The manifestation of oxygen contamination in ErD 2 thin films
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Erbium dihydride Er(H,D,T)2 is a fluorite structure rare-earth dihydride useful for the storage of hydrogen isotopes in the solid state. However, thermodynamic predictions indicate that erbium oxide formation will proceed readily during processing, which may detrimentally contaminate Er(H,D,T)2 films. In this work, transmission electron microscopy (TEM) techniques including energy-dispersive x-ray spectroscopy, energyfiltered TEM, selected area electron diffraction, and high-resolution TEM are used to examine the manifestation of oxygen contamination in ErD2 thin films. An oxide layer 30–130 nm thick was found on top of the underlying ErD2 film, and showed a cube-oncube epitaxial orientation to the underlying ErD2. Electron diffraction confirmed the oxide layer to be Er2O3. While the majority of the film was observed to have the expected fluorite structure for ErD2, secondary diffraction spots suggested the possibility of either nanoscale oxide inclusions or hydrogen ordering. In situ heating experiments combined with electron diffraction ruled out the possibility of hydrogen ordering, so epitaxial oxide nanoinclusions within the ErD2 matrix are hypothesized. TEM techniques were applied to examine this oxide nanoinclusion hypothesis.
I. INTRODUCTION
The use of metal hydrides is under consideration for the solid-state storage of hydrogen for alternative energy applications,1–3 where precedence is given to storage density and ease of hydrogen loading and unloading.4,5 However, other applications require that the hydrogen stay in solid form for long-term storage. One such application would be to store tritium (3H; T) or deuterium (2H; D) for neutron tube targets,6–9 where deuterium ions are accelerated down a vacuum tube into metal deuteride or metal tritide targets to produce high-energy neutrons via the D + D or D + T fusion reactions. Erbium is particularly suitable for long-term hydrogen storage, as it readily forms Er(H, D,T)2 compounds and does not dissociate easily.10 Erbium dihydrides can be grown by evaporating a thin film of Er metal onto a substrate, and then converting the metal film to an Er(H,D,T)2 film by subsequent elevated-temperature loading of the metal with the selected hydrogen isotope. Erbium metal and erbium dihydride thin-film growth have been accomplished on many different substrates: Mo,7,11–13 thin carbon films,14 rocksalt,14–16 sapphire,17,18 glass,19,20 Cr on Cu,6 and Mo on Si.21,22 In addition to polycrystalline thin films, it has been shown that epitaxial films can also be grown.15,16 a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0217
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http://journals.cambridge.org
J. Mater. Res., Vol. 24, No. 5, May 2009 Downloaded: 19 Mar 2015
Because of its dissociation-resistance, metal-like electrical conductivity,20,23 and thin-film compatibility, erbium dihydride is attractive for various applications. However, one outstanding problem with Er as a hydride material is that it is highly susceptible to oxidation. Indeed, the enthalpy of for
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