Formation of High-Field Pinning Centers in Superconducting MgB2 Wires by Using High Hot Isostatic Pressure Process

D. Gajda,T. Cetner,İ. Belenli,F. Karaboğa,A. Morawski,M. Akdoğan,A. J. Zaleski,H. Yetiş

doi:10.1007/s10948-017-4161-y

Abstract

This paper demonstrates the effects of hot isostatic pressure (HIP) on the structure and transport critical parameters of in situ MgB2 wires without a barrier. Our results show that only HIP and nano-boron allow the formation of more high-field pinning centers, which lead to the increase in critical current density (Jc) at high applied magnetic fields. Nano-boron and annealing at a low pressure increase the Jc in the low magnetic field. This indicates that nano-particles create more high-field pinning centers. In addition, the results show that nano-boron improves the connection between the grains. Scanning electron microscope results show that HIP increases the reaction rate between Mg and B, density, and homogeneity of the MgB2 material. Additionally, HIP allows to create a structure with small grains and voids and eliminates the significance of the number of voids. High isostatic pressure allows to obtain high Jc of 10 A/mm2 (at 4.2 K) in 10 T and increases irreversible magnetic field (Birr) and upper critical field (Bc2). Measurements show that these wires have high critical temperature of 37 K.

Highlights

The MgB2 material has many advantages, such as having a simple and easy structure, high critical temperature, low resistivity in the normal state, high critical current density at low magnetic fields, low anisotropy, and low-cost components and being suitable for the powder-in-tube (PIT) method [1,2,3,4,5]
We show that annealing the MgB2 wires by using the hot isostatic pressure (HIP) process eliminates these problems and allows the formation of a greater number of high-field pinning centers in the MgB2 wires with the contribution of the use of nano-sized boron
This indicates that HIP accelerates the rate of reaction in the MgB2 material and leads to the formation of more superconducting phases

Summary

Introduction

The MgB2 material has many advantages, such as having a simple and easy structure, high critical temperature, low resistivity in the normal state, high critical current density at low magnetic fields, low anisotropy, and low-cost components and being suitable for the powder-in-tube (PIT) method [1,2,3,4,5]. Pure Mg has several advantages such as high ductility and malleability These factors allow to form the structure of an unreacted MgB2 wire by cold working, e.g., cold drawing [10], cold rolling [10], mechanical alloying [11], and cold isostatic pressure [12]. Some chemical additives accelerate the reaction rate (e.g., Sn [17], because it has a lower melting point) This leads to an increase in the number of superconducting phases, the formation of small MgB2 grains, and the improvement in grain boundary pinning. We show that annealing the MgB2 wires by using the HIP process eliminates these problems and allows the formation of a greater number of high-field pinning centers in the MgB2 wires with the contribution of the use of nano-sized boron

Experimental Details

Results and Discussion

Conclusions