Al-alloyed MgB2: correlation of superconducting properties, microstructure and chemical composition

Al-alloyed MgB2 was prepared by solid state reaction of Mg1-xAlx alloy and crystalline B powder particles. The nominal composition of the samples was Mg1-xAlxB2, with x = 0, 0.1, 0.2, 0.3. Residual resistivity (rho(0)) of the samples measured by four-probe technique is comparable to the single crystals, indicating the excellent quality of these samples. The lattice parameters c and a decrease at a rate of 1.15 pm and 0.17 pm per at.% Al, respectively. Superconducting transition temperature (T-c) determined by measuring the resistive superconducting transition, decreases with increasing Al alloying at a rate of 1.56 K/at.% Al. Microstructure and chemical composition have been investigated by scanning electron microscopy (SEM), electron probe microanalysis (EPMA), and transmission electron microscopy (TEM). Al is incorporated into MgB2 grains of similar to 1 mu m size by substitution of Mg lattice sites with Al. Al was found to be distributed inhomogenously which partially explains the broadening of the superconducting transition width (Delta T-c) with increasing Al mole fraction. Al alloyed samples contain large (similar to 15 mu m) and small (similar to 2 mu m) secondary phases embedded in the (Mg, Al)B-2 matrix. The composition of large secondary phases is found to be (Mg,Al)B7+delta, with the mole fraction of excess boron (delta) increasing from 0.99 at.% in pure sample to 4.14 at.% in highest alloyed sample. The Al to Mg mole fraction ratio in these large secondary phases is about half of that in the matrix. The size and density of the large secondary phases increased with increase in Al mole fraction. The secondary phases constitute less than 4% of the total sample volume and are thus not likely to affect the bulk superconductivity.