Physica C 295 Ž1998. 39–46
Preparation parameters governing microstructure and grain
coupling of Bi 2 Sr2 CaCu 2 O xrAg tapes prepared by partial melting
Ryoji Funahashi
a
b
a,)
, Ichiro Matsubara a , Hiroshi Ohashi b, Kazuo Ueno a ,
Hiroshi Ishikawa a
Osaka National Research Institute, AIST, Midorigaoka, Ikeda, Osaka 563, Japan
Osaka Electro-Communication UniÕersity, Hatcho, Neyagawa, Osaka 572, Japan
Received 3 March 1997; revised 4 June 1997; accepted 4 July 1997
Abstract
Bi 2 Sr2 CaCu 2 O x ŽBi-2212. tapes have been prepared using the isothermal partial melting ŽIPM. method, in which partial
melting and solidification are carried out at the same temperature but in different atmospheres. Maximum critical current
density Ž Jc . at 4.2 K under 0 T is 3.0 = 10 5 Arcm2 and 1.9 = 10 5 Arcm2 for the tapes prepared at 8658C and 8308C,
respectively. This difference in Jc is attributed to Bi-2212 grain size, which is larger in the former tape than the latter. Large
grains and a rough distribution of solid phases ŽBi-free and Cu-free phases. in a partially melted state lead to large Bi-2212
grains. Amount of impurities decreases and grain coupling becomes stronger with increasing solidification time Ž ts .. Jc is
saturated in tapes solidified for more than a certain time, the length in which is dependent on melt processing temperature.
More time is needed to achieve saturated Jc in tapes melted at higher temperature, because it takes longer for the large solid
grains to react completely with a liquid phase in the partially melted state. It is clear that Bi-2212 grain size can be
controlled by melt processing temperature, and that both amount of impurities and grain coupling strength can be controlled
by solidification time. q 1998 Elsevier Science B.V.
Keywords: Bi 2 Sr2 CaCu 2 O xrAg tapes; Isothermal partial melting method; Preparation parameter; Microstructure; Grain coupling
1. Introduction
There have been many studies on the power
applications of superconducting oxides, especially
those of Bi–Sr–Ca–Cu–O and Y–Ba–Cu–O superconductors, some of which have reported the successful preparation of Bi 2 Sr 2 CaCu 2 O x ŽBi2212.rAg tapes w1–3x with critical current density
)
Corresponding author. Tel.: q81 727 519541; Fax: q81 727
519622; E-mail: fune@onri.go.jp
Ž Jc . of more than 10 4 Arcm2 at 77 K under zero
magnetic field w1x. However, since the superconducting transition temperature of the Bi-2212 phase is
close to 77 K and there are few effective pinning
centers over 20 K, the power applications of Bi-2212
tapes under magnetic fields are restricted to low
temperatures. On the other hand, the upper critical
field Ž Hc2 . of such tapes is very high Ž) 20 T. at 4.2
K, which is favorable for application under high
magnetic fields. Bi-2212 superconducting magnets
which generate a magnetic field of up to about 3 T at
4.2 K have been produced w4,5x.
0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.
PII S 0 9 2 1 - 4 5 3 4 Ž 9 7 . 0 1 5 9 0 - 6
40
R. Funahashi et al.r Physica C 295 (1998) 39–46
Most Bi-2212 tapes with high Jc are prepared by
partial melting. Since the Bi-2212 phase is an incongruent melting system, a liquid phase with a Bi-rich
composition and solid phases with compositions of
Sr–Ca–Cu–O ŽBi-free phase. and Bi–Sr–Ca–O
ŽCu-free phase. are formed in the partially melted
state w2,6,7x. The Bi-2212 phase is precipitated and
grows by peritectic reaction between the liquid and
the solid phases w8x. If the peritectic reaction is not
completed, impurities such as the Bi-free phase, the
Cu-free phase, and the Bi 2 Sr2 CuO x ŽBi-2201. phase
originating in the liquid phase, remain in the tapes
and depress Jc . Moreover, because Bi-2212 tapes are
polycrystalline ceramics, Jc is also determined by
grain size and grain coupling strength at grain
boundaries. That is to say, for high Jc , control of
microstructure and grain coupling strength are important. Under the conventional partial melting
method, because the Bi-2212 grains are solidified by
slow cooling, the optimal temperature range for the
solidification reaction of Bi-2212 grains is maintained for only a short time. Bi-2201, Bi-free and
Cu-free phases therefore remain as impurities. In this
study, Bi-2212 tapes have been prepared using an
isothermal partial melting ŽIPM. method to reduce
the amount of impurities w9,10x. Since solidification
is carried out by controlling atmospheres at a constant temperature, the optimal temperature range for
solidification of Bi-2212 grains can be maintained
indefinitely. At a given atmosphere, this method
involves only two preparation parameters, melt pro-
cessing temperature and solidification time. The effect of these parameters on microstructure and grain
coupling of Bi-2212 tapes can therefore be observed
independently in this method. This paper investigates
that which of the preparation parameters of the IPM
method governs Bi-2212 grain size, amount of impurities, and grain coupling strength in Bi-2212 tapes.
2. Experimental
Bi-2212 superconducting tapes were prepared using the IPM method, in which partial melting and
solidification were performed at the same temperature but in different atmospheres w10x. This method
makes use of changes in the melting point of the
Bi-2212 phase accompanying change in oxygen partial pressure Ž pO 2 .. The composition of the precursor Bi-2212 powder was Bi 2.0 Sr2.4 Ca 0.7 Cu 2.0 O x . The
melting points of this powder measured using differential thermal analysis ŽDTA. were 8108C in an N2
atmosphere Ž pO 2 s 0%. and 8708C in pO 2 s 20%.
The Bi-2212 powder was suspended in an organic
binder and the suspension deposited on an Ag sheet
Ž150 mm thick.. After evaporation of the binder,
another Ag sheet was placed over the deposit. The
specimens were pressed to densify under 240 MPa at
room temperature. The green tapes were heated at
5008C in air in order to eliminate the organic binder
completely, then partially melted at 8308C or 8658C
for 5 min in an N2 gas flow Žpurity: 5 N, O 2
Fig. 1. SEM photographs of tapes solidified at 8308C Ža. and 8658C Žb. for 24 h.
R. Funahashi et al.r Physica C 295 (1998) 39–46
41
mined by X-ray diffraction ŽXRD, Cu K a source.
and electron probe micro-analysis ŽEPMA.. Microstructural observation was carried out using a
scanning electron microscope ŽSEM.. For observation of the partially melted microstructure, tapes
partially melted for 5 min in an N2 gas flow were
quenched in water. In order to estimate grain coupling strength, AC susceptibility was employed using
a SQUID magnetometer ŽQuantum Design..
3. Results and discussion
3.1. Bi-2212 grain size
Fig. 2. ts dependence of average Bi-2212 grain size for tapes
solidified at 8308C Ž`. and 8658C ŽI..
contamination - 5 ppm.. Solidification was subsequently carried out at the same temperature in pO 2
s 20% Žbalanced with N2 gas. for 1–36 h Žsolidification time: ts .. After heat treatment, the tapes were
allowed to cool down to 7508C in the furnace and
then removed.
Zero resistivity temperature ŽTc,zero . and Jc were
measured using a standard four-probe method. Jc
measurement was performed at 4.2 K under applied
magnetic fields of 0–8 T. The criterion for the
determination of Jc was 1.0 mVrcm. Composition
and amount of impurities in the tapes were deter-
SEM photographs of the tapes solidified at 8308C
Ža. and 8658C Žb. for 24 h are shown in Fig. 1.
Average Bi-2212 grain size is about 65 mm and 130
mm for the tapes prepared at 8308C and 8658C,
respectively, and is independent of ts for the tapes
treated at 8308C with ts G 3 h and at 8658C with
ts G 12 h ŽFig. 2.. Holesinger et al. w9x have also
reported that Bi-2212 grain size increases with melt
processing temperature. The Bi-2212 phase is solidified by peritectic reaction between the solid Bi-free
or Cu-free phases and the liquid Bi-rich phase formed
by partial melting. Microstructural observation in the
partially melted state is therefore indispensable to
discuss the Bi-2212 grain size.
Fig. 3 shows back scattering images for the tapes
quenched from the partially melted state at 8308C Ža.
Fig. 3. Back scattering images for tapes quenched from partially melted state at 8308C Ža. and 8658C Žb..
R. Funahashi et al.r Physica C 295 (1998) 39–46
42
and 8658C Žb.. The Bi-2212 phase melts incongruently so that the solid Bi-free and Cu-free phases and
the liquid Bi-rich phase are formed. The dark spots
in Fig. 3 correspond to the solid phases. ŽSr,Ca.CuO 2
and ŽSr,Ca. 2 CuO 3 as the Bi-free phases and
Bi 2 ŽSr,Ca. 3 O6 as the Cu-free phase are observed in
the tapes melted at both 8308C and 8658C. It is clear
that the grains of the solid phases in the tape melted
at 8658C are larger and more roughly dispersed than
in the tape melted at 8308C. Since the grain size of
the solid phases is smaller in the tape melted at
8308C, peritectic reaction is completed earlier. As
the result, zero resistivity is not observed in the tape
treated at 8658C with ts - 6 h, but is noted at 85 K in
the tape solidified at 8308C even for 1 h. However,
the large grains and the rough dispersion of the solid
phases lead to larger Bi-2212 grains, because the
grain growth of the Bi-2212 phase progresses peritectically at the edge of Bi-2212 grains through
supply of cations from the solid Bi-free or Cu-free
phases. It is clear from the above results that Bi-2212
grain size can be controlled by melt processing
temperature, with which it increases.
3.2. Amount of impurities
A diffraction peak of the Bi-free phase,
ŽSr,Ca.CuO 2 , appears around 2 u s 35.68 in the tapes
prepared at 8658C ŽFig. 4.. The intensity of this peak
Fig. 4. XRD patterns for tapes solidified at 8658C.
Fig. 5. Back scattering image for tape solidified at 8658C with
ts s12.
in the tapes with ts of 12 and 18 h is greater than in
the tapes with ts of 24 and 36 h. This result indicates
that the amount of Bi-free phase remaining in the
former is larger than in the latter.
Bi-free grains can be observed in back scattering
images ŽFig. 5. in all tapes prepared at 8658C.
Bi-free grain size decreases with increasing ts . Fig. 6
shows the ts dependence of average Bi-free grain
size for the tapes prepared at 8658C. Average Bi-free
grain size rapidly decreases in 18 - ts - 24 h. This
indicates that the amount of impurities can be controlled by ts . Bi-free grains are surrounded by the
solid Bi-2212 phase during the solidification stage
ŽFig. 5.. Sr, Ca and Cu ions have to diffuse through
the solid Bi-2212 phase to react with the liquid phase
at the edge of the Bi-2212 grains. In consequence, it
takes a longer time to obtain large Bi-2212 grains.
No Bi-free grains are found in the tapes solidified at
8308C for 3 h, because the peritectic reaction is
almost completed. The value of ts at which the
peritectic reaction is completed depends on melt
processing temperature and becomes longer with increasing grain size of the solid phases in the partially
melted state.
Because of the peritectic reaction, the amount of
remaining Bi-2201 phase is proportional to the
amount of Bi-free phase remaining in the tapes.
Diffraction peaks due to the Bi-2201 phase are so
weak in XRD patterns that the ts dependence of the
amount of Bi-2201 phase is not clear. However, the
amount of Bi-2201 phase seems to decrease with
R. Funahashi et al.r Physica C 295 (1998) 39–46
Fig. 6. ts dependence of average Bi-free grain size for tapes
solidified at 8658C.
increasing ts . As mentioned above, Bi-2212 grains
are surrounded by the liquid phase. The Bi-2201
phase originating in the liquid phase is present at the
grain boundaries and makes the grain coupling weak.
The relationship between the amount of impurities
and the grain coupling strength at the grain boundaries is dealt with in Section 3.3.
3.3. Grain coupling strength
Jc at 4.2 K under 0 T and transport superconducting transition width, DTc , depend on ts ŽFig. 7.. It is
possible to classify tapes into two groups with borderlines at 18 - ts - 24 h and at 1 - ts - 3 h for the
tapes treated at 8658C and 8308C, respectively. These
43
values of ts correspond with those beyond which
impurities decrease rapidly. Transition width reflects
grain coupling strength w11x. A wide transition indicates that grain coupling is weak at the grain boundaries. Because grain coupling is weak in the tapes
with ts of 12 and 18 h at 8658C or with ts of 1 h at
8308C, which have large DTc , Jc for these tapes is
lower than for tapes solidified for a longer time.
This finding regarding weak coupling is supported
by AC susceptibility measurement. Fig. 8 shows
temperature dependence of AC susceptibility. A
broad peak is observed in x Y for all tapes. This peak
is due to AC field penetration into the grain boundaries w12–14x. With increasing ts peak temperature
rises and magnetic superconducting transition in x X
becomes sharper. Lower peak temperature is observed in tapes with weaker grain coupling.
The x Y peak shifts downward in temperature
with increasing AC amplitude, h ŽFig. 9.. Peak
temperature decreases rapidly with increasing h in
tapes affected seriously by weak grain coupling w12–
14x. This indicates conversely that grain coupling
becomes strong with increasing ts . It is clear that
grain coupling strength can be controlled by ts . As
mentioned above, because the Bi-2212 phase grows
by peritectic reaction, the amount of remaining Bifree and Cu-free phases is proportional to the amount
of liquid phase Žpreceding Bi-2201 phase. remaining
at the grain boundaries. Although more detailed experiments regarding location and amount of the Bi2201 phase are necessary, grain coupling is thought
to be weakened by the Bi-2201 phase. That is to say,
tapes including a large amount of Bi-free phase are
affected seriously by weak coupling and their Jc is
Fig. 7. ts dependence of Jc at 4.2 K under 0 T ŽI. and DTc Ž`. for tapes solidified at 8308C Ža. and 8658C Žb..
44
R. Funahashi et al.r Physica C 295 (1998) 39–46
Fig. 8. Temperature dependence of AC susceptibility for tapes
solidified at 8658C for 12 h Ž`., 18 h ŽI. and 36 h Ž^..
Temperature is normalized by magnetic transition onset temperaX
ture ŽTc,mag . and susceptibility by absolute value of x at 5 K for
each tape.
limited to low values. It is reported that large tilt
angle grain boundaries also make the grain coupling
weak w15,16x. In this study, however, it is clear from
XRD measurement that the c-axis alignment is comparable in the tapes treated at 8658C with ts ) 12 h.
Therefore, the grain boundary angle seems to change
little with ts .
Because the h dependence of the peak temperature of x Y is almost the same in the tapes solidified
for 24 h at 8658C and 8308C, grain coupling strength
is comparable in such tapes. The difference in Bi2212 grain size causes a difference in Jc , which is
3.0 = 10 5 Arcm2 and 1.9 = 10 5 Arcm2 at 4.2 K
under 0 T for the tapes solidified for 24 h at 8658C
and 8308C, respectively. The increase in Jc is due to
a decrease with increasing Bi-2212 grain size in the
number of grain boundaries through which superconducting current flows.
A schematic model of the solidification of the
Bi-2212 phase is shown in Fig. 10. First, the solid
Bi-free and Cu-free phases and the liquid phase are
formed in the partially melted state by incongruent
melting of the Bi-2212 phase. The composition of
the Bi-free and Cu-free phases depends on melt
processing temperature w17,18x. In this study, howev er, Ž S r,C a . C u O 2 , Ž S r,C a . 2 C u O 3 , an d
Bi 2 ŽSr,Ca. 3 O6 are observed as solid phases in the
tapes melted and quenched at both 8308C and 8658C.
The grain size of the solid phases increases with melt
processing temperature. After heat treatment, the
larger solid grains lead to larger Bi-2212 grains.
Solidification starts with increasing pO 2 , and the
Bi-2212 phase is precipitated and grows by peritectic
reaction. In the first stage of peritectic reaction, a
product of the reaction is precipitated between the
liquid and the solid phases w8,19x. Fig. 11 is a back
scattering image for the tape solidified at 8658C for 6
h. The Bi-2212 phase is precipitated between the
Bi-free solid phase and the Bi-2201 phase originating in the liquid phase. This photograph shows the
first stage of peritectic reaction w8x. Starting time of
the first stage of the peritectic reaction after increasing pO 2 is thought to depend on the composition of
the solid phases. For the growth of the Bi-2212
grains, Sr, Ca and Cu ions diffuse through the solid
Bi-2212 phase to react with the liquid phase at the
edge of the Bi-2212 grains. Before the completion of
peritectic reaction, the Bi-free, the Cu-free, and the
liquid phases remain. In particular, the liquid phase
which precedes the Bi-2201 phase is present at the
grain boundaries and makes grain coupling weak.
Even a small amount of Bi-2201 phase, which produces very weak diffraction peaks in XRD, affects
Y
Fig. 9. h dependence of peak temperature of x normalized by
Tc,mag for tapes solidified at 8658C for 12 h Ž`., 18 h ŽI., 24 h
Ž^. and 36 h Že..
R. Funahashi et al.r Physica C 295 (1998) 39–46
45
grain coupling. The completion time of peritectic
reaction depends on the grain size of the Bi-free and
the Cu-free phases in the partially melted state. A
long time is needed for the large solid grains to react
with the liquid phase completely. After the completion of peritectic reaction, impurities disappear and
grain coupling becomes strong. In this study, the
Bi-free grains are observed in the tape solidified at
8658C even for 36 h. It seems that the composition
of the precursor powder has to be optimized to
remove Bi-free grains completely.
4. Conclusion
Fig. 10. Schematic model of solidification of Bi-2212 phase.
Bi-2212 tapes have been prepared using the IPM
method, in which partial melting and solidification
are carried out at the same temperature but in different atmospheres.
Maximum Jc is 3.0 = 10 5 Arcm2 and 1.9 = 10 5
Arcm2 for the tapes prepared at 8658C and 8308C,
respectively. This difference in Jc is attributed to
microstructural difference, i.e. Bi-2212 grains are
larger in the former tape than the latter. It is clear
that large grains and rough distribution of solid
phases in the partially melted state lead to larger
Bi-2212 grains.
Bi-2212 grain size can be controlled by melt
processing temperature while both amount of impurities and grain coupling strength can be controlled by
ts . In order to obtain tapes with high Jc , melt
processing temperature should be as high as possible
and solidification time as long as possible.
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