Increased flux pinning force and critical current density in MgB₂ by nano-La₂O₃ doping

MgB₂ superconducting wires and bulks with nano-La₂O₃ addition have been studied. A series of MgB₂ superconducting bulk samples with nano-La₂O₃ addition levels of 0, 5, 7, 18wt% were prepared. AC resistivity data showed slight increases of Bc and unchanged B for the bulk samples with doping levels lower than 7 wt% and decreased critical fields for the heavily doped (18 wt%) bulk. X-ray diffraction (XRD) showed the presence of LaB₆ in the nano-La₂O₃ doped MgB₂ bulk samples and decreased MgB₂ grain size in nano-La₂O₃ doped bulks. Monocore powder-in-tube (PIT) MgB₂ wires without and with 5 wt% nano-La₂O₃ addition (P-05) were prepared for transport property measurement. 2mol%C-doped Specialty Materials Inc. (SMI) boron powder was used for wire P-05 and previously prepared control wires (control wires were made without the addition of nano-La₂O₃ powder, W-00 and P2). Low field magnetic properties were obtained from magnetization loop (M-H), transport critical current density (J ) was measured at 4.2 K for the nano-La₂O₃ doped PIT wire (P-05) and the control samples (P2 and W-00). The transport critical current density J (B) of P-05 at 4.2 K and 8 T (6.0 ×10⁴ A/cm²) was twice that of the control wire. The critical magnetic fields (Bc and B ) of P-05 and the control sample P2 were compared. The critical fields of P-05 were slightly less than those of P2. Kramer-Dew-Hughes plots indicated a change from surface pinning to a mixture of volume pinning and surface pinning. It is shown that enhancement of P-05's transport properties is due to additional flux pinning by the fine-size rare-earth borides rather than enhanced Bc or B .