Clark Sheldon Snow

First principles site occupation and migration of hydrogen, helium, and oxygen in β-phase erbium hydride

First principles density functional methods were used to investigate the atomistic behavior of hydrogen, helium, and oxygen in β-phase ErH2. The ground state for hydrogen was indeed determined to be the tetrahedral position as commonly assumed, but if the surrounding tetrahedral sites are filled, any additional hydrogen will occupy the octahedral site. Only a small amount of thermally generated tetrahedral-vacancy octahedral-occupancy pairs are predicted at equilibrium since the formation energy is 1.21 eV. Other possible scenarios that result in octahedral hydrogen occupation include a H/Er ratio >2.0 and the presence of oxygen in the lattice. Our calculations indicate that oxygen impurities will reside in tetrahedral sites, even if that site is already occupied and hydrogen must be displaced into a neighboring octahedral site. Oxygen will migrate at moderate temperatures by jumping between tetrahedral and octahedral sites. The extent of hydrogen self-diffusion will depend on the concentration of tetrahe...

UNIT CELL EXPANSION IN ErT2 FILMS

XRD analysis of plasma-vapor-deposited ErT2 films during aging (T decay to 3 He) reveals an hkl-dependent unit-cell expansion in which (200) grains expand out-of-plane as much as 0.01 A more than (111) out-of-plane grains. Texture analysis of an aged ErT2 film reveals a bimodal (111)/(200) out-of-plane preferred orientation. Sin 2 ψ analysis reveals significant in-plane macrostrain due to 3 He formation/growth. The mechanistic origins regarding these observations are also discussed.

Development of bubble microstructure in ErT2 films during aging

Helium bubbles form in metal tritide films as tritium decays into H3e, influencing mechanical properties and long-term film stability. The bubble nucleation and growth mechanisms comprise an active research area, but there has been only one previous systematic experimental study of helium bubble growth in metal tritides, on zirconium tritides. There have been no such studies on tritides such as ErT2 that form platelike bubbles and lack a secondary bubble population on a network of line dislocations, and yet such a study is needed to inform the modeling of helium bubble microstructure development in a broader range of metal tritides. Transmission electron microscopy has been used to study the growth and evolution of helium bubbles in ErT2 films over a four-year period. The results have been used to test the present models of helium bubble nucleation and growth in metal tritides, particularly those forming platelike bubbles. The results support the models of Trinkaus and Cowgill. The observations of nonunif...

Activation of erbium films for hydrogen storage

Hydriding of metals can be routinely performed at high temperature in a rich hydrogen atmosphere. Prior to the hydrogen loading process, a thermal activation procedure is required to promote facile hydrogen sorption into the metal. Despite the wide spread utilization of this activation procedure, little is known about the chemical and electronic changes that occur during activation and how this thermal pretreatment leads to increased rates of hydrogen uptake. This study utilized variable kinetic energy X-ray photoelectron spectroscopy to interrogate the changes during in situ thermal annealing of erbium films, with results confirmed by time-of-flight secondary ion mass spectrometry and low energy ion scattering. Activation can be identified by a large increase in photoemission between the valence band edge and the Fermi level and appears to occur over a two stage process. The first stage involves desorption of contaminants and recrystallization of the oxide, initially impeding hydrogen loading. Further he...

X-ray powder diffraction data for ErH2-xDx.

We report X-ray powder diffraction data for ErH2-xD x formed by hydrogen (i.e. protium)-deuterium loading of Er metal. Lattice parameters for the varying hydrogen-deuterium compositions followed Vergards law behavior. The cubic lattice parameter at room temperature for ErH2-xD x obeys a linear relationship according to the formula a = 5.1287 -1.1120 10-4 x; where a = the lattice parameter of the fluorite-type structure and x = the mole percent of deuterium. Microstrain measurements suggest a possible ordering of hydrogen and deuterium in the composition ErH1D1.

Summary of Sandia research on metal tritides : FY 2007.

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. Sandias research into metal tritides began in the early 1960s 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.

Mechanical properties of metal dihydrides

First-principles calculations are used to characterize the bulk elastic properties of cubic and tetragonal phase metal dihydrides,

In-situ time-of-flight neutron diffraction of ErD2 (beta phase) formation during D2 loading.

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