Enhanced critical current properties observed in Na2CO3-doped MgB2

A significant improvement of the critical current properties in MgB2 bulk has been attained by sodium carbonate doping. A series of Mg1-2xB2(Na2CO3)x bulk samples with x = 0-0.1 were prepared by solid-state reaction in sealed stainless tubes. Both the critical current density, Jc, and irreversibility field, Hirr, at 20 K were systematically improved up to x = 0.055 and decreased monotonically by excess doping, while their Tcs were continuously decreased with an increase of x. The sample with x = 0.055, having a slightly decreased Tc of 37.6 K, recorded the best critical current performance at 20 K with Jc of 3.8 x 105 A cm-2 in self-field and μ0Hirr of approximately 6 T. Both small particles of MgO and a carbon-containing local region, Mg(B,C)2, are believed to act as effective pinning centres, resulting in an enhancement of the flux pinning force. In addition, the coherence length ξ of the MgB2 was dramatically shortened by sodium carbonate doping, and consequently the μ0Hc2 was enhanced to approximately 29 T at the highest doping level of x = 0.10