A novel route to prepare bulk superconductors : Spark Plasma Sintering and Texturing
Résumé
The unconventional sintering process called Spark Plasma Sintering (SPS) was used to prepare superconducting MgB2 cryo-magnets. The role of the starting powder on the superconducting properties of MgB2 has been investigated. Three sets of bulk MgB2 material were processed from : (i) commercial available powder, (ii) a mixture of Mg metal and amorphous B using a single-step solid-state reaction process and (iii) a mixture of amorphous boron coated with carbon and Mg metal. The samples were prepared by varying different SPS processing conditions such as temperature, dwell time, applied pressure and atmosphere. The structural and microstructure of the samples were investigated and correlated to their superconducting properties. The best sample was prepared at 850°C. At 20K its critical current density was Jc = 500 kA/cm2 , while the trapped field measured at the surface of a 20 mm diameter disk was equal to 3.9 T. The dependence on temperature of the levitation force was investigated on MgB2 disks with various diameters and thicknesses. The levitation force took the same values from 17 to 32 K supporting the claim by E.H. Brandt [1] that the screening currents in disks flow at first along the outside edge of the samples. Otherwise, the Tc determined from these measurements was equal to 38 K, in good agreement with measurements by other techniques. Overall these results suggest that bulk MgB2 superconductors could be a viable variant for magnetic levitation and cryo-magnet applications.
The measurement of the properties of single phase Bi2Ca2Sr2CuO8 superconductor ceramics consolidated using new route referred as “Spark Plasma Texturing” (SPT) is in progress and will be also discussed.
[1] E.H.Brandt, Phys.Rev.58, 6506 (1998)