Publications

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Erbium metal thin-films have been deposited on molybdenum-on-silicon substrates and then converted to erbium dideuteride (ErD{sub 2}). Here, we study the effects of deposition temperature ({approx}300 or 723 K) and deposition rate (1 or 20 nm/s) upon the initial Er metal microstructure and subsequent ErD{sub 2} microstructure. We find that low deposition temperature and low deposition rate lead to small Er metal grain sizes, and high deposition temperature and deposition rate led to larger Er metal grain sizes, consistent with published models of metal thin-film growth. ErD{sub 2} grain sizes are strongly influenced by the prior-metal grain size, with small metal grains leading to large ErD{sub 2} grains. A novel sample preparation technique for electron backscatter diffraction of air-sensitive ErD{sub 2} was developed, and allowed the quantitative measurement of ErD{sub 2} grain size and crystallographic texture. Finer-grained ErD{sub 2} showed a strong (1 1 1) fiber texture, whereas larger grained ErD{sub 2} had only weak texture. We hypothesize that this inverse correlation may arise from improved hydrogen diffusion kinetics in the more defective fine-grained metal structure or due to improved nucleation in the textured large-grain Er.

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Erbium dihydride Er(H,D,T){sub 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){sub 2} films. In this work, transmission electron microscopy (TEM) techniques including energy-dispersive x-ray spectroscopy, energy-filtered TEM, selected area electron diffraction, and high-resolution TEM are used to examine the manifestation of oxygen contamination in ErD{sub 2} thin films. An oxide layer {approx}30-130 nm thick was found on top of the underlying ErD{sub 2} film, and showed a cube-on-cube epitaxial orientation to the underlying ErD{sub 2}. Electron diffraction confirmed the oxide layer to be Er{sub 2}O{sub 3}. While the majority of the film was observed to have the expected fluorite structure for ErD{sub 2}, 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 ErD{sub 2} matrix are hypothesized. TEM techniques were applied to examine this oxide nanoinclusion hypothesis.

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In an effort to better understand the structural changes occurring during hydrogen loading of erbium target materials, we have performed D{sub 2} loading of erbium metal (powder) with simultaneous neutron diffraction analysis. This experiment tracked the conversion of Er metal to the {alpha} erbium deuteride (solid-solution) phase and then on to the {beta} (fluorite) phase. Complete conversion to ErD{sub 2.0} was accomplished at 10 Torr D{sub 2} pressure with deuterium fully occupying the tetrahedral sites in the fluorite lattice. Increased D{sub 2} pressure (up to 500 Torr at 450 C) revealed {approx}10 % deuterium occupation of the octahedral sites. Subsequent vacuum pumping of the sample at 450 C removed octahedral site occupancy while maintaining tetrahedral deuterium occupancy, thereby yielding stoichiometric ErD{sub 2.0} {beta} phase.

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Sandia National Laboratories has cradle to grave responsibility for all neutron generators in the US nuclear weapons stockpile. As such, much research effort is exerted to develop a comprehensive understanding of all the major components of a neutron generator. One of the key components is the tritium containing target. The target is a thin metal tritide film. Sandia's research into metal tritides began in the early 1960's with a collaboration with the Denver Research Institute (DRI) and continues to this day with a major in house research effort. This document is an attempt to briefly summarize what is known about the aging of erbium tritide and to review the major publications conducted at Sandia in FY 07. First, a review of our knowledge of helium in erbium tritide will be presented. Second, executive summaries of the six major SAND reports regarding neutron tube targets published in FY07 by Department 2735, the Applied Science and Technology Maturation Department, and research partners are presented.

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Er(D,T){sub 2-x} {sup 3}He{sub x}, erbium di-tritide, films of thicknesses 500 nm, 400 nm, 300 nm, 200 nm, and 100 nm were grown and analyzed by Transmission Electron Microscopy, X-Ray Diffraction, and Ion Beam Analysis to determine variations in film microstructure as a function of film thickness and age, due to the time-dependent build-up of {sup 3}He in the film from the radioactive decay of tritium. Several interesting features were observed: One, the amount of helium released as a function of film thickness is relatively constant. This suggests that the helium is being released only from the near surface region and that the helium is not diffusing to the surface from the bulk of the film. Two, lenticular helium bubbles are observed as a result of the radioactive decay of tritium into {sup 3}He. These bubbles grow along the [111] crystallographic direction. Three, a helium bubble free zone, or 'denuded zone' is observed near the surface. The size of this region is independent of film thickness. Four, an analysis of secondary diffraction spots in the Transmission Electron Microscopy study indicate that small erbium oxide precipitates, 5-10 nm in size, exist throughout the film. Further, all of the films had large erbium oxide inclusions, in many cases these inclusions span the depth of the film.

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XRD analysis of plasma-vapor-deposited ErT2 films during aging (T decay to He3) reveals an hkl-dependent unit-cell expansion in which (200) grains expand out-of-plane as much as 0.01 Å more than (111) out-of-plane grains. Texture analysis of an aged ErT2 film reveals a bimodal (111)/(200) out-of-plane preferred orientation. Sin2 ψ analysis reveals significant in-plane macro-strain due to He3 bubble formation/growth. The mechanistic origins regarding these observations are also discussed. © 2007 International Centre for Diffraction Data.

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