Study of amorphous Ni–S(La) alloy used as HER cathode in alkaline medium

The hydriding combustion synthesis (HCS) of Mg₂FeH₆ and Mg₂Ni_1−xFe_x hydrides was systematically studied by changing the value of x from 0.25 to 1.0. This study aimed at improving the Mg₂FeH₆ yield in production and examining the effect of the addition of nickel to the hydrogen storage properties of the Mg₂FeH₆ hydride. In synthesizing metallic hydrides, the raw materials in metallic powders were mechanically activated by a ball mill before the HCS treatment. As a result, the ball-milled 2Mg + Fe recorded as much as 5 mass% in hydrogen storage capacity during the HCS treatment, and the final product successfully indicated a high purity Mg₂FeH₆. Interestingly, the deformation enthalpies of the Mg₂Ni_1−xFe_x hydrides were larger by 10% or more than Mg₂NiH₄ and Mg₂FeH₆, taken individually. This was collateral evidence to prove that the HCS of Mg₂FeH₆ and Mg₂Ni_1−xFe_x hydrides with ball milling (BM) produced a new structure of the Mg–Ni–Fe–H system due to the synergy effect.     

Mechanically milled Mg composites for hydrogen storage the transition to a steady state composition

Magnesium alloys are potentially the best materials for gaseous hydrogen storage. However, their practical use is limited by poor hydrogen absorption and desorption kinetics. This problem can be resolved by mixing Mg alloys with other materials to form composites. We present an investigation of the initial hydriding characteristics, as well as the compositional transformation of composites made of La₂Mg₁₇ + LaNi₅ mechanically milled in a 2:1 weight ratio. Composites produced with varying durations and intensities of milling were tested. Those milled to the greatest extent proved to have the best initial hydrogen absorption and desorption kinetics. The kinetics of the most heavily milled composite were superior to those of La₂Mg₁₇. This composite absorbed 90% of its full hydrogen capacity (3.5 wt.% H₂) in less than 1 min at 250°C and desorbed the same quantity of hydrogen in 6 min. Under the same conditions pure La₂Mg₁₇ took 2.5 h to absorb and 3 h to desorb 90% of its full hydrogen capacity (4.9 wt.% H₂). Scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction were used to characterize the mechanically milled powders before and after hydriding. The unhydrided powders consisted of LaNi₅ grains surrounded by a fractured LaMg₁₇ matrix. Hydrogen cycling, at temperatures up to 350°C, induced phase changes, segregation, and disintegration of the composites. The resulting fine powder (less than 1 μm) consisted primarily of Mg, Mg₂Ni, and La phases.     

Mg6Ir2H11, a new metal hydride containing saddle-like [IrH4] and square-pyramidal [IrH5] hydrido complexes

Mg₆Ir₂H₁₁ has been synthesised by both hydrogenation of the intermetallic compound Mg₃Ir at 20 bar and 300 °C, and sintering of the elements at 500 °C under 50 bar hydrogen pressure. Neutron powder diffraction on the deuteride indicates a monoclinic structure (space group P2₁/c, Mg₆Ir₂D₁₁: a=10.226(1), b=19.234(2), c=8.3345(9) Å, β=91.00(1)°, T=20 °C) that is closely related to orthorhombic Mg₆Co₂H₁₁. It contains a square-pyramidal [IrH₅]⁴⁻ complex and three saddle-like [IrH₄]⁵⁻ complexes of which one is ordered and two are disordered. Five hydride anions H⁻ are exclusively bonded to magnesium. The compound has a red colour, is presumably non-metallic and decomposes under 3 bar argon at 500 °C into Mg₃Ir, iridium and a previously unreported intermetallic compound of composition Mg₅Ir₂.     

Microstructural evolution of some MgO–TiO2 and MgO–Al2O3 powder mixtures during high-energy ball milling and post-annealing studied by X-ray diffraction

High-energy dry ball-mill and post-anneal processing were applied to synthesize MgTiO₃ and Mg₂TiO₄ single crystalline phases from the predetermined compositions of MgO–TiO₂ powder mixtures. Also, the experiments were performed to show that it is possible to prepare MgAl₂O₄ single crystalline phase from the predetermined composition of MgO–Al₂O₃ powder mixture only by employing high-energy dry ball milling, i.e. without post-annealing the milled samples. In contrast, fully developed single crystalline powders of MgTiO₃ and Mg₂TiO₄ were obtained after post-annealing the milled samples for 1 h at 900 and 1200 °C, respectively.     

Remarkable improvement of hydrogen sorption kinetics in magnesium catalyzed with Nb2O5

Kinetics of hydrogen absorption and desorption reactions was investigated on the MgH₂ composite doped with 1 mol% Nb₂O₅ as a catalyst by ballmilling. The composite after dehydrogenation at 200 °C absorbed gaseous hydrogen of 4.5 mass% even at room temperature under lower pressure than 1 MPa within 15 s and finally its capacity reached more than 5 mass%. On the other hand, the catalyzed MgH₂ after rehydrogenation desorbed 6 mass% hydrogen at 160 °C under purified He flow, which followed the first order reaction. From the Kissinger plot, the activation energy for hydrogen desorption was estimated to be 71 kJ/mol H₂, indicating the product was significantly activated due to the catalytic effect of Nb₂O₅.     

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.     

LiMg2RuH7, a new quaternary metal hydride containing octahedral [Ru(H)H6] complex anions

LiMg₂RuH₇ and its deuteride were synthesized by sintering mixtures of LiH, magnesium and ruthenium powders at 500–550 °C and a hydrogen (deuterium) pressure of 120–155 bar, and characterized by X-ray and neutron powder diffraction. The yellow powder crystallizes with hexagonal symmetry (space group, P6₃/mmc; hydride - A = 4.7060(1) Å, C = 10.6960(2) Å; deuteride - A = 4.6998(1) Å, C = 10.6674(3) Å). The structure is an ordered substitution variant of Mg₃ReH₇. It contains a nearly regular octahedral 18-electron [Ru(II)D₆]⁴⁻ complex with bond distances [Ru-6D1] = 1.704(7) Å, and a deuteride anion D⁻ coordinated in a trigonal bipyramidal configuration by two close magnesium and three distant lithium cations with bond distances [D2-2Mg] = 1.852(6) Å and [D2−3Li] = 2.7134(1) Å respectively.     

Crystallization of magnesium niobate from mechanochemically derived amorphous phase

The effect of high-energy ball milling and subsequent annealing on the mixture of MgO and Nb₂O₅ has been investigated. X-ray diffraction (XRD) measurement indicates that an amorphous phase is produced after milling for 5 h, while traces of MgNb₂O₆ crystallized from the amorphous phase during prolonged milling. Significant crystallization of MgNb₂O₆ from the amorphous state is observed after annealing at 500 °C, while the reaction of the remaining MgO and Nb₂O₅ does not take place at this temperature. Single phase MgNb₂O₆ can be achieved for all the milled samples at 700 °C. No significant grain growth is observed when the milled powders were annealed at temperature below 900 °C. Almost fully dense MgNb₂O₆ ceramics are obtained after annealing at 1100 °C from the as-milled powders.     

Hydrogen storage properties of MgH2/SWNT composite prepared by ball milling

A systematic investigation was performed on the hydrogen storage properties of mechano-chemically prepared MgH₂/single-walled carbon nanotube (SWNT) composites. It is found that the hydrogen absorption capacity and hydriding kinetics of the composites were dependent on the addition amount of SWNTs as well as milling time. A 5 wt.% addition of SWNTs is optimum to facilitate the hydrogen absorption and desorption of MgH₂. The composite MgH₂/5 wt.% SWNTs milled for 10 h can absorb 6.7 wt.% hydrogen within about 2 min at 573 K, and desorb 6 wt.% hydrogen in about 5 min at 623 K. Prolonging the milling time over 10 h leads to a serious degradation on hydrogen storage property of the MgH₂/SWNT composite, and property/structure investigations suggest that the property degradation comes from the structure destruction of the SWNTs.     

Toughened ZrO2 ceramics sintered with a La2O3-rich glass as additive

In this study, the influence of the glass addition and sintering parameters on the densification and mechanical properties of tetragonal zirconia polycrystals (3Y-TZP) ceramics were evaluated. High-purity tetragonal ZrO₂ powder and La₂O₃-rich glass were used as starting powders. Two compositions based on ZrO₂ and containing 5 wt.% and 10 wt.% of La₂O₃-rich glass were studied in this work. The starting powders were mixed/milled by planetary milling, dried at 90 °C for 24 h, sieved through a 60 mesh screen and uniaxially cold pressed under 80 MPa. The samples were sintered in air at 1200 °C, 1300 °C, 1400 °C for 60 min and at 1450 °C for 120 min, with heating and cooling rates of 10 °C/min. Sintered samples were characterized by relative density, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Hardness and fracture toughness were obtained by Vickers indentation method. Dense sintered samples were obtained for all conditions. Furthermore, only tetragonal-ZrO₂ was identified as crystalline phase in sintered samples, independently of the conditions studied. Samples sintered at 1300 °C for 60 min presented the optimal mechanical properties with hardness and fracture toughness values near to 12 GPa and 8.5 MPa m^1/2, respectively.     

Properties of copper matrix reinforced with various size and amount of Al2O3 particles

Copper matrix was reinforced with Al₂O₃ particles of different size and amount by internal oxidation and mechanical alloying accomplished using high-energy ball milling in air. The inert gas-atomised prealloyed copper powder containing 1 wt.% Al as well as a mixture of electrolytic copper powder and 3 wt.% commercial Al₂O₃ powder served as starting materials. Milling of Cu-1 wt.% Al prealloyed powder promoted formation of fine dispersed particles (1.9 wt.% Al₂O₃, approximately 100 nm in size) by internal oxidation. During milling of Cu-3 wt.% Al₂O₃ powder mixture the uniform distribution of commercial Al₂O₃ particles has been obtained. Following milling, powders were treated in hydrogen at 400 °C for 1 h in order to eliminate copper oxides formed at the surface during milling. Compaction was executed by hot-pressing. Compacts processed from 5 to 20 h-milled powders were additionally subjected to high-temperature exposure at 800 °C in order to examine their thermal stability and electrical conductivity. Compacts of Cu-1 wt.% Al prealloyed powders with finer Al₂O₃ particles and smaller grain size exhibited higher microhardness than compacts of Cu-3 wt.% Al₂O₃ powder mixture. This indicates that nano-sized Al₂O₃ particles act as a stronger reinforcing parameter of the copper matrix than micro-sized commercial Al₂O₃ particles. Improved thermal stability of Cu-1 wt.% Al compacts compared to Cu-3 wt.% Al₂O₃ compacts implies that nano-sized Al₂O₃ particles act more efficiently as barriers obstructing grain growth than micro-sized particles. Contrary, the lower electrical conductivity of Cu-1 wt.% Al compacts is the result of higher electron scatter caused by nano-sized Al₂O₃ particles.     

Dielectric properties of sintered aluminum nitride

In the present work, the effects of sintering additives on dielectric loss tangent (tan δ) of AlN ceramics were explored. Different amounts of Y₂O₃ and Mg₃N₂ were respectively added as sintering additives to AlN powders, and pressureless-sintering was performed at 1900 °C for 2 h in a nitrogen flow atmosphere. The resulted AlN ceramics became denser due to the addition of Mg₃N₂, and nearly fully dense sample was obtained with a relative density of 0.998. tan δ decreased with increasing Mg₃N₂ amount and it was found to depend on the density of the AlN ceramic. The best tan δ value of 3.9 × 10⁻⁴ was obtained by adding 1 mol% of Y₂O₃ and Mg₃N₂ together.     

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.     

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.     

Improvement in hydrogen sorption properties of Mg by reactive mechanical grinding with Cr2O3, Al2O3 and CeO2

We tried to improve the hydrogen sorption properties of Mg by mechanical grinding under H₂ (reactive mechanical grinding) with oxides Cr₂O₃, Al₂O₃ and CeO₂. The hydriding rates of Mg are reportedly controlled by the diffusion of hydrogen through a growing Mg hydride layer. The added oxides can help pulverization of Mg during mechanical grinding. A part of Mg is transformed into MgH₂ during reactive mechanical grinding. The Mg+10wt.%Cr₂O₃ powder has the largest transformed fraction 0.215, followed in order by Mg+10wt.%CeO₂ and Mg+10wt.%Al₂O₃. The Mg+10wt.%Cr₂O₃ powder has the largest hydriding rates at the first and fifth hydriding cycle, followed in order by Mg+10wt.%Al₂O₃ and Mg+10wt.%CeO₂. Mg+10wt.%Cr₂O₃ absorbs 5.87wt.% H at 573 K, 11 bar H₂ during 60 min at the first cycle. The Mg+10wt.%Cr₂O₃ powder has the largest dehydriding rates at the first and fifth dehydriding cycle, followed by Mg+10wt.%CeO₂ and Mg+10wt.%Al₂O₃. It desorbs 4.44 wt.% H at 573 K, 0.5 bar H₂ during 60 min at the first cycle. All the samples absorb and desorb less hydrogen at the fifth cycle than at the first cycle. It is considered that this results from the agglomeration of the particles during hydriding–dehydriding cycling. The average particle sizes of the as-milled and cycled powders increase in the order of Mg+10wt.%Cr₂O₃, Mg+10wt.%Al₂O₃ and Mg+10wt.%CeO₂. The quantities of hydrogen absorbed or desorbed for 1 h for the first and fifth cycles decrease in the order of Mg+10wt.%Cr₂O₃, Mg+10wt.%Al₂O₃ and Mg+10wt.%CeO₂. The quantities of absorbed or desorbed hydrogen increase as the average particle sizes decrease. As the particle size decreases, the diffusion distance shortens. This leads to the larger hydriding and dehydriding rates. The Cr₂O₃ in the Mg+10wt.%Cr₂O₃ powder is reduced after hydriding–dehydriding cycling. The much larger chemical affinity of Mg than Cr for oxygen leads to a reduction of Cr₂O₃ after cycling.     

Orthorhombic Mg4IrD5 with disordered deuterium distribution

Mg₄IrH₅ and its deuteride was synthesized by the reaction of magnesium and iridium powders with hydrogen (deuterium) at high pressure (41–100 bar) and high temperature (723–783 K). The structure was determined by X-ray and neutron powder diffraction. This compound crystallizes with a new structure type with orthorhombic symmetry (space group Imma, cell parameters a = 4.8110(3) Å, b = 8.9624(6) Å, c = 10.8970(8) Å, Z = 4, hydride, T = 298 K). It contains two deuterium sites: one is disordered with an occupancy of 75% and surrounds iridium in a distorted square planar configuration with distances [Ir---D] = 1.69 Å; the other is ordered and is coordinated by magnesium in a distorted tetrahedral configuration.     

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.     

Hydriding and electrochemical properties of amorphous rich MgxNi100−x nanomaterial obtained by mechanical alloying starting from Mg2Ni and MgNi2

The hydrogen desorption properties of Mg-rich alloys are significantly improved by nanostructure formation. This effect is examined for a melt-spun and nanocrystallized Mg–Ni–La alloy by pressure–composition isotherm (PCT) and thermal desorption spectrum (TDS) measurements. This alloy exhibits fast desorption kinetics and favorable PCT characteristics with an H-capacity of 4.6 wt%. TDS measurements reveal a definite peak ascribable to release of hydrogen from nanoboundaries, in addition to those associated with decompositions of hydrides such as MgH₂, Mg₂NiH₄ and LaH₃. Hydrogen transport along the nanoboundaries appears to facilitate the desorption kinetics in this alloy.     

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.     

Hydrogen-generating solid-state hydride/hydroxide reactions

Solid-state reactions of ionic hydrides with alkaline hydroxides are shown to produce hydrogen gas and metal oxides. These reactions are analogous to the well-known hydrolysis reactions of ionic hydrides. Both classes of reactions are generally exothermic and are thermodynamically favored; ΔG° < 0 near room temperature. However, solid-state hydride/hydroxide reactant mixtures are kinetically stable at room temperature and can be prepared by mechanical milling without appreciable reaction. Thus, optimally stoichiometric mixtures are possible and nearly theoretical amounts of hydrogen can be generated. Reaction occurs upon heating with H₂ evolution beginning at 50 °C and complete reaction occurring by 200–300 °C. The reaction rate can be enhanced with additives such as TiCl₃. Specifically, we discuss the reactions LiH + LiOH, 2LiH + NaOH, LiBH₄ + 4LiOH, and 3LiBH₄ + 4LiOH·H₂O. The 3LiBH₄ + 4LiOH·H₂O reaction generates approximately 10 wt.% hydrogen with more than 5 wt.% produced at temperatures below 100 °C.