Browsing by Subject "Metallic oxides"

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Magnetic, thermoelectric, and electronic properties of layered oxides and carbon materials
(2007) Caudillo, Roman, 1977-; Goodenough, John B.; Jose-Yacaman, Miguel
The structure and physical properties of layered oxides and carbon materials were studied. Two layered carbon materials were studied: carbon nanotubes (CNTs) synthesized by electron irradiation from amorphous carbon in situ in a transmission electron microscope (TEM) and a carbon and silver nanocomposite consisting of graphitic carbon nanospheres encapsulating Ag nanoparticles. In the CNT experiments, the effect of electron irradiation in the TEM is shown to alter drastically their structure and properties, even being able to transform amorphous carbon into a CNT. This suggests a possible alternative synthesis technique for the production of CNTs, in addition to providing a method for tailoring their properties. The structure and magnetic properties of the carbon and silver nanocomposite was characterized with x-ray diffraction, scanning and transmission electron microscopy techniques, and magnetic susceptibility measurements with a superconducting quantum interference device (SQUID) magnetometer. While the sp² bonding gives a grapheme sheet its mechanical properties, the p[subscript pi] electrons are responsible for its electronic and magnetic properties. In a flat graphene sheet the p[subscript pi] electrons are itinerant, but in a narrow p[subscript pi] band. The introduction of curvature to the graphene sheets that encapsulate the Ag nanoparticles is demonstrated to narrow the p[subscript pi] band sufficiently to result in "ferromagnetic" behavior. A model that is able to explain spin localization and ferrimagnetic spin-spin interactions in graphitic materials with positive curvature is presented. Layered oxides from the family of the P2 Na[subscript x]CoO₂ structure were synthesized and their properties studied. Na[subscript x]CoO₂ has a rich phase diagram ranging form a promising Na-rich thermoelectric composition to the hydrated Na-poor composition Na[subscript 0.33]CoO₂· 1.3H₂O that is superconductive. Intermediate to these two Na compositions exists an insulating phase with x [approximately equal to] 0.5 that presents a variety of interesting structural, magnetic, thermoelectric, and electronic behavior. Investigations of Na[subscript x]CoO₂ that probe the role of H₂O in the superconductive Na[subscript 0.33]CoO₂· 1.3H₂O are presented and conclude that H₂O plays a more active role than a passive lattice spacer. The relationship between Na ordering and an interesting magnetic behavior observed with [chi](T) measurements of annealed NaxCoO₂ and Sr[subscript x/2]CoO₂ samples is determined and found to correspond to a (2a x 2a) superstructure. The properties of NaxCoO₂ (x [approximately equal to] 0.5) are reviewed and thermoelectric S(T) measurements are made in order to develop a model that is able to explain the salient features of the NaxCoO₂ (x [approximately equal to] 0.5) phase.
The reactive and dissociative behavior of size-selected clusters and ion complexes in the gas phase
(1992) Maleknia, Simin Dokht; Brodbelt, Jennifer S.
Molecular complexation properties of several types of size-selected ions are examined in the gas phase by mass spectrometric techniques. Transition metal oxide clusters of (MoO₃)[subscript n]⁻ where n = 2-12 and (WO₃)[subscript n]⁻ extending to n = 8 are generated by field and laser desorption, and electron capture in the negative ionization mode. The structures of these clusters are characterized by low energy collision induced dissociation (CID). The clusters dissociate by the neutral loss of metal trioxide units. Reactions of ligands such as ethylene oxide and cyclohexene sulfide are examined to distinguish the structural variations of the clusters. The systematic evaluation of the size effect in molecular recognition processes is examined by association reactions of crown ether macrocyclic hosts with alkali metal ions and primary amine substrates. Ion complexes of crown ethers with two different alkali metals are formed by liquid secondary ion mass spectrometry. These complexes produce crown ether/alkali metal adduct ions upon gas-phase isolation and high energy dissociation. The abundance ratios of product ions are correlated to the alkali metal ion selectivites of the crown ether. The selectivity of 18-crown-6 follows the order of Na⁺ ≥ K⁺ > Li⁺ > Rb⁺ > Cs⁺, and the concept of 'maximum contact point' (MCP) describes this order. In contrast, the high energy activation of alkali metal ion adducts of crown ethers results in homolytic cleavages of C-C or C-O bonds. The metastable ion dissociation of hydrogen bonded complexes of crown ethers with ammonium or methyl-ammonium shows a stability order of 21-crown-7 > 18-crown-6 > 15-crown-5. High energy CID of the ion complexes proceeds by cavity size dependent intramolecular ring opening reactions of the host. Complexes of crown ethers with methyl- and tosyl-hydrazinium ions dissociate by covalent bond cleavages of the N-N or N-S bonds of the guest substrates. Finally, reactions of perfluorinated cyclic or bicyclic ether anions with O₂ are examined as models of oxygen transport by perfluorinated compounds. These complexes show binding strengths that reflect both the topological and chemical nature of the hosts.