Thickness control of solution deposited YBCO
superconducting films by use of organic polymeric additives
S. Morlens, N. Romà, S. Ricart,
a)
A. Pomar, T. Puig, and X. Obradors
Institut de Ciència de Materials de Barcelona (ICMAB) C.S.I.C., Campus Universitat Autònoma de
Barcelona, 08193 Bellaterra, Spain
(Received 16 February 2007; accepted 8 May 2007)
We show that the thickness of yttrium–barium–copper–oxide (YBCO) superconducting
films grown from trifluoroacetate precursors can be strongly modified using polymeric
additives, while deposition conditions by spin or dip coating remain unchanged. A
screening of different families of organic additives has been performed, and the best
results have been achieved using polymers having an oxygen functionalized backbone.
Two different polymeric additives, polyvinyl pyrrolidone (PVP) and poly(ethylene
glycol) (PEG), have been more thoroughly investigated, and thermal analysis suggests
that PEG is the most promising alternative because the pyrolysis step of the new
complex precursors remains sharp and narrow and hence the final homogeneity of the
film is preserved. The combination of anhydrous trifluoroacetic acid (TFA) solutions
and poly(ethylene-glycol) (PEG
8000
) as additive can produce an increase of the YBCO
film thickness up to 300%, while keeping a fast pyrolysis process and high critical
current densities.
I. INTRODUCTION
Superconducting power systems are regarded as a very
promising application of high-temperature oxide super-
conductors. From this viewpoint, high-temperature su-
perconducting thin/thick films, especially YBa
2
Cu
3
O
7-x
(YBCO), deposited on various substrates, have attracted
much attention in recent years because of their unique
properties of high epitaxial film, high critical temperature
(T
c
), and high critical current density (J
c
).
Physical deposition techniques, such as pulsed laser
deposition (PLD) methodology,
1–4
sputtering,
5
metal
evaporation,
6
or BaF
2
process
7
and chemical deposition
methods such as metalorganic chemical vapor deposition
(MOCVD)
8
or metalorganic deposition (MOD)
9
are ef-
fectively used to produce thick films. In all cases, it is
desirable to achieve great film thickness without decreas-
ing the critical current density, an issue that is still under
investigation in all the different deposition methodolo-
gies.
The implementation of low-cost deposition techniques
for high critical current YBCO-coated conductors is one
of the major objectives to achieve widespread use of
superconductivity in power applications. Chemical depo-
sition techniques appear to be a very promising route to
achieve this goal.
10
The use of trifluoroacetic (TFA) pre-
cursors,
11
avoiding the formation of deleterious BaCO
3
phase, has already been shown to be very competitive
and has to be considered one of the most valuable alter-
natives for achieving low-cost chemically based coated
conductors.
12–16
The investigation of methodologies allowing tuning
and control of film thickness using the TFA-MOD proc-
ess is a very important issue to enhance the industrial
scalability of the process. A close analysis and compre-
hension of the relationship among metalorganic precur-
sor composition, solution deposition methodology, py-
rolysis process, and growth conditions are required for
this purpose. Up to now, few studies have been devoted
to the problem of controlling film thickness of YBCO–
TFA films, and so maximum thickness of 0.8 to 1.0 m
has been demonstrated in a single deposition.
9
Multide-
position processes can lead to films with thickness up to
∼2 m,
17,18
although in this case much lower thickness
per step is often used, and hence up to nine deposition
steps are needed.
10,14
Recent progress in the understand-
ing of the pyrolysis process
19
now allows further inves-
tigation of the use of modified TFA precursors with the
purpose of tuning the film thickness in each deposition
step and hence leading to an enhanced flexibility of the
deposition and pyrolysis steps.
In the present work, a new approach to improving the
thickness of epitaxial YBCO layers is proposed. Taking
into account the wide flexibility in precursors, solvents,
and additives presented by the chemical MOD route to
YBCO layers, a study related to the influence of organic
additives on their final properties is endeavored. In this
way, a screening of different additives is realized, and
a)
Address all correspondence to this author.
e-mail: ricart@icmab.es
DOI: 10.1557/JMR.2007.0296
J. Mater. Res., Vol. 22, No. 8, Aug 2007 © 2007 Materials Research Society 2330