D. Buhl et al., PHASE-COMPOSITION AND GRAIN ALIGNMENT IN PARTIAL MELT-PROCESSED BI-2212 THICK-FILMS, Applied superconductivity, 4(7-8), 1996, pp. 299-317
Citations number
34
Categorie Soggetti
Material Science","Physics, Applied","Physics, Condensed Matter
The melting and solidification of Bi2.2Sr2.05CaO0.95Cu2O8+delta (Bi-22
12) thick films on silver substrates and bulk material during the part
ial melting process was investigated. The Bi-2212 formation via a liqu
id phase is known as a peritectic reaction resulting in a multiphase m
icrostructure after. cooling. Therefore, special emphasis was laid on
the examination of the Bi-2212 phase reformation during processing. Th
e three crucial steps of the heat treatment were heating up to a maxim
um temperature above the solidus (875 degrees C), cooling with a defin
ed rate to 850 degrees C and annealing at 850 degrees C, all done in p
ure oxygen (p(O-2)=1 atm). This lead to a microstructure consisting of
Bi-2212 with minor amounts of a Bi-free and a Cu-free secondary phase
of the stoichiometries Sr14-xCaxCu24Oy (014x24) and Bi9Sr11Ca5Ox, (91
150). The one-layer compound Bi11Sr9Cu5O6+delta (Bi-11905) was also pr
esent as minority phase or as intergrowths in Bi-2212 grains. The micr
ostructure of the material depending on the processing steps was inves
tigated. The absolute volume fraction of the four solid phases was onl
y slightly changed when varying the processing parameters. However, an
inhomogeneous microstructure resulted when heating higher than 10 deg
rees C above the solidus upon partial melting and cooling slower than
5 degrees C/h. Macroscopic areas of distortions increased in size and
number that could not be eliminated by post-processing long-time annea
ling. In some areas of the samples, liquid disappeared due to capillar
y forces acting on the framework of liquid, Bi-free and Cu-free second
ary phases in the partially molten state. In the liquid poor areas, th
e network of the secondary phases could not resolve upon cooling due t
o the lack of liquid but remained as an open porous structure. The ove
rall phase composition was independent of the sample thickness. The gr
ains were aligned with their (a, b) planes parallel to the lateral sam
ple dimensions only in films thinner than 150 mu m. In mu m thick samp
les, an average misalignment of +/-5 degrees was found. This degree of
texturing got lost when increasing the thickness. Above a sample thic
kness of 200 mu m, the grains-were randomly distributed. (C) 1997 Else
vier Science Ltd.