Effects of sintering on composite metal hydride alloy of Mg2Ni and TiNi synthesized by mechanical alloying

As-milled composite metal hydrides composed of Mg₂Ni and TiNi phases were cold-pressed under a pressure of 490 MPa and sintered for 1 h at 5×10⁻⁶ Torr and 300 °C. Electrochemical characteristics of the sintered composite metal hydride electrode were investigated. The maximum discharge capacity of the sintered composite alloy electrode was 125 mAh/g at a discharge current density of 100 mA/g. This value was similar to that of the as-milled one before sintering. However, the sintered electrode retained 80% of the maximum discharge capacity after 150 cycles, while the as-milled electrode retained only 55%. This is because after the sintering process an interface between Mg₂Ni and TiNi plays a role similar to a diffusion layer of hydrogen. In the sintered composite electrode, when a discharging step proceeds, hydrogen absorbed in a Mg₂Ni particle can move into a TiNi phase through the bonded-interface between Mg₂Ni and TiNi, then discharges at the interface between TiNi and the electrolyte. Also, the electrochemical impedance spectroscopy (EIS) tests showed that the composite alloy electrodes had a lower charge-transfer resistance and a higher hydrogen diffusion coefficient than those in single-phase Mg₂Ni. This indicates that TiNi particles in the composite are the active sites for redox reaction of hydrogen and the pathway for the diffusion of hydrogen     

Influence of tin on the electrochemical and gas phase hydrogen sorption in Mg2−xSnxNi (x = 0, 0.1, 0.3)

Mg_2−xSn_xNi (x = 0, 0.1, 0.3) alloys were synthesized by reactive ball milling under protective Ar atmosphere and liquid n-heptane. The microstructure and the morphology of the powders were determined by X-ray diffraction and scanning electron microscopy. The as-milled alloys consist of Mg₂Ni nanocrystals with an average grain size in the range 3–7 nm, depending on the alloy composition. Sn containing phases were not detected even in the Sn-rich alloy. Obviously, Sn is dissolved in the Mg₂Ni intermetallic compound. Gas phase sorption of hydrogen was not observed in the alloys containing Sn (Mg_2−xSn_xNi; x = 0.1, 0.3). It was suggested that Sn impedes the process of hydrogen molecules decomposition. The as-milled alloys absorbed reversibly hydrogen electrochemically. Mg₂Ni alloy showed the highest discharge capacity of 300 mAh/g. The capacity of Mg_1.9Sn_0.1Ni and Mg_1.7Sn_0.3Ni was about 260 mAh/g. It was found that Sn improved the cycle life of the electrode.     

Microstructural evolution during controlled ball milling of (Mg2Ni+MgNi2) intermetallic alloy

Nearly dual-phase Mg–Ni alloy fabricated by ingot metallurgy (IM) and comprising 30 vol% Mg₂Ni and 61 vol% MgNi₂ intermetallic compounds (remaining 9 vol% of unreacted Mg) was mechanically (ball) milled under controlled shearing for 10, 30, 70 and 100 h. The majority of the medium- and small-sized powder particles exhibited a relatively homogeneous microstructure of milled Mg₂Ni and MgNi₂. A fraction of large-sized particles developed the ‘core and mantel’ microstructure after milling for 70 and 100 h. The ‘core’ contains poorly milled MgNi₂ particles and the ‘mantel’ is a thoroughly milled mixture of Mg₂Ni, MgNi₂ and, possibly, residual Mg. X-ray diffraction provides evidence of nanostructurization and eventual amorphization of a fraction of a heavily ball milled Mg₂Ni phase. The remnant Mg₂Ni developed a nanocrystalline/submicrocrystalline structure. The co-existing MgNi₂ phase developed a submicrocrystalline structure within the powder particles. The results are rationalized in terms of enthalpy effects by the application of Miedema’s semi-empirical model to the phase changes in ball milled intermetallics.     

Synthesis of fine composite particles for hydrogen storage, starting from Mg-YNi2 mixture

We synthesized new composite particles for hydrogen storage on the basis of an idea of “particle designing”. As starting materials, powders of Mg and YNi₂ were selected. Fine composite particles containing mainly Mg₂Ni could be designed by repetitive hydriding and dehydriding cycles at 673 K. In the synthesis process of the composite particles, the following two points were found to be essential for this technique. The first point is that, after being activated by the sequential processes of hydrogenation, amorphization and disproportionation, YNi₂ reacts effectively with Mg. The second point is that evaporated Mg, which occurs during dehydriding, adheres to the surface of the activated YNi₂ and accelerates a diffusion reaction to form Mg₂Ni at the interface. In these composite particles, Mg₂NiH₄ is formed, even at 373 K, under a hydrogen pressure of 5 MPa.     

Electrochemical characteristics of Mg2−xZrxNi (x = 0–0.6) electrode alloys prepared by mechanical alloying

The electrode alloys Mg_2−xZr_xNi (x = 0, 0.15, 0.3, 0.45 and 0.6) were prepared by mechanical alloying (MA). Mg in the alloy was partially substituted with Zr in order to improve the electrochemical characteristics of the Mg₂Ni-type alloy. The microstructures and the electrochemical characteristics of the experimental alloys were measured systemically. The effects of substituting Mg with Zr and MA technique on the microstructures and electrochemical performances of the alloys were investigated in detail. The results obtained by XRD, SEM and TEM show that the substitution of Zr is favourable for the formation of an amorphous phase. For a fixed milling time, the amorphous phase in the alloy grows with increasing Zr content. The electrochemical measurement indicates that the substitution of Zr can dramatically enhance the discharge capacity with preferable cycle stability, and it markedly improves the discharge voltage characteristic of the alloys. For x ≤ 0.3, the discharge capacity of the alloys monotonically increases with milling time. But for x > 0.3, it has a maximum value with the change of milling time.     

TiNiNb钎焊Cf/SiC与TC4接头组织结构

文中在钎焊温度980℃、钎焊时间15 min的条件下,采用Ti_54.8Ni_34.4Nb_10.8（原子分数,%）共晶合金粉末真空钎焊C_f/SiC复合材料与TC4钛合金.用SEM,EDS及差热分析法（DTA）观察测定了钎料组织、成分及熔点,分析了钎焊接头的微观组织结构.结果表明,Ti_54.8Ni_34.4Nb_10.8共晶钎料由Ti₂Ni及Ti（Nb,Ni）化合物组成,实际熔点为935℃.钎焊过程中,Ti和Nb元素与复合材料反应形成TiC和NbC混合反应层;钎料中的镍与TC4中的镍发生互扩散,在TC4钛合金侧形成扩散层;连接层由弥散分布的Ti（Nb,Ni）化合物和Ti₂Ni相组成.C_f/SiC与连接层界面为接头最薄弱环节,此处易形成裂纹.     

连接温度对TiAl/Ti3AlC2扩散焊接头界面结构及性能的影响

采用Ti/Ni复合中间层实现了TiAl合金和Ti₃AlC₂陶瓷的扩散连接,利用SEM,XRD等分析方法对接头界面结构进行了分析.结果表明,TiAl/Ti₃AlC₂接头典型界面结构为TiAl/Ti₃Al+Al₃NiTi₂/Ti₃Al/α-Ti+Ti₂Ni/Ti₂Ni/TiNi/Ni₃Ti/Ni/Ni₃(Ti,Al)/Ni₃Al+TiC_x+Ti₃AlC₂/Ti₃AlC₂.随着连接温度的升高,TiAl/Ti界面处的Ti_ss层逐渐减小,Ti₃Al化合物层逐渐变厚;TiNi化合物层厚度显著增加,Ti₂Ni和Ni₃Ti层厚度基本保持不变.接头抗剪强度随连接温度升高先增加后减小,当连接温度为850℃时,接头的抗剪强度最高可达到85.3 MPa.接头主要在Ni/Ti₃AlC₂界面及Ti₃AlC₂基体处发生断裂.     

Hydriding combustion synthesis of Mg–CaNi5 composites

The composites of Mg–x wt.% CaNi₅ (x = 20, 30 and 50) were prepared by hydriding combustion synthesis (HCS) and the phase evolution during HCS as well as the hydriding properties of the products were investigated. It was found that Mg reacted with CaNi₅ forming Mg₂Ni and Ca during the heating period of HCS. Afterwards, the resultant Mg₂Ni and Ca as well as the remnant Mg reacted with hydrogen during the cooling period. The lower platform in the P–C isotherms corresponds to the hydriding of Mg, and the higher one corresponds to Mg₂Ni. With the increase of the content of CaNi₅ from 20 to 50 wt.%, the hydrogen content of the HCS products increases at first and then decreases. The Mg–30 wt.% CaNi₅ composite has the maximum hydrogen capacity of 4.74 wt.%, and it can absorb 3.51 wt.% of hydrogen in the first hydriding process without activation.     

Activation of Zr0.8Ti0.2Mn0.4V0.6Ni electrode by hot-charging treatment and its cycling performance for Ni-MH secondary battery

An electrode with a composition Zr_0.8Ti_0.2Mn_0.4V_0.6Ni was activated by hot-charging treatment at various current densities and for different times, and then its cycling performance was examined. The first discharge capacity increases as the hot-charging times and the current density increase. The hot-charging and the charge–discharge cycling create cracks on the surface of the particles and make the particles smaller, leading to the exposure of the clean surfaces of the particles.

The form of the cycling performance curve of the alloy with small and large particles is suggested. The surface of the electrode, analyzed by energy-dispersive spectrometry (EDS), shows an gradual increase in the atomic ratio of Ni as the number of cycle increases. The electrolyte after 50 cycles analyzed by ICP have relatively large concentrations of V and Zr.     

Functionally graded Mg2Si/Al composite produced by an electric arc remelting process

A graded composite of Mg₂Si/Al has successfully been fabricated by an electric arc remelting process. The results show that the graded microstructure of the composite consists of a large amount of Mg₂Si primary particles surrounded by clouds of -Al particles. The primary Mg₂Si particles gradually increase in size, while there is a decrease in length of the dendrites with distance from the bottom. In addition, the primary Mg₂Si dendritic crystals have certain orientations tending to be identical to the direction from top to bottom, and the dendritic crystals in the same array are almost parallel. The morphology of eutectic Mg₂Si is identified as fine and short fibrous like. The microhardness of graded material along the graded microstructure gradual increased with increasing distance from the top part.     

Nanocrystalline LaNi4−xMn0.75Al0.25Cox electrode materials prepared by mechanical alloying (0≤x≤1.0)

Polycrystalline hydrogen storage alloys based on lanthanum (La) are commercially used as negative electrode materials for the nickel–metal hydride (Ni–MH_x) batteries. In this paper, mechanical alloying (MA) was used to synthesize nanocrystalline LaNi_4−xMn_0.75Al_0.25Co_x (x=0, 0.25, 0.5, 0.75 and 1.0) hydrogen storage materials. XRD analysis showed that, after 30 h milling, the starting mixture of the elements decomposed into an amorphous phase. Following the annealing in high purity argon at 700 °C for 0.5 h, XRD confirmed the formation of the CaCu₅-type structures with a crystallite sizes of about 25 nm. The nanocrystalline materials were used as negative electrodes for a Ni–MH_x battery. Cobalt substituting nickel in LaNi₄Mn_0.75Al_0.25 greatly improved the discharge capacity and cycle life of the LaNi₅ material. For example, in the nanocrystalline LaNi_3.75Mn_0.75Al_0.25Co_0.25 powder, discharge capacities up to 258 mA h g⁻¹ (at 40 mA g⁻¹ discharge current) were measured. Mechanical alloying is a suitable procedure to obtain LaNi₅-type alloy powders for electrochemical energy storage.     

The structural and kinetic characteristics of Mg1.9Al0.1Ni alloy synthesized by mechanical alloying

The structural and kinetic characteristics of the mechanically alloyed Mg_1.9Al_0.1Ni were investigated. It was found that Mg_1.9Al_0.1Ni can absorb/desorb about 3.55/3.44 mass% H at a high rate and it has a hexagonal crystal structure as Mg₂Ni. The hydriding/dehydriding (H/D) rates in the two-phase (–β) region of Mg_1.9Al_0.1Ni were measured and studied at temperatures ranging from 553 to 623 K under an approximately isobaric condition. The obtained data of H/D rates indicated that hydrogen diffusion was the rate-controlling step through the hydride phase. A new model was successfully used to calculate the kinetic experimental results. It can be seen that theoretical calculation agrees well with experimental data. The corresponding activation energies are 47 600 and 54 500 J/mol H₂ for H/D processes, respectively.     

Ti600和Ni-25%Si合金钎焊接头性能及失效机理分析

采用Ti-Zr-Ni-Cu非晶钎料对高温钛合金Ti600和Ni-25%Si (原子分数,%)合金进行钎焊试验,重点研究了钎焊温度对镍硅与钛合金接头组织及性能的影响,结合接头组织特征及断口结构分析阐明了Ti600和Ni-25%Si合金钎焊接头的失效机理. 结果表明,钎缝内部包含多个区域,随着连接温度从900 ℃上升至980 ℃,包含(Ti,Zr)₂Si和Ti₂Ni相的区域逐渐消失,包含Ti₅Si₃和Ti₂Ni相的区域逐渐变厚,最终占据全部钎缝. 力学性能分析表明,随着钎焊温度的升高,接头抗剪强度先增大后降低. 当钎焊温度为960 ℃时,接头的抗剪强度能够达到峰值177 MPa. 在脆性Ti₂Ni相基体上弥散分布的Ti₅Si₃相颗粒破坏了Ti₂Ni相的连续性,阻碍了裂纹在钎缝内部的扩展是钎焊接头抗剪强度提升的根本原因.     

Direct synthesis of MgCNi3 by mechanical alloying

MgCNi₃, an intermetallic compound with superconductivity, was synthesized from the Mg (or Mg₂Ni), Ni and graphite powders by mechanical alloying (MA). It is shown that the preliminary condition for the formation of MgCNi₃ is that Mg₂Ni must form in advance of MgCNi₃ in the MA process or be the starting component.     

Hydrogen absorption properties of the slurry system composed of liquid C6H6 and F-treated Mg2Ni

The hydrogenation characteristics of the slurry composed of the NH₄F solution treated Mg₂Ni and liquid C₆H₆ were studied. The F-treatment results in a net-shaped MgF₂ surface and higher nickel content in the sub-layer. It is found that the hydride of the NH₄F treated alloy has a much higher activity for the hydrogenation of benzene. The catalytic activity for hydrogenation of the alloy depended strongly on the surface properties of the catalyst. At 483 K and under a hydrogen pressure of 4.0 MPa, the alloy absorbed hydrogen first, transformed into hydride and then the benzene was hydrogenated to cyclohexane with the hydride as the catalyst. The hydrogen absorption capacity of slurry system composed of 20 wt.% treated alloy and benzene reached 6.4 wt.% and the hydrogenation completed in 20 min. Results of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) analysis on the crystal structure, surface composition and surface morphology of the untreated and treated alloy are presented and discussed.     

Ingenious Interlacement of CoNiO2 on Carbon Nanotubes for Highly Stable Lithium-Ion Batteries

Nickel-cobalt oxide is considered as a promising anode for lithium-ion battery, owing to its high specific capacity, simple synthesis process and high safety. However, like most transition metal oxide anode materials, nickel-cobalt oxide suffers from poor conductivity, easy agglomeration and large volume expansion in the charging and discharging process, causing an inferior cycling lifespan. Here we report a structure design that CoNiO₂ particles are ingeniously interlaced on carbon nanotubes by a simple solvothermal method. These nanotubes are irregularly intertwined to obtain an independent electrode structure with high electronic conductivity, which can also alleviate the notorious volume expansion. Consequently, the corresponding lithium-ion battery shows superior electrochemical performance. It provides a discharge capacity of 1213.7 mAh g⁻¹ at 0.5 A g⁻¹, and can be stable over 100 cycles with a capacity retention of 96.45%. Furthermore, the battery can also deliver a reversible capacity of 544.8 mAh g⁻¹ at the high current density 3 A g⁻¹. This work provides a unique idea for the performance improvement of nickel-cobalt oxide anode for lithium-ion batteries.     

Phase Stability in Mechanically Alloyed Mg–Ni System Studied by Experiments and Thermodynamic Calculations

In this study, microstructural evolution of Mg–Ni alloy during mechanical alloying(MA) was investigated.Also, a thermodynamic approach was utilized to predict the most stable phases formed in Mg–Ni alloy after MA. The phase composition and microstructural properties of Mg–Ni alloy were assessed by X-ray diffractometry, high-resolution field emission scanning electron microscopy and high-resolution transmission electron microscopy. The results showed that ball milling of magnesium and nickel powder mixture for 70 h yields nanostructural Mg2Ni compound with an average grain size of ~20 nm. Thermodynamic calculations revealed that in the composition ranges of 0.0 \ XMg\ 0.03(at.%)and 0.97 \ XMg\ 1, there is no driving force for amorphous phase formation. In the composition range of 0.07 \ XMg\ 0.93, the change of Gibbs free energy for amorphous phase formation was more negative than solid solution.While for XMg= 0.66(nominal composition of Mg2Ni intermetallic phase), the change of Gibbs free energy for intermetallic phase was found to be more negative than both amorphous and solid solution phases indicating that Mg2Ni intermetallic compound is the most stable phase, in agreement with the experimental observations.     

Crystal structures of VSn2, NbSn2 and CrSn2 with Mg2Cu-type structure and NbSnSb with CuAl2-type structure

Several binary stannides of the early transition metals T have been reported with the composition T₂Sn₃ previously. However, the present structure refinements from single-crystal X-ray data show that they have the compositions VSn₂, NbSn₂ and CrSn₂ (R = 0.028, R = 0.018 and R = 0.021 with 17 variable parameters and 828, 512 and 440 structure factors respectively). Their orthorhombic Mg₂Cu-type structure is closely related to the structures of MoSn₂ (Mg₂Ni type) and CuAl₂. The latter structure type was confirmed for NbSnSb by a structure refinement from single-crystal data (R = 0.010 for eight variables and 254 F values). Electrical conductivity measurements show CrSn₂ and MoSn₂ to be metallic conductors.     

Improving the performance of hydrogen storage electrodes based on mechanically alloyed Mg61Ni30Y9

Amorphous Mg₆₁Ni₃₀Y₉ powder was produced by mechanical alloying using a Retsch planetary ball mill under liquid nitrogen cooling. Additional gentle milling with graphite powder resulted in a thin graphite coating of powder particles. Further milling with a high energy SPEX mill transferred the alloy into a fully nanocrystalline state. The morphological and microstructural changes were followed by means of XRD, SEM, TEM and DSC. Hydrogen storage electrodes based on those alloy powders were fabricated and their cathodic and anodic polarization behaviour and their charge–discharge cycling behaviour in 6 M KOH solution were investigated. It was found that the alloy modification from a non-defective amorphous to a highly defective nanocrystalline state is more effective for improving the hydrogen sorption properties of the alloy than the graphite coating, but is detrimental for the alloy passivation. Accordingly, a SPEX-milled powder electrode exhibits with C_max = 570 mAh/g a higher maximum discharge capacity than a coated Retsch-milled powder electrode with C_max = 435 mAh/g, but degrades faster during repeated cycling. Using graphite powder supporting material for electrode preparation on a nickel foam carrier was found to be much more beneficial than nickel powder for achieving maximum discharge performance.     

Synthesis and characterization of 0.3 Vf TiC–Ti3SiC2 and 0.3 Vf SiC–Ti3SiC2 composites

In this work, two composite compositions—one with 30% (v/v) SiC, the other with 30% (v/v) TiC, balance Ti₃SiC₂—were synthesized and characterized. Fully dense samples were fabricated by hot isostatically pressing Ti, SiC and C powders for 8 h at 1500 or 1600 °C and a pressure of 200 MPa. Both TiC and SiC lower grain boundary mobility in Ti₃SiC₂. Coarsening of the SiC particles was also observed. At comparable grain sizes, all composites tested were weaker in flexure than the unreinforced Ti₃SiC₂ matrix, with the reduction in strength being the worst for the SiC composites. This reduction in strength is most probably due to thermal expansion mismatches between the matrix and reinforcement phases. The composite samples were exceptionally damage tolerant; in one case a 100 N Vickers indentation (in a 1.5-mm thick bar) did not reduce the flexural strength as compared to an unindented or as-fabricated samples. The same is true for thermal shock resistance; quenching samples from 1400 °C in room temperature water, resulted in strength reductions that were 12% at best and 50% at worst. In the 25–1000 °C temperature range, the thermal expansion coefficients of the two composites were indistinguishable at 8.2×10⁻⁶ K⁻¹. The Vickers hardness values depended on load; at 100 N, the hardnesses were ≈15 GPa; at 300 N, they asymptote to 7–8 GPa. For the most part, very few cracks emanate from the corners of the Vickers indents even at loads as high as 500 N. In the few cases where cracks did initiate, fracture toughness values were crudely estimated to lie in the 5–7.5 MPa √m range.