Hydrogenation behaviour of Ce-based AB5 intermetallic compounds

In the present work, the effect of Zr on the hydrogenation behaviour of CeNi₅ intermetallic compound has been studied. All intermetallic compounds have been synthesized by arc melting method in the argon atmosphere and well characterized by the means of XRD and SEM. The elemental composition of each sample has also been investigated using EDX technique. EDX spectrum confirms the elements present in the sample and the exact quantitative composition of these elements. The lattice constant and volume was found to increase with Zr substitution. Pressure composition isotherm has been carried out for CeNi_5−xZr_x (x = 1, 2) alloys in the temperature and pressure ranges of 293 ≤ T ≤ 333 K and 0.5 ≤ P ≤ 35 bar, respectively. The plateau pressure was found to reduce significantly, which makes these alloys more useful for practical applications in comparison to parent alloy (CeNi₅). The enthalpy and entropy for the systems has been calculated using Van’t Hoff plot.     

Phase relations and hydrogenation behavior of Sr(Al1−xMgx)2

The phase relations and hydrogenation behavior of Sr(Al_1−xMg_x)₂ alloys were studied. The pseudobinary C36-type Laves phase Sr(Al,Mg)₂ was found as a structural intermediate between the Zintl phase and the C14 Laves phase. The single-phase regions for the Zintl phase, C36 phase and C14 phase, were determined to be x=0–0.10, 0.45–0.68 and 0.80–1, respectively. The Mg-substituted Zintl phase Sr(Al_0.95Mg_0.05)₂ can be hydrogenated to Sr(Al,Mg)₂H₂ at about 473 K. However, the Sr(Al,Mg)₂H₂ directly decomposes into SrH₂ and Sr(Al,Mg)₄ starting at 513 K. When the temperature is 573 K, the C36 Laves phase Sr(Al_0.5Mg_0.5)₂ can be hydrogenated into SrMgH₄ and Al, while the C14 Laves phase Sr(Al_0.1Mg_0.9)₂ is hydrogenated into SrMgH₄, Mg₁₇Al₁₂ and Mg.     

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.     

Investigation of hydrogen absorption/desorption properties of ZrMn0.85−xFe1+x alloys

The crystal structures and hydrogen absorption/desorption properties of the ZrMn_0.85−xFe_1+x alloys (x = 0, 0.2, 0.4) were investigated systematically. The pressure–composition (PC) isotherms and absorption kinetics were measured at 273–333 K by the volumetric method. Besides the crystal structure, the plateau pressure and the hydrogen intake capacity, this article also discussed the absorption kinetics, the pulverization resistance and the thermodynamic properties. XRD patterns revealed that ZrMn_0.85Fe and ZrMn_0.65Fe_1.2 were formed as hexagonal C14 laves phase structure while ZrMn_0.45Fe_1.4 possessed cubic C15 laves phase structure. With the increase of Fe and decrease of Mn, the plateau pressure increased while the hydrogen intake capacity lowered and the hydrogen absorption kinetics degraded. On the other hand, the hysteresis alleviated, the pulverization resistance improved and the stability of the hydrides decreased. The decomposition pressure was increased to more than 160 times for ZrMn_0.85Fe and more than 2500 times for ZrMn_0.65Fe_1.2 compared with that of the ZrMn₂ alloy.     

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.     

Effects of cold roll and heat treatments on anomalous thermal behavior in Fe100−xAlx(x = 5–30) alloys

DSC measurements were carried out for various Fe_100−xAl_x(x = 5–30 at%) alloys to clear the effects of cold roll and quenching rate from 1173 K. In the case of cold roll free specimens, an exothermic peak was observed at around 530–560 K in quenched specimens and no peaks in slowly cooled specimens. The peak temperature and its exothermic heat depended on the alloy composition. The maximum exothermic heat was obtained for a 25 at% Al alloy and its value were about 1200 J/mol. The peak in a 5 at% Al alloy was remained as a future work. The exothermic heat was affected by the quenching temperature in alloys above 15 at% Al. The peak temperature was decreased by decreasing the quenching temperature. In a 15 at% Al alloy, the peak became negligibly small by quenching from 1023 K. The activation energies in cold roll free specimens were evaluated from the Kissinger analysis and they were 134, 108, 133 and 110 kJ/mol for 15 at% Al, 20 at% Al, 25 at% Al and 30 at% Al alloys, respectively. On the other hand, cold rolled specimens showed an exothermic peak at around 470 K, independently of the cooling rate. Their exothermic heats and temperatures were comparable order to those of furnace cooled and water quenched specimens. The present results suggested that origin of exothermic peaks of all alloys were same in nature and atomic ordering may be related to the exothermic behavior at relatively low temperatures.     

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.     

Effect of partial substitution of Co with Fe on the properties of LaNi3.55Mn0.4Al0.3Co0.75−xFex (x=0, 0.15, 0.55) alloys electrodes

The effect of iron substitution on the electrochemical behaviour of LaNi_3.55Mn_0.4Al_0.3Co_0.75−xFe_x compounds (x=0, 0.15, 0.55) has been studied by chronopotentiometry and cyclic voltammetry techniques. The maximum capacity decreases linearly from 308 to 239 mAhg⁻¹ when the iron content increases from 0 to 7.3 wt.% (x=0.55). This decrease can be explained by the corrosion of the alloy in the aqueous KOH electrolyte. In spite of this decrease and of the long time needed for the activation, a good stability of discharge capacity was observed in LaNi_3.55Mn_0.4Al_0.3Co_0.75−xFe_x compounds. The reversibility of the electrochemical redox reaction of LaNi_3.55Mn_0.4Al_0.3Co_0.75−xFe_x alloy electrodes has been observed in the alloys least rich in iron. The hydrogen diffusivity in LaNi_3.55Mn_0.4Al_0.3Co_0.75−xFe_x alloy electrodes decreases when increasing the iron content. The obtained values of the hydrogen diffusion coefficient D_H, varies between 2.1×10⁻⁷ and 8.2×10⁻⁹ cm² s⁻¹ depending on the iron content of the electrode.     

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.     

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.     

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.     

Structural, electrical and magnetic properties of Cu1−xZnxFe2O4 ferrites (0 ≤ x ≤ 1)

Copper–zinc ferrites bearing chemical formula Cu_1−xZn_xFe₂O₄ for x ranging from 0.0 to 1.0 with the step increment of 0.2 were prepared by the standard solid-state technique. The variation of Zn substitution has a significant effect on the structural, electrical and magnetic properties. Lattice parameters ‘a’ increased from 8.370 to 8.520 Å. Dielectric constant decreased up to 311 with the increase in frequency from 80 Hz to 1 MHz at room temperature. All the samples follow the Maxwell–Wagner's interfacial polarization. Saturation magnetization, magnetic moment and Yafet–Kittel angles were also determined. The possible reasons responsible for the change in density related, electrical and magnetic properties with the increase in Zn concentration are undertaken.     

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.     

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.     

Structural, thermodynamic, and electrochemical properties of TixZr1−x(VNiCrMnCoAl)2 C14 Laves phase alloys

The crystal structure of the monoclinic phase η-Al₁₁Cr₂ of the space group C2/c, a ≈ 1.76 nm, b ≈ 3.05 nm, c ≈ 1.76 nm, β ≈ 90° [L.A. Bendersky, R.S. Roth, J.T. Ramon, D. Shechtman, Metall. Trans. A 22A (1991) 5] has been determined by single-crystal X-ray diffraction. The structure model, refined to a final R value of 0.0441, has the composition of Al_83.8Cr_16.2. a = 1.77348(10) nm, b = 3.04555(17) nm, c = 1.77344(10) nm, monoclinic angle β = 91.0520(12)°. There are 80 (66Al + 14Cr) independent atomic positions in a unit cell, of which all Cr atom sites and 8 Al atom sites have icosahedral coordination. These icosahedra are interconnected forming icosahedral chains along , (1 0 1) icosahedral layer blocks as well as a three-dimensional icosahedral structure.     

Influence of hydrogenation on structure and magnetic properties of HoFe11−xCoxTi

Magnetic properties and crystal structure of the hydrides of ferromagnetic compounds HoFe_11−xCo_xTi (x = 1, 2, 4, 6, 7, 11) are investigated. The crystal structure was determined by X-ray diffraction (XRD) analysis and the magnetization was measured in applied magnetic fields up to 10 T and at temperatures ranging from 5 K to room temperature. Results show that the crystal structure of the hydrides is the same as for parent compounds but with a moderate unit cell increase. Other properties such as saturation magnetization are affected by H insertion within the lattice. The effect of hydrogenation on magnetic anisotropy energy leads to disappearance of the FOMPs observed in the parent compounds.     

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.     

Electrochemical properties of the MmNi3.55Mn0.4Al0.3Co0.75−xFex (x = 0.55 and 0.75) compounds

The hydrogen storage alloys MmNi_3.55Mn_0.4Al_0.3Co_0.75−xFe_x (x = 0.55 and 0.75) were used as negative electrodes in the Ni-MH accumulators. The chronopotentiommetry and the cyclic voltammetry were applied to characterize the electrochemical properties of these alloys. The obtained results showed that the substitution of the cobalt atoms by iron atoms has a good effect on the life cycle of the electrode. For the MmNi_3.55Mn_0.4Al_0.3Co_0.2Fe_0.55 compound, the discharge capacity reaches its maximum of 210 mAh/g after 12 cycles and then decreases to 190 mAh/g after 30 charge–discharge cycles. However, for the MmNi_3.55Mn_0.4Al_0.3Fe_0.75 compound, the discharge capacity reaches its maximum of 200 mAh/g after 10 cycles and then decreases to 160 mAh/g after 30 cycles.

The diffusion behavior of hydrogen in the negative electrodes made from these alloys was characterized by cyclic voltammetry after few activation cycles. The values of the hydrogen coefficient in MmNi_3.55Mn_0.4Al_0.3Co_0.2Fe_0.55 and MmNi_3.55Mn_0.4Al_0.3Fe_0.75 are, respectively, equal to 2.96 × 10⁻⁹ and 4.98 × 10⁻¹⁰ cm² s⁻¹. However, the values of the charge transfer coefficients are, respectively, equal to 0.33 and 0.3. These results showed that the substitution of cobalt by iron decreases the reversibility and the kinetic of the electrochemical reaction in these alloys.     

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.     

Electrical, magnetic and elastic properties of La1.2(Sr1−xCax)1.8Mn2O7 (0.0 ≤ x ≤ 0.4)

Polycrystalline bulk samples of double layered manganite system La_1.2(Sr_1−xCa_x)_1.8Mn₂O₇ (0.0 ≤ x ≤ 0.4) were prepared by sol–gel method. After characterizing the samples using XRD and SEM, their electrical, magnetic and elastic properties were investigated. The lattice parameters and cell volume show a monotonous decrease with increase of Ca content, whereas the grain size is found to increase with increasing Ca content. The value of T_IM is found to decrease with Ca content up to x = 0.3 and then a slight increase of T_IM is observed. The low temperature upturn of resistivity is attributed to the spin-glass-like behavior, which is also evidenced by the irreversibility observed between ZFC and FC magnetizations. The conduction mechanism above T_IM can be explained by Mott VRH model. The present magnetization and ultrasonic studies indicate that the system shows a secondary transition at T^*, which decreases with increasing Ca content. Further, the T^* seems to be intrinsic to the present double layered manganite system.