2Mg-Fe Mixtures Research Articles

The Effect of Air Exposure on the Hydrogenation Properties of 2Mg-Fe Composite after Mechanical Alloying and Accumulative Roll Bonding (ARB)

In this study, we successfully obtained a 2Mg-Fe mixture through mechanical alloying (MA) and processed it via accumulative roll bonding (ARB) (MA+ARB). Our primary focus was to analyze the impact of ambient air exposure while also evaluating the processing route. Some powder samples were exposed to air for 12 months (stored in a glass desiccator with an average yearly temperature and relative humidity of ~27 °C and 50.5%) before undergoing ARB processing. The Mg samples obtained after ARB processing exhibited a (002)-type texture. Our results demonstrate that all samples, including those processed via ARB, could rapidly absorb hydrogen within a matter of minutes despite considerable differences in surface area between powders and rolled samples. Grain size reduction by MA and ARB processing and texturing may have influenced this behavior. ARB-processed samples reached approximately 60% (~1.8 wt.%) of their maximum acquired capacity within just 24 min compared to powders (~2.2 wt.%) stored for a year, which took 36 min. In addition, the desorption temperatures (~300 °C) were lower than those of MgH2 (~434 °C). The absorption and desorption kinetics remained fast, even after prolonged exposure to air. Although there were minor variations in capacities, our overall findings are promising since scalable techniques such as ARB have the potential to produce hydrogen storage materials that are both safe and cost-effective in a highly competitive market.

Molecular Dynamics Investigation into the Hydrogen Adsorption Mechanism of the Magnesium-Based Material 2Mg–Fe Mixture

Molecular Dynamics Investigation into the Hydrogen Adsorption Mechanism of the Magnesium-Based Material 2Mg–Fe Mixture

Hydrogen storage properties of 2 Mg–Fe mixtures processed by hot extrusion: Effect of ram speeds

Hot extrusion processing was used for producing bulk samples of 2MgFe mixtures adequate to hydrogen storage. High-energy ball milling was used to prepare 2MgFe powder mixtures, which were cold-pressed into cylindrical pre-forms. Such pre-forms were then processed by hot extrusion (at 300 °C and extrusion ratio of 3/1) to produce bulk samples. In this work, it was analyzed the influence of the ram speed (1, 5, and 10 mm/min) on the microstructure and hydrogen sorption properties of obtained bulks. Nanograins, which resulted from the milling process, remained in the nanosize after hot extrusion conditions. More porous samples were produced at a ram speed of 1 mm/min, which also kept the smaller grain size. These features led the sample processed at 1 mm/min to absorb about 4.8 wt% of H, which was higher than precursor powders that absorbed around 4.3 wt% of H. This behavior was assigned to a redistribution of Fe during extrusion, which was kept agglomerated in the powders. The results also showed that desorption temperatures of bulks were very similar to that of 2MgFe powders. Such behavior is very interesting, considering the lower surface area of bulks. When compared to MgH2 powders, samples processed in this work presented much lower desorption temperatures. Except for the sample processed at 10 mm/min, all other samples fully desorbed in less than 8 min at 350 °C, while commercial MgH2 would take much more than 40 min for complete desorption.

Hydrogen storage properties of 2Mg-Fe mixtures processed by hot extrusion at different temperatures

2Mg-Fe mixtures produced by high-energy ball milling were consolidated into bulk form by hot extrusion at different processing temperatures (573 K (300 °C), 623 K (350 °C) and 673 K (400 °C)), aiming to evaluate their influence on the structure and microstructure of bulk materials and their consequent influence on the hydrogen sorption properties. In spite being in the nanosize range, the highest the processing temperature, the larger the grain sizes. However, the nanometric grain size remained after any hot extrusion condition, as estimated by Rietveld refinement. The pinning effect of Fe on Mg grain boundaries explained this effect. In the first absorption (activation), powders showed a hydrogen storage capacity of ∼4.53 wt%, while the extruded samples (bulk materials) reached almost the same capacity during the period of hydrogenation (∼94% of the maximum hydrogen storage capacity for Mg2FeH6 - 5.5 wt%). The smallest crystallite sizes and highest surface area for hydrogenation explain the good performance of powders. However, when comparing only extruded samples, it was observed that the highest capacity and the lowest incubation times were mainly related to grain sizes and to the favorable texture along (002) plane of αMg. The desorption temperature of bulk materials was very similar to that of powders, which is good considering the lower surface area of bulk materials.

Hydrogen storage properties of 2Mg–Fe mixtures processed by hot extrusion: Influence of the extrusion ratio

Abstract Hot extrusion processing was employed to produce 2Mg–Fe bulk mixtures for hydrogen storage. 2Mg–Fe powder mixtures were prepared by high-energy ball milling. These mixtures were cold pressed into cylindrical pre-forms, which were then processed by hot extrusion (at 300 °C) to produce bulks. In this work, we analyzed the influence of the extrusion ratio (3/1, 5/1 and 7/1) on the sorption properties of the bulks. The nanometric grain size remained unaltered after all hot extrusion conditions. More porous bulks were produced at an extrusion ratio of 3/1. In the first cycle of hydrogenation, the sample processed at 3/1 absorbed more hydrogen (4 wt% of H) than the precursor powders (3 wt% of H). The results showed that the desorption temperature of bulks were very similar to that of 2Mg–Fe powders, which is good considering the lower surface area of bulks, and that samples with Fe in excess presented lower desorption temperatures.

Mg2FeH6-based nanocomposites with high capacity of hydrogen storage processed by reactive milling

The compound Mg2FeH6 was synthesized from a 2Mg-Fe mixture in a single process through high-energy ball milling under hydrogen atmosphere at room temperature. The complex hydride was prepared from Mg powder and granulated or powdered Fe using a planetary mill. The phase evolution during different milling times was performed by X-rays diffraction technique. The dehydrogenation behavior of the hydride was investigated through simultaneous thermal analyses of differential scanning calorimetry and thermogravimetry coupled with mass spectrometer. The use of powdered iron as starting material promoted conversion to complex hydride at shorter milling times than when granulated iron was used, nevertheless, after 24 hours of milling the 2Mg-Fe (powdered or granulated) mixtures presented similar dehydrogenation behavior. The hydrogen absorption during milling was on average 3.2 wt. (%), however, changing the proportions of the reagents to 3Mg-Fe a Mg2FeH6-MgH2 based nanocomposite with higher density of hydrogen (5.2 wt. (%)) was obtained.

High-Yield Direct Synthesis of Mg<sub>2</sub>FeH<sub>6</sub> from the Elements by Reactive Milling

Magnesium complex hydrides as Mg2FeH6 are interesting phases for hydrogen storage in the solid state, mainly due to its high gravimetric and volumetric densities of H2. However, the synthesis of this hydride is not trivial because the intermetallic phase Mg2Fe does not exist and Mg and Fe are virtually immiscible under equilibrium conditions. In this study, we have systematically studied the influence of the most important processing parameters in reactive milling under hydrogen (RM) for Mg2FeH6 synthesis: milling time, ball-to-powder weight ratio (BPR), hydrogen pressure and type of mill. Low cost 2Mg-Fe mixtures were used as raw materials. An important control of the Mg2FeH6 direct synthesis by RM was attained. In optimized combinations of the processing parameters, very high proportions of the complex hydride could be obtained.

Hydrogen Sorption Properties of the Complex Hydride Mg<sub>2</sub>FeH<sub>6</sub> Consolidated by HPT

In the present work, we have processed 2Mg-Fe mixtures by reactive milling (RM) under hydrogen atmosphere to synthesize Mg2FeH6 phase in the powder form which were then systematically processed by High Pressure Torsion (HPT) to produce bulk samples. The bulk samples were characterized in terms of microstructural and structural analyses and of hydrogen desorption properties. The hydrogen sorption properties after HPT processing was evaluated in comparison with the Mg2FeH6 powder obtained by RM and with commercial MgH2. HPT processing of Mg2FeH6 can produce bulks with a high density of defects that drastically lower the activation barrier for hydrogen desorption. Therefore, the bulk nanocrystalline Mg2FeH6 samples show endothermic hydrogen decomposition peak at a temperature around 320°C. In addition, when compared with the Mg2FeH6 and MgH2 powders, the Mg2FeH6 HPT disks showed the same results presented by the Mg2FeH6 powders and certainly decreases the onset transition temperature by as much as 160°C when compared with the MgH2 powders.

Thermodynamic and kinetic studies of Mg–Fe–H after mechanical milling followed by sintering

Abstract The transition metal complex hydride Mg2FeH6 has been successfully synthesized utilizing mechanical milling of a 2Mg–Fe mixture followed by heating at 673 K under 6 MPa of hydrogen pressure, without pressing step. The obtained yield of Mg2FeH6 was about 50%. Hydrogen storage properties of the Mg–Fe–H system, i.e. capacities, absorption/desorption kinetics and thermodynamic parameters, were examined. The pressure-composition isotherm (PCI) measurements of the samples at 548–673 K showed that the alloys possessed good cyclic stability and reversibility. Enthalpies and entropies of decomposition of the Mg–Fe–H system were evaluated by van’t Hoff plots. The absorption/desorption rates at 573 K were very fast in comparison with the reported data. The non-isothermal desorption of hydrogen was found greatly dependent on the thermal history of the sample.

Mechanical alloying and electronic simulations of 2Mg–Fe mixture powders for hydrogen storage

2Mg–Fe powder mixture was milled in hydrogen atmosphere to prepare materials for hydrogen storage. The microstructure and dehydrogenating property of the milled system were examined. Then energy and electronic structure of the hydride phase were investigated by using a first-principles plane-wave pseudopotential method to explain the experimental results from XRD, DSC and TGA analysis. By calculating the heats of formation of MgH 2, (MgFe)H 2 solid solutions and Mg 2FeH 6 compound, the forming process of Mg 2FeH 6 was assumed as that magnesium was first dissolved in some H atoms to form MgH 2, second, iron atom progressively dissolved into MgH 2 lattice to form (MgFe)H 2 solid solutions, finally the (MgFe)H 2 solid solution can be changed into Mg 2FeH 6 phase. For DSC curves as a function of temperature for all the powders milled for different time, a drastic change of dissociation temperature is never found, this kind of relatively sluggish change can be explained using the idea on forming process of Mg 2FeH 6. Since MgH 2 phase is still the major phase in the coexistence mixtures of two types of hydrides β-MgH 2 and Mg 2FeH 6, Mg 2FeH 6 reduces the negative formation heat of the mixture compared with that of MgH 2, which indicates that Mg 2FeH 6 lowers the desorption temperature of MgH 2 through a catalytic effect and the milled powder containing duplex β-MgH 2 + Mg 2FeH 6 exhibits the most rapid desorption rate among all the powders studied from TGA analysis.

Formation of Nanocrystalline and Amorphous Hydrides during Controlled Reactive Mechanical Alloying (CRMA) of 2Mg-Fe Mixture

Formation of Nanocrystalline and Amorphous Hydrides during Controlled Reactive Mechanical Alloying (CRMA) of 2Mg-Fe Mixture

Environmental degradation by hydrolysis of nanostructured β-MgH 2 hydride synthesized by controlled reactive mechanical milling (CRMM) of Mg

Controlled reactive mechanical milling (CRMM) of Mg powder under “ shearing” mode in hydrogen gas for 26.5, 50, 70 and 100 h results in the formation of nanostructured β-MgH 2 hydride. Based on the comparison of integrated X-ray diffraction (XRD) intensities of Mg and β-MgH 2 peaks, it is postulated that during reactive milling up to 50 h the crystalline Mg is mostly consumed to form β-MgH 2. However, from 50 to 100 h the crystalline Mg mostly forms an amorphous phase with only a small fraction of it being consumed for the β-MgH 2 creation. The formation of amorphous Mg was also observed in the 2Mg–Fe mixture subjected to controlled reactive mechanical alloying (CRMA) in hydrogen. A massive formation of Mg(OH) 2 hydroxide is observed by XRD in the powders reactively milled in hydrogen and subsequently exposed to the ambient environment for about 4 months. The formation of Mg(OH) 2 occurs due to hydrolysis of nanostructured β-MgH 2. Abnormally high weight losses on the order of ∼16–24 wt.% are observed during thermogravimetric analysis (TGA) of powders containing Mg(OH) 2 which confirm the release of water from Mg(OH) 2 upon heating. Also, differential scanning calorimetry (DSC) curves show endothermic peaks corresponding to the release of water from Mg(OH) 2 which are in excellent agreement with DSC peaks corresponding to the release of water from Mg(OH) 2 in the 2Mg–Fe 10 h reactively milled and “aged” mixture.

Formation and environmental stability of nanocrystalline and amorphous hydrides in the 2Mg–Fe mixture processed by controlled reactive mechanical alloying (CRMA)

The synthesis of the Mg 2FeH 6 complex hydride has been attempted by controlled reactive mechanical alloying (CRMA) of 2Mg–Fe powder mixture under hydrogen using shearing mode in a magneto-mill Uni-Ball-Mill 5. The first batch of 2Mg–Fe powder was initially mechanically pre-alloyed by milling under argon with the addition of small amount of toluene and subsequently reactively milled under hydrogen. The second batch was directly milled under hydrogen. No formation of Mg 2FeH 6 has been observed even after the longest milling duration of 59 h. In the powders reactively alloyed directly under hydrogen some quantity of nanocrystalline β-MgH 2 hydride was formed during the first 10–15 h of milling. However, the X-ray diffraction (XRD) peaks due to β-MgH 2 disappeared after further milling up to 59 h. In addition, the XRD peaks corresponding to the nanocrystalline Mg completely disappeared after 20–24 h of CRMA under hydrogen in both batches of powders while strong but broadened peaks from Fe were still visible. The absorption of hydrogen from the milling vial occurred continuously up to 20–24 h of milling. Energy dispersive spectroscopy (EDS) analysis clearly showed that Mg was still present in the microstructure of powder particles after milling up to 59 h for both batches of 2Mg–Fe powder. These results clearly indicate that amorphous Mg and amorphous hydrides are being formed after prolonged CRMA under shearing mode. It has been shown for the first time that β-MgH 2 undergoes hydrolysis into Mg(OH) 2 when it is exposed for several months to the moisture in the environment. This is associated with abnormally high weight losses (∼10 wt.%) measured by thermogravimetric analysis (TGA) and the appearance of two endothermic differential scanning calorimetry (DSC) peaks related to the desorption of hydrogen from β-MgH 2 and release of water from Mg(OH) 2. Other powders after prolonged milling showed TGA weight losses on the order of 2–4 wt.% and only a single DSC peak, which provides additional strong evidence that amorphous hydrides were formed during CRMA. During DSC tests up to 500 °C the amorphous Mg crystallized into the nanocrystalline Mg phase.