The behaviour of highly over-stoichiometric LaNi5 Mn2 alloy as negative electrode for Ni/MH batteries

Key electrochemical properties of highly over-stoichiometric LaNi₅Mn₂ alloy with CaCu₅-type structure have been measured and compared with those of stoichiometric LaNi₄Mn alloy. LaNi₅Mn₂ is obtained by mechanical alloying of two-phase La(Ni,Mn)₅+(Ni,Mn) alloy previously produced by induction melting. The as-milled alloy was thermally annealed at 450°C for one hour to crystallise milling-induced amorphous phase. As derived from DSC and XRD measurements, further annealing at 550°C produces segregation of minor NiMn-phase (21 wt%), which leaves stoichiometric LaNi₄Mn alloy (79 wt%) as a major phase. Electrochemical cycle-life and potential equilibrium measurements for both LaNi₅Mn₂ and LaNi₄Mn alloys show that the over-stoichiometric alloy has much lower discharge capacity than the stoichiometric one (135 mAh/g and 260 mAh/g, respectively). Conversely, the over-stoichiometric alloy exhibits much better cycle-life than the stoichiometric one (5% and 25% decay capacity after 55 cycles, respectively). These results demonstrate that stoichiometry is an effective parameter for tuning the discharge capacity and cycle-life of LaNi_5+x -type electrodes to the performances required by a particular application.     

Effect of substituting Ni with Cu on the cycle stability of La0.7Mg0.3Ni2.55−xCo0.45Cux (x = 0–0.4) electrode alloy prepared by casting and rapid quenching

In order to improve the cycle stability of the La–Mg–Ni system (PuNi₃-type) hydrogen storage alloy, Ni in the alloy was partly substituted by Cu. Hydrogen storage alloys La_0.7Mg_0.3Ni_2.55−xCo_0.45Cu_x (x = 0, 0.1, 0.2, 0.3, 0.4) were prepared by casting and rapid quenching. The effects of substituting Ni with Cu and the quenching rate on the microstructures and the cycle stability of the as-cast and quenched alloys were investigated in detail. The results obtained by X-ray diffraction show that the as-cast and quenched alloys have a multiphase structure, including the (La, Mg)Ni₃ phase, the LaNi₅ phase and the LaNi₂ phase, and the amount of the LaNi₂ phase increased with the increase of the Cu content. The substitution and rapid quenching have an inappreciable influence on the phase compositions of the alloys, but both obviously changed the phase abundances of the alloys. The results derived by transmission electron microscopy confirm that the substitution of Cu for Ni is favourable for the formation of an amorphous phase in the as-quenched alloys. The results obtained by the electrochemical measurement indicate that substituting Ni with Cu improved the cycle stability. When the Cu content increases from 0 to 0.4, the cycle lives of the as-cast and rapidly solidified alloys increased from 72 cycles to 88 cycles and from 100 cycles to 122 cycles, respectively.     

Influence of the substituting Ni with Fe on the cycle stabilities of as-cast and as-quenched La0.7Mg0.3Co0.45Ni2.55 − xFex (x = 0–0.4) electrode alloys

The electrode alloys La_0.7Mg_0.3Co_0.45Ni_2.55 ? xFe_x (x = 0, 0.1, 0.2, 0.3, 0.4) are fabricated by casting and rapid quenching techniques. The effects of the substitution of Fe for Ni on the cycle stabilities as well as the structures of the alloys have been investigated thoroughly. The results indicate that the substitution of Fe for Ni significantly enhances the cycle stability of the alloys. Furthermore, the positive impact of such substitution on the cycle stability has been observed to be more pronounced for the as-quenched alloy as compared to that for the as-cast one. Scanning electron microscopy (SEM) studies demonstrate that all the alloys exhibit a multiphase structure comprising of two major phases (La, Mg)Ni₃ and LaNi₅ along with a residual phase of LaNi₂. The substitution of Fe for Ni has been observed to facilitate the formation of a like amorphous structure in the as-quenched alloy. With an increase in Fe contents, a significant grain refinement of the as-quenched alloy and an obvious enlargement in the lattice constants and the cell volumes of the alloys have been noticed.     

Effect of rapid quenching on the microstructures and electrochemical characteristics of La0.7Mg0.3Ni2.55−xCo0.45Alx (x = 0–0.4) electrode alloys

A new type of hydrogen storage alloy La_0.7Mg_0.3Ni_2.55−xCo_0.45Al_x (x = 0, 0.1, 0.2, 0.3, 0.4) with a PuNi₃-type structure (R− 3m) were prepared by casting and rapid quenching. The effects of the rapid quenching on the microstructures and electrochemical performances of the specimen alloys were investigated in detail. The results obtained by XRD, SEM and TEM show that the as-cast and quenched alloys have a multiphase structure, including the (La, Mg)Ni₃ phase, the LaNi₅ phase and the LaNi₂ phase. The rapid quenching had an inappreciable influence on the phase composition of the alloys, but it obviously changed the phase abundance of the alloys. The rapid quenching can significantly improve the compositional homogeneity and markedly decrease the grain size of the alloys. The results obtained by the electrochemical measurements indicate that the rapid quenching obviously enhanced the cyclic stability of the alloys, but it decreased the discharge capacity and activation capability of the alloys.     

Study on the microstructure and electrochemical kinetic properties of MmNi4.50−xMnxCo0.45Al0.30 (0.25 ≤ x ≤ 0.45) hydrogen storage alloys

The phase structures, surface morphologies and electrochemical kinetic properties of MmNi_4.50?xMn_xCo_0.45Al_0.30 (Mm is the mischmetal, x = 0.25, 0.30, 0.35, 0.40 and 0.45) hydrogen storage alloys have been investigated in this paper. The X-ray diffraction (XRD) shows that all the alloys mainly consist of LaNi₅ phase with CaCu₅-type structure, which belongs to P6/mmm space group (central symmetry). Scanning electron microscopy (SEM) tests indicate that there are partial element segregations in the alloys. Meanwhile, energy dispersive spectrum (EDS) results display that the elements constituting Mm exist in the matrix phase in relatively larger proportion, while Mn, Al and Co tend to appear in precipitate phase. For the alloy with x = 0.35, the electrochemical performances, including discharge capacity, high-rate dischargeability (HRD) and cycling life, of the alloy electrode are better than that of other alloy electrodes. With the increase of Mn content, the exchange current density (I₀) of the alloy electrodes first increases and then decreases, the hydrogen diffusion coefficient (D) of alloy electrodes gradually decreases. There is a linear correlation between HRD at a discharge current density of 1500 mA/g and I₀.     

Electrochemical Characteristics of LaNi_(4.5)Al_(0.5) Alloy Used as Anodic Catalyst in a Direct Borohydride Fuel Cell

Fuel cells using borohydride as the fuel have received much attention because of high energy density and theoretical working potential.In this work,LaNi4.5Al0.5 hydrogen storage alloy used as the anodic material has been investigated.It was found that the increasing operation temperature was helpful to the open-circuit potential,the discharge potential and the power density,but showed a negative effect on the utilization of the fuel due to the accelerated hydrogen evolution.The high KOH concentration was favorable for high-rate discharge capability.The adsorption and transformation of hydrogen on LaNi4.5Al0.5 alloy electrode has been observed,but its contribution to the discharge capability during a high-rate discharge was small.     

Structures and electrochemical performances of as-spun RE-Mg-Ni-Co-Al alloys applied to Ni-MH battery

The La-Mg-Ni-Co-Al-based AB₂-type La_0.8-xCe_0.2Y_xMgNi_3.4Co_0.4Al_0.1 (x = 0, 0.05, 0.1, 0.15, 0.2) alloys were prepared by melt spinning. The effects of Y content on the structures and electrochemical hydrogen storage characters were thoroughly studied. The structures of the experimental samples were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). It is shown that there are a main phase LaMgNi₄ and a second phase LaNi₅ in the experimental samples. The variation of Y content incurs obvious changes of the phase abundance without changing phase composition. Namely, with the increase of Y content, the LaMgNi₄ phase increases and LaNi₅ phase decreases. Furthermore, melt spinning and the replacement of Y for La also lead to the grains refinement of the alloy. The electrochemical tests display that the as-spun alloys possess excellent activation properties, and obtain the maximums of discharge capacity at the first cycling. The replacement of Y for La can visibly facilitate the discharge potential characteristics, however,diminish the discharge capacity. The electrochemical kinetics, involving in the high rate discharge ability (HRD), hydrogen diffusion coefficient (D), limiting current density (I_L) and charge transfer rate, increases firstly and then decreases with the increase of Y content. The cyclic stability is greatly improved by melt spinning and the replacement of Y for La, which is derived from the improvement of the anti-corrosion, oxidation-resistance and the anti-pulverization abilities.     

Effect of substituting B for Ni on electrochemical kinetic properties of AB5-type hydrogen storage alloys for high-power nickel/metal hydride batteries

In this paper, the structure and electrochemical kinetic properties of MmNi_3.70−xMn_0.35Co_0.60Al_0.25B_x (x = 0.00, 0.10, 0.15, 0.20, 0.25) hydrogen storage alloys prepared by inductive melting have been systematically studied. The X-ray diffraction (XRD) shows that the alloys with B have not only LaNi₅ phase, but the secondary phase with CeCo₄B-type structure. The amount of the secondary phase and the plateau pressure of pressure-composition (P-C) isotherms gradually increase with the increase of B content. As x increases from 0.00 to 0.25, the high-rate dischargeability (HRD) of alloy electrodes first increases and then decreases. When x = 0.20, the HRD value reaches the maximum—63.1% at 1500 mA/g discharge current density. Electrochemical kinetic measurements indicate that the superior HRD of alloy electrodes is ascribed to their high surface electrocatalytic activity and fast hydrogen transfer in the bulk of alloys. The substitution of Ni with B in suitable amount could improve the kinetic properties of rare earth-based AB₅-type alloys because of the formation of the secondary phase.     

Measuring the gas permeability of a metal hydride bed of the LaNi<Subscript>5</Subscript> type alloy

We have tested the neutral, against the sorption material, gas (nitrogen) in the RKhO-8 metal hydride reactor containing 1 kg of the LaNi_4.8Mn_0.3Fe_0.1 alloy and have calculated the viscous permeability coefficient: k = 0.42 ± 0.08 μm².     

Production of Functional Nanocrystalline Materials in Hydrogen

On the example of KS25 and KS37 samarium–cobalt-base commercial alloys and LaNi_4.5Al_0.5 alloy, we show the possibility, in principle, of obtaining functional materials in the nanocrystalline state with the help of a planetary mill in hydrogen medium. Milling with a rotational speed of 600 rpm during 24 h leads to the disproportionation of KS25 and KS37 alloys into samarium hydride and iron–cobalt (cobalt) and of LaNi_4.5Al_0.5 into Ni₃Al and amorphous products. After vacuum annealing up to 1181 K, the main phases of samarium–cobalt materials are recombined. The crystallite sizes after annealing are 58–72 and 70 nm for KS25 and KS37, respectively. We established that LaNi_4.5Al_0.5 alloy is not recombined in vacuum, and the nanocrystalline state in it can be reached by milling up to 30 min. The crystallite sizes constitute 45–78 nm.     

Investigation of quench sensitivity and transformation kinetics during isothermal treatment in 6082 aluminum alloy

The quench sensitivity of 6082 aluminum alloy was investigated by time–temperature–property (TTP) curves. The sensitive temperature of quenching ranges from 250 °C to 440 °C in 6082 Al-alloy, and the nose temperature is about 360 °C. During isothermal treatment process, the Mg₂Si particles precipitate from the supersaturated solid solution, and the precipitation rate is the highest at the nose temperature. A number of rod-shaped β′ and β particles precipitate in the early stage of isothermal treatment at 360 °C. Prolonging the holding time leads to more and coarser β particles in the matrix. Both the precipitation of β′ and β particles results in loss of solute and decreasing of the subsequent age hardening effect. Also, the important coefficients k₂–k₅ and critical cooling rate for 6082 Al-alloy are identified, and the properties after different rates of cooling were predicted using quench factor analysis.     

Effect of high dilution on the in situ synthesis of Ni–Zr/Zr–Si(B,C) reinforced composite coating on zirconium alloy substrate by laser cladding

High dilution of transition metals was employed as a new idea for in situ synthesis of Ni–Zr/Zr–Si(B, C) reinforced composite coatings by high power diode laser (HPDL) cladding Ni–Cr–B–Si powders on zirconium substrate. Microstructure, phase composition, the mechanism of in situ synthesis reinforcement and the microhardness of coatings were investigated by means of optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and micro-sclerometer. The results reveal that the morphologies and phase constituents are related to the content of alloying elements in powders. In low alloy coatings, the matrix was mainly composed of intermetallic compounds including NiZr and Ni₁₀Zr₇, while the reinforcements consisted of Zr₅Si₄, β-ZiSi, α-ZrSi and ZrC. At the top of high alloy coatings, the matrix was partially comprised of Zr-based amorphous phase with the reinforcements containing ZrB₂. It is thermodynamically favorable for ZrB₂ ceramic reinforcement to form compared to ZrC phase. The microstructure evolution was dependent on the contribution of the high dilution zirconium alloy substrate to the in situ reinforcement synthesis. The microhardness of the coating showed clear improvement compared with zirconium alloy substrate, although high variability was also found.     

Interfacial microstructures and properties of aluminum alloys/galvanized low-carbon steel under high-pressure torsion

A new composite processing technology characterized by hot-dip Zn–Al alloy process was developed to achieve a sound metallurgical bonding between Al–7 wt% Si alloy (or pure Al) castings and low-carbon steel inserts, and the variations of microstructure and property of the bonding zone were investigated under high-pressure torsion (HPT). During hot-dipping in a Zn–2.2 wt% Al alloy bath, a thick Al₅Fe₂Zn_x phase layer was formed on the steel surface and retarded the formation of Fe–Zn compound layers, resulting in the formation of a dispersed Al₃FeZn_x phase in zinc coating. During the composite casting process, complex interface reactions were observed for the Al–Fe–Si–Zn (or Al–Fe–Zn) phases formation in the interfacial bonding zone of Al–Si alloy (or Al)/galvanized steel reaction couple. In addition, the results show that the HPT process generates a number of cracks in the Al–Fe phase layers (consisting of Al₅Fe₂ and Al₃Fe phases) of the Al/aluminized steel interface. Unexpectedly, the Al/galvanized steel interface zone shows a good plastic property. Beside the Al/galvanized steel interface zone, the microhardnesses of both the interface zone and substrates increased after the HPT process.     

LaNi1−x Co x O3 as interconnection materials

Thermo Gravimetric Analyis (TGA) shows that incorporation of Co³⁺ considerably reduces the oxygen loss at high temperatures in LaNiO₃. Electrical resistivity of LaNi_1?x Co _x O₃ (x?0·2) is essentially independent of oxygen partial pressure in the 600–1000 K range.     

Studies on the oxidation behaviour of intermetallic compounds

The oxidation behaviour of the AB₅ type intermetallic compounds CaNi₅ and LaNi₅ has been studied at different temperatures ranging from 100 to 800° C. The kinetic results indicate that the element A (Ca, La) is first oxidized rapidly followed by slower oxidation of nickel. X-ray diffractograms of the oxidized LaNi₅ show the formation of ternary phases like LaNiO₃ and La₂NiO₄ at temperature as low as 400° C. A comparison between the oxidation behaviour of the two compounds reveals that CaNi₅ is more resistant to oxidation and decomposition than the LaNi₅ system.     

Hydrides of LaNi compounds

Hydrides of various compounds in the La-Ni system have been prepared by gas absorption at temperatures from 25 to 200°C and pressures from 50 to 100 atmospheres of H₂. The compositions of the resulting hydrides are: La₇Ni₃H₂₁, LaNiH_3.6, LaNi₂H_4.5, LaNi₃H₅, La₂Ni₇H₁₀ and LaNi₅H_6.5. X-ray diffraction verified that LaNiH_3.6 and LaNi₅H_6.5 are ternary compounds, while LaNi₂H_4.5, LaNi₃H₅ and La₂Ni₇H₁₀ form in an amorphous state when hydrided at temperatures near 25°C. La₇Ni₃ forms primarily LaH₃ during hydrogen absorption. There were no indications of superconductivity or magnetic order down to 1.8K in any of these materials.     

Experimental and computational investigations of LaNi5-xAlx (x = 0, 0.25, 0.5, 0.75 and 1.0) tritium-storage alloys

Although already scientists in recent years have reported some experimental and theoretical results of LaNi-Al series of tritium-storage alloys, several key aspects remain the subject of considerable debate. In an effort to interpret some of these unknowns, we have performed experimental and theoretical investigations for LaNi_(5-x)Al_x(x = 0, 0.25, 0.5, 0.75 and 1.0) tritium-storage alloys. Firstly, the XRD characterization indicates that the unit cell volumes of LaNi_(5-x)Al_x increase with Al content in alloys. Secondly, the PCisotherm measurement of LaNi_(5-x)Al_xalloys shows that their hydrogen absorption/desorption plateau pressures reduce with the increase of Al content while their plateau widths narrow simultaneously. The deuterium absorption/desorption plateaus have a similar trend to hydrogen's except for their plateaus being higher than hydrogen's. To explain the above experimental findings, a series of calculations based on density functional theory(DFT) and frozen phonon approach have been performed. The results manifest that:(1) the partial substitutions of Al for Ni reduce the hydrogen formation energies of LaNi_(5-x)Al_xH and the number of available interstitial sites, and therefore lead to the absorption/desorption plateau pressures being reduced and the plateau widths being narrowed down at the same experimental temperatures;(2) the covalent interaction between H and Ni is an important factor for estimating the stability of LaNi_(5-x)Al_x-H system;(3) since the calculated enthalpy change H is generally more accurate than the calculated entropy change S with respect to the corresponding experimental value for each LaNi_(5-x)Al_xH(or D), the curves of H vs. hydrogen storage capacity instead of Van't Hoff relation, can be used to predict the experimental plateau pressures of LaNi_(5-x)Al_x-H(D or T) at a given temperature;(4) the hydrogen isotope effect of LaNi_(5-x)Al_x-H(D or T) system can be quantitatively described as a linearity relation between ⊿ZPE + ⊿H~(vib) and 1/√mQ(Q = H, D, T). From the good agreement between the predicted and experimental ln(P_H/P_0) and ln(P_D/P_0), it is deduced that predicting ln(P_T/P_0) of LaNi_(5-x)Al_x T is feasible. The procedure of pre-computing and comparing curves of H vs. hydrogen storage capacity proposed in this paper provided an attractive tool to increase the efficiency of experimental alloying design of hydrogen(deuterium or tritium) storage materials.     

Superconductivity in LaNi2Ge2 and related intermetallics

In this note, we report the discovery of superconductivity in LaNi₂Ge₂ (T_c = 0.69 K), isostructural to several new superconductors we have previously reported. In addition, we report limited solid solubility of Sn in several of the latter compounds and that Sn modestly raises T_c of these materials. Ni in LaNi₂Ge₂ is nonmagnetic.     

Electrochemical properties of (La1−xTix)0.67Mg0.33Ni2.75Co0.25 (x = 0-0.20, at%) hydrogen storage alloys

(La_1−xTi_x)_0.67Mg_0.33Ni_2.75Co_0.25 (x = 0, 0.05, 0.10, 0.15 and 0.20, at%) alloys are synthesized by arc-melting and subsequent heat solid-liquid diffusing techniques, and the crystalline structures and electrochemical properties of the alloys are investigated systematically. The structural analysis results show that all the alloys mainly consist of (La, Mg)Ni₃ phase with the rhombohedral PuNi₃-type structure and LaNi₅ phase with the hexagonal CaCu₅-type structure. However, when the Ti content is higher than 0.10, a little amount of TiNi₃ phase start to form. Electrochemical measurements show that the alloy electrodes could be activated to their maximum discharge capacity within four cycles, the maximum discharge capacity is around 321.9-384.6 mAh g⁻¹, both the cyclic stability and the high-rate discharge ability first increased and then decrease with increasing x. All the results show that a little amount of Ti substitution for La in AB₃-type hydrogen storage alloys is effective to the improvement of the overall electrochemical properties.     

Effects of the substitution of Al for Ni on the structure and electrochemical performance of La0.7Mg0.3Ni2.55 − x Co0.45Al x (x = 0–0.4) electrode alloys

In order to improve the cycling stability of La–Mg–Ni system (PuNi₃-type) hydrogen storage alloy, Ni in the alloy was partially substituted by Al, and La_0.7Mg_0.3Ni_2.55 − x Co_0.45Al _x (x = 0, 0.1, 0.2, 0.3, 0.4) electrode alloys were prepared by casting and rapid quenching. The effects of the substitution of Al for Ni on the structure and electrochemical performance of the as-cast and quenched alloys were investigated in detail. The results obtained by XRD, SEM and TEM show that the substitution of Al for Ni has an inappreciable influence on the abundance of the LaNi₂ phase in the as-quenched alloy, while it increases the amount of the LaNi₂ phase in the as-cast alloys. In addition, the substitution of Al for Ni is unfavourable for the formation of an amorphous in the as-quenched alloy. The results obtained by the electrochemical measurement indicate that the cycling stabilities of the as-cast and quenched alloys are significantly ameliorated with increasing Al content. When Al content increases from 0 to 0.4, the cycle life of the as-cast and quenched (30 m/s) alloys enhances from 72 to 132 cycles and from 100 to 136 cycles, respectively.