Fluoride doping and Boron particle size effect on MgB2−xFx superconductor

Although electron doping is not favorable for hole charge carrier superconductors, in this article we report the effect of most electronegative dopant F-anion on MgB2 to show what are the limitations of this kind of doping. The samples of the general formula MgB2−xFx where x=0.0, 0.04, 0.08, and 0.16 mol% were prepared via elemental solid state reaction of stoichiometric amounts at a sintering temperature of 970 °C under argon pressure. All starting materials were taken as pure elements except F-ion taken as MgF and extra magnesium compensates the lack of evaporated and unreacted magnesium during diffusion mechanism producing doped-MgB2. The X-ray measurements proved that the MgB2−xFx system mainly belongs to a hexagonal single phase with P6/mmm space group. The lattice parameters a and c-axes exhibited length elongation with increasing Fluoride doping from x=0.04→0.16, respectively. Scanning electron microscopy was investigated and proved that decreasing starting material particle size for the boron increases the average grain boundaries in the boron bulk and consequently enhances the diffusion mechanism of Mg and F-ions through the B-matrix. The DC-electrical resistivity measurements of the parent MgB2 was found to be ∼38.6 K. At the meantime the doped samples showed a slight decrease in Tcs with x=0.04 mol% (Tc=31 K) and sharp decrease with 0.08≤x≤0.16 mol% (Tcs=22 and 16 K), respectively.