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Equilibrium and non-equilibrium metal-ceramic interfaces

Description: Metal-ceramic interfaces in thermodynamic equilibrium (Au/ZrO{sub 2}) and non-equilibrium (Au/MgO) have been studied by TEM and HREM. In the Au/ZrO{sub 2} system, ZrO{sub 2} precipitates formed by internal oxidation of a 7%Zr-Au alloy show a cubic ZrO{sub 2} phase. It appears that formation of the cubic ZrO{sub 2} is facilitated by alignment with the Au matrix. Most of the ZrO{sub 2} precipitates have a perfect cube-on-cube orientation relationship with the Au matrix. The large number of interfacial steps observed in a short-time annealing experiment indicate that the precipitates are formed by the ledge growth mechanism. The lowest interfacial energy is indicated by the dominance of closed-packed (111) Au/ZrO{sub 2} interfaces. In the Au/MgO system, composite films with small MgO smoke particles embedded in a Au matrix were prepared by a thin film technique. HREM observations show that most of the Au/MgO interfaces have a strong tendency to maintain a dense lattice structure across the interfaces irrespective of whether the interfaces are incoherent of semi-coherent. This indicates that there may be relatively strong bond between MgO and Au.
Date: January 1, 1991
Creator: Gao, Y. & Merkle, K.L.
Partner: UNT Libraries Government Documents Department

TEM characterization of Al/Al{sub 2}O{sub 3} composite fabricated by reactive metal infiltration

Description: The microstructure of Al/{alpha}-Al{sub 2}0{sub 3} composites made by infiltrating Al into dense mullite preforms has been characterized using transmission electron microscopy. Observations revealed that the formation of the Al/Al{sub 2}0{sub 3} composites involves three stages. Initially, Al infiltrates into a dense mullite preform through grain boundary diffusion, and reacts with mullite at grain boundaries to form a partial reaction zone. Then, a complete reaction takes place in the reaction region between the partial reaction zone and the full reaction zone to convert the dense mullite preform to a composite of {alpha}-Al{sub 2}0{sub 3} (matrix) and an Al-Si phase (thin channels). Finally, the reduced Si from the reaction diffuses out of the Al/Al{sub 2}0{sub 3} composite through the metal channels, whereas Al from the molten Al pool is continuously drawn to the reaction region until the mullite preform is consumed or the sample is removed from the molten Al pool. Based on the observed microstructure, infiltration mechanisms have been discussed, and a growth model of the composites is proposed in which the process involves repeated nucleation of Al{sub 2}0{sub 3} grains and grain growth.
Date: December 31, 1994
Creator: Gao, Y.; Jia, J.; Loehman, R.E. & Ewsuk, K.G.
Partner: UNT Libraries Government Documents Department

Strain, structure and electronic states in MBE grown (Nb,Ti)O{sub 2} mixed rutile

Description: We have grown and characterized epitaxial Nb{sub x}Ti{sub 1-x}O{sub 2} on TiO{sub 2}(11) and (100) for the purpose of investigating the role of chemically-inequivalent metal atoms on the thermal and photocatalytic properties of TiO{sub 2}. Our goal is to introduce, in a highly controlled fashion, a Group VA transition metal into the lattice of a Group IVA transition metal oxide without altering the crystallographic structure. So doing would alter the electronic structure in interesting and potentially useful ways by the addition of one valence electron per substituted metal atom. However, strain builds in the film as more Nb is added at a rate which depends on the crystallographic orientation of the growth direction. Films grown along (110) can accommodate Nb mole fractions as high as {approximately}0.3 without forming misfit, dislocations, whereas those grown along (100) are limited to {approximately}10 at. % Nb. Nb-O bond lengths in Nb{sub x}Ti{sub 1-x}O{sub 2} are the same as Ti-O bond lengths in pure TiO{sub 2} prior to the onset of dislocation formation. The extra 4d valence electron per Nb atom forms a nonbonding band which is degenerate with bonding states in the valence band region.
Date: October 1996
Creator: Chambers, S. A.; Gao, Y. & Thevuthasan, S.
Partner: UNT Libraries Government Documents Department

Highly-Efficient Laser with Self-Aligned Waveguide and Current Confinement by Selective Oxidation

Description: We report highly efficient, low-threshold-current edge-emitting lasers where both the optical waveguide and lateral current confinement are achieved by lateral selective oxidation of AlGaAs. External differential quantum efficiency in excess of 95% and 40% wall-plug efficiency are demonstrated in 600 {micro}m-long devices without facet coatings. Shorter, 300-{micro}m-long, uncoated devices have <6 mA threshold currents. This high-performance is a combined result of placement of the oxide layers so as to achieve the minimum optical mode volume and bi-parabolic grading of the Al{sub x}Ga{sub 1{minus}x}As heteroepitaxy for minimum height/potential barriers, less than 15 meV, created by the wide-energy-gap layers required for selective wet oxidation. Since the initial development of wet AlGaAs oxidation methods, a number of oxidized edge-emitting laser concepts have been tried. The most successful of these have used lateral selective oxidation of AlGaAs layers between 100 and 300 nm thickness. These layers have been used as current restricting apertures or for both current restriction and lateral waveguiding. Use of an oxide layer above and below the laser active region offers the ability to create a self-aligned waveguide with current apertures on both sides of the pn-junction in a process requiring only one epitaxial growth step. Previous use apertures for these dual purposes resulted multi-moded lasers with reduced efficiency and elevated threshold current density due to non-ideal formation of the waveguide and possibly excess stress caused by the thick (300 nm) oxide layer.
Date: July 27, 1999
Creator: Allerman, A.; Blum, O.; Gao, Y. & Vawter, G.A.
Partner: UNT Libraries Government Documents Department

Atomic structure of heterophase interfaces

Description: High-resolution electron microscopy (HREM) has been used to study internal interfaces between dissimilar materials, notably ceramic/metal interfaces. Structures observed for system with small and large misfits are compared in metal/metal, metal/ceramic, and ceramic/ceramic boundaries. The interfaces were prepared by a variety of techniques, including internal reduction, internal oxidation, and epitaxial growth by MOCVD and special thin-film techniques. While interfaces produced by internal oxidation and reduction in fcc systems typically form boundaries on (111) planes, non-equilibrium boundaries have also been generated using special thin film techniques. All boundaries can be characterized by their tendency to form coherent structures. While it appears that the amount of misfit and the bond strength primarily determine the degree of coherency, kinetic factors and substrate defects also seem to play an important role in determining the local defect structure at the boundary and the type of misfit localization. 24 refs., 9 figs., 1 tab.
Date: January 1, 1991
Creator: Merkle, K.L.; Buckett, M.I. & Gao, Y.
Partner: UNT Libraries Government Documents Department

Cross-sectional TEM studies of DIGM in irradiated Au-Cu bilayers

Description: Cross-sectional transmission electron microscopy was used to study diffusion-induced gain boundary migration (DIGM) in irradiated and annealed Au/Cu bilayers. Using this technique, in combination with small probe x-ray energy dispersive spectroscopy, DIGM alloyed zones in Au were identified in an irradiated sample.
Date: November 1, 1992
Creator: Gao, Y.; Alexander, D.E. & Rehn, L.E.
Partner: UNT Libraries Government Documents Department

The effect of DIGM and irradiation-induced grain growth on interdiffusion in bilayer ion-beam mixing experiments

Description: Experiments were performed demonstrating that ion irradiation enhances diffusion-induced grain boundary migration (DIGM) in polycrystalline Au/Cu bilayers. Here, a model is presented relating film-averaged Cu composition in Au with treatment time, grain size and film thickness. Application of this model to the experimental results indicates that irradiation enhances DIGM by increasing the grain boundary velocity. Effects of DIGM and irradiation-induced grain growth on the temperature dependence of ion mixing in bilayers are discussed.
Date: November 1, 1992
Creator: Alexander, D.E.; Rehn, L.E.; Baldo, P.M. & Gao, Y.
Partner: UNT Libraries Government Documents Department

In situ real-time studies of oxygen incorporation in complex oxide thin films using spectroscopic ellipsometry and ion scattering and recoil spectrometry

Description: The surface termination of c-axis oriented YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} (YBCO) and the oxygen incorporation mechanism has been investigated using a unique combination of spectroscopic ellipsometry (SE) and time of flight ion scattering and recoil spectrometry (ToF-ISARS). The high surface sensitivity of the ToF-ISARS technique combined with the bulk oxygen sensitivity of SE are shown to yield complimentary information. The SE provided the film orientation and quality, while ToF-ISARS supplied surface compositional and structural information and enabled isotopic {sup 18}O tracer studies. It was determined that the O content of the film had little effect on the surface termination of the film, indicating a lack of labile Cu(1) sites at the c-axis oriented YBCO surface. Also, strong evidence for a Ba or BaO terminated structure is shown. The data related to the {sup 18}O tracer studies indicate that O from the reaction ambient incorporates only into the labile Cu(1) sites during both deposition and annealing, while stable O sites were populated with O from the sputtered target, indicating either the need for sputtered atomic O or sputtered YCuO complexes to occupy the stable Cu(2) sites.
Date: May 25, 2000
Creator: Mueller, A. H.; Gao, Y.; Irene, E. A.; Auciello, O.; Krauss, A. R. & Achultz, J. A.
Partner: UNT Libraries Government Documents Department

Tilt grain boundaries in YBa sub 2 Cu sub 3 O sub 7-x thin films

Description: Grain boundaries in YBa{sub 2}Cu{sub 3}O{sub 7-x} superconductor thin films grown on (001) MgO by metal-organic chemical vapor deposition (MOCVD) have been characterized using transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). It was found that the YBa{sub 2}Cu{sub 3}O{sub 7-x} thin films were highly textured with the c axes, or (001) orientation, nearly parallel between grains and perpendicular to the MgO substrate. A majority of the grain boundaries are low-angle boundaries with a tilt angle, {theta}, less than 15{degree}. The low-angle boundaries appear to be strongly faceted on an atomic scale in such a way that the boundary planes tend to be parallel to the (100), (010), or (110) lattice planes in one of the adjacent grains. Almost all of the lattice planes, except for a number of distorted regions along the boundaries, are continuous across the boundaries from one grain to another, accommodating the misorientation with a slight bending of the lattice planes. The small-angle boundaries are shown to consist of arrays of dislocations. A domain structure, formed by the interchange of a and b axes has been observed in large grains. The domain boundaries are strongly faceted with the (100) and (010) lattice planes parallel to the boundaries. These observations on the atomic structure of boundaries, are used to discuss the effect of grain boundaries on superconductor properties in YBa{sub 2}Cu{sub 3}O{sub 7-x} thin films. 15 refs., 9 figs.
Date: July 1, 1990
Creator: Gao, Y.; Bai, G.; Chang, H.L.M.; Merkle, K.L. & Lam, D.J.
Partner: UNT Libraries Government Documents Department

Fundamental Study of Low-Nox Combustion Fly Ash Utilization

Description: This study is principally concerned with characterizing the organic part of coal combustion fly ashes. High carbon fly ashes are becoming more common as by-products of low-NOx combustion technology, and there is need to learn more about this fraction of the fly ash. The project team consists of two universities, Brown and Princeton, and an electrical utility, New England Power. A sample suite of over forty fly ashes has been gathered from utilities across the United States, and includes ashes from a coals ranging in rank from bituminous to lignite. The characterizations of these ashes include standard tests (LOI, Foam Index), as well as more detailed characterizations of their surface areas, porosity, extractability and adsorption behavior. The ultimate goal is, by better characterizing the material, to enable broadening the range of applications for coal fly ash re-use beyond the current main market as a pozzolanic agent for concretes. The potential for high carbon-content fly ashes to substitute for activated carbons is receiving particular attention. The work performed to date has already revealed how very different the surfaces of different ashes produced by the same utility can be, with respect to polarity of the residual carbon. This can help explain the large variations in acceptability of these ashes as concrete additives.
Date: November 1, 1997
Creator: Suuberg, E. M.; Kuloats, I.; Smith, K.; Sabanegh, N.; Hurt, R. H.; Lilly, W. D. et al.
Partner: UNT Libraries Government Documents Department

Fundamental Study of Low NOx Combustion Fly Ash Utilization

Description: This study is principally concerned with characterizing the organic part of coal combustion fly ashes. High carbon fly ashes are becoming more common as by-products of low-NOx combustion technology, and there is need to learn more about this fraction of the fly ash. The project team consists of two universities, Brown and Princeton, and an electrical utility, New England Power. A sample suite of over forty fly ashes has been gathered from utilities across the United States, and includes ashes from a coals ranging in rank from bituminous to lignite. The characterizations of these ashes include standard tests (LOI, Foam Index), as well as more detailed characterizations of their surface areas, porosity, extractability and adsorption behavior. The ultimate goal is, by better characterizing the material, to enable broadening the range of applications for coal fly ash re-use beyond the current main market as a pozzolanic agent for concretes. The potential for high carbon-content fly ashes to substitute for activated carbons is receiving particular attention. The work performed to date has already revealed how very different the surfaces of different ashes produced by the same utility can be, with respect to polarity of the residual carbon. This can help explain the large variations in acceptability of these ashes as concrete additives.
Date: May 1, 1997
Creator: Suubert, E. M.; Kuloats, I.; Smith, K.; Sabanegh, N.; Hurt, R.H.; Lilly, W. D. et al.
Partner: UNT Libraries Government Documents Department

In situ x-ray diffraction studies of a new LiMg{sub 0.125}Ni{sub 0.75}O{sub 2} cathode material

Description: A Synchrotron x-ray source was used for In Situ x-ray diffraction studies during charge on a new LiMg{sub 0.125}Ti{sub 0.125}Ni{sub 0.75} cathode material synthesized by FMC Corp. It had been demonstrated by Gao that this new material has superior thermal stability than LiNiO{sub 2} and LiCo{sub 0.2}Ni{sub 0.8}O{sub 2} at over-charged state. In this current paper, studies on the relationship between the structural changes and thermal stability at over-charged state for these materials are presented. For the first time, the thermal stability of these materials are related to their structural changes during charge, especially to the formation and lattice constant change of a hexagonal phase (H3). The spectral evidence support the hypothesis that the improvement of thermal stability is obtained by suppressing the formation of H3 phase and reducing the shrinkage of its lattice constant c when charged above 4.3 V.
Date: July 1, 1999
Creator: Yang, X.Q.; Sun, X.; McBreen, J.; Gao, Y.; Yakovleva, M.V.; Xing, X.K. et al.
Partner: UNT Libraries Government Documents Department

Studies of ferroelectric heterostructure thin films and interfaces via in situ analytical techniques.

Description: The science and technology of ferroelectric thin films has experienced an explosive development during the last ten years. Low-density non-volatile ferroelectric random access memories (NVFRAMs) are now incorporated in commercial products such as ''smart cards'', while high permittivity capacitors are incorporated in cellular phones. However, substantial work is still needed to develop materials integration strategies for high-density memories. We have demonstrated that the implementation of complementary in situ characterization techniques is critical to understand film growth and interface processes, which play critical roles in film microstructure and properties. We are using uniquely integrated time of flight ion scattering and recoil spectroscopy (TOF-ISARS) and spectroscopic ellipsometry (SE) techniques to perform in situ, real-time studies of film growth processes in the high background gas pressure required to growth ferroelectric thin films. TOF-ISARS provides information on surface processes, while SE permits the investigation of buried interfaces as they are being formed. Recent studies on SrBi{sub 2}Ta{sub 2}O{sub 9} (SBT) and Ba{sub x}Sr{sub 1{minus}x}TiO{sub 3} (BST) film growth and interface processes are discussed.
Date: August 30, 1999
Creator: Auciello, O.; Dhote, A.; Gao, Y.; Gruen, D. M.; Im, J.; Irene, E. A. et al.
Partner: UNT Libraries Government Documents Department

Studies on Relationship Between Structure of Over-Charge State and Thermal Stability for LiNiO(sub 2) Based Cathode Materials

Description: A synchrotrons x-ray source was used for In Situ x-ray diffraction studies on cathode materials during charge and discharge. Two new cathode materials, LiNi{sub 0.75}Mg{sub 0.125}Ti{sub 0.125}O{sub 2} and LiNi{sub 0.56}Co{sub 0.25}Mg{sub 0.05}Ti{sub 0.05}O{sub 2}, were studied in comparison with LiNiO{sub 2}, and LiCo{sub 0.2}Ni{sub 0.8}O{sub 2}. The relationship between the structural changes and thermal stability at over-charged state has been investigated. For the W time, The thermal stability of these materials are related to their structural changes during charge, especially to the formation of a hexagonal phase H3 with collapsed lattice along ''c'' axis. A hypothesis is proposed that through suppressing the formation of H3 phase when charged above 4.3 V, the thermal stability of the cathode materials can be improved.
Date: October 17, 1999
Creator: Sun, X.; Yang, X. Q.; McBreen, J.; Gao, Y.; Yakovleva, M. V.; Xing, X. K. et al.
Partner: UNT Libraries Government Documents Department

TEM characterization of GaN nanowires

Description: Transmission electron microscopy was applied to study GaN nanowires grown on carbon nanotube surfaces by chemical reaction between Ga{sub 2}O and NH{sub 3} gas in a conventional furnace. These wires grew in two crystallographic directions, <2{und 11}0> and <01{und 1}0> (fast growth directions of GaN), in the form of whiskers covered by small elongated GaN platelets. The morphology of these platelets is similar to that observed during the growth of single crystals from a Ga melt at high temperatures under high nitrogen pressure. It is thought that growth of nanowires in two different crystallographic directions and the arrangement of the platelets to the central whisker may be influenced by the presence of Ga{sub 2}O{sub 3} (based on the observation of the energy dispersive x-ray spectra), the interplanar spacings in the wire, and the presence of defects on the interface between the central part of the nanowire and the platelets surrounding it.
Date: February 21, 2002
Creator: Liliental-Weber, Zuzanna; Gao, Y.H. & Bando, Y.
Partner: UNT Libraries Government Documents Department

Electron microscopy study of MOCVD-grown TiO sub 2 thin films and TiO sub 2 /Al sub 2 O sub 3 interfaces

Description: TiO{sub 2} thin films grown on (11{bar 2}0) sapphire at 800{degree}C by the MOCVD technique have been characterized by transmission electron microscopy. The TiO{sub 2} thin films are single crystalline and have the rutile structure. The epitaxial orientation relationship between the TiO{sub 2} thin films (R) and the substrate (S) has been found to be: (101)(0{bar 1}0){sub R}{parallel}(11{bar 2}0)(0001){sub S}. Growth twins in the films are commonly observed with the twin plane {l brace}101{r brace} and twinning direction {l angle}011{r angle}. Detailed atomic structures of the twin boundaries and TiO{sub 2}/{alpha}-Al{sub 2}O{sub 3} interfaces have been investigated by high-resolution electron microscopy (HREM). When the interfaces are viewed in the direction of (0{bar 1}0){sub R}/(0001){sub S}, the interfaces are found to be structurally coherent in the direction of ({bar 1}01){sub R}/(1{bar 1}00){sub S}, in which the lattice mismatch at the interfaces is about 0.5%. 8 refs., 4 figs.
Date: November 1, 1990
Creator: Gao, Y.; Merkle, K.L.; Chang, H.L.M.; Zhang, T.J. & Lam, D. J.
Partner: UNT Libraries Government Documents Department

Atmosphere control during preparation of YBa sub 2 Cu sub 3 O sub 7- x magnet windings

Description: Large coils of YBa{sub 2}Cu{sub 3}O{sub 7-x} can be fired successfully if the furnace atmosphere is carefully controlled. Organics added during processing produce CO{sub 2} during the initial portions of the firing schedule. Transmission electron microscopy of material fired in atmospheres containing various levels of CO{sub 2} clearly shows the extent of grain boundary degradation caused by CO{sub 2}. Coils with acceptable critical current density can be produced if the rate of CO{sub 2} removal is adequate. 9 refs., 4 figs., 1 tab.
Date: October 1, 1990
Creator: Poeppel, R.B.; Dorris, S.E.; Picciolo, J.J.; Balachandran, U.; Lanagan, M.T.; Zhang, C.Z. et al.
Partner: UNT Libraries Government Documents Department

Studies of ferroelectric heterostructure thin films, interfaces, and device-related processes via in situ analytical techniques.

Description: The science and technology of ferroelectric thin films has experienced an explosive development during the last ten years. Low-density non-volatile ferroelectric random access memories (NVFRAMS) are now incorporated in commercial products such as ''smart cards'', while high permittivity capacitors are incorporated in cellular phones. However, substantial work is still needed to develop materials integration strategies for high-density memories. We have demonstrated that the implementation of complementary in situ characterization techniques is critical to understand film growth and device processes relevant to device development. We are using uniquely integrated time of flight ion scattering and recoil spectroscopy (TOF-ISARS) and spectroscopic ellipsometry (SE) techniques to perform in situ, real-time studies of film growth processes in the high background gas pressure required to growth ferroelectric thin films. TOF-ISARS provides information on surface processes, while SE permits the investigation of buried interfaces as they are being formed. Recent studies on SrBi{sub 2}Ta{sub 2}O{sub 9} (SBT) and Ba{sub x}Sr{sub 1{minus}x}TiO{sub 3} (BST) film growth and interface processes are discussed. Direct imaging of ferroelectric domains under applied electric fields can provide valuable information to understand domain dynamics in ferroelectric films. We discuss results of piezoresponse scanning force microscopy (SFM) imaging for nanoscale studies of polarization reversal and retention loss in Pb(Zr{sub x}Ti{sub 1{minus}x})O{sub 3} (PZT)-based capacitors. Another powerful technique suitable for in situ, real-time characterization of film growth processes and ferroelectric film-based device operation is based on synchrotrons X-ray scattering, which is currently being implemented at Argonne National Laboratory.
Date: June 29, 1999
Creator: Aggarwal, S.; Auciello, O.; Dhote, A. M.; Gao, Y.; Gruen, D. M.; Im, J. et al.
Partner: UNT Libraries Government Documents Department

Degradation of properties of YBa sub 2 Cu sub 3 O sub x superconductors sintered in CO sub 2 -containing atmosphere

Description: Stability of the YBa{sub 2}Cu{sub 3}O{sub x} (YBCO) superconductor toward reacting with CO{sub 2} in CO{sub 2}/O{sub 2} gas mixtures during sintering was investigated as a function of the partial pressure of CO{sub 2} and temperature. The transport critical current density, J{sub c}, of the superconductor decreased drastically with increasing partial pressure of CO{sub 2} was increased, J{sub c} became zero (at 77 K) even though the major phase of the sample was still a superconductor as determined by magnetic susceptibility measurements. Microstructures and compositions of the samples were investigated by transmission electron microscopy in conjunction with energy-dispersive x-ray spectroscopy. Two types of grain boundaries were observed: {approximately}10% of the grain boundaries contained a second phase; the regions near the remaining grain boundaries were tetragonal. At high partial pressures of CO{sub 2}, the YBCO completely decomposed to BaCO{sub 3}, Y{sub 2}BaCuO{sub 5}, and CuO. 11 refs., 3 figs.
Date: September 1, 1990
Creator: Balachandran, U.; Zhang, C.; Xu, D.; Gao, Y.; Merkle, K.L.; Mundy, J.N. et al.
Partner: UNT Libraries Government Documents Department

Effects of atmosphere and heating rate during processing of a ceramic superconductor

Description: Properties of ceramic superconductors depend strongly on the temperature, heating rate, pressure, and atmosphere used during synthesis and fabrication. We have developed a process for synthesizing orthorhombic YBa{sub 2}Cu{sub 3}O{sub x} (123) superconducting powders by calcining the precursor powders under reduced total oxygen pressure. The resultant 123 powders are mixed with organics, and wires and coils are fabricated by extrusion. The wires and coils are fired at a reduced total pressure in flowing O{sub 2} to reduce the concentrations of CO{sub 2}, CO, and H{sub 2}O and thus prevent decomposition of the 123. Transport critical current density of the superconductor decreases drastically with increasing concentrations of CO{sub 2} in the gas mixture. Transmission electron microscopy of materials sintered in O{sub 2} atmospheres containing various levels of CO{sub 2} clearly shows the extent of grain boundary degradation. 29 refs., 9 figs., 2 tabs.
Date: January 1, 1991
Creator: Balachandran, U.; Xu, D.; Zhang, C.; Dorris, S.E.; Russell, R.A.; Dusek, J.T. et al.
Partner: UNT Libraries Government Documents Department

Control of YBa sub 2 Cu sub 3 O sub 7 minus x degradation during heat treatment

Description: The importance of closely controlling the furnace atmosphere during processing of YBa{sub 2}Cu{sub 3}O{sub 7-x} (YBCO) was examined. The stability of YBCO during sintering was studied as a function of CO{sub 2} partial pressure in CO{sub 2}/O{sub 2} gas mixtures. The zero-field critical current density, J{sub c}, decreased with increasing CO{sub 2} partial pressure, and ultimately reached zero, even though Meissner effect measurements showed that the bulk of the samples with zero transport J{sub c} remained superconducting. Examination of the microstructure and composition of the samples by TEM, AEM, and SIMS showed the presence of BaCuO{sub 2} and BaCu{sub 2}O{sub 2} at a minority of the grain boundaries. Near other grain boundaries, where second phases were not readily evident, the structure was found to be tetragonal for several tens of nm up to the grain boundaries, whereas the grain interiors were found to be orthorhombic. At high partial pressures of CO{sub 2}, YBCO completely decomposed to BaCO{sub 3}, Y{sub 2}BaCuO{sub 5} (211), and CuO. In the firing of plastically processed superconductors, such as extruded YBCO coils, it was found that the use of reduced total pressure prevents the decomposition of YBCO. 13 refs., 5 figs., 2 tabs.
Date: October 1, 1990
Creator: Dorris, S.E.; Poeppel, R.B.; Picciolo, J.J.; Balachandran, U.; Lanagan, M.T.; Zhang, C.Z. et al.
Partner: UNT Libraries Government Documents Department