Jaarboek no. 89. 2010/2011 - Koninklijke Maatschappij voor ...

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Natuurkundige voordrachten I Nieuwe reeks 89 Schakelbare spiegels: een samenspel van licht en waterstof 30 (a) (b) (c) Figure 1 (a)-(c) Photographs of an Yttrium-hydride switchable mirror in the as-deposited, dihydride and trihydride states. The metal-insulator transition, which is occurs at low hydrogen pressures (at room temperature less than 1 bar), is not of structural origin. It occurs within the hexagonal phase at a composition H/Y≈2.86. have characteristic colors: for example, YH 3 is yel- lowish, LaH 3 red, while some fully hydrogenated alloys containing magnesium are colorless. One of the most surprising results of these early measurements was, however, that the films retained their structural integrity even though they expanded by typically 15% during hydrogenation of the pure parent metal to the trihydride. This meant that for the first time physical properties, such as electrical resistivity, Hall effect, optical transmission, reflection and absorption, were amenable to experimental investigation. This led to the discovery of new phenomena in the electrical, optical and mechanical properties of these materials. Furthermore, the possibility to fine-tune their properties by alloying and the ease of continuously changing their hydrogen content made them especially attractive for fundamental condensed matter physics. Soon after their discovery it became clear that switchable metal hydride films would pose intriguing questions. Their transparent appearance in the fully loaded state posed a serious problem to theorists. In particular, the yellow appearance of YH3 seemed to be in contradiction with state-of-the-art band structure calculations that predicted a metallic state for this material. The physics of rare-earth switchable mirrors is thus closely related to that of electron correlation and metal-insulator transitions. Three families of switchable mirrors So far we have only described the properties of Yttrium and Lanthanum hydrides. It turns out that all Rare-Earth (RE) hydrides exhibit an optical switchability. Two other families of hydrogen-based switchable mirrors have been discovered. We have thus the following families of switchable mirrors: - rare-earth switchable mirrors - color neutral magnesium-rare-earth (Mg-RE) switchable mirrors - magnesium-transition-metal (Mg-TM) switchable mirrors An example of the reversible optical switching of a Mg-Gd layer is shown in figure 2 and that of a Mg-Ti layer in figure 3. The Mg-RE and the Mg-TM switchable mirrors both exhibit the three fundamental optical states of matter, i.e. shiny metallic, transparent and highly absorbing, when their hydrogen content is modified. The microscopic mechanisms responsible for their ‘black state’ are, however, due to their structure on a nanoscale. There is by now a large body of articles on the remarkable properties of switchable metal-hydride mirrors. 5, 6 Recently it was also shown that alloys and multilayered samples can be used to fine-tune all the essential properties of switchable mirrors and induce new phenomena. 7
Applications Beside their purely fundamental interest, switchable mirrors offer also attractive possibilities as smart coatings in electrochromic devices, as hydrogen indicators for catalytic and diffusion investigations, as the active layer in fiber optic hydrogen sensors and as hydrogen absorption detectors in a combinatorial search for new lightweight hydrogen storage and membrane materials. a. Hydrogenography Gremaud et al. 8 recently developed Hydrogenography, a combinatorial technique that allows to simultaneously measure the hydrogen absorption isotherms of thousands of thin film metal-hydrides, Natuurkundige voordrachten I Nieuwe reeks 89 Schakelbare spiegels: een samenspel van licht en waterstof Figure 2 Stills of a movie showing the switching from metallic reflecting to transparent and back to the metallic state in a Magnesium-Gadolinium thin layer. After introduction of hydrogen the layer switches within a few seconds to the transparent state. Introduction of oxygen (or simply ambient air) brings the mirror back to its original reflective state within a few minutes. Figure 3 Optical appearance of a thin layer of Magnesium-Titanium. Without hydrogen, the layer is reflective while it is strongly absorbing in presence of hydrogen. The residual reflectance is due to the glass substrate on which the Mg-Ti film is sputtered. by following the optical changes occurring upon hydrogenation. In a Hydrogenography experiment the amount of light transmitted through a thin film is recorded as a function of the hydrogen pressure in equilibrium with the sample. According to the Beer- Lambert law, the logarithm of the optical transmission is directly proportional to the hydrogen concentration in the material. The Pressure-Optical Transmission-Isotherms obtained with hydrogenography, in which the hydrogen pressure is plotted as a function of the logarithm of the optical transmission, are therefore equivalent to the standard Pressure-Composition-Isotherms measured in conventional metal-hydride research. The advantage of Hydrogenography is that it allows to measure the 31
Page 1: NATUURKUNDIGE VOORDR ACHTEN 2010 -
Page 4 and 5: ISBN 978 90-72644-23-7 Drukkerij Vi
Page 7 and 8: INHOUD Diligentiaprijs voor Scholie
Page 9 and 10: VERSLAG VAN DE KONINKLIJKE MAATSCHA
Page 11 and 12: Oprichting in 1793 Het Gezelschap t
Page 13 and 14: N.Th. Michaelis 1898-1904 Dr. E.H.
Page 15 and 16: ALFABETISCH REGISTER VAN DE VOORDRA
Page 17 and 18: Natuurkundige voordrachten I Nieuwe
Page 23 and 24: Elektriciteitopslag in batterijen m
Page 25 and 26: De vermogensdichtheid van een batte
Page 27 and 28: Figuur 3 Het effect dat vacatures (
Page 29: Schakelbare spiegels: een samenspel
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Page 35 and 36: Gammaflitsen: extreem nieuws uit de
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Page 39 and 40: Figuur 2 Als we voor alle 2704 gamm
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Page 53 and 54: Figuur 5 De economische haalbaarhei
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