YBCO Encapsulation By Diamond Like Carbon Films Deposited By Laser Ablation Technique
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YBCO ENCAPSULATION BY DIAMOND LIKE CARBON FILMS DEPOSITED BY LASER ABLATION TECHNIQUE.
L. GANAPATHI, S. GILES AND RAMA RAO Excel Superconductor, Inc., 140-29, Keyland Ct. Bohemia, NY-11716.
ABSTRACT We have investigated the deposition and superconducting properties of YBCO thin films on single crystal LaA10 3, MgO and polycrystalline alumina substrates followed by the DLC (diamond like carbon) encapsulation [1]. DLC films were deposited at pressures ranging from high vacuum to 100 mT He. Substrate temperature was varied from 21oC to greater than 100oC. The process compatibility of Laser ablation technique for depositing both the materials was found convenient to sequentially deposit YBCO and DLC films from high purity stoichiometric targets. Epitaxial YBCO films on (100) LaA1O 3 , (Tc-zero = 89K) and (100) MgO (Tczero = 85K) substrates showed identical superconducting transitions before and after encapsulation by a DLC layer. The encapsulated films showed no degradation due to acid treatment or aging over a period of 45 days. Similar results were obtained for YBCO films on polycrystalline alumina substrate. A buffer layer of YSZ was essential to obtain good superconducting properties on alumina substrate. A 200 nm thick YSZ provided the barrier necessary6 for minimizing the interface reaction between A120 3 and YBCO. Typically a Tce-8 K was obtained for these films. These films were textured with c-axis perpendicular to the substrate surface. Likewise, there was no degradation of superconducting properties of these films after deposition of the DLC films or chemical attack by dilute HNO 3 . INTRODUCTION
Degradation of high T, superconducting films on exposure to atmospheric water vapor [2-5] has been one of the continuing concerns limiting the effective use of these materials. Recently, optimization of deposition conditions on epitaxially matched, well characterized substrates have rendered the films somewhat resistant to degradation. However, this does not guarantee protection against acids even at high dilutions and other degrading agents. Mechanical wear and tear is another issue which has to be addressed. In view of these considerations, passivation of the superconducting surface against environmental degradation is mandatory for any practical exploitation of these materials for device fabrication. Current studies indicate several approaches that may lead to successful passivation of these surfaces. Chin-An Chang et al [6,7] observed substantial differences in the degradation characteristics of films with and without silver incorporated into the high Tc material. Composite material films were grown by sandwiching layers of the starting materials for the superconductor and silver. The electron beam deposited sandwich is then processed to obtain the superconducting films. Critical current density at 77K, Tc-•ero, contact resistance at low temperatures, and room temperature resistance were monitored to study effectiveness of passivation. Silver was found to distribute itself on the surface and along grain bound
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