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Materials Express
2158-5849/2014/4/105/010
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doi:10.1166/mex.2014.1153
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Critical current density improvement by
intermediate deformation for the fabrication of
Bi2Sr2Ca2Cu3O10+/Ag round wires
Peng Xie1 , Timing Qu1, 2, ∗ , Kaite Huang1 , Feng Feng1, 3 , and Zhenghe Han1
1
Applied Superconductivity Research Center, Department of Physics, Tsinghua University, Beijing 100084, China
Key Laboratory for Advanced Materials Processing Technology, Department of Mechanical Engineering, Tsinghua
University, Beijing 100084, China
3
Division of Advanced Manufacturing Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China
2
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Multi-filamentary Bi2 Sr2 Ca2 Cu3 OIP:
/Ag round wires
were11fabricated
the powder-in-tube method. Both drawOn: Fri,
Aug 2017by
16:19:37
10+5.10.31.210
Copyright:
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ing and cold isostatic pressing methods
wereAmerican
used to densify
thePublishers
filament cores of the Bi2 Sr2 Ca2 Cu3 O10+ /Ag
round wires during intermediate deformation after the first heat treatment. Critical current measurements show
that both methods can increase the self-field critical current density at 77 K. By optimizing the drawing process, the core density of the first heat treated round wires was increased significantly, resulting in a more than
doubled critical current density improvement of the Bi2 Sr2 Ca2 Cu3 O10+ /Ag round wires. Increasing the drawing
stress using a high strength metal sheath increased the critical current density further to about 1 × 104 A/cm2 .
By carrying out the intermediate deformation using the cold isostatic pressing method, the optimal critical
current density was 8.6 × 103 A/cm2 .
Keywords: Powder in Tube Method, Superconductor, Drawing, Cold Isostatic Pressing, Core Density, Critical
Current Density.
1. INTRODUCTION
High temperature superconducting (HTS) materials including Bi2 Sr2 Ca2 Cu3 O10+ /Ag (Bi-2223/Ag) wires, Bi2 Sr2
Ca1 Cu2 O8+ /Ag (Bi-2212/Ag) wires and YBa2 Cu3 O7−
(YBCO) coated conductors have been widely used in various applications, such as cables, magnets, motors, transformers, fault current limiters, superconducting magnetic
energy storages, etc.,1–4 due to their high critical current
density (Jc ) and feasible operation at liquid nitrogen temperature. Most of the HTS wires were fabricated into a
tape geometry having a large aspect ratio of 15–40. However, there were at least two disadvantages which restricted
∗
Author to whom correspondence should be addressed.
Email: [email protected]
the application of these HTS tapes. (1) Electro-magnetic
anisotropy: usually the Jc of the tape under the perpendicular field was much lower than that under the parallel field,5–7 which arose from the intrinsic flux pinning
anisotropy of the cuprate superconductors.8 (2) Mechanical anisotropy: the HTS tapes can only be bent along the
tape surface due to both their high aspect ratio and their
brittle ceramic core characteristics. Therefore, for many
applications, round HTS wires are preferable. Both Bi2212/Ag and MgB2 round wires have been developed for
applications operating below 20–30 K.9–13 However, for
operating temperatures above 30 K, studies of YBCO or
Bi-2223/Ag round wires are few in number.14–16 For Bi2223/Ag tape, it is easy to increase the superconducting
core density and enhance the filament texture. However,
Mater. Express, Vol. 4, No. 2, 2014
105
Article
ABSTRACT
Article
Materials Express
Critical current density improvement for Bi-2223/Ag round wires
Xie et al.
for round wires, it is much more difficult. In order to keep
drawing die, and thus the diameter of the wire was
the round geometry, flat rolling or uniaxial pressing proreduced. The wire cross sectional area reduction per drawcesses usually used in tape fabrication are not suitable.
ing pass was about 7%, and the die angle was 13.5 .
Drawing could only yield an upper limit of core density
The drawing was carried out at a speed of 0.5 m/s without
(∼ 70% of the theoretical core density) for lacking of large
lubrication.
stress imposed on the powder.17 Besides, the filament texDrawing with metal sheaths was also carried out. As
ture also cannot be significantly improved for lacking of
shown in Figure 2, brass sheath of 1.7 mm/0.8 mm
flat rolling process.18 These are the reasons for very few
(outer diameter of 1.7 mm and inner diameter of 0.8 mm)
reports on the Bi-2223/Ag round wires over the years. As
and stainless steel sheath of 1.2 mm/0.8 mm were
far as we know, the highest Jc of Bi-2223/Ag round wires
adopted, respectively. After drawing, the outer sheath of
is 3.2 × 104 A/cm2 reported by Fujikami, and consist of a
brass or steel was corroded by a 30 wt.% FeCl3 solution
concentric arrangement of filaments.16 It has low packing
with heating in a water bath at 80 C for about 1 hour.
ratio (∼ 12%) of Bi-2223 powder and high ratio of the silThe CIP process was carried out at a pressurization rate
ver matrix in the cross section of the wire. As a result, the
of 5 MPa/s, and held for 60 seconds upon reaching the
Ic of the wire is low too. Xiaodong Su’s work showed that
target pressure to obtain the CIP samples. Water was used
the intermediate drawing process is supposed to be necesto transmit the pressure during the CIP process. To prevent
sary to increase the critical current (Ic ) of round wires.15
permeation of water into the superconducting cores from
Recently, the cold isostatic pressing (CIP) method was
the two ends of the wire, which can cause a failure of the
introduced into the fabrication of Bi-2212/Ag wires and
CIP process, the HT1 treated samples were inserted into
the critical current density of Bi-2212/Ag wires can be
the 0.1 mm thick polyethylene terephthalate (PET) bag,
doubled.19
and then the bag was vacuumized and thermoplastically
In this work, various methods including drawing, drawsealed to keep the pressure in the bag below 0.1 atm.
ing with high strength sheath sleeving and cold isostatic
A Nikon ME600 optical microscope was used to
pressing were used to increase the core density, hoping to
observe cross sections of the deformed round wires.
increase the Jc of the round wire. Both the core density and
Micrographs and elemental analysis were obtained on a
deforthe Jc were increased by optimizing the mechanical
Leo-1530
electronic microscope (SEM) equipped
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?
mation process. Meanwhile, an effective
way
to
enhance
with
an
Oxford
energy
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Publishers
the local 2223 filament texture was explored.
tem in the
backscattered electron (BSE) imaging mode.
Self-field Ic values of fully-processed wires were obtained
using the standard four-probe method with a criterion of
2. EXPERIMENTAL DETAILS
1.0 V/cm over the temperature range 77 K to 4.2 K. InThe traditional powder-in-tube (PIT) method was used to
field Ic was measured with applied field up to 8 T at 4.2 K
fabricate 61 filaments Bi-2223/Ag green round wire havand 0.075 T at 77 K, respectively. The image processing
ing a diameter of 0.8 mm. The heat treatment procesoftware-Photoshop was used to calculate the filling facdure is shown in Figure 1. The first heat treatment (HT1)
tor, which is defined as the ratio of the core cross sectional
was carried out at 830 C for 20 hours in an atmosphere
area to the total wire cross sectional area.
with a reduced oxygen partial pressure (pO2 = 008 bar).
Intermediate mechanical deformation was carried out by
3. RESULTS AND DISCUSSION
either the drawing method or the CIP method. Then, the
main heat treatment (MHT) was carried out to obtain the
3.1. Intermediate Drawing Process
fully-processed round wires by sintering at 830 C with
3.1.1. The Critical Current Performance
a subsequent slow-cooling process in an atmosphere with
In the fabrication of Bi-2223/Ag tapes, it has been found
pO2 = 008 bar.
that intermediate mechanical deformation is crucial for
During the intermediate drawing process, for each drawobtaining high Jc performance.20 21 Thus, in this work,
ing pass, the HT1 treated wire was drawn through a
the intermediate drawing process was introduced between
the two heat treatments to improve the Jc of the Bi2223/Ag round wires. The drawing samples were obtained
by drawing the HT1 treated samples to different diameters. The relation between the Jc values and the diameter
of the drawing samples is shown in Figure 3. The original
diameter of the green wire was 0.80 mm. The Jc value
of the reference sample without intermediate drawing was
around 3600 A/cm2 (shown by the solid square point in
Fig. 3). As the diameter of drawing samples decreased,
the Jc value first increased and then decreased. The Jc
Fig. 1. The fabrication procedure of Bi-2223/Ag round wire.
reached its maximum value of about 8300 A/cm2 when
106
Mater. Express, Vol. 4, 2014
Critical current density improvement for Bi-2223/Ag round wires
Xie et al.
Materials Express
Fig. 2. The optical photos of the samples: (a) the cross section of the Bi-2223/Ag round wire; (b) the cross section of the Bi-2223/Ag round wire
with brass sheath sleeving; (c) the cross section of the Bi-2223/Ag round wire with steel sheath sleeving; (d) the appearance of the three kinds of
wires for intermediate deformation.
the diameter was around 0.71 mm. Therefore, Jc of round
wires was more than doubled by introducing the intermediate drawing process. If the diameter of the drawing
sample was reduced further still, the Jc of the wire would
Fig. 3. The Jc of the drawing samples (green wire of 0.80 mm)
variation with different diameter. The error bars represent the standard
deviation of the Jc data. The samples fabricated without intermediate
drawing were shown in solid symbol. The samples fabricated with intermediate drawing were shown in hollow symbols.
Mater. Express, Vol. 4, 2014
decline. The similar result was found in Bi-2223/Ag tapes
by Grasso.22 who pointed out that large deformation led
to the formation of cracks in the superconducting cores
and thus suppressed Jc .
For the drawing sample having a diameter of 0.71 mm,
the relation between the Ic (self field) and the temperature is shown in Figure 4(a). The Ic of the Bi-2223/Ag
round wire was 15 A at 77 K, and it became larger as
the temperature decreased. When the sample was cooled
to 4.2 K, Ic reached 80 A. The Ic increased by a factor of
5 as the temperature decreased from 77 K to 4.2 K, which
was similar to Bi-2223/Ag tape results.23 The in-field
Ic results are shown in Figure 4(b). Ic decreased rapidly
as the applied field increased when the applied field was
below 0.1 T at 77 K. Even at 4.2 K, Ic reduced significantly until the applied field reached 1 T. The total Ic of
the Bi-2223 tape can be seen as the sum of the weak link
critical current-Icw and the strong link critical current-Ics .
At 77 K, Icw decreases fast even before 101 –102 mT, while
the Ics retains until several tesla. The transport capacity
of the weak-link can be easily reduced by the increasing external field. Therefore, the fast decrease of Ic below
0.1 T at 77 K and 1 T at 4.2 K indicated that large amount
107
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Article
Materials Express
Critical current density improvement for Bi-2223/Ag round wires
Xie et al.
The details of the drawing sample having an optimum
diameter— 0.71 mm can be observed in Figures 5(d)
and (e). Many outgrowth Bi-2223 grains are observed to
have grown into the silver matrix, as shown in Figure 5(d),
and some adjacent filaments are connected by these outgrowth grains. A similar phenomenon in Bi-2212/Ag wires
has been investigated by Shen,24 who pointed out that
the growth of Bi-2212 grains between filaments can form
filament to filament bridging composed of two types of
bridges: one having a potential high-Jc path and the other
not. We considered that the outgrowths observed in our
work led to bridging similar to the latter one, which did
not carry much current.
As shown in Figure 5(d), at the inner part of the filaments, many thin Bi-2223 grains formed with random
orientation, which were not well textured and connected.
Between these randomly oriented Bi-2223 grains, some
several m-sized CuO or SrO grains can be observed.
These non superconducting phases and the residual voids
in the filaments can block the current flow and cause a
Jc decrease. The weak-link existed due to the low core
density and the irregular alignment of Bi-2223 grains compared to the microstructure of Bi-2223/Ag tapes, which
contributed to the fast decrease of Ic in low fields, as
shown in Figure 4(b). In each filament, however, along the
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superconducting-silver
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Aug 2017 16:19:37 interface, a Bi-2223 layer with a
thicknessPublishers
of 2–4 m formed with good texture, as shown
Copyright: American Scientific
in Figure 5(e), which can provide good supercurrent paths.
Moreover, it is interesting to point out that, as the intermediate drawing proceeded, many Bi-2223 layers along
the superconducting-silver interfaces intersected to form
acute angles at the corners of the filaments, as shown
in Figures 5(c) and (d). In these local regions, the two
Fig. 4. (a) Self-field Ic of 0.71 mm Bi-2223/Ag round wire (optimized drawing sample), as the temperature changing from 77 K to
adjacent Bi-2223 layers in the filament could grow com4.2 K. (b) In-field Ic of Bi-2223/Ag round wire (being 0.71 mm with
pactly to form a 10 m thick Bi-2223 region, as shown
filling factor of 43.5%) at 4.2 K and 77 K.
in Figure 5(f), and the angle between the two adjacent
Bi-2223 layers was small at these corners. Because the Jc
through the grain boundaries is very sensitive to the orienof weak-link existed in the present round wire. The intation of adjacent crystallites and increases exponentially
field Ic can be further improved if the weak links could be
with decreasing misorientation angle,25 these acute correduced.
ners of the filaments with large Bi-2223 grains and small
angle grain boundaries could be potential high-Jc supercur3.1.2. The Micro Structure Evolution
rent paths. As discussed above, the Jc of the Bi-2223/Ag
SEM micrographs of fully-processed drawing samples
round wires could be further improved by increasing the
having a diameter in the range 0.80 mm to 0.71 mm are
superconducting-silver interfaces and the local acute angle
shown in Figure 5. A number of voids can be observed in
regions of the superconducting filaments.
Figure 5(a) for the sample fabricated without intermediate drawing. For the samples fabricated with intermediate
3.1.3. The Core Density Variation
drawing, shown in Figures 5(b) and (c), the voids were
As the diameter of round wires decreased during the drawlargely reduced as the diameter of the drawing samples
ing process, the filling factor–rsc first decreased and then
decreased. Meanwhile, the filament shapes changed, and
stopped decreasing and stabilized with some fluctuation,
a number of silver-superconducting interfaces intersected
as shown in Figure 6. Any given section of Bi-2223/Ag
to form acute angles at the corners of the filaments as the
wire is a composite of two parts: one is the superconductdiameter decreased, which will be discussed later in this
subsection.
ing filament array and the other is the silver matrix, as
108
Mater. Express, Vol. 4, 2014
Critical current density improvement for Bi-2223/Ag round wires
Xie et al.
Materials Express
Article
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Fig. 5. The SEM micrographs of the cross section of the fully-processed drawing samples. (a) The sample of 0.80 mm fabricated without
intermediate drawing; (b) the sample fabricated with intermediate drawing to 0.75 mm; (c) the sample fabricated with optimum intermediate
drawing to 0.71 mm; (d) the detail of several filaments (sample of 0.71 mm); (e) the Bi-2223 texture along the superconducting-sliver interface;
(f) the Bi-2223 phase at the corners of the filaments.
shown in Figure 2(a). During the fabrication process, the
following relations are always satisfied:
Ag Sr Ag l = MAg
sc Sr sc l = Msc
(1)
where Ag and sc are the mass densities of the silver
matrix and superconducting cores, respectively. MAg and
Msc are the mass of the silver matrix and superconducting
cores, respectively, rAg is the ratio of the silver matrix to
the total round wire cross sectional area, rsc is the ratio
of the superconducting cores to the total round wire cross
sectional area, where these two ratios satisfy rAg + rsc =
100%. S is the total cross sectional area and l is the length
of the wire. By solving Eq. (1), we obtain:
sc = k ·
1 − rsc
rsc
where the coefficient k is given by:
k = Ag · Msc /MAg
Mater. Express, Vol. 4, 2014
(2)
During the fabrication process, the silver matrix mass
and the superconducting core mass are constants, and also
the density of silver matrix is almost unchanged. Therefore, the coefficient k is a constant. As shown in Eq. (2),
the core density–sc depends only on the filling factor—rsc .
For green round wires with no heat treatment, we calculated the filling factor when drawing the wire. The rsc
value stabilized at 50% when the diameter of the green
wire was from 1.2 mm to 0.8 mm. However, as
indicated in Figure 6, for the drawing samples which had
already been sintered for 20 hours at HT1, the rsc reduced
from 50% to 43% after the intermediate drawing process.
Therefore, from Eq. (2) the core density sc increased by
32.5% after the intermediate drawing process. This could
be explained by that: during the green wire drawing process, if the drawing parameters were fixed, there existed
an upper limit for the core density.17 26 However, during sintering, the voids formed throughout the aggregate,
as shown in Figure 5(a). Drawing after sintering could
109
Materials Express
Article
Fig. 6. The rsc variation with the diameter of drawing samples.
The sample fabricated without intermediate drawing was shown in solid
symbol. The samples fabricated with intermediate drawing were shown
in hollow symbols. The dash line was assisted to observation.
Critical current density improvement for Bi-2223/Ag round wires
Xie et al.
Fig. 7. The relation between the Je and the diameter of Bi-2223/Ag
wires. The error bars represent the standard deviation of the Je data.
“”: Intermediate drawing with no sleeving for comparison; “”:
Intermediate drawing with brass sheath sleeving; “”: Intermediate
drawing with steel sheath sleeving. As a comparison, the samples
fabricated without intermediate drawing were shown in solid symbols.
The samples fabricated with intermediate drawing were shown in hollow symbols.
further decrease these voids and therefore increase the core
density.
As shown in Figure 6, the relationship between rsc and
the diameter in the drawing process can be divided into
two regions: (1) the rsc decreases when drawing the wire
metal sheaths were corroded to make oxygen diffusion
from 0.80 mm to 0.71 mm, corresponding to an
possible in the subsequent heat treatment.
increase in the core density; (2) the rsc remains stable when
The relationship between the engineering critical curDelivered
to: ?
the diameter is below 0.71 mm, corresponding
to a coreby Ingenta
rent
density
e ) and the diameter of fully-processed brass
Aug
2017(J16:19:37
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the theoreticalOn:
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or
steel
sleeved
samples is shown in Figure 7. The Je
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sity by the estimation in our experiment.
SimilarAmerican
results Scientific
of the brass sleeved samples and steel sleeved samples
were found in the Bi-2223/Ag tape study conducted by
both increased with decreasing diameter. However, the
Han et al.17 They proposed a “powder-flow” model to
Je of the brass sleeved samples were no better than the
explain such results. Powder flow will stop when the fricordinary drawing samples without sleeving. Whereas, for
tion between powder grains is equal to the driving force on
steel sleeved samples, the Je could be improved to 4.02 ×
the powder, thus the powder packing density reaches a crit103 A/cm2 , and the rsc was about 40.7%. Because Jc =
ical value. Further density increase requires an increase of
Je /rsc , the maximum Jc value reached 9.88 × 103 A/cm2 .
the drawing stress by modifying the drawing parameters.
As far as we know, this is a Jc record for 61 filaments
As indicated in Figures 3 and 6, both the Jc and the
Bi-2223/Ag round wires, although it is lower than the Jc
core density of the round wire increased as the diameter
(3.2 × 104 A/cm2 ) of the round wire composed of condecreased, which demonstrates again that the Jc is related
centrically arranged filaments reported by Fujikami.16
to the core density.27 To increase the Jc further, the superWe speculate that the higher Jc value reported for wire
conducting core should be further densified to form more
with concentrically arranged filaments is owing to betcompact Bi-2223 grains.
ter texture relative to our samples. As we mentioned in
3.1.4. Drawing with High Strength Metal Sheaths
During the intermediate drawing process, a compressive
stress was imposed on the Bi-2223 powder via the silver matrix which contains an outer silver alloy sheath and
inner pure silver filament walls. Considering that the silver
matrix is ductile and its yield strength is not high enough,
the stress imposed on the powder was limited. As a result,
the possible increase of the core density was limited.
In this work, drawing with high strength metal sheaths
was used to further densify the core. Brass and stainless
steel sheaths were used to sleeve the HT1 treated round
wire, respectively. Then the sleeved Bi-2223/Ag wires
were drawn by different passes, and finally, the sleeved
110
Section 3.1.2, the grain alignment of our wire could be
improved by carrying out the intermediate deformation.
The final goals of improving Bi-2223 texture in the present
work and in the literature (16) are consistent. Improving
the texture of the round wire further should be our next
target. Nevertheless, the multifilament Bi-2223/Ag round
wires shown in this work are more easily produced on an
industrial scale, and it have a larger packing ratio, which
means the amount of silver can be greatly reduced for the
present round wires.
During the drawing process, the maximum stress
applied on the powder is limited by the strength of the
outer sheath. In previous studies, using Ag alloys rather
than pure Ag may have made it possible to apply a
Mater. Express, Vol. 4, 2014
Critical current density improvement for Bi-2223/Ag round wires
Xie et al.
Materials Express
authors.33 34 By carrying out CIP at different stages of
tape fabrication, the core density has been raised and the
critical current density also enlarged. However, as far as
Yield
Tensile
we know, fabrication of Bi-2223/Ag round wire using
Material
strength (MPa)
strength (MPa)
intermediate CIP processing has not yet been reported.
Pure silver
6529
17030
In this work, the CIP process on samples of HT1 treated
31
Silver alloy (AgMg01 Ni01 )
160–180
230–240 (self test)
Bi-2223/Ag
round wires was conducted. A sample fabriBrass (UNS C26000)
145
310–420
cated without CIP treatment is shown in Figure 8(a), in
Stainless steel (UNS S30400)
240
585
which a number of voids are observed. As the CIP pressure
was increased to 200 MPa, the voids were only slightly
reduced, as shown in Figure 8(b). However, Figures 8(c)
larger drawing stress and therefore increase the final core
and (d) indicate that the voids in the filaments were greatly
density.17 Therefore, we considered that the cores might
reduced for both samples fabricated with a CIP pressure
be further densified by drawing with other metal sheaths,
of 400 MPa and 590 MPa, respectively.
of which the mechanical strengths are stronger than that of
The relationship between the diameter of CIP samples
silver alloys. Generally, the outer sheath goes through plasand
the CIP pressure is shown in Figure 9. The origitic deformation during the drawing process, and the stress
nal
diameter
was 0.805 mm and it slightly reduced to
required to sustain plastic deformation of the material is
28
0.803
mm
when
the pressure was increased to 100 MPa.
the flow stress, which is a function of the plastic strain.
The
wire
diameter
reduced sharply from 0.803 mm to
The flow stress equals the yield strength of the material at
0.758
mm
as
the
pressure
increased from 100 MPa to
which plastic flow is initiated, and in most cases it rises as
300
MPa.
As
the
CIP
pressure
continued to increase,
the plastic strain increases due to work hardening of the
the
diameter
reduction
rate
slowed,
and finally, the diammaterial. Because the initial stress and the maximum stress
eter
reduced
to
0.746
mm
when
the pressure reached
during plastic deformation are respectively determined by
590
MPa.
Although
the
diameter
of
the wire could be
the yield strength and the tensile strength, the flow stress
reduced
remarkably
as
the
CIP
pressure
increased, no elonusually resides in between.
gation
of
the
wire
was
observed
after
the
CIP process (the
Delivered
Table I lists the yield strength and the tensile
strengthby Ingenta to: ?
original
length
was
12
cm).
During
the
CIP
process, the
IP: in
5.10.31.210
On: Fri, 11 Aug 2017 16:19:37
values of the metal materials used
our experiment.
wire
always
satisfies
the
following
conditions:
Copyright:
American
Scientific
Publishers
The yield strength values of the silver and silver alloy
are about 65 MPa and 160 MPa–180 MPa, respectively,
SAg + Ssc = d 2 /4
SAg
+ Ssc = d 2 /4
which determine the minimum stress required to cause
where SAg and Ssc are the cross sectional areas of the
plastic deformation of the wire. The flow stress genersilver
matrix and the superconducting cores of the HT1
ated by the brass sheath during drawing was close to that
treated
wire, respectively. SAg
and Ssc are the cross secof the silver alloy. While the flow stress generated by
tional areas of the silver sectional areas of the silver matrix
the steel sheath was much larger than that of the silver
and the superconducting cores of the CIP treated wire,
alloy. As a result, the stress enhancement might be limited
respectively. Besides, d and d are the diameters of the
by drawing with a brass sheath. However, a much larger
HT1 treated wire and the CIP treated wire, respectively.
compressive stress could be imposed on the powder by
The volume of the silver matrix was assumed to be condrawing with a stainless steel sheath to increase the core
stant throughout the CIP process, and there was no elondensity so as to increase the Jc . It should be noted that
gation. Consequently, the cross sectional area of silver
the optimal Jc of steel sleeved samples was very close to
matrix was constant, indicating that SAg = SAg
. Thus, we
4
2
1 × 10 A/cm , almost 1/6–1/3 of the Jc of commercial Bican obtain:
2223/Ag tapes. A possible way to further enhance the Jc
SAg /Ssc + 1
is drawing with even higher strength sheath materials like
= d/d 2
(3)
32
SAg /Ssc + Ssc /Ssc
Hastelloy C276, which is commonly used as a substrate
for coated conductors.
As mentioned in 3.1.3, the rsc of the HT1 treated wire
was 50%, and, as a result, SAg /Ssc equaled to 1. By sub3.2. Cold Isostatic Pressing (CIP)
stitution into Eq. (3), we obtain:
As shown above, core densification by introducing the
Ssc
d/d 2
intermediate drawing process was crucial for Jc improve=
(4)
Ssc
2 − d/d 2
ment of Bi-2223/Ag round wires. Another effective way
to increase the core density is the CIP method. During
As the superconducting core density increase is inversely
the CIP process, a controlled uniform pressure is applied
proportional to its area reduction if the wire length is fixed,
simultaneously to the entire external surface of the sample.
thus, we obtain:
S
Improving of the critical current density of Bi-2223/Ag
Nsc = sc = sc
(5)
tapes by the CIP method has been reported by some
sc
Ssc
Table I. The yield strength (0.2% offset) and tensile strength of the
materials used in this study.
111
Article
Mater. Express, Vol. 4, 2014
Materials Express
Critical current density improvement for Bi-2223/Ag round wires
Xie et al.
Article
Fig. 8. The SEM micrographs of the CIP samples with different CIP pressure. (a) The HT1 treated sample with no CIP treatment; (b) the sample
of 200 MPa CIP treatment after HT1; (c) the sample of 400 MPa CIP treatment after HT1; (d) the sample of 590 MPa CIP treatment after HT1.
where Nsc is the relative core density normalized to the
The Jc value of the HT1 treated wire without intermediate
was around 3500 A/cm2 . As the CIP
core density of the HT1 treated wire. Combining
Eqs.
(4)
Delivered by Ingentadeformation
to: ?
pressure
increased,
Jc increased slowly when the pressure
and (5), we deduce:
IP: 5.10.31.210 On: Fri, 11
Aug 2017
16:19:37
was below
100 MPa, then it increased rapidly over the
Copyright: American Scientific
Publishers
d/d 2
pressure range of 100 MPa–300 MPa and stabilized at
Nsc =
(6)
2 − d/d 2
8.6 × 103 A/cm2 in the pressure range 300 MPa–590 MPa.
The effect of pressure on the Jc demonstrated in
The calculated variation in Nsc with CIP pressure is also
Figure 9 was much smaller when the CIP pressure was
shown in Figure 9. When the pressure was below 100 MPa,
below 100 MPa than when the pressure was between
the Nsc was almost unchanged. After that, the Nsc increased
100 MPa and 300 MPa. This can be explained by
with increasing CIP pressure and reached its maximum,
the fact that the yield strength of the silver alloy
a 39.8% density increase compared with the HT1 treated
(AgNi01 Mg01 wt.% in this work) was 160–180 MPa.31
cores, when the pressure was 590 MPa. The Jc variation
A CIP pressure lower than the yield strength of the outer
with the CIP pressure was nearly simultaneous to the Nsc
sheath could not produce a plastic deformation of the
variation, as shown in Figure 9.
sheath. As a result, there was no obvious effect on the
core densification, and the improvement of the Jc was not
significant. When the CIP pressure was above 200 MPa,
which exceeded the yield strength of the silver alloy,
an effective plastic deformation of the wire took place,
corresponding to the significant wire diameter reduction
observed. Thus, the superconducting cores were densified and the Jc of the round wire was rapidly improved.
However, the Jc value of the 590 MPa sample was not
higher than that of the 300 MPa sample. A possible reason could be a lack of shear stress between grains during
the CIP process,17 33 which could limit the Jc improvement. Besides, the maximum CIP pressure in this work
was 590 MPa, which is already close to the triple point
of water at high pressure and room temperature (liquid,
Fig. 9. “”: The Jc of the CIP samples variation with the CIP presice V and ice VI phases coexist in thermodynamic equisure. “”: The diameter of the CIP samples variation with the CIP
pressure “”: The normalized core density Nsc variation with the CIP
librium at 618.4 MPa and 0.16 C).35 We considered that
pressure. The error bars represent the standard deviation of the data.
CIP treatment with even larger pressure should increase
112
Mater. Express, Vol. 4, 2014
Critical current density improvement for Bi-2223/Ag round wires
Xie et al.
Materials Express
the core density further. Therefore, another pressure transmitting medium should be used to obtain a CIP pressure
above 618.4 MPa. More work on the CIP treatment of
Bi-2223/Ag round wires will be carried out in the future.
Mater. Express, Vol. 4, 2014
113
Article
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Acknowledgment: This work is supported Delivered
by the Pro-by Ingenta
ment of a compact 3 T MgB2 Magnet; IEEE Trans. Appl. Supercond.
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On: Fri, 11 Aug
2017 16:19:37
gram of International S&T Cooperation
(2010DFA64510)
22, 4400604 (2012).
Copyright: American Scientific Publishers
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and the Fundamental Research Program of Science
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Article
Received: 1 November 2013. Accepted: 29 December 2013.
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