Ag clad dip-coated superconductor tapes were fabricated from Tl-1212 superconducting powders of Tl0.8Bi0.2Sr2Ca0.8Y0.2Cu2O7 starting composition derived via two different methods; the conventional Solid-State (SS) and coprecipitation (CP) methods. The effects of using superconducting powders derived from SS and CP methods on phase formation, microstructure and critical current density were investigated for tapes annealed at 850°C for 60 min in flowing O2. XRD analysis of the tapes oxide cores indicated a lower volume percentage of 1212 phase for the CP tape (84%) compared to the SS tape (97%). However, the transport Jc of the CP tape (4670 A cm-2) was significantly higher than the SS tape (560 A cm-2). The SEM micrograph of the CP tapes oxide core revealed partially melted microstructure with fused grains in contrast to the microstructure of the SS tapes core which shows porous microstructure with irregularly shaped grains. The higher Jc for the CP tape is suggested to be due to improved inter-grain connectivity as a result of the partial melting.
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Superconductivity in thallium-based (Tl-based) materials was first reported by Sheng and Hermann (1988) and since then there have been numerous effort to fabricate these materials into superconducting wires and tapes for applications in superconducting motors, transformers, transmission lines and magnets (Selvamanickam et al., 1996; Goyal et al., 1995). Amongst Tl-based superconductors, much interest have been focused on single Tl-O layered Tl1223 and Tl1212 materials as they have been suggested to show better in-field behavior compared to multiple Tl-O layers system (Jergel et al., 1996). However, Tl1212 is expected to produce higher transport critical current density (Jc) and better performance in magnetic fields compared to Tl1223 due to reduced anisotropy through its stronger interlayer coupling and less severe thermally activated flux motion (Lao et al., 2000). Some research have been reported on Tl1212/Ag tapes using Powder-In-Tube (PIT) method (Hamid and Abdul Shukor, 2001) but no report was found for Tl1212 using Dip-Coating (DC) which is a faster and simpler method of tape fabrication. A key factor in processing Tl1212 tapes to achieve high Jc is to optimize not only the processing details of the tapes but also the quality of the superconductor powders used. For Tl-based superconductor system, synthesis of single-phased samples is difficult due to the constrain imposed by Tl2O3 that melts and evaporates at 717°C at atmospheric pressure whereas formation of the high-Tc phase typically takes place at higher temperatures (Abdul Shukor, 1993). Conventional solid-state method using oxide powders has employed the two-step method, which used non Tl-containing precursor in the first stage before addition of Tl in the final sintering stage to reduce Tl-loss. On the other hand, chemical coprecipitation method offers unique advantages over conventional solid-state method such as lower temperature synthesis, precise composition control, high purity and chemical homogeneity due to the fact that in the process the mixing of components occurs at the atomic level (Segal, 1997; Rao et al., 1993). Interestingly, single-step synthesis of bulk Tl1212 using Tl-containing coprecipitated precursors have been recently reported (Salleh et al., 2005). However, the use of the coprecipitation derived powder in fabrication of dip-coated tapes has not been reported. In this paper, we report on Tl1212/Ag dip-coating tapes fabrication using powders prepared by SS and CP methods. Results of powder X-ray Diffraction (XRD) and Scanning Electron Microscope (SEM) investigations on the tapes are presented and analyzed.
MATERIALS AND METHODS
In the first stage, bulk Tl0.8Bi0.2Sr2Ca0.8Y0.2Cu2O7 superconductor was prepared via two different routes, i.e., the coprecipitation route and the conventional solid-state route. The bulk SS sample was prepared by a two-step solid-state reaction, as described in previous research (Yahya et al., 2004). In the coprecipitation route, a similar synthesis process as described by Salleh et al. (2005) was used. Reagent-grade Tl, Bi, Sr, Ca, Y and Cu acetates with stoichiometric ratios of 0.8:0.2:2:0.8:0.2:2 were dissolved in glacial acetic acid and was reacted with excess oxalic acid at 50°C under continuous stirring, yielding precipitation of the mixed oxalates. The precipitate was filtered and dried overnight at 80°C and then was ground to powdery state. Calcination of the powder was done in air at 600°C, to avoid loss of Tl for a duration of 20 h with heating and cooling rates of 1°C min-1 following which the powder was reground and sintered at 1000°C for 6 min in flowing O2 followed by furnace cooling to room temperature. A slurry solution was then prepared by mixing the SS (and CP) superconductor powder with appropriate amounts of organic binder, dispersant and solvent (Sung et al., 2000). Strips of silver substrate were dipped into the slurry and coated films of 0.25-0.40 mm in thickness were achieved and monitored. The coated silver strips were dried at 80°C for 2 days and were separately wrapped using silver foil and were heated at 450°C in air for 3 days, after which the tapes were deformed by cold rolling to 45-55% deformation in tapes thickness. The tapes were annealed in flowing O2 at 850°C for 60 min annealing duration.
Electrical resistance (dc) measurements for both tape samples were carried out using the standard four-point-probe method with silver paint contacts in a Janis model CCS 350ST cryostat in conjunction with a closed cycle refrigerator from CTI Cryogenics model 22. The phase characterization was done by powder XRD using Bruker D8 Advanced diffractometer with Cu-Kα source. Transport critical current densities (Jc) were measured in zero magnetic fields using standard four-point-probe method and evaluated using the electric field criterion of 1 μV cm-1. SEM micrographs were recorded using JEOL model JSM-6360LA scanning electron microscope.
RESULTS AND DISCUSSION
XRD patterns for both fabricated tapes are shown in Fig. 1. The volume fractions of 1212 and 1201 phase and SrCO3 impurity are estimated using the same technique employed by Matsushita et al. (1994). The normalized resistances versus temperature curves of the bulk and tape samples are shown in Fig. 2. All bulk and tape samples exhibit metallic behavior in its normal state with Tc onset of between 85-103 K.
|Fig. 1:||Powder X-ray diffraction pattern for tape samples (a) SS and (b) CP. Peaks due to 1201 phase and SrCO3 impurities are indicated by (*) and (+), respectively|
|Fig. 2:||Normalized resistance versus temperature curve for (a) SS bulk, (b) CP bulk, (c) CP tape and (d) SS tape samples|
Based on the Tc onset values, it is suggested that 1212 phase is responsible for the observed superconductivity as superconductivity due to 1201 phase is reported to be below 50 K (Sheng et al., 1991). Table 1 show sample, 1212:1201:SrCO3 phase Vol. %, zero-resistance critical temperature (Tc zero) of bulk samples, zero-resistance critical temperature (Tc zero) of tape samples and transport critical current density (Jc) at 40 K of tapes prepared in this study.
|Fig. 3:||Microstructures of bulk and tapes (a) SS bulk, (b) SS tape, (c) CP bulk and (d) CP tape|
|Table 1:|| |
Sample, 1212:1201:SrCO3 phase Vol. % of tape samples, zero-resistance critical temperature (Tc zero) of bulk samples, zero-resistance critical temperature (Tc zero) of tape samples and transport critical current density (Jc) at 40 K of tape samples
In this study, higher Jc has been obtained for the CP tape compared to the SS tape. In view of the fact that the 1212 volume fraction for CP tape was only 84%, it is possible that Jc can be further increased if the 1212 volume fraction is higher and the amount of impurity is lower. Work on addition of a sintering aid to the Tl-containing precursor that results in SrCO3-free superconducting sample has been carried out and will be reported elsewhere. Because the volume fraction of the 1212 phase in the CP tape was lower than that of the SS tape (Table 1), it is suggested that the higher magnitude of Jc for the CP tape is not due to 1212-phase volume fraction. In addition, differences in thermal and mechanical treatments of the tapes may not be the reason as both tapes were subjected to the same thermomechanical processes during fabrication. The minor 1201 phase and the SrCO3 impurity observed for the CP tape are also not know to introduce magnetic pinning effects in high-temperature superconductors. As such, the reason for the differences in Jc can be due to other reasons, such as, the microstructure of the tapes core.
Figure 3 shows SEM micrographs of fractured surface of the bulk and superconducting oxide core of the tape samples. The micrograph of CP bulk revealed elongated plate-like grains with sizes between 7-15 μm, which is remarkably different from the micrograph of SS bulk that revealed slightly rounded grains of 3-4 μm in average size. Interestingly, the micrograph of CP tapes core displayed partially melted and fused grains in contrast to micrograph of SS tapes core which revealed porous microstructure consisting of irregular shaped grains of 1-4 μm in average size. No particular texturing was observed from the SEM micrograph of the tapes core for both tapes (Fig. 3a-d).
From the microstructural observations, it is possible that the partially melted microstructure with fused grains and the accompanying reduction in porosity of the CP tapes has lead to improved connectivity between grains which may be the reason for the higher Jc compared to the SS tape. While, reduction in sample porosity is known to cause increase in sample Jc (Jeong and Sohn, 1998), partially melted microstructure with fused grains have shown higher Jc values for Tl1212/Ag tapes fabricated via the powder-in-tube method (Hamid and Abdul Shukor, 2001).
Tl0.8Bi0.2Sr2Ca0.8Y0.2Cu2O7 tapes employing powders derived from the conventional Solid-State (SS) and Tl-containing coprecipitation (CP) methods were fabricated using dip-coating technique on silver substrate. The tape fabricated using Tl-containing coprecipitation derived oxide powder has superior transport Jc than the tape that used conventional solid-state derived powder. The enhanced Jc of the CP tape compared to the SS tape is attributed to improved inter-grains connectivity as a result of partial melting.
This research has been supported by the Malaysian Ministry of Science, Technology and Innovation under IRPA grant No. 09-02-01-007-EA007.
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