Critical current versus strain research at the University of Twente

Abstract

At the University of Twente a U-shaped spring has been used to investigate the mechanical properties of a large variety of superconducting tapes and wires. Several mechanisms are responsible for the degradation of critical current as a function of applied strain. A change in its intrinsic parameters causes a reversible critical current dependence in Nb 3 Sn. The critical current reaches a maximum at a wire-dependent tensile strain level, and decreases when this tensile strain is either released or further increased. In Bi-based tapes the critical current is virtually insensitive to tensile strain up to a sample-dependent irreversible strain limit. When this limit is exceeded, the critical current decreases steeply and irreversibly. This behaviour is attributed to microstructural damage to the filaments. This cracking of the filaments is verified by a magneto-optical strain experiment. Recent experiments suggest that in MgB 2 the degradation of critical current is caused by a change in intrinsic properties and damage to the microstructure. Magneto-optical imaging can be used to investigate the influence of applied strain on the microstructure of MgB 2 , as is done successfully with Bi-based tapes. In all these conductors the thermal precompression of the filaments plays an important role. In Nb 3 Sn it determines the position of the maximum and in Bi-based and MgB 2 conductors it is closely related to the irreversible strain limit.

Distribution of normalized critical current of bent multifilamentary Bi2223 composite tape

“…[3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] Under no applied strain, the critical current is different from specimen to specimen and from position to position within a specimen. [18][19][20][21][22][23][24] Under applied strain, the damage evolution behavior is also different from specimen to specimen and from position to position within a specimen, resulting in wider distribution of critical current.…”

Residual and fracture strains of Bi2223 filaments and their relation to critical current under applied bending and tensile strains in Bi2223∕Ag∕Ag alloy composite superconductor

“…1-27 For description of damage behavior of Bi2223 filaments that transport superconducting current, two important parameters have been mentioned. 4,13,14,16,19,21,27 One is the intrinsic fracture strain Bi,f and another is the residual strain Bi,r . If the Bi2223 filaments are tested alone, they fracture at the intrinsic fracture strain Bi,f .…”

A direct tensile device to investigate the critical current properties in superconducting tapes

“…[4][5][6][7][8][9] So far, one of the most challenging is to protect the conductor and magnet from I c degradation with a high electromagnetic force caused by high magnetic field and the transport of current. 10 For the low critical temperature superconductors, several instruments such as U-spring, Pacman, C-ring, Walters spring, and their improvabilities [11][12][13][14] have been reported to investigate the behavior of I c in superconducting strands (e.g., Nb 3 Sn, NbTi) as a function of magnetic field, temperature, and strain. In the instruments mentioned above, the short length of specimen (U-spring) and/or complicated structure (spiraled shape) are not suitable for the study of superconducting tapes.…”