Mark D. Teplitsky

Enhancement of the 77 K irreversibility field and critical current density of (Bi,Pb)2Sr2Ca 2Cu3Ox tapes by manipulation of the final cooling rate

By manipulating the cooling rate from the final heat treatment, we have raised the 77 K, self‐field critical current density (Jc) of multifilament (Bi,Pb)2Sr2Ca2Cu3Ox (2223) tapes by a factor of 3, and the irreversibility field (H*) by more than 50%. The Jc of samples cooled in 7.5% O2 from their reaction temperature of 825 °C increased from ∼8 to ∼24 kA/cm2 and H*(77 K) increased from ∼120 to ∼200 mT as the cooling rate was decreased from 5 to 0.016 °C/min. The results unambiguously show that the flux pinning properties of 2223 tapes can be improved by simple changes in wire processing.

An environmental scanning electron microscope study of the Ag/Bi-2223 composite conductor from 25 to 840°C

An environmental scanning electron microscope (ESEM) equipped with a hot stage was used to examine phase evolution and microstructural transformations in a partially de-sheathed Ag/Bi-2223 composite conductor as it was heated from ambient temperature to well above 800°C in an oxygen-containing environment. Coarsening of the powder, the opening and eventual filling in of gaps in the powder, recrystallization of the silver sheath, a granular-to-platy transition, platelet-like growth of the layered bismuthates, the formation and spreading of a transient liquid phase, and (at the highest temperatures) partial melting, accompanied by shifting of grain colonies, were all observed in the electron microscope images during various stages of the heating process.

Connectivity and flux pinning improvements in Ag-clad BSCCO-2223 tapes produced by changes in the cooling rate

The rate at which Ag-clad (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x tapes are cooled from their final reaction heat treatment influences both the intergranular connectivity and intragranular flux pinning strength of the polycrystalline filaments. As the cooling rate from 825 °C to 730 °C in 7.5% O 2 was decreased over a range of 5 °C/min to 0.005 °C/min, J c (77 K, 0 T) increased from ∼8 to ∼24 kA/cm 2 , and the irreversibility field increased from, ∼120 to, ∼200 mT. The J c (4.2 K, 0 T) increased in a similar fashion. Cooling slowly also sharpened the critical temperature transition and increased the critical onset temperature from 107 K to 109 K. These improvements in the superconducting properties occurred despite partial decomposition of the (Bi, Pb) 2 Sr 2 Ca 2 Cu 3 O x phase into non-superconducting impurity phases during the slow cooling. A microstructural basis for these multiple effects is described.

Spatial variations in composition in high-critical-current-density Bi-2223 tapes

A detailed compositional analysis of high-critical-current-density (J{sub c}) (55 and 65 kA/cm2 at 77 K) (Bi, Pb){sub 2}Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub y} (Bi-2223) tapes was undertaken by energy dispersive spectroscopy in the transmission electron microscope. Structural features were coupled with characteristic compositions of the Bi-2223 phase. The average of all compositional measurements of the Bi-2223 phase was determined to be Bi{sub 1.88}Pb{sub 0.23}Sr{sub 1.96}Ca{sub 1.95}Cu{sub 2.98}O{sub y}. However, spatial variations in the Bi-2223 composition and differing phase equilibria were found throughout the filament structure. In particular, a considerable range of Bi-2223 compositions can be found within a single tape, and the lead content of the Bi-2223 phase is significantly depressed in the vicinity of lead-rich phases. The depletion of lead in the Bi-2223 phase around the 3221 phases may be a current-limiting microstructure in these tapes. (c) 2000 Materials Research Society.