Structure and hydrogen storage characteristics of as-spun Mg-Y-Ni-Cu alloys

Experimental alloys with compositions of Mg_25-xY_xNi₉Cu (x = 0, 1, 3, 5, 7) have been successfully prepared through melt spinning method. The phase compositions and microstructures were measured by X-Ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The de-/hydrogenation properties were measured by utilizing Sievert apparatus, differential scanning calorimetry (DSC) and thermal gravimetric analyzer (TGA) connected with a H₂ detector. The Arrhenius and Kissinger methods were adopted to calculate their dehydrogenation activation energies. The results show that hydrogen absorption kinetics of the alloys notably decline while their hydrogen desorption kinetics conspicuously improve with spinning rate increasing. The dehydrogenation activation energy markedly decreases with spinning rate increasing, which makes the hydrogen desorption kinetics improve. The thermodynamic parameters (ΔH and ΔS absolute values) clearly decrease with spinning rate increasing. The hydrogen absorption capacity exhibits different trends with spinning rate rising. Specifically, hydrogen absorption capacity increases at the beginning and declines later for Y₁ alloy, but that of Y₇ alloy always decreases with spinning rate growing.     

Thermodynamics and kinetics of MgH<Subscript>2</Subscript>–nfTa<Subscript>2</Subscript>O<Subscript>5</Subscript> composite for reversible hydrogen storage application

The thermodynamics and kinetics of hydrogen absorption–desorption of nfTa₂O₅–Mg–MgH₂—composite (nf stands for nano-flakes) have been studied. The nfTa₂O₅–Mg composite could absorb hydrogen at room temperature (17 °C). The hydrogen desorption of nfTa₂O₅–MgH₂ composite starts at 200 °C. The remarkably improved hydrogen absorption–desorption of catalyzed Mg–MgH₂ could be attributed to the nano-engineered surface by nfTa₂O₅. The enthalpies of hydrogen absorption–desorption were found to be 80 ± 2, and 76 ± 3 kJ/mol respectively. The activation energy of hydrogen absorption was evaluated as 49 ± 5 kJ/mol which is same as the energy barrier for diffusion of hydrogen in Mg matrix. The apparent activation of hydrogen desorption of nfTa₂O₅–MgH₂ was found to be 74 ± 7 kJ/mol. The nfTa₂O₅–MgH₂ composite has shown cyclic stability up to fifty hydrogen absorption–desorption without significant changes in the kinetics and hydrogen storage capacity.     

Hydrogenation characteristics of the proton conducting oxide-hydrogen storage alloy composite

Hydrogen absorption-desorption characteristics of a proton conducting oxide, SrCe_0.95Yb_0.05O_3−δ, were investigated using an electrochemical technique for the first time. It is suggested that the oxide can electrochemically repeat hydrogen absorption and desorption like hydrogen storage alloys. Subsequently, we attempted to improve activation properties of a hydrogen storage alloy, MmNi_3.72Co_0.60Mn_0.45Al_0.32, by mixing the proton conducting oxide. As a result, the oxide-alloy composite was found to enhance the activation performance of anode in Ni-metal hydride battery. The proton conducting oxide lying on the surface corrosion layer of alloy particles plays a role as a new path for hydrogen atoms to go into the alloy.     

Hydrogen storage properties of Mg-based mixtures elaborated by reactive mechanical grinding

The hydrogen sorption properties of Mg + 10 wt% WO₃ and Mg + 5 wt% Cr₂O₃ mixtures made by reactive (under hydrogen) mechanical grinding were studied and compared with those of elemental Mg subjected to a similar preparation procedure. It was observed that both oxides have an important catalytic effect on hydrogen absorption and desorption. Moreover, in the case of Cr₂O₃ addition, both milling speed (i.e, milling energy and milling mode) and ball to powder weight ratio influence drastically the hydrogen sorption kinetics.     

Metal Hydride Nanoparticles with Ultrahigh Structural Stability and Hydrogen Storage Activity Derived from Microencapsulated Nanoconfinement

Metal hydrides (MHs) have recently been designed for hydrogen sensors, switchable mirrors, rechargeable batteries, and other energy‐storage and conversion‐related applications. The demands of MHs, particular fast hydrogen absorption/desorption kinetics, have brought their sizes to nanoscale. However, the nanostructured MHs generally suffer from surface passivation and low aggregation‐resisting structural stability upon absorption/desorption. This study reports a novel strategy named microencapsulated nanoconfinement to realize local synthesis of nano‐MHs, which possess ultrahigh structural stability and superior desorption kinetics. Monodispersed Mg₂NiH₄ single crystal nanoparticles (NPs) are in situ encapsulated on the surface of graphene sheets (GS) through facile gas–solid reactions. This well‐defined MgO coating layer with a thickness of ≈3 nm efficiently separates the NPs from each other to prevent aggregation during hydrogen absorption/desorption cycles, leading to excellent thermal and mechanical stability. More interestingly, the MgO layer shows superior gas‐selective permeability to prevent further oxidation of Mg₂NiH₄ meanwhile accessible for hydrogen absorption/desorption. As a result, an extremely low activation energy (31.2 kJ mol^–1) for the dehydrogenation reaction is achieved. This study provides alternative insights into designing nanosized MHs with both excellent hydrogen storage activity and thermal/mechanical stability exempting surface modification by agents.     

Cr3C2 incorporation into an Inconel 625 laser cladded coating: Effects on matrix microstructure,mechanical properties and local scratch resistance

There is an increasing industrial demand for metal alloys with high wear resistance under severe operating conditions. Ni-based alloys, such as Inconel superalloys, are an excellent option for these applications; however, their use is limited by their high cost. Ni-based coatings deposited onto carbon steel substrates are being developed to achieve desired surface properties with reduced cost. Laser cladding deposition has emerged as an excellent method for processing Ni based coatings. In this work, microstructure, mechanical properties and local wear behaviour have been investigated in response to the addition of Cr₃C₂ ceramic particles into an Inconel 625 alloy deposited onto a ferritic steel substrate by laser cladding. Using this deposition technique, a homogeneous distribution of Cr₃C₂ particles was observed in the coating microstructure. The addition of ceramic particles to the starting powder resulted in the formation of hard precipitates in the coating microstructure. The partial dissolution of Cr₃C₂ particles during the laser cladding process increased the hardness of the Inconel 625 matrix. Depth sensing indentation and scratch tests were performed to study the local wear behaviour and scratch resistance of the cermet matrix compared with the conventional Inconel 625 alloy. Finally, the effect of Cr₃C₂ on mechanical properties was correlated with the observed microstructure modifications.     

Preparation and properties of hydrogen-storage composites in the MgH2-C system

The hydriding properties of magnesium have been studied at temperatures from 670 to 720 K and pressures from 19 to 55 MPa. The results suggest that the hydrogen pressure and hydrogen absorption/desorption cycles influence the nucleation process in the Mg-H₂ system. Using mechanical activation of MgH₂ + graphite and MgH₂ + graphene nanofiber (GNF) mixtures, we have prepared MgH₂/graphite and MgH₂/NGF composites. Investigation of their hydriding properties has shown that mechanical activation markedly accelerates hydrogen sorption/desorption processes and reduces the thermal stability of the material compared to unmilled magnesium hydride. The mechanisms of hydrogen desorption from the composites in different stages of their thermal decomposition are analyzed.     

Analysis of DC and AC properties of a humidity sensor based on polyaniline–chromium oxide composites

Polyaniline–Chromia (PANI–Cr₂O₃) composites were synthesized by in situ polymerization. The prepared composites were characterized by scanning electron microscopy, X-ray diffraction and Fourier transforms infrared spectroscopy. The structural studies confirm the polymerization of aniline over Cr₂O₃ particles which results into strong interaction between PANI and Cr₂O₃ particles. Direct current conductivity of composites increases with increase in temperature. Among all composites, 30 wt% shows high conductivity. The humidity sensing mechanism of the PANI–Cr₂O₃ composites is studied and change in its resistance with respect to percentage relative humidity ranging from 20 to 95 % is recorded. The humidity sensing studies shows that the change in the resistance is due to the uncurling of polymer chains by the absorption of water vapor which leads to increase in conduction paths. The results indicate better humidity sensing response by the addition of Cr₂O₃ particles to PANI, among all the composites, 30 wt% composite shows higher sensitivity.     

Structural,morphological and electrical properties of Cr2O3 nanoparticles

Chromium oxide (Cr₂O₃) nanoparticles have been prepared by chemical precipitation followed by calcination at high temperatures. The influence of calcination temperature on the particle size, microstructure, surface area and morphology was examined by X-ray diffraction, Fourier transform infrared, N₂ adsorption–desorption isotherms, transmission electron microscopy and thermal analysis techniques. The results indicate the formation of a nanosized single Cr₂O₃ phase. The particles possess high specific surface area and mesoporous structure, and their sizes increase with increasing the calcinations temperature. DC conductivity was measured in the temperatures range of 170–475 K. For the high temperature region, the conduction was found to be due to small polaron hopping of holes. While for the low temperatures region, the conduction was attributed to variable range hopping mechanism of holes. The temperature dependence of the AC conductivity and dielectric constant was investigated in the same temperature range at four test frequencies. In addition, the impedance spectra of these nanoparticles were investigated only at temperatures above 350 K.     

Canonic-Like HER Activity of Cr1–xMoxB2 Solid Solution: Overpowering Pt/C at High Current Density

Abundant transition metal borides are emerging as substitute electrochemical hydrogen evolution reaction (HER) catalysts for noble metals. Herein, an unusual canonic-like behavior of the c lattice parameter in the AlB₂-type solid solution Cr_1–xMo_xB₂ (x = 0, 0.25, 0.4, 0.5, 0.6, 0.75, 1) and its direct correlation to the HER activity in 0.5 M H₂SO₄ solution are reported. The activity increases with increasing x, reaching its maximum at x = 0.6 before decreasing again. At high current densities, Cr_0.4Mo_0.6B₂ outperforms Pt/C, as it needs 180 mV less overpotential to drive an 800 mA cm⁻² current density. Cr_0.4Mo_0.6B₂ has excellent long-term stability and durability showing no significant activity loss after 5000 cycles and 25 h of operation in acid. First-principles calculations have correctly reproduced the nonlinear dependence of the c lattice parameter and have shown that the mixed metal/B layers, such as (110), promote hydrogen evolution more efficiently for x = 0.6, supporting the experimental results.     

Hydrogen gas sensing performance of Pd-Ni alloy thin films

We investigated the hydrogen (H₂) sensing properties of palladium (Pd)-nickel (Ni) alloy films with varying Ni content and discussed them in light of structural deformations. The Pd-Ni alloys operated reversibly upon H₂ absorption and desorption and their sensitivities decreased linearly with Ni content added to Pd. This was attributed to reduction in the lattice constant and interstitial volume caused by the Ni addition, allowing fewer hydrogen atoms to penetrate into the Pd-Ni alloy with higher Ni content. Interestingly, the response time of the Pd-Ni alloys was much shorter than that of pure Pd, presumably due to the fast permeation of hydrogen atoms through microscopic imperfections in the alloys. Unlike pure Pd, the Pd-Ni alloys showed an almost linear relationship between the sensitivity and H₂ concentration without hysteretic behaviors, enabling the detection of low concentration of H₂ down to 0.01%. These results provide a significant understanding of the role of Ni in the Pd-Ni thin films for improving the H₂ sensing properties of the Pd-based alloy film sensors.     

Preparation of MgH2–Ni, MgH2–Ni–Si and MgH2–Ni–Ni2Si composites by mechanochemical method and their hydrogen absorption and desorption properties

To improve kinetics for hydrogen absorption of Mg and hydrogen desorption of MgH₂, ternary composites were prepared from MgH₂, Ni and Si or Ni₂Si by a mechanochemical technique. The X-ray diffraction spectra of the resultant ternary composites after the initial hydrogen desorption treatment at 400 °C in vacuum suggested that in all composites the nanocrystalline Mg₂Ni would be formed in the intergrain region between Mg and Ni components, while in the MgH₂–Ni–Si composite the nanocrystalline Mg₂Si would also be formed in the intergrain region between Mg and Si components. However, Ni₂Si did not form any alloys with Mg. The hydrogen absorption rate at 250 °C for the MgH₂–Ni–Ni₂Si composite was comparable to that for the MgH₂–Ni and MgH₂–Ni–Si composites, while the hydrogen desorption rate at 250 °C decreased in the order of MgH₂–Ni > MgH₂–Ni–Ni₂Si > MgH₂–Ni–Si. In contrast, the hydrogen desorption rate at 220 °C for the MgH₂–Ni–Ni₂Si composite was faster than that for the MgH₂–Ni composite, suggesting that Ni₂Si was a key material in the improvement of hydrogen desorbability at lower temperatures. Moreover, the most plausible reaction model and the rate-determining process for hydrogen absorption and desorption at 250 °C were determined.     

机械合金化(Mg+Mg2 Ni)+TiO2合金的储氢性能

用机械合金化法合成了(Mg Mg2Ni) TiO2储氢合金,借助XRD分析了TiO2的加入对合金的物相结构的影响,SEM考察了合金的形貌.TiO2在合金的吸放氢过程中起到很好的催化作用,降低合金放氢温度并且提高合金储氢量,(Mg Mg2Ni) 10wt%TiO2合金在573K下的储氢量是5.84wt%.     

Self-reducing coal-derived carbon/Ni3Fe magnetic composites with frequency-dependent microwave absorption performance

Coal-derived carbon/Ni₃Fe magnetic composites with frequency dependent microwave absorption performance were prepared at low temperatures (750–850 °C) using coal as the carbon source. The Ni₃Fe alloy was successfully formed due to the carbothermal reaction and reducing gas. SEM images indicate the surface becomes rougher and the number of interlayer of the composites increases with increasing reaction temperature. Consistently, high degree of graphitization of the coal-derived carbon was confirmed by using Raman spectroscopy. Specifically, coal-derived carbon/Ni₃Fe magnetic composites exhibit frequency-dependent microwave absorption characteristics at 2–18 GHz, that is, as the reaction temperature rises from 750 °C to 850 °C, the minimum reflection loss gradually shifts to low frequencies. Among them, CC/Ni₃Fe_(8 0 0)-0.4 exhibits a minimum reflection loss of ?60.76 dB at 16.64 GHz, while the thickness is only 1.28 mm. Such a clean strategy provides experience for the environmental application of coal and microwave absorption. Meanwhile, a lightweight, stable and efficient microwave absorber has been developed.     

Effects of Si,Mn, and water vapour on the microstructure of protective scales grown on Fe–20Cr in CO2 gas

Abstract

Model alloys Fe–20Cr–0.5Si and Fe–20Cr–2Mn (wt-%) were exposed to Ar–20CO₂ and Ar–20CO₂–20H₂O at either 818 or 650°C. In dry gas, protective scales on Fe–20Cr–0.5Si consisted of an outer Cr₂O₃ layer and an inner SiO₂ layer. In wet gas, additional chromia whiskers were formed on top of the duplex scale. Chromia grains formed in wet gas were much smaller than those in dry gas. A TEM analysis revealed that phase constitutions of the protective scale on Fe–20Cr–2Mn were not uniform: Mn₃O₄ and MnCr₂O₄ above alloy grain boundaries and Mn₃O₄, Cr₂O₃ and MnCr₂O₄ on alloy grains. Formation of different oxides and morphologies are discussed in terms of changes in diffusion paths and thermodynamics caused by the presence of carbon and hydrogen.     

Evolution of oxide scale on a Ni–Mo–Cr alloy at 900 °C

The cyclic oxidation behavior of a Ni–Mo–Cr alloy was studied in air at 900 °C for exposure periods of up to 1000 h. The morphology, microstructure and composition of the oxide scale was characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. Oxidation kinetics was determined by weight gain measurements. The results show that steady state oxidation was achieved within 1 h of exposure. During transient oxidation, the alloy grain boundaries intersecting the alloy surface became depleted in Ni and enriched in Mo and Cr. The scale initially formed at the surface was NiO which grew outwardly. However, a protective Cr₂O₃ layer developed, rapidly retarding the rate of oxidation. Formation of NiMoO₄ was also observed. The presence of Mo in the alloy facilitated the formation of a Cr₂O₃ layer at an early stage of oxidation. The alloy exhibited considerable oxide spalling during prolonged exposure.     

Effect of grain boundary precipitation and δ-ferrite formation on surface defect of low nickel austenitic stainless steels

The present study elaborately discussed the effect of grain boundary precipitation and delta (δ) ferrite formation on surface defects of low nickel austenitic stainless steels. Several specimens were collected from different locations (i.e. centre and side) of cast slabs and rolled plates with variable nitrogen and carbon contents. The alloy segregation, precipitation and δ-ferrite formations were analyzed using several characterization tools. It was observed that the segregation of alloying elements along the grain boundaries is solely responsible for variation in precipitation and δ-ferrite formation among the specimens. Type of precipitation is mainly composition dependant. Relatively higher nitrogen and low carbon containing specimens show Cr₂N and/or Cr₃C_1.52N_0.48 rich continuous precipitation along the grain boundaries. On the other hand, decrease in nitrogen content along with the increase in carbon content results in discontinuous or cellular type of precipitation. Continuous grain boundary precipitates were found more detrimental compare to discontinuous or cellular precipitations. Micro-cracks are probably generated at the stress concentrated areas of precipitation–matrix interfaces and propagate along the interfaces or grain boundaries towards the stress free surface. Again, additional driving forces towards the surface crack formation and propagation are facilitated by the δ-ferrite and austenite interfaces during hot rolling.     

Synthesis of Nanostructured Mg-Ni Alloy and Its Hydrogen Storage Properties

Nanostructured Mg-Ni alloy with the particle size in the range of 40-50 nm was synthesized by the thermal decomposition of bipyridyl complexes of Mg and Ni metals at 773 K for 24 h under dry argon gas ambient. The as-prepared nano-alloy was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for compositional and structural analysis. The alloy exhibited superior hydrogen absorption and desorption behavior with 3.2 wt% absorption within 1 min at 573 K and about 3 wt% desorption within 5-10 min at 573 K. This favorable behavior of Mg-Ni compound for the hydrogen storage is due to the specific nanostructure of the material. The low activation energy values and favorable thermodynamics indicate that the prepared Mg-Ni alloy is an attracting material for hydrogen storage applications.     

Phase structure and hydrogen absorption property of LaMg2Cu

LaMg₂Cu alloy was prepared by inductive melting and then was annealed at 723 K for 10 h in 0.1 MPa argon atmospheres. X-ray diffraction (XRD) and scanning electron microscopy (SEM) showed that the alloy consisted of LaMg₂Cu₂ phase, LaMg₃ phase and a few of unknown phases. The annealing treatment improved the equilibrium pressure and hydrogen absorption capacity of LaMg₂Cu alloy. The hydrogen absorption capacity of the as-cast and annealed alloys at 473 K were 2.86 and 3.33 wt.%, and the equilibrium pressure were 1 and 3 MPa, respectively. The enthalpy and entropy of LaMg₂Cu–H hydriding reaction were determined. LaMg₂Cu alloy could absorb hydrogen with rapid hydriding kinetics, the hydrogen absorption rate of LaMg₂Cu increased from 423 K to 498 K and the uptake time for hydrogen content to reach 90% of the maximum storage capacity for annealed alloy was less than 550 s at 498 K. The experimental curves of hydrogen absorption kinetics could be fitted with good accuracy by Jander equation. It suggested that the hydriding of LaMg₂Cu alloy was a three-dimensional diffusion-controlled process. And the activation energy and pre-exponential factor of LaMg₂Cu alloy were also calculated.     

Structural relaxation and embrittlement of Fe74Co10B16 and Fe74Co5Cr5B16 metallic glasses

Structural relaxation and annealing embrittlement behaviour of Fe₇₄Co₁₀B₁₆ and Fe₇₄Co₅Cr₅B₁₆ metallic glasses has been studied by differential scanning calorimetry (DSC) and bend ductility measurements. A pre-anneal technique was employed with DSC to determine activation energies of relaxation at various temperatures. Activation energies of embrittlement were derived from measurements of the embrittlement kinetics. The results obtained for both the alloys are compared to ascertain the effects of chromium addition. A spectrum of activation energies is obtained corresponding to structural relaxation and embrittlement. The activation energies of structural relaxation are found to be slightly higher for the chromium-containing alloy than for the ternary alloy. This observation is consistent with the higher crystallization temperature of the chromium-containing alloy as reported earlier. The ductile-brittle transition temperature of the Fe₇₄Co₁₀B₁₆ glass, however, decreases by 50 K (for 15 min anneal) on addition of 5 at% chromium. Activation energies for embrittlement of the chromium-containing alloy are also considerably smaller than for the ternary alloy. It is concluded that despite increasing the thermal stability, chromium reduces the mechanical stability of Fe₇₄Co₁₀B₁₆ glass.