Effect of Heat Treatments under High Isostatic Pressure on the Transport Critical Current Density at 4.2 K and 20 K in Doped and Undoped MgB2 Wires.

Daniel Gajda,Matt Rindfleisch,Konstantin Nenkov,Md Shahriar A Hossain,Andrzej J Zaleski,Malgorzata Małecka,Tomasz Czujko,Andrzej J Morawski

doi:10.3390/ma14185152

Abstract

Annealing undoped MgB2 wires under high isostatic pressure (HIP) increases transport critical current density (Jtc) by 10% at 4.2 K in range magnetic fields from 4 T to 12 T and significantly increases Jtc by 25% in range magnetic fields from 2 T to 4 T and does not increase Jtc above 4 T at 20 K. Further research shows that a large amount of 10% SiC admixture and thermal treatment under a high isostatic pressure of 1 GPa significantly increases the Jtc by 40% at 4.2 K in magnetic fields above 6 T and reduces Jtc by one order at 20 K in MgB2 wires. Additionally, our research showed that heat treatment under high isostatic pressure is more evident in wires with smaller diameters, as it greatly increases the density of MgB2 material and the number of connections between grains compared to MgB2 wires with larger diameters, but only during the Mg solid-state reaction. In addition, our study indicates that smaller wire diameters and high isostatic pressure do not lead to a higher density of MgB2 material and more connections between grains during the liquid-state Mg reaction.

Highlights

MgB2 superconductors have many advantages in that they are cheap components [1] and have a low specific weight [2], low anisotropy [3], high critical temperature [1], low resistivity in the normal state [4], and high Bc2 [5]
The results showed that annealing under high isostatic pressure (HIP) significantly improved the structure of the MgB2 material in wires made by using the powder in tube (PIT) method [34,35,36,37,38,39]
Microstructure of Undoped MgB2 Wires The results in Figure 1a,b show that thermal treatment at 700 ◦C under a low isostatic pressure of 0.1 MPa leads to the presence of a large number of large voids with a non-uniform distribution and to a decrease in the MgB2 material density in superconducting wires

Summary

Introduction

MgB2 superconductors have many advantages in that they are cheap components [1] and have a low specific weight [2], low anisotropy [3], high critical temperature [1], low resistivity in the normal state [4], and high Bc2 [5]. The analysis of the MgB2 structure shows that the volume decreases by 25% after a reaction [6] This in turn causes a reduction in the connections between grains and creates surface pinning centers that significantly decrease the critical current density (Jc) in middle and high magnetic fields [7,8]. A 2002, an investigation of SiC showed that the addition of nanometer-scale SiC can effectively increase Bc2 and transport the critical current density (Jct) at high temperatures and fields and decrease the anisotropy [11,13,18,19]. The results for the SiC-doped MgB2 material showed that a lower heating time improves Jc while slightly degrading Tc [27]

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Conclusion