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THE EFFECT OF AGEING ON YBA2CU3O7-X
SUPERCONDUCTING PELLETS
P.M. Nikolic1, S.S. Vujatovic1, S.Djuric1, B. Stojanovic2, K. Radulovic3,
D. Vasiljevic-Radovic3, M. Miletic1, C. R. Foschini2
1)
Institute of Technical Sciences of SASA, P.O.Box 745, 11000 Belgrade, Yugoslavia
2)
Instituto de Química-UNESP- Araraquara, CP 355, 14801-970, Brasil
3)Center
for multidiciplinary studies, University of Belgrade, Yugoslavia
ABSTRACT
In this work, a twelve years old superconducting YBa 2Cu3O7-x samples were
examined and their properties compared with freshly made superconducting samples
of the same composition, using a modified non-contact inductance bridge method. It
has been shown that the old sample exhibited decreased superconducting properties
but still kept about a third of its superconducting phase. Also, there were no
important changes in either the onset or critical temperature with ageing. Using the
EDS method, the content of oxygen in freshly made and 12 year old samples was
compared. For the old sample, the content of oxygen was calculated to be 6.55 per
one atom of yttrium, while it was 6.9 for the freshly made sample. X-ray
investigations YBa2Cu3O7-x samples after storage for twelve years showed no
changes in structure.
Key-words: Superconductor, YBaCu, Microstructure, Electrical Properties
INTRODUCTION
Georg Bednorz and Karl Alex Müler
(1)
discovered superconductivity at over
30 K in the class of ceramic oxides containing lanthanum barium and copper in early
1986.
Soon
afterwards
superconductivity at 94 K
these
(2).
results
were
verified
in
YBa2Cu3O7-x
with
Since then much work has been published concerning
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this high Tc superconductor. For example, Kamimura and Sano
Kamimura
(4)
Almond et al.
(3)
and Namura and
recently studied tight binding Hamiltonians for this superconductor.
(5)
investigated the modulated optical reflectance characterization of
superconducting YBa2Cu3O7-x thin films and microwave devices. The influence of
oxygen vacancies in CuO chains is studied in the framework of the extended two –
sublattice Mitsual model where pseudospins variables are used for the description of
two possible positions of oxygen ions
synthesized in 1987
(7)
(6).
In our laboratory YBa2Cu3O7-x pellets were
and their superconducting properties examined
(8).
Some of
these samples were kept and after twelve year we were able to measure their
superconducting properties again. In this work we compared some new and twelve
years old data for the same samples in order to detect differences due to ageing.
EXPERIMENTAL PROCEDURE
Transport properties of superconducting YBa 2Cu3O7-x pellets, produced
twelve years ago, were examined again. Their critical temperature was measured
using the same modified noncontact method
(8)
as
twelve years ago. It consisted of an inductance
bridge whose scheme is given in Figure 1. An
oscillator was employed as a generator whose
frequency could be changed between 10 Hz and
200 kHz. This oscillator fed two coils L1 and L2
connected in a series, and wound in the opposite
way. Coils L1 and L2 were inductively coupled with
coils L3 and L4, respectively. Resistors R1 and R2,
each of 22 k, and a variable resistor R3 of 10 k
were inserted in this bridge. Both coils L2 and L4
had soft ferrite cores, which were used to increase
the sensitivity of the bridge. During the measuring
Figure 1: Modified noncontact
inductance
bridge
for
critical
temperature measurements.
procedure, the superconducting sample was placed
between coils L2 and L4. Then Eddy currents were
induced in the sample, by coils L2 and L4, producing a magnetic field, which
modified self-inductances of both L2 and L4 coils and also the mutual inductance
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(L24) of these coils. The response of this inductance bridge was measured using a
phase detector. Finally the output of the phase detector was connected to a personal
computer using an AD converter. The temperature of the sample was measured with
a copper – constantan thermocouple which was attached to the sample with a
thermoconductive paste. Both old and new superconducting samples were pellets 8
mm in diameter and 2 mm thick, and they were placed in a sample holder made of a
pertinax insulator. This sample holder containing the superconducting sample was
always put in the same position in the narrow space between coils L2 and L4. This
was ensured by a special lead. The measuring procedure was rather simple.
Measurements were first made at room temperature in the sample in (U) and the
sample out (Uo) positions. Afterwards the superconducting sample with its holder and
the copper – constantan thermocouple were cooled down by immersion in liquid
nitrogen. After several minutes the cold superconducting sample with its holder, was
put again between coils L2 and L4, as quickly as possible (about 1 and 2 seconds).
The sample was then warmed up relatively slowly, up to 10 seconds, and the change
of the voltage from the bridge was registered by the PC as a function of time. The
time dependency of the relative bridge response (U/U o) for the 12 years old
superconducting sample is given in Figure 2, with crosses. We have recently
produced
fresh
superconducting
samples
of
YBa2Cu3O7-x using the same technology and quality
of starting compounds (Y2O3, Cu2O and BaCO3) as
explained elsewhere
(7).
In the same Figure the
change of the relative bridge response for a new
freshly synthesized sample is given with open
circles. The response of the inductance bridge was
registered every 10 milliseconds. In Figure 2 we
have shown every tenth measuring point, to make
the picture more clear.
At the same time, every 10 ms, the
temperature of the sample was registered. One end
of the copper – constantan thermocouple was stuck Figure 2: The relative bridge
response vs. time for the 12
to the sample, while the other end was permanently years old (x) and for new (o)
superconducting sample.
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immersed in a Dewar vessel with liquid nitrogen. The measuring signal for this
thermocouple was also connected to a computer using the same AD converter.
The obtained thermal characteristics of our twelve – year – old
superconducting sample could be easily compared with the results reported in 1988.
for the same sample (8).
RESSULTS AND DISCUSSION
The relative bridge response diagrams (U/Uo) versus time, for both old
(aged) and freshly synthesised samples had similar shapes (Figure 2). There was a
sudden change of level after the sample was cooled down to liquid nitrogen
temperature, then the level remained almost unchanged for several seconds until it
started to increase almost linearly reaching an inset after another few seconds. The
main
difference
between
these
two
diagrams
is
the
distance
between
superconducting and nonsuperconducting levels. For the freshly made sample this
distance was about 76% while it was only about 24 % for the twelve years old
sample. We might conclude that the superconducting relative ratio of the aged and
new freshly made sample is about three. It is obvious that after twelve years the
superconducting YBa2Cu3O7-x sample did not lose its superconducting properties, but
they decreased greatly in degree down to about one third of the initial value.
Changes in the superconducting relative ratio might depend on the storing method of
a particular sample. Our samples were kept in an ordinary laboratory cupboard only
protected against dust and mechanical injuries.
X-ray work was also done on both samples. No differences in Bragg's
reflection pattern were detected between the old and new samples. Also, for the old
sample no differences were found in either Bragg’s reflection patterns or the ratio of
their intensities, for diffractograms made twelve years ago and now. In our opinion xray measurements are not sensitive enough for the purpose of detecting such small
changes in structure. Finally, we may add that the modified non-contact inductance
bridge arrangement is a fast method, which can easily be used to measure the
critical temperature of high - Tc ceramic superconductors. This method can also be
employed to detect either the relative proportion of the superconducting phase in
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high – Tc ceramic samples or to follow changes in superconducting properties with
ageing.
Figure 3: Microstructure of the old
superconducting YBa2Cu3O7-x sample.
Figure 4: Microstructure of the freshly
made superconducting YBa2Cu3O7-x
sample.
Figure 3 shows the microstructure of the old superconducting sample. We
can see that some grains are rather large, bigger than 10 m. Unfortunately we didn’t
record the microstructure twelve years ago so we can not make any comparison. For
the new, freshly made YBa2Cu3O7-x sample the microstructure is given in Figure 4,
where it can be seen that the crystal grains are much smaller.
Obviously, there is a possibility that during twelve years the grain size of our
sample probably increased, but we can not be sure about that.
Using the EDS method we could compare the content of oxygen in the new
and 12 years old sample. In Figure 5 the EDS diagram is given for the old sample,
where the content of oxygen was calculated to be 6.55 per one atom of yttrium. For
the freshly made sample, whose EDS diagram is given in Figure 6. the ratio of
oxygen versus one yttrium atom was about 6.9. This means that the old sample lost
some oxygen and that is perhaps the main reason of the decrease of the
superconducting relative ratio.
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Figure 5: The EDS diagram of the old
superconducting YBa2Cu3O7-x sample.
Figure 6: The EDS diagram of the freshly
made
superconducting
YBa2Cu3O7-x
sample.
It is interesting to mention that YBa2Cu3O7-x has an orthorhombic structure
given in Figure 7.
It is well known that the distances between copper atoms Cu2 and Cu1 and
oxygen atom O4, is very important. In Fig. 8a and
Fig. 8b the changes of Cu1 – O4 and Cu2 – O4
distance
versus
Yba2Cu3Ox
(9)
the
content
of
oxygen
in
are given, respectively. One can see
that for x  6.5 the distance between Cu1 – O4 is
about 1.8 Å and for x  6.9, Cu1 –O4 is about 1.87
Å. It is possible to conclude that when the oxygen
content decreases, in the YBa2Cu3Ox sample, then
the distance Cu2-O4 increases and the distance
Cu1-O4 decreases. In our further work we intend to
do further analysis and probably confirm these
prediction.
Figure 7: The
YBa2Cu3O7-x.
structure
of
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Figure 8: The changes of Cu1-O4 (a) and Cu2-O4
(b) distances vs. the content of oxygen in
YBa2Cu3O7-x.
CONCLUSION
A twelve years old superconducting YBa2Cu3O7-x samples were examined
using a modified non – contact inductance bridge method. Thermal characteristics of
our twelve – year old superconducting sample were compared with the results
reported in 1988. and also with results obtained for recently produced fresh
superconducting samples using the same technology and quality of starting
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compounds. It has been shown that the old sample exhibited reduced
superconducting properties but still kept about a third of its superconducting phase.
We can also conclude that there were no important changes in either the onset or
critical temperature with ageing. X-ray work showed no changes in structure of the
examined YBa2Cu3O7-x samples after storage for twelve years. Analysis using the
EDS method showed that the content of oxygen decreased from about x=6.9 to 6.55
per atom of yttrium.
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