Evan MacA. Gray

Versatile in situ powder X-ray diffraction cells for solid–gas investigations

Two multipurpose sample cells of quartz (SiO2) or sapphire (Al2O3) capillaries, developed for the study of solid–gas reactions in dosing or flow mode, are presented. They allow fast change of pressure up to 100 or 300 bar (1 bar = 100 000 Pa) and can also handle solid–liquid–gas studies.

The hydrogen activation of LaNi5

Abstract Changes to the hydridable intermetallic compound LaNi 5 and its hydriding behaviour during the first few hydrogen absorption—desorption cycles (activation) have been studied. A number of bulk (manometric hydrogen content measurements, optical microscopy), structural (X-ray and neutron diffraction) and microstructural (scanning and transmission electron microscopy) techniques provide strong evidence that the major part of the observed reduction in hysteresis after activation is due to the formation of extended two-dimensional defects in the alloy. Evidence is presented that these defects also contribute to the observed increase in the rate of hydride formation after activation.

Stability of the hydrogen absorption and desorption plateaux in LaNi5—H.: Part 4: thermal history effects

Abstract We consider the effects of elevated (uniform) temperatures on the pressure hysteresis of LaNi 5 H, concentrating on two main aspects: (i) promotion of γ-phase formation and (ii) annealing effects. Hysteresis data taken at temperatures up to 110°C and at 30°C after annealing at temperatures up to 400°C are presented and discussed. A strictly ascending sequence of measuring and annealing temperatures was employed. From the discontinuities in the absorption and desorption plateaux, the formation of the γ phase at 90°C in absorption and 60°C in desorption was inferred. Both these temperatures are lower than in the original studies during an ascending temperature sequence. Critical perusal of published data suggests that detailed thermal history, rather than measuring temperature, determines the temperature at which the γ phase appears. Hydrogen cycling at 110°C caused the γ phase to also form during the immediately following absorption-desorption cycle executed at room temperature. It is tentatively proposed that the microstructural modification required to form the γ phase at room temperature is an ordering of dislocations.

Dynamic micromapping of CO2 sorption in coal.

We have applied X-ray and neutron small-angle scattering techniques (SAXS, SANS, and USANS) to study the interaction between fluids and porous media in the particular case of subcritical CO2 sorption in coal. These techniques are demonstrated to give unique, pore-size-specific insights into the kinetics of CO2 sorption in a wide range of coal pores (nano to meso) and to provide data that may be used to determine the density of the sorbed CO2. We observed densification of the adsorbed CO2 by a factor up to five compared to the free fluid at the same (p, T) conditions. Our results indicate that details of CO2 sorption into coal pores differ greatly between different coals and depend on the amount of mineral matter dispersed in the coal matrix: a purely organic matrix absorbs more CO2 per unit volume than one containing mineral matter, but mineral matter markedly accelerates the sorption kinetics. Small pores are filled preferentially by the invading CO2 fluid and the apparent diffusion coefficients have been estimated to vary in the range from 5x10(-7) cm2/min to more than 10(-4) cm2/min, depending on the CO2 pressure and location on the sample.

A surface analytical study of SO2 stabilisation of LaNi5Hx surfaces

Abstract The stabilisation of LaNi 5 H 6 by SO 2 poisoning, under conditions approximating those expected in real engineering applications of the hydride, has been investigated with several complementary techniques — SEM, TEM, XPS, XAES and in-situ XRD. The effects of hydrogen loading and SO 2 exposure, and of the ambient conditions include: the formation of a 1–2 monolayer thick oxide film at the interface between the gaseous ambient atmosphere and the solid; the formation of a 25–100 nm thick decomposition layer below the oxide film; and the retention of the alloy below the decomposition layer. The decomposition layer is found to be depleted in Ni, according to XPS analyses, while TEM reveals the presence of nano-crystallites (most likely Ni) embedded in an amorphous matrix; the latter consists of mixed oxide, hydroxide and carbonate species. The relationships between the data have resulted in the formulation of a model which proposes that the nano-crystallites promote dissociative/associative H 2 chemisorption/desorption at surface sites which under normal circumstances are protected against poisoning by the surrounding amorphous matrix, which is permeable to H 2 . The effect of SO 2 exposure is apparently for the S to react at the surface sites of the nano-crystallites, to form a sulphide, thus stabilising the fully loaded hydride by prevention of associative desorption. There is little, if any, penetration by S of the metal hydride substructure. Also, we find that long-term stability is promoted by the presence of trace amounts of SO 2 in the ambient atmosphere; this suggests that the stabilisation is, at least in part, a dynamic process.

Ordering of deuterium in PdD0.65 at 54 K

Neutron powder diffraction studies on PdD0.65 demonstrate that the 50 K anomaly is due to an order-disorder transition of deuterium within octahedral interstitial sites. The slow transition to the ordered phase involves diffusion of deuterium from the nearest-neighbour deuterium positions to the second-nearest-neighbour positions. The ordered crystal structure remains cubic, and is accurately described in space group Pm3n by doubling the cell constant relative to the disordered structure (space group Fm3m).

A high-temperature high-pressure gas-handling cell for neutron scattering experiments.

In this article, we describe a high-pressure gas-handling cell at high temperatures developed for neutron scattering studies. The cell has been developed from a pre-existing autoclave design, that was developed for high-pressure high-temperature synthesis of ternary alkali-metal–transition-metal hydrides.

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.

Nanostructured Metal Hydrides for Hydrogen Storage Studied by In Situ Synchrotron and Neutron Diffraction

This work was focused on studies of the metal hydride materials having a potential in building hydrogen storage systems with high gravimetric and volumetric efficiencies of H storage and formed / decomposed with high rates of hydrogen exchange. In situ diffraction studies of the metal-hydrogen systems were explored as a valuable tool in probing both the mechanism of the phase-structural transformations and their kinetics. Two complementary techniques, namely Neutron Powder Diffraction (NPD) and Synchrotron X-ray diffraction (SR XRD) were utilised. High pressure in situ NPD studies were performed at D 2 pressures reaching 1000 bar at the D1B diffractometer accommodated at Institute Laue Langevin, Grenoble. The data of the time resolved in situ SR XRD were collected at the Swiss Norwegian Beam Lines, ESRF, Grenoble in the pressure range up to 50 bar H 2 at temperatures 20-400°C. The systems studied by NPD at high pressures included deuterated Al-modified Laves-type C15 ZrFe 2-x Al x intermetallics with x = 0.02; 0.04 and 0.20 and the CeNi 5 -D 2 system. D content, hysteresis of H uptake and release, unit cell expansion and stability of the hydrides systematically change with Al content. Deuteration exhibited a very fast kinetics; it resulted in increase of the unit cells volumes reaching 23.5 % for ZrFe 1.98 Al 0.02 D 2.9(1) and associated with exclusive occupancy of the Zr 2 (Fe,Al) 2 tetrahedra. For CeNi 5 deuteration yielded a hexahydride CeNi 5 D 6.2 (20°C, 776 bar D 2 ) and was accompanied by a nearly isotropic volume expansion reaching 30.1% (∆a/a=10.0%; ∆c/c=7.5%). Deuterium atoms fill three different interstitial sites including Ce 2 Ni 2 , Ce 2 Ni 3 and Ni 4 . Significant hysteresis was observed on the first absorption-desorption cycle. This hysteresis decreased on the absorption-desorption cycling. A different approach to the development of H storage systems is based on the hydrides of light elements, first of all the Mg-based ones. These systems were studied by SR XRD. Reactive ball milling in hydrogen (HRBM) allowed synthesis of the nanostructured Mg-based hydrides. The experimental parameters (P H2 , T, energy of milling, ball / sample ratio and balls size), significantly influence rate of hydrogenation. The studies confirmed (a) a completeness of hydrogenation of Mg into MgH 2 ; (b) indicated a partial transformation of the originally formed -MgH 2 into a metastable -MgH 2 (a ratio / was 3/1); (c) yielded the crystallite size for the main hydrogenation product, -MgH 2 , as close to 10 nm. Influence of the additives to Mg on the structure and hydrogen absorption/desorption properties and cycle behaviour of the composites was established and will be discussed in the paper.

Apparatus for time- and frequency-domain measurements of metal-hydride reaction kinetics

The traditional method of studying reaction kinetics in metal hydrides, by applying a substantial pressure step and monitoring the amount of gas absorbed vs time, suffers from undesirable temperature changes due to the strongly exothermic/endothermic absorption/desorption reactions. This paper describes a new apparatus suitable for small-signal measurements in both the time and frequency domains. Measurements in the frequency domain promise considerable practical benefits, including in principle the resolution of rate-limiting and other reaction steps. Preliminary results obtained in the frequency domain for the LaNi5H2 system are presented.