Samples of Al-alloyed MgB2 were prepared by the solid-state reaction of Mg1-xAlx and crystalline B powder. The nominal composition of the samples was Mg1-xAlxB2, with x = 0, 0.1, 0.2 or 0.3. The microstructure and chemical composition were investigated by using scanning electron microscopy, electron probe microanalysis and transmission electron microscopy. The Al concentration of the matrix was close to, but less than, the nominal Al concentration and the difference increased with increasing nominal Al concentration. The Al was incorporated into MgB2 grains of about 1μm in size by the substitution of Mg lattice sites with Al. The latter was found to be distributed inhomogeneously, which partially explained a broadening of the superconducting transition width, ΔTC, with increasing Al mole fraction. The Al-alloyed samples contained large (about 15μm) and small (about 2μm) secondary phases embedded in the (Mg,Al)B2 matrix. The composition of large secondary phases was found to be (Mg,Al)B7+δ, with the mole fraction of excess B (δ) increasing from 0.99at% in the pure sample to 4.14at% in the highest alloyed sample. The Al to Mg mole fraction ratio in these large secondary phases was about half of that in the matrix. The size and density of the large secondary phases increased with increasing Al mole fraction. The secondary phases constituted less than 4% of the total sample volume, and were thus not likely to affect the bulk superconductivity. The diffusion of Mg, and evaporation, governed the formation of secondary phases and could be explained by considering the effects of annealing temperature, annealing time and B powder particle size. The residual resistivity of pure MgB2 (x = 0) and Al-alloyed MgB2 (x = 0.1, 0.2, 0.3), as measured by using the 4-probe technique, was comparable to that of single crystals; thus indicating the excellent quality of the samples. The lattice parameters, c and a, decreased at the rate of 1.15pm and 0.17pm/at%Al, respectively. The superconducting transition temperature, as determined by measuring the resistive superconducting transition decreased, with increasing Al alloying, at a rate of 1.56 K/at%Al.

Al-Alloyed MgB2 - Correlation of Superconducting Properties, Microstructure and Chemical Composition. B.Birajdar, T.Wenzel, P.Manfrinetti, A.Palenzona, M.Putti, O.Eibl: Superconductor Science and Technology, 2005, 18, 572-81