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Photoferroelectric effects in BaTiO{sub 3} crystals

Description: Intrinsic photo-induced changes in the hysteresis response in BaTiO{sub 3} single crystals have been observed by the application of ultra-violet (UV) light in combination with an applied bias. This UV/bias treatment suppresses the amount of switchable polarization by over 90%. The photoferroelectric effects observed are reproducible, reversible, and stable. Electron paramagnetic resonance measurements were also made to characterize the charge trapping associated with these hysteresis effects. The EPR results suggest that isolated Fe impurity sites are charge trapping centers in these crystals, and follow trends similar to those measured electrically. These results also re-enforce the idea that charge trapping centers can lock certain domain configurations.
Date: July 1, 1994
Creator: Warren, W. L.; Dimos, D. & Smyth, D. M.
Partner: UNT Libraries Government Documents Department

In-situ TEM crystallization of sol-gel derived BaTiO{sub 3} thin films

Description: Plan view and cross sectional view samples of pyrolyzed Ba-Ti methoxypropoxide films on (100)Si were prepared in the amorphous form. In-situ hot stage experiments were conducted in a Philips CM30 to follow the crystallization process. With time at 750C, large grains nucleated mostly at the BaTiO{sub 3} surface; the regions near the Si did not crystallize.
Date: June 1, 1994
Creator: Mecartney, M. L.; Gust, M. C. & Evans, N. D.
Partner: UNT Libraries Government Documents Department

Preparation and Properties of Refractory Sulfides

Description: From abstract: "The methods of preparation of the sulfides of cerium, thorium, uranium, and barium are discussed. Where necessary, the apparatus are described. Procedures for the handling and grinding of the sulfides and hydrides which are used for their preparation are given...The physical, mechanical, chemical, and thermodynamic properties are given for various sulfide refractories which have been developed at the Berkeley, California chemical laboratories."
Date: 1948
Creator: Brewer, Leo; Bromley, Leroy; Gilles, Paul & Lofgren, Norman
Partner: UNT Libraries Government Documents Department

Structure, stoichiometry and stability in magnetoplumbite and {beta}-alumina type ceramics. Technical progress report

Description: Work has continued on the computer simulation of atomic scale defect complexes in polyaluminates with the magnetoplumbate and {beta}-alumina structures. More stringent criteria have been developed for determining the viability of interatomic potential models for these complex structured materials, details of the cation coordination having been found to be important. It appears that the energetics of point defect complex formation is the reason why Ba hexa-aluminate does not adopt the magnetoplumbate structure, in contrast to Sr hexa-aluminate, which does.
Date: December 1, 1992
Creator: Cormack, A. N. & Park, J. G.
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

Mechanism(s) for the suppression of the switchable polarization in PZT and BaTiO{sub 3}

Description: Switchable polarization can be significantly suppressed in ferroelectric (FE) materials by optical, thermal, and electrical processes. The thermal process can occur by either annealing the FE in a reducing environment or by heating it in air to 100 C while impressing a bias near the switching threshold. The optical process occurs while biasing the FE near the switching threshold and illuminating with bandgap light. And the electrical suppression effect occurs by subjecting the FE to repeated polarization reversals. Using electron paramagnetic resonance, polarization-voltage measurements, and charge injection scenarios, the authors have been able to elucidate both electronic and ionic trapping effects that lead to a suppression in the amount of switchable polarization in FE materials. The relative roles of electronic and ionic effects in the same material can depend on the stress condition. For instance, in oxidized BaTiO3 crystals, optical and thermal suppressions occur by electronic domain pinning; electrical fatigue in the BaTiO{sub 3} crystals also appears to involve electronic charge trapping, however, it is suggested that these electronic traps are further stabilized by nearby ionic defects. In sol-gel PZT thin films with either Pt, RuO{sub 2}, or La-Sr-Co-O electrodes it appears that the polarization suppression induced by electrical fatigue, a temperature/bias combination, or a light/bias combination are all primarily due to the trapping of electronic charge carriers to first order.
Date: April 1995
Creator: Warren, W. L.; Dimos, D.; Tuttle, B. A.; Pike, G. E.; Raymond, M. V.; Nasby, R. D. et al.
Partner: UNT Libraries Government Documents Department

Effect of flux flow on current distribution and heat generation in composite superconductors during a thermal disturbance

Description: An analytical investigation of current distribution and heat generation rate in composite superconductors, incorporating the effects of flux flow during disturbances, is carried out. Equations describing current density in the superconductor and the heat generation rate per unit volume of the composite conductor in the current sharing regime are derived. The results show that when the superconductor is in the flux-flow state, the current density and the heat generation rate depend only on a dimensionless parameter {phi}{sub f} = ({rho}{sub n}/{rho}{sub st})[H/H{sub c2(0)}](l-{lambda})/{lambda}. When the thermal disturbance is relatively small and {phi}{sub f} >> 1, the current density in the superconductor remains at the critical current density with all the excess transferred to the stabilizer and the heat generation rate is equal to that usually employed for low temperature superconductors. When the thermal disturbance is large and {phi}{sub f} >> 1, the current density in the superconductor can be greater than the critical current density and the heat generation rate equals the critical generation rate, independent of whether the superconductor is in the flux-flow state or the normal state. For moderate and large thermal disturbances and {phi}{sub f} =1, which is applicable to high-temperature superconductors because of high H{sub c2}(0), the heat generation rate is q = q{sub c}/2 if the Superconductor is in the flux-flow state and q = q{sub c} if the superconductor is in the normal state. An argument is provided to indicate when and Linder what circumstances will all the excess current be transferred to the stabilizer while the current in the superconductor remains at the critical current during a thermal disturbance. The differences between hi-h- and low-temperature superconductors and its implication for cryogenic stability are discussed. Data on critical currents and thermal runaway of sintered YBa{sub 2}Cu{sub 3}O{sub 7} with unoriented grains are presented.
Date: September 1, 1994
Creator: Cha, Y.S.; Hull, J.R. & Askew, T.R.
Partner: UNT Libraries Government Documents Department

Fabrication and modification of metal nanocluster composites using ion and laser beams

Description: Metal nanocluster composites have attractive properties for applications in nonlinear optics. However, traditional fabrication techniques -- using melt-glass substrates -- are severely constrained by equilibrium thermodynamics and kinetics. This paper describes the fabrication of metal nanoclusters in both crystalline and glassy hosts by ion implantation and pulsed laser deposition. The size and size distribution of the metal nanoclusters can be modified by controlling substrate temperature during implantation, by subsequent thermal annealing, or by laser irradiation. The authors have characterized the optical response of the composites by absorption and third-order nonlinear-optical spectroscopies; electron and scanning-probe microscopies have been used to benchmark the physical characteristics of the composites. The outlook for controlling the structure and nonlinear optical response properties of these nanophase materials appears increasingly promising.
Date: December 1, 1994
Creator: Haglund, R. F., Jr.; Osborne, D. H., Jr.; Magruder, R. H., III; White, C. W.; Zuhr, R. A.; Townsend, P. D. et al.
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

Heteroepitaxy of perovskite ferroelectrics on silicon: a path to silicon-integrated ferroelectrics

Description: We have studied the thin-film heteroepitaxy between simple perovskite oxides and Si as it is accomplished with molecular beam epitaxy growth. Interface chemistry and electrostatics play critical roles in determining whether a commensurate interface develops. Submonolayer silicides are the required precursors for the transition to commensurate epitaxial oxide structures in these systems. This approach is illustrated with BaTiO{sub 3}; high crystalline perfection is achieved and resistivities as high as 10{sup 13} ohm-cm can be developed for BaTiO{sub 3} MOS capacitors on (001) Si. The epitaxy is (100)BaTiO{sub 3}//(001)Si with [0001]BaTiO{sub 3} aligned in plane with the polar axis along the [110] of Si. This epitaxy is dominated by a strong propensity for Ba to order along the [110] of Si during the submonolayer silicide formation. This commensurate heteroepitaxial growth leads to a truly monolithic crystal and attendant properties. 9 figs, 23 refs.
Date: October 1, 1996
Creator: McKee, R.A.; Walker, F.J. & Chisholm, M.F.
Partner: UNT Libraries Government Documents Department

The interaction of vortices with twin boundaries

Description: Twin boundaries provide a convenient defect for investigating vortex dynamics in superconductors. They occur naturally in YBa{sub 2}Cu{sub 3}O{sub 7}, with well defined planar geometries. They are a strong pinning defect, and at the temperature of the melting transition they are the dominant pinning sites in clean crystals. Twin boundaries are easily seen in polarized light, making their location and number directly observable without special equipment. Finally, they present a highly anisotropic pinning potential to the vortices, creating the possibility of interesting new behavior. In this paper, we describe experiments using twin boundaries to probe two effects in vortex dynamics: a new peak effect in the critical current occurring in the solid state just below the melting transition, and anisotropic pinning by planar defects.
Date: May 1, 1995
Creator: Crabtree, G. W.; Kwok, W. K.; Welp, U.; Fendrich, J.; van der Beek, C. J.; Vlasko-Vlasov, V. K. 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

Thin-film perovskites-ferroelectric materials for integrated optics

Description: Optical guided wave (OGW) devices, based on LiNbO{sub 3} or GaAs. are commercially available products with established markets and applications. While LiNbO{sub 3} presently dominates the commercial applications, there are several drivers for the development of improved electro-optic (EO) materials. If the appropriate crystal quality could be obtained for thin-film BaTiO{sub 3} supported on MgO for example, or for an integrated BaTiO{sub 3}/Mg0 structure on silicon or GaAs, then the optimum OGW device structure might be realized. We report on our results for the growth of optical quality, epitaxial BaTiO{sub 3} and SrTiO{sub 3} on single-crystal MgO substrates using source shuttering molecular beam epitaxy (MBE) techniques. We also discuss how these materials can be integrated onto silicon. Our MBE studies show that, for this important class of perovskite oxides, heteroepitaxy between the perovskites and alkaline earth oxides is dominated by interfacial electrostatics at the first atomic layers. We have been able to demonstrate that a layer-by-layer energy minimization associated with interfacial electrostatics leads to the growth of high quality thin films of these materials. We have fabricated waveguides from these materials, and the optical clarity and loss coefficients have been characterized and found to be comparable to in-diffused waveguide structures typically represented by Ti drifted LiNbO{sub 3}.
Date: December 31, 1995
Creator: Walker, F. J. & McKee, R. A.
Partner: UNT Libraries Government Documents Department

Fabrication of High Current YBa<sub>2</sub>Cu<sub>3</sub>O(sub>7-y</sub>) Coated Conductors Using Rolling-Assisted Biaxially Textured Substrates

Description: High critical current YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-y</sub> (referred to as YBCO) coated conductors were fabricated with a layer sequence of YBCO/YSZ/CeO<sub>2</sub>/Ni. The cube (100) texture in the starting Ni substrates was obtained by cold rolling followed by recrystallization. A thin CeO<sub>2</sub> (Cerium Oxide) layer with a thickness of 100-200 Å was grown epitaxially on the biaxially textured-Ni substrates using an e-beam evaporation technique. This was followed by the growth of a thick (<= 0.77 µm) YSZ (Yttria Stabilized Zirconia) layer using either e-beam evaporation or rf magnetron sputtering. The e-beam CeO<sub>2</sub> film had a dense microstructure. The microstructure of the e-beam YSZ film was porous whereas the sputtered YSZ film was dense. The YBCO films were grown by pulsed laser deposition on both e-beam and sputtered YSZ layers. A transport critical current density of ~ 1 x l0<sup>6</sup> A/cm<sup>2</sup> at 77 K was obtained for ~ 0.8 µm thick YBCO Rims on both YSZ surfaces in zero field. To demonstrate the quality and compatibility of the e-beam CeO<sub>2</sub> layers; YBCO films were also grown on CeO<sub>2</sub>-buffered YSZ (100) single crystal substrates using e-beam co-evaporated Y-BaF<sub>2</sub>-Cu precursors followed by a post-annealing process. A transport critical current density of over 1 x lO<sup>6</sup>A/cm<sup>2</sup> at 77 K was obtained on a ~ 0.3 µm thick YBCO film in zero field.
Date: June 1, 1999
Creator: Christen, D.K.; Feenstra, R.; Kroeger, D.M.; Lee, D.F.; List, F.A.; Martin, P.M. et al.
Partner: UNT Libraries Government Documents Department

Demonstration of High Current Density YBCO Coated Conductors on RE<sub>2</sub>O<sub>3</sub>-Buffered Ni Substrates with Two New Alternative Architectures

Description: In continuation of our effort to develop single buffer layer architectures for YBCO (YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-<font face="symbol">g</font>) coated tape conductors, we have studied RE<sub>2</sub>O<sub>3</sub> (RE = Y, and rare earths) as candidate materials. Three types of crystal structures including the preferred cubic phase are known for the rare earth oxides. High quality simple cubic RE<sub>2</sub>O<sub>3</sub> buffer layers were grown epitaxiahy on {100}<001> textured Ni substrates using both reactive evaporation and sol-gel processing. Detailed X-ray studies have shown that the Y<sub>2</sub>O<sub>3</sub>, Eu<sub>2</sub>O<sub>3</sub>, Gd<sub>2</sub>O<sub>3</sub>, and Yb<sub>2</sub>O<sub>3</sub> were grown with a single epitaxial orientation. SEM micrographs indicated that both e-beam and sol-gel grown films were dense, continuous and crack free. High J<sub>c</sub> YBCO films were grown on RE<sub>2</sub>O<sub>3</sub>-buffered Ni substrates with sputtered cap layers. Two new alternative buffer layer architectures were developed. A high J<sub>c</sub> of 1.8 MA/cm<sup>2</sup> at 77 K and self-field was obtained on YBCO films with a layer sequence of YBCO (pulsed laser deposition)/Yb<sub>2</sub>O<sub>3</sub> (sputtered)/Y<sub>2</sub>O<sub>3</sub> (e-beam)/Ni. Also, a high J<sub>c</sub> of over 1 MA/cm<sup>2</sup> at 77 K and self-field was obtained on YBCO films with a layer sequence of YBCO (ex-situ BaF<sub>2</sub> process)/CeO<sub>2</sub> (sputtered)YSZ sputtered)/RE<sub>2</sub>O<sub>3</sub> (sol-gel or e-beam)Ni. The performance of sol-gel grown buffers approached the quality of e-beam grown buffers.
Date: July 12, 1999
Creator: Beach, D. B.; Chirayil, T. G.; Christen, D. K.; Cui, X.; Feenstra, R.; Goyal, A. et al.
Partner: UNT Libraries Government Documents Department

Proximity detector circuits: an attractive alternative to tunnel diode oscillators for contactless measurements in pulsed magnetic fields

Description: A new radio frequency oscillator circuit based on a proximity detector integrated circuit is described as an alternative for the traditional tunnel diode oscillator used for pulsed magnetic field measurements at low temperatures. The new circuit has been successfully applied to measure the superconducting upper critical field in Ba{sub 0.55}K{sub 0.45}Fe{sub 2}As{sub 2} single crystfl.ls up to 60 T. The new circuit design avoids many of the problems associated with tunnel diode circuits while keeping the advantages of contact less measurements in pulsed magnets.
Date: January 1, 2009
Creator: Altarawneh, Moaz M & Mielke, Charles H
Partner: UNT Libraries Government Documents Department

Grain boundaries in complex oxides. Technical progress report

Description: A quantitative comparison was achieved between space charge theory and segregation at grain boundaries in the model system TiO{sub 2}. The ionic space charge can be titrated from negative to positive potential. A space charge model was developed that includes the lattice defect chemistry. Defect formation energies in the Frenkel pair for TiO{sub 2} were studied. Grain boundary enthalpy was measured during coarsening. It was shown that it is the barium vacancy that forms near surfaces in donor-doped BaTiO{sub 3} electroceramics during oxidative-cooling.
Date: May 1, 1993
Creator: Chiang, Yet-Ming
Partner: UNT Libraries Government Documents Department

Columnar defect pinning in untwinned single crystal YBa{sub 2}Cu{sub 2}O{sub 7}

Description: The pinning by columnar defects produced by 1.0 GeV U ion bombardment along the c-axis of an untwinned YBa{sub 2}Cu{sub 3}O{sub 7} single crystal is characterized. The accommodation angle is nearly 90{degrees} at low field, the resistivity anisotropy is reversed, and the onset of pinning occurs well into the fluctuation regime. The anisotropy and strength of the pinning are compared to that of twin boundaries.
Date: May 1, 1994
Creator: Crabtree, G. W.; Fendrich, J.; Kwok, W. K. & Glagola, B. G.
Partner: UNT Libraries Government Documents Department

A review of metalorganic chemical vapor deposition of high-temperature superconducting thin films

Description: A status report is given on the metalorganic chemical vapor deposition (MOCVD) of high-temperature superconducting thin films. The advantages of MOCVD processing manifest themselves in the quality of the films produced, and in the economy of the process. Metalorganic precursor requirements, deposition parameters and film properties are discussed. Also difficulties have been identified in making MOCVD a manufacturing technology. To solve these problems, future research directions are proposed.
Date: December 31, 1989
Creator: Erbil, A.; Zhang, K.; Kwak, B. S. & Boyd, E. P.
Partner: UNT Libraries Government Documents Department

Density of states and tunneling characteristics of layered superconductors

Description: We have studied the structure of the density-of-states (DOS) curves and tunneling characteristics of layered superconductors with two distinct layers in a unit cell. In general, the peaks of the DOS curves do not correspond to energy gaps of each layer, but depend on the gaps and the interlayer hopping strengths in a complex manner. This makes the interpretation of tunneling data of layered superconductors much less straightforward than isotropic superconductors. Our simulated tunneling characteristics bear certain resemblance to experimental results.
Date: April 1, 1993
Creator: Liu, S. H. & Klemm, R. A.
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