M.P. Pitt

Thermal stability of Li2B12H12 and its role in the decomposition of LiBH4

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).

Neutron diffraction study of the LaNi5–D system during activation

Abstract 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.

Evolution of microstructure in the LaNi5–D system during the early absorption–desorption cycles

Abstract 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.

Time-of-flight neutron powder diffraction with a thick-walled sample cell

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.

Crystalline Al1 − x Ti x phases in the hydrogen cycled NaAlH4 + 0.02TiCl3 system

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 reflections, and they are modeled in the A1 structure type. Quantitative phase analysis indicates that the Al3Ti proportion continues to increase with further H cycles. The formation of Ti-poor Al1 −  x Ti x (x < 0.25) phases in later H cycles is detrimental to hydrogenation kinetics, compared to the starting Ti-richer near-surface Al2Ti/NaAlH4 interface present during the first absorption of hydrogen.