Kunio Kamiya

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.

Metal organic deposition of epitaxial Y123 films using a low-cost vacuum technique

Epitaxial YBa/sub 2/Cu/sub 3/O/sub 7/ (Y123) films were obtained by metal-organic deposition (MOD) using a simple, low-cost vacuum technique without gas flowing. The total pressure and oxygen partial pressure in the furnace were controlled to be 10 kPa and 10 Pa, respectively, by evacuation with a rotary pump followed by refilling with a mixture of O/sub 2/ and N/sub 2/ (O/sub 2/ content: 1000 ppm). XRD analyses exhibited that c-axis-oriented epitaxial Y123 films have successfully been obtained on CeO/sub 2/-buffered YSZ (100) (CbZ) and CeO/sub 2/-buffered sapphire (012) (CbS) substrates. In-plane alignments of these films were as high as that of the CeO/sub 2/ buffer layer. The film on the CbZ demonstrated a high critical current density J/sub c/ of /spl sim/2.1 MA/cm/sup 2/ at 77 K all over the film; the fluctuation of J/sub c/ being within /spl plusmn/10% of the average. Inductive T/sub c/ measurement showed a very sharp peak with T/sub c/=90.5 K. Inductive-J/sub c/ and T/sub c/ of the Y123 film on CbS were 1.1 MA/cm/sup 2/ and 89.6 K, respectively, whereas the T/sub c/ of the Y123 film on LaAlO/sub 3/ was lower than 77 K owing to the occurrence of the a-axis grains.

Effect of adsorbed water contained in heat-treated silica particles on electrorheology

Electrorheology (ER) of heat-treated monodispersed silica particles in silicon oil was investigated using a rotational viscometer. The effect of adsorbed water contained in the silica particles on the ER effect was clarified. The silica particles were prepared by hydrolyzing tetraethylorthosilicate (TEOS). The silica particles were heated at various temperatures and suspended in silicon oil, followed by measuring the ER effect in the silicon oil at room temperature. The heat treatment temperature ranged from 25 to 700°C. The adsorbed water was characterized by thermogravimetry and IR spectroscopy. The ER effect became maximum at some heat treatment temperature between room temperature and 200°C. Physically adsorbed water on silica which was removed with heat treatment below 200°C affected the ER effect. A large quantity of physically adsorbed water, however, decreased the ER effect. The ER effect became negligibly small with heat treatment above 200°C. A proper quantity of physically adsorbed water was found to be necessary for an appreciable ER effect. The effect of adsorbed water on the ER is discussed.

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}

The 5-cm-diameter double-sided YBa 2 Cu 3 O 7 (YBCO) films were prepared by metal organic deposition (MOD) using a commercially available metal-naphthenate coating solution. Firstly, YBCO film was prepared by MOD on one side of a double-side-polished 5-cm-diameter LaAlO 3 substrate. Secondly, another side was similarly coated with YBCO by MOD. After the latter processing, degradation of average J c value in the first side was not observed; but the fluctuation of critical current density J c slightly increased. The double-sided YBCO films showed average J c values of 0.8-1.0 MA/cm 2 at 77 K and microwave surface resistances R s (12 GHz) of 0.86-1.07 mΩ at 70 K.

-Buffered YSZ Prepared by Fluorine-Free MOD

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.

Preparation of Double-Sided YBCO Films on LaAlO 3 by MOD Using Commercially Available Coating Solution

It has been difficult to prepare Y123 films with a thick ness greater than 400 nm by using a single coating-preflring procedure in metal organic deposition (MOD), since shrinkage of the film causes cracks in the pyrolysis process. In the present study, we developed a novel coating solution for crack-free films with thicknesses greater than 500 nm. We used a Y-, Ba-, and Cu-acetylacetonate-based solution (metal content: ~2mol metal/L) with various additives for the coating solution. We prepared prefired films on CeO2-buffered YSZ (CbYSZ) or CeO2-buffered SrTiO3 (CbSTO) substrates using the single coating-preflring process and this solution. When using some additives, we could obtain crack-free preflred films with thicknesses corresponding to 350-800 nm after crystallization to Y123. These prefired films were heat-treated at high-temperatures, resulted in epitaxial Y123 films. The Jc and Ic of the film on CbSTO was ~2.3 MA/cm2 and 143 A/cm, respectively.

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

Double-sided YBa 2 Cu 3 O 7 (YBCO) films were successfully prepared on 50-mm-diameter CeO 2 -buffered sapphire substrates by metalorganic deposition (MOD) process using an acetylacetonate coating solution. Mapping analysis of superconducting current densities (J c ) at 77.3 K revealed that J c values of the double-sided films indicated in excess of 2 MA/cm 2 in the center parts with a small decrease of J c at the outer side of the specimens. The J c values of one side (A) are higher than those of the other side (B). Microwave surface resistance (R s ) of sides A and B of the film exhibited 0.57 and 0.60 mΩ, respectively, at 70 K (12 GHz). The difference in the R s values should be attributed to the slight difference in the J c values, which arose from the surface morphology of the CeO 2 buffer layer and heat treatment conditions during the firing process in MOD.

Preparation of Y123 Thick Films by Fluorine-Free MOD Using a Novel Solution

12 cm-long rectangular MOD-YBa₂Cu₃O₇ (YBCO) films were prepared by MOD on R-plane sapphire substrates with an evaporated-CeO₂ buffer layer. The substrates were coated with a fluorine-free solution by two methods; spin coating and wire-bar coating. Distribution of inductive J_c of the spin-coated and wire-bar-coated Alms was investigated. In the inductive measurement of superconducting YBCO films, the data are presumed to be affected by film edges if the width is comparable to the diameter of the probe coil. However, the present results showed that the edge effect is less significant if the coil does not cross the film edges and that the inductive J_c agrees well with the transport J_c . YBCO films with high J_c were obtained by both the coating methods. At present, the J_c values and uniformity of J_c in YBCO films prepared by spin coating were higher than those by wire-bar coating.