Hiroyuki Akata

Superconducting property of Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub x/ films formed on silver substrates by continuous chemical vapor deposition technique

A new CVD system equipped with a triple-reactor was developed to improve the forming rate of the superconducting tape. Three-layered Y123 films with good crystallinity covering the total thickness were obtained directly on roll milled non-textured Ag tapes by using this triple-stage CVD system. 11 m long Y123 tape with a uniform J/sub c/ of 3.0/spl times/10/sup 4/ A/cm/sup 2/ at 77 K, 0 T was successfully prepared on roll milled non-textured Ag tape at a forming rate of 3.0 m/h, Y123 films formed directly on the cube-textured Ag[100] tapes showed two types of orientation, cube-on-cube and diagonal-on-cube, between Y123 and Ag. On the other hand, Sm123 and Nd123 films showed the simple orientation, cube-on-cube, so these films were expected to improve the in-plane alignment of Y123 film by inserting them as the intermediate layer.

Biaxially oriented NdBa/sub 2/Cu/sub 3/O/sub 7/ films prepared on {100} textured Ag tapes without any buffer layers

We successfully obtained biaxially oriented NdBa/sub 2/Cu/sub 3/O/sub 7/ superconducting films with the T/sub c/ of 89 K on {100} textured Ag tapes without any buffer layers. The lower substrate temperature seems to be favorable for the better biaxial orientation of the NdBa/sub 2/Cu/sub 3/O/sub 7/ grains; the film deposited at 650/spl deg/C had an excellent biaxial crystal orientation, while on the textured Ag tape heated at 720/spl deg/C, NdBa/sub 2/Cu/sub 3/O/sub 7/ phase grew in two orientations, 45/spl deg/ apart.

In-plane alignment of YBa/sub 2/Cu/sub 3/O/sub 7-y/ films realized by CeO/sub 2/ buffer layer on textured Ag{100} tapes

Homogeneous in-plane aligned YBa/sub 2/Cu/sub 3/O/sub 7-y/ (YBCO) films on long metallic substrates are required for power applications. In order to establish in-plane aligned YBCO films, we have been studying Ag{100}<001> textured tapes buffered with a [001]-oriented CeO/sub 2/ layer. The YBCO and CeO/sub 2/ films were deposited by pulsed laser deposition (PLD). The CeO/sub 2/ deposition rate was 900 /spl Aring//min. The CeO/sub 2/ films on Ag{100}<001> textured substrates mere [100]-oriented and highly in-plane aligned. The YBCO films deposited by PLD on CeO/sub 2/ buffered Ag substrates also have c-axis oriented and good in-plane alignment. T/sub c/ and J/sub c/ values of the YBCO films were 81 K and 1/spl times/10/sup 4/ A/cm/sup 2/ at 77 K, respectively.

YBCO-coated conductor on buffered textured metallic tape by hot-wall type MOCVD

We have prepared YBa/sub 2/Cu/sub 3/O/sub 7-y/ thin films on {100}<001> cube textured Ag tape (CUTE-Ag tape) by metal organic chemical vapor deposition (MOCVD). The obtained films are highly c axis oriented, and the surfaces of the films were smooth. However, the in-plane alignment of the films was not pure four-fold symmetric. Then we examined preparation of YBCO films on Y/sub 2/O/sub 3/ buffered CUTE-Ag tapes. The Y/sub 2/O/sub 3/ layer was deposited by MOCVD and the YBCO films on that layer was c axis oriented with good in-plane alignment. Furthermore after the YBCO films growth was terminated, the peak of Y/sub 2/O/sub 3/ almost disappeared. We considered that the Y/sub 2/O/sub 3/ layer was incorporated into the YBCO film during deposition. We also deposited YBCO films on {110}<001> textured Ag tape for which good in-plane alignment of YBCO film was expected from the near-coincidence site lattice (NCSL) model. The critical temperature of both YBCO films on the CUTE-Ag tape and on the {110}<001> textured Ag tape were T/sub c(zero)/=80 K.

Superconducting Property of Y1Ba2Cu3Ox Films Formed on Silver Substrates by Continuous Chemical Vapor Deposition

Good c-axis oriented YBCO films were formed directly on both non-textured and cube- textured Ag tapes. 11m long YBCO tape with Jc (lμV/cm) of 3.0x104A/cm2 at 77K, 0T was prepared on roll milled non-textured Ag tape at a forming rate of 3.0m/h. YBCO films formed on cube-textured Ag{100} tapes showed two types of the location, cube-on-cube and diagonal- on-cube between YBCO and Ag, however, no inter-diffusion between YBCO and Ag was analyzed by TEM-EDS study, and also the epitaxial growth of YBCO layer grew on Ag{l00} was observed.

Evidence of Mobile Ag and Growth Mechanism of YBa2Cu3O7-y Films Deposited on Cube Textured Ag Tape by Chemical Vapor Deposition

YBa2Cu3O7-y (YBCO) films were prepared on {100} cube textured Ag tape (CUTE-Ag tape) by metal organic chemical vapor deposition (MOCVD) using a liquid state metal organic (MO) source. Numerous spiral steps accompanied by screw dislocations were observed and many particles were observed on the terrace surface of the spiral steps by atomic force microscopy (AFM). We observed a change in surface morphology before and after heat treatment in vacuum at 550°C. After heat treatment, these particles became smaller and concentrated at the edge of the spiral steps. We confirmed by scanning Auger microscopy that the main composition of these particles was Ag. We propose a growth mechanism on a Ag substrate in which Ag segregates to the surface and acts as surfactant for YBCO growth.

Rapid Preparation of YBCO Coated Conductors Using Ag Tapes by Liquid Phase Epitaxy

We have succeeded in YBa2Cu3O7-d(YBCO) thick film growth on seeded single crystals of silver plates and on seeded cube-textured silver tapes by liquid phase epitaxy (LPE). In order to decrease the processing temperature far from the melting point of silver (941 °C in the ambient atmosphere), we use a novel solvent which is composed of BaO-CuO-BaF2-Ag. The typical processing temperature is about 860–900°C. The YBCO films formed on the seeded Ag(110) single crystalline substrates show very flat surface morphology and good in-plane orientation as well as good c-axis orientation perpendicular to the substrate surface. The good in-plane alignment of YBCO // Ag is also observed. These films show superconductivity at about 90K. The films formed on cube-textured tapes of Ag{100} also show complete coverage and c-axis orientation. We have proved a possibility of fabricating long superconducting tapes using textured silver tapes by the LPE processing, which is superior to the conventional vapor phase deposition for the rapid and lower cost fabrication of thick films.

LPE growth of YBCO on silver substrates

The YBCO thick films were formed by LPE on the polycrystalline silver substrates, silver (110) single crystalline substrates and {100} cube textured silver tapes. The films on these substrates were grown at such a low growth temperature as 867 °C. For the polycrystalline silver substrates, c-axis oriented YBCO films were observed on particular grains which have specific orientation. On the silver (110) single crystalline substrates, epitaxially grown YBCO covered all over the surface. Critical temperature of 90 K was measured by SQUID but resitivity drop was also observed at 40 K with the measurement current density of ∼104A / cm2. Epitaxially grown YBCO were observed all over the surface of (100) |001]| cube textured silver tapes and thickness of YBCO film is about 2 μ m.

Preparation of YBa2Cu3O7-y Thin Film on the Cube Textured Silver Tape by MOCVD

In order to improve the in-plane alignment of YBCO films on the {100} textured Ag tape, we examined to prepare the Y2O3 buffer layer on the MgO substrate and {100} Ag tape by metal organic chemical vapor deposition (MOCVD). The {n00} texturing of Y2O3 layer on the MgO became predominantly at 680°C and the YBCO films on that layer showed T c(zero) = 90K The YBCO film on the Y2O3 buffered {100} Ag tape was also c-axis oriented and good in-plane alignment, however, the Y2O3 layer was not {n00} oriented. Therefore it did not show good superconducting properties. It indicated that the texturing of Y2O3 was important to obtain high superconducting properties.

Growth mechanism of YBa/sub 2/Cu/sub 3/O/sub 7-y/ thin films on the metallic tapes by MOCVD

We fabricated a YBa/sub 2/Cu/sub 3/O/sub 7-y/ (YBCO) thin film on an MgO single crystalline substrate and cube textured (CUTE-Ag)-tapes using metal organic chemical vapor deposition (MOCVD). It was confirmed that a c-axis oriented YBCO film grew on these substrates. The terraced surface and spiral growth pattern are clearly visible. Many Ag particles were observed on the terrace surface of the spiral steps. After heat treatment these particles concentrated at the edge of the spiral steps. The dependence of the terrace width on the substrate temperature is drastically changed at the certain temperature range between 750 and 800/spl deg/C. At high temperatures, the terrace width of the films on MgO increases and reaches about 90 nm at 800/spl deg/C, whereas those on CUTE-Ag tape have a terrace width of about 160 nm at the same temperature. The values of the supersaturation ratios are estimated as 50 and 15 from the terrace widths on MgO and CUTE-Ag. The results from the surface morphology and the microstructure suggest that there is a difference in the growth mechanism between the YBCO films on MgO and those on CUTE-Ag in the higher temperature region. We speculate that the quasi-liquid layer is more stable on the CUTE-Ag at higher temperatures than 750/spl deg/C, and the growth mechanism of the YBCO films on the CUTE-Ag is mixed with the Vapor-Liquid-Solid (VLS) growth mode and the surface-active species (Surfactants) growth mode.