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

Abstract 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 °C–680 °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 °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 °C to 700 °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 MgB 2 wires after annealing at 700 °C for 40 min under an isostatic pressure of 1.1 GPa.