(Georgia Institute of Technology, 1995-08-14)
Thompson, J. R.; Paul, David; Wang, Z. L. (Zhong Lin); Kroeger, D. M.; Christen, D. K.
This work establishes the stability at elevated temperatures of columnar defects, artificially formed in the Bi-based cupric oxide superconductor Bi₂Sr₂Ca₁Cu₂O₈ for enhanced vortex pinning. Isochronal anneals, conducted in air, led to losses of critical current density in two stages. The defects were relatively stable up to ~ 550 °C, where second stage annealing began; above this, the pinning diminished rapidly. The recrystallization and loss of columnar defects were corroborated by transmission electron microscopy.
(Georgia Institute of Technology, 1995-05)
Goyal, A.; Wang, Z. L. (Zhong Lin); Sun, Y. R.; Kroeger, D. M.; Thompson, J. R.; Chou, Y. T.; Yun, Y.; Harmer, A. P.
Composites containing 25 vol% Ag
were compressed at room temperature to over 110% at
850°C in air. Measurement of the strain rate sensitivity
yielded a value of 0.5, characteristic of superplastic
deformation. As deformed materials had sub-micron
grain size and significant c-axis texture parallel to the
pressing direction. TEM examination showed that the
grains were highly defected and that the grain
boundaries were clean. The T[subscript c] was however low with
an onset of 50K and a width of ~10K. Annealing studies
were carried out with an aim to "fully oxygenate" the
material and anneal out a minimal number of defects to
obtain higher transition temperatures, at the same time
retaining a significant defect density for enhanced fluxpinning.
Magnetization measurements were performed
after most anneals in order to evaluate intragranular
and intergranular properties. Results indicate the
presence of unusually high J [subscript c]'s at low temperatures
after the final anneal (T[subscript c] onset ~90°K). The
observations may be explained by highly superior
intragranular properties coupled with increased local
current loop size.
(Georgia Institute of Technology, 1993-10)
Wang, Z. L. (Zhong Lin); Brynestad, J.; Kroeger, D. M.; Sun, Y. R.; Thompson, J. R.; Williams, R. K.
Magnetic-susceptibility and grain-boundary-chemistry data were obtained on dense YBa[subscript 2]Cu[subscript 4]O[subscript 8] (Y1:2:4) samples. Transmission-electron microscopy showed that the grain boundaries were free of second
phases and the dislocation density was low. Cation content, oxygen composition, and hole density were
determined by the combined techniques of nanoprobe energy-dispersive x-ray spectroscopy and
electron-energy-loss spectroscopy. A total of 25 pairs of grains was analyzed and the results indicated
that grain boundary and bulk compositions do not differ. The relative orientations of the crystallites
were determined, and the results show that a wide variety of misorientations was sampled. Almost all of
the grain boundaries were fully oxygenated and there are no hole deficiencies, but the magnetic susceptibility measurements showed that the material is granular weak linked. These results indicate that
Y 1:2:4 is a material in which clean, stoichiometric boundaries still form weak links.