Transport critical current densities and n factors in mono- and multifilamentary MgB2/Fe tapes and wires using fine powders

Mono- and multifilamentary MgB₂/Fe tapes and wires with high transport critical current densities have been prepared using the powder-in-tube (PIT) process. The fabrication details are described. The effect of powder grain sizes and recrystallization temperature on j_c has been investigated. At 25 K and 1 T, j_c values close to 10⁵ A/cm² were measured, while j_c of 10⁶ A/cm² were extrapolated for 4.2 K/0T in our monofilamentary tape. MgB₂/Fe tapes exhibit high exponential n factors for the resistive transition: n≈80 and 40 were found at 5 T and 7 T, respectively. The highest transport j_c values obtained so far in MgB₂/Fe wires with 7 filaments were 1.1×10⁵ A/cm² at 4.2 K and in a field of E2 T, which is still lower than for monofilamentary tapes. The function F_p∝b^p·(1-b)^q has been established over the whole field range, and exhibits a maximum at F_p≅0.18. Improved deformation and recovering processing is expected to lead to higher j_c values.