W. Kondo

Strong flux pinning due to dislocations associated with stacking faults in Y Ba2Cu3O7 − δ thin films prepared by fluorine-free metal organic deposition

The magnetic-field angle dependence of the critical current density Jc(H, θ) was measured in Y Ba2Cu3O7 − δ (YBCO) thin films with strong flux pinning (Jc ≥ 2.5xa0MAxa0cm − 2 at 77.3xa0K) prepared by a fluorine-free (FF) metal organic deposition (MOD) method and by thermal co-evaporation. Steep Jc(θ) peaks around were observed in FF-MOD films, and anisotropic scaling analysis showed that the pinning is mainly due to small random (point) pins and ab-plane-correlated pins. Few small precipitates with diameter less than 10xa0nm were observed by transmission electron microscopy (TEM); instead, a high density of stacking faults parallel to the ab-plane was observed in some areas in cross-sectional TEM images. We hypothesize that at 77xa0K most stacking faults are weak planar pinning centers by themselves and that (partial) dislocations formed at the boundary between stacking faults and the YBCO matrix become strong linear pinning centers parallel to the ab-plane. The linear pin acts as an ab-plane-correlated pin when it is perpendicular to the current direction, and acts as a small random pin in other cases, which well explains the observed Jc(H, θ) of FF-MOD YBCO films.

Preparation of 2-inch-diameter double-sided YBa2Cu3O7 films by coating-pyrolysis process

Two-inch-diameter and double-sided YBa 2 Cu 3 O 7 (Y123) films were prepared on LaAlO 3 (LAO)(001) and LaAlO 3 -Sr 2 AlTaO 6 (LSAT)(001) substrates by a coating-pyrolysis process through single annealing in an 80-mm-diameter tube furnace. Mapping analyses of composition and crystallinity showed that the films comprised highly homogeneous and strongly c-axis-oriented Y123 grains. Inductive-J c scanning at 77.3 K revealed that an average J c value >1 MA/cm 2 was obtained for both sides of the films. The films on LSAT showed slightly higher X-ray diffraction intensities and J c values than those on LAO, probably because LSAT is twin-free and has a smaller lattice misfit with Y123.

2-D large-size YBCO films on sapphire for FCL prepared by coating-pyrolysis process

Abstract Large-size, two-dimensional YBCO films such as 5-cm-∅ circles and 12xa0×xa03 cm 2 rectangles were successfully prepared by a chemical solution-based coating-pyrolysis process on sapphire substrates with an evaporated-CeO 2 buffer layer. Two types of furnace systems were adopted in the final heat treatment depending on the sample dimensions. A 5-cm-∅ YBCO film annealed in an infrared image furnace showed an average inductive- J c value of 1.3 MA/cm 2 at 77 K. While, a 12xa0×xa03 cm 2 rectangular film annealed in a large-diameter tube furnace demonstrated much more uniform inductive- J c distribution, i.e., 85% of the observations fall within ±20% of the average although the J c value was approximately 0.4 MA/cm 2 at 77 K.

Preparation of CeO/sub 2/-buffer layers for large-area MOD-YBCO films (10/spl times/30 cm/sup 2/) with high-J/sub c/

Epitaxial cerium oxide (CeO/sub 2/; thickness: 40 nm) buffer layers were deposited on large area (maximum size 10/spl times/30 cm/sup 2/) R-cut single-crystal sapphire (/spl alpha/-Al/sub 2/O/sub 3/) substrates by double electron-beam guns. Substrate heater-and-holder system was modified to maintain the substrate temperature uniform over the large area during deposition. Oxygen gas of a pressure 4/spl times/10/sup -2/ Pa was introduced to keep radio frequency plasma. When the substrate temperature was controlled in the range 640/spl deg/-700/spl deg/C, the CeO/sub 2/ buffer showed a complete [001] orientation and very smooth surfaces by x-ray diffraction analysis and atomic force microscopic observations, respectively, in the whole area of the large size substrates. These buffer layers without post-deposition annealing were quite suitable for preparing YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) films by a metal-organic deposition (MOD) using a metal acetylacetonate-based solution. The average critical current density of the 210-nm-thick MOD-YBCO film was in excess of 2.6 MA/cm/sup 2/ measured by an inductive method at 77.3 K.

Structural aspect of high-Jc MOD-YBCO films prepared on large area CeO2-buffered YSZ substrates

YBa2Cu3O7 (YBCO) film was prepared by fluorine-free metalorganic deposition (MOD) on a 2-inch-diameter CeO2-buffered yttria-stabilized zirconia (CbYSZ) substrate. The MOD-YBCO films on a flat CbYSZ prepared using a metal acetylacetonate-based coating solution demonstrated high critical current density (Jc), the average being higher than 4 MA/cm2 at 77.3 K and in a self field using an inductive method. The Jc distribution was uniform in the whole area of the MOD-YBCO films on the large size substrates. Cross sectional transmission electron microscopic images of the MOD-YBCO film exhibited a very smooth CeO2/ YSZ and CeO2 buffer layer/ YBCO film interfaces. Both the buffer and the YBCO layers were dense; however, a small amount of (100)-oriented YBCO occurred inside the matrix of (001)-oriented YBCO layers. Also, some distinct dislocations were observed at the CeO2/ YBCO interface. From around the stepped surface of the CeO2 buffer layer there seemed to grow a slightly distorted (001)-oriented YBCO domain, however, the domain restored its (001)-oriented periodicity in the region at some distance above the steps. Such growth mode is considered to correlate the high Jc of the MOD-YBCO film with very small average roughness of CeO2 buffer layer.

Characterization of 50-mm-diameter Y123 films prepared by a coating-pyrolysis process using an infrared image furnace

Abstract Epitaxial YBa2Cu3O7−y (Y123) films were prepared on 50-mm-diameter LaAlO3 (LAO)(0xa00xa01), NdGaO3 (NGO)(1xa01xa00) and CeO2-buffered Al2O3(0xa01xa02) substrates by a chemical solution-based coating-pyrolysis process using a large-area infrared image furnace for high-temperature heat treatment. The furnace provides uniform temperature distribution over the film specimens and high irradiation power density that enables a high heating rate (∼200 °C/min). An average inductive-Jc (77 K) in excess of 2 MA/cm2 was reproducibly obtained for 0.3–0.7-μm-thick c-axis-oriented Y123 films on LAO(0xa00xa01), of which the variation was fairly small all over the films. Microwave surface resistance of the film was 0.48 m Ω at 12 GHz and 70 K by the sapphire rod resonator method. Moreover, an average Jc (77 K )=0.5 MA/cm2 and Tc=89 K were attained in a 0.3-μm-thick film on CeO2-buffered Al2O3(0xa01xa02). Despite being subjected to rapid thermal annealing, the Y123 film on NGO(1xa01xa00) was a-axis oriented and resulted in lower Jc (77 K )=0.15 MA/cm2 and Tc=84 K.

Microstructure of Epitaxial Y123 Films on

Epitaxial YBa2Cu3O7 (Y123) films have been prepared by fluorine-free metalorganic deposition (MOD) using a metal acetylacetonate-based coating solution on cerium oxide (CeO2: 40 nm)-buffered yttria-stabilized zirconia (CbYSZ) substrates. The 210-nm-thick Y123 film demonstrated a high critical current density (Jc), the average Jc being 4.5 MA/cm2 at 77.3 K using an inductive method. However, with increasing thickness the Jc values decreased; the average Jc was 3.4 and 2.2 MA/cm2 for 400-nm- and 550-nm-thick films, respectively. In low-magnification cross sectional transmission electron microscopy (XTEM) images for the films, the stripe-like contrast due to the c-axis orientation of Y123 was clearly seen from the interface with CeO2 to the surface. The whole c-axis-oriented Y123 layers were very dense even in the 550-nm-thick film. Full width at half maximum (FWHM) of Deltaomega for the 308/038 reflections in XRD omega - 2thetas scanning decreased with increasing film thickness. High resolution XTEM images of the 550-nm-thick film showed wavy c-planes of Y123 due to the short stacking faults near the interface with CeO2 buffer and near the surface whereas the c-planes were very flat but contained long stacking faults in the middle part of the film. It is suggested that the absence of wavy stacking faults, which probably work as pinning centers, decreased Jc in the thicker films. Further decrease in Jc in the 550-nm-thick film is presumably caused by the roughness in the outer layer near the surface.

{\rm CeO}_{2}

Distribution of J/sub c/ in the two-dimensional large-size YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) film was investigated by an inductive method for the application to fault current limiter elements. The YBCO films were prepared by metal-organic deposition (MOD) using a fluorine-free acetylacetonate-based complex solution on R-plane sapphire with an evaporated-CeO/sub 2/ buffer layer. The average J/sub c/ values of 1-3 MA/cm/sup 2/ at 77 K were obtained for 1 cm /spl times/ 12 cm-, 3 cm /spl times/ 12 cm-, and 10 cm /spl times/ 30 cm-rectangular YBCO films as well as on 2-inch-diameter films. In contrast with the TFA-MOD method via BaF/sub 2/ precursor, the fluctuation of J/sub c/ in the present films was fairly small; no significant size effects or shape effects were recognized on the fluctuation of J/sub c/. Instead, fluctuation of J/sub c/ depended on the average J/sub c/ values; higher-J/sub c/ films tended to exhibit more uniform J/sub c/ distribution.

-Buffered YSZ Prepared by Fluorine-Free MOD

Large-size, two-dimensional Y Ba2Cu3O7 (YBCO) films were successfully prepared by a chemical solution-based coating pyrolysis (CP) process on 30 × 10 cm2 R-plane sapphire substrates with a vacuum-deposited CeO2 buffer layer. The CeO2 layer was very smooth and uniform with an average roughness (Ra) of 0.48 nm by atomic force microscopic observations and highly (001)-oriented by large-area x-ray diffraction θ–2θ scans. The c-axis-oriented YBCO films were prepared by a CP process using a fluorine-free, metal acetylacetonate-based coating solution. A 30 × 10 cm2-rectangular c-axis-oriented YBCO film annealed in a large-diameter tube furnace demonstrated high inductive-Jc values, for which the distribution was fairly uniform, with an average 1.0 MA cm−2 at 77 K.

Distribution of inductive J/sub c/ in two-dimensional large-size YBCO films prepared by fluorine-free MOD on CeO/sub 2/-buffered sapphire

We developed 500 Vrms/ 200 Arms superconducting thin-film fault current limiter (FCL) modules that can withstand high electric fields (E>30 Vrms cm−1) by using large-area YBa2Cu3O7 (YBCO) thin films with high-resistivity Au–Ag alloy shunt layers. Au–Ag alloy films about 60 nm thick were sputter-deposited on YBCO/CeO2/sapphire films (2.7 cm × 20 cm) prepared using a fluorine-free MOD method. Each 20 cm long Au–Ag/YBCO film was then divided into three segments (each ~5.7 cm long) by four Ag electrodes deposited on the Au–Ag layer, resulting in an effective length of 17 cm. The 500 V/200 A FCL modules were then fabricated by first connecting two of the segmented films in parallel using Ag-sheathed Bi-2223 superconducting tapes and then connecting in parallel an external resistor and a capacitor for each segment to protect the Au–Ag/YBCO film from hot spots. Switching tests using a short-circuit generator revealed that all the modules carried a superconducting ac current of ≥237 Arms and that modules prepared with YBCO films having a relatively homogeneous critical current Ic distribution successfully withstood ≥515 Vrms for five cycles without any damage. These results demonstrate that (a) the FCL modules fabricated here successfully achieved the rated current of 200 Arms and rated voltage of 500 Vrms and (b) total area of the YBCO films on sapphire substrates required for the 500 V/200 A (100 kV A) module was less than one-third that for conventional thin-film FCL modules that use gold shunt layers, leading to the significantly reduced cost of thin-film FCLs. Film damage due to hot spots depended on the difference in Ic between the two parallel-connected films and on the inhomogeneity of the Ic distribution in the film, and is most probably due to nonlinear current flows at the moment of quenching that cause local overheating.