Publications

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Tantalate materials play a vital role in our high technology society: tantalum capacitors are found in virtually every cell phone. Furthermore, electronic characteristics and the incredibly inert nature of tantalates renders them ideal for applications such as biomedical implants, nuclear waste forms, ferroelectrics, piezoelectrics, photocatalysts and optical coatings. The inert and insoluble nature of tantalates is not fundamentally understood; and furthermore poor solubility renders fabrication of novel or optimized tantalates very difficult. We have developed a soft chemical route to water-soluble tantalum oxide clusters that can serve as both precursors for novel tantalate materials and ideal models for experimental and computational approaches to understanding the unusually inert behavior of tantalates. The water soluble cluster, [Ta6O19]8- is small, highly symmetric, and contains the representative oxygen types of a metal oxide surface, and thus ideally mimics a complex tantalate surface in a simplistic form that can be studied unambiguously. Furthermore; in aqueous solution, these highly charged and super-basic clusters orchestrate surprising acid-base behavior that most likely plays an important role in the inertness of related oxide surfaces. Our unique synthetic approach to the [Ta6O19]8- cluster allowed for unprecedented enrichment with isotopic labels (17O), enabling detailed kinetic and mechanistic studies of the behavior of cluster oxygens, as well as their acid-base behavior. This SAND report is a collection of two publications that resulted from these efforts.

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The sintering behavior of Sandia chem-prep high field varistor materials was studied using techniques including in situ shrinkage measurements, optical and scanning electron microscopy and x-ray diffraction. A thorough literature review of phase behavior, sintering and microstructure in Bi{sub 2}O{sub 3}-ZnO varistor systems is included. The effects of Bi{sub 2}O{sub 3} content (from 0.25 to 0.56 mol%) and of sodium doping level (0 to 600 ppm) on the isothermal densification kinetics was determined between 650 and 825 C. At {ge} 750 C samples with {ge}0.41 mol% Bi{sub 2}O{sub 3} have very similar densification kinetics, whereas samples with {le}0.33 mol% begin to densify only after a period of hours at low temperatures. The effect of the sodium content was greatest at {approx}700 C for standard 0.56 mol% Bi{sub 2}O{sub 3} and was greater in samples with 0.30 mol% Bi{sub 2}O{sub 3} than for those with 0.56 mol%. Sintering experiments on samples of differing size and shape found that densification decreases and mass loss increases with increasing surface area to volume ratio. However, these two effects have different causes: the enhancement in densification as samples increase in size appears to be caused by a low oxygen internal atmosphere that develops whereas the mass loss is due to the evaporation of bismuth oxide. In situ XRD experiments showed that the bismuth is initially present as an oxycarbonate that transforms to metastable {beta}-Bi{sub 2}O{sub 3} by 400 C. At {approx}650 C, coincident with the onset of densification, the cubic binary phase, Bi{sub 38}ZnO{sub 58} forms and remains stable to >800 C, indicating that a eutectic liquid does not form during normal varistor sintering ({approx}730 C). Finally, the formation and morphology of bismuth oxide phase regions that form on the varistors surfaces during slow cooling were studied.

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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 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.

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.

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.

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