Magnetic Field Induced Texture in High-Tc Superconductors

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IEEE
TRANSACTIONS
ON
APPLIED SUPERCONDUCTIVITY,
VOL. 9,
NO.
2,
JUNE
1999
2231
Magnetic Field Induced Texture in High-Tc Superconductors
P.
J.
Ferreira, H.B. Liu and J.B.Vander Sande
Massachusetts Institute
of
Technology
Department
of
Materials Science
and
Engineering, Cambridge,
MA,
02
139, USA
Abstract-
Bi-2212 superconductor thick films and tapes were
melt-grown under a zero and
10 T
magnetic field. In the latter
case the degree of alignment and consequent transport critical
current densities were enhanced. Melt-processing tapes or thick
films in a 10
T
magnetic field seems to produce uniformly
textured grains throughout a large thickness.
A
theoretical
model developed to quantify the degree of texture achieved
during various processing conditions suggests that the degree of
alignment is obtained through grain rotation during the early
stages
of
grain growth from the liquid.
I.
INTRODUCTION
The principal limitation to technological applications of
High-T, polycrystalline oxide superconductors is the low
transport critical current density
(JJ
found in these materials.
This limitation is strongly correlated with the misorientation
between the grains. Hence, to minimize the number of
intergranular weak links, a high degree of crystallographic
texture must be obtained. One possible route by which a
strong crystallographic texture can be produced is to melt-
process the material under the effect of an elevated magnetic
field. The driving force for grain alignment is provided by the
anisotropic paramagnetic susceptibility exhibited by the
superconductor grains. When a superconductor grain is
placed in a magnetic field, the axis of maximum
susceptibility aligns
with
the magnetic field direction. As a
result, in the case of superconductor materials, such as Bi-
2212, Bi-2223 and YBCO the grains should align with the c-
axis parallel to the external magnetic field [2],[3].
Despite the fact that grain alignment in high-Tc
superconductors induced by a magnetic field has been
confirmed by various groups [4]-[ lo], little attention has been
paid to the case of BSCCO/Ag thick films or tapes processed
under the same conditions. Recently, however, Ma and Wang
[
111 achieved a high degree of texture in Bi-2223 tapes melt
processed in a 4.5 T magnetic field, and Liu et a1.[12]
reported an enhancement in grain alignment of Bi-2212 thick
films processed under the influence of an elevated magnetic
field.
In
the case of melt-grown Bi-2212 tapes or thick films, it
remains a challenge to prepare well texture tapes or thicks
with reasonable thicknesses. However, processing thick films
or tapes by a partial melting method in a high magnetic field
may produce uniformly textured grains throughout the whole
oxide.
In this work we will show that the grain orientation
of
Bi-
2212lAg thick films and tapes can be controlled by the
application of a 10 T magnetic field during high temperature
processing.
Manuscript received September
14,
1998.
This
work
was
supported by the
US.
Department
of
Energy
under
Grant
No.
DE-FG02-85ER45 179.
In
addition, the degree of texture and consequent Jc were
enhanced by the application of a magnetic field. In order to
quantify the degree of texture achieved during processing, a
theoretical model has been developed.
11. EXPERIMENTAL
PROCEDURE
The starting materials were first prepared by solid state
reaction. Highly pure (99%) Bi2O3, SrCO3, CaC03 and CuO
are weighed according to the normal composition
Bi2Sr2CaCu208. The mixed powders were first reacted at
800°C for 12 hours
in
air. The samples were then ground in
an agate mortar and pestle, then pressed into pellets and
sintered at 86OOC for 24 hours. In the case of thick films, the
pellets were finally ground into fine particles and the particles
were deposited
on
silver foil in isopropanol with different
thicknesses. The thick films were then dried at 100°C for
several hours. In the case of tapes, the pellets were ground
into powder and tapes with dimensions 12~2~0.127
mm
were
fabricated by a standard powder-in-tube method.
The thick films and tapes were placed in a high
temperature furnace, which is positioned vertically in the
52mm bore of a superconducting magnet. Magnetic fields of
up
to
10
T are parallel to the long axis
of
the furnace. The
processing cycle started by increasing the magnetic field to a
pre-determined level, followed by the thermal cycle and
subsequent cooling at 10 C/hour to room temperature under
an elevated magnetic field (Fig. 1). The surface of the film
was perpendicular to the magnetic field. After processing
under a high magnetic field, the thick films were annealed in
zero field at 8OOOC for 24 hours.
Microstructural observation and microanalysis were
performed in a JEOL 6320 FEGSEM. Back-scattered electron
images were used to produce contrast from the different
phases. The critical current density Jc was measured at 4.2
K,
zero field by a standard four-probe method. The Jc was
determined from I-V curves using a criterion of 1 pV/cm.
111.
EXPERIMENTAL
RESULTS
A.
Bi-2212 Thick
Films
SEM backscattered images of polished cross sections of
various thick films with
two
different thicknesses, processed
in zero field and a 10 T magnetic field, are shown in Fig. 2
and Fig. 3, respectively. When the films were processed
under a zero magnetic field, the degree of texture decreased
with increasing thickness of the film (Fig.2). In the case of
the thick films melt-grown under a
10
T magnetic field, the
degree of texture remains high with an increase in thickness
'
1051-8223/99$10.00
0
1999
IEEE
2232
(Fig.3).
In
this process, the
grains
align with the
crystallographic c-axis (the direction of lowest growth rate)
parallel
to
the magnetic field through the entire thickness of
the film.
a)
Bi-2212
Thick
Films
885
p i q c/~ n
lH
OT
b)
Bi-2212
Tapes
Fig.2: SEM images of BI-22 12 cross-sections melt-processed
Fig.
1:
Thermal sequences for melt-grown Bi-2212/Ag thick
films and tapes. T, is the maximum processing temperature.
In
Fig.4, the transport critical current densities of the
films,
measured at 4.2
K,
are plotted as a function of their thickness
for films processed under a zero and a 10 T magnetic field.
When the thickness of the film increases, the Jc values
decrease for both groups of films. However, the Jc values for
the films processed under a
10
T magnetic field are higher
than those obtained from the samples processed under zero
field (Fig.4).
A.
Bi-2212
Tapes
Transport critical current densities of tapes processed under a
OT
and
10
T magnetic
field
as
a
function
of
the
maximum
processing temperature Tm are shown in Fig.
5.
It is clear
that an increase in Jc for the taped processed under a
magnetic field. In addition, the optimum processing
temperatures T, goes through a maximum around 883-885 C.
SEM images of polished cross sections of the tapes
processed under
0
and 10 T magnetic fields are shown in
Fig.6. For the tapes processed under
a
zero magnetic field,
many of the grains grow with their c-axis randomly oriented
with respect to the Bi-2212/Ag, whereas for the tapes melt-
grown under a 10 T magnetic field, a high degree
of
alignment
is
evident across the whole thickness.
Iv.
THE
MODEL
In this
model we are assuming that the magnetic field does
not affect the process of nucleation and growth from the
liquid [13]. Instead, we suggest that rotation of
superconductor grains in the early stages of growth under the
presence of a magnetic field may be the cause for the increase
in
alignment.
Fig.3: SEM images of BI-22 12 cross-sections melt-processed
under a 10 T magnetic field.
878 880 882 884 886 888 890
T,
(C)
Fig.4: Maximum temperature Tm dependence
of
Jc at 4.2
K,
zero field, for the tapes processed following thermail
sequences under zero and
10
T magnetic fields.
2233
moment
Mc
and
Mob
are paramagnetic moments and thus,
we can rewrite
(1)
as
"i o
40
$0
~ l ~ l ~ l i o
Thickness
(prn)
Fig.5: Transport current density at 4.2K, zero field, as a
function of thickness for films processed under a
0
and 10 T
magnetic field.
Fig.6:
SEM
images of cross-sections of Bi-2212 tapes melt-
grown under a
0
T and 10 T magnetic field
.
This situation would be very similar to a rotation of particles
in
a
free
medium, since most
of
the.materia1 would be in the
liquid state and thus particles can rotate without interacting.
A.
The Magnetic Energy
of
a Grain
Assuming that an anisotropic grain with a volume
V
is
placed in a magnetic field
H,
the change in magnetic energy
of
the grain with a change in magnetic field can be written as
dE,
=
-GV&
=
-(Mc
COS0
+Mab
sin8)VdH
,
(1)
where
M
is the magnetic moment per unit volume, which can
be resolved in the two directions c and ab, and
8
is the angle
between the magnetic field and the c-axis of the grain. For
high-Tc superconductors in their normal state, the magnetic
where
X c
is the paramagnetic susceptibility along the
c'
direction and
Xab
is the paramagnetic susceptibility normal
to the ab plane.
On
integrating (2) we obtain for the magnetic
energy of a grain the expression
Rearranging gives
where
Ax
is the difference in the volume susceptibilities of
the grain.
B. Early Stage
of
Growth
Subsequent to the nucleation event, nuclei will
start
their
growth under the influence of a magnetic field.
In
the early
stages of growth, the grains will be completely surrounded by
a liquid phase and thus, we shall treat the grains as small
particles rotating in a free medium without interactions.
Let us start by considering the probability
f(8)
that a grain
has an orientation with angle
8
under the influence
of
a
magnetic field.
This
can be expressed, according to classic
Boltzman statistics, as
0
Let us now imagine a situation where the total number of
grains is
n.
Thus, the mean number of grains with an
orientation between
8
and 8+d8 can be given by
The distribution
n(8)
can be thus be related to an alignment
parameter which can be used to quantify the degree of texture
in melt-processed superconductor materials under the
influence of a magnetic field. Let us define this alignment
parameter F, such that
F=l
for a completely aligned structure
and F=O for a completely random structure, in the form
2234
1.0
0.8
8
06-
Y-
0.4
0.2
0.0
LL
where
s2
is the variance of the distribution for a particular
processing condition and
sH=o
is the variance of the
distribution in the absence of a magnetic field.
In
Fig. 7, the F factor is plotted as a hnction of the
magnetic field for different grain sizes and a temperature of
875
C,
which is approximately five degrees below the
melting point of Bi-2212. The anisotropy in molar magnetic
susceptibility
A X ~ O'~
is
approximately 22.5~10" cm3/mol [2],
[3]. The anisotropy in volume magnetic susceptibility
AX
used in
(9)
is then 1.5~10-~, if we assume a density of 5.5
g/cm3
[
141 for the superconductor.
As
depicted
in
Fig.
7,
when the magnetic field increases,
there is a tendency for the texture to increase, except in the
cases where the grain size is too small. Therefore a high
degree of alignment can be obtained by increasing the
magnetic field and the grain size. It is also evident that
in
the
case of larger grain sizes, the magnetic field tends to saturate
and thus increasing the magnetic field has only a negligible
effect on the degree of texture.
2
-
-
-
-
-
V.
DISCUSSION
The improvement in Jc for the thick films and tapes
processed under a
10
T magnetic field is a consequence of the
enhancement in the degree of texture, which is confirmed by
the
SEM
micrographs shown in Fig. 2,3 and
6.
The fact that
the employed processing temperatures are above the melting
point of Bi-2212 lead
us
to suggest that the mechanism for
alignment seems to be closely related to the early stages of
growth of superconductor grains from the liquid. In this
regime the grains are still surrounded by a liquid phase, and
thus the magnetic energy will rotate the c-axis of the grains
towards a direction parallel to the magnetic field. Additional
growth of the grains occurs mainly perpendicular to the c-
axis, but with the c-axis aligned parallel
to
the field. Thus,
when the magnetic field is applied during the early growth of
superconductor grains, a high degree of alignment is
produced. As shown in Fig.7, grains of the order of
600
A
can be highly aligned by the application of a
10
T magnetic
field.
The reason
for
a
Jc
decrease with increasing thickness is
probably due to the fact that for greater thicknesses, the
number of grains not aligned with the magnetic field will
more likely grow longer in the case of thicker films which
deteriorates the process of alignment, In addition, it might be
that as the thickness increases, there is a larger amount of
grain boundary area, which if poorly connected, affect the
J,
performance.
The model described here is only valid if the
superconductor material is processed under a magnetic field
at a temperature where there is a large fraction of liquid
phase. In the case where the superconductor is never
processed above the melting point in the presence of a
magnetic field, or when the magnetic field is applied during
the late stages of growth, an interaction energy between the
grains needs to be considered
[
131. Under these conditions,
875C
Mg-&ic
Fidd
(Tesla)
Fig.7: F factor as a function of magnetic field
for
various
grain sizes. The processing temperature is 875
C.
the degree of alignment predicted by
(6)
is not achieved and
becomes a function of the distance and angle between the
grains, and the grain aspect ratio [13].
VI.
CONCLUSIONS
The degree
of
texture and transport critical current density
i n
Bi-2212 thick films and tapes are enhanced by the application
of a magnetic field during melt-growth.
During
early growth
of superconductor grains, the magnetic field can induce grain
rotation and thus, a high degree of alignment may be
obtained. However, once the grains begin to interact,
t he
degree of alignment decreases.
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