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Effects of different reactions on composition homogeneity and superconducting properties of Al-doped MgB2

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A primary difference between pure MgB2 and its alloyed forms is that the former is a line compound and, once formed, has the same composition everywhere, whereas the latter is a solid solution and requires diffusion to move alloying elements. Since defect energies are high, this opens up the possibility that alloying elements might not be distributed homogeneously, which could have important consequences for the observed superconducting properties. To address this issue, two sets of Mg1-xAlxB2 samples, with 0≤x≤0.45, were prepared from elements using reaction temperatures and times at opposite extremes of those typically reported in the literature. Sample set A was given a reaction of 1 h at 850 °C, which stopped just short of completion, while sample set B was reacted at temperatures as high as 1200 °C and thoroughly annealed for over 80 h. The trace reactants remaining after reaction A indicated that Al is taken up more slowly than Mg, thereby making compositional gradients likely. Indeed, Williamson–Hall analyses of x-ray diffraction peaks showed that set A had higher crystalline strain than set B when x>0 but not when x = 0. Since the presence of Al correlated with increased strain only for set A, it was concluded that reaction A produced substantial Al gradients across the individual grains while reaction B did not. Magnetization and heat capacity measurements indicated good bulk superconducting properties for all samples despite their structural differences, and consistent trends were observed when each sample set was considered alone. However, when both sets were considered together, their behaviour was distinct when plotted versus x (e.g. two Tc(x) curves), with trends for set A being shifted toward higher x than for set B. On the other hand, all of the data merged (e.g. one Tc(v) curve) when analysed in terms of the unit cell volume v. Thus, while the first analysis might suggest that the different reactions produced different superconducting behaviour, the second analysis, which captures the average Al content actually present inside the grains, shows that the samples have common behaviour intrinsic to the addition of Al. Moreover, these analyses show that it is important to coordinate structural and property characterizations to remove artifacts of composition gradients and uncover the intrinsic trends. Because the standard characterizations of the superconducting properties above gave no clear indication that the two sample sets had different homogeneities, the structural information was vital to make a correct assessment of the effects of Al doping on superconductivity. Since many investigations have used reactions similar to reaction A and did not analyse data in terms of structural changes, previous results should be interpreted cautiously.

Document Type: Research Article

DOI: http://dx.doi.org/10.1088/0953-2048/18/11/001

Publication date: November 1, 2005

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