Review of a chemical approach to YBa₂Cu₃O_7 x-coated superconductors—metalorganic deposition using trifluoroacetates

Abstract

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.

High-<TEX>$I_c$</TEX> single-coat YBCO films prepared by the MOD process

“…From these results, the critical thickness of as-pyrolized films for avoiding the crack formation is between 1.6 and 1.9 μm, which correspond to the viscosity values of 212 and 254 mPa⋅sec, respectively. The cracks most probably originate from an excessive drying stress caused by the shrinkage during the pyrolysis heat treatment [6].…”

“…The TFA-MOD process had several drawbacks, including a long duration period (typically over 10 h) at the pyrolysis stage (normally up to 400°C) and a limited thickness (normally up to~0.5 µm) of a single-coat YBCO film due to a serious crack formation [6] during the pyrolysis stage. By employing a fluorine-reduced precursor solution, the pyrolysis stage could be greatly shortened within 3 h [8,9].…”

On the Mechanism of Cuprate Crystal Growth: The Role of Mixed Metal Carbonates

“…These syntheses however invariably result in the formation of large crystallites of metal carbonates that are stable throughout calcination that results in the formation of unwanted associate phases and also alters the fi nal stoichiometry of the superconductor. [ 12 ] It has been postulated that calcium may be present in the synthesis of Bi-2212 at elevated temperatures as calcium carbonate, which aids in the melting of SrCO 3 by forming a low melting point eutectic mixture, [ 11 ] which would ensure complete melting and subsequent availability of calcium and strontium ions for Bi-2212 growth, but this has never been conclusively determined.…”

Highly oriented LiFePO4 thin film electrodes via chemical solution deposition

“…The appropriate coating solution was achieved by the solvent-in-gel (SIG) processing route [31], which successfully resolved the issue of the insolubility of metal phosphates in methanol. The morphology, the crystal structure, and the electrochemical behaviour of the LiFePO 4 thin films prepared on two different substrates and with various precursor concentrations were studied.…”