Influence of Tl-Containing Coprecipitation Derived Precursor Powder on Microstructure and Transport Properties of Tl_0.8Bi_0.2Sr₂Ca_0.8Y_0.2Cu₂O₇ Superconducting Dip-Coated Tapes

MATERIALS AND METHODS

In the first stage, bulk Tl_0.8Bi_0.2Sr₂Ca_0.8Y_0.2Cu₂O₇ 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-K_α source. Transport critical current densities (J_c) 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 T_{c 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 T_{c 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 (T_{c zero}) of bulk samples, zero-resistance critical temperature (T_{c zero}) of tape samples and transport critical current density (J_c) 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 J_c 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 J_c 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 J_c 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 J_c 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 tape’s core displayed partially melted and fused grains in contrast to micrograph of SS tape’s 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 J_c compared to the SS tape. While, reduction in sample porosity is known to cause increase in sample J_c (Jeong and Sohn, 1998), partially melted microstructure with fused grains have shown higher J_c values for Tl1212/Ag tapes fabricated via the powder-in-tube method (Hamid and Abdul Shukor, 2001).

CONCLUSION

Tl_0.8Bi_0.2Sr₂Ca_0.8Y_0.2Cu₂O₇ 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 J_c than the tape that used conventional solid-state derived powder. The enhanced J_c of the CP tape compared to the SS tape is attributed to improved inter-grains connectivity as a result of partial melting.

ACKNOWLEDGMENT

This research has been supported by the Malaysian Ministry of Science, Technology and Innovation under IRPA grant No. 09-02-01-007-EA007.