Fabrication and characterization of melt-processed YBCO Page: 4 of 5
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beneficial in reducing the carbon content and processing time while
maintaining small particle size .
The Ndi+xBa2-xCu3Oy single crystals which were used as seeds
are grown using standard flux growth method. The details have
been reported earlier .
YBa2Cu3Or, Y2BaCuOs (-20-25 wt %) and PtO2 (0-0.5 wt %)
were mixed homogeneously by ball milling overnight (- 8-10 hr.).
The mixed powder were then pressed in to a desired shape and
seeded with a Nd+xBa2-XCu30y single crystal and melt-processed in
a temperature gradient as reported earlier . In brief, the samples
are heated to about 1050 *C and then slowly cooled through the
peritectic temperature to about 950 *C and then finally cooled to
the room temperature at a rate of 100 *C/hr.. The samples
(levitators) are then oxygenated between 450 -700 *C for about 8-
10 days in a flowing oxygen atmosphere.
Microstructural development was investigated by using a
scanning electron microscopy (SEM), with a energy dispersive x-
ray spectroscopy (EDS) and optical microscopy.
III RESULTS AND DISCUSSIONS
Although, considerable progress in increasing the size of the
domain in melt-processed YBCO has been achieved through
seeding methods, the diameter of the domain is typically limited to
roughly about 1.5 inches. Only with sizable temperatures gradient
and/or compositional gradient larger samples can be produced.
This size limitation is believed to be in part due to the high
supercooling along the direction of growth, which often leads to
disorderly production of nuclei at the later stage of crystal growth
resulting in multi-domained YBCO and partly due to the lower
temperature at the edges of the sample where grain nucleation can
occur resulting in multiple impinging domains. The nucleation at
the edges of the sample can be minimized by coating the edges with
Yb203. Because of the considerably low melting temperature of
YbBa2Cu30. ( - 920 *C), the edge nucleation can be reduced.
Figure 1 shows a photograph of a 2 inch diameter YBCO levitator
fabricated by coating the edges of the sample with Yb2O3.
Due to the differences in the growth mechanism in the a/b axis
and the c-axis two distinct microstructures results in seeded YBCO
levitators. The nucleation of the YBa2Cu30X phase at the surface of
the Nd+,Ba2-xCu3Oy single crystal occurs after the slow cooling
process has started. The crystal begins growing epitaxially in form
of a thin, square platelet, with well developed (100), (010) and
(001) planes. The pedestal that develops below the seed is formed
by well defined growth steps. These steps are relatively high close
to the seed but much less pronounced at the edge of pellet. The
growth stops when the growth front reaches the reaction layer that
forms at the interface between the crucible and the sample (Fig. 2).
The reaction layer is usually 1-3 mm thick and consists of a large
amount of Y2BaCuO5 embedded in Ba-Cu-O phases. Small grains
of YBa2Cu3OX and some complex Y-Ba-Cu-AI-Pt-O phases are also
observed in these regions.
As mentioned earlier, two distinct regions due to the growth in
a/b axis and c axis are observed in these levitators. Just under the
seed the region grows mainly due to the growth along the c-axis. In
this region, generally a pyramid shaped volume (Fig. 3) develops
with low Y2BaCuOs concentration (Fig 4) and a blocky sub-domain
structure (Fig. 5). The sub-domains are mostly square-like shape
with sizes up to 1 mm2. The c-axis misalignment between adjacent
sub-domains is usually well below five degree. The volume
Fig. 2. Vertical cross-section of a YBCO levitator. A thin reaction zone can be
seen at the bottom of the sample.
Fig. 3. Vertical cross section of a YBCO levitator under a polarized optical
microscope at a magnification of 50 X.
Fig. 4. Microstructure of the c-axis growth region showing the distribution of
of the Y2BaCuO5 phase in this area directly below the seed is
usually around 10 vol. % and increases with decreasing height.
Surrounding the inner region (the pyramid shaped region) the
solidification mostly occurs by ab plane growth. The transition
from the c-axis growth to the ab plane growth region is well
defined, as can be revealed by the abrupt change in Y2BaCuOs
density. This region shows a high Y2BaCuOs concentration and a.
strip - sub-domain structure (Fig. 5). The strip like sub-domain
structure consists mostly of rectangular sub domains of very small
sizes ( < 0.1 mm2). The volume fraction of Y2BaCuO5 in this area
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Sengupta, S.; Corpus, J.; Gaines, J. R., Jr.; Todt, V. R.; Zhang, X. F.; Miller, D. J. et al. Fabrication and characterization of melt-processed YBCO, report, November 1996; Indiana. (digital.library.unt.edu/ark:/67531/metadc679139/m1/4/: accessed October 21, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.