Significant improvement in critical current densities of C-doped MgB ₂ tapes made by high-energy ball milling

C-doped MgB ₂ tapes were fabricated using crystalline boron as one of the ball milled precursor powders. It was found that the ball milling process can strongly enhance the J _c values of MgB ₂ tapes and reduce the anisotropy of the J _c. At 4.2K, the transport J _c of the 80h milled C-doped MgB ₂ tapes sintered at 800°C reaches 4.3×10 ⁴Acm ² at 10T and 2.2×10 ⁴Acm ² at 12T, respectively. These values are even higher than for tapes made from amorphous B powders. Ball milling can improve not only the upper critical field, by increasing the C substitution level in the MgB ₂ lattice, but also the flux pinning, by increasing grain boundaries, both of which result in the enhancement of J _c performance. Moreover, the high MgB ₂ phase fraction and large density of milled MgB ₂ tapes are beneficial to the active area. These results indicate that the use of crystalline B in combination with a ball milling process may be an economic method to harness the excellent properties of MgB ₂ for practical applications.