The inverse levitation of a high temperature superconductor polymer composite consisting of powdered quench melt growth Ba2YCu3O(7-delta) and cyanoacrylate is reported. Magnetic hysteresis loop measurements for the composite are compared to those measured for the bulk material prior to powdering. Differences in the flux pining capability between the two material forms are small but significant

Bennett, L. H.

Chiang, C. K.

Shull, R. D.

Swartzendruber, L. J.

Turchinskaya, M.

English

NASA Technical Reports Server

N92-21630
LEVITATION OF SUPERCONDUCTING COMPOSITES
C. K. Chiang, M. Turchinskaya, L. J. Swartzendruber,
R. D. Shull, and L. H. Bennett
Material Science and Engineering Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899
ABSTRACT
The inverse levitation of a high temperature superconductor-polymer
composite consisting of powdered quench-melt-growth Ba2YCu3OT_ _ and
cyanoacrylate is reported. Magnetic hysteresis loop measurements for the
composite are compared to those measured for the bulk material prior to
powdering. Differences in the flux pining capibility between the two material
forms are small but significant.
INTRODUCTION
The recently developed polycrystalline Ba2YCu307_ high temperature superconductor made
by the quench-melt-growth technique (QMG) possesses very effective mechanisms for pinning
magnetic flux lines. 1,2 This material requires no external additive such as silver, and still
posseses a very large critical current and displays a sizeable levitation effect. The source of flux
pinning is attributed to the small non-superconducting Ba2YCuO 5 phase which is trapped inside
the QMG superconductor. The present investigation was initiated to study sample size effects
on macroscopic properties and to determine the effectiveness of the QMG superconductor as a
component in a superconductor-polymer composite.
EXPF_,RIMENTS
A polyc_rystalline Ba2YCu307.¢ (8 - 0) sample was prepared by the quench-melt-growth
technique. 1,2 A large grain, 5 mm x 3 mm x 1 mm, was chosen from the sample and used for
this study. The grain was split into two parts of equal weight. One part was set aside for
comparison (hereafter referred to as the bulk sample). The other part was ground into powder
using a mortar and pestle. The particle size of the powder was distributed from less than 1 #m
to as large as 500 _m. The average size of the particles was estimated to be 70 _m. The
powder was made into superconductor-polymer composite by infiltrating the powdered
superconductor (80 weight percent) with an insulating liquid cyanoacrylate (20 weight percent).
The composite was curred at abient temperature, (hereafter referred to as the composite sample).
The volume of the composite sample was slightly larger than the bulk sample.
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Low alternating field (ac) susceptibility measurements were performed on both samples using
a Hartskom-type bridge operated at 0.50e rms and at 1.68 kHz. Electrical resistivity of the
bulk sample was measured using a standard four-probe dc technique. Magnetic hysteresis
measurements were done at 77 K using a vibrating sample magnetometer after cooling the
samples in zero applied field.
RF_ULTS AND DISCUSSION
Figure 1 shows the temperature dependence for the electrical resistivity and the ac
susceptibility of the bulk QMG Ba2YCu307. 4 sample. The data suggest that the bulk sample is
of high quality. The ac susceptibility data indicates a large decrease in magnetic susceptibility
on decreasing temperature at 92 K. This is consistent with the observed decrease in electrical
resistivity. Both results indicate a transition width about 1 K. The ac susceptibility data also
indicate the fraction of superconductor was near 100% for the bulk sample at 77K. (there are
small fractions of non-superconducting Ba2YCuO 5 and CuO.)
Two experiments were performed using the bulk and the composite samples. First a
permanent magnet was used to test both the levitation and inverse levitation phenomena. 3
Figure 2 shows a photograph of the composite suspended below a small magnet. There appeared
to be no qualitative difference in either the levitation or inverse levitation properties between the
bulk and the composite samples.
1
0
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t--
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>. -7
__. -8
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-12
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-15
(_ -16
-17
-18
1' 1' 1' I I
_L _J. .L
60 80
TEMPERATURE
! I
IO0
(K)
120
Figure la Magnetic susceptibility of the bulk QMG Ba2YCu3074 _ as a function of temperature.
182
.4 ! ! " ! I ! ! I I I I ! ! I !
..-. 0.3
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0
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E
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>-
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I I0 , ,c _ i_ .,, . _. ^JL__ I I ! I I I , I
85 90 95 100
TEMPERATURE (K)
Figure lb Electrical resistivity of the bulk QMG Ba2YCu3OT. _ as a function of temperature.
ORIGINAL PA_
_LACK AND W,.'HITE Pt-iOTOGRAPH
Figure 2 Inverse levitation of a QMG Ba2YCu307. _ superconductor/polymer composite.
183
Figure 3 shows a comparison of the magnetic hystersis loops for the bulk QMG Ba2YCu307_
and the composite sample. A loop for a typical sintered Ba2YCu3OT_ _ sample is also included
for comparison. The loop for the bulk and the composite are similar, both possesing a large
remanent magnetization and a large amount of enclosed area. This indicates a significant amount
of magnetic flux pinning. It also indicates that powdering the bulk QMG Ba2YCu3OT_ _ sample
and isolating each particle in the polymer does not destroy the inherent flux pinning capibility
of the superconductor. The small differences which are apparent in the hysteresis loops for these
two forms of samples, are likely caused by differences in sample shape, size and texture. These
parameters were not controlled in the present study. However, both of these hysteresis loops are
significantly different from that typical of a sintered sample, which has little flux pinning
capibility.
300
-300 t J
-1 0.5 0 0.5 1
Applied Field (kOe)
Figure 3 Comparison of the magnetic hystersis loops for the bulk QMG Ba2YCu307__,
the composite and a sintered sample.
In the case of the inverse levitation effect, the balance of forces can be written as 3
M(H)[dH/dz].zo = g < 1 >
where M(H) is magnetization per unit volum,_ dH/dz is the field gradient where the sample is
located and g is the acceleration of gravity. _,_ 'ion < 1 > requires positive non-zero value
for M(H) in the first quadrant of the hysteresis loop for the inverse levitation to be possible.
184
Figure 3 showsthatthepowderingof abulk sampleonly hasa limited effecton reducingM(H)
in thisquadrant. Theparticlesizeeffecton M(H) maybeestimatedfrom the field dependence
of the magnetizationof a superconductor.From Bean'smodel4 we have
Jc = Km/a < 2>
or equivalently,
m = _K <3>
where m is the vertical width of the hysteresisloop, Jc is the critical current for the
superconductor,a is the particle diameterand K is a constantdependingon the geometryof
sample. For a given material theJc is fixed for the tempeatureof measurement,and m will
dependon the sizeof samplethroughtheparametersa anf K. If we takea cubic bulk sample
of magnitizationmb cut into n3 equalcubic particles,the magnetizationof eachparticle will be
on the order of mn = mb/n. The total magnetizationper unit volume achievableby the n3
particlesis thusreducedby a factor of n, whereasthevolumeof eachparticlehasbeenreduced
by a factor of n3. This simpleanalysissuggeststhat the reductionof the magneticmomentis
much slower than the reductionof the volume of particles. For the simple study here, the
changeof totalmagneticmomentfrom thebulk sampleto thecompositesampleis muchsmaller
than thefactor l/n, suggestingthatthat eitherthebulk samplemeightbe somewhatgranularin
natureor thegeometricfactor K is not the samefor thebulk sampleandthecompositesample,
or a combinationof both. Obviously, the useof compositesfor levitation must be weighed
againstmanyother factors, suchassamplesize, synthesismethodandengineeringdesigns.
It is also notedthat theparticle sizeeffect is reflectedin the shapeof the hysteresisloop.
The loop for the compositesampleis not identical to that of the bulk. The compositeloop
appearsto reacha saturationat a lower field that of bulk. While the penetrationof the field
occursat a higher field for the bulk sample. Consequently,upontheapplicationof field, the
compositewhich is composedof smaller superconductingparticles will begin to lose its
superconductivitybeforethebulk Ba2YCu3OT__does. However, for somepracticalapplication
involving levitationusingsuperconductingmaterials,thisreducedperformanceat highfieldsmay
not bea critical factor.
REFERENCE
.
2.
3.
4.
M. Murakami, M. Morita, K. Doi, and K. Miyamoto, Jpn, J. Appl. Phys. 28, 1189,
(1989).
M. Murakami, M. Morita, N. Koyama, Jpn, J. Appl. Phys. 28, L1125, (1989).
C.Y.Huang, Y. Shapira, E. J. McNiff, Jr., P. N. Peters, B. B. Schwartz, M. K. Wu,
R.D.ShuI1, and C. K. Chiang, Mod. Phys. Lett., B2, 869, (1988).
C. P. Bean, Rev. Phys. Lett., 8, 250, (1962); Rev. Mod. Phys., 36, 31, (1964)
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SECTION 2:
FLUX DYNAMICS
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Levitation of superconducting composites

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