Search

A thermodynamic model is presented to aid the selection of compatible pairs of hydrogen storage alloys for service in a multi-stage metal-hydride compressor. The model is built around the concept of an ideal compressor in which all pairs of hydrides operate between the same working temperatures. The key feature of the model is a link between the thermodynamic characteristics of the hydrides in each pair (the entropy change ΔS and enthalpy change ΔH when the hydride is formed from the metal) that results from requiring that the heated low-pressure hydride is able to desorb to the cooled high-pressure metal at a unique intermediate pressure. This necessary linking places severe constraints on the choice of alloys, since ΔS and ΔH cannot be freely chosen and are in fact strongly correlated for hydrides with similar operating temperatures. The model is based on the van ‘t Hoff relation, which is derived from first principles and shown to be only approximately valid at the high hydrogen pressures of interest for vehicle filling stations. Pressure hysteresis and plateau slope are incorporated into the model. A case study of a two-stage compressor is presented, based on the curved van ‘t Hoff line and the experimentally determined effects of pressure hysteresis and plateau slope.

To calculate a gas sorption isotherm from measurements made using the manometric (Sieverts) technique requires accurate knowledge of the sample cell void volume, the volume able to be occupied by the free gas. For many combinations of material and morphology, this volume changes as the sample expands or compresses, swells, undergoes chemical change or adsorbs gas. For gas adsorption measurements on porous materials, the volume of the adsorbate is often accepted as the pore volume, and the adsorbate density changes as the material adsorbs. For materials exhibiting IUPAC type I isotherms, including many carbons, the uptake saturates at high pressures and additional information about the volume (adsorption space) and density of the adsorbate can be extracted at high pressure, provided adsorption has finished. For hydrogen, this typically requires cryogenic temperatures at pressures up to 10 MPa. In this work, hydrogen isotherms are conducted at pressures up to 200 MPa, in order to investigate the properties of the adsorbate at ambient temperatures. In addition, the manometric method gas uptake equations for absolute, excess and net isotherms are revisited and extrapolated to high pressure in order to extract information on the adsorbate volume and density, and enable the construction of the absolute isotherm.

For a successful transition to a renewable energy economy, optimisation of renewable energy systems must evolve to take into account not only technical and/or economic objectives, but also environmental and socio-political objectives. A Normalised Weighted Constrained Multi-Objective (NWCMO) meta-heuristic optimisation algorithm has been proposed for achieving a compromise between mutually conflicting technical, economic, environmental and socio-political objectives, in order to simulate and optimise a renewable energy system of any configuration. In this paper, an optimisation methodology based on these four classes of objective is presented. The methodology was implemented using Particle Swarm Optimisation and tested against data from the literature. In the Case Study the original results could be reproduced, but the newly-implemented algorithm was further able to find a more optimal design solution under the same constraints. The influence of additional quantified socio-political inputs was explored.

An intrinsically microporous high surface area (772 m2 g–1) hexaazatrinaphthylene-based network polymer has been synthesized. Additionally, the material was postmodified via the introduction of palladium ions to determine the effect of metalation on the properties of the assembly. Both the unmodified as-synthesized and metal-modified polymers were characterized using elemental analysis, Fourier transform infrared spectroscopy, solid-state 13C cross-polarization-magic-angle spinning NMR spectroscopy, Brunauer–Emmett–Teller surface area analysis, X-ray photoelectron spectroscopy, X-ray diffraction analysis (XRD), small-angle X-ray scattering, and thermogravimetric analysis techniques. The hydrogen uptake capacities of these materials were determined for a wide range of pressures up to 2000 bar. The palladium functionalization changed the intermolecular chain distances in the polymer, enhanced the microporosity, and increased the hydrogen uptake at ambient temperature.

A study of the effect of the ball-milling gas environment on the kinetic enhancement of MgH2 with different additives was conducted using argon and hydrogen. The as-sourced MgH2 was milled for 20 h and then milled for a further 2 h after adding 1–2 mol% of one of the additives titanium isopropoxide, niobium oxide or carbon buckyballs, varying the gas environment for both ball-milling stages. The milling environment had little or no effect on the desorption kinetics in most cases. However, in some cases, the absorption uptake differed by up to 2 wt%, depending on the gas used. This effect was not consistent among the composite samples surveyed, demonstrating the importance of reporting all information about the ball-milling processes used, including the gas environment.

This paper describes an ultra-high-vacuum thermal desorption spectrometer, designed to study hydrogen in steels at ∼1 wt ppm (∼50 atomic ppm) concentration. The high sensitivity achieved also facilitates the analysis of surface phenomena. The instrument was evaluated with model materials and provided good measurements of diffusible hydrogen. A hydrogen peak at ∼350 °C was identified for steels exposed to water during hydrogen charging, and attributed to water molecules adsorbed on the sample surface for samples exposed to the laboratory atmosphere for times as short as 1 min. Recommendations are made for precautions to be taken when handling the samples.

A model has been developed for a metal-hydride tank for hydrogen storage, based on linked modular mathematical models in Simulink®. The objective of the work was a tank-level model suitable for incorporation into a whole-of-system model, implying modest computing demands and reduced complexity. Finite-element analysis was not used. Because the apparent kinetics of a practicable metal-hydride tank is dominated by heat flow originating in the enthalpy of hydrogen absorption/desorption, particular attention was paid to modelling the effective thermal conductivity, based on a detailed description of the thermal resistance between hydride particles. The model was tested against our own experimental data for a pair of tanks with 6.4 kg total hydrogen capacity, and compared with a published 2D model and published experimental data. In all cases, the new model performed very well. The incorporation of a physical model of the effective thermal conductivity means that the tank model can also be used as a research tool to investigate ways of improving tank performance by altering the physical characteristics of the metal-hydride itself to achieve an optimal thermal design and enhanced reaction kinetics.

Large photovoltaic arrays are becoming common as the world moves to replace fossil-fuelled electricity generators. As the array size and project cost increase, it becomes increasingly important to know accurately what the array’s performance will be before it is built. Large arrays inevitably contain modules with a spread of performance characteristics such as short-circuit current and open-circuit voltage, and suffer from temperature differences between modules. In this first study of these problems, a model has been developed that accurately predicts the behaviour of a photovoltaic array subject to variability between modules and inhomogeneity of cell temperature across the array. The model was applied to a real rooftop array consisting of 912 modules (298 kW nominal peak power). Based on measured string currents, the predicted average string temperature was compared the temperature measured by a radiometric survey using a drone-mounted IR camera and matched very well. The five-parameter model of cell I - V characteristics was fitted to manufacturer’s data, with highest weighting given to the region around the maximum-power point (MPP) where a real array should operate via active MPP tracking. The model was used to explore separately the effects of a spread in module characteristics arising in the manufacturing process and of temperature inhomogeneity across the array. The current in each module of a string was constrained to be the same, and the voltage of every parallel-connected string was also constrained to be the same. These constraints lead to greater power loss than is predicted based on an average module at an average temperature. Compared to a hypothetical array assembled from identical average modules at the same average temperature, variability caused a loss of power of about 2%, depending on the detailed form of the distribution function chosen to represent the spread of characteristics in the manufacturer’s tolerance band. As a rule of thumb, de-rating the maximum power to the lower end of the manufacturer’s tolerance band is recommended to account for module variability during the design phase. The effect of temperature inhomogeneity is more serious, because temperature affects V oc strongly, causing parallel-connected strings to be pulled away from their ideal operating points to obey the constraint of equal voltage. A modest 10 °C temperature gradient across the studied array was predicted to cause about a 4% loss of power at the MPP. Much higher real temperature differences could be expected in summer and were observed. The study confirmed that temperature inhomogeneity poses a serious design problem for large arrays, requiring careful thermal design to achieve not only acceptably low average array temperature, but also the least possible temperature spread.

An enhanced one-dimensional model has been developed for an alkaline electrolyser cell for hydrogen production, based on linked modular mathematical models in Simulink®. Where possible, the model parameters were derived on a physical basis and related to the materials of construction and the configuration of its components. This means that the model can be applied to many alkaline electrolyser cells, whereas existing semi-empirical models were generally developed for a specific cell. In addition to predicting the overall equilibrium electrolyser cell performance, the model is a powerful tool for understanding the contributions to cell voltage from the various internal components. It is thus useful as a guide to researchers aiming for improved performance through modified geometry and enhanced electrode materials. The model performed very well when compared to published models tested against the same sets of experimental data.

Electricity generation presents the biggest opportunity to lower CO 2 emissions and it is foreseen that hydrogen energy technology will play an important role in realising the scenario to cap global warming at 2 °C through replacement of fossil fuels with renewables. The transition to electric power for transport in battery- and fuel-cell-electric vehicles will further increase the need for low-carbon electricity generation. For a successful transition to a renewable energy economy the traditional approach of designing energy systems to meet only goals related to the technology (capacity, availability, reliability) and economics (return on investment, cost to the consumer) must evolve to take on a more holistic viewpoint and be able to take into account other goals addressing environmental and social considerations. This paper reviews approaches for integrating hydrogen energy technology into hybrid energy systems, emphasising electricity generation using a hydrogen fuel cell. Integration of energy storage, sizing methodologies, energy flow management and their associated optimization algorithms and software implementation are addressed. Few published case studies go beyond technical considerations. This reality is discussed in the light of available software packages. A four-dimensional multi-objective meta-heuristic function is proposed with weighting of technical, economic, environmental and socio-political factors to suit the design goals for the energy system.

A high-pressure Sieverts apparatus, specifically designed to investigate carbon and other low-density materials for hydrogen storage, has been constructed and used to investigate potential storage materials for which the volume of the sample is uncertain or difficult to define. The apparatus can be managed from a computer via a graphical interface and used to measure gas sorption isotherms from 77 K to 873 K, utilising a computer driven piston pressure booster to compress the gas to 340 bar. Based on measurements of activated carbons and graphene-like samples, this article demonstrates the low sensitivity of the apparatus to uncertainty in the sample volume and the quality of data obtained for H2 adsorption isotherms measured at 296 K and 77 K at pressures up to 300 bar.

An in-situ neutron investigation of the spatial variation in hydride composition of LaNi5 after a single absorption pressure step was performed. Compositional inhomogeneities are formed due to the strong temperature gradients created by the rapid absorption process coupled with the pressure and temperature hysteresis of the metal–hydrogen interaction. The hydride fraction of LaNi5 in a cylindrical cell was mapped using the ENGIN-X stress/strain instrument and quantitative phase analysis performed using the Rietveld technique. The material was observed to preferentially absorb hydrogen close to the edges of the cell where heat transfer out of the material was more efficient. This spatial variation was maintained even after thermal equilibration. The experimental results are compared to predictions of a 3D multiphysics model solved by the software package COMSOL. The good agreement achieved demonstrates the suitability of this model for optimisation of metal hydride tank systems.

The effect of titanium isopropoxide (TTIP) on the kinetics of absorption and desorption of magnesium hydride was compared to the benchmark additive niobium oxide (Nb2O5). A systematic survey was performed for ball-milled MgH2 mixed with up to 2 mol% TTIP or Nb2O5 additive for different ball-milling times. It was found that TTIP was just as effective as Nb2O5 for the enhancement of the kinetics of absorption and desorption and that 0.5 mol% was sufficient. The TTIP composite sample had superior hydrogen capacity compared to the Nb2O5 composite sample. As TTIP is a liquid at room temperature, 30 min ball-milling with the magnesium hydride was sufficient time for mixing. Hand mixing the TTIP with MgH2 demonstrated excellent desorption kinetics but the absorption kinetics were poorer compared to ball-milling.

An advanced PEM fuel cell mathematical model is described and realised in four ancillaries in the Matlab–Simulink environment. Where possible, the model is based on parameters with direct physical meaning, with the aim of going beyond empirically describing the characteristics of the fuel cell. The model can therefore be used to predict enhanced performance owing to, for instance, improved electrode materials, and to relate changes in the measured performance to internal changes affecting influential physical parameters. Some simplifying assumptions make the model fairly light in computational demand and therefore amenable to extension to simulate an entire fuel-cell stack as part of an energy system. Despite these assumptions, the model emulates experimental data well, especially at high current density. The influences of pressure, temperature, humidification and reactant partial pressure on cell performance are explored. The dominating effect of membrane hydration is clearly revealed.

The addition of transition metal nanoparticles to carbon nanostructures has been shown to increase the hydrogen storage capacity of carbon nanostructures by dissociating molecular hydrogen and allowing adsorption via chemical means, a process known as hydrogen spillover. This paper is an overview of experimental and theoretical studies on hydrogen storage on transition metal doped carbon nanostructures via the spillover mechanism and the prospects for achieving practical hydrogen storage targets. The most promising materials are found to be high surface area hexagonal system carbons for which the π-conjugation is broken by well dispersed oxygen functional groups or lattice dopants.

This review focuses on a new generation of polymers with intrinsic microporosity (PIMs) and their hydrogen storage applications. PIMs are a novel microporous material attracting attention due to their potentially useful functional properties. In this review, the methods of synthesis, suitable characterisation techniques and applications are briefly described. Importantly, the hydrogen capacity of PIMs that have been investigated recently is highlighted and the hydrogen uptake is compared to other microporous materials such as hypercrosslinked polymers (HCPs), metal organic frameworks (MOFs) and activated carbons. This review brings H2 storage-applicable PIMs that have been reported in different studies together for the first time, providing a reference for better comparison of H2 storage in PIMs.

We present an overview of hydrogen-containing superconductors, including metallic, cuprate, pnictide and carbon-based materials. Particularly in BCS-like superconductors, hydrogen introduces new phonon modes and these may lead to better or worse superconducting properties, with anharmonicity of the H potential being a decisive factor. The availability of the deuterium isotope, in addition to protium, makes hydrogen an extremely useful extra probe of mechanisms of superconductivity. While hydrogen modification has not so far led to any confirmed remarkable improvement in already existing metallic superconductors, hydrogen plays an important role in the synthesis of novel iron-based superconductors and as a substitutional impurity appears to increase the transition temperature. Most recently, a new record for the superconducting transition temperature of 203 K has been claimed for a metallic hydride under high pressure, H 2 S. We survey elemental hydrides and superconductors, discuss techniques for introducing hydrogen into materials, then review and summarise published knowledge about hydrogen-containing superconductors.

Metal hydrides, particularly those based on intermetallic alloys, are effective, safe and efficient for storage of hydrogen. While their low gravimetric energy density is unsuited to mobile applications, this is seldom a problem for stationary hydrogen storage. In this review we focus on developments in the newest AB3 class of intermetallic hydrides, in the context of a renewable energy system in which hydrogen is supplied at relatively low pressure and delivered to a fuel cell. AB3 alloys are of interest because their characteristics are roughly speaking intermediate between those of the long-established AB5 and AB2 types. AB3 alloys offer promise of coming closer to satisfying important performance criteria than do the classic types, through tuning of their hydrogen absorption characteristics. Modification of the nominal composition by element substitution, leading to structural changes affecting all hydriding properties, is the most effective approach. After establishing performance criteria for the present application, conventional elemental and alloy hydrides are briefly compared, followed by a comprehensive survey of research on AB3 hydrides and the opportunities to tune their performance.

Metal-hydrides have been of great interest as one of the most promising materials for hydrogen storage applications. For widespread use, the most appropriate container and thermal management systems to minimise the time of absorption and desorption and maximise the amount of stored hydrogen must be designed. In recent years, many attempts have been made to identify the relationship between different operating and design variables and the resultant performance of metal-hydride systems. In this review, features of mathematical models of metal-hydride reactors including the assumptions, different applied equations and solution methods which have been developed in previous studies are presented. The evolution of the reactor geometries and configurations of cooling systems as well as different effective factors of metal-hydride performance are also discussed.

Palladium hydride was discovered more than 150 years ago and remains one of the most-studied interstitial metal hydrides because of the richness of its physical behaviours, which include ordered phases and anomalous properties at temperatures below 100 K, a superabundant-vacancy (SAV) phase with stoichiometry Pd3H4 formed at high temperature and pressure, and quenching of the enhanced Pauli paramagnetism of palladium. One of the most fascinating properties of palladium hydride is superconductivity at about 10 K without external pressure, in contrast to the newly-discovered polyhydride room-temperature superconductors that require megabar pressures. Moreover, the superconductivity exhibits an inverse isotope effect. Remarkably, modern first-principles approaches are unable to accurately predict the superconducting transition temperature by calculating the electron–phonon coupling constant within Migdal-Eliashberg theory. Anharmonicity of the hydrogen site potential is a key factor and poses a great challenge, since most theoretical approaches are based on the harmonic approximation. This review focuses on the electron and phonon band structures that underpin all such calculations, with palladium as a reference point. While the electron band structures of palladium and its monohydride are uncontroversial, the phonon band structure of palladium hydride in particular is problematic, with a realistic treatment of anharmonicity required – and largely yet to be achieved – to reproduce the results of inelastic neutron scattering experiments. In addition to the monohydride and SAV phases, possible higher hydrides are surveyed and the origin of the famous “50-K” anomaly in specific heat and other physical properties is critically reviewed.

An enhanced model of a PEM electrolyser cell is presented, based on linked modular mathematical models of the anode, cathode, membrane and cell voltage. In addition to predicting the overall equilibrium electrolyser cell performance, based on parameters related to the materials of construction and the configuration of its components, the model is a powerful tool for exploring control strategies and understanding the contributions to cell voltage from the various internal components as a guide to researchers aiming for improved performance.

A systematic survey of the catalytic effect of up to 10 wt% C 60 carbon buckyballs on the kinetics of hydrogen desorption and subsequent absorption of magnesium hydride has been made. It was found that there was an optimum amount of C 60 additive for the best desorption kinetics as well as an optimum time of ball-milling, improving the desorption kinetics over that of ball-milling alone. A comparison with the additive Nb 2 O 5 under the same conditions indicated that desorption kinetic enhancement was relatively minor. The subsequent absorption kinetics were always poorer than the un-catalysed samples.

An in-situ structural study of three Mg–Zn samples with nominal composition Mg0.9963Zn0.0037, Mg0.89Zn0.11 and Mg0.73Zn0.27 was performed using neutron diffraction to investigate what phases are formed during the hydrogen absorption process. It was found that at the temperatures needed for reasonable kinetics, a Mg–Zn liquid phase was present. This aspect of the reaction mechanism has not been revealed by previous ex-situ investigations in the literature. The only hydride formed was tetragonal MgH2. No Mg–Zn–H hydride phase was found and the Zn did not appear to play any part in the reaction.

The increasing interest in enhanced coal bed methane (ECBM) production and reporting of greenhouse gas emissions from mines has spotlighted gas diffusion and gas-induced swelling in coals. Although it is generally agreed that carbon dioxide (CO2) diffuses faster than methane (CH4) in coal and that diffusion rates decline as the pressure is increased, there is no general agreement on the physical mechanism responsible. In this paper kinetic data for sorption-induced swelling for five Australian bituminous coals have been analysed. Gases investigated included CO2 and CH4, Xe and ethane. It was found that swelling rates are influenced by gas type. CH4 swelled coal more slowly than CO2 at the same pressures. Other swelling kinetic data showed that, of the gases investigated, xenon was the slowest, followed by ethane. CO2 swelling was found to be the fastest overall. The swelling rate was found to depend on maceral composition, with the slower-swelling coals being vitrinite-rich. However, the swelling rate of different coals varied differently with increasing pressure: the slowest swelling coal showed an increase in swelling rates with an increase in system pressure, whereas the fastest swelling coals showed a decrease in the swelling rates at high pressures.

A full analysis of the uncertainties in the calculation of the amount of gas absorbed or desorbed by a sample using the gravimetric technique has been made. The gravimetric technique provides a near direct measurement of the uptake of gas by continuously measuring the weight of the sample plus absorbed gas and is frequently used to characterise the uptake of hydrogen in potential hydrogen storage materials. These calculations provide the capability to determine the uncertainty in the final gas uptake measurement given knowledge of the measurement uncertainties. Uptake of hydrogen gas on activated carbon has been modelled to allow individual constants such as the buoyancy volumes as well as measurement uncertainties to be changed while determining the effect on the uptake uncertainty. The measurement uncertainties critical to minimising the overall uptake uncertainty have been identified and a comparison with another popular gas uptake method, the Sieverts technique, has been made.

The Sieverts technique is a popular method of measuring the uptake of gas on a sample and is an important tool for the characterisation of hydrogen uptake by potential hydrogen storage materials. An analysis of the consequences of errors in the calibration of volumes in this technique has been made using hydrogen absorption on an AB2 alloy as an example. Trends in the error in the uptake as a function of errors in the volumes have been explained in terms of the equations for uptake. Significant advantages are found for calibration techniques for which the ratio of volumes is an intrinsic measurement.

An analysis of the uncertainties in the calculation of the amount of gas absorbed or desorbed by a sample has been made for the manometric or Sieverts technique, commonly used for the determination of hydrogen uptake on potential hydrogen storage materials. The analytical derivations provide a means to calculate the uncertainty in each step of a hydrogen absorption isotherm. In addition, a Sieverts system is modelled to allow a detailed investigation of the effect of volumes and other parameters as well as the relative impact of the different measurement uncertainties. Reliable uncertainty estimates are especially important for authoritative measurements of gas uptake by carbons and other nanostructured materials used for hydrogen storage.

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.

Diffraction-based methods offer unique advantages for elucidating the pathways by which materials absorb and desorb hydrogen, especially when a phase change or the formation of new compounds is involved. In this case, the hydriding reaction may be followed via the changing crystallography of the phases involved in response to a change in temperature or hydrogen pressure. By using a fast diffractometer, the reaction kinetics may also be correlated to environmental conditions and the degree of completion of the reaction. In this paper we consider and model quantitatively the essential elements of a successful in-situ diffraction experiment with neutrons or X-rays under hydrogen pressures up to several kilobars: a gas manifold to accurately measure hydrogen uptake; a pressure cell designed for maximum detected intensity; means to exclude scattering arising in the cell as much as possible; methodology to correct for attenuation and subtract background intensity from the cell and environment.

The “standard” procedure for calibrating the Vesuvio eV neutron spectrometer at the ISIS neutron source, forming the basis for data analysis over at least the last decade, was recently documented in considerable detail by the instrument's scientists. Additionally, we recently derived analytic expressions of the sensitivity of recoil peak positions with respect to fight-path parameters and presented neutron–proton scattering results that together called into question the validity of the “standard” calibration. These investigations should contribute significantly to the assessment of the experimental results obtained with Vesuvio. Here we present new results of neutron–deuteron scattering from D 2 in the backscattering angular range ( θ > 90 ° ) which are accompanied by a striking energy increase that violates the Impulse Approximation, thus leading unequivocally the following dilemma: (A) either the “standard” calibration is correct and then the experimental results represent a novel quantum dynamical effect of D which stands in blatant contradiction of conventional theoretical expectations; (B) or the present “standard” calibration procedure is seriously deficient and leads to artificial outcomes. For Case (A), we allude to the topic of attosecond quantum dynamical phenomena and our recent neutron scattering experiments from H 2 molecules. For Case (B), some suggestions as to how the “standard” calibration could be considerably improved are made.

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.

Spin relaxation due to polar optical phonon scattering in semiconductors was investigated. The relaxation of the electron spin was found to increase with increasing the strength of the electric field. However, a high field completely depolarized the electron spin due to an increase of the spin precession frequency of the hot electrons, suggesting that high field transport conditions might not be desirable for spin-based technology with these semiconductors. It was also found that spin relaxation decreases with increasing moderately n -doping density or decreasing temperature. The results were discussed in comparison with the data available in the literature.

The effects of small changes in flight-path parameters (primary and secondary flight paths, detector angles), and of displacement of the sample along the beam axis away from its ideal position, are examined for an inelastic time-of-flight (TOF) neutron spectrometer, emphasising the deep-inelastic regime. The aim was to develop a rational basis for deciding what measured shifts in the positions of spectral peaks could be regarded as reliable in the light of the uncertainties in the calibrated flight-path parameters. Uncertainty in the length of the primary or secondary flight path has the least effect on the positions of the peaks of H, D and He, which are dominated by the accuracy of the calibration of the detector angles. This aspect of the calibration of a TOF spectrometer therefore demands close attention to achieve reliable outcomes where the position of the peaks is of significant scientific interest and is discussed in detail. The corresponding sensitivities of the position of peak of the Compton profile, J(y), to flight-path parameters and sample position are also examined, focusing on the comparability across experiments of results for H, D and He. We show that positioning the sample to within a few mm of the ideal position is required to ensure good comparability between experiments if data from detectors at high forward angles are to be reliably interpreted.

Magnesium silicide (Mg2Si) has attracted interest as a hydrogen storage material due to favorable thermodynamics (ΔHdesorption = 36 kJ/mol H2) for room temperature operation. To date, direct hydriding of Mg2Si under hydrogen gas to form MgH2 and Si has only been attempted at low pressure and has been hindered by poor kinetics of absorption. In this paper we study the dehydrogenation reaction with in-situ neutron powder diffraction and present results of our attempts to hydrogenate Mg2Si under both hydrogen and deuterium gas up to temperatures of 350 °C and pressures of 1850 bar. Even under these extreme absorption conditions Mg2Si does not absorb any measureable quantity of hydrogen or deuterium.

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.

The use of intermittent renewable energy sources for power supply to off-grid electricity consumers depends on energy storage technology to guarantee continuous supply. Potential applications of storage-guaranteed systems range from small installations for remote telecoms, water-pumping and single dwellings, to farms and whole communities for whom grid connection is too expensive or otherwise infeasible, to industrial, military and humanitarian uses. In this paper we explore some of the technical issues surrounding the use of hydrogen storage, in conjunction with a PEM electrolyser and PEM fuel cell, to guarantee electricity supply when the energy source is intermittent, most typically solar photovoltaic. We advocate metal-hydride storage and compare its energy density to that of Li-ion battery storage, concluding that a significantly smaller package is possible with metal-hydride storage. A simple approach to match the output of a photovoltaic array to an electrolyser is presented. The properties required for the metal-hydride storage material to interface the electrolyser to the fuel cell are discussed in detail. It is concluded that relatively conventional Mischmetal-based AB5 alloys are suitable for this application.

We investigated the inter-band transition effects in spin generation in semiconductors by measuring the time-resolved differential transmission of the samples. The degree of polarization pumped with the single-photon excitation of electrons was found to decay with a time constant of 210 ps, giving a spin relaxation time of 420 ps, and to be enhanced with decreasing temperature. We also found that the polarization depends strongly on the excitation photon energy. The results are discussed based on the selection rules governing optical transitions from heavy-hole, light-hole and split-off states to conduction band states in a bulk sample.

Spin polarization in the presence of an external magnetic field and electric bias in quantum confined semiconductor structures has been studied by time- and polarization-resolved spectrometry. From measurements with angular variations of the magnetic field from the Voigt configuration (VC) it was found that both the frequency (Ω) and decay rate (β) of the oscillatory component of the polarization increase with variation of the angle from the VC. Their dependences are discussed based on the electron spin dephasing related to the spread of the electron g-factor (ge) (i.e. unequal values of the longitudinal (ge||) and transverse (ge⊥) components of ge) and the exchange interaction between the electron and hole spins. It is demonstrated that the increase in Ω upon deviation of the magnetic field from the VC relates to the anisotropy of ge (ge|| and ge⊥) resulting from the quantum confinement effect. However, the angular dependence on β is related to the residual exchange interaction between the electron spin and rapidly relaxing hole spin.

We study theoretically the propagation and distribution of electron spin density in semiconductors within the drift–diffusion model in an external electric field. From the solution of the spin drift–diffusion equation, we derive the expressions for spin currents in the down-stream ( DS ) and up-stream ( US ) directions. We find that drift and diffusion currents contribute to the spin current and there is an electric field, called the drift–diffusion crossover field, where the drift and diffusion mechanisms contribute equally to the spin current in the DS direction, and that the spin current in the US direction vanishes when the electric field is very large. We calculate the drift–diffusion crossover field and show that the intrinsic spin diffusion length in a semiconductor can be determined directly from it if the temperature, electron density and both the temperature and electron density, respectively, are known for nondegenerate, highly degenerate and degenerate systems. The results will be useful in obtaining transport properties of the electron’s spin in semiconductors, the essential information for spintronic technology.

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.

An experimental and theoretical study on the optically stimulated spin transport in zinc-blende semiconductors is presented. The first part of the paper describes an experiment which investigates the effect of a longitudinal electric field on the spin-polarized carriers induced by a circularly polarized light. Since the photo-generated hole spins relaxation is extremely fast, the experiment observes only the effect resulting from spin-polarized electrons accumulating at the transverse edges of the sample, as a result of left-right asymmetries in scattering for spin-up and spin-down electrons in the presence of spin-orbit (SO) interaction. It is found that the effect depends on the longitudinal electric field and doping density as well as on temperature. The results are discussed. The second part of the paper deals with a theoretical investigation using norm-conserving pseudopotential and Green function formalism to analyse the SO mechanism responsible for the light-induced Hall voltage. The findings resulting from the investigation are discussed and are compared with experimental data.

Random substitutional Ti2.08Zr exhibits zero Bragg scattering and alloys with this nominal composition are therefore used to fabricate pressure cells for neutron scattering measurements. Because Ti and Zr alloys are well known to be subject to hydrogen embrittlement, the embrittlement of Ti2.001Zr by deuterium gas was investigated to assess its suitability for use in neutron diffraction experiments on samples under deuterium pressure. At 77°C, the alloy survived 15 days in 689 bar D2 with no apparent effect on mechanical properties. At 100°C, however, embrittlement was initiated by 607-bar D2, and at 200°C rapid decrepitation occurred, producing ϵ-(TiZr)D x . It is therefore recommended that Ti≈ 2Zr should not be considered for structural use with deuterium gas above room temperature.