The effect of strain and temperature on the critical current density of high-field superconductors

Abstract

A J(B,ϵ) probe has been designed and built to measure the critical current density, J(_c), versus strain on short superconducting wires and tapes. Measurements can be made from 2K to 4.2K in high magnetic fields in our 17T magnet system and at 77K for strains up to ±1.0%.The effect of strain on J(_c) has been measured on a 0.37mm diameter Nb(_3)Sn wire at 4.2K and 2.9K from l0T up to 14T. The compressive pre-strain of the wire measured at 4.2K is found to be about 0.3%. At 4.2K, the value of B(_c2) maximum is 19.6T and at 2.9K is 21.2T. The functional form of the flux pinning of the sample at 4.2K can be expressed as F(_p)=4.20±0.57xl0(^8)[B(_c2)(ϵ)](^1.67±0.7b(^1/2)(l-b)(^2); and for the sample at 2.9K it is F(_p)=1.37±0.05xl0(^10)[B(_c2)(ϵ)](^0.54±0.16b(^1/2)(l-b)(^2). From temperature scaling, the functional form is F(_p)=3.63 ±0.62xl0(^8)[B(_c2)(T)](^1.72±1.3)b(^1/2)(l-b)(^2). The difference in the values of n between strain scaling and temperature scaling shows that n only parameterises the change in J(_c) with change in strain or temperature. The effect of strain on J(_c) has been measured on five short sections of an Ag-sheathed BSCCO(2223) tape at 4.2K and in magnetic fields up to 12T. J(_c) degradation starts to occur between 0.18%-0.25% strain. The appearance of secondary peaks in the critical current distribution is attributed to inter-grain J(_c) and infra-grain J(_c). J(_c) measurements have been made on bulk PbMo(_6)S(_8)(PMS) samples. J(_c) increased by a factor of «7 when the sample was hot isostatically pressed(HIP). Measurements have also been made on series of HIP'ed gadolinium(Gd) doped Pbi(_1-x)Gd(_x)Mo(_6)S(_8) samples. The highest T(_c) value is 14.55K when x=0.1. The highest J(_c) is when x=0.2. The irreversibility field B(_irr)(0) from J(_c) measurements has been calculated to be 53.9T, 45.5T, 34.IT and 33.5T for the x=0, 0.1,0.2 and 0.3 samples, respectively.