Increase of the critical current density of MgB2 superconducting bulk samples by means of methylene blue dye additions

Highlights: • MgB₂ bulk samples were doped with C using methylene blue trihydrate. • All the carbon present in the additive is introduced in the MgB₂ lattice. • The critical current density is improved with optimum at 1 wt.% addition. • The main flux pinning mechanism at high field might be modified by doping. -- Abstract: MgB₂ bulk samples were made by means of reaction of elemental Mg and B powders with or without additions of methylene blue trihydrate, a carbon source, which melts at high temperature upon decomposition. The T_c of the MgB₂ phase as well as its a-axis parameter were decreased as a result of carbon doping, whereas the c-axis parameter only shows a weak tendency for increasing. The critical current density of MgB₂ was improved both in self-field and under applied magnetic field for T ≤ 25 K, the best results being obtained for 0.5–1.0 wt.% methylene blue trihydrate addition. Based on normalized flux pinning force (f(b)) versus reduced field (b) plots, it was found that the flux pinning mechanism at low reduced field is similar for all samples, with a behavior close to the predictions of the point pinning model. However, for b larger than 1, the behavior of f(b) is significantly altered in the methylene blue trihydrate added samples and tends towards the surface pinning model. Methylene blue trihydrate is an efficient carbon source and furthermore provides a liquid phase between 200 °C and 450 °C, which may have some influence on the critical current density improvements at low field.