Bong Ki Ji

Texture and surface analysis of NiO buffer deposited on biaxially textured Ni tapes by a MOCVD method

NiO buffer layers for YBCO coated conductors were deposited on textured Ni substrates by a metal-organic chemical vapor deposition(MOCVD) method. Processing variables were the oxygen partial pressure and substrate temperature. The degree of texture and the surface roughness of the deposited NiO surface were analyzed by X-ray pole figure, atomic force microscopy (AFM), and scanning electron microscope (SEM). The (200) textured NiO layer was formed at 450 /spl sim/ 470 /spl deg/C and oxygen partial pressure of 1.67 Torr. Out-of-plane(/spl omega/-scan) and in-plane(/spl Phi/-scan) texture were 10.34/spl deg/ and 10.00/spl deg/, respectively. The surface roughness estimated by atomic force microscopy was in the range of 3.1 /spl sim/ 4.6 nm which was much smoother than that prepared by an oxidation method. We discuss the development of the (200) texture in the MOCVD-NiO films in terms of processing variables.

Characterization of the thermal conductivity and mechanical properties of sheath alloy materials for Bi-2223 superconductor tapes

We evaluated the effects of adding alloying-element to the Ag sheath on the thermal conductivity and mechanical properties of Bi-2223 tapes. The thermal conductivity of Ag and Ag-alloys was evaluated by using the thermal integral method in the temperature range of 10 to 100 K, and correlated to the indirectly-measured values obtained from the density, specific heat, and thermal diffusivity. It was observed that the additions of Au, Pd, and Mg to the Ag sheath significantly decreased thermal conductivity at low temperatures, probably due to the presence of alloying-elements. Specifically, the thermal conductivity of the Ag/sub 0.92/Pd/sub 0.06/Mg/sub 0.02/ and Ag/sub 0.973/Au/sub 0.025/Mg/sub 0.002/ alloys at 30 K was 28.9 and 59.2 (W/(m/spl middot/K)), respectively, which is about 17 to 35 times lower than that of Ag (997.2 (W/(m/spl middot/K))). At the same time, these additions to the Ag sheath, improved its mechanical strength. It is believed that this improvement is related to the presence of dispersed alloying-elements which leads to a smaller grain size.

Superconducting joint between Bi-Pb-Sr-Ca-Cu-O superconductor tapes

We evaluated the effects of joining processes such as contact method, shape of joined area and pressure on the electrical and mechanical properties of Bi-2223 superconducting tape. Specifically, the current carrying capacity and bonding strength of the lap-joined tape were measured as a function of uniaxial pressure and correlated to the microstructural evolution. It was observed that the current carrying capacity was reduced in the transition region of the jointed tape and was significantly dependent on the uniaxial pressure. The jointed tape, fabricated with a pressure of 1,600 MPa, showed the highest value of current capacity 90% of the tape itself. It is believed that the highest value of current capacity results from improvements in core density, contacting area and grain alignment. In addition, bonding strength of the jointed tape was evaluated and correlated to the microstructural evolution.

Characterization of thermal conductivity and mechanical properties of Ag-alloy sheathed Bi(Pb)-Sr-Ca-Cu-O superconductor tape

We evaluated the effect of alloying additions to Ag on thermal conductivity and mechanical properties of Ag-alloy sheathed Bi-2223 (BSCCO) superconductor tape. The tapes were made with combinations of Ag alloys such as Ag-Mg, Ag-Sb, and Ag-Au for inner and outer sheath. Thermal conductivity of the tapes was evaluated by using thermal integral method at 10-120 K. It was observed that the addition of alloys reduced remarkably thermal conductivity and improved mechanical strength. The thermal conductivity for Ag-Mg, Ag-Sb, and Ag-Au at 40 K was measured to be 411.4, 142.3, and 109.7 W/(m/spl middot/K), respectively, which is approximately 2 to 9 times lower than that of Ag (1004.6 W/(m/spl middot/K)). In addition, the thermal conductivity of alloy-sheathed tape significantly depended on their thermal conductivity of sheath materials. For Ag-alloy sheathed tapes, the thermal conductivity was much lower (i.e., 5 -18 times lower) than that of the Ag sheathed tape The mechanical property of alloy-sheathed tape was also evaluated and correlated to the microstructural evolution.

Development of textured Ni substrates for coated conductor prepared by powder metallurgy and plasma arc melting method

We fabricated the textured Ni substrate and evaluated the effects of processing variables on microstructural evolution and texture transformation. Ni-rods as an initial specimen were prepared by two different methods, i.e., powder metallurgy (P/M) and plasma arc melting (PAM). To evaluate the effect of two preparation methods, the initial specimens were prepared to be of same size and experienced by same rolling conditions. The texture of the substrate was characterized by pole-figure and surface condition was evaluated by atomic force microscopy. It was observed that the texture of substrate made by P/M did not significantly varied with annealing temperature of 800 /spl sim/ 1200/spl deg/C and the full-width at half-maximums (FWHM) of both in-plane and out-of-plane were 9/spl deg/ /spl sim/ 10/spl deg/. On the other hand, the texture of substrate made by PAM was more dependent on the annealing temperature and the FWHM of in-plane texture was 9/spl deg/ /spl sim/ 13/spl deg/ at the temperature range. In addition, twin texture, (221), was formed as the temperature increased further. OM micrographs showed that the grain size of substrate made byP/M was smaller than that made by PAM and this difference was correlated to the microstructure of initial specimens.

Deposition of CeO/sub 2/ buffer layers for YBCO coated conductors on biaxially textured Ni substrates by MOCVD technique

CeO/sub 2/ buffer layers for YBa/sub 2/Cu/sub 3/O/sub 7/(YBCO) coated conductors were deposited on biaxially textured Ni substrates by metalorganic chemical vapor deposition (MOCVD) method. The variables were the oxygen partial pressure (P/sub O2/), deposition temperature and time. The [200] texture of CeO/sub 2/ was formed at T = 500 /spl deg/C - 520 /spl deg/C, t = 3 - 15 min and P/sub O2/ = 2.30 torr, while the [111] and [200] texture were competitively formed at other condition. The surface roughness of CeO/sub 2/ films was as good as 5 - 15 nm up to 500 /spl deg/C, while it rapidly increased as a result of grain growth of the CeO/sub 2/ at T /spl ges/ 520 /spl deg/C. The surface roughness of the CeO/sub 2/ films also increased as the deposition time increased. The growth rate of the CeO/sub 2/ films at T = 520 /spl deg/C and P/sub O2/ = 2.30 torr was 200 nm/min, which is much higher than those prepared by other physical deposition methods.

Microstructural and texture analysis of BSCCO-2223 superconducting tape

We evaluated the microstructural evolution and the degree of texture of single filamentary BSCCO-2223 superconductor tape. Specifically, in order to characterize the positional degree of texture inside the BSCCO core quantitatively, a pole figure analysis was taken on the surfaces at different depths of superconducting core, and correlated to the microstructural investigation. Pole figure analysis indicated that the degree of texture varied with depth of the superconductor core, and was higher near the interface than inside the core. In addition, EPMA analysis showed that the second phases were randomly distributed and there is no remarkable difference in the distribution of second phases inside the BSCCO core.

Interface irregularity and texture evolution of Bi-Sr-Ca-Cu-O superconductor tape processed by PIT method

We evaluated the degree of texture and the interface irregularity between the Ag sheath and the superconductor core of Bi-Sr-Ca-Cu-O (BSCCO) superconductor tape during drawing and rolling processes. The degree of texture and the interface irregularity were characterized by pole figure analysis and the coefficient of variation in thickness (COV) factor, respectively. It was observed that the interface became gradually irregular during the drawing and rolling processes. On the other hand, the degree of texture improved significantly during rolling process, but little during the drawing process. The critical current of the tape depended remarkably on the combined effects of the interface irregularity and the degree of texture. As the dimension of the wire/tape was changed from a diameter of 3.25 mm to a thickness of 0.20 mm, the critical current increased by 10 times. Microstructural investigation showed that grain alignment was locally degraded by the existence of both second phases and interface irregularity. It was observed that larger grain size and better texturing developed near the relatively straight interface compared to those inside the superconducting core.

Fabrication and characterization of cube textured Ni substrate for YBCO coated conductors

We fabricated Ni-substrates for YBCO coated conductors applying by powder metallurgy technique and evaluated the effects of pressing and annealing time on the texture. Compacts were prepared applying uniaxial or isostatic pressure. The texture of the substrate, made by applying cold isostatic pressure (CIP), was stronger than that by uniaxial pressure. We attribute this to the fact that the CIP method provided higher density and more uniform density distribution. The texture of the substrate made by CIP had a strong 4-fold symmetry and [111]/spl par/ND texture after annealing at a temperature of 1000 /spl deg/C. It was noted that the degree of texture was almost independent of annealing time and the full-width at half-maximum (FWHM) of in-plane and out-of-plane were measured to be in the range of 9.55/spl deg/ - 10.53/spl deg/ and 8.57/spl deg/ - 9.85/spl deg/, respectively. The development of strong cube texture and high fraction of low angle grain boundaries of Ni-substrates made by powder metallurgy technique in our study was considered to be useful in the deposition of YBCO coated conductors.

Bi-axially textured Ni tapes fabricated by a cold rolling process of nickel powder compacts

Powder metallurgy has been widely used to make high purity metals and alloys. We applied the powder metallurgy route to the fabrication of the bi-axially textured Ni and Ni-W alloy tapes for YBCO coated conductors. The Ni and Ni-W powder compacts were cold-rolled into tapes. The Ni powder used in this study was 5 /spl mu/m in size and 99.99% in purity. To densify them, the Ni and Ni-W rods were sintered at 1100 /spl deg/C for 6 h in 96% Ar-4% H/sub 2/ atmosphere, and cold-rolled into thin tapes of 80-100 /spl mu/m thickness. Heat-treatment condition for the development of cube texture were established for the tapes. The out-of plane and in-plane texture of the Ni and Ni-W tapes were about 8/spl deg/. In this paper, we discuss the effect of the processing variables on the texture development of the Ni and Ni-W tapes.