Peculiarities in the carbon substitution of MgB2

To examine the role of kinetics on C substitution into MgB2, heat treatments at 900 degreesC on stoichiometric quantities of Mg, B and C forming the MgB2-xCx series, have been carried out for various times and for different carbon fractions, under an external Ar pressure of 50 Bar. The variation in the superconducting transition temperatures on account of C incorporation into the MgB2 matrix has been measured using the onset of diamagnetism and the appearance of zero resistance using the a.c. susceptibility and resistivity measurements carried out in the 4-300 K temperature range. The phase purity of the samples and the lattice parameters have been measured by XRD measurements. The microstructure has been characterized by scanning electron microscope observations. The heat treatments carried out on MgB2-xCx for a nominal composition of x = 0.3, indicate that for shorter periods, the superconducting transitions are broad, and with increase in the heat treatment time the transition widths narrow and the onset of diamagnetism shifts to higher temperature reaching a value of 35 K at 50 h. The a-lattice parameter shows a corresponding increase with heat treatment time and also attains a constant value between the 24 and 50 h treatments. These results reveal that the extent of carbon substituted into the MgB2 matrix at shorter times is large whereas for longer annealing times the matrix is depleted in carbon content, with carbon content in the superconducting volume becoming more uniform. These prolonged heat treatments were carried out for several nominal compositions of carbon and the measured T-C decreases to less than 20 K for large carbon contents. The T-C versus a-lattice parameter obtained from this work, shows a correlation in agreement with similar measurements on single crystals. (C) 2004 Elsevier B.V. All rights reserved.