Mechanisms for Improving Critical Current in Powder-in-Tube MgB2 Wires and Tapes

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

Vienna

Conferencia; 7th European Conference on Applied Superconductivity; 2005

EUCAS

The strong potential for commercial applications of MgB$_2$ is due to a unique combination of characteristics, such as the high transition 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 in polycrystalline samples allows the use of simple powder in tube (PIT) methods to fabricate wires and tapes. Several groups have achieved an improvement of the transport properties in a magnetic field that is substantially larger than the low irreversibility field ($H_{irr}$) of the clean basic compound, making MgB$_2$ a candidate to replace NbTi or Nb$_3$Sn in magnets. The introduction of more pinning centers and the connectivity enhancement of grains are both necessary to increase $H_{irr}$ and the critical current density ($J_c$) at intermediate temperatures, which can be reached by both liquid He as well as commercial cryocoolers. These features can be achieved in many ways, i.e. by doping, nanoparticle addition, effective heat treatments such as hot isostatic pressing and combinations of these methods. In this work we present the results of the optimization of several processing parameters to fulfill these requirements not only in short wires and tapes but also in the fabrication of coils. We characterized MgB$_2$ with different additions (e.g.C, Al and SiC) prepared as wires and tapes by PIT, with commercial stainless steel and Ti sheaths. Both sheath materials have high hardness and chemical compatibility with MgB$_2$, necessary to obtain high $J_c$ values, while Ti also has the lightweight advantage. The magnetization and transport properties were measured at temperatures between 4 and 26 K. A series of samples were prepared by different mechanic deformation procedures and sintered at different temperatures between 700 and 950$^{circ}$C. The superconducting properties such as $J_c$ and $H_{irr}$ were correlated with the fabrication conditions.