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Dive into the research topics where P. L. Karas is active.

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Featured researches published by P. L. Karas.


Physica C-superconductivity and Its Applications | 1993

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

J. A. DeLuca; P. L. Karas; John Eric Tkaczyk; Peter J. Bednarczyk; M. F. Garbauskas; C. L. Briant; Donald Bruce Sorensen

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.


Applied Physics Letters | 1992

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

John Eric Tkaczyk; J. A. DeLuca; P. L. Karas; Peter J. Bednarczyk; M. F. Garbauskas; Ronald H. Arendt; K. W. Lay; Jagadeesh S. Moodera

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.


Applied Physics Letters | 1993

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

John Eric Tkaczyk; J. A. DeLuca; P. L. Karas; Peter J. Bednarczyk; D. K. Christen; C.E. Klabunde; H. R. Kerchner

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.


Applied Physics Letters | 1993

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

Dean J. Miller; J.G. Hu; J. D. Hettinger; K. E. Gray; John Eric Tkaczyk; J. A. DeLuca; P. L. Karas; J.A. Sutliff; M. F. Garbauskas

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.


Journal of Materials Research | 1992

Critical current and microstructure of uniaxially aligned, polycrystalline YBa 2 Cu 3 O 7−δ

John Eric Tkaczyk; C. L. Briant; J. A. DeLuca; E. L. Hall; P. L. Karas; K.W. Lay; E. Narumi; D.T. Shaw

Three processing routes that generate uniaxial alignment but otherwise yield very different microstructure and critical current are compared. Fine grain size and {ital c}-axis alignment are obtained in magnetically aligned ceramics, pyrolyzed thick films, and {ital in} {ital situ} deposited thin films. The dense, well aligned microstructure of the {ital in} {ital situ} process produces the highest zero field critical current {ital J}{sub {ital c}}{gt}10{sup 4} A/cm{sup 2} at 77 K. However, the critical current is suppressed in low magnetic field, suggesting that uniaxial alignment is not sufficient to avoid Josephson-type intergranular coupling. Above 1 T, the critical current of the aligned ceramic dominates in spite of its less ideal microstructure. The critical current in this high field region is one to two orders of magnitude greater than that of nonaligned material. This result implies the existence of a 3-d percolative network of strong links.


Journal of Materials Research | 1995

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

C. L. Briant; J. A. DeLuca; P. L. Karas; M. F. Garbauskas; J.A. Sutliff; Amit Goyal; D. M. Kroeger

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.


Superconductivity and its applications | 2008

‘‘1223’’ Tl‐Ca‐Ba‐Cu‐oxide thick films prepared by a silver‐addition process

J. A. DeLuca; P. L. Karas; John Eric Tkaczyk; C. L. Briant; M. F. Garbauskas; Peter J. Bednarczyk

Progress is reported in the understanding and development of the silver‐addition process for the preparation of ‘‘1223’’‐TCBCO thick films. Described are improvements in film homogeneity achieved via modification of the spray deposition procedure, increases in Jc achieved via post synthesis heat treatment in oxygen, and the results of studies of the film growth mechanism which indicate that presence of silver, barium oxide, and thallium oxide are all necessary for the development of a liquid phase responsible for the accelerated film formation obtained by this technique. Also reported are studies of the preparation of films on metallic substrates which include Jc vs Field data at 4.2 K and 77 K for ‘‘1223’’ film on silver foil exhibiting Tc(0)=105 K and Jc(zf‐77 K)=10,000 A/cm2.


Archive | 1993

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

John Eric Tkaczyk; J. A. DeLuca; P. L. Karas; Peter J. Bednarczyk; M. F. Garbauskas; J.A. Sutliff

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.


Journal of Materials Research | 1991

The synthesis of superconducting Tl–Ca–Ba–Cu–oxide films by the reaction of spray deposited Ca–Ba–Cu–oxide precursors with Tl2O vapor in a two-zone reactor

J. A. DeLuca; M. F. Garbauskas; Roger B. Bolon; James G. McMullen; Winifred E. Balz; P. L. Karas


Archive | 1996

Apparatus for fabrication of superconductor

John Eric Tkaczyk; J. A. DeLuca; P. L. Karas

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K. E. Gray

Argonne National Laboratory

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Amit Goyal

Oak Ridge National Laboratory

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C.E. Klabunde

Oak Ridge National Laboratory

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D. K. Christen

Oak Ridge National Laboratory

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