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

Abstract

In this paper we present results of transport critical current density (J c ) at 20 K and 4.2 K, irreversible magnetic field (B irr ), upper critical field (B c2 ), critical temperature (T c ), pinning force (F p ), scanning pinning force scaling results (F p /F pmax and B/B irr ) and electron microscope (SEM) images of un-doped MgB 2 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 MgB 2 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 T c above 27 K and increases T c and B irr below 25 K. Un-doped MgB 2 wire annealed in 1 GPa for 15 min at 680°C at has a 20 K, 4.5 T J c of 100 A/mm 2 in and a B irr of 7 T. At 4.2 K, this wire has J c of 100 A/mm 2 at 10.5 T. Scaling results show that the dominant pinning mechanism is point pinning for undoped MgB 2 wires under 1 GPa pressure and annealed at 680°C (at 20 K).

Experimental research of high field pinning centers in 2% C doped MgB2 wires at 20 K and 25 K

“…This is confirmed by the study of Serquis et al 18 We believe that this effect decreases the T c and the critical parameters above 25 K and increases the critical parameters below 16 K. Based on the results in Fig. 2 and the results for the undoped MgB 2 wires, 19 we can deduce that a higher level of C doping and a higher pressure will cause a further reduction of critical parameters at higher temperatures (20 K-39 K). This indicates that a pressure of 1 GPa and a small amount of dopant are the optimum conditions.…”

“…It also leads to a reduction in the T c . Our study on the undoped MgB 2 wires showed that 1 GPa pressure decreases the T c by 1.5 K, increases the critical parameters in the range of 10 K to 27 K, and decreases the critical parameters above 27 K. 19 The similar influence of the isostatic pressure to that of C doping on the T c may indicate that the isostatic pressure creates a lattice distortion. This is confirmed by the study of Serquis et al 18 We believe that this effect decreases the T c and the critical parameters above 25 K and increases the critical parameters below 16 K. Based on the results in Fig.…”

“…Only a hot isostatic pressure (HIP) makes it possible to obtain a homogeneous distribution of dislocations. 18,19 A homogeneous distribution of dislocations is very important, because it allows us to obtain a homogeneous distribution of the high field pinning centers. This leads to a high critical current density in the high magnetic fields.…”