26 Matching Results

Search Results

Advanced search parameters have been applied.

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

Sol-gel process for preparing YBa{sub 2}Cu{sub 4}O{sub 8} precursors from Y, Ba, and Cu acidic acetates/ammonia/ascorbic acid systems

Description: Sols were prepared by addition of ammonia to acidic acetate solutions of Y{sup 3+}, Ba{sup 2+}, and Cu{sup 2+}. Ascorbic acid was added to a part of the sol. The resultant sols were gelled to a shard, a film, or microspheres by evaporation at 60 C or by extraction of water from drops of emulsion suspended in 2-ethylhexanol-1. Addition of ethanol to the sols facilitated the formation of gel films, fabricated by a dipping technique, on glass or silver substrates. At 100 C, gels that were formed in the presence of ascorbic acid were perfectly amorphous, in contrast to the crystalline acetate gels. Conversion of the amorphous ascorbate gels to final products was easier than for the acetate gels. The quality of coatings prepared from ascorbate gels was superior to that of acetate gel coatings.
Date: August 1, 1995
Creator: Deptula, A.; Lada, W.; Olczak, T.; Goretta, K.C.; Di Bartolomeo, A. & Casadio, S.
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

Modulated spin and charge densities in cuprate superconductors

Description: Neutron scattering experiments have played a crucial role in characterizing the spin and charge correlations in copper-oxide superconductors. While the data are often interpreted with respect to specific theories of the cuprates, an attempt is made here to distinguish those facts that can be extracted empirically, and the connections that can be made with minimal assumptions.
Date: August 1, 1997
Creator: Tranquada, J.M.
Partner: UNT Libraries Government Documents Department

LOADING AND ELUTION CHARACTERISTICS OF SOME NATURAL AND SYNTHETIC ZEOLITES

Description: The cation exchange characteristics of several commercially available zeolites were evaluated, Equilibrium loading characteristics and exchange capacities were determined experimentally in the binary systems cesium-lithium, cesium-- sodium, cesium--potassium, cesium-Tubidium, cesium -- hydrogen, cesium-- ammonium, strontium-- magnesium, strontium-- calcium, strontium-- barium, and strontium-- hydrogen for several zeolites. Exchangeable cation fractions in the equilibrium solutions were varied from 0.2 to 0.00001, while the total normality of the system constituents was held constant. Results were plotted as equivalent fraction of the traced cation on the zeolite at equilibrium vs. the fraction of traced cation remaining in the equilibrium solution. If an influent composition is known, and falls in the system listed above, zeolite column loading may be read directly in univalent-univalent or divalent-divalent systems. However, in the strontium-sodium or strontium-- hydrogen systems, a k value must be used because squared terms appear in the equilibrium expression as follows: 2 Na- zeolite plus or minus Sr-solution in equilibrium 2 Nasolution + Sr-zeolite, or k = (Sr-zeolite)(Na-solution)/ (Na-zeolite)2(Sr-solution). The k for a given cation is a function of zeolite loading with that cation. Several column 50% loading capacities were determined to randomly test the validity of the above curves to predict column capacities, Several columns loaded with known amounts of cesium and strontium radioisotopes were heated to 800 deg C for 24 hours to simulate a zeolite packaging dehydration cycle. Losses of cesium and strontium activity during the 24-hr period were determined. Elution of the remaining activity on the columns was accomplished with various eluting agents including dilute HNO/sub 3/, NH/sub 4/NO/sub 3/, CaCl/sub 2/ and La(NO/su b 3/)/sub 3/, and elution curves were calculated. (auth)
Date: August 10, 1962
Creator: Ames, L.L. Jr. & Knoll, K.C.
Partner: UNT Libraries Government Documents Department

Studies of nonlinear electrodynamics of high-temperature superconductors

Description: Nonlinear electrodynamics of high-{Tc} superconductors are studied both theoretically and experimentally. For powdered samples, a novel model is presented in which the metallographically observed superconducting grains in the powder are modeled as superconducting current loops of various areas with weak links. Surprising harmonic generation behavior in an arc field, H{sub 1} cos({omega}t), is predicted by the model; the power at high harmonics show sharp dips almost periodic in a superposing dc magnetic field, revealing flux quantization in the prototype loops in the model. Such oscillation of the harmonic power in dc magnetic field P{sub nf}(H{sub dc}), is indeed experimentally observed in powdered YBa{sub 2}Cu{sub 3}O{sub 7}. Other experimental aspects also agree with model predictions. For bulk sintered cylindrical samples, a generalized critical state model is presented. In this model, the nonlinear electrodynamics are due to flux-pinning, somewhat similar to low-temperature type-II superconductors, but with a more generalized critical current densities' dependence on magnetic field -- J{sub c}(H){approximately}H{sub local}{sup -{beta}}, with {beta} being an adjustable parameter. Experiments in ac and dc magnetic fields on a sintered cylindrical rod of YBa{sub 2}Cu{sub 3}O{sub 7} yield unambiguous evidence of independent inter- and intragranular contributions to the complex harmonic permeability {tilde {mu}}{sub n} = {mu}{prime}{sub n} -i{mu}{double prime}{sub n}. Temperature- dependence measurements reveal that, while the intragranular supercurrents disappear at {Tc}{ge}91.2 K, the intergranular supercurrents disappear at T{ge}86.6 K. This result is, to our knowledge, the first clear measurement of the phase-locking temperature of the 3-D matrix formed by YBa{sub 2}Cu{sub 3}O{sub 7} grains, which are in electrical contact with one another through weak links.
Date: August 1, 1991
Creator: Lam, Quan-Chiu H.
Partner: UNT Libraries Government Documents Department

SCALING OF ZIRCONIUM AND ZIRCONIUM ALLOYS. Technical Progress Report No. 1

Description: Zirconium-base binary alloys were prepared using the elements Mo, Fe, Co, Cr, Ni. Cu, Ag, Ta, Nb, Bt. Pa, Os, Ir, Rh, Ru, V, Si, Mn, Sn, Pb, W, Sb, Ge, Al, Ti, Mg, Zn, La, and Ba. Also, Zr-base ternary alloys were prepared using the elements Cu, Ni, Nb, Ta, V, Sn, Sb, and W. The maximum total alloy content of the alloys was five per cent. The Zr used was low-Hf sponge,and the alloys were prepared-by arc melting in an Ar atmosphere. The alloys were tested for scaling resistance at the temperatures 1200, 1500, and 1800 deg F in a multi-specimen furnace for periods up to about 300 hours. Scaling Propensity of the alloys was evaluated on the basis of the weight increase per unit area as compared to that of unalloyed Zr. The binary alloys containing Ni and Cu showed the greatest scaling resistance of all the binary alloys at all temperatures. Ternary alloys showed little or no improvement over the most scaling resistant binary alloys. The number of alloys more resistant than unalloyed Zr was about the same at 1200 and 1500 deg F, but was considerably more at 1800 deg F. In general, there was some change in the order of relative scaling propensity based on the average weight increase per average. unit area per hour at the three temperatures. A logarithmic plot of weight increase versus time for most of the alloys showed a sharp increase in the scaling rate after various periods of time, whereas very few showed a decrease. (auth)
Date: August 1, 1957
Creator: Burka, J.A.; Crouse, C.S. & Swift, R.E.
Partner: UNT Libraries Government Documents Department

Nitric acid leaching of radium and other significant radionuclides from uranium ores and tailings

Description: Nitric acid leaching of representative uranium ores and mill tailings from the western U.S. mining districts removes up to 98% of the /sup 226/Ra and /sup 230/Th, yielding a residue containing 17 to 60 pCi of radium per gram. At best, this is an order of magnitude greater than that in surrounding soils, but about the same level as a standard proposed for building materials in the United Kingdom. Data are also presented on the water penetration and leaching of tailings, the solubility of BaSO/sub 4/, and radon emanation coefficients of ores, tailings, and nitric acid-leached residues.
Date: August 1, 1977
Creator: Ryon, A. D.; Hurst, F. J. & Seeley, F. G.
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

Energy Gaps in the Failed High-Tc Superconductor La1.875Ba0.125CuO4

Description: A central issue in high-T{sub c} superconductivity is the nature of the normal-state gap (pseudogap) in the underdoped regime and its relationship with superconductivity. Despite persistent efforts, theoretical ideas for the pseudogap evolve around fluctuating superconductivity, competing order, and spectral weight suppression due to many-body effects. Recently, although some experiments in the superconducting state indicate a distinction between the superconducting gap and pseudogap, others in the normal state, either by extrapolation from high-temperature data or directly from La{sub 1.875)Ba{sub 0.125}CuO{sub 4} (LBCO-1/8) at low temperature, suggest the ground-state pseudogap is a single gap of d-wave form. Here, we report angle-resolved photoemission data from LBCO-1/8, collected with improved experimental conditions, that reveal the ground-state pseudogap has a pronounced deviation from the simple d-wave form. It contains two distinct components: a d-waev component within an extended region around the node and the other abruptly enhanced close to the antinode, pointing to a dual nature of the pseudogap in this failed high-T{sub c} superconductor that involves a possible precursor-pairing energy scale around the node and another of different but unknown origin near the antinode.
Date: August 11, 2011
Partner: UNT Libraries Government Documents Department

Specific heat evidence for strong coupling in YBa sub 2 Cu sub 3 O sub 7

Description: Specific heat data for YBa{sub 2}Cu{sub 3}O{sub 7} are consistent with {gamma} = 15 {plus minus} 3 mJ/mole{center dot}K{sup 2} and 2{Delta}{sub 0}/k{Tc} = 6.8 {plus minus} 0.6. These results indicate that strong-coupling effects are present but that the coupling is unlikely to be predominantly a conventional electron-phonon interaction. 8 refs., 1 fig.
Date: August 1, 1990
Creator: Gordon, J.E.; Fisher, R.A.; Kamin, S. & Phillips, N.E.
Partner: UNT Libraries Government Documents Department

New materials for thick-film electronics

Description: Certain ceramic compounds such as rare-earth perovskites and transition-metal diborides that exhibit metallic conductivity are readily available or can be easily synthesized. Some are stable in air firing temperatures typical of hybrid-circuit processing. The work described is part of an exploratory program to determine whether these materials can be used as low-cost alternatives to precious metals for thick-film conductor and low-resistor inks or as substitutes for copper in nitrogen-fireable compositions. 8 refs., 13 figs., 3 tabs.
Date: August 1, 1990
Creator: Hoffheins, B.S. & Lauf, R.J.
Partner: UNT Libraries Government Documents Department

Flux motion and dissipation in high-temperature superconductors

Description: The effects on flux motion and dissipation of interlayer coupling of the Cu-O planes along the c-axis are considered for the high- temperature superconductors (HTS). It is argued that for the highly-anisotropic HTS, the weak interlayer coupling plays a dominant role that can be described by incoherent Josephson tunneling between superconducting Cu-O bi- or tri-layers. In YBa{sub 2}Cu{sub 3}O{sub 7}, the layers are strongly coupled, presumably because the conducting Cu-O chains short circuit the Josephson tunneling, so that these effects are weak or missing.
Date: August 1, 1991
Creator: Gray, K.E. & Kim, D.H.
Partner: UNT Libraries Government Documents Department

Magnetic hysteresis and flux creep of YBa sub 2 Cu sub 3 O sub x grown by the melt-powder-melt-growth (MPMG) process

Description: Magnetic hysteresis and flux creep of melt-powder-melt-growth (MPMG) YBa{sub 2}Cu{sub 3}O{sub x} containing nominal 0, 25 and 40 mole% Y{sub 2}BaCuO{sub 5} (21 1) were measured in the temperature range of 5 to 80 K and in magnetic fields up to 5 T. With the introduced fine dispersion of second phase 211 particles, the critical magnetization current density J{sub c} shows a weak field dependence over a wide range of temperature, and the effective pinning energy U{sub eff} is much enhanced. From these results, a functional expression U{sub eff}(J,T) = {minus} U{sub o} G(T) (J {vert bar}J{sub i}){sup n} is obtained, where G(T) = (1 {minus} (T{vert bar}T{sub x}){sup 2}){sup 2} with Tx = 82.5 K near the irreversibility temperature. The observed power-law relationship of U{sub eff}(J, T) clearly demonstrates two of three regimes as predicted by the theory of collective flux creep, namely n = 3/2 and 7/9 for J < J{sub c} and J {much lt} J{sub c}, respectively. In addition, the divergence of U{sub eff} at low current densities also suggests the existence of a vortex-glass state.
Date: August 24, 1992
Creator: Kung, P.J.; McHenry, M.E. (Los Alamos National Lab., NM (United States) Carnegie-Mellon Univ., Pittsburgh, PA (United States). Dept. of Materials Science and Engineering); Maley, M.P.; Willis, J.O. (Los Alamos National Lab., NM (United States)); Murakami, M. & Tanaka, S. (International Superconductivity Technology Center, Tokyo (Japan). Superconductivity Research Lab.)
Partner: UNT Libraries Government Documents Department

Effect of substrate-twin-induced microstructure on transport properties of epitaxial Tl sub 2 Ba sub 2 CaCu sub 2 O sub x films in a magnetic field

Description: The role of substrate-induced microstructure on transport properties in c-axis oriented epitaxial Tl{sub 2}Ba{sub 2}CaCu{sub 2}O{sub x} films grown on LaAlO{sub 3} and SrTiO{sub 3} has been studied. For a magnetic field parallel to the Cu-0 planes, resistivity and the critical current density, J{sub c}, have been measured as a function of angle {theta} between the applied field and the direction of transport current. Resistivity dips and enhancement of the critical current density, J{sub c}, were observed for magnetic fields applied parallel to the substrate twins in LaAlO{sub 3} in high fields ({ge} 2 T). Meanwhile for films on SrTiO{sub 3}, resistivity and J{sub c} were Lorentz-force independent.
Date: August 1, 1992
Creator: Kim, D. H.; Miller, D. J.; Hettinger, J. D.; Gray, K. E.; Landis, P. G.; Sharping, J. E. (Argonne National Lab., IL (United States)) et al.
Partner: UNT Libraries Government Documents Department

High resolution-angle resolved photoemission studies of high temperature superconductors

Description: Recent photoemission studies of Y 123 and Bi 2212 performed with high energy and angular resolution have provided detailed information on the nature of the states near the Fermi level. Measurements of the superconducting gap, band dispersion, and the density of states near the Fermi level in the normal state all support a Fermi liquid description of these materials. 5 refs., 4 figs.
Date: August 1, 1989
Creator: Olson, C.G.; Liu, R.; Lynch, D.W.; Veal, B.W.; Chang, Y.C.; Jiang, P.Z. et al.
Partner: UNT Libraries Government Documents Department

Current transport across grain boundaries in TLBa sub 2 Ca sub 2 Cu sub 3 O sub x thick polycrystalline films and YBa sub 2 Cu sub 3 O sub 7 step edges

Description: We have performed transport measurements in magnetic fields (H//c) up to 10T on a series of YBa{sub 2}Cu{sub 3}O{sub 7} grain boundaries induced by epitaxial growth on a substrate containing a series of step edges. For this material we find an activation energy which goes as U(T){approximately}(1-t). Electric field versus current density curves taken at a constant temperature while varying the magnetic field allows a determination of the magnetic field dependence of the activation energy using the Ambagaokar-Halperin Model. These values agree with results extracted from the resistive transitions. Similar characterizations of a sample of TlBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub x} are also presented. The temperature dependence of the activation energy for this system may be summarized as U(T){approximately}(1-t){sup 2}, suggesting that the grain boundaries behave as SNS juctions.
Date: August 1, 1992
Creator: Hettinger, J.D.; Kim, D.H.; Miller, D.J.; Hu, J.G.; Gray, K.E.; Sharping, J.E. (Argonne National Lab., IL (United States)) et al.
Partner: UNT Libraries Government Documents Department

Design of high-T[sub c] superconducting bolometers for a far infrared imaging array

Description: The design of high-[Tc] superconducting bolometers for use in a far infrared imaging array from wavelengths 30--100[mu]m is discussed. Measurements of the voltage noise in thin films of YBa[sub 2]CU[sub 3]O[sub 7-[var sigma]] on yttria-stabilized zirconia buffer layers on silicon substrates are used to make performance estimates. Useful opportunities exist for imaging and spectroscopy with bolometer arrays made on micro-machined silicon membranes. A circuit on each pixel which performs some signal integration can improve the sensitivity of large two-dimensional arrays of bolometers which use multiplexed readout amplifiers.
Date: August 1, 1992
Creator: Verghese, S.; Richards, P. L. (Lawrence Berkeley Lab., CA (United States)); Fork, D. K. (Xerox Palo Alto Research Center, CA (United States)); Char, K. (Conductus, Inc., Sunnyvale, CA (United States)) & Geballe, T. H. (Stanford Univ., CA (United States). Dept. of Applied Physics)
Partner: UNT Libraries Government Documents Department

Thin-film high temperature superconducting flux transformers coupled to SQUIDS

Description: Using photolithographic patterning and laser deposition, we have constructed a sensitive thin-film SQUID magnetometer from YBa{sub 2}Cu{sub 3}O{sub 7-x} (YBCO). The device consists of a flux transformer, deposited on one substrate, which is coupled to a thin-film YBCO SQUID, made with bi-epitaxial junctions, deposited on another substrate. At 77K, the magnetometer has attained a sensitivity of 1.8pTHz{sup {minus}1/2} at 1 Hz, and has been used to measure the magnetic signal from the human heart.
Date: August 1, 1991
Creator: Wellstood, F.C.; Miklich, A.H.; Kingston, J.J.; Ferrari, M.J.; Clarke, J. (Lawrence Berkeley Lab., CA (United States)); Colclough, M.S. et al.
Partner: UNT Libraries Government Documents Department

Flux pinning by 211 precipitates in melt-processed YBa[sub 2]Cu[sub 3]O[sub x]

Description: Magnetic hysteresis data have been taken at 4.2 and 50 K over a wide range of magnetic fields on melt-textured (MT) and quench-melt-growth-processed (QMGP) YBa[sub 2]Cu[sub 3]O[sub x] samples with 6 vol.% Of Y[sub 2]BaCuO[sub x] (211). In order to obtain accurate comparisons of the intragranular critical current densities, the magnetization measurements were performed on finely powdered samples, the average particle size of which was less than the grain size of the original sample. The QMGP samples exhibited substantially larger hysteresis, indicating significantly enhanced flux pinning at these temperatures. At 4.2 K and 5 T, J[sub c] increase from 4 [times] 10[sup 4] A/cm[sub 2] in the MT sample to 5.5 [times] 10[sup 5] A/cm[sup 2] in the QMGP sample. A possible pinning mechanism related to the observed hysteretic behavior is discussed.
Date: August 1, 1992
Creator: Shi, D.; Sengupta, S.; Smith, M.; Wang, Z.; Biondo, A. C.; Balachandran, U. et al.
Partner: UNT Libraries Government Documents Department

Bicrystal YBCO dc squids with low noise

Description: We have fabricated 12 dc SQUIDs by laser depositing YBa[sub 2]Cu[sub 3]O[sub 7-x] on a SrTiO[sub 3] bicrystal substrate with a misorientation angle of 24*. At 77K all twelve devices had acceptable values of critical current, resistance and voltage modulation produced by an external magnetic field. The white noise energy of one device with an estimated inductance of 41 pH was 1.8 [times] 10[sup [minus]30] JHz[sup [minus]1]. The noise power scaled as 1/f at frequencies below about 1kHz, however, by using a bias current reversal scheme we were able to reduce this noise by two orders of magnitude at 1 Hz, to a value of about 1.5 [times] 10[sup [minus]29] JHz[sup [minus]1]. We made a magnetometer by coupling the SQUID to a flux transformer with a 5-turn input coil. The measured magnetic field gain was 60, and the white noise 36fT Hz[sup [minus][1/2]]. However, the transformer produced relatively large levels of 1/f flux noise, not reduced by the bias reversal scheme, that limited the noise at 1 Hz to 1.7 pT Hz[sup [minus][1/2]]. A single-layer magnetometer with a single-turn pick up loop is briefly described.
Date: August 1, 1992
Creator: Miklich, A.H.; Koelle, D.; Dantsker, E.; Nemeth, D.T.; Kingston, J.J.; Kromann, R.F. et al.
Partner: UNT Libraries Government Documents Department

High energy resolution ARPES measurements of the normal and superconducting states of Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}}

Description: The near-E{sub F} electronic structure and Fermi surface of Bi2212 has been mapped out with ARPES. A key feature of our measured bandstructure is the existence of an extended region of flat CuO{sub 2}-derived bands at E{sub F}. Comparative analysis of this data with that from NdCeCuO and YBCO{sub 7} suggests that many of the anomalous (normal) physical properties of Bi2212 and YBCO{sub 7} (NdCeCuO) may be related to the existence (absence) of such bands at E{sub F}. Superconducting gap anisotropy at least an order of magnitude larger than that of the conventional superconductors has been observed in the a-b plane of Bi2212 in ARPES. For samples with {Tc} of 88K, the gap size reaches a maximum of approximately 20 MeV along the Cu-O bond direction, and a minimum of much smaller or vanishing magnitude 450 away. The experimental data is discussed within the context of various theoretical models. In particular, a detailed comparison with what is expected from a superconductor with a d{sub x2-y2} order parameter is carried out, yielding a consistent picture.
Date: August 1, 1993
Creator: Dessau, D. S.; King, D. M. & Shen, Z. X.
Partner: UNT Libraries Government Documents Department

Suppression of magnetic flux noise in YBa sub 2 Cu sub 3 O sub 7-x flux transformers

Description: We have constructed hybrid magnetometers by coupling a flux transformer of YBa{sub 2}Cu{sub 3}O{sub 7-x} to a Nb-PbIn SQUID. The low frequency (1/f) noise is dominated by fluctuations in the supercurrent circulating in the transformer, driven by vortex motion. The application of a small static magnetic field induces a supercurrent that reversibly suppresses the noise, in quantitative agreement with a model of thermally activated vortex hopping between symmetrical pairs of pinning sites. A persistent current can be used to reduce the low frequency noise power of high-{Tc} flux transformers by at least one order of magnitude. 11 refs., 4 figs., 1 tab.
Date: August 1, 1991
Creator: Ferrari, M.; Wellstood, F.C.; Kingston, J.J.; Johnson, M. & Clarke, J.
Partner: UNT Libraries Government Documents Department