Effect of ball milling time on the substitution of carbon in glucose doped MgB2 superconductors: Dispersion behavior of glucose

The effect of the ball milling time (BMT) on the substitution of the carbon in the glucose doped MgB2 samples is investigated here. Using in situ solid state reaction, four different doped samples of Mg(B.98C.02)2 were prepared by mixing powders of Mg, boron and glucose for 2 h, 4 h, 8 h and 12 h using planetary ball milling. A reference sample of un-doped MgB2 was also prepared under same conditions. The particle size distribution of the un-reacted samples show a decrease in the particle size as the BMT is increased. Both the average particle size as well as the standard deviation show a substantial decrease with the increase in the milling time up to 8 h. After 8 h, the size reduction is rather insignificant. From the XRD data, the crystallite size of the doped MgB2 computed using the Scherrer formula was found to decrease with the increasing BMT, showing a saturation level after 8 h of the milling time. TEM images also confirm the crystallite size obtained from the XRD data. The substitution of the C in the MgB2 lattice, measured from the change in the c/a ratio, increases with increasing BMT. The maximum carbon substitution is achieved at approximately 8 h of BMT. Moreover, a systematic enhancement of the residual resistivity and a decrease in TC with an increasing BMT further confirms a progressive substitution of the carbon in the MgB2. These results suggest that a minimum ball milling time is necessary to disperse the glucose uniformly for a maximum substitution of nano C in the B plane of MgB2 lattice. The optimum BMT is found to be 8 h. Thus, the decrease in the particle size due to the ball milling enhances the dispersion of the constituent materials thereby favoring a greater substitution of the dopant in the MgB2 during the solid-state reaction.

► The effect of ball milling time on the dispersion of glucose in MgB2 was systematically studied using XRD, SEM and TEM.
► Establish the ball milling time for the substitution of C in MgB2 using glucose and quantify its amount.
► Uniform dispersion of the glucose in MgB2 is necessary to get the optimum benefit of the C substitution.