Gasinduziertes optisches Schaltverhalten dünner Magnesium-Nickel-Schichten

This thesis adresses the phase transformations of thin magnesium nickel films upon hydrogen absorption and desorption. The reactions were induced by hydrogen and oxygen containing gases. The films were evaporated onto glass substrates and were covered with a thin catalyst top layer, commonly palladium. This layer catalyses the gas phase reactions and protects the MgNi layer from oxidation. Upon hydrogen absorption the investigated metallic MgNi alloys show a reversible phase transformation to semiconducting hydrides (switchable mirrors). This leads to a transition from a reflecting to a transparent state. Therefore such film systems may be used for smart windows with a high contrast ratio between the two states and an effective glare protection. However, up to now the cycling stability of the layers is too low. To improve the cycling stability, first the switching mechanism and its dependence on film properties are explored. The characteristics and the kinetics of the transformation reactions were investigated by measurements of the transmittance and reflectances of the film system during the phase transitions. These measurements were compared with calculations assuming various transformation pathways. With these tools the characteristics of the phase transformations that is nucleation and growth could be examined. Using methods like REM, AES, optical spectroscopy, x-ray diffraction and solid state NMR-spectroscopy the film systems were characterized in both states of the MgNi film, metallic and semiconducting. The transformation properties show a strong dependence on structural and interfacial properties of the film system. During absorption of hydrogen the specific interfacial properties lead to the effect that the nucleation of the hydride phases is surprisingly favoured at or near the substrate interface. Therefore the formation of an hydrogen impermeable hydride layer at the catalyst/MgNi interface is suppressed. This effect explains the good hydriding characteristics of the layers with high transmittance values in the semiconducting state, which is crucial for applications like smart windows. However, in detail the hydriding characteristics show a dependence on composition of the MgNi layers, which is mainly due to structural properties. In this respect, only layers within a defined compositonal region with an x-ray-amorphous structure may be completely hydrided. In contrast the dehydriding reaction in oxygen containing atmosphere follows the expected mechanism with nucleation of the metallic phase at the catalyst/MgNi-interface but commonly with slower kinetics. With the knowledge of these characteristics the mechanism of the degradation were investigated using the methods developed in the upper part. Degradation mechanisms are poisoning of the palladium-surface, diffusion of palladium in the magnesium-nickel layer as well as oxidation of the magnesium-nickel film. The film system was improved by inclusion of additional layers such as an platinum overlayer and an vanadium interlayer between the palladium and the magnesium-nickel layer. With this film system the poisoning of the catalyst surface as well as the diffusion of palladium in the active layer could be prevented whereas the oxidation of the magnesium-nickel layer could be reduced. The cycling stability was improved from about 20 cycles at the beginning to about 300 cycles with similar degradation levels. In conclusion this thesis provides basic findings to improve the layer system of switchable mirrors