materials which change their properties under different conditions are every designer's dream. From simple quartz crystals to heat-sensing devices, a list of their potential uses can run into hundreds of pages. In the past, a lot of effort went into designing materials that can become transparent or reflective as desired. Such materials would find extensive applications in energy-saving buildings. Recently, Peter Duine of the Phillips Research Laboratories, Eindhoven, Netherlands, has demonstrated a magnesium-based device which can do exactly this ( Nature , Vol 391, No 15 January).
All these devices function thanks to the curious optical properties of the alkali -- alkaline-earth and rare metals. These become translucent and insulating from their original reflective, metallic state when converting the metal into its ionic hydride (a compound with hydrogen). To make this happen, a thin film of the metal, typically yttrium or lanthanum, is deposited on glass. These are then sealed in an air-tight cell after coating with a film of palladium (to prevent the metals from oxidation). Hydrogen, then introduced into the cell, reacts with the metal and forms the required hydride.
However, it has been known for some time now that yttrium-based films are not suitable since yttrium is not fully reflective, but shows a selective reflection of colours. Duine is experimenting with magnesium. Using an alloy of magnesium and gadolinium, he and his team got a film which is completely reflective. The two metals combined give the optical properties of magnesium while retaining the switchablity of the rare earth gadolinium. The film can be switched between the two states in about one second.
However, the problem with the device really lies in the hydrogen. Pumping hydrogen is not easy and for commercial use, it needs to be derived from a fuel cell. At the moment, the Phillips team is using a wet electrochemical cell but a solid state cell, in which the hydrogen can be transported through a solid electrolyte, is what they need. Till that is cheaply available, the commercial use of smart window panes will not really take off.