Plasma synthesized doped B powders for MgB2 superconductors

Gas phase plasma synthesis methods have been used to prepare nano-scale particles of C doped boron powder that are well suited for powder-in-tube (PIT) fabrication of superconducting wire. Mixtures of H-2, BCl3, and CH4 gas are injected into an induction plasma where doped boron powder is formed and collected on a stainless steel screen. The particles typically are similar to 20 nm in size and collect into lacy agglomerates in the 1-50 mu m size range. Pressed pellets of these powders heated in a Mg atmosphere are transformed to the MgB2 phase at temperatures ranging from 600 degrees C to 1200 degrees C. Critical current densities, J(c), at 5 K run from about 10(6) A/cm(2) at self-field to 4 x 104 A/cm(2) at 7 T. At 20 K, Jc ranges from 5 x 10(5) A/cm(2) at self-field to about 10(4) A/cm(2) at 3 T. Samples of pure B powder and B with 7.4%C powder were prepared as PIT wires having a combination Fe inner jacket and a cupro-nickel outer jacket. These PIT J(c) values are lower than, but comparable to the pressed pellet values. The PIT samples can be reacted in times on the order of 10 min and still exhibit greater than 10(5) A/cm(2) at 5 K and 2 T. (c) 2005 Elsevier B.V. All rights reserved.