Growth, microstructure and hydrogenation of Pd-catalyzed complex metal hydride thin films

Groei, microstructuur en waterstof-absorptie door Pd-gekatalyseerde, complexe metaal hydride dunne films Schakelbare spiegels Het onderzoek dat in dit proefschrift gepresenteerd wordt is gebaseerd op experimenten met een bepaald type schakelbare spiegels. Huiberts e.a. toonde het schakelbare effect van dunne metaal lagen voor het eerst aan in 1995 aan de Vrije Universiteit Amsterdam. Een schakelbare spiegel bestaat uit een dunne metaal film die is aangebracht op een glasplaatje, het substraat. De eerste schakelbare spiegels bestonden uit de metalen lanthaan (La) en yttrium (Y). Het andere element dat nodig is voor het functioneren van een schakelbare spiegel is waterstof (H2). Een waterstof molecuul bestaat uit twee waterstof atomen en wanneer deze moleculen gesplitst worden kunnen de waterstof atomen opgenomen worden door de metaal laag. Op deze metaal laag wordt doorgaans een palladium (Pd) laag aangebracht die de film beschermt tegen oxidatie en de waterstof opname katalyseert. De waterstof atomen, die tussen de metaal atomen door diffunderen kunnen dan met de metaal atomen een hydride fase vormen. Wanneer dit gebeurt dan verandert de film van een spiegelende metaal film naar een transparante isolator (vergelijkbaar met een normale ruit). Het bijzondere van deze reactie is dat deze omkeerbaar is door het waterstof te verwijderen wordt de transparante film weer een metaal film.Griessen, R.P. [Promotor]Dam, B. [Copromotor

Effect of the strong metal-support interaction on hydrogen sorption kinetics of Pd-capped switchable mirrors

The morphology and electronic structure of Pd clusters grown on oxidized yttrium surfaces are investigated by scanning tunneling microscopy and ultraviolet photoelectron spectroscopy. The hydrogen sorption mediated by the Pd clusters is determined from the optically monitored switching kinetics of the underlying yttrium film. A strong thickness dependence of the hydrogen uptake is found. The electronic structure of the as-grown Pd clusters depends only weakly on their size. Strong changes of the photoemission spectra are found after hydrogenation, in particular the oxide peak shifts and the Pd peaks vanish. Both phenomena are due to a strong metal-support interaction (SMSI) state, characterized by a complete encapsulation of the clusters by a reduced yttrium oxide layer. Scanning tunneling spectroscopy confirms the SMSI state of small Pd clusters after hydrogen exposure. The SMSI effect is less important with increasing Pd thickness. This explains the critical thickness for the catalyzed hydrogen uptake by the Pd/Y

Structural and optical properties of Mg2NiHx switchable mirrors upon hydrogen loading

The structural, thermodynamic and optical properties of M

Huge compact flux avalanches in superconducting Nb thin films

Using a magneto-optical technique we perform a quantitative analysis of magnetic flux penetration in superconducting niobium thin films on A-plane (112̄0) sapphire substrates as a function of temperature (1.5 K ≤ T ≤

Optical hydrogen sensors based on metal-hydrides

For many hydrogen related applications it is preferred to use optical hydrogen sensors above electrical systems. Optical sensors reduce the risk of ignition by spark formation and are less sensitive to electrical interference. Currently palladium and palladium alloys are used for most hydrogen sensors since they are well known for their hydrogen dissociation and absorption properties at relatively low temperatures. The disadvantages of palladium in sensors are the low optical response upon hydrogen loading, the cross sensitivity for oxygen and carbon, the limited detection range and the formation of micro-cracks after some hydrogen absorption/desorption cycles. In contrast to Pd, we find that the use of magnesium or rear earth bases metal-hydrides in optical hydrogen sensors allow tuning of the detection levels over a broad pressure range, while maintaining a high optical response. We demonstrate a stable detection layer for detecting hydrogen below 10% of the lower explosion limit in an oxygen rich environment. This detection layer is deposited at the bare end of a glass fiber as a micro-mirror and is covered with a thin layer of palladium. The palladium layer promotes the hydrogen uptake at room temperature and acts as a hydrogen selective membrane. To protect the sensor for a long time in air a final layer of a hydrophobic fluorine based coating is applied. Such a sensor can be used for example as safety detector in automotive applications. We find that this type of fiber optic hydrogen sensor is also suitable for hydrogen detection in liquids. As example we demonstrate a sensor for detecting a broad range of concentrations in transformer oil. Such a sensor can signal a warning when sparks inside a high voltage power transformer decompose the transformer oil over a long period

Effect of hydrogen uptake and substrate orientation on the flux penetration in NbHx thin films

By absorbing various amounts of hydrogen in the same niobium sample we are able to change the local superconducting properties thus introducing quenched disorder in a controlled manner. Its effect on the magnetic flux penetration in Nb

A thin-film device for detecting hydrogen

The present invention relates to a thin-film device, to a method for producing a thin-film device, to a protective layer for shielding an oxygen, moisture and/or carbon monoxide sensitive surface, to a method for shielding such a surface, to a method for forming a metal framework material, to a hydrogen sensor and to an electro-magnetic transformer comprising said sensor.ChemE/Chemical EngineeringApplied Science

Hydrogen sorption mechanism of oxidized nickel clusters

Using an optical technique to measure hydrogen sorption kinetics the catalytic activity of the NiOx clusters is determined. The thus measured temperature dependence reveals an activation energy of 0.58 eV. The catalytic activity of NiOx clusters is studied as a function of the oxygen concentration. The surface properties are analyzed by Auger-electron spectroscopy. It appears that the catalytic hydrogen sorption originates from the dissociative chemisorption of hydrogen on O: Ni, which is strongly suppressed by the presence of oxides. © 2004 American Institute of Physics

Double layer formation in Mg-TM switchable mirrors (TM : Ni, Co, Fe)

The optical properties of Mg-TM (TM: Ni, Co, Fe; Mg:TM ≈ 2) thin films during hydrogenation are investigated using reflection and transmission measurements. Mg-Ni and Mg-Co show an unusual behavior upon exposure to hydrogen. The nucleation of the hydride starts at the film substrate interface and not - as intuitively expected - at the Pd covered top side of the film. Consequently, a double layer structure is formed. Reflection measurements are used to identify the loading behavior as the double layering yields to an remarkable optical black state at intermediate hydrogen concentrations. Photometric measurements on samples loaded at