TEM analysis of the microstructure in TiF3-catalyzed and pure MgH2 during the hydrogen storage cycling

We utilized transmission electron microscopy (TEM) analysis, with a cryogenically cooled sample stage, to detail the microstructure of partially transformed pure and titanium fluoride-catalyzed magnesium hydride powder during hydrogenation cycling. The TiF₃-catalyzed MgH₂ powder demonstrated excellent hydrogen storage kinetics at various temperatures, whereas the uncatalyzed MgH₂ showed significant degradation in both kinetics and capacity. TEM analysis on the partially hydrogen absorbed and partially desorbed pure Mg(MgH₂) revealed a large fraction of particles that were either not transformed at all or were completely transformed. On the other hand, in the MgH₂+TiF₃ system it was much easier to identify regions with both the hydride and the metal phase coexisting in the same particle. This enabled us to establish the metal hydride orientation relationship (OR) during hydrogen absorption. The OR was determined to be (1 1 0)MgH₂ || (?1 1 0 ?1)Mg and [?1 1 1]MgH₂ || [0 1 ?1 1]Mg. During absorption the number density of the hydride nuclei does not show a dramatic increase due the presence of TiF₃. Conversely, during desorption the TiF₃ catalyst substantially increases the number of the newly formed Mg crystallites, which display a strong texture correlation with respect to the parent MgH₂ phase. Titanium fluoride also promotes extensive twinning in the hydride phase.     

Superior catalytic effect of TiF3 over TiCl3 in improving the hydrogen sorption kinetics of MgH2: Catalytic role of fluorine anion

TiF₃ shows a superior catalytic effect over TiCl₃ in improving the hydrogen sorption kinetics of MgH₂. Combined phase analysis and microstructure characterization suggest that both titanium halide additives react with host MgH₂ in a similar way. However, systematic X-ray photoelectron spectroscopy studies reveal that the incorporated fluorine (F) differs significantly from its analog chlorine (Cl) in terms of bonding state. The asymmetry of F 1s spectra and the sputtering-induced peak shift suggest that, in addition to the Mg–F bond, a new Ti–F–Mg bonding is formed in the TiF₃-doped MgH₂. In contrast, only one stable binding state of Cl is identified in the form of MgCl₂ for the TiCl₃-doped MgH₂. In combination with the designed experiments, these findings suggest that the generation of active F-containing species may be responsible for the advantage of TiF₃ over TiCl₃ in improving both the absorption and desorption kinetics of MgH₂. Fundamentally, it emphasizes the functionality of F anion in tuning the activity of compound catalyst.     

Improving hydrogen storage performance of Li–Mg–N–H system by adding niobium hydride

Hydrogen storage properties of 2LiNH2 – MgH2 –xNbH(x = 0 and 0.05) composites and the catalysis of NbH on hydrogen sorption reaction of the Li–Mg– N–H system were investigated. Hydrogen sorption properties of 2LiNH2 –MgH2 system are effectively improved by adding NbH. Temperature programmed desorption results show the addition of NbH reduces the dehydriding onset temperature of 2LiNH2 –MgH2 system by 21 K. Approximate 3.62 wt% hydrogen in 2LiNH2 –MgH2 – 0.05NbH composite is released following a 500 min at 433 K, whereas the amount of hydrogen desorption is only *3.16 wt% for the pristine system under the same condition. The sample with NbH exhibits higher dehydriding rate compared with the pristine one. Moreover, hydrogen absorption rate increases by adding NbH into the 2LiNH2 – MgH2 system. Hydrogen absorption capacity of the samples with NbH is 3.23 wt% within 400 min, which is higher than that of pristine sample. Fine NbH particles homogeneously distribute in the 2LiNH2 –MgH2 –0.05NbH composite, and catalyze the hydrogen sorption reaction rather than reacts as a reactant into new compound.     

Microstrucure and strengthening of Al–Li–Cu–Mg alloys and MMCs: I. Analysis and Modelling of Microstructural changes

A complete and detailed analysis of the microstructural development during ageing in an 8090 (Al–2.3Li–1.2Cu–1Mg–0.1Zr) alloy, an 8090/20 wt% SiC_p MMC, an Al–1.5Li–Cu–Mg MMC and an Al–Cu–Mg MMC (all with similar Cu and Mg contents) has been performed. Volume fractions of all precipitates relevant for precipitation strengthening of the alloys (δ′ phase, S′ phase and GPB zones) have been determined using a recently derived method based on differential scanning calorimetry (DSC). The volume fractions have subsequently been successfully fitted using a novel model for transformation kinetics. The sizes of these precipitates have been analysed using newly derived expressions consistent with the latter model. As a result of dislocation generation around misfitting SiC particles the volume fractions of both GPB zones and S′ phase depend strongly on the presence of these particles. Also the amount of Li present in the alloys influences the volume fractions of the phases significantly. The sizes of S′ are similar for the four alloys.     

Improvement of hydrogen-storage properties of MgH2 by addition of Ni and Ti via reactive mechanical grinding and a rate-controlling step in its dehydriding reaction

In a shift from prior work, MgH₂, instead of Mg, was used as a starting material in this work. A sample with a composition of 86 wt% MgH₂-10 wt% Ni-4 wt% Ti was prepared by reactive mechanical grinding. Activation of the sample was completed after the first hydriding cycle. The effects of reactive mechanical grinding of Mg with Ni and Ti were discussed. The formation of Mg₂Ni increased the hydriding and dehydriding rates of the sample. The addition of Ti increased the hydriding rate and greatly increased the dehydriding rate of the sample. The titanium hydride, TiH_1.924, was formed during reactive mechanical grinding. This titanium hydride, which is brittle, is thought to help the mixture pulverized by being pulverized during reactive mechanical grinding and further to prevent agglomeration of the magnesium by staying as a hydride among Mg particles. A rate-controlling step for the dehydriding reaction of the hydrided MgH₂-10Ni-4Ti was analyzed by using a spherical moving boundary model on an assumption that particles have a spherical shape with a uniform diameter.     

Hydrogen sorption properties of a Mg–Y–Ti alloy

The catalytic effect of titanium on the hydrogen sorption properties of a Mg–Y–Ti alloy has been investigated. The alloy is formed by a majority phase Mg_24+xY₅, a minor phase of solid solution of Y in Mg and Ti clusters randomly dispersed in the sample. During the first hydrogen absorption cycle 5.6 wt.% hydrogen was absorbed at temperatures above 613 K. The alloy decomposed almost completely to MgH₂ and YH₃. After hydrogen desorption pure Mg and YH₂ were formed. For further absorption/desorption cycles the material had a reversible hydrogen capacity of 4.8 wt.%. The MgH₂ decomposition enthalpy was determined to ?68 kJ/mol H₂, and the calculated activation energy of hydrogen desorption of MgH₂ was 150(±10) kJ/mol.     

Advancement in the Hydrogen Absorbing and Releasing Kinetics of MgH<Subscript>2</Subscript> by Mixing with Small Percentages of Zn(BH<Subscript>4</Subscript>)<Subscript>2</Subscript> and Ni

Zn(BH₄)₂ made in our former investigation and Ni were mixed with MgH₂ to promote the hydrogen absorption and release features of Mg. A 96 w/o MgH₂ + 2 w/o Ni + 2 w/o Zn(BH₄)₂ sample [named MgH₂–4NZ] was prepared by milling in a planetary ball mill in a hydrogen atmosphere. The proportion of the additive was small (4 w/o) in order to increase hydrogen absorbing and releasing rates without majorly sacrificing the hydrogen-storage capacity. The hydrogen absorption and release features of the MgH₂–4NZ were inspected in detail and compared with those of 99 w/o MgH₂ + 1 w/o Zn(BH₄)₂ [named MgH₂–1Z] and 95 w/o MgH₂ + 2.5 w/o Ni + 2.5 w/o Zn(BH₄)₂ [named MgH₂–5NZ] samples. The activation of the MgH₂–4NZ was not required. The MgH₂–4NZ had a useful hydrogen-storage capacity (the quantity of hydrogen absorbed after 60 min) of about 5.5 w/o at the first cycle. At the first cycle, the MgH₂–4NZ absorbed 3.84 w/o hydrogen after 5 min and 5.47 w/o hydrogen after 60 min at 593 K in 12 bar hydrogen. The MgH₂–4NZ had a higher releasing rate, larger amounts of hydrogen absorbed and released after 60 min, and a better cycling capability than the MgH₂–1Z. Staying of Ni (as Mg₂Ni) and a larger amount of Zn among particles is believed to have led to the better cycling capability of the MgH₂–4NZ.     

Superior hydrogen storage kinetics of MgH2 nanoparticles doped with TiF3

MgH₂ nanoparticles were obtained by hydriding ultrafine magnesium particles which were prepared by hydrogen plasma–metal reaction. The X-ray diffraction (XRD) and transmission electron microscopy (TEM) results show that the obtained sample is almost pure MgH₂ phase, without residual magnesium and with an average particle size of ∼300 nm. Milled with 5 wt.% TiF₃ as a doping precursor in a hydrogen atmosphere, the sample desorbed 4.5 wt.% hydrogen in 6 min under an initial hydrogen pressure of ∼0.001 bar at 573 K and absorbed 4.2 wt.% hydrogen in 1 min under ∼20 bar hydrogen at room temperature. Compared with MgH₂ micrometer particles doped with 5 wt.% TiF₃ under the same conditions as the MgH₂ nanoparticles, it is suggested that decrease of particle size is beneficial for enhancing absorption capacity at low temperatures, but has no effect on desorption. In addition, the catalyst was mainly responsible for improving the sorption kinetics and its catalytic mechanism is discussed.     

Modification of Fe and Al elemental powders’ sintering with addition of magnesium and magnesium hydride

An attempt to modify sintering of iron and aluminium elemental powders with use of small additions of Mg and MgH₂ was presented in this paper. The kinetics of such modified sintering was investigated using DSC technique, XRD analysis and SEM observations. Significant changes in the mechanism of exothermal formation reaction of Fe–Al intermetallic phases in compositions doped with magnesium and its hydride was observed. Initiation temperature of Self-propagating High-temperature Synthesis (SHS) reaction was pronouncedly shifted to lower value as compared with undoped composition. Influence of additions on the SHS reaction kinetics parameters was also calculated with use of the JMA model and changes of the Avrami exponent value of specific phase formation was noticed. Positive effect of MgH₂ addition on partial homogeneity of final product was also studied.     

The Microstructure and Tensile Properties of a Newly Developed Mg–Al/Mg<Subscript>3</Subscript>Sb<Subscript>2</Subscript> In Situ Composite in As-Cast and Extruded Conditions

The microstructures and tensile properties of Mg–x wt%Al–y wt%Sb alloys have been studied where x/y ratio was 1 and Sb(Al) contents were 5, 10, 15 and 20 wt%, respectively. The results indicated that by increasing Sb(Al) content, not only the crystals of primary Mg₃Sb₂ alter from small flake-like particles to polygonal or needle-like morphology, but also the eutectic structure changes from semi-continuous network in Mg–5Al–5Sb to continuous network in Mg–20Sb–20Al alloy. The results obtained from thermal analysis revealed different peaks related to the formation of Mg₃Sb₂ as primary phase and eutectic structure containing Mg₁₇Al₁₂?+?Al₃Mg₂ intermetallic phases. Further results also revealed that Sb(Al) additions change the solidification performance of the material by depressing the Mg₃Sb₂ nucleation temperature, reducing solidification range and widening eutectic area. Tensile testing results showed that with the increase in Sb (Al) content, ultimate tensile strength (UTS) and elongation values of the alloys are decreased in as-cast condition. But, significant improvement in the UTS and elongation values of the extruded specimens was attributed to the severe fragmentation of intermetallic phases and well distributed fine particles in the matrix which provided proper obstacles for dislocation motion. It was interesting to note that the fracture behavior of intermetallic particles was found to be different, while Mg₃Sb₂ was ductile, intermetallic compounds in eutectic regions were brittle.     

Hydrogen desorption studies of the Mg24Y5–H system: Formation of Mg tubes,kinetics and cycling effects

The current study focuses on the hydrogen desorption properties of hydrogenated Mg₂₄Y₅. Recently, we have reported the formation of unidirectional MgH₂ structures by hydrogen absorption and induced disproportionation of Mg₂₄Y₅. During hydrogen desorption, a complex voiding phenomenon produces Mg tubes and carved particles with nano-sized walls. The selected area electron diffraction patterns demonstrate that the Mg tubes are single crystals. A harmonized picture of the unidirectional growth based on different Mg vapor models is proposed. The kinetic properties of hydrogen desorption are improved as compared with commercial MgH₂. Hydrogenation/dehydrogenation cycling lowers the thermal stability of the hydrogen desorption at the expense of the total desorbed hydrogen capacity. Both whiskers and microparticles are depleted into clusters of nanoparticles after extensive cycling.     

Catalytic effect of Ni nanoparticles on the desorption kinetics of MgH2 nanoparticles

Mg and Ni nanoparticles were prepared by hydrogen plasma-metal reaction (HPMR). MgH₂ nanoparticles were obtained by hydriding the Mg nanoparticles. Hydrogen storage kinetics of the MgH₂ nanoparticles doped with different amount of Ni nanoparticles was investigated by differential scanning calorimetry (DSC) and hydrogen desorption rate measurements. The obtained samples show superior hydrogen storage kinetics. 6.1 wt% hydrogen is desorbed in 10 min at 523 K under an initial pressure of 0.01 bar of H₂ when the proportion of Ni nanoparticles is 10 wt%. The desorption rate increases when enhancing the amount of catalyst. However, the activation energy of desorption does not decrease any more when the amount of Ni exceeds a value. The enhanced desorption kinetics are mainly attributed to the accelerated combination process of hydrogen atoms by the Ni nanoparticles on the surface of MgH₂.     

基于镍基固溶体催化的氢化镁储氢性能

采用高能球磨制备Ni?25％X(X=Fe,Co,Cu,摩尔分数)固溶体,然后将其掺杂于MgH2体系中.与球磨纯MgH2相比,MgH2/Ni?25％X复合体系初始放氢温度降低近90℃,其中,Ni?25％Co固溶体呈最佳催化效果.球磨MgH2/Ni?25％Co复合体系在300℃、10 min内可释放5.19％(质量分数)氢...     

Effect of Y addition on microstructure and mechanical properties of Mg–Zn–Mn alloy

The effects of Y on the microstructure and mechanical properties of Mg–6Zn–1Mn alloy were investigated. The results show that the addition of Y has significant effect on the phase composition, microstructure and mechanical properties of Mg–6Zn–1Mn alloy. Varied phases compositions, including Mg₇Zn₃, I-phase (Mg₃YZn₆), W-phase (Mg₃Y₂Zn₃) and X-phase (Mg₁₂YZn), are obtained by adjusting the Zn to Y mass ratio. Mn element exists as the fine Mn particles, which are well distributed in the alloy. Thermal analysis and microstructure observation reveal that the phase stability follows the trend of X>W>I>Mg₇Zn₃. In addition, Y can improve the mechanical properties of Mg–Zn–Mn alloy significantly, and the alloy with Y content of 6.09% has the best mechanical properties. The high strength is mainly due to the strengthening by the grain size refinement, dispersion strengthening by fine Mn particles, and introduction of the Mg–Zn–Y ternary phases.     

Thermodynamic modeling of Al–Ce–Mg phase equilibria coupled with key experiments

The ternary Al–Ce–Mg phase diagram was calculated using the Calphad method and investigated with selected key experiments. Arc melted alloys were annealed at 400 °C for 500 h and the phases were analyzed using quantitative X-ray powder diffraction (XRD). Differential thermal analysis (DTA) was also performed on an alloy with a composition near the ternary phase Al₁₃CeMg₆ (τ). Temperatures above 1000 °C could be attained due to a special sealing of the sample under argon by welding in a tantalum crucible to avoid evaporation and oxidation. Only with this procedure could reproducible and reliable DTA signals be obtained. The present experimental investigation and the consistent thermodynamic calculation show that the “ternary phase” Ce(Mg,Al)₂, seemingly isolated in the ternary at 400 °C, can be rationalized as a single solid solution phase between the binary end members if a larger temperature range and a solid state miscibility gap is considered. It is demonstrated that previously reported low values of ternary liquidus temperatures must be related to other phase equilibria. The actually found ternary liquidus temperatures are much higher and widely governed by the high melting compound Ce(Al,Mg)₂ and also by Al₁₁Ce₃ with primary solidification fields stretching far into the ternary system.     

Ab initio investigation of structures, electronic and thermodynamic properties for Li-Mg-H ternary system

A systematic consideration of the compounds made up of Li, Mg and H has been taken with respect to the structural, electronic, and thermodynamic properties, by means of density functional theory (DFT). Through the database mining approach, the ground state structures of LiMgH₃ and Li₂MgH₄ are identified to be R3c and Pbam, respectively. The Li-Mg-H ternary hydrides are insulators dominated by ionic bonds besides some covalent components between Mg and H. Energies of different formation pathways have been calculated at finite temperature. Hydrides synthesized from Li, Mg and H₂ possess obvious energetic advantage, but may be inhabited kinetically by pure phase separation. Thermodynamically reversible decomposition to LiH and MgH₂ brings about another issue for the actual preparation and stable existence of the ternary hydrides. Inserting H atoms to the sites of the ordered alloys with high electric density has been taken as another way to explore possible structures of this system. As H uptakes stepwise, the resulted compounds turn from conductors to insulators. The present results shed light on the design of Li-Mg-H ternary hydrides.     

Influence of aging treatments on microstructure and electrochemical properties in Mg–8.8Hg–8Ga (wt%) alloy

All precipitate morphologies in Mg–8.8%Hg–8%Ga alloy for a range of aging temperatures are investigated in detail using SEM, TEM and OM. The results show that Mg₂₁Ga₅Hg₃ are the dominant precipitate in Mg–8.8%Hg–8%Ga alloy. Mg₂₁Ga₅Hg₃ phase precipitate in dispersed particles. There are few papers focuses on the relationship between the aging behavior and the electrochemical and corrosion properties in Mg–8.8%Hg–8%Ga alloy. This study elaborates on the morphological evolution of Mg₂₁Ga₅Hg₃ precipitates as a function of aging time and temperature and investigates the associated second phase morphology-electrochemical and corrosion response. The corrosion behavior of the Mg–8.8%Hg–8%Ga alloy is also discussed.     

Influence of the microstructure on the machinability of heat-treated Al–10.8% Si cast alloys: Role of copper-rich intermetallics

This study was conducted with the intention of investigating a new experimental alloy, namely the 396 alloy which belongs to the Al–Si near-eutectic cast alloy group and contains about 10.8%Si. In the light of the above, the main purpose of the work is to report on the changes observed in the mechanical and machinability criteria resulting from the effects of the presence of two levels of Cu, namely 2.25% and 3.5%; and of the effects of two levels of Mg, namely 0.3% and 0.6%. In addition to the preceding, the effects of Mg-free alloys and Sr-modification on these same alloys were also investigated.The results demonstrate that the increase in the levels of Cu and/or Mg in the 396-T6 alloy has a detrimental effect on drill life. Such an effect may be attributed to the formation of large amounts of the coarse blocklike Al₂Cu phase, together with the formation of thick plates of the Al–Si–Cu–Mg phase. The Mg-free experimental alloy displays the lowest cutting force and moment in addition to producing the highest number of holes in the alloys studied. This observation may be explained by the cooperative precipitation of the Al₂Cu, Mg₂Si, Al₂CuMg, and Al₅Si₆Cu₂Mg₈ hardening phases in Mg-containing alloys which confer greater strength on the alloy than would be the case with the precipitation of only the Al₂Cu phase in the Mg-free alloy. A comparison of the non-modified alloy and the Sr-modified alloy (containing the same level of Mg and Cu additions) in terms of the number of holes drilled, reveals that the morphology of Si particles has a noticeable effect in governing the tool life of near-eutectic Al–Si alloys. The chip breakability of the alloys containing the Al₂Cu phase is superior to that of the alloys containing Mg₂Si. Thus, combined additions of Cu and Mg are expected to further refine the size of the chips produced.