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Fabrication and Characterization of Multi-layer
YBCO Coated Conductors on a Curved Surface
Y.L. Cheung, E.F. Maher, J.S. Abell and I.P. Jones

Abstract— Biaxially textured YBCO films and various buffer
layers were prepared in situ on rotating cylindrical surfaces using
highly cube textured pure Ni and Ni-5at%W RABiTS tapes by
pulsed laser deposition (PLD). The superconducting and
crystallographic properties of the curved composite were similar
to those achieved on flat surfaces. The cube textures of the curved
architecture were characterized by electron back-scattered
patterns (EBSP). Cross-sectional TEM and Scanning
transmission electron microscopy (STEM) were used to determine
the chemical analysis, diffusion characteristics, microstructures
and interfaces of the curved multilayer samples.
Index Terms—coated conductors, EBSP, STEM, YBCO
I. INTRODUCTION
O
BTAINING higher JE (Engineering Current Density) and
good performance in high magnetic field are parts of
major research work in superconducting wires or coil
applications. Multi-YBCO layers and STO insulating layers
have been successfully deposited on the standard buffer layer
architecture of RABiTS (CeO2/YSZ/CeO2) by pulsed laser
deposition on a curved Ni substrate to demonstrate an
innovative integrated fabrication technology in High
Temperature Superconducting coil applications [1-3]. This
approach is proposed to use a combination of thin film
deposition and patterning techniques in order to prepare a
multi-turn, multi-layered cylindrical coil (coated conductor
cylinder) without the requirement of producing long lengths of
coated conductor tape. If this novel technology is successful,
only one layer of base substrate would be required to provide
the initial texture, thus the cost will be dramatically reduced
and JE will be significantly increased by proportionately
increasing the YBCO cross-sectional area. Moreover,
producing a coil is simply by a multi-layer in situ deposition
sequence onto a rotating cylindrical former [2-3]. It will
overcome the requirements of reproducible manufacture of
kilometre lengths within a vacuum chamber.
Manuscript received October 5, 2004.
Y.L. Cheung is with the School of Metallurgy and Materials, The
University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
(corresponding author phone: 44-121-4145209; e-mail: ylc263@bham.ac.uk).
E.F. Maher is with Coated Conductor Consultancy Ltd 4 High Street,
Watlington, Oxon, OX9 5PS, UK (e-mail: enquires@3-3s.co.uk).
J.S. Abell and I.P. Jones are with the School of Metallurgy and Materials,
The University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK (email: j.s.abell@bham.ac.uk, i.p.jones@bham.ac.uk).
Electron back-scattered diffraction patterns have been used
to study the cube architecture of curved coated conductor
samples. On the other hand, the detailed interface
microstructure and chemical analysis of curved coated
conductor samples has been characterized by scanning
transmission electron microscopy (STEM) in this paper.
II. EXPERIMENTAL
YBCO multi-layer and buffer layers were grown on a
curved Ni RABiTS substrate from Oxford Instruments or
curved Ni-5at%W RABiTS substrates from IFW Dresden. All
layers were prepared in situ by a Lambda Physik LPX200i
Excimer laser running at 248nm (KrF). A special rotating
cylindrical heated former (3cm diameter) was used in this
research in order to produce in situ formation of multi-turn and
multi-layered HTS coils. The deposition parameters were
chosen to be the same as for optimised flat tapes [3]. The
target-substrate distance was 5.5 - 6cm, laser fluence of 1.5 – 2
J/cm2, substrate temperature of 780oC, oxygen pressure of
0.6mbar, cooling down at 10oC/min in 700mbar oxygen after
the ablation. To reduce the possibility of substrate oxidation
and to remove oxide on the substrate, 0.15 mbar of reducing
gas: 4% H2 in argon, was introduced to the chamber during
heating of the substrate.
Electron Backscattered Diffraction (EBSD) patterns were
obtained from JEOL 840A Scanning Electron Microscope
with an Oxford Instruments INCA detector. The diffraction
pattern can be used to measure the crystal orientation and the
grain boundary misorientations which are indexed from the
Kikuchi bands.
To prepare the cross-sectional TEM samples, a Gatan model
691 Precision Ion Polishing System (PIPS) was used to mill
the sample. During ion milling, the ion-milling holder was
rotated and two ion guns were used. A typical ion milling
condition was at a 10o tilt angle and 5keV ion beam energy.
When coloured fringes were observed, the ion milling
condition was changed into an ion polishing procedure. The
ion polishing condition was at a 8o tilt angle and 3keV of ion
beam energy. The electron micro-images were observed on a
transmission electron microscope (Model Tecnai F20, FEI)
with a field emission gun (FEG) and energy dispersive
spectrometer which can operate in scanning mode as Scanning
Electron microscope (STEM).
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III. RESULTS AND DISCUSSION
A. Electron Backscatter Diffraction
Standard buffer architecture (CeO2/YSZ/CeO2) were grown
both on curved pure Ni and Ni-5at%W tapes using the same
parameters in order to determine the surface texture quality of
that buffer architecture. Grain boundary data from those two
samples are shown in Fig. 1 and Fig. 2. For the buffered Ni-W
tape, grains were significantly smaller than the grains on pure
Ni tape. Furthermore, more grains were misoriented between
2-10 degree compared to the buffered Ni, which indicated a
better cube texture in order to grow good quality YBCO layers
on Ni-W tapes[5].
Fig. 2. Grain boundary data measured by EBSD from the final CeO 2 layer
deposited on curved buffered Ni-5%W. (Up) Grain boundary positions
superimposed on the pattern quality image. (Down) The boundaries are
colour coded according to the histogram of misorientation angle.
Fig. 1. Grain boundary data measured by EBSD from the final CeO 2 layer
deposited on curved buffered Ni. (Up) Grain boundary positions
superimposed on the pattern quality image. (Down) The boundaries are
colour coded according to the histogram of misorientation angle.
B. Scanning Transmission Electron Microscopy
Triple YBCO layers were grown on a buffered curved Ni
substrate using SrTiO3 insulating layers. High-resolution
images and diffraction patterns of the interfaces were
presented in a previous paper from a Tecnai TEM [3]. Further
microanalysis of the sample is discussed here by STEM
operation with the same TEM.
TEM bright field image and STEM dark field images of
the triple YBCO layers are shown in Fig. 3 and Fig. 4. No
secondary phase particles or reaction phases were found within
the films. However, from Fig. 3, the surface of the sample
showed outgrowth formation (white arrows) with
approximately 300nm in diameter. It is evident that the
outgrowth formation was due to the surface roughness of the
second YBCO layer and second STO insulating layer.
Moreover, droplet shapes of porosity (black arrows) were
found in the YBCO layer as shown in Fig. 3 and Fig. 4. The
thicker the YBCO layer, the more the pores have been
observed. Porosity content in YBCO is related to the film
thickness [6].
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st
1 YBCO
70
nd
STO
STO
2 YBCO
rd
3 YBCO
Ba
60
counts
50
40
Cu
30
O
20
10
Sr
0
Ti
0
200
400
600
800
1000
1200
position(nm)
Fig. 3. Cross-sectional transmission electron micrograph of triple YBCO
layers on curved buffered Ni tapes. Outgrowths are indicated by white
arrows and pores are indicated by black arrows.
Fig. 5. STEM quantmap from the first YBCO layer to third YBCO layer.
clean. It proves that there was no significant diffusion between
the YBCO layers and STO insulating layers. SrTiO3 is a good
diffusion barrier and insulating layer in this research.
Moreover, it can provide a very good template for patterning
YBCO layers on curved surface.
IV. CONCLUSIONS
200nm
Fig. 4. Dark field STEM image of triple YBCO layers on curved buffered
Ni tapes. Black arrow indicates the porosity in the first YBCO layer.
Sharp and cube EBSD patterns of CeO2/YSZ/CeO2 on
curved Ni and NiW tapes were successfully acquired. The
grains on pure Ni tapes had large misorientation; however, the
misorientation of the grains on NiW tapes were mostly
between 2-10 degree which provides a good template for
growing YBCO layers.
Outgrowths and pores were observed in the YBCO layers.
TEM micrographs showed that the occurrence of pores and
surface roughness were dependent on YBCO thickness. STEM
quantmap linescan showed that there was no significant
diffusion between the YBCO layers and STO insulating layers.
STO is a good diffusion barrier and insulating layer for
separating YBCO layers in this research.
ACKNOWLEDGMENT
A STEM quantmap linescan was acquired from the first
YBCO layer to the third YBCO layer shown in Fig. 5 which
performs a quantitative analysis at each point on the line. The
Y signal is relatively low and has been omitted from Fig. 5 for
the sake of clarity. The elements showed even counts in each
layer and the interfaces between the layers were sharp and
The authors gratefully acknowledge Dr. B. Holzapfel and
Dr. J. Eickemeyer of IFW-Dresden for providing the Ni5at%W tapes and helpful discussions. The authors would like
to thank R.I. Chakalova for preparing the samples on Ni tapes.
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