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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

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

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

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

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

The MBE growth and optical quality of BaTiO{sub 3} and SrTiO{sub 3} thin films on MgO

Description: High quality epitaxial BaTiO{sub 3} and SrTiO{sub 3} have been grown on MgO; stabilized at a one unit cell height; and grown to film thicknesses of 0.5--0.7 {mu}m. These relatively thick films remain adherent when thermally cycled between growth temperatures and room temperature, are crack free with high optical quality, and have both in-plane and out-of-plane X-ray rocking curves of 0.3--0.5{degree}. These films have been grown using molecular beam epitaxy (MBE) methods starting with the TiO{sub 2} layer of the perovskite structure. The TiO{sub 2}-layer/MgO interface uniquely satisfies electrostatic requirements for perovskite heteroepitaxy and provides the template structure that leads to the high quality films that are obtained. Wavelength dependence of optical loss has been characterized between 475 nm and 705 nm with loss coefficients < l dB/cm being obtained at the He-Ne wavelength.
Date: May 1, 1994
Creator: McKee, R. A.; Specht, E. D.; Alexander, K. B. & Walker, F. J.
Partner: UNT Libraries Government Documents Department

Defect characterization of electronic conducting pseudo-perovskite systems. Final report

Description: The goal of the program has been to study the interrelationships between electrical conductivity, oxidation-reduction kinetics, defect structure, and composition of n- and p-type binary and ternary transition metal oxides. The stimulus for making these studies was the observation that both conducting n and p type oxides displayed a dependence on oxygen activity that was not predicted by the defect chemistry of their majority defects. The project has focused primarily on the understanding of electronic and ionic conduction in the REBO{sub 3} oxides, where RE is a rare earth ion and B is a transition metal ion. This is being done by studying the interrelationships between the electronic and ionic conductivity, the electronic structure of the B site transition metal ion, and the acceptor concentration. The dependence of these characteristics on the oxygen activity, the temperature, and the defect chemistry of the oxide system is being determined. Theoretical mechanisms and models are being developed from the body of experimental results to provide a predictive tool. The effect of dopants and impurities, processing, electrical and thermal stability as a function of oxygen activity and temperature and their relationship to defect chemistry of the perovskite type oxides have been studied extensively by this research team. As a result of the difference in mobility between electronic and ionic defects, the electrical conductivity changes as the concentration of electronic defects changes. Thus, electrodes or resistors consisting of such oxides are susceptible to instabilities in resistance as they are cycled into temperature regimes where thermodynamic equilibrium may be attained. Many of the new energy conversion systems which use such oxides are encountering difficulties as a result of this instability in resistivity.
Date: December 31, 1994
Creator: Anderson, H. U.; Nasrallah, M.; Sparlin, D. M. & Parris, P. E.
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