Significant improvement in critical current densities of C-doped MgB2 tapes made by high-energy ball milling

C-doped MgB2 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 Jc values of MgB2 tapes and reduce the anisotropy of the Jc. At 4.2 K, the transport Jc of the 80 h milled C-doped MgB2 tapes sintered at 800 C reaches 4.3 X 10^4 A cm^2 at 10 T and 2.2X10^4 A cm^2 at 12 T, 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 MgB2 lattice, but also the flux pinning, by increasing grain boundaries, both of which result in the enhancement of Jc performance. Moreover, the high MgB2 phase fraction and large density of milled MgB2 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 MgB2 for practical applications.

The effect of ball milling time on the grain size, microstructure, Jc, flux pinning and Bc2 of MgB2 tapes made from crystalline B is investigated. It is found that the long milling time can decrease the MgB2 grain size, which is proved by the results of XRD and TEM. At the same time, the XRD analysis shows that the substitution level of B by C in the MgB2 lattice is increased. Subsequently, the pinning force and Bc2 are improved. The 80 h milled C-doped MgB2 tape samples sintered at 800 C have the highest in-field Jc values, which reach 2.2 X 10^4 A cm^2 at 4.2 K and 12 T, six times larger than that of the 10 h samples. The significantly enhanced Jc performance of the ball milled MgB2 tapes is mainly attributed to the improved Bc2 and pinning force. Furthermore, the SEM images show that lengthy ball milling can result in a high MgB2 phase fraction and large density of MgB2 tapes, both of which are beneficial for the grain linkage of the ball milled MgB2.

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