Field cooling of a MgB2 cylinder around a permanent magnet stack: prototype for superconductive magnetic bearing

The behaviour of bulk superconductors as levitators of permanent magnets (PMs) has been extensively studied for the textured YBCO high-temperature superconductor material, in the temperature range lower than 77 K, obtaining extremely high trapped fields but also experiencing limitations on the mechanical characteristics of the material and on the possibility to produce large objects. Alternatively, bulk MgB2, even if it is superconducting at lower temperatures, has fewer mechanical problems, when fully densified, and presents stable magnetization in the temperature range between 10 and 30 K. With the reactive Mg-liquid infiltration technique we have produced dense MgB2 bulk cylinders of up to 65 mm diameter and 100 mm height. This kind of cylinder can be consider as a prototype of a passive magnetic bearing for flywheels or other rotating electrical machines. We have conductively cooled one of these superconducting cylinders inside a specially constructed cryostat, and the levitation forces and stiffness, with respect to axial movements of various arrangements of the PM, have been measured as a function of the temperature below T-c. We verified the very stable characteristics of the induced magnetization after several cycles of relative movements of the PM and the superconducting cylinder.