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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