Spark Plasma Synthesis and Sintering of Superconducting MgB2-Based Materials

Superconducting (SC) and mechanical properties of spark plasma (or SPS) produced MgB2 -based materials allow their efficient applications in fault current limiters, superconducting electromotors, pumps, generators, magnetic bearings, etc. The synthesized from Mg and B at 50 MPa, 1050 degrees C for 30 min material has a density of 2.52 g/cm(3), critical current density, j(c) = 7.1.10(5) A/cm(2) at 10 K, 5.4.10(5) A/cm(2) at 20 K, and 9.10(4) A/cm(2) at 35 K in zero magnetic field; at 20 K its field of irreversibility B-irr(20)=7 T and upper critical field B-c2(20)=11 T; microhardness H-V=10.5 GPa and fracture toughness K-IC =1.7 MPa.m(1/2) at 4.9 N-load. SPS-manufactured in- situ MgB2-based materials usually have somewhat higher j(c) than sintered ex-situ. The pressure variations from 16 to 96 MPa during the SPS-process did not affect material SC characteristics significantly; the jc at 10-20 K was slightly higher and the material density was higher by 11%, when pressures of 50-96 MPa were used. The structure of SPS-produced MgB2 material contains Mg-B-O inclusions and inclusions of higher borides (of compositions near MgB4, MgB7, MgB12, MgB17, MgB20), which can be pinning centers. The presence of higher borides in the MgB2 structure can be revealed by the SEM and Raman spectroscopy.