Large-area, uniform, high critical current density (J c ) YBa 2 Cu 3 O 7−x (YBCO) superconductor films are now routinely obtained by metalorganic deposition using trifluoroacetates (TFA-MOD). This method does not require any expensive vacuum apparatus at any time during the whole process. Thus, TFA-MOD is regarded as one of the most suitable candidates for fabricating a YBCO tape for many high-power applications. This method originated from an electron beam process using BaF 2 developed by Mankiewich et al. Afterwards, Gupta et al reported using TFA-MOD to prepare a similar precursor film. These two ex situ processes used fluorides instead of BaCO 3 to avoid the fatal deterioration in J c , which is caused in the resulting films through metal carboxylic groups. Fluorides not only avoid such deterioration but also lead to perfectly c-axis-oriented epitaxial crystal growth. In conventional metalorganic deposition, nucleation in the precursor film causes random orientation in the resulting film. However, in TFA-MOD, nanocrystallites in the precursor film never cause such disorder. Furthermore, during the firing process of TFA-MOD, water and HF gas diffuse quickly between the film surface and growth front of the YBCO layer. This diffusion never limits the growth rate of YBCO. What distinguishes TFA-MOD from conventional metalorganic deposition? What happens during heat treatment? In this paper, we discuss all the TFA-MOD processes and the peculiar growth scheme of the YBCO layer in TFA-MOD using the model of a quasi-liquid network. In addition, we review the history of TFA-MOD and recent results and discuss the prospects of future applications.
Ultrahigh-Jc YBa2Cu3O7-x (YBCO) films have been successfully fabricated by the
metalorganic deposition method using a trifluoroacetate coating solution
which is prepared by a newly developed purification technique, the
solvent-into-gel (SIG) method. The prepared pure coating solution has
less than 0.25% impurities and has a wide flexibility in process conditions to
obtain high-Jc YBCO film. Using this feature, we have successfully
formed 50 mm diameter YBCO films, which have a critical current density over 10 MA cm-2 (77 K, 0 T) on
LaAlO3 single crystalline substrates.
We have succeeded in enhancing the critical current density
(Jc)
of melt-textured Y–Ba–Cu–O superconductors by employing ultra-fine Y211
powder as precursor. The Y211 starting powders were reduced down to 110 nm
in diameter with ball-milling. Single-grain Y–Ba–Cu–O samples up to 33 mm in
diameter were fabricated from the precursors containing finely milled Y211
powders. The Y211 inclusions in the melt-textured samples could be refined by
decreasing the particle size of the Y211 starting powders. The maximum value of
Jc at 77 K
was 1.1 × 105 A cm−2
in a self-field. However, the size reduction of Y211 led to an inhomogeneous distribution of
Y211 particles, in which the volume fraction of the Y211 phase greatly decreased near a
seed crystal. An Ag-added bulk sample 33 mm in diameter exhibited a high trapped
magnetic field of 1.6 T at 77 K.
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