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Imprinted spiral structures as neutron polarizers.

Description: Neutron diffraction from magnetic spiral structures is governed by strong selection rules for the polarization of the outgoing beam. When the sample is entirely of one chirality--for instance a right handed spiral--the neutrons diffracted by some Bragg reflections are fully polarized. While the scattering theory has been formulated long ago, attempts to controllably modify the population of left handed and right handed spiral domains in natural magnetic structures (which for instance occur in some rare earth metals) have been largely unsuccessful. In contrast, we have been able to imprint helical magnetic structures in La/Fe multilayers (each layer approximately 30 {angstrom} thick) simply by rotating the growing sample in a weak external field (30e). A first estimate is given of the efficiency of these multilayers as polarizers of neutron beams.
Date: October 7, 1998
Creator: Lohstroh, W.
Partner: UNT Libraries Government Documents Department

Advanced materials for solid oxide fuel cells

Description: The purpose of this research is to improve the properties of the current state-of-the-art materials used for solid oxide fuel cells (SOFCs). The objectives are to: (1) develop materials based on modifications of the state-of-the-art materials; (2) minimize or eliminate stability problems in the cathode, anode, and interconnect; (3) Electrochemically evaluate (in reproducible and controlled laboratory tests) the current state-of-the-art air electrode materials and cathode/electrolyte interfacial properties; (4) Develop accelerated electrochemical test methods to evaluate the performance of SOFCs under controlled and reproducible conditions; and (5) Develop and test materials for use in low-temperature SOFCs.
Date: December 31, 1995
Creator: Armstrong, T. & Stevenson, J.
Partner: UNT Libraries Government Documents Department

Structure and properties of metal hydrides prepared by mechanical alloying

Description: Our research examines the structure and reversible hydrogen storage capacity of alloys based on the LaNi{sub 5} intermetallic. The alloys are prepared by mechanical alloying (MA), a technique particularly useful when alloying LaNi{sub 5} with low melting point elements such as tin and calcium. In LaNi{sub 5-y}Sn{sub y}, x-ray diffraction and Rietveld analysis show that tin preferentially occupies the Ni(3g) sites in the LaNi{sub 5} structure, and the unit cell volume increases linearly with tin content to a maximum tin solubility of 7.33 atomic percent (LaNi{sub 4.56}Sn{sub 0.44}). The addition of tin to LaNi{sub 5} causes (a) a logarithmic decrease in the plateau pressures for hydrogen absorption and desorption, which is consistent with the corresponding increase in the volume of the LaNi{sub 5} unit cell; (b) a decrease in the hysteresis between the pressures for hydride formation and decomposition, which is in agreement with a recent theoretical model for the effect; and (c) a linear decrease in the hydrogen storage capacity. Effect (c) is explained by a rigid-band model whereby electrons donated by the tin atoms occupy holes in the 3d band of LaNi{sub 5}, which could otherwise be occupied by electrons donated by the hydrogen atoms. Thermodynamic van`t Hoff analysis for these alloys show an increase in hydride formation enthalpy and no change in entropy with increasing tin concentration. LaNi{sub 5} with calcium additions shows enhanced kinetics of hydrogen absorption/desorption. The powder particles prepared by MA have a larger surface area than particles of the same overall size prepared by arc casting. All LaNi{sub 5}-based alloys prepared by MA in an inert environment require no activation for hydrogen absorption and suffer less comminution upon hydriding/dehydriding.
Date: September 1, 1995
Creator: Wasz, M.L. & Schwarz, R.B.
Partner: UNT Libraries Government Documents Department

Electrochemical properties of mixed conducting La{sub 1{minus}x}M{sub x}Co{sub 1{minus}y}Fe{sub y}O{sub 3{minus}{delta}} (M = Sr,Ca,Ba) perovskites

Description: At elevated temperatures, some ABO{sub 3} perovskite type oxides having Co and Fe as B site cations exhibit substantial mixed (anionic and electronic) conductivity. Because of this behavior, they are candidate materials for applications such as solid oxide fuel cell cathodes and oxygen separation membranes. The purpose of the present study is to increase the understanding of the effects of composition, temperature, and environment on the electrochemical properties of selected materials within the La{sub 1{minus}x}M{sub x}Co{sub 1{minus}y}Fe{sub y}O{sub 3{minus}{delta}} (M=Sr,Ca,Ba) system in order to evaluate their applicability for the above-mentioned applications. Characterization techniques include XRD, SEM, TGA, dc conductivity, dilatometry, oxygen permeation measurements, and iodometric titration.
Date: October 1, 1995
Creator: Stevenson, J.W.; Armstrong, T.R.; Pederson, L.R. & Weber, W.J.
Partner: UNT Libraries Government Documents Department

Role of intergrowths in the properties of the naturally layered manganites.

Description: The structural and magnetic properties of the two-layered Ruddlesden-Popper phase SrO(La{sub 1{minus}x}Sr{sub x}MnO{sub 3}){sub 2} with x = 0.3 and x = 0.4 are investigated. These naturally layered manganites exhibit a colossal magnetoresistance, a magnetic anisotropy which is strongly composition-dependent, almost no remanence, and a non-vanishing magnetization in an extended temperature range above the Curie temperature (T{sub c}). The magnetization in this temperature range is not intrinsic to the crystal, but is attributed to intergrowths. These two-dimensional lattice imperfections consist of additional or missing SrO layers and have been observed in transmission electron micrographs. The magnetic properties of the intergrowths differ from the bulk crystal which results in unusual magnetic behavior.
Date: October 27, 1998
Creator: Bader, S.D.; Berger, A.; Jiang, J.S.; Miller, D.J.; Mitchell, J.F. & Osgood, R.M. III
Partner: UNT Libraries Government Documents Department

Transport and magnetism correlations in thin-film ferromagnetic oxides

Description: In order to determine the {Tc}-dependence of the colossal magnetoresistance (MR) exhibited by the ferromagnetic La{sub 0.7}M{sub 0.3}MnO{sub 3+{sigma}} (M = Ba, Ca, Sr) system, the authors examine the magnetic-field and temperature dependent resistivity and magnetization of a series of thin films that were grown via pulsed-laser deposition. The films had magnetic ordering temperatures (T{sub C}) ranging from 150 to 350 K; all samples displayed a large negative MR that is largest near {Tc}. The magnitude of a given sample`s MR at {Tc} inversely correlates with {Tc}; samples with a low {Tc} display significantly larger MR values than do samples with large {Tc}`s. The quantity {rho}({Tc})/{rho}(4 K), the amount by which the resistivity is reduced by full ferromagnetic order, is an activated function of {Tc} with an activation energy E{sub a} = 0.1 eV. These results indicate that the magnitude of the CMR effect in a given specimen is controlled not by {rho}({Tc}), but by {Tc} via the ratio {rho}({Tc})/{rho}(4 K). Phenomenological scaling relationships are also reported that link {rho}(H,T) to both H and M(H, T).
Date: September 1, 1995
Creator: Hundley, M.F.; Neumeier, J.J.; Heffner, R.H.; Jia, Q.X.; Wu, X.D. & Thompson, J.D.
Partner: UNT Libraries Government Documents Department

LaNiO(3) Buffer Layers for High Critical Current Density YBa(2)Cu(3)O(7-delta) and Tl(2)Ba(2)CaCu(2)O(8-delta) Films

Description: We demonstrate high critical current density superconducting films of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} (YBCO) and Tl{sub 2}Ba{sub 2}CaCu{sub 2}O{sub 8{minus}{delta}} (Tl-2212) using LaNiO{sub 3} (LNO) buffer layers. YBCO films grown on an LNO buffer layer have only a slightly lower J{sub c} (5K, H=0) than films grown directly on a bare LaAlO{sub 3} substrate. It is noteworthy that YBCO films grown on LNO buffer layers exhibit minor microstructural disorder and enhanced flux pinning. LNO-buffered Tl-2212 samples show large reductions in J{sub c} at all temperatures and fields compared to those grown on bare LaAlO{sub 3}, correlating to both a-axis grain and nonsuperconducting phase formation. With additional optimization, LNO could be a promising buffer layer for both YBCO and Tl-based superconducting films, perhaps ideally suited for coated conductor applications.
Date: August 24, 1999
Creator: Carlson, C.M.; Parilla, P.A.; Siegal, M.P.; Ginley, D.S.; Wang, Y.-T.; Blaugher, R.D. et al.
Partner: UNT Libraries Government Documents Department

Magnetization studies of oxides related to the high temperature cuprate superconductors

Description: The magnetic properties related to the following high temperature superconductors were measured utilizing a Faraday magnetometer: BaCuO{sub 2+x}, La{sub 2} CuO{sub 4}, Sr{sub 2} RhO{sub 4}, Sr{sub 2} VO{sub 4}, and Sr{sub 2} CuO{sub 3}. Neutron diffraction, magnetic susceptibility, and heat capacity measurements are discussed.
Date: June 19, 1995
Creator: Wang, Z.
Partner: UNT Libraries Government Documents Department

Development of metal hydride composites

Description: Most of current hydride technology at Savannah River Site is based on beds of metal hydride powders; the expansion upon hydridation and the cycling results in continued breakdown into finer particles. Goal is to develop a composite which will contain the fines in a dimensionally stable matrix, for use in processes which require a stable gas flow through a hydride bed. Metal hydride composites would benefit the advanced Thermal Cycling Absorption process (hydrogen isotope separation), and the Replacement Tritium Facility (storage, pumping, compression, purification of hydrogen isotopes). These composites were fabricated by cold compaction of a mixture of metal hydride granules and coarse copper powder; the porosity in the granules was introduced by means of ammonium carbonate. The composite pellets were cycled 138 times in hydrogen with the loss of LANA0.75 (LaNi{sub 4.25}Al{sub 0.75}) limited to the surface. Vacuum sintering can provide additional strength at the edges. Without a coating, the metal hydride particles exposed at the pellet surface can be removed by cycling several times in hydrogen.
Date: December 1, 1992
Creator: Congdon, J.W.
Partner: UNT Libraries Government Documents Department

Magnetic correlations in doped transition metal oxides

Description: The authors review recent reactor- and spallation-source-based neutron scattering experiments on the magnetic fluctuations and order in a variety of doped transition metal oxides. In particular, data are shown for the NiO chain compound, Y{sub 2{minus}x}Ca{sub x}BaNiO{sub 5}, the two-dimensional cuprate superconductors La{sub 2{minus}x}Sr{sub x}CuO{sub 4} and YBa{sub 2}Cu{sub 3} O{sub 6+x}, and the classical three-dimensional ``Mott-Hubbard`` system V{sub 2{minus}y}O{sub 3}.
Date: February 15, 1995
Creator: Aeppli, G.; Bao, W. & Broholm, C.
Partner: UNT Libraries Government Documents Department

Measurement of lanthanum and technetium in uranium fuels by inductively coupled plasma atomic emission spectroscopy.

Description: An important parameter in characterizing an irradiated nuclear fuel is determining the amount of uranium fissioned. By determining the amount of uranium fissioned in the fuel a burnup performance parameter can be calculated, and the amount of fission products left in the fuel can be predicted. The quantity of uranium fissioned can be calculated from the amount of lanthanum and technetium present in the fuel. Lanthanum and technetium were measured in irradiated fuel samples using an Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) instrument and separation equipment located in a shielded glove-box. A discussion of the method, interferences, detection limits, quality control and a comparison to other work will be presented.
Date: June 10, 1999
Creator: Carney, K.; Crane, P.; Cummings, D.; Krsul, J. & McKnight, R.
Partner: UNT Libraries Government Documents Department

The effect of temperature, growth kinetics, and substrate on the microstructure of RF off-axis sputter deposited YBa{sub 2}Cu{sub 3}O{sub 7} thin films

Description: YBa{sub 2}Cu{sub 3}O{sub 7} (YBCO) thin films were grown on single crystal substrates by RF off-axis sputter deposition under different growth condition, systematically varied to change the kinetic and thermodynamic processes that determine both the film microstructure and its crystallographic orientation. The effect of substrate temperature, position, and material, total chamber pressure, and RF power on the final film structure was examined. The growth matrix was bounded by temperatures from 640 to 780C, pressures from 50 to 200 mtorr, and power from 50 to 100 watts. Higher growth rates were achieve by increasing the power setting, lowering the total pressure, and moving the substrate farther into the plasma. Care was taken to stay within these parametric stability limits defining the 123-YBCO growth regime. The resulting films were characterized, primarily, by scanning tunneling and atomic force microscopy, x-ray diffraction, and eddy-current measurements. At the extremes of the growth conditions used in this study, ex-situ examination of the films revealed that they spanned the extremes from all c-axis normal to all a-axis normal material, with mixed a-/c-axis normal material in between extremes in deposition parameter limits, with the mixed films containing a-axis material overlaying a c-axis layer. Smooth single orientation films, suitable for multilayer device structure, could be routinely obtained by controlled growth. Highest surface roughness measurements were usually due to incomplete or non uniform a-axis coverage on c-axis films. Combined data obtained from different studies showed that lower temperatures and higher rates favored a-axis growth under the conditions used in these studies. The YBCO films were more unstable to a cross-over from c-axis to a-a-axis growth when deposited on NdGaO{sub 3} and LaAIO{sub 3} than on SrTiO{sub 3}. Under conditions favoring c-axis perpendicular growth, higher temperatures and rates resulted in rounder islands.
Date: July 1, 1995
Creator: Hawley, M.; Houlton, R.J.; Garzon, F.H. & Raistrick, I.D.
Partner: UNT Libraries Government Documents Department

Overview of recent magnetic studies of high T{sub c} cuprate parent compounds and related materials

Description: Recent studies of the magnetic properties of several high superconducting transition temperature (T{sub c}) cuprate parent compounds and related materials will be reviewed. The observations of a Heisenberg to XY-like crossover upon cooling below {approximately}300K towards the Neel temperature T{sub N} = 257 K and a subsequent magnetic field-induced XY-like to Ising-like crossover near TN in single crystals of the K{sub 2}NiF{sub 4} type spin 1/2 model compound Sr{sub 2}CuO{sub 2}Cl{sub 2} will be described.
Date: December 1995
Creator: Johnston, D. C.; Ami, T. & Borsa, F.
Partner: UNT Libraries Government Documents Department

Effect of Ce composition on the structural and electronic characteristics of some metal hydride electrodes: A XANES and EXAFS investigation

Description: Substitution of the B component in the prototype AB{sub 5} type (LaNi{sub 5}) metal hydride alloys have resulted in their increased acceptance as anodes for rechargeable alkaline batteries. Recently substitution of the A component (La) for imparting properties such as increased corrosion resistance has received attention. This investigation deals with the role of Ce as a substituent for the La and its effect in terms of corrosion resistance. The alloys chosen have the general composition of La{sub x}Ce{sub 1-x}B{sub 5} (x = 1, 0.8, 0.5 and 0.25) where B is Ni{sub 3.55}CO{sub 0.75}Mn{sub 0.4}Al{sub 0.3} together with alloys containing the mischmetal (Mm) as the A component (both synthetic and commercial). Electrochemical cycling results show that Ce lowers the capacity loss in the alloys and that this effect is not a simple function of the extent of lattice expansion during hydriding as was previously suggested. Correlation of the electrochemical and XAS results show that capacity loss is directly related to the extent of Ni corrosion. Effect of Ce substitution seems to result in a stable Ce oxide hydroxide coating which imparts the corrosion resistance.
Date: December 31, 1994
Creator: Mukerjee, S.; McBreen, J.; Reilly, J.J.; Johnson, J.R.; Adzic, G.; Kumar, M.P.S. et al.
Partner: UNT Libraries Government Documents Department

Direct imaging of the first order spin flop transition in the layered manganite La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7}.

Description: The spin-flop transition in the antiferromagnetic layered manganite La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7} was studied using magnetization measurements and a high-resolution magneto-optical imaging technique. We report the direct observation of the formation of ferromagnetic domains appearing at the first order spin-flop transition. The magnetization process proceeds through nucleation of polarized domains at crystal defect sites and not through the expansion of polarized domains due to domain wall motion. A small magnetic hysteresis is caused by the difference between the mechanisms of nucleation and annihilation of domains in the mixed state. These results establish a direct link between the magnetic structure on the atomic scale as seen in neutron scattering and the macroscopic properties of the sample as seen in magnetization and conductivity measurements.
Date: August 31, 1999
Creator: Berger, A.; Gray, K. E.; Miller, D. J.; Mitchell, J. F.; Vlasko-Vlasov, V. K. & Welp, U.
Partner: UNT Libraries Government Documents Department

The phase diagrams and doped-hole segregation in La{sub 2}CuO{sub 4+{delta}} and La{sub 2-x}Sr{sub x}CuO{sub 4+{delta}} (x {le} 0.15, {delta} {le} 0.12)

Description: The magnetic and structural phase diagrams of the La{sub 2}CuO{sub 4+{delta}} system and the La{sub 2-x}Sr{sub x}CuO{sub 4+{delta}} are reviewed, with emphasis on recent results obtained from magnetic and structural neutron diffraction, thermogravimetric analysis, iodometric titration, magnetic susceptibility {chi}(T), and {sup 129}La nuclear quadrupole resonance (NQR) measurements.
Date: October 31, 1993
Creator: Johnston, D.C.; Borsa, F. & Canfield, P.C.
Partner: UNT Libraries Government Documents Department

Synthesis and characterization of metal hydride electrodes. Interim report

Description: The objective of this project is to elucidate the compositional and structural parameters that affect the thermodynamics, kinetics and stability of alloy hydride electrodes and to use this information in the development of new high capacity long life hydride electrodes for rechargeable batteries. The work focuses on the development of AB{sub 5} alloys and the application of in situ methods, at NSLS, such as x-ray absorption (XAS), to elucidate the role of the alloying elements in hydrogen storage and corrosion inhibition. The most significant results to date are: The decay of electrode capacity on cycling was directly related to alloy corrosion. The rate of corrosion depended in part on both the alloy composition and the partial molar volume of hydrogen, V{sub H}. The corrosion rate depended on the composition of the A component in AB{sub 5} (LaNi{sub 5} type) alloys. Partial substitution of La with Ce in AB{sub 5} alloys substantially inhibits electrode corrosion on cycling. Recent results indicate that Co also greatly inhibits electrode corrosion, possibly by minimizing V{sub H}. The AB{sub 5} alloys investigated included LaNi{sub 5}, ternary alloys (e.g. LaN{sub 4.8}Sn{sub 0.2} and La{sub 0.8}Ce{sub 0.2}Ni{sub 5}), alloys with various substitutions for both La and Ni (e.g. La{sub 0.8}Ce{sub 0.2}Ni{sub 4.8}Sn{sub 0.2}) and mischmetal (Mm) alloys of the type normally used in batteries, such as MmB{sub 5} (B = Ni{sub 3.55}Mn{sub 0.4}A1{sub 0.3}Co{sub 0.75}). A major effort was devoted to the effects of La substitution in the A component. Both in situ and ex situ XAS measurements are used to study the electronic effects that occur on the addition of various metal substitutions and on the ingress of hydrogen.
Date: October 1, 1995
Creator: McBreen, J. & Reilly, J.J.
Partner: UNT Libraries Government Documents Department

A summary of the fission product metabolism studies up to March 1944

Description: The metabolic properties of the carrier free long lived fission products in the adult rat have been investigated at different intervals ranging from one to sixty-eight days following administration of the radioactive material. The radio elements studied include the most important long lived fission products, namely, Sr, Y, Zr, Cb, Ra, I, Xe, Cs, Ba, La, Ce, Pr, Np, and the unseparated uranium free fission mixture. The routes of administration included intraperitoneal, intramuscular, oral and intrapulmonary. The tissues assayed for the distribution of the administered radio elements included heart, liver, kidney, testes, spleen, muscle, skin, stomach, small intestine, large intestine, bone, blood, lungs, brain, fat, adrenals, lymph nodes. Urine and feces were collected separately at daily intervals for assay. Data acquired is provided in tabular form. For the fission products which were poorly absorbed from the site of both intramuscular arid intraperitoneal injection (namely, Y, Zr, Cb, Ru, La, Ce, Pr, Np, and the uranium free fission mixture) only the intramuscular data are given here and it is expressed in terms of the distribution of the fraction absorbed from the site of injection. For the radio elements which are completely absorbed following intraperitoneal and intramuscular injection or are absorbed in the digestive tract the distribution is expressed in terms of the administered dose. Due to the difficulty of introducing an exactly known dose into the lungs the distribution in this series of experiments is expressed as distribution of retained activity within the animal. A total of 510 animals were used and 10,500 samples assayed with at least two measurements made on each sample. The average measured recovery of the administered dose in the tissue and excreta was 103.6%.
Date: March 30, 1944
Partner: UNT Libraries Government Documents Department

Synthesis, characterization, phase diagrams and superconducting and normal state magnetic properties of La{sub 2{minus}x}Sr{sub x}CuO{sub 4} (0 {le} x {le} 0.08) and electrochemically oxidized La{sub 2{minus}x}Sr{sub x}CuO{sub 4+{delta}} (0 {le} x {le} 0.33, 0 {le} {delta} {le} 0.12)

Description: La{sub 2{minus}x}Sr{sub x}CuO{sub 4} (0 {le} x {le} 0.15) can all be intercalated with oxygen by a novel electrochemical oxidation method. Bulk superconductivity is found with an onset {Tc} {approx} 40 K for the whole range 0.01 {le} x {le} 0.15; for x = 0.25 and 0.33, the electrochemical oxidation did not improve the superconducting properties. The magnetic susceptibility {chi}(T = 50--320 K) data for La{sub 2}CuO{sub 4.11} and La{sub 1.92}Sr{sub 0.08}CuO{sub 4.07} are nearly identical with those of conventionally prepared La{sub 1.85}Sr{sub 0.15}CuO{sub 4}, indicating that the hole doping level (p) in the CuO{sub 2} planes of the three compounds is nearly the same. Combined thermogravimetric analysis and iodometric titration experiments indicate that part of the intercalated oxygen has a formal valence close to {minus}1. The maximum doped-hole concentration in the CuO{sub 2} planes that can be achieved from combined Sr-doping and electrochemical oxygen doping for 0 {le} x {le} 0.15 is p {approx} 0.16 holes/formula unit. Oxygen can also intercalate into single crystal La{sub 2}CuO{sub 4} through a slow electrochemical oxidation process. The required low current and long time for the charging process reflects that the oxygen intercalation for a single crystal is limited by its small specific surface area and long diffusion distance. The anisotropic superconducting, magnetic and transport properties are summarized and compared with those of polycrystalline La{sub 2}CuO{sub 4+{delta}} as well as of YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} and La{sub 2{minus}x}Sr{sub x}CuO{sub 4} single crystals. The single crystal La{sub 2}CuO{sub 4+{delta}} has a maximum {Tc} {approx} 40 K, which is lower than that ({Tc} {approx} 42--45) of the corresponding polycrystalline samples. The magnetic phase diagram of La{sub 2{minus}x}Sr{sub x}CuO{sub 4} in the antiferromagnetic (AF) regime (0 {le} x {le} 0.02) has been derived from {sup 139}La NQR studies from 4 to 250 K.
Date: August 1, 1993
Creator: Chou, F.
Partner: UNT Libraries Government Documents Department

Imprinting artificial magnetic structures.

Description: Recently we created La/Fe multilayers with a helical magnetic structure imprinted from the conditions of growth rather than by the magnetic interactions between layers. Each sublayer was 30{angstrom} thick, and during deposition the sample was rotated in an external field of 3 Oe. a field strong enough to magnetize the Fe layer being deposited but not sufficient to perturb the magnetization of the Fe layers already grown. As a result adjacent Fe layers formed a helical structure with a chirality and periodicity determined by the rotational direction and speed of the substrate and the rate of deposition. Following this discovery, an extensive set of experiments (mainly using Kerr effect magnetometry and polarized neutron reflectivity) was undertaken to ascertain the stability of imprinted magnetic structures, and to understand the onset of magnetization during growth. La/Fe imprinted helical magnetic structures (of different La and Fe thicknesses) were found to be stable in time and to be permanently erased only by magnetic fields larger than 90 Oe.
Date: September 25, 1998
Creator: Lohstroh, W.
Partner: UNT Libraries Government Documents Department

The role of rare-earth dopants in nanophase zirconia catalysts for automotive emission control.

Description: Rare earth (RE) modification of automotive catalysts (e.g., ZrO{sub 2}) for exhaust gas treatment results in outstanding improvement of the structural stability, catalytic functions and resistance to sintering at high temperatures. Owing to the low redox potential of nonstoichiometric CeO{sub 2}, oxygen release and intake associated with the conversion between the 3+ and 4+ oxidation states of the Ce ions in Ce-doped ZrO{sub 2} provide the oxygen storage capacity that is essentially to effective catalytic functions under dynamic air-to-fuel ratio cycling. Doping tripositive RE ions such as La and Nd in ZrO{sub 2}, on the other hand, introduces oxygen vacancies that affect the electronic and ionic conductivity. These effects, in conjunction with the nanostructure and surface reactivity of the fine powders, present a challenging problem in the development of better ZrO{sub 2}-containing three-way catalysts. We have carried out in-situ small-to-wide angle neutron diffraction at high temperatures and under controlled atmospheres to study the structural phase transitions, sintering behavior, and Ce{sup 3+} {leftrightarrow} Ce{sup 4+} redox process. We found substantial effects due to RE doping on the nature of aggregation of nanoparticles, defect formation, crystal phase transformation, and metal-support interaction in ZrO{sub 2} catalysts for automotive emission control.
Date: July 16, 1999
Creator: Loong, C.-K. & Ozawa, M.
Partner: UNT Libraries Government Documents Department

Enhancement of methane conversion using electric fields. Quarterly report, December 1994--March 1995

Description: The goal of this project is the development of novel, economical, processes for the conversion of natural gas to more valuable projects such as methanol, ethylene and other organic oxygenates or higher hydrocarbons. The methodologies of the project are to investigate and develop low temperature electric discharges and electric field-enhanced catalysis for carrying out these conversions. In the case of low temperature discharges, the conversion is carried out at ambient temperature which in effect trades high temperature thermal energy for electric energy as the driving force for conversion. The low operating temperature relax and thermodynamic constraints on the product distribution found at high temperature and also removes the requirements of large thermal masses required for current technologies. With the electric field-enhanced conversion, the operating temperatures are expected to be below those currently required for such processes as oxidative coupling, thereby allowing for a higher degree of catalytic selectivity while maintaining high activity.
Date: April 1, 1995
Creator: Mallinson, R.G. & Lobban, L.L.
Partner: UNT Libraries Government Documents Department

Short range spin correlations in the CMR material La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7}.

Description: The (La{sub 1{minus}} x Sr{sub x}){sub 3}Mn{sub 2}O{sub 7} compounds are layered materials that exhibit higher magneto-resistance than the corresponding 3D manganite perovskites. Quasi-elastic neutron scattering on a polycrystalline sample of La{sub 1.4}Sr{sub 1.6}Mn{sub 2}O{sub 7} shows that the spin fluctuation spectrum of the these layered CMR materials is qualitatively similar to those found in the perovskite manganites (La,Ca)MnO{sub 3}; their concentration, lifetime, and coherence length increase as T decreases to T{sub c}. Unlike the perovskites we find a lower spin-diffusion constant above T{sub c} of {approximately}5 meV {angstrom}{sup 2}.
Date: March 23, 1998
Creator: Kelley, T. M.
Partner: UNT Libraries Government Documents Department