Selected Publications

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By David D. Allred (et al.)
Abstract:

Oxidation of a 1000 Å sputter-deposited thorium thin film at 150 °C in 100 ppm of flowing oxygen in argon produces the long-sought solid form of thorium monoxide. Changes in the scattering length density (SLD) distribution in the film over the 700-min experiment measured by in-situ, dynamic neutron reflectometry (NR) shows the densities, compositions and thickness of the various thorium oxides layers formed. Screened, hybrid density-functional theory calculations of potential thorium oxides aid interpretation, providing atomic-level picture and energetics for understanding oxygen migration. NR provided evidence of the formation of substoichiometric thorium oxide, ThOy (y < 1) at the interface between the unreacted thorium metal and its dioxide overcoat which grows inward, consuming the thorium at a rate of 2.1 Å/min while y increases until reaching 1:1 oxygen-to-thorium. Its presence indicates that kinetically-favored solid-phase ThO can be preferentially generated as a majority phase under the thermodynamically-favored ThO2 top layer at conditions close to ambient.

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By Matthew R. Jones, Dale R. Tree, and David D. Allred (et al.)
Abstract: An optical fiber thermometer consists of an optical fiber whose tip is coated with a highly conductive, opaque material. When heated, this sensing tip becomes an isothermal cavity that emits like a blackbody. This emission is used to predict the sensing tip temperature. In this work, analytical and experimental research has been conducted to further advance the development of optical fiber thermometry. An inexpensive optical fiber thermometer is developed by applying a thin coating of a high-temperature cement onto the tip of a silica optical fiber. An FTIR spectrometer is used to detect the spectral radiance exiting the fiber. A rigorous mathematical model of the irradiation incident on the detection system is developed. The optical fiber thermometer is calibrated using a blackbody radiator and inverse methods are used to predict the sensing tip temperature when exposed to various heat sources.
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By Lawrence K. Barrett, Dallin J. Barton, Steven G. Noyce, David D. Allred, Richard R. Vanfleet, and Robert C. Davis
Abstract: High-aspect-ratio metallic microstructures have a variety of potential applications in sensing and actuation. However, fabrication remains a challenge. We have fabricated nickel microstructures with over 20:1 aspect ratios by electroplating patterned carbon-coated carbon-nanotube forests using a nickel chloride bath. Pulse plating allows nickel ions to diffuse into the interior of the forest during off portions of the cycle. Done properly, this solves the problem of the formation of an external crust, which otherwise blocks nickel deposition in the interior of the structures. Thus, densities of 86 ± 3% of bulk Ni for the composite structures are achieved. Cantilever structures do not yield under load, but break. Measurements of the material properties of this composite material indicate an elastic modulus of ~42 GPa and a strength of 400 MPa. We demonstrate the utility of this method with an external field magnetic actuator consisting of a proof mass and two flexures. We achieved 1-mN actuation forces. [2014-0274]
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By David D. Allred (et al.)
Abstract: Detecting and understanding the complex signatures of species for attribution of highly enriched uranium, HEU, is challenging even though these compounds have been intensively studied for 65 years. Attempts to obtain, for example, chemical speciation signatures on uranium oxides are frustrated by the presence of extremely diverse phases, complex structures, and their tendency to form solid solutions with the coexistence of many nonstoichiometric oxides. More importantly, the spectroscopic signatures of many of these oxides, using common techniques such as X-ray diffraction or Raman scattering, are remarkably similar with each other. On the other hand, the effort to understand the U-O system also exhibits some of the most intriguing and challenging properties in theoretical and computational chemistry. This is due to the spatial extent between localization and delocalization of the 5f orbitals of the uranium atom. In this article, spectroscopic ellipsometry (SE) measurements and a comparison of six fitting methods as well as theoretical calculations are combined to examine the intrinsic electronic structure and the corresponding band gap of uranium oxides to determine the chemical speciation in a,102 nm thick reactively sputtered uranium oxide film. The SE results reveal that the UOx film exhibits two absorption edges, a primary absorption edge slightly above 2.6 eV and a secondary absorption at 1.7-1.8 eV. The optical band gaps compared with the theoretical calculations performed on UO2, U4O9, U3O7, alpha-U3O8, alpha-UO3, delta-UO3, and gamma-UO3 suggest that the UOx film is composed of at least two components; the primary absorption is caused by the alpha-UO3 sublayer, which is superimposed on top of an adjacent alpha-U3O8 sublayer that is hypothesized to be heteroepitaxial growth of alpha-U3O8 along the UOx/substrate interface. Comparison to the ellipsometry measurements shows that the DFT+U and hybrid (HSE) calculations predict the correct trend for band gaps as a function of oxidation state and crystallography but they fail to capture the exact gaps. However, they provide important information for interpretation of the experimental results and highlight some of the structural complexity that prevails in the UOx compounds. The combination of theoretical and experimental methods to examine the intrinsic electronic structure and the band gap of the corresponding uranium oxides could benefit from the development of new methods for better distinguishing chemical speciation in uranium oxides. In addition, the experimental measurement of the indirect band gap of alpha-U3O8, is, to our knowledge, reported for the first time.
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By D. D. Allred (et al.)
Abstract: A unique approach to detect chemical speciation and distribution on nanometer-scale nuclear materials has been achieved by the combination of neutron reflectometry and shell-isolated surface-enhanced Raman spectroscopy. Both surface and underlying layers of the uranium oxide materials were determined with angstrom-level resolution. Our results reveal that the UOx film is composed of three sublayers: an, similar to 38 angstrom thick layer of U3O8 formed along the UOx/substrate interface; the adjacent sublayer consists of an similar to 900 angstrom thick single phase of alpha-UO3, and the top layer is gamma-UO3 with a thickness of similar to 115 angstrom.
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By Kellen Moulton, Nicholas B. Morrill, Adam M. Konneker, Brian D. Jensen, Richard R. Vanfleet, David D. Allred, and Robert C. Davis
Abstract: This paper examines the effect of iron catalyst thickness on the straightness of growth of carbon nanotubes (CNTs) for microelectromechanical systems fabricated using the CNT-templated-microfabrication (CNT-M) process. SEM images of samples grown using various iron catalyst thicknesses show that both straight sidewalls and good edge definition are achieved using an iron thickness between 7 and 8 nm. Below this thickness, individual CNTs are well aligned, but the sidewalls of CNT forests formed into posts and long walls are not always straight. Above this thickness, the CNT forest sidewalls are relatively straight, but edge definition is poor, with significantly increased sidewall roughness. The proximity of a device or feature to other regions of iron catalyst also affects CNT growth. By using an iron catalyst thickness appropriate for straight growth, and by adding borders of iron around features or devices, a designer can greatly improve straightness of growth for CNT-MEMS.