Abstract
In the present study, a novel choice of sheath materials for drawing long superconducting MgB2 wire by using the powder-in-tube technique (PIT) is reported. This would eliminate the need for an intermediate strain-relieving annealing process and would reduce the time and cost of fabrication. Our strategy involved the use of a composite sheath instead of a sheath made of a single material. The relatively inert Fe constituted the inner sheath around the MgB2 powder while the Cu—which is capable of efficient heat dissipation—was used as the outer sheath. Important mechanical properties of the wire such as elastic modulus, ultimate tensile strength, yield strength, hardness, and microstructure were carefully studied at different stages of the drawing process using tensile and microhardness tests. To clearly delineate the effect of Cu cladding on the ductile behavior of the iron sheath, another MgB2 wire with only an Fe sheath was prepared; its mechanical properties were measured and compared with those of the composite Cu–Fe-sheathed MgB2 wire. After a few drawing steps, the composite Cu–Fe-sheathed wire showed a lower elastic modulus and tensile strength than those of its Fe sheath counterpart. While both types of wires showed an increase in hardness as the drawing process progressed, the composite-sheath wire consistently showed a lower hardness than that of its counterpart, implying its lower susceptibility to fracture; it can therefore be safely drawn to small diameters without the need for intermediate annealing during the wire drawing process.
Highlights
The discovery of superconductivity in MgB2 in 2001 has opened new avenues of research and application of this material due to its relatively high TC, light weight, the low cost of its constituent materials, and the absence of weak links [1,2,3,4,5,6,7]
We report the mechanical properties of the sheath as functions of diameter which depends upon the number of drawing steps, i.e., the amount of cold work done on reduction, which depends upon the number of drawing steps, i.e., the amount of cold work the material
In order to examine the suitability of our technique of using such a composite sheath, we carefully carried out tests of tensile strength, microhardness and microstructure and evaluated key parameters of mechanical properties such as elastic modulus, yield strength, ultimate tensile strength and hardness
Summary
The discovery of superconductivity in MgB2 in 2001 has opened new avenues of research and application of this material due to its relatively high TC , light weight, the low cost of its constituent materials, and the absence of weak links [1,2,3,4,5,6,7]. Copper, which has a much higher thermal conductivity (401 W/(m·K)) than that of iron (80.4 W/(m·K)) [14], can efficiently dissipate heat if used as a sheath metal This aspect of copper as an ideal sheath material is severely compromised due its high reactivity with the MgB2 superconducting powder [15]. In light of the above, we set out to develop a novel cost-effective wire processing technique for the reduction of size to as low as 1 mm without breaking which does not require intermediate strain-relief annealing To this end, we devised a composite sheath by covering the inner iron sheath around the powder with a copper cladding. Even during an actual operation, MgB2 wire with a composite copper/iron sheath will be very effective in dissipating heat
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