TRANSPORT PROPERTIES OF CaZrO3-DOPED YBa2Cu3O7 FILMS

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TRANSPORT PROPERTIES OF CaZrO3-DOPED YBa2Cu3O7
FILMS
Noel A. Rutter, John H. Durrell, Mark G. Blamire
and Judith L. MacManus-Driscoll
Department of Materials Science and Metallurgy,
University of Cambridge
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1. Introduction
The critical current of a grain boundary
decreases exponentially with increasing grain
boundary angle. Doping with Ca was shown to
increase Jc for HAGBs. This improvement was
reproduced in LAGBs, especially at low T and high
B.
Schmehl et al. 1
Daniels et al. 2
BaZrO3 nanoparticles have been shown by
MacManus-Driscoll et al. to enhance the field
dependence of Jc in YBCO coated conductor films 3.
YBCO/BaZrO3
Goyal et al. found that CaZrO3 significantly
reduced Tc when used to dope YBCO films 4.
pure YBCO
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2. Sample Preparation
Films on single crystal substrates were deposited
by PLD from a YBCO target doped with 5 CaZrO3.
RABiTS tapes were fabricated by the AMSC MOD
method and were doped by depositing a CaZrO3
overlayer which is diffused into the YBCO.
Diffusion along a semi-infinite bar, from a
finite source
Diffusion constants of Ca in YBCO at 500?C 5
DIG 3x10-7 mm2s-1
DGB 3x10-5 mm2s-1
Cx,t (arbitrary)
t1800s
t7200s
Cx,t (arbitrary)
These calculations assume planar boundaries,
which is unlikely to be the case in MOD RABiTS 6.
boundaries
grains
T 500?C
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3. Critical Current Measurement
The YBCO films were patterned using optical
lithography and chemical etching. Tracks in the
single-crystalline films were 20 mm wide whilst
those in the coated conductor films were 125 mm
wide in order to ensure that the region measured
contained multiple grains.
Critical current measurements were carried out as
a function of temperature, field and field
orientation using a 2-axis goniometer.
Transport current direction is q90?, f0?. Film
c-axis direction is q0?.
f90?
q90?
c-axis
B
q
q0?
f0?
f
current
f-90?
5
4. Bulk Doping Tc
All films grown from the CaZrO3-doped target have
a significantly reduced Tc. We varied the oxygen
partial pressure used during the annealing step
in order to attempt to increase Tc above 75 K.
No significant dependence of Tc on oxygen
annealing pressure was seen for oxygen partial
pressures above 200 mbar. Below this pressure, Tc
is reduced and the transition is wider. Post
anneals in very low oxygen partial pressures
produced non-superconducting films.
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5. Bulk Doping Jc
2.5
T60 K
Out-of-plane field scans show relatively low
anisotropy and no evidence of preferential
enhanced c-axis pinning. The shape of the scans
around q90? suggests there may be an extra
pinning contribution in the plane of the film.
2
B
1.5
I
1
5
T60 K B1 T
The graph to the right shows out-of-plane scans
of field orientation at various in-plane angles.
This shows the full range of field-orientation
dependence of Jc at 1 T. The ratio between the
intrinsic (f90?) and force-free (f0?) peaks is
around 1 2.4.
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6. Overlayer Doping of RABiTS Tapes
The RABiTS tapes used in these experiments are
not very sharply textured. Hence there should be
a number of grain boundaries with misorientations
in the 5-10? range which it may be possible
to beneficially dope.
(103) YBCO pole figure
After depositing the overlayer, Ca is diffused in
by annealing in O2 at 500?C.Tc reduces slightly
with increased anneal time and also the
superconducting transition is broadened. However
Tc is still significantly higher than that of
bulk-doped samples.
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7. Overlayer Doping - Jc
T77 K self field
There is a significant reduction in the critical
current of the doped RABiTS tapes as the calcium
is diffused for longer periods of time. This
indicates that Ca may well be diffusing into the
grains (particularly near the top of the film)
even during the shortest anneals.
T80 K
Two samples were annealed at 500?C for just 30
minutes and then quenched to room temperature in
order to prevent excessive diffusion. These
samples have similar Jc values to those of
undoped tapes.
B
I
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8. In-plane Field Measurements
When the field was swept in the plane, the dip in
Jc seen for single grain boundaries was not
apparent in RABiTS tapes7, due to the fact that
the field is never aligned with all the grain
boundaries across the track. In 1 T field, the
undoped sample has Jc values between those of the
doped samples, but Ca-doping has a more
beneficial effect at f90?. Doping is most
beneficial in higher magnetic fields but has a
less significant effect at 60 K 8.
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9. Comparison with Undoped Samples
It is noticeable that the doped RABiTS samples
have Jc vs f curves which are flatter than those
of undoped samples. The graph below shows how
such scans compare with IBAD tapes and films
grown on single crystals when measured at 80 K
and 1 T. The single crystal has a very
anisotropic field-angle dependence whilst the
IBAD conductor lies between the two extremes.
We conclude that these results indicate that the
degree of anisotropic response to field depends
on the degree of current percolation in the
sample9. Hence it appears that non-uniform
Ca-doping of RABiTS may lead to increased
percolation.
T80 K B 1T
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• Summary
• Films deposited from a 5 CaZrO3-doped YBCO
  target have low Tc values of around 75 K.
• RABiTS tapes which are doped from a CaZrO3
  overlayer have Tc above 90 K indicating that the
  Ca does not diffuse significantly into the
  grains.
• Self-field Jc is degraded by the Ca-doping,
  though the field dependence is improved.
• In narrow tracks, Ca-doping shows improved
  results for large magnetic fields oriented in the
  plane of the film.
• The doped samples show a reduced anisotropic
  dependence on in-plane field angle. This suggests
  that the non-uniformity of the doping may
  increase the level of percolation.
• Doped and undoped RABiTS samples are
  considerably more isotropic than IBAD coated
  conductors and single crystalline films.

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References 1. A. Schmehl et al. Europhys. Lett.
47, p.110 (1999) 2. G.A. Daniels et al. Appl.
Phys. Lett. 77, p.3251 (2000) 3. J.L.
MacManus-Driscoll et al. Nature Materials 3, 439
(2004) 4. A. Goyal et al. US DOE Annual Peer
Review 2004 5. A. Berenov et al. Solid State
Ionics, 64 (2003) 6. D.M. Feldmann et al. Appl.
Phys. Lett. (2005) In press 7. J.H. Durrell et
al. Phys. Rev. Lett., 90 247006 (2003) 8. N.A.
Rutter et al., IEEE Trans. Appl. Supercond.
(2005) In press 9. N.A. Rutter et al. In
preparation
Acknowledgements This research was funded by the
UK Engineering and Physical Sciences Research
Council. NAR and JHD would also like to thank the
Royal Academy of Engineering for financial
support to present this work. The authors would
like to acknowledge American Superconductor
Corporation who provided the RABiTS tapes used in
these experiments.