Amorphous Ni-Zr Alloys Research Articles

Evolution of local atomic structure accompanying devitrification of amorphous Ni-Zr alloy thin films

Thin film metallic glasses undergoing devitrification can form partially crystallized or fully crystallized materials with novel structural and magnetic properties. The development of desired and tunable properties of such systems drives the need to understand the mechanism of their thermal evolution at the atomic level. Co-sputtered amorphous Ni-Zr alloy thin films which were thermally annealed in steps of 200°C from room temperature up to 800°C, were observed to undergo an amorphous-to-crystalline transformation. Evolutions in local atomic structure, including oxide formation and depletion during this devitrification process, were determined using a combination of X-ray reflectivity (XRR), grazing incidence X-ray diffraction (GIXRD) and Extended X-ray Absorption Fine Structure (EXAFS) techniques. The most probable phase of the alloy was determined as Ni7Zr2; undergoing a polymorphous transformation. The slight oxide content in the films was also noted to decrease rapidly as annealing proceeded; leaving the crystallization pathway unhindered. modeling and analyses of XRR and GIXRD data showed that film thickness decreased with annealing while long range order increased. Pair distribution function peak widths were observed to decrease with annealing, indicating a control over structural disorder in the system as it transitioned from the amorphous to crystalline state. Partial atomic distribution in the environment of each constituent was examined at every stage of annealing through EXAFS measurements, which gave proper insight into atomic scale changes to the order present. Detailed analyses through these techniques gave coherent results, thus providing a true picture of the thermal evolution process of devitrification in this technologically useful material.

Skeletal Ni Catalysts Prepared from Amorphous Ni-Zr Alloys: Enhanced Catalytic Performance for Hydrogen Generation from Ammonia Borane.

Skeletal Ni catalysts were prepared from Ni-Zr alloys, which possess different chemical composition and atomic arrangements, by a combination of thermal treatment and treatment with aqueous HF. Hydrogen generation from ammonia borane over the skeletal Ni catalysts proceeded efficiently, whereas the amorphous Ni-Zr alloy was inactive. Skeletal Ni prepared from amorphous Ni30 Zr70 alloy had a higher catalytic activity than that prepared from amorphous Ni40 Zr60 and Ni50 Zr50 alloys. The atomic arrangement of the Ni-Zr alloy also strongly affected the surface structure and catalytic activities. Thermal treatment of the amorphous Ni-Zr alloys at a temperature slightly lower than the crystallization temperature led to an increase of the number of surface-exposed Ni atoms and an enhancement of the catalytic activities for hydrogen generation from ammonia borane. The skeletal Ni catalysts also showed excellent durability and recyclability.

Investigations of the mechanical properties of nanoimprinted amorphous Ni–Zr alloys utilizing the molecular dynamics simulation

In this work, the molecular dynamics simulation is used to investigate the mechanical behavior of amorphous Ni–Zr alloys during the nanoimprinting process. These simulations include the alloy composition, working temperature, radial distribution function, and the recovery ratio. It has been shown that the hardness and the reduced modulus of amorphous Ni–Zr alloy grow with an increasing proportion of Zr atoms. Moreover, due to the kinetic energy of atoms the deformation degree of amorphous Ni–Zr alloys can be enhanced by using the higher sample temperature. The short-range order of individual fractal-like amorphous Ni–Zr alloy deformation mechanism has been also demonstrated. In addition, it has been shown that the pileup index also increases with temperature, i.e. atoms bonding energy at higher temperatures decreases, resulting in the material softening and the thermal expansion. And, the recovery ratio is found to be the alloy composition loading dependent.

Active skeletal Ni catalysts prepared from Ni–Zr amorphous alloys by oxygen treatment

Skeletal Ni catalysts were prepared from an amorphous Ni40Zr60 alloy (a-NiZr) by the selective extraction of Zr moieties by the HF treatment, followed by heating at various temperatures under vacuum, oxygen and hydrogen atmosphere. The HF treatment selectively extracted Zr moieties from a-NiZr, which resulted in the formation of high-surface-area and skeletal Ni metals. The oxygen treatment at 423K and following hydrogen treatment at 473K enhanced the catalytic activities of the skeletal Ni catalyst in the hydrogenation reaction of 1-octene with molecular H2. The temperatures and atmosphere of the pretreatment strongly affected to the catalytic performances of the skeletal Ni catalysts.

Diffusion of Hydrogen in Amorphous Ni-Zr Alloys

Amorphous alloys Ni64Zr36 and Ni36Zr64 structure and hydrogen mobility are researched by the molecular dynamics method. The analysis of structure factors and partial distribution functions of atoms revealed hydrogen affects the short order parameters of the disordered systems. Diffusion coefficients of hydrogen are shown to depend on its concentration.

Effect of Continuous Hydrogen Charging on Tensile and Fatigue Properties of Amorphous Ni-Zr Alloy Membranes

Effect of hydrogen charging on mechanical properties in tensile and fatigue tests was investigated for the Ni-Zr amorphous alloy membranes with and without palladium plating. As a result of the tensile test, it was shown that both tensile strength and fracture strain decreased by continuous hydrogen charging in all the specimens. It was also found that palladium plating reduced effectively the hydrogen embrittlement sensitivity. Fatigue properties were also lowered by hydrogen charging. In the specimen without hydrogen charging, fatigue limit was about 600-700 MPa, while in the specimen with hydrogen charging, no clear fatigue limit was observed. Fracture morphology was changed from the vein-like patterns to the shell patterns by hydrogen charging, both after the tensile test and the fatigue test.

Magnetic susceptibility contributions and electronic density of states in(Ti,Zr)100−x(Ni,Cu)xmetallic glasses and crystalline compounds

Available experimental data on the magnetic susceptibility of melt-quenched amorphous TE-TL alloys ($\mathrm{TE}=\mathrm{Ti}$ or Zr; $\mathrm{TL}=\mathrm{Ni}$ or Cu) and the corresponding crystalline counterparts are reviewed. In order to analyze the composition dependence of the magnetic susceptibility in these systems, the individual contributions (element-resolved Langevin diamagnetic, Van Vleck and Pauli spin susceptibility, as well as the Landau susceptibility) were determined directly by theoretical calculations for these TE-TL alloys in a face-centered-cubic (fcc) structure. The total susceptibility, both experimental and calculated, was found to decrease with increasing TL content up to about $60--70\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$. This variation was mainly ascribed to corresponding changes of the Van Vleck contribution and the spin susceptibility. The composition dependence of the latter term is in line with the previously established trend of variation of the density of states at the Fermi level $n({E}_{F})$ upon alloying. As in previous reports on electronic band-structure calculations, it turned out that to get agreement with experiments on $n({E}_{F})$ in amorphous Zr-Ni alloys, at least some partial chemical ordering should be taken into account. Available experimental data of crystalline stoichiometric Zr-Ni compounds beyond $70\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$ Ni show a gradual increase of the Pauli susceptibility, indicating an approach toward the onset of ferromagnetic order observed previously experimentally around $90\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$ Ni in amorphous Zr-Ni alloys. The susceptibility calculations for fcc Ti-Ni and Zr-Ni alloys indicate a strong increase of the spin susceptibility component above $70\phantom{\rule{0.3em}{0ex}}\mathrm{at.}\phantom{\rule{0.2em}{0ex}}%$ Ni, and the calculated Stoner enhancement for ${\mathrm{Zr}}_{10}{\mathrm{Ni}}_{90}$ fulfills the condition of ferromagnetism in agreement with the experimental observation.

Phase stratification in disordered metallic systems

Phase stratification in a series of binary melts (AlBi, AlCd, AlIn, AlPb, CdGa, CrSn, and CuTl) and an amorphous NiZr alloy was studied. Equations describing the main stratification parameters in the liquid phase (the enthalpy ΔH, the relative volume change of melt formation ΔV/V 0, and the highest temperature T k at which phase stratification occurs) were obtained. An equation describing the concentration dependence of the relative integral Gibbs energy in the amorphous (liquid) phase was found. This equation took into account the influence of volume changes caused by melt formation on its entropy. The equations obtained were shown to qualitatively correctly describe phase stratification and the subsequent growth of crystalline phases in NiZr system alloys.

Crystallization of amorphous Ni-Zr alloys during heating with molecular dynamics simulations

The heating processes of amorphous Ni xZr 100-x( x=10, 16.7, 33.3) alloys were investigated with molecular dynamics simulations. The simulation results show that the crystallization of amorphous alloys during heating is controlled by the heating rate and the alloy's composition. The slower heating rate depresses the crystallizing temperatures and the melting temperatures of the amorphous alloys. Crystallization can be eliminated at rapid heating rates, the critical value of which decreases with increasing Ni content of the studied amorphous Ni-Zr alloys. Different crystalline structures formed during crystallizing depend on the heating rate, and the transition between crystalline structures was observed in the heating processes.

Molecular Dynamics Study of Icosahedral Clusters in Ni–ZrAmorphous Alloys

Formation of icosahedral clusters in rapidly solidified binaryamorphous NixZr100−x (x = 15, 33.3, 50, 66.7, 85) isstudied by using molecular dynamics simulation methods. A large numberof icosahedral clusters with 13 atoms (Ih13) were observed inNixZr100−x alloys, and most of them, even those inZr-rich alloys, are found to be Ni-centred. Studies on the structuresof Ni33.3Zr66.7 obtained at different cooling ratesdemonstrate that most of iscosahedral clusters enhanced by decreasingcooling rates are also Ni-centred. The essentials of Ni atomspreferring to be the core of icosahedral clusters are illustrated withthe criterion of energy minimization and the equilibrium interatomicdistances between different atoms.

Effect of tetragonal ZrO 2 on the catalytic activity of Ni/ZrO 2 catalyst prepared from amorphous Ni–Zr alloys

In zirconia-supported nickel catalysts, the influence of the ZrO 2 phase on the catalytic activity for CO 2 methanation has been investigated by TPD and TPR measurements. Ni/ZrO 2 catalysts with various amounts of the tetragonal ZrO 2 polymorph were prepared from amorphous Ni–Zr alloys by an oxidation–reduction treatment. The fraction of tetragonal ZrO 2 in the Ni/ZrO 2 catalysts increased with increasing nickel content of the amorphous alloy precursors. The methanation activity of Ni/ZrO 2 was strongly influenced by the ZrO 2 phase change. The tetragonal zirconia-supported nickel catalysts showed a higher turnover frequency for the methanation reaction and a greater CO 2 adsorption than the monoclinic zirconia-supported nickel catalysts.

Concentration Dependence of Hydrogen Diffusivity in Amorphous Zr-Ni Alloys at Elevated Temperatures

Concentration Dependence of Hydrogen Diffusivity in Amorphous Zr-Ni Alloys at Elevated Temperatures

Heat Capacity and Absolute Entropy of the Ni–Zr Amorphous Alloys

In this work, we carried out comprehensive measurements of the heat capacity of the amorphous and crystalline alloys Ni 0.667 Zr 0.333 and Ni 0.333 Zr 0.667 in the temperature range from ~12 K to the crystallization points for the former and from ~12 to 900 K for the latter. The resulting data, along with data on the high-temperature thermodynamic properties of a liquid solution and crystalline phases [6], were used for calculating the absolute entropy of the amorphous state. The Ni 0.333 Zr 0.667 and Ni 0.667 Zr 0.333 amorphous alloys as bands 2.5‐3.0 mm wide and 0.02 mm thick were obtained by the spinning method on a copper disk in a vacuum setup under a purified helium atmosphere. The initial ingots were prepared from components with a purity of no less than 99.8 wt %. The composition of the first alloy corresponds to the intermetallic compound NiZr 2 , whereas the second alloy belongs to the twophase Ni 21 Zr 8 + Ni 10 Zr 7 region of the Ni‐Zr phase diagram. Preliminary studies of the heat behavior of the alloys at high temperatures by differential scanning calorimetry (SETARAM DSC 111) showed that the amorphous alloys Ni 0.667 Zr 0.333 and Ni 0.333 Zr 0.667 crystallize in the temperature ranges 817‐874 and 628‐686 K with maxima at 841 and 655 K, which is accompanied by exotherms equal to 3.58 and 2.91 kJ/mol, respectively. Thus, the alloys were converted into the crystalline state by heating of the samples under dynamic vacuum conditions to temperatures 100 K higher than those of the crystallization peak maxima. The conversion was monitored by repeat DSC analysis.

Phonon dispersion in amorphous Zr–Ni alloys

Longitudinal and transverse phonon dispersion curves for binary amorphous Zr x Ni 100− x ( x=17, 33, 50, 67, 83) alloys have been obtained using two theoretical approaches, namely, Takeno and Goda's self-consistent phonon theory and Bhatia and Singh (BS) model for a hypothetical one-component metallic glass. The effective pair potentials for the alloys are obtained within the Wills and Harrison formulation, employing the concept of an effective atom. The phonon dispersion curves from BS model exhibit the chief characteristics of both the longitudinal and transverse modes of propagation obtained by the molecular dynamics (MD) simulation. e.g. the longitudinal mode shows oscillatory behaviour while the same is absent in the transverse mode. It suggests the suitability of the effective pair potential used in the calculation. The elastic constants and elastic Debye temperatures are calculated from the phonon dispersion curves. The results are found to be very close to those derived from MD and to the available experimental value.

Improvement of electrode performances of Mg 2Ni by mechanical alloying

Nanocrystalline and amorphous Mg 2Ni-based hydrogen storage alloys for Ni–MH batteries had been synthesized by mechanical alloying. The surface modification and Zr addition had also been carried out for improvement of its electrode performance. In comparison with the arc-melted polycrystalline one, the nanocystalline Mg 2Ni phase showed a higher discharge capacity. By increasing milling time from 120 to 160 h, the grain size of Mg 2Ni phase was more refined and the discharge capacity was raised from 180 to 370 mAh g −1. The discharge capacity of the 160-h milled Mg–Ni–Zr amorphous alloys also reached 530 mAh g −1. XPS (X-ray photoelectron spectroscopy) analysis showed that Mg2p spectra were shifted to lower binding energy implying the enhancement of the hydrogen diffusion and charge transfer reaction, and resulted in increasing the discharge capacity in amorphous alloys. To prevent the rapid degradation, the alloy powders were also coated with Ni and graphite by additional ball milling. The Ni and graphite protected the Mg from oxidation, and the coated powders showed a better cyclic stability. After 50 cycles, the degradation of bare electrode was 94% of maximum capacity, but that of coated electrode with Ni and graphite was 45 and 76% of maximum capacity, respectively.

Advanced materials for global carbon dioxide recycling

CO 2 emission increase inducing global warming occurs mostly with the growth of the economic activity. Global CO 2 recycling can prevent global warming and supply abundant renewable energy. Global CO 2 recycling consists of three district: The electricity is generated by solar cells on deserts. At coasts close to the deserts, the electricity is used for hydrogen production by seawater electrolysis and hydrogen is used for methane production by the reaction with CO 2. Methane (CH 4) is liquefied and transported to energy consuming districts where after CH 4 is used as a fuel CO 2 is recovered, liquefied and transported to the coasts close to the deserts. Key materials necessary for the global CO 2 recycling are the anode and cathode for seawater electrolysis and the catalyst for CO 2 conversion. All of them have been tailored by us. Amorphous and nanocrystalline nickel alloys are active cathodes for hydrogen production in seawater electrolysis. Anodically deposited nanocrystalline Mn–Mo and Mn–W oxides are the unique substance which can evolve oxygen with 100% efficiency without evolving chlorine in seawater electrolysis. Amorphous Ni–Zr alloys are excellent precursors of catalysts for conversion of CO 2 into CH 4 by the reaction with hydrogen at 1 atm. A prototype CO 2 recycling plant to supply clean energy preventing global warming has been built on the roof of our Institute (IMR) in 1996 using these key materials and has been operating successfully.

Correlation of electronic structure and catalytic activity of Zr–Ni amorphous alloys for the hydrogen evolution reaction

The electrocatalytic activity of amorphous Zr–Ni alloys with respect to the hydrogen evolution reaction (h.e.r.) was studied in relation to both composition and active surface area. Kinetic parameters of the h.e.r. were evaluated by electrochemical and impedance spectroscopy techniques in 1-M NaOH solution at room temperature. Intrinsic activity of the investigated metallic glasses was related to the real exchange current density, j 0 and the reciprocal value of charge transfer resistance, R ct −1. The surface roughness factor was deduced from impedance measurements. Electrocatalytic activity of two investigated Zr 100− y Ni y alloys, with y=33; and y=60 was directly related to the split-band electronic structure of Zr–Ni alloys. An enhanced electrocatalytic activity observed for the h.e.r. with increasing y in the Zr–Ni alloy was associated with a rapid increase of electronic density of states at Fermi energy level, D ( E F) of the 3d Ni band with increasing Ni concentration. The formation of Ni-hydrides during hydrogen evolution was prevented and the high activity of the 3d Ni band for the h.e.r. was preserved. It was shown that alloying of Zr–Ni metallic glasses with varying individual component contents could be explored to optimize the electrocatalytic properties of Zr–Ni glasses and their use in water electrolysis, as electrode materials of long term stability. The highest catalytic activity could be expected for y=65, when a maximum hybridization of 3d–4d orbitals and a rapid decrease i.e. minimum density of 4d Zr states at the Fermi level were observed.

Oxidation of amorphous Ni–Zr alloys studied by XPS, UPS, ISS and XRD

Amorphous Ni 36Zr 64 and Ni 24Zr 76 alloys were treated under conditions close to those used for activating Ni–Zr alloys in catalytic reactions. The composition and the structure of the surface region were investigated by X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), ion scattering spectroscopy (ISS) and X-ray diffraction (XRD). The results show that exposing the amorphous alloys to 500 mbar O 2 at 523 K led to oxidized surfaces consisting of zirconia and different oxidic nickel species. On the oxidized Ni 24Zr 76 surface, only small amounts of NiO and Ni 2O 3 were found, while Ni 2O 3 was detected as the major Ni-containing species on the oxidized Ni 36Zr 64 surface. Oxidation of Ni in the alloys appeared to be inhibited by the presence of metallic Zr and started only after complete oxidation of the surrounding Zr. Annealing the oxidized alloys to 773 K led to a reduction of the surface oxides. The liberated oxygen atoms presumably diffused into the bulk, releasing metallic Ni. At the topmost layer, zirconia was reduced by carbon atoms originating from the bulk to form a ZrC-like surface compound. The exchange of Ni and Zr atoms between the surface and the bulk was enhanced by the oxidation and the annealing. The preferential oxidation of zirconium, which attracts Zr atoms from the bulk to the surface, provides the driving force for such processes.

Compositional dependence of the CO 2 methanation activity of Ni/ZrO 2 catalysts prepared from amorphous Ni [sbnd]Zr alloy precursors

Finely grained Ni/ZrO 2 catalysts were prepared from amorphous Ni Zr alloy precursors by oxidation and subsequent reduction pretreatment, and the catalytic activity for CO 2 methanation was examined as a function of precursor alloy composition and temperature. The catalysts thus prepared produce exclusively methane, apart from water as a by-product. The conversion of CO 2 increases with temperature in the range of 373–573 K. Among the catalysts examined, the maximum methanation rate is obtained on the catalysts prepared from the amorphous alloy precursors containing 40 and 50 at% zirconium. Further, the methanation rates of all the catalysts prepared from the amorphous alloy precursors are higher than that of a 3 at% Ni/ZrO 2 catalyst prepared by wet impregnation. The number of surface nickel atoms, determined by hydrogen chemisorption, increases with zirconium content in the catalysts, while, interestingly, the turnover number decreases with increasing zirconium content. In the catalysts prepared from the amorphous alloys, two types of zirconia are present: metastable tetragonal and stable monoclinic zirconia. The former zirconia phase is present predominantly in the catalyst prepared from the Ni-30 at% Zr alloy, but the relative amount of this oxide phase, with respect to the total amounts of zirconia, gradually decreases with an increase in zirconium content of alloys. Thus, the higher turnover number of the catalysts with higher nickel content can be attributed to nickel supported on metastable tetragonal zirconia. Increasing nickel content of the precursor alloys leads to an increase in tetragonal zirconia and to a decrease in the number of surface nickel atoms on the catalysts. This is responsible for the fact that the maximum conversion appears at medium contents of zirconium in the precursor alloys.

The effect of structural heterogeneity on the pitting corrosion behavior of melt-spun amorphous NiZr alloys

The pitting behavior of amorphous NiZr alloys prepared by melt-spinning was thoroughly investigated by placing an emphasis on the effect of the microstructure and surface composition. It was found that the alloy ribbons showed different pitting corrosion behavior along the thickness direction from the dull side to the shiny side in de-aerated 0.5 M H 2SO 4 solution with 0.1 M NaCl. The dull side surface was most susceptible to pitting corrosion, but it became difficult to occur if the as-prepared dull side surface was removed by mechanical polishing. The shiny side surface of all the alloys passivated spontaneously and the pitting potential increased with an increase in alloy zirconium content. TEM observation indicated that all the shiny side surfaces were wholly amorphous, while some microcrystalline phases existed in the amorphous matrix in the dull side. The crystalline phases formed in the dull side were identified as Ni 3Zr and NiZr 2 for the Ni30Zr and Ni70Zr alloys, respectively. The presence of both the microcrystalline phases in the amorphous matrix and the irregular surface morphology of the dull side were responsible for the origin of the pitting corrosion of the dull side surface. The high corrosion resistance of the shiny side surface was attributed to the high protectiveness of the passive film and the structural homogeneity of the alloy.