Mechanism Of Mg Research Articles

Effect of cations in chloride solutions on low-temperature hydrolysis performances and mechanisms of Mg–Ca-based hydride

Hydrogen generation via hydrolysis can provide fuel cell with the on-site and controllable supply. However, the current hydrolysis materials that exhibit poor performances at low temperatures are not suitable for portable devices and military. In this study, influence of different cations (Na+, Ca2+, Mg2+ and NH4+) in chloride solutions on low-temperature (253, 263, 273, 283K) hydrolysis performances of Mg–Ca based hydride (MCH) and mechanisms of interaction are investigated. The results demonstrate that Mg2+ and NH4+ in chloride solutions significantly enhance the rate of hydrolysis reaction between MCH and water. In NH4Cl solution, MCH exhibits the best initial hydrolysis performance and the highest H2 yield, 1198 mL g−1(MCH) of H2 is generated within the first 5 min and the final conversion rate reaches 89% at 273 K. MCH displays the best hydrolysis dynamics in MgCl2 solution, where the activation energy is low, only 3.56 kJ mol−1. This can be attributed to the fact that the types of hydroxide products and the corresponding physical states with respect to the hydrolysis of MCH in various chloride solutions differ with the variations of OH− affinity toward Mn+ cations (i.e. pK/(n+1)) when pH values of chloride solutions are altered. Thus, different low-temperature hydrolysis performances can be observed. Above all, the remarkable low-temperature hydrolysis performances of MCH–MgCl2 and MCH–NH4Cl systems indicate that both systems are promising candidates for supplying H2 for the portable and military fuel cells at various remote locations, especially in areas with low temperatures.

PEG-assisted synthesis and formation mechanism of Mg(OH)2 nanostructures using natural brine

In this study, magnesium hydroxide nanoflakes (MHF) were synthesized using poly(ethylene glycol) (PEG 4000) surfactant by exploiting chemical precipitation method from an impure brine enriched by large amounts of CaCl2 for the first time. Synthesized samples were characterized by X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Transmission Electron Microscopy (TEM), Energy Dispersive X-ray spectroscopy (EDS), Fourier Transform Infrared Spectroscopy (FTIR) and UV–Vis absorption spectroscopy. The effect of PEG on structural and morphological characteristics of synthesized samples, as well as the formation mechanism of MH nanostructures from the brine in the presence or absence of PEG (0, 5 and 10 mL), were precisely investigated. Obtained results implied the positive effect of the used surfactant on the uniformity of the morphology, the agglomeration behavior of MH nanoplates, and the decrease of the crystallite size (from 25.26 to 19.35 nm) of synthesized nanoparticles from the brine, which the optimum amount of this surfactant was 5 mL. In addition, UV–Vis spectroscopy results and investigation of optical properties showed that the presence of PEG led to an increasing in the value of the optical bandgap energy of MHF from 4.7 to 5 eV, indicating their ability to be used in optoelectronic nano-devices.

Thermochemical transformation and reversible performance of Mg(NH2)2–NaMgH3 system

Reversible hydrogen uptake of Mg(NH2)2–MgH2 system has been achieved by the introduction of Na for the first time. The thermochemical and mechanochemical transformations of Mg(NH2)2-xNaMgH3 (x = 0.5, 1 and 2) composites have been investigated through structural characterizations and gas desorption analyses. The hydrogenation property and reaction mechanism of Mg(NH2)2–NaMgH3 have been discussed. Experimental results indicate that the replacement of NaMgH3 for MgH2 can extensively suppress ammonia emission and improve the hydrogen desorption property of Mg(NH2)2–MgH2 system. For Mg(NH2)2–NaMgH3 system, its thermochemical reaction at low temperature is quite similar to the reaction under mechanochemical process. At high temperature (190 °C), the formation of NaMg(NH2)(NH) is detected. The introduction of Na alters the reaction route and enhances the reaction enthalpy of Mg(NH2)2–MgH2 system. The compound of NaMg(NH2)(NH) is synthesized by the reaction of MgNH and NaNH2 and it can reversibly store 1.8 wt% of hydrogen at 10 atm and 190 °C. The reversible cycling between NaMg(NH2)(NH) and Mg(NH2)2 is responsible for the partial reversible hydrogenation of Mg(NH2)2–NaMgH3 system.

Magnesium decreases aluminum accumulation and plays a role in protecting maize from aluminum-induced oxidative stress

Acidic soils with a pHwater below 5.5 occupy up to 40% of world’s arable land. Aluminum (Al) toxicity and magnesium (Mg) deficiency often coexist in acidic soils, limiting crop production. In this study, we investigated a role of Mg in alleviation of Al stress in maize (Zea mays) by characterizing the growth responses, Al accumulation and the antioxidant properties of plants cultured hydroponically. Physiological and molecular analyses were used to investigate the mechanisms of Mg governing the alleviation of Al-induced ROS production and root growth inhibition. Aluminum (50 μM) decreased root growth and induced oxidative stress. Exogenously added millimolar concentrations of Mg significantly alleviated Al toxicity as evidenced by restoration of plant growth, suppression of Al uptake, and a decline in root H2O2 concentration. Furthermore, the addition of Mg to the Al treatment solution enhanced the activities and expression of genes encoding superoxide dismutase, catalase and peroxidase compared to the Al-only treatment. The results indicate that Mg plays a role in alleviation of Al toxicity, reduction of Al accumulation and protection from Al-induced oxidative stress through activation of antioxidative enzymes.

The creep behavior of Mg–9Al–1Si–1SiC composite at elevated temperature

Abstract The creep properties of as-cast Mg–9Al–1Si alloy and Mg–9Al–1Si–1SiC composite were compared. The results show that Mg–9Al–1Si–1SiC composite performs a better creep resistance than that of Mg–9Al–1Si alloy at constant temperature and stress (473 K,70 MPa). Besides, the creep behavior of Mg–9Al–1Si–1SiC composite at various temperature from 448 K to 498 K and under stresses of 70–90 MPa were systematically investigated. The Mg–9Al–1Si–1SiC composite exhibited a stress exponent from 5.5 to 6.9 and the creep activation energy fell within the range of 86–111 kJ/mol. The results showed that the creep mechanism of Mg–9Al–1Si–1SiC composite was mainly attributed to the effects of secondary phase strengthening mechanism and dislocation climb mechanism.

The roles of native defects and transition metal additives in the dehydrogenation mechanism of Mg(AlH4)2

The roles of native defects and transition-metal additives (Ti, Sc and Ni) in the dehydrogenation of Mg(AlH4)2 hydride were investigated by First-principles calculations based on density functional theory. The elementary native defects including Hi−, VH+, VH−, (H2)i0, VMg2−, Mgi2+, and VAlH4+ in Mg(AlH4)2 were identified. Based on the formation and migration of dominant defects, we proposed a dehydrogenation mechanism of Mg(AlH4)2. The formation of the dominant defect VAlH4+ is the rate-limiting step in the dehydrogenation process. The highly mobile Hi− diffuses into the lattice and binds with Mg2+ to produce MgH2 phase. In the transition metal doped hydrides, interstitial defects Tii and Sci cause the Fermi-level shift to the left and lead to the decrease of the formation energy and activation energy of VAlH4+, which is beneficial to the dehydrogenation of Mg(AlH4)2. This study presents an in-depth understanding on the roles of native defects and transition metal additives in the dehydrogenation process of Mg(AlH4)2 hydride.

Factors affecting the leaching mechanism of Mg bearing oxide impurities in Hydrogen Assisted Magnesiothermic Reduction (HAMR) of TiO2

Titanium production by a direct TiO2 reduction route which is highly energy-efficient and environment-friendly compared to commercial Kroll's process, known as HAMR (Hydrogen Assisted Magnesiothermic Reduction) process has been developed in recent times to meet the growing global demands of low-cost pure Ti. Removal of the Mg compounds after each reduction and de-oxygenation step in the HAMR process by leaching, is essential to ensure good purity and mechanical properties of the final Ti powder. In this study, we investigate the effects of temperature, particle size, and pH on the dissolution kinetics of Mg from Mg-bearing compounds such as MgO and MgCl2 that remain after the HAMR Process. We also report the underlying mechanism and rate-controlling steps of Mg removal using HCl in connection with the HAMR process. The dissolution kinetics followed a logarithmic rate kinetic model for diffusion through a porous medium that is commonly used for battery research. Kinetics of leaching were significantly improved using finer particles with 99.96 wt.% Mg removal and corresponding Ti loss of only 1.8 wt. %. The rate of dissolution exhibited a weak dependence with the pH and did not abide by the rate-controlling steps of dissolution of ionic oxides like MgO proposed by previous kinetic models.

The preparation of high purity MgO and precision separation mechanism of Mg and Ca from dolomite

High purity MgO is widely used in various fields as an important magnesium compound. The separation of Mg and Ca is of great importance for the preparation of high purity MgO from dolomite. In this study, the key effects of carbonization temperature and nano-calcium carbonate agglomeration on the separation mechanism of calcium and magnesium have been investigated. The results indicated that the reaction in the carbonization process was stepwise. Precise control of the carbonization temperature was essential for the formation of Mg(HCO3)2 due to its low reaction efficiency at lower temperatures or decomposition markedly at higher temperatures. Meanwhile, there was a common-ion effect in solutions containing Mg2+ and Ca2+. Nano-CaCO3 particles were formed during carbonization and agglomerated with the prolongation of static time. A product with a purity of 99.56% MgO and 0.16% CaO was obtained under the optimized conditions: ratio of liquid–solid 30 L/g, carbonization temperature 22–25 °C, carbonization time 1 h, and refining time 4 h. This work has important guiding significance for the preparation of high purity magnesium oxide from dolomite.

Research on the evolution mechanism of pinned particles in welding HAZ of Mg treated shipbuilding steel

The large heat input welding performance of shipbuilding steel could be improved effectively by Mg treated in steelmaking process, but the mechanism of Mg on the nano-scale pinned particles had not been figured out yet, and this is also the bottleneck for Mg oxide metallurgy developing. In this research, with Mg-treated shipbuilding steel produced by industrial trials, the systematic experiments and theoretical analysis of thermodynamics and crystallography had been done. The results showed that, with increased welding energy, as the core pinned particles, the quantity of particles between 40 nm and 120 nm was stable when the welding energy was <250 kJ/cm. During welding process, a number of reversible chemical reactions involving [Mg] and MgO generation or consumption occurred in the heat affect zone for Mg treated shipbuilding steel, and the temperature for more MgO formation just meet the thermodynamic conditions that the solid solution behavior of pinned particles TiN started to accelerate. Due to the extremely low disregistry value between MgO and TiN, the newly generated MgO which was determined by the [Mg] content in steel was highly likely to precipitate on the original TiN particles, delayed the solid solution of TiN, and generated more stable pinned particles.

Nanowear Mechanisms of Mg Alloyed with Al and Y at Elevated Temperatures

We examine the role of important alloying elements (Al, Y), on friction and wear mechanisms of Mg as a function of temperature. Friction and wear tests of Mg–5%Al and Mg–5%Y alloys performed at room temperature, 100 °C and 150 °C reveal that addition of Al and Y significantly reduces the coefficient of friction. However, despite higher hardness of Mg–5%Y alloy, its wear rate was found to be higher than Mg–5%Al alloy at elevated temperatures. We show that this behaviour, observed for Mg–5%Y alloy at elevated temperature, can be attributed to its higher surface energy than Mg–5%Al and pure Mg, leading to enhanced adhesive wear, which is detrimental to tribological applications.

Dual phases strengthening behavior of Mg–10Gd–1Er–1Zn–0.6Zr alloy

Abstract The microstructure evolution and strengthening mechanisms of Mg–10Gd–1Er–1Zn–0.6Zr (wt.%) alloy were focused in the view of the size parameters and volume fraction (fp) of dual phases (long period stacking ordered (LPSO) structures and β′ precipitates). Results show that two types of LPSO phases with different morphologies formed, and the morphology and size of both LPSO phases varied with the solution conditions. However, the volume fraction decreased monotonously with increasing solution temperature, which in turn raised the volume fraction of β′ phase during aging. The alloy exhibited an ultimate tensile strength of 352 MPa, a yield strength of 271 MPa, and an elongation of 3.5% after solution treatment at 500 °C for 12 h and aging at 200 °C for 114 h. In contrast to the LPSO phase, the β′ phase seems to play a more important role in enhancing the yield strength, and consequently, a decreased fLPSO/fβ′ ratio results in an increased yield strength.

Magnesium alleviates aluminum toxicity by promoting polar auxin transport and distribution and root alkalization in the root apex in populus

Populus can tolerant high concentration Al stress. However, the mechanisms of Mg alleviation to Al toxicity in populus remain unknown. In the present study, adequate Mg was supplied to poplar under high concentration Al toxicity stress, and the pH gradients of root surface, Mg and Al uptake from root apex were detected using electrophysiological analysis, fluorescence staining, etc. Furthermore, auxin distribution and transport dynamic kinetics in root tips were also detected. In the present study, we found that adequate supply of Mg alleviated high concentration Al toxicity to populus root growth. Mg promoted the pH gradients of root surface in populus, leading to alkalization in the root transition zone, which prevented Al toxicity to root elongation. The auxin distribution in root tips of populus was detected using DR5:GFP reporter. The transport and distribution of polar auxin in root transition zone were inhibited by Al toxicity, but partially alleviated by Mg supply. Further electrophysiological experiments using auxin transporter mutant pin-formed 2 (pin2) showed that Mg alleviated Al toxicity by PIN2-based polar auxin transport, which regulated root surface alkalization in the transition zone. Transcriptomic analysis and yeast complementation experiments using mrs2 mutants also reveals same relationships among Mg, auxin and Al. The present study suggests that Mg promotes polar auxin transport and distribution, which leads to the elevated root surface pH regulation, and further alleviates the Al toxicity. Our results partially clarify the mechanism of Mg in alleviating the toxicity of high concentration Al in woody plants.

Tuning the crystal and electronic structure of Li4Ti5O12 via Mg/La Co-doping for fast and stable lithium storage

“Zero-strain” Li4Ti5O12 has become one of the most promising anode materials for lithium-ion battery but its low electronic and ionic conductivity lead to the poor rate capability. Herein, the high-rate performance and the cycling stability of Li4Ti5O12 have been largely enhanced by replacing Li and Ti with a little amount of Mg and La, respectively. The synergistic modulation mechanism of Mg and La co-doping on the crystal/electronic structure and electrochemical performances has been unveiled. Firstly, Mg and La co-doping enlarges the lattice parameters, unit cell volume and Li1–O bond. These facilitate the lithium-ion migration and enhance the rate capability. Secondly, Ti–O bond is shortened which enhances the structure stability and cyclic performance. Thirdly, the first-principles calculations further confirm that Mg/La co-doping modulates the electronic density of states and decreases the Li+ migration barrier. The polarization and charge transfer impedance are effectively alleviated. Moreover, the diffusion coefficient of lithium ions is further improved because of the reduction of much more Ti3+. At 10C, it delivers a discharge capacity of 107.8mAh/g after 500cyles which reserves 92.2% of the initial capacity. This study provides some insights into optimizing the electrochemical performances of Li4Ti5O12 by tuning the crystal and electronic structure with lattice doping.

Texture and Lattice Strain Evolution during Tensile Loading of Mg–Zn Alloys Measured by Synchrotron Diffraction

To explore the effect of neodymium (Nd) on the deformation mechanisms of Mg–Zn alloys, texture and lattice strain developments of hot-rolled Mg–Zn (Z1) and Mg–Zn–Nd (ZN10) alloys were investigated using in situ synchrotron diffraction and compared with elasto-viscoplastic self-consistent simulation under tensile loading. The Nd-containing ZN10 alloys show much weaker texture after hot rolling than the Nd-free Z1 alloy. To investigate the influence of the initial texture on the texture and lattice strain evolution, the tensile tests were carried out in the rolling and transverse direction. During tension, the {002}&lt;100&gt; texture components develop fast in Z1, which was not seen for ZN10. On the other hand, &lt;100&gt; fiber // loading direction (LD) developed in both alloys, although it was faster in ZN10 than in Z1. Lattice strain investigation showed that &lt;101&gt; // LD-oriented grains experienced plastic deformation first during tension, which can be related to basal slip activity. This was more apparent for ZN10 than for Z1. The simulation results show that the prismatic slip plays a vital role in the plastic deformation of Z1 directly from the beginning. In contrast, ZN10 plastic deformation starts with dominant basal slip but during deformation prismatic slip becomes increasingly important.

Microstructure and strengthening mechanism of Mg—5.88Zn—0.53Cu—0.16Zr alloy solidified under high pressure

Mg—5.88Zn—0.53Cu—0.16Zr (wt.%) alloy was solidified at 2—6 GPa using high-pressure solidification technology. The microstructure, strengthening mechanism and compressive properties at room temperature were studied using SEM and XRD. The results showed that the microstructure was refined and the secondary dendrite spacing changed from 35 μm at atmospheric pressure to 10 μm at 6 GPa gradually. Also, Mg(Zn,Cu)2 and MgZnCu eutectic phases were distributed in the shape of network, while under high pressures the second phases (Mg(Zn,Cu)2 and Mg7Zn3) were mainly granular or strip-like. The solid solubility of Zn and Cu in the matrix built up over increasing solidification pressure and reached 4.12% and 0.32% respectively at 6 GPa. The hardness value was HV 90 and the maximum compression resistance was 430 MPa. Therefore, the grain refinement strengthening, the second phase strengthening and the solid solution strengthening are the principal strengthening mechanisms.

The mechanism of Mg2+ conduction in ammine magnesium borohydride promoted by a neutral molecule.

Light weight and cheap electrolytes with fast multi-valent ion conductivity can pave the way for future high-energy density solid-state batteries, beyond the lithium-ion battery. Here we present the mechanism of Mg-ion conductivity of monoammine magnesium borohydride, Mg(BH4)2·NH3. Density functional theory calculations (DFT) reveal that the neutral molecule (NH3) in Mg(BH4)2·NH3 is exchanged between the lattice and interstitial Mg2+ facilitated by a highly flexible structure, mainly owing to a network of di-hydrogen bonds, N-Hδ+-δH-B and the versatile coordination of the BH4- ligand. DFT shows that di-hydrogen bonds in inorganic matter and hydrogen bonds in bio-materials have similar bond strengths and bond lengths. As a result of the high structural flexibiliy, the Mg-ion conductivity is dramatically improved at moderate temperature, e.g. σ(Mg2+) = 3.3 × 10-4 S cm-1 at T = 80 °C for Mg(BH4)2·NH3, which is approximately 8 orders of magnitude higher than that of Mg(BH4)2. Our results may inspire a new approach for the design and discovery of unprecedented multivalent ion conductors.

Role of Cu current collector on electrochemical mechanism of Mg–S battery

Development of magnesium sulfur battery is accompanied with all known difficulties present in Li–S batteries, however with even more limited choice of electrolytes. In the present work, the influence of current collector on electrochemical mechanism was investigated in light of different reports where improved behavior was ascribed to electrolyte. Notable differences in cycling behavior are reported when Al current collector is replaced by Cu current collector independent of electrolyte. The initial reduction of sulfur follows similar reaction path no mater of current collector, but formation of MgS can be in competition with formation of CuS in the presence of Cu cations. With the subsequent cycling cells prepared from cathodes deposited on Cu current collector show decrease in the voltage and formation of single plateau during cycling. The change corresponds to the involvement of Cu into the reaction and formation of redox couple Mg/CuS as determined by Cu K-edge XANES measurements. Corrosion of Cu foil is identified by SEM and serves as a source of Cu cations for the chemical reaction between Cu and polysulfides. Mg/CuS redox couple shows improved cycling stability, but theoretical energy density is severely reduced due to substitution of S with CuS as cathode active material.

The relation between two ductility mechanisms for Mg alloys revealed by high-throughput simulations

Understanding the deformation mechanisms of Mg has been a long-standing and significant problem for materials scientists. Currently, there are two widely acknowledged ductility-improving mechanisms, i.e., the dislocation nucleation mechanism and dislocation mobility mechanism. Since the two mechanisms were previously investigated using very different solute sets, hitherto there is no clear understanding of the relation between them. Based on the newly proposed descriptors for the mechanisms, high-throughput calculations are performed for 63 solutes. Our results confirm that the two mechanisms are strongly correlated. In most of the cases, the same solute has the same effect on the deformation through both mechanisms.

Predicting Twin Nucleation in a Polycrystalline Mg Alloy Using Machine Learning Methods

Twinning is an important deformation mechanism for Mg and other hexagonal metals. While twin nucleation is known to depend on the size and crystal orientation of the parent grain, so far there is no satisfactory criterion to predict twin nucleation in a polycrystalline Mg alloy prior to its deformation. In this work, machine learning is employed to tackle this problem. From a single-phase, polycrystalline Mg-0.47 wt pct Ca extruded alloy, three micro-tensile specimens, E-0, E-45, and E-90, were fabricated with their tensile axis being 0, 45, and 90 deg from the extrusion direction. Each specimen was deformed by 4 pct tensile strain in a scanning electron microscope. Six hundred thirty-six grains from E-0, 572 grains from E-45, and 840 grains from E-90 were characterized by electron backscattered diffraction (EBSD) before and after deformation. Twin nucleation was identified in 27, 150, and 220 grains in E-0, E-45, and E-90, respectively. Eighteen attributes that can influence twin nucleation, such as grain diameter, c-axis direction, and Schmid factors, were computed for each grain. Five machine learning algorithms, including decision tree, tree ensemble (XGBoost), artificial neural network (ANN), support vector machine (SVM), and naive Bayes, were used to build models to predict twin nucleation according to a grain’s 18 attribute values, with E-45 as the training set and the other two specimens as the test sets. The ANN and SVM models show the best performance, both achieving ~ 87 pct prediction accuracy for specimens E-45 and E-90. None of the models perform well for E-0 because of the imbalanced class distribution in this specimen. The unpredicted twin nucleation events in E-90 mostly originate from triple junctions or twin–twin transmission at grain boundaries.

A novel evolution mechanism of Mg–Al-oxides in liquid steel: Integration of chemical reaction and coalescence-collision

Understanding the evolution of non-metallic inclusions is important for optimizing steel properties. Conventional mechanisms propose that evolution of inclusions from solid to liquid is governed by chemical reactions. Meanwhile, conventional approaches of evolution description mainly focus on thermodynamic. The present work proposed that thermodynamic consideration is insufficient to fully describe the evolution of inclusions from solid to liquid. The kinetic interactions among inclusions provide significant influence on the inclusion evolution as well. Here we present a novel mechanism of inclusion evolution by combining both thermodynamic and kinetic techniques. The evolution of the Mg–Al-oxides in steel was studied by performing an industrial trial. The time-dependent compositions of steel, slag and inclusions were thermodynamically predicted with an “effective equilibrium reaction zone (EERZ)” model. The kinetic interactions among solid and liquid inclusions were studied based on a film-drainage concept and a phase-field method. The results show that the formation of the Ca–Al-(Mg)-oxide liquid layers on the Mg–Al-oxides is due to both impacts of chemical reaction and coalescence-collision. The liquid Ca–Al-(Mg)-oxides have high coalescence efficiency (E2) that is independent of inclusion size. Small solid Mg–Al-oxides can be much more easily attached by large liquid Ca–Al-(Mg)-oxides, which lead to the evolution of inclusions. A large initial contact area between solid and liquid inclusions facilitates the initial attachment after coalescence-collision. The presented theory fills the gap of kinetic influence on inclusion evolution from solid to liquid. The proposed mechanism and methodology can be extended to study general system of solid-liquid inclusions.