Evidence of Point Pinning Centers in Un-Doped Mgb2 Wires at 20 K after HIP Process

Gajda D,Morawski A

doi:10.4172/2169-0022.1000244

Abstract

In this paper we present results of transport critical current density (Jc) at 20 K and 4.2 K, irreversible magnetic field (Birr), upper critical field (Bc2), critical temperature (Tc), pinning force (Fp), scanning pinning force scaling results (Fp/Fpmax and B/Birr) and electron microscope (SEM) images of un-doped MgB2 wires of 0.63 mm diameter. All wires were annealed at pressures ranging from 0.1 MPa to 1 GPa for 15 min between 680°C to 740°C. SEM images show that 1 GPa pressure yields small grains, higher MgB2 material density, and small voids. The results obtained by a physical properties measurement system (PPMS) show that high pressure (1 GPa) and 700°C annealing slightly decreases Tc above 27 K and increases Tc and Birr below 25 K. Un-doped MgB2 wire annealed in 1 GPa for 15 min at 680°C at has a 20 K, 4.5 T Jc of 100 A/mm2 in and a Birr of 7 T. At 4.2 K, this wire has Jc of 100 A/mm2 at 10.5 T. Scaling results show that the dominant pinning mechanism is point pinning for undoped MgB2 wires under 1 GPa pressure and annealed at 680°C (at 20 K).

Highlights

MgB2 superconductors has many advantages, namely high critical temperature (39 K) [1], low resistivity, simple structure, low anisotropy and high critical field [2,3]
We show that the HIP process produces a dominant point pinning mechanism in un-doped MgB2 wires, leading to an increase in Jc and Fp at 20 K
These results show that low temperature annealing produces small grains (50-100 nm), a small quantity and size of voids and higher MgB2 material density

Summary

Introduction

MgB2 superconductors has many advantages, namely high critical temperature (39 K) [1], low resistivity, simple structure, low anisotropy and high critical field [2,3]. The high critical temperature of MgB2 enables these superconductors to operate using liquid hydrogen or cryocoolers [4] This significantly reduces the cost of the use and application of these wires. MgB2 in un-doped form has several weaknesses which inhibit its use: lack of sufficient point pinning centers (normal areas of thickness similar to the length of coherence and dislocations) needed for high Jc in medium and high magnetic fields [5], and large voids with an inhomogeneous distribution and weak connections between grains [6]. HIP creates dislocations, eliminates voids, produces small grains and small normal area, increases connections between grains, and increases the density and homogeneity of the MgB2 material [13,14]

Results

Discussion

Conclusion