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Electron transfer and physical and chemical processes at low temperatures

Description: We summarize some phenomena that occur at temperatures of the order of 15K, and are dominated by quantum mechanical tunneling. Although electron tunneling dominates many conduction processes at low temperatures, we discuss evidence that phenomena like oxidation, as well as the solution of alkali metals in ammonia, can also be dominated by electron tunneling. Both phenomena demonstrate that the chemical potential of a metastable system can equilibrate at low temperatures by electron tunneling. The case of alkali metal clusters covered with ammonia is contrasted to covering the clusters with Xe. In this case changes in the activated conduction are observed which are consistent with the dielectric constant of the rare gas.
Date: October 1, 1995
Creator: Strongin, M.; Xia, B. & Jacobsen, F.M.
Partner: UNT Libraries Government Documents Department

Development of 0.5-5 W, 10K Reverse Brayton Cycle Cryocoolers - Phase II Final Report

Description: Miniature cryocoolers for the 8-30 K range are needed to provide 0.5-5 w of cooling to high sensitivity detectors (for long-wave-length IR, magnetism, mm-wave, X-ray, dark matter, and possibly y-ray detection) while maintaining low mass, ultra-low vibration, and good efficiency. This project presents a new approach to eliminating the problems normally encountered in efforts to build low-vibration, fieldable, miniature cryocoolers. Using the reverse Brayton Cycle (RBC), the approach applies and expands on existing spinner technology previously used only in Nuclear Magnetic Resonance (NMR) probes.
Date: October 15, 2001
Creator: Doty, F. D.; Boman, A.; Arnold, S.; Spitzmesser, J. B.; Jones, D.; McCree, D. et al.
Partner: UNT Libraries Government Documents Department

Thermal oscillation smoothing of DT solid layers for HAPL and NIF scale targets

Description: Deuterium-Tritium (D-T) solid fuel layers must meet stringent roughness specifications for both the ICF and IFE laser fusion programs and native beta-layering alone is unable to provide sufficient solid layer smoothing to meet these specifications at 18.3 K or below. Consequently, several supplemental smoothing options have been proposed to resolve this issue, including a technique called 'Thermal Breathing'. This technique consists of oscillating the temperature of the solid D-T layer about its equilibration temperature for a period of one to several hours. Recently, thermal oscillations have been used to successfully smooth rough solid D{sub 2} in spherical targets. In order to study this particular smoothing technique, we examined the effects of thermal oscillations on equilibrated D-T solid layers, using both ICF and IFE scale layering cells and layer thicknesses. The D-T solid layers that were Subjected to thermal breathing in these studies were equilibrated at temperatures ranging from 16.0 K to 19.25 K, followed by 1.5 to 2 hours of temperature oscillations. During the HAPL scale experiments the amplitude and period of the oscillations were both varied to examine parametric effects of these variables on final layer roughness. In both sets of experiments, once the oscillations completed we allowed the layers to 'relax' at their initial equilibration temperature for another 1 to 2 hours, to observe any 'rebounding' or re-roughening that might occur. The rCF scale experiments were performed using a 2 mm beryllium torus, for which the layer was free from optical distortions that were observed in our IFE scale cell (a 4 mm dia. sapphire sphere-cylinder). Our results showed a temperature dependent smoothing effect ofthe DT solid layer ranging from 20% to 35% over the temperature range of 17.3 K to 19.0 K for the rCF-scale, 2-mm celL The final RMS roughness for layers grown in this 2-mm ...
Date: January 1, 2009
Creator: Sheliak, John D; Geller, Drew A & Hoffer, James K
Partner: UNT Libraries Government Documents Department

Polarization orientation dependence of the far infrared spectra of oriented single crystals of 1,3,5,-trinitro-s-triazine (RDX) using terahertz-time-domain spectroscopy

Description: The far infrared spectra of (100), (010), and (001)-oriented RDX single crystals were measured as the crystal was rotated about the axis perpendicular to the polarization plane of the incident radiation. Absorption measurements were taken at temperatures of both 20 K and 295 K for all rotations using terahertz time-domain spectroscopy. A number of discrete absorptions were found ranging from 10-100 cm(-1) (0.3-3 THz). The absorptions are highly dependent on the orientation of the terahertz polarization with respect to crystallographic axes.
Date: January 1, 2008
Creator: Whitley, Von H; Hooks, Dan E; Ramos, Kyle J; O' Hara, John F; Azad, A K; Taylor, A J et al.
Partner: UNT Libraries Government Documents Department

Onset of bulk pinning in BSCCO single crystals

Description: The long growth defects often found in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8} single crystals effectively weaken the geometrical barrier and lower the field of first flux penetration. This means that the intrinsic (bulk) magnetic properties can be more easily accessed using magnetic measurements. Thus, the onset of strong bulk flux pinning in the sample bulk is determined to lie at T {approximately} 40 K, independent of whether the field strength is above or below the field of the second peak in the magnetization.
Date: December 1, 1996
Creator: Beek, C.J. van der; Indenbom, M.V.; Berseth, V.; Benoit, W. & Li, T.W.
Partner: UNT Libraries Government Documents Department

Surface roughness statistics and temperature step stress effects for D-T solid layers equilibrated inside a 2 mm beryllium torus

Description: Solid D-T layers are equilibrated inside a 2 mm diameter beryllium toroidal cell at temperatures ranging from 19.0 K to 19.6 K, using the beta-layering process. The experimental runs consists of multiple cycles of rapid- or slow-freezing of the initially liquid D-T charge, followed by a lengthy period of beta-layering equilibration, terminated by melting the layer. The temperature was changed in discrete steps at the end of some equilibration cycles in an attempt to simulate actual ICF target conditions. High-precision images of the D-T solid-vapor interface were analyzed to yield the surface roughness {sigma}{sub mns} as a sum of modal contributions. Results show an overage {sigma}{sub mns} of 1.3 {+-} 0.3 {micro}m for layers equilibrated at 19.0 K and show an inverse dependence of {sigma}{sub mns} on equilibration temperature up to 19.525 K. Inducing sudden temperature perturbations lowered {sigma}{sub mns} to 1.0 {+-} 0.05 {micro}m.
Date: December 31, 1998
Creator: Sheliak, J.D. & Hoffer, J.K.
Partner: UNT Libraries Government Documents Department

Magnetoresistance behavior of UNiGe

Description: The authors have measured the temperature dependences of the magnetoresistance of single-crystalline UNiGe for both parallel (i//B//c-axis) and perpendicular configurations (i//a-axis, B//c-axis) in magnetic fields up to 18 T. Both configurations yield similar magnetoresistance behavior, which emphasizes the strong magnetic contribution to the resistivity in all directions. Crossing magnetic-phase boundaries causes anomalies in the magnetoresistance, which allowed a completion of the previously proposed magnetic phase diagram.
Date: May 1, 1995
Creator: Nakotte, H.; Lacerda, A.; Purwanto, A.; Havela, L.; Sechovsky, V.; Prokes, K. et al.
Partner: UNT Libraries Government Documents Department

Specific Heat Measurements of TiB(2)[subscript] and (6)[superscript]LiF from 0.5 to 30K

Description: The specific heats of TiB{sub 2} and {sup 6}LiF have been measured from 0.5 to 30 K as part of a larger project in the construction of a neutron spectrometer. For this application, the measured specific heats were used to extrapolate the specific heats down to 0.1 K with lattice, electronic, and Schottky equations for the respective samples. The resultant specific heat values at 0.1 K for TiB{sub 2} and {sup 6}LiF are 4.08 x 10{sup -4} {+-} 0.27 x 10{sup -4} J/K/mol and 9.19 x 10{sup -9} {+-} 0.15 x 10{sup -9} J/K/mol, respectively.
Date: March 28, 2005
Creator: Lang, B.E.; Marcus, H.D.; Woodfield, B.F.; Burger, A; Utupal, N.R; Lamberti, V. et al.
Partner: UNT Libraries Government Documents Department

YBa{sub 2}Cu{sub 3}O{sub 7-x} 45{degree} [001] tilt grain boundaries induced by controlled low-energy sputtering of MgO substrates: Transport properties and atomic-scale structure

Description: Grain boundaries can act as weak links in the high {Tc} materials. If properly controlled, these grain boundaries can be used in various device applications. We have been able to reproducibly form 45{degree} [001] tilt grain boundary junctions in YBa{sub 2}Cu{sub 3}0{sub 7-x} thin films. The films were grown on MgO substrates using a pre-growth substrate treatment. A low energy broad beam Argon ion source was used to irradiate a select region of (100) MgO substrates. The film on the milled portion of the substrate grows predominantly with a grain orientation rotated 45{degree} about the c-axis with respect to the grain on the unmilled portion. Backscattered electron Kikuchi patterns have been used to confirm that the rotation occurs across the entire milled portion of the substrate. Transport properties of these films are discussed and related to high resolution electron microstructural and microchemical analyses of the grain boundaries. This technique has potential use in device applications as a method for controlled grain boundary engineering.
Date: October 1, 1994
Creator: Vuchic, B.V.; Merkle, K.L.; Funkhouser, J.W.; Bucholz, D.B.; Dean, K.A.; Chang, R.P.H. et al.
Partner: UNT Libraries Government Documents Department

Weak ferromagnetism in [kappa]-(ET)[sub 2]Cu[N(CN)[sub 2]]Cl

Description: We present magnetization measurements at ambient pressure on [kappa]-(ET)[sub 2]Cu[N(CN)[sub 2]]Cl, an organic salt that is superconducting under pressure ([Tc] = 12.8 K at 0.3 kbar). An antiferromagnetic transition near 45 K and, for the first time in this class of materials, a transition near 22 K to a state displaying weak ferromagnetic hysteresis with a saturation moment of 8 x 10 [sup [minus]4][mu][sub B]/formula are observed. This low temperature state is characterized by a sequence of first order magnetization jumps.
Date: October 1, 1992
Creator: Welp, U.; Kwok, W.K.; Crabtree, G.W.; Carlson, K.D.; Wang, H.H.; Geiser, U. et al.
Partner: UNT Libraries Government Documents Department

Influence of Proton Irradiation on Angular Dependence of Second Generation (2G)HTS

Description: In the Facility for Rare Isotope Beams (FRIB) the quadrupoles in the fragment separator are exposed to very high radiation and heat loads. High Temperature Superconductors (HTS) are a good candidate for these magnets because they can be used at {approx}30-50 K and tolerate higher heat generation than Nb-Ti magnets. Radiation damage studies of HTS wires are crucial to ensure that they will survive in a high radiation environment. HTS wires from two vendors were studied. Samples of 2G HTS wires from SuperPower and American Superconductor (ASC) were irradiated with a 42 {mu}A, 142 MeV proton beam from the Brookhaven Linac Isotope Producer (BLIP). The angular dependence of the critical current was measured in magnetic fields at 77K.
Date: May 1, 2011
Creator: Shiroyanagi, Y.; Greene, G.; Gupta, R. & Sampson, W.
Partner: UNT Libraries Government Documents Department

Local and average structures of the spin-glass pyrochlore Y2Mo2O7 from neutron diffraction and neutron pair distribution function analysis

Description: The observation of canonical spin-glass behavior in the pyrochlore oxide Y{sub 2}Mo{sub 2}O{sub 7} has been a subject of considerable interest as the original structural studies were interpreted in terms of a well-ordered crystallographic model. It is widely held that the stabilization of the spin-glass state requires some level of positional disorder along with frustration. Recent reports from local probe measurements, extended x-ray-absorption fine structure (EXAFS) and {sup 89}Y NMR, have been interpreted in terms of disorder involving the Mo-Mo distances (EXAFS) and multiple Y sites (NMR). This work reports results from temperature-dependent (15--300 K) neutron diffraction (ND) and neutron pair distribution function studies which can provide from the same data set information on both the average and local structures. The principal findings are that: (1) there is no crystallographic phase transition over the temperature region studied within the resolution of the ND data; (2) the diffraction data are well fitted using a fully ordered model but with large and anisotropic displacement parameters for three of the four atomic sites; (3) the pairwise real-space correlation function G(r) shows clear evidence that the principal source of disorder is associated with the Y-O1 atom pairs rather than the Mo-Mo pairs, in disagreement with the interpretation of the EXAFS results; (4) fits to the G(r) improve significantly when anisotropic displacements for all sites are included; (5) inclusion of a split-site position parameter for O1 improves, slightly, both the G(r) fits and the Rietveld fits to the ND data; and (6) for all models the fits become worse as the temperature decreases and as the fitting range decreases. These results are qualitatively consistent with the {sup 89}Y NMR observations and perhaps recent muon-spin-relaxation studies. The issue of static versus dynamic disorder is not resolved, definitively. An estimate of the distribution of exchange constants due ...
Date: January 1, 2009
Creator: Proffen, Thomas Ernst; Kim, Hyunjeong; Greedan, John; Gout, Delphine; Lozano - Gorrin, A D; Derahkshan, Shahab et al.
Partner: UNT Libraries Government Documents Department

Selected studies of magnetism at high pressure

Description: Most previous studies of magnetism in various compounds under extreme conditions have been conducted over a wide pressure range at room temperature or over a wide range of cryogenic temperatures at pressures below 20 GPa (200 kbar). We present some of the most recent studies of magnetism over an extended range of temperatures and pressures far beyond 20 GPa, i.e., in regions of pressure-temperature (P-T) where magnetism has been largely unexplored. Recent techniques have permitted investigations of magnetism in selected 3d transition metal compounds in regions of P-T where physical properties may be drastically modified; related effects have often been seen in selected doping studies at ambient pressures.
Date: September 1, 1995
Creator: Hearne, G.R.; Pasternak, M.P. & Taylor, R.D.
Partner: UNT Libraries Government Documents Department

Cryotribology: Development of cryotribological theories and application to cryogenic devices

Description: High-performance superconducting solenoids are susceptible to premature quenches, or superconducting to normal state transitions, due to abrupt conductor movements within the winding. Abrupt motions involving 5{approximately}10{mu}m conductor displacements dissipate sufficient energy to trigger a quench. Sliding and mechanical behaviors of materials at cryogenic temperatures have been experimentally examined. After accounting for changes in the sliding materials' low-temperature strength properties, we have found that the adhesion theory of friction and wear remains applicable at cryogenic temperatures. The adhesion friction theory suggests two methods for controlling unsteady sliding motions. The first involves the selection of sliding materials whose friction coefficients increase with increasing sliding speed. A number of material pairs have been examined for positive friction-velocity characteristics. This materials-based approach to frictional stabilization does not seem a viable option at 4.2 K. The second altemative is to preprogram the force conditions within high-risk regions of the winding to regulate the occurrence of unsteady sliding motions. Structural models are proposed to account for unsteady conductor motions on a variety of dimensional scales. The models are used to design a small superconducting solenoid. Performance of this solenoid suggests that force-based motion control is a potentially viable design approach for achieving successful dry-wound magnets.
Date: September 15, 1992
Creator: Iwasa, Y.; Michael, P. (Massachusetts Inst. of Tech., Cambridge, MA (United States)) & Rabinowicz, E. (Massachusetts Inst. of Tech., Cambridge, MA (United States) Massachusetts Inst. of Tech., Cambridge, MA (United States). Francis Bitter National Magnet Lab.)
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

Polarized neutron reflection study of the unidirectional magnetic anisotropy of permalloy on Ni[sub 0. 5]Co[sub 0. 5]O

Description: Couples of ferromagnetic permalloy/antiferromagnetic metal oxide exhibit unidirectional magnetic anisotropy along a magnetic field applied during cooling. Polarized neutron reflection has been used to check if the effect is due to a bias of the antiferromagnetic configuration in the oxide, where the layer immediately adjacent to the permalloy is polarized parallel to the easy direction of magnetization. The measurements were made on a sample consisting of permalloy/500[Angstrom] Ni[sub 0.05]Co[sub 0.5]O. The polarized neutron reflectivities taken at the four corners of the magnetic hysteresis loop at 20K. A faint magnetic signal consistent with the unidirectional bias of the F/AF structure is also observed. This behavior is discussed in the light of the current theories.
Date: August 1, 1992
Creator: Felcher, G.P.; Huang, Y.Y. (Argonne National Lab., IL (United States)); Carey, M. & Berkowitz, A. (California Univ., San Diego, La Jolla, CA (United States). Center for Magnetic Recording Research)
Partner: UNT Libraries Government Documents Department

Temperature and ion-mass dependence of amorphization dose for ion beam irradiated zircon (ZrSiO[sub 4])

Description: The temperature dependence of amorphization dose for zircon under 1.5 MeV Kr ion irradiation has been investigated using the ANL HVEM-Tandem Facility. Three regimes were observed in the amorphization dose-temperature curve. In the first regime (15 to 300 K), the critical amorphization dose increased from 3.06 to 4.5 ions/nm[sup 2]. In the second regime (300 to 473 K), there is little change in the amorphizationdose. In the third regime (> 473 K), the amorphization dose increased exponentially to 8.3 ions/nm[sup 2] at 913 K. This temperature dependence of amorphization dose can be described by two processes with different activation energies (0.018 and 0.31 eV respectively) which are attributed to close pair recombination in the cascades at low temperatures and radiation-enhanced epitaxial recrystallization at higher temperatures. The upper temperature limit for amorphization of zircon is estimated to be 1100 K. The ion-mass dependence of the amorphization dose (in dpa) has also been discussed in terms of the energy to recoils based on data obtained from He, Ne, Ar, Kr, Xe irradiations and a [sup 238]Pu-doped sample.
Date: December 1, 1992
Creator: Wang, L.M.; Ewing, R.C. (New Mexico Univ., Albuquerque, NM (United States). Dept. of Geology); Weber, W.J. (Pacific Northwest Lab., Richland, WA (United States)) & Eby, R.K. (Toronto Univ., ON (Canada). Dept. of Geological Sciences)
Partner: UNT Libraries Government Documents Department

Transport and magnetization critical current densities in TlBa sub 2 Ca sub 2 Cu sub 3 O sub x tapes

Description: The powder in tube process was used to produce silver-sheathed tapes of TlBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub 8+x} (Tl-1223). The powder was produced by thalliating a precursor powder mixture to produce the Tl-2223 phase and then beating to drive off excess Tl and reach the Tl-1223 stoichiometry. The tapes were rolled and pressed, each step followed with a 3 h sintering. The 200 {mu}m thick tapes show little sign of texturing; however, the critical current shows a small ({approximately}50%) dependence on the direction of the applied magnetic field. Both transport and magnetization measurents indicate relatively strong pinning at high temperatures. The 75 K self field critical current density is 62 MA/m{sup 2}. Transport measurements reveal the presence of weak links at all temperatures, but with a relatively weak field dependence above {approx}0.1T.
Date: January 1, 1992
Creator: Willis, J.O.; Maley, M.P.; Kung, P.J.; Coulter, J.Y.; Peterson, D.E.; Wahlbeck, P.G. et al.
Partner: UNT Libraries Government Documents Department

Apical Cu-O bond in YBa[sub 2]Cu[sub 3]O[sub 7-[delta]] superconductors by XAFS

Description: Careful Cu K-edge X-ray absorption fine structure (XAFS) measurements on oriented powders of YBa[sub 2]CU[sub 3]O[sub 7[minus]] were made as a function of temperature, of [delta], [Tc] method of preparation and grain size. The XAFS data were analyzed for the Cu(1) - O(4) apical bond distance distribution. AU samples indicated evidence for a double-peak in the distribution with a splitting of 0.1[Angstrom], with the clearest case for the [delta] [approx] 0 samples occurring for the [Tc] = 89K 20[mu]m grain samples and the least clear one for the 5[mu]m grain [Tc] = 93K sample. There is no significant temperature dependence of the splitting. Because the XAFS evidence for the splitting depends on a small signal, it is prudent to verify its existence by other independent measurements.
Date: January 1, 1993
Creator: Stern, E.A.; Qian, M. (Washington Univ., Seattle, WA (United States). Dept. of Physics); Yacoby, Y. (Hebrew Univ., Jerusalem (Israel). Racah Inst. of Physics); Heald, S.M. (Brookhaven National Lab., Upton, NY (United States)) & Maeda, H. (Okayama Univ. (Japan). Dept. of Chemistry)
Partner: UNT Libraries Government Documents Department