Enhanced critical current properties observed in Na2CO3-doped MgB2

A significant improvement of the critical current properties in MgB 2 bulk has been attained by sodium carbonate doping. A series of Mg 1−2 x B 2 (Na 2 CO 3 ) x bulk samples with x = 0–0.1 were prepared by solid-state reaction in sealed stainless tubes. Both the critical current density, J c , and irreversibility field, H irr , at 20 K were systematically improved up to x = 0.055 and decreased monotonically by excess doping, while their T c s were continuously decreased with an increase of x . The sample with x = 0.055, having a slightly decreased T c of 37.6 K, recorded the best critical current performance at 20 K with J c of 3.8 × 10 5 A cm −2 in self-field and μ 0 H irr 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 MgB 2 was dramatically shortened by sodium carbonate doping, and consequently the μ 0 H c 2 was enhanced to approximately 29 T at the highest doping level of x = 0.10.