Search

The purpose of this study is to compare the thermal and structural stability of single phase Li2B12H12 with the decomposition process of LiBH4. We have utilized differential thermal analysis/thermogravimetry (DTA/TGA) and temperature programmed desorption-mass spectroscopy (TPD-MS) in combination with X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy to study the decomposition products of both LiBH4 and Li2B12H12 up to 600 °C, under both vacuum and hydrogen (H2) backpressure. We have synthesized highly pure single phase crystalline anhydrous Li2B12H12 (Pa-3 structure type) and studied its sensitivity to water and the process of deliquescence. Under either vacuum or H2 backpressure, after 250 °C, anhydrous Li2B12H12 begins to decompose to a substoichiometric Li2B12H12-x composition, which displays a very broad diffraction halo in the d-spacing range 5.85-7.00 Å, dependent on the amount of H released. Aging Pa-3 Li2B12H12 under 450 °C/125 bar H2 pressure for 24 h produces a previously unobserved well-crystallized β-Li2B12H12 polymorph, and a nanocrystalline γ-Li2B12H12 polymorph. The isothermal release of hydrogen pressure from LiBH4 along the plateau and above the melting point (Tm = 280 °C) initially results in the formation of LiH and γ-Li2B12H12. The γ-Li2B12H12 polymorph then decomposes to a substoichiometric Li2B12H(12-x) composition. The Pa-3 Li2B12H12 phase is not observed during LiBH4 decomposition. Decomposition of LiBH4 under vacuum to 600 °C produces LiH and amorphous B with some Li dissolved within it. The lack of an obvious B-Li-B or B-H-B bridging band in the FTIR data for Li2B12H(12-x) suggests the H poor B12H(12-x) pseudo-icosahedra remain isolated and are not polymerized. Li2B12H(12-x) is persistent to at least 600 °C under vacuum, with no LiH formation observable and only a ca. d = 7.00 Å halo remaining. By 650 °C, Li2B12H(12-x) is finally decomposed, and amorphous B can be observed, with no LiH reflections. Further studies are required to clarify the structural symmetry of the β- and γ-Li2B12H12 polymorphs and substoichiometric Li2B12H(12-x).

The hydrogen (H) cycled planetary milled (PM) NaAlH4 + 0.02TiCl3 system has been studied by high resolution synchrotron X-ray diffraction and transmission electron microscopy during the first 10 H cycles. After the first H absorption, we observe the formation of four nanoscopic crystalline (c-) Ti-containing phases embedded on the NaAlH4 surface, i.e. Al2Ti, Al3Ti, Al82Ti18 and Al89Ti11, with 100% of the originally added Ti atoms accounted for. Al2Ti and Al3Ti are observed morphologically as a mechanical couple on the NaAlH4 surface, with a moderately strained interface. Electron diffraction shows that the Al82Ti18 phase retains some ordering from the L12 structure type, with the observation of forbidden (100) ordering reflections in the fcc Al82Ti18 lattice. After 2 H cycles the NaAlH4 + 0.02TiCl3 system displays only two crystalline Ti-containing phases, Al3Ti and Al89Ti11. After 10 H cycles, the Al89Ti11 is completely converted to Al85Ti15. Al89Ti11, Al85Ti15 and Al3Ti do not display any ordering refle...

The hydrogen cycled (H) planetary milled (PM) NaAlH4 + xM (x TiN > TiC > Ti, with rapid hydrogenation kinetics of ca. 23 wt.% H/hour for TiO2 enhanced NaAlH4, equivalent to TiCl3 enhanced NaAlH4. The TiN and TiC are partially reduced by ca. 7 and 22% respectively, and the TiO2 is completely reduced. The location of the reduced Ti cannot be discerned by X-ray diffraction at these minor proportions.

We have studied the complex decomposition mechanism of cubic γ-Mg(BH4)2 (Ia3d, a = 15.7858(1) A) by in-situ synchrotron X-ray diffraction, temperature-programmed desorption, visual observation of the melt, and Fourier transform infrared (FTIR) spectroscopy. The decomposition and release of hydrogen proceeds through eight distinct steps, including two polymorphic transitions before melting, with a new e-Mg(BH4)2 phase at ca. 150 °C. After melting, strong changes in sample color from yellow to brown to gray are consistent with the unknown Mg–B–H phase(s) (that diffract with high d-spacing halos) in the sample changing from an average composition of MgB2H5.3 at 325 °C, to MgB2.9H3.2 at 350 °C, and to MgB4.0H3.7 by 450 °C. From 350 to 450 °C, the crystalline Mg proportion increases. No combination of previously assigned anionic BnHm species (including MgB12H12 and Mg(B3H8)2) can account for the average composition of the unknown proportion of the sample. This is supported by FTIR spectra showing an absence o...

This study elucidates the role of transition metal (TM) additives in enhancing hydrogen (H) reversibility and hydrogenation kinetics for the NaAlH4 system. The isothermal hydrogen absorption kinetics of the planetary milled (PM) NaAlH4 + xTMCln (TM row 1 = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu; row 2 = Zn, Y, Zr, Nb, Mo, Ru, Rh Pd; row 3 = Pt; 2 < n < 5) and cryo-milled (CM) NaAlH4 + xTMCln (TM row 1 = Ti, V, Cr, Fe, Co, Ni, Cu; 2 < n < 3) systems have been measured at 140 °C and 150 bar system pressure. The variation in hydrogenation kinetics across the TM series for NaAlH4 + xTMCln is strongly dependent on the TM species and additive level, milling technique, and the type, structure, and morphological arrangement of nanoscopic Al1–xTMx phases that are embedded on the NaAlH4 surface. In the most interesting case, the surface-embedded Al1–xTix phases in the TiCl3-enhanced NaAlH4 system perform a dual catalytic function, where the outer Al1–xTix surface performs dissociation/recombination of molecular H2 and t...

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 MgH2 + 0.02Ti-additive system (additives = 35 nm Ti, 50 nm TiB2, 40 nm TiC

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.

Isotopic Mg11B2 has been deuterated at 400 °C and 800 bar, with the production of β-Mg(11BD4)2 observed by in situ neutron diffraction. A natural MgB2 sample has been deuterated under similar condi...

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.

Magnesium hydride nanoparticles were synthesized within a carbon aerogel (CA) scaffold using a dibutylmagnesium precursor. The synthesis reaction was tracked using small-angle X-ray scattering (SAXS) to analyze the structural evolution during MgH2 formation. The CA/MgH2 composite was also investigated using X-ray diffraction (XRD) and transmission electron microscopy (TEM) to provide a better representation of the physical system. The CA has a large quantity of 2 nm pores as shown by nitrogen adsorption data. Both SAXS and TEM investigations confirm that MgH2 does form within the 2 nm pores but XRD proves that there is also a significant quantity of larger MgH2 particles within the system. Variations between hydrogen desorption isotherms from the CA/MgH2 composite and bulk MgH2 are detected that are indicative of changes in the decomposition properties of the small fraction of 2 nm MgH2 nanoparticles within the CA/MgH2 composite, changes which match theoretical predictions.

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.

Transmission electron microscopy has been used to study ball milled and H cycled NaAlH4 with 10 mol% TiCl3. Isolated from the main phases in this hydrogen storage system, nanocrystalline aggregates of fcc TiH x (0≤x

The time-of-flight diffraction techniques that are normally practiced at pulsed neutron sources afford opportunities that are not readily available at continuous fixed-wavelength sources. The present work concerns the increasing trend in materials science to study samples in complex non-ambient environments, such as high gas pressure. Taking the example of a sample cell in which a material is studied under fluid pressure, the optimization of the cell design for best data collection rate is considered. The design of primary- and scattered-beam masks for eliminating background scattering from the sample cell and the correction of the data for cell and sample attenuation are addressed. The outputs of this work include a simple expression for the optimum wall thickness of a thick-walled sample cell, a procedure for accurately determining the required mask aperture width for any scattering angle, more compact expressions for some of the results of the work of Paalman & Pings [J. Appl. Phys.(1962),33, 2635–2639] on absorption corrections, and guidance as to the correction of diffraction profiles for cell and background effects. Examples are given, drawn from studies of materials under hydrogen gas pressures up to 1800 bar in cells constructed from Ti2.1Zr and Inconel.

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.

The NaH + Al + 0.02CeCl3 system has been studied by high-resolution X-ray synchrotron diffraction and transmission electron microscopy (TEM), after planetary milling under hydrogen and hydrogen (H) cycling. Isothermal absorption kinetics were determined at 150 °C, and compared with the NaH + Al + 0.02TiCl3 system, indicating that CeCl3 and TiCl3 are equally effective additives, with CeCl3 preferred on the basis of hydrogen storage capacity. After milling, AlCe contains 100% of the Ce. After the first H absorption, we observe two Al1−xCex phases. The first, AlCe, contains ca. 60% of the originally added Ce atoms. The AlCe phase observed after milling and H cycling is chemically disordered, with complete exchange between the Al and Ce sublattices occurring, yielding zero intensity in ordering reflections such as (100). In the absorbed state after H cycling, the remaining 40% of Ce atoms are contained in a cubic Al1−xCex phase not previously observed in the Al-Ce phase diagram. Indexing yields a primitive cubic unit cell of dimension 7.7111 A, in space group P23. Lineshape analysis indicates the AlCe and unknown cubic Al1−xCex phases are ca. 35 nm and 30 nm in dimension respectively. High resolution TEM imaging confirms that both Al1−xCex phases are embedded on the NaAlH4 surface, and localised energy dispersive X-ray spectroscopy (EDS) indicates a ca. 2:1 Al:Ce ratio for the unknown cubic Al1−xCex phase.

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.