M. Kemali

Magnetic properties of commercial metal hydride battery materials

The magnetic properties of two commercial LaNi5-based materials and their variation as a function of hydrogen cycles have been studied using magnetisation measurements. Pure LaNi5 is a Pauli paramagnet, but it has been shown that after hydrogen cycling Ni clusters are formed on its surface. At room temperature these clusters were found to act as superparamagnetic particles. Magnetisation curves have been obtained for a treated LaMM(Ni3.43Mn0.38Al0.29Co0.67)4.77 sample and similar superparamagnetic behaviour has been identified. Measurements have also been made on a modified alloy, of commercial interest, at a range of high and low temperatures. This sample displays ferromagnetic, as well as paramagnetic and superparamagnetic, behaviour. It has been shown that the response at low field increases dramatically upon heating, demonstrating a large growth in the number of superparamagnetic particles present on its surface.

An empirical potential for interstitial hydrogen in some C-15 Laves phase compounds from IINS measurements

The potential wells seen by hydrogen in several interstitial sites in the C-15 Laves phase compounds, ZrTi2 (ZT), ZrCr2 (ZC) and TiCr1.85 (TC), have been investigated using incoherent inelastic neutron scattering (IINS) data. Parameters describing the harmonic terms in the H-potential well in the three compounds are obtained from the first localized hydrogen vibrations as measured by the IINS technique. The H-potential well is created by the sum of the pairwise potentials between the hydrogen and the four metallic atoms forming the different types of tetrahedral site which are labelled g (A2B2), e (AB3) or b (B4) in the C-15 structure. A Born-Mayer potential has been used to model these pairwise interactions, with parameters determined from the IINS. The resulting overall potential energy surface describes well the observed site occupancies and the diffusional behaviour of H in the ZC and TC compounds. On the other hand, due to the higher H content of the ZT sample, H-H interactions have to be included in the model for a proper description of the energy potential well in this compound. The potential energy surface enables us to identify the different diffusion paths of H in the C-15 structure and hence to infer the different time scales of H diffusion (localized and long-range diffusion) that have recently been shown to exist in these compounds (Skripov A V et al 1996 J. Phys.: Condens. Matter 8 L319-24; 1997 J. Alloys Compounds 253/254 432-4).

Water dynamics in Na zeolite P by QENS

Abstract Quasi-elastic neutron-scattering (QENS) measurements have been made using the IRIS spectrometer (ISIS, UK) on water in synthetic Na zeolite P. The quasi-elastic broadening observed is assigned to a rotation of the water molecules.

Interstitial site occupation in α-LaNi5-H studied by deep inelastic neutron scattering

Abstract Deep inelastic neutron scattering (DINS) from hydrogen is a new technique made possible by the advent of accelerator-based neutron sources such as ISIS at the Rutherford Appleton Laboratory. DINS measures both the concentration and the mean kinetic energy of hydrogen in the sample. H concentrations of less than 1% relative to the metal can be measured, since the hydrogen signal is well separated from the metal signal. In situ measurements on un-cycled α-LaNi 5 -H revealed that the sample contained trapped hydrogen after desorption. The changes in H concentration measured with DINS were in good agreement with manometric measurements. The H kinetic energy did not change significantly with concentration, suggesting that the trapped and untrapped H atoms occupy essentially the same interstices.

The use of small angle neutron scattering in the study of hydrogen trapping at defects in metals

Small angle neutron scattering (SANS) techniques have been used to investigate the trapping of hydrogen and deuterium on dislocations in palladium. Calculations of the expected form of this scattering are presented. It is shown that the different scattering lengths of H and D, respectively negative and positive, can be exploited to prove explicitly that the hydrogen is being trapped at the feature that is causing the SANS in the metal sample. In particular, at an edge dislocation, we can expect the H/D to be trapped in the region of lattice dilation below the edge. Thus, because the scattering length of palladium is positive, deuterium trapping will compensate for the reduction in the scattering density in the host lattice and hence will reduce the intensity of the SANS progressively as D is added, until the local scattering density deviation becomes positive. At this point, the overall SANS cross-section passes through a minimum. On the other hand, increasing H trapping will continuously increase the SANS intensity. The model for the expected scattering is described and measurements of the SANS intensities from dislocations in palladium are presented for a series of D concentrations. A minimum is observed in the SANS at about 1% atomic concentration, in reasonable agreement with the theory.

Quasi-elastic neutron scattering study of the hydrogen diffusion in the C15 Laves structure, TiCr1.85

Abstract High-resolution quasi-elastic neutron scattering measurements have been performed on the hydride form of the C15 Laves phase compound, TiCr 1.85 , using the IN10 backscattering spectrometer at the ILL, France. The broadened spectra were observed over the temperature range 313–442 K and over a range of momentum transfers from 0.15 to 1.82 A −1 for a concentration of [H]/[M]=0.15. Only a single Lorentzian convoluted with the resolution function was necessary to fit the data in the whole Q range, suggesting that the diffusion occurs via a single jump mechanism. Values of the tracer diffusion coefficient and the activation energy for diffusion have been extracted from the low- Q sections of the data, and the mean jump length has been determined.

Inelastic neutron-scattering measurements on the C15 Laves phase compounds HfV2Hx

Abstract The C15 Laves phase compounds have been the subject of several investigations due to their ability to absorb large amount of hydrogen, which make them a good candidates as the negative electrode material for metal hydride batteries. The inelastic neutron scattering technique was used on the TFXA spectrometer at RAL, to measure the vibrational frequency of hydrogen in the Laves phase compound HfV2Hx. The measurements were made for different hydrogen concentration and different temperatures. The data obtained indicate that the site occupancy depends on the hydrogen concentration, first the 2Hf-2V site is filled and then when the concentration is increased the 1Hf-3V is also occupied, which is consistent with previous diffraction data.

A Born-Mayer approach to the hydrogen potential in C-15 Laves phase compounds

The potential wells seen by hydrogen in several interstitial sites in the C-15 Laves phase compounds ZrTi 2 , ZrCr 2 and TiCr 1.85 have been investigated using incoherent inelastic neutron scattering (IINS) data. Parameters describing the harmonic terms in the H-potential well in the three compounds are obtained from the first localised hydrogen vibrations as measured by IINS. The H-potential well is created by the sum of the pairwise potentials between the hydrogen and the four metallic atoms forming the different types of tetrahedral sites which are labelled (A 2 B 2 ), (AB 3 ) or (B 4 ) in the C-15 structure. A Born-Mayer potential has been used to model these pairwise interactions with parameters determined from the IINS data. It has been shown that the hydrogen potential well can be described by parameters characteristic of each metal.

Incoherent inelastic neutron scattering from the C-15 Laves phase ZrTi2H3.6

Abstract Incoherent inelastic neutron scattering data were obtained for the C-15 Laves phase compound ZrTi 2 H 3.6 using the time-focused crystal analyzer spectrometer at ISIS, Rutherford–Appleton Laboratory, UK. A doublet and a singlet were observed in the first harmonic region of the localized hydrogen vibrations. This result is compatible with hydrogen occupying the e-type (3Ti–1Zr) site. The measured spectra are discussed in terms of an interatomic potential of the Born–Mayer type.

Neutron diffraction measurements on the C15 Laves phase TaV2Dx

Abstract The C15 Laves phase compounds have been much investigated due to their ability to absorb large amounts of hydrogen reversibly, which make them good candidates for the negative electrode in metal hydride batteries. Neutron diffraction measurements were performed on the C15 Laves phases compound TaV2Dx to identify the site occupancy of deuterium and to investigate the 100 K anomaly observed by Skripov using specific heat measurements. The refinement of the diffraction data as well as a comparison with previous neutron diffraction measurements will be described.