16 Matching Results

Search Results

Advanced search parameters have been applied.

Synthesis of nanocrystalline barium-hexaferrite from nanocrystalline goethite using the hydrothermal method: Particle size evolution and magnetic properties

Description: To characterize particle size/magnetic property relationships, 9 to 50 nm in diameter barium hexaferrite, BaFe{sub 12}O{sub 19} (BHF), particles were prepared using a new synthesis route. By replacing the conventional 50 to 100 nm particles of goethite with nanocrystalline goethite produced via the microwave anneal method of Knight and Sylva, nanocrystalline BHF was synthesized using the hydrothermal method. Evolution of particle size and morphology with respect to concentration and heat treatment time is reported. Hysteresis properties, including coercivity (0.2--1.0 kOe), magnetization saturation (0.1--33.4 emu/g), and magnetization remanence (0.004--22.5 emu/g) are discussed as a function of particle size. The magnetization saturation and remanence of the 7 nm particles is nearly zero, suggesting the superparamagnetic threshold size for BHF is around this size. In addition, the equilibrium morphology of BHF crystals was calculated to be truncated hexagonal prisms which was verified by experiment, and the isoelectric point, pH of 4.1, was measured for 18 nm BHF particles.
Date: March 1, 1997
Creator: Penn, R.L.; Banfield, J.F. & Voigt, J.
Partner: UNT Libraries Government Documents Department

Prediction of metal sorption in soils

Description: Radionuclide transport in soils and groundwaters is routinely calculated in performance assessment (PA) codes using simplified conceptual models for radionuclide sorption, such as the K{sub D} approach for linear and reversible sorption. Model inaccuracies are typically addressed by adding layers of conservativeness (e.g., very low K{sub D}'s), and often result in failed transport predictions or substantial increases in site cleanup costs. Realistic assessments of radionuclide transport over a wide range of environmental conditions can proceed only from accurate, mechanistic models of the sorption process. They have focused on the sorption mechanisms and partition coefficients for Cs{sup +}, Sr{sup 2+} and Ba{sup 2+} (analogue for Ra{sup 2+}) onto iron oxides and clay minerals using an integrated approach that includes computer simulations, sorption/desorption measurements, and synchrotron analyses of metal sorbed substrates under geochemically realistic conditions. Sorption of Ba{sup 2+} and Sr{sup 2+} onto smectite is strong, pH-independent, and fully reversible, suggesting that cation exchange at the interlayer basal sites controls the sorption process. Sr{sup 2+} sorbs weakly onto geothite and quartz, and is pH-dependent. Sr{sup 2+} sorption onto a mixture of smectite and goethite, however, is pH- and concentration dependent. The adsorption capacity of montmorillonite is higher than that of goethite, which may be attributed to the high specific surface area and reaction site density of clays. The presence of goethite also appears to control the extent of metal desorption. In-situ, extended X-ray absorption fine structure (EXAFS) spectroscopic measurements for montmorillonite and goethite show that the first shell of adsorbed Ba{sup 2+} is coordinated by 6 oxygens. The second adsorption shell, however, varies with the mineral surface coverage of adsorbed Ba{sup 2+} and the mineral substrate. This suggests that Ba{sup 2+} adsorption on mineral surfaces involves more than one mechanism and that the stability of sorbed complexes will be affected by substrate ...
Date: March 2, 2000
Creator: WESTRICH,HENRY R.; ANDERSON JR.,HAROLD L.; ARTHUR,SARA E.; BRADY,PATRICK V.; CYGAN,RANDALL T.; LIANG,JIANJIE et al.
Partner: UNT Libraries Government Documents Department

Formation of artificially-layered high-temperature superconductors using pulsed-laser deposition

Description: Pulsed-laser deposition has been used to synthesize artificially-layered high-temperature superconductors. Novel thin-film SrCu0{sub 2}/BaCu0{sub 2} superlattices have been synthesized which superconduct at temperatures as high as 70 K. These results demonstrate that pulsed-laser deposition and epitaxial stabilization can be effectively used to engineer artificially-layered thin-film materials.
Date: March 1, 1995
Creator: Norton, D.P.; Chakoumakos, B.C. & Budai, J.D.
Partner: UNT Libraries Government Documents Department

Structures of 90{degrees} domain walls in ferroelectric barium titanate ceramics

Description: Ferroelectric domain walls in tetragonal ferroelectric barium titanate ceramics are studied by means of electron microscopy. SEM and TEM observations are consistent with domain configuration already proposed. Conventional TEM measurements on SADP agree very well with twin-related model currently admitted for ferroelectric domains. In spite of the very small lattice parameter variation during cooling (involving a small spontaneous strain) of BaTiO{sub 3} ceramics, displacements of specific features associated with atomic column positions are measured across domain walls on high resolution images. Using a dedicated image analysis software, these displacements are calculated with a high precision. 2D vector-maps of the atomic displacements show different kinds of atomistic structure for different domain walls.
Date: March 1995
Creator: Normand, L.; Thorel, A. & Kilaas, R.
Partner: UNT Libraries Government Documents Department

Carbon-14 immobilization via the Ba(OH)/sub 2/. 8H/sub 2/O process

Description: The airborne release of /sup 4/C from varous nuclear facilities has been identified as a potential biohazard due to the long half-life of /sup 14/C (5730 y) and the ease with which it may be assimilated into the biosphere. At ORNL, technology has been developed for the removal and immobilization of this radionuclide. Prior studies have indicated that /sup 14/C will likely exist in the oxidized form as CO/sub 2/ and will contribute slightly to the bulk CO/sub 2/ concentration of the gas stream, which is airlike in nature (approx. 330 ppmv CO/sub 2/). The technology that has been developed utilizes the CO/sub 2/-Ba(OH)/sub 2/.8H/sub 2/O gas-solid reaction with the mode of gas-solid contacting being a fixed bed. The product, BaCO/sub 3/, possesses excellent thermal and chemical stability, prerequisites for the long-term disposal of nuclear wastes. For optimal process operation, studies have indicated that an operating window of adequate size does exist. When operating within the window, high CO/sub 2/ removal efficiency (effluent concentrations < 100 ppbv), high reactant utilization (> 99%), and an acceptable pressure drop across the bed (3 kPa/m at a superficial velocity of 13 cm/s) are possible. This paper addresses three areas of experimental investigation: (1) microscale studies on 150-mg samples to provide information concerning surface properties, kinetics, and equilibrium vapor pressures; (2) macroscale studies on large fixed beds (4.2 kg of reactant) to determine the effects of humidity, temperature, and gas flow rate upon bed pressure drop and CO/sub 2/ breakthrough; and (3) design, construction, and initial operation of a pilot unit capable of continuously processing a 34-m/sup 3//h (20-ft/sup 3//min) air-based gas stream.
Date: March 1, 1983
Creator: Haag, G.L.; Nehls, J.W. Jr. & Young, G.C.
Partner: UNT Libraries Government Documents Department

Thermodynamics of geothermal fluids

Description: A model to predict the thermodynamic properties of geothermal brines, based on a minimum amount of experimental data on a few key systems, is tested. Volumetric properties of aqueous sodium chloride, taken from the literature, are represented by a parametric equation over the range 0 to 300{sup 0}C and 1 bar to 1 kbar. Density measurements at 20 bar needed to complete the volumetric description also are presented. The pressure dependence of activity and thermal properties, derived from the volumetric equation, can be used to complete an equation of state for sodium chloride solutions. A flow calorimeter, used to obtain heat capacity data at high temperatures and pressures, is described. Heat capacity measurements, from 30 to 200{sup 0}C and 1 bar to 200 bar, are used to derive values for the activity coefficient and other thermodynamic properties of sodium sulfate solutions as a function of temperature. Literature data on the solubility of gypsum in mixed electrolyte solutions have been used to evaluate model parameters for calculating gypsum solubility in seawater and natural brines. Predictions of strontium and barium sulfate solubility in seawater also are given.
Date: March 1, 1981
Creator: Rogers, P.S.Z.
Partner: UNT Libraries Government Documents Department

Processing and properties of silver-clad Tl-Ba-Ca-Cu-O wires and tapes

Description: TlBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub x} and Tl{sub 2}Ba{sub 2}Ca{sub 2}Cu{sub 3}O{sub x} powders were synthesized, loaded into Ag tubes, and worked into wires and tapes by drawing and rolling. All processing outside of furnaces was in a dry-N{sub 2} glovebox. All heat treatments were performed in O{sub 2}. The Ag-clad wires fabricated from these powders exhibited onset of superconductivity at {approx}118 K and critical current densities at 77K of 2{times}10{sup 3} to 4{times}10{sup 3} A/cm{sup 2}.
Date: March 1, 1992
Creator: Goretta, K.C.; Wu, C.T.; Lanagan, M.T.; Boling, M.A.; Shi, D.; Miller, D.J. et al.
Partner: UNT Libraries Government Documents Department

Simulation of vortex-line pinning by defects in the Y-Ba-Cu-O system

Description: The temperature properties of a two-dimensional flux lattice are studied by Monte Carlo simulation, with particular attention to the effects of twin-boundaries. The parameters selected are appropriate for the YBa{sub 2}CU{sub 3}0{sub 7} high-temperature superconducting system. The intrinsic properties of the vortex state are investigated by monitoring system evolution at fixed temperature and applied magnetic field. By varying the temperature, the loss of type-II superconductivity via fluxoid lattice melting is also examined. The introduction of model defects induces the creation of metastable and glassy states which reduce overall hexane order but are found to enhance system resistance to flux-lattice melting.
Date: March 1, 1992
Creator: Jamison, R.; Burmester, C. P.; Gronsky, R. (Lawrence Berkeley Lab., CA (United States)) & Wille, L. T. (Florida Atlantic Univ., Boca Raton, FL (United States). Dept. of Physics)
Partner: UNT Libraries Government Documents Department

Submillimeter and microwave residual losses in epitaxial films of Y-Ba-Cu-O and Tl-Ca-Ba-Cu-O

Description: We have used a novel bolometric technique and a resonant technique to obtain accurate submillimeter and microwave residual loss data for epitaxial thin films of YBa{sub 2}Cu{sub 3}O{sub 7}, Tl{sub 2}Ca{sub 2}Ba{sub 2}Cu{sub 3}O{sub 10} and Tl{sub 2}CaBa{sub 2}Cu{sub 2}O{sub 8}. For all films we obtain good agreement between the submillimeter and microwave data, with the residual losses in both the Y-Ba-Cu-O and Tl-Ca-Ba-Cu-O films scaling approximately as frequency squared below {approximately} 1 THz. We are able to fit the losses in the Y-Ba-Cu-O films to a weakly coupled grain model for the a-b plane conductivity, in good agreement with results from a Kramers-Kronig analysis of the loss data. We observe strong phonon structure in the Tl-Ca-Ba-Cu-O films for frequencies between 2 and 21 THz, and are unable to fit these losses to the simple weakly coupled grain model. This is in strong contrast to the case for other high {Tc} superconductors such as YBa{sub 2}Cu{sub 3}O{sub 7}, where phonon structure observed in ceramic samples is absent in epitaxial oriented films and crystals because of the electronic screening due to the high conductivity of the a-b planes.
Date: March 1, 1992
Creator: Miller, D.; Richards, P. L. (Lawrence Berkeley Lab., CA (United States)); Garrison, S. M.; Newman, N. (Conductus, Inc., Sunnyvale, CA (United States)); Eom, C. B.; Geballe, T. H. (Stanford Univ., CA (United States). Dept. of Applied Physics) et al.
Partner: UNT Libraries Government Documents Department

An outline of positron measurements of superconducting oxides

Description: Positron measurements on superconducting oxides have gone through an evolution from divergent results of low statistical precision on samples of suspect quality to convergent results of higher statistical precision on high quality samples. We outline the elements affecting the progress of these experiments and questions that can be addressed at our present state of the art. 11 refs.
Date: March 1, 1991
Creator: Howell, R.H. & Fluss, M.J.
Partner: UNT Libraries Government Documents Department

Statics and kinetics of oxygen ordering in the oxide superconductor YBa sub 2 Cu sub 3 O sub z

Description: Oxygen ordering in the high-temperature superconductor YBa{sub 2}Cu{sub 3}O{sub z} is well described by a two-dimensional Ising model with anisotropic next-nearest neighbor effective pair interactions, repulsive along the a direction, attractive along b. Monte Carlo simulation shows that this marked anisotropy gives rise, particularly at stoichiometric index z close to 7, to large deviations from unity of the ratio D{sub 11}{sup *}/D{sub 22}{sup *} of tracer diffusion tensor components in the a and b directions. Oxygen ordering kinetics are shown to evolve in a manner which parallels those of the experimentally observed increase with aging time of the superconducting transition temperature of z<7.
Date: March 1, 1992
Creator: McCormack, R.; de Fontaine, D. (Lawrence Berkeley Lab., CA (United States)); Ceder, G. (Massachusetts Inst. of Tech., Cambridge, MA (United States)) & Salomons, E. (CEN Saclay, Gif-sur-Yvette, (France))
Partner: UNT Libraries Government Documents Department

Microstructure and associated properties of YBa[sub 2]Cu[sub 3]O[sub x] superconductors prepared by melt-processing techniques

Description: From the standpoint of applications, melt-processed bulk YBa[sub 2]Cu[sub 3]O[sub x] (YBCO) superconductors are of considerable interest. We have studied the microstructure and levitation force of melt-processed YBCO, YBCO plus Y[sub 2]BaCuO[sub 5], and YBCO plus Pt samples. Large single crystalline samples, grown using a seeding technique, were also studied. The levitation force is highest in melt-processed samples made by the seeding technique. 6 figs, 24 refs.
Date: March 1, 1993
Creator: Balachandran, U.; Zhong, W.; Youngdahl, C.A. & Poeppel, R.B.
Partner: UNT Libraries Government Documents Department

Defect structure of semiconducting and insulating epitaxial oxides. Progress report, May 1, 1993--April 30, 1994

Description: The investigation has focused on epitaxial growth of BaSrTiO{sub 3} over the entire solid solution range, point defects in epitaxial BaTiO{sub 3} using temperature-dependent conductivity and deep-level optical spectroscopy, and their nonlinear optical properties.
Date: March 1, 1994
Creator: Wessels, B. W.
Partner: UNT Libraries Government Documents Department

Recent developments in high-speed, non-sampling electromagnetic calorimetry

Description: Brief discussions are given of the work that has been done on the suppression of the slow component in BaF{sub 2}, the developments in the understanding of undoped CsI, and the properties of the new scintillator CeF{sub 3}. The properties of the Cherenkov radiator PbF{sub 2} along with test beam results are presented. Monte Carlo simulations indicate that, with the addition of a small amount of scintillator, PbF{sub 2}, can be made hadron compensating off-line so that the resolution of a compensating hadron calorimeter will not be degraded by its presence. 14 refs., 6 figs., 4 tabs.
Date: March 1, 1990
Creator: Anderson, D.F. & Ramberg, E.J.
Partner: UNT Libraries Government Documents Department

SYNTHESES OF SOME ALKALI-METAL p-ETHYLBENZENESULFONATES

Description: Methods were developed for the syntheses of alkalimetal p--bar ethylbenzenesulfonates. P--bar ethylbenzenesulfonic acid is isolated and is purified as barium p--bar -ethylbenzenesulfonate which is converted to the Na, K, or Cs p-bar -ethylbenzenesulfonate by means of a slight excess of the corresponding alkali-metal carbonate. Lithium sulfate is used to form the lithium p-bar -ethylbenzenesulfonate. (auth)
Date: March 30, 1962
Creator: White, J.C. & Holsopple, H.L.
Partner: UNT Libraries Government Documents Department

Nonlinear materials for frequency conversion

Description: Two figures of merit, the threshold power (P/sub th/) and the limiting volume (V/sub min/) can be used to compare the relative efficiency and economy of new harmonic generating crystals. The properties of barium metaborate and L-Arginine phosphate are used to illustrate the effect of nonlinearity, birefringence, and damage threshold on these figures of merit.
Date: March 22, 1988
Creator: Velsko, S.P. & Eimerl, D.
Partner: UNT Libraries Government Documents Department