Search

Applied physics letters 120(20), 202203 (2022). doi:10.1063/5.0083256, We report generation of ultra-broadband longitudinal acoustic coherent phonon wavepackets in SrTiO$_3$ (STO) with frequency components extending throughout the first Brillouin zone. The wavepackets are efficiently generated in STO using femtosecond infrared laser excitation of an atomically flat 1.6 nm-thick epitaxial SrRuO$_3$ film. We use femtosecond x-ray diffraction at the European X-Ray Free Electron Laser Facility to study the dispersion and damping of phonon wavepackets. The experimentally determined damping constants for multi-THz frequency phonons compare favorably to the extrapolation of a simple ultrasound damping model over several orders of magnitude., Published by American Inst. of Physics, Melville, NY

A mixture of 3NaH/AlB2 was prepared by ball-milling; its hydriding reaction was studied between 375–425 °C and 25–50 bar hydrogen pressure by means of volumetric titration. The hydriding reaction was characterized by means of in situ synchrotron radiation powder X-ray diffraction and high-pressure differential scanning calorimetry. Hydriding reaction took place at the molten state, and its reaction products were NaBH4 and Al. The scanning electron microscopy images of the material revealed that the material morphology changes after hydriding. A maximum hydrogen uptake of 4.7 wt % was registered for the hydriding experiment at 425 °C and 50 bar hydrogen pressure. Dehydriding reaction was studied by means of volumetric titration and differential scanning calorimetry. The dehydriding reaction at 425 °C and 1 bar argon pressure registered a release of 2.4 wt %. The low dehydriding level was attributed to the reduction of the available particle surface upon melting of the material during the hydriding reaction.

Hydrogen is recognized as a possible future energy carrier, which can be produced from renewable energy and water. A major challenge in a future ‘hydrogen economy’ is the development of safe, compact, robust, and efficient means of hydrogen storage, in particular for mobile applications. The present review focuses on light metal boron based hydrides, for which the general interest has expanded significantly during the past few years. Synthesis methods, physical, chemical and structural properties of novel boron based hydrides are reviewed along with new approaches for improving kinetic and thermodynamic properties: (i) anion substitution, (ii) reactive hydride composites and (iii) nanoconfinement of hydrides and chemical reactions. The light metal borohydrides reveal a fascinating structural chemistry and have the potential for storing large amounts of hydrogen. A combination of the different approaches may provide a new route to a wide range of interesting energy storage materials in the future.

A composite of Ca(BH4)2−MgF2 is proposed as a reversible hydrogen storage system. The dehydrogenation and rehydrogenation reaction mechanisms are investigated by in situ time-resolved synchrotron radiation powder X-ray diffraction (SR-PXD) and Raman spectroscopy. The formation of an intermediate phase (CaF2−xHx) is observed during rehydrogenation. The hydrogen content of 4.3 wt % is obtained within 4 h during the first dehydrogenation at isothermal and isobaric conditions of 330 °C and 0.5 bar H2, respectively. The cycling efficiency is evaluated by three release and uptake cycles together with absorbed hydrogen content in the range 5.1−5.8 wt % after 2.5 h (T = 330 °C and p(H2) = 130 bar). The kinetic properties on the basis of hydrogen absorption are comparable for all cycles. As compared to pure Ca(BH4)2 and Ca(BH4)2−MgH2 composite, Ca(BH4)2−MgF2 composite reveals the kinetic destabilization and the reproducibility of hydrogen storage capacities during cycling, respectively.

Complex hydrides are very promising candidates for future light-weight solid state hydrogen storage materials. The present work illustrates detailed characterization of such novel hydride materials on different size scales by the use of synchrotron radiation and neutrons. The comprehensive analysis of such data leads to a deep understanding of the ongoing processes and mechanisms. The reaction pathways during hydrogen desorption and absorption are identified by in situ X-ray diffraction (XRD). Function and size of additive phases are estimated using X-ray absorption spectroscopy (XAS) and anomalous small-angle X-ray scattering (ASAXS). The structure of the metal hydride matrix is characterized using (ultra) small-angle neutron scattering (SANS/USANS). The hydrogen distribution in tanks filled with metal hydride material is studied with neutron computerized tomography (NCT). The results obtained by the different analysis methods are summarized in a final structural model. The complementary information obtained by these different methods is essential for the understanding of the various sorption processes in light metal hydrides and hydrogen storage tanks.

The influence of additives on the reaction kinetics and microstructure refinement in LiBH4–MgH2 composites is investigated in detail. Indications of the rate-limiting processes during the reactions are obtained by comparison of the measured reaction kinetics with simulations with one specific rate-limiting process. The kinetics of the sorption reactions are derived from volumetric measurements as well as from in situ X-ray diffraction measurements. During desorption, the hydrogen is released at a constant rate, which is possibly correlated with the one-dimensional growth of MgB2 platelets. In contrast, the kinetic curves of the absorption reactions exhibit the typical shape of contracting-volume controlled kinetics. The microscopical interpretation of kinetic measurements are supported by transmission electron microscopy images confirming the formation of additive-nanostructures in the grain boundaries upon cycling. The present investigations underline the importance of the additives as nucleation substrates and the influence of microstructure on the reaction kinetics.

This work demonstrates that hydrogen can be reversibly stored in a composite of NaF and Al. NaF and Al reacts to a mixture of Na3AlF6 and NaAlH4 via hydridofluoride phases of the form Na3AlH6-xFx. The analysis of thermodynamics based on literature standard enthalpies of formation yields the technically favourable enthalpy of reaction of roughly 35 kJ/mol H-2 for a theoretical gravimetric hydrogen storage capacity of 3.3 wt%. Reaction mechanisms are discussed with respect to substitution of hydrogen by fluorine in complex hydrides. (C) 2008 Elsevier B.V. All rights reserved.

Lithium tetrahydridoboranate is among the materials with the highest hydrogen content and has great potential as a possible H-2-storage material, although, the release and uptake of H-2 is not fully understood. In this work, LiBH4 was studied by in situ synchrotron radiation powder X-ray diffraction (PXD) and solid state CP/MAS NNIR both at variable temperatures. This study revealed two new phases observed during dehydrogenation of LiBH4. Phase I is hexagonal, a = 4.93(2) and c = 13.47(3) angstrom and is observed in the temperature range -200-300 degrees C, and phase II is orthorhombic, a = 8.70(1), b = 5.44(1) and c = 4.44](8) angstrom and is observed in the temperature range similar to 300-400 degrees C applying a constant heating rate of 5 degrees C/min. Apparently, I transforms into II, e.g. at a constant temperature of T= 265 degrees C after 5 h. Furthermore, a third phase, III, is observed in the temperature range RT to 70 degrees C, and is caused by a reaction between LiBH4 and water vapor from the atmosphere. Hydrogen release is associated with the decomposition of III at ca. 65 degrees C. (C) 2007 Elsevier B.V. All rights reserved.

A detailed analysis of the reaction mechanism of the reactive hydride composite (RHC) MgH2 + 2LiBH(4) MgB2 + 2LiH + 4H(2) was performed using high-pressure differential scanning calorimetry (HP-DSC) measurements and in situ synchrotron powder X-ray diffraction (XRD) measurements along with kinetic investigations using a Sievert-type apparatus. For the desorption the following two-step reaction has been observed: MgH2 + 2LiBH(4) Mg + 2LiBH(4) + H-2 MgB2 + 2LiH + 4H(2). However, this reaction is kinetically restricted and proceeds only at elevated temperatures. In contrast to the desorption reaction, LiBH4 and MgH2 are found to form simultaneously under fairly moderate conditions of 50 bar hydrogen pressure in the temperature range of 250-300 degrees C. As found in pure light metal hydrides, significant improvement of sorption kinetics is possible if suitable additives are used. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. (Less)

The kinetics of the reversible reaction of hydrogen gas with magnesium forming MgH2 is enhanced significantly by the addition of transition metals oxide catalysts and by using nanostructured powders. Hydrogen absorption and desorption properties of such systems were studied by differential scanning calorimetry (DSC) under hydrogen atmosphere, from which the heat of hydride formation and decomposition is determined. Apart from Mg and stoichiometric MgH2, we find an additional, slightly destabilized phase, which is formed prior to MgH2 upon hydrogenation. The amount of this phase depends on the degree of nanostructuring and the used additive. X-ray diffraction measurements confirm the compared to MgH2 slightly different lattice parameters of the intermediate phase. The results correspond to recent neutron diffraction measurements, by which a new MgH2−δ phase was found. We propose that this destabilized phase acts as a gateway for de-hydrogenation of MgH2.

In-situ synchrotron radiation powder X-ray diffraction (SR-PXD) technique is a powerful tool to gain a deeper understanding of reaction mechanisms in crystalline materials. In this paper, the implementation of a new in-situ SR-PXD cell for solid–gas reactions is described in detail. The cell allows performing measurements in a range of pressure which goes from light vacuum (10−2 bar) up to 200 bar and temperatures from room temperature up to 550 °C. The high precision, with which pressure and temperature are measured, enables to estimate the thermodynamic properties of the observed changes in the crystal structure and phase transformations.

Chemical reactions between LiF and MgB2 with different molar ratios of 1:1 and 4:1 under hydrogen atmosphere were studied by high-pressure differential scanning calorimetry (HP-DSC) and in situ syn...

The decomposition pathway in LiBH4−MgH2 reactive hydride composites was investigated systematically as a function of pressure and temperature. Individual decomposition of MgH2 and LiBH4 is observed at higher temperatures and low pressures (T ≥ 450 °C and p(H2) ≤ 3 bar), whereas simultaneous desorption of H2 from LiBH4 and formation of MgB2 was observed at 400 °C and a hydrogen backpressure of p(H2) = 5 bar. The simultaneous desorption of H2 from LiBH4 and MgH2 without intermediate formation of metallic Mg could not be observed. In situ X-ray diffraction (XRD) and infrared (IR) spectroscopy reveal the present crystalline and amorphous phases.

An exhaustive microstructural characterization is reported for the LiBH4-MgH2 reactive hydride composite (RHC) system with and without titanium isopropoxide additive. X-ray diffraction with Rietveld analysis, transmission electron microscopy coupled to energy dispersive X-ray analysis, selected-area electron diffraction and electron energy loss spectroscopy are presented in this paper for the first time for this system for all sorption steps. New data are reported regarding average crystallite and grain size, microstrain, phase formation and morphology; these results contribute to the understanding of the reaction mechanism and the influence of the additives on the kinetics. Microstructural effects, related to the high dispersion of titanium-based additives, result in a distinct grain refinement of MgB2 and an increase in the number of reaction sites, causing acceleration of desorption and absorption reactions. Considerations on the stability of phases under electron beam irradiation have also been reported. European Commission MRTN-CT-2006-035366 Ministerio de Ciencia e Innovación CTQ2009-13440

Reactive hydride composites (RHCs) are very promising hydrogen storage materials for future applications due to their reduced reaction enthalpies and high gravimetric capacities. At present, the materials\' functionality is limited by the reaction kinetics. A significant positive influence can be observed with addition of transition-metal-based additives. To understand the effect of these additives, the chemical state and changes during the reaction as well as the microstructural distribution were investigated using x-ray absorption near-edge structure (XANES) spectroscopy and anomalous small-angle x-ray scattering (ASAXS). In this work, zirconium- and vanadium-based additives were added to 2LiBH4–MgH2 composites and 2LiH–MgB2 composites and measured in the vicinity of the corresponding absorption edge. The measurements reveal the formation of finely distributed zirconium diboride and vanadium-based nanoparticles. The potential mechanisms for the observed influence on the reaction kinetics are discussed.

In this work, high-energy ball-milled magnesium hydride samples used for hydrogen storage are investigated using small- and ultra-small-angle neutron scattering (SANS) as well as small-angle X-ray scattering (SAXS). Size distributions of inhomogeneities with dimensions from 10 A up to more than 10 mm, corresponding to crystallite and particle sizes obtained by X-ray diffraction and electron microscopy, are determined as a function of milling time, milling tool material and added metal oxide catalysts in order to study the influence of the microstructure on the sorption kinetics. Significant changes of the volume fraction distributions are found for samples containing the catalyst chromium oxide, particularly when the catalyst particles are nanometre-sized. Cr2O3 is an effective agent for breaking up particles during the milling process. The comparison of SANS and SAXS curves give some of the first information about the distribution of hydrogen-containing structures. Using anomalous small-angle X-ray scattering, an energy-dependent scattering is found for an MgHx sample with 1 mol% Fe2O3. From the separated scattering curve a size distribution of hard spheres is obtained with a size range which is expected for crystallite and particle sizes of the Fe2O3 catalyst. Chemical shifts in the absorption spectra give information about the stability of the metal oxide catalysts during the milling process.

Diphenylthallium hydroxide reacts very smoothly in organic solvents with catechols, pyrogallols and 1,2-diketones to stable paramagnetic diphenylthalliumsemiquinone and diphenylthalliumsemidione complexes respectively. This reaction has been investigated with 43 different ligands in numerous solvents. Therefore its general application is proofed. The ESR-spectra of these solutions show the hydrogen hyperfine structure of the ligands and an unusual large coupling which we assign to magnetic interaction of the free electron with the 203Tl and 205Tl nuclei. The thallium splittings observed depend remarkably on the ligands used. We refer this to different solvation phenomena. Semiquinones of the pyrogallol type show on principle two different radical types. Small concentrations of diphenylthallium hydroxide produce 1 1 complexes. These can be converted into 1 2 complexes by addition of further amounts of diphenyl thallium hydroxide. The ESR-spectra of the semiquinone-and the semidione-thallium complexes exhibit an unusually strong solvent dependence of the thallium coupling and of the g-factor. It is possible to explain both variations uniformly by the different donor ability of the solvents used. All complexes investigated indicate remarkable temperature dependence of the thallium splitting and the g-factor. The thallium couplings show a positive temperature gradient whereas the g-factor decrease with increasing temperature as expected. Based on the sum of the observed effects, we assume that the radicals observed by us are ion pairs in which a diphenylthallium cation interacts with the semiquinone- or semidione-anion, respectively.