Reactive Co-Evaporation of YBCO for 2G HTS Tapes
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1001-M14-02
Reactive Co-Evaporation of YBCO for 2G HTS Tapes Jonathan Storer, Jens H‰nisch, Chris Sheehan, and Vladimir Matias Superconductivity Technology Center, Los Alamos National Laboratory, TA-3 4200 drop point 01U, Los Alamos, NM, 87545 ABSTRACT We present a new reel-to-reel method for growth of high temperature superconducting (HTS) films by reactive co-evaporation on flexible metal tapes. We have demonstrated proof of principle for this method with a small laboratory-scale setup using 8 cm long tape pieces. YBa2Cu3O7-δ is deposited on ion-beam assisted deposition textured MgO layers on top of flexible polycrystalline metal tapes. Critical current densities at 75.5 K of over 2 MA/cm2 have been achieved in HTS films with over 2 µm in thickness, yielding a self field critical current of 450 A/cm-width. A 4.5 µm thick film had a self field critical current of 590 A/cm. We discuss some practical possibilities for manufacturing of superconducting wire using this process and present new areas of research that are still needed. INTRODUCTION Among a number of possible thin-film deposition processes, reactive coevaporation has been used extensively in the past for deposition of RBa2Cu3O7 films [1]. A particularly attractive evaporation technique utilizing a heater with an oxygen pocket has been applied successfully to deposition on large area substrates [2]. This technique benefits from a black-body type heater, which keeps the substrates at uniform temperature in an oxygen atmosphere, and a low oxygen pressure where the evaporant sources are located. The technique has been extended to long metal tapes by several groups [3, 4]. We suggest a new reel-to-reel mechanism that could benefit from such a process to produce long samples of high-Jc superconducting tapes. We also present preliminary results using this process to produce short coated conductor tapes. EXPERIMENT Apparatus We use elemental sources of yttrium, copper, and barium. The former two are heated using a differentially-pumped Pierce-type electron gun which is skipped between them and the latter is conveniently heated using 400 kHz RF induction. Atomic absorption is used to measure the vapor densities just beneath the tapes. These data are used in real time to adjust the RF power, as well as the electron beam scan patterns and residence times. Atomic absorption permits the accurate control of the film composition during extended operation at high rate as there are no quartz crystal parts which are consumed. The chemical composition of our films is about 5 atomic % Ba-deficient compared to 1:2:3 cation stoichiometry.
As shown Fig. 1, our samples alternately move from a low pressure region where they receive a deposit from the sources to a high pressure region where they are oxidized and thermodynamically stable. The stability region is shown in Fig. 1 as the gray region, which has been determined by Lindemer et al [5]. A rotation speed of 5 Hz results in a coating time of 20 ms and an oxidation time of 180 ms. The ratio of coating time to oxidation time
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