 100 MPa and between −80 and 120 °C). This provided clear experimental evidence of the downward curvature of the van’t Hoff plot in the high-pressure region, which arises from the non-ideal behavior of hydrogen. In this paper, we also show that the high-sorption pressures can be well estimated by the van’t Hoff equation using low-pressure data. Ultimately, this study was combined with synchrotron radiation X-ray diffraction (SR-XRD) performed in an effort to monitor the structural evolution of the compound in situ under ultra-high hydrogen pressure. We found that even under extreme conditions of 2 GPa and 200 °C, both the main AB2 phase and the secondary CsCl-type AB phase were fully hydrogenated and their structures were conserved. The existence of a critical temperature for the AB2-H2 system was demonstrated from the evolution of the SR-XRD patterns recorded for several temperatures during the desorption process.

Coarse-grained versus fully atomistic machine learning for zeolitic imidazolate frameworks.

Zeolitic imidazolate frameworks are widely thought of as being analogous to inorganic AB2 phases. We test the validity of this assumption by comparing simplified and fully atomistic machine-learning models for local environments in ZIFs. Our work addresses the central question to what extent chemical information can be "coarse-grained" in hybrid framework materials.

Activation of Ti–Fe–Cr alloys containing identical AB2 fractions

TiFe-based alloys are solid-state hydrogen storage materials operated at room temperature (RT). The current study presents a systematic approach for solving the activation issue (difficulty in the first hydrogenation), one of the major obstacles to the practical application of TiFe alloys, via the use of a secondary AB2 phase. Based on the Ti–Fe–Cr ternary phase diagram, Ti–Fe–Cr alloys containing 80 at% Ti(Fe,Cr) (AB phase) and 20 at% Ti(Fe,Cr)2 (AB2 phase) were designed; the Cr concentrations in the AB and AB2 phase were systematically varied while maintaining fixed phase fractions. Activation at RT was achieved when the overall Cr concentration was higher than 9.7 at%. Analysis of the activation characteristics of the individual phases revealed that the AB2 phase readily absorbed hydrogen, thereby initiating activation of AB + AB2 alloys. Notably, higher Cr concentrations enable the AB phase to absorb hydrogen at RT during the activation process, although the kinetics are much slower than that of the co-existing AB2 phase. The equilibrium hydrogen pressures from the pressure-composition isotherms decrease as the Cr concentration increases, indicating that Cr stabilizes hydrides. Increased hydride stability may also promote the kinetics of the initial hydride formation in both the AB and AB2 phases. An optimal composition for Ti–Fe–Cr alloys can be designed given the conditions of easy activation at RT and maximum reversible capacity within an operating pressure range.

Effects of cerium on the hydrogen absorption-desorption properties of rare earth-Mg-Ni hydrogen-absorbing alloys

Abstract The influence of Ce addition on the phase constitution, microstructure, hydrogen absorption/desorption properties and battery performances of newly developed rare earth (RE)-Mg-Ni hydrogen-absorbing superlattice alloys for negative electrode materials in Ni-metal hydride (MH) batteries were investigated. The partial substitution of RE (La and Nd) with Ce results in a higher discharge performance and a lower cycle life in the battery. The Ce addition greatly affects the phase constitution, which is mainly characterized by increased formation of the AB2 phase (A = RE or Mg and B = Ni or Al). The existence of the AB2 phase is found to accelerate alloy pulverization and oxidation when the alloys are used as negative electrode materials in Ni-MH model cells. The accelerated pulverization and oxidation are considered to be responsible for the observed higher discharge performance and lower cycle life in the batteries, respectively.

Effect of annealing treatment on the anti-pulverization and anti-corrosion properties of La0.67Mg0.33Ni2.5Co0.5 hydrogen storage alloy

The La0.67Mg0.33Ni2.5Co0.5 hydrogen storage alloy was prepared by the vacuum intermediate frequency induction furnace followed by annealing treatment. The pulverization degree of both the as-cast and annealed alloy powders after gaseous hydriding and dehydriding cycle was investigated and the discovery was that annealing treatment could hardly ameliorate their anti-pulverization ability. The element content of La, Mg, Ni and Co existing in electrolyte before and after the electrochemical cycles by using ICP-AES technology was also analyzed and it showed that a large amount of La and Mg were dissolved in the electrolyte, but the amount of dissolution for La and Mg significantly declined when the alloy was annealed. The XRD analysis revealed that all the alloys consisted of two main phases AB3 and AB2 and a residual phase AB5 while annealing treatment made the AB2 phase decrease slightly. Furthermore, the anti-corrosion abilities of various elements in different phases of the as-cast and annealed alloy samples were studied by analyzing the element (La, Mg, Ni, Co) change with the corrosion time in phases AB3 and AB2 by means of EDS. It turned out that the element of La was mainly corroded out from the phase AB2 while not easily from the phase AB3. However, the element of Mg was both easily corroded out from the phases AB2 and AB3, but the corrosion was more obvious in the phase AB3. Therefore, annealing improved the anti-corrosion performances of La and Mg in the phase AB2.

Different failure modes for V-containing and V-free AB2 metal hydride alloys

Failure modes of a V-containing and a V-free AB2 Laves phase-based metal hydride alloy were studied by the combination of X-ray diffractometer, scanning electron microscope, X-ray energy dispersive spectroscopy, inductively coupled plasma, Soxhlet extraction, and magnetic susceptibility measurement. Cells with the V-containing alloy exhibited less capacity degradation up until venting occurred in the cells, after which the capacity rapidly degraded. Cells with the V-free alloy remained linear in capacity degradation throughout the cycle life test. The failure mechanism for the V-containing alloy is related to the formation of an oxide layer that penetrates deeper into the alloy particles due to high V leaching and impedes gas recombination, while the failure mechanism for the V-free alloy is related to the continuous pulverization of the main AB2 phase.

Improvement in high-rate dischargeability, activation, and low-temperature performance in multi-phase AB2 alloys by partial substitution of Zr with Y

The changes in structure, gaseous phase hydrogen storage, and electrochemical properties due to the incorporation of Y in the alloy formula have been studied on two series of AB2 alloys, with an originally C14- (relatively higher Cr-content) and a C15- (relatively higher Co-content) predominated microstructure, respectively. In each series, 1–4 at.% of Y was added to the designed formula and partially replaced Zr. A majority of the added Y went into the secondary phases, such as YNi, (Y, Sn)-rich, and Y2O3, and only a small amount being found in the main AB2 phase. The lattice constants and the unit cell volume decreased due to the reduction in Zr-content. The C15 phase abundances of the Y-containing alloys were higher than those of the Y-free alloys in both series. As the Y-content increased, the gaseous phase hydrogen storage capacity measured at 30 °C decreased in the high-Cr series and increased in the high-Co series. Both the hydrogen mid-point pressures and the electrochemical discharge capacities of the Y-containing alloys were lower than those in the Y-free alloys in both series. Both the activation and high-rate dischargeability (HRD) improved with the increase in Y-content. The −40 °C resistance was substantially reduced with the incorporation of Y in both series.

Structural and electrochemical properties of Ti 1.5Zr 5.5V xNi 10− x

Quaternary alloys with the formula Ti 1.5Zr 5.5V x Ni 10− x ( x between 0 and 3.0) were studied as a potential replacement for Laves phase alloys used as the negative electrode active material in nickel metal hydride batteries. The V-containing alloys all show multi-phase structures. The major phase shifts from a Zr 7Ni 10 structure to a Zr 9Ni 11 structure and finally to a C14 structure as the vanadium content increases. Other minor phases with C15 and ZrNi crystal structures are also present. The solubility of vanadium is high in AB2 phases (both C14 and C15), moderate for the ZrNi phase and very low for Zr 7Ni 10 and Zr 9Ni 11 phases. The bulk hydrogen transport property of the alloys is dominated by synergetic effects between major and minor phases. Electrochemical testing shows that the highest discharge capacity, 357 mAh/g, was obtained from an alloy with a chemical composition of Ti 1.5Zr 5.5V 2.5Ni 7.5 and mainly C14 structure. Testing also shows the high rate dischargeability is controlled by the surface reaction and Ti 1.5Zr 5.5V 0.5Ni 9.5 has the best high rate dischargeability.

Studies of Sn, Co, Al, and Fe additives in C14/C15 Laves alloys for NiMH battery application by orthogonal arrays

The effects of Sn, Co, Al, and Fe as additives to C14/C15 Laves phase alloy in nickel metal hydride battery are studied with modified L-16 orthogonal arrays. Sixteen alloys with the general formula T i 9 Z r 2 − w V 5 N i 38 C r 5 − x M n 16 − y − z S n w C o x A l y F e z were prepared and made into sealed cells. The main components of these alloys are C14 and C15 structured AB2 phases with significant difference in Ni-content. Other minor phases found are Zr 2Ni 2Sn, ZrNi, Zr 7Ni 10, and Zr 9Ni 11. In the gas phase study, while the variations in pressure–concentration isotherm hysteresis and slope factor are small, the detrimental effects to storage capacity from Sn and Al are more evident than those from Co and Fe. Al lowers the equilibrium pressure plateau while the other three additives raise it. In the electrochemical study, a higher level of Sn promotes a greater content of the Zr 2Ni 2Sn phase which is less stable in an alkaline environment and has a detrimental effect on both charge retention and cycle life performance. Sn is also not desirable for applications with a demanding request of high-rate and low-temperature performance. The addition of Sn is to ease the activation and to reduce raw material cost by allowing the use of less expensive zirconium source. Adequate amounts of Co and Al addition improve activation, low temperature, and specific power of the battery. Addition of Fe makes activation more difficult and decreases specific power, but improves both charge retention and cycle life. We propose two recipes; one level composed of 1.5% Co and 0.4% Al of additives for power applications and the other with 1% Co and 0.4% Fe for a good balance among other battery properties.

Identification and Optimization of AB2 Phases Using Principal Component Analysis, Evolutionary Neural Nets, and Multiobjective Genetic Algorithms

Available data for a large number of AB2 compounds were subjected to a rigorous study using a combination of Principal Component Analysis (PCA) technique, multiobjective genetic algorithms, and neural networks that evolved through genetic algorithms. The identification of various phases and phase-groups were very successfully done using a decision tree approach. Since the variable hyperspaces for the different phases were highly intersecting in nature, a cumulative probability index was defined for the formation of individual compounds, which was maximized along with Pauling's electronegativity difference. The resulting Pareto-frontiers provided further insight into the nature of bonding prevailing in these compounds.

Catalytic effects of pseudo AB2 phases on hydrogen sorption

Abstract The compound GdNi 4 Mg in its amorphous and crystalline state has been investigated as catalyst for hydrogen sorption of magnesium. Addition of only 0.3 at.% of catalyst by ball milling resulted in an important improvement of hydrogen sorption kinetics. Optimum milling time has been found to be 5 h. The amorphous and crystalline GdNi 4 Mg compounds give almost identical kinetics. Both absorption and desorption have been improved and the kinetics obtained are almost the same as other metal catalysts. No evidences of decomposition during ball milling or hydrogenation have been found so that it should be concluded that the intermetallic itself acts as a catalyst.

Structural and Magnetic Properties of New Zr(Fe<sub>0.8</sub>Cu<sub>0.2</sub>)<sub>2</sub> and Zr(Fe<sub>0.8</sub>Cu<sub>0.1</sub>Co<sub>0.1</sub>)<sub>2</sub> Hydrogen Storage Materials

In this work we investigate the effect of hydrogen absorption/desorption at room temperature on the structural, magnetic and microstructural characteristics of Zr(Fe0.8Cu0.2)2 and Zr(Fe0.8Co0.1Cu0.1)2. Hydrating kinetics of the as cast bulk samples have been examined while in both samples an anomalous behavior at the absorption part of the P-C curve has been observed. Co doped sample absorbs higher hydrogen content at lower pressure. Crystal structure analysis has been performed by using the Rietveld method. Co free sample has saturation magnetization of 50.85 Am2/kg while Co doped sample has 54.04 Am2/kg at external field of 1.8 T. After hydrogenation the magnetization decreases. Thermomagnetic analysis in the range of 4.2 to 1100 K reveals that the Curie temperature of AB2 phases is in the range of 510 to 550 K. Scanning electron microscopy with energy dispersive x-ray spectroscopy was used in order to examine the composition and surface morphology of the bulk samples. The grain size reduces due to hydrogenation and this is attributed to the more hysteretic magnetization curve.

Effects of La addition on the electrochemical behavior and F treatment of Zr[sbnd]Ti[sbnd]V[sbnd]Ni alloys

A new class of materials and new treatment method are proposed to improve the electrocatalytic activity of the AB2-type alloys without making use of highly electrocatalytic precious metals such as Pd or Pt. The effects of La on the AB2 hydriding compounds Zr0.5Ti0.5V0.75Ni1.5 were investigated in terms of alloy microstructure, initial activation, kinetics, hydrogen capacity and F treatment effect. Various analytical techniques such as electron probe micro analysis, electron spectroscopy for chemical analysis, inductively coupled plasma spectroscopy and X-ray diffraction are employed to characterize the alloy structure and surface. It was found that La addition of the ZrTiVNi alloys benefit electrode kinetics, and F-treated alloys exhibit easy activation. La is not soluble in the AB2 phase but rather forms discrete La-rich inhomogeneous sites in the microstructure. These sites remain on the surfaces of crushed powder particles and serve as H-transparent windows through the less transparent AB2 oxide layer, providing for the fast electrochemical hydrogen reaction. An Ni-rich layer with microcave structure appeared on the F-treated alloy surface and is of benefit to alloy activation. After F treatment the LaNi phase on the La added alloy surface disproportioned into LaF3 and Ni clusters. The Ni clusters in loose LaF3 layer catalyze the electrochemical reaction and accelerate alloy activation.

F-treatment effect on the hydriding properties of the La-substituted AB2 compound (Ti, Zr)(Mn, Cr, Ni)2

The La-substitution effects of AB2 hydriding compounds (Laves phase) Ti0.5−x/2Zr0.5−x/2LaxMn0.8Cr0.8Ni0.4 (x = 0.1-0.1) on the hydriding properties including initial activation characteristics, pressure-composition-temperature property and long storability in air and water have been investigated and compared with those of F-treated alloys. It was found that a small La substitution could dramatically improve the initial activation and surface protection against air or water exposure, especially after F-treatment. La could not be substituted into the AB2 phase but formed La-rich particles which served as catalytic sites for hydrogen uptake and protected against the normally damaging of F-treatment on the AB2 alloy surface. No significant deterioration of the surface reactivity was observed for F-treated 0.05La alloy even after 6 months exposure to air and water. The optimum La content should be in the range of 0.03–0.05.

Phase composition and susceptibility to intercrystalline corrosion of steel 000Kh16N15M3 (� I844)

1. It was found that aging of steel 000Kh16N15M3 at 650–750°C for 100 and 1000 h results primarily in precipitation of M23C6 cubic carbide. 2. Reducing the carbon content below 0.01% and also alloying with silicon and niobium favor formation of the hexagonal AB2 phase. 3. No relationship was found between ICC susceptibility and an increase of the chromium concentration in the carbide or the amount of chromium-containing phase.