Optical, Electrical and Switching Properties of Gadolinium - Gadolinium Hydride / Palladium Silver System
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Optical, Electrical and Switching Properties of Gadolinium - Gadolinium Hydride / Palladium Silver System H. Schmitt, E. Shalaan Technical Physics, Saarland University, D-66123 Saarbruecken, Germany
ABSTRACT Gadolinium (Gd)- and gadolinium/magnesium (Gd/Mg)-films as well as top layers of catalytic films were prepared by rf-sputtering technique. To improve the optical properties, optical transparency, color and switching time, the Gd films were modified with Mg. By change of the sputtering parameters, the grain size of the Gd-films was varied in the range from 29 nm to 9.5 nm. To improve the catalytic properties, palladium-films (Pd) were modified with silver (Ag). To investigate the influence of the structure and the microstructure on the optical and the switching properties of the films, X-ray diffraction measurements and scanning electron microscope studies were performed. It is shown that the optical transmission of the GdH3 films can be improved by the modification of the gadolinium-films with magnesium. Further, a significant influence of the grain size is found. If the grains size of the films is reduced to 9.5 nm, the optical transmission of the films increases. The reduced grain size, leading to a larger inner surface, decreases the optical and the electrical switching time and the optical transmission. A reduction of the switching time is attained by the modification of the catalytic palladium films with silver. INTRODUCTION Since the oil crisis in 1973, but especially recently there is a large discussion about the conservation of energy. One possibility is to utilize solar radiation for heating by use of adequate architectural arrangements such as large windows, which are oriented to the sun. But, if the solar power is too large, some protection is necessary. This can be obtained by mechanical sunblind. A more qualified method, however, is a window system whose optical transmittance could be switched electronically. To build up such a smart window, different material systems can be used. Besides organic polymers, the oxides of transition metals, e.g. WO3 can be used. They can incorporate hydrogen (H) or alkali metal ions to drastically change their optical behavior. The topic of this presentation is the transformation of gadolinium films to gadoliniumhydrides. J. N. Huilberts et al. found, that the formation of YH3 makes the reflecting Y film optically transparent. The color of the hydride film can be reduced if one replaces Y by another rare earth metal, the gadolinium (Gd) [2]. Chemically, the reaction can be described by equation (1): Gd + 1 H2 => GdH2 + ½ H2
Gd H3
(1)
At room temperature, the first part of the reaction is irreversible, the second part of the reaction from Gd-dihydride to Gd-trihydride is reversible and can be used in a smart window arrangement. The advantage of hydrides instead of transition metal oxides is the much higher optical reflectivity of the metallic films. The use of gadoliniumhydride films (GdH3) instead of YH3 shifts the absorption edge to lower wavelengths. Ad
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