Homogeneity range and crystal structure of Ni substituted Mg6(Pd,Ni) complex intermetallic compounds

The solubility of nickel in Mg₆Pd compound has been studied by chemical and structural methods. The solubility limit at 673 K attains 9 at% Ni, i.e. more than four times the value previously reported. Therefore, a re-determination of the ternary Mg-Pd-Ni phase diagram in the Mg-rich corner is desirable. Mg₆(Pd,Ni) compounds have the same crystal structure as the binary Mg₆Pd compound. The unit-cell composition at the solubility limit is Mg₃₄₄₍₂₎Pd₁₂₍₁₎Ni₃₆₍₁₎ with 75% of Ni atoms substituting Pd ones. Ni substitution occurs on the three crystallographic sites exclusively occupied by Pd in Mg₆Pd. The substitution is not fully random. Ni atoms have a slight preference for site with low coordination number. The results reported here enlarge the compositional range in which Mg₆(Pd,Ni) compounds can be used as reversible hydrogen storage materials.     

Fast hydrogen sorption kinetics for ball-milled Mg2Ni alloys☆

The sorption kinetics of Mg₂Ni alloys are strongly improved using two different surface treatments. One consists of the preparation of tailor-made Mg₂Ni/C composites by ball-milling with previously ground carbons. The strong reducing character of carbon, allowing for the reduction of NiO initially present at the alloy surface, leads to the partial removal of the oxide layer that strongly hinders the hydrogen migration throughout the alloy surface on desorption, whereas resulting Ni particles act as catalysts during the absorption process. The second surface treatment deals with the deposition of Pd particles on the alloy surface using the polyol process. The catalytic effect of Pd is responsible for an important enhancement of the absorption kinetic, whereas Pd particles probably act as hydrogen pumps, during the desorption process, leading to a faster hydrogen release. By combining both techniques, desorption rates as high as 2.7 wt% in 60 min and 2.9 wt% in 30 min are obtained at 150 and 200 °C, respectively.     

Hydrogen sorption in titanium alloys with a symmetric Σ5(310) tilt grain boundary and a (310) surface

The hydrogen sorption in intermetallic B2 TiM (M = Ni, Co, Pd) with a symmetric ??5(310) tilt grain boundary and a (310) surface is studied by density functional theory methods. The effect of hydrogen on the electronic characteristics of the alloys is analyzed as a function of a sorption position at the interfaces. The hydrogen sorption energy is shown to depend on the local environment of hydrogen; on the whole, hydrogen at the interfaces prefers titanium-rich positions. The hydrogen sorption energy in metal-rich positions decreases when the d shell of the second alloy component is filled with electrons. The grain-boundary energy, the surface energy, and the hydrogen segregation energies to the interfaces are calculated. Hydrogen sorption in titanium alloys is shown to decrease Griffith work and to favor brittle fracture along tilt grain boundaries.     

Mössbauer study of Mg–Ni(Fe) alloys processed as materials for solid state hydrogen storage

Mg–Ni–Fe magnesium-rich intermetallic compounds were prepared following two distinct routes. A Mg₈₈Ni₁₁Fe₁ sample (A) was prepared by melt spinning Mg–Ni–Fe pellets and then by high-energy ball milling for 6 h the obtained ribbons. A (MgH₂)₈₈Ni₁₁Fe₁ sample (B) was obtained by high-energy ball milling for 20 h a mixture of Ni, Fe and MgH₂ powders in the due proportions. A SPEX8000 shaker mill with a 10:1 ball to powder ratio was used for milling in argon atmosphere. The samples were submitted to repeated hydrogen absorption/desorption cycles in a Sievert type gas–solid reaction controller at temperatures in the range 520?÷?590 K and a maximum pressure of 2.5 MPa during absorption. The samples were analysed before and after the hydrogen absorption/desorption cycles by X-ray diffraction and Mössbauer spectroscopy. The results concerning the hydrogen storage properties of the studied compounds are discussed in connection with the micro-structural characteristics found by means of the used analytical techniques. The improved kinetics of hydrogen desorption for sample A, in comparison to sample B, has been ascribed to the different behaviour of iron atoms in the two cases, as proved by Mössbauer spectroscopy. In fact, iron results homogeneously distributed in sample A, partly at the Mg₂Ni grain boundaries, with catalytic effect on the gas–solid reaction; in sample B, instead, iron is dispersed inside the hydride powder as metallic iron or superparamagnetic iron.     

Hydrogen storage characteristics of nanograined free-standing magnesium–nickel films

Free-standing magnesium–nickel (Mg–Ni) films with extensive nanoscale grain structures were fabricated using a combination of pulsed laser deposition and film delaminating processes. Hydrogen sorption and desorption properties of the films, free from the influence of substrates, were investigated. Oxidation of the material was reduced through the use of a sandwiched free-standing film structure in which the top and bottom layers consist of nanometer-thick Pd layers, which also acted as a catalyst to promote hydrogen uptake and release. Hydrogen storage characteristics were studied at three temperatures, 296, 232, and 180°C, where multiple sorption/desorption cycles were measured gravimetrically. An improvement in hydrogen storage capacity over the bulk Mg–Ni target material was found for the free-standing films. As shown from a Van’t Hoff plot, the thermodynamic stability of the nanograined films is similar to that of Mg₂Ni. These results suggest that free-standing films, of which better control of material compositions and microstructures can be realized than is possible for conventional ball-milled powders, represent a useful materials platform for solid-state hydrogen storage research.     

Electronic structure and stability of Mg<Subscript>6</Subscript>Ti<Emphasis Type="Italic">M</Emphasis> (<Emphasis Type="Italic">M</Emphasis> = Mg,Al, Zn) and their hydrides

The influence of Al and Zn atoms on the stability of the Mg₇Ti compound and its hydride Mg₇TiH₁₆ has been studied theoretically. The calculations have been carried out in terms of the full-potential linearized augmented plane wave method. It has been demonstrated that the partial substitution of Al or Zn atoms for magnesium atoms leads to a decrease in the stability of the hydride and to an increase in the stability of the intermetallic compound.     

Bulk and surface properties of liquid Al-Mg, Au-Sn, and Mg-Tl compound forming alloys

We have used phenomenological models based on statistical mechanics to study the bulk and surface properties of Au-Sn, Al-Mg, and Mg-Tl binary liquid alloys. Our study reveals that the three alloys are weakly ordered systems with the most stable intermetallic complexes at temperatures of 823 K, 1073 K and 923 K been AuSn, Al₃Mg₂ and Mg₄Tl, respectively. An analysis of Warren-Cowley short-range order parameter indicates that the weakest intermetallic compound is Al-Mg while Au-Sn is observed to be more chemically ordered than Mg-Tl. Furthermore, our surface properties calculations shows that Mg-atom and Sn-atom segregate to the surface over the whole concentration range of Al-Mg and Au-Sn alloys, respectively. In Mg-Tl, there is a competing effect between Tl-atom being drawn into the bulk and at the same time Tl-atom wanting to segregate to the surface over the whole concentration range.     

Preparation of thin films of the ternary heavy fermion system CeNi2Ge2

2 Ge₂ layers on W(110). In order to produce well-ordered and atomically clean surfaces of the Ce-based intermetallic system the growth was performed under UHV conditions (p<2×10^-11 mbar). Both the polycrystalline CeNi₂Ge₂ compound and the individual elements Ce, Ni, and Ge were used as evaporants. The characterisation of the layers was made with LEED, SEM, and XPS. We find a significant influence of the substrate temperature and the evaporation power on the growth characteristics. The compound material CeNi₂Ge₂ exhibits complicated behaviour when evaporated. Under carefully selected growth conditions we obtain well-ordered films with a stoichiometry of Ce:Ni:Ge=1:2:2 and a (001) oriented surface of the body-centered tetragonal ThCr₂Si₂-type structure. The k_∥ dispersion and binding energies of the valence bands of these layers were determined with ARUPS. Received: 26 October 1997/Accepted: 27 October 1997     

Hydrogenation of Zr2Ni

The intermetallic compound Zr₂Ni has been found to take up hydrogen on charging at room temperature. Zr₂(NiFe) H_4.7 and Zr₂(NiFe) H_4.5 show the same structure (CuAl₂ type) as the uncharged compound but with an expanded lattice.Analysis of room temperature spectra in zero and applied fields indicates that the⁵⁷Fe atoms occupy Ni sites in Zr₂(Ni ⁵⁷Fe). Volume expansion effects account for about one third of the increase in isomer shift ( +0.58 mms^–1) observed on hydrogenation. The distribution of hydrogen atoms around the probe³⁷Fe atoms also causes a decrease of 0.23 mms^–1 in the mean value for the quadrupole splitting compared Rith uncharged Zr₂Ni.     

Structural, vibrational and thermodynamic properties of Mg2 FeH6 complex hydride

Mg₂FeH₆, which has one of the highest hydrogen storage capacities among Mg based 3d-transitional metal hydrides, is considered as an attractive material for hydrogen storage. Within density-functional perturbation theory (DFPT), we have investigated the structural, vibrational and thermodynamic properties of Mg₂FeH₆. The band structure calculation shows that this compound is a semiconductor with a direct X-X energy gap of 1.96 eV. The calculated phonon frequencies for the Raman-active and the infrared-active modes are assigned. The phonon dispersion curves together with the corresponding phonon density of states and longitudinal-transverse optical (LO-TO) splitting are also calculated. Findings are also presented for the temperature-dependent behaviors of some thermodynamic properties such as free energy, internal energy, entropy and heat capacity within the quasi-harmonic approximation based on the calculated phonon density of states.     

Dy-Ni二元系合金相图

木文用X射线粉末衍射法配合差热分析法,确证了金属间化合物DyNi₄的存在,发现了一种新的金属间化合物Dy₄Ni₁₇,并作出了这个二元系合金相图。此合金系中共存在着十种金属间化合物,它们是:Dy₃Ni,Dy₃Ni₂,DyNi,DyNi₂,DyNi₃,Dy₂Ni₇,DyNi_{4关键词：}     

Impact of amorphization on the magnetic state and magnetocaloric properties of Gd3Ni

Rapid quenching and ball milling have been used to modify the magnetic state and magnetocaloric properties of the intermetallic compound Gd₃Ni. The melt-spun and ball-milled Gd₃Ni samples are found to exhibit a soft ferromagnetic-like behavior below 120 K, whereas in the crystalline state, the Gd₃Ni compound is an antiferromagnet with a Néel temperature of about 99 K. The reduced value of the saturation magnetization observed in amorphous Gd₃Ni samples is ascribed to the appearance of a magnetic moment on Ni atoms. After amorphization, the Gd₃Ni samples exhibit substantially improved magnetocaloric properties in a low field region in comparison with crystalline Gd₃Ni.     

Positron annihilation study of the influence of doping on the 3<Emphasis Type="Italic">d</Emphasis> electron states in the Ni<Subscript>3</Subscript>Al intermetallic compound

The 3d electron states in Ni₃Al single crystals doped with Fe, Co, and Nb have been investigated using angular correlation of annihilation radiation (ACAR). The ACAR spectra contain information on the momentum distribution of valence electrons and strongly bound 3d electrons of the intermetallic compound. It has been established that the positrons in the Ni₃Al crystals predominantly annihilate in the nickel sublattice from delocalized states. The doping of the compound by the third element leads to a variation in the momentum distribution of Ni 3d electrons due to the change in the character of interatomic bonds. An analysis of the momentum distribution has demonstrated that the niobium atoms increase the covalent component of the chemical bond as compared to the binary compound due to the d _Nb-d _Ni hybridization. The doping with cobalt atoms also enhances the tendency toward the formation of the covalent bond. At the same time, iron atoms have a weak effect on the electronic structure of the intermetallic compound.     

A quantitative interpretation of the surface segregation in air-exposed intermetallic compounds. A test case UNiAl

The first quantitative interpretation of the surface segregation in an air-exposed intermetallic compound, namely UNiAl, has been proposed. The analysis of the experimental data indicates that the driving force for the segregation is associated with the surface free energy of the pure metal constituents of the intermetallic compound. The slope of the surface abundance vs surface energy curve differs by approximately a factor of 8 from the slope predicted by a Boltsmann-type distribution function. The same slope is also derived by analyzing the segregation properties of Zr, Cr and Mn in ZrCr₂ and ZrMn₂, previously studied by other investigators.     

Catalytic generation of hydrogen by applying fluorinated-metal hydrides as catalysts

Metal–hydrogen complexes such as NaAlH₄, KBH₄, and NaBH₄ are known as high H-content materials. The highly reactive natures of these materials against moist air and water can be easily stabilized in aqueous KOH and NaOH solutions. Accordingly, it is required to develop catalysts suitable for generating hydrogen from the stabilized metal–hydrogen complexes in alkaline solutions. This work is aimed at developing catalysts that can generate hydrogen from such solutions with considerably high kinetics under moderate temperature and pressure conditions. We have found that Mg₂Ni, a typical high-temperature hydriding alloy, exhibits excellent functions as a catalyst for the hydrolysis of BH₄ ^--ion-containing solutions. The fluorination-treatment (F-treatment) effects on granular particles of Mg₂Ni and Mg₂NiH₄ are reported in this paper. Received: 13 November 2000 / Accepted: 14 November 2000 / Published online: 9 February 2001     

Solid-phase synthesis in Ni-Mg and Ni-Mg-C systems upon ball milling of powder mixtures

It is shown by X-ray diffraction analysis and differential scanning calorimetry that phase formation in mechanochemical synthesis (MS) of two-component mixtures in the Ni-Mg system occurs through amorphization. The final product of the MS of Ni₇₅Mg₂₅ mixture is the nonequilibrium nanocrystalline Ni-7.5 at % Mg solid solution with a lattice constant a = 0.3602 nm (a _Ni = 0.3526 nm). Milling of a mixture with a higher magnesium content (Ni_66.7Mg_33.3) yields a two-phase mixture composed of an amorphous phase and Ni(Mg) solid solution. MS of three-component mixtures in the Ni-Mg-C system, regardless of the form of the initial components (Ni₆₀Mg₂₅C₂₀ mixtures of elementary components or a mixture of Ni₂Mg intermetallic compound, Ni powder, and graphite), yields double cubic carbide of antiperovskite type, Ni₃MgC_0.75 (E2₁ structure type), with a lattice constant of 0.3782 nm. It is shown that the initial milling stage for a mixture of this elementary components in the presence of graphite is characterized by the distortion of Mg particles with the formation of texture in the (002) basic plane.     

High-temperature studies of the magnetic susceptibility of samarium and the Al<Subscript>2</Subscript>Sm compound

The magnetic susceptibility of metallic samarium and the Al₂Sm intermetallic compound has been experimentally studied by the Faraday method in the temperature range of 300–1800 K. It has been shown that the temperature dependences of the magnetic susceptibility of Sm and Al₂Sm in a crystalline state can be described in the framework of Van Vleck paramagnetism theory taking into account variable valence and the contribution from the conduction electrons. Using this theoretical interpretation of the data, the effective valence of samarium in the metallic state and in the Al₂Sm intermetallic compound has been estimated as a function of the temperature.     

Analysis of hydrogen adsorption in the bulk and on the surface of magnesium nanoparticles

The stability of magnesium hydride (MgH _x ) nanoparticles (x = 0.5, …, 2) is investigated using ab initio calculations. It is shown that for a nanoparticle diameter of D ～ 5 nm, the internal pressure becomes lower than 3 kbar; for this reason, the structure of hydride nanoparticles coincides with the structure of this hydride in crystalline form. It is found that the phase of partly saturated MgH _x hydrides (x < 2) must decompose into the phase of pure hcp magnesium and the α phase of MgH₂. The frequencies of jumps of hydrogen atoms within the hcp phase of magnesium and in the α phase of MgH₂ are calculated; it is shown that slow diffusion of hydrogen in magnesium is due to the large height of potential barriers for motion of hydrogen within MgH₂. To attain high diffusion rates, the structures of Mg₅₃Sc and Mg₅₃Ti crystals and their hydrides are calculated. It is found that the frequency of jumps of H atoms in Mg₅₃ScH₁₀₈ near the Sc atoms does not noticeably change as compared to the frequency of jumps in the α phase of MgH₂, while the frequency of jumps in Mg₅₃TiH₁₀₈ near Ti atoms is higher by approximately a factor of 2.5 × 10⁶. This means that diffusion in manganese hydride with small admixtures of titanium atoms must be considerably eased. Chemical dissociation of hydrogen molecules on the (0001) surface of hcp magnesium, on the given surface with adjoined individual Ti atoms, and on the surface of a one-layer titanium cluster on the given surface of magnesium is investigated. It is found that dissociation of hydrogen at solitary titanium atoms, as well as on the surface of a Ti cluster, is facilitated to a considerable extent as compared to pure magnesium. This should also sharply increase the hydrogen adsorption rate in magnesium nanoparticles.     

Microstructure and tribological properties of TiCu2Al intermetallic compound coating

TiCu₂Al ternary intermetallic compound coating has been in situ synthesized successfully on pure Ti substrate by laser cladding. Tribological properties of the prepared TiCu₂Al intermetallic compound coating were systematically evaluated. It was found that the friction coefficient and wear rate was closely related to the normal load and sliding speed, i.e., the friction coefficient of the prepared TiCu₂Al intermetallic compound coating decreased with increasing normal load and sliding speed. The wear rate of the TiCu₂Al intermetallic compound coating decreased rapidly with increasing sliding speed, while the wear rate first increased and then decreased at normal load from 5 to 15 N.     

NdNi2Ge2化合物的结构和电磁输运性质

利用非自耗真空电弧熔炼法制备了NdNi₂Ge₂化合物样品,采用X射线粉末衍射技术和Rietveld全谱拟合分析方法测定了其晶体结构. 结果显示该化合物的空间群为I4/mmm,点阵参数为:a=4.120(1),c=9.835(0),Z=2,Nd原子占据2a晶位,Ni原子占据4d晶位,Ge原子占据4e晶位. NdNi₂Ge₂化合物呈现顺磁性,应用居里-外斯定律拟合计算得到居里-外斯常数为25.8,居里-外斯温度为6.24 K. 有效势磁矩为3.69μ_B,这与理论计算Nd³⁺的磁矩相符,表明磁矩主要源于Nd³⁺. 电阻率变化范围为0.3 Ω ·μm^-1-1 Ω ·μm,电阻曲线拟合显示NdNi₂Ge₂呈半金属性. 关键词： 2Ge₂')" href="#">NdNi₂Ge₂ Rietveld结构精修 电磁输运