MgH_2-Nb体系解氢能力与电子结构的第一原理计算

采用第一原理赝势平面波方法,构建了一个NbH0.6/MgH2相界模型,研究了Nb合金化对MgH2解氢能力与电子结构的影响.结果显示:NbH0.6/MgH2相界的结构稳定性比MgH2相差,表明Nb合金化利于提高MgH2相的解氢能力;Nb对MgH2相解氢能力增强的主要原因在于Nb-H间电子相互作用比Mg-H间强,有利于促进NbH0.6相形核,并且α-Mg在MgH2-Nb体系的NbH0.6/MgH2相界中形核比在MgH2相中容易.     

An Analytical Bond Order Potential for Mg−H Systems

Magnesium-based materials provide some of the highest capacities for solid-state hydrogen storage. However, efforts to improve their performance rely on a comprehensive understanding of thermodynamic and kinetic limitations at various stages of (de)hydrogenation. Part of the complexity arises from the fact that unlike interstitial metal hydrides that retain the same crystal structures of the underlying metals, MgH₂ and other magnesium-based hydrides typically undergo dehydrogenation reactions that are coupled to a structural phase transformation. As a first step towards enabling molecular dynamics studies of thermodynamics, kinetics, and (de)hydrogenation mechanisms of Mg-based solid-state hydrogen storage materials with changing crystal structures, we have developed an analytical bond order potential for Mg−H systems. We demonstrate that our potential accurately reproduces property trends of a variety of elemental and compound configurations with different coordinations, including small clusters and bulk lattices. More importantly, we show that our potential captures the relevant (de)hydrogenation chemical reactions 2H (gas)→H₂ (gas) and 2H (gas)+Mg (hcp)→MgH₂ (rutile) within molecular dynamics simulations. This verifies that our potential correctly prescribes the lowest Gibbs free energies to the equilibrium H₂ and MgH₂ phases as compared to other configurations. It also indicates that our molecular dynamics methods can directly reveal atomic processes of (de)hydrogenation of the Mg−H systems.     

Influence of a Second Cation (M = Ca2+, Mg2+) on the Phase Evolution of (BaxM1–x)F2 Starting from Amorphous Deposits

A second type of cation (Mg²⁺, Ca²⁺) was introduced into BaF₂ by low‐temperature atomic beam deposition. The structure evolution from low‐temperature (–150 °C) amorphous deposits to high‐temperature (< 1000 °C) annealed crystalline phases was studied by in‐situ transmission electron microscopy and X‐ray diffraction. Amorphous (Ba_0.5, Ca_0.5)F₂ crystallizes in a first step to metastable solid solution phase (fluorite‐type), which then decomposes into the pure phases of CaF₂ and BaF₂ at higher temperature. The crystallization behavior of amorphous (Ba_xMg_1–x)F₂ is completely different. When the Mg/Ba atomic ratio is around 1:1, the mixture transforms to the ternary compound BaMgF₄ at annealing, and no decomposition occurs by further heating up to 1000 °C. When the Ba concentration is below 15 % in atomic ratio (x < 0.15), the mixture forms a solid solution phase (rutile type) with the lattice expanded by +1 % compared to rutile type MgF₂. The difference between the phase evolutions of the two mixture systems is discussed.     

Superionic transport in solid fluoride solutions with a fluorite structure

Results of systematic research into ionic transport in solid solutions M_{1 ? x} R_xF_{2 + x} and Na_{0.5 ? x} R_{0.5 + x} F_{2 + 2x} (M = Ca, Sr, Ba, Cd, Pb; R = rare-earth elements) with the fluorite structure are reviewed. The mechanism of anionic conduction is analyzed. The link between ionic transport and defects in the structure of these crystals is discussed. Potentialities of their application in electrochemical devices are considered.     

Structural and mechanical properties of alkali hydrides investigated by the first-principles calculations and principal component analysis

The structural and mechanical properties of alkali hydrides (LiH, NaH, KH, RbH, and CsH) were investigated via first-principles calculations which cover the optimized structural parameters. The density functional theory in combination with the generalized gradient approximation (GGA) were used in this study. From the present study, one could note that alkali hydrides are brittle materials and mechanically stable. It was found that stiffness and shear resistance are greater in LiH than in other hydrides. It is more brittle in nature, and comparatively harder than the other materials under study; it also presents a high degree of anisotropy. The results were then investigated and analyzed with principal component analysis (PCA), which is one of the most common techniques in multivariate analysis, was used to explore the correlations among material properties of alkali hydrides and to study their trends. The alkali hydrides obtained by the first-principles calculations were also compared with the alkaline-earth metal hydrides (BeH₂, MgH₂, CaH₂, SrH₂, and BaH₂) and discussed in this work.     

Improvement in Hydrogen Desorption from β‐ and γ‐MgH2 upon Transition‐Metal Doping

A thorough study of the structural, electronic, and hydrogen‐desorption properties of β‐ and γ‐MgH₂ phases substituted by selected transition metals (TMs) is performed through first‐principles calculations based on density functional theory (DFT). The TMs considered herein include Sc, V, Fe, Co, Ni, Cu, Y, Zr, and Nb, which substitute for Mg at a doping concentration of 3.125 % in both the hydrides. This insertion of TMs causes a variation in the cell volumes of β‐ and γ‐MgH₂. The majority of the TM dopants decrease the lattice constants, with Ni resulting in the largest reduction. From the formation‐energy calculations, it is predicted that except for Cu and Ni, the mixing of all the selected TM dopants with the MgH₂ phases is exothermic. The selected TMs also influence the stability of both β‐ and γ‐MgH₂ and cause destabilization by weakening the Mg?H bonds. Our results show that doping with certain TMs can facilitate desorption of hydrogen from β‐ and γ‐MgH₂ at much lower temperatures than from their pure forms. The hydrogen adsorption strengths are also studied by density‐of‐states analysis.     

Molecular Magnesium Hydrides

Solid magnesium hydride [MgH₂]_∞ has been pursued as a potential hydrogen‐storage material. Organic chemists were rather interested in soluble magnesium hydride reagents from mid‐20th century. It was only in the last two decades that molecular magnesium hydride chemistry received a major boost from organometallic chemists with a series of structurally well‐characterized examples that continues to build a whole new class of compounds. More than 40 such species have been isolated, ranging from mononuclear terminal hydrides to large hydride clusters with more than 10 magnesium atoms. They provide not only insights into the structure and bonding of Mg?H motifs, but also serve as models for hydrogen‐storage materials. Some of them are also recognized to participate in catalytic transformations, such as hydroelementation. Herein, an overview of these molecular magnesium hydrides is given, focusing on their synthesis and structural characterization.     

Spectroscopic study of polycrystalline TiO2 doped with vanadium

The structure of coordination sites (V⁴⁺ ions) and their spatial distribution in the polycrystalline titanium dioxide (rutile) lattice were studied by ESR. It was found that at low degrees of doping, at [V⁴⁺] < 0.5 at.%, the vanadium ions are isotropically distributed in the rutile lattice. At [V⁴⁺] > 0.5 at.% a new microphase with the mixed composition {TiO₂—VO²} is formed. The mixed microphase has a noticeably narrower band gap than the initial TiO₂. Comparison of the photocurrent spectra and the plots of the integral photocurrent vs. vanadium content with the structural data obtained using ESR spectroscopy showed that the formation of the {TiO₂—VO²} microphases deteriorates the photoelectrochemical properties of the modified photoelectrodes. Synthetic procedures interfering the formation of such microphases in the doped rutile are discussed.     

Separation of Ba and Pb from aqueous solutions of Sr with commercially available “precipitated active manganese dioxide”

Ba and Pb radionuclides can be removed from Sr in aqueous solution in both sodium acetate and acetic acid, containing 20 mg Sr carrier, by stirring with small (0.1–0.5 g) amounts of solid manganese dioxide. In tracer experiments⁸⁵Sr was separated with only small losses from¹³³Ba and²¹⁰Pb by separation factors of 87 and 135, respectively. The separation factor is defined here as the % of the initial⁸⁵Sr activity/ % of the initial¹³³Ba or²¹⁰Pb activity remaining in the aqueous phase after MnO₂ contact. The applicability of this technique for removing Ba and Pb radionuclides in the analysis of⁹⁰Sr in environmental samples (especially milk) is discussed.     

Effect of ZrC Nanopowders on Enhancing the Hydro/Dehydrogenation Kinetics of MgH2 Powders

Hydrogen has been receiving great attention as an energy carrier for potential green energy applications. Hydrogen storage is one of the most crucial factors controlling the hydrogen economy and its future applications. Amongst the several options of hydrogen storage, light metal hydrides, particularly nanocrystalline magnesium hydride (MgH₂), possess attractive properties, making them desired hydrogen storage materials. The present study aimed to improve the hydrogen storage properties of MgH₂ upon doping with different concentrations of zirconium carbide (ZrC) nanopowders. Both MgH₂ and ZrC were prepared using reactive ball milling and high-energy ball milling techniques, respectively. The as-prepared MgH₂ powder was doped with ZrC (2, 5, and 7 wt%) and then high-energy-ball-milled for 25 h. During the ball milling process, ZrC powders acted as micro-milling media to reduce the MgH₂ particle size to a minimal value that could not be obtained without ZrC. The as-milled nanocomposite MgH₂/ZrC powders consisted of fine particles (~0.25 μm) with a nanosized grain structure of less than 7 nm. Besides, the ZrC agent led to the lowering of the decomposition temperature of MgH₂ to 287 °C and the reduction in its apparent activation energy of desorption to 69 kJ/mol. Moreover, the hydrogenation/dehydrogenation kinetics of the nanocomposite MgH₂/ZrC system revealed a significant improvement, as indicated by the low temperature and short time required to achieve successful uptake and release processes. This system possessed a high capability to tackle a long continuous cycle lifetime (1400 h) at low temperatures (225 °C) without showing serious degradation in its storage capacity.     

Renewed Insight into the Promoting Mechanism of Magnesium Hydride on Ammonia Borane

Our previous study found that mechanically milling with magnesium hydride (MgH₂) could dramatically improve the dehydrogenation property of ammonia borane (AB). Meanwhile, it appears that the MgH₂ additive maintains its phase stability in the milling and subsequent heating process. In an effort to further the mechanistic understanding of the AB/MgH₂ system, we reinvestigated the property and structure evolution in the hydrogen release process of the AB/0.5MgH₂ sample. Property examination using volumetric method and synchronous thermal analyses showed that the AB/0.5 MgH₂ sample releases ～13.8 wt % hydrogen after being heated at 300 °C. This hydrogen amount is in excess of that available from AB, indicative of the participation of a faction of MgH₂ in the dehydrogenation process of AB. Structural and chemical state analyses using Fourier transformation infrared spectroscopy and solid‐state ¹¹B nuclear magnetic resonance techniques further showed that part of MgH₂ participates in the dehydrogenation process of AB from the first step, resulting in the formation of Mg? B? N? H intermediate species. The incorporation of Mg in AB is believed to be a crucial event leading to dehydrogenation property improvements, particularly for the release of the last equivalent of H₂ in AB at relatively moderate temperature. These findings have provided renewed insight into the promoting mechanism of MgH₂ on the hydrogen release from AB.     

Ti掺杂的MgH2和Mg2NiH4的放氢性能

以TiF₃和Ti(OBu-n)₄为催化剂, 研究了Ti离子掺杂对MgH₂和Mg₂NiH₄放氢性能的影响. 结果表明, 未掺杂的MgH₂起始放氢温度为420 ℃, 掺杂TiF₃和Ti(OBu-n)₄后分别降低到360和410 ℃; Mg₂NiH₄在掺杂TiF₃后放氢温度由230 ℃降低到220 ℃, 而掺杂Ti(OBu-n)₄后没有变化. 可见无论对MgH₂或Mg₂NiH₄, 在降低放氢温度方面TiF₃都明显优于Ti(OBu-n)₄. 另外, 研究还发现, TiF₃掺杂对MgH₂放氢动力学有显著的提高, 但对Mg₂NiH₄没有明显的提高. 结合XRD和FTIR的测试分析, 我们认为: 催化作用很大程度上取决于氢化物自身的晶体结构和催化剂的电子结构; 降低氢化物放氢温度和提高动力学性能的原因是催化剂与氢化物之间的相互作用削弱了氢化物中Mg—H或Ni—H键, 使得活泼的H…H原子对容易形成, 从而有利于H₂的释出.     

Mössbauer study on the CMR double perovskite AFe<Subscript>0.5</Subscript>Mo<Subscript>0.5</Subscript>O<Subscript>3 </Subscript>with A=(Ba,Sr) or (Sr,Ca): Chemical pressure effect

Summary The double perovskites, AFe_0.5Mo_0.5O₃with A=(Ba,Sr) or (Sr,Ca), were prepared by a sol-gel method, and the substitution effect at site A was studied by M?ssbauer spectrometry. In the M?ssbauer spectra of the double perovskite (Ba, Sr)Fe_0.5Mo_0.5O₃, the isomer shifts decreased fromδ=0.72 mm/s to δ=0.4 mm/s and the internal magnetic fields increased with the increase of the Sr content. The Ba-rich samples were shown to contain superparamagnetic components under the same preparation conditions. Better crystallinity and larger hyperfine fields were obtained when 5% of the Sr-content of SrFe_0.5Mo_0.5O₃was substituted by Ca as compared with substitution by Ba. Phonon density of states (DOS) of SrFe_0.5Mo_0.5O₃substituted with Ca or Ba were obtained by nuclear inelastic scattering. The peaks of phonon DOS were shifted, depending on chemical compression/expansion of the lattice. The chemical pressure effect could be observed in the M?ssbauer spectra and the phonon DOS spectra.     

Cation disorder and phase transitions in the four-layer ferroelectric Aurivillius phases ABi4Ti4O15 (A=Ca, Sr, Ba, Pb)

Crystal structures of a series of bi-layered compounds ABi₄Ti₄O₁₅ (A=Ca, Sr, Ba, Pb) have been investigated using a combination of synchrotron X-ray and neutron powder diffraction data. All four oxides adopt an orthorhombic structure at room temperature and the structures have been refined in space group A2₁am. This orthorhombic structure is a consequence of a combination of rotation of the TiO₆, resulting from the less than optimal size of the A-type cation, and displacement of the Ti atoms towards the Bi₂O₂ layers. There is partial disorder of the Bi and A-type cations over two of the three available sites, which increases in the order Ca<Sr and Pb<Ba.     

A New Carrier for Direct Spectrographic Determination of Trace Alkalies and Alkaline Earths in High Purity Thorium Oxide

Abstract

A d.c. arc emission spectrographic method is described for the determination of sub ppm levels of Ba, Li and Sr and ppm levels of Cs, K and Na in high purity thorium oxide. The method which employs RbCl as carrier is simple and rapid and does not involve the use of any special atmosphere for exciting the sample. It was observed that RbCl shows definite improvement over AgCl in carrier action. The carrier distillation technique reported makes possible the following range of estimations: Li: 0.02 – 5.0 ppm, Ba, Sr: 0.5 – 20 ppm, Cs: 2–50 ppm, K,Na: 5–100 ppm. The detection limit obtained by this method for Li and Sr is lower than that obtained by the earlier carrier-distillation methods reported as well as other direct sensitive techniques of Zeidel and Avni for the analysis of Tho₂. The precision of the method ranges from 8% for Cs to 17% for Ba. Volatilization studies on the choice of carrier material are presented.     

Preparation of Barium Titanate and Related Materials by the Alkoxide-Hydroxide Route

Formation of perovskites MTiO₃ (M = Ba, Sr) at low temperature (< 100°C) by heterogeneous reactions of M(OH)₂ aqueous solutions with hydrolysis products of Ti(OBu)₄ is discussed. Reaction products are characterized by means of chemical and X-ray analysis, and electron microscopy. Reaction rate is enhanced by hydroxide concentration and reaction temperature, and increases drastically when Sr(OH)₂ is used instead of Ba(OH)₂. Particles shape and size of the perovskites formed is independent of morphology of Ti(OBu)₄ hydrolysis product. The main factor of perovskites morphology regulation lies in the rate of M(OH)₂ absorption. Lowering the absorption rate allows us to prepare uniform perovskite powders which can be readily sintered to ceramic bodies with high dielectric permittivity.     

Influence of Defects on the Stability and Hydrogen-Sorption Behavior of Mg-Based Hydrides

This review deals with the destabilization methods for improvement of storage properties of metal hydrides. Both theoretical and experimental approaches were used to point out the influence of various types of defects on structure and stability of hydrides. As a case study, Mg, and Ni based hydrides has been investigated. Theoretical studies, mainly carried out within various implementations of DFT, are a powerful tool to study mostly MgH₂ based materials. By providing an insight on metal-hydrogen bonding that governs both thermodynamics and hydrogen kinetics, they allow us to describe phenomena to which experimental methods have a limited access or do not have it at all: to follow the hydrogen sorption reaction on a specific metal surface and hydrogen induced phase transformations, to describe structure of phase boundaries or to explain the impact of defects or various additives on MgH₂ stability and hydrogen sorption kinetics. In several cases theoretical calculations reveal themselves as being able to predict new properties of materials, including the ways to modify Mg or MgH₂ that would lead to better characteristics in terms of hydrogen storage. The influence of ion irradiation and mechanical milling with and without additives has been discussed. Ion irradiation is the way to introduce a well-defined concentration of defects (Frankel pairs) at the surface and sub-surface layers of a material. Defects at the surface play the main role in sorption reaction since they enhance the dissociation of hydrogen. On the other hand, ball-milling introduce defects through the entire sample volume, refine the structure and thus decrease the path for hydrogen diffusion. Two Severe Plastic Deformation techniques were used to better understand the hydrogenation/dehydrogenation kinetics of Mg- and Mg₂Ni-based alloys: Equal-Angular-Channel-Pressing and Fast-Forging. Successive ECAP passes leads to refinement of the microstructure of AZ31 ingots and to instalment therein of high densities of defects. Depending on mode, number and temperature of ECAP passes, the H-sorption kinetics have been improved satisfactorily without any additive for mass H-storage applications considering the relative speed of the shaping procedure. A qualitative understanding of the kinetic advanced principles has been built. Fast-Forging was used for a “quasi-instantaneous” synthesis of Mg/Mg₂Ni-based composites. Hydrogenation of the as-received almost bi-phased materials remains rather slow as generally observed elsewhere, whatever are multiple and different techniques used to deliver the composite alloys. However, our preliminary results suggest that a synergic hydrogenation / dehydrogenation process should assist hydrogen transfers from Mg/Mg₂Ni on one side to MgH₂/Mg₂NiH₄ on the other side via the rather stable a-Mg₂NiH_0.3, acting as in-situ catalyser.     

Determination of calcium in silicate rocks by potentiometric titration with ethyleneglycol-bis-(2-Aminoethylether)tetraacetic acid and a calcium-selective electrode

The method reported for the determination of calcium in silicate rocks involves titration with ethyleneglycol-bis(2-aminoethylether)tetraacetic acid (EGTA) to a potentiometric end-point. A Crytur calcium-selective electrode is used; the selectivity constants (K_ca,x) are less than 10^-3 for Mg, Ba, Sr, K and Na. The method can be used for calcium contents of 2% or higher (and exceptionally for lower contents), in the presence of up to 60% Mg, 33% AI, 17% Fe, 5% Ba, 5% Ti, and 2.5% Mn. Al, Fe and Ti are masked with sulphosalicylic acid, barium is precipitated as its sulphate, and manganese is bound as its cyanide complex. If the magnesium content is less than that of calcium, EDTA can be used as titrant, magnesium being masked with acetylacetone.     

Chemical bonding in ternary magnesium hydrides

The electronic structure of various alkali and alkaline–earth magnesium‐based hydrides was investigated in detail. These types of crystalline compounds show MgH₄ or MgH₆ units ordered within a light‐metal framework. We investigated the nature of the chemical bonding in these units by means of quantum chemical calculations of several related clusters. The properties of the charge density of the clusters, within the framework of the theory of atoms in molecules, was analyzed. A further set of computations of the band structure of the solid hydrides was conducted using a state‐of the‐art density functional‐based method and the mechanism of stabilization of the Mg? H units is discussed. It was found that the properties obtained at the molecular level correlate well with those of the solid crystals, indicating the molecular nature of the extended systems in which the units MgH_x, x = 4, 6, are stabilized by means of Mg? H closed‐shell interactions. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 150–164, 2003     

Transformation of Metallic Ti to TiH2 Phase in the Ti/MgH2 Composite and Its Influence on the Hydrogen Storage Behavior of MgH2

The current work explores the in-situ formation of TiH₂ additive in a Ti/MgH₂ nanocomposite system. Mild mechanical milling leaves Ti chemically unchanged, while formation of stable TiH_2–x occurs upon strong mechanical milling. TiH_2–x further transforms to TiH₂ upon recycling the powder (dehydrogenation and subsequent hydrogenation) and lowers the activation energy of MgH₂ to 89.4 kJ (mol H₂)⁻¹ [E_a of as-received MgH₂ is 153 kJ (mol H₂)⁻¹]. This work also reiterates that metallic Ti additive mixed MgH₂ requires strong mechanical milling for better H₂ ab/de-sorption performance. The current observations support the view that lattice strain may be an important factor in the catalysis of additives incorporated MgH₂ hydrogen storage systems.