Investigations on the synthesis and microstructural characteristics of Hg(Bi)-Ba-Ca-Cu-O HTSC in relation to critical current density

摘要

In the present study, we have investigated the effect of Bi doping at the Hg site in the oxygen deficient HgO_(delta) layer of HgBa_2Ca_2Cu_3O_(8+delta) cuprate superconductor in relation to the occurrence of substructures in the form of long period polytypoid (LPP), defect matrix substructures, stacking faults and their influence on superconducting properties, particularly the critical current density. Bulk polycrystalline samples have been prepared by solid state reaction process through precursor route. It has been observed that the as grown Hg_(1-x)Bi_xBa_2Ca_2Cu_3O_(8+delta), variants (with x = 0.05, 0.10, 0.15, 0.20, 0.25) correspond to the Hg(Bi): 1223 phase. Transmission electron microscopic (TEM) observations reveal the presence of long period polytypoid-like structures, associated defect matrix substructure and stacking faults. A correlation between the details of defect substructures associated with LPP and the transport critical current density has been found to exist. It is suggested that this correlation arises due to defect matrix substructure and stacking faults providing effective flux pinning sites. The optimum transport critical current density J_(ct) of approx 1.05 X 10~3 A/cm~2 at 77K has been observed for the composition Hg_(0.85)Bi_(0.15)Ba_2Ca_2Cu_3O_(8+delta). TEM studies reveal that the phase with this composition has the highest density of ordered LPP structures and associated defect matrix substructures. Such an observation bringing out a correlation between transport critical current density and LPP together with defect matrix substructure appears to be the first of its type. Evaluation of intra-grain critical current density through magnetic measurements for the optimum phase Hg_(0.85)Bi_(0.15)Ba_2Ca_2Cu_3O_(8+delta) revealed that this phase also has corresponding highest J_c (intra-grain) approx 4.7 X 10~4 A/cm~2 at 77K.