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, J(c), and irreversibility field, Hi,, 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 x 10(5) A cm(-2) in self-field and mu(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 of the MgB2 was dramatically shortened by sodium carbonate doping, and consequently the mu(0)H(c2) was enhanced to approximately 29 T at the highest doping level of x = 0.10.