Search

The twice hydrogen (H) cycled planetary milled (PM) NaAlH 4 + x TMCl n (transition metal (TM) = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zr, Pd, Pt; 2 n 4 + x TMCl n (TM = Ti, V, Cr, Fe, Ni; 2 n x 1− x TM x ( x 1− x TM x phases. Very broad reflections in the 2 A d-spacing region for Cr, Mn, Co, and Pd are identified as partially ordered body centred cubic (bcc) A2 and B2 Al 1− x TM x type structures. The amorphous (a-) Al 1− x V x phase observed in the H cycled PM NaAlH 4 + x VCl 3 system ranges in composition from a-Al 90 V 10 ( x = 0.02) up to a-Al 72 V 28 ( x = 0.1). Across the TM series, the Al 1− x TM x particle size ranges as Sc-V 4-25 nm, Cr, Mn, Co 4 phase is observed in the H cycled PM NaAlH 4 + 0.1ScCl 3 system, with unit cell dimensions of a = 4.9995(2) A and c = 11.2893(1) A, compared to the average dimensions of ‘normal’ NaAlH 4 across the TM and rare earth (RE) series with a = 5.0228(1) A and c = 11.3516(1) A.

The twice hydrogen (H) cycled planetary milled (PM) and cryo milled (CM) NaAlH 4 + x TMCl 3 (transition metal (TM) = Ti, V, Fe) systems ( x > 0.1) have been studied by high resolution synchrotron X-ray diffraction, and high resolution transmission electron microscopy (TEM). Intense primary amorphous (a-) Al 1− x TM x halos are evident in diffraction data of PM samples for V and Fe, and in CM samples for Ti, V, and Fe. Weaker primary amorphous Al 1− x Ti x halos are evident in PM samples for Ti. The Ti poor a-Al 1− x Ti x phase observed for NaAlH 4 + x TiCl 3 ( x > 0.1) ranges in composition from a-Al 86.5 Ti 13.5 → a-Al 92 Ti 8 . High resolution TEM studies of the Al 1− x V x phases in the H cycled PM NaAlH 4 + 0.1VCl 3 system demonstrates that a nanoscopic composite morphology can exist between face centred cubic (fcc) crystalline (c-) Al 1− x V x and a-Al 1− x V x phases, with the c-Al 1− x V x /a-Al 1− x V x composite embedded on the NaAlH 4 surface. The amorphous Al 1− x V x reaches ca. 28 at.% V.

The NaAlH4 + xTiCl3 (x < 0.1) system has been studied by a combination of X-ray synchrotron and neutron diffraction, and isotopic H2/D2 scrambling after the completion of the milling process, and the first thermal release of hydrogen (H). An in situ X-ray synchrotron diffraction study of the isochronal release of hydrogen from planetary milled (PM) NaAlH4 + 0.1TiCl3 shows that crystalline (c-) Al1−xTix phases do not form until almost all H is released from the sample, demonstrating that the surface embedded nanoscopic crystalline Al/amorphous (a-) Al50Ti50 composite facilitates the release of H during the very first thermal desorption. Planetary milled (PM) NaAlH4 + xTiCl3 is observed to disproportionate at room temperature, with no NaAlH4 remaining after ca. 200 days. A complete lack of ambient hydrogen release from PM NaAlH4 + 0.1Al (80 nm) measured over 200 days suggests that the nanoscopic a-Al50Ti50 phase is entirely responsible for the hydrogen release during thermal desorption of milled NaAlH4 + xTiCl3. Isotopic H/D exchange has been observed by combined neutron and X-ray synchrotron diffraction on a PM NaAlD4 + 0.04TiCl3 sample, after exposing the milled sample to 20 bar H2 at 50 °C for ca. 6 days. Under these pressure/temperature (P/T) conditions, disproportionation of NaAlD4 is avoided, and ca. 32% of D atoms are exchanged with H atoms. Asymmetrically broadened reflections in the synchrotron data show peak splitting into two unit cell types, one expanded with H, the other remaining close to pure D based unit cell dimensions. The 2-phase model when fitted to the neutron data demonstrates that ca. 56% of D atoms in ca. 58% of all unit cells are exchanged with H, yielding a NaAl(H0.56D0.44)4 composition for the expanded unit cells. HD scrambling (1 bar mixture of H2 and D2 at 23 °C) performed on desorbed H empty PM NaAlH4 + 0.1TiCl3 shows classic H2 + D2 ↔ 2HD equilibrium mixing, demonstrating that nanoscopic Ti containing Al1−xTix surface embedded phases perform a H2 dissociation/recombination function that unadulterated NaAlH4 cannot.

The NaAlH 4 + x TiCl 3 ( x 4 mosaic size (coherence length) determined from the X-ray synchrotron diffraction lineshape, and measurement of the external powder grain dimensions of ca. 250–300 particles by TEM, reveals that after the completion of the milling process, 〈1 1 0〉 {1 1 1} edge dislocated 2–20 nm Al crystallites are dispersed in a Ti rich amorphous (a-)Al 1− x Ti x (0.3 x 50 Ti 50 composite is embedded on the surface of single crystalline NaAlH 4 powder grains. The NaAlH 4 single crystal powder grains are moderately defected with uncorrelated defects, induced from the milling process.

The Zr 2 FeD 5 system has been annealed to 680 °C under ultra high vacuum, and studied in situ by neutron diffraction. The system disproportionates through three distinct regions in temperature. Initially, the tetragonal Zr 2 FeD 5 ( P 4/ ncc ) is retained up to 330 °C, while steadily depleted of D. From 330 °C to 530 °C, a complex multi-phase disproportionation occurs, with the production of cubic ZrD 2 , tetragonal ZrD 2 , tetragonal Zr 2 FeD 5 ( I 4/ mcm ), and growth of the intermetallic ZrFe 2 . At the beginning of the 330–530 °C period, the total atom count from quantitative phase analysis (QPA) indicates the formation of amorphous (a-) Zr 56 Fe 44 . By 530 °C, QPA and peak breadth analysis indicate that ca. 2/3rd of the sample is consumed as very small nanocrystals ( 2 . From 530 °C to 680 °C, the cubic ZrD 2 is almost entirely consumed and depleted of D to form the final mixture of the intermetallic phases Zr 3 Fe and ZrFe 2 . QPA of the final intermetallic mixture yields a Zr:Fe ratio greater than that observed in either the arc melted alloy or the initial Zr 2 FeD 5 deuteride, indicating that a ca. Zr 71 Fe 29 amorphous component was present in the initial arc melted alloy. According to the total atom count by QPA, crystallisation of the Fe richer amorphous Zr 56 Fe 44 phase formed at 330 °C begins at ca. 530 °C, and later by 680 °C, all amorphous phases have completely crystallised to yield a 70.77:26.75:2.47 mol.% mixture of Zr 3 Fe:ZrFe 2 :ZrD 2− x .

High-energy synchrotron in situ X-ray powder diffraction has been used to elucidate the mechanism of the hydriding phase transformation in a LaNi5 model hydrogen storage intermetallic in real time. The transformation proceeds at 10 °C via the transient growth of an interfacial phase, the γ phase, with lattice parameters intermediate between those of the α (dilute solid solution) and β (concentrated hydride) phases. The γ phase forms to partially accommodate the 24% change in unit cell volume between the α and β phases during hydriding and dehydriding. The α, γ and β phases coexist at the nanoscopic level.

Possibilities and limitations in using transmission electron microscopy to characterize pure NaAlH 4 and transition metal enhanced NaAlH 4 have been investigated in detail. NaAlH 4 is extremely sensitive to O 2 and H 2 O 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 NaAlH 4 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, TiCl 3 and FeCl 3 enhanced NaAlH 4 . It is demonstrated that a large amount of the added Ti or Fe is located at the surface of the NaAlH 4 grains as a combination of crystalline and amorphous Al 1− x TM x (TM = Ti, Fe) nanoparticles.

A mechanochemical synthesis process has been used to synthesise aluminium nanoparticles. The aluminium is synthesised via a solid state chemical reaction which is initiated inside a ball mill at room temperature between either lithium (Li) or sodium (Na) metal which act as reducing agents with unreduced aluminium chloride (AlCl 3 ). The reaction product formed consists of aluminium nanoparticles embedded within a by-product salt phase (LiCl or NaCl, respectively). The LiCl is washed with a suitable solvent resulting in aluminium (Al) nanoparticles which are not oxidised and are separated from the by-product phase. Synthesis and washing was confirmed using X-ray diffraction (XRD). Nanoparticles were found to be ∼25–100 nm from transmission electron microscopy (TEM) and an average size of 55 nm was determined from small angle X-ray scattering (SAXS) measurements. As synthesised Al/NaCl composites, washed Al nanoparticles, and purchased Al nanoparticles were deuterium (D 2 ) absorption tested up to 2 kbar at a variety of temperatures, with no absorption detected within system resolution.

The intrinsic ageing of the CaNi 5 D x system has been studied at 80 °C and 9 bar over a period of 13 days. The system displays a distinct two-phase mixture of the intermediate α′ and β phases, whose phase proportions approach 40 and 50 wt.%, respectively at the end of the ageing period. The change in proportion of each intermediate phase and free fcc Ni suggests two periods of Ni depletion. The amount of free Ni in the system is doubled through the ageing period. The orthorhombic symmetry of the α′ and β phase is preserved, albeit with a strong depletion of three of the four Ni positions from the β phase during the second half of the ageing period. That one half of the hexagonally equivalent 2 c Ni position does not release Ni indicates that low symmetry geometrical ordering in orthorhombic symmetry is responsible for the rapid loss of recoverable hydrogen capacity in this system.

The crystallography of the Pd-Dx system has been studied by in situ neutron powder diffraction at 309 °C, in the supercritical region, and, after quenching in the pure β phase to 50 °C, in the two-phase region at 50 °C. Rietveld profile analysis of the supercritical diffraction patterns showed that 14% of D interstitials were occupying tetrahedral interstices, in sharp contrast to previous studies at lower temperatures. Tetrahedral occupancy was maintained through the two-phase region at 50 °C. These results are discussed in the light of first-principles total-energy calculations of hydrogen states in palladium.

During the first few hydrogenation–dehydrogenation cycles of virgin LaNi 5 , the absorption plateau pressure drops sharply, accompanied by powdering of the starting intermetallic. We looked for an explanation of this phenomenon in the microstructural features of the metal that can be studied via diffraction. Five complete absorption–desorption cycles were conducted in an unbroken sequence, with neutron powder diffraction patterns being recorded in cycles 1, 2 and 5. We found that, after activation, the drop in absorption plateau pressure correlates with increasing coherency of the phase transformation in the c -direction, translating to lower elastic strain energy associated with the transformation. The total microstrain in the dehydrided metal is essentially constant after cycle 1. These findings afford a partial understanding of the pressure behaviour in the first few cycles, but the mechanism by which the lattice coherency develops remains to be explained.

The microstructural changes occurring during the initial absorption of deuterium by virgin LaNi 5 at 40°C have been investigated using in-situ neutron powder diffraction. Rietveld profile refinement was used to determine the α and β phase proportions, lattice parameters and microstrains. In absorption, we found that in the two-phase region (i) the lattice parameters of the α and β phases were (within resolution) independent of the phase proportions; (ii) the α-phase diffraction peaks remained essentially unbroadened relative to the virgin metal; (iii) the β-phase peaks were relatively broad with the usual anisotropy of breadth. These findings imply that, as nuclei of β phase form for the first time in a particle that is wholly α phase, the lattice expansion causes pure β crystallites containing a very high density of lattice defects to fracture off the particle, i.e., decrepitate. Hence the nanoscale mixing and strong mechanical interaction between the α and β phases noted in multiply cycled material are not observed during the initial absorption of D atoms, because the lattice parameter misfit cannot be accommodated. In desorption, and subsequently, there is sufficient accommodation of the lattice parameter mismatch between the α and β phases for them to coexist in the same powder particle.

The thermal decomposition of anhydrous Pa3[combining macron] Li2B12H12 was studied in situ by high resolution synchrotron X-ray diffraction. A first-order phase transition can be observed at 355 °C where the unit cell volume expands by ca. 8.7%. The expanded β-Li2B12H12 polymorph simultaneously decomposes to a hydrogen poor γ-Li2B12H12-x phase. Expansion of the unit cell across the discontinuity is consistent with reorientational motion of B12H12(2-) anions, and the presence of a frustrated Li(+) lattice indicating Li ion conduction.

International audience; A metastable niobium hydride, believed to trigger rapid H desorption from the MgH2 matrix, was detected in the course of real time synchrotron diffraction studies of the desorption kinetics of hydrogen in MgH2 with about 5% NbH nanoparticles. In order to completely characterize this metastable phase, we performed in situ neutron diffraction experiments during hydrogenation / dehydrogenation of a magnesium-niobium nanocomposite. At 250°C (under 1 bar of H pressure) a hydride phase NbHx appears. Unfortunately, the available low pressure together with the low q-range did not allow to propose a structural arrangement for this phase. We thus switched to standard neutron diffraction (that is at constant pressure and room temperature) of ball-milled Mg-Nb-H nanocomposites for which various NbH hydrides were detected including the solid solution. It turns out that the formation of these niobium hydrides (metastable or not) is closely related to the milling time and depends on the quality of the starting metal.

Transmission electron microscopy (TEM) has been used to characterize NaAlH4 doped with Ti, after H cycling. NaAlH4 was shown to be highly unstable under the electron beam, and 'knock on' damage lead to a decomposition of NaAlH4 with Na and H evaporating from the sample. All Ti containing phases were stable under the electron beam. After H cycling, the Ti was present as a mixture of amorphous and crystalline Al1-xTix.