Laves Phase Intermetallic Compounds Research Articles

Elaboration and hydriding behavior characterization of a magnesium matrix composite containing intermetallic particles

The AB2 Laves phase intermetallic compounds, as well as magnesium metal (among a great variety of other metals and alloys) present hydriding properties of interest for practical applications in hydrogen storage devices through the formation of metallic hydrides. This fact has encouraged numerous experimental works concerning the characterization of materials by different available techniques. The application of internal friction measurements to study the hydriding processes of intermetallic compounds is precluded because of the fast decrepitation of the material upon few cycles of formation and decomposition of the hydrid phase. However, a composite where the intermetallic particles are included within a ductile magnesium matrix, is suitable to be studied by internal friction techniques, and possible of obtaining information about the interaction between hydrogen atoms, particles, and matrix. In the present work, it is described the elaboration of a composite material obtained by sintering a pellet prepared from a mixture of equal amounts in weight of Mg and Zr(Fe0.7Cr0.3)2 particles, ranging in sizes up to 250 μm and 50 μm respectively. Details on some experimental procedures and discussion about the material composition are considered and correlated with morphological characteristics determined by SEM, and also assessed from the observed hydriding behavior and internal friction results.

Anisotropic Magnetization in DyCo2Single Crystal

Intermetallic Laves phase compounds RCo2 (R=rare earth element) exhibit various interesting magnetic properties. YCo2 and LuCo2 are exchange enhanced paramagnets, which show an itinerant metamagnetic transition at 70T. When R is one of the magnetic elements, Co 3d-moment is induced by a large internal field from the rare earth 4f -moment. In light rare earth compounds, PrCo2 and NdCo2 are ferromagnets, 2) where the coupling between rare earthand Co-moments is parallel. On the other hand, in heavy rare earth compounds, GdCo2, TbCo2, DyCo2, HoCo2, ErCo2 and TmCo2 are ferrimagnets, where the coupling between rare earthand Co-moments is antiparallel. CeCo2 is an exceptional one due to the tetravalence of Ce-ion, and is a paramagnet down to 1.5K below which it exhibits superconductivity. The magnetocrystalline anisotropy of this system, especially in heavy rare earth RCo2 series, was investigated intensely by mean of magnetization,5-8) neutron scattering,9-11) and Mossbauer spectroscopy measurements. 13) However, for DyCo2, there is no magnetic anisotropy data except Mossbauer study with polycrystalline sample, in which only the easy of magnetization is inferred as the 〈100〉. Until now, a single crystal growth of DyCo2 has not been succeeded presumably because of the difficulty due to its incongruent phase. In this note, we report the first successful growth of the single crystalline DyCo2, on which magnetic anisotropy has been directly investigated by magnetization measurements. For comparison with the experimental results, we calculated Dy-moment based on the crystalline electric field (CEF) effect. A single crystal of DyCo2 was prepared by Czochralski pulling method in an induction furnace. The raw materials were 3N (99.9% pure)-Dy and 4N-Co. In order to reduce impurity phase, e.g. DyCo3, Dy-rich composition was selected, namely the starting composition was DyCo1.8. No other peak except DyCo2 was observed in X-ray powder diffraction measurement. The residual resistivity ratio (RRR) of the present single crystal was 20. We also prepared a polycrystalline sample whose RRR was 128 for comparison. The single crystal ingot was cut into a cube by a spark cutter. The weight of the sample was 9.2mg. Magnetization measurements were carried out up to 5.5T using a SQUID magnetometer axis (Quantum Design MPMS). The temperature dependence of magnetic susceptibility χ under 0.01T is shown in Fig. 1. With decreasing temperature, χ increases rapidly below Curie temperature Tc. Tc of the present single crystal is found to be 136K, which agrees with the previous report and our data on a polycrystalline sample. χ of a polycrystalline sample follows a Curie-Weiss law above Tc. However, for the single crystalline sample, χ shows deviation from the Curie-Weiss law above Tc, which is considered to be due to magnetic impurities (Co and/or DyCo3). The estimated amount of the impurities is of the order of ∼ 1%, whose influence on the high field magnetization measurement is negligible. The field dependence of magnetization for fields along the each principle direction at 4.2K is shown in Fig. 2. The correction by subtracting the demagnetizing field is made in Fig. 2. The easy axis is found to be the 〈100〉. The absolute value of magnetization for each direction at 1T and 5T is listed in Table I. The ratio of these absolute values among the 〈100〉-, the 〈110〉and the 〈111〉directions is close to 1 : cos 45◦(= 0.71) : cos 54◦(= 0.59); the ratio of magnetizations projected to the field directions assuming the magnetic moment of Dy aligns to the

Development of novel intermetallic compounds and solid solution systems as hydrogen storage devices

One of the limitations surrounding the exploitation of hydrogen as a fuel is the difficulty in storing it economically and conveniently. At present, hydrogen is typically stored as a compressed gas or as a cryogenic liquid. However, neither method is very appealing. Gaseous storage requires a large volume and/or weight of storage vessels, while liquid hydrogen requires a rather expensive liquification process and some energy as well. Another possibility for storage that is currently being studied and utilized is metal hydrides. The hydrogen reacts with the metals and forms metal hydrides. Storing hydrogen as a metal hydride has many advantages. Firstly, hydrogen can be stored in metal hydrides more efficiently (in terms of volume) than as liquid, or even solid, hydrogen. Secondly, the formation reaction is easily reversible. The formation of the hydride is an exothermic and usually spontaneous reaction, but the hydrogen can easily be recovered by heating the hydride. Therefore, the metal hydride can be classified as a safe method of storing hydrogen since the hydrides are generally stable below their dissociation temperatures. Moreover, the self-cooling effect will slow down and eventually stop any loss of hydrogen if a leak is developed in the storage system. Several metal hydrides have been developed, but none of them satisfies all these requirements. Some of the more common drawbacks of the currently available materials are that (a) hydrogen-metal ratios are too low, (b) metals involved are too costly, (c) the absorption or release of hydrogen is difficult, slow or sensitive to poisoning phenomena and (d) significant hysteresis is exhibited. The available materials for hydrogen storage are briefly reviewed and a more detailed overview is given of two classes of materials that are being developed in the author's laboratory, namely zirconium-based Laves phase intermetallic compound and body-centred cubic solid solutions based on niobium.

55MnNMR study of the hexagonal Laves phase compoundThMn2: Coexistence of magnetic and nonmagnetic sites of Mn

The magnetic ground state of the hexagonal Laves phase intermetallic compound ${\mathrm{ThMn}}_{2}$ was investigated by ${}^{55}\mathrm{Mn}$ nuclear magnetic resonance. Both paramagneticlike resonance at the zero-Knight-shift position and antiferromagnetic resonance at high frequencies were found to be consistent with the coexistence of magnetic and nonmagnetic Mn atoms, i.e., an antiferromagnetic state with partially nonmagnetic Mn sites, proposed from neutron diffraction experiments. The hyperfine field at the magnetic $6h$ site was estimated to be 138 kOe, which is larger than that observed for another antiferromagnetic Laves phase compound ${\mathrm{YMn}}_{2}$ (114 kOe) despite the $6h$-site magnetic moment estimated by neutron diffraction experiments $(1.6{\ensuremath{\mu}}_{\mathrm{B}})$ being smaller than the moment of ${\mathrm{YMn}}_{2} (2.7{\ensuremath{\mu}}_{\mathrm{B}}).$ Possible interpretations are discussed.

NMR measurements in milled GdCo 2 and GdFe 2 intermetallic compounds

We have used the nuclear magnetic resonance technique to study the magnetic and structural properties of the Gd–Co and Gd–Fe metallic systems, starting with the C15 Laves phase intermetallic compounds, and submitting them to a high-energy milling process. This leads to the amorphization of the samples, as determined by the X-ray diffraction spectra. For the Gd–Co system the NMR study used the 59 Co nucleus; in the Gd–Fe system, 155,157 Gd and 57 Fe were used. Both systems showed segregation of the pure elements, after a few hours of milling. In the Gd–Co system, a single line, of increasing width, was observed in the 59 Co spectrum. In the Gd–Fe system, the 155 Gd and 157 Gd resonances show three lines, arising from electric quadrupole interaction. With increasing milling time, the lines broaden, and extra lines appear attributed to a cubic phase of Gd; this interpretation is supported by the X-ray analysis of the samples. The 57 Fe NMR spectrum of this system also informs on the direction of magnetization of the samples in the early stages of milling. From 1 to 7 h of milling, a spectrum of α-Fe was observed. The study of the NMR line intensity as a function of radio frequency (RF) power in GdCo 2 suggests the existence of regions of the samples with different degrees of disorder. We have observed the persistence of NMR signals from the original intermetallic compounds in the samples with up to 10 and 7 h of milling, respectively, for GdCo 2 and GdFe 2.

On the nature of the magnetic phase transition of the HoCo2 intermetallic

In this work we calculate the itinerant and localized magnetization curves of the Laves phase intermetallic compound HoCo2 and discuss the nature of its magnetic phase transition. The numerical results were calculated from the self-consistent solution of magnetic coupled equations obtained through the combination of functional integral methods with molecular field approximation.

Epitaxial growth of cubic Laves phase intermetallic compounds (YCo2, TbCo2) and new epitaxial relationship between fcc (111) and bcc (110)

YCo2 and TbCo2 Laves phase compounds were epitaxially grown on (110) W buffer layers by pulsed laser deposition at a base pressure of 5×10−10 Torr and a substrate temperature of 500 °C. The films were characterized in situ by reflection high-energy electron diffraction, and after deposition by grazing-incidence x-ray diffraction and scanning electron microscopy. The relative orientation of the RCo2(111), where R=Y or Tb, and the W(110) planes was found to be unlike any previously observed relationship between fcc and bcc materials. Two in-plane crystallographic domains of RCo2(111) have been pointed out and are discussed in terms of geometrical lattice matching.

Iron-bearing precipitates in Zircaloys: a Mössbauer spectroscopy study

Mössbauer spectroscopy of 57Fe was used to study the chemical form and distribution of iron in Zircaloy-2 and Zircaloy-4. The spectra of Zircaloy-4 consist of two overlapping quadrupole-split doublets, which can be ascribed to precipitates of the pseudobinary Laves-phase intermetallic compound Zr(Cr,Fe) 2, which has a hexagonal MgZn 2 (C14) type structure and cubic MgCu 2 (C15) type structure. In Zircaloy-2, a large fraction of iron was also found in the form of Zr 2(Fe,Ni) precipitates, having a tetragonal Al 2Cu (C16) type structure. Characteristic room-temperature values of Mössbauer spectral parameters were determined for the three types of intermetallic precipitates. At liquid helium temperature, both alloys exhibit a poorly resolved magnetic hyperfine splitting, attributed to the low-temperature ferromagnetism of cubic Zr(Cr,Fe) 2 Laves-phase particles. This effect is more pronounced in Zircaloy-4 than in Zircaloy-2. The possibility of determining volume fractions of different phases from their relative spectral areas is discussed.

Ｃ１５とＣ１４型二元系ラーベス相合金の格子定数支配因子とその寄与の相違

The kind and extent of the contributions of dominant factors for a lattice constant were studied concerning AB2 Laves phase intermetallic compounds with a cubic-C15 and a hexagonal-C14 type structure containing IV or V groups as A atom. It was found that the dominant factors were atomic radii, electronegativities, and valence electron numbers of A and B atoms. It was possible to replace these dominant factors with the following ones: Atomic radii of A and B atoms, by the average atomic radius rav and the atomic radius ratio rA/rB; Electronegativities of A and B atoms, by the average electronegativity χav and the difference of electronegativity of A and B atom ΔχB-A; The valence electron numbers of A and B atoms, by the average valence electron Dav and the difference of the valence electron of A and B atoms ΔDB-A. On the other hand, the extent of the contribution of dominant factors for the C14 type structure differed from the C15 type. In C15-type compounds, the contribution of the atomic radius of A and B atoms was very large; the contributions of electronegativity and the valence electron of A and B atoms were not more than 10%. In contrast, in C14 type compounds the contribution of the atomic radius was about 50%, that of electronegativity was about 35%, and that of the valence electron about 15%. Consequently, C14 type compounds have more ionic and covalent bond character than the C15-type.

Hydrogen diffusion in DyFe2 and its effect on magnetic properties

For the DyFe2 Laves phase intermetallic compound, the nature of hydrogen-induced amorphization, structural changes and the dynamic behavior of hydrogen atoms were studied by means of X-ray diffraction, magnetic after-effect and thermal desorption spectrometry. DyFe2 can be amorphized by hydrogen charging within the narrow temperature range between 523 K and 593 K, even at a low hydrogen pressure of 0.1 MPa. At lower temperatures, DyFe2 forms a crystalline hydride, whereas it decomposes into α-Fe and DyH2 at higher temperatures. From the relaxation spectra obtained in magnetic after-effect measurements, it is found that hydrogen atoms in the crystalline hydride occupy the tetrahedral interstitial sites consisting of DyFe3 and Dy2Fe2 with activation energies of short-range diffusion determined as 0.24 eV and 0.35 eV respectively. For the amorphous hydride, hydrogen atoms additionally occupy Dy3Fe sites with an activation energy of 0.43 eV, and the distribution of activation energies becomes broader.

TEM investigation of the low temperature phase of HfV2

C15 Laves phase intermetallic compounds are potential high temperature structural materials, of which C15 HfV2 is particularly attractive. At room temperatures (RT), the C15 Laves phase has a fcc-based structure with a lattice parameter a=7.4 Å, as shown in Fig. 1. Sensitive specific heat measurements indicate that HfV2 undergoes a structural transformation at 115 K, as shown in Fig. 2. The crystal structure of the low temperature (LT) phase of HfV2 is not unambiguously known, although some studies have been done. A HfV2 alloy was made by arc-melting. The alloy was homogenized at 1200°C for 120h. In a preliminary study we have concentrated on selected area diffraction patterns (SADs) along different zone axes of C15 structure at 83 K, using a Philips CM 30 microscope with a liquid nitrogen cold stage.A <100>C15 SAD at RT is shown in Fig. 3 (a). At 83 K, SADs along some <100>C15 show (100), (200), and (300) superlattice spots, as shown in Fig 3 (b), while SADs along other <100>C15 indicate (110) and (200) superlattice spots, as shown in Fig. 3 (c).

The effects of high-energy milling on the structural and hyperfine properties of YFe 2

Mössbauer spectroscopy, X-ray diffraction and nuclear magnetic resonance have been used to study the effects of severe milling on the atomic and magnetic properties of the ferromagnetic intermetallic Laves phase compound YFe 2. The Mössbauer effect and nuclear magnetic resonance were carried out on the Fe site and on the Y site, respectively. Consistently, after 30 h of milling, the results indicate that the milling process induces structure and atomic disorder. The X-ray diffraction pattern shows no crystalline reflection planes, indicating that the structure is amorphous within the experimental resolution. The Curie temperature reduces from 542 K to less than 100 K after 50 h milling, although at 4.2 K the average 57Fe magnetic hyperfine field value remains near the same as in the original crystalline compound.

Concentration and temperature dependence of electrical resistivity in Zr1−xHoxCo2 (0≤x≤1) intermetallic compounds and their hydrides

The electrical resistivity studies have been carried out on the C 15-type Laves phase intermetallic compounds Zr1−xHoxCo2 (0≤x≤1) and their hydrides in the temperature range 16–300 K. The first-order magnetic transition occurring for x=1 disappears for lower Ho concentrations. Anomalies in the form of resistivity minima observed for the intermediate x values are attributed to the spin fluctuations in the vicinity of the magnetic ordering temperature. On hydrogen absorption, the first-order transition observed for x=1.0 as well as the anomalies observed for the other x values disappear at higher hydrogen concentrations. A concentration-dependent metal-semiconductor-like transition has been observed in all cases. This may be a consequence of charge transfer taking place between hydrogen and the 3d band, which leads to a strong decrease in the conduction electron density. The activation energy is evaluated from the ln ρ vs 1/T plots.

Study of itinerant electron metamagnetism in Y(Co 1−xM x) 2, (M = Ni, Fe) in very high magnetic fields

Effects of Ni and Fe substitutions on the metamagnetic property of the Laves phase intermetallic compound YCo 2 are studied in pulsed magnetic fields up to 110T. A linear increase in the critical field B c is observed for the Ni substitution, whereas the Fe doping is found to lower B c. The results are qualitatively in accordance with the band picture of metamagnetism.

Metamagnetism and quenching of spin fluctuation scattering in the Laves phase intermetallic compounds

For a number of Y(Co, Al) 2 and (R, Y)Co 2 compounds, the effects of external and internal (molecular) fields on the resistivity are accentuated. The results show that the resistivity is initially enhanced by applied field. A sharp decrease in resistivity, which is possibly due to the quenching of the spin fluctuations, can only be observed just at the metamagnetic transition. The different roles of the external and molecular fields on this phenomenon are also discussed.

Anomalous giant magnetostriction of amorphous (SmFe2) (1−x) at. % B (x) at. % alloy in a low magnetic field

RFe2 laves phase intermetallic compounds are promising materials for magnetostrictive applications. To obtain a larger magnetostriction in a low magnetic field, the influence of boron addition on the giant magnetostriction of an amorphous (SmFe2) (1−x) at. % B (x) at. % alloy has been examined. In the amorphous alloy, the saturation magnetostriction anomalously increases, while the saturation magnetization decreases with the increase in x. The highest saturation magnetostriction of −670×10−6 at 10 kOe and its effective magnetostriction of −490×10−6 at 0.3 kOe can be obtained for amorphous (SmFe2) 99.26 at. % B 0.74 at. % alloy. This effective giant magnetostriction obtained in a low magnetic field is larger than those reported in previous researches. I attribute this anomalous giant magnetostriction in a low magnetic field to the increment of elastic energy in the amorphous (SmFe2) (1−x) at. % B (x) at. % alloy.

Functional integral approach to the rare-earth-transition-metal Laves phase intermetallic compounds

The functional integral formalism is used to describe the magnetic properties at finite temperatures of rare-earth-transition-metal Laves phase intermetallic compounds RB 2. The self-consistent equations involve both itenerant d-electrons and localized f-spins. An illustration is made for the case of GdFe 2.

Substitutional solutes in the laves phase intermetallic compounds Cr 2Zr and Cu 2Mg—solution hardening and defect structures

This paper reports that hardness and strength can be attained in pure metals by solid-solution hardening. Although the primary mechanisms of solid-solution hardening are though to be known in dilute metal alloys, the mechanisms in intermetallic compounds are not established for most structure types. The mere existence of solution hardening is not well demonstrated. There are, however, data for the B2 compounds AgMg and, recently, AlCo, the L1[sub 2] structure Ni[sub 3]Al, and preliminary results for the C15 cubic Laves phase Cu[sub 2]Mg. The plan here is to examine further the Cu[sub 2]Mg data and present new results for defects, moduli, density, and hardening in a high-temperature C15 compound, Cr[sub 2]Zr (melting temperature 1673[degrees]C). A special feature, and complication, of intermetallics is that off-stoichiometry leas to constitutional defects--substitutions or vacancies that are created by deviations from stoichiometry. Thus, in order to understand hardening, we need to identify the sites of constitutional solutes in off-stoichiometric intermetallics and to quantify the elastic interactions with dislocations of both these solutes and of ternary additions.

Metamagnetism and quenching of spin fluctuations in nearly ferromagnetic Laves phase intermetallic compounds

Temperature variation of the magnetization and the critical field of typical itinerant electron metamagnetic compounds Y(Co 1- x Al x ) 2 ( x= 0.1, 0.11) are examined indetail in steady and pulsed high magnetic fields. Thermodynamic analysis of the results clearly indicates that there is a substantial reduction in γ associated with the metamagnetic transition, possibly due to a quenching of the spin fluctuations.

Hyperfine field studies in Laves phase intermetallic compounds

Hyperfine field measurements in Tb 0.3Dy 0.7Fe 2− x Co x indicate that the easy direction of magnetization changes from the [111] axis to the [100] axis in going from 295 to 77 K for most of the Co concentrations studied. The easy direction remains along the [111] axis for concentrations x⩾0.9 (in the measured temperature range between 4.5 and 295 K).