Mechanisms for improving critical currents in powder-in-tube MgB2 wires and tapes

A.SERQUIS; L.CIVALE; G.SERRANO; B.MAIOROV; M. T. MALACHEVSKY

Viena, Austria

Conferencia; European Conference on Applied Superconductivity EUCAS; 2005

The strong potential for commercial applications of MgB$_2$ is dueto a unique combination of characteristics, such as the hightransition temperature $T_c \sim 39~$K, the chemical simplicity,the lightweight and the low cost of raw materials. In addition,the absence of weak-link behavior at grain boundaries inpolycrystalline samples allows the use of simple powder in tube(PIT) methods to fabricate wires and tapes. Several groups haveachieved an improvement of the transport properties in a magneticfield that is substantially larger than the low irreversibilityfield ($H_irr$) of the clean basic compound, making MgB$_2$ acandidate to replace NbTi or Nb$_3$Sn in magnets. The introductionof more pinning centers and the connectivity enhancement of grainsare both necessary to increase $H_irr$ and the critical currentdensity ($J_c$) at intermediate temperatures, which can be reachedby both liquid He as well as commercial cryocoolers. Thesefeatures can be achieved in many ways, i.e. by doping ornanoparticle addition, and effective heat treatments such as hotisostatic pressing. In this work we present the results of theoptimization of several processing parameters to fulfill theserequirements not only in short wires and tapes but also in thefabrication of coils. We characterized MgB$_2$ with differentadditions (e.g.C, Al and SiC) prepared as wires and tapes by PIT,with commercial stainless steel and Ti sheaths. Both sheathmaterials have high hardness and chemical compatibility withMgB$_2$, necessary to obtain high $J_c$ values, while Ti also hasthe lightweight advantage. The magnetization and transportproperties were measured at temperatures between 4 and 26 K. Aseries of samples were prepared by different mechanic deformationprocedures and sintered at different temperatures between 700øCand 950øC. The superconducting properties such as $J_c$ and$H_irr$ were correlated with the fabrication conditions.