Effect of Thermal Cycling and Mechanical Deformation on the Superconducting Properties of Ag/Yba 2 Cu 3 O 7 -X Wires
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EFFECT OF THERMAL CYCLING AND MECHANICAL DEFORMATION ON THE SUPERCONDUCTING PROPERTIES OF Ag/YBazCusO7-x WIRES. M.A.
OCAMPO
,
E.
ORGAZ
AND T. AKACHI
"Centro de Investigaci6n y Desarrollo CONDUMEX, Blvd. Cervantes Saavedra 255, Mexico, D.F. 11520, M~xico "*Instituto de Investigaciones en Materiales, UNAM, Apdo Postal 70-360,
M~xico,
D.F.
04510,
Mexico
ABSTRACT Preforms made by filling silver tubes with YBa2Cu3O7-x (YBCO) powder were drawn to obtain wires with diameters in the 0.25mm to 1.00mm range. Straight samples of these wires were sintered and oxygenated. The observed changes in Tc and Jc while the samples were thermaly cycled or mechanically deformed are reported.
INTRODUCTION The interest for large scale applications of the high temperature ceramic superconductors led us to investigate several systems with wire or tape geometries. In particular, wires obtained by drawing silver tubes with YBCO powders had received some attention. This system has to support significant stress during the cooling down to liquid nitrogen temperature due to differential thermal contraction between the ceramic superconducting nucleus and the metallic dressing, and the superconducting properties can suffer eventual degradation. In this paper we describe the changes in the Tc and the Jc of such wires produced by thermally cycling them between room and liquid nitrogen temperatures, and those produced by mechanical deformation.
SAMPLES PREPARATION AND MEASUREMENTS Silver tubes of 4.35mm of inner diameter, wall thickness of 1.00mm, and 99.9% purity, were filled with YBCO powder, with a Tc of 92K when pelletized, sintered and oxygenated. The filled tubes were drawn to obtain wires with diameters in the 0.25mm to 1.00mm range. Straight samples, 10cm long, were cut, and some of them were introduced in 50% diluted nitric acid to reduce the silver wall. Samples were sintered at 900C in flowing oxygen at atmospheric pressure, and then oxygen annealed at 400C in the same environment. Resistance vs temperature curves for the samples were gotten using a closed cycle helium refrigerator to cool the Mat. Res. Soc. Symp. Proc. Vol. 169.
1990 Materials Research Society
1264
samples and their resistances were measured using a four lead configuration and-s low frecuency modulation technique with a were ohm. For the Jc determination samples sensitivity of 10 immersed in liquid nitrogen. The current-voltage characteristics were gotten using a programable current source and a voltmeter with 10 ?volt sensitivity. the samples were abruptly For thermal cycling studies, immersed in liquid nitrogen and warmed up in a dry environment, 1,2,4,8, Tc and Jc measurements were done after completion of and 16 cycles. Mechanical deformation studies were done by continually on cylinders with succesively smaller rolling the samples diameters between measurements. Since the deformation process cycling effect is was done on the same samples a thermal combined with this results.
RESULTS The main effect of thermal cycling in the R-T curves is the in the cu
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