AB5 Alloys Research Articles

On the melting of interstitial alloys FeH, FeSi and FeC with a body-centred cubic structure under pressure

Taking the model of interstitial alloy AB with a body-centred cubic structure and the condition of absolute stability for the crystalline state, we derive analytic expression for the temperature of the limit of absolute stability for the crystalline state, the melting temperature, and the equation for the melting curve of this alloy using the statistical moment method. The results allow us to determine the melting temperature of alloy AB under pressure as well as at zero pressure. In limit cases, we obtain the melting theory of main metal A with a body-centred cubic structure. The theoretical results are numerically applied for alloys FeH, FeSi and FeC using different potentials.

Thermal expansion coefficient of BCC defective substitutional alloy AB with interstitial atom C

The analytic expressions for the free energy, the concentration of equilibrium vacancies, the isothermal compressibility, the thermal expansion coefficient and the heat capacities at constant volume and at constant pressure of BCC substitutional alloy AB with interstitial atom C and with vacancy under pressure are derived by the statistical moment method. The theoretical results are applied to the thermal expansion coefficient of alloy FeCrSi in the interval of temperature from 700 K to 1100 K, in the interval of substitutional atom concentration from 0% to 10%, in the interval of interstitial atom concentration from 0% to 5% and in the interval of pressure from 0 GPa to 10 GPa. Our calculated results for main metal Fe are compared with the experimental data.

Study on the Melting of the Defective Interstitial Alloys TaSi and WSi with BCC Structure

The statistical moment method is used to study the melting of defective interstitial AB alloys, where A is the main element and B is an interstitial atom, with a body-centered-cubic (BCC) structure. The melting temperature of the AB alloy with defects is obtained from the temperature of absolute stability for the crystalline state and the equilibrium vacancy concentration. Numerical calculations are performed for the interstitial alloys TaSi and WSi. Our calculated results are in good agreement with other calculations.

STUDY ON ELASTIC DEFORMATION OF INTERSTITIAL ALLOY FeCWITH BCC STRUCTURE UNDER PRESSURE

The analytic expressions of the free energy, the mean nearest neighbor distance between two atoms, the elastic moduli such as the Young modulus E, the bulk modulus K, the rigidity modulus G and the elastic constants C11, C12, C44 for interstitial alloy AB with BCC structure under pressure are derived from the statistical moment method. The elastic deformations of main metal A is special case of elastic deformation for interstitial alloy AB. The theoretical results are applied to alloy FeC under pressure. The numerical results for this alloy are compared with the numerical results for main metal Fe and experiments.

Hydrogenation and crystallization of amorphous phase: A new mechanism for the electrochemical capacity and its decay in milled Mg[sbnd]Ni alloys

MgNi-based alloys have been regarded as very promising anode material for nickel-metal hydride battery owing to their electrochemcial capacity much higher than that of AB5 alloys. However, MgNi alloy anodes generally suffer very serious capacity decay in cycling, which is previously ascribed to the corrosion of Mg in alkali electrolyte in the past decades. To further reveal the capacity fading mechanism of MgNi-based alloys, the electrochemcial hydrogen storage properties of Mg2Ni and amorphous Mg50Ni50 alloys and their microstructural evolution during cycling were comparatively studied. It has been firstly demonstrated that it is the amorphous MgNi phase instead of Mg2Ni phase contributes to electrochemical discharge capacity, because the Mg2NiH4 hardly released hydrogen under electrochemcial conditions. Then, it has been found that the charge input level has significant effect on the cyclic performance of amorphous Mg50Ni50 alloy anode. At full charge of 500 mAh g−1, the amorphous MgNi phase easily crystallized to the nanocrystalline Mg2NiH4, which leaded to the capacity decay immediately. Comparatively, with the low charge input of 100 mAh g−1, the discharge capacity of amorphous Mg50Ni50 alloy remained almost unchanged for 425 cycles. The excellent cyclability is attributed to the less hydrogenation-induced crystallization at low charge input. This work clearly demonstrates that the hydrogenation-induced crystallization is a dominating reason for the electrochemical capacity decay in the milled amorphous MgNi anode. It provides a new approach to improve the electrochemcial properties of MgNi alloy electrode by inhibiting the hydrogenation-induced crystallization.

Thermodynamic properties of BCC interstitial alloy FeH under pressure

The analytic expressions of the mean nearest neighbor distance, the free energy and the thermodynamic quantities for interstitial alloy AB with body-centered cubic (BCC) structure under pressure are derived by the statistical moment method. The theoretical results are applied to interstitial alloy FeH. Our calculated results for Fe as main metal are in good agreement with experiments.

Thermodynamic properties of FCC interstitial alloy AuSi with defects

The analytic expressions for free energy, concentration of the equilibrium vacancies and thermodynamic quantities such as the isothermal compressibility, thermal expansion coefficient and heat capacities at constant volume and at constant pressure of FCC interstitial alloy AB with defects under pressure are derived by the statistical moment method. The theoretical results are applied to AuSi interstitial alloy. Our calculated results for main metal Au are compared with the experimental data.

Study on the melting of interstitial alloys FeH and FeC with BCC structure under pressure

From the model of interstitial alloy AB with BCC structure and the condition of absolute stability for crystalline state we derive analytic expression for the temperature of absolute stability for crystalline state, the melting temperature and the equation of melting curve of this alloy by the way of applying the statistical moment method. The obtained results allow us to determine the melting temperature of alloy AB at zero pressure and under pressure. In limit cases, we obtain the melting theory of A main metal with BCC structure. The theoretical results are numerically applied for alloys FeH and FeC with using different potentials.

Enhanced kinetics of hydrogen electrosorption in AB5 hydrogen storage alloy decorated with Pd nanoparticles

The surface of an AB5 alloy (LaMmNi4.1Al0.3Mn0.4Co0.45) was modified with Pd nanoparticles using a microwave-assisted polyol method. Cyclic voltammetry (CV) and chronoamperometry (CA) were used to characterize the electrochemical behavior of the resulting composite in 6 M KOH electrolyte. The microwave-assisted polyol method makes it possible to obtain Pd nanoparticles of irregular shapes and sizes, unevenly distributed on the AB5 surface. CV and CA results showed that the kinetics of hydrogen absorption/desorption processes were enhanced after modification of the AB5 surface with Pd nanoparticles. Furthermore, no electrochemical activation is required in the case of the Pd-modified AB5 alloy.

Study on elastic deformation of substitution alloy AB with interstitial atom C and BCC structure under pressure

The analytic expressions of the free energy, the mean nearest neighbor distance between two atoms, the elastic moduli such as the Young modulus E, the bulk modulus K, the rigidity modulus G and the elastic constants C11, C12, C44 for substitution alloy AB with interstitial atom C and BCC structure under pressure are derived from the statistical moment method. The elastic deformations of main metal A, substitution alloy AB and interstitial alloy AC are special cases of elastic deformation for alloy ABC. The theoretical results are applied to alloy FeCrSi. The numerical results for alloy FeCrSi are compared with the numerical results for main metal Fe, substitution alloy FeCr, interstitial alloy FeSi and experiments.

(Invited) Design of Electrocatalysts with Ad-Atoms, Single Crystals and Supported Nano-Particles for the Applications to Fuel Cells

For the wide applications of any types of fuel cells, the performance, durability and cost-reduction are essential. We have worked on developments of the materials, structures and the relating mechanisms for DMFC, PAFC, SOFC and PEFC. In this presentation, the author will briefly review first some major discoveries noted topics 1 and 2 below, but mainly discuss on the recent results relating to topic 3, obtained by e. g. “HiPer-FC Project”, targeted the development of high performance & durable electrocatalysts and hydrocarbon type PEMs based on mechanistic studies of the enhancements or degradations. Development of catalysts for direct methanol fuel cells (DMFCs) DMFCs are attractive by the high volumetric power-density, but they were accompanied with a large potential-loss at the anode even at Pt with the highest activity among pure catalysts. We discovered that in the classification of alloying components into relatively active A-metals (Pt, Ir, Pd, Rh etc.) and inert B-metals (Ru, Os, Re, etc.), AB alloys exhibit a noticeable enhancement and its optimum composition of about 1/1 for the methanol oxidation and AA alloys showed just a dilution effect. We proposed the “Bi-Functional Mechanism” on the catalysis, where each A and B atom contributes to the enhancement by playing a different role in the reaction scheme and generalized the theory by extending to several types “Electrocatalysis by Ad-Atoms” as convenient tools for new catalyst developments. The preparation of nano-sized Pt-Ru supported on carbon black (CB), invented by us, and the performance have widely been referred and the same catalyst has been uniquely applied to both of commercialized DMFCs and polymer electrolyte fuel cells (PEFCs) in residential co-generation systems. Development of less-expensive Pt-alloy catalysts for anode/cathode Much tolerance to the poisoning by CO remained in reformed H2 fuel is needed to the anode Pt catalyst for commercialized PEFCs. On the other hand, a management of the improvement of performance and the reduction of Pt loading at the cathode are essential because O2 reduction reaction (ORR) is much slower than that of H2 oxidation reaction (HOR). We discovered that a superior tolerance even to 100ppm CO for HOR can be achieved by alloys between non-precious transient-metal groups such as Fe, Co, Ni etc. with the other precious metal groups such as Pt, Pd, Rh etc. by the lowered CO adsorbing strength and coverage on their surfaces. The alloys exhibit a positive core-level shift in Pt4f binding energy, which is inversely proportional to Pt-CO bonding energy, vice vasa at non-CO tolerant alloys. Thus, a guiding principle for the development of CO tolerant anode catalysts is established. On the other hand, we also discovered an enhanced ORR at Pt alloyed with Co etc, which is more than one order of magnitude at their optimum compositions of 30-50%, compared with pure Pt. By efficient and unique applications of the electrochemical method (EC) together with modern experimental systems, we have clarified that the alloys are covered with a “Pt skin” consist of 1-2 atomic-layer modified in the electronic structure by that of underlying alloy, resulting in the noticeable enhancement of ORR as mentioned above. Development of highly dispersed catalysts and the performances and durability Reduction of Pt loading to less than 1/10 from the presently used level, corresponding to the amounts for the treatment of exhausting-gas from ICE cars, i.e., 10gPt/100kW, is essential for the wide penetration to fuel cell vehicles (FCVs) market. We aimed to overcome the difficult hurdle, i.e., the increase of the mass activity MA which is proportional to the specific surface area S and the specific area activity J, by nano-sizing (to ca. d=2nm in diameter) and alloying (as mentioned above) of the advanced catalysts, respectively. We proposed an original “Territory Theory” and demonstrated experimentally there is no reduction in the MA caused by the so-called “Particle Size Effect” even at d=2nm as long as the inter-crystallite distance is kept larger than ca. 15nm on the supporting substrates, resulting in the MA increased proportionally to increased S-value. Based on the above results, we invented a “Nano-Capsule Method” for the preparation of nano-sized pure and alloy catalysts, which are controlled strictly in the size, composition and dispersion state on the supporting materials and has been demonstrating their extremely high performance and durability in comparison with widely commercial catalysts and the same level durability as that of larger-size ones, independent of the particle sizes. We have developed HC-type PEMs and already demonstrated single PEFC operations for 5,000 hrs with these advanced PEMs.

Influence of co-milling with palladium black on hydrogen sorption performance and poisoning tolerance of surface modified AB5-type hydrogen storage alloy

This work is focused on the effect of Pd introduction into AB5-type hydrogen storage alloy through ball milling, in combination with conventional electroless plating technique, on hydrogen sorption performance and poisoning tolerance of the material. The AB5 substrate alloy was milled with small (0–5 wt%) amounts of Pd black. The effect of co-milling of the AB5 substrate with Pd on structure/phase composition, morphology, and hydrogen storage performance of the material was investigated.It was shown that co-milling of the AB5-type alloy powder with ≤1 wt% of Pd black does not result in the improvement of the material activation performance and poisoning tolerance while increasing Pd additive content produced insignificant improvements. At the same time, ball milled samples, further subjected to a standard procedure of electroless autocatalytic deposition of Pd, exhibited similar (co-milling with <1 wt% Pd) or better (co-milling with ≥1 wt% Pd) hydrogen uptake kinetic performances at lower Pd surface loading as compared to the non-milled AB5 alloy powder plated with Pd at similar conditions. The observed features were credited to the higher specific surface area of the ball milled material where Pd species introduced in the course of the ball milling played a role as nucleation centres during subsequent deposition of Pd. Conversely, the Pd-plated material obtained by co-milling of AB5 with >2 wt% of Pd black exhibited lower maximum hydrogen sorption capacities, as compared to the non-milled surface modified reference material. The most probable reason for that is in the distortion and disproportionation of the parent structure of the AB5 substrate during its co-milling with Pd black.

Study on the melting of interstitial alloy AB with FCC structure under pressure

From the model of interstitial alloy AB with FCC structure and the condition of absolute stability for crystalline state we derive analytic expression for the temperature of absolute stability for crystalline state, the melting temperature and the equation of melting curve of this alloy by the way of applying the statistical moment method. The obtained results allow us to determine the melting temperature of alloy AB at zero pressure and under pressure. In limit cases, we obtain the melting theory of main metal A with FCC structure. The theoretical results are numerically applied for alloys AuSi and AgSi.

The Influence of Air Exposition of the Zr-Mn-Cr-Ni-Al Alloy on Cycle Life

It was found by scanning electron microscopy method that the ZrMn0.5Ni1.2Cr0.18А0.1 alloy has a dendritic structure, and the shooting of a typical section of the surface of the metallographic sample in characteristic radiation determined its chemical heterogeneity. The X-ray diffraction method has found that the C15 and C14 are the main phases of the alloy. In addition, the alloy contains Ni10Zr7 and Ni11Zr9 secondary phases. The method of potentiometric cycling has established that the air exposition of ZrMn0.5Ni1.2Cr0.18А0.1 alloy powder results in an increase in the electrochemical stability of the electrodes pressed from this powder and causes a significant increase of their cycle life. It is important that the cycle life of the AB2 alloy doped simultaneously with chromium and aluminum increased. Such doping is usually carried out in order to increase the cycle life due to the creation of hydrogen penetrating stable oxide films. Alloys with the same content of the Ni10Zr7 phase have the same activation rate of the initial electrodes. According to investigations, corrosion of the material originates on the interphase surface and begins to spread along it, indicating its pitting nature, and the surface of the pitting itself has the form of flake.

The cluster-plus-glue-atom models of solid solution CuNi alloys: A first-principles study

The solid solution alloys containing short-range order (SRO) structure, such as CuNi alloys, are not well structurally described using conventional quasi-disordered models. Based on the first-principles calculations, the optimal cluster-plus-glue-atom (CPGA) models of six typical solid solution CuNi alloys were demonstrated. These CPGA models are better to be used to describe the microstructure of the six CuNi alloys at the atomic scale from the viewpoint of energy. Our further study shown that the stabilities of CPGA models are highly related to the symmetry of initial CPGA models and the aggregation degree of solute atoms in CPGA models. In the condition of low solute Ni concentration, the stability of CPGA model is mostly determined by the symmetry of initial CPGA model, the CPGA model with high symmetry is more stable. As the concentration of solute Ni increasing, the aggregation degree of solute Ni atoms in CPGA models plays more and more important role. The CPGA model with higher aggregation degree of solute Ni is more stable. The simple selection method of optimal CPGA models, therefore is proposed, and which can be extended to other bielement solid solution AB alloys with positive mixing enthalpy of A-B. Using the selected CPGA models, we found the third additive atoms having negative mixing enthalpy with solvent and solute atoms prefer to reside the central site of characteristic principal cluster (CPC) of CPGA, and thereby enhancing the stability of alloys. Our studies provide a distinctive perspective to select the optimal CPGA models of solid solution alloys containing SRO structure and design alloy using the optimal CPGA model.

Performance Comparison between AB5 and Superlattice Metal Hydride Alloys in Sealed Cells

High-power cylindrical nickel metal/hydride batteries using a misch metal-based Al-free superlattice alloy with a composition of La11.3Pr1.7Nd5.1Mg4.5Ni63.6Co13.6Zr0.2 were fabricated and evaluated against those using a standard AB5 metal hydride alloy. At room temperature, cells made with the superlattice alloy showed a 40% lower internal resistance and a 59% lower surface charge-transfer resistance compared to cells made with the AB5 alloy. At a low temperature (−10 °C), cells made with the superlattice alloy demonstrated an 18% lower internal resistance and a 60% lower surface charge-transfer resistance compared to cells made with the AB5 alloy. Cells made with the superlattice alloy exhibited a better charge retention at −10 °C. A cycle life comparison in a regular cell configuration indicated that the Al-free superlattice alloy contributes to a shorter cycle life as a result of the pulverization from the lattice expansion of the main phase.

Effects of Alkaline Pre-Etching to Metal Hydride Alloys

The responses of one AB5, two AB2, four A2B7, and one C14-related body-centered-cubic (BCC) metal hydrides to an alkaline-etch (45% KOH at 110 °C for 2 h) were studied by internal resistance, X-ray diffraction, scanning electron microscope, inductively coupled plasma, and AC impedance measurements. Results show that while the etched rare earth–based AB5 and A2B7 alloys surfaces are covered with hydroxide/oxide (weight gain), the transition metal–based AB2 and BCC-C14 alloys surfaces are corroded and leach into electrolyte (weight loss). The C14-predominated AB2, La-only A2B7, and Sm-based A2B7 showed the most reduction in the internal resistance with the alkaline-etch process. Etched A2B7 alloys with high La-contents exhibited the lowest internal resistance and are suggested for use in the high-power application of nickel/metal hydride batteries.

Comparative study of hydrogen electrosorption from alkali metals electrolytes and hydrogen sorption from gas phase in AB5 alloy

AB5-type metal alloy (LaMm–Ni4.1Al0.3Mn0.4Co0.45) was investigated in alkali metal hydroxide electrolytes (LiOH, NaOH, KOH, RbOH, CsOH) at various temperatures (0–30°C) with no binding materials used. Limited volume electrode approach (LVE) was used in all electrochemical measurements. PCT Sievert’s method was used to determine maximum hydrogen capacity of the alloy to be 4.7at.H/f.u. (289mAh/g). The highest electrochemical hydrogen capacity, nearly reaching the maximum value, was obtained in 6M KOH solution at 30°C (275.1mAh/g) and in mixed 6M LiOH/KOH solution at 30°C (276.1mAh/g). Temperature influence on electrochemical capacity of the alloy was investigated. In the temperature range of 0–30°C the capacity is high and stable in 6M KOH solution and mixed 6M LiOH/KOH and 6M NaOH/KOH solutions. It is possible to improve the capacity of the alloy by designign mixed alkali metal hydroxide solutions.

Stress effect on the swelling/shrinking behavior of an AB2 alloy during hydrogenation cycles

The stress influence on a Ti0.85Zr0.15Mn1.33V0.3 alloy during hydrogenation cycles has been studied in a special design test cell for mechanical studies during hydrogenation. The alloy was synthesized in an induction melting furnace with an inert Ar atmosphere. The full basic characterization of the material is presented, from a structural and morphological point of view by X-ray powder diffraction (XRPD), Scanning Electron Microscopy (SEM), Energy Dispersion X-ray spectroscopy (EDX) and laser grain size analysis, and, from a thermodynamic point of view by the measure of the Pressure composition Isotherms (P-c-I) at 23 and 80 °C. Besides, in order to measure the mechanical properties of the material, volume variations measurements during hydrogenation cycles were performed. The samples were milled up to sizes beneath 1.25 mm, and afterwards placed into a cylindrical die under three different levels of axial stress, 0.0325 MPa, 0.12 MPa and 0.84 MPa, for more than 100 cycles. For higher stress levels the powder bed shrinks as cycles continue, while for lower stresses the powder bed swells. A natural tendency for the powder to decrepitate and agglomerate was observed, as the size distribution of the particles reduces and the shape become uneven, generating an increase of the porosity and volume in the case of the lower stress sample. As the stress increases, the grains rearrange easily and a polydisperse distribution of particles facilitates the accommodation of smaller grains between bigger ones, producing a reduction in the porosity and volume of the hydride bed after several cycles.

In operando neutron diffraction study of LaNdMgNi9H13 as a metal hydride battery anode

La2MgNi9-related alloys are superior metal hydride battery anodes as compared to the commercial AB5 alloys. Nd-substituted La2-yNdyMgNi9 intermetallics are of particular interest because of increased diffusion rate of hydrogen and thus improved performance at high discharge currents. The present work presents in operando characterization of the LaNdMgNi9 intermetallic as anode for the nickel metal hydride (Ni-MH) battery. We have studied the structural evolution of LaNdMgNi9 during its charge and discharge using in situ neutron powder diffraction. The work included experiments using deuterium gas and electrochemical charge-discharge measurements. The alloy exhibited a high electrochemical discharge capacity (373 mAh/g) which is 20% higher than the AB5 type alloys. A saturated β-deuteride synthesized by solid-gas reaction at PD2 = 1.6 MPa contained 12.9 deuterium atoms per formula unit (D/f.u.) which resulted in a volume expansion of 26.1%. During the electrochemical charging, the volume expansion (23.4%) and D-contents were found to be slightly reduced. The reversible electrochemical cycling is performed through the formation of a two-phase mixture of the α-solid solution and β-hydride phases. Nd substitution contributes to the high-rate dischargeability, while maintaining a good cyclic stability. Electrochemical Impedance Spectroscopy (EIS) was used to characterize the anode electrode on cycling. A mathematical model for the impedance response of a porous electrode was utilized. The EIS showed a decreased hydrogen transport rate during the long-term cycling, which indicated a corresponding slowing down of the electrochemical processes at the surface of the metal hydride anode.