Thin-film metal hydrides

Abstract

Thin film metal hydrides have traditionally been studied (as a model system) for hydrogen storage applications. Meanwhile, other applications including optical switchable mirrors, optical hydrogen sensors, and the use of thin film metal hydrides to study diffusion have emerged. From a more fundamental point of view, surface effects and the fact that thin film metal hydrides have to obey constraints on their volumetric expansion which may lead to high internal stresses. This ensures that thin film metal hydrides may have a significantly different response to hydrogen as their bulk counter parts. This can lead to fascinating phenomena such as suppression of phase transitions, altering the nature of phase transitions and shifting of plateau pressures. This chapter discusses the thermodynamics of thin film metal hydrides, common experimental techniques, and several thin film metal hydrides grouped by application. First, we discuss thin film metal hydrides for hydrogen storage applications focussing on magnesium and magnesium alloys. Next, we consider switchable mirrors based on rare-earth metal hydrides. Third, we discuss metal hydrides including palladium-based and tantalum-based materials for hydrogen sensing applications. In addition, we discuss experimental techniques including hydrogenography and neutron reflectometry that are specifically suited to studying metal hydride thin films as well as how thin film metal hydrides can be applied to study hydrogen diffusion.