Sr2YSbO6 as a buffer layer for YBa2Cu3O7−δ superconducting films

Article (PDF Available)inJournal of Materials Science 46(3):688-692 · February 2011with37 Reads
DOI: 10.1007/s10853-010-4794-8
Abstract
We report the epitaxial growth of Sr2YSbO6 films over a MgO single crystal using magnetron sputtering technique. Sr2YSbO6 films were used as a buffer layer for growth of YBa2Cu3O7−δ films by DC sputtering. It was experimentally determined that the films evidence superconducting properties, such as a critical temperature of 90 K and a self-field critical current density at 77 K of 0.8 MA/cm2. These results, as well as good lattice matching and chemical stability between Sr2YSbO6 and YBa2Cu3O7−δ, suggest that Sr2YSbO6 is an ideal material to be applied as buffer layer for high-performance superconductor coatings. For instance, Sr2YSbO6 material can be used as a single buffer layer in coated conductor tapes based on ion beam assisted deposition of MgO template (IBAD-MgO).
Sr
2
YSbO
6
as a buffer layer for YBa
2
Cu
3
O
72d
superconducting
films
O. Ortiz-Diaz
W. Saldarriaga
W. Lopera
D. Reyes
A. Cortes
J. M. Caicedo
D. A. Landinez Tellez
J. Roa-Rojas
Received: 29 November 2009 / Accepted: 24 July 2010 / Published online: 7 August 2010
Ó Springer Science+Business Media, LLC 2010
Abstract We report the epitaxial growth of Sr
2
YSbO
6
films over a MgO single crystal using magnetron sputtering
technique. Sr
2
YSbO
6
films were used as a buffer layer for
growth of YBa
2
Cu
3
O
7-d
films by DC sputtering. It was
experimentally determined that the films evidence super-
conducting properties, such as a critical temperature of
90 K and a self-field critical current density at 77 K of
0.8 MA/cm
2
. These results, as well as good lattice matching
and chemical stability between Sr
2
YSbO
6
and YBa
2
Cu
3
O
7-d
,
suggest that Sr
2
YSbO
6
is an ideal material to be applied as
buffer layer for high-performance superconductor coatings.
For instance, Sr
2
YSbO
6
material can be used as a single
buffer layer in coated conductor tapes based on ion beam
assisted deposition of MgO template (IBAD-MgO).
Introduction
During the past decade, significant advances in the per-
formance levels of high-temperature superconducting
(HTS) wire have made it suitable for commercially viable
applications such as electric power cables, fault current
limiters, motors, and generators [1, 2]. For instance, both
the United Sates Department of Energy and private
industry have been developing a key superconductor cable
and fault current limiter projects [1], and there is a 5-year
Japanese national project for materials and power appli-
cations of coated conductors, which was started in 2008
[2]. These power applications share a common require-
ment: that the superconducting material be formed into a
long, strong, and flexible conductor so that it can be used
like the copper wire it is intended to replace. And this is
where the problems began, because the HTS materials are
ceramics that are more like a piece of chalk than the ductile
metal copper [3].
The first solution to this problem, the so-called first
generation wire, was a tape that was made packing Bi–Sr–
Ca–Cu–O (BSCCO) superconducting powder into a silver
tube, following a series of rolling and heating steps [4].
In spite of successful applications, this type of conductor is
expensive for most commercial applications due to the use
of silver.
Further, BSCCO is not suitable for applications such as
motors and magnets at liquid nitrogen temperature; it loses
its ability to carry super current in a magnetic field [3, 5].
The alternative approach, known as the second generation
wire, uses the epitaxial growth of a YBa
2
Cu
3
O
7-d
super-
conducting coating on a thin metal tape. The advantages of
this wire are that very little silver is needed, making it
inexpensive, and that the compound YBa
2
Cu
3
O
7-d
retains
much higher current-carrying ability in a magnetic field.
O. Ortiz-Diaz D. A. L. Tellez J. Roa-Rojas
Grupo de
´
sica de Nuevos Materiales (GFNM), Departamento
de
´
sica, Universidad Nacional de Colombia,
Bogota
´
DC, Colombia
e-mail: oortizd@bt.unal.edu.co
O. Ortiz-Diaz
Grupo de
´
sica de Materiales (GFM), Escuela de
´
sica,
Universidad Pedago
´
gica y Tecnolo
´
gica de Colombia,
Tunja, Colombia
W. Saldarriaga
Universidad Nacional de Colombia, Medellı
´
n, Colombia
W. Lopera D. Reyes A. Cortes J. M. Caicedo
Thin Films Group, CENM, Universidad del Valle, Cali,
Colombia
Present Address:
J. Roa-Rojas (&)
A. A. 14490 Bogota
´
DC, Colombia
e-mail: jroar@unal.edu.co
123
J Mater Sci (2011) 46:688–692
DOI 10.1007/s10853-010-4794-8
Despite these advantages of superconductors, the ability to
carry current without loss is limited to current densities
lower than a critical value, J
c
. In order to carry a higher
current in a wire, the objective of research efforts is to
increase J
c
.
In this context, the preparation of biaxially textured
substrates and subsequent epitaxial buffer layers is very
important for the realization of long-length YBa
2
Cu
3
O
7-d
-
coated conductors. The buffer layer should not only
satisfy chemical stability, but structural matching with
YBa
2
Cu
3
O
7-d
as well because the alignment of the
superconductor is required for high J
c
values [3, 6].
Different oxide materials have been successfully used as
a buffer layer to fulfill these requirements [3, 714].
However, most of them are really a multilayer architec-
tures, which significantly increase the complexity as well
as the cost of production [6]. Therefore, the development of
a single buffer layer is of great interest, as this might
simplify the preparation process and lead to a more cost-
effective fabrication of coated conductors. To fabricate
templates of great length, the most promising approach is,
generally, with ion beam assisted deposition, IBAD yttria-
stabilized zirconia (YSZ), Gd
2
Zr
2
O
7
or MgO [1518].
Of these, the best is IBAD-MgO because very good biaxial
texture can be obtained with films only 10 nm thick, which
reduces production costs [3, 16, 17].
Sr
2
YSbO
6
appears to be a promising material for fulfill
these criteria, as it has a lattice parameter exhibiting a good
lattice match with YBa
2
Cu
3
O
7-d
(mismatch between a and
b YBa
2
Cu
3
O
7-d
parameters and a of Sr
2
YSbO
6
is *5%).
In this article, it has been applied effectively as a buffer
layer for YBa
2
Cu
3
O
7-d
film growth by DC sputtering.
Schematic representation of the resultant architecture
YBa
2
Cu
3
O
7-d
–Sr
2
YSbO
6
–MgO is shown in Fig. 1.This
superconducting film has a J
c
value *10
3
times that grown
on MgO. Sr
2
YSbO
6
films that were deposited over MgO
single-crystal substrate, because Sr
2
YSbO
6
has a good
match with MgO, which is the material of the IBAD-MgO
tapes. Other applications for the Sr
2
YSbO
6
material are in a
Josephson junction because they are an insulating material
for the deposition of superconductor films in microwave
applications, and for the elaboration of crucibles for the
preparation of superconductors due to their chemical non
reactivity with YBa
2
Cu
3
O
7-d
.
The material Sr
2
YSbO
6
was chosen because we had
been working to find new substrates for YBa
2
Cu
3
O
7-d
within the perovskite family A
2
BB
0
O
6
0
since by means of
substitutions they permit adjusting the lattice parameters
[19]. Previous studies of the material in polycrystalline
form showed that it was viable as a substrate for the growth
of YBa
2
Cu
3
O
7-d
films [20], which is effectively shown in
this article.
Experimental details
The deposition of Sr
2
YSbO
6
buffer layers, on MgO (1 0 0)
substrate, was performed by magnetron sputtering (13.56
MHz, 70 W) using a polycrystalline target, which was
fabricated by the solid state reaction method, based on SrO,
Sb
2
O
3
, and Y
2
O
3
powder oxides. Detailed Sr
2
YSbO
6
syn-
thesis processing conditions, and study of structural
ordering, can be found elsewhere [19, 21]. YBa
2
Cu
3
O
7-d
target was prepared by the solid state reaction method such
is described in reference [19].
X-ray diffraction patterns, both for the polycrystalline
target and for the films, were recorded by a PHILLIPS
PW1710 diffractometer using Cu-Ka radiation (k =
1.5406 A
˚
).
The substrate temperature and oxygen pressure for the
Sr
2
YSbO
6
growth were kept at 800 °C and 7 9 10
-3
mbar,
respectively. YBa
2
Cu
3
O
7-d
films were deposited on these
buffer layers by sputtering DC (* 30 W) at an optimized
substrate temperature of 850 °CanO
2
pressure of 3.5 mbar
for 1 h, followed by cooling up to 550 °C in 30 min at O
2
pressure of *850 mbar and, therefore, were annealed at
550 °C for 30 min at the same O
2
pressure.
The superconducting properties of YBa
2
Cu
3
O
7-d
films
were determined by measurements of the transition tem-
perature (T
c
) and critical current density (J
c
)at77Kin
self-field, by means of ACT measurements (bias AC cur-
rent of 30 Hz) with the four-probe method, using the PPMS
system of Quantum Design. These measurements were
performed on YBa
2
Cu
3
O
7-d
micro bridges, with 20 lmof
width and 100 nm of thickness, which were prepared by
UV photolithography.
Results and discussion
The X-ray diffraction pattern of Sr
2
YSbO
6
polycrystalline
target is shown in Fig. 2. The Rietveld refinement, per-
formed with the EXPGUI and GSAS programs [22, 23],
reveals that the Sr
2
YSbO
6
has the expected typical
Fig. 1 Schematic representation of YBa
2
Cu
3
O
7-d
films on Sr
2
YSbO
6
buffer layer
J Mater Sci (2011) 46:688–692 689
123
structural ordering of a complex cubic perovskite with
lattice parameter a = 8.249 A
˚
.
Figure 3 shows the X-ray diffraction pattern for the film
of Sr
2
YSbO
6
, with 2 h of deposition for 2h between 10°
and 80°. It consists of strong peaks (2 0 0) of MgO and (4 0
0) of Sr
2
YSbO
6
. Figure 4 shows a short detailed scan for
2h between 41 and 46. It shows the MgO peak in 2h = 43°
and the Sr
2
YSbO
6
peak in 2h = 43.1°. This result reveals
the epitaxial growth of Sr
2
YSbO
6
films on MgO (1 0 0)
substrate.
The X-ray diffraction pattern for the film of
YBa
2
Cu
3
O
7-d
growth over Sr
2
YSbO
6
buffer layer, for 2h
between 10° and 90°, is shown in Fig. 5. It consists of
peaks (0 0 l) of YBa
2
Cu
3
O
7-d
, besides the MgO and
Sr
2
YSbO
6
peaks, such as is detailed in Fig. 6. These result
reveals the epitaxial growth of YBa
2
Cu
3
O
7-d
over
Sr
2
YSbO
6
/MgO buffered substrate.
5020 30 40 60 70 80
2θ (degrees)
0
5000
10000
Intensity (counts)
experimental
calculated
(400)
Fig. 2 XRD pattern of Sr
2
YSbO
6
policrystalline target
0
1e+05
2e+05
3e+05
4e+05
5e+05
6e+05
Intensity (counts)
5020 30 40 60 70 80
2θ (degrees)
Fig. 3 XRD of Sr
2
YSbO
6
films, for 2h between 10° and 90°
41 42 43 44
45
46
2θ (degrees)
0
5e+05
1e+06
1,5e+06
2e+06
Intensity (counts)
(200) MgO
(400) Sr2YSbO6
Fig. 4 Detailed XRD pattern of Sr
2
YSbO
6
films
20
0
5e+05
1e+06
Intensity (counts)
50
30 40 60 70 80 90
2θ (degrees)
YBCO (005)
(200) MgO
(400) Sr2YSbO6
(006) YBCO
Fig. 5 XRD pattern of YBa
2
Cu
3
O
7-d
buffered films
41 42 43 44
0
5e+05
1e+06
Intensity (counts)
45
2θ (degrees)
(200) MgO
(400) Sr2YSbO6
(006) YBCO
Fig. 6 Detailed XRD pattern of YBa
2
Cu
3
O
7-d
films
690 J Mater Sci (2011) 46:688–692
123
Figure 7 shows the behavior of YBa
2
Cu
3
O
7-d
film’s
resistance as a function of temperature. For films growth
over Sr
2
YSbO
6
the curve exhibits linear behavior up to a
transition temperature T
c
of 90 K. In the same figure the
measurements corresponding to a YBa
2
Cu
3
O
7-d
films
growth over MgO and on SrTiO
3
, with the same condi-
tions, are shown. The T
c
of YBa
2
Cu
3
O
7-d
films over MgO
is 86 K, and over SrTiO
3
is 90 K. Although the T
c
values
are similar for films over STO and over Sr
2
YSbO
6
buffer
layer, the resistance in the normal zone is less for the
YBa
2
Cu
3
O
7-d
growth on buffer layer; also, the extrapo-
lated residual resistance for this film (0.62 X) is less than
the residual resistance (4.86 X) of YBa
2
Cu
3
O
7-d
film
growth on SrTiO
3
.
Results of measurements for voltage V in function of
current I (IV curves) are shown in Figs. 8 and 9 for the
films of YBa
2
Cu
3
O
7-d
over MgO and over buffered sub-
strate Sr
2
YSbO
6
/MgO, respectively. Based on IV data,
with the 1 lV/cm criteria, the critical current values were
determined in 0.013 and 16.5 mA, respectively. So, the
critical current density value for YBa
2
Cu
3
O
7-d
films
growth over Sr
2
YSbO
6
buffer layer is J
c
*0.8 MA/cm
2
,
which is 1269 times the J
c
of YBa
2
Cu
3
O
7-d
/MgO films.
The J
c
value for YBa
2
Cu
3
O
7-d
/Sr
2
YSbO
6
/MgO films
growth over buffer layer appears to be less than those
reported in the literature (J
c
*10
7
A/cm
2
). However, it is
worth saying that the value for YBa
2
Cu
3
O
7-d
/MgO is less
too in comparison with references (J
c
*10
6
A/cm
2
). Thus,
the sputtering deposition conditions perhaps are not yet
optimized, and we believe that with other methods of
deposition, such as PLD, we could improve the J
c
results.
The results reported in the literature are for films deposited
in wealthy laboratories that have optimized deposition
conditions.
Conclusion
Although the optimal deposition conditions for Sr
2
YSbO
6
films are not yet known, we have shown that Sr
2
YSbO
6
can
be succesfully used as a buffer layer for the epitaxial
growth of YBa
2
Cu
3
O
7-d
films with high density current
value in self-field at 77 K. Taking into account that there is
evidence of the effect of buffer layer thickness on J
c
[9]
and that the YBa
2
Cu
3
O
7-d
deposition conditions can be
improved, our values of J
c
could be greater than those
reported in this article.
It is worth saying that the layer of Sr
2
YSbO
6
material
that was used plays the roll of a buffer layer because the
negative effects of MgO over superconducting properties
of YBa
2
Cu
3
O
7-d
films were eliminated, and because it has
an excellent structural matching with YBa
2
Cu
3
O
7-d
and
0
50
100
150
Resistance (Ω)
150
0 30 60 90 120 180 210 240 270 300
Temperature (K)
Over STO
Over MgO
Over Sr2YSbO6
(a)
(b) (c)
Fig. 7 Resistance in function of temperature for YBa
2
Cu
3
O
7-d
films
over different substrates: (a) over MgO single crystal; (b) over SrTiO
3
single crystal; and (c) over Sr
2
YSbO
6
/MgO buffered substrate
0
0,5
1
Current (mA)
0
0,001
0,002
0,003
Voltage (V)
Fig. 8 IV curves for YBa
2
Cu
3
O
7-d
/MgO film at 77 K
02468101214161820
Current (mA)
-1e-06
0
1e-06
2e-06
3e-06
4e-06
5e-06
6e-06
7e-06
8e-06
9e-06
Voltage (V)
Fig. 9 IV curves for YBa
2
Cu
3
O
7-d
film over Sr
2
YSbO
6
/MgO
buffered substrate at 77 K. The dashed line is a guide for the eye.
It shows the voltage value of 1 lV
J Mater Sci (2011) 46:688–692 691
123
with MgO. These results show that Sr
2
YSbO
6
can be an
excellent substrate material for the YBa
2
Cu
3
O
7-d
layers in
coated conductors, using the IBAD-MgO templates.
Besides which, with the Sr
2
YSbO
6
material the architec-
ture of the coated conductors can be simplified, because
only one buffer layer is needed with Sr
2
YSbO
6
.
Acknowledgements The authors wish to thank Universidad Peda-
go
´
gica y Tecnolo
´
gica de Colombia (UPTC), The National Council for
Scientific and Technological Development (CNPq) of Brazil, and
Universidad Nacional de Colombia for their special support. This
study was partially supported by Universidad Nacional de Colombia
(DIB 20301007460, code 8003080) and Centro de Excelencia en
Nuevos Materiales CENM, contract 043–2005. The authors wish to
thank to Direccio
´
n de Investigaciones (DIN) of Universidad Peda-
go
´
gica y Tecnolo
´
gica de Colombia (UPTC), and The National
Council for Scientific and Technological Development (CNPq) of
Brazil for their special support.
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