Influence of the lamella structure and high isostatic pressure on the critical current density in in situ MgB2 wires without a barrier

In this article, the significant impact of a lamellar (layered) structure and a high isostatic pressure on the normal state resistance (R-n), critical temperature (T-c), irreversible magnetic field (B-irr), upper magnetic field (B-c2) and critical current densities (J(c)) at 4.2 K and 20 K was presented. Our research showed that annealing at temperatures in the range of 630 degrees C-680 degrees C (above the melting point of Mg) at atmospheric pressure (0.1 MPa) did not create a lamellar (layered) structure. This led to low T-c, J(c) and B-irr, and high R-n. The analysis, made by using scanning electron microscopy (SEM), showed that the annealing temperature increased up to 700 degrees C under a pressure of 0.1 MPa, which created a lamellar structure. This led to significant growth of T-c, J(c) and B-irr and a slight increase of R-n. Moreover, the measurements showed that annealing at temperatures from 630 degrees C to 700 degrees C did not change the B-c2 value. In comparison to pressureless heat treatment, annealing under the high isostatic pressure of 1.1 GPa obtained a lamellar structure with layers of lower thickness and higher density. This led to significant increases in J(c) and B-irr and a visible reduction of R-n. SEM analysis showed that the increase of isostatic pressure up to 0.3 GPa created a lamellar structure with thicker layers and lower density. This microstructure led to lower J(c) and B-irr and significantly higher R-n. On the other hand, the SEM analysis showed that annealing under 0.8 GPa did not cause the formation of a layered structure, and as a result, it led to significant reductions in B-irr and J(c) (4.2 K and 20 K) and higher R-n. The increase of the isostatic pressure from 0.1 MPa to 1.1 GPa did not affect T-c (B = 0 T) and B-c2. The results indicated that the layered structure obtained a high density of pinning centers, which were particularly effective at higher magnetic fields. J(c) of 100 A/mm(2) in 8 T at 4.2 K was obtained in in situ undoped MgB2 wires after annealing at 700 degrees C for 40 min under an isostatic pressure of 1.1 GPa. (C) 2018 Elsevier B.V. All rights reserved.