Publications

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Pre-oxidized and glass-to-metal (GtM) sealed austenitic stainless steels were found to display a ferritic layer near the metal/oxide interface, as determined by electron backscatter diffraction (EBSD). Electron probe microanalysis (EPMA) showed that this layer was depleted in alloying elements due to the oxidation and sealing process. Characterization of the morphology suggested that it formed through the martensite transformation mechanism. Moreover, this observed layer was correlated to the composition gradient through published empirical relationships for martensite-start (Ms) temperatures. Due to Cr, Mn, and Si depletion during pre-oxidation and glass sealing, Ms temperatures near room temperature are possible in this surface region. Further support for a martensitic transformation was provided by thermochemical modeling. Possible detrimental ramifications of bulk composition, surface depletion, and phase transformations on GtM sealing are discussed. Copyright © 2007 MS&T'07®.

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The properties of energetic thin films considered for alternative braze[1] techniques are investigated. Vapor-deposited Ni/Ti multilayer foils having a net 1:1 stoichiometry exhibit self-propagating, high temperature combustion reactions. The rate of reaction depends on Ni/Ti multilayer design with steady-state propagation speeds of freestanding foils measured from 0.2 to 1.0m/s. Transmission electron microscopy and x-ray diffraction further show that NiTi films reacted in a self-propagating mode have a fine-grain, polycrystalline microstructure. All films are composed of cubic B2 and monoclinic B19' phases with some having NiTi2 or Ni3Ti precipitates. Copyright © 2006 ASM International®.

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High-purity AlPt thin films prepared by self-propagating, high temperature combustion synthesis show evidence for a new rhombohedral phase. Sputter deposited Al/Pt multilayers of various designs are reacted at different rates in air and in vacuum, and each form a new trigonal/hexagonal aluminide phase with unit cell parameters a = 15.571(8) {angstrom}, c = 5.304(1) {angstrom}, space group R-3 (148), and Z, the number of formula units within a unit cell, = 39. The lattice is isostructural to that of the AlPd R-3 lattice as reported by Matkovic and Schubert (Matkovic, 1977). Reacted films have a random in-plane crystallographic texture, a modest out-of-plane (001) texture, and equiaxed grains with dimensions on the order of film thickness.

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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.

In the crystal structure of the title compound, C{sub 4}H{sub 4}N{sub 2}O{sub 3}, the packing is dominated by intermolecular carbonyl-carbonyl interactions and N-H...O hydrogen bonds.

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The impacts of small niobium additions to processing, microstructure, and electrical properties in the Zr-rich lead zirconate titanate ceramics (PZT 95/5) were investigated. The influence of niobium content on dielectric responses and the characteristics of ferroelectric behaviors, as well as the relative phase stability and the hydrostatic pressure induced ferroelectric-to- antiferroelectric phase transformation are reported. Results indicate that increasing the niobium concentration in the solid solutions enhances densification, refines the microstructure, decreases dielectric constant and spontaneous polarization, and stabilizes the ferroelectric phase. The stabilization of ferroelectric phase with respect to the antiferroelectric phase near PZT 95/5 composition dramatically increases the pressure required for the ferroelectric-to-antiferroelectric phase transformation. These observations were correlated to the creation of A-site vacancies and a slight modification of the crystal structure. The importance of these composition-property relationships on device application will be presented.

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The outline of this report is: (1) structures of hexagonal Er meal, ErH{sub 2} fluorite, and molybdenum; (2) texture issues and processing effects; (3) idea of pole figure integration; and (4) promising neutron diffraction work. Summary of this report are: (1) ErD{sub 2} and ErT{sub 2} film microstructures are strongly effected by processing conditions; (2) both x-ray and neutron diffraction are being pursued to help diagnose structure/property issues regarding ErT{sub 2} films and these correlations to He retention/release; (3) texture issues are great challenges for determination of site occupancy; and (4) work on pole-figure-integration looks to have promise addressing texture issues in ErD{sub 2} and ErT{sub 2} films.

The electrical properties of lead zirconate titanate ceramics near the 95/5 composition are extremely sensitive to the chemical composition and processing conditions. To precisely control the lead stoichiometry in a solid solution has been a challenge because of lead volatility during high temperature sintering. In this study, we investigated the effect of the amount of lead in the solid solution on crystal structure, dielectric behavior, and phase transformation characteristics for chemically prepared niobium modified PZT 95/5 ceramics. Implications are important for process control and assurance of material performance.

Exploration of the fundamental chemical behavior of the AlCl{sub 3}/SO{sub 2}Cl{sub 2} catholyte system for the ARDEC Self-Destruct Fuze Reserve Battery Project under accelerated aging conditions was completed using a variety of analytical tools. Four different molecular species were identified in this solution, three of which are major. The relative concentrations of the molecular species formed were found to depend on aging time, initial concentrations, and storage temperature, with each variable affecting the kinetics and thermodynamics of this complex reaction system. We also evaluated the effect of water on the system, and determined that it does not play a role in dictating the observed molecular species present in solution. The first Al-containing species formed was identified as the dimer [Al({mu}-Cl)Cl{sub 2}]{sub 2}, and was found to be in equilibrium with the monomer, AlCl{sub 3}. The second species formed in the reaction scheme was identified by single crystal X-ray diffraction studies as [Cl{sub 2}Al({mu}-O{sub 2}SCl)]{sub 2} (I), a scrambled AlCl{sub 3}{center_dot}SO{sub 2} adduct. The SO{sub 2}(g) present, as well as CL{sub 2}(g), was formed through decomposition of SO{sub 2}CL{sub 2}. The SO{sub 2}(g) generated was readily consumed by AlCl{sub 3} to form the adduct 1 which was experimentally verified when 1 was also isolated from the reaction of SO{sub 2}(g) and AlCl {sub 3}. The third species found was tentatively identified as a compound having the general formula {l_brace}[Al(O)Cl{sub 2}][OSCl{sub 2}]{r_brace}{sub n}. This was based on {sup 27}Al NMR data that revealed a species with tetrahedrally coordinated Al metal centers with increased oxygen coordination and the fact that the precipitate, or gel, that forms over time was shown by Raman spectroscopic studies to possess a component that is consistent with SOCl{sub 2}. The precursor to the precipitate should have similar constituents, thus the assignment of {l_brace}[Al(O)Cl{sub 2}][OSCl{sub 2}]{r_brace}{sub n}. The precipitate was further identified by solid state {sup 27}Al MAS NMR data to possess predominantly octahedral A1 metal center which implies {l_brace}[Al(O)Cl{sub 2}][OSCl{sub 2}]{r_brace}{sub n} must undergo some internal rearrangements. A reaction sequence has been proposed to account for the various molecular species identified in this complex reaction mixture during the aging process. The metallurgical welds were of high quality. These results were all visually determined there was no mechanical testing performed. However, it is recommended that the end plate geometry and weld be changed. If the present weld strength, based on .003' - .005' penetration, is sufficient for unit performance, the end plate thickness can be reduced to .005' instead of the .020' thickness. This will enable the plug to be stamped so that it can form a cap rather than a plug and solve existing problems and increase the amount of catholyte which may be beneficial to battery performance.

In situ neutron diffraction analysis was employed to study the behavior of the cathode and anode materials in a commercial Li-ion cell (Saehan Enertech, Inc) using the exact configuration of the commercial product. Accurate lattice parameters were refined for the LiCoO2 type cathode based on measurements collected as a function of the state of charge, Simultaneous structural characterization was possible on the graphitic anode as well. The simultaneous direct correlation of structural information for both the anode and cathode with the electrochemical data provided a highly detailed picture of the behavior of the active cell materials that ultimately underlie the cell performance. © 2003 The Electrochemical Society. All rights reserved.

The purpose of this program was to investigate methods to characterize the colloidal stability of nanoparticles during the synthesis reaction, and to characterize their organization related to interparticle forces. Studies were attempted using Raman spectroscopy and ultrasonic attenuation to observe the nucleation and growth process with characterization of stability parameters such as the zeta potential. The application of the techniques available showed that the instrumentation requires high sensitivity to the concentration of the system. Optical routes can be complicated by the scattering effects of colloidal suspensions, but dilution can cause a lowering of signal that prevents collection of data. Acoustic methods require a significant particle concentration, preventing the observation of nucleation events. Studies on the dispersion of nanoparticles show that electrostatic routes are unsuccessful with molecular surfactants at high particle concentration due to electrostatic interaction collapse by counterions. The study of molecular surfactants show that steric lengths on the order of 2 nm are successful for dispersion of nanoparticle systems at high particle concentration, similar to dispersion with commercial polyelectrolyte surfactants.

Polyoxoniobate chemistry, both in the solid state and in solution is dominated by [Nb{sub 6}O{sub 19}]{sup 8-}, the Lindquist ion. Recently, we have expanded this chemistry through use of hydrothermal synthesis. The current publication illustrates how use of heteroatoms is another means of diversifying polyoxoniobate chemistry. Here we report the synthesis of Na{sub 8}[Nb{sub 8}Ti{sub 2}O{sub 28}] {center_dot} 34H{sub 2}O [{bar 1}] and its structural characterization from single-crystal X-ray data. This salt crystallizes in the P-1 space group (a = 11.829(4) {angstrom}, b = 12.205(4) {angstrom}, c = 12.532(4) {angstrom}, {alpha} = 97.666(5){sup o}, {beta} = 113.840(4){sup o}, {gamma} = 110.809(4){sup o}), and the decameric anionic cluster [Nb{sub 8}Ti{sub 2}O{sub 28}]{sup 8-} has the same cluster geometry as the previously reported [Nb{sub 10}O{sub 28}]{sup 6-} and [V{sub 10}O{sub 28}]{sup 6-}. Molecular modeling studies of [Nb{sub 10}O{sub 28}]{sup 6-} and all possible isomers of [Nb{sub 8}Ti{sub 2}O{sub 28}]{sup 8-} suggest that this cluster geometry is stabilized by incorporating the Ti{sup 4+} into cluster positions in which edge-sharing is maximized. In this manner, the overall repulsion between edge-sharing octahedra within the cluster is minimized, as Ti{sup 4+} is both slightly smaller and of lower charge than Nb{sup 5+}. Synthetic studies also show that while the [Nb{sub 10}O{sub 28}]{sup 6-} cluster is difficult to obtain, the [Nb{sub 8}Ti{sub 2}O{sub 28}]{sup 8-} cluster can be synthesized reproducibly and is stable in neutral to basic solutions, as well.

Combined XRD/neutron Rietveld refinements were performed on PbZr{sub 0.30}Ti{sub 0.70}O{sub 3} powder samples doped with nominally 4% Ln (where Ln = Ce, Nd, Tb, Y, or Yb). Resulting refined structural parameters indicated that the lattice parameters and volume changes in the tetragonal perovskite unit cell were consistent with A and/or B-site doping of the structure. Ce doping is inconsistent with respect to its rather large atomic radius, but is understood in terms of its oxidation to the Ce{sup +4} oxidation state in the structure. Results of the B-site displacement values for the Ti/Zr site indicate that amphoteric doping of Ln cations in the structure results in superior properties for PLnZT materials.