Speeding up Film Deposition Rate: Its Effects on Microstructures of YBa 2 Cu 3 O y Superconducting Thick Films
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Speeding up film deposition rate: Its effects on microstructures of YBa2 Cu3 Oy superconducting thick films X. F. Zhang,a) H. H. Kung, S. R. Foltyn, Q. X. Jia, E. J. Peterson, and D. E. Peterson Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (Received 29 June 1998; accepted 24 September 1998)
˚ Two very different pulsed laser deposition rates, 192 and 6 Ays, were used to produce 1 mm thick superconducting YBa2 Cu3 Ox (YBCO) films on (001) SrTiO3 single-crystal substrates at 790 ±C. Transmission electron microscopy (TEM) was used to characterize and compare microstructures between the two films. It has been found that the high deposition rate led to a slight deviation from the expected epitaxial orientations, and extra stress was induced in the films by increased lattice mismatch between the films and the substrates. In addition, misoriented YBCO grains were formed in the high-rate films after a thickness of about 150 nm. Postannealing in oxygen had no visible influence on these defects, although superconducting properties were improved significantly. In contrast to the high-rate films, overall epitaxial orientations have been formed in the low-rate films, and no misoriented YBCO grains were found. However, variations in lattice parameters and columnar voids were observed, although their existence apparently does not have considerable influence on superconducting current density sJc d. Cation disorder was observed in both films. A two-step film growth mechanism is concluded which is responsible for the formation of some defects in the high-deposition rate films.
I. INTRODUCTION
Many potential applications of high-transition temperature sTc d superconductors such as power transmission lines, magnetic separators, electric motors, transformers, etc. require long wires which must be flexible, capable of high supercurrent, and affordable. The difficulty in producing such wires arises from the fact that high-Tc superconductors are inherently brittle ceramics. The polycrystalline nature of bulk high-Tc superconducting materials leads to a large number of high angle (.10±) grain boundaries, inducing intergranular weak links which can lower supercurrent considerably.1–3 An early effort to produce high-Tc superconducting wires is a process called oxide-powder-in-tube (OPIT), which has been used to fabricate high quality Bi–Sr–Ca–Cu– O (BSCCO) wires.4–6 Kilometer long BSCCO/OPIT tapes are now routinely produced with Jc values on the order of 104 Aycm2 . However, although BSCCO/OPIT wires have an advantage in self-grain alignment during processing, the wires lose supercurrentcarrying ability dramatically in magnetic fields, and therefore these first generation wires are not useful at liquid nitrogen temperature in many applications.
a)
Address all correspondence to this author. Current address: Materials Science Division, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, Berkeley, California 94720.
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J. Mater.
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