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Ductile grinding of Ba(Sr)TiO{sub 3} detector substrates

Description: Several precision machining techniques have been applied to barium strontium titanate wafers as deterministic replacements for lapping, polishing and reticulation. Surface finishes approaching polish quality have been achieved in less time than normally required, leading to potentially lower manufacturing costs for uncooled FPA`s.
Date: August 1, 1994
Creator: Kahl, W.K.
Partner: UNT Libraries Government Documents Department

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

Nanocrystalline BaTiO{sub 3} from gas-condensation process

Description: Nanocrystalline BaTiO{sub 3} can be prepared by gas condensation, with an average particle size as small as 18 nm. The stoichiometry of nanocrystalline BaTiO{sub 3} particles can be controlled precisely and reproducibly. Nanocrystalline BaTiO{sub 3} powders, fabricated by a novel e-beam evaporation method, show good sintering behavior which can be sintered to a high density at a temperature as low as 1200 C and exhibit a relatively large dielectric constant than that of coarse-grained ceramics. A thermal analysis was also carried out to assert the lowest limit temperature for forming nanostructured BaTiO{sub 3} from Ba/Ti oxidized clusters at ambient pressure.
Date: December 31, 1996
Creator: Li, S.; Eastman, J.A.; Thompson, L.J.; Bjormander, C. & Foster, C.M.
Partner: UNT Libraries Government Documents Department

(Ba,Sr)TiO{sub 3} dielectrics: Relationship between bulk and thin film properties

Description: Thin films of complex perovskites have a number of potentially important applications. Of major scientific and practical concern is the scaling of properties as film dimensions are reduced. This paper describes a satisfactory relationship between bulk and thin film dielectric properties of (Ba,Sr)TiO{sub 3}. Relative contributions of strain, A:B cation stoichiometry, and interface are separated to explain temperature dependent dielectric behavior.
Date: December 22, 1999
Creator: Kingon, A. I.; Streiffer, S. K.; Parker, C. B. & Stemmer, S.
Partner: UNT Libraries Government Documents Department

MOCVD growth and characterization of (Ba{sub x}Sr{sub 1{minus}x})Ti{sub 1+y}O{sub 3+z} thin films for high frequency devices

Description: The authors have investigated the structural and electrical characteristics of (Ba{sub x}Sr{sub 1{minus}x})Ti{sub 1+y}O{sub 3+z} (BST) thin films. The BST thin films were deposited at 650 C on platinized silicon with good thickness and composition uniformity using a large area, vertical liquid-delivery metalorganic chemical vapor deposition (MOCVD) system. The (Ba+Sr)/Ti ratio of the BST films was varied from 0.96 to 1.05 at a fixed Ba/Sr ratio of 70/30, as determined using x-ray fluorescence spectroscopy (XRF) and Rutherford backscattering spectrometry (RBS). Patterned Pt top electrodes were deposited onto the BST films at 350 C through a shadow mask using electron beam evaporation. Annealing the entire capacitor structure in air at 700 C after deposition of top electrodes resulted in a substantial reduction of the dielectric loss. Useful dielectric tunability as high as 2.3:1 was measured.
Date: January 18, 2000
Creator: Baumann, P. K.; Streiffer, S. K.; Im, J.; Baldo, P.; McCormick, A.; Auciello, O. et al.
Partner: UNT Libraries Government Documents Department

Study of the structure of ferroelectric domain walls in barium titanate ceramics

Description: Structure of 90{degree} ferroelectric domain boundaries in barium titanate ceramics has been studied by means of Transmission Electron Microscopy and High Resolution TEM. Tilts of specific fringes across domain walls are measured on HREM images and Selected Area Diffraction Patterns. They are in a good agreement with the twin model admitted for these domain boundaries. A computerized method has been developed to give access to quantitative information about atomic displacements across these ferroelectric domain walls. The so calculated displacement field is then compared with Landau-Ginzburg based theoretical predictions.
Date: February 1, 1995
Creator: Normand, L.; Thorel, A.; Kilaas, R. & Montardi, Y.
Partner: UNT Libraries Government Documents Department

Stoichiometry and Deposition Temperature Dependence of the Microstructural and Electrical Properties of Barium Strontium Titanate Thin Films

Description: Barium Strontium Titanate (BST) was deposited on Pt/ZrO2 / SiO2/Si substrates using liquid source metal organic chemical vapor deposition. A stoichiometry series was deposited with various GrII/Ti ratios (0.658 to 1.022) and a temperature series was deposited at 550 to 700°C. The thin films were characterized using transmission electron microscopy. Both series of samples contained cubic perovskite BST and an amorphous phase. The grain size increased and the volume fraction of amorphous phase decreased with increasing deposition temperature. The electrical and microstructural properties improved as the GrII/Ti ratio approached 1 and deteriorated beyond 1. This research demonstrates that BST thin films are a strong candidate for future MOS transistor gate insulator applications.
Date: May 1998
Creator: Pena, Piedad
Partner: UNT Libraries

Synchrotron x-ray diffraction study of the surface layer in poled ceramic BaTiO{sub 3}

Description: Both unpoled and poled BaTiO{sub 3} were studied by laboratory and synchrotron x-ray sources. The shorter-wavelength synchrotron radiation was used to probe deeper below the surface. Diffraction patterns revealed a distinctive surface layer of the same tetragonal structure as the main BaTiO{sub 3} fraction. A sin{sup 2}{Psi} analysis of peak shift confirmed that no measurable change of elastic strain occurs upon poling, indicating that poling may induce excessive strain that was relieved by microcracking or that a majority of domains reverse orientation. However, domain switching and possible microcracking may induced inhomogeneous strain (microstrain) and alter domain-size distribution. Line-broadening analysis showed large anisotropy of both coherently diffracting domain size and microstrain. The poled specimen shows a larger microstrain and smaller average domain size, which indicates possible effects of microcracking and additional defects created during poling.
Date: January 1, 1997
Creator: Ledbetter, H.; Balzar, D.; Stephens, P.W.; Li, J. & Dunn, M.L.
Partner: UNT Libraries Government Documents Department

Spiral order in Ba{sub 2}CuGe{sub 2}O{sub 7}

Description: The quasi 2-D square-lattice antiferromagnet Ba{sub 2}CuGe{sub 2}O{sub 7} was studied by neutron scattering and bulk magnetic techniques. An incommensurate magnetic spiral structure with the propagation vector (1+{xi}, 1+{xi}, 0)({xi}=0.027) was observed below T{sub N}=3.26 K. Spin dynamics can be adequately described by conventional spin-wave theory with two exchange constants: nearest- neighbor in-plant antiferromagnetic coupling J{sub 1}{approx}0.48 meV and interplane ferromagnetic interaction J{sub 3}{approx}0.013 meV. This set of exchange parameters apparently fails to explain the spiral order. The non-centrosymmetric crystal structure suggests that the incommensurate phase may be the result of a Dzyaloshinskii-Moriya instability of the Neel ground state.
Date: August 1, 1996
Creator: Zheludev, A.; Shirane, G.; Sasago, Y.; Koide, N. & Uchinokura, K.
Partner: UNT Libraries Government Documents Department

Development of pair distribution function analysis

Description: This is the final report of a 3-year LDRD project at LANL. It has become more and more evident that structural coherence in the CuO{sub 2} planes of high-{Tc} superconducting materials over some intermediate length scale (nm range) is important to superconductivity. In recent years, the pair distribution function (PDF) analysis of powder diffraction data has been developed for extracting structural information on these length scales. This project sought to expand and develop this technique, use it to analyze neutron powder diffraction data, and apply it to problems. In particular, interest is in the area of high-{Tc} superconductors, although we planned to extend the study to the closely related perovskite ferroelectric materials andother materials where the local structure affects the properties where detailed knowledge of the local and intermediate range structure is important. In addition, we planned to carry out single crystal experiments to look for diffuse scattering. This information augments the information from the PDF.
Date: September 1, 1996
Creator: Vondreele, R.; Billinge, S.; Kwei, G. & Lawson, A.
Partner: UNT Libraries Government Documents Department

Combustion synthesis and engineering of nanoparticles for electronic, structural and superconductor applications. Annual progress report, August 20, 1993--May 30, 1994

Description: Fully dense, nanocrystalline ceramic articles were prepared by the new nanofabrication process developed in this research program. The process consists of two steps: synthesis of ceramic nanoparticles and fabrication of dense, nanocrystalline ceramic parts. The synthesis step produced 10-nanometer-diameter crystallites and is capable of being scaled up to kilogam/hour production rates. The fabrication step produced dense articles at significantly reduced sintering temperatures and times-representing a factor of 10-100 reduction in process energy requirements. The process was demonstrated by producing ultrafine-gained yttria-doped ZrO{sub 2}, an important material with a variety of energy-related applications (e.g., solid electrolytes, oxygen sensors, electrode materials, thermal barrier coatings, etc.); BaTiO{sub 3} (for capacitor applications); and YBa{sub 2}Cu{sub 3}O{sub 7-x}(a high-temperature superconductor with uses, e.g., in magnetic flux trapping and high-speed capacitor applications). Results from this reporting period (September 1993--May 1994) clearly illustrate the capabilities of this energy-efficient and directly commercializable process for producing dense, nanocrystalline, multicomponent oxide ceramics.
Date: May 30, 1994
Creator: Stangle, G.C.; Amarakoon, V.R.W. & Schulze, W.A.
Partner: UNT Libraries Government Documents Department

Combustion synthesis and engineering nanoparticles for electronic, structural and superconductor applications. Final report, May 31, 1992--May 30, 1996

Description: Dense, nanocrystalline ceramic articles of doped ZrO{sub 2} (for use in solid electrolytes, oxygen sensors, electrode materials, thermal barrier coatings, etc.), BaTiO{sub 3} (for capacitor applications), and YBa{sub 2}Cu{sub 3}O{sub 7-x} (a high-temperature superconductor with uses, e.g., in magnetic flux trapping and high-speed capacitor applications) were prepared by the new nanofabrication process that has been developed in this research program. The process consists of two steps: synthesis of ceramic nanoparticles, and fabrication of dense ceramic articles that possess nanocrystalline features. The synthesis step is capable of producing 10-nanometer-diameter crystallites of doped ZrO{sub 2}, and of being scaled up to kilogram/hour production rates. The fabrication step produced dense, ultrafine-grained articles at significantly reduced sintering temperatures and times--representing a factor of 10-100 reduction in process energy requirements. The process has thus been shown to be technically feasible, while a preliminary engineering cost analysis of a pilot plant-scale version of the process indicates that it is both a cost- and an energy-efficient method of producing nanoparticles and nanocrystalline ceramics from those nanoparticles. One U.S. patent for this process has been allowed, and an additional five (continuation-in-part) applications have been filed. Technology transfer efforts have begun, through ongoing discussions with representatives from three manufacturing concerns.
Date: May 30, 1996
Creator: Stangle, G.C.; Schulze, W.A. & Amarakoon, V.R.W.
Partner: UNT Libraries Government Documents Department

Pseudogap and Incommensurate Magnetic Fluctuations in YBa{sub 2}Cu{sub 3}O{sub 6.6}

Description: Unpolarized inelastic neutron scattering is used to study the temperature and wave vector dependence of the dynamical magnetic susceptibility, {xi}`` (q,{omega}), of a well characterized single crystal YBa{sub 2}Cu{sub 3}O{sub 6.6} (T{sub c} = 62.7 K). We find that a pseudogap opens in the spin fluctuation spectrum at temperatures well above T{sub c}. We speculate that the appearance of the low frequency incommensurate fluctuations is associated with the opening of the pseudogap. To within the error of the measurements, a gap in the spin fluctuation spectrum is found in the superconducting state.
Date: July 24, 1997
Creator: Dai, P., Mook, H.A.,
Partner: UNT Libraries Government Documents Department

Neutron scattering studies of the magnetic fluctuations in YBa{sub 2}Cu{sub 3}O{sub 7-{delta}}

Description: Neutron scattering measurements have been made on the spin fluctuations in YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} for different oxygen doping levels. Incommensurability is clearly observed for oxygen concentrations of 6.6 and 6.7 and is suggested for the 6.93. Measurements of the resonance for the O6.6 concentration show that it exists in a broadened and less intense form at temperatures much higher than Tc.
Date: November 5, 1997
Creator: Mook, H.A.; Dai, P.; Hunt, R.D. & Dogan, F.
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
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

Fabrication of spherical and irregularly shaped powders of Li and Ba titanates from titanium tetrachloride by inorganic sol-gel process.

Description: The authors report preparation procedures for some lithium and barium titanate compounds. The procedures utilize inexpensive, commercial TiCl{sub 4} in an aqueous HCl solution. The main preparation steps consist of chloride elimination by distillation with nitric acid, addition of hydroxides of Li (or Ba), evaporation of sols to dry powders, and thermal treatment to titanates. Thermal and X-ray analyses indicate that the formation temperatures are lower than the corresponding temperatures necessary for conventional solid-state reactions. These temperatures can be further decreased by addition of ascorbic acid during formation of Ti nitrate sols. Spherical powders (with particle diameters of < 100 {micro}m) are obtained by solvent extraction of water from sol drops emulsified in 2-ethylheksanol-1. Final correction of the composition of the spherical powders is carried out by an impregnation process.
Date: April 26, 2002
Creator: Deptula, A.; Olczak, T.; Lada, W.; Sartowska, B.; Chmielewski, A. G.; Alvani, C. et al.
Partner: UNT Libraries Government Documents Department

Effect of Lamination Conditions on the Gas Permeability and Adhesion Strength of Green Ceramic Tapes

Description: The gas permeability and adhesion strength of laminated green ceramic tapes were determined for samples comprised of barium titanate as the dielectric, and poly(vinyl butyral) and dioctyl phthalate as the main components of the binder mixture. The green tapes were laminated for times of 2-10 min, pressures of 1.8-7 MPa, and temperatures of 35-85C. The adhesion strength, which was measured by a peel test, increased with increasing lamination time, temperature, and pressure. The permeability, which was determined from gas flux measurements, decreased with increasing lamination time, temperature, and pressure. The dependence of the permeability and adhesion strength on lamination time, temperature, and pressure is qualitatively consistent with a mechanistic description of the lamination process as one of binder flow in porous media
Date: August 31, 2007
Creator: Krueger, D.
Partner: UNT Libraries Government Documents Department

Neutron scattering investigation of the magnetic order in single crystalline BaFe2As2

Description: The magnetic structure of BaFe{sub 2}As{sub 2} was determined from polycrystalline neutron diffraction measurements soon after the ThCr{sub 2}Si{sub 2}-type FeAs-based superconductors were discovered. Both the moment direction and the in-plane antiferromagnetic wavevector are along the longer a-axis of the orthorhombic unit cell. There is only one combined magnetostructural transition at {approx}140 K. However, a later single-crystal neutron diffraction work reported contradicting results. Here, we show neutron diffraction results from a single-crystal sample, grown by a self-flux method, that support the original polycrystalline work.
Date: January 1, 2008
Creator: Bao, Wei; Qiu, Y; Kofu, M; Lee, S - H; Chang, S; Wu, T et al.
Partner: UNT Libraries Government Documents Department