2492 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 15, NO. 2, JUNE 2005 Effect of Combined Bending Strain and Thermal Cycling on the Voltage-Current Characteristic Curves of Bi-2223 Tapes Carlos Y. Shigue, Carlos A. Baldan, Ulisses R. Oliveira, Francisco J. H. Carvalho, Jr., and E. R. Filho Abstract—High critical temperature superconductors are evolving from a scientific research subject into large-scale ap- plication devices. In order to meet this development demand they must withstand high current capacity under mechanical loads arising from thermal contraction during cooling from room temperature down to operating temperature (usually 77 K) and due to the electromagnetic forces generated by the current and the induced magnetic field. Among the HTS materials, the compound imbedded in an Ag/AgMg sheath has shown the best results in terms of critical current at 77 K and tolerance against mechanical strain. Aiming to evaluate the influence of thermal stress induced by a number of thermal shock cycles we have evaluated the V-I characteristic curves of samples mounted onto semicircular holders with different curvature ra- dius (9.75 to 44.5 mm). The most deformed sample showed the largest reduction of critical current (40%) compared to the undeformed sample and the highest sensitivity to thermal stress . The V-I characteristic curves were also fitted by a potential curve displaying -exponents varying from 20 down to 10 between the initial and last thermal shock cycle. Index Terms—Bending strain, Bi-2223 tapes, critical current, degradation, thermal shock. I. INTRODUCTION S UPERCONDUCTING conductors based on the HTS (Bi-2223) compound imbedded in an Ag matrix have been developed in an industrial production scale in which a combination of small diameter filaments and the ductile metal matrix results in a conductor with low crack formation and high tolerance against strain [1], [2]. The influence of mechanical stress-strain on critical current has been the object of extensive studies in the literature [3]–[9]. By applying strains above the tensile fracture strain of the Bi-2223 filaments (0.1%), the critical current of the Bi-2223/Ag com- posite conductor is severely reduced [8]. Most of modern commercial conductors use an AgMg alloy instead of pure Ag as the matrix element in order to enhance the mechanical Manuscript received October 4, 2004. This work was supported in part by Companhia Paulista de Força e Luz under Contract 46000002059. C. Y. Shigue, U. R. Oliveira, and F. J. H. Carvalho Jr. are with the Department of Materials Engineering, Faculdade de Engenharia Química de Lorena—DEMAR/FAENQUIL, Caixa 12607-970, Lorena—SP, Brazil (e-mail: cyshigue@demar.faenquil.br). C. A. Baldan is with the DEMAR/FAENQUIL, Lorena—SP, Brazil, and also with Faculdade de Engenharia de Guaratinguetá—FEG/UNESP, Guaratinguetá—SP, Brazil (e-mail: cabaldan@demar.faenquil.br). E. R. Filho is with the Faculty of Electrical and Computing Engineering, Uni- versidade Estadual de Campinas—UNICAMP, Campinas—SP, Brazil (e-mail: ruppert@fee.unicamp.br). Digital Object Identifier 10.1109/TASC.2005.847497 TABLE I CHARACTERISTICS OF THE Bi-2223/Ag TAPE properties of the conductor aiming at increasing the tolerance against degradation caused by the filament strain. The mechanical test essay of tolerance of critical current against thermal cycling is of great interest in the long term relia- bility evaluation of superconducting power apparatuses because of the need of operation in temperatures ranging between 77 K and room temperature. Ochiai et al. [9] analyzed the influence of thermal cycling on the critical current of B(Pb)SCCO/Ag conductors based only on thermal stresses arising from thermal expansion caused by heating to room temperature. In this work we will study the influence of thermal cycling on voltage-cur- rent (V-I) characteristic curves of Bi-2223/Ag/AgMg composite samples initially subjected to bending strain. The analysis will be done in terms of evaluation of the critical current and the power law exponent of the V-I curve [10]. II. EXPERIMENTAL PROCEDURE The test was conducted on commercial Bi-2223 supercon- ducting tapes. The matrix is composed of pure Ag-sheath and AgMg alloy. Table I presents its main characteristics. Five sample holders with each applying different bending strain (Fig. 1) was calculated following (1). The calculated bending strain values are shown in Table II. In order to deter- mine the actual bending strain we have measured the strain in two samples by using a strain gage bonded on the tape surface. The measured value was within 95% of the calculated one. The bending strain is calculated from: (1) where is the sample thickness, and the bending radius. The V-I curves were measured by the four-point probe method using a programmable current source Lambda model EMI 30 V/333 A capacity and a Keithley DMM model 195A 1051-8223/$20.00 © 2005 IEEE