Effect of ball milling time on the substitution of carbon in glucose doped MgB 2 superconductors: Dispersion behavior of glucose

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The effect of the ball milling time (BMT) on the substitution of the carbon in the glucose doped MgB 2 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 MgB 2 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 MgB 2 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 MgB 2 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 T C with an increasing BMT further confirms a progressive substitution of the carbon in the MgB 2. 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 MgB 2 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 MgB 2 during the solid-state reaction.