Structural and optical properties of Mg_xAl_1-xH_y gradient thin films: a combinatorial approach

The structural, optical and dc electrical properties of MgxAl1-x (0.2≤x≤0.9) gradient thin films covered with Pd/Mg are investigated before and after exposure to hydrogen. We use hydrogenography, a novel high-throughput optical technique, to map simultaneously all the hydride forming compositions and the kinetics thereof in the gradient thin film. Metallic Mg in the MgxAl1-x layer undergoes a metal-to-semiconductor transition and MgH₂ is formed for all Mg fractions x investigated. The presence of an amorphous Mg-Al phase in the thin film phase diagram enhances strongly the kinetics of hydrogenation. In the Al-rich part of the film, a complex H-induced segregation of MgH₂ and Al occurs. This uncommon large-scale segregation is evidenced by metal and hydrogen profiling using Rutherford backscattering spectrometry and resonant nuclear analysis based on the reaction ¹H(¹⁵N,αγ)¹²C. Besides MgH₂, an additional semiconducting phase is found by electrical conductivity measurements around an atomic [Al]/[Mg] ratio of 2 (x=0.33). This suggests that the film is partially transformed into Mg(AlH₄)₂ at around this composition

Hydrogen Dynamics in Lightweight Tetrahydroborates

The high hydrogen content in complex hydrides such as M[AlH4]x and M[BH4]x (M = Li, Na,K, Mg, Ca) stimulated many research activities to utilize them as hydrogen storage materials. An understanding of the dynamical properties on themolecular level is important to understand and to improve the sorption kinetics. Hydrogen dynamics in complex hydrides comprise long range translational diffusion as well as localized motions like vibrations, librations or rotations. All the different motions are characterized by their specific length- and timescales. Within this review we give an introduction to the physical properties of lightweight complex hydrides and illustrate the huge variety of dynamical phenomena on selected example

Synthesis Mechanism of Alkali Borohydrides by Heterolytic Diborane Splitting

Similar to alane in alanates, borane species are assumed to be the mass transport intermediate in the hydrogen storage reaction MH + B + 3/2H2 MBH4 with M = Li and Na. One possible substep of this reaction is the interaction of diborane with the alkali hydride. In this paper, we unravel the synthesis mechanism of alkali borohydrides by solid-gas reaction of alkali hydrides and diborane gas by H/D isotope labeling of the reaction educts (e.g., LiD + B2H 6). The labeling enables us to trace the hydrogen/deuterium atoms in the borohydride product by Raman scattering and in the gas by infrared spectrometry measurements. We conclude that, during the LiBH4 synthesis from LiH, the entire BH4- unit is transferred from the diborane to the Li+ cation. This provides clear evidence for the heterolytic splitting of diborane on alkali hydrides and implies exchange of BH4- with H- ions of the underlying hydride. The detection of Li-H bonds at the surface of newly formed LiBH4 confirms the importance of H- defects for the synthesis of borohydrides. © 2010 American Chemical Society

Micro discharges in HVDC gas insulated systems

ISSN:1070-9878ISSN:1558-413

Single Quality Factor for Enthalpy-Entropy Compensation, Isoequilibrium and Isokinetic Relationships

Enthalpy-entropy compensation (EEC) is very often encountered in chemistry, biology and physics. Its origin is widely discussed since it would allow, for example, a very accurate tuning of the thermodynamic properties as a function of the reactants. However, EEC is often discarded as a statistical artefact, especially when only a limited temperature range is considered. We show that the likeliness of a statistical origin of an EEC can be established with a compensation quality factor (CQF) that depends only on the measured enthalpies and entropies and the experimental temperature range. This is directly derived from a comparison of the CQF with threshold values obtained from a large number of simulations with randomly generated Van ‘t Hoff plots. The value of CQF is furthermore a direct measure of the existence of a genuine isoequilibrium or isokinetic relationship

Single Quality Factor for Enthalpy‐Entropy Compensation, Isoequilibrium and Isokinetic Relationships

Enthalpy-entropy compensation (EEC) is very often encountered in chemistry, biology and physics. Its origin is widely discussed since it would allow, for example, a very accurate tuning of the thermodynamic properties as a function of the reactants. However, EEC is often discarded as a statistical artefact, especially when only a limited temperature range is considered. We show that the likeliness of a statistical origin of an EEC can be established with a compensation quality factor (CQF) that depends only on the measured enthalpies and entropies and the experimental temperature range. This is directly derived from a comparison of the CQF with threshold values obtained from a large number of simulations with randomly generated Van ‘t Hoff plots. The value of CQF is furthermore a direct measure of the existence of a genuine isoequilibrium or isokinetic relationship