In this paper, we investigated the influences of Na substitution on the enhancements of critical current density (J c) in the BiPbSrCaCuO (BPSCCO) superconductors. The Bi 1.6 Pb 0.4 Sr 2 Ca 2¹x Na x Cu 3 O 10+¤ (where x was ranged from 0 to 0.1) polycrystalline samples were fabricated using solid state reaction technique. The zero electrical critical temperature (T c) of Na-substituted BPSCCO samples was found to be slightly higher than that of Na-free BPSCCO one. Magnetization J c data measured at 65 K with field applied parallel to the c-axis of the samples showed that J c of Na-substituted BPSCCO samples were enhanced. Possible reasons for the J c enhancements were attributed to the effective flux pinning by crystal defects at Ca layers generated by Na substitution. Improvements of volume pinning force (F p) in terms of magnitude and position of F pmax were also obtained.

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Abstract:In this work, the effects of sintering temperature and Na subtitution on zero critical temperature (T c,0 ) in Bi-Pb-Sr-Ca-Cu-O superconductors were investigated. The Bi 1.6 Pb 0.4 Sr 2 Ca 2-x Na x Cu 3 O 10+ bulk samples with x = 0.00 and 0.05 (BPSCCO) were prepared by using the solid state reaction technique at elevated sintering temperatures of 840 and 850 o C. To examine the improvements of structural and superconducting properties of the BPSCCO samples, the X-ray diffraction (XRD), scanning electronic microscope (SEM) and DC resistance versus temperature (R-T) measurements were carried out. The experimental findings exhibit a co-existence of the high T c (Bi-2223) and low T c (Bi-2212) phases in all samples. The estimated volume fraction of the Bi-2223 phase was increased from 71.4 to 75.7%, and the average size of the Bi-2223 grains was enlarged from 4.5 to 9.8 m as changing the sample preparation conditions. The improved connectivities between the Bi-2223 grains were also obtained by quantitively analyzing the relation between T c,0 and residual resistance ratio (RRR). These improvements were likely to be reasons for the enhancements of T c,0 in the BPSCCO samples.