D.K. Ross

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.

Monte Carlo simulation of quasielastic neutron scattering from localised and long-range hydrogen motion in C15 Laves phase intermetallic compounds

A number of cubic C15 Laves phase intermetallic compounds are able to absorb large amounts of hydrogen. At low solute concentrations, the protons occupy tetrahedral interstitial sites, which form a network of hexagons. It is believed that the motion of hydrogen occurs on two distinct time-scales; a rapid localised motion around the hexagons, coupled with a slower long-range diffusion between hexagons. Results from Monte Carlo simulations of hydrogen diffusion in a Laves phase compound are presented. It is demonstrated that the incoherent quasielastic neutron scattering function, obtained from the simulations, can be interpreted in terms of a broad and a narrow Lorentzian component, emanating from hydrogen motion on the two time scales. The narrower component follows a Chudley-Elliot model, indicative of long-range diffusion, whilst the broader component can be interpreted in terms of localised motion. The calculated effective jump length for long-range diffusion is significantly in excess of the actual jump-length, in agreement with experimental observation. The model is briefly discussed in relation to hydrogen diffusion in ZrV2H1.1.

Inelastic neutron scattering study on different grades of palladium of varying pretreatment

The inelastic incoherent neutron scattering (IINS) technique has been utilized to study the way in which various kinds of palladium retain hydrogen after different pretreatments. Cold-rolled palladium foil, coarse palladium powder and finely divided palladium samples of different particle size and pretreatment were compared after controlled hydrogenation at room temperature and subsequent short-term dehydrogenation at 20, 50 and 70 °C. In a set of experiments under constant conditions, the incomplete decomposition of the β- and α-hydride phases at the given temperatures were found to be uncorrelated to the morphology of the samples under investigation or to the presence of strongly adherent carbonaceous degradation products at the surface of some of the samples. The dominant factor was the effect of hydrogen trapping due to mechanical pretreatment. It was shown that this effect may also be of some relevance at elevated temperatures. It was also found that IINS is a suitable technique for the analysis of strongly adherent carbons at the surface of metals and metal hydrides and to discriminate between simple molecular structures and organic species of extended size. This is even the case for partial coverage of finely divided powders, the strong electromagnetic absorption and high electrical conductivity raises problems in utilizing other analytical techniques.

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.

Observations of twinning in YBa2Cu3O6+x, 0 < x < 1, at high temperatures

We report here on the nature of the twinning associated with the tetragonal to orthorhombic I (T/OI) phase transition in YBCO, for temperatures between 500 and 600oC. In situ neutron diffraction measurements show the coexistence of the T and split OI peaks in the vicinity of the transition. However, by direct numerical simulation, we show that this is to be expected from the twinned OI structure when the separation of adjacent twinning planes and the orthorhombicity both approach zero. It is demonstrated that the observed hkl-dependent peak broadening is consistent with this interpretation. A comparison of the measured and simulated peak shapes enables us to make an approximate estimate of the actual twin widths for a given orthorhombicity.

An in situ neutron time-of-flight diffraction study of LaMM(NiCoAlMn)5-x battery electrode materials and their deuterides, for x=0 and x=0.2

The structural properties of two mechanically activated commercial LaNi5 type battery materials and their deuterides have been investigated by means of in situ neutron powder diffraction analysis. A discussion of the hysteresis between the absorption and desorption isotherms is given. Using standard Rietveld refinement procedures, information on the variation of the deuterium site occupancy, the lattice symmetry and the cell volume are also presented. In particular, we report on the variation of line-broadening with concentration as well as on the complex time-dependent changes in the lattice parameters and the line-broadening following a step change in the deuterium pressure. We then show that these relatively novel data provide valuable information on the rather different phase transition processes observed for the two materials and offer an explanation for the superior kinetics of the AB4.8 alloy.

Magnetic properties of the YCo3–H system

The YCO 3 -H system is an interesting system, which shows a dramatic influence of hydrogen absorption on its magnetic properties. The β-hydride phase has been divided into β 1 and β 2 phases originally due to their different magnetic properties. Our measurement of the absorption isotherm at room temperature, however, shows clearly that the β phase is actually separated into two phases. Thus a new hydride phase has been identified. Magnetisation measurements have also been made using a VSM. The results show that the β 1 phase is most likely to be paramagnetic. We also observe that the β 2 phase seems to have two Curie temperatures. Because there are three types of Co atoms in the YCo 3 structure, we would infer that the magnetic ordering of the different Co atoms takes place at different temperatures in this phase.