Large domain YBa2Cu3O(x) levitators have been fabricated using a seeded melt processing technique. Depending upon the seed, either a single or five domained sample can be obtained. The grain boundaries separating each domains in the five domain levitator are found to be 90 degrees. Similar levitation forces can be observed for single and five domained samples. After thermal cycling, however, a small decrease in the levitation force of the five domain levitator was observed as a function of thermal cycles while nearly no change in force was observed in the single domain levitator. Finally, it is shown that both, single and five domain YBCO, behave similarly as a function of sample thickness

Corpus, J.

Gaines, J. R., Jr.

Miller, D. J.

Sengupta, S.

Todt, V. R.

Zhang, X.

English

NASA Technical Reports Server

FABRICATION OF LARGE DOMAIN YBa2Cu30 x FOR MAGNETIC
SUSPENSION APPLICATIONS
S. Sengupta, J. Corpus, J. R. Gaines, Jr.
Superconductive Components, Inc.
Columbus, OH
,/_i _
V. R. Todt, X. Zhang, and D. J. Miller
Materials Science Division
Argonne National Laboratory
Argonne, IL
ABSTRACT
Large domain YBa2Cu3Ox levitators have been fabricated using a seeded melt
processing technique. Depending upon the seed, either a single or five domained sample
can be obtained. The grain boundaries separating each domains in the five domain
levitator are found to be 90 degrees. Similar levitation forces can be observed for single
and five domained samples. After thermal cycling, however, a small decrease in the
levitation force of the five domain levitator was observed as a function of thermal cycles
while nearly no change in force was observed in the single domain levitator. Finally it is
shown that both, single and five domain YBCO, behave similarly as a function of sample
thickness.
INTRODUCTION
A stable levitation or suspension of a magnet is one of the fascinating properties of a
superconductor. Unlike active suspension between magnets, the suspension of a magnet
by a superconductor is completely passive. A magnet can be suspended above as well as
below a type II superconductor (Fig. 1 & 2). A large number of applications are
envisioned utilizing this unique property. These applications include rotary motion
bearings, cryopumps, cryocoolers, cryoflowmeters, energy storage devices, contactless
transportation, and vibration isolators. 0)
The stable suspension of superconducting materials was first demonstrated by
Arkadiev. _2'3) Although the levitation by the superconducting material was studied in
679
https://ntrs.nasa.gov/search.jsp?R=19960050145 2020-06-16T03:14:02+00:00Z
Figure 1. A ring magnet is suspended above
a group of large domain levitators.
i
Figure 2. A magnet attached to a weight is
suspended below a single large domain
levitator (in cup).
680
1950s and 60s, the necessity of using expensive liquid helium (Temp. = 4.2 K) greatly
hindered further developmentJ 4) The discovery of a high Tc superconductor with
transition temperatures above liquid nitrogen (77 K) renewed the interest in
superconducting levitation, tS) Devices based on the principle of superconducting
levitation are now being developed at various laboratories all over the world. _6q°) The
levitation force on a magnet due to a superconductor is an integrated parameter which
depends on the characteristics of the magnet as well as the superconductor. These factors
include:
1. the distribution and the intensity of the magnetic field.
2. the local critical current density, Jc, of the superconductor, and
3. the characteristic length scale, d, of the induced current loop in the
superconductor.
For a given magnet and for superconductors with similar dimensions, the levitation
force can be enhanced by increasing Jc and/or d of the superconductor. The critical
current density can be improved by introducing defects that can act as flux pinning
centers, and d can be increased by increasing the size of the strongly linked regions.
The melt processing technique offers an attractive way to fabricate strongly linked
YBa2Cu3Ox (YBCO). (11"i9) Moreover, defects like YzBaCuO5 inclusions and other
structural defects (like dislocations and stacking faults) can be incorporated during
processing that can further enhance the current carrying capability in these materials. °7-19)
In a typical melt processing method, YBCO is heated above its peritectic point where it
melts incongruently into Y2BaCuO5 and a Ba- and Cu-rich liquid. The semi-solid melt is
cooled slowly to obtain aligned grains of YBCO. The grains are aligned, however, only in
small regions called domains. Within the domain the superconductor is mostly strongly
linked. The domains are separated by large angle grain boundaries which act as weak
links, thereby reducing the current carrying capability in the presence of magnetic fields
(Only under some special situations like 90 degree grain boundaries, may the boundary
act as strongly linked and be able to carry currents comparable to that within a domain).
In a melt processed YBCO sample the levitation force can be enhanced by increasing
the domain size or by developing samples with strongly linked large angle boundaries.
One way to increase the domain size is by initiating grain growth by using a seed crystal.
In presence of a favorable temperature gradient the seed not only ensures a single
nucleation site but also permits controlled orientation of the grains. Using the seeding
technique along with a controlled temperature gradient YBCO domains as large as that of
the sample size (single domain) can be fabricated. Seeding with a single crystal of MgO,
A1203, SmBa2Cu30 x and Ndl+xBa2.xCu3Oy have been reported by various groups. (2°-24)
The seed can be added to the coldest point of the sample prior to heating or after an extra
melting step. The addition of the seed prior to heating is preferable as it reduces the
difficulty of mass production.
681
In this paper, fabrication of large domain levitators using a Ndl+xBa2.xCu3Oy seed is
considered. A five domain or a single domain levitator can be produced based on the
type of seed crystal used. The properties of the five domain as well as single domain
levitators are also considered.
PREPARATION AND PROPERTIES OF THE SEED CRYSTALS
Ndl+xBa2.xCu30y crystals are used as seeds in order initiate grain growth in melt
processed YBCO. The crystals are prepared by using a self flux method. Specifically, a
Barium and Copper rich mixture of Nd203, BaCO3, and CuO are mixed and heated in
AI203 crucibles above the peritectic point of NdBa2Cu3Ox. The melt was then slowly
cooled and the excess liquid drained after adequate crystal growth. The crystals were
then collected and cleaned for seeding.
Two types of seeds were obtained depending upon the amount of flux in the melt.
When the amount of flux is low, the crystals obtained have a cubic appearance. These
crystals are, however, not single crystals and do have a unique microstructure as shown in
Figure 3. The crystal clearly shows five different regions. The central region is
surrounded by four differently oriented crystals of the same crystal structure.
Transmission electron microscopic studies have revealed that the surrounding crystals are
rotated 90 degrees with respect to the central crystal. Furthermore, the surrounding
crystals are also rotated by 90 degrees with respect to its adjacent crystal. The flat seeds
on the other hand are single crystals. No special microstructure was observed as shown
in Figure 4.
Figure 5 shows the X-ray diffraction of the crushed seeds (both cubic and flat) and
near phase pure NdBa2Cu3Ox produced by calcination in low partial pressure O2
atmosphere. As evident from the figure, both the cube and the fiat seeds have lattice
structure similar to that of NdBa2Cu3Ox powder.
Energy dispersive x-ray analysis showed an average cation composition of Nd:Ba:Cu
of 1.25:1.85:2.4 with about 6 % AI contamination from crucible reactions for fiat crystals.
A composition of Nd:Ba:Cu of 1.35:1.75:2.8 with about 2 % A1 contamination for the
center of the cube shaped crystal was found, while the surrounding crystals showed a
composition of Nd:Ba:Cu of 1.3:1.7:2.5 with about 6 % A1 contamination. These
compositional differences, especially those in Al-contamination, can be explained by
differences in the initial phase composition and by processing during heat treatment.
During the growth of the fiat seeds a relatively larger contact between the crucible and its
content existed. For the growth of cubic seeds, the contact of the melt with the crucible
wall was less, leading to a reduction of contamination of the central crystal. The reason
for the stronger contamination of the surrounding crystals may be due to the prolonged
682
Figure3. Photomicrograph of a "cubic" seed
consisting of a central crystal and four
surrounding single crystals.
Figure 4. SEM photo of a flat, single crystal
seed.
683
processing at the elevated temperatures. This in turn may account for the increased AI
dissolution and diffusion through the content of the crucible.
A crystal can only work as a seed to initiate grain growth in melt processed YBCO, if
the seed has a melting point higher than that of YBCO. Figure 6 shows the DTA of
crushed flat and cube crystals. As evident from the figure, the crystal melts at about 1120
°C, which is considerably higher than the melting point of YBCO (~ 1010 °C).
Interestingly, near phase pure NdBa2Cu30 x prepared by calcination in low partial
pressure 02 atmosphere melts at about 1070 °C. This difference in the melting point may
be related with the surface energy of the faceted structure of the seeds. The higher
melting point of both the cube and fiat seeds thus open the possibility of using them as
seeds for melt processing NdBa2Cu3Ox itself.
FABRICATION AND PROPERTIES OF THE LEVITATORS
A top seeding method is used to melt process YBCO levitators. As mentioned earlier
both cubic and fiat shaped crystal were used for seeding. The details of the method have
been reported previously. (24'25) Figure 7 shows some of the melt -processed YBCO
levitators. Samples seeded with cubes as well as fiat crystals showed similar appearance.
However, a difference in the structure was obvious after polishing. A five domain
structure was observed when the levitator was seeded with a cube seed (Fig. 8). Seeding
with flat seeds, however, resulted in a single domain sample (Fig. 9). The melt processed
YBCO levitators are therefore observed to replicate the feature of the seed used to initiate
grain growth.
Transmission electron microscopy has been used to study the grain boundaries
developed between two domains in a five domain levitator. Two kinds of grain
boundaries were observed as shown in Figures 10 & 11. The first kind of grain boundary
was observed between two domains that are rotated 90 degrees about a common
[100/010] axis with both [001] axes lying in the plane of the paper. The boundary plane
is always very sharp, free of secondary phases and shows macroscopical faeetting. The
boundary plane varies macroscopically from a symmetrical configuration - predominantly
in the upper part of the pellet - to configurations where the grain boundary lies
perpendicular to the [001 ] direction of one of the domains. The grain boundary is often
inclined and meandering in the third dimension (into the paper plane) as well. The
second kind of grain boundary is observed between two domains that are rotated 90
degrees about a common [ 100/010] axis with one [001 ]-axis normal to the plane of the
paper and one [001 ]-axis contained within the plane of the paper. This type of grain
boundary has been reported to support a high critical current density in thin film
superconductors.
684
140.0
120.0
100.0
-- 80.0
4)
n.
60.0
40.0
20.0
0.0
20 30
I Nd123 powder
Flat single crystal (powdered)
-_ Cube shaped crystal (powdered)
40 50
2 Theta
60 70 80
Figure 5. X-ray diffraction of NdBa2Cu30 ×
powder, crushed single crystals, and crushed
cubed crystals. (in order from top to bottom)
r-
t/)
t--
Cube
,,,I,,,I,iLl,,,I,,,I,,,
t-
a
Illl,,,I,,Lll,ll,,,I,,I
,,, I,, iI,, , I,,, I,,, I , , ,
700 800 9DO ]O00 1100 1200 1300
Temperature(°C)
Figure 6. Differential thermal analysis of
cubed crystals, flat single crystals, and Nd]23
powder.
685
Figure 7. Photo of two melt-9rocessed YBCO
levitators.
Figure 8. Photo of two polished, five domain
levitators processed using cube seeds.
686
Figure 9. Photo of a single domain levitator
processed using flat seeds.
687
Figure 10. Transmission electron micrograph
of two domains with both [001 ] axes lying in
the plane of the paper.
Figure 11. Transmission electron micrograph
of two domains with one [001 ]-axis normal to
the plane of the paper and one [001 ]-axis
contained within the plane of the paper.
688
The levitation forces of a single domain and five domain levitator are plotted in
Figure 12. As can be seen from the figure, similar levitation forces were observed for
both samples. This result suggests that the 90 degree grain boundaries in the five domain
levitator may be acting as strongly linked boundaries. Recent transport measurements
across the these boundaries further confirms our resultsJ TM
In order to further investigate the properties of these two types of levitators (five
domain and single domain) the levitation forces as a function of thermal cycling were
studied. The levitators were cooled to liquid nitrogen temperature and then heated
rapidly by using a copper block as a heat source. Figure 13 shows the zero distance
levitation force of a five domain and a single domain levitator as a function of thermal
cycling. As can be seen from the figure, only a small decrease (-10 %) in the levitation
force of the five domain levitator was observed after 60 thermal cycles. Nearly no
change was observed for the single domain sample. The result can be explained by
considering the anisotropic nature of the thermal expansion in YBCO. This anistropy
may lead to the development of microcracks during thermal cycling in a five domain
levitator thereby reducing the levitation force.
Figure 14 shows the levitation force as a function of sample thickness for a five
domain and a single domain levitator. In both cases the samples are sliced from the
bottom. As evident from the figure, a small decrease in the levitation force is observed
for both levitators with decreasing thickness. The force however drops rapidly once the
thickness is reduced beyond 0.5 cm. The single domain levitators also show different
behavior depending upon the position of the cut. The forces decrease much more rapidly
when the levitator is cut from the top in comparison to that of the levitator cut from the
bottom. This result shows the deleterious effect of the reaction from the crucible.
CONCLUSIONS
Large domain YBa2Cu30 x levitators have been fabricated using a seeded melt
processing technique. Two types of seed crystals were prepared by a self flux method.
The levitator was found to replicate the type of seed used to initiate grain growth. Based
on the seed type, the levitators were either single domained or five domained. The
levitation force for the single as well as five domain levitators were found to be similar.
The grain boundaries separating two domains in the five domain levitator were found to
be 90 degrees. Only a small change in the levitation force was observed as a function of
thermal cycling in a five domain levitator. Nearly no change in the force was observed
for the single domain levitator. The result can be explained by the anisotropic nature of
the thermal expansion in YBCO. Finally, both single domain and five domain YBCO
showed similar behavior as a function of thickness.
689
10.0
8.0
A
z
v
o 6.0
r-
0
_, 4.0
>
o
J
2.0
0.0
0
u o
I I I I I
• Five domain material
• Single domain
IJ
no
•o
m
I , , , I = , I
2 4 6
Distance (mm)
12
Figure 12. Levitaion force as a function of
distance is plottted for a single and five domain
levitator.
1,5
t_
1.3(J
1.1
2
N_ 0.9
"Oq)
"_ 0.7
Single domain
Five domain
0
Z 05. ,,, I , , , , ,,,. t,,, I,,. ,
0 10 20 30 40 50 60
Cycles
70
Figure 13. Zero distance levitation force as a
function of thermal cycling is plotted for a
single and five domain levitator.
690
1.2
._ 1.0
_ o.9
"_ 0.8
_ 0.7
0.6
0.3
Five domain
Single domain
P, , , I I , i = I = J = I i i ,
0.4 0.5 0.6 0.7 0.8
Thickness (cm)
Figure 14. Levitation force as a function of
thickness is plotted for a single and five domain
levitator
691
ACKNOWLEDGMENTS
The work at Argonne National Laboratory is partially supported by the US
Department of Energy, Division of Basic Energy Science-Materials Sciences,
Conservation and Renewable Energy Utility Concepts-Superconductivity Technology
Program, under Contract # W-31-109-ENG-38.
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Fabrication of Large Domain YBa2Cu3O(x) for Magnetic Suspension Applications

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