Analyses of hydrogen sorption kinetics and thermodynamics of magnesium–misch metal alloys

The multi-component Mg–x wt.% Mm alloys are synthesized using the mechanical ball-milling technique and their hydrogen storage capacities, absorption/desorption kinetics, and thermodynamic parameters are quantified. The analysis of kinetic properties presented here is based on the method of exponential peeling. It is seen that the presence of misch metal (Mm) in the alloy samples dramatically decreases their rate of decrepitation and increases their cyclic stability. However, the increased concentration of misch metal in the samples has an adverse effect on their hydrogen storage capacity and their reaction rate. The hydrogen storage properties also vary with reaction temperature. The best hydrogen absorption kinetics are observed at temperatures around 300 °C and the desorption kinetics are quite fast at temperatures of 400 °C and above. The hydrogen desorption activation energy of Mg–x wt.% Mm hydride is much lower than that of MgH₂. The pressure–composition–isotherm (P–C–T) plots of the samples at 300–420 °C indicate that the alloys possess good cyclic stability but very poor reversibility, making the study of their thermodynamic properties difficult. The P–C–T plots also indicate that the increase of the concentration of the misch metal's rare earths leads to an increase of the hydrogen equilibrium pressure and decrease of hydrogen storage capacity.     

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.     

Computational thermodynamics and the kinetics of martensitic transformation

To assist the science-based design of alloys with martensitic microstructure, a multicomponent database kMART (kinetics of MARtensitic Transformation) encompassing the components Al, C, Co, Cr, Cu, Fe, Mn, Mo, N, Nb, Ni, Pd, Re, Si, Ti, V, and W has been developed to calculate the driving force for martensitic transformation. Built upon the SSOL database of the Thermo-Calc software system, a large number of interaction parameters of the SSOL database have been modified, and many new interaction parameters, both binary and ternary, have been introduced to account for the heat of transformation, T ₀ temperatures, and the composition dependence of magnetic properties. The critical driving force for face-centered cubic (fcc) → body-centered cubic (bcc) heterogeneous martensitic nucleation in multicomponent alloys is modeled as the sum of a strain energy term, a defect-size-dependent interfacial energy term, and a composition-dependent interfacial work term. Using our multicomponent thermodynamic database, a model for barrierless heterogeneous martensitic nucleation, a model for the composition and temperature dependence of the shear modulus, and a set of unique interfacial kinetic parameters, we have demonstrated the efficacy of predicting the fcc → bcc martensitic start temperature (M _s ) in multicomponent alloys with an accuracy of ± 40 K over a very wide composition range.     

Computational thermodynamics and the kinetics of martensitic transformation

To assist the science-based design of alloys with martensitic microstructure, a multicomponent database kMART (kinetics of MARtensitic Transformation) encompassing the components Al, C, Co, Cr, Cu, Fe, Mn, Mo, N, Nb, Ni, Pd, Re, Si, Ti, V, and W has been developed to calculate the driving force for martensitic transformation. Built upon the SSOL database of the Thermo-Calc software system, a large number of interaction parameters of the SSOL database have been modified, and many new interaction parameters, both binary and ternary, have been introduced to account for the heat of transformation, T ₀ temperatures, and the composition dependence of magnetic properties. The critical driving force for face-centered cubic (fcc) → body-centered cubic (bcc) heterogeneous martensitic nucleation in multicomponent alloys is modeled as the sum of a strain energy term, a defect-size-dependent interfacial energy term, and a composition-dependent interfacial work term. Using our multicomponent thermodynamic database, a model for barrierless heterogeneous martensitic nucleation, a model for the composition and temperature dependence of the shear modulus, and a set of unique interfacial kinetic parameters, we have demonstrated the efficacy of predicting the fcc → bcc martensitic start temperature (M _s ) in multicomponent alloys with an accuracy of ± 40 K over a very wide composition range.     

Integrated design of Nb-based superalloys: Ab initio calculations,computational thermodynamics and kinetics,and experimental results

An optimal integration of modern computational tools and efficient experimentation is presented for the accelerated design of Nb-based superalloys. Integrated within a systems engineering framework, we have used ab initio methods along with alloy theory tools to predict phase stability of solid solutions and intermetallics to accelerate assessment of thermodynamic and kinetic databases enabling comprehensive predictive design of multicomponent multiphase microstructures as dynamic systems. Such an approach is also applicable for the accelerated design and development of other high performance materials. Based on established principles underlying Ni-based superalloys, the central microstructural concept is a precipitation strengthened system in which coherent cubic aluminide phase(s) provide both creep strengthening and a source of Al for Al₂O₃ passivation enabled by a Nb-based alloy matrix with required ductile-to-brittle transition temperature, atomic transport kinetics and oxygen solubility behaviors. Ultrasoft and PAW pseudopotentials, as implemented in VASP, are used to calculate total energy, density of states and bonding charge densities of aluminides with B2 and L2₁ structures relevant to this research. Characterization of prototype alloys by transmission and analytical electron microscopy demonstrates the precipitation of B2 or L2₁ aluminide in a (Nb) matrix. Employing Thermo-Calc and DICTRA software systems, thermodynamic and kinetic databases are developed for substitutional alloying elements and interstitial oxygen to enhance the diffusivity ratio of Al to O for promotion of Al₂O₃ passivation. However, the oxidation study of a Nb–Hf–Al alloy, with enhanced solubility of Al in (Nb) than in binary Nb–Al alloys, at 1300 °C shows the presence of a mixed oxide layer of NbAlO₄ and HfO₂ exhibiting parabolic growth.     

Modeling thermodynamics,kinetics, and phase transformation morphology while heat treating titanium alloys

This article summarizes researchwork on computer modeling of thermodynamics and kinetics of phase transformation behavior in a number of titanium alloys, carried out in collaboration with TIMET UK. The goal is to model the microstructural evolution of the alpha/beta phases as a function of heat treatment and alloy chemistry including five major titanium alloys. The results of the research include time-temperature-transformation and continuous cooling diagrams. Experimental validation, by measurement of the kinetics of growth and dissolution of alpha and other phases, has been conducted.     

Enhancement of Ti-Cr-V BCC alloys on the dehydrogenation kinetics of Li-Mg-N-H hydrogen storage materials

The hydrogen storage properties of a Li-Mg-N-H material doped by a 4 mol.% Ti₃Cr₃V₄ body centre cubic (BCC) alloy hydride and prepared with a ball-milling method were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and Sievert’s technology test. The results show that the Ti₃Cr₃V₄ BCC alloy hydride/Li-Mg-N-H composite has good reversible hydrogen storage properties. The dehydrogenation kinetics of the Li-Mg-N-H system can be greatly improved by doping the Ti₃Cr₃V₄ BCC alloy hydride. The composite desorbed 4.1 wt.% hydrogen in the first 60 min at 473 K under 0.1 MPa pressure, but when without the BCC alloy addition, only 3.0 wt.% hydrogen was desorbed under the same dehydrogenation condition. It can be deduced that the Ti₃Cr₃V₄ BCC alloy uniformly distributed in the Li-Mg-N-H substrate could decrease the activating energy of hydrogen molecules to H atoms and increase H diffusion paths in the composite, enhancing the dehydrogenation kinetics of the Li-Mg-N-H system.     

预氧化钒钛磁铁精矿的固态还原动力学及其机理

研究预氧化钒钛磁铁精矿固态还原反应的动力学,采用XRD、SEM和EDS等手段研究还原产物的显微结构和物相变化,在此基础上,对其固态还原机理进行研究。结果表明：以煤为还原剂,在还原温度为950~1100°C时,预氧化钒钛磁铁精矿的固态还原受界面化学反应控制,反应的表观活化能为67.719 k J/mol;预氧化钒钛磁铁精矿的还原历程可描述为：预氧化钒钛磁铁精矿→钛铁晶石→钛铁矿→亚铁板钛矿（Fe Ti2O5）→（FenTi1-n）Ti2O5。预氧化钒钛磁铁精矿在1050°C还原60 min后,还原产物中会形成M3O5型（M为Fe、Ti、Mg、Mn等）固溶体,存在于M3O5固溶体中铁的难还原性是限制预氧化钒钛磁铁矿还原的主要原因。     

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.     

考虑热力学和动力学相关性的二元合金凝固界面动力学建模

在同时考虑碰撞限制生长模式和短程扩散限制生长模式的情况下,提出一个更加完善且具有可变动力学前因子的二元合金固?液界面动力学模型.与上述两种生长模式相耦合,提出4种潜在的热力学和动力学相关性,并将其应用于平界面迁移和枝晶凝固.其中,有效热力学驱动力与有效动力学能垒间的线性相关性更符合物理实际.基于此线性热力学和动力学相关...     

Co替代Ni改善纳米晶和非晶Mg2Ni型合金的贮氢动力学

为了改善Mg2Ni型合金的贮氢动力学性能,用Co部分替代合金中的Ni,用快淬技术制备了Mg2Ni1？xCox（x=0,0.1,0.2,0.3,0.4）贮氢合金。用XRD、HRTEM表征了快淬态合金的微观结构,用自动控制的Sieverts设备测试了合金的吸放氢动力学性能,用程控电池测试仪测定了合金薄带的电化学贮氢动力学。结果表明：Co替代Ni提高了Mg2Ni型合金的非晶形成能力,合金的非晶化程度随着Co含量的增加而增加。此外,Co替代Ni显著地改善了合金的贮氢动力学,当Co含量从0增加到0.4时,快淬态（15m/s）合金在5min内的吸氢饱和率从81.2%增加到84.9%,20min的放氢率从17.60%增加到64.79%,氢扩散系数从1.07×10-11cm2/s增加到2.79×10？11cm2/s,极限电流密度从46.7mA/g增加到191.7mA/g。     

肯尼亚天然金红石矿沸腾氯化热力学和动力学

以焦炭作还原剂,利用天然金红石和氯气制备四氯化钛。在1173到1273 K的温度范围内,对肯尼亚天然金红石颗粒在间歇式流化床中的氯化反应热力学和动力学进行研究。体系的热力学分析显示：生成CO的反应在高温下占主导地位;若生成CO反应的比率η越大,且随着体系反应受温度的影响加剧,高温下反应的吉布斯自由能越负;同时,η增大,产物中TiCl4分压对反应的吉布斯自由能的影响趋于减小。应用气-固多相反应理论和流化床两相模型建立反应的数学模型,计算了气泡相和乳相中模型的各种参数,床层高度范围内氯气的平均浓度为0.3 mol/m^3。结果表明：天然金红石氯化反应主要在乳相中进行,反应速率主要由界面反应控制。     

Structural and hydrogen storage capacity evolution of Mg2FeH6 hydride synthesized by reactive mechanical alloying

Mg-based metal hydrides are promising as hydrogen storage materials for fuel cell application. In this work, Mg2FeH6 complex hydride phase was synthesized by controlled reactive ball milling of 2Mg-Fe (atomic ratio) powder mixture in H2. Mg2 FeH6 is confirmed to be formed via the following three stages= formation of MgH2 via the reaction of Mg with H2, incubation stage and formation of Mg2 FeH6 by reaction of fully refined MgH2 and Fe.The incubation stage is characterized by no traces of Mg or hydride crystalline phase by XRD. On the other hand,Mg is observed uniformly distributed in the milled powder by SEM-EDS. Also, almost the same amount of H2 as the first stage is detected stored in the powders of the second stage by DSC and TGA.     

AlH3 as a hydrogen storage material:recent advances,prospects and challenges

Aluminum hydride (AlH3) is a covalently bon-ded trihydride with a high gravimetric (10.1 wt％) and volumetric (148 kg.m-3) hydrogen capacity.AlH3 decomposes to A...     

A comparative study on the hydriding kinetics of Zr-based AB2 hydrogen storage alloys

Two kinetic models (Jander model and Chou model) are used to investigate the hydrogen absorption kinetic mechanism of Zr-based AB₂ type Laves phase alloys (Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.5Co_0.5, Ti_0.1Zr_0.9(Mn_0.9V_0.1)_1.1Fe_0.5Ni_0.5 and Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.55Ni_0.55). The analysis shows that the rate-controlling step is the diffusion process at high temperatures in the range from 673 K to 923 K with a low hydrogen concentration (solid solution phase). Both models can well describe the experimental data but Chou model is preferred. Chou model is simpler and easier to use for analyzing the experimental results. The activation energies calculated using Chou model with the least square method are 29.3 kJ/mol H₂ for Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.5Co_0.5, 43.8 kJ/mol H₂ for Ti_0.1Zr_0.9(Mn_0.9V_0.1)_1.1Fe_0.5Ni_0.5 and 48.5 kJ/mol H₂ for Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.55Ni_0.55, which are close to the values reported in the literature (28.3 kJ/mol H₂ for Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.5Co_0.5 and 40.3 ± 1.5 kJ/mol H₂ for both Ti_0.1Zr_0.9(Mn_0.9V_0.1)_1.1Fe_0.5Ni_0.5 and Ti_0.1Zr_0.9Mn_0.9V_0.1Fe_0.55Ni_0.55).     

Synthesis and hydrogen storage properties of Mg-based alloys

Mg-based alloys have been made by mechanical alloying Mg with some transition and non-transition elements. The thermal stability and hydrogen storage properties have been investigated. It was found that mechanical alloying results in a supersaturated solid solution of some elements in the Mg phase. Thermal annealing and/or hydrogenation cause irreversible decomposition of supersaturated solid solution leading to a composite of Mg or MgH₂ with other phase(s) depending on the composition and contents. Therefore, the plateau pressure or thermodynamic properties of hydrogen absorption/desorption of the supersaturated solid solution are no different from that of the Mg composite. While in some equilibrium systems, the formation of Mg solid solution is reversible upon hydrogenation/dehydrogenation. The plateau pressure of the hydrogenation/dehydrogenation is increased due to the interaction of the alloying elements with the Mg lattice in the solid solution. The Mg–Li system is an exception because of the formation of stable LiH upon hydrogenation of Mg(Li) solid solution. No interaction takes place between Mg or MgH₂ with LiH, therefore, no destabilization of MgH₂ is observed.     

Reaction thermodynamics and kinetics on in-situ Al2O3/Cu composites

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TiCrMo三元储氢合金的结构和性能

采用XRD、SEM和PCT法研究Mo取代TiCr1.8合金中的部分Cr对其结构和储氢性能的影响。XRD测试结果表明,随着合金中Mo取代Cr的量的增加,合金的相组成首先转变为MoCr体心立方(BCC)相与Laves相共存,Laves相的含量逐渐降低;当Mo含量超过一定量时,合金的相组成转变为MoCrBCC相与微量的α-Ti相共存;MoCrBCC相的晶胞参数随合金中Mo含量的增加而增大。合金的压力组分温度(PCT)测试结果表明,合金的吸氢量随合金中Mo含量的增加而减少。     

Structures and properties of La-Mg-Ni system hydrogen storage alloys

The double-roller rapid quenching technology was successfully used to prepare La-Mg-Ni system hydrogen storage alloys. The effects of magnesium content and heat-treatment process on the alloys properties were studied. When the alloy with 1.09%（mass fraction） Mg is heat treated at 900℃ for 4 h, its discharge capacity is more than 380 mA.h/g at 0.2C, and the cyclic life is beyond 500 counts at 2C. By XRD and PCI analyzing, the results show that the alloys are composed of LaNis and LaNi3 phase. The hydrogen absorption/desorption pressure of the alloy increases, so does the slope of plateau, and the plateau becomes broad first and narrow again as Mg content increases. This method is simple to be suitable for production on a large scale.