Highly ameliorated gaseous and electrochemical hydrogen storage dynamics of nanocrystalline and amorphous LaMg12-type alloys prepared by mechanical milling

Nanocrystalline and amorphous LaMg12-type alloy-Ni composites with a nominal composition of LaMg11 Ni+x wt.% Ni (x=100, 200) were synthesized via ball milling.The influences of ball mill-ing duration and Ni adding amount x on the gaseous and electrochemical hydrogen storage dynamics of the alloys were systematically studied.Gaseous hydrogen storage performances were studied by a differential scanning calorimeter and a Sievert apparatus.The dehydrogenation activation energy of the alloy hydrides was evaluated by Kissinger method.The electrochemical hydrogen storage dynam-ics of the alloys was investigated by an automatic galvanostatic system.The H atom diffusion and ap-parent activation enthalpy of the alloys were calculated.The results demonstrate that a variation in Ni content remarkably enhances the gaseous and electrochemical hydrogen storage dynamics perform-ance of the alloys.The gaseous hydriding rate and high-rate discharge (HRD) ability of the alloys ex-hibit maximum values with varying milling duration.However, the dehydriding kinetics of the alloys is always accelerated by prolonging milling duration.Specifically, rising milling time from 5 to 60 h makes the hydrogen desorption ratio (a ratio of the dehydrogenation amount in 20 min to the saturat-ed hydrogenation amount) increase from 57% to 66% for x=100 alloy and from 57% to 70% for x=200.Moreover, the improvement of gaseous hydrogen storage kinetics is attributed to the descending of dehydrogenation activation energy caused by the prolonging of milling duration and growing of Ni content.     

Effect of scandium and zirconium alloying on microstructure and gaseous hydrogen storage properties of YFe3

RM₃ compounds (R = rare earth metals, M = transition metals) have rarely been studied for gaseous hydrogen storage applications because of unfavorable thermodynamics. In this work, the hydrogen storage properties of a single-phase YFe₃ alloy were improved by non-stoichiometric composition and alloying with Sc and Zr. Only the Y_1.1–ySc_yFe₃ (y = 0.22, 0.33) alloys consist of a single rhombohedral phase. The Sc substitution for Y leads to the reduction in the unit cell volume of the YFe₃ phase, and thus significantly increases the dehydriding equilibrium pressure and decreases the dehydrogenation temperature. The alloy Y_0.77Sc_0.33Fe₃ delivers a decomposition enthalpy change of 33.54 kJ/mol and a lowest dehydrogenation temperature of 135 °C, in comparison with 38.99 kJ/mol and 165 °C for the alloy Y_1.1Fe₃. The Zr substitution causes a similar thermodynamic destabilization effect, but the composition and microstructure of Y–Zr–Fe alloys need to be further optimized.     

Synthesis of Mg-based composite material with in-situ formed LaH3 and its hydrogen storage characteristics

In this work, a Mg-based composite material with in-situ formed LaH₃, Mg₂NiH₄-LiBH₄ + 20 wt% LaH₃, was prepared by ball milling LiBH₄ and hydrogenated LaMg₂Ni and Mg₂Ni powder mixture, followed by heat treatment at 573 K. The onset dehydrogenation temperature of the composite is reduced by 50 K compared with that of Mg₂NiH₄-LiBH₄. The LaH₃-doped composite shows faster kinetics, absorbing 1.43 wt% hydrogen within 100 s at 423 K, which is 6.5 times faster than Mg₂NiH₄-LiBH₄. Moreover, the composite releases 1.24 wt% hydrogen within 500 s at 573 K, 0.69 wt% higher than Mg₂NiH₄-LiBH₄. The activation energy of the composite is reduced by 8.2 and 80 kJ/mol compared with that of Mg₂NiH₄-LiBH₄ and commercial MgH₂, respectively. The improvement in hydrogen storage properties is attributed to the fact that LaH₃ promotes the generation of nano-sized spongy Mg structure, which has good catalytic activity during the subsequent hydrogenation/dehydrogenation process.     

Microstructure and electrochemical properties of melt-spun Nd_(0.8)Mg_(0.2)(Ni_(0.8)Co_(0.2))_(3.8) hydrogen storage alloy

The effects of rapid solidification on the microstructure and electrochemical properties of Nd0.8Mg0.2(Ni0.8Co0.2)3.8 alloy were systematically investigated.The microstructure of alloys was characterized by scanning electron microscopy(SEM),X-ray diffractometer(XRD) and transmission electron microscopy(TEM).It was found that the melt-spun Nd0.8Mg0.2(Ni0.8Co0.2)3.8 ribbons became thinner and the average grain size of the ribbons became smaller with increasing wheel speed.A fraction of amorphous phase was observed for the ribbons melt-spun at high wheel speed(≥20 m/s).Microstructural characterization revealed that two phases:(Nd,Mg)2(Ni,Co)7 main phase(Ce2Ni7 type structure) and NdNi5 second phase(CaCu5 type structure),existed in the samples in cast state and melt-spun.The cycle stability of the melt-spun alloys was significantly enhanced as compared with cast alloy,and the sample prepared at wheel speed of 20 m/s exhibited good comprehensive electrochemical properties.     

Microstructure and improved hydrogen storage properties of Mg based alloy powders prepared by modified milling method

Abstract

In this study, the modified preparation method of combining planetary and vibratory ball milling was proposed to prepare Mg based hydrogen storage alloy powders. The comparison of micromorphology and hydrogen storage behaviour between Mg₂Ni prepared using the modified and conventional preparation methods were investigated experimentally. The comparison results showed that the combination of first planetary and then vibratory ball milling has more favourable effect on improving both the kinetics and the thermodynamics of ball milled Mg₂Ni alloys. The sample synthesised by first planetary milling for 40 h and then vibratory milling for 30 h has faster hydrogen absorption kinetics and lower dehydriding onset temperature than those prepared by the single method of planetary or vibratory milling and hydriding combustion synthesis owing to its popcorn-like microstructure. Moreover, this kind of modified method reduces the reaction enthalpy and activation energy by up to ～18 and 22% respectively.     

Manufacturing Process,Electrochemical Properties and Structures of Ml(NiMnTiCo)_5 Hydrogen Storage Alloys

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Phase structure and hydrogen storage properties of LaMg3.93Ni0.21 alloy

The phase structure and hydrogen storage property of LaMg3.93Ni0.21 alloy were studied.XRD and SEM results exhibited that LaMg3.93Ni0.21 alloy consisted mainly of LaMg3,La2Mg17 and LaMg2Ni phases;after hydriding/dehydriding process,all the three phases transformed,La3H7 phase existed and the actual hydrogen absorption phases were Mg and Mg2Ni phases.Pressure-composition-temperature (P-C-T)measurement showed that the reversible hydrogen storage capacity of LaMg3.93Ni0.21 alloy was 2.63 wt.%,and the absorption time for reaching 90%of the storage capacity was 124 s at 523 K,and it was 1850 s for deabsorbing 90%of the maximum dehydrogen capacity.The hydriding process of LaMg3.93Ni0.21 alloy followed the nucleation and growth mechanisms.The enthalpy and entropy for hydriding and dehydriding reactions of the Mg phase in LaMg3.93Ni0.21 alloy were calculated to be-66.38±1.10 kJ/mol H2,-100.96±1.96 J/(K·mol)H2 and 68.50±3.87 kJ/mol H2,98.28±5.48 J/(K·mol)H2,respectively.A comparison of these data with those of MgH2(-74.50 kJ/mol H2,-132.30 J/K·mol H2)suggested that the hydride of LaMg3.93Ni0.21 alloy was less stable than MgH2.The existence of La hydride and synergetic effect of multiphase led to higher reversible hydrogen storage capacity and better kinetic property at lower temperature for LaMg3.93Ni0.21 alloy.     

Gaseous hydrogen storage properties of Mg-Y-Ni-Cu alloys prepared by melt spinning

For purpose of promoting the hydrogen absorption and desorption thermodynamics and kinetics properties of Mg-Ni-based alloys, partially substituting Y and Cu for Mg and Ni respectively and melt spinning technique were applied for getting Mg_(25-x)Y_xNi_9 Cu(χ = 0-7) alloys. Their microstructures and phases were characterized with the help of X-ray diffraction and transmission electron microscopy. Their hydrogen absorbing and desorbing properties were tested by a Sievert apparatus, DSC, and TGA, which were connected with a H_2 detector. In order to estimate the dehydrogenation activation energy of alloy hydride, both Arrhenius and Kissinger methods were applied for calculation. It is found that their hydriding kinetics notably declines, however, their hydrogen desorption kinetics conspicuously improves, with spinning rate and Y content increasing. Their hydrogen desorption activation energy markedly decreases under the same constraint, and it is found that melt spinning and Y substituting Mg improve the real driving force for dehydrogenation. As for the tendency of hydrogen absorption capacity,it presents an elevation firstly and soon after a decline with the rising of spinning rate, however, it always lowers with Y content growing. With Y content and spinning rate increasing, their thermodynamic parameters(△H and △S absolute values) visibly decrease, and the starting hydrogen desorption temperatures of alloy hydrides obviously lower.     

Hydrogen storage thermodynamic and kinetic characteristics of PrMg12-type alloys synthesized by mechanical milling

To improve the hydrogen storage performance of PrMg_(12)-type alloys,Ni was adopted to replace partially Mg in the alloys. The PrMg_(11)Ni+x wt.% Ni( x = 100,200) alloys were prepared via mechanical m illing. The phase structures and m orphology of the experim ental alloys were investigated by X-ray diffraction and transm ission electron microscopy. The results show that increasing milling time and Ni content accelerate the form ation of nanocrystalline and am orphous structure. The gaseous hydrogen storage properties of the experim ental alloys were determ ined by differential scanning calorim etry( DSC) and Sievert apparatus. In addition,increasing milling time makes the hydrogenation rates of the alloys augment firstly and decline subsequently and the dehydrogenation rate always increases. The maximum capacity is 5. 572 wt. % for the x = 100 alloy and 5. 829 wt. % for the x = 200 alloy,respectively. The enthalpy change( ΔH),entropy change( ΔS) and the dehydrogenation activation energy( E_k~(de)) markedly lower with increasing the milling time and the Ni content due to the generation of nanocrystalline and amorphous structure.     

Mechanism of the enhancement of discharge rate property for Nd_(0.8)Mg_(0.2)(Ni_(0.8)Co_(0.2))_(3.8) hydrogen storage alloy by magnetic annealing

For further improving the rate properties of Nd0.8Mg0.2(Ni0.8Co0.2)3.8 hydrogen storage alloy,the alloy was annealed in high mag-netic fields(10 T).The results showed that the electrochemical properties and magnetic properties of Nd0.8Mg0.2(Ni0.8Co0.2)3.8 hydrogen storage alloy were considerably enhanced.And there was no change of the phase composition.The lattice constants of c and c/a ratio increased for the sample magnetically annealed,yielding uniaxial magnetic anisotropy along the c axis.The increase o...     

Enhancing (de)hydrogenation kinetics properties of the Mg/MgH_2 system by adding ANi_5(A=Ce,Nd,Pr,Sm,and Y) alloys via ball milling

Magnesium(Mg)-based alloys have already been widely studied as the hydrogen storage materials because of their high reversible hydrogen storage capacity,low cost,light weight,etc.However,the poor de/hydrogenation kinetic properties dramatically hinder the practical applications.In this work,the MgH_2-ANi_5(A=Ce,Nd,Pr,Sm,and Y) composites were prepared by a high-energy ball milling method.which can effectively refine the particle size thus improving the kinetic properties.Experimental results reveal that the MgH_2-ANi_5 composites mainly consist of Mg_2 NiH_4,MgH_2 and rare earth(RE) hydride,which will be dehydrogenated to form Mg_2 Ni,Mg and stable RE hydride reversibly.Accordingly,the asmilled MgH_2-ANi_5(A=Ce,Nd,Pr,Sm,and Y) composites with various A-elements can respectively contribute to a reversible hydrogen storage capacity of 6.16 wt%,5.7 wt%,6.21 wt%,6.38 wt%,and 6.5 wt%at a temperature of 300℃,and show much better kinetic properties in comparison to the pure MgH_2 without any additive.In-situ formed Mg_2 Ni and stable RE hydride(such as CeH_(2.73) and YH_2) might act as effective catalysts to significantly improve the hydrogen storage properties of MgH_2.The present work provides a guideline on improving the kinetic properties of the Mg-based hydrogen storage alloys.     

Structure and oxygen storage capacity of Pr-doped Ce_（0.26）Zr_（0.74）O_2 mixed oxides

Binary Ce-Zr(CZ),Pr-Zr(PZ) and ternary Ce-Zr-Pr(CZP) mixed oxides were prepared by an ammonia-aided co-precipitation method,and were aged in a steam/air flow at 1050 °C.X-ray diffraction(XRD),Raman spectra,X-photon spectra(XPS) and CO temperature programmed reduction(TPR) were carried out to characterize the micro-structure and reducibility of catalysts.The oxygen storage capacity(OSC) was evaluated with CO serving as probe gas.The results showed that a pseudo cubic structure was formed for the Zr-rich ceria-zirconia mixed oxides with Pr doping.The insertion of Pr prevented the phase segregation of the mixed oxides during the hydrothermal ageing.The Pr doped samples showed better redox performances in comparison with CZ,and the sample doped with 5 wt.% Pr showed the most remarkably promoted dynamic oxygen storage capacity.This phenomenon was closely related to both the reducibility and oxygen mobility of the mixed oxides.The introduction of praseodymium into ceria-zirconia could accelerate the oxygen migration by increasing the amount of oxygen vacancies,although it was difficult for Pr3+ ions themselves to participate in the oxygen exchange process.     

Enhanced maximum energy product of(Sm_(1-y)Y_y)_2Fe_(17)N_x caused by abundant yttrium doping

Replacement of samarium(Sm) with abundant yttrium(Y) can help solve the potential shortage of Sm in the preparation of promising Sm_2 Fe_(1)7 N_x magnets.In this article,phase composition,microstructure and magnetic properties of(Sm_(1-y)Yy)_2 Fe_(17)N_x(y=0,0.2,0.4,0.6,0.8,1.0) were investigated.Maximum energy product(BH)_(max) is improved when less than 40 at% Y is doped in(Sm_(1-y)Y_y)_2 Fe_(17)N_x powder.In particular,when 20 at% Y replaces Sm,(BH)_(max) of(Sm_(1-y)Y_y)_2 Fe_(17)N_x powder increases by 15.1% from 131.7 to151.6 kJ/m~3.The effect of annealing temperature on the structural properties of high Y doping(Sm_(0.6)Y_(0.4))_2 Fe_(17) and the magnetic properties of the corresponding nitrides were subsequently investigated.In the RE_2 Fe_(17) phase grain combination process,the interlaced structure of the rhombohedral Th_2 Zn_(17)-type structural phase and the hexagonal Th_2 Ni_(17)-type structural phase is formed.Due to shortrange exchange coupling,the nitride with the highest content of two interlaced RE_2 Fe_(1)7 phases has the highest magnetic properties:B_r=1.23 T,H_(cJ)=443.9 kA/m and(BH)_(max)=197.6 kJ/m~3.     

The variation of microstructures,spectral characteristics and catalysis effects of Fe~(3+) and Zn~(2+) co-doped CeO_2 solid solutions

Fe³⁺and Zn²⁺ions were doped into the lattice of CeO₂ via the hydrothermal method.The micro structure and spectra features were analyzed systemically.XRD results show that the solid solubility of Fe³⁺and Zn²⁺ions in Ce_1-x(Fe_0.5Zn_0.5)_xO₂ can be identified as x=0.16.The cell volumes are decreased by increasing the doped content.The TEM graphs prove that the grain size of the sample is about...     

Microstructure evolution,mechanical properties and creep mechanisms of Mg-12Gd-1MM-0.6Zr (wt%) magnesium alloy

In this research, the microstructure evolution, mechanical properties, and creep mechanisms of Mg-12Gd-1MM-0.6Zr (wt%) alloy under different conditions were systematically studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and tensile creep tests. Regarding the microstructure of the as-cast sample, the average grain size is about 42 μm, and the eutectic compounds were determined to be Mg₅(Gd_0.8MM_0.2). During homogenization, these eutectic compounds gradually dissolve, and Mg₁₂MM particles are precipitated. During hot extrusion, complete dynamic recrystallization (DRX) occurs, resulting in equiaxial grains with an average grain size of about 12 μm and the formation of streamlines consisting of Mg₁₂MM particles along the extrusion direction (ED). After T5 treatment (225 °C for 7 h), a large number of β′(Mg₇Gd) phases are precipitated on the {11–20}α habit plane and are interconnected, forming an interlaced network structure. The ultimate tensile strength (R_m = 405 MPa) and yield strength (R_P0.2 = 288 MPa) of the T5 sample are significantly higher than those of the as-extruded sample (R_m = 289 MPa, R_P0.2 = 185 MPa), but the elongation (A = 4%) was remarkably lower than that of the as-extruded sample (A = 18%). When crept at 225 °C under 100 MPa, the steady-state creep rates of the as-cast, as-extruded, and T5 samples are 1.59 × 10^–8, 1.08 × 10^–8, and 1.40 × 10^–8 s^–1, respectively, and their total strains within 100 h are respectively breaking, 0.81%, and 0.92%, indicating that the as-extruded alloy exhibits the best creep resistance. TEM analysis reveals that, during the creep process of the T5 sample, the β′ particles coarsen and the precipitate-free zones (PFZs) widen, which increase the steady-state creep rate and the total strain within 100 h as compared with the as-extruded sample.     

A comparison study of hydrogen storage performances of SmMg11Ni alloys prepared by melt spinning and ball milling

The melt spinning(MS) and ball milling(BM) technologies are thought to be efficient to prepare nanostructured Mg and Mg-based alloys for improving their hydrogen storage performances. In this paper, two technologies, viz. melt spinning and ball milling, were employed to fabricate the SmMg_(11)Ni alloy. The structure and hydrogen storage performance of these two kinds of alloys were researched in detail. The results reveal that the as-spun and milled alloys both contain nanocrystalline and amorphous structures. By means of the measurement of PCT curves, the thermodynamic parameters of the alloys prepared by MS and BM are ΔN_(Ms)(des) = 82.51 kJ/mol and ΔH_(BM)(des) = 81.68 kJ/mol, respectively, viz.ΔH_(MS)(des) ΔH_(BM)(des). The as-milled alloy shows a larger hydrogen absorption capacity as compared with the as-spun one. The as-milled alloy exhibits lower onset hydrogen desorption temperature than the as-spun one. As to the as-milled and spun alloys, the onset hydrogen desorption temperatures are557.6 and 565.3 K, respectively. Additionally, the as-milled alloy shows a superior hydrogen desorption property than the as-spun one. On the basis of time that required by desorbing hydrogen of 3 wt% H_2, the as-milled alloy needs 1488.574,390 and 192 s corresponding to hydrogen desorption temperatures 593,613,633 and 653 K, while the as-spun alloy needs 3600,1020,778 and 306 s corresponding to the same temperatures. The dehydrogenation activation energies of the as-milled and spun alloys are 100.31 and105.56 kJ/mol, respectively, the difference of which is responsible for the much faster dehydriding rate of the as-milled alloy.     

Fabrication of Ce-doped(Gd_2Y)Al_5O_(12)/Y_3Al_5O_(12) composite-phase scintillation ceramic

In this paper, a novel Ce(Gd_2 Y)Al_5O_(12)/Ce:Y_3Al_5O_(12)(Ce:GYAG/Ce:YAG) composite scintillation ceramic was designed and fabricated by a solid-state reaction method. The phase, luminescence and scintillation properties were investigated. The Ce:GYAG/Ce:YAG composite ceramic consisting of two-phase has a broad emission band ranging from 500 to 750 nm. The total mass attenuation coefficient of Ce:GYAG/Ce:YAG is 0.3864 cm~(-1), in between those of Ce:YAG and Ce:GYAG ceramics. In addition, the composite ceramic had a high light yield of 20430 ph/MeV. By controlling the ratio of GYAG and YAG, the composite ceramic can realize a spectrum design and total mass attenuation coefficient control to meet the requirements for wide-X-ray-energy-range detectors.     

Room temperature compressive properties and strengthening mechanism of Mg_(96.17)Zn_(3.15)Y_(0.50)Zr_(0.18) alloy solidified under high pressure

The microstructures of Mg_(96.17)Zn_(3.15)Y_(0.50)Zr_(0.18) alloys solidified under 2-6 GPa high pressure were investigated by employing SEM(EDS) and TEM.The strengthening mechanism of experimental alloy solidified under high pressure is also discussed by analyzing the compressive properties and compression fracture morphology.The results show that the microstructure of experimental alloy becomes significantly fine-grained with increasing GPa level high pressure during solidification process,and the secondary dendrite arm spacing reduces from 40 μm at atmospheric pressure to 10 μm at 6 GPa pressure.The morphology of the second phases changes from the net structure by the lamellar-type eutectic structure at atmospheric pressure to discontinuous thin rods or particles at 6 GPa pressure.Besides,the solid solubility of Zn in the Mg matrix is improved with the increase of the solidification pressure.Compared with atmospheric-pressure solidification,high-pressure solidification can improve the strength of the experimental alloy.The compressive stre ngth is improved from 263 to 437 MPa at 6 GPa.The fracture mechanism of the experimental alloy changes from cleavage fracture at atmospheric pressure to quasi-cleavage fracture at high pressure.The main mechanism of the strength improvement of the experimental alloy includes the grain refinement strengthening caused by the refinement of the solidification microstructure,the second phase strengthening caused by the improvement of the morphology and distribution of the second phases,and solid solution strengthening caused by the increase of the solid solubility of Zn in the Mg matrix.     

Process optimization of electroless copper plating and its influence on electrochemical properties of AB5-type hydrogen storage alloy

The amount of Cu coating by chemical plating was investigated based on quadratic regression orthogonal experimental design being adapted to the variation law of temperature,pH value and Ni2+concentration,and the relevant regression equation was expressed as y=2.1609+0.5295×10-3T2-0.0342P2-0.0265N2+0.0023TP+0.0020TH+0.0199PN-0.0959T+0.3814P-0.2073N.The results showed that the deposition rate augmented with the increasing in temperature,pH value and Ni2+concentration.The experimental parameters of the optimal coating were temperature 75 ℃,pH value 8.5 and Ni2+concentration 1.2 g/L.The electrochemical tests indicated that the cycle stability increased from 60.66% to 75.58%,indicating that the treated alloy exhibited better corrosion resistance.     

MmNi5-x(CoAlMn)x/Mg纳米晶复合储氢合金的吸氢性能

研究了机械合金化制备的ＭｍＮｉ５－ｘ（ＣｏＡｌＭｎ）ｘ／Ｍｇ纳米晶复合储氢合金的吸氢特性。通过测定不同镁含量的ＭｍＮｉ５－ｘ（ＣｏＡｌＭｎ）ｘ／Ｍｇ纳米晶复合储氢合金的吸氢ＰＣＴ曲线,考察了镁含量对其吸氢特性的影响。根据对吸氢速度的测定研究了机械合金化对储氢合金的吸氢动力学特性的影响。机械合金化制备的纳米晶复合储氢合金的活化性能与ＭｍＮｉＭ５－ｘ（ＣｏＡｌＭｎ）ｘ铸态合金相比有较大的提高,不需活化或只需一次活化即可吸氢,吸氢量及吸氢的动力学性能与铸态合金相比也有很大的提高。最后对机械合金化形成的纳米晶吸氢特性进行了分析。