J. A. DeLuca

VUV fluorescence of Nd3+‐, Er3+, and Tm3+‐doped trifluorides and tunable coherent sources from 1650 to 2600 Å

Broad‐band vacuum ultraviolet (VUV) fluorescence has been observed in Nd3+‐, Er3+‐, and Tm3+‐doped LaF3, YF3, LiYF4, and LuF3 by electron‐beam and VUV excitation. The VUV excitation spectra indicate a large Stokes shift (≳5000 cm−1) and a high fluorescence quantum yield of 0.8. The threshold power for laser action in these crystals by optical pumping has been estimated. Using a molecular H2 laser as a pumping source, a laser system which is tunable from 1650 to 2600 A could be constructed.

The preparation of “1223” Tl-Ca-Ba-Cu-oxide superconducting films via the reaction of silver-containing spray deposited Ca-Ba-Cu-Oxide with thallium oxide vapor

Abstract A process is described for the preparation of superconducting films of “1223” TlxCa2Ba2Cu3Oy (0.65 10 000 A/cm2 was measured at 60 K-2 T with the magnetic field applied parallel to the crystallographic c-axis of the film.

Transport critical currents in spray pyrolyzed films of TlBa2Ca2Cu3Oz on polycrystalline zirconia substrates

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 77 K, zero field, and Jc≳104 at 60 K in a 2 T field applied along the c‐axis. The observed behavior is attributed to a large degree of uniaxial alignment of platelike grains, and to superior intragranular flux pinning. These results are consistent with recent theories concerning the nature of vortices in highly anisotropic (layered) superconductors and ‘‘brick wall’’models of intergranular current transport.

uv fluorescence of cerium‐doped lutetium and lanthanum trifluorides, potential tunable coherent sources from 2760 to 3220 Å

Broad‐band uv flurescence is reported for LaF2 : Ce3+ (2760–3120 A) and LuF3 : Ce3+ (2880–3220 A). The five‐peak excitation spectrum expected for the 5d state of Ce3+ in a noncubic crystal field is observed. The fluorescence quantum yields for LaF3 : 1% Ce3+ and LuF3 : 0.1% Ce3+ are 0.9 and 0.82, respectively. Estimates of the threshold power for lasing action suggest that a laser system tunable from 2760 to 3220 A is feasible with noble‐gas–halide lasers as pumping sources.

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.

Enhanced transport critical current at high fields after heavy ion irradiation of textured TlBa2Ca2Cu3Oz thick films

Spray pyrolyzed T1(1223) films deposited on polycrystalline YSZ substrates are characterized before and after heavy ion irradiation. A factor of 2 decrease in zero field critical current is observed. However, significantly improved critical current is found at fields above 1 T, where intragranular effects dominate. The irreversibility line at 5 T is shifted by ∼20 K to higher temperatures. Scaling of the data before and after irradiation and at different temperatures is modeled by an expression which interpolates between single vortex pinning and collective creep.

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.

Superconducting thallium oxide films via an electro-deposited precursor

Abstract Electro-deposited precursor films of BaCaCuO (BCCO) doped with Ag were prepared on Ag foil. The electrodeposited films were processed using a two-zone anneal, which employs a Tl source zone and a separate sample zone, both within an O 2 ambient. The annealed films showed c -axis oriented 1223 as the major phase. At 77 K and no magnetic field, the critical current density was 4.42×10 4 A/cm 2 using the field criterion of 1 μV/cm. The field dependence for the Tl-1223 film was equally impressive, showing 8.2×10 3 A/cm 2 at 5.5 T (77 K) for the field oriented parallel to the ab plane ( H ‖ ab ) and parallel to the applied current.

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.

Crystallization of colonies of locally aligned grains during thallination of spray‐pyrolyzed TlBa2Ca2Cu3Ox thick films

Recent microstructural studies of high‐Jc, Tl‐1223, spray‐pyrolyzed thick films have shown that the high‐Jc and reduced weak‐link behavior can be ascribed to the presence of a unique microstructure, wherein the film is divided into ‘‘colonies’’ of biaxially aligned regions. In an effort to understand the nucleation and growth of Tl‐1223 as well as the formation of the colony microstructure, several sets of films quenched from various stages of the high‐temperature thallination process were examined using electron microscopy, x‐ray diffraction and electron backscatter diffraction (EBSD). It is found that the crystallization of the 1223 phase is associated with formation of a liquid phase which contains significant amounts of all oxides. Furthermore, it is found that the film nucleates at the substrate on the side closest to the thallium source. The growth front then moves up through the thickness and along the length of the film. X‐ray microdiffraction measurements show that local biaxial alignment or the ...