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In situ laser raman spectroscopy during sequential oxidizing and reducing conditions for a vanadium-phosphorous-oxide catalyst

Description: A VPO catalyst prepared in an organic medium has been studied by in situ laser Raman spectroscopy under reaction conditions for n-butane oxidation to maleic anhydride. Data were obtainable at low laser power and short collection times. Raman characterization during continuous flow (steady state) revealed that the (VO){sub 2}P{sub 2}O{sub 7} phase was present. Sequential oxidizing (10% O{sub 2} in N{sub 2}) and reducing (2% n-butane in N{sub 2}) conditions were explored at 350{degree} and 400{degree}C. Cycling (unsteady state) revealed enhancement of {alpha}{sub II}-VOPO{sub 4}, {beta}-VOPO{sub 4}, {gamma}-VOPO{sub 4}, and {delta}-VOPO{sub 4} during oxidizing conditions; intensity of Raman bands due to (VO){sub 2}P{sub 2}O{sub 7} increased during reducing conditions.
Date: December 31, 1995
Creator: Soejarto, A.D.; Schrader, G.L. & Coulston, G.W.
Partner: UNT Libraries Government Documents Department

Insulator to correlated metal transition in V_1-xMo_xO_2

Description: The change from metallic to insulating states is one of the most dramatic transitions that solids undergo on cooling or chemical doping. Many materials display this transition, but only a handful have the right combination of crystal structure and physical properties to serve as model systems. VO{sub 2} is one of those materials. Using Mo as a chemical dopant in VO{sub 2}, we find unanticipated phenomenology for both the electronic and structural characteristics of the resulting insulator to metal transition. The results support a complex, previously proposed scenario involving the coexistence of both electron repulsion and electron pairing for yielding an insulator in VO{sub 2}, but not simply; many issues are raised about local versus itinerant behavior and structure-property correlations in this most iconic ofdoped correlated electron systems.
Date: January 1, 2009
Creator: Klimczuk, Tomasz; Ronning, Filip; Holman, Katherine; Mcqueen, Tyrel M; Williams, Anthony J; Stephens, P W et al.
Partner: UNT Libraries Government Documents Department

Structural and electrochemical potential simulation for the cathode material Li{sub 1+x}V{sub 3}O{sub 8}

Description: The structure and electrochemical potential of monoclinic Li{sub 1+x}V{sub 3}O{sub 8} were calculated within the local-density-functional-theory framework by use of plane-wave-pseudopotential methods. Special attention was given to the compositions 1+x=1.2 and 1+x=4, for which x-ray diffraction structure refinements are available. The calculated low-energy configuration for 1+x=4 is consistent with the three Li sites identified in x-ray diffraction measurements and predicts the position of the unobserved Li. The location of the tetrahedrally coordinated Li in the calculated low-energy configuration for 1+x=1.5 is consistent with the structure measured by x-ray diffraction for Li{sub 1.2}V{sub 3}O{sub 8}. Calculations were also performed for the two monoclinic phases at intermediate Li compositions, for which no structural information is available. Calculations at these compositions are based on hypothetical Li configurations suggested by the ordering of vacancy energies for Li{sub 4}V{sub 3}O{sub 8} and tetrahedral site energies in Li{sub 1.5}V{sub 3}O{sub 8}. The internal energy curves for the two phases cross near 1+x=3. Predicted electrochemical potential curves agree well with experiment.
Date: December 1, 1997
Creator: R. Benedek, R.; Thackeray, M.M. & Yang, I.H.
Partner: UNT Libraries Government Documents Department

Excitations and possible bound states in the S = {1/2} alternating chain compound (VO){sub 2}P{sub 2}O{sub 7}

Description: Magnetic excitations in an array of (VO){sub 2}P{sub 2}O{sub 7} single crystals have been measured using inelastic neutron scattering. Until now, (VO){sub 2}P{sub 2}O{sub 7} has been thought of as a two-leg antiferromagnetic Heisenberg spin ladder with chains running in the a-direction. The present results show unequivocally that (VO){sub 2}P{sub 2}O{sub 7} is best described as an alternating spin-chain directed along the crystallographic b-direction. In addition to the expected magnon with magnetic zone-center energy gap {Delta} = 3.1 meV, a second excitation is observed at an energy just below 2{Delta}. The higher mode may be a triplet two-magnon bound state. Numerical results in support of bound modes are presented.
Date: July 30, 1997
Creator: Tennant, D.A.; Nagler, S.E. & Sales, B.C.
Partner: UNT Libraries Government Documents Department

Magnetic excitations in (VO)HPO{sub 4}{center_dot}{1/2}H{sub 2}O

Description: The magnetic excitations of an antiferromagnetic spin dimer system, (VO)HPO{sub 4}{center_dot}{1/2}H{sub 2}O, are examined using inelastic neutron scattering. A dispersionless mode is found, consistent with expectations for a dimer excitation. The intensity variation of the mode reveals a V{sup 4+} - V{sup 4+} dimer separation of 4.43{angstrom}, almost 50% larger than the originally expected length.
Date: July 11, 1997
Creator: Garrett, A.W.; Nagler, S.E. & Tennant, D.A.
Partner: UNT Libraries Government Documents Department

Optical Spectroscopy of the Structural Phase Transition of VO{sub 2} Nanocrystals in Optical Fibers and Thin Films

Description: The optical spectroscopy of VO{sub 2} nanocrystals in planar and fiber-waveguide geometries shows a size- and doping-dependent structural semiconductor-to-metal phase transition and a shape-dependent surface plasmon resonance, with unusual linear and nonlinear applications possibilities.
Date: December 4, 2001
Creator: Lopez, R.
Partner: UNT Libraries Government Documents Department

Tungsten-vanadium oxide sputtered films for Electrochromic Devices

Description: Mixed vanadium and tungsten oxide films with compositions ranging from 0 to 100% vanadium (metals basis) were prepared by reactive sputtering from metallic vanadium and tungsten targets in an atmosphere of argon and oxygen. The vanadium content varied smoothly with the fraction of total power applied to the vanadium target. Films containing vanadium were more color neutral than pure tungsten oxide films, tending to gray-brown at high V fraction. The electrochromic switching performance of these films was investigated by in situ monitoring of their visible transmittance during lithium insertion/extraction cycling in a non-aqueous electrolyte (1M LiClO{sub 4} in propylene carbonate). The solar transmittance and reflectance was measured ex situ. Films with vanadium content greater than about 15% exhibited a marked decrease in switching range. Coloration efficiencies followed a similar trend.
Date: October 1, 1998
Creator: Michalak, F.; Richardson, T.; Rubin, M.; Slack, J. & von Rottkay, K.
Partner: UNT Libraries Government Documents Department

Optical switching of coherent VO{sub 2} precipitates embedded in sapphire

Description: In this work, we report the formation of a new type of active or {open_quotes}smart{close_quote} surface that is produced by ion implantation and thermal processing. By co-implanting vanadium and oxygen into a single- crystal sapphire substrate and annealing the system under appropriate conditions, it was possible to form buried precipitates of vanadium dioxide that were crystallographically oriented with respect to the host Al{sub 2}O{sub 3} lattice. The implanted VO{sub 2} precipitate system undergoes a structural phase transition that is accompanied by large variations in the optical transmission which are comparable to those observed for thin films of VO{sub 2} deposited on sapphire. Co-implantation with oxygen was found to be necessary to ensure good optical switching behavior.
Date: December 31, 1995
Creator: Gea, L.A.; Boatner, L.A.; Budai, J.D. & Zuhr, R.A.
Partner: UNT Libraries Government Documents Department

Structure and electrochemical potential simulation for the cathode material Li(1+x)V(3)O(8).

Description: The structure and electrochemical potential of monoclinic Li{sub 1+x}V{sub 3}O{sub 8} were calculated within the local-density-functional-theory framework by use of plane-wave-pseudopotential methods. Special attention was given to the compositions 1+x=1.2 and 1+x=4, for which x-ray diffraction structure refinements are available. The calculated low-energy configuration for 1+x=4 is consistent with the three Li sites identified in x-ray diffraction measurements and predicts the position of the unobserved Li. The location of the tetrahedrally coordinated Li in the calculated low-energy configuration for 1+x=1.5 is consistent with the structure measured by x-ray diffraction for Li{sub 1.2}V{sub 3}O{sub 8}. Calculations were also performed for the two monoclinic phases at intermediate Li compositions, for which no structural information is available. Calculations at these compositions are based on hypothetical Li configurations suggested by the ordering of vacancy energies for Li{sub 4}V{sub 3}O{sub 8} and tetrahedral site energies in Li{sub 1.5}V{sub 3}O{sub 8}. The internal energy curves for the two phases cross near 1+x=3. Predicted electrochemical potential curves agree well with experiment.
Date: December 5, 1997
Creator: Benedek, R.; Thackeray, M. M. & Yang, L. H.
Partner: UNT Libraries Government Documents Department

NMR Studies of the Vanadium Spin Dynamics and Spin Structure in LiV2O4, CaV2O4, and (LixV1-x)3BO5 (x is almost equal to 0.33, 0.40)

Description: Strong electron correlation is believed to be an essential and unifying factor in diverse properties of condensed matter systems. Ground states that can arise due to electron correlation effects include Mott insulators, heavy fermion, ferromagnetism and antiferromagnetism, spin glasses, and high-temperature superconductivity. The electronic systems in transition metal oxide compounds are often highly correlated. In this thesis, the author presents experimental studies on three strongly correlated vanadium oxide compounds: LiV{sub 2}O{sub 4}, (Li{sub x}V{sub 1-x}){sub 3}BO{sub 5}, and CaV{sub 2}O{sub 4}, which have completely different ground states.
Date: December 1, 2007
Creator: Zong, Xiaopeng
Partner: UNT Libraries Government Documents Department

Coherent V{sub 2}O{sub 3} precipitates in {alpha}-Al{sub 2}O{sub 3} co-implanted with vanadium and oxygen

Description: The oxides of vanadium VO{sub 2} and V{sub 2}O{sub 3} are of fundamental and practical interest since they undergo structural phase transitions during which large variations in their optical and electronic properties are observed. In the present work, the authors report the formation of buried precipitates of V{sub 2}O{sub 3} in sapphire by ion implantation and thermal annealing. It was found that the co-implantation of oxygen and vanadium was required in order to form nanophase V{sub 2}O{sub 3} precipitates. Additionally, these precipitates, which formed only following an anneal of the co-implanted sample under reducing conditions, are coherent with the sapphire lattice. Two epitaxial relationships were observed: (0001)V{sub 2}O{sub 3}//(0001) {alpha}-Al{sub 2}O{sub 3} and (11--20)V{sub 2}O{sub 3}//(0001) {alpha}-Al{sub 2}O{sub 3}. This finding is in agreement with results obtained elsewhere for thin films of V{sub 2}O{sub 3} deposited on c-axis-oriented sapphire.
Date: December 31, 1994
Creator: Gea, L.A.; Boatner, L.A.; Budai, J.D. & Rankin, J.
Partner: UNT Libraries Government Documents Department

Optically active surfaces formed by ion implantation and thermal treatment

Description: Embedded VO{sub 2} precipitates have been formed in single-crystal sapphire by the ion co-implantation of vanadium and oxygen and subsequent thermal annealing. The embedded VO{sub 2} particles have been shown to exhibit an optical switching behavior that is comparable to that of continuous thin films. In this work, the mechanisms of formation of these optically active particles are investigated. It is shown that precipitation of the vanadium dioxide phase is favored when the thermal treatment is performed on an ion-damaged but still crystalline (rather than amorphized) Al{sub 2}O{sub 3} substrate. The best optical switching behavior is observed in this case, and this behavior is apparently correlated with a more-favorable dispersion of VO{sub 2} small particles inside the matrix.
Date: August 1, 1996
Creator: Gea, L.A.; Boatner, L.A.; Evans, H.M. & Zuhr, R.
Partner: UNT Libraries Government Documents Department

Selective methane oxidation over promoted oxide catalysts. Quarterly report, June--August 1995

Description: The partial oxidation of methane to oxygenates involves a complex network of heterogeneous and homogeneous free radical reactions, particularly under higher temperatures and pressures. It is generally difficult to separately study the relative importance of each of these types of reactions during the reaction process by experimental means. However, in order to maximize the oxygenate yields, better reactor/reaction designs, such as the double-bed reactor configuration explored previously in this project, are needed. For these tasks, it is often necessary to understand the extent to which each of these two different types of reactions contributes to the reaction process. Therefore, during this quarter, efforts were made in this aspect through a kinetic modeling approach. With such an approach, one can readily examine the contribution of each and every reaction step in the reaction process, simply by including or excluding it from the model.
Date: October 1, 1995
Partner: UNT Libraries Government Documents Department

LiV{sub 2}O{sub 4}: A heavy fermion transition metal oxide

Description: The format of this dissertation is as follows. In the remainder of Chapter 1, brief introductions and reviews are given to the topics of frustration, heavy fermions and spinels including the precedent work of LiV{sub 2}O{sub 4}. In Chapter 2, as a general overview of this work the important publication in Physical Review Letters by the author of this dissertation and collaborators regarding the discovery of the heavy fermion behavior in LiV{sub 2}O{sub 4} is introduced [removed for separate processing]. The preparation methods employed by the author for nine LiV{sub 2}O{sub 4} and two Li{sub 1+x}Ti{sub 2{minus}x}O{sub 4} (x = 0 and 1/3) polycrystalline samples are introduced in Chapter 3. The subsequent structural characterization of the LiV{sub 2}O{sub 4} and Li{sub 1+x}Ti{sub 2{minus}x}O{sub 4} samples was done by the author using thermogravimetric analysis (TGA), x-ray diffraction measurements and their structural refinements by the Rietveld analysis. The results of the characterization are detailed in Chapter 3. In Chapter 4 magnetization measurements carried out by the author are detailed. In Chapter 5, after briefly discussing the resistivity measurement results including the single-crystal work by Rogers et al., for the purpose of clear characterization of LiV{sub 2}O{sub 4} it is of great importance to introduce in the following chapters the experiments and subsequent data analyses done by his collaborators. Heat capacity measurements (Chapter 6) were carried out and analyzed by Dr. C.A. Swenson, and modeled theoretically by Dr. D.C. Johnston. In Chapter 7 a thermal expansion study using neutron diffraction by Dr. O. Chmaissem et al. and capacitance dilatometry measurements by Dr. C.A. Swenson are introduced. The data analyses for the thermal expansion study were mainly done by Dr. O. Chmaissem (for neutron diffraction) and Dr. C.A. Swendon (for dilatometry), with assistances by Dr. J.D. Jorgensen, Dr. D.C. Johnston, and S. Kondo the ...
Date: February 12, 1999
Creator: Kondo, Shinichiro
Partner: UNT Libraries Government Documents Department

VO{sub 2} precipitates for self-protected optical surfaces

Description: A method for forming crystallographically coherent precipitates of vanadium dioxide in the near-surface region of sapphire and the resulting product is disclosed. Ions of vanadium and oxygen are stoichiometrically implanted into a sapphire substrate (Al{sub 2}O{sub 3}), and subsequently annealed to form vanadium dioxide precipitates in the substrate. The embedded VO{sub 2} precipitates, which are three-dimensionally oriented with respect to the crystal axes of the Al{sub 2}O{sub 3} host lattice, undergo a first-order monoclinic-to-tetragonal (and also semiconducting-to- metallic) phase transition at {minus}77 C. This transformation is accompanied by a significant variation in the optical transmission of the implanted region and results in the formation of an optically active, thermally switchable surface region on Al{sub 2}O{sub 3}.
Date: December 1, 1997
Creator: Gea, Laurence A. & Boatner, Lynn A.
Partner: UNT Libraries Government Documents Department

Final report. Superconducting materials

Description: Our group has discovered a many body effect that explains the surprising divergence of the spin susceptibility which has been measured by neutron scattering experiments on high temperature superconductors and vanadium oxide metals. Electron interactions on nested - i.e., nearly parallel paths - have been analyzed extensively by our group, and such processes provide a physical explanation for many anomalous features that distinguish cuprate superconductors from ordinary metals.
Date: September 11, 1999
Creator: Ruvalds, John
Partner: UNT Libraries Government Documents Department

Magnetic correlations in a classic Mott system

Description: The metal-insulator transition in V{sub 2}O{sub 3} causes a fundamental change in its magnetism. While the antiferromagnetic insulator (AFI) is a Heisenberg localized spin system, the antiferromagnetism in the strongly correlated metal is determined by a Fermi surface instability. Paramagnetic fluctuations in the metal and insulator represent similar spatial spin correlations, but are unrelated to the long range order in the AFI. The phase transition to the AFI induces an abrupt switching of magnetic correlations to a different magnetic wave vector. The AFI transition, therefore, is not a conventional spin order-disorder transition. Instead it is accounted for by an ordering in the occupation of the two degenerate d-orbitals at the Fermi level.
Date: July 1997
Creator: Bao, W.; Broholm, C.; Aeppli, G.; Carter, S. A.; Dai, D.; Frost, C. D. et al.
Partner: UNT Libraries Government Documents Department

Chemical and sonochemical approaches to the formation of VO{sub 2} films and VO{sub 2}-impregnated materials

Description: A new chemical and chemical/ultrasonic approach to the preparation of VO{sub 2} films and VO{sub 2}-impregnated bulk materials has been developed. In this approach, a V{sub 2}O{sub 5} sol prepared by quenching is used to coat SiO{sub 2} substrates. The resulting gel-film is heat treated in a reducing atmosphere to form a film identified as VO{sub 2} from the results of X-ray diffraction and both optical and resistivity measurements, which reveal the phase transition characteristic of vanadium dioxide. The advantage of this approach to the formation of VO{sub 2} is that the V{sub 2}O{sub 5} sol can be used to impregnate porous materials, which are then heated treated to form an optically active composite material. The switching properties of the VO{sub 2} films are investigated using optical and resistivity measurements, and the results are compared to those obtained for VO{sub 2}-films prepared by more-conventional methods.
Date: November 1, 1997
Creator: Keppens, V.; Mandrus, D. & Boatner, L.A.
Partner: UNT Libraries Government Documents Department

Hadamard transform spectrometry: A new analytical technique; Progress report, Second year, March 15, 1992--November 15, 1992

Description: The document is divided into 4 parts: Hadamard transform photoacoustic spectrometry and depth profiling; Hadamard transform imaging with a 2D Hadamard encoding mask (Raman image using pararosaniline hydrochloride); Hadamard transform Raman spectrometry; and work on the growth of VO{sub 2}(s) crystals for Hadamard masking material. 13 figs, refs.
Date: November 15, 1992
Creator: Fateley, W.G. & Hammaker, R.M.
Partner: UNT Libraries Government Documents Department

Preparation of Uranium-Niobium Alloys by Bomb Reduction; PREPARATION D'ALLIAGES URANIUM-NIOBIUM PAR REDUCTION EN BOMBE

Description: Uranium--niobium alloys were prepared by direct reduction with calcium of urardum tetrafluoride and niobium pentoxide. The reduction of potassium oxyfluoniobate did not give satisfactory results. Alloys containing up to 20% niobium have been successfully achieved. Ternary alloys containing chromium and vanadium have been prepared by simultaneous reduction of chromium anhydride and vanadium pentoxide. The preparation of the alloys is described and the results are discussed. (auth)
Date: March 13, 1959
Creator: Huet, J.J. & Massaux, H.
Partner: UNT Libraries Government Documents Department