Superior hydrogen storage kinetics of MgH2 nanoparticles doped with TiF3

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

Cycling hydrogen desorption properties and microstructures of MgH2-AlH3-NbF5 hydrogen storage materials

Magnesium hydride (MgH2) is a candidate material for hydrogen storage.MgH2-AlH3 composite shows superior hydrogen desorption properties than pure MgH2.However,t...     

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

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

Changes in microstructure and absorption-desorption pressures during hydrogen cycling in some intermetallic compounds

Changes in microstructure and their effects on hydrogen sorption properties have been investigated as a function of the number of hydrogen sorption cycles in LaNi₅ and FeTi. For LaNi₅, while the desorption pressure does not significandy depend on the number of cycles, the absorption pressure drastically decreases between the first and second cycles. In addition to severe cracking and pulverization, numerous edge dislocations with Burgers vectors of the l/3<1210>-type are introduced during the first absorption cycles. Both the introduction of numerous lattice defects and the formation of severe cracks seem to be completed during the first cycle, resulting in the reproducible P (pressure)-C (composition) characteristics for the subsequent cycles. In contrast, FeTi exhibits ever-changing P-C characteristics with cycle number. The introduction of dislocations with Burgers vectors of the <100>-type in the form of tangles and cracks occurs during each of absorption cycles with the extent to which this occurs being considerably smaller in each of absorption cycles of FeTi than in the first absorption cycle of LaNi₅. The difference in material response to hydrogénation seems to be closely related to the difference in ductility of these two intermetallics.     

Improvement in hydrogen sorption kinetics of MgH2 with Nb hydride catalyst

Nanocrystalline MgH₂ with fine and evenly dispersed Nb hydride was prepared by ball milling a mixture of MgH₂ and 1 mol.% NbF₅. This NbH-catalyzed MgH₂ desorbed 6.3 wt.% H₂ in 15 min and absorbed more than 90% of its initial hydrogen capacity within 5 min at 573 K. Moreover, this fast sorption kinetics was maintained after 10 cycles. Based on X-ray diffraction and transmission electron microscopy/energy-dispersive spectroscopy analyses, it is suggested that NbF₅ melts during high-energy ball milling and this promotes the formation of extremely fine, film-like Nb hydride preferentially along the grain boundaries of nanocrystalline MgH₂ by a liquid/solid reaction. This unique nanostructured Nb hydride is believed to suppress the grain growth of MgH₂ quite effectively and thus maintain its initial catalytic effect throughout repeated hydrogenation–dehydrogenation cycles.     

超细纯镁及镁钛粉体的制备及储氢性能

采用直流电弧等离子体方法制备超细纯Mg及Mg-Ti粉体。运用X射线衍射（XRD）,透射电子显微镜（TEM）,压力成分温度（PCT）方法和TG/DTA技术研究粉体吸放氢前后的相组成、微观结构和吸放氢性能。结果表明,大部分超细Mg和Mg-Ti颗粒呈六角形,颗粒大小在50700nm范围内。根据范特霍夫方程计算由PCT曲线获得的吸氢平台压力,Mg-Ti粉的氢化焓约为67kJ/mol H2,显著高于纯镁粉的氢化焓78.6kJ/mol H2。TG/DTA分析表明,氢化后Mg-Ti粉的放氢起始温度为386°C,低于氢化纯镁粉的放氢温度（423°C）。通过电弧蒸发法直接向Mg中添加Ti而获得的Mg-Ti超细粉体可以显著改善镁的储氢热力学性能。     

TEM analysis of the microstructure of thermal barrier coatings after isothermal oxidation

Thermal barrier coatings (TBCs) are extensively used to protect metallic components in applications where the operating conditions include an aggressive environment at high temperatures. The most important factor controlling TBC durability is the nucleation, and subsequent thickening, of a thermally grown oxide (TGO) layer which is formed during high-temperature oxidation. For this reason, the aim of this work is to analyse the TGO microstructure evolution during isothermal oxidation to explain the macroscopic oxidation behaviour. To this end, transmission electron microscopy (TEM) was used to evaluate the TGO fine microstructure. ZrO₂(Y₂O₃) top coat and NiCrAlY bond coating were air plasma sprayed onto an Inconel 600 Ni base alloy. The TBCs were isothermally oxidized in air at 950 and 1050 °C for 24, 48, 72, 144 and 336 h and the principal differences in TGO composition were analysed. α-Al₂O₃ was the main TGO constituent in the TBC treated at 950 °C. On the other hand, Al was rapidly consumed in the TBC oxidized at 1050 °C leading to the formation of NiAl₂O₄ spinels, after 72 h exposure, and NiO, after 336 h. The TGO growth kinetics followed a power law, controlled by Al³⁺ diffusion, in the samples treated at 950 °C. However, two different power laws fitted the TGO growth kinetics in the coatings treated at 1050 °C as the diffusion of Ni²⁺ is relevant after 72 h exposure.     

Analysis of deformation twins and the partially dehydrogenated microstructure in nanocrystalline magnesium hydride (MgH2) powder

Cryo-stage transmission electron microscopy (TEM), supported by Density Functional Theory (DFT), is employed to explore the microstructure of magnesium hydride (MgH₂) powders. Mechanical milling results in deformation twinning of the hydride. The crystallography of the twins is established. DFT analysis shows that the twin unit cell is just as thermodynamically stable as the undeformed α-MgH₂ matrix. It is hypothesized that the twins contribute significantly to the observed milling-induced kinetic enhancement by acting as high diffusivity paths for hydrogen. Energy-filtered TEM analysis on partially desorbed MgH₂ demonstrates that nucleation and growth of metallic magnesium occurs non-uniformly. Larger powder particles are a composite of isolated magnesium grains heterogeneously nucleated on the remaining hydride. Smaller particles are either fully transformed to magnesium or remain entirely a hydride. There is little evidence for any “core–shell” structure. It is also shown that in situ hydrogen desorption in the TEM is not representative of the elevated-temperature ex situ sequence.     

Enhancing hydrogen sorption in MgH2 by controlling particle size and contact of Ni catalysts

A comparative study based on MgH2 ball-milled with altered Ni particle sizes under different conditions was conducted to reveal the effect of close contact betw...     

Boundary conditions during the electropermeation of hydrogen through pure iron

The boundary conditions prevailing at the entry side of pure iron during galvanostatic (G) or potentiostatic (P) mode of charging in 0.1 N H₂SO₄ have been studied by comparing the normalised permeation transients with the dimensionless theoretical curves constructed for different cases. In the case of vacuum-annealed pure iron, irrespective of the mode of charging, the condition for constant hydrogen concentration at the entry side is satisfied, whereas in the case of hydrogen-annealed specimens, the constant hydrogen flux condition is valid. The diffusivity values for hydrogen in pure iron, pre-treated by different methods, have been computed and compared.     

Analysis of microstructure and texture evolution in pure magnesium during symmetric and asymmetric rolling

Asymmetric rolling of commercially pure magnesium was carried out at three different temperatures: room temperature, 200 °C and 350 °C. Systematic analysis of microstructures, grain size distributions, texture and misorientation distributions were performed using electron backscattered diffraction in a field emission gun scanning electron microscope. The results were compared with conventional (symmetric) rolling carried out under the same conditions of temperature and strain rate. Simulations of deformation texture evolution were performed using the viscoplastic self-consistent polycrystal plasticity model. The main trends of texture evolution are faithfully reproduced by the simulations for the tests at room temperature. The deviations that appear for the textures obtained at high temperature can be explained by the occurrence of dynamic recrystallization. Finally, the mechanisms of texture evolution in magnesium during asymmetric and symmetric rolling are explained with the help of ideal orientations, grain velocity fields and divergence maps displayed in orientation space.     

温度对Mg-3Ni-2MnO2储氢材料吸放氢过程相转变行为的影响

利用充氢反应球磨工艺制备氢化态Mg-3Ni-2MnO2储氢复合材料,测试材料的吸放氢动力性能,并利用Avrami指数研究储氢材料吸放氢过程中相转变行为特征.结果表明:在研究的温度范围内,温度越高,越有利于提高Mg-3Ni-2MnO2储氢复合材料的吸放氢速度;在150～200 ℃范围内吸氢时,其Avrami指数由初始阶段的1.0～1.5很快变为0.5,即储氢材料很快进入已形成相的增厚阶段;在150～200 ℃范围内,温度变化对吸氢相转变影响不大,但影响相转变速率;放氢过程中,根据Avrami 指数的变化,相转变基本过程为形核长大阶段和新相继续稳定长大阶段(无新的晶核形成),温度变化同样影响其放氢速率,但对其放氢过程的相转变规律影响不大.     

稀土Dy和Sm对贮氢合金微观结构和电化学性能的影响

在AB5型贮氢合金MINi3.55Co0.75Al0.3Mn0.4基础上,通过B侧加入Cu、Fe、Zn和Cr、A侧加入稀土元素Dy和Sm的方法制备低Co贮氢合金,并对合金的放电容量、循环稳定性以及微观结构进行了研究。研究表明,加Dy低Co贮氢合金不仅可明显地改善充放电循环稳定性,而且可以获得较高的放电容量。加Sm低Co贮氢合金虽然具有较高的放电容量及较少的活化次数,但充放电循环稳定性略有下降。     

Surface analysis, hydrogen adsorption and electrochemical performance of alkali-reduce treated hydrogen storage alloy

The hydrogen storage alloy powders (M1Ni4.0Co0.6Al0.4, M1 = rich-La mischmetal) were treated in a hot 6 mol/L KOH 0.02 mol/L KBH4 solution, the surface compositions and chemical states of the treated and untreated alloys were analyzed by XPS and EDX, the hydrogen adsorption on the surface of these alloys was evaluated by thermal desorption spectroscopy (TDS), the effects of the surface treatment on the electrochemical performances of the alloy electrodes were investigated. The results show that the hydrogen adsorption is greatly strengthened by the surface modification, and hence leads to marked improvement in the electrocatalytic activity, the treated alloy exhibits higher exchange current density and lower apparent activation energy for the hydrogen electrode reaction than the untreated alloy.     

钐对无钴AB5型稀土贮氢合金微观结构和电化学性能的影响

用真空感应熔炼铸锭以及快淬工艺制备过化学计量比无钴AB5型稀土贮氢合金La0.85-xCe0.15 SmxNi4.55 Mn0.40Al0.30和La0.69Ce0.31-xSmxNi4.55Mn0.40Al0.30(x=0,0.08,0.16).研究了钐含量及快淬对无钴AB5型稀土贮氢合金的微观结构、吸放氢性能、电化...     

Effects of annealing treatment on the microstructure and electrochemical properties of low-Co hydrogen storage alloys containing Cu and Fe

The effects of annealing treatment on the microstructure and electrochemical properties of low-Co LaNi3.55Mno.35Co0.20Al0.20Cuo.75Fe0.10 hydrogen storage alloys were investigated.X-ray diffraction (XRD) analysis indicated that annealing treatment remarkably reduced the lattice strain and defects,and increased the unit-cell volume.The optical microscope analysis showed that the as-cast alloy had a crass dendrite microstructure with noticeable composition segregation,which gradually disappeared with increasing annealing temperature,and the microstructure changed to an equiaxed structure after annealing the alloy at 1233 K.The electrochemical tests indicated that the annealed alloys demonstrated much better cycling stability compared with the as-cast one.The capacity retention at the 100th cycle increased from 90.0％(as-cast) to 94.7％ (1273 K).The annealing treatment also improved the discharge capacity.However,the high rate dischargeability (HRD)value of the annealed alloy slightly dropped,which was believed to be ascribed to the decreased exchange current density and the hydrogen diffusion coefficient in alloy bulk.     

Pr替代Ti对TiFe系储氢合金微观结构和电化学性能的影响

采用感应熔炼法制备Ti1.1-xPrxFe0.6Ni0.3Mn0.2(x = 0,0.04,0.06,0.08)合金,通过X射线衍射仪(XRD)、扫描电子显微镜(SEM)、蓝电电池测试等手段对合金的显微组织结构和电化学性能进行分析.XRD结果表明:合金基体相为TiFe,以及少量的ZrMn2第二相,添加Pr未改变合金相组...     

Evolution of microstructure and thermomechanical properties during superelastic compression cycling in Cu–Al–Ni single crystals

The aim of this work is to relate the macroscopic evolution of the compression superelastic effect in Cu–Al–Ni shape memory alloy single crystals with the evolution of the microstructure during cycling. The analysis has been carried out as a function of the number of cycles, the maximum reached deformation and the kind of induced martensite. Moreover, the new microstructure after mechanical cycling and the evolution of the thermal transformation have been also studied. The presence of two new families of dislocations created by different mechanisms has been observed and the influence of each one on the stress-induced and thermal transformations has been analyzed. In the samples where both kinds of dislocations are present at the same time, the observed behaviour is a combination of their effects in proportion to their density.