Date of Award
Doctor of Philosophy
Materials Science and Engineering
Philip D. Rack
Anthony J. Pedraza, Thomas T. Meek, James R. Thompson, David K. Christen
BaF2-based precursors were deposited on various substrates by electron beam co- evaporation and trifluoroactates-involved metal organic deposition (TFA-MOD) for fabricating biaxially-textured YBa2Cu3O7-δ (YBCO) films. A low-pressure processing system was established for the conversion of the precursors. High critical current densities near 4 MA/cm2 (77 K, self field) were obtained for the first time through the ex-situ conversion under the total pressures as low as 10 mTorr. The viability of the low- pressure conversion may enable a large-scale, cost-effective batch production of YBCO- based high temperature superconductor (HTS) wires.
The effects of the processing conditions on the film structure and properties were studied. It was found that the structure of YBCO films is very sensitive to the processing parameters. The optimal processing window for making high Jc films from the as- deposited e-beam co-evaporated precursors was relatively small and the conversion rate was limited to less than 1 Å/sec.
The e-beam co-evaporated precursor and the TFA-MOD precursor were compared in terms of the precursor characteristics and the conversion behavior. It was found that the precursor crystallanity and chemistry have strong effect on conversion. It was demonstrated that an intermediate heat treatment could drastically change the precursor conversion. The modified e-beam co-evaporated precursor can be converted at an effective conversion rate of 12 Å/sec and in a much wider processing window. The result suggested that the precursor structure and chemistry can be further tailored to facilitate fast conversion and high Jc film fabrication.
Conversion of fluorine-free precursors made by pulsed laser deposition (PLD) from an YBCO target was investigated to probe an alternative ex-situ route for fabricating biaxially-textured YBCO films. Epitaxial YBCO film with critical temperature Tc of 88 K and critical current density Jc (77 K, self field) of ~ 1 MA/cm2 was obtained. Nearly perfect c-axis YBCO structure exposed by scanning transmission electron microscopy (STEM) suggested that oxygen deficiency and a lack of defects for flux pinning were the main reasons for the relatively lower Tc and Jc. Therefore, appropriate defects need to be introduced into the films.
Zhang, Yifei, "Ex-situ Epitaxial YBa2Cu3O7-δ Films: Precursor Conversion and Film Characterization. " PhD diss., University of Tennessee, 2006.