Critical current density improvements in MgB₂ superconducting bulk samples by K₂CO₃ additions  

MgB₂ bulk samples with potassium carbonate doping were made by means of reaction of elemental Mg and B powders mixed with various amounts of K₂CO₃. The T_c of the superconducting phase as well as its a-axis parameter were decreased as a result of carbon doping. Potassium escaped the samples during reaction. The critical current density of MgB₂ was improved both in self field and under applied magnetic field for T ≤ 30 K, with optimum results for 1 mol% K₂CO₃ addition. The normalized flux pinning force (f(b)) shows that the flux pinning mechanism at low field is similar for all samples, following the predictions of the point pinning model. In contrast the behavior of f(b) is significantly altered at reduced fields (b) larger than unity by K₂CO₃ additions, tending towards surface pinning. Besides providing carbon, another effect of K₂CO₃ may originate from the presence of a transient liquid phase that appears to improve the crystallinity and thus the critical current density at low field.