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2-1-1992
Effect of CO2 on the Processing of Y-Ba-Cu-O Superconductors Effect of CO2 on the Processing of Y-Ba-Cu-O Superconductors
C. Zhang
San Jose State University
Guna S. Selvaduray
San Jose State University
, gunas@email.sjsu.edu
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Recommended Citation Recommended Citation
C. Zhang and Guna S. Selvaduray. "Effect of CO2 on the Processing of Y-Ba-Cu-O Superconductors"
Journal of Materials Research
(1992): 283-291. https://doi.org/10.1557/JMR.1992.0283
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Effect
of
C02
on the processing
of
Y
-Ba-Cu-0
superconductors
G. Selvaduray and
C.
Zhang
Department
of
Materials Engineering, San Jose Stare University, San Jose, California 95192-0086
U. Balachandran,
Y.
Gao, K. L. Merkle, H. Shi, and R. B. Foeppel
Materials and Components Technology Division, Argonne National Laboratory,
9700
South Cass Avenue, Argonne, Illinois 60439-4838
(Received 26 June 1991; accepted 10 September 1991)
The
superconducting properties
of
YBa
2
Cu
3
0
6
+x
reacted with various
known
ratios
of
02
/C
0
2
gas
mixtures during sintering at different temperatures
were
studied.
lc
was
found
to decrease drastically upon reaction with
C0
2
,
becoming
zero
at certain
C0
2
activities.
The
stability region for
the
123
superconductor, as a function
of
C0
2
activity
and
temperature,
was
empirically formulated as follows: log Pco
2
< (
-45,000)/T
+ 33.4.
The
grain boundaries
in
sintered samples with l c = 0
were
investigated with
HRTEM
in
conjunction with EDS.
Two
distinct types
of
grain boundaries
were
observed.
Approximately
10%
of
the grain boundaries
were
wet
by a thin layer
of
a second phase,
deduced to
be
BaCu0
2
.
The
remaining boundaries
were
sharp grain boundaries.
The
grain
structure near the sharp grain boundaries
was
tetragonal.
These
two
types
of
grain
boundaries are thought to
be
responsible for
lc
being
zero.
I.
INTRODUCTION
There
are
two potential sources
of
C0
2
when
YBa
2
Cu
3
06+x
(123) superconductors are processed by
solid-state sintering.
One
source
is the
C0
2
contained
in the oxygen
gas
used during sintering and/or
annealing.
Another
source
is
the
C0
2
derived from
the
decomposition
of
BaC0
3
during
the calcination step.
Each
source
affects
the
quality
of
the final product in
different
ways
and
can
lead to a drop in the critical
current density,
l
c.
During
calcination,
the
Y
2
0
3
,
BaC0
3
,
and CuO
powders ideally react according to reactions (1) and (2),
to form 123.
BaC03
(s)
+
CuO
(s)--+
C02
(g)+
BaCu0
2
(s)
(1)
4BaCu0
2
(s) +
Y20
3
(s)
+
2Cu0
(s)--+
2YBa
2
Cu
3
0.,
(s)
(2)
When
BaC0
3
decomposes, according to reaction (1),
C0
2
is
released.
The
localized concentration
of
C0
2
can quickly reach its equilibrium value, and stagnant
regions
of
C0
2
can
form, even in systems
with
flowing
gases.
This
localized
C0
2
pressure, depending
on
the
temperature
and
other
thermodynamic conditions, can
cause
other
decomposition reactions, thereby forming
undesired phases,
which
in
turn contribute to reduce
the
critical current density
of
the final product.
Fjellvag
1
studied
the
interaction between
C0
2
and
YBa
zCu
3
06+x
at
a total pressure
of
1 atm,
with
0.999 atm
of
0
2
and
0.001 atm
of
C0
2
,
and
reported two reaction
mechanisms:
YBa2Cu306+x
(s)
+
2C0
2
(g)
--+
2BaC0
3
(s)
+
~Y20
3
(s)
+
3Cu0
(s)
+
(2x
-1)/40
2
(g)
(3)
YBa2Cu306+x
(s)
+
2C02
(g)--+
2BaC0
3
(s)
+
~Y2Cu
2
0
5
(s)
+
2Cu0
(s) +
(2x
-1)/40
2
(g)
(4)
Reaction (3)
was
reported to
occur
at
temperatures
below
730
±
10
°C,
and
reaction (4) at temperatures
above
730
±
10
°C.
Both
of
these reactions lead to the
formation
of
nonsuperconducting phases.
As
can
be
seen
from these two reactions, the partial pressure
of
C0
2
will
determine
the
extent to
which
the nonsuperconducting
reaction products are formed.
The
effect
of
C0
2
partial
pressure on
the
oxide-carbonate equilibrium,
as
a func-
tion
of
temperature,
was
also
derived
by
Fjellvag, and
is
shown
in Fig. 1. Analytically,
the
equilibrium partial
pressure
of
C0
2
can
be
represented by the following
expression:
-8900
log
Pco
2
=
-y
+
5.7
(5)
where
Pco
2
is in atmospheres and the temperature T
is
above
730
°C.
The
partial pressure
of
C0
2
can
also affect
the
partial pressure
of
0
2
,
which in turn affects
the
oxygen
content
of
the
123
compound
formed, and thus
the
phase
composition
of
the
123.
The
superconducting properties
of
123
compounds,
as
a function
of
oxygen
content,
have
been
studied by
4
other
investigators.
2
-
For
x
::::::::
1,
YBa
2
Cu
3
0
6
+X
is
in
the
orthorhombic phase and exhibits superconductivity
at
90
K.
As
the
oxygen
content,
which
is
dependent
on
both
temperature
and
oxygen
partial pressure, decreases,
the transition temperature
of
the
orthorhombic phase
J.
Mater. Res., Vol. 7, No.
2,
Feb 1992 © 1992 Materials Research Society
283
~
~
cg
'b
•
•
t(OC)
900
800
700
600
•
-2
-3
E
0
N
0
..,
c...
-4
0'
0
-5
8 9 10
11
liT
·10
4
(
K-
1
)
G.
Selvaduray et
at.:
Effect
of
C0
2
on
the processing of
Y-Ba-Cu-0
superconductors
FIG. 1. Stability
of
YBa
2Cu30b+x with respect to carbonate formation
(Pco
2
+ 1 atm).
1
decreases
to
60 K, at 0.6 < x < 0.7. At x ~ 0.4, the
material becomes tetragonal, and superconductivity is
destroyed. The transition
of
the orthorhombic phase to
the tetragonal phase occurs at an oxygen content
of
x ~ 0.5.
The
equilibrium constant for reactions (3) and
(4) is given by:
4
K=
{lPo2]
(
2
x - l
)/
} /
{ [
Pco2l
2
}
(6)
Equation (6) shows the effect
of
partial pressure
of
C0
2
on the partial pressure
of
0
2
,
and thus on the structure
of
the 123 compound synthesized.
This investigation was undertaken to determine the
effect
of
the presence
of
C0
2
in atmospheres used for
processing
123 compounds. The specific aims
of
the
investigation included (a) determining process conditions
necessary for avoiding the formation
of
undesir-
able phases, and (b) establishing processing-structure-
properties relationships.
II. EXPER IMENTAL PROCED URE
Appropriate amounts
of
Y
2
0
3
,
BaC0
3
,
and CuO
were mixed as a 400 gm batch and wet milled for 15 h in
methanol
in
polyethylene
jars
containing
Zr0
2
grinding
media. The resultant slurry was pan dried and screened
through a
30
mesh sieve.
The
screened powder was cal-
cined for 4 h in flowing 0
2
with a pressure
of
~2
mm
Hg
at a temperature
of
~
850
oc.
During cooling,
the
vacuum was discontinued, and ambient pressure 0
2
was
passed through. A 3 h hold at
450
°C
was incorpo-
rated into the cooling schedule to promote oxygenation
of
the powder.
The
calcined powders were ground,
milled, and checked for phase purity by x-ray diffraction
(XRD) and differential thermal analysis (DTA). The
calcination procedure used in this work gives essentially
phase-pure orthorhombic
123 powder.
5
The calcined
powder was cold-pressed into
~ 1 em diameter pellets
at
~ 140 MPa, subdivided into groups, and sintered
for 5 h.
Four groups, each made up
of
four samples, were
sintered at temperatures
of
910, 940, 970, and 1000 °
C,
respectively, with oxygen gas at ambient pressure passed
through the furnace during the entire process. Sixteen
groups, each made up
of
four samples, were also sin-
tered at the same temperatures, but with
C0
2
/0
2
gas
mixtures, at ambient pressure.
The
C0
2
concentration
in
the mixtures ranged from
~50
ppm
to
5%.
Bars
of
1
mm
2
cross section and 10
mm
long were
cut from each pellet for critical current density
(Jc)
measurements. A criterion
of
1 f
..
N
/e
m was applied
to
Jc measurements. Representative samples were used for
measuring the ·critical transition temperature
(T
c)
by
resistivity and low field
RF
SQUID magnetometer.
Samples were also mounted and polished, using a
diamond paste, down to 1
J.Lm,
for optical and scanning
electron microscopy.
The
mean grain size
of
each sample
was measured using the line intercept method. High reso-
lution transmissi
on
electron microscopy (HRTEM) was
used for studying the microstructure and composition of
the grains and grain boundaries. Phase composition of
the sintered samples was studied by XRD.
Finally, the samples that had been sintered
in
50 ppm,
500
ppm, and 0.5%
C0
2
containing atmo-
spheres were reannealed in pure oxygen flow, at 450
oc
for 15
h,
to check the reversibility
of
the second phases
formed. Samples that had been sintered in pure oxygen
flow were reannealed in oxygen flow, with
5%
C02
for
10
h, to check for formation
of
sc::cond
phases.
Ill. RESULTS
A . Sintering temperat
ur
e and atmosphere
The effects
of
sintering temperature and atmosphere
on
the density and the critical current density
of
the
samples tested are shown
in
Figs. 2 and 3, respectively.
Figure 2 shows that the sintering temperature has a major
effect on the density
of
the sample, as can
be
expected.
However, the sintering atmosphere, within the range
investigated
in
this work, does not appear to affect
the
density.
The
critical current density,
on
the other
hand,
J . Mater. Res.,
Vol.
7,
No.
2.
Feb 1992
284
--
--
•
•
•
• •
G.
Selvaduray
eta/.:
Effect of
C0
2
on the processing of
Y-Ba-Gu-0
superconductors
100
--<>--
100%02
SO
ppm
C02
----
500
ppm
C02
~
90
e..
0.5%C02
~
Slnterlng
time
Sh
Annealing Time
12h
~
u
a
80
u
~
~
70
60+-~
--.-
~-,--~~--r--r~
--
,--T~
900 920
940
960
980
1000
1
020
Sinterln
g
Te
mp
e r
at
u
re
("C)
FIG
.
2.
The
effect
of
sintering temperature
on
density.
was found to
be
affected by
both
sintering
temperature
and
C0
2
content in
the
sintering
atmosphere
,
as
shown
in
Fig. 3.
fc
values decreased
with
decreasing
sinter-
ing temperature and increasing
C0
2
content.
Figure
4
summarizes these results and presents them in terms
of
processing conditions,
namely
Pco
2
and
T,
necessary
for production
of
superconductors.
The
solid line sep-
arates
the
stability
regions
of
superconductors
and
semi-
conductors.
An
experimental
relationship between
Pco,
and T
was
derived from this
diagram
,
and
found
to
be
e
·-
.s
Jf
Jf
in
)-
c
es
~n
·or
ere
the
:ly.
tjor
te
d.
oge
the
lf)d,
as follows:
log
Pco
2
=
C/T
+ D
where C = -
45
,00
0 K
and
D = 33.4.
B. M
icrostructure
and
atmosphere
(7)
The
mic
rostructures
of
two typical samples are
shown in Figs. 5
and
6. In
Fig
. 5
the
microstructures
of samples sintered
at
910,
940
, 970,
and
1000
°C, and
100% 0
2
are
shown.
The
grain
size
as
determined
by
400
--.-
5000
ppm
C02
500
ppm
C02
-
50
ppm
C02
300
-
-o-
.......
10
0
o/o
02
..
a
CJ
s
'
200
":1
0
100
o+-~~~
~~~~
-,--
~~
--~-r
--r-~
900
920
940
960
980
1000
1020
Sin t
erlng
T e
mp
er
atur
e (°C)
FIG.
3. The effect
of
sintering temperature on
lc.
8.5
-
-
E
8.3
~
"'
0
8.1
u
c.
bJ)
..::
7.9
Si
nt
eri
n g
Te
mp
er
atu
re
(C
0
)
1000
970
940
910
Cl
"
"
semiconductor
•
c
superconductor
7.
7
+-~--
--r
--
~-,--~--.-~
--.-
--~-i
-5.3
-4.3
-3.3
-2.3 -1.3
-0.3
FIG.
4.
Stability of 123 compounds as a function
of
Pco
2
and
temperature.
the
li
ne
intercept
method
is pl
otted
in
Fig_ 7.
Large
grain
growth
is
observed
at
970
°
C,
and
the
grain
size
data
are
in
good
agreement
with
the density
and
J ,
data
reported
earlier in
Figs
. 2
and
3.
The
denser
samples
have
larger
grain
sizes,
and
the l c
increase
is
t
hought
to result from
the
increase
in
connectivity
between
the
grains.
From
Fig. 6
it
can
also
be
seen
that
the
crystals
of
the
samples
sintered
in
lower
C0
2
atmospheres
are
twinned
,
whereas
those sintered in
5%
C0
2
do
not
show
any
twinning.
The
tw
in structure in 123
determines
its
orthorhombic
superconducting
properties,
6
and
it
can
be
concluded that
the
samples
sintered
in
5%
C0
2
are
not
orthorhombic.
C l
ose
examination
of
the
microstructure also
shows
that
the
extent
to
which
the
orthorhombic
structure
is
present
decreases
with
increasing
C0
2
presence
in
the
s
int
ering
atmosphere
.
C.
Relationship
between
Tc
and
atmosp
here
The
critical temperature, T
c,
of
the
samples
was
mea-
sured
by
both
resistivity
and
magnetization techniques,
and
are
shown
in
Figs
. 8
and
9,
re
s
pectivel
y.
The
data
in
F ig. 8(a),
for
a
sample
sintered
at
940
°C
and
100
% 0
2
,
exhibits
onset
of
transition
at
90
K
and
zero
resistance
at
82 K. A
linear
temp
erature
dependence
of
r
es
ist
ivity
(R),
wi
th
8Rf
8T > 0
before
the
onset
of
superconductivity,
indicates that this
sample
is a
good
superconductor.
Fi
gure
8(b)
shows
the
data
for
a
sample
sintered
in
an
atmosphere
containing
50
ppm
C0
2
•
Two
transition
te
m
pe
ratures,
one
at
90
K
and
one
at
80
K ,
are
observed
,
indicating
the
presence
of
a
second
superconducting
tran-
sition
(or
phase),
and
zero
re
sistance is
observed
at
78
K.
For
samples
sintered
in
an
atmosphere
containing
0.5%
J. Mater. Res
.,
Vol.
7,
No
. 2, Feb 1992
285
G.
Selvaduray
eta/.:
Effect of
C0
2
on
the processing of
Y-Ba-Cu-0
superconductors
FIG. 5. Optical micrographs for samples sintered under 100% 0
2
at (a) 910
°C,
(b) 940 °
C,
(c)
970
° C, and (d) 1000
°C.
C0
2
,
6R/6T
< 0 [Fig. 8(c)], and the sample shows
detrimental, despite the fact that the
major
phases are still
marked semiconducting characteristics. Increasing
C0
2
superconducting.
The
decrease in superconducting prop-
contents in the sintering atmosphere were found to in-
erties is found to
be
directly related to the concentration
crease the electrical resistivity
of
the sample.
These
of
C0
2
in
the sintering atmosphere. It
can
be
inferred
findings are
in
good agreement with the findings
of
Wang
that
C0
2
leads to partial carbonation
of
the sample, and
et
al.,1 Nakazawa and Ishikawa,
8
and
Kwok
et
a/.
9
that these carbonated phases may accumulate along the
The
diamagnetism observed in the data reported
in
grain boundaries. Thus, although these samples consist
Fig. 9 indicates that the
major
phase
of
these samples is
of
superconducting grains, these grains would then be
still superconducting.
The
presence
of
C0
2
is found to
be
isolated by nonsuperconducting second phases along
J.
Mater. Res.,
Vol.
7,
No. 2, Feb 1992
286