Rare-earth hydride thin films

Growth and Microstructure of Rare-Earth (Hydride) Thin Films... 

One of the most important aspects of the discovery of switchable mirrors is the fact that in the form of thin films rare-earth hydrides (REH ) are amenable to a whole series of experiments, which were often impossible with bulk samples since hydrogen absorption resulted in a total disintegration of bulk samples. 

The growth and microstmcture of switchable mirror thin fllrns involves specific aspects that need to be discussed before describing their physical properties. We focus first on Y, La and rare-earth films (for convenience all abbreviated as RE films), deposited in both their metallic and hydride forms. Some references to second-generation mirrors (Mg-RE) are included and third-generation switchable mirrors (Mg-TM) are shortly discussed separately. 

The discrepancy in the measured hydriding rates of La+H may originate from differences in the behavior of massive metal versus thin-film samples. Kinetic results are reported by Atkinson et al. (1976) for the reactions of several rare-earth metal films with hydrogen at low pressures (10 to 10 bar) and low temperatures (— 196 to 100°C),... 

In transmission EELS the specimens are confined to very thin slices of the material or thin edges of crushed powders. In the case of rare earths this restricts samples to evaporated thin films. Due to the high reactivity of the rare earths, oxide, hydroxide or hydride phases pose serious problems in the conventional vacua of many transmission spectrometers. More flexibility in the nature of samples is possible in reflection EELS. Evaporated thin films, foils or massive bulk samples can all be readily investigated, but the sample surfaces have to be cleaned thoroughly by often tedious procedures. Some surface cleaning and preparation techniques of rare earth materials have been compiled by Netzer and Bertel (1982), and more recently by Netzer and Matthew (1986). 

From mass spectrometry studies Curzon (1984) reports that the reduction of CO, inside the vacuum chamber, leads to the formation of CH4 (and consequently of CH3, CH2 and CH). The results indicate that the CH4 (and also H2) arises from the reduction of the CO (and also H2O, which reacts with CO). That is another way to explain the formation of hydrides with rare earth thin films. The adsorption of CsHg, C2H2 and CH4 on clean polycrystalline Dy films at 295 K has been studied by Cemy and Smutek (1990). The experiments suggest that at low doses, the gases are completely dissociated into C and H atoms. The bonding of these atoms to Dy is assumed to be equivalent to that which... 

When thin rare earth metal films were begun to be studied in electron microscopes with poor vacuums it quickly became evident that one was studying the characteristics of thin oxide or hydride films. Such studies began with Murr (1%7) and were continued by Kumar et al. (1970) and have become a torrent from Caro and coworkers (see section 2.2.3). Kumar et al. (1970) found the films to oxidize by a linear rate law in contrast to Murr (1967) who observed a logarithmic rate. 

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