Nucleation and growth of YBa 2 Cu 3 O x on SrTiO 3 and CeO 2 by a BaF 2 postdeposition reaction process
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The nucleation and growth of the c-axis-aligned YBa2Cu3Ox on SrTiO3 and CeO2, from precursor films, were studied by examining quenched and fully processed specimens using transmission electron microscopy techniques. The precursor films, a stoichiometric mixture of fine-grained Y, Cu, and BaF2, were deposited using physical vapor deposition methods. An Y–Ba oxy-fluoride formed from the precursor contributed to the nucleation of YBa2Cu3Ox, while a liquid layer between the unreacted precursor and the YBa2Cu3Ox layer played an important role in the growth of YBa2Cu3Ox. However, the process of nucleation of YBa2Cu3Ox on SrTiO3 and CeO2 were significantly different. I. INTRODUCTION
Significant progress in conductor development with high critical temperature superconductors has been made over the last few years. Currently, composite tapes of Bi cuprates, Bi2Sr2CaCu2O8 and Bi2Sr2Ca2Cu3O10, in a Ag matrix are commercially manufactured in kilometer lengths. Such tapes, carrying 100 A or more in self-fields at 77 K,1 have been used to construct large models for electrical power devices. Unfortunately, the practical applications of these tapes in magnetic fields at temperatures near that of liquid nitrogen are very limited due to highly anisotropic electronic properties of the compounds. In the case of YBa2Cu3O7 (YBCO) excellent magnetic field properties have been achieved in melt textured bulk specimens and in thin films. However, it is a great challenge to fabricate long lengths of high current YBCO. Misorientation angles between adjacent grains greater than a few degrees result in low critical current densities.2 However, a few years ago, it was found that one could grow a biaxially aligned YBCO layer on a metallic tape if an appropriate biaxially aligned oxide buffer layer was deposited on the metallic substrate.3 This was accomplished by utilizing an ion-beam-assisted deposition technique for the buffer layer, followed by a pulsed laser deposition technique for YBCO. In response to this achievement, an extensive effort was initiated world-wide for the development of thick YBCO films on a metallic substrate. Presently, meter lengths of YBCO tapes made using this process can sustain critical currents of >100 A (10 kA/mm2) in self-field at 75 K.4 Although this is a very encouraging result, it is considered to be only a proof-of-principle test. This method, particularly the deposition of a thick (> 0.5 m) yttria-stabilized ZrO2, J. Mater. Res., Vol. 16, No. 10, Oct 2001
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YSZ, layer for a textured buffer layer, is very time consuming and is not likely to be adapted for commercialscale production of kilometer length tapes. Thus, alternative methods for fabrication of textured buffer layers on a metallic substrate have been developed. In one of these methods, a metallic tape, such as Ni with a (100) texture, is prepared. Then a multilayered diffusion barrier, e.g., CeO2/Y–ZrO2/CeO2, is epitaxially deposited prior to the application of the YBCO layer.5 It has been shown th
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