Bending of Bi(2223) - Ag tapes at 77 and 300 K

Abstract

An experimental study of Bi( 2223)-Ag tapes undergoing bending and straightening at 300 and 77 K has been performed. The special test equipment allows us to measure changes of the critical current in a short element of the tape during continual decreasing or increasing of the bending diameter at both temperatures mentioned above. Comparison of 300 and 77 K results shows the effect of the Ag sheath strength on the bending sensitivity of Bi( 2223)-Ag composite tape. Remarkable lowering of critical current degradation by bending at 77 K was observed. This offers the possibility of winding small-diameter coils with Bi(2223)-Ag tape pre-cooled by liquid nitrogen.

Differences in applied axial strain andIcdegradation of Bi(2223)/Ag tapes

“…The pre-strain can reach considerable values. The thermal pre-compression in the Bi-2223 filaments of Ag sheathed tape can be then evaluated as [6,18]:…”

“…The difference in the thermal contraction is ε = 2.2 × 10 −3 and E Ag = 77 GPa. Different values of E Bi have been used (E Bi = 10 GPa [6], E Bi = 35 GPa [18]) for the calculation of the thermal compression. A Young's moduli study by Oduleye et al [22] has shown that E Bi depends exponentially on the porosity of Bi-2223 filaments.…”

“…They may be subjected to strains resulting from different thermal contractions of the tape's elements, from tension and bending during the winding, and finally from electromagnetic forces acting in the coils. Therefore several authors have tried to study the influence of the mechanical strain (mainly bending and tension) on the transport properties of Bi(2223)/Ag [1][2][3][4][5][6][7][8][9][10][11][12][13][14]. The results of all the studies show that during bending or tensile tests, the critical current decrease is rather small, up to so-called irreversible strain.…”

“…The level of the irreversible strain varys with the number of filaments [1,3,9,15], the mechanical strength of the Ag (Ag-alloy) sheath [11,12] and the filaments quality (density) [16]. The possibility of critical current recovery in Ag-sheathed Bi(2223) tapes after bending and straightening has been explained by a gliding of Bi(2223) grains caused by compression of the Ag sheath when cooling down to 77 K [6]. Recovery of the I c when the strain was relieved was also observed [10].…”

The effects of hydrostatic pressure on the BSCCO compound for the OPIT procedure

“…It is so, because during wire-drawing deformation penetrates the material from the circumference towards the axis (small reduction per pass (RPP), < 10%, higher strains closer to the circumference), whereas during HE the deformation zone covers the full cross section, causing higher strains closer to the axis (high RPP, > 95%). It has been shown that, for wire-drawing and rolling, despite the deformation-sintering schedule the resulting Vickers hardness of about 160 is the maximum available [7,11,15,26,27]. This value reflects the ceramic core density of about 5.0 g cm −3 , namely about 80% of the theoretical density.…”