EFFECTS OF STRAIN AND ANNEALING TEMPERATURE ON
CRITICAL CURRENT DENSITIES IN Cu/MgB2 and Ni/MgB2
SUPERCONDUCTORS
V. Beilin*, M. Roth*, E. Dul’kin*, E. Yashchin*, Y. Lapides**, J. Greenberg**,
E. Mojaev*, M. Tsindlekht***, E. Galstyan***,Y. Felner***
* Department of Applied Physics
** Institute of Inorganic Chemistry
*** The Racah Institute of Physics
The Hebrew University of Jerusalem, Jerusalem 91904, Israel
ABSTRACT
Critical current density, Jc, ac susceptibility (real part, χ’) and X-ray diffraction of
Cu/MgB2 and Ni/MgB2 wires and tapes were studied in their as-deformed and thermally
processed (annealed) states. The Jc increase and degradation throughout the wires/tapes
deformation (drawing, rolling) routes was observed. Annealing of Cu/MgB2 tapes in the
temperature range of 250°C to 850°C caused Jc degradation (down to a negligible level at
elevated temperatures). Sintering of Ni/MgB2 tapes in the 850°C - 950°C range resulted in
achieving high Jc > 1.3x105A/cm2 at 4.2K, while thermal processing at the lower
temperatures of 750-800°C caused the degradation of core connectivity and, thus, of J c that
dropped below its value in non-treated tapes. Two-step annealing consisting of low- and
high-temperature steps did not lead to Jc degradation (JD). JD in Ni/MgB2 tapes was
explained by the formation of thin MgB4 layers around MgB2 granules leading to their
decoupling. Another possible origin of this effect might be stress relaxation in the MgB2
core during Ni sheath softening.
1. INTRODUCTION
Since the recent discovery of superconductivity in MgB2 [1], extensive research activities
have been focused on the superconductivity mechanism as well as on the methods of
manufacturing practically applicable conductors of this material. It has become quite
evident that some specific characteristics of this compound, such as its low cost, high
critical current density, relatively high critical temperature, Tc, of 39K (the highest among
low-temperature superconductors), etc. make MgB2 a very attractive material for high
power applications. Significant advantage of MgB2 over higher-Tc superconductors, in
particular over Bi-2223, is the absence of weak links at grain boundaries [2], at least in the
thermally-processed state. The particularly important feature of granular MgB2 (e.g. in the
form of mechanically processed powder within a metallic sheath) is its ability to carry high
superconducting currents in an unsintered state [3]. At the same time, as-deformed wires,
although exhibiting Jc as high as about 105 A/cm2 at 4.2K, demonstrate weak-link-like
behavior [4] that may depend on their mechanical prehistory. Evolution of deformationinduced intergranular weak links in course of subsequent thermal treatment is an issue of
high practical significance. Accordingly, the choice of the deformation path and thermal
processing required for the core sintering must be addressed in developing metal-clad
MgB2 wires and tapes. Meanwhile, the influence of deformation prehistory in combination
with subsequent heat treatment on the wire or tape performance still has not been studied
adequately. In the present work, the Jc, ac susceptibility of Cu/MgB2 and Ni/MgB2 wires
and tapes have been studied in as-deformed states as well as after heat treatment at various
temperatures. We have also performed the X-ray diffraction (XRD) and the first acoustic
emission characterization of as-sintered metal-clad MgB2 tapes.
2. EXPERIMENTAL
Cu/MgB2 and Ni/MgB2 wires and tapes were fabricated by the powder-in-tube method
using MgB2 precursor powder of Alfa Aesar (ex-situ route) The details of the wires and
tapes preparation and processing could be found elsewhere [4,5]. Powder-packed Cu and
Ni tubes were drawn to various diameters down to 1.2mm and then rolled into the tapes of
various thickness down to 0.18 mm. Cu/MgB2 wires and tapes were fabricated as well by
the in-situ route, where MgB2 phase was synthesized in the process of wires and tape
thermal processing. Mixture of Mg and B powders with an atomic ratio close to MgB2
stoichiometry was used as a precursor in this case. Deformation degree, , in the both
drawing and rolling processes were calculated as the ratio of initial cross section area at the
beginning of the process, Fin, to that at the given processing step, Fps :  = Fin/ Fps. Thermal
operations were carried out in flowing high-purity Ar in the temperature range of 250°C to
950°C. XRD characterization used for the core phase analysis was carried out with Philips
PW-1710 diffractometer using K radiation. Jc measurements were performed by the fourpoint probe method with current and voltage contacts formed by soldering at the surface of
metallic sheaths. Maximal current we could measure using an available current source did
not exceed 240 and, sometimes, 120A. Therefore, in the cases where we did not reach the
critical state in the samples due to these current limitations or due to excessive heating of
current contacts, only the “lower-limit” Jc values were presented below. The ac
susceptibility was measured in the range of 5-50K at the frequency of 770 Hz in the
driving magnetic field of about 0.08Oe. In this work, the acoustical emission (AE) method
[6] was for the first time applied to probe the sintering of MgB2 superconducting core. AE
measurements were carried out in the process of tape bending at the room temperature
around a cylindrical rod of 6-8 mm in diameter with bending angles varying from 10o to
90o. Generation of AE signals was considered as an indication of nucleation and growth of
stress-induced cracks which would only be observed in the sintered (solid) powdered core.
3. RESULTS AND DISCUSSION
Jc values of as-deformed wires and tapes vs. cross-section area reduction  = Fin/ Fps are
shown in Table1 along with the corresponding data for tapes thermally treated at various
temperatures, Tann. It can be seen that the Jc growth during the drawing process is followed
by severe Jc degradation down to about zero value along with the  growth. An interesting
result has been that rolling the as-drawn Ni/MgB2 wire with negligible Jc has resulted in Jc
recovery followed by its significant growth with further  increase. Jc of Cu/MgB2 tapes at
high  values has decreased with  growth.
Figs. 1,2 show the real part of the ac susceptibility, ’, of Cu/MgB2 and Nu/MgB2 wires
and tapes.  = (T) traces in as-deformed state demonstrate a two-step superconducting
transition with two transition temperatures (Fig.1 and curves 1,4 in Fig. 2). The higher
transition temperature, Tonset, is 39K (in accordance with the published data for MgB2) in
all cases, and it features the strong-link intragranular behavior. The lower transition
temperature, TL, corresponds to the weak-link-like intergrnular coupling in as-deformed
wires and tapes.
0
10
20
30
40
50
60
-6
-6
1.0x10
1.0x10
1
2
3
4
5
ac susceptibility', a.u.
0.0
-6
-1.0x10
-6
-2.0x10
0.0
-6
-1.0x10
4
-6
-2.0x10
-6
-6
-3.0x10
-3.0x10
-6
-6
-4.0x10
-4.0x10
3
-6
-6
-5.0x10
-5.0x10
1
-6
-6.0x10
5
-6
-6.0x10
2
-6
-6
-7.0x10
0
10
20
30
40
-7.0x10
60
50
Temperature, K
Fig.1. ’ vs. T dependence for as-deformed with different cross section area reductions δ
and annealed Cu/MgB2 wires and tapes: 1- round wire, δ = 13.3; 2-round wire, δ = 15.2; 3tape, δ = 35.8; 4- tape, δ = 50.3; 5- tape 3 subjected to annealing at 600°C; arrows indicate
TL temperatures.
0
10
20
30
40
50
ac susceptibility ', a.u.
0.0
0.0
1
-7
-4.0x10
-7
-4.0x10
-7
-7
-8.0x10
-8.0x10
4
-6
-1.2x10
-6
-1.2x10
1
2
-6
-1.6x10
-6
-1.6x10
3
0
10
20
30
40
50
Temperature, K
Fig.2. ’ vs. T dependence for as-deformed and thermally treated Ni/MgB2 wires and tapes
1- as-rolled tape, δ = 15.7; 2- tape treated at 750°C; 3 - tape as-sintered at 950°C; 4- asdrawn wire, δ = 7.9; arrows indicate TL temperatures.
Jc reduction after initial growth along with  growth during drawing (Table 1) implies that
the powder density (PD) exceeds its limiting value (critical packing density [7]) under the
given deformation conditions. Therefore, further wire straining and thus MgB2 powder
movement would proceed along with the core disintegration resulting in the observed J c
reduction.
Table 1. Experimentally determined parameters of Cu/MgB2 and NiMgB2 wires and tapes
a
Material
Shape
State
Cu/MgB2
Cu/MgB2
Cu/MgB2
Cu/MgB2
Cu/MgB2
Cu/MgB2
Cu/MgB2
Cu/MgB2
Cu/MgB2
Cu/MgB2
Cu/MgB2
Cu/MgB2
Ni/MgB2
Ni/MgB2
Ni/MgB2
Ni/MgB2
Ni/MgB2
Ni/MgB2
Ni/MgB2
Ni/MgB2
Ni/MgB2
Ni/MgB2
Ni/MgB2
Ni/MgB2
Ni/MgB2
Ni/MgB2
Ni/MgB2
Ni/MgB2
Wire
Wire
Tape
Tape
Wire
Wire
Wire
Tape
Tape
Tape
Wire
Tape
Wire
Wire
Wire
Wire
Wire
Tape
Tape
Tape
Tape
Tape
Tape
Tape
Tape
Tape
Tape
Tape
as-drawn
as-drawn
as-rolled
as-rolled
Tanna- 650 oC, 0.5h
Tanna- 750 oC, 0.5h
Tanna- 850 oC, 0.5h
Tannb- 600 oC, 0.5h
Tannb- 250 oC, 1h
Tannb- 400 oC, 1h
in-situ, 700oC
in-situ, 700oC
as-drawn
as-drawn
as-drawn
as-drawn
as-drawn
as-rolledc
as-rolledc
as-rolledc
as-rolledc
Tannd- 650 oC, 1h
Tannd- 750 oC, 1h
Tann d -800 oC, 1h
Tann d -850 oC, 0.5h
Tannd -900 oC, 0.5h
Tannd -950 oC, 0.5h
Tann -915 oC, 1h
Strain
degree
13.3
15.2
35.8
50.3
3.3
6.25
7.9
10.2
14.3
15.7
17.1
19.7
26.9
24.7
Jc, A/cm2
at 4.2K
10,000
210
<25
<25
<25
<25
3300
300
115,000
195,000
450
1,100
TL, K
30
28
33
28
22
28
270
<100
32
4,350
14,800
22,900
15,700
520
1,300
>32,700
>32,700
>32,700
>130,000
-
- round wires with the strain degree of 13.9
- tapes with the strain degree of 35.8
c
- tapes rolled out of the round wire with the strain degree of 14.3 prior to rolling.
d
- tapes with the strain degree of 17.3
TL decrease as well as Jc drop with  increase for as-deformed Cu/MgB2 tapes (Fig.1,
Table 1) were supposedly caused by the same mechanism.
b
The Jc and TL growth under subsequent rolling of as-drawn wires (Table 1, curves 1,3 in
Fig. 1) are the indications of recovery and further enhancement in the core connectivity by
powder redistribution. Different behavior of the core connectivity along the drawing and
rolling paths of the PIT procedure originates supposedly from different stress states in the
deformation zone for both processes.
The main goals of thermal processing (annealing) the MgB2 tapes are sintering the core to
provide high current-carrying ability of the tapes, as well as sheath softening to make
possible further operations e.g. the tape bending for coil preparation. In order to probe the
MgB2 core sintering, we have employed the AE characterization. In the AE experiments,
Ni/MgB2 tapes subjected to annealing in the temperature range of 650°C to 950°C have
been studied. It is found that the tapes treated at Tann < 800°C did not generate AE signals
under bending tests. AE signals only appear starting from TT of 800°C. At Tann 850°C to
950°C AE signals was observed even at minimal bending angles. Accordingly 850°C has
been accepted as the temperature where the core sintering is well-established (sintering
threshold, Tst ) in the studied samples.
It is shown in Table 1 that annealing of Cu/MgB2 tapes fabricated by the ex-situ route,
results in Jc degradation down to a negligible level in the temperature range of 250°C to
850°C. Accordingly, we have observed a broadened superconductive transition with
lowered TL temperature in the ’ vs. T curve in Fig.1 for the Cu/MgB2 tape as-annealed at
600°C (curve 5). Further increase in annealing temperature would not result in Jc
enhancement due to strong Cu-Mg interaction. Contrary to that, the in-situ route at 700°C
provides the high current-carrying ability of Cu/MgB2 tapes (Table 1). Hereafter we
describe just the results for in-situ tapes. Annealing the as-deformed Ni/MgB2 tapes at
900°C-950°C has resulted in a sharp one-step superconducting transition and drastic Jc
enhancement (see Table 1 and Fig. 2, curve 3). These data indicate the suppression of the
weak-link behavior and the establishment of a long-range strong-link connectivity over the
tape core. It means that thermal processing at this temperature provides the necessary core
sintering and may be used, in principle, as a finishing operation in the process of Ni/MgB2
tapes fabrication. At the same time, this temperature is too high as leading to strong coresheath interaction with the formation of a reactive layer at the core-sheath interface.
Therefore, we have studied the tape annealing at lower temperatures and found that high
core connectivity as well as the corresponding high Jc are maintained down to Tann of
850°C (Table 1) in accordance with the AE data. Meanwhile, further reduction in Tann has
resulted in severe Jc degradation as compared to the as-rolled tapes.
The χ(T) trace for the tape treated at 750°C demonstrates a one-step behavior (without the
intergranular step) with a broadened transition (curve 2 in Fig. 2) indicating the
deterioration of the core connectivity. The samples of two other batches subjected to 1h
annealing at 650°C to 900°C also show. Jc degradation, but with the differences in
temperature windows between all three batches (see Table 1 for batch 1 and Fig. 3 for all
three batches).
There are three possible processes which may be responsible for the observed Jc
degradation in Ni/MgB2 tapes at the intermediate Tann, namely: a) chemical interaction
between the MgB2 core and Ni sheath resulting in the formation of low-temperature Nicontaining phases, b) MgB2 decomposition during annealing and c) mechanical interaction
between the sheath and core. With regard to the first factor, XRD analysis reveals indeed
some differences in phase compositions (not presented here) at the Ni/core interface
between the tapes treated at 750°C and 950°C. When studying the “a” scenario we have
used samples subjected to thermal processing consisting of two steps, I and II, at different
temperatures: i) 750°C (I) + 950°C (II), ii) 950°C (I)+ 750°C (II). If the low-temperature
(750°C) annealing virtually causes the formation of the phases (no matter-stable or
nonstable ones) responsible for the Jc degradation found, we are prone to observe such
effect in one of these samples. However both samples have demonstrated high J c similarly
to that in the sample subjected to one-step annealing at 950°C.
In order to check the possibility of the “b” scenario, the equilibrium composition of MgB2
core in Ar atmosphere has been calculated for a closed system in a wide temperature range
using ASTRA software [8] and the IVTANTERMO Thermodynamic Database [9]. The
results of the calculations are presented in Table 2. Apparently, the main equilibrium
species formed due to MgB2 decomposition are gaseous Mg and solid MgB4. It may be
anticipated that in case of an unsintered tape (for Tann < Tst), Mg evaporation from the tape
core will be dominated by evaporation from the surfaces of granules (rather than from
their volumes) throughout the core volume.
Table 2. Equilibrium composition of MgB2 in 1 atm Ar*.
Phase
Species
B2
B
Gaseous
Mg
Mg2
MgB2
MgB4
Condensed MgB12
B
Mg
* Partial pressures of the
in mass fractions.
Temperature, K
873
973
1073
0
0
0
0
0
6.81x10-21
0.0000346 0.0004461 0.0035404
1.46x10-12 1.84x10-10 8.997x10-9
0.909084
0.908998
0.908348
-6
5.31x10
0.0000686 0.0005461
0
0
0
0
0
0
0
0
0
gases are given in atm, concentrations
1173
1273
0
0
-18
1.68x10
1.74x10-16
0.0195775 0.0821073
2.184x10-7 3.112x10-6
0.904917
0.890392
0.0030694 0.0137508
0
0
0
0
0
0
of the condensed phases -
Therefore, the MgB4 phase may be nucleated inhomogeneously over the granule volume,
preferably within a near-surface volume. In order to explain the obtained results, a
simplified picture has been considered where all MgB4 is located inside a thin single-phase
surface layer. It is possible to expect then the core decoupling into separate MgB2 granules
surrounded by insulating MgB4 shells and thus Jc degradation, if the thickness of this
intergranular barrier layer,Δbl, fulfills the following condition: Δbl>λ, where λ is a
coherence length. A similar approach has been used in [10] for MgO layers around powder
particles in unsintered Cu/MgB2 tapes. The characteristic particle size, Sbl, corresponding
to the equality of Δbl ≈ λ can be roughly estimated using the data on MgB4 concentrations,
CMgB4, as follows:
Sb l= k*λ / CMgB4 (T),
(1)
where for spherical particles Sbl is a sphere of radius R and k = 3, while for plate-like
particles Sbl is a plate of thickness t, and k = 1. Taking into account the λ anisotropy and
the scatter of published data [11] as well as moderate c-axis core texturing in MgB2 tapes
[5], we have adopted for our estimations λ ≈ 5 nm.
Based on the calculated data of Table 2, the estimated R value for T = 800°C is about 20
μm and t ≈ 7 μm. In the above estimations, we have taken the mass density ratio between
MgB2 and MgB4 phases as 1.5:1 based on their atomic compositions. As already noted
above, the data of Table 2 are calculated for a closed system, albeit in real MgB2 tapes
some Mg losses have been observed [12]. Meanwhile, as it has been found in [13], in an
open system the MgB4 concentration just below 900°C may be as high as 80%, i.e. more
than three orders of magnitude higher than in the closed system. Therefore, even minor
deviations from closeness will give rise to a significant CMgB4 enhancement and thus to Sbl
reduction and/or to Δbl growth. If we assume that the Ceq value increases up to 1% due to
minor Mg losses, it results in Δbl of about 20λ.
High-temperature processing, e.g. at 950oC, facilitates particle sintering (fusing) in the
solid phase and, consequently, a drastic reduction of the core internal surface. In this case
Mg will evaporate mainly from the outer core surface that would not cause the
deterioration of intergrain connectivity and thus the Jc degradation. In addition, the
decrease of evaporation surface area would result in the reduction of evaporation rate.
Results of the experiments with two-step annealing can be easily understood in the
frameworks of the above model. MgB4 barrier layers formed at the first step at 750°C in
“i” experiment, will be fast eliminated at the sintering step by diffusion-induced core
homogenezation at the higher temperature of 950°C. In “ii” experiment this layer would
not be formed at all, because the core will be sintered already at the first high-temperature
(950°C) step. The second step at 750°C will be applied to the as-sintered core, where Mg
evaporates predominantly from outer core surface not giving rise to the formation of
intergranular barrier layers.
The third possible scenario of Jc degradation may be connected with the softening of
metallic sheath in the process of annealing at temperatures Tann, which are sufficiently high
to provide this softening, e.g. by recrystallization, and they are still below Tst. In this case,
stresses accumulated inside the MgB2 core during the drawing and rolling processes will be
relaxing that would result in the looseness of as-densified core followed by degradation of
its connectivity. This effect is responsible for Jc degradation in Cu/MgB2 tapes (see
[10,14]) that starts at temperatures as low as 250°C (see Tabl.1) where no MgB2
decomposition can take place.
At higher temperatures, the contribution of Cu-Mg interaction becomes dominating. As to
the Ni/MgB2 tapes, the observation of two Jc-degradation windows at “low” and “high”
temperatures in batch 3 (Fig.3) may indicate the synergy of both Mg decomposition and
stress relaxation mechanisms. Accordingly, differences between Jc vs. Tann data between
the Ni/MgB2 samples of different batches may originate from minor differences in the tape
fabrication paths influencing the granule sizes as well as the sheath strength and core
density.
Normalized critical current density, J c /Jmax
0
200
400
600
800
1.0
1000
1.0
batch 1
batch 2
batch 3
0.8
0.6
0.8
0.6
0.4
0.4
0.2
0.2
0.0
0.0
0
200
400
600
Annealing temperature, T
800
1000
ann
Fig.3. Normalized critical current densities of Ni/MgB2 tapes subjected to annealing at
various temperatures; Jmax - maximal current density applied to a sample at maximal Tann.
4. SUMMARY
Core densification and subsequent disintegration of MgB2 wires leads to growth and a
following drastic drop in the critical current, Jc, along the drawing path. Rolling of the asdrawn wire causes the increase of core connectivity and, thus, of Jc due to powder
redistribution.
Annealing Cu/MgB2 tapes in the temperature range of 250oC to 850oC results in Jc
degradation (down to a negligible level at elevated temperatures) caused by stress
relaxation in the core due to Cu softening at low temperatures and presumably by Cu-Mg
interaction as well at high temperatures.
Sintering of as-deformed Ni/MgB2 tapes at 900-950°C suppresses the weak links formed
during tape rolling and results in establishing long-range strong connectivity throughout
the tape core and high Jc. Severe Jc degradation upon annealing of Ni/MgB2 tapes at 750oC
- 800oC has been revealed. The Jc degradation effect presumably originates from the
formation (at intermediate temperatures) of thin insulating MgB4 layers around MgB2
granules which cause their decoupling. Another possible reason of this effect is stress
relaxation in the MgB2 core due Ni sheath softening, that may also cause granules
decoupling and thus connectivity reduction.
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ACKNOWLEDGMENT
This work was supported by the grant of the Ministry of the National Infrastructures of
Israel.