Synthesis of NaAlH4-based hydrogen storage material using milling under low pressure hydrogen atmosphere

The present study highlights the advantages of milling NaH/Al under moderate hydrogen pressure as a favourable production step for NaAlH₄-based hydrogen storage materials. Firstly, it is demonstrated that NaAlH₄ can be obtained by applying a moderate hydrogen pressure (6–12 bar) during milling of NaH and Al with and without the presence of an inexpensive catalyst (TiCl₄). The yield of NaAlH₄ depends critically on process parameters, such as hydrogen pressure and milling time. A fully converted product is capable of reversible hydrogen storage without any activation procedure. Under optimized conditions, a capacity of 4.2 wt.% was achieved and kinetics in the first desorption are comparable to NaAlH₄ doped with TiCl₃. Secondly, the synthesis has been optimized towards shorter milling times. By applying a few absorption/desorption cycles to material that was partially converted during milling, almost full reversible storage capacity can be reached. In addition, kinetics is extremely enhanced. For example, such material exhibits an optimum capacity already after two sorption cycles at 100 bar and 125 °C and allows to absorb 80% of the reversible hydrogen content within a few minutes.     

Investigation of hydrogen discharging and recharging processes of Ti-doped NaAlH4 by X-ray diffraction analysis (XRD) and solid-state NMR spectroscopy

The processes occurring in the course of two sequential hydrogen discharging and recharging cycles of Ti-doped sodium alanate were investigated in parallel using XRD analysis and solid-state NMR spectroscopy. Both methods demonstrate that in hydrogen storage cycles (Eq. (1)) the majority phases involved are NaAlH₄, Na₃AlH₆, Al and NaH. Only traces of other, as yet unidentified phases are observed, one of which has been tentatively assigned to an Al–Ti alloy on the basis of XRD analysis. The unsatisfactory hydrogen storage capacities heretofore observed in cycle tests are shown to be due entirely to the reaction of Na₃AlH₆ with Al and hydrogen to NaAlH₄ (Eq. (1), 2nd hydrogenation step) being incomplete. Using XRD and NMR methods it has been shown that a higher level of rehydrogenation can be achieved by adding an excess of Al powder.     

Reversible hydrogen storage properties of NaAlH4 catalyzed with scandium

A systematic study was carried out to compare the reversible hydrogen storage properties of Sc-doped NaAlH₄ with Ti-doped NaAlH₄. Uncycled and cycled (five times) samples of 1, 2 and 4 mol% Sc-doped NaAlH₄ and 4 mol% Ti-doped NaAlH₄ were examined. Temperature programmed desorption, constant temperature desorption, and constant temperature cycling results revealed the following: Sc-doped NaAlH₄ realized very fast hydrogenation kinetics, reasonably fast dehydrogenation kinetics and minimal hydrogen capacity losses during cycling over a broad range of temperatures without any additives or co-dopants. Sc-doped NaAlH₄ also exhibited comparable kinetics and a higher hydrogen storage capacity compared to Ti-doped NaAlH₄, depending on the temperature, metal dopant level, and cycling. Sc exhibited this superior performance over Ti perhaps because Sc is an earlier transition metal than Ti.     

Comparison of hydrogen cycling kinetics in NaAlH4-carbon aerogel composites synthesized by melt infusion or ball milling

This work compares the initial desorption and hydrogen cycling kinetics of NaAlH₄ melt infused into carbon aerogel with NaAlH₄-carbon aerogel composite synthesized by ball milling. Samples having comparable carbon content (47.4 wt%) prepared by either method yield virtually identical desorption and cycling behavior. Furthermore, the ball milled material can be made with lower carbon content and still maintain only slightly reduced kinetic improvements. Surprisingly, there is no evidence for mixed infused-like and bulk-like behavior even for carbon content as low as 9.1 wt%. Unlike melt infused NaAlH₄, where co-infusion of TiCl₃ catalyst has proven difficult, the ball milled material can easily accommodate co-doping with both carbon and TiCl₃. The inclusion of carbon with TiCl₃ results in a modest but significant improvement in kinetics compared to NaAlH₄ doped with TiCl₃ alone, especially for rehydrogenation. Ball milling with activated carbon produces an improvement very similar to that of carbon aerogel, whereas graphene and graphite have smaller effects, in that order. During cycling of the second stage NaAlH₄ reaction, i.e., Na₃AlH₆ ↔ 3NaH + Al + (3/2)H₂, addition of either activated carbon or carbon aerogel at 23.1 wt%, but without TiCl₃, results in kinetic performance as good as or better than NaAlH₄ doped with TiCl₃.     

TEM characterization of pure and transition metal enhanced NaAlH4

Possibilities and limitations in using transmission electron microscopy to characterize pure NaAlH₄ and transition metal enhanced NaAlH₄ have been investigated in detail. NaAlH₄ is extremely sensitive to O₂ and H₂O and must be handled under inert atmosphere at all times. Furthermore, it is highly unstable under the electron beam and only basic techniques such as diffraction contrast imaging and selected area diffraction that can be performed with a low flux electron beam can be used without the NaAlH₄ decomposing. By comparison, phases containing transition metal additive are very stable under the electron beam. The latter are investigated by a combination of high resolution imaging, electron diffraction and spectroscopy to determine distribution, composition, crystal structure and defect content in ball milled and hydrogen cycled, TiCl₃ and FeCl₃ enhanced NaAlH₄. It is demonstrated that a large amount of the added Ti or Fe is located at the surface of the NaAlH₄ grains as a combination of crystalline and amorphous Al_1−xTM_x (TM = Ti, Fe) nanoparticles.     

X-ray diffraction study on formation of ErNbO4 powder

The formation of ErNbO₄ powder, prepared by calcining an Er₂O₃ (50 mol%) and Nb₂O₅ (50 mol%) powder mixture at 1100 and 1600 °C for different durations, was investigated by using X-ray diffraction. The experimental results have displayed that although the solid-state reaction had started to some extent when the mixture was pre-calcined at 1100 °C for a duration of 13 h, the two original phases Er₂O₃ and Nb₂O₅ still dominated the mixture. When the duration of the calcination reaction was increased to 120 h at the same temperature, the resultant mixture experienced a nearly complete phase transformation. Accordingly, the ErNbO₄ phase was dominant phase in the mixture. Nevertheless, a small portion of the raw powder still existed in the mixture. When the calcining temperature was elevated to 1600 °C, ErNbO₄ powder with higher purity could be obtained for a relatively much shorter duration (only up to several tens of hours). A simple formation mechanism of ErNbO₄, an elevated-temperature-assisted solid-state chemical reaction: Er₂O₃+Nb₂O₅2ErNbO₄, is suggested. In addition, the present experimental results offer important evidence for the formation of the additional phase ErNbO₄ induced in Er:LiNbO₃ crystals by vapour transport equilibration (VTE) treatment.     

Structure and hydrogen storage properties of MgH2 catalysed with La2O3

The catalytic effect of the addition of lanthanum oxide (La₂O₃), in the range 0.5–2.0 mol%, on the hydrogen storage properties of MgH₂ prepared by ball milling has been studied. The addition of La₂O₃ reduces the formation during milling of the metastable orthorhombic γ-MgH₂ phase. The desorption rate of samples with 1 and 2 mol% La₂O₃ comes out to be about 0.010 wt% per second at 573 K under an hydrogen pressure of 0.3 bar, better than for sample with 0.5 mol% La₂O₃. The presence of LaH₃ after hydrogenation/dehydrogenation cycles has been observed in all samples. The sample with 1 mol% of La₂O₃ gives a lower hysteresis factor compared with sample with 2 mol%.     

Structural study on Zr0.39Ni0.61 and (Zr0.39Ni0.61)D0.59 amorphous alloys by neutron and X-ray diffraction

Neutron and X-ray diffraction and reverse Monte Carlo (RMC) simulation were performed on Zr_0.39Ni_0.61 and (Zr_0.39Ni_0.61)D_0.59 amorphous alloys to investigate the rearrangement of metal atoms due to the deuterium absorption. The RMC models showed that the Zr–Zr distances slightly increase but the Zr–Ni and Ni–Ni distances remain virtually unchanged after the deuterium absorption. Additionally, the Voronoi polyhedral analysis of the RMC configurations showed that there is not much difference in the local structure around Zr and Ni between Zr_0.39Ni_0.61 and (Zr_0.39Ni_0.61)D_0.59 amorphous alloys.     

Investigation of reaction between LiNH2 and H2

The reactions for LiNH₂ under a H₂ and an Ar flow were investigated, respectively. The results showed that LiNH₂ can be converted into LiH and NH₃ by reacting with H₂ under a H₂ flow condition, whereas LiNH₂ is converted into Li₂NH and NH₃ by decomposition under an Ar flow. Moreover, the reaction between LiNH₂ and H₂ can be accelerated by mixing LiNH₂ with LiH as well as doping LiNH₂ with TiCl₃. The confirmation of reaction between LiNH₂ and H₂ is helpful for the deeper insight in the systems of Li–N–H and Li–Mg–N–H for hydrogen storage materials.     

Microstructural evolution of some MgO–TiO2 and MgO–Al2O3 powder mixtures during high-energy ball milling and post-annealing studied by X-ray diffraction

High-energy dry ball-mill and post-anneal processing were applied to synthesize MgTiO₃ and Mg₂TiO₄ single crystalline phases from the predetermined compositions of MgO–TiO₂ powder mixtures. Also, the experiments were performed to show that it is possible to prepare MgAl₂O₄ single crystalline phase from the predetermined composition of MgO–Al₂O₃ powder mixture only by employing high-energy dry ball milling, i.e. without post-annealing the milled samples. In contrast, fully developed single crystalline powders of MgTiO₃ and Mg₂TiO₄ were obtained after post-annealing the milled samples for 1 h at 900 and 1200 °C, respectively.     

Crystal structure of Nax(MoO)5(P2O7)4 studied by synchrotron X-ray diffraction

The crystal structure of sodium pentamolybdyl tetradiphosphate Na_x(MoO)₅(P₂O₇)₄ has been determined from synchrotron diffraction data collected at 293 K on two microcrystals. The compound crystallizes in a monoclinic space group I 1 1 2/a (no. 15, setting 11), with unit cell parameters a = 22.905(3), b = 23.069(2), c = 4.8537(2) Å, γ = 90.641(9)° and a = 22.898(3), b = 23.056(2), c = 4.8551(2) Å, γ = 90.82(1)°, for crystals I and II, respectively. The structure is pseudo-tetragonal, and the crystals are pseudo-merohedrally twinned by 90° rotation around the c-axis. The structure closely resembles the previously reported Li-deintercalated Mo_1.3OP₂O₇ [V.V. Lisnyak, N.V. Stus, P. Popovich, D.A. Stratiychuk, Ya. Filinchuk, V.M. Davydov, J. Alloys Compd. 360 (2003) 81–84]. Comparison of the two structures led us to conclude that the Mo₂ and Mo₃ clusters were erroneously identified in Mo_1.3OP₂O₇. A revised structure of Mo_1.3OP₂O₇ contains a fully occupied oxygen site instead of the 16% occupied Mo(2) site, thus the revised formulae for the Li-deintercalated compound is (MoO)₅(P₂O₇)₄. In both structures, the (MoO)₅(P₂O₇)₄ framework strongly resembles the one in the earlier reported Ag(MoO)₅(P₂O₇)₄, while the location of Na and Ag atoms differ.     

TiCrV hydrogen storage alloy studied by positron annihilation spectroscopy

We have studied the structural changes of Ti₂₄Cr₃₆V₄₀ alloy prepared by arc-melting using positron annihilation spectroscopy and X-ray diffraction (XRD) as functions of the number of hydrogen pressure swing cycles and degassing temperature. As the hydrogen storage capacity decreased with the number of pressure swing cycles, both positron lifetime and XRD peak width increased. Upon hydriding, the crystal structure changed from bcc to bct with increased lattice constants. The increase in positron lifetime is due to the volume expansion caused by hydride formation. After degassing at 500 °C, the hydrogen storage capacity recovered to 95% of the initial level, and the XRD peak width and the lattice constants nearly completely returned to their initial values. However, the positron lifetime was still longer than the initial level suggesting the survival of dislocations. The degradation of hydrogen storage capacity is probably caused by both hydride formation and the generation of dislocations.     

Microstructural evolution during controlled ball milling of (Mg2Ni+MgNi2) intermetallic alloy

Nearly dual-phase Mg–Ni alloy fabricated by ingot metallurgy (IM) and comprising 30 vol% Mg₂Ni and 61 vol% MgNi₂ intermetallic compounds (remaining 9 vol% of unreacted Mg) was mechanically (ball) milled under controlled shearing for 10, 30, 70 and 100 h. The majority of the medium- and small-sized powder particles exhibited a relatively homogeneous microstructure of milled Mg₂Ni and MgNi₂. A fraction of large-sized particles developed the ‘core and mantel’ microstructure after milling for 70 and 100 h. The ‘core’ contains poorly milled MgNi₂ particles and the ‘mantel’ is a thoroughly milled mixture of Mg₂Ni, MgNi₂ and, possibly, residual Mg. X-ray diffraction provides evidence of nanostructurization and eventual amorphization of a fraction of a heavily ball milled Mg₂Ni phase. The remnant Mg₂Ni developed a nanocrystalline/submicrocrystalline structure. The co-existing MgNi₂ phase developed a submicrocrystalline structure within the powder particles. The results are rationalized in terms of enthalpy effects by the application of Miedema’s semi-empirical model to the phase changes in ball milled intermetallics.     

Mg6Ir2H11, a new metal hydride containing saddle-like [IrH4] and square-pyramidal [IrH5] hydrido complexes

Mg₆Ir₂H₁₁ has been synthesised by both hydrogenation of the intermetallic compound Mg₃Ir at 20 bar and 300 °C, and sintering of the elements at 500 °C under 50 bar hydrogen pressure. Neutron powder diffraction on the deuteride indicates a monoclinic structure (space group P2₁/c, Mg₆Ir₂D₁₁: a=10.226(1), b=19.234(2), c=8.3345(9) Å, β=91.00(1)°, T=20 °C) that is closely related to orthorhombic Mg₆Co₂H₁₁. It contains a square-pyramidal [IrH₅]⁴⁻ complex and three saddle-like [IrH₄]⁵⁻ complexes of which one is ordered and two are disordered. Five hydride anions H⁻ are exclusively bonded to magnesium. The compound has a red colour, is presumably non-metallic and decomposes under 3 bar argon at 500 °C into Mg₃Ir, iridium and a previously unreported intermetallic compound of composition Mg₅Ir₂.     

Another unusual phenomenon for Zr7Ni10: structural change in hydrogen solid solution and its conditions

Zr₇Ni₁₀ has three hydrogen occlusion phases, , β and γ, and the following unusual features are known for the phase transitions in the Zr₇Ni₁₀–H₂ system: (1) The intermediate hydride phase (β) appears only during dehydrogenation but not during hydrogenation, and (2) The continuous hydrogen solid solution phase () exhibits a much higher hydrogen solubility during hydrogenation than during dehydrogenation. In order to clarify the mechanism about the difference in the hydrogen solubility of the phase, the relation between the pressure-composition isotherms and corresponding structural change has been examined by a conventional volumetric method and X-ray diffraction. Through the examination, we discovered that the crystal structure of the phase, which undergoes hydrogenation followed by dehydrogenation, is different from that of its pure metal phase, where the crystal structure of the dehydrogenated phase changes from an orthorhombic structure to a tetragonal structure. The conditions causing the structural change were then examined, and it has been found that the phase maintains its original orthorhombic structure as long as it is hydrogenated so as not to absorb enough hydrogen to change it to the hydride with a higher hydrogen content (γ). The phenomenon can be understood as one of the hydrogen-assisted phase transitions such as hydrogen-induced amorphization (HIA) in the sense that the phase transition requires hydrogenation under special conditions.     

Synthesis and conductivity of GdPO4 nanorods: Impacts of particle size and Ca doping

A series of Gd_1−xCa_xPO₄·nH₂O nanorods were prepared using a simple hydrothermal reaction which was optimized by tuning the pH values of the precursor. The resulted nanorods were characterized by X-ray diffraction, transmission electron microscopy, Fourier transformation infrared spectroscopy, and alternative current impedance technique. It is demonstrated that all Gd_1−xCa_xPO₄·nH₂O nanorods crystallized in a pure hexagonal structure. For x = 0, the particle dimension decreased with increasing the pH value. For x > 0, the solid solution limit of Ca²⁺ in GdPO₄·nH₂O nanorods was about 3 mol%, below which the lattice volume increased with increasing the doping level of Ca²⁺. The conductivities of nanorods were highly dependent on both the particle size and Ca²⁺ concentration, as indicated by the increased conductivity as particle size reduces or Ca²⁺ doping level increases. These observations were understood in terms of the dehydration and the introduction of HPO₄²⁻ defects by Ca²⁺ doping.     

LiAlD4 with VCl3 additives: Influence of ball-milling energies

The reduction reaction of VCl₃ mixed with LiAlD₄ is studied for different ball-mill energies. From low to high ball-milling energies VCl₃ is reduced by the formation of Li–V–Cl metastables phases, LiCl and free Al and V or Al–V phases. It is also shown that the enhancement of the kinetics reaches a limit with increased ball-mill energy. From measurements of the released hydrogen with a Sieverts apparatus and X-ray and neutron diffraction, it is shown that even under mild conditions, at or close to room temperature, the two first steps of the decomposition of VCl₃-enhanced LiAlD₄ occurs during the first weeks after milling at temperatures in the range 20–50 °C.     

Ba4In2−x(CO3)1+xO6−2.5x and Ba4In0.8Cu1.6(CO3)0.6O6.2—new indium oxycarbonates closely related to n=3 Ruddlesden–Popper phases

Investigations of phase relations in the Ba-rich part of the In₂O₃–BaO(CO₂)–CuO pseudo-ternary system at 900 °C have revealed the existence of new indium–copper oxycarbonate – Ba₄In_0.8Cu_1.6(CO₃)_0.6O_6.2. Rietveld refinement of the X-ray powder diffraction data combined with infrared studies gives evidence that this phase is a oxycarbonate crystallising in the tetragonal structure (space group I4/mmm) with unit cell parameters: a=4.0349(1) Å and c=29.8408(15) Å. In the binary part of the In₂O₃–BaO(CO₂) system we have identified the occurrence of Ba₄In_2−x(CO₃)_1+xO_6−2.5x oxycarbonate solid solution showing a crystal structure also described by I4/mmm space group, but with the unit cell parameters: a=4.1669(1) Å and c=29.3841(11) Å for x=1. The existence range of this phase, −0.153<x<0.4, includes chemical compositions of earlier found phases: Ba₅In_2+xO_8+0.5x with 0≤x≤0.45 (known as the -solid solution), as well as the binary Ba₄In₂O₇ phase. The crystal structures of both new oxycarbonates are isomorphic and related to n=3 member of the Ruddlesden–Popper family.     

CVD掺硅金刚石残余应力的X射线衍射和拉曼光谱分析(英文)

采用X射线衍射(XRD)和拉曼光谱2种方法测量了不同硅碳比的CVD掺硅金刚石薄膜的残余应力。采用偏压增强热丝化学气相沉积装置在硬质合金基底上制备了掺硅金刚石薄膜,将正硅酸乙酯以不同的体积比溶解在丙酮中以使得反应气体中的硅碳比从0.1%变化到1.4%,从而控制掺硅金刚石薄膜的掺杂浓度。SEM和XRD的表征结果显示,随着硅掺杂浓度的增加,金刚石薄膜的晶粒尺寸减小,而金刚石(110)的晶面则逐渐占优。XRD法是测量入射角从0°到45°变化时对应的金刚石(220)面XRD衍射峰,并采用sin2ψ方法计算掺硅金刚石薄膜的残余应力。拉曼谱法则是通过检测金刚石特征峰偏移1332cm1位置的偏移量来测量残余应力。2种方法测得的残余应力随着硅掺杂含量的升高显示出良好的一致性,所有的硅掺杂金刚石的残余应力均为压应力,Si/C摩尔比为0.1%的薄膜具有最高的残余应力,为~1.75GPa(拉曼谱法)或~2.3GPa(XRD法)。随着硅掺杂浓度的进一步升高,薄膜的残余应力则稳定在~1.3GPa左右。