YBCO-Coated Conductors Manufactured by High-Rate Pulsed Laser Deposition
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YBCO-Coated
Conductors Manufactured by High-Rate Pulsed Laser Deposition
Alexander Usoskin and Herbert C. Freyhardt Abstract High-temperature superconductors of the second generation—coated conductors— are based on an architecture of YBCO films deposited on a well-textured substrate tape. The deposition technique used in the processing of YBCO films is responsible not only for both the resulting critical currents in the conductors and the cost efficiency of the employed production route, but also for the ultimate viability of the chosen technology. This article describes an advanced deposition method for YBCO films using high-rate pulsed laser deposition (HR-PLD). An elaborate variable azimuth ablation allows target roughening to be considerably reduced in the course of continuous deposition, and as a result, the integral deposition speed and speed stability can be increased to technologically interesting high values. Well-selected process parameters have been demonstrated to yield high currents of up to 480 A/cm-width in short tapes and 360 A/cm-width in 6-m-long tapes. Together with quasi-equilibrium heating, the HR-PLD method allows the processing of long-length YBCO-coated conductors and offers a cost-efficient route for their production on an industrial scale.
Introduction Significant progress has been achieved recently in the improvement of critical current (Ic) values of tapes coated with hightemperature superconductors (HTS),1–6 especially those manufactured by means of high-rate pulsed laser deposition (HR-PLD).3 The PLD technique based on excimer lasers has been an excellent tool for manufacturing high-temperature superconductor (HTS) films with the highest critical current density (Jc) on short substrates (1–10 cm) with a low-volume deposition speed of 2 nm m2/h (i.e., a 2-nm-thick film deposited on 1 m2 unit area per hour). For large-scale production, speeds of 50 nm m2/h are needed, and therefore, this low deposition speed seemed to rule out PLD as an attractive candidate for scaled-up fabrication. Another of the general problems of conventional PLD, particularly connected to
MRS BULLETIN/AUGUST 2004
YBCO films, is that PLD must deposit a film 0.1 nm thick per deposition pulse in order to ensure good film stoichiometry. Because of the relatively small divergence of the plasma plume, only a small substrate area of several square centimeters is coated during a single deposition pulse. On the other hand, an increase in the pulse repetition rate f is limited by a drop of Jc , which is found to commence for f 10–20 Hz. This results in low-rate film processing with a deposition speed of only 0.5–2 nm m2/h, which implies a processing time of 5 h to produce a 1 m 10 mm tape coated with a 1-m-thick film. Therefore, the aim of HR-PLD is to enable both a substantial increase in the integral deposition speed (the time-averaged speed of film deposition on a macroscopic tape surface) and large-area deposition. In order to achieve this goal, one has to overcome a
number of specific problems, for examp
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