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.
Interfaces are a critical determinant of the full range of materials properties, especially at the nanoscale. Computational and experimental methods developed a comprehensive understanding of nanograin evolution based on a fundamental understanding of internal interfaces in nanocrystalline nickel. It has recently been shown that nanocrystals with a bi-modal grain-size distribution possess a unique combination of high-strength, ductility and wear-resistance. We performed a combined experimental and theoretical investigation of the structure and motion of internal interfaces in nanograined metal and the resulting grain evolution. The properties of grain boundaries are computed for an unprecedented range of boundaries. The presence of roughening transitions in grain boundaries is explored and related to dramatic changes in boundary mobility. Experimental observations show that abnormal grain growth in nanograined materials is unlike conventional scale material in both the level of defects and the formation of unfavored phases. Molecular dynamics simulations address the origins of some of these phenomena.
Most engineering materials are inherently inhomogeneous in their processing, internal structure, properties, and performance. Their properties are therefore statistical rather than deterministic. These inhomogeneities manifest across multiple length and time scales, leading to variabilities, i.e. statistical distributions, that are necessary to accurately describe each stage in the process-structure-properties hierarchy, and are ultimately the primary source of uncertainty in performance of the material and component. When localized events are responsible for component failure, or when component dimensions are on the order of microstructural features, this uncertainty is particularly important. For ultra-high reliability applications, the uncertainty is compounded by a lack of data describing the extremely rare events. Hands-on testing alone cannot supply sufficient data for this purpose. To date, there is no robust or coherent method to quantify this uncertainty so that it can be used in a predictive manner at the component length scale. The research presented in this report begins to address this lack of capability through a systematic study of the effects of microstructure on the strain concentration at a hole. To achieve the strain concentration, small circular holes (approximately 100 {micro}m in diameter) were machined into brass tensile specimens using a femto-second laser. The brass was annealed at 450 C, 600 C, and 800 C to produce three hole-to-grain size ratios of approximately 7, 1, and 1/7. Electron backscatter diffraction experiments were used to guide the construction of digital microstructures for finite element simulations of uniaxial tension. Digital image correlation experiments were used to qualitatively validate the numerical simulations. The simulations were performed iteratively to generate statistics describing the distribution of plastic strain at the hole in varying microstructural environments. In both the experiments and simulations, the deformation behavior was found to depend strongly on the character of the nearby microstructure.
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.
{sm_bullet}Mixing from some thermal process steps thought to drive H,D,T loss - This does not appear to be a problem with the Mo/Er occluder stacks {sm_bullet}Diffusion barriers investigated to prevent mixing
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.
The silicon microelectronics industry is the technological driver of modern society. The whole industry is built upon one major invention--the solid-state transistor. It has become clear that the conventional transistor technology is approaching its limitations. Recent years have seen the advent of magnetoelectronics and spintronics with combined magnetism and solid state electronics via spin-dependent transport process. In these novel devices, both charge and spin degree freedoms can be manipulated by external means. This leads to novel electronic functionalities that will greatly enhance the speed of information processing and memory storage density. The challenge lying ahead is to understand the new device physics, and control magnetic phenomena at nanometer length scales and in reduced dimensions. To meet this goal, we proposed the silicon nanocrystal system, because: (1) It is compatible with existing silicon fabrication technologies; (2) It has shown strong quantum confinement effects, which can modify the electric and optical properties through directly modifying the band structure; and (3) the spin-orbital coupling in silicon is very small, and for isotopic pure {sup 28}Si, the nuclear spin is zero. These will help to reduce the spin-decoherence channels. In the past fiscal year, we have studied the growth mechanism of silicon-nanocrystals embedded in silicon dioxide, their photoluminescence properties, and the Si-nanocrystal's magnetic properties in the presence of Mn-ion doping. Our results may demonstrate the first evidence of possible ferromagnetic orders in Mn-ion implanted silicon nanocrystals, which can lead to ultra-fast information process and ultra-dense magnetic memory applications.
The stress evolution during electrodeposition of NiMn from a sulfamate-based bath was investigated as a function of Mn concentration and current density. The NiMn stress evolution with film thickness exhibited an initial high transitional stress region followed by a region of steady-state stress with a magnitude that depended on deposition rate, similar to the previously reported stress evolution in electrodeposited Ni [S. J. Hearne and J. A. Floro, J. Appl. Phys. 97, 014901-1 (2005)]. The incorporation of increasing amounts of Mn resulted in a linear increase in the steady-state stress at constant current density. However, no significant changes in the texture or grain size were observed, which indicates that an atomistic process is driving the changes in steady-state stress. Additionally, microstrain measured by ex situ x-ray diffraction increased with increasing Mn content, which was likely the result of localized lattice distortions associated with substitutional incorporation of Mn and/or increased twin density.