112 Matching Results

Search Results

Advanced search parameters have been applied.

Pressure as a Probe of the Physics of Relaxor Ferroelectrics

Description: Pressure studies have provided new insights into the physics of compositionally-disordered ABO{sub 3} oxide relaxors. Specifically results are presented and discussed on a pressure-induced ferroelectric-to-relaxor crossover phenomenon, the continuous evolution of the energetics and dynamics of the relaxation process, and the interplay between pressure and electric field in determining the dielectric response.
Date: August 9, 1999
Creator: Samara, George A.
Partner: UNT Libraries Government Documents Department

Electrical Conductivity of the Lower-Mantle Ferropericlase

Description: Electrical conductivity of the lower-mantle ferropericlase-(Mg{sub 0.75},Fe{sub 0.25})O has been studied using designer diamond anvils to pressures over one megabar and temperatures up to 500 K. The electrical conductivity of (Mg{sub 0.75},Fe{sub 0.25})O gradually rises by an order of magnitude up to 50 GPa but decreases by a factor of approximately three between 50 to 70 GPa. This decrease in the electrical conductivity is attributed to the electronic high-spin to low-spin transition of iron in ferropericlase. That is, the electronic spin transition of iron results in a decrease in the mobility and/or density of the charge transfer carriers in the low-spin ferropericlase. The activation energy of the low-spin ferropericlase is 0.27 eV at 101 GPa, similar to that of the high-spin ferropericlase at relatively low temperatures. Our results indicate that low-spin ferropericlase exhibits lower electrical conductivity than high-spin ferropericlase, which needs to be considered in future geomagnetic models for the lower mantle. The extrapolated electrical conductivity of the low-spin ferropericlase, together with that of silicate perovskite, at the lower mantle pressure-temperature conditions is consistent with the model electrical conductivity profile of the lower mantle.
Date: April 19, 2007
Creator: Lin, J F; Weir, S T; Jackson, D D; Evans, W J; Vohra, Y K; Qiu, W et al.
Partner: UNT Libraries Government Documents Department

Subband Structure of a Two-Dimensional Electron Gas Formed at the Polar Surface of the Strong Spin-Orbit Perovskite KTaO3

Description: We demonstrate the formation of a two-dimensional electron gas (2DEG) at the (100) surface of the 5d transition-metal oxide KTaO{sub 3}. From angle-resolved photoemission, we find that quantum confinement lifts the orbital degeneracy of the bulk band structure and leads to a 2DEG composed of ladders of subband states of both light and heavy carriers. Despite the strong spin-orbit coupling, we find no experimental signatures of a Rashba spin splitting, which has important implications for the interpretation of transport measurements in both KTaO{sub 3}- and SrTiO{sub 3}-based 2DEGs. The polar nature of the KTaO{sub 3}(100) surface appears to help mediate formation of the 2DEG as compared to non-polar SrTiO{sub 3}(100).
Date: March 1, 2012
Creator: King, P.D.C.
Partner: UNT Libraries Government Documents Department

Interface Structure and Transport of Complex Oxide Junctions

Description: The interface structure and magnetism of hybrid magnetic tunnel junction-spin filter devices have been investigated and correlated with the transport behavior exhibited. Magnetic tunnel junctions made of theoretically predicted half-metallic electrodes (perovskite La0.7Sr0.3MnO3 and spinel Fe3O4) sandwiching a spinel NiMn2O4 tunnel barrier exhibit very high crystalline quality as observed by transmission electron microscopy. Structurally abrupt interfaces allow for the distinct magnetic switching of the electrodes as well as large junction magnetoresistance. The change in the magnetic anisotropy observed at the spinel-spinel interface supports the presence of limited interdiffusion and the creation of a magnetically soft interfacial layer, whose strong exchange coupling to the Fe3O4 electrode likely accounts for the low background magnetoresistance observed in these junctions, and the successful spin filtering when the barrier layer is ferrimagnetic.
Date: February 1, 2008
Creator: Nelson-Cheeseman, B. B.; Wong, F.; Chopdekar, R. V.; Chi, M.; Arenholz, E.; Browning, N. D. et al.
Partner: UNT Libraries Government Documents Department

Changes of the spin dynamics in perovskite and bilayer manganite.

Description: The authors report a series of {micro}SR measurements performed on single crystals of La{sub 1-x}Sr{sub x}MnO{sub 3} (0 {le} x {le} 0.125) and on a polycrystalline bilayer manganite La{sub 2-2x}Sr{sub 1+2x}Mn{sub 2}O{sub 7} (x = 0.52). They find that the temperature dependence of the spin-lattice relaxation rate is strongly dependent on the hole doping and the associated changes in the magnetic structure. The systems have competing, anisotropic interactions, leading to a complex interplay of charge, spin and orbital order. This has a significant influence on the spin dynamics, both for perovskite and layered manganites.
Date: July 2, 2002
Creator: Coldea, A. I.; Blundell, S. J.; Steer, C. A.; Pratt, F. L.; Prabhakaran, D. & Mitchell, J. F.
Partner: UNT Libraries Government Documents Department

Low Loss Composition of BA{sub X}SR{sub Y}CA{sub 1-X-Y}TIO{sub 3}:BA{sub 0.12-0.25}SR{sub 0.35-0.47}CA{sub 0.32-0.53}TIO{sub 3}

Description: A dielectric thin-film material for microwave applications, including use as a capacitor, the thin-film comprising a composition of barium strontium calcium and titanium of perovskite type (Ba{sub x}Sr{sub y}Ca{sub 1-x-y})TiO{sub 3}. Also provided is a method for making a dielectric thin film of that formula over a wide compositional range through a single deposition process.
Date: October 19, 1999
Creator: Xiang, Xiao-Dong; Chang, Hauyee & Takeuchi, Ichiro
Partner: UNT Libraries Government Documents Department


Description: Neutron inelastic scattering measurements of the polar TO phonon mode in the cubic relaxor Pb(Mg{sub 1/3}Nb{sub 2/3})O{sub 3}, at room temperature, reveal anomalous behavior similar to that recently observed in Pb(Zn{sub 1/3}Nb{sub 2/3}){sub 0.92}Ti{sub 0.08}O{sub 3} in which the optic branch appears to drop precipitously into the acoustic branch at a finite value of the momentum transfer q = 0.2 {angstrom}{sub {minus}1}, measured from the zone center. By contrast, a recent neutron study indicates that PMN exhibits a normal TO phonon dispersion at 800 K. The authors speculate this behavior is common to all relaxor materials, and is the result of the presence of nanometer-scale polarized domains in the crystal that form below a temperature T{sub d}, which effectively prevent the propagation of long wavelength (q = 0) phonons.
Date: March 9, 2000
Partner: UNT Libraries Government Documents Department

Development of proton-conducting membranes for separating hydrogen from gas mixtures

Description: Dense ceramic membranes made from mixed protonic/electronic conductors are permeable only to hydrogen, and in principle, provide a simple efficient means of separating hydrogen from gas mixtures. At a time when world demand for hydrogen is growing, such proton- conducting membranes have the potential to significantly alter the economics of hydrogen separation and purification processes and thus improve the economic viability of processes that utilize hydrogen, such as some refinery operations and direct and indirect coal liquefaction. This paper describes a recently initiated program to develop materials and fabrication processes to separate hydrogen with dense ceramic membranes in a non-Galvanic mode of operation (i.e., without electrodes or external power supply).
Date: June 1, 1996
Creator: Dorris, S.E. & Balachandran
Partner: UNT Libraries Government Documents Department

Ion-Beam-Induced Defects and Defect Interactions in Perovskite-Structure Titanates

Description: Ion-beam irradiation of perovskite structures results in the production and accumulation of defects. Below a critical temperature, irradiation also leads to a crystalline-to-amorphous transformation. The critical temperature for amorphization under 800 keV Kr{sup +} ion irradiation is 425,440 and 550 K for SrTiO{sub 3}, CaTiO{sub 3} and BaTiO{sub 3}, respectively. The results of ion-channeling studies on SrTiO{sub 3} irradiated with 1.0 MeV Au{sup 2+} ions suggest that the crystalline-to-amorphous transformation is dominated by the accumulation and interaction of irradiation-induced defects. In SiTiO{sub 3} irradiated with He{sup +} and 0{sup +} ions at 180 K, isochronal annealing studies indicate that there is significant recovery of defects on both the oxygen and cation sublattices between 200 and 400 K. These results suggest that defect recovery processes may control the kinetics of amorphization. A fit of the direct-impact/defect-stimulated model to the data for SrTiO{sub 3} suggests that the kinetics of amorphization are controlled by both a nearly athermal irradiation-assisted recovery process with an activation energy of 0.1 plus or minus 0.05 eV and a thermal defect recovery process with an activation energy of 0.6 plus or minus 0.1 eV. In SrTi0{sub 3} implanted with 40 keV H{sup +} to 5.0 x 10{sup 16} and 1.0 x 10{sup 17} ions/cm{sup 2}, annealing at 470 K increases the backscattering yield from Sr and Ti and is mostly likely due to the coalescence of H{sub 2} into bubble nuclei. Annealing at 570 K and higher results in the formation of blisters or large cleaved areas.
Date: August 23, 1999
Creator: Boatner, L.A.; Jiang, W.; Meldrum, A.; Thevuthasan, S.; Weber, W.J. & Williford, R.E.
Partner: UNT Libraries Government Documents Department


Description: During the second semiannual period we have carried out a series of QM calculations on the BaZr{sub 1-x}Y{sub x}H{sub x}O{sub 3} proton conductor to determine the equilibrium proton positions and corresponding energies. We find that the size of the crystal cell is very important for the identification of the equilibrium proton positions. To derive ReaxFF parameters for Pt-surfaces and for H-transfer in Y-doped BaZrO{sub 3}, we carried out QM-calculations on the structures and energies of relevant condensed phases and cluster systems. These data then served as a training set to optimize the ReaxFF parameters. ReaxFF for various bulk metals (Pt, Zr, and Y) and metal clusters (Pt) was developed. BaZr{sub 1-x}Y{sub x}O{sub 3-{delta}} with x = 0.2, 0.3, 0.4, 0.5 was synthesized using a modified Pechini method and characterization (X-ray diffraction, scanning electron microscopy, and impedance spectroscopy) of this material was performed. We find that a single perovskite phase is formed in BaZr{sub 1-x}Y{sub x}O{sub 3-{delta}}. The proton conductivity of BaZr{sub 1-x}Y{sub x}O{sub 3-{delta}} with x = 0.5 is significantly lower than the conductivity of the samples with Y-dopant concentrations of 0.1 and 0.2.
Date: October 30, 2003
Creator: Merinov, Boris; Dorso, Claudio O.; III, William A. Goddard; Wu, Jian & Haile, Sossina
Partner: UNT Libraries Government Documents Department


Description: Dense thin films of SrCe{sub 0.95}Tm{sub 0.05}O{sub 3-{delta}} (SCTm) with perovskite structure were prepared on porous alumina or SCTm substrates by the methods of (1) polymeric-gel casting and (2) dry-pressing. The polymeric-gel casting method includes preparation of mixed metal oxide gel and coating of the gel on a macroporous alumina support. Micrometer thick SCTm films of the perovskite structure can be obtained by the polymeric-gel casting method. However, the deposited films are not hermetic and it may require about 50 coatings in order to obtain gas-tight SCTm films by this method. Pd-Cu thin films were synthesized with elemental palladium and copper targets by the sequential R.F. sputter deposition on porous substrates. Pd-Cu alloy films could be formed after proper annealing. The deposited Pd-Cu films were gas-tight. This result demonstrated the feasibility of obtaining an ultrathin SCTm film by the sequential sputter deposition of Sr, Ce and Tm metals followed by proper annealing and oxidation. Such ultrathin SCTm membranes will offer sufficiently high hydrogen permeance for practical applications. Thin gas-tight SCTm membranes were synthesized on porous SCTm supports by the dry-pressing method. In this method, the green powder of SCTm was prepared by wet chemical method using metal nitrates as the precursors. Particle size of the powder was revealed to be a vital factor in determining the porosity and shrinkage of the sintered disks. Small particle size formed the dense film while large particle size produced porous substrates. The SCTm film thickness was varied from 1 mm to 0.15 mm by varying the amount of the target powder. A close match between the shrinkage of the substrate and the dense film led to the defect free-thin films. The selectivity of H{sub 2} over He with these films was infinite. The chemical environment on each side of the membrane influenced the H{sub ...
Date: November 1, 2004
Creator: Lin, Jerry Y.S.; Gupta, Vineet & Cheng, Scott
Partner: UNT Libraries Government Documents Department

Increase of magnetic transition temperatures by reduction of local disorder for perovskite manganites.

Description: We report the synthesis of Sr{sub 1-x}Ca{sub x}MnO{sub 3} and La{sub 0.5}Ba{sub 0.5}MnO{sub 3} perovskites over extended cation and oxygen composition ranges and describe the dependence of their phase stability on the tolerance factor t = t(x,T,{delta}) that is a function of composition, temperature, and oxygen content. We show that magnetic transition temperatures depend strongly on the tolerance factor and charge disorder while dependence on the structural disorder is less important. By reducing charge and structural disorder we have significantly increased the Curie and Neel temperatures for perovskite manganites.
Date: July 2, 2002
Creator: Dabrowski, B.; Chmaissem, O.; Mais, J.; Kolesnik, S.; Jorgensen, J. D. & Short, S.
Partner: UNT Libraries Government Documents Department


Description: In the present quarter, the possibility of using a more complex interfacial engineering approach to the development of reliable and stable oxygen transport perovskite ceramic membranes/metal seals is discussed. Experiments are presented and ceramic/metal interactions are characterized. Crack growth and fracture toughness of the membrane in the reducing conditions are also discussed. Future work regarding this approach is proposed are evaluated for strength and fracture in oxygen gradient conditions. Oxygen gradients are created in tubular membranes by insulating the inner surface from the reducing environment by platinum foils. Fracture in these test conditions is observed to have a gradient in trans and inter-granular fracture as opposed to pure trans-granular fracture observed in homogeneous conditions. Fracture gradients are reasoned to be due to oxygen gradient set up in the membrane, variation in stoichiometry across the thickness and due to varying decomposition of the parent perovskite. The studies are useful in predicting fracture criterion in actual reactor conditions and in understanding the initial evolution of fracture processes.
Date: January 1, 2003
Creator: Bandopadhyay, Dr. Sukumar & Nagabhushana, Dr. Nagendra
Partner: UNT Libraries Government Documents Department

The Stability and Electrical Properties of High Temperature Proton Conductors

Description: The morphological and electrical properties of Ba{sub 1-x}Sr{sub x}Ce{sub 0.8}Y{sub 0.2}O{sub 3-{delta}} with x varying from 0 to 1 prepared by a modified Pechini method were investigated as potential high temperature proton conductors. Dense microstructures were achieved for all the samples upon sintering at 1500 C for 5 h. The phase structure analysis indicated that perovskite phase was formed for 0 {le} x {le} 0.2, while for x larger than 0.5, impurity phases of Sr{sub 2}CeO{sub 4} and Y{sub 2}O{sub 3} appeared. The tolerance to H{sub 2}O for the samples improved with the increase in Sr content when exposed to boiling water, while the electrical conductivity decreased from x = 0 to 1. However, the resistance to CO{sub 2} attack at elevated temperatures was not improved within the whole x range studied.
Date: July 6, 2010
Creator: Brinkman, K.
Partner: UNT Libraries Government Documents Department

The Structural Disorder and Lattice Stability of (Ba,Sr)(Co,Fe)O3 Complex Perovskites

Description: The structural disorder and lattice stability of complex perovskite (Ba,Sr)(Co,Fe)O3, a promising cathode material for solid oxide fuel cells and oxygen permeation membranes, is explored by means of first principles DFT calculations. It is predicted that Ba and Sr ions easily exchange their lattice positions (A-cation disorder) similarly to Co and Fe ions (B-cation disorder). The cation antisite defects (exchange of A- and B-type cations) have a relatively high formation energy. The BSCF is predicted to exist in an equilibrium mixture of several phases and can decompose exothermically into the Ba- and Co-rich hexagonal (Ba,Sr)CoO3 and Sr- and Fe-rich cubic (Ba,Sr)FeO3 perovskites.
Date: May 1, 2011
Creator: S.N.Rashkeev
Partner: UNT Libraries Government Documents Department

Primary carbonatite melt from deeply subducted oceanic crust

Description: Partial melting in the Earth's mantle plays an important part in generating the geochemical and isotopic diversity observed in volcanic rocks at the surface. Identifying the composition of these primary melts in the mantle is crucial for establishing links between mantle geochemical 'reservoirs' and fundamental geodynamic processes. Mineral inclusions in natural diamonds have provided a unique window into such deep mantle processes. Here they provide exper8imental and geochemical evidence that silicate mineral inclusions in diamonds from Juina, Brazil, crystallized from primary and evolved carbonatite melts in the mantle transition zone and deep upper mantle. The incompatible trace element abundances calculated for a melt coexisting with a calcium-titanium-silicate perovskite inclusion indicate deep melting of carbonated oceanic crust, probably at transition-zone depths. Further to perovskite, calcic-majorite garnet inclusions record crystallization in the deep upper mantle from an evolved melt that closely resembles estimates of primitive carbonatite on the basis of volcanic rocks. Small-degree melts of subducted crust can be viewed as agents of chemical mass-transfer in the upper mantle and transition zone, leaving a chemical imprint of ocean crust that can possibly endure for billions of years.
Date: July 1, 2008
Creator: Walter, M.J.; Bulanova, G.P.; Armstrong, L.S.; Keshav, S.; Blundy, J.D.; Gudfinnesson, G. et al.
Partner: UNT Libraries Government Documents Department


Description: Low-energy-electron diffraction, Auger-electron spectroscopy, electron-energy-loss, and ultraviolet-photoelectron spectroscopies were used to study the structure, composition, and electron energy distribution of a clean single-crystal (111) face of strontium titanate (perovskite). The dependence of the surface chemical composition on the temperature has been observed along with corresponding changes in the surface electronic properties. High-temperature Ar-ion bombardment causes an irreversible change in the surface structure, stoichiometry, and electron energy distribution. In contrast to the TiO{sub 2} surface, there are always significant concentrations of Ti{sup 3+} in an annealed ordered SrTiO{sub 3} (111) surface. This stable active Ti{sup 3+} monolayer on top of a substrate with large surface dipole potential makes SrTiO{sub 3} superior to TiO{sub 2} when used as a photoanode in the photoelectrochemical cell.
Date: December 1, 1977
Creator: Lo, Wei Jen & Somorjai, G. A.
Partner: UNT Libraries Government Documents Department

Dual Phase Membrane for High Temperature CO2 Separation

Description: Dual-phase membranes consisting of stainless steel supports infiltrated with molten carbonate have been shown to be selective to CO{sub 2} at high temperatures (400-650 C). However, over time at high temperatures, the formation of iron oxides on the surface of the stainless steel supports render the membranes ineffective. This report details synthesis and characteristics of dual-phase carbonate membrane with an oxidation resistant perovskite type ceramic (lanthanum-strontium-cobaltite-iron; LSCF) support. Porous LSCF supports were prepared from its powder synthesized by the citrate method. Both steady state permeation and mercury porosimetry confirmed that the LSCF membrane sintered at 900 C has pores large enough to absorb molten carbonate, yet small enough to retain the molten carbonate under high pressure conditions. Results of XRD analysis have shown that LSCF and the molten carbonate mixture do not react with each other at temperatures below 700 C. Four-point method conductivity tests indicate that the support material has sufficiently high electronic conductivity for this application. Li-Na-K carbonate was coated to the porous LSCF support by a liquid infiltration method. Helium permeance of the support before and after infiltration of molten carbonate are on the order of 10{sup -6} and 10{sup -10} moles/m{sup 2} {center_dot} Pa {center_dot} s respectively, indicating that the molten carbonate is able to sufficiently infiltrate the membrane. Preliminary high temperature permeation experiments indicate that the membrane does separate CO{sub 2} in the presence of O{sub 2}, with a maximum flux of 0.623 ml/cm{sup 2} {center_dot} min obtained at 850 C.
Date: September 29, 2006
Creator: Lin, Jerry Y.S. & Anderson, Matthew
Partner: UNT Libraries Government Documents Department

Effect of A-site Non-stoichiometry on LSCF Cathodes

Description: LSCF Cathodes were explored when effected with A-site non-stoichiometry. At 700-800 C, the operating temperatures of intermediate temperature (IT-) SOFCs have enabled the use of stainless steels in the SOFC framework and current collectors, allowing significant reductions in cost. However, the lower operating temperatures of IT-SOFC's also result in significant decreases in power densities of cells with LSM cathodes due to their high activation energies. LSCF is a mixed ionic electronic conducting perovskite that exhibits higher performance than LSM/YSZ composites and shows potential as a replacement cathode. This study investigates the effect of A-site stoichiometry on the performance of LSCF cathodes. Cell tests showed that A-site and Sr-deficient LSCF cathodes consistently outperformed stoichiometric LSCF cathodes, exhibiting up to 10% higher cell power densities. It was also observed that all stoichiometric, A-site, and Sr-deficient LSCF cathodes degraded over time at similar rates. Contributions of ohmic and electrode polarization losses to cell degradation rates were similar regardless of cathode composition.
Date: September 1, 2011
Creator: Templeton, Jared W.; Lu, Zigui; Stevenson, Jeffry W. & Hardy, John S.
Partner: UNT Libraries Government Documents Department

Oxygen Transport Ceramic Membranes

Description: The present quarterly report describes some of the investigations on the structural properties of dense OTM bars provided by Praxair and studies on newer composition of Ti doped LSF. The in situ electrical conductivity and Seebeck coefficient measurements were made on LSFT at 1000 and 1200 C over the oxygen activity range from air to 10{sup -15} atm. The electrical conductivity measurements exhibited a p to n type transition at an oxygen activity of 1 x 10{sup -10} at 1000 C and 1 x 10{sup -6} at 1200 C. Thermogravimetric studies were also carried out over the same oxygen activities and temperatures. Based on the results of these measurements, the chemical and mechanical stability range of LSFT were determined and defect structure was established. The studies on the fracture toughness of the LSFT and dual phase membranes exposed to air and N{sub 2} at 1000 C was done and the XRD and SEM analysis of the specimens were carried out to understand the structural and microstructural changes. The membranes that are exposed to high temperatures at an inert and a reactive atmosphere undergo many structural and chemical changes which affect the mechanical properties. A complete transformation of fracture behavior was observed in the N{sub 2} treated LSFT samples. Further results to investigate the origin of the slow kinetics on reduction of ferrites have been obtained. The slow kinetics appear to be related to a non-equilibrium reduction pathway that initially results in the formation of iron particles. At long times, equilibrium can be reestablished with recovery of the perovskite phase. Recent results on transient kinetic data are presented. The 2-D modeling of oxygen movement has been undertaken in order to fit isotope data. The model is used to study ''frozen'' profiles in patterned or composite membranes.
Date: February 1, 2005
Creator: Bandopadhyay, S.; Nithyanantham, T.; Zhou, X.-D; Sin, Y-W.; Anderson, H.U.; Jacobson, Alan et al.
Partner: UNT Libraries Government Documents Department


Description: The morphological and electrical properties of yttrium (Y) and indium (In) doped barium cerate perovskites of the form BaIn{sub 0.3-x}Y{sub x}Ce{sub 0.7}O{sub 3-{delta}} (with x=0-0.3) prepared by a modified Pechini method were investigated as potential high temperature proton conductors with improved chemical stability. The sinterability increased with the increase of In-doping, and the perovskite phase was found in the BaIn{sub 0.3-x}Y{sub x}Ce{sub 0.7}O{sub 3-{delta}} solid solutions over the range 0 {le} x {le} 0.3. The conductivities decreased (from x to x, insert quantitative values) while the tolerance to wet CO{sub 2} improved for BaIn{sub 0.3-x}Y{sub x}Ce{sub 0.7}O{sub 3-{delta}} samples with an increase of In-doping.
Date: February 18, 2010
Creator: Brinkman, K.
Partner: UNT Libraries Government Documents Department