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.     

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.     

Influence of tin on the electrochemical and gas phase hydrogen sorption in Mg2−xSnxNi (x = 0, 0.1, 0.3)

Mg_2−xSn_xNi (x = 0, 0.1, 0.3) alloys were synthesized by reactive ball milling under protective Ar atmosphere and liquid n-heptane. The microstructure and the morphology of the powders were determined by X-ray diffraction and scanning electron microscopy. The as-milled alloys consist of Mg₂Ni nanocrystals with an average grain size in the range 3–7 nm, depending on the alloy composition. Sn containing phases were not detected even in the Sn-rich alloy. Obviously, Sn is dissolved in the Mg₂Ni intermetallic compound. Gas phase sorption of hydrogen was not observed in the alloys containing Sn (Mg_2−xSn_xNi; x = 0.1, 0.3). It was suggested that Sn impedes the process of hydrogen molecules decomposition. The as-milled alloys absorbed reversibly hydrogen electrochemically. Mg₂Ni alloy showed the highest discharge capacity of 300 mAh/g. The capacity of Mg_1.9Sn_0.1Ni and Mg_1.7Sn_0.3Ni was about 260 mAh/g. It was found that Sn improved the cycle life of the electrode.     

Microstructures and properties of Mg–7Gd alloy containing Y

Mg–7 mass%Gd–x mass%Y (x = 0, 1, 3 and 5) alloys were prepared by casting method, and the microstructures, age hardening behavior and mechanical properties have been investigated. The results show that the addition of Y to the binary Mg–7Gd alloy could reduce the grain size of the as-cast alloys, and enhance the age hardening response and improve mechanical properties during the investigated temperature range. The Mg–7Gd–5Y alloy exhibits maximum ultimate tensile strength and yield strength at peak hardness, and the values are 258 and 167 MPa at room temperature, and 212 and 140 MPa at 250 °C, respectively, which is about 1.8 times as high as the Mg–7Gd binary alloy. When x is more than 3, the amount of Mg₅(Gd,Y) phase is observed at the peak hardness of aged alloys. The significant improvement of the tensile strength at peak hardness is mainly attributed to the fine dispersion of the β-Mg₅(Gd,Y) precipitate.     

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.     

Synthesis of fine composite particles for hydrogen storage, starting from Mg-YNi2 mixture

We synthesized new composite particles for hydrogen storage on the basis of an idea of “particle designing”. As starting materials, powders of Mg and YNi₂ were selected. Fine composite particles containing mainly Mg₂Ni could be designed by repetitive hydriding and dehydriding cycles at 673 K. In the synthesis process of the composite particles, the following two points were found to be essential for this technique. The first point is that, after being activated by the sequential processes of hydrogenation, amorphization and disproportionation, YNi₂ reacts effectively with Mg. The second point is that evaporated Mg, which occurs during dehydriding, adheres to the surface of the activated YNi₂ and accelerates a diffusion reaction to form Mg₂Ni at the interface. In these composite particles, Mg₂NiH₄ is formed, even at 373 K, under a hydrogen pressure of 5 MPa.     

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.     

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.     

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.     

Improved durability of hydrogen storage alloys

An effective and durable hydrogen storage module was required to fuel micro-power systems. Two primary specifications for the hydrogen fuel module in this application were a high volumic storage capacity and rapid hydrogen storage and release under atmospheric pressure or lower at room temperature. In addition, the hydrogen module should be operable for thousands of cycles with fast hydriding and dehydriding rates and be resistant to deactivation on exposure to air for many months and longer. In our prior work, mechanical grinding a small amount of palladium with the hydrogen storage alloys was shown to greatly improve the hydrogen storage performance. The palladium treatment of three intermetallic alloys, AB₅ type LaNi_4.7Al_0.3 and CaNi₅, and A₂B type Mg₂Ni, lowered the activation pressure to sub-atmospheric pressure at room temperature and also significantly increased the hydrogen absorption and desorption rates. This work focused on the durability of hydrogen absorption and desorption performances after exposure of the storage materials to air. The palladium treated hydrogen storage alloys retained both low activation pressures and fast absorption and desorption rates even after more than 2 years air exposure.     

The effect of Nd content on the electrochemical properties of low-Co La–Mg–Ni-based hydrogen storage alloys

The low-Co content La_0.80−xNd_xMg_0.20Ni_3.20Co_0.20Al_0.20 (x = 0.20, 0.30, 0.40, 0.50, 0.60) alloys were prepared by inductive melting and the effect of Nd content on the electrochemical properties was investigated. XRD shows that the alloys consist mainly of LaNi₅ phase, La₂Ni₇ phase and minor LaNi₃ phase. The electrochemical P–C–T test shows hydrogen storage capacity increases first and then decreases with increasing x, which is also testified by the electrochemical measurement that the maximum discharge capacity increases from 290 mAh/g (x = 0.20) to 374 mAh/g (x = 0.30), and then decreases to 338 mAh/g (x = 0.60). The electrochemical kinetics test shows exchange current density I₀ increases with x increasing from 0.20 to 0.50 followed by a decrease for x = 0.60, and hydrogen diffusion coefficient D increases with increasing x. Accordingly high rate dischargeability increases with a slight decrease at x = 0.60 and the low temperature dischargeability increases with increase in Nd content. When x is 0.50, the alloy exhibits a better cycling stability.     

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 electrode characteristics of the new composite alloys Mg2Ni-xwt.% Ti2Ni

A new composite alloy Mg₂Ni-xwt.% Ti₂Ni has been successfully synthesised using a ‘particle inlaying’ method. Scanning electron microscopy and energy dispersive spectroscopy revealed that very fine Ti₂Ni particles were inlaid onto the surface of Mg₂Ni particles by mechanical treatment and sintering. XRD showed the composite alloys were composed of primary alloys Mg₂Ni, Ti₂Ni and new phases TiNi, Ti---Mg formed in the composite procedure. The electrode characteristics of Mg₂Ni-xwt.% Ti₂Ni alloys in an alkaline solution have been investigated and compared with those of Mg₂Ni. The discharge capacity of the alloy electrode was effectively improved from 8 mA h g¹ of Mg₂Ni to 165 mA h g¹ of Mg₂Ni-40wt.% Ti₂Ni at ambient temperature, which is almost comparable with that of Ti₂Ni electrode (170 mA h g¹). It is believed that the fine Ti₂Ni particles inlaid on the surface of Mg₂Ni particles play a two-fold role: firstly, they hydride-dehydride as hydrogen storage materials themselves: secondly, they provide active sites and pathways for Mg₂Ni hydriding-dehydriding. This is supported by analysis of discharge behaviour and electrochemical impedance spectra studies.     

Fabrication and dry sliding wear behavior of in situ Al–K2ZrF6–KBF4 composites reinforced by Al3Zr and ZrB2 particles

Aluminum matrix composites reinforced by Al₃Zr and ZrB₂ particles were fabricated from Al–x wt.%(K₂ZrF₆–KBF₄) (x = 5, 10, 15, 20, 25) systems via magnetochemistry in situ reaction and the dry sliding wear properties and behavior of the composites were investigated. XRD and SEM analysis show that ZrB₂ and Al₃Zr reinforcement phases have been obtained and been distributed uniformly in the aluminum matrix. Wear test results show that the values of wear weight loss of the composites decrease with the increase of x under all identical wear conditions, and that of the relative wear resistance R_relat. increases under the applied load of 100 N. Especially, when x = 25, the wear weight loss (under a sliding time of 120 min and an applied load of 100 N), which is 0.245 to that of the A356 alloy, and the R_relat. (under the intermediate wear-sliding stage and an applied load of 100 N) is 4.772, which is 1.513 to that of the primary stage, respectively. Two modes of the wear mechanisms of the as-prepared composites were identified.     

The effect of sequential and continuous high-energy impact mode on the mechano-chemical synthesis of nanostructured complex hydride Mg2FeH6

The effect of sequential and continuous high-energy impact mode in the magneto-mill Uni-Ball-Mill 5 on the mechano-chemical synthesis of nanostructured ternary complex hydride Mg₂FeH₆ was studied by controlled reactive mechanical alloying (CRMA). In the sequential mode the milling vial was periodically opened under a protective gas and samples of the milled powder were extracted for microstructural examination whereas during continuous CRMA the vial was never opened up to 270 h duration. MgO was detected by XRD in sequentially milled powders while no MgO was detected in the continuously milled powder. The abundance of the nanostructured ternary complex hydride Mg₂FeH₆, produced during sequential milling, and estimated from DSC reached 44 wt.% after 188 h, and afterwards it slightly decreased to 42 wt.% after 210 and 270 h. In contrast, the DSC yield of Mg₂FeH₆ after continuous CRMA for 270 h was 57 wt.%. Much higher yield after continuous milling is attributed to the absence of MgO. This behavior provides strong evidence that MgO is a primary factor suppressing formation of Mg₂FeH₆. The DSC hydrogen desorption onset temperatures are close to 200 °C while the desorption peak temperatures for all powders are below 300 °C and the desorption process is completed within the range 10–20 min. Within the investigated nanograin size range of 5–13 nm, the DSC desorption onset and peak temperatures of β-MgH₂ and Mg₂FeH₆ do not exhibit any trend with nanograin (crystallite) size of hydrides. TPD hydrogen desorption peaks from the powders containing a single ternary complex hydride Mg₂FeH₆, are very narrow, which indicates the presence of small but well-crystallized hydride particles. Their narrowness provides good evidence that the phase composition, bulk hydrogen distribution and hydride particle size distribution are very homogeneous. The overall amount of hydrogen desorbed in TPD from single-hydride Mg₂FeH₆ powders is somewhat higher than that observed in DSC and TGA desorption.

The powder milled sequentially for 270 h and desorbed in a Sieverts-type apparatus at 250 and 290 °C, yielded about a half of the hydrogen content obtained during DSC and TGA tests. No desorption of hydrogen was detected in a Sieverts-type apparatus at 250 and 290 °C after 128 and 70 min, respectively, from the powder continuously milled for 270 h. The latter easily desorbed 3.13 and 2.83 wt.% hydrogen in DSC and TGA tests, respectively.     

Electrochemical behaviour assessment of novel Mg-rich Mg–Al–RE alloys (RE = Ce, Er)

The electrochemical behaviour of new Mg–Al–RE (RE = Ce, Er) alloys AE91 was investigated in 0.01 M NaCl electrolyte (pH = 12) and compared with that of the most commonly used Mg alloy in the automotive field, the AZ91D. Scanning electron microscopy and quantitative electron probe microanalysis were used to characterize the samples prior to the electrochemical tests. AE91 alloys showed very similar microstructures characterized by a three-phase appearance: a Mg-based solid solution containing only Al and two intermetallic phases γ(Mg₁₇Al₁₂) and (Al_1 − xMg_x)₃Ce or (Al_1 − xMg_x)₂Er. Free corrosion potential measurements, potentiodynamic polarization curves and electrochemical impedance spectroscopy revealed improved passivity behaviour compared to AZ91D alloy. The apparent presence of trace amounts of rare earth oxides in the passive film is presumed to be the reason for the enhanced corrosion resistance of AE91 alloys in the aggressive environment considered.     

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.     

Effects of Ce and Mm additions on the glass forming ability of Al–Ni–Si metallic glass alloys

The effects of Ce and Mm contents on the glass forming ability (GFA) of melt-quenched Al_89−xNi₈Ce_xSi₃ and Al_89−xNi₈Mm_xSi₃ (x = 0, 1, 3, 5, 7 at.%) alloys have been systematically investigated by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). According to the XRD and DSC results, both Ce and Mm elements can enhance the GFA and thermal stability of the Al–Ni–Si alloys. Moreover, only the x = 5 and x = 7 alloys are totally amorphous in both systems quenched at the wheel speed of 36.6 m/s. Compared with amorphous Al₈₄Ni₈Ce₅Si₃ alloy at different cooling rates, amorphous Al₈₄Ni₈Mm₅Si₃ alloy has higher GFA which is considered to have relation to the different atomic structure of the amorphous alloy.     

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.