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

Abstract

The behaviour of bulk superconductors as levitators of permanent magnets (PMs) has beenextensively studied for the textured YBCO high-temperature superconductormaterial, in the temperature range lower than 77 K, obtaining extremely hightrapped fields but also experiencing limitations on the mechanical characteristics ofthe material and on the possibility to produce large objects. Alternatively, bulkMgB2, even if it is superconducting at lower temperatures, has fewer mechanical problems, whenfully densified, and presents stable magnetization in the temperature range between 10 and30 K. With the reactive Mg-liquid infiltration technique we have produced denseMgB2 bulk cylinders of up to 65 mm diameter and 100 mm height. This kind of cylindercan be consider as a prototype of a passive magnetic bearing for flywheels orother rotating electrical machines. We have conductively cooled one of thesesuperconducting cylinders inside a specially constructed cryostat, and thelevitation forces and stiffness, with respect to axial movements of variousarrangements of the PM, have been measured as a function of the temperature belowTc. We verified the very stable characteristics of the induced magnetization afterseveral cycles of relative movements of the PM and the superconducting cylinder.