n-values of commercial YBCO tapes before and after irradiation by... M.Chudy , Z. Zhong

n-values of commercial YBCO tapes before and after irradiation by...  M.Chudy , Z. Zhong
n-values of commercial YBCO tapes before and after irradiation by fast neutrons
M.Chudy1,2, Z. Zhong2, M.Eisterer3, T. Coombs2
1
Graduate School of Technology Management, University of Pretoria, Lynnwood Road, Hatfield, Pretoria 0001,
South Africa
2
Department of Engineering, University of Cambridge, 9 JJ Thomson Avenue, Cambridge CB3 0FA, UK
3
Vienna University of Technology – Atominstitut, Stadionallee 2, 1020 Vienna, Austria
Abstract
The n-value is an important superconducting parameter, which represents the homogeneity of
characterized superconductor as well as thermally activated depinning. In addition n-values are
important for the evaluation of pinning mechanisms and pinning forces. n-values are crucial input
parameters for the numerical simulations of superconducting tapes, coils and other complicated
superconducting applications where E-J power law applies. In this publication, complex measurement
data of n-values from different 2nd generation of high temperature superconducting (2G HTS) tapes
are presented and analysed. In addition, 2G HTS tapes were step by step irradiated by fast neutron
fluences up to 1x1022 m-2 . n-values of the irradiated tapes, containing additional randomly distributed
pinning centres, are presented, analysed and compared with unirradiated samples. Special attention is
placed on the underlying physics resulting in power-law part of the I-V curve and on the correlation
between critical currents and n-values. The measurements are performed within the temperature
range of 50 K-85 K and magnetic fields up to 15 T.
I Introduction
Each I-V curve of a superconducting sample contains a power-law part close to the transition to the
dissipative state. This part of the I-V curve can be described by a simple equation:
V/Vc=(I/Ic)n
(1)
where V and I are the measured voltage and current, Vc is the voltage criterion, Ic is the critical
current and the n-value the exponent n. Even though the n-value is an important superconducting
parameter, results on 2 G HTS tapes are available only in a few recent publications [1-5]. Therefore,
a comprehensive overview of experimental n-value data would be beneficial when dealing with
numerical modelling of superconductors. In low-temperature superconductors, the exponential
relationship (power-law part) in I-V is usually explained by the Jc nonuniformity. Warnes and
Larbalestier [6] and Plummer and Evetts [7] successfully developed models based on the Jc
nonuniformity, but such a mechanism is not so successful in high temperature superconductors.
Magnetic flux creep is a mechanism introduced by Anderson and Kim [8,9] and describes a concept
of thermally activated flux lines released from pinning centres. This phenomenon is mainly observed
at higher temperatures where HTS operate. The rate of this process (R) is exponential, depending on
the temperature of the superconductor according to the relation:
(2)
where Ue represents activation energy, k – Boltzmann‟s constant, T – absolute temperature. This
process leads to the redistribution of flux lines resulting in a decrease of the magnetic moment of the
superconductor, which is known as magnetic relaxation. Magnetic relaxation in HTS was reviewed by
Y. Yeshurun et al. [10], where the direct relation between flux creep and the power law relationship in
the I-V curve was pointed out.
Finally, studies by J.Z Sun [11] and R. Griessen [12] provided
evidence that the power-law relationship is a consequence of the flux creep. However, both effect
,nonuniformity and the flux creep, do not exclude each other and can collaboratively contribute to the
exponential part of the I-V curve at the same time. Nowadays, flux creep is considered as the primary
reason of the power-law relationship for high temperature superconductors even though some older
publications suggested otherwise (i.e. nonunifortmity or inhomogeneity) [13,14].
It means that even
a “perfectly uniform” HTS would have the power law part of the I-V curve. The n-value of the
“perfectly uniform” HTS can be even relatively low in the case of efficient pinning centres with small
activation energy. Nonuniformity is not necessarily present in large scale in modern 2G HTS tapes.
Generally, the influence of the uniformity factor to the n-value is more important at lower
temperatures. With increasing temperatures and magnetic fields the uniformity factor becomes less
important and importance of flux creep increases. As assumed by the uniformity based models
[13,14], critical currents are limited by grain boundaries and not grains. The transition between grain
boundary and grain limited currents could be a threshold, where nonuniformity is not the main reason
of the power-law relationship. In any case, the n-value always strongly depends on the efficient
pinning mechanism in the superconductor.
Measurements of n-values after introducing additional pinning centres require special attention.
Irradiation by fast neutrons introduces randomly distributed spherical pinning centres with diameters
of a few nm [15-20] into a superconductor. It is a proven method of introducing efficient pinning
centres into HTS, resulting in critical currents enhancements. Significant critical current enhancement
was reported in rather stronger magnetic fields and lower temperatures for several types of tapes [2022].
Regarding the HTS applications, high n-values reduce the losses by operation close to the J c and are
necessary to operate magnets in the persistence mode. At the same time, high n-values are also
associated with unstable behaviour which can cause premature quenching in superconducting
machines [23]. Usually, high n-values are considered as an asset of superconductors and rapid n-value
reduction at higher magnetic fields can make them unsuitable for any applications. The n-value can
vary significantly in different kind of tapes as well as after irradiation. Therefore, measurements were
performed on commercial 2G HTS tapes from three different manufacturers and also after irradiation
by fast neutrons.
II Samples
Standard tapes from three different manufacturers were used. The first is the 2G HTS SCS 4045 tape
from SuperPower, which is made by Metal Organic Chemical Vapor Deposition (MOCVD) on an Ion
Beam Assisted Deposition (IBAD) made MgO template [24]. The YBCO layer is 1µm thick. Two set
of samples are characterized, both sets are the SCS 4045 tapes manufactured in 2008 and 2012
respectively. The size of YBCO grains in this kind of tapes is ~ 1 µm. The second series of samples
are from American Superconductor (AMSC). These tapes have RABiTS® (Rolling Assisted Bi-axially
Textured Substrates) substrate and Metal Organic Deposited (MOD) YBCO layer (~1µm). The 4 mm
wide tape is marked as 344 and the 12 mm wide tape as Amperium 8612, with a double HTS Layer
[25]. The usual size of YBCO grains in RABiTS® tapes is 20-50 µm [26]. The third series of samples
is coming from Shanghai Superconductor Technology Co.[27]. In these tapes, the MgO template is
made by IBAD and Pulsed Laser Deposition (PLD) is used for 1µm thick YBCO layer. The best
tested 4 mm wide sample reached the highest value of critical current. More information about all the
samples are listed in Table 1. All the used 4 mm wide samples were 26 mm long and the 12 mm wide
samples were 80 mm long.
III Instrumentation
All the presented instruments are adjusted for transport current characterization of short samples (apx.
3cm length) by a standard 4 point method. As data for this study are obtained from several
experiments performed by different devices in different laboratories (Low temperature and
superconductivity laboratory at Vienna University of Technology, Atominstitut, and EPEC
superconductivity laboratory at the University of Cambridge). Most of the samples were characterized
by several experimental set-ups. A brief description of experimental instrumentation is given below:
The electromagnet set-ups
Two electromagnet measurement set-ups were employed for angle-resolved transport measurements,
the first one was in Vienna (Atominstitut) and the second one in Cambridge (EPEC superconductivity
group). The first set-up consisted of a 1.4 T water cooled electromagnet and the second of a 800mT
electromagnet where bigger area of homogenous field can be achieved. As for cryostats, a simple
tubeshaped vacuum vessel (flask) and a polystyrene box were used. All the measurements were
performed in liquid nitrogen. Both set-ups are designed for angle-resolved transport measurements in
the maximal Lorentz force configuration. The main difference between the two measurement set-ups
is the rotation mechanism. While on the first set-up the holder is statically mounted between the
rotating magnet poles, while the holder in the second set-up is rotating and the magnet poles are
stable. Maximum achievable resolution on both systems is 0.5°. Both sample holders are equipped
with Hall sensors and they produce identical results. The Cambridge was used preferred for wider
samples reaching very high currents and the irradiated samples were characterized exclusively in
Vienna due to safety certificates.
6 T measurement set-up
The 6 T measurement set-up is a helium gas flow cryostat equipped with a 6 T split coil. The main
advantage of this cryostat is a wide temperature range of measurements from about 4.2 K to 150 K.
The horizontal magnetic field allows to perform angle-resolved transport measurements with a
rotating sample holder. The rotating sample holder is equipped with a sensitive Hall probe, a Cernox
temperature sensor and a fine rotating mechanism with a stepper motor (0.1° precision). Characterized
samples can have lengths up to 30 mm. Indium press contacts are typically used for current and the
conductive silver glue is used for the voltage contacts.
17 T measurement set-up
The 17 T measurement set-up is a helium flow cryostat which is equipped with superconducting coils,
generating magnetic field up to 17T in vertical direction. The Variable Temperature Inset (VTI) has
an inner diameter of about 3 cm and the magnetic field is homogeneous in a vertical length of about
30 mm. These parameters limit the maximum samples lengths to 30 mm. Two sample holders
designed for short tapes characterization are available. One places the sample with the ab plane and
the other with the c-axis parallel to the magnetic field. A 300 A current source was available for the
transport characterization.
Triga Mark II reactor
The TRIGA Mark II was used as an irradiation facility in this work. It is a pool type research reactor
that is used for training, research and isotope production (TRIGA - Training, Research, Isotope
production, General Atomic) [28]. The reactor has a maximum continuous thermal power of 250 kW,
though the power can be increased up to 250 MW for about 40 ms in the pulse regime. The fuel is in
the form of an uniform mixture of 8 wt% uranium, 1 wt% hydrogen and 91 wt% zirconium, where the
zirconium-hydride is being the main moderator. The maximum neutron flux density of 1017 m-2 s-1 at
250 kW is reached in the Central Irradiation Facility (CIF). The sample‟s temperature is estimated to
remain below 50 °C during the irradiation procedure.
IV Results
Tapes in low magnetic fields
All the measurements in this section are performed by the electromagnetic measurement set-ups in
liquid nitrogen in fields below or equal to 400 mT. As all studied commercial tapes have good grain
alignment, the transition from grain boundary limited currents to grain limited currents must occur in
this field range, according to [22,29] even well below 400mT. As at low fields the grain boundary
limited currents may occur, both nonuniformity and flux creep must be taken into account as the
reasons of the exponential part of the I-V curve.
It is also important to mention that all the
measurements are evaluated by a voltage criterion of 1 µV/cm as Uc is a locked parameter by fitting
with eq.(1). 4 mm tapes and wider 12 mm tapes from the three different manufacturers were
characterized. In addition, two sets of 4mm wide samples from SuperPower were characterized.
Critical currents as well as n-values of all types of characterized tapes are listed in table 1. Results
from both, newer (2012) and older (2008) tapes are available in figure 1. The results from 4mm
AMSC and SHSC tapes are shown in figures 2 and 3, respectively. All the angle-resolved
measurements were performed in one of the introduced electromagnet measurement set-ups. The
results of the n-values are shown together with figures of critical currents for a better illustration of
critical current behaviour. Figures with n-values are generally noisier than the figures with critical
currents as a consequence of fitting algorithm. n-values and critical currents were calculated from the
power law fit of the exponential part of the IV curve. The beginning of the exponential part of the IV
curve was determined by first three points above the noise level. The same algorithm was applied for
all the measurements from all used measurement set-ups.
Table 1: Critical currents and n-values of the samples at 77 K and self-field.
Sample
SuperPower(2008) 4 mm
SuperPower(2012) 4mm
SuperPower(2012) 12mm
AMSC 4mm
AMSC 12mm
SHSC 4mm
Critical current 77 K self-field
98 A
114 A
389 A
92.5 A
534 A
167.5 A
90
50 mT
100 mT
200 mT
300mT
400 mT
80
10
2.00x10
10
28
50mT
100mT
200mT
300mT
400mT
26
1.75x10
10
60
1.50x10
10
50
1.25x10
10
40
1.00x10
10
30
7.50x10
9
16
20
5.00x10
9
14
10
2.50x10
9
24
22
Jc (Am )
20
-2
n value
70
Ic (A)
2.25x10
n-value
28.8
30.5
30.1
36
52.2
42.14
18
12
10
-20
0
20
40
60
80
100
120
-30 -20 -10 0
10 20 30 40 50 60 70 80 90 100 110 120 130
angle(deg)
a)
angle(deg)
b)
110
30mT
50mT
70mT
100mT
34
2.8x1010
32
100
2.5x1010
90
2.3x1010
80
10
2.0x10
70
1.8x1010
30
Jc (Am-2)
Ic (A)
3.0x1010
30mT
50mT
70mT
100mT
n value
120
28
26
24
22
20
-20
0
20
40
60
80
100
120
-20
angle(deg)
c)
d)
0
20
40
60
80
100
120
angle(deg)
Figure 1: a) b) Critical current and n-values of the SuperPower tape (2008) at 77 K, c) d) critical current and nvalues of the SuperPower tape (2012) at 77 K.
By comparing of the result at 100 mT between SuperPower(2008) and SuperPower(2012), the
increase in critical current is between 45 % (at ~ 90°) and 86% (at 0°). However n-values at 90°
stayed practically unchanged (difference of ~5%) and the difference at 0° is only about ~33%.
100
2.50x1010
90
2.25x1010
80
2.00x1010
70
10
1.75x10
60
1.50x1010
50
1.25x1010
40
1.00x1010
42
30mT
50mT
100mT
200mT
300mT
40
38
36
n value
Jc (Am-2)
Ic (A)
34
32
30
28
26
24
22
20
-20
0
a)
20
40
60
80
100
120
-20
angle(deg)
0
20
40
60
80
100
120
angle(deg)
b)
Figure 2: 4mm AMSC tape at 77K: a) critical currents b) n-values
Surprisingly, n-values of the AMSC tape are not dependent on the angle of magnetic field, even
though some correlation between n-values and critical current exists also in RABiTS AMSC tape as
presented in [2]. Figures 2 and 5 are even showing signs of inverse J c - n-value correlation, which will
be discussed later in this paper. In numerous experimental studies, it has been observed that n-values
are usually correlated with critical currents [1,2,30], which causes that n-value varies also with
external magnetic field. However, if the correlation is a consequence of relation presented by Zeldov
et al. [31], then:
(
)
(3)
where U0 is the J = 0 activation energy (pinning energy), J is current density and J c0 is the critical
current density in absence of thermal activation. From (2):
(4)
where dM/dT is the rate of magnetization change. Thus, a simple substitution:
(
)
( (
)
)
(
)
(5)
(6)
It shown that n-value is directly proportional to the pinning energy of flux and not to the J c. Thus Jc = f
(U0), according to (3):
(
)
(7)
Uec is activation energy at the critical current. It can be assumed that (Jc0 –Jc )is higher at higher
temperatures and converging to zero at very low temperatures.
If Jc is a function of (ϕ,H,T)
according to (7), then U0 is also function of (ϕ,H,T), however, the angular dependence of U0 in
external magnetic field can be quite different than the one for the Jc. This can be a consequence of
flux lines deformation e.g. into staircase like shapes [3,4]. Nonuniformity might play a role in the
uncorrelated dependence as well.
4.0x1010
160
3.5x1010
140
30mT
44
50mT
42
100mT
40
200mT
3.0x1010
100
2.5x1010
80
2.0x1010
60
10
38
36
n value
120
Jc (Am-2)
Ic (A)
300mT
34
32
30
28
26
24
1.5x10
22
10
40
1.0x10
-20
0
20
a)
40
60
angle(deg)
80
100
20
120
-20
b)
0
20
40
60
80
100
120
angle(deg)
Figure 3: SHSC 4mm wide tape at 77 K a) critical currents b) n-values
The SHSC sample has shown superior properties. Critical currents were significantly higher than in
the other characterized tapes. Although n-values were very high at 30 mT, in higher fields they were
comparable with tapes from other manufacturers. Anisotropy of the n-values is obvious only from
fields above 200 mT.
In the next step, characterization of 12mm wide tapes from SuperPower(2012) and AMSC was
performed (figures 4,5). According to the manufacturers [24, 25], the 12mm tape from SuperPower
should have identical structure as the 4 mm tape. Therefore, approximately three times higher critical
currents and similar n-values would be expected compared to the 4mm tape. The AMSC 12 mm tape
contains two HTS layers. The additional layer causes that critical currents should be about 6 times
higher than in the case of the 344, 4mm wide tape. It is relatively difficult to predict the n-values of
this tape according to the results of the 344 AMSC 4 mm wide tape. In the case of the
SuperPower(2012) tape, n-values are slightly higher than for the 4mm tape. This difference seems to
be insignificant and most likely it is just an effect of different voltage criterion. Wider tape creates
more voltage for the same electric field E and therefore critical electric field Ec, for the wider 12mm
tape is smaller than Ec of 4mm tape at the same voltage criterion. For this reason, n-values were
calculated for different voltage criteria [32]. The original 1µV/cm criterion, but also higher 3 µV/cm
and 6 µV/cm voltage criteria were used. Although, n-values calculated with higher criterion are
slightly lower, no significant difference can be seen between figures 4b and 4c. Even though n-values
calculated by this enlarged voltage criterion are very similar to the 4 mm SuperPower(2012) tape, the
used enlargement of the voltage criterion is not completely justified due to for example the edge effect
of the tapes.
A clear ab peak shift can be observed in figure 4. This phenomenon was observed in other references
[5,33]. It is interesting to note that the shift of the ab peak is obvious also in the case of n-values as
well (figure 4b). Change of the criterion seems to be without too much effect by the 12 mm wide
AMSC tape with two YBCO layers. This tape has shown very high n-values for both voltage criteria.
The measured n-values are significantly higher especially in low fields if compared to the 344 AMSC
4 mm wide tape. (figure 5) . Most likely, other more complex mechanisms are involved in this
phenomenon where identifying of these mechanisms would be a pure speculation at this stage.
450
35
400
50mT
100mT
200mT
300mT
400mT
350
250
30
n value
Ic(A)
300
200
25
20
150
15
100
-20
a)
0
20
40
60
angle(deg)
80
100
120
-20
0
20
40
60
80
100
80
100
120
angle(deg)
b)
32
50mT
100mT
200mT
300mT
400mT
30
28
n value
26
24
22
20
18
16
-20
c)
0
20
40
60
120
angle(deg)
Figure 4: 12mm wide SuperPower(2012) tape a) critical currents b) n-values with criterion 1µV/cm c) n-values with
criterion 3µV/cm
55
50mT
100mT
200mT
300mT
400mT
500
50
45
400
Ic(A)
n value
40
300
35
30
25
200
20
-20
a)
0
20
40
60
angle(deg)
80
100
120
-20
0
20
40
60
80
100
120
angle (deg)
b)
50mt
100mt
200mt
300mt
400mt
55
50
45
n value
40
35
30
25
20
15
-20
c)
0
20
40
60
80
100
120
angle(deg)
Figure 5:12 mm wide double HTS layer Amperium tape a) critical currents b) n-values with criterion 1µV/cm c) nvalues with criterion 6µV/cm
Tapes in high magnetic fields and after irradiation
In this part, results in higher magnetic fields (B ≥ 1 T) are presented. The tapes were irradiated several
times by fast neutron fluences and re-measured. The presented graphs (figure 6) consist of results
after each irradiation step. The type of artificial defects induced by neutron irradiation strongly
depends on the kinetic energy of neutrons. Fast neutrons are neutrons with higher energies (E ≥0.1
MeV) and they produce spherical defects of amorphous material with a diameter of a few nm (so
called collision cascades)[15,17-19]. The point defects and clusters of point defects are created by
neutrons with lower energies and they can act as effective pinning as well [34]. The Triga Mark
reactor II in Vienna (CIF) was used as an irradiation facility in this work. The irradiation levels of fast
neutrons applied to the samples were up to a fluences of 1x1022 m-2. The other irradiation levels were:
2x1021 m-2 and 4x102 m-2. Since the irradiation procedure is a very time consuming process, only the 4
mm wide SuperPower(2008) tapes were characterized. The main time delays are caused by the fact
that the samples are becoming radioactive emitters after the neutron irradiation procedure. In order to
perform the measurements without any health and safety hazards, it is necessary to wait until the
radioactivity of the sample is decayed to the acceptable limits.
The measurements were performed in the 6 T and 17 T measurement set-ups at various temperatures
from 50 K up to 85 K. Similar studies focused on only critical current enhancement/reduction of 2 G
HTS tapes after irradiation are already available [20-22]. In figure 6 are shown the n-value results
together with corresponding critical currents.
unirr
21 -2
irr 2x10 m
21 -2
irr 4x10 m
22 -2
irr 1x10 m
77 K, 3 T
3.5x10
9
3.0x10
9
10
2.5x10
9
8
2.0x10
9
6
1.5x10
9
4
1.0x10
9
5.0x10
8
10
8
-2
Jc (Am )
Ic (A)
12
77 K, 3 T
12
n value
14
6
2
-20
0
20
40
60
80
100
120
4
-20
140
0
20
40
60
80
100
120
140
angle(deg)
angle(deg)
a)
b)
9
28
7.0x10
unirr
21 -2
irr 2x10 m
21 -2
irr 4x10 m
22 -2
irr 1x10 m
26
24
18
9
77 K, 1 T
6.5x10
9
6.0x10
16
9
5.5x10
18
4.5x10
16
4.0x10
14
3.5x10
9
9
-2
5.0x10
n value
9
20
Jc(A.m )
Ic (A)
22
77 K , 1 T
14
12
9
9
12
3.0x10
10
2.5x10
8
2.0x10
10
9
9
-20
0
20
40
60
80
100
120
140
-20
0
20
40
angle(deg)
c)
60
80
100
120
140
angle(deg)
d)
16
1.3x1010
50
unirr
irr 2x1021m-2
irr 4x1021m-2
irr 1x1022m-2
e)
1.1x1010
1.0x1010
35
8.8x109
30
7.5x109
25
6.3x109
20
5.0x109
0
20
40
60
angle(deg)
80
100
120
n value
14
40
-20
64 K, 4 T
64 K, 4 T
Jc(A.m-2)
Ic (A)
45
12
10
-20
140
0
20
40
60
80
100
120
140
angle (deg)
f)
Figure 6: SuperPower(2008) tape characterized after fast neutron irradiation in the 6 T measurement set-up: a)
critical currents 77 K, 3 T b) n-values 77 K, 1T c) critical currents 77 K, 1T d)n-values 77 K, 1T e) critical currents
64 K, 4 T f) n- values 64 K, 4 T.
A significant reduction of n-values over the whole scale of measured angles, fields and temperatures
is observed at the highest fast neutron fluence (1x10 22m-2). At the lower irradiation levels, there are
some regions with increase and also with reduction of the n-values. At 64 K, the position of the Jc
peak corresponds to the deep drop of the n-value curves which is in literature known as inverse
correlation to the Jc (figure 6f) [3-5]. Despite of this drop, the anisotropy of n-values at 64 K is rather
low. The inverse correlation of the n-values by the Jc peak has been observed also at 50 K. The
inverse n-value and Jc correlation is commonly related to the staircase flux lines in the superconductor
[3,4]. Staircase flux lines are usually created in periodic pinning structures of high densities. Intrinsic
pinning could be considered as this pinning structure at certain temperatures. Intrinsic pinning is
characteristic by the small activation energy of the pinned flux; however, the high density of these
pinning centres makes them very efficient at low temperatures when thermal depinning becomes less
important. The change of the n-value curve between 77 K and 64 K indicates some transition in the
role of intrinsic pinning in this range. This transition could cause change of parameters in relation (7),
or even transition to a different relation. It was noticed in [31] that the relation between J c and n-value
can be even linear at certain low current conditions. Nevertheless, it is important to note that all the
presented measurements in this section are performed at high fields where critical currents are
controlled by grains only and not the grain boundaries as shown in [22,29]. This excludes models
explaining n-values assuming grain boundary controlled currents and nonuniformity from [13,14].
Even the explanation by the staircase flux lines theory is limited to the low fields, as the flux lines
they remain mostly straight due to their high numbers in fully penetrated superconductor in higher
fields.
After the fast neutron irradiation, randomly distributed efficient pinning centres are introduced into
superconducting grains. They are very efficient at wide range orientations of external magnetic fields
except orientations where very high density of efficient pinning is present e.g. close to the ab peak.
The fast neutron introduced pinning centres can have higher activation energies than original pinning
and can enhance n-values. This enhancement is observed in figure 6, especially at 77 K (figures 6b,d).
A drop of n-values is observed at the fluence of 1x10 22m-2. The drop of n-values looks like a
consequence of very high density pinning centres. All the situations with very high density of efficient
pinning centres (high neutron fluences, intrinsic pinning) have always shown smaller n-values. There
is probably a threshold value of efficient pinning centres concentration after which the n-value starts
to be reduced with increasing concentration. This threshold value is obviously strongly dependant on
field and temperature. High irradiation fluences cause damage of the crystal structure which can be its
reason of loss or partial loss of ability of flux pinning. The consequences are reduced Jc values close
to the ab peak in all the presented measurements (figures 6a,c,e) ,especially at the highest fluence.
The other consequence is probably partial n-value recovery at the ab peak at 64 K, 4T, at the highest
neutron fluence, where the flux pinning abilities of the intrinsic pinning centres could be significantly
reduced.
None of the mentioned effects is a consequence of nonuniformity in the HTS, which is experimentally
proven in the following section of the paragraph. It is well known that neutron irradiation introduces
disorder into the YBCO crystals. The commonly observed consequence of this disorder, which is
mostly a consequence of already mentioned point defects in the oxygen
sublattice, is critical
temperature reduction of irradiated samples [15,33]. However if fast neutron irradiation creates
nonuniformity in HTS, particular YBCO grains would have different T c causing wider transition to
the normal state. Therefore, fine Tc measurements after each irradiation step were performed. The
result (figure 7) shows no influence of fast neutron irradiation on the broadening the transition. It
means that the point defects caused by neutron irradiation are homogenously distributed and no
additional nonuniformity is created. The homogenous distribution is possible due to their mobility
and also relatively high neutron fluences.
8x10-7
7x10-7
6x10-7
U (V)
5x10-7
4x10-7
3x10-7
2x10-7
unirradiated
irr 2x1021m-2
irr 4x1021m-2
irr 1x1022m-2
1x10-7
0
-1x10-7
86
88
90
92
T (K)
Figure 7: Tc transition transport measurements after each irradiation step performed in 17 T measurement set-up.
The SuperPower(2008) tape was characterized also in high fields up to 15 T (17 T measurement setup) in two main magnetic field directions. This measurement should confirm results from previous
angle-resolved measurements with additional information of n-values in very high fields. Results of
these measurements together with measurements after irradiation by fast neutron fluences of 4x1021m2
are shown in figure 8. It is worth to note that at lower temperatures such as 64 K and 50 K, at
magnetic fields parallel to the ab planes (H II ab - represents 90° in figure 6), n-values are not
reduced with increasing magnetic field (after the initial drop at low fields).
125
unirr
35
H II c
H II c
30
100
21
-2
irr 4x10 m
50K
64K
77K
85K
25
n value
Ic (A)
75
50
20
15
10
25
5
0
0
0
2
4
6
8
10
12
14
0
16
Field (T)
a)
2
4
6
8
10
b)
35
unirr
H II ab
30
H II ab
n value
25
Ic (A)
14
21
-2
irr 4x10 m
50K
64K
77K
85K
100
50
12
Field (T)
20
15
10
0
5
-1
c)
0
1
2
3
4
5
6
7
8
-1
9 10 11 12 13 14 15 16
Field (T)
d)
0
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15
Field (T)
Figure 8: critical currents and n-values at different external magnetic fields, temperatures and irradiation levels:
a),b) HIIab, c),d) HIIc.
V Summary and conclusions
n-values of different 4mm and 12 mm wide commercial 2 G HTS tapes at various magnetic fields and
temperatures were presented. The performance of the HTS tapes characterized in higher fields is still
relatively low. In addition reduction of n-values would enforce them to be operated even at lower
currents, which makes them not suitable for most of the possible applications where high fields are
required.
Higher n-values and relatively low anisotropy of n-values (compared to Jc) was observed in recent 2G
HTS tapes. n-values were studied also after fast neutron irradiation, which introduced randomly
distributed spherical defects into the superconductor. Two main factors, nonuniformity and flux creep,
are considered as a source the exponential relationship of the I-V curve. Nonuniformity was excluded
as the primary factor at higher temperatures and higher fields, where the currents are limited by grains
and not by the grain boundaries. In addition, it was shown that all the n-value changes after fast
neutron irradiation cannot be consequence of nonuniformity as irradiation defects were distributed
homogenously. Another studied phenomenon was correlation between n-values and Jc. It was shown
that n-value is proportional to pinning energy which usually results into correlation with Jc. However,
inverse correlation between n-values and Jc was found under circumstances, where high densities of
efficient pinning centres are present.
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