Mechanical and superconducting properties of PIT-processed MgB₂wire after heat treatment

Abstract

The mechanical and superconducting properties of MgB 2 wires heat treated at various conditions have been investigated. Superconducting MgB 2 wires, fabricated by the powder-in-tube (PIT) method, show a transition temperature T c at 32.4 K for the as-rolled wire as shown by dc-resistivity measurement. It is found that the T c is improved (37.3-38.4 K) by subsequent Ar annealing treatment under various sintering temperatures and times. Remarkably, the annealed samples show significant improvement in the overall quality of the samples. On annealing at 1323 K for 0.6 ks, we obtained the best quality sample with a J c of 470 A mm −2 at 2.0 T. In order to find out the irreversible field (B irr ), the technique of the Kramer plot was applied. The B irr obtained was 7.6 and 9.6 T, for the as-rolled as well as the annealed samples at 1073 K for 3.6 ks, respectively. The mechanical properties were measured under tensile load. The yield stress was 450 MPa for the as-rolled sample and decreased by annealing at 1323 K for 0.6 ks to 110 MPa. After the wires were heat treated at temperatures between 1073 and 1323 K and tensile-stressed up to their yield stress at room temperature. The critical current measurements were carried on such samples at 4.2 K under applied magnetic fields of 2 and 5 T. The stress level for the 95% tolerance of critical current was determined to 97 MPa.

Flux pinning force in bulkMgB2with variable grain size

“…The peak of the f p curves takes place at b peak ϳ 0.2 for all samples, which seems to be intrinsic to bulk MgB 2 . This position of the peak has been observed in MgB 2 in many other works [13][14][15][16][17] and also in some LTSs, such as the A-15 Nb 3 Sn ͑Refs. 18-20͒ and the Chevrel-phase PbMo 6 S 8 , 21 characterized by F p ϰ b 1/2 ͑1−b͒ 2 .…”

“…20,21 The values of m = 1.6-2.1 have also been obtained in MgB 2 superconductor by another group. 14 The flux-line shear mechanism has been studied in detail and several authors proposed the expressions for F p based on the flux-lattice shearing. Kramer derived an expression for the pinning force, F p ϰ b 1/2 ͑1−b͒ 2 , assuming that the flux motion occurs by synchronous shear of the flux lattice around the line pins that are too strong to allow local depinning of the magnetic flux: Although the predicted function F p ϰ b 1/2 ͑1−b͒ 2 is observed in many superconductors, this formula has two main drawbacks, since it predicts a weak dependence on the grain size, which is in disagreement with experimental observations, 5,18,19 and it uses an expression for c 66 , which is only valid at high fields, 1 − b Ӷ 1.…”

In situinvestigations of phase transformations in Fe-sheathed MgB2wires

“…It has been widely reported that a reaction layer forms during heat treatment at the Fe/MgB 2 interface in wires and tapes prepared both by the in situ [36][37][38][39] and the ex situ [40][41][42][43][44][45] preparation routes.…”

Modelling of Critical Current Density of Sintered Ag Added Bulk MgB2

“…In addition, the critical current density for the silver added samples are improved as compared to the silver free sample, where the sample added 5 wt% of silver showed highest critical current density 40 kA/cm 2 at 20 K. These values are very low as compared to the present results which clearly indicates that both optimum silver content and optimum sintering temperature are crucial to get high performance bulk MgB 2 material. Similarly, improvements had reported that Ag doping significantly enhances MgB 2 formation rate in MgB 2 wires with enhanced properties [22]. On the other hand, these results indicate that Ag addition to MgB 2 can be one of the ways to improve not only mechanical properties [17,[21][22][23][24] but also flux pinning; the critical temperature remains almost unchanged.…”

“…Similarly, improvements had reported that Ag doping significantly enhances MgB 2 formation rate in MgB 2 wires with enhanced properties [22]. On the other hand, these results indicate that Ag addition to MgB 2 can be one of the ways to improve not only mechanical properties [17,[21][22][23][24] but also flux pinning; the critical temperature remains almost unchanged. It is also positive that Ag does not react with MgB 2 and can help to reduce cracks density like in cuprates [25].…”