•HPT processing was conducted on bulk specimens thicker than the usual thin-disks.•The Al alloy (A5086) and commercial purity magnesium samples were compared.•Distributions of strain and microhardness were evaluated in the radial and axial direction.•Plastic deformation is highly localized in the middle plane at outer edge in both materials.•Different DRX rates governed the differences in microstructure and hardening behavior.Two distinct bulk light metals were opted to study the shear strain evolution and associated heterogeneities in texture/microstructure development during torsional straining by high pressure torsion (HPT): a face centered cubic Al alloy (A5086) and a hexagonal commercial purity Mg. Relatively thick disk samples - four times thicker than usually employed in HPT process - were processed to 180° and 270° rotations. With the help of X-ray tomography, the shear strain gradients were examined in the axial direction. The results showed strongly localized shear deformation in the middle plane of the disks in both materials. These gradients involved strong heterogeneities in texture, microstructure and associated hardness, in particular through the thickness direction at the periphery of the disk where the interplay between significant strain hardening and possible dynamic recrystallization could occur.

•Atomized micro-sized and condensed ultrafine Mg powders were processed by HPT.•From atomized powder, the micro-HPT product contained 1 μm equiaxed grains.•From condensed powder, a nano-HPT MgO + elongated Mg composite was obtained.•The nano-HPT product showed faster absorption kinetics than the micro-HPT one.•The micro-HPT had higher storage capacity and faster desorption kinetics.While severe plastic deformation (SPD) on bulk samples has been widely applied for modifying the H-sorption properties, there has been little attention towards the use of SPD on powder materials. In this context, the aim of the present work was to compare the H-storage properties of high-pressure torsion (HPT) consolidated products obtained from two distinct Mg powder precursors: atomized micro-sized and condensed ultrafine powder particles. The results showed that the nature of the initial powder precursor had a pronounced effect on the H-sorption behavior. The HPT product obtained from the condensed ultrafine powder showed faster absorption kinetics than the consolidated product obtained from the atomized powder. However, the HPT product obtained from atomized powder could absorb more hydrogen and showed faster desorption kinetics corresponding to a lower activation energy. These results are discussed by taking into account the effectiveness of the HPT process to refine the grain sizes and differences in the dispersion of fine MgO oxide particles.

•Core-shell structured Mg-MFx nano-composites were prepared using arc plasma method.•MFx addition reduces the agglomeration tendency of Mg ultrafine particles.•NiF2 addition can remarkably improve the hydrogen absorption kinetics of Mg particles.•The VF3 addition can enhance hydrolysis properties of MgH2 particles.In this work, core-shell structured Mg-MFx (M = V, Ni, La and Ce) nano-composites are prepared by using arc plasma method. The particle size distribution, phase components, microstructures, hydrogen sorption properties of these composites and hydrolysis properties of their corresponding hydrogenated powders are carefully investigated. It is shown that the addition of MFx through arc plasma method can improve both the hydrogen absorption kinetics of Mg and the hydrolysis properties of corresponding hydrogenated powders. Among them, the Mg-NiF2 composite shows the best hydrogen absorption properties at relatively low temperatures, which can absorb 3.26 wt% of H2 at 373 K in 2 h. Such rapid hydrogen absorption rate is mainly due to the formation of Mg2Ni and MgF2 on Mg particles during arc evaporation and condensation. In contrast, measurements also show that the hydrogenated Mg-VF3 composite has the lowest peak desorption temperature and the fastest hydrolysis rate among all the hydrogenated Mg-MFx composites. The less agglomeration tendency of Mg particles and VO2 covered on MgH2 particles account for the reduced hydrogen desorption temperature and enhanced hydrolysis rate.

•Core-shell structured Mg@TM (TM = Co, V) composites were synthesized.•Hydrogen sorption properties of the ternary composite was better than those of binary ones.•Co and V containing hydrides coated on Mg particles account for the improved hydrogen sorption properties.Target improving the hydrogen sorption properties of Mg, core-shell structured Mg@TM (TM = Co, V) composites were synthesized via an approach combining arc plasma method and electroless plating. The core-shell structures with the MgH2 core and V or Co containing hydride shells for hydrogenated Mg@TM particles were observed through HAADF-STEM and HRTEM techniques. The measured hydrogenation enthalpy (ΔHabs = −70.02 kJ/mol H2) and activation energy (Ea = 67.66 kJ/mol H2) of the ternary Mg@Co@V composite were lower than those of binary composites and the pure Mg powder. In addition, the onset dehydrogenation temperature for the hydrogenated ternary composite measured from DSC was 323 °C, about 60 °C lower than that of pure MgH2. On one hand, these improved properties can be attributed to the core-shell structure which may introduce more contacts between catalysts and Mg, thus providing more nucleation sites for hydrogen sorption. On the other hand, the co-effect of MgCo hydrides (Mg2CoH5&Mg3CoH5) acting as “hydrogen pump” and V2H accelerating the dissociation of H2 might also contribute to the improved hydrogen sorption properties of Mg.

•γ-Fe(Ni) nano powder was prepared via arc plasma evaporation of Fe–Ni mixture.•Mg2Fe(Ni)H6 was produced using coarse grained Mg and γ-Fe(Ni) nano powder.•Formation kinetics of Mg2FeH6 was improved using γ-Fe(Ni) as precursor.•The Mg2Fe(Ni)H6 has a “tangled nanowire” microstructure.•The Mg2Fe(Ni)H6 shows better hydrogen sorption properties over Mg2FeH6.In this work, Mg2Fe(Ni)H6 was synthesized under relatively mild conditions using precursors of coarse-grained Mg powder and Fe(Ni) composite containing γ-Fe(Ni) nano particles prepared through arc plasma method. The microstructure, composition, phase components and the hydrogen storage properties of the Mg–Fe(Ni) composite were carefully investigated. It is observed that the Mg2Fe(Ni)H6 formed from the Mg–Fe(Ni) composite has a “tangled nanowire” morphology with the wire diameter of 100–240 nm. In contrast, a typical columnar morphology was observed for the Mg2FeH6 produced from hydrogenation of coarse-grained Mg and pure α-Fe nano particles. A promotion effect on the synthesis of Mg2FeH6 is found for the utilization of γ-Fe(Ni) precursor instead of pure α-Fe. Indeed, γ-Fe(Ni) has the same fcc lattice as Mg2FeH6, which may remarkably shorten the diffusion distance of Fe during the formation of Mg2FeH6 from MgH2 and Fe. Meantime, the existence of Ni in Fe would catalyze the hydrogenation and influence the growing of Mg2Fe(Ni)H6/γ-Fe(Ni) interface, leading to the formation of Mg2Fe(Ni)H6 tangled nanowires. For hydrogen release, the peak desorption temperature of Mg2Fe(Ni)H6 is 593 K, which is 22 K lower than that of the Mg2FeH6. The absorption and desorption enthalpies of Mg2Fe(Ni)H6 were measured to be −68.8 ± 3.0 and 69.2 ± 3.2 kJ/mol H2, respectively. The improvements in both hydrogen sorption kinetics and thermodynamics can be attributed to destabilization effect of Ni substitution for Fe in the Mg2FeH6.Without any high-energy pre-treatments, Mg2Fe(Ni)H6, which has a “tangled nanowire” morphology, can be successfully synthesized under relatively mild conditions using precursors of coarse grained Mg powder and Fe(Ni) composite containing γ-Fe(Ni) nano particles. The improved hydrogen sorption kinetics and thermodynamics of Mg2Fe(Ni)H6 can be attributed to the destabilization of Mg2FeH6 through doping with Ni.

In the present work, hydrogen sorption behaviors of some of the 3NaBH4–LnF3 (Ln = Ce, Sm, Gd and Yb) composites were investigated and the mechanisms associated with different effects of LnF3 (Ln = La, Ce, Pr, Nd, Sm, Gd, Ho, Er and Yb) on reversible hydrogen sorption in NaBH4 were proposed based on careful comparisons. The key factors controlling the properties of 3NaBH4–LnF3 can be summarized as follows: (i) electronegativity χp of Ln3+ determines the thermodynamic stability of 3NaBH4–LnF3 composites with their χp in the range of 1.23–1.54 being suitable for reversible hydrogen storage; (ii) the electronic configuration of Ln3+ influences rehydrogenation behaviors: more stable the oxidation state of the Ln3+ is, the better is the rehydrogenation performance of NaBH4; (iii) the unique crystal structure of the Ln–B phase formed during dehydrogenation, and geometrical configuration of B in Ln–B, provide dangling bonds for hydrogen atoms to embed in, consequently modifying the rehydrogenation kinetics. Because Gd3+ possesses a combination of suitable electronegativity, stable oxidation state and favorable geometric structure in GdB4, the 3NaBH4–GdF3 composite exhibits the best overall hydrogen storage properties among all the studied 3NaBH4–LnF3 composites, with high cycling stability up to 51 cycles along with fast kinetics. This understanding provides us with criterions to design new borohydride-based hydrogen storage systems and to optimize their hydrogen storage properties.

In the present work, two LiBH4 based hydrogen storage composites, namely, 3LiBH4/graphene and 3LiBH4/graphene–10 wt% CeF3, were prepared through ball milling and their hydrogen sorption behaviors were investigated. It was shown that the dehydrogenation kinetics of 3LiBH4/graphene was improved by doping 10 wt% CeF3, which was attributed to the formation of CeB6. The rehydrogenation of the 3LiBH4/graphene composite could be achieved under 440 °C with 3.3 MPa hydrogen pressure and the hydrogen absorption capacity reached 7.40 wt% to LiBH4 after 10 h. Further addition of CeF3 did not improve the rehydrogenation kinetics but reduced the absorption capacity. SEM observations suggested that LiBH4 was confined in a graphene wrapper after rehydrogenation, forming “graphene capsules” that helped the regeneration of LiBH4.

•A Mg based Mg@Ni nano-composite is prepared through a duplex treatment.•Mg@Ni powder shows better hydrogen sorption kinetic properties than Mg–Ni powder.•The Ni layer is replaced by Mg2Ni layer after hydriding/dehydriding cycles.•The Mg2Ni layer improves hydrogen sorption properties of Mg ultrafine particles.For the first time, a nano Ni decorated Mg ultrafine powder, Mg@Ni nano-composite powder, was prepared through arc plasma evaporation of pure Mg followed by the electroless plating of Ni on the Mg ultrafine particles in a NiCl2 solution. The phase components, microstructure and hydrogen sorption properties of the Mg@Ni composite powder were carefully investigated. It is observed that the composite is mainly composed of ultrafine Mg particles decorated by nano Ni grains introduced during electroless plating. Based on the Pressure-Composition-Temperature measurements, the hydrogenation enthalpy of the Mg@Ni composite is determined to be −73.6 kJ/mol H2, close to that of pure Mg. Meantime, the hydrogen absorption kinetics can be improved and the hydrogen desorption temperature of the hydrogenated Mg@Ni nano-composite can be reduced as compared to those of the pure Mg ultrafine powder. The improved hydrogen sorption properties can be mainly attributed to the gateway effects from the Mg2Ni/Mg2NiH4 phases formed on Mg/MgH2 ultrafine particles after re-/dehydrogenation cycles. The results showed that through arc plasma method followed by electroless plating of transition metals, the hydrogen storage properties of Mg ultrafine powders can be effectively improved.

In the present work, two new reversible hydrogen storage composites, NaBH4 + LaF3 and NaBH4 + LaH2, have been prepared through a mechanical milling method with the aim of comparatively studying the effects of La fluoride and La hydride on the hydrogen sorption behaviors of NaBH4. Experimental investigations have shown that both La fluoride and La hydride enable reversible hydrogen sorption in NaBH4. In particular, LaF3 exhibits a superior promoting effect than LaH2, which agrees well with theoretical predictions. Surprisingly, better hydrogen sorption properties can be achieved in both systems through undergoing de-/rehydriding cycles. The reversible hydrogen storage capacity reaches up to 3.0 wt% at 238 °C and 2.9 wt% at 326 °C in NaBH4 + LaF3 and NaBH4 + LaH2 systems after the 6th dehydrogenation, respectively. In both cases, the formation of La boride plays the major role in the reversible hydrogen sorption in NaBH4. The superior promoting effect of La fluoride than La hydride upon modifications of thermodynamics and kinetics of NaBH4 should be ascribed to the following factors: (i) the formation of a thermodynamically more stable compound NaF instead of NaH reduces the overall enthalpy changes of re/de-hydriding reactions in NaBH4 + LaF3 to −31.8 kJ mol−1 H2 and 72.5 kJ mol−1 H2, respectively; (ii) the ion exchange of F− for H− leads to the reduction of the onset dehydrogenation temperature of NaBH4 to 160 °C in the NaBH4 + LaF3 composite; (iii) the F− anion favors the formation of LaB6 while H− favors the formation of LaB4. The role of functional anions and cations, de-/rehydrogenation mechanisms and nucleation modes in the two reversible hydrogen storage composites have been proposed based on experimental and theoretical analyses. The comparison study carried out in this work helps to design and search for new metal borohydride based composites for reversible hydrogen storage.

•Rare earth fluorides REF3(RE = Y, La, Ce) were added into LiAlH4 through ball milling.•The dehydrogenation properties of REF3 doped LiAlH4 were studied.•CeF3 as a catalyst showed superior destabilization effects on LiAlH4 than YF3 or LaF3.•Mechanisms for dehydrogenation behaviors in REF3 doped LiAlH4 were proposed.The catalytic effects of rare earth fluoride REF3 (RE = Y, La, Ce) additives on the dehydrogenation properties of LiAlH4 were carefully investigated in the present work. The results showed that the dehydrogenation behaviors of LiAlH4 were significantly altered by the addition of 5 mol% REF3 through ball milling. The destabilization ability of these catalysts on LiAlH4 has the order: CeF3>LaF3>YF3. For instance, the temperature programmed desorption (TPD) analyses showed that the onset dehydrogenation temperature of CeF3 doped LiAlH4 was sharply reduced by 90 °C compared to that of pristine LiAlH4. Based on differential scanning calorimetry (DSC) analyses, the dehydriding activation energies of the CeF3 doped LiAlH4 sample were 40.9 kJ/mol H2 and 77.2 kJ/mol H2 for the first and second dehydrogenation stages, respectively, which decreased about 40.0 kJ/mol H2 and 60.3 kJ/mol H2 compared with those of pure LiAlH4. In addition, the sample doped with CeF3 showed the fastest dehydrogenation rate among the REF3 doped LiAlH4 samples at both 125 °C and 150 °C during the isothermal desorption. The phase changes in REF3 doped LiAlH4 samples during ball milling and dehydrogenation were examined using X-ray diffraction and the mechanisms related to the catalytic effects of REF3 were proposed.

•A Reversible hydrogen sorption of 2.3 wt% is achieved in a 3NaBH4/HoF3 composite.•A dual-cation borohydride NaHo(BH4)4 formed during first dehydrogenation process.•Onset dehydriding temperature of NaBH4 is lowered down to 86 °C by the HoF3 addition.•The ion exchanges play an important role for the reversible hydrogen sorption.In this work, the complex hydrogen sorption behaviors in a 3NaBH4/HoF3 composite prepared through mechanical milling were carefully investigated, including the reactions occurred during ball milling and de-/rehydrogenation processes. Different from other rear earth fluorides, the HoF3 can react with NaBH4 during ball milling, leading to the formations of Na–Ho–F and Na–Ho–BH4 complex compounds. The first dehydriding of the 3NaBH4/HoF3 composite can be divided into 4 steps, including the ion exchange between H− and F−, the formation of NaHo(BH4)4, the decomposition of NaHo(BH4)4 and reaction of NaBH4 with Na–Ho–F compounds. The final products, HoB4, HoH3 and NaF, can be rehydrogenated to generate NaBH4 and NaHoF4 with an absorption capacity of 2.3 wt% obtained at 400 °C. Based on the Pressure–Composition–Temperature measurements, the de-/rehydrogenation enthalpies of the 3NaBH4/HoF3 composite are determined to be 88.3 kJ mol−1 H2 and −27.1 kJ mol−1 H2, respectively.

•A method combining arc plasma evaporation and high pressure torsion is developed.•Bulk Mg based nano-composite containing dispersed MgO is successfully prepared.•Bulk Mg composite shows better H-storage properties over the ultrafine Mg powder.•MgO improves the hydrogen sorption kinetics of the bulk Mg nano-composite.A nanostructured Mg based bulk material containing Mg nano-grains and dispersed nanosized MgO was prepared through arc plasma method followed by high pressure torsion (HPT). In comparison to pure Mg ultrafine powders prepared using only arc plasma method, the hydrogenation enthalpy was slightly increased and the absorption activation energy was reduced through the HPT treatment. The structure refinement and the fragmentation of the MgO layer induced by the HPT account for the improved hydrogen sorption thermodynamic and kinetic properties.

In this work, a reversible hydrogen storage composite, 3NaBH4/NdF3, has been prepared using a mechanical milling method. The de-/rehydrogenation properties as well as the mechanisms of reversible hydrogen sorption in the composite were carefully investigated. Based on the pressure–temperature–composition measurements, the de-/rehydrogenation enthalpies of the 3NaBH4/NdF3 composite are determined to be 86.4 kJ mol−1 H2 and −13.2 kJ mol−1 H2, respectively. The onset dehydriding temperature of the composite is determined to be 413 °C in 0.1 MPa Ar atmosphere and can be as low as 80 °C under vacuum conditions. Analyses revealed that NdB6, Nd2H5 and NaF were formed after the decomposition of the composite, which can be hydrogenated to produce NaBH4 and NaNdF4. The formation of NaNdF4 instead of NdF3 in the hydrogenated products is believed to be responsible for the reduced hydrogen storage capacity, while the intermediate formation of B, Nd and NdB4 during dehydrogenation accounts for the asymmetric hydriding/dehydriding behaviors in the 3NaBH4/NdF3 composite.

In the present study, a new hydrogen storage system, being able to reversibly absorb/desorb hydrogen at fairly low temperatures, was developed based on a 3NaBH4–PrF3 composite. It is shown that 3 wt% of reversible hydrogen sorption can be achieved in the 3NaBH4–PrF3 composite at 400 °C with fast kinetics. After the addition of 5 mol% VF3, the dehydrogenation kinetics of the 3NaBH4–PrF3 composite can be significantly improved. The onset dehydriding temperature is lowered down to 46 °C in vacuum, and the dehydrogenation finishes in 2 min at 400 °C. Both the dehydrogenation enthalpy and activation energy of 3NaBH4–PrF3 can be lowered down through the addition of VF3. In particular, the dehydrogenation products of the 3NaBH4–PrF3–5 mol% VF3 composite can be rehydrogenated at a temperature as low as 48 °C with the regeneration of NaBH4. At 84 °C, a reversible hydrogen sorption of about 1.2 wt% can be achieved in the 3NaBH4–PrF3–5 mol% VF3 composite. The improvement in hydrogen sorption properties can be mainly attributed to the formation of the VB2 phase during dehydrogenation as an efficient catalyst, which maintains well its catalytic effect in the re-/dehydrogenation cycles. Based on a series of controlled experiments and phase analyses, the de-/rehydrogenation mechanisms of the 3NaBH4–PrF3 composite without and with VF3 addition are proposed and discussed in detail.

Mg based Mg–Rare earth (RE) hydrogen storage nano-composites were prepared through an arc plasma method and their composition, phase components, microstructure and hydrogen sorption properties were carefully investigated. It is shown that the Mg–RE composites have special metal-oxide type core–shell structure, that is, ultrafine Mg(RE) particles are covered by nano-sized MgO and RE2O3. In comparison to pure Mg powders prepared using the same method, the hydrogen absorption kinetics can be significantly improved through minor addition of RE to Mg. In addition, the Mg–RE composite powders show better anti-oxidation ability than pure Mg powders, resulting in the increased hydrogen storage capacity of Mg–RE powders over pure Mg powders. In particular, the hydrogenation enthalpy can be increased and the dehydriding temperature can be reduced through minor addition of Er. The experimental results show that both the RE in solid solution state in Mg and the RE2O3 nano-grains covered on Mg particles contribute to the improved hydrogen storage thermodynamic, kinetic and anti-oxidation properties of Mg ultrafine particles.Highlights► RE (RE = Nd, Gd, Er) doped Mg based composites are prepared by arc plasma method. ► Mg–RE powders are composed of Mg(RE) ultrafine particles covered by MgO/RE2O3. ► Both RE in Mg and RE2O3 on Mg particles improve hydrogen sorption properties. ► Mg–RE composite powers show higher anti-oxidation ability than Mg powders.

•Mg–TM–La ternary composite powders are prepared directly through arc plasma method.•Both TM and La2O3 improve hydrogen sorption properties of Mg particles.•Mg–TM–La ternary powders show better properties than Mg based binary powders.•Mg–Ni–La powder is able to absorb hydrogen with fairly rapid kinetics at 303 K.For the first time, Mg based Mg–Transition metal (TM) –La (TM = Ti, Fe, Ni) ternary composite powders were prepared directly through arc plasma evaporation of Mg–TM–La precursor mixtures followed by passivation in air. The composition, phase components, microstructure and hydrogen sorption properties of the composite powders were carefully investigated. Composition analyses revealed a reduction in TM and La contents for all powders when compared with the compositions of their precursors. It is observed that the composites are all mainly composed of ultrafine Mg covered by nano La2O3 introduced during passivation. Based on the Pressure–Composition–Temperature measurements, the hydrogenation enthalpies of Mg are determined to be −68.7 kJ/mol H2 for Mg–Ti–La powder, −72.9 kJ/mol H2 for Mg–Fe–La powder and −82.1 kJ/mol H2 for Mg–Ni–La powder. Meantime, the hydrogen absorption kinetics can be significantly improved and the hydrogen desorption temperature can be reduced in the hydrogenated ternary Mg–TM–La composites when compared to those in the binary Mg–TM or Mg–RE composites. This is especially true for the Mg–Ni–La composite powder, which can absorb 1.5 wt% of hydrogen at 303 K after 3.5 h. Such rapid absorption kinetics at low temperatures can be attributed to the catalytic effects from both Mg2Ni and La2O3. The results gathered in this study showed that simultaneous addition of 3d transition metals and 4f rare earth metals to Mg through the arc plasma method can effectively alter both the thermodynamic and kinetic properties of Mg ultrafine powders for hydrogen storage.

The quantitative evolution of the residual stress states in the surface layers of an AISI D2 steel after Low Energy High Current Pulsed Electron Beam (LEHCPEB) treatment has been investigated by using X-ray diffraction technique. The initial material contained mainly ferrite plus carbides and the ferrite had a compressive stress of about 560 MPa. After the LEHCPEB treatment, the residual stress of the ferrite in the surface layers became tensile in nature, reaching values as high as 730 MPa. A residual tensile stress also existed in the austenite formed at the surface. The stress increased from 170 MPa after 5 pulses to 700 MPa after 25 pulses of treatment. The evolution of the residual stress state in the surface layers can be explained by taking into account the fast thermal cycle, deformations induced by the dynamic thermal stress at the surface together with melting and phase transformations generated by the LEHCPEB treatment.Highlights► Evolution of residual stress in LEHCPEB treated material is studied. ► Stress state in the treated surface is tensile in nature. ► Stress strain states are related to phase transformations and melting.

A 3NaBH4/YF3 hydrogen storage composite was prepared through ball milling and its hydrogen sorption properties were investigated. It is shown that NaBH4 does not react with YF3 during ball milling. The dehydrogenation of the composite starts at 423 °C, which is about 100 °C lower than the dehydrogenation temperature of pure NaBH4, with a mass loss of 4.12 wt%. Pressure–Composition–Temperature tests reveal that the composite has reversible hydrogen sorption performance in the temperature range from 350 °C to 413 °C and under quite low hydrogenation plateau pressures (<1 MPa). Its maximum hydrogen storage capacity can reach up to 3.52 wt%. The dehydrogenated composite can absorb 3.2 wt% of hydrogen within 5 min at 400 °C. Based on the Pressure–Composition–Temperature analyses, the hydrogenation enthalpy of the composite is determined to be −46.05 kJ/mol H2, while the dehydrogenation enthalpy is 176.76 kJ/mol H2. The mechanism of reversible hydrogen sorption in the composite involves the decomposition and regeneration of NaBH4 through the reaction with YF3. Therefore, the addition of the YF3 to NaBH4 as a reagent forms a reversible hydrogen storage composite.Highlights► 3NaBH4/YF3 system is a found to be a reversible hydrogen storage composite. ► The composite can absorb 3.2 wt% of hydrogen within 5 min at 400 °C. ► NaBH4 can be regenerated at moderate temperatures and low hydrogen pressures.

The formation of highly-twinned ultrafine austenite on a cold rolled AISI 316L steel induced by low energy high current pulsed electron beam (LEHCPEB) treatment has been investigated. It is shown that rapid phase transformation, recrystallization and recovery can be achieved simultaneously through pulsed electron beam treatments under heating mode without melting the surface. The extremely fast thermo-mechanical cycles induced by the LEHCPEB account for the surface austenization and the formation of ultrafine twins. The results demonstrated that pre-deformation combined with LEHCPEB treatment under heating mode can be used to create ultrafine microstructures with special boundary characteristics in the surface layer.Highlights► A duplex treatment involving cold rolling and electron beam annealing is developed. ► The duplex treatment can create ultrafine structures with special boundaries. ► Ultrafine twinned austenite formed on a 316L steel after the duplex treatment. ► Phase transformation, recrystallization and recovery occurred during the treatment.

•Core-shell structured binary Mg@Ti and ternary Mg@Ti@Ni composites were synthesized.•Hydrogen sorption properties of the ternary composite was better than those of binary systems.•Ti and Ni containing hydrides covered on Mg particles account for the improved hydrogen sorption properties.Core-shell structured binary Mg@Ti and ternary Mg@Ti@Ni composites were synthesized using an arc plasma method followed by electroless plating in solutions. Their microstructures and hydrogen storage properties were systematically investigated. The hydrogenated composites with core–shell structures containing MgH2 core and Ti or MgNi hydrides shells were observed. Based on the Pressure-Composition-Temperature measurements, the hydrogen absorption enthalpy (−67.12 kJ/mol H2) of the ternary composite was slightly higher than that of the binary composite (−77.20 kJ/mol H2). In addition, the hydrogenation activation energy (63.7 kJ/mol H2) of the ternary composite and the peak dehydrogenation temperature (642 K) of the hydrogenated ternary composite were also lower than those of the binary composite, respectively. These improvements in hydrogen sorption properties of Mg can be mainly attributed to the co-effect of TiH2 acting as the activation site and Mg2Ni acting as the “hydrogen pump”.Targeting the improvement in hydrogen sorption properties of ultrafine Mg powder, a ternary Mg@Ti@Ni composite powder with core–shell structure was systhesized. The dehydriding temperature of the hydrogenated ternary composite is lower than that of the binary composites and pure Mg due to the synergetic effects from Mg2Ni and TiH2 covered on Mg particles.

•A Mg-La-Fe-H composite was prepared through reactive ball milling.•An unsaturated hydride LaH2.3 formed in the Mg-La-Fe-H composite.•Mg grains preferentially nucleate at the interface of MgH2/α-Fe during dehydrogenation.•The Mg-La-Fe-H shows better hydrogen sorption properties over the Mg-La-H and Mg-Fe-H.In the present work, a Mg-La-Fe-H nano-composite was prepared through reactive ball milling of the mixture of MgH2, Fe and La powders. The phase components, microstructure and hydrogen sorption properties of the composite powders were carefully investigated. After milling, an unsaturated hydride LaH2.3 formed in the Mg-La-Fe-H composite. It is observed that since Mg(102) and α-Fe(110) planes have the similar interplanar spacing, Mg grains preferentially nucleate at the interface of MgH2/α-Fe when MgH2 transforms to Mg. Based on the Pressure-Composition-Temperature measurements, the hydrogenation enthalpy of the Mg-La-Fe-H composite is determined to be −72.0 kJ/mol H2. Meantime, the hydrogen absorption kinetics can be significantly improved and the hydrogen desorption temperature can be reduced in the Mg-La-Fe-H composite when compared to those in the Mg-La-H or Mg-Fe-H composites. This is especially promising for the Mg-La-Fe-H composite, which can absorb 5.0 wt% of hydrogen within 8 h at room temperature and desorb hydrogen at 463 K. Such rapid absorption kinetics at low temperatures can be attributed to the synergetic catalytic effects from both LaH2.3 and α-Fe. The above results indicated that simultaneous addition of La and Fe to MgH2 through the reactive ball milling can effectively improve the hydrogen sorption kinetic properties of MgH2.

In the present study, a new hydrogen storage system, being able to reversibly absorb/desorb hydrogen at fairly low temperatures, was developed based on a 3NaBH4–PrF3 composite. It is shown that 3 wt% of reversible hydrogen sorption can be achieved in the 3NaBH4–PrF3 composite at 400 °C with fast kinetics. After the addition of 5 mol% VF3, the dehydrogenation kinetics of the 3NaBH4–PrF3 composite can be significantly improved. The onset dehydriding temperature is lowered down to 46 °C in vacuum, and the dehydrogenation finishes in 2 min at 400 °C. Both the dehydrogenation enthalpy and activation energy of 3NaBH4–PrF3 can be lowered down through the addition of VF3. In particular, the dehydrogenation products of the 3NaBH4–PrF3–5 mol% VF3 composite can be rehydrogenated at a temperature as low as 48 °C with the regeneration of NaBH4. At 84 °C, a reversible hydrogen sorption of about 1.2 wt% can be achieved in the 3NaBH4–PrF3–5 mol% VF3 composite. The improvement in hydrogen sorption properties can be mainly attributed to the formation of the VB2 phase during dehydrogenation as an efficient catalyst, which maintains well its catalytic effect in the re-/dehydrogenation cycles. Based on a series of controlled experiments and phase analyses, the de-/rehydrogenation mechanisms of the 3NaBH4–PrF3 composite without and with VF3 addition are proposed and discussed in detail.

In the present work, two new reversible hydrogen storage composites, NaBH4 + LaF3 and NaBH4 + LaH2, have been prepared through a mechanical milling method with the aim of comparatively studying the effects of La fluoride and La hydride on the hydrogen sorption behaviors of NaBH4. Experimental investigations have shown that both La fluoride and La hydride enable reversible hydrogen sorption in NaBH4. In particular, LaF3 exhibits a superior promoting effect than LaH2, which agrees well with theoretical predictions. Surprisingly, better hydrogen sorption properties can be achieved in both systems through undergoing de-/rehydriding cycles. The reversible hydrogen storage capacity reaches up to 3.0 wt% at 238 °C and 2.9 wt% at 326 °C in NaBH4 + LaF3 and NaBH4 + LaH2 systems after the 6th dehydrogenation, respectively. In both cases, the formation of La boride plays the major role in the reversible hydrogen sorption in NaBH4. The superior promoting effect of La fluoride than La hydride upon modifications of thermodynamics and kinetics of NaBH4 should be ascribed to the following factors: (i) the formation of a thermodynamically more stable compound NaF instead of NaH reduces the overall enthalpy changes of re/de-hydriding reactions in NaBH4 + LaF3 to −31.8 kJ mol−1 H2 and 72.5 kJ mol−1 H2, respectively; (ii) the ion exchange of F− for H− leads to the reduction of the onset dehydrogenation temperature of NaBH4 to 160 °C in the NaBH4 + LaF3 composite; (iii) the F− anion favors the formation of LaB6 while H− favors the formation of LaB4. The role of functional anions and cations, de-/rehydrogenation mechanisms and nucleation modes in the two reversible hydrogen storage composites have been proposed based on experimental and theoretical analyses. The comparison study carried out in this work helps to design and search for new metal borohydride based composites for reversible hydrogen storage.

In the present work, hydrogen sorption behaviors of some of the 3NaBH4–LnF3 (Ln = Ce, Sm, Gd and Yb) composites were investigated and the mechanisms associated with different effects of LnF3 (Ln = La, Ce, Pr, Nd, Sm, Gd, Ho, Er and Yb) on reversible hydrogen sorption in NaBH4 were proposed based on careful comparisons. The key factors controlling the properties of 3NaBH4–LnF3 can be summarized as follows: (i) electronegativity χp of Ln3+ determines the thermodynamic stability of 3NaBH4–LnF3 composites with their χp in the range of 1.23–1.54 being suitable for reversible hydrogen storage; (ii) the electronic configuration of Ln3+ influences rehydrogenation behaviors: more stable the oxidation state of the Ln3+ is, the better is the rehydrogenation performance of NaBH4; (iii) the unique crystal structure of the Ln–B phase formed during dehydrogenation, and geometrical configuration of B in Ln–B, provide dangling bonds for hydrogen atoms to embed in, consequently modifying the rehydrogenation kinetics. Because Gd3+ possesses a combination of suitable electronegativity, stable oxidation state and favorable geometric structure in GdB4, the 3NaBH4–GdF3 composite exhibits the best overall hydrogen storage properties among all the studied 3NaBH4–LnF3 composites, with high cycling stability up to 51 cycles along with fast kinetics. This understanding provides us with criterions to design new borohydride-based hydrogen storage systems and to optimize their hydrogen storage properties.

In this work, a reversible hydrogen storage composite, 3NaBH4/NdF3, has been prepared using a mechanical milling method. The de-/rehydrogenation properties as well as the mechanisms of reversible hydrogen sorption in the composite were carefully investigated. Based on the pressure–temperature–composition measurements, the de-/rehydrogenation enthalpies of the 3NaBH4/NdF3 composite are determined to be 86.4 kJ mol−1 H2 and −13.2 kJ mol−1 H2, respectively. The onset dehydriding temperature of the composite is determined to be 413 °C in 0.1 MPa Ar atmosphere and can be as low as 80 °C under vacuum conditions. Analyses revealed that NdB6, Nd2H5 and NaF were formed after the decomposition of the composite, which can be hydrogenated to produce NaBH4 and NaNdF4. The formation of NaNdF4 instead of NdF3 in the hydrogenated products is believed to be responsible for the reduced hydrogen storage capacity, while the intermediate formation of B, Nd and NdB4 during dehydrogenation accounts for the asymmetric hydriding/dehydriding behaviors in the 3NaBH4/NdF3 composite.