Chakrapani Varanasi

Flux pinning behavior of incomplete multilayered lattice structures in YBa2Cu3O7−d

Magnetization results of YBa2Cu3O7−d films processed with interlayers of CeO2 inclusions are presented. Unexpected flux pinning results that are different from previous observations with nanoparticulate layered inclusions were observed. Flux pinning was found to be in some cases either slightly improved at either low fields 8T although degraded, sometimes severely, at interim magnetic fields. Most unexpectedly, the pinning performance of the various samples rapidly converges as the temperature is reduced from 77 to 65K, causing all films to have similar Jc(H) behavior at 65K even though dramatically different at 77K.

Biaxially textured copper and copper?iron alloy substrates for use in YBa2Cu3O7?x coated conductors

Copper and Cu?Fe (Fe ~2.35?wt%) alloy substrates were thermo-mechanically processed and the biaxial texture development, magnetic properties, yield strength, and electrical resistivity were studied and compared to determine their suitability as substrates for high-temperature superconducting coated conductor applications. Average full width half maximum (FWHM) of 5.5? in Phi scans (in-plane alignment), and 6.6? in omega scans (out-of-plane alignment) was obtained in copper samples. Cu?Fe samples showed 5.9? FWHM in Phi scans and 5.9? in omega scans. Even with the presence of 2.35% Fe in the Cu-alloy, the saturation magnetization (Msat) value was found to be 4.27?emu?g?1 at 5?K, which is less than in Ni samples by an order of magnitude and comparable to that of Ni?9?at.%?W substrates. The yield strength of the annealed Cu?Fe alloy substrate was found to be at least two times higher than that of similarly annealed copper substrates. The electrical resistivity of Cu?Fe alloy was found to be an order of magnitude higher than that of pure copper at 77?K.

Low AC loss structures in YBCO coated conductors with filamentary current sharing

Architectural design improvements, such as filamentation, to YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) coated conductors can result in a more ac-tolerant version of the conductor. However, finely made striations in the conductor make filament breakage more probable. In this case, weakly linking the filaments can enable current sharing among the filaments of striated coated conductors while maintaining reduced hysteretic losses. Data is presented for a YBCO sample divided into superconducting filaments separated such that the transverse critical current density of the striation is significantly less than the longitudinal critical current density along the filaments. A LAO substrate was physically scribed with parallel incisions to adversely affect the subsequent epitaxial growth of the YBCO layer between the striations. Vibrating sample magnetometry measurements verified a reduction in hysteretic loss compared to a control sample of epitaxially grown YBCO on an unscribed LAO substrate. Since filamentation requires a twist in the conductor for practical applications, a discussion is also given outlining an alternate means of accomplishing this by placing a twist in the coated conductor architecture itself.

YBa2Cu3O7−x–Ag thick films deposited by pulsed laser ablation

Abstract Ag-doped YBa 2 Cu 3 O 7− x films, with thickness ranging from 0.06 to 2.5 μ m, were deposited by pulsed laser ablation onto (100) LaAlO 3 single-crystal substrates. The target was YBa 2 Cu 3 O 7− x with 5 wt.% Ag addition. The presence of Ag in the films in concentrations of ∼1 at.% was detected by X-ray fluorescence and secondary ion mass spectrometry (SIMS) analysis. Biaxial alignment of the films was indicated by φ scans with full-width-half-maximum (FWHM) spread of 1–2° for various thicknesses. Utilizing a standard deposition process, most films showed a critical transition temperature ( T c ) >90 K as measured by the ac susceptibility technique. Film critical current densities ( J c ) on the order of 10 6 A/cm 2 were measured at 77 K with a four-probe technique on a 100- μ m-wide patterned microbridge.

Spectral-component monitoring and control of pulsed laser deposition of YBCO films

We examine the time-resolved spectral components emitted at approximately 327 nm and approximately 550 nm in YBCO plumes during pulsed laser deposition of thin films using a KrF excimer laser at (lambda) equals 248 nm. The studied emission signals last for approximately 20 microsecond(s) ec, and show variations when process parameters such as laser power, laser excitation voltage, beam focus, chamber pressure, substrate temperature, pulse repetition rate, and target rotation rate are changed. These signals are also dependent on other factors such as target wear and age of the laser gas mixture. Spectral-component monitoring is a supplementary method of real-time plume evaluation, and allows observation of changes both prior to deposition and during the actual deposition. Adjustments can be made to the process parameters to make the plume conform to criteria necessary for the growth of films with specific qualities. The use of these spectral components as real-time process- control state variables for more reproducible fabrication of high quality thin films will be assessed.

Addition of alternate phase nanoparticle dispersions to enhance flux pinning of Y-Ba-Cu-O thin films

Nanoparticle dispersions of various phases were added to YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO or 123) thin films by multilayer pulsed laser deposition, to determine their effect on flux pinning. The different pinning materials examined include Y/sub 2/BaCuO/sub 5/ (Y211 or green-phase), La/sub 2/BaCuO/sub 5/ (La211 or brown-phase), Y/sub 2/O/sub 3/, CeO/sub 2/, and MgO, with lattice constant mismatches varying from 0.5% to 12% with respect to YBCO. Y211 and Y/sub 2/O/sub 3/ provided significant pinning increases at temperatures of 65 K and 77 K, however other phases provided enhancements only at 65 K (for CeO/sub 2/ and La211) for limited range of applied field strengths. An interesting correlation between T/sub c/ transition widths and pinning strengths was observed. The additions produced markedly different nanoparticle and film microstructures, as well as superconducting properties.

Nanoparticulate Flux Pinning Centers for YBa

YBa2Cu3O7-delta high temperature superconductors can maintain fairly high critical current densities (Jc) with increasing magnetic field. This in-field performance can be further improved upon by incorporating nanoparticulate magnetic flux pinning centers into the superconductors. This short paper briefly discusses and compares recent efforts by the U.S. Air Force Research Laboratory to incorporate insulating nanoparticles into the YBCO superconducting thin films by pulsed laser deposition.

_{2}

In this paper, we are reporting a dielectric oxide buffer Ba/sub 2/YNbO/sub 6/ (BYNO) and its performance on various substrates for a potential buffer layer for the growth of YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) coated conductors. Ba/sub 2/YNbO/sub 6/ is a moderate dielectric. Using pulsed laser deposition, epitaxial BYNO films were grown at 850/spl deg/C with an oxygen pressure of 200 mTorr on single crystal MgO (100) substrate and ion beam assisted sputter deposited MgO buffered hastelloy metal substrates. The surface morphology of the BYNO films reveals out growths even though the average surface roughness is only 2-8 nm. The texture of BYNO films is /spl sim/8/spl deg/ and thickness of these layers /spl sim/100 nm on metal substrates. Highly c-axis oriented YBCO films were deposited on BYNO buffered substrates. Critical transition temperatures (T/sub c0/) determined from electrical transport measurements vary between 88-89 K and corresponding critical current densities (J/sub c/) ranging from 0.5-1 MA/cm/sup 2/ at 77 K.

Cu

Pr doped YBa 2 Cu 3 O 7-d targets with composition Y 1-x Pr x Ba 2 Cu 3 O 7-d where × = 0.0001, 0.001, 0.01, and 0.1 were prepared from oxide powders and were used to deposit thin films by pulsed laser deposition using conditions previously optimized for pure YBa2Cu3O7-d. The Pr dopant was found to be dispersed throughout the film by secondary ion mass spectrometry and found to have an increased density of nanoparticles on the surface. The pinning force of the doped samples was found to decrease with increasing concentration of Pr; however, at 0.01% concentration the doped film displayed a significant enhancement over pure YBa 2 Cu 3 O 7-d for nearly the full range of 0 – 9 T.