Seok Hern Jang

Measurement of joint properties of Bi(Pb)-Sr-Ca-Cu-O (2223) tapes by field decay technique

We joined 19-multifilamentary Bi-2223 superconductor tapes and fabricated double-pancake coils by using resistive- and superconducting-joint methods. The critical current ratio (CCR) of the jointed tape and the decay characteristics, joint resistance, and n-value of the pancake coils were evaluated. The joint resistance of the coils was characterized by the field decay technique. It was observed that the CCR was higher in the joined tape made by the resistive-joint method, compared to that by the superconducting-joint method. On the other hand, joint resistance was measured to be 4 orders of magnitude smaller in the superconducting-joint coil; approximately 40% of critical current was retained in the persistent current mode and the joint resistance was 0.18 n/spl Omega/. Better and longer retention of the magnetic field in the superconducting-joint coil is believed to be due to the direct connection between the superconducting cores.

Effect of W addition on the microstructure and properties of Ni-W substrates for coated conductors

We fabricated Ni and Ni-W alloys for use as substrates in YBCO coated conductor applications and evaluated the effects of the W addition on the texture, microstructure, and mechanical and magnetic properties of the substrate. Pure Ni and Ni-W (2, 3, and 5 at.%) alloys were prepared by plasma arc melting and then cold rolled and annealed in the temperature range of 600-1300/spl deg/C. The texture of the substrates was evaluated by pole-figure and orientation distribution function (ODF) analysis. The magnetic properties were also evaluated using the physical property measurement system (PPMS). It was observed that the Ni-W substrates had a stronger cube texture and a wider annealing temperature range in which the cube texture became stable than those of the pure Ni substrate. The full-width at half-maximums (FWHMs) of in-plane texture for the Ni-W substrate were 4.42/spl deg/-5.57/spl deg/ at an annealing temperature of 800/spl deg/C-1300/spl deg/C, while that of pure Ni was 9.5/spl deg/ at 800/spl deg/C. Therefore, it is considered that the addition of W enhances the formation of the cube texture and improves the texture stability at higher annealing temperatures. In addition, the Ni-W substrates had a smaller grain size and higher mechanical strength and hardness, as compared to those of the pure Ni substrate. These improvements are probably due to various strengthening mechanisms, such as solid solution hardening and/or grain size strengthening. PPMS analysis showed that the addition of W effectively reduced the saturation magnetization in an applied magnetic field, as well as the Curie temperature.

Fabrication and characteristics of the joint properties in (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x/ closed double pancake coil

We fabricated two superconducting closed coils and evaluated the joint resistance of the coils by a field decay technique at 77 K. In the process (Bi, Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub x//Ag tape was wound in the form of double-pancake coils, and the ends of the tape were joined to each other by a resistive-joint method. Joint resistance of the coils was evaluated as a function of critical current of the coil, contact length, sweep time, and external magnetic flux density. Joint resistance was observed to be almost independent of critical current, sweep time, and external magnetic flux density, i.e., resistances were within the range of 7.8 /spl times/ 10/sup -9/ /spl Omega/ to 12.6 /spl times/ 10/sup -9/. On the other hand, the joint resistance was dependent on the contact length of the closed coil. The initially induced current/magnetic flux was observed to be dependent on critical current and contact resistance of the coil.

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.

The Effects of the Humidity and Thickness on YBCO Film Prepared Using the TFA-MOD Method

This study examined the epitaxial growth of YBa₂Cu₃O_7-x (YBCO) films on (00l) LaAlO₃ substrates prepared by metal organic deposition (MOD) using a trifluoroacetate (TFA) solution. The effects of humidity (0-20% of P_H2O) and film thickness (0.4-2.1 mum) on the phase formation, texture, and microstructures of the YBCO films were examined by X-ray diffraction, pole-figure, and scanning electron microscopy (SEM). The microstructure and resultant critical properties varied remarkably with humidity. With increasing humidity, the amount of the second phase (BaF₂) decreased, the degree of texture was enhanced, and consequently the critical current (I_C) and temperature (T_C ) were improved. In the case of multi-coated films, the I_C value increased from 35 to 152 A/cm-width with an increasing number of coatings from one to three, while the corresponding critical current density (J_C) was in the range of 0.9-1.5 MA/cm² . Both the I_C and J_C decreased when an additional coating was applied due to microstructural degradation.

A Study on the Processing Variables of the BSCCO-2212 Bulk Superconductors

Bi-2212/SrSO4 bulk superconductors were fabricated using a casting process, and the effects of the powder mixing method, annealing temperature, and SrSO4 content on the texture, microstructure, and critical properties were evaluated. The Bi-2212 powders were mixed with SrSO4 either by hand-mixing (HM) or planetary ball milling (PBM), followed by melting at 1100degC-1200degC, solidification, and annealing. The rod produced by PBM had a more homogeneous microstructure and smaller grain size of SrSO4 and minor phases, resulting in increased Ic, than the rod made by HM. The Ic of the rod was also dependent on the annealing temperature with the highest Ic of 200 A being obtained at an annealing temperature of 810degC. This was attributed to the moderate density and 2212 texture as well as the smaller and less minor phase than that observed at higher temperature. In addition, the optimum SrSO4 content was found to be 6 wt.% for an increased Ic. SEM, EPMA, and DTA analyses indicated that the differences in microstructural evolution caused the variations of the Ic with the powder mixing method, annealing temperature, and SrSO4 addition.

Effect of twisting on microstructure, critical current, and AC losses of Bi-2223 superconductor tape

Abstract We evaluated the effect of twisting on microstructure, critical current, and AC losses of Bi-2223 superconductor tapes. It was observed that grain size and grain alignment were reduced with decreasing twist pitch probably due to the formation of an irregular interface between Ag and filaments. The critical current of the tapes decreased with decreasing pitch. For the tape having a twist pitch of 10 mm, ≈50% of the critical current was maintained compared to that of the non-twisted tape. The reduction of critical current is believed to be due to irregular interface, poor grain alignment, small grain size, and existence of second phases, etc. In addition, it was observed that AC losses of the tapes were reduced as the pitch decreased probably due to the combined effect of lower critical current and electrically decoupled filament of twisted tape.

Effects of Firing Temperature and Film Thickness on the Critical Properties of YBCO Film by the '211 Process'

We fabricated by metal organic deposition (MOD) using trifluoroacetic acid (TFA) via the “211 process”, and then evaluated the phase formation, texture evolution, and critical properties as a function of the firing temperature and film thickness. In the fabrication process, Y2Ba1Cu1Ox and Ba3Cu5O8 powders were used as precursors instead of Y-, Ba- and Cu-based acetates. The films were fired in the temperature range of 750°C -800°C and the film thickness of films was controlled by repeating the number of coating cycles. The microstructure varied significantly with the firing temperature. The grains size increased, the film became denser, and the degree of the texture and phase purity varied with increasing firing temperature. The films fired at 775°C after calcining at 4005 showed the highest critical current (Ic) of 18 A/cm-width, which corresponds to a critical current density (Jc) of 0.9 MA/cm2. For multi-coated films, the Ic increased from 18 in the singly coated film to 100 A/cm-width with the triply coated film. In contrast, the corresponding Jc were in the range of 0.9-1.2 MA/cm2. Both Ic and Jc values decreased as the coating number repeated further as a result of the degraded microstructure.

Fabrication of TFA-MOD YBCO Films Using the Y2Ba1Cu1Ox and Ba3Cu5O8 Powders

We fabricated YBCO film using a TFA-MOD method and evaluated the phase formation, texture evolution, and critical properties as a function of the firing temperatures. In order to enhance the reaction kinetics and to control the formation of the second phases, Y2Ba1Cu1Ox and Ba3Cu5O8 powders were used as precursors (the so called “211 process”), instead of Y-, Ba-, and Cu-based acetate, and dissolved in trifluoroacetic acid (TFA). The films were calcined at 460°C and then fired at 750°C-800°C in a 12.1% humidified Ar-O2 atmosphere. We found that the microstructure varied significantly with the firing temperature; the grain grew further and the film became denser as the firing temperature increased. The textures of all of the films were similar and mainly biaxial. On the other hand, the intensity of the major and minor texture components differed from each other. For the film fired at 775°C, the critical current was obtained to be 39 A/cm-width (corresponding critical current density is 2.0 MA/cm2), which was probably attributed to such factors as the enhanced phase purity and out-of-plane texture, the moderate film density and grain size, and crack-free surface.