Microstructure and phase development in yttrium barium(2) copper(3) oxygen(7) superconducting films prepared from carboxylate precursors
Chu, Peir-Yung
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https://hdl.handle.net/2142/23316
Description
Title
Microstructure and phase development in yttrium barium(2) copper(3) oxygen(7) superconducting films prepared from carboxylate precursors
Author(s)
Chu, Peir-Yung
Issue Date
1992
Doctoral Committee Chair(s)
Buchanan, Relva C.
Department of Study
Engineering, Materials Science
Discipline
Engineering, Materials Science
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Engineering, Materials Science
Language
eng
Abstract
Superconducting $\rm YBa\sb2Cu\sb3O\sb{7-x}$ thin films, 0.1-1.5 $\mu$m thick, were prepared from carboxylate precursors by reactive liquid phase sintering. Processing effects on phase development, microstructure evolution, and important film properties, such as transition temperature, critical current density, and weak-link behavior, were investigated. The precursor trimethylacetate salts were dissolved in propionic acid/amylamine mixed solvent system, resulting in stable solutions suitable for coatings. Thin films were deposited onto LaAlO$\sb3$ single crystal substrates by spin-coating and were fired at 740-950$\sp\circ$C under O$\sb2$, N$\sb2$, or mixed $\rm N\sb2/O\sb2$ atmosphere.
The phase formation map for BaO-CuO binary compound was constructed for the films processed in flowing N$\sb2$ atmosphere, showing liquid phase formation at 640$\sp\circ$C. Molecular level mixing in the precursor solution and processing under flowing N$\sb2$ were the two contributing factors for this low temperature liquid phase formation. The use of N$\sb2$ atmosphere also facilitated the decomposition of BaCO$\sb3$ and the formation of the 123 phase. A two-step firing program was developed to optimize the processing based on the reactive liquid phase sintering. The films were first fired at 800$\sp\circ$C/40 min in N$\sb2$ for crystallization and grain growth, and then at 920$\sp\circ$C/10 min in a low P$\sb{{\rm O}\sb2}$ atmosphere (P$\sb{{\rm N}\sb2}$/P$\sb{{\rm O}\sb2}$ = 100) for densification and orientation by capillary forces. Based on the experimental results and observations, reaction mechanisms and densification models were developed for the reactive liquid phase sintering process.
The films prepared by reactive liquid phase sintering showed essential epitaxial growth and excellent preferred orientation with T$\sb{\rm c(zero)}$ = 87.8 K and $\rm J\sb{c}\ > 10\sp5$ A/cm$\sp2$ at 77 K. Due to capillary action, any residual liquid phase or second phase materials were extruded from grains and located on the film surface, resulting in clean grain boundaries. SIMS analysis indicated uniform cation concentration profile throughout the films and insignificant film/substrate interdiffusions.
Major findings in this work include: (1) successful development in the solution chemistry for thin film fabrication based on trimethylacetate precursors, (2) construction of phase formation map, showing low temperature liquid phase formation in N$\sb2$ during the processing of the 123 films, and (3) development of a new operative reactive liquid phase sintering mechanism to prepare high quality 123 thin films in controlled atmosphere.
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