Kevin C. Ott

Regeneration of Ammonia Borane Spent Fuel by Direct Reaction with Hydrazine and Liquid Ammonia

A method to regenerate lightweight, hydrogen-rich ammonia borane improves its prospects as a vehicular fuel source. Ammonia borane (H3N-BH3, AB) is a lightweight material containing a high density of hydrogen (H2) that can be readily liberated for use in fuel cell–powered applications. However, in the absence of a straightforward, efficient method for regenerating AB from dehydrogenated polymeric spent fuel, its full potential as a viable H2 storage material will not be realized. We demonstrate that the spent fuel type derived from the removal of greater than two equivalents of H2 per molecule of AB (i.e., polyborazylene, PB) can be converted back to AB nearly quantitatively by 24-hour treatment with hydrazine (N2H4) in liquid ammonia (NH3) at 40°C in a sealed pressure vessel.

Target modification in the excimer laser deposition of YBa2Cu3O7−x thin films

Under conditions used for pulsed laser deposition (308 nm, 20 ns, 2–8 J/cm2) of Y‐Ba‐Cu‐O superconducting thin films, we have measured a pronounced decrease in deposition rate with cumulative laser exposure of the target. This decay in rate is accompanied by evolution on the target surface of microscopic columnar structures, having yttrium‐enriched surfaces, which are aligned in the direction of the incident laser beam (45°). Neither the vapor plume direction nor film stoichiometry is affected by the presence of these oriented, chemically altered surface features.

Synthesis of YBa2Cu3O7−γ and YBa2Cu4O8 by aerosol decomposition

Abstract The synthesis of submicron YBa 2 Cu 3 O 7−γ (1-2-3) and composite 1-2-3/CuO powders using aerosol decomposition of nitrate solutions and the behavior of those powders during subsequent processing were investigated. Transmission Electron Microscopy (TEM) revealed that 1-2-3 and 1-2-3/CuO particles produced at 800°C (residence time of 15 s) were polycrystalline and hollow, while solid particles were produced at temperatures greater than 900°C. Solvent evaporation led to a hollow particle morphology which was retained unless the reactor temperature exceeded 900°C, where densification was rapid enough to result in collapse of the hollow particles within the reactor residence time. Single-phase 1-2-3 powder with a critical temperature ( T c ) of 92K was produced using reactor temperatures of 900–1000°C. Dry-pressed 1-2-3 pellets densified rapidly at 800–900°C to 89% of theoretical density, which is considerably below the temperatures required for coarser powders produced by other techniques. 1-2-3/CuO particles produced at 800°C consisted of 25–50 nm crystallites of 1-2-3 and CuO, and pellets formed from that powder could be converted to single-phase YBa 2 Cu 4 O 8 (1-2-4) by heating in atmospheric-pressure oxygen at 750°C for 24 h, followed by 800°C for 24 h. Larger grained 1-2-3/CuO powder (100–250 nm) produced at 900 and 1000°C showed no conversion to 1-2-4 with the same treatment, demonstrating the critical importance of the small grain sizes obtained by aerosol decomposition for rapid conversion of 1-2-3/CuO to 1-2-4.

Nanocrystalline YBa2Cu3O7−δ/Ag composite particles produced by aerosol decomposition

Abstract YBa 2 Cu 3 O 7−δ (1-2-3) powders containing silver have been prepared by aerosol decomposition for the first time. The primary phase observed by X-ray diffractometry and transmission electron microscopy (TEM) is 1-2-3. Metallic Ag grains have also been identified by using energy-dispersive spectroscopy and diffraction pattern analysis in the TEM. For reactor operating temperatures of 1000° C, each YBa 2 Cu 3 O 7−δ particle has associated with it a single Ag grain which does not wet the superconductor. Thus, the Ag is contained in discrete grains dispersed uniformly throughout the powder. Both materials are nanocrystalline with a grain size on the order of 10 to 80 nm.

Non-covalent immobilization of homogeneous cationicchiral rhodium–phosphine catalysts on silica surfaces

Non-covalent immobilization of [(R,R)-Me-(DuPHOS)Rh(COD)]OTf by interaction of the triflate counter ion with surface silanols of silica supports leads to an active, stable, enantioselective, asymmetric hydrogenation catalyst.

Large-scale synthesis of CexLa1−xF3 nanocomposite scintillator materials

Transparent nanocomposites have been developed which consist of nanocrystals embedded in an organic matrix. The materials are comprised of up to 60% by volume of 7–13 nm crystals of the phosphor CexLa1−xF3, and are greater than 70% transparent in the visible region at a thickness of 1 cm. Consistencies of the nanocomposites range from a solid polymer to a wax to a liquid, depending on the workup conditions of the nanoparticle synthesis. These transparent nanophosphor composite materials have potential applications in radiation detection as scintillators, as well as in other areas such as imaging and lighting, and can be produced on large scales up to near-kilogram quantities at near ambient conditions, much lower in temperature than typical nanoparticle syntheses.

LaF3:Ce nanocomposite scintillator for gamma-ray detection

Nanophosphor LaF3:Ce has been synthesized and incorporated into a matrix to form a nanocomposite scintillator suitable for application to γ-ray detection. Owing to the small nanocrystallite size (sub-10 nm), optical emission from the γ / nanophosphor interaction is only weakly Rayleigh scattered (optical attenuation length exceeds 1 cm for 5-nm crystallites), thus yielding a transparent scintillator. The measured energy resolution is ca. 16% for 137Cs γ rays, which may be improved by utilizing brighter nanophosphors. Synthesis of the nanophosphor is achieved via a solution-precipitation method that is inexpensive, amenable to routine processing, and readily scalable to large volumes. These results demonstrate nanocomposite scintillator proof-of- principle and provide a framework for further research in this nascent field of scintillator research.

Synthesis of dealuminated zeolites NaY and MOR and characterization by diverse methodologies: 27Al and 29Si MAS NMR, XRD, and temperature dependent 129Xe NMR

Zeolites NaY and mordenite have been dealuminated by SiCl4 and steaming methods and carefully characterized during and after the various synthetic stages by 27Al and 29Si magic angle spinning nuclear magnetic resonance (NMR) and X-ray diffraction (XRD) to ensure sample quality. 129Xe NMR spectra for xenon adsorbed in NaX, siliceous Y, mordenite, and dealuminated mordenite have been obtained for very low xenon loadings and as a function of temperature. Data obtained for faujasites indicate that 129Xe NMR chemical shifts are very sensitive to the Si/Al ratio reflecting the presence or absence of strong adsorption sites. 129Xe NMR chemical shift data for the mordenite samples show that Xe dwells preferentially in the side pockets which do not seem to be affected by the dealumination process. In addition, the 129Xe NMR data clearly reveal information not available from 29Si and 27Al NMR or XRD regarding structural integrity of the zeolites. The findings presented here point to the need for characterization of siliceous zeolites by several techniques before conclusions regarding zeolite structural integrity can be drawn.

Neutron powder diffraction study of Cu-site preference of Li dopants in YBa2 (Cu1 − xLix)3O6 + δ

We report structural refinements for three Li-doped samples of the YBa 2 Cu 3 O 6 + δ superconductor, with 1.90% to 6.5% of the Cu replaced by Li, using neutron powder diffraction data. The superconducting transition temperature falls dramatically with doping level, falling to ≈60 K at 1.90% Li, to less than 10 K, if at all, at 5.3% and 6.5% Li. In contrast, the structure changes slowly with increasing dopant levels, and remains orthorhombic for all samples. The oxygen stoichiometry decreases slowly from δ ≈0.97 for an undoped sample to 0.82 as the Li content is increased to 6.5%, roughly a decrease of one O 2− for every two Li 1+ introduced, so as to maintain an approximately constant hole concentration. At low doping levels, the Li shows a strong preference for the chain Cu(1) sites; but at higher levels, this preference shifts to the plane Cu(2) sites. This behavior suggests that the solubility of Li in the chain sites is only about 5%, and that increasing the dopant level above this amount then forces the excess Li into the plane sites. The preferential doping of Li, first into the chain sites and then into the plane sites, provides a unique opportunity to study the effect of hole doping into the CuO 2 planes, the role played by the Cu(1)-O(1) charge reservoir, and the O(4) ions that couple it to the planes. The observed changes in T c with doping are discussed in this context.

Influence of Beam and Target Properties on the Excimer Laser Deposition of Yba2Cu3O7−x Thin Films

Target modification by excimer laser exposure has been investigated. Under conditions typical in the fabrication of superconducting thin films, deposition rate decreases with exposure and significant physical and chemical modifications occur on the target surface. These modifications do not inhibit congruent evaporation.