J.A. Sutliff

Local texture and percolative paths for long‐range conduction in high critical current density TlBa2Ca2Cu3O8+x deposits

A possible microstructural origin of the high critical current densities which have been obtained in c‐axis‐aligned, polycrystalline TlBa2Ca2Cu3O8+x deposits has been identified. The results of x‐ray diffraction determinations of basal plane texture of Tl‐1223 deposits prepared by spray pyrolysis are observed to depend on the size of the x‐ray beam. Furthermore, most grain boundaries were found from transmission electron microscopy to have small misorientation angles. It is concluded that although overall the basal plane orientations are nearly random, there is a high degree of local texture indicative of colonies of similarly oriented grains. The spread in a‐axis orientation within a colony is ∼10°–15°. Intercolony conduction, it is suggested, may be enhanced by a percolative network of small‐angle grain boundaries at colony interfaces.

X-ray microdiffraction analysis of mm-scale orientational correlations in Tl-1223 high-critical-current, high-temperature superconducting films

Abstract Tl 1− y Ba 2 Ca 2 Cu 3 O x , y c -axes normal to the film; a - and b -axes are randomly oriented on a macroscopic scale. We describe X-ray pole-figure measurements from 0.1 mm regions of Tl-1223 films. For samples with the highest critical currents, the a -axes of the grains are locally aligned over distances up to 1 mm. Regions of higher critical current are associated with regions of high a -axis alignment.

Brick‐wall structure in polycrystalline TlBa2Ca2Cu3Ox thick films with high critical currents

Microstructural studies of TlBa2Ca2Cu3Ox(Tl‐1223) thick films that exhibit high critical current densities (Jc) for nonepitaxial polycrystalline materials show that these films possess a ‘‘brick‐wall’’ structure that may be partly responsible for high current densities. The magnetic field dependence of Jc is similar to that reported for Bi‐Sr‐Ca‐Cu‐O materials that exhibit this structure. Structural analyses indicate a high degree of c‐axis alignment but little in‐plane texture, suggesting that high‐angle [001] tilt boundaries are prevalent. Scanning and transmission electron microscopy reveal that the microstructure consists of overlapping layers reminiscent of the brick‐wall structure. These results suggest that high critical current densities may be achieved by percolative transport through this structure.

Dependence of critical current density on microstructure and processing of high-T c superconductors

Microstructural origins for reduced weak-link behavior in high-Jc melt-processed YBCO, spray pyrolyzed thick films of Tl-1223, metallic precursor Y-124 polycrystalline powder-in-tube (PIT) wires and PIT Bi-2212/2223 are discussed. Since the materials studied are the highest Jc, polycrystalline, high-Tc superconductors fabricated worldwide, the results provide important guidelines for further improvements in superconducting properties, thereby enabling practical applications of these materials. It is found that strongly linked current flow within domains of melt-processed 123 occurs effectively through a single crystal path. In c-axis oriented, polycrystalline Tl-1223 thick films, local in-plane texture has been found to play a crucial role in the reduced weak-link behavior. Formation of “colonies” of grains with a common c-axis and modest in-plane misorientation was observed. Furthermore, a colony boundary in general has a varying misorientation along the boundary. Large regions comprised primarily of low angle boundaries were observed. Percolative transport through a network of such small angle boundaries appears to provide the nonweak-linked current path. Although powder-in-tube BSCCO 2212 and 2223 also appear to have a “colony” microstructure, there are some important differences. Colonies in BSCCO consist of stacks of grains with similar c-axis orientation in contrast to colonies in Tl-1223 films where few grains are stacked on top of one another. Furthermore, most grains within a colony in BSCCO have the same lateral dimensions as that of the colony, resulting largely largely in “twist” boundaries. Further microstructural characterization of high-Jc PIT 2212 and 2223 is currently underway. In the case of Y-124 wires, weak macroscopic in-plane texture is found. Additional measurements are underway to determine if a sharper, local in-plane texture also exists. It is found that in three of the four types of superconductors studied, reduced weak-link behavior can be ascribed to some degree of biaxial alignment between grains, either on a “local” or a “global” scale.

The effect of colonies of aligned grains on critical current in high-temperature superconductors

Abstract X-ray microdiffraction has been used to map the orientational distribution of grains in a 4 × 5mm 2 polycrystalline film of Tl 1− y Ba 2 Cu 2 Cu 3 O x (Tl-1223) grown on yttira-stabilized zirconia (YSZ). The film consists of “colonies”, each containing ∼ 10 4 grains with their a -axes aligned; the c -axes of all grains are aligned normal to the film, but there is no overall a -axis alignment. While the grain boundaries within a colony are small angle, colonies are separated by large-angle tilt boundaries. The typical colony size is 0.4 mm. Measured critical currents are compared with simulations based on observed distributions of grain alignment; we find that variations in grain alignment account for the observed variations in the critical current density. Simulations over longer distances indicate that a colony structure can substantially increase the critical current, especially in magnetic fields.

The microstructural evolution of a silver-containing spray deposited 1223 Tl–Ca–Ba–Cu oxide superconductor

This paper reports a study of the microstructural evolution of Ag-containing 1223 Tl--Ca--Ba--Cu oxide superconductors in spray-deposited films. The films were formed by spray depositing nitrates of Ca, Ba, Cu, and Ag onto a polycrystalline yttria-stabilized zirconia substrate. These deposits were then converted to a mixture of oxides (calcia, calcium-copper oxide, and barium cuprate) and metallic silver by heating in oxygen. When thallium oxide vapor was passed over the film, the thallium was incorporated into the film and the 1223 phase was formed. The evidence strongly suggests that the development of the 1223 superconductor involves the formation of a liquid phase. Our analysis suggests that the initial phase to form a liquid is CaO that contains thallium, barium, copper, and silver. Once this initial liquid is formed, it incorporates more thallium which, in turn, allows it to dissolve other types of oxides present in the film. In this way the liquid spreads across the surface. The equilibrium 1223 phase precipitates from it.

Texture and transport in spray pyrolyzed TlBa 2 Ca 2 Cu 3 O 9 thick films

©1995 Materials Research Society. The original publication is available at: http://www.mrs.org/

In x Ga 1-x As (x=0.25–0.35) grown at low temperature

InxGa1−xAs (x=0.25–0.35) grown at low temperature on GaAs by molecular beam epitaxy is characterized by Hall effect, transmission electron microscopy, and ultrafast optical testing. As with low temperature (LT) GaAs, the resistivity is generally higher after a brief anneal at 600°C. High-resolution transmission electron microscopy shows all the as-grown epilayers to be heavily dislocated due to the large lattice mismatch (2–3%). When the layers are annealed, in addition to the dislocations, precipitates are also generally observed. As with LT-GaAs, the lifetime shortens as growth temperature is reduced through the range 300–120°C; also, the lifetime in LT-InxGa1−xAs is generally shorter in as-grown samples relative to annealed samples. Metal-semiconductor-metal photodetectors fabricated on the material exhibit response times of 1–2 picoseconds, comparable to results reported on GaAs grown at low temperature, and the fastest ever reported in the wavelength range of 1.0–1.3 μm.

Local texture and percolative paths for long-range conduction in high critical current density TlBa[sub 2]Ca[sub 2]Cu[sub 3]O[sub 8+][sub [ital x]] deposits

A possible microstructural origin of the high critical current densities which have been obtained in c‐axis‐aligned, polycrystalline TlBa2Ca2Cu3O8+x deposits has been identified. The results of x‐ray diffraction determinations of basal plane texture of Tl‐1223 deposits prepared by spray pyrolysis are observed to depend on the size of the x‐ray beam. Furthermore, most grain boundaries were found from transmission electron microscopy to have small misorientation angles. It is concluded that although overall the basal plane orientations are nearly random, there is a high degree of local texture indicative of colonies of similarly oriented grains. The spread in a‐axis orientation within a colony is ∼10°–15°. Intercolony conduction, it is suggested, may be enhanced by a percolative network of small‐angle grain boundaries at colony interfaces.

Field Dependence of the Transport Critical Current in Polycrystalline TlBa 2 Ca 2 Cu 3 O z Films

Large critical currents with a relatively weak magnetic field dependence are obtained in thick films of TlBa2Ca2Cu3Oz. Transport measurements indicate Jc > 105 A/cm2 at 77K, zero field, and Jc>104 at 60K in a 2T field applied along the c-axis. Reduced weak link behavior at low fields is attributed enhanced intergranular coupling which accompanies the uniaxial aligned, plate-like microstructure. The improved performance at high fields is attributed to strong intragranular flux pinning. Scaling of the high field critical current at different temperatures is observed and the data can be fit to an expression which interpolates between single vortex pinning and collective creep regime.