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In this work, we investigated the influence of isostatic pressure (of 0.1 MPa and 1.1 GPa) during heat treatment, doping, size and shape of crystalline boron grains, and annealing time at low annealing temperature (570 °C) on the formation of high-field pinning centers in MgB2 wires. The results indicate that high isostatic pressure (1.1 GPa) at low annealing temperature (570 ° C) and annealing time of 120 min significantly increases the density of high-field pinning centers in MgB2 wires with small and large boron grains. Transport measurements show that an increase of annealing time from 120 to 210 min at 1.1 GPa slightly decreases the critical current density (Jc) and irreversibility field (Birr) in MgB2 wires with large boron grains, suggesting that longer annealing time weakly affects the density of high-field pinning centers. On the other hand, for MgB2 wires with small boron grains, the annealing time of 210 min significantly reduces J c and B irr . This indicates that the longer annealing time at 1.1 GPa significantly reduces the density of high-field pinning centers. Our studies indicate that dislocations created by the hot isostatic pressure process significantly increase B irr in the temperature range from 5 to 12 K. On the other hand, strains due to the shrinkage of MgB2 material increase B irr in the temperature range from 12 to 34 K. The results show that small grains of crystalline boron and high isostatic pressure lead to a high density of dislocations and strains during transformation to the MgB2 phase. [ABSTRACT FROM AUTHOR]

We show transport results of the critical temperature ( T c ), irreversible magnetic field ( B irr ), upper critical field ( B c2 ), critical current density ( J c ) and the pinning force ( F p ) for undoped MgB 2 wires without barriers. The results of SEM show that the hot isostatic pressure process increases the density and uniformity of MgB 2 materials. Hot isostatic pressure allows for obtaining small grains and increasing the number of connections between the grains, accelerated reaction rate and limits the diffusion of Fe atoms into MgB 2 material from the adjacent sheath. The pressure of 1.1 GPa increases T c quite significantly in high magnetic field range of 6–12 T, improves the B irr and B c2 and increase the J c at 4.2 K and 20 K of about three times. Our results may be important for this field since many MgB 2 wires are made from big grains of boron greater than 1 μm. Moreover, our research can be important for the production of MgB 2 wires for the International Thermonuclear Experimental Reactor with boron 11 B. [ABSTRACT FROM AUTHOR]