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 H2, 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 ∼20 nm in size and collect into lacy agglomerates in the 1-50 μm size range. Pressed pellets of these powders heated in a Mg atmosphere are transformed to the MgB2 phase at temperatures ranging from 600 °C to 1200 °C. Critical current densities, Jc, at 5 K run from about 106 A/cm2 at self-field to 4 × 104 A/cm2 at 7 T. At 20 K, Jc ranges from 5 × 105 A/cm2 at self-field to about 104 A/cm2 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 Jc 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 105 A/cm2 at 5 K and 2 T.