Superconducting composites

In this orthorhombic structure, superconductivity is not observed, and two inequivalent Bi sites are observed. A cubic, but non-superconducting specimen has been reported (10) for x =. 13. In this structure, all Bi sites are equivalent, and a metallic or delocalized electronic structure would be expected to exist. This structure apparently persists into the superconducting compositions, where it has been observed for x =. 374 (4) which displays a Tc of 30.5 K. A cubic phase has also been reported for x =. 6, with Tc = 34K. The limit of this solid solution which may be at or beyond this x =. 6 composition, may be dependent upon synthesis temperature and technique. Indications of phase disproportionation at these high x values have been published by Jones et al. (11). 

In summary, the superconducting compositions appear to have a tetragonal structure for BaPbj Bi Og, and a cubic structure for Ba K BiOg. Dopant atom ordering has not been reported in structural studies. 

In the region of superconducting compositions in BaPbx.x-B Og, oxygen vacancies have been found to decrease Tc (46)(47). This has definite implications for preparation of high Tc material, but also indicates that the Te may be tuned to any desired transition temperature below the maximum Tc by careful technological treatments. 

Sleight, A.W. and Cox, D.E., Symmetry of Superconducting Compositions in the BaPbj xBixOs System. Solid State Comm. 58(6) 347 (1986). 

Considerable effort has been expended in growing single crystals of compositions in the solid solution, with particular emphasis on the superconducting composition. A variety of methods have been employed, all flux type, as the solid solution members are not congruently melting. There have been a few detailed reports on growth of single crystals from melts in the... 

The current transfer problem that had been identified with low temperature superconducting composites deserves additional mention for the high temperature superconductors, that in the bulk material are frequently not fully dense. Making the electrical connection in such a manner as to obtain uniform current distribution throughout the cross section of the material is difficult. The method described by Jin, et al. (24) with embedded wires or particles may provide for a significant improvement but the present techniques used to determine the critical current by a surface contact on the ceramic sample are subject to this problem. A discussion for the multifilamentary wire of NbsSn is provided by Goodrich and Fickett (30) and this discussion is likely to be similar to the high temperature materials that are not fully dense. 

Nanocrystalline materials comprising sub-100 / metal particles, when compressed to 50% of their bulk density, show properties (specific heat, thermal conductivity, saturation magnetization and critical temperature for superconductivity) provocatively different from those of their crystalline or glassy counterparts.(48) It is well known that the interfaces of mechanically reduced composites are effective in interacting with dislocations and with flux lines in superconducting composites. Precursor materials for the preparation of ultrafine filamentary composites can also be imagined. Here the combinations of interphasial boundaries and dislocations can... 

T s derived from resistivity vs. temperature measurements are summarized In Table I. The starting Y-123 shows zero resistance at 92K. All samples except those with the highest bismuth concentration (x-0.429) show a sudden drop In resistance with an onset temperature of 88K. All the composite samples that show this drop reach a measured zero resistance at 83K (see, for example. Figure 5A). This Is strong evidence for the persistence of an Interconnected superconducting phase. Three of the coprecipitated samples show a resistance vs. temperature curve with a similar shape to the superconducting composite samples. However, In these materials, the... 

Plasma-Chemical Production of Oxide Powders for Synthesis of High-Temperature Superconducting Composites... 

W. Y. Chen and J. R. Purcell, Dynamic Simulation of Normal Zone Evolution in a Superconducting Composite, submitted to /. Appl. Phys. 

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