An optical method to determine the thermodynamics of hydrogen absorption and desorption in metals

Hydrogenography, an optical high-throughput combinatorial technique to find hydrogen storage materials, has so far been applied only to materials undergoing a metal-to-semiconductor transition during hydrogenation. We show here that this technique works equally well for metallic hydrides. Additionally, we find that the thermodynamic data obtained optically on thin Pd-H films agree very well with Pd-H bulk data. This confirms that hydrogenography is a valuable general method to determine the relevant parameters for hydrogen storage in metal hydrides. © 2007 American Institute of Physics

Interplay of diffusion and dissociation mechanisms during hydrogen absorption in metals

Kinetic measurements of gas-solid reactions, in particular hydrogen-sorption kinetics, are usually interpreted according to single rate-limiting step models. However, recent studies gave clear evidence for the interdependence of fundamentally different steps involved in hydrogen sorption. This interdependence is explored in a one-dimensional continuum model in order to estimate the time, temperature, and pressure dependences of the sorption kinetics and the relevance of several materials parameters involved in the process. Quantitative descriptions of the extraction of physical parameters for various scenarios are given. The model is successfully tested for several experimental cases, ranging from model systems (thin films) to practical systems (LaNi5 Hx and MgH2 powder samples). © 2008 The American Physical Society

Opto-mechanical characterization of hydrogen storage properties of Mg-Ni thin film composition spreads

Thin film composition spreads of Mg-Ni were deposited by co-sputtering on micromachined Si-cantilevers. The investigated compositions range from about M

Combinatorial thin film methods for the search of new lightweight metal hydrides

The search for new lightweight metal hydride storage materials is essentially like looking for a needle in a haystack. Over the years, a number of combinatorial methods have been developed to scan the properties of materials in an efficient way. We demonstrate that combinatorial techniques are also applicable for the search of suitable hydrogen storage materials. This applies especially to hydrogenography, a novel optical screening method that measures simultaneously the enthalpy of hydride formation of thousands of materials on a single thin film wafer. © 2007 Acta Materialia Inc

Highly destabilized Mg-Ti-Ni-H system investigated by density functional theory and hydrogenography

Using hydrogenography, we recently mapped the thermodynamic properties of a large range of compositions in the quaternary Mg-Ti-Ni-H system. The enthalpy of hydride formation of Mg-Ni alloys is significantly altered upon Ti doping. For a small range of compositions, we find a hydrogenation enthalpy ΔH=-40 kJ (mol H2) -1, which is the desired enthalpy for hydrogen storage at moderate temperature and pressure. This enthalpy value is surprising since it is significantly less negative than the ΔH of the Mg-Ni and Mg-Ti hydrides. The nanostructure of the Mg-Ti-Ni-H films hinders a direct determination of the hydride phases involved by x-ray diffraction. Using density functional theory calculations for various hydrogenation reaction paths, we establish that the destabilization of the Mg-Ni-H system by Ti doping is due to the formation of Mg2 Ni and Ti-Ni intermetallics in the as-deposited state, which transform into a metastable Ti-doped Mg2 Ni H4 phase upon hydrogenation. The Ti-doped Mg2 Ni H4 phase can be considered as a heavily doped semiconductor. © 2008 The American Physical Society

Chemical short-range order and lattice deformations in MgyTi1-yHx thin films probed by hydrogenography

A multisite lattice gas approach is used to model pressure-optical- transmission isotherms (PTIs) recorded by hydrogenography on Mgy Ti1-y Hx sputtered thin films. The model reproduces the measured PTIs well and allows us to determine the chemical short-range order parameter s. The s values are in good agreement with those determined from extended x-ray absorption fine structure measurements. Additionally, the PTI multisite modeling yields a parameter L that accounts for the local lattice deformations with respect to the average Mgy Ti1-y lattice given by Vegard's law. It is thus possible to extract two essential characteristics of a metastable alloy from hydrogenographic data. © 2008 The American Physical Society

Effect of H-induced microstructural changes on pressure-optical transmission isotherms for Mg-V thin films

Although neither V nor Ti is supposed to mix with Mg, in thin films some alloy formation is observed. On hydrogenation, these alloys show clearly different behaviors. Films of Mg V disproportionate during repeated absorption and desorption of hydrogen, in contrast to Mg Ti thin films which are structurally reversible upon several hydrogenation cycles. Using Hydrogenography on co-sputtered Mg V thin films, we observe the progressive segregation of Mg and V atoms induced by the formation of the respective hydrides. A simple multi-site lattice-gas model with long range H H interaction reproduces well the optically measured isotherms and allows to determine the evolution of the chemical short-range order parameter s with cycling. The complex features observed in the isotherms are related to microstructural changes in the film. This paper shows how hydrogenography, being an optical technique, allows us to determine thermodynamic and structural/compositional information for a very large number of metal hydrides simultaneously. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved

Ti-catalyzed Mg(AlH4)(2): A reversible hydrogen storage material

Mg–Al thin films with a compositional gradient are co-sputtered from off-centered magnetron sources and capped with a thin Pd layer. We study their hydride formation by monitoring their optical transmission during hydrogenation under defined pressure and temperature conditions. We find that Mg(AlH₄)₂ is already formed from the elements at p(H₂)=1 bar and T=100 ° C. A thin layer of Ti acts as a catalyst. Doping of Mg–Al with Ti has a negative influence on hydrogen absorption

Critical composition dependence of the hydrogenation of Mg2 +/-delta Ni thin films

The hydrogenation of metallic M

High-throughput concept for tailoring switchable mirrors

The optical properties, the switching kinetics and the lifetime of hydrogen switchable mirrors based on Mg-Ni alloys are determined with particular regard to the composition of the optically active metal-hydride layer in combination with the thickness of the catalytic capping layer. For this, a high-throughput experiment is introduced. The switching kinetics and the reversibility of switchable mirrors are strongly thickness dependent, though the details hinge on the fine structure of the clustered capping layer. Therefore, the kinetics is correlated with the surface structures of Pd on M