Fabrication of High-Quality Ion-Beam Deposited Cubic Oxide Template Films on Meter-Length Substrates
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ABSTRACT Ion-beam assisted deposition was used to fabricate biaxially aligned cubic zirconia on flexible metal substrates. These films were used as templates for heteroepitaxially deposited YBa 2Cu 307-8 (YBCO) films. The quality of the crystalline texture of the template films has a direct influence on the superconducting properties of the final YBCO films. We describe our efforts to fabricate high-quality cubic zirconia templates on short-length substrates processed in stationary mode and meter-long substrates processed in continuous mode. Our best phi scan FWHM values for the films on the meter-long tapes were 12.6 degrees for the template and 6.1 degrees for the overcoated YBCO. This meter-long tape had a self-field, 75 K, superconducting critical current of 122 amps. INTRODUCTION Coated conductors of YBCO on flexible metal substrates are of interest because of their ability to achieve critical current densities (Jc) of I MA/cm 2 on meter-length tapes at liquid nitrogen temperature [1]. Achieving such high performance levels requires the YBCO films to be grown by heteroepitaxial deposition techniques on highly textured template layers. One method employed to fabricate such template layers is ion-beam assisted deposition (IBAD) of biaxially aligned yttria-stabilized zirconia (YSZ). Initially, this method was used for fabrication of templates on stationary, small-area substrates [2,3,4]. The ability to deposit template films on meter-length substrates requires that the substrates be moving through the deposition zone. Also, a linear ion gun is employed to expand the template film deposition area and to shorten the time necessary to grow the template YSZ films on the meter-length tapes [5]. Further optimization of substrate surface finish, substrate shielding, and assist gun and vapor fluence deposition parameters are necessary to fabricate the highest quality template films. This work delineates how these latter parameters have been optimized on stationary tapes, prior to high-quality template film depositions on moving tapes.
EXPERIMENT The substrates used in this work were nickel-based alloys of Inconel 625 or Hastelloy C276. The substrates were 1 cm wide and either 50 or 100 gim thick. Before processing, the substrates were spot welded into 1.1 m long loops. These loops were polished using a rotating buffing wheel loaded with polishing compound. The surface finish was characterized by scanning force microscopy. After polishing, the tapes were ultrasonically cleaned with soap and water, rinsed with deionized water, and blow-dried with dry nitrogen gas. The average surface roughness was 2-5 nm, for scan lengths of 1 gim. Prior to mounting in the deposition system, the loops were baked for 10 minutes, in vacuum, at 900 TC. 67
Mat. Res. Soc. Symp. Proc. Vol. 585 © 2000 Materials Research Society
Two tape loops were mounted side-by-side on a mechanical drive in the MBAD YSZ vacuum deposition system. The loops were kept taut between a drive wheel and spring tensioned idler wheels. In continuous coating mode, the
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