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.     

Hydrogenation of CaLi2−xMgx (0 ≤ x ≤ 2) with C14 Laves phase structure

CaLi_2−xMg_x (0 ≤ x ≤ 2) which has the C14-type Laves phase structure has been successfully synthesized and hydrogenated. The C14-type Laves phase structure was kept after hydrogenation of CaLi_2−xMg_x (x = 0.2, 0.5, 1). After hydrogenation of CaLi₂ and CaMg₂, the Laves phase disappeared. The CaH₂ and LiH phases were formed from CaLi₂ and the CaH₂ and Mg phases from CaMg₂, respectively. CaLi_2−xMg_x (0 < x < 2) ternary alloys formed stable hydride phases with the C14-type Laves phase structure in contrast to CaLi₂ and CaMg₂ binary alloys.     

Cycle stabilities of the La0.7Mg0.3Ni2.55−xCo0.45Mx (M = Fe, Mn, Al; x = 0, 0.1) electrode alloys prepared by casting and rapid quenching

In order to improve the cycle stability of La–Mg–Ni system (PuNi₃-type) hydrogen storage alloy, Ni in the alloy was partly substituted by Fe, Mn and Al, and the electrode alloys La_0.7Mg_0.3Ni_2.55−xCo_0.45M_x (M = Fe, Mn, Al; x = 0, 0.1) were prepared by casting and rapid quenching. The effects of the substitution of Fe, Mn and Al for Ni and rapid quenching on the microstructures and electrochemical properties of the alloys were investigated in detail. The results obtained by XRD, SEM and TEM indicate that element substitution has no influence on the phase compositions of the alloys, but it changes the phase abundances of the alloys. Particularly, the substitution of Al and Mn obviously raises the amount of the LaNi₂ phase. The substitution of Al and Fe leads to a significant refinement of the as-quenched alloy's grains. The substitution of Al strongly restrains the formation of an amorphous in the as-quenched alloy, but the substitution of Fe is quite helpful for the formation of an amorphous phase. The effects of the substitution of Fe, Mn and Al on the cycle stabilities of the as-cast and quenched alloys are different. The positive influence of the substitution elements on the cycle stabilities of the as-cast alloys is in proper order Al > Fe > Mn, and for as-quenched alloys, the order is Fe > Al > Mn. Rapid quenching engenders an inappreciable influence on the phase composition, but it markedly enhances the cycle stabilities of the alloys.     

Characterization and hydrogenation of CeNi5−xCrx (x = 0, 1, 2) alloys

The effect of partial substitution of Ni by Cr in CeNi₅ intermetallic compound has been studied by pressure–composition isotherm measurements for different temperatures. The samples were prepared of high purity materials using the standard arc melting technique in argon atmosphere. The structure and the elemental composition of different alloys have been investigated by means of XRD, SEM and EDX techniques. The unit cell volume of the alloy was found to increase with increasing Cr content. In order to calculate the hydrogen storage capacity pressure–composition isotherm has been investigated for CeNi_5−xCr_x (x = 1, 2) alloys in the temperature and pressure ranges of 293 ≤ T ≤ 333 K and 0.5 ≤ P ≤ 35 bar, respectively. The P–C–T isotherm for different alloys clearly shows the presence of three regions ,  + β and β. The enthalpy and entropy for the systems has also been calculated using Van’t Hoff plot. The variation of enthalpy and entropy with hydrogen content has also been studied.     

On the crystal structure of new series of compounds, RPt2+xSb2−y (x = 0.125, y = 0.25; R = La, Ce, Pr)

A new compound CePt_2+xSb_2−y (x = 0.125, y = 0.25) was synthesized by arc-melting of the elements. The chemical and structural characterizations were carried out at room temperature on as-cast samples using X-ray diffractometry, metallographic analysis and EDS-microanalysis. According to the results of X-ray single crystal diffraction this antimonide crystallizes in I4cm space group (no. 108), Z = 32, ρ = 12.19 Mg/m³, μ = 89.05 mm⁻¹ (a = 12.5386(3) Å, c = 21.4692(6) Å (crystal I) and a = 12.5455(2) Å, c = 21.4791(5) Å (crystal II)). The structure and composition were confirmed by powder X-ray diffraction (a = 12.4901(2) Å, c = 21.3620(4) Å) and EDS-microanalysis respectively. Isotypic compounds were observed with La and Pr from X-ray powder diffraction of as-cast alloys at room temperature (a = 12.6266(4) Å, c = 21.4589(6) Å for LaPt_2+xSb_2−y and a = 12.5184(5) Å, c = 21.4178(7) Å for PrPt_2+xSb_2−y). The CePt_2+xSb_2−y structure is derived from CaBe₂Ge₂ (a = 2a₀ − 2b₀, b = 2a₀ + 2b₀, c = 2c₀) and comprises a new atomic arrangement with both vacancy on 4(b) pyramidal site and substitution of antimony atoms (X) by platinum (B) in the B–XX–B layers (referring to the subcell structure) forming two B––1/2B1/2XX–3/4B and two X–BB–X layers per cell. The structure of CePt_2+xSb_2−y is compared with those reported before for URh_1.6As_1.9 and CeNi_1.91As_1.94.     

AC susceptibility study of Bi1.66Pb0.34Sr2Ca2−xMgxCu3Oy (x = 0, 0.2 and 0.4) superconductor systems

A systematic study of the intergranular properties of Bi_1.66Pb_0.34Sr₂Ca_2−xMg_xCu₃O_y (x = 0, 0.2 and 0.4) samples has been done, using the AC susceptibility technique. The samples were prepared by conventional solid state reaction method. It was found that Mg substitution in place of Ca reduces the intergranular coupling of Bi-2223 system. Analysis of the temperature dependence of the AC susceptibility near the transition temperature (T_c) has been done employing Bean's Critical State Model. The observed variation of intergranular critical current densities (J_c) with temperature indicates that the critical current density decreases by increasing the amount of Mg. The higher electronegativity of Mg in the unit cell promotes more intake of oxygen in the material, and the grain boundaries are in more over-doped regime. These over-doped regions reduce the intergranular coupling and increases weak link behavior of Mg doped samples.     

Influences of the substitution of Fe for Ni on structures and electrochemical performances of the as-cast and quenched La0.7Mg0.3Co0.45Ni2.55−xFex (x = 0–0.4) electrode alloys

In order to improve the cycle stability of the La–Mg–Ni system PuNi₃-type hydrogen storage electrode alloys, Ni in the alloy was partially substituted by Fe. The La_0.7Mg_0.3Co_0.45Ni_2.55−xFe_x (x = 0, 0.1, 0.2, 0.3, 0.4) hydrogen storage alloys were prepared by casting and rapid quenching. The effects of the substitution of Fe for Ni on the structures and electrochemical performances of the as-cast and quenched alloys were investigated in detail. The results of the electrochemical measurement indicate that the substitution of Fe for Ni obviously decreases the discharge capacity, high rate discharge capability (HRD) and discharge potential of the as-cast and quenched alloys, but it significantly improves their cycle stabilities, and its positive impact on the cycle life of as-quenched alloy is much more significant than on that of the as-cast one. The microstructure of the alloys analyzed by XRD, SEM and TEM show that the as-cast and quenched alloys have a multiphase structure which is composed of two major phases (La, Mg)Ni₃ and LaNi₅ as well as a residual phase LaNi₂. The substitution of Fe for Ni helps the formation of a like amorphous structure in the as-quenched alloy. With the increase of Fe content, the grain sizes of the as-quenched alloys significantly reduce, and the lattice constants and cell volumes of the alloys obviously increase.     

Study of the structural, thermodynamic and electrochemical properties of LaNi3.55Mn0.4Al0.3(Co1−xFex)0.75 (0 ≤ x ≤ 1) compounds used as negative electrode in Ni-MH batteries

This study concerns the influence of iron for cobalt substitution on the structural, thermodynamic and electrochemical properties of the hydrides of poly-substituted LaNi_3.55Mn_0.4Al_0.3(Co_1−xFe_x)_0.75 (0 ≤ x ≤ 1) alloys used as material for negative electrode in Ni-MH batteries. The Fe substitution leads to an increase of the cell parameter, this increase is linear according to the rate of substitution, and a decrease of the equilibrium pressure in agreement with the geometric law. Nevertheless, it is observed that the Fe substitution leads to a deviation from the linear variation between the logarithm of the pressure and the cell volume observed for Co, Mn and Al for Ni substitution. The Fe for Co substitution leads also to a decrease of the solid–gas and electrochemical capacity.     

Preparation and structures of the La1−xKxCo1−xNbxO3 (x = 0–l) system

The La_1−xK_xCo_1−xNb_xO₃ system was performed by conventional solid state reaction technique using metal oxides. By DSC analysis, the activation energy of crystallization of the powders with x = 0.3 is 388.4 kJ/mol. The crystal structure of the compound reveals a transition from rhombohedral to cubic, and then to orthorhombic structure as the amount of the potassium niobate (KNbO₃) increases. It is found that the structure of the samples with x < 0.3 is similar to that of lanthanum cobaltate (LaCoO₃), while at the compositions with 0.7 ≥ x ≥ 0.3, the structure transforms to cubic. Finally, with x ≥ 0.7, the structures were similar to that of KNbO₃. According to the results of selected-area-diffraction (SAD) patterns and X-ray diffraction (XRD) identifications, the lattice parameters were calculated. The direction of superlattice structure along [2 1 0] was found for x = 0.5 as identified from SAD patterns. The dielectric constants were measured with cubic structure. Dielectric constant (K) decreases with increasing x.     

Hydrogen storage properties of Mg100−xNix (x = 5, 11.3, 20, 25) composites prepared by hydriding combustion synthesis followed by mechanical milling (HCS + MM)

We reported the structure and the notable hydrogen storage properties of the composites Mg_100−xNi_x (x = 5, 11.3, 20, 25) prepared from metallic powder mixtures of magnesium and nickel by the process of HCS + MM, i.e., the hydriding combustion synthesis (HCS) followed by mechanical milling (MM). X-ray diffraction (XRD) and scanning electron microscopy (SEM) results demonstrated that mechanical milling led to drastic pulverization and grain refinement of the composite produced by HCS. All the composites with different compositions showed a remarkable decline in dehydriding temperature comparing with that of the hydride mixtures prepared only by HCS. Furthermore, the hydriding rates of these composites were excellent. At 313 K the composite Mg₈₀Ni₂₀ showed the highest hydrogen capacity of 2.77 wt.% within 600 s among these four composites. The Mg₉₅Ni₅ showed maximum capacity of 4.88 wt.% at 373 K and 5.41 wt.% at 473 K within only 100 s. Some factors contributing to the improvement in hydriding rates were discussed in this paper.     

Structural, magnetic and electrical transport properties for a series of La1−xSrxFe1−xMnxO3 (0.3 ≤ x ≤ 0.7) compounds

The structural, electrical transport and magnetic properties have been studied for compounds: La_1−xSr_xFe_1−xMn_xO₃ (0.3 ≤ x ≤ 0.7). The lattice parameter, a, first decreases with x, and followed by an increase when Sr²⁺ and Mn⁴⁺ was continuously doped. The cell parameters, b and c, slightly decrease with coupled substitution of Sr²⁺ for La³⁺ and Mn⁴⁺ for Fe³⁺. In the paramagnetic temperature range, formation of magnetic clusters is suggested; the sizes of clusters decrease with x up to 0.5, following that they increase sharply with continuing doping. The electrical behaviors of all specimens demonstrate insulators and the electrical resistivity increases with content of Mn⁴⁺ and Sr²⁺ ions doped. A variable range hopping model is suitable to describe electrical transport process for the compounds at low temperature. At high temperature the electrical transport process can be described by bipolaron model for all compounds.     

Effect of Yb substitution on microstructure, physical and mechanical properties of negative thermal expansion Zr1−xYbxWMoO8−x/2 (x = 0–0.05) ceramic

Cubic Zr_1−xYb_xWMoO_8−x/2 (x = 0–0.05) ceramic was first fabricated by a polymorphous precursor transition method. X-ray diffraction experiment indicates that samples with x ≤ 0.05 are single phase solid solution. The measured bulk density, microstructure, maximal compression strength and Young's modulus are obviously sensitive to Yb substitution level, while none of such sensitivity was found for the lattice parameters, negative thermal expansion coefficients and Vickers hardness. Drilling tests on Zr_0.96Yb_0.04WMoO_7.98 ceramic indicate good machinability, which is often required for quality and shape control in engineering applications.     

Synthesis and characterization of LiGaxMn2−xO4 (0 ≤ x ≤ 0.05) by triethanolamine-assisted sol–gel method

Spinel LiGa_xMn_2−xO₄ (0 ≤ x ≤ 0.05) cathode materials with phase-pure particles and nano-sized distribution were synthesized by sol–gel method using triethanolamine as the chelating agent. The effects of heat treatment on the physicochemical properties of the spinel LiGa_xMn_2−xO₄ powders were examined with thermogravimetric and differential thermal analysis (TG/DTA), powder X-ray diffraction (XRD) and scanning electron micrograph (SEM). The LiGa_xMn_2−xO₄ (0 ≤ x ≤ 0.05) electrodes were characterized electrochemically by charge/discharge experiments under a current rate of 0.5C at 55 °C. Although the Ga-doped spinel electrode showed smaller initial discharge capacity, it exhibited better cycling performance than the undoped-LiMn₂O₄ electrode. The dQ/dV versus potential plots at 55 °C revealed that the improvement in cycling performance of the Ga-doped spinel electrode is attributed to stabilization of the spinel structure by the presence of gallium ion.     

Influence of stress on magneto-impedance in Co71−xFexCr7Si8B14 (x = 0, 2) amorphous ribbons

Systematic measurements of stress-impedance (SI) have been carried out using Co-rich amorphous ribbons of nominal composition Co_71−xFe_xCr₇Si₈B₁₄ (x = 0, 2) at various excitation frequencies and bias fields and at room temperature. The impedance, Z(σ) for both the samples was found to be very sensitive functions of applied tensile stresses (up to 100 MPa) exhibiting a maximum SI ratio as much as 80% at low frequency 0.1 MHz. The nature of variation of SI changes with the excitation frequency especially at higher frequencies in MHz region where it exhibits a peak. Magnetization measurements were also performed to observe the effects of applied stresses and magnetization decreased with the application of stress confirming the negative magnetostriction coefficient of both the samples. Both the samples exhibited negative magneto-impedance (MI) when the variation of Z is observed with the applied bias magnetic field, H. The impedance as functions of applied magnetic field, Z(H), decreases with the application of stress thus making the MI curves broader. Maximum MI ratio as large as 99% has been observed for both the samples at low fields 27 Oe.     

Martensitic transformation and anomalies in resistivity of (Ti–50Ni)1−xCx (x = 0.1, 0.5 at.%) shape memory alloys

Phase transformation of solid solution (Ti–50Ni)_1−xC_x (x = 0.1, 0.5 at.%) alloys have been studied by using differential scanning calorimetry, physical property measurement system and optical microscope. The transformation temperature decreases due to the existence of titanium carbide (TiC) particles compared with that of near-equiatomic Ti–Ni shape memory alloy. The resistivity vs. temperature curves show hysteresis. Thermoelastic martensitic transformation occurred in two alloys despite the difference in TiC content. Nevertheless, the resistivity results show different martensitic transformation routes. A one-step B2 → B19′ transformation occurred in the low TiC content alloy and an R transformation appeared in another alloy, suggesting that the martensitic transformation routes depended on the TiC content. The cumulative effect of the TiC particles causes the local stress field and lattice distortion to restrain the transformation of the B19′. On the other hand, the TiC content has an effect on the temperature coefficient of electrical resistivity (TCR) of alloys. The Ti–Ni–0.5C alloy shows a negative TCR in the range 100–300 K during which transformation occurs. Another alloy shows the opposite result. The cause of the negative TCR is briefly discussed.     

Influence of Co substitution on magnetoelastic properties of Er2Fe14−xCoxB (x = 1, 3 and 5) intermetallic compounds

The magnetostriction and thermal expansion of Er₂Fe_14−xCo_xB (x = 1, 3 and 5) intermetallic compounds were measured, using the strain gauge method in the temperature range 75–450 K under applied magnetic fields up to 1.5 T. For all samples the longitudinal magnetostriction (λ_l) undergoes an anomaly around the spin reorientation temperature (T_SR). It is also observed that λ_l decreases with increasing the Co content. All compounds show saturation type behaviour in their anisotropic magnetostriction curves at different temperatures and applied fields. The saturation behaviour of the compound with x = 3 occurs at higher temperatures than with x = 1 and 5. The volume magnetostriction strongly increases below μ₀H = 0.3 T, then monotonically rises with applied field up to the spin reorientation temperature. An invar type behaviour is observed above 200 K in the compound with x = 1. The results are discussed based on the temperature dependence of magnetocrystalline anisotropy of compounds below and above their T_SR.     

Electrical behaviour of hydridofluorides of potassium–calcium KCaH3−xFx with x = 1, 1.5, 2, 2.5

This paper reports the results of the electrical conductivity measurements for KCaH_3−xF_x series with (x = 1, 1.5, 2, 2.5) in the temperature range 298–503 K.The activation energy of the electrical conductivity for the studied compounds depends on hydrogen amount and Reau's criteria. Differential thermal analysis curves were measured in the same temperature range 298–503 K.Possible correspondence between preferential order given by X-ray diffraction, thermal behaviour and electrical properties are discussed.     

Investigation on the structures and electrochemical performances of La0.75−xZrxMg0.25Ni3.2Co0.2Al0.1 (x = 0–0.2) electrode alloys prepared by melt spinning

In order to improve the electrochemical cycle stability of the La–Mg–Ni system A₂B₇-type electrode alloys, La in the alloy was partially substituted by Zr and the melt-spinning technology was used for preparing La_0.75−xZr_xMg_0.25Ni_3.2Co_0.2Al_0.1 (x = 0, 0.05, 0.1, 0.15, 0.2) electrode alloys. The microstructures and electrochemical performances of the as-cast and quenched alloys were investigated in detail. The results obtained by XRD, SEM and TEM showed that the as-cast and quenched alloys have a multiphase structure which is composed of two main phases (La, Mg)Ni₃ and LaNi₅ as well as a residual phase LaNi₂. The substitution of Zr for La leads to an obvious increase of the LaNi₅ phase in the alloys, and it also helps the formation of a like amorphous structure in the as-quenched alloy. The results of the electrochemical measurement indicated that the substitution of Zr for La obviously decreased the discharge capacity of the as-cast and quenched alloys, but it significantly improved their cycle stability. The discharge capacity of the alloys (x ≤ 0.1) first increased and then decreased with the variety of the quenching rate. The cycle stability of the alloys monotonously rose with increasing quenching rate.     

Synthesis, structural, magnetic and electrical properties of La1−xCdxMnO3 manganites (0.1 ≤ x ≤ 0.5)

Cd-substituted LaMnO₃ compounds have been prepared using sol–gel method at a relatively low temperature of 900 °C. The crystal structure examined by X-ray powder diffraction indicates that the samples were single phase and crystallize in a rhombohedral () structure with increase the Cd content. The Rietveld refinement of the structure shows that the substitution of La by Cd in the La site changes the structure parameters, such as MnO bond length and MnOMn angles. Magnetic measurements reveal that the magnetization of these compounds exhibit a ferromagnetic to paramagnetic transition with an increasing in the Curie temperature T_C when x increases from 0.1 to 0.5. The resistivity measurements reveal that the samples exhibit a metallic to semiconductor transition and the electrical temperature transition T_P present a similar trend of T_C, with increasing the Cd content.     

Structural and magnetic properties of single-crystalline spinel systems ZnCr2−xAlxSe4 (x = 0.15 and 0.23)

Single crystals of the spinel solid solutions ZnCr_2−xAl_xSe₄ with x = 0.15 and 0.23 were grown and studied by X-ray diffraction and macroscopic magnetic measurements. The solubility of Al³⁺ ions in the parent compound ZnCr₂Se₄ is very limited and only weakly substituted single crystals can be obtained. The spinel structure is hardly modified by this admixture and regular cation distribution is preserved. Alike ZnCr₂Se₄, the two compounds investigated order antiferromagnetically at low temperatures. Both the Néel temperatures and the paramagnetic Curie temperatures are similar to that reported for the parent selenide. Also the effective magnetic moments are close to that of ZnCr₂Se₄ and compatible with trivalent Cr ions. In contrast, the saturation magnetic moments measured above the metamagnetic phase transitions in strong magnetic fields and calculated per one Cr atom, appear to be strongly affected by the Al-substitution, being rapidly suppressed with rising Al-content.