The influence of the partial Ca substitution on the microstructure of YBCO tapes

Abstract

YBCO tapes were obtained by the OPIT (Oxide Powder in Tube) method. As a superconductor two type precursors (YBa2Cu3Oz and Y0.7Ca0.3Ba2Cu3Oz) were used. Calcium substitution increases the number of carriers and pinning centres in the superconducting material. Hot-rolling can provide texturing of the superconducting core. The microstructure was investigated by scanning electron microscopy, X-ray microanalysis and energy dispersive spectroscopy. It was established that hot-rolling increases the critical current density of Ca substituted YBCO tapes.

Full text

JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS Vol. 9, No. 2, February 2007, p. 453 - 455
The influence of the partial Ca substitution on the
microstructure of YBCO tapes
S. TERZIEVA*, A. STOYANOVA-IVANOVA, V. MIKLIa, A. ZAHARIEV, CH. ANGELOVb
Y. DIMITRIEVc, V. KOVACHEV
Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria
a Centre for Materials Research, Tallin Technical University, Ehitajate 5, Tallin 79086, Estonia
b Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences, 72 Tzarigradsko Chausee
Blvd., 1784 Sofia, Bulgaria
c Department of Silicate Technology, University of Chemical Technology and Metallurgy, Blvd. Kl. Ohriski 8,
1756 Sofia, Bulgaria
YBCO tapes were obtained by the OPIT (Oxide Powder in Tube) method. As a superconductor two type precursors
(YBa2Cu3Oz and Y0.7Ca0.3Ba2Cu3Oz) were used. Calcium substitution increases the number of carriers and pinning centres
in the superconducting material. Hot-rolling can provide texturing of the superconducting core. The microstructure was
investigated by scanning electron microscopy, X-ray microanalysis and energy dispersive spectroscopy. It was established
that hot-rolling increases the critical current density of Ca substituted YBCO tapes.
(Received November 1, 2006; accepted December 21, 2006)
Keywords: YBCO tapes, Hot-rolling, Calcium substitution, Microstructure
1. Introduction
Recently, Y-123 type superconductors with Ca doping
have been attracting much attention, because of their
strong pinning properties in magnetic field at liquid
nitrogen temperature [1,2,3]. From a practical point of
view, YBa2Cu3Oz (YBCO) is expected to be utilized for
the wire material as well as the bulk material. Chemical
doping generates substitutional defects and increases the
number of the carriers and pinning centres in the
superconducting material [3-5]. In our previous reports, it
was established that the superconducting properties in
REBa2Cu3Oz (RE-rare element, briefly REBCO) ceramics
with Ca-substitution depend strongly on the phase purity,
porosity, alignment of crystallites and properties of the
grain-boundaries [3,6,7]. It is well known that the Oxide
Powder in Tube (OPIT) method including several cycles
of heat treatment and mechanical deformation steps, leads
to a higher critical current density (Jc) in Ag- sheathed
Bi(Pb)SCCO tapes [8-10]. On the other hand using the
hot-rolling leads to a purer crystal structure and better
textured superconductors [11]. The YBCO tapes were
obtained by the OPIT method and two different regimes of
rolling (cold and hot) with two types of precursors
(YBa2Cu3Oz and Y0.7Ca0.3Ba2Cu3Oz). The best results were
obtained for hot-rolled YBCO tapes with Ca substitution
[12]. Additional investigations on the microstructure of
such tapes could be very useful to accumulate new
knowledge about the processing conditions of
superconducting tapes.
In this work, we investigate the influence of hot-
rolling and calcium substitution on the phase formation,
texture and structure of YBCO tapes.
2. Experimental
2.1. Precursor preparation
Y1Ba2Cu3Oz and Y0.7Ca0.3Ba2Cu3Oz samples were
synthesized by a optimized ceramic method, including
solid-state reaction and sintering [13].
It used the triple heat treatment regime. The first step
included the mixing and milling of appropriate amounts of
Y2O3, CuO, BaCO3, CaCO3 and calcinations in flowing
oxygen at 900 oC for 21h. The second step of the heat
treatment was at 930 oC for 21h in the same atmosphere,
followed by annealing at 450 oC for 2h. The last step
started with grindings and pressing the powder into pellets,
followed by sintering at 950 oC for 23 h, slow cooling to
450 oC and holding at that temperature for 23 h.
2.2. Tapes preparation by the OPIT method
The tube used was made by pressing and drawing of
Ag with 4N purity. The dimensions of the tube were: inner
diameter -3.6mm; thickness -1.1mm. The OPIT method
including filling up and mechanical packing of the
superconducting powder in the silver tube, rolling of the
tube until the forming of the tape, thermal treatment and
sintering of the 1:2:3 phase – the basic superconducting
phase in the YBCO system. For deformation of filled Ag
tubes, we chose longitudinal rolling as described in [12].
The rolling was realized in six stages of deformation with
equal steps (Fig. 1). The total degree of deformation was
220% for rolled tapes. During “hot rolling” the material
was heated in advance up to 825 oC for 1h followed by

S. Terzieva, A. Stoyanova-Ivanova, V. Mikli, A. Zahariev, Ch. Angelov, Y. Dimitriev, V. Kovachev
454
rolling at room temperature. In this way, obtained tapes
were subjected to thermal treatment in oxygen under
optimal conditions – heating up to 890 oC for 21h with
isothermal delay, cooling slowly to 450 oC and keeping at
that temperature for 23h.
123456
0
10
20
30
40
50
εa−ε0[%]
Number of rolling stage
Y0.7Ca0.3Ba2Cu3Oz
YBa2Cu3Oz
Fig. 1. Difference between the assigned (εa) and
obtained (εo) deformation for each rolling stage.
2.3. Sample analyses
The phase formation of the samples was studied by
X-ray powder diffraction analysis using a Bruker D5005
diffractometer with Cu Kα radiation. The microstructure
of the tapes was studied with a Jeol JSM-840A scanning
electron microscopy (SEM). The chemical composition of
the samples was determined by the X-ray microanalyses
and using energy dispersive spectroscopy (EDS) on a
LINK Analytical AN10000 system. The qualitative and
quantitative analyses were carried out at an accelerating
voltage of 20 kV. The distribution of the elements inside
the superconducting core was visualized by an X-ray
mapping technique. Optical images were taken with
polarized light, using Nikon, Microphot-FX optical
microscopy (OM).
3. Result and discussion
The X-ray diffraction patterns for Y1Ba2Cu3Oz and
Y0.7Ca0.3Ba2Cu3Oz tapes (Fig. 2) show that they have
orthorhombic structures and there are no extra peaks from
the impurity phases. Scanning electron and optical
micrographs in polished samples of 1:2:3 from a region
containing the grain boundary phases was shown in Fig. 3.
The micrographs reveal a plate-like structure of the 1:2:3
grains. Their average sizes ranges from 10 µm to 40 µm
for the Y1Ba2Cu3Oz tape (Fig. 3A). It can be seen in Fig.
3B that the structure of the superconducting core of the
Y0.7Ca0.3Ba2Cu3Oz tape has finer grains and both types of
tapes are without pores. From the data in Table1, one can
see that powders and YBCO tapes have the stoichiometric
1:2:3 superconducting phase.
Table 1. Integral EDX data for elements contents in
atomic percentages (at%).
Samples Y Ca Ba Cu
Y1Ba2Cu3Oz
powder
7.2 0 12.4 21.0
Y1Ba2Cu3Oz
tape
8.9 0 16.0 22.8
Y0.7Ca0.3Ba2Cu3Oz
powder
5.7 2.0 12.7 20.8
Y0.7Ca0.3Ba2Cu3Oz
tape
6.9 2.4 16.5 22.4
The transition temperature (Тс) was defined from AC
current magnetic measurements and the critical current
density (Jс) was measured by the four contact method. The
data from all measurements are given in Table 2. The
Y0.7Ca0.3Ba2Cu3Oz tape has the highest critical current
density.
Intensitv. (a.u.)
0
100
200
300
400
2-Theta (Degree)
11
20 30 40 50 60
A
B
Fig. 2. XRD patterns using Cu-Ka radiation for
Y1Ba2Cu3Oz (A) and Y0.7Ca0.3Ba2Cu3Oz (B) tapes. The
symbol (
z
) indicates the main peaks of the 123 phase.
Fig. 3. SEM (up) and optical micrographs (down) of
Y1Ba2Cu3Oz (A) and Y0.7Ca0.3Ba2Cu3Oz (B) tapes.

The influence of the partial Ca substitution on the microstructure of YBCO tapes
455
Table 2. The critical temperature (Tc, [K]) and critical
current density (Jc, [A/cm2]) of Y1Ba2Cu3Oz and
Y0.7Ca0.3Ba2Cu3Oz tapes.
Samples
Tc
Jc
Jc
After 3
months
Y1Ba2Cu3Oz
bulk sample
91 7.3 7.3
Y1Ba2Cu3Oz
hot-rolled tape
90 98.8 144.9
Y0.7Ca0.3Ba2Cu3Oz
bulk sample
81 6.3 6.3
Y0.7Ca0.3Ba2Cu3Oz
hot-rolled tape*
81 614.0 733.0
* The Jc of Y0.7Ca0.3Ba2Cu3Oz tapes was measured after
one year and the value was 965 A/cm2.
4. Conclusion
It is established that hot-rolling technology is suitable
for making Y0.7Ca0.3Ba2Cu3Oz tapes. The results show that
hot-rolled tapes with and without Ca-substitution have
orthorhombic 1:2:3 structures with no impurity phases.
The superconducting core of the Y0.7Ca0.3Ba2Cu3Oz tape
has a finer grain structure than the Y1Ba2Cu3Oz tape.
It is shown that the critical current density (Jc)
continues to increase with the time, reaching up to
965 A/cm
D. Dimitrov, J. Mat. Scien. and Tech. 5, 41 (1997).
2 after one year.
Acknowledgements
This study was supported by Grant No.ТН- 1525 from
the National Science Fund Ministry of Education and
Science of Bulgaria.
References
[1] M. Morakami, Melt Processed High Temperature
Superconductors, Word Scientific, Singapore 1993,
p. 21.
[2] M. Bindi, A. Botarelli, A. Gauzzi, L. Gianni,
S. Ginocchio, B. Holzapfel, A. Baldini,
S. Zannella, Supercond. Sci. Technol. 17, 512 (2004).
[3] E. K. Nazarova, A. J. Zaleski, A. L. Zahariev, A. K.
Stoyanova – Ivanova, K. N. Zalamova, Physica C
403, 283 (2004).
[4] H. Hatada, H. Shimizn, Physica C 83, 304 (1998).
[5] T. Harada, K. Yoshida, Phisica C 383, 48 (2002).
[6] S. Terzieva, A. Stoyanova-Ivanova, K. Zalamova,
V. Mikli, Ch. Angelov, V. Kovachev,
J. Optoelectron. Adv. Mater. 7, 477 (2005).
[7] A. K. Stoyanova-Ivanova , S. D. Terzieva, A. D.
Staneva, V. Mikli, R. Traksmaa, Y. B. Dimitriev, V.
T. Kovachev Central European J. Chem 4, 167
(2006).
[8] K. Osamura, S. Oh, S. Ochiai, Supercond. Sci.
Technol. 3, 143 (1990).
[9] Y. Yamada, K. Jikihara, T. Hasebe, T. Yanagiya, Jap.
J. Appl. Phys 29, 456 (1990).
[10] S. R. Shukla, D. K. Pandya, Y. S. Reddy, N. Kumar,
S. K. Sharma, R. G. Sharma, Physica C 219, 483
(1994).
[11] V. Lovchinov, A. Stoyanova, I. Iordanov, H. Ignatov,
[12] A. Stoyanova-Ivanova, S. Terzieva, A. Zahariev,
V. Mihailov, H. Ignatov, Proceedings of advansed
studies on superconducting engineering, Budapest
University of Technol., Hungary, (2004), p. 56.
[13] C. N. R. Rao, R. Nagarajan, R. Vijayaraghavan,
Supercond. Sci. Technol. 6, 1 (1993)
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*Corresponding author: mira@issp.bas.bg