Metal hydrides for smart window and sensor applications
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Introduction The discovery of Y switchable mirrors, thin films that switch between an optically reflective metallic state and a transparent semiconducting state, opened up a whole new range of applications.1 It revealed that a metal hydride thin film remains remarkably stable on repeated hydrogenation, despite the large volume changes involved.2 While these volume changes can hardly be avoided, it was nevertheless possible to show that the metal-insulator transition observed on hydrogenation is induced only by the hydrogen concentration and not by the volume change or any other structural transition.3,4 The optical changes observed on hydrogenation allow for the development of effective combinatorial methods.5,6 Especially in thin films, one can quickly scan thousands of materials systems and identify the most attractive compositions for hydrogen storage and other metal hydride applications. In metal hydrides displaying a metal–insulator transition, the optical changes are easily observed in the visual part of the spectrum, for which the term “hydrogenography” was coined.6 However, the same method can also be applied to interstitial hydrides, such as Pd-based hydrides.7 While the focus of metal hydride thin-film research has been on the discovery of new material compositions, it can also be used to study the effect of doping,8 interface energy,9 and the elastic and plastic effects of clamping a nanomaterial to a matrix.10
Besides hydrogen storage, metal hydrides are used in heat storage applications,11 separation membranes,12 hydrogen sensors,13 and smart window applications.14,15 Here, we focus on the last two applications. The application of both thin-film devices depends critically on microstructural stability during cycling. First, we discuss the present status and potential advantage of the use of switchable mirrors as elements of smart window devices. In the second part of this review, we will present various ways to use the optical changes in metal hydrides to develop a hydrogen sensor.
Smart windows Smart windows are seen as an important technology to reduce power consumption in automotive and building sectors.16 The current state-of-the-art smart window is an electrochromic window using WO3 as a switching layer.17 This material has a transparent and dark blue state, depending on the hydrogen content, for instance. However, the shading performance is limited in such an absorption-switching device, because some part of the absorbed energy is re-emitted as heat. Hence, a device that uses a change in reflective properties is desired. The presence of a reflective, metallic state is an important advantage of metal hydrides and makes them highly attractive for smart window applications. While conventional smart windows switch between a transparent and a dark blue absorbing state, metal hydrides can be switched between a transparent and a reflective mirror state.
K. Yoshimura, Advanced Industrial Science and Technology, Nagoya, Japan; [email protected] C. Langhammer, Department of Applied Physics, Chalmers Univers
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