Workshop on metal hydride films for optoelectronic applications

Last week we had a workshop in our department, bringing together international experts within the field of metal hydrides, thin films and electrochromism in metal hydrides and oxides:

Participants at the workshop, from left: Dag Noreus (Stockholm University), Darius Milcius (LEI, Lithuania), Henrik Fahlquist (Stockholm University), Bernard Dam (TU Delft, The Netherlands), Smagul Karazhanov (IFE), Arve Holt (IFE), Kazuki Yoshimura (AIST, Japan), Sean Erik Foss (IFE), Aline Rougier (CNRS, France), Stefano Deledda (IFE), Trygve Mongstad (IFE), Yasusei Yamada (AIST, Japan), Jan Petter Mæhlen (IFE) and Volodomyr Yartys (IFE).

It was interesting to be able to discuss the opportunities with regards to these relatively new applications of metal hydride films. The two main points of interest was the recent advances in Mg-based metal films for electrochromic and gasochromic windows and the opportunities and questions that the recent discovery of photochromic effect in yttrium hydride films present.

Electrochromic oxides

Aline Rougier from CNRS in France set the background of the workshop with a survey of the status of electrochromism in metal oxide films. Metal oxides are more well-established than metal hydrides as chromogenic materials, and has been applied e. g. in automatic dimming of rear-view mirrors in cars, a technology commercialized by Gentex. Rougier has long experience with WO3 as an optically active element, and emphasized also that the synthesis of transparent conducting films is an integral part of the research challenges for electrochromics. The main challenge for implementation of electrochromics in smart window applications is the durability after thousands of switching cycles and the challenge of producing films that are uniform over a large area. Currently, a large research project under the 7th framework programme in the EU, Innoshade, is working on the development of oxides and organic materials for smart window applications.

Smart windows and hydrogen sensing

Kazuki Yoshimura and Yasusei Yamada from the AIST in Japan represent the currently most active research group on metal hydride films. Metal hydride films have the advantage that they become reflective rather than absorbing in the opaque state, which is favorable for windows that control the indoor solar irradiation and energy flow. In recent reports, the group of Yoshimura has demonstrated several impressive advances on gasochromic and electrochromic metal hydride-based devices. Among them are development of a "4th generation" of metal hydride films (Mg-alkaline earth) for switchable windows, durability tests and enhancement of cyclability from a few hundred cycles to several thousand and a full-scale experiment to test the energy performance of metal-hydride-based switchable mirrors with respect to static windows. The latter demonstrated 34% reduction on energy use for a room with electrochromic windows during a typical Japanese summer day. Several spin-off activities are on the way to be commercialized, as optical hydrogen sensors and a simple but innovative hydrogen detection sheet.

Phase transitions in Mg2HiH4

Dag Noreus from Stockholm university has long experience with Mg-based metal hydrides, working on structure determination of hydrides. In a collaboration with Darius Milcius from the Lithuanian Energy Institute, he has done work with films of Mg2NiH4, with the intention of demonstrating a pressure-sensitive metal-hydride switchable window. The idea was that the allotropes of Mg2NiH4 has very different optical properties, and experiments on powders have shown that phase transitions can be driven by applying mechanical pressure. However, no such response to mechanical pressure has yet been demonstrated for metal hydride films.

From switchable windows to nano-thermodynamics

Bernard Dam from TU Delft in the Netherlands has long experience with optical properties of thin-film metal hydrides, coming from the research group of Ronald Griessen who initiated this activity in 1996. Currently, there are several researchers in his group working on metal hydride films for optical hydrogen sensors. The activity directly related to switchable windows has been relatively low in the recent years. Work of Dam's group has further refined the thin-film metal hydride systems as a smart way to investigate nano-scale effects on the thermodynamics and stability of metal hydrides.

Photochromism in metal hydride films

As mentioned, the photochromic effect of yttrium hydride films was one of the main points of interest at the workshop. This effect was discovered in our group, reported in 2011. The effect is interesting on many levels: Firstly, it demonstratrates a new effect in metal hydrides which is of fundamental interest, something we should definitely learn more about. The properties of the effect suggests that the physical mechanism is different from what is found for photochromic metal oxides. Secondly, there are many reasons why this reaction is relevant for technological applications. It is probable that further research will lead to technological innovation. We hope to be able to further study this effect. Bernard Dam, who has been involved from the start of the work on photochromic yttrium hydride is also very eager to continue the studies.